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ScienceDirect

Geochimica et Cosmochimica Acta 249 (2019) 17–41

A melt inclusion study on volatile abundances in the lunar mantle

Peng Ni (倪鹏) a,⇑, Youxue Zhang (张有学) a, Sha Chen (陈沙) a, Joel Gagnon b

aDepartment of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USAbDepartment of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada

Received 22 December 2017; accepted in revised form 23 December 2018; Available online 08 January 2019

Abstract

Earth’s Moon was thought to be highly depleted in volatiles due to its formation by a giant impact. Over the last decade,however, evidence has been found in apatites, lunar volcanic glass beads, nominally anhydrous minerals and olivine-hostedmelt inclusions, to support a relatively ‘‘wet” Moon. In particular, based on H2O/Ce, F/Nd, and S/Dy ratios, recent meltinclusion (MI) work estimated volatile (H2O, F, and S) abundances in lunar rocks to be similar to or slightly lower thanthe terrestrial depleted mantle. Uncertainties still occur, however, in whether the limited numbers of lunar samples studiedare representative of the primitive lunar mantle, and whether the high H2O/Ce ratio for pyroclastic sample 74220 is due tolocal heterogeneity. In this paper, we report major element, trace element, volatile, and transition metal data in MIs for 5mare basalt samples (10020, 12040, 15016, 15647 and 74235) and a pyroclastic deposit (74220).

With our new lunar MI data, H2O/Ce ratios are still found to vary significantly among different lunar samples, from �50for 74220, to �9 for 10020, �3 for 74235, 1.7 to 0.9 for 12008, 15016, and 15647, and 0.5 for 12040. H2O/Ce ratios for thesesamples show positive correlation with their cooling rates, indicating a possible effect of post-eruptive loss of H on their H2O/Ce variations. It is evident that most other lab and lunar processes, including loss of H2O during homogenization, mantlepartial melting, magma evolution, and ingassing during or post eruption are unlikely the causes of high H2O/Ce variationsamong different lunar samples. By comparing ratios of F/Nd, S/Dy, Zn/Fe, Pb/Ce, Cs/Rb, Rb/Ba, Cl/K, Na/Sr, Ga/Lu,K/Ba, and Li/Yb between 74220 and other lunar samples, the possibility of 74220 originating from a volatile-enriched hetero-geneity in the lunar mantle can also be excluded. With all the above considerations, we think that the H2O/Ce ratio for 74220best represents the pre-degassing lunar basaltic melt and primitive lunar mantle, either because it was formed by a rapid erup-tion process, or it was sourced from a deeper part of the lunar mantle that experienced less degassing H2O loss during lunarmagma ocean crystallization. With an H2O/Ce ratio of �50, the primitive lunar mantle is estimated to contain �84 ppm H2O.

Comparing volatile abundances in melt inclusions, glassy embayments, and glass beads in 74220 yields the followingvolatility trend for volcanic eruptions on the lunar surface:

H2O� Cl � Zn � Cu � F > S � Ga � Cs > Rb � Pb > Na > K � Li.Using the melt inclusion data obtained thus far, the volatile depletion trend for the Moon from a MI perspective is esti-

mated. Our results show that most of the volatile elements in the lunar mantle are depleted relative to the bulk silicate Earthby a factor of 2 to 20, however, a good correlation between half condensation temperature and the volatile depletion trend isnot observed. The relatively flat pattern for the lunar volatile depletion trend requires a lunar formation model that can rec-oncile the abundances of these volatiles in the lunar mantle.� 2018 Elsevier Ltd. All rights reserved.

Keywords: Moon; Melt inclusions; Volatiles; Water; Lunar mantle

https://doi.org/10.1016/j.gca.2018.12.034

0016-7037/� 2018 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Geophysical Laboratory, CarnegieInstitute of Washington, Washington, DC 20015, USA.

E-mail address: pni@carnegiescience.edu (P. Ni).

18 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

1. INTRODUCTION

Early geochemical data and other considerations led tothe hypothesis that the Moon was formed by a giant impactbetween the proto-Earth and a large (probably Martiansize) planetary body (e.g. Hartmann and Davis, 1975;Cameron and Ward, 1976). In the past ten years, magmaticwater has been detected and reported in variable types oflunar samples, including volcanic glass beads (e.g. Saalet al., 2008, 2013; Chen et al., 2015), apatites (e.g. Boyceet al., 2010, 2014; McCubbin et al., 2010a,b; Greenwoodet al., 2011; Barnes et al., 2014; Tartese et al., 2014),anorthosites (Hui et al., 2013; Hui et al., 2017), andolivine-hosted melt inclusions (hereafter referred to asMIs, e.g. Hauri et al., 2011, 2015; Chen et al., 2015; Niet al., 2017a). Among these types of samples, lunar meltinclusions are able to provide direct evidence of high H2Oconcentrations in the pre-eruptive magma (up to1410 ppm, Hauri et al. 2011). In addition, based on H2O/Ce, F/Nd, and S/Dy ratios in lunar MIs, Chen et al.(2015) estimated the primitive lunar mantle to contain110 ppm H2O, 5.3 ppm F, and 70 ppm S, similar to orslightly lower than the terrestrial depleted mantle. Despitethe powerful role of lunar MIs in supporting a relatively‘‘wet” Moon, uncertainties still exist in lunar MI studiesthat complicate the interpretation of volatile abundancesin the lunar mantle. In particular, Chen et al. (2015) foundthat H2O/Ce ratios are much higher for natural MIs from74220 than homogenized MIs from other lunar samples.To rule out the possible complexity of diffusive H2O lossfrom MIs during homogenization in the lab, Ni et al.(2017a) conducted a systematic study and concluded thatsuch an effect is minimal if the homogenized MIs are largerthan �45 mm in diameter. After excluding the effect of dif-fusive H2O loss during homogenization, the large variationsbetween 74220 and other lunar samples still exist. On theother hand, 74220 is currently the only lunar sample withhigh H2O/Ce ratios. This sample, however, has been knownas a ‘‘volatile-rich” pyroclastic deposit since its return bythe Apollo 17 mission (e.g. Meyer et al., 1975; Butler andMeyer, 1976; Moynier et al., 2006; Paniello et al., 2012).Therefore, some studies (Albarede et al., 2013; Albaredeet al., 2015) argued that 74220 might be a local anomalyoriginated from a volatile-enriched part of the lunar mantle,which should not be used to represent the bulk Moon. Toresolve the controversy, it is necessary to investigate meltinclusions from a larger collection of lunar samples, andto verify whether 74220 is sourced from a local heterogene-ity in the lunar mantle.

The discovery of primitive H2O in the Moon is having asignificant impact on the Giant Impact Hypothesis for itsorigin. Before 2013, the high H2O concentration was foundin mare basalts younger than 3.9 Ga, which permits a‘‘bone-dry” Moon at the time of formation from the GiantImpact because there was sufficient time for H2O to begradually added after the Giant Impact (Hauri, 2013;Tartese and Anand, 2013). At the time Hui et al. (2013)published results that the lunar magma ocean containedhundreds of ppm H2O, it was thought to be difficult tomaintain the Giant Impact Hypothesis for Moon formation

(Hauri, 2013). However, it turned out that the GiantImpact Hypothesis is resilient and flexible enough toaccommodate the new discoveries. For example, one sug-gested solution is that right after the Giant Impact, therewas a gas disk enveloping the newly formed proto-Moon,and dissolution of H species (mostly OH) from the gas diskinto the lunar magma ocean is enough to establish lunarH2O abundance (Pahlevan et al., 2016; Sharp, 2017).Another suggested solution is that the upper parts of theMoon-forming disk are dominated by an atmosphere ofheavy atoms or molecules, leading to inefficient diffusion-limited H loss, allowing the Moon to retain H2O(Nakajima and Stevenson, 2018). A third solution isasteroidal/cometary bombardment during the lunar magmaocean stage that replenished H2O in the lunar mantle(Hauri et al., 2015, 2017; Barnes et al., 2016). These devel-opments highlight the importance of establishing the abun-dances of not only H2O but also other volatiles in the Moonin setting stringent constraints on the origin of the Moon.

In this paper, we extend previous lunar melt inclusionstudies to a broader collection of lunar mare basalt andpyroclastic deposit samples (10020, 12040, 15016, 15647,74220, and 74235) to better understand volatile abundancesin the lunar mantle. One main purpose is to address theprimitive H2O/Ce ratio for the Moon. Furthermore, inaddition to volatile elements (H2O, F, Cl, S) that are typi-cally analyzed in lunar melt inclusions, effort was also madeto precisely measure moderately volatile elements (Li, Na,K, Cu, Zn, Ga, Rb, Cs, and Pb) for a systematic compar-ison between 74220 and other lunar samples, as well as abroader understanding of the volatile depletion trend forthe Moon, to provide further constraint on the origin ofthe Moon.

2. SAMPLE PREPARATION AND METHODS

2.1. Lunar samples studied

Olivine-hosted melt inclusions in three low-Ti basalts(12040, 15016 and 15647), two high-Ti basalts (10020 and74235), and one pyroclastic deposit (74220) investigatedin this work are briefly described below (Lunar SampleCompendium unless otherwise noted). A summary of theirbulk compositions is also shown in Table 1. More detailedbackground information about these samples can be foundfrom the Lunar Sample Compendium: https://www-cura-tor.jsc.nasa.gov/lunar/lsc/index.cfm.

10020,49: A fine-grained (with an average grain size of�200 mm) low-K, high-Ti ilmenite basalt containing olivinephenocrysts with a composition of Fo77-59 with partiallyglassy MIs (Chen et al., 2015).

12040,199: A slowly cooled low-Ti olivine basalt with anaverage grain size of �1 mm and olivine compositions rang-ing from Fo63 to Fo45 and averaging Fo58. Evidence ofaccumulation of olivine has been reported (e.g. Walteret al., 1971; Walker et al., 1976; Bombardieri et al., 2005),which indicates that the whole rock composition of12,040 might not be representative of its parental liquid.

15016,47: A highly vesicular (�50% vesicles in volume)and medium-grained olivine-normative basalt returned

Tab

le1

Whole

rock

data,

majorelem

ents,vo

latile

elem

ents,non-volatile

traceelem

ents,an

dtran

sitionmetal

dataforallthemeltinclusionsan

dglassbeadsan

alyzed

inthisstudy.

Moderatelyvo

latile

elem

entconcentrationsbylaser-ab

lationIC

P-M

Sarereported

separatelyin

Tab

le2.

Lunar

sample

#

1002

012

040

1501

615

421

1564

774

220

Typ

eWhole

rock

*

Natural

Whole

rock

*

Homogenized

Whole

rock

*

Homogenized

Glass

bead

Whole

rock

*

Homogenized

Whole

rock

*

Natural

Embay

ment

Glass

bead

HomoT

(�C)

N/A

1210

1200

1180

1140

1160

1160

1270

1300

1287

1300

1280

1280

N/A

Homot

(min)

N/A

108

1010

1010

210

210

102

N/A

Sam

ple#

NMI5

OL13

OL15

aOL16

bOL36

bOL41

OL43

OL8

OL10

OL12

aOL19

GL7

OL3

OL6

OL11

OL8b

Emb

OldOL2E

mb

GL5

Host

ol

Fo#

6951

5455

4457

4765

7057

6560

4481

8080

Host

ol

dia(mm)

402

324

596

515

274

153

553

228

517

223

253

368

443

250

330

394

MIdia

(mm)

2334

7022

2831

3525

2931

4945

4062

SiO

2(w

t

%)

38.57

54.57

41.68

44.88

41.57

48.03

46.28

46.49

45.43

43.56

48.01

48.3

45.12

44.26

45.79

44.4

46.83

42.12

38.57

38.3

39.13

38.83

39.23

TiO

210

.69

3.6

2.66

46.17

2.79

3.08

3.68

4.21

2.09

2.13

1.94

2.13

2.04

0.4

2.44

1.76

3.24

8.81

10.29

11.73

9.77

8.78

Al 2O

39.23

13.46

7.45

7.93

7.9

9.26

8.42

10.6

9.4

8.07

9.77

9.12

9.2

8.15

7.15

97.74

6.71

6.32

8.09

7.57

6.14

5.50

FeO

19.34

5.55

23.05

25.71

26.04

22.47

25.34

20.33

24.28

23.29

17.22

19.04

23.64

24.5

20.37

22.36

24.26

32.82

22.04

22.75

22.79

23.18

22.56

MnO

0.27

0.09

0.27

0.25

0.27

0.27

0.27

0.23

0.25

0.28

0.21

0.24

0.25

0.34

0.27

0.28

0.26

0.35

0.3

0.31

0.31

0.3

0.28

MgO

7.89

1.68

16.40

6.4

6.81

4.71

4.34

5.94

5.28

11.76

7.87

8.6

6.01

8.1

16.8

10.04

8.52

4.53

14.44

6.68

6.04

10.97

14.72

CaO

11.25

19.23

8.03

8.95

8.74

10.91

9.42

11.1

9.3

9.33

11.31

10.41

10.04

9.05

8.17

9.65

8.96

8.16

7.68

9.97

10.22

8.07

7.05

Na 2O

0.39

0.56

0.21

0.26

0.32

0.33

0.36

0.46

0.39

0.25

0.26

0.24

0.27

0.35

0.11

0.22

0.23

0.24

0.36

0.38

0.42

0.31

0.31

K2O

0.06

0.09

0.04

0.06

0.18

0.05

0.09

0.1

0.14

0.04

0.04

0.03

0.05

0.04

0.01

0.04

0.05

0.08

0.09

0.07

0.08

0.06

H2O

(ppm)

–43

4–

13.4

14.2

7.1

14.4

12.2

14.0

–12

.012

.015

.130

.1–

–11

15.4

–99

420

–

F–

108.0

–38

521

014

950

157

833

1–

63.2

48.0

57.3

41.4

––

27.7

82.9

–45

.18.3

–

P–

709

–53

995

134

741

949

829

1–

311

286

397

281

––

227

382

–12

2.8

119.9

–

S†

–49

5–

867

2788

960

1113

1269

926

–90

783

490

583

3–

–74

515

07–

602

375

–

Cl

–10

.98

–3.83

10.75

4.29

5.18

7.38

3.61

–3.44

3.19

3.5

3.17

––

2.44

5.57

–3.44

0.39

–

Li�

–10

.2–

8.6

4.7

–5.1

4.8

4.5

––

2.7

4.2

27.5

––

4.0

6.2

–11

.510

.1–

Na

–44

72–

2301

2676

–30

6534

8430

79–

–20

9220

3127

73–

–17

6319

96–

3051

2877

–

K–

843

–37

711

94–

635

718

1037

––

209

288

211

––

319

580

–52

956

8–

Sr

–17

5–

9618

8–

128

145

116

––

9394

92–

–85

105

–22

126

122

4–

Y–

150.2

–17

1.1

122.0

–50

1.0

196.5

197.4

––

29.3

28.8

32.0

––

24.5

43.6

–54

.468

.758

.3–

Zr

–32

9–

125

376

–16

118

313

2–

–10

111

610

6–

–88

183

–20

926

722

7–

Nb

–21

.4–

7.9

22.4

–9.9

11.7

7.8

––

7.2

7.0

6.4

––

5.5

10.9

–14

.720

.316

.8–

Ba

–12

4–

6119

2–

8595

62–

–42

5850

––

4280

–66

8979

–

La

–15

.1–

5.7

16.4

–9.4

9.5

6.4

––

5.3

6.2

6.3

––

4.5

9.8

–7.1

9.4

7.3

–

Ce

–46

.3–

17.2

46.8

–28

.627

.318

.6–

–15

.719

.317

.4–

–12

.025

.7–

21.3

28.0

23.0

–

Pr

–7.4

–2.7

7.3

–5.5

4.4

3.0

––

2.0

2.2

2.8

––

1.8

3.8

–3.28

4.74

3.95

–

Nd

–40

.9–

15.2

36.3

–40

.124

.418

.1–

–12

.212

.013

.3–

–8.5

19.7

–21

2923

–

Sm

–15

.6–

6.7

11.1

–27

.310

.98.9

––

4.1

2.7

3.7

––

2.4

6.5

–6.6

10.2

8.3

–

Eu

–3.2

–1.2

1.6

–1.9

1.2

2.0

––

0.7

3.2

0.7

––

0.4

0.6

–2.1

3.6

3.0

–

Gd

–23

.6–

13.7

16.6

–53

.918

.215

.6–

–4.8

6.3

6.9

––

3.8

7.6

–9.2

11.9

7.9

–

Tb

–3.3

–3.4

2.9

–12

.94.5

3.6

––

0.9

1.0

0.8

––

0.7

1.4

–1.3

2.3

1.7

–

Dy

–29

.9–

28.4

21.2

–95

.633

.830

.3–

–6.4

6.3

5.7

––

4.9

8.4

–12

.514

.411

.7–

Ho

–6.2

–5.9

4.5

–20

.07.7

7.4

––

1.2

0.9

1.5

––

0.87

1.6

–2.0

3.3

2.1

–

Er

–20

.2–

19.8

13.5

–52

.020

.921

.6–

–3.4

3.4

2.9

––

2.3

4.8

–5.4

7.6

6.2

–

Tm

–2.7

–2.4

1.6

–5.8

2.6

3.5

––

0.4

0.23

0.5

––

0.31

0.60

–0.8

1.2

0.8

–

Yb

–16

.6–

17.0

11.0

–36

.815

.721

.8–

–2.8

3.4

2.3

––

1.8

3.7

–5.5

6.9

5.0

–

Lu

–2.8

–2.3

1.8

–4.8

2.4

3.0

––

0.6

0.7

0.2

––

0.43

0.44

–0.7

1.0

1.0

–

V–

––

–70

.4–

3063

18.9

––

141.6

133.5

115.8

––

150.7

111

––

––

–

Cr

––

––

2918

–17

3525

1611

36–

–88

3565

2564

39–

–89

4143

08–

––

––

Mn

––

––

3301

–21

2921

6122

22–

–28

6028

2338

33–

–31

2338

91–

––

––

Co

––

––

73–

2221

43–

–28

46.5

58.4

––

6712

9–

––

––

Ni

––

––

21–

2.1

1.1

2.2

––

–15

.256

––

25.4

18–

––

––

Cu

––

––

52–

1615

24–

–14

.026

.64.3

––

28.5

72–

––

––

P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41 19

Lunar

sample

#74

235

Typ

eWhole

rock

*Homogenized

Natural

Interstitial

glass

HomoT

(�C)

1285

1285

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

Homot(m

in)

22

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

N/A

Sam

ple

#OL2

OL4

NMI1a

NMI1b

NMI1b’

NMI3b

NMI4

EMB1

EMB3

NMI5

NMI6

NMI7

NMI11

Matrix1

aMatrix1

bMatrix2

@1

Matrix2

@3

Matrix2

@4

Host

olFo#

7272

7272

7272

7572

7271

7169

71

Host

oldia(mm)

237

240

192

192

192

278

541

147

133

185

130

105

165

MIdia

(mm)

5145

2136

3614

5845

3714

1235

24

SiO

2(w

t%)

38.62

39.12

38.04

50.36

43.21

–53

.15

50.33

49.46

51.69

51.17

52.39

50.97

52.18

56.56

56.56

––

–

TiO

212

.17

11.77

12.45

3.23

12.13

–3.07

6.15

3.94

3.6

5.77

3.2

4.34

3.99

82.34

2.34

––

–

Al 2O

38.61

8.45

8.18

13.46

11.17

–14

.15

12.89

12.58

12.94

13.55

13.95

12.59

13.51

16.15

16.15

––

–

FeO

19.32

20.16

22.16

10.1

15.5

–8.38

9.57

11.14

10.33

98.12

10.71

9.59

12.37

12.37

––

–

MnO

0.28

0.27

0.31

0.21

6–

0.14

0.14

–0.17

0.15

0.14

0.19

0.19

0.18

0.18

––

–

MgO

8.35

7.24

6.53

2.39

2.74

–2.27

3.08

3.17

2.72

2.08

2.39

2.29

2.31

1.78

1.78

––

–

CaO

10.7

10.21

9.95

17.4

13.4

–17

.22

17.12

17.21

16.72

17.37

17.98

17.03

16.73

9.8

9.8

––

–

Na 2O

0.4

0.46

0.32

0.56

0.51

–0.65

0.63

0.55

0.56

0.6

0.58

0.57

0.63

1.13

1.13

––

–

K2O

0.07

0.07

0.06

0.05

0.1

–0.11

0.1

0.11

0.09

0.09

0.1

0.09

0.11

0.18

60.18

6–

––

H2O

(ppm)

–21

.253

.572

.976

.175

.857

.957

.257

9158

.663

.988

.468

108

125

8811

511

6

F–

46.4

48.9

65.1

64.0

68.1

54.8

51.0

53.9

54.8

62.0

57.9

75.7

8810

0.1

138.1

9110

810

9.1

P–

172

199

365

319

347

299

317

354

298

329

299

356

317

347

361

320

373

375

S†

–15

2616

4314

9095

2754

763

1145

1566

1338

1037

810

1187

675

–1138

––

–

Cl

–3.70

–3.8

3.64

3.81

3.23

3.52

3.66

4.8

4.15

3.69

4.06

3.38

5.5

6.3

5.2

6.8

7.28

Li�

–62.6

7.7

6.3

5.3

––

6.1

7.0

8.8

––

–11

.811

.612

.711

.511

.310

.2

Na

–41

5436

6737

1434

88–

–53

6350

8350

12–

––

5255

8008

7873

5156

5547

4760

K–

554

614

636

570

––

885

869

981

––

–81

211

7311

3778

589

773

5

Sr

–18

917

718

820

1–

–24

523

721

4–

––

241

371

320

312

288

285

Y–

99.2

102

111

128

––

144.6

141

110

––

–15

313

4.7

140.6

105.9

104.2

115.8

Zr

–30

625

427

032

1–

–37

436

838

2–

––

384

405

506

296

293

299

Nb

–28

.028

.129

.524

.9–

–30

.428

.061

––

–30

.342

.157

.733

.035

.734

.7

Ba

–89

6591

95–

–11

111

010

4–

––

108

179

149

121

121

118

La

–6.1

6.5

7.4

9.0

––

10.0

11.0

7.7

––

–11

.214

.712

.212

.311

.011

.6

Ce

–22

.225

.927

.629

.8–

–32

.835

.228

––

–35

.944

.939

.038

.735

.639

.3

Pr

–4.0

4.4

4.8

5.8

––

6.3

5.7

4.5

––

–6.9

7.3

6.8

6.5

5.9

7.1

Nd

–24

.323

.629

34–

–37

.635

28–

––

3842

.941

.135

.534

.339

Sm

–9.6

8.4

1113

––

14.8

1512

––

–17

14.5

14.6

13.5

11.7

13.6

Eu

–0.9

3.0

20.8

––

2.3

43

––

–5

1.9

3.5

2.9

2.8

2.5

Gd

–14

.912

.016

17–

–18

.719

19–

––

2119

.419

.615

.513

.916

.4

Tb

–2.5

2.7

3.4

4.0

––

4.0

3.7

3.6

––

–4.1

3.6

3.9

2.8

2.7

3.3

Dy

–19

.619

.623

.826

.0–

–26

.825

.424

––

–31

.925

.326

.819

.720

.021

.8

Ho

–4.1

3.9

4.9

5.7

––

6.1

4.7

4.8

––

–6.2

5.0

6.0

3.7

4.3

4.3

Er

–11

.910

.814

.816

.3–

–15

.616

.613

.3–

––

17.0

15.2

16.6

12.1

11.8

12.9

Tm

–1.8

1.8

1.6

2.3

––

2.1

2.3

2.0

––

–2.3

2.2

2.17

1.8

1.9

2.0

Yb

–11

.611

.214

.913

.8–

–14

.914

.212

.0–

––

17.0

15.4

16.7

12.3

11.9

12.9

Lu

–2.0

1.4

2.3

2.1

––

2.0

2.6

2.4

––

–2.7

2.0

2.6

2.0

1.7

2.1

V–

58.5

141

9.5

16.3

––

25.0

26.6

52–

––

–56

17.9

––

–

Cr

–32

9851

6999

762

2–

–14

7912

0627

78–

––

–21

3315

96–

––

Mn

–33

7332

2319

3614

75–

–27

4617

7224

51–

––

–29

1230

36–

––

Co

–7.6

2310

23–

–39

1817

––

––

1727

––

–

Ni

–1.1

0.3

0.6

2–

–2

0.9

0.8

––

––

0.7

0.8

––

–

Cu

–7.9

10.6

4.3

4–

–2.9

54

––

––

117

––

–

*Whole

rock

dataarefrom

Rhodes

etal.(197

4),Rhodes

etal.(197

6),Ryd

eran

dSchuraytz(200

1),an

dHalliset

al.(201

4).

†SconcentrationsfornaturalM

Isin

74235could

beaff

ectedbysulfideglobulesin

theMIs.ItalicizedSconcentrationsarecorrectedbyexcludingthecycles

affectedbysulfides

andareless

reliab

le.

�Italicized

Liconcentrationsareforhomogenized

MIs,which

havetheconcern

ofcontamination

duringheating.

Lithium

concentrationsforoneMIin

7423

5areevidentlyaff

ected

by

contaminationan

dhence

marked

inred.ThisMIisclose

toolivinesurface,

hence

more

easily

affectedbyLicontaminationan

dH

loss

duringhomogenization.

20 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41 21

from Apollo 15. Olivine crystals identified in this samplehave compositions ranging from Fo69 to Fo58.

15647,22: A relatively coarse-grained olivine basalt fromApollo 15 containing anhedral olivine crystals that are<1 mm in diameter and have compositions of Fo62 to Fo45.

74220,892: A soil sample containing abundant orangeand black glass beads, and individual grains of olivine withcompositions of �Fo80. Hauri et al. (2011), Saal et al.(2013), Chen et al. (2015), and Ni et al. (2017a). In thisstudy, one additional MI, two embayments, and a glassbead from 74220 are analyzed.

74235,22: A vitrophyric high-Ti basalt from Apollo 17that contains thin blades of ilmenite and phenocrysts of oli-vine within a matrix of glass and feathery minerals. Thissample represents a rapidly quenched volcanic liquid. Com-positions of olivine crystals in 74235 are within a narrowrange of Fo75-70.

In addition to the lunar samples described above, onegreen glass bead from 15421 is also studied for comparison.

2.2. Sample preparation methods

For 74220 and 15421, no crushing was required andglass beads or olivine crystals were directly picked. Eachof the other mare basalt samples was gently crushed in astainless-steel crusher, and olivine grains were manuallypicked from the crushed samples using an optical micro-scope. Among the samples, a small number of MIs identi-fied in 10020 and 74235 were partially glassy. In addition,interstitial glasses with measurable sizes (>10 mm by10 mm) were found in the matrix of 74235. These sampleswere directly polished to expose the target MI or interstitialglass, and prepared into indium mounts for electron micro-probe (EMP), and secondary ion mass spectrometry(SIMS) analyses. The glass beads from 74220 and 15421(250 mm and 480 mm in diameter, respectively) were directlypolished to expose the center of the beads and then pressedinto indium mounts for EMP and laser ablation inductivelycoupled plasma mass spectrometry (LA-ICP-MS) analyses.

All melt inclusions found in 12040, 15016, and 15647 arehighly crystalline, requiring homogenization experiments toproduce a homogeneous glass phase for EMP and SIMSanalyses. Therefore, homogenization experiments were con-ducted on MIs from 12040, 15016, and 15647 in a gas-mixing furnace. Crystalline inclusions from 74235 were alsohomogenized using the same procedure. Each host olivinegrain containing unexposed MIs was placed in a graphitecrucible, and gradually inserted into the hot spot of the fur-nace. The olivine crystal was protected during heating usinga constant flow of 99.9999% pure N2 to avoid being oxi-dized by O2 or contaminated by H2O in the ambient atmo-sphere. The use of a graphite crucible also ensures areducing environment (IW-1.9 to IW-2.6, Ni et al.,2017a), slightly more reducing than that for mare basalts(�IW-1, Sato, 1976; Weitz et al., 1997). Such a reducingcondition during homogenization is slightly below the sta-bility line for olivine (IW-0.3 to IW-1.1, Nitsan, 1974).Reduction of the olivine, however, was found to be slow,with the reduced layer being only 10 to 25 mm thick after30 minutes at 1330 �C (Ni et al., 2017a), mitigating the

concern of olivine reduction in our short homogenizationexperiments (2 to 10 min). One small olivine crystal in74235 with visible reduction was discarded. By insertingthe crucible slowly, heating rate for the sample was con-trolled at <200 �C/min to prevent olivine cracking. Thehomogenization temperatures (1140 to 1300 �C) were cho-sen to be near the liquidus of the mare basalts. The dura-tions (2–10 min) were chosen as a compromise tominimize H2O loss (short duration) and to equilibrate themelt inclusions with the host olivine (long duration). Afterhomogenization, the crucible was quickly removed from thefurnace and immediately quenched in water. Direct mea-surements show that the crucibles are quenched to below200 �C in less than 20 seconds. The olivine grain was recov-ered from the graphite crucible, polished to expose the meltinclusion interior, and pressed into an indium mount forsubsequent analyses.

2.3. Analytical methods

Major element compositions of MIs and olivine crystalswere determined using a CAMECA SX-100 electron micro-probe at the University of Michigan, with a 10 nA focusedbeam at the accelerating voltage of 15 kV. The analyticalprocedure is similar to that described in Ni et al. (2017a).

Volatile and trace element compositions for the MIswere determined using a CAMECA IMS 7-f GEO SIMSat Caltech during four visits. A total of 33 elements wasanalyzed for the MIs in three separate sessions: volatiles(H reported as H2O, F, S, and Cl) and P, transition metals(V, Cr, Mn, Co, Ni, and Cu), and other trace elements (Li,Na, K, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb and Lu). Analyses were conducted fol-lowing procedures as in Chen et al. (2015) and Ni et al.(2017a), which are briefly described below. For any givenMI, volatiles were analyzed in the first session, followedby other trace elements, and then the transition metals, withall three analytical sessions conducted during the same lab-oratory visit.

In the first session, 12C, 16O1H, 18O, 19F, 30Si, 31P, 32S,and 35Cl were sequentially analyzed using a 3–5 nA, 15-mm-diameter Cs+ primary ion beam. A set of seven MPI-DING glass standards (GOR128-G, GOR132-G, KL2-G,ML3B-G, StHs6/80-G, T1-G, ATHO-G, Jochum et al.,2006) and a mid-ocean ridge basalt glass (MORB) was usedfor calibration of H2O, F, Cl, P, and S. Concentrations ofH2O in the reference glasses are from Chen et al. (2015)and Ni et al. (2017a). For F, Cl, P, and S, reference valuesfor MPI-DING reference glasses from Jochum et al. (2006)were used. A synthetic basaltic glass, Et1 (Ni and Zhang,2016) with 12.8 ± 0.6 ppm H2O was used to assess 16O1-H�/18O� ratios for low H2O samples across differentindium blocks and during different analytical sessions.The sample blocks were placed in the high vacuum chamberfor one or two days prior to volatile measurements, andbackground 16O1H�/18O� ratios of 1 � 10�3 to 3 � 10�3

were achieved. To make sure the beam was on the meltinclusion instead of its host olivine, beam position was care-fully adjusted based on 27Al16O� secondary ion imagesbefore acquisition of each data point. Each position was

22 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

pre-sputtered for 60–120 s to remove possible surface con-tamination before the data acquisition was started. Twentycycles of data were collected at each point and in each cycle,and every ion species was counted for 1 s. Uncertainties areapproximately 20% (2r) for H2O and Cl, and approxi-mately 10% (2r) or lower for F, P and S, as indicated byanalyses of standard glasses Et1 and GOR 128 under stablevacuum and comparable analytical conditions. In addition,two analyses on the same MI (74235 NMI1b) yieldedresults that are within 10% difference for H2O, F, P, andCl (Table 1), supporting the reproducibility of our analysis.

In the second session of analyses, twenty-two trace ele-ments (Li, Na, K, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm,Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were analyzedwith a 10 to 14 nA O� and 15 mm diameter primary ionbeam using the energy-filtering technique (c.f. Zinner andCrozaz, 1986). The position of trace element analysis foreach MI was carefully located on or near the previous spotof volatile measurements with the help of 27Al+ ion images.In addition to ion species for the target trace elements, 28Siwas also monitored as the internal standard. Two glassstandards (NIST 610 and NIST 612, Pearce et al., 1997)from the National Institute of Standards and Technologywere used for the trace element analyses. Average 2r ana-lytical errors given by the instrument are less than 10%for Li, Na, K, Sr, Y, Zr, Ba, and Ce, 10 to 40% for Nb,La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Yb, approxi-mately 50% for Tm and Lu, and approximately 100% forEu. Multiple analyses on NIST 612 during different sessionsreproduced concentrations to <10% difference for Li, K, Zr,Nb, Sm, Eu, and Gd, and to <5% for the other trace ele-ments analyzed. During one of the three visits, one addi-tional NIST standard (NIST 614, Gao et al., 2002) andtwo MPI-DING glass standards (GOR128-G and KL2-G,Jochum et al., 2006) were used as secondary standards toverify our analyses, and the results are within 2r analyticalerrors compared to the reference values (Ni et al., 2017a).

Six transition metal elements (V, Cr, Mn, Co, Ni, andCu) were analyzed in the third session using a 12 or17 nA O� primary ion beam that was 15 mm in diameter.A mass resolution power (MRP) of 6000 V was used inthe analysis to separate target ion species from possibleinterferences (e.g. 47Ti16O� on 63Cu� and 49Ti16O� on65Cu�). 28Si� or 30Si� was used as the internal standard,and two isotopes for Ni (60Ni and 62Ni) and Cu (63Cuand 65Cu) were monitored to check for possible mass inter-ferences. Three MPI-DING glasses (GOR128-G, GOR132-G and T1-G), which are more similar to our MIs in majorelement composition, were used as the standards for transi-tion metal measurements, because strong matrix effectswere observed when analyzing Co, Ni and Cu (Supplemen-tary Fig. S1). Average 2r analytical errors are 5% or lowerfor V, Cr and Mn, approximately 10% for Co and Cu, andapproximately 30% for Ni. Repeated analyses on glass stan-dard NIST 612 during different SIMS sessions reproducedall transition metal concentrations to about 5%.

Concentrations of Li, Be, Na, Ni, Cu, Zn, Ga, Rb, Cs,and Pb for one melt inclusion (74220 OL11), one embay-ment (74220 OldOL2Emb), and two glass beads (74220GL5 and 15421 GL7) were analyzed by LA-ICP-MS at

the University of Windsor using a PhotonMachines Ana-lyte Excite 193 nm, short-pulse-width (sub 4 ns), Ar-F exci-mer laser ablation system coupled with an Agilent 7900,quadrupole ICP-MS. A laser beam with an energy densityof 5.19 J/cm2, and a repetition rate of 25 Hz was used forthe analyses. Spot size of the laser beam was 40 mm forthe melt inclusion and glass beads. The ellipsoidal-shapedembayment, which measured �280 mm along the long axis,was analyzed using an array of 8 spots, each with a diame-ter of 25 mm, situated along a linear traverse. Quadrupoledwell time was 10 ms per mass for 7Li, 9Be, 23Na, 60Ni,63Cu, 66Zn, 71Ga, 85Rb, 133Cs and 208Pb. Signal intensitiesfor 44Ca and 25Mg were also measured during the analyses,with 44Ca used as the internal standard to correct for differ-ences in ablation rate between standard reference materialsand unknowns, and 25Mg being used to monitor when thelaser beam penetrated the MI and into the host olivine crys-tal. Additional details about the operating conditions of theLA-ICP-MS can be found in Ni et al. (2017b). A time-resolved signal for the LA-ICP-MS analysis of the meltinclusion (74220 OL11) is shown in SupplementaryFig. S2. The laser beam penetrated the host olivine after�28 s of analysis on the MI, which is clearly resolved bythe increase in 25Mg and 60Ni counts, and the decrease in44Ca and 23Na counts in the spectra. A series of 5 MPI-DING glasses (GOR128-G, GOR132-G, KL2-G, ML3B-G, T1-G, Jochum et al., 2006) plus one NIST standard(NIST 614, Gao et al., 2002) were used for external calibra-tion to bracket concentrations in the unknowns and correctfor matrix effects. Calibration curves for all 10 elements areshown in Supplementary Fig. S3. Average 2r analyticaluncertainties are <10% for Na, Ni, and Ga, <15% for Li,Cu, and Rb, <30% for Be and Zn, and <50% for Cs andPb. For a melt inclusion 74220 OL11, Li and Na were mea-sured by both SIMS (Ni et al., 2017a) and LA-ICP-MS,and the results are within 10% relative difference.

3. COMPOSITIONS OF THE MELT INCLUSIONS

AND GLASS BEADS

Homogenized melt inclusions in 15016, 12040, 15647,and 74235 are usually one single glassy phase (Fig. 1aand b), but may contain a bubble, or a particle of Fe metal,or occasionally a spinel crystal. One natural MI in 10020was identified to contain a glassy fraction that is largeenough (e.g. >10 mm) for SIMS analysis (Fig. 1c). Natural,partially glassy MIs found in 74235 are oval or elongated inshape, and usually contain ilmenite crystals andsubmicrometer-sized sulfide globules. Major elements, vola-tiles, non-volatile trace elements, and transition metal con-centrations measured for the homogenized and natural MIsare reported in Table 1, with whole rock data from pub-lished literature for comparison.

3.1. Major elements

In general, major element compositions of the homoge-nized MIs are relatively close to their whole rock composi-tions, except for 12040, whose whole rock composition hasbeen affected by olivine accumulation (e.g. Walter et al.,

Fig. 1. Back-scattered electron images for two homogenized (a, b) and two natural (c, d) MIs in this study. (a) The homogenized MI 12040OL41 is essentially one single glassy phase; (b) Homogenized MI 15016 OL10 contains a tiny Fe metal particle; (c) Natural partially glassy MI10020 NMI5 contains a spinel and a plagioclase crystal; (d) Natural partially glassy MIs discovered in 74235 usually contain ilmenite crystalsand submicrometer-sized sulfide globules.

P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41 23

1971; Walker et al., 1976; Bombardieri et al., 2005). Con-centrations of SiO2 in the homogenized MIs range from39.1 wt% to 48.3 wt%, while MgO concentrations vary from4.3 to 8.6 wt%. On the other hand, major element compo-sitions for glass in partially glassy MIs in 10020 and74235 are significantly more evolved than their parentalrocks. For example, glass in a partially glassy MI (10020OL15) contains 54.6 wt% SiO2 and 1.7 wt% MgO, com-pared to 40.8 wt% SiO2 and 7.0 wt% MgO in the wholerock (Rhodes and Blanchard, 1980), indicating a highdegree of post-entrapment crystallization. Glass in partiallyglassy MIs in 74235 contains 43.2–52.4 wt% SiO2 and 2.1–3.2 wt% MgO compared to 38.6 wt% SiO2 and 8.4 wt%MgO in the whole rock (Rhodes et al., 1976). Post-entrapment crystallization of ilmenite has a significantimpact on TiO2 concentrations for MIs in 74235, resultingin as low as 3.1 wt% TiO2 in the glass of the natural MIs,compared to the whole rock with 12.2 wt% TiO2. Intersti-tial glasses found in the matrix of 74235 are even moreevolved in chemical composition, with approximately 56.6wt% SiO2, 1.8 wt% MgO, and 2.3 wt% TiO2.

To avoid possible complications caused by homogeniza-tion experiments on the melt inclusion composition, it isimportant to check whether homogenized MIs reachedequilibrium with their host olivines. One way to evaluatewhether equilibrium is reached between an MI and its hostolivine is to calculate the apparent KD values (KD = [Fe/Mg]OL/[Fe/Mg]MI). According to Xirouchakis et al.(2001), the KD between olivine and silicate melt at equilib-rium is �0.33 for low-Ti basalts, and �0.28 for high-Tibasalts. Apparent KD values for the homogenized MIscan depart from the equilibrium values for at least the fol-

lowing reasons: a) inadequate homogenization tempera-ture/duration could result in residual crystal phases in theMIs and cause the apparent KD to be higher or lower thanthe equilibrium value, depending on whether the residualphase is more enriched in Mg or Fe; b) overheating cancause olivine dissolution into the MI, resulting in elevatedKD values; c) post-entrapment loss of Fe through the hostolivine can also result in an elevated apparent KD betweenMI and olivine (Danyushevsky et al., 2000; Bombardieriet al., 2005). As a quality control procedure, only homoge-nized MIs within a small range of KD values close to equi-librium (0.25–0.44 for low-Ti basalts, 0.21 to 0.37 for high-Ti basalts) were selected for SIMS analyses. Additionalhomogenized MIs with KD values outside of the expectedrange were only measured by EMP, and the results arereported in Supplementary Table S1 as a reference.

3.2. Non-volatile trace elements

Bulk silicate Earth (BSE) normalized rare earth element(REE) patterns of all lunar samples in this study are pre-sented in the Supplementary Fig. S4. The patterns are rela-tively flat or bow-shaped, which differ from terrestrialbasalts but are similar to other lunar mare basalts(Shearer et al., 2006). Fig. S4 also shows that various lunarsamples studied here have more divergence in heavy REEthan in light REE, which is very different from terrestrialbasalts. Modeling of the lunar REE patterns (e.g. Nyquistet al., 1977, 1979) indicated that mare-basalts are derivedfrom a fractionated, non-chondritic source, and are mostlikely mafic cumulates from the lunar magma ocean(Shearer et al., 2006). Therefore, the more divergent HREE

24 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

concentrations among different lunar basalts could be a fea-ture inherited from their lunar mantle sources formed dur-ing lunar magma ocean crystallization. Close examinationof Sm/Dy and La/Yb ratios shows that melt inclusions inthe same lunar sample can vary by a factor of 4 in theseREE ratios (Fig. S5). The large variations in Sm/Dy andLa/Yb ratios for an individual sample cannot be explainedby simply crystal fractionation, as the commonly trivalentREE, with a few exceptions (i.e., Ce4+ and Eu2+), are rela-tively similar in compatibility. A more likely explanation isthe mixing of melts with different degrees of partial meltingduring lunar basalt formation, similar to the findings byShimizu (1998) and Slater et al. (2001) for terrestrialbasalts.

Spidergrams for all measured melt inclusions in thisstudy are plotted in Fig. 2. In general, trace element pat-terns for MIs in 10020, 15016, 15647, and 74235 are consis-tent with their whole rock compositions. Trace elementconcentrations for MIs in 12040, however, are significantlyelevated compared to the whole rock composition of 12040(Fig. 2b), which might be caused by olivine accumulation inthe whole rock (e.g. Walter et al., 1971; Walker et al., 1976;Bombardieri et al., 2005). Most samples studied in thiswork show depletion in K, Sr, P, F, and Eu when normal-ized to BSE, which is typical for lunar samples. The deple-tion of K and F agrees with the volatile-depleted nature ofthe Moon (discussed later). One exception is sample 12040,whose MIs do not show obvious depletion in F comparedto terrestrial samples. Depletion of P and other siderophileelements in the Moon was explained by segregation of asmall Fe-rich core (Newsom, 1986). Depletion of Eu andSr, on the other hand, was explained by plagioclase removalfrom the mantle source of mare basalts (e.g. Taylor et al.,2006). The widespread Eu anomaly in essentially all lunarmare basalts has been interpreted as support for a global

Fig. 2. Spidergrams of all measured melt inclusions in this study. Wholeblack or blue solid circles. All other filled symbols are for homogenized Minterstitial glasses in 74235 are plotted as crosses in (d). BSE compositio

lunar magma ocean (Taylor et al., 2006). For the twohigh-Ti basalt samples (10020 and 74235), positive anoma-lies for Ti and Nb are observed, which are attributed toilmenite addition during the formation of high-Ti basalts(Schearer et al., 2006; Chen et al., 2015). Natural partiallyglassy MIs in 10020 and 74235, however, show lowerdegrees of enrichment in Nb and Ti (sometimes depletionfor Ti), indicating precipitation of ilmenite during post-entrapment crystallization (even though ilmenite cannotbe seen in Fig. 1c). For sample 74235, the homogenizedMI shows essentially the same degrees of enrichment ofNb and Ti as the whole rock (Fig. 2d).

3.3. Volatiles (H2O, F, Cl, and S)

Concentrations of H2O, F, Cl, and S for all MIs andglasses measured in this study are plotted in Fig. 3. Thehighest H2O concentration determined in this study is434 ppm, which was found in the natural partially glassyMI in 10020. The other group of partially glassy MIs in74235 contained 51–88 ppm H2O. Interestingly, the moreevolved interstitial glasses in the matrix of 74235 contain91 to 138 ppm of H2O, slightly higher than in the partiallyglassy MIs in 74235. The glasses in the matrix of 74235 alsocontain higher concentrations of non-volatile incompatibleelements than the MIs. These observations together indi-cate retention of H2O in the final stage of magma evolutionfor 74235. All the other homogenized MIs are relativelydry, with less than �30 ppm H2O, similar to the low H2Oconcentrations measured in homogenized MIs in 12008and 15016 by Chen et al. (2015). The H2O/F ratio in natu-ral 74220 MIs is similar to but other MIs have lower H2O/Fratios than terrestrial MIs and the depleted mantle.

In terms of other volatiles in the melt inclusions, Cl andS concentrations are in approximately the same range

rock data from Neal (2001) and Hallis et al. (2014) are plotted inIs, while the open symbols are for natural unheated MIs. Data forn is from McDonough and Sun (1995).

Fig. 3. Comparison of H2O, F, Cl and S concentrations in lunar MIs from this study to those in lunar MIs and glass beads from the literature,and to those in terrestrial MIs. The concentrations and ratios in the depleted mantle (DMM, Salters and Stracke, 2004), CI chondrites andbulk silicate earth (CI and BSE, McDonough and Sun, 1995) are also plotted for comparison. Lunar data from the literature are plotted ingrey, and terrestrial data are plotted in light grey to be distinguished from data obtained in this study. *74220 MI data are from Hauri et al.(2011), Chen et al. (2015) and Ni et al. (2017a). Data for ‘‘Other MIs” in the literature are from Chen et al. (2015). A compilation of literaturelunar MI data is provided in Supplementary Table S2. Data for lunar glass beads are from Hauri et al. (2015). Volatiles data for the terrestrialMIs are from GeoRoc (http://georoc.mpch-mainz.gwdg.de/), except for the MORB MIs, which are from Saal et al. (2002).

P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41 25

(mostly �3 to 11 ppm Cl and �440 to 1500 ppm S) withpublished lunar MI data for 74220 and other lunar samples,and the Cl/S ratios are close to the depleted mantle

(Fig. 3b). Fluorine concentrations in MIs in 10020, 15016,15647, and 74235 are in the range of �30 to �140 ppm,which is also similar to previously published lunar MI data.

26 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

Fluorine concentrations in MIs in 12040, however, are 150to 580 ppm, which are significantly higher than MIs inother lunar samples. MIs in 12040 yield elevated F/S ratiosand lowered Cl/F ratios, as can be seen in Fig. 3b and c.Compared to terrestrial MIs, F concentrations for MIs in

3

12040 are higher than MIs from mid-ocean ridge basalt(MORB) and similar to MIs in ocean island basalt (OIB),suggesting enrichment of F in the parental liquid of 12040.

3.4. Transition metals

Concentrations of first-row transition metals (V, Cr,Mn, Co, Ni, and Cu) in lunar MIs are plotted in Fig. 4and compared with their whole rock and MORB composi-tions. As can be seen in the figure, transition metal concen-tration patterns for the MIs are often consistent with theirassociated whole rock compositions. For 12040, however,Ni concentrations in the MIs are lower than in the wholerock by almost two orders of magnitude (Fig. 4a). Such adifference is likely due to both post-entrapment crystalliza-tion of olivine from the MIs, and the presence of �25%modal olivine in the whole rock of 12040 (Walker et al.,1976). Concentrations of V, Cr and Mn in lunar wholerocks are close to 1 when normalized to BSE. When com-pared to MORB and OIB (shaded region in each panel ofFig. 4), there is typically significant Cr enrichment, minorMn enrichment, and significant Cu depletion in lunar meltinclusions. The minor Mn enrichment is likely related to Feenrichment in lunar basalts compared to terrestrial basalts,which is reflected in the constant Mn/Fe ratios. The signif-icant Cr enrichment in lunar melt inclusions is likely due tothe lower f(O2) on the Moon, so that Cr is less likelyremoved or buffered by spinel. Assuming the transitionmetal abundances in the MIs represent the pre-eruptivemelt composition, the ubiquitous depletion of Cu in lunarmelt inclusions is likely related to the volatile depletion oflunar rocks (discussed in more detail later). Significant Nidepletion and minor V depletion are also occasionallyobserved in lunar MIs, which is likely due to olivinefractionation.

Among the transition metal elements measured in thisstudy, Cu, Co and Ni are chalcophile (e.g. Rajamani andNaldrett, 1978), and their abundances could be affected sig-nificantly by sulfides. For the four lunar samples whosetransition metal abundances were analyzed, sulfides wereonly observed in 74235. Sulfide crystallization caused largesulfur concentration variations in MIs in 74235 (Table 1)and resulted in significantly lower Cu concentrations com-pared to the whole rock (Fig. 4d). The concentrations ofCo and Ni seem less affected by sulfide crystallization, with

Fig. 4. Transition metal concentrations in lunar MIs measured inthis study compared to their concentrations in the whole rock andin terrestrial basaltic glasses. Whole rock data for the lunar samplesare from Morrison et al. (1970), Compston et al. (1971), and Neal(2001). The red and blue shadowed regions show 1r variation inMORB and OIB data, respectively. Global MORB glass data arefrom PetDB (http://www.earthchem.org/petdb) and OIB glass dataare from GeoRoc (http://georoc.mpch-mainz.gwdg.de/). Wholerock data in the databases are excluded to avoid the possible effectof olivine accumulation on Ni concentrations. BSE concentrationsare from McDonough and Sun (1995). The elements are sequencedso that MORB and OIB trend is smooth from slightly incompatibleto highly compatible.

Table 2Concentrations (in ppm) of moderately volatile elements for a melt inclusion (74220 OL11), an embayment (74220 OldOL2Emb) and a glassbead in 74220, as well as a green glass bead in 15421. Average concentrations reported in Hauri et al. (2015) for glass beads in 74220 are alsoincluded as a comparison. Half condensation temperatures (50% Tc) are from Lodders (2003).

Element Zn Pb Cs Rb Na Ga K Cu Li

50%Tc (K) 726 727 799 800 958 968 1001 1037 114274220 OL11 8.4 0.23 0.04 0.77 2954 5.8 529 8.0 12.674220 OldOL2Emb* 1.6 0.21 n/a 0.57 2518 3.2 n/a 2.4 12.074220 GL5 1.5 0.15 0.02 0.47 2146 2.8 n/a 1.17 11.074220 (Hauri15) n/a n/a n/a n/a n/a n/a 515 n/a 11.78GB/MI** 0.22 0.80 0.61 0.75 0.89 0.59 1.19 0.18 1.0715421 GL7 0.36 0.037 b.d. 0.131 788 1.49 n/a 0.93 3.2

* Average of 8 analyses.** GB/MI: ratio of concentration in the glass bead (GL5) to that in the melt inclusion (OL11) after a post-entrapment crystallizationcorrection of 22.5%. Potassium concentration for the glass beads is from Hauri et al. (2015).

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the complication of olivine crystallization, which could alsodeplete Co and Ni concentrations in the residual melt.Residual sulfide phases in the source mantle during partialmelting could control chalcophile element abundances (e.g.Peach et al., 1990). According to recent experiments byDing et al. (2018), however, mare basalts are under-saturated in sulfide during partial melting, thus eliminatingthe potential for residual sulfides to control the chalcophileelement signatures for mare basalts.

3.5. Moderately volatile elements

Concentrations for moderately volatile elements mea-sured in 74220 OL11, 74220 OldOL2Emb and two glassbeads are summarized in Table 2. The orange and greenglass beads have similar K, Na, Cu, Zn, Ga, and Li abun-dances compared to those reported in Albarede et al.,(2015) and Hauri et al. (2015). The melt inclusion containshigher abundances of Zn, Pb, Cs, Rb, Na, Ga, and Cu thanthe glass beads, indicating an effect of degassing loss onmoderately volatile elements in glass beads.

4. DISCUSSION

4.1. Volatile loss during volcanic eruptions on the lunar

surface

Loss of volatiles during lunar volcanic eruptions can beexamined by comparing glassy melt inclusions, glassyembayments and glass beads in 74220, which experiencedvarious degrees of degassing during eruption. Before mak-ing the comparison, volatile concentrations in the glassyMI and embayments were corrected for their degrees ofpost-entrapment crystallization (22.5% for 74220 OL11,24.1% for 74220 OL8bEmb and 8.6% for 74220 OldO-L2Emb) following the procedure in Hauri et al. (2011).The corrected concentrations were then used to comparevolatile abundances in the degassed glass beads and embay-ments to the relatively un-degassed melt inclusion from74220 (Fig. 5). Almost all the volatiles measured in thisstudy show increasing degrees of depletion from glassy meltinclusions to glassy embayments and glass beads (Fig. 5),

which is consistent with expectations. From the figure, thevolatility trend for volcanic eruption on the Moon can becategorized as:

H2O � Cl � Zn � Cu � F > S � Ga � Cs > Rb

� Pb > Na > K � Liðno lossÞ:The observed high degree of loss for Zn and Cu in glass

beads and embayments compared to MIs suggests cautionin using literature Zn and Cu data to estimate their concen-trations in pre-eruptive basalts.

The above results are inconsistent with half condensa-tion temperatures (Lodders, 2003), indicating that thevolatility scale for lunar volcanic eruptions is different fromthe condensation sequence in a solar nebular. The aboverelative volatility trend also differs from the volatility trendobserved in recent experimental work by Norris and Wood(2017). At one bar and approximately the iron-wustite oxy-gen buffer, the experimentally derived relative volatilitytrend was: Sb > Cd > Ge > Bi > Tl � Ag > Sn > Cu �Pb > Zn > In > Cr > Ga (no loss). Both volatile solubilityand diffusivity in lunar melts at near vacuum conditionsseem to be the main control on volatility during basalticeruptions. The lack of existing theory and experimentaldata in quantitatively explaining volatile degassing duringlunar volcanic eruptions indicate that more work is neces-sary to improve understanding of volatile behavior at lunarconditions.

Copper and zinc concentrations measured in the meltinclusion from 74220 are �8 ppm, which is higher than inthe glass beads (�1 ppm), but significantly lower than pre-vious measurements of the bulk composition of glass beadsamples (�30 ppm Cu and �300 ppm Zn, Wanke et al.,1973; Duncan et al., 1974). We infer that the concentrationsin the melt inclusion are close to the pre-eruptive concentra-tions in this lunar basalt, and the concentrations in the glassbead interior from micro-beam measurements demonstratevolatile degassing, whereas the high concentrations in bulkglass bead samples likely reflect vapor condensates on beadsurfaces (Meyer et al., 1975), as supported by their rela-tively light Cu and Zn isotopic compositions (Moynieret al., 2006). The Zn/Fe ratio in the MI from 74220(OL11) is 4.7 � 10�5, which is about one order of magni-

Fig. 5. Volatile element concentrations for glass beads, glassy embayments and a melt inclusion in 74220. Elements (H reported as H2O) onthe x-axis are ordered by decreasing half condensation temperatures (Lodders, 2003) from left to right. The filled symbol is for the meltinclusion. Open symbols are for embayments and crosses are for glass beads. Data from the literature are plotted in grey symbols.Concentrations in the glassy embayments and melt inclusion are corrected for post-entrapment crystallization.

28 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

tude higher than those for the orange glass beads (this studyand Albarede et al., 2015). The degassing loss of moderatelyvolatile elements (e.g. Cu, Zn, Pb) in the glass beads indi-cates that whole rock measurements of these elements forlunar basalts could also be affected by degassing loss.Therefore, using whole rock data of lunar basalts to discusslunar mantle composition for moderately volatile elements(e.g., O’Neill, 1991) may need reevaluation.

4.2. Volatile abundances in the primitive lunar mantle

To obtain the volatile depletion trend for the Moon, it isnecessary to estimate the abundances of multiple volatileelements, ideally covering a large range of volatilities, inthe primitive lunar mantle. For studies on terrestrialbasalts, volatile concentrations are often paired with non-volatile trace elements that show similar geochemicalbehaviors during partial melting or crystal fractionationto assess volatile abundances in their mantle source (e.g.Michael, 1988; Saal et al., 2002; Plank et al., 2009;Koleszar et al., 2009). In this study, we employ a similarmethod to estimate the abundances of H2O, F, Cl, S, Li,Na, K, Cu, Zn, Pb, Cs, Rb, and Ga in the primitive lunarmantle. Estimates of the bulk silicate Moon compositionfor these elements have been done previously using certainvolatile/refractory ratios for mare basalts and volcanic glassbeads (e.g. O’Neill, 1991; Taylor and Wieczorek, 2014;Hauri et al., 2015; Albarede et al., 2015). Our estimationof their abundances, however, is mostly based on meltinclusion data, with whole rock data for mare basalts andvolcanic glass beads as a reference. Compared to similarwork in the literature, our approach helps avoid possibleconcerns related to data for volcanic glass beads and basaltwhole rock measurements (e.g. degassing loss, surfacecontamination).

The selection of appropriate volatile/refractory elementpairs is based on: a) their relative degrees of compatibilityin the terrestrial mantle (e.g. Sun and McDonough, 1989;Zhang, 2014); and b) the use of such ratios in previous stud-ies for terrestrial and lunar basalts (e.g. O’Neill, 1991;McDonough and Sun, 1995; Michael, 1995; Salters and

Stracke, 2004; Taylor and Wieczorek, 2014; Albaredeet al., 2015; Chen et al., 2015; Hauri et al., 2015). Becausethe lunar mantle sources for mare basalts are unlikely thesame as terrestrial mantle peridotite in composition (e.g.Nyquist et al., 1977; 1979), preferred volatile/refractoryratios for terrestrial studies might not be the best choicesfor the Moon. Therefore, multiple ratios were tested foreach volatile element with existing lunar MI and basaltdata, and the one with the best correlation in lunar datawas adopted, similar to the approach in Albarede et al.(2015). As a result, we focus on ratios of H2O/Ce, F/Nd,Cl/K, S/Dy, Li/Yb, Na/Sr, K/Ba, Cu/Mn, Zn/Fe, Pb/Ce,Cs/Rb, Rb/Ba, and Ga/Lu in this study. Similar resultswere obtained using alternate ratios, such as Cl/Ba, Na/Ho, K/La, Li/Dy, and Cu/Fe, which support our estima-tion based on the selected ratios.

4.2.1. H2O/Ce ratios

Ratios of H2O/Ce have been employed in the literaturefor studying terrestrial mid-ocean ridge basalts (e.g.Michael, 1988; Saal et al., 2002), as well as lunar basalts(Chen et al., 2015; Hauri et al., 2015). The H2O/Ce ratiosare roughly constant for terrestrial submarine MORBMIs with a wide range of MgO concentrations (5–13 wt%,Fig. S6b), supporting the use of H2O/Ce ratios to compen-sate for the effect of partial melting and igneous differenti-ation. Note that the submarine samples erupted atsufficiently high pressures and cooled rapidly, which sup-pressed H2O loss, whereas no such luxury is possible instudying lunar samples. Nonetheless, lunar MIs also showthat the H2O/Ce ratio in a given sample is identical betweenthe high-MgO homogenized MIs and the low-MgO glass inpartially glassy MIs: meaning that crystal fractionationitself does not change the H2O/Ce ratio (see later discus-sions on 10020 and 74235).

For the homogenized MIs, there is a concern of diffusiveH2O loss from the MIs during homogenization, whichwould lower the H2O/Ce ratios. In Chen et al., 2015,homogenized lunar MIs show systematically lower H2O/Ce compared to the natural MIs from 74220, promotingsuch a concern. Ni et al. (2017a) conducted a series of

P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41 29

experiments on lunar melt inclusions, and concluded thatdiffusive H2O loss during homogenization could be signifi-cant within minutes, but the effect is small for large MIsunder most conditions. For simplification, homogenizedMIs larger than 45 mm in diameter will be prioritized whendiscussing H2O/Ce ratios. Below we briefly review all H2O/Ce data for lunar MIs in the literature and from this study,which are also plotted in Fig. 6a after excluding homoge-nized MIs smaller than 45 mm in diameter.

H2O/Ce ratios in lunar MIs have been studied for fourlow-Ti mare basalts (12008, 12040, 15016, and 15647),two high-Ti mare basalts (10020 and 74235), and onehigh-Ti pyroclastic deposit (74220) (Chen et al., 2015;Hauri et al., 2015; Ni et al., 2017a). All MI data from thisstudy are summarized in Table 1, and a compilation of lit-erature lunar MI data is provided in SupplementaryTable S2.

For low-Ti basalt 12008, six homogenized MIs havebeen studied, with SiO2 ranging from 41 to 49 wt%, MgOfrom 6.8 to 10.7 wt%, and a narrow range of H2O/Ce ratiosfrom 1.5 to 1.8 (Chen et al., 2015). One of the six homoge-nized MIs with a diameter of 74 lm has an H2O/Ce ratio of1.6.

For low-Ti basalt 12040, six homogenized MIs havebeen studied, with SiO2 ranging from 41 to 48 wt%, MgO

Fig. 6. Concentrations of volatile versus refractory elements in terreshomogenized MIs data for MIs < 45 mm in diameter from this study andloss during homogenization. Submarine MORB and OIB MI data are fret al. (2002), and Dixon et al. (2002). Lunar MI and glass data from the lifor the lunar literature data can be found in Fig. 3. In the legend, ‘‘HMrespectively.

from 4.3 to 6.8 wt%, and H2O/Ce ratio from 0.3 to 0.8 (thisstudy). Most of the homogenized MIs have similar H2Oconcentrations of 11 to 14 ppm, and the variations inH2O/Ce ratio are mostly due to their variable Ce concentra-tions (Table 1). Similar trends have been reported for ter-restrial MIs from Iceland and Galapagos, possiblyindicating H2O re-equilibration (Fig. S6, Koleszar et al.,2009; Neave et al., 2014). As a result, the largest MI witha diameter of 70 mm turns out to have the lowest H2O/Ceratio of 0.3 because of its high Ce concentration. Therefore,an average H2O/Ce ratio of 0.5 for all homogenized MIs isused to represent sample 12040.

For vesicular low-Ti basalt 15016, five homogenizedMIs were investigated (Chen et al., 2015 and this study),with SiO2 ranging from 44 to 48 wt%, and MgO from 6.0to 9.4 wt%. H2O/Ce ratio in three MIs ranges from 0.8 to1.7 (this study). For the other two homogenized MIs,REE concentrations were not measured and hence H2O/Ce ratios are unavailable. One large MI with a diameterof 49 mm has a H2O/Ce ratio of 1.7.

For low- to very-low-Ti basalt 15647, two homogenizedMIs have been examined (this study). The whole rock con-tains 10.0 wt% MgO, and the two homogenized MIs con-tain 8.5 and 4.5 wt% MgO, respectively, indicatingvarious degrees of crystal fractionation. The H2O/Ce ratio

trial submarine and lunar olivine-hosted melt inclusions. In (a),Chen et al. (2015) are excluded to reduce the concern of diffusive Hom GeoRoc and Michael (1995), Danyushevsky et al. (2000), Saalterature are marked with stars and plotted in grey. Detailed sourcesI” and ‘‘NMI” stand for homogenized and natural melt inclusions,

30 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

ranges from 0.9 to 0.6 (this study). One large MI (45 mm indiameter) from this sample has a H2O/Ce ratio of 0.9.

For high-Ti basalt 10020, data for H2O and Ce are avail-able for three MIs from Chen et al. (2015) and this study:two for partially glassy natural MIs, and one for a homog-enized MI (with a diameter of 83 mm). As shown in Fig. 6a,the glassy parts of two natural MIs (on the high end of10020 group) are very similar in composition, with�50 ppm Ce, and �400 ppm H2O. The elevated Ce concen-trations in these two MIs from 10020 are consistent with thehighly evolved nature of the melt inclusions, which contain�55 wt% SiO2. The homogenized MI from 10020 containsa factor of three lower H2O and Ce abundances (131 ppmH2O, 16.1 ppm Ce), and a similar H2O/Ce ratio (Fig. 6a).The high consistency in H2O/Ce ratios for the three MIsfrom 10020 indicate that: (a) H2O was not lost from thishomogenized MI during heating; (b) the H2O/Ce ratio doesnot vary significantly even after �67% crystal fractionation.H2O/Ce ratio in the MIs of this sample is �9.

The pyroclastic deposit sample 74220 contains abundantorange glass beads originated from fire fountain eruptions(Heiken et al., 1974), as well as olivine fragments bearingessentially glassy MIs. Both the glass beads and especiallythe glassy MIs have been extensively studied (Saal et al.,2008; Chen et al., 2011; Hauri et al., 2011; Hauri et al.,2015; Ni et al., 2017a) because glassy lunar MIs are rare.The major oxide compositions of these glassy MIs in olivineare similar to the glass beads or may be related to themthrough crystal fractionation (Hauri et al., 2011; Chenet al., 2015; Ni et al., 2017a). Saal et al. (2008) reportedH2O in glass beads and Hauri et al. (2011) reported H2Oin MIs. Chen et al. (2015), Hauri et al. (2015), and Niet al. (2017a) reported H2O/Ce ratios in natural andhomogenized MIs. As Ni et al. (2017a) pointed out, H2Oconcentrations in natural MIs in 74220 show positive corre-lation with MI diameter, indicating post-eruptive H2O lossfrom the MIs. Therefore, H2O/Ce ratios recorded by thelargest natural MIs are more representative of the pre-eruptive H2O/Ce ratio for 74220. Ni et al. (2017a) foundthat H2O concentrations in MIs in 74220 are systematicallydifferent between those reported in Hauri et al. (2015) andthose reported in Chen et al. (2015) and Ni et al. (2017a),either due to heterogeneities among different subsamplesof 74220, or due to differences in analytical configurationand calibration. For consistency, here we adopt H2O/Ceratios from the work of Chen et al. (2015) and Ni et al.(2017a), which reported H2O/Ce ratios between 25.9 and55.5 for natural MIs, with an H2O/Ce of �50 for the largesttwo MIs.

For high-Ti basalt 74235, H2O and Ce concentrations inhomogenized MIs, natural partially glassy MIs and intersti-tial glasses were measured using SIMS (this study). As men-tioned earlier, the interstitial glasses in the matrix of 74235contain higher H2O concentrations (88–125 ppm) than thenatural partially glassy MIs (57–91 ppm). By comparingthem in the H2O-Ce plot (Fig. 6a), however, it can be seenthat H2O/Ce ratios for the natural MIs and interstitialglasses are very similar, and the relatively high H2O concen-trations in interstitial glasses are due to their more evolvedcompositions compared to the natural partially glassy MIs.

This conclusion is also supported by the major elementcomposition, because the interstitial glasses contain higherSiO2 (�57 wt%) and incompatible elements compared tothe glassy parts of natural MIs (43 to 52 wt% SiO2) andare lower in Mg#. The fact that interstitial glasses from74235 preserved similar H2O/Ce ratios as the MIs is a bitsurprising and might suggest that the matrix of 74235evolved as a closed system with respect to H2O at the finalstages of basalt solidification, when these MIs and intersti-tial glasses formed. Compared to terrestrial MIs that oftencontain weight percent levels of H2O, the low concentra-tions (�100 ppm) of H2O in 74235 are easier to preservein a lava. Even one meter of basalt lava on the Moon wouldprovide sufficient pressure to keep 100 ppm H2O under-saturated in the magma. Also, relatively rapid cooling (evi-denced by glass in the MIs and matrix) would help to pre-vent H2O loss. One homogenized MI has a slightly lowerH2O/Ce ratio, as well as an elevated Li concentration, likelybecause this MI is close to the surface of the olivine grain(Supplementary Fig. S7), allowing effective exchange ofH2O and Li between the MI and the environment duringhomogenization. Despite the large variations in major ele-ment compositions (38 to 57 wt% SiO2 and 1.8 to 6.5 wt% MgO) for homogenized MIs, partially glassy MIs, andinterstitial glasses, their H2O/Ce ratios all fall in a smallrange from 1.6 to 3.3, indicating similar behaviors forH2O and Ce during crystal fractionation (Fig. 6a). TheH2O/Ce ratio of 1.6–3.3 for 74235 is approximately a factorof 20 lower than MIs from 74220, and a factor of 3 lowerthan MIs from 10020.

Overall, H2O/Ce ratios vary by two orders of magnitudeamong different lunar samples, from 50 for 74220 to 9 for10020, 3 for 74235, 1.6 for 12008 and 15016, 0.9 for15647, and 0.5 for 12040. The variation in H2O/Ce ratioshas been reported previously by Chen et al. (2015), andthe authors argued that the naturally glassy MIs best main-tain pre-eruptive H2O/Ce ratio due to rapid quenchingcompared to the homogenized crystalline MIs. On the otherhand, Albarede et al. (2015) argued that the high H2O/Ceratio in 74220 is a local feature and cannot be used to rep-resent the lunar mantle. With a wider range of sample stud-ied, and a larger set of elements measured in this study, thehigh H2O/Ce ratio for 74220 is interpreted to be a betterrepresentative for the primitive lunar mantle. A detaileddiscussion of the H2O/Ce ratio variation in lunar sampleswill be provided after going through all the volatile/refrac-tory ratios. Using a BSE abundance of Ce (1.675 ppm,McDonough and Sun, 1995), and a H2O/Ce ratio of 50for 74220, the primitive lunar mantle is estimated to contain84 ppm H2O.

4.2.2. F/Nd ratios

Ni et al. (2017a) showed that loss of F, Cl or S from MIsduring short-duration homogenization experiments is negli-gible. Therefore, homogenized MIs are not distinguishedfrom natural MIs in the discussion of F/Nd, Cl/K, or S/Dy ratios below. F/Nd ratios have been used to estimateF abundances in the lunar mantle by Chen et al. (2015)and Hauri et al. (2015). The same ratio is used here for con-sistency. Ratios of F/Nd for most MIs analyzed in this

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study (Fig. 6b) fall in the range of 1.4 to 4, which is consis-tent with results from Chen et al. (2015). MIs from 12040,however, show much higher F/Nd ratios (6–25, �20 formost MIs), about the same as for BSE, DMM, and terres-trial OIB MIs (�22, Chen et al., 2015). Plotting F versusREEs other than Nd does not remove the large scatter inthe data. MIs in 12040 show other compositional peculiar-ities, including relatively high P, S and Cl concentrations inone MI (but P enrichment is not correlated with F enrich-ment, indicating that apatite addition is not the explana-tion), and relatively high heavy REE concentrations inanother MI. The elevated F/Nd ratios in 12040 are anenigma and might reflect enriched F in the source lunarmantle of 12040 or secondary enrichment of F during theformation of 12040. Using a BSE abundance of 1.25 ppmNd, a F/Nd ratio of 4 corresponds to a F abundance of5 ppm in the primitive lunar mantle based on most lunarsamples studied. If the relatively high F/Nd ratio of �20for MIs from 12040 is used, the lunar mantle would contain�26 ppm F, which is very similar to BSE. Sample 12040 iscurrently the only mare basalt with a high F abundancesimilar to BSE. Because 12040 is thought to have beenformed by olivine accumulation and might be geneticallyrelated to other Apollo 12 basalts (e.g. 12018, 12004,12021 and 12051; Walter et al., 1971), it is recommendedthat future studies examine F/Nd ratios for MIs in thesesamples to verify the high F/Nd ratios measured for12040, and to better understand the formation of Apollo12 basalts. Even though there is large variation in F/Ndratio in MIs from 12040 and other lunar basalts, the totalvariation of about one order of magnitude is still muchsmaller than the two orders of magnitude variation in theobserved H2O/Ce ratio in MIs of lunar basalts.

4.2.3. Cl/K ratios

In Fig. 6c, Cl is compared to K for lunar melt inclusions.Although Cl is commonly compared with K in the litera-ture, Cl/K ratio in DMM (0.009; Salters and Stracke,2004) is lower than in BSE (0.07; McDonough and Sun,1995) by a factor of �8, meaning that this ratio is fraction-ated significantly during magma processes on Earth and Clis more incompatible than K in the terrestrial mantle. Ter-restrial MI data are consistent with Cl being more incom-patible than K as evidenced by Cl/K ratios in MORBMIs being very close to the DMM value of 0.009, whereasthe OIB MIs from Samoa, Iceland, and Hawaii show higherand more variable Cl/K ratios, averaging �0.045 and withvalues greater than 0.07. In the lunar MI data, Cl concen-trations correlate well with K concentrations, indicatingsimilar degrees of incompatibility during crystal fractiona-tion and partial melting on the Moon. The Cl/K ratiosfor lunar MIs are in a narrower range, between 0.0035and 0.015, with an average of �0.009. The average Cl/Kratio of �0.009 for lunar MIs is similar to that in theDMM and is 8 times lower than the BSE ratio of 0.07(Fig. 5c). Estimation of the abundance of Cl in the lunarmantle, however, is hindered by the fractionated Cl/Kratios between DMM and BSE. Assuming the source man-tle for mare basalts studied here did not experience signifi-cant depletion or enrichment processes that would alter the

Cl/K ratio, a Cl/K ratio of �0.008 for lunar MIs wouldcorrespond to a concentration of 0.39 ppm Cl in primitivelunar mantle sources (using a K abundance of 49.2 ppmbased on K/La ratios, discussed later).

4.2.4. S/Dy ratios

Concentrations of S are plotted versus Dy for lunar MIsin Fig. 6d. Unlike H2O/Ce, F/Nd, and Cl/K in lunar sam-ples, S/Dy ratios can be affected not only by degassing lossof S, but also sulfide segregation in the melt. For green andorange glass beads, the degassing effect is more dominant ingenerating variations in the S/Dy ratios (Fig. 6d). For thenatural partially glassy MIs and interstitial glasses, how-ever, sulfide saturation may play a more important role infractionating S/Dy ratios. For example, the homogenizedMIs from 74235 have S/Dy ratios of 78 to 84, but S/Dyratios in natural MIs from the same sample can be fraction-ated to values < 25. The low S/Dy ratio for the natural MIfrom 10020 (�17) is also likely due to the formation of sul-fides, based on its highly evolved composition (Table 1).Because S/Dy ratios could be lowered due to the abovetwo factors, the highest S/Dy ratio for each sample isassumed to be more representative of its parental melt.For low-Ti basalts (12008, 12040, 15016 and 15647), thehighest S/Dy ratio in each sample ranges from 118 to179. For the high-Ti basalts (10020, 74220 and 74235),the highest S/Dy ratio in each sample ranges from 84 to95 (Fig. 7). This difference between low- and high-Ti marebasalts in terms of S/Dy is about 20% to 50%, which needsto be confirmed using more lunar MI data. If this differenceis accurate, however, it might provide insights on the for-mation of low- and high-Ti mare basalts (e.g. the mineralmode of sulfides in their source mantle). Assuming a BSEabundance of 0.67 ppm for Dy (McDonough and Sun,1995), the estimated S abundance in the source mantlewould be 52 to 64 ppm for the high-Ti basalts, and 79 to120 ppm for the low-Ti basalts. The estimated S abundancefor source mantle of high-Ti basalt is about a factor of 2lower than DMM (119 ppm, Salters and Stracke, 2004),but the estimated S abundance for the low-Ti basalt is verysimilar to DMM.

4.2.5. Li/Yb, Na/Sr, K/Ba, and Cu/Mn ratios

In this study, concentrations of moderately volatile ele-ments Na (50% Tc = 958 K, Lodders, 2003), K (50%Tc = 1001 K, Lodders, 2003), Li (50% Tc = 1135 K,Lodders, 2003) and Cu (50% Tc = 1037 K, Lodders, 2003)were also measured by SIMS for most MIs, enabling usto estimate their abundances in the lunar mantle based onlunar MI data. For these elements, Li/Dy, Li/Yb, Li/Lu,Na/Ti, K/U, K/La, K/Ba, Cu/Sc, and Ti/Cu ratios havebeen used in the literature for their abundances in the ter-restrial or lunar mantle (O’Neill, 1991; McDonough andSun, 1995; Salters and Stracke, 2004; Hauri et al., 2015).Here we adopt Li/Yb and K/Ba ratios from previous stud-ies for Li and K. For Na, Na/Sr is used because a bettercorrelation between Na and Sr is observed for lunar data(Fig. 8a). For Cu, we use Cu/Mn instead of Cu/Sc becauseMn also has similar incompatibility compared to Cu, andSc data are not available in our dataset. In addition, ratios

Fig. 7. S/Dy ratios in olivine-hosted melt inclusions plotted versus the TiO2 concentrations in their host rock. Host rock TiO2 concentrationsare used because their concentrations in the melt inclusions could have been fractionated by post-entrapment crystallization. S/Dy ratios ineach lunar sample can be significantly lowered due to degassing or ilmenite fractionation. Data sources for the melt inclusions and glass beadsare the same as in Fig. 6. References for host rock concentrations are listed in Table 1.

Fig. 8. Plots of moderately volatile elements versus refractory elements. (a) Na versus Sr; (b) K versus Ba. (c) Li versus Yb; (d) Cu versus Mn.Lunar MI data for Na and K in (a) and (b) are all measured by SIMS. In the legend, ‘‘HMI” stands for homogenized melt inclusions and‘‘NMI” stands for natural melt inclusions. Terrestrial MORB data are from PetDB (http://www.earthchem.org/petdb). OIB data are fromGeoRoc (http://georoc.mpch-mainz.gwdg.de/), screened to those with 35 to 52 wt% SiO2, and 6 to 20 wt% MgO. Lunar basalt data are fromthe MoonDB (http://www.moondb.org) and Wiesmann (1975). Terrestrial MI data in (b) are from the same sources as noted in Fig. 6. *74220MIs, orange and glass beads data are from Hauri et al. (2011), Chen et al. (2015) and Ni et al. (2017a).

32 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41 33

such as Li/Dy, Na/Ho, K/La and Cu/Fe ratios are alsoused to verify our estimation, and similar results wereobtained.

Lithium data for all homogenized MIs and copper datafor natural MIs from 74235 are excluded when estimatingLi and Cu abundances in the lunar mantle. For Li, wenoted that Li concentrations in homogenized MIs couldbe affected by contamination from the crucible duringhomogenization. In olivine, Li diffusivity is higher than Hdiffusivity (Chakraborty, 2010), making it possible for Licontamination to happen in relatively short time periods.For Cu, as discussed in Section 3.4, natural MIs from74235 contain visible sulfide grains, which could decreaseCu concentrations in the residual glass significantly. There-fore, these Li and Cu data are excluded to avoidcomplications.

In general, ratios of Na/Sr, K/Ba, and Li/Yb for lunarMIs overlap well with the lunar basalt data (Fig. 8), indicat-ing no significant loss of Na, K, and Li in lunar basalts rel-ative to MIs. This conclusion is consistent with our resultsindicating less than 10% degassing loss of Na, K, and Liin orange glass beads and glassy embayments compared tothe melt inclusion from 74220 (Fig. 5). Ratios of Na/Sr inlunar MIs range from 11.5 to 30, with a geometric average

of 20 1.2 (1r error, Fig. 8a). Lunar MI K/Ba ratios are

mostly between 4.2 and 9.4, with a geometric average of

6.9 1.2 (Fig. 8b). Relatively more scatter is seen in the

Li-Yb plot (Fig. 8c). Highly evolved interstitial glasses andMIs from 74235 and 10020 have Li/Yb ratios a factor of 2to 3 lower than the less evolved green and orange glassesand 74220 MIs. If Li/Yb data for the highly evolved glassesand MIs are excluded, the rest of the data gives approxi-

mately consistent Li/Yb ratios, which averaged 2.4 1.1

(Fig. 8c). Cu/Mn ratios in lunar MIs also correlate well withlunar basalt data, however, the scatter is more than one

order of magnitude greater, with an average of 0.011 1.4

(Fig. 8d). The higher scatter in Cu/Mn ratios might be dueto the higher volatility for Cu in lunar volcanism as con-cluded above, but these ratios could also be affected by therole of sulfides during lunar magma processes.

Assuming a BSE abundance for the refractory elementsin the lunar mantle (19.9 ppm Sr, 6.6 ppm Ba, 0.441 ppmYb, and 1045 ppm Mn, McDonough and Sun, 1995;Hauri et al., 2015), Na, K, Li, and Cu abundances in thelunar mantle can be calculated to be 398 ppm, 45.5 ppm,1.1 ppm, and 11 ppm, respectively.

4.2.6. Zn/Fe, Pb/Ce, Cs/Rb, Rb/Ba and Ga/Lu ratios

Analyses of Zn, Pb, Cs, Rb, and Ga concentrations inlunar melt inclusions by SIMS is a challenging task dueto their low abundances, low ionization rate, or strong massinterferences. Therefore, no data for these elements havebeen previously reported for lunar MIs. In this study, how-ever, we were able to analyze these elements by LA-ICP-MS for one MI, and use the data to estimate Zn, Pb, Cs,Rb and Ga abundances in the lunar mantle.

For these elements, ratios of Zn/Sc, Zn/Fe, Pb/Ce, Cs/Rb, Cs/Ba, Cs/La, Rb/Ba, Rb/La, and Ga/Al have been

used in the literature to estimate their abundances in theterrestrial or lunar mantle (Newsom, 1986; O’Neill, 1991;McDonough and Sun, 1995; Salters and Stracke, 2004;Taylor and Wieczorek, 2014; Albarede et al., 2015; Hauriet al., 2015). Here we focus on Zn/Fe, Pb/Ce, Cs/Rb, andRb/Ba ratios for Zn, Pb, Cs, and Rb. For Ga, which showssimilar degree of incompatibility compared to the heavyREE (Zhang, 2014), multiple ratios are tested and Ga/Luis selected for this study.

Individual plots for Zn/Fe, Pb/Ce, Cs/Rb, Rb/Ba, andGa/Lu ratios of terrestrial basalt, lunar basalt and lunarMI data are in Supplementary Fig. S8. Based on our meltinclusion measurement of Zn, Pb, Cs, Rb, and Ga concen-trations for 74220, we get Zn/Fe = 4.7 � 10�5, Pb/Ce = 0.011, Cs/Rb = 0.052, Rb/Ba = 0.012, and Ga/Lu = 8.5, which are roughly within the range for lunarbasalt data (Supplementary Fig. S8). Assuming BSE-likeconcentrations of 6.26 wt% Fe, 1.675 ppm Ce, 6.6 ppmBa, and 0.0675 ppm Lu for the lunar mantle(McDonough and Sun, 1995), we obtain 2.9 ppm Zn,0.018 ppm Pb, 0.079 ppm Rb, 0.0041 ppm Cs, and0.57 ppm Ga for the primitive lunar mantle.

4.3. H2O/Ce variation among different lunar samples

In Chen et al. (2015), large variations in melt inclusionH2O/Ce ratios between 74220 (with glassy MIs) and otherlunar samples (with mostly homogenized MIs) were firstnoticed, which remain an uncertainty in interpreting H2Oabundance for the lunar mantle. Ni et al. (2017a) studiedthe effect of diffusive H loss during homogenization onH2O/Ce variation and concluded that this effect is insignif-icant compared to the range of variations observed. Hence,the H2O/Ce variation among different lunar samples is notan artifact due to the homogenization experiments, but areal signal possibly recording a complicated history thathas shaped volatile behavior on the Moon. Albaredeet al. (2015) interpreted the high H2O/Ce ratio for 74220to be a local feature that is not representative of the lunarmantle. While Chen et al. (2015) explained the H2O/Ceratio variation as a result of post-eruptive cooling loss ofH2O, and that the naturally glassy MIs best maintain pre-eruptive H2O/Ce ratio due to rapid quench. Based on ourcurrent understanding of Moon formation and evolution,below we provide a detailed discussion on possible pro-cesses and reasons that could have caused heterogeneousH2O/Ce distribution in different lunar igneous samples.

4.3.1. Diffusive loss of H2O during cooling on the lunar

surface

As shown by numerous hydration and dehydrationexperiments on terrestrial (Hauri, 2002; Portnyagin et al.,2008; Chen et al., 2011; Gaetani et al., 2012) and lunarMIs (Ni et al., 2017a), a melt inclusion is not a perfectlyclosed system with respect to H2O. Rapid H diffusionthrough the host olivine could potentially decrease H2O/Ce ratios in MIs during cooling on the lunar surface. Oneimplication of the effect of cooling on H2O/Ce variationamong lunar samples is the correlation between H2O/Ceratio and the apparent cooling rate. Based on MI data

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available so far, samples with more glassy MIs tend to havehigher H2O/Ce ratios. Sample 74220 has the highest H2O/Ce ratio (�50), and most MIs found in this sample areglassy. For samples with intermediate H2O/Ce ratios (�9for 10020, �3 for 74235), some of the MIs found in thesesamples are partially glassy. While for samples with thelowest H2O/Ce ratios (1.7–0.5, 15016, 15647, 12040, and12008, this study and Chen et al., 2015), virtually all MIswere found to be highly crystalline.

For a more quantitative understanding of the role ofcooling rate on H2O/Ce variations, the diffusive H lossmodel from Ni et al. (2017a) is employed to model H2O/Ce ratio decrease in lunar MIs as a function of cooling rate.In this model, a spherical melt inclusion is assumed to be atthe center of a spherical olivine, and H2O inside the MI isgradually drained by diffusive loss of H across the host oli-vine. Main parameters needed for this model include diam-eter of the olivine, diameter of the MI, boundary conditionfor H, and the temperature history. For the diameter of oli-vine, a value of 200 mm is assumed for all the MIs.Although the actual diameter for the olivines could varyfrom 100 mm to 600 mm in different samples, Ni et al.(2017a) has shown that the effect of varying olivine diame-ter is relatively small. For the diameter of MI, we assume arange of 20 mm to 60 mm, which approximates most MIsstudied in lunar samples (Table 1). Ideally, the boundarycondition for H should be the evolving H2O concentrationin the host magma during degassing. But because suchinformation is unavailable, the extreme case with an exte-rior concentration of zero H2O is assumed. The tempera-ture history of cooling is assumed to start at 1200 �C or1300 �C, and cools asymptotically to the final temperaturewith a given initial cooling rate (q). Equations and otherdetails about the model can be found in Ni et al. (2017a).

In terms of the cooling rate for low-Ti basalts, 12008and 12040 have been estimated to have cooling rates of10–100 K/h, and < 15 K/h, respectively (Donaldson et al.,1975). For quantitative comparison and plotting, 15 K/hwill be used for 12040. Quantitative cooling rates are notavailable in the literature for 15016 or 15647. But Takedaet al. (1975) estimated that 15016 is one of the fastest cooledsamples among many Apollo basalts, and cooled morerapidly than 12040. 15647 likely cooled at a slower ratecompared to 15016 because this rock is a coarse-grainedbasalt, whereas 15016 is a medium-grained basalt.

For the two high-Ti basalts, 74235 was estimated tohave a cooling rate of 150–250 K/h (Usselman et al.,1975), while quantitative cooling rate data have not beenreported for 10020. Based on the presence of glass in natu-ral MIs and absence of glass in the matrix, the cooling ratefor 10020 is likely slightly lower than 74235, which hasquenched glass in the MIs and also in the matrix.

The cooling rate of 74220 glass beads has been estimatedto be �20000 K/h by Uhlmann et al. (1974) and �6060 K/hby Hui et al. (2018), and that of MI-bearing olivine crystalshas been estimated to be �3600 K/h by Ni et al. (2017a).Considering the model used here is for H loss fromolivine-hosted MIs, the estimated cooling rate of�3600 K/h for MI-bearing olivine crystals is adopted.Using the higher cooling rate estimated by Uhlmann

et al. (1974) does not affect our model significantly becausethe range of estimated cooling rates (3600–20000 K/h) for74220 is high enough to ensure limited H2O loss aftereruption.

As shown in Fig. 9, H2O/Ce ratios for different lunarsamples are plotted versus their estimated cooling rates.The H2O/Ce ratios show positive correlations with coolingrates in Fig. 9, in accordance with our observations basedon the occurrence of glassy melt inclusions. Qualitativecooling rate data are also consistent with Fig. 9. For exam-ple, 15016 cooled more rapidly than 12040 (Takeda et al.,1975), and the H2O/Ce ratio in 15016 is about 2 times thatin 12040. Also, 15016 has a higher H2O/Ce ratio comparedto 15647, and the latter likely cooled more slowly as dis-cussed earlier. The only exception seems to be 10020, whichlikely cooled at a slower rate than 74235 as discussed ear-lier, however, has a higher H2O/Ce ratio than 74235. Evenwith this exception, the overall trend of higher H2O/Ceratio in MIs for more rapidly cooled lunar basalts isconvincing.

Modeled trends of H2O/Ce ratios in Fig. 9 show that thedecrease in H2O/Ce is highly dependent on cooling rate,especially for the range of cooling rates that are typicalfor mare basalts. Although our current model is limitedby the evolution of H2O abundance in the host magma,and a detailed understanding of their thermal history aftereruption, it unambiguously shows that H2O/Ce in lunarMIs can be efficiently decreased due to diffusive H lossunder typical time scales of mare basalt cooling. Therefore,diffusive loss of H2O could potentially be an important pro-cess that caused large H2O/Ce variations in lunar MIs.

4.3.2. Ingassing contamination that increased H2O/Ce in

74220 during eruption

Because an olivine-hosted melt inclusion is not a per-fectly closed system (e.g., Hauri, 2002; Portnyagin et al.,2008; Chen et al., 2011; Gaetani et al., 2012; Ni et al.,2017a), there is also a possibility of H2O diffusing into theMIs just before eruption, thus resulting in a higher H2O/Ce for 74220 compared to other lunar samples (Albaredeet al., 2015). This possibility, however, is not favored forthe following reasons: (1) For ingassing to occur, the meltinclusions would have had to be surrounded by a sourcewith high H2O fugacity. Such a wet environment with highH2O fugacity is common on Earth (such as hydrothermalfluid, and alteration by liquid water), but unknown on theMoon. Contamination from water-rich carbonaceous chon-drites during eruption is also unlikely because such contam-ination would also result in elevated Zn, Cl, F, or Pb in74220, which is not observed (discussed further in 4.3.3and in Fig. 10); and (2) more importantly, according tothe modeling and experiments by Ni et al. (2017a) amongothers, smaller MIs are more vulnerable to ingassing anddegassing compared to larger MIs. If the MIs are affectedby ingassing during eruption, a negative correlationbetween H2O content and MI size is expected. The observedtrend, however, is opposite to this expectation with smallerMIs having lower H2O concentrations and lower H2O/Ceratio in 74220 (Ni et al., 2017a), indicating an effect of‘‘degassing loss” instead of ‘‘ingassing gain” for H2O during

Fig. 9. Correlation between H2O/Ce ratio and sample cooling rate (q) for lunar MIs studied so far. Modeled H2O/Ce ratios as a function ofcooling rate assuming an initial temperature (T0) of 1200 �C and 1300 �C for MIs with a diameter of 20–60 mm have been plotted in green andblue patches, respectively. Error bars for the cooling rate represent the range of estimation in the literature.

Fig. 10. Comparing volatile/refractory element ratios in MIs from 74220 to other samples. Besides H2O/Ce, MIs from 74220 show no sign ofvolatile enrichment relative to other lunar MIs or basalts. Error bars in the figure represent 1 standard deviation variation of the data. Glassbeads data in the figure is from Hauri et al. (2015). Lunar basalt data are from MoonDB.

P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41 35

eruption. Therefore, it is unlikely that the high H2O/Ceratio for 74220 is the result of late stage water ingassing.

4.3.3. Is sample 74220 a volatile-enriched local

heterogeneity?

As the lunar sample with the highest H2O/Ce ratios,some studies have proposed that 74220 could be originatedfrom a volatile-enriched local heterogeneity inside the lunarmantle, and therefore cannot be used to represent theMoon (e.g. Albarede et al., 2013, Albarede et al., 2015).If this is the case, sample 74220 should also show enrichedabundances of other volatiles, such as F, Cl, S, Cu, Zn, Pb,among others. In Chen et al. (2015), it was shown that MIsfrom 74220 have similar F/Nd, Cl/Ba and S/Dy ratios com-pared to MIs from lunar mare basalt samples. For the mod-erately volatile elements, previous bulk analyses werealmost always affected by a thin coating of surface con-

densed volatiles, and gave elevated Zn, Cu, and Pb abun-dances, as well as relatively low 238U/204Pb ratios, andlight Cu and Zn isotopes (Tera and Wasserburg, 1976;Moynier et al., 2006). Such observations led to the concernthat 74220 was sourced from a volatile-enriched localheterogeneity in the lunar mantle (e.g. Paniello et al.,2012), however, these bulk measurements might not repre-sent the pre-eruptive chemical and isotopic compositionsfor 74220. With more MI data for F, Cl and S, andin situ analyses of moderately volatile elements, it is possibleto compare a larger set of volatile/refractory ratios for MIsin 74220 to other lunar MIs and lunar basalts to test thepossibility of a local heterogeneity source for 74220. Asplotted in Fig. 10, with the exception of H2O/Ce, volatile/refractory element ratios for MIs from 74220 are mostlywithin error compared to MIs, glass beads, and marebasalts from other lunar samples. The consistent ratios of

36 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

S/Dy, Zn/Fe, Pb/Ce, F/Nd, Cs/Rb, Rb/Ba, Cl/K, Na/Sr,Ga/Lu, K/Ba and Li/Yb between 74220 and other lunarsamples indicate that 74220 is unlikely originated from avolatile-enriched source in the lunar mantle, which justifiesthe use of its MI data to estimate H2O abundance for theMoon.

4.3.4. Partial melting of the lunar mantle and magma

evolution on H2O/Ce

As mentioned earlier, H2O/Ce ratios in lunar MIs areused to estimate H2O abundances in the lunar mantle. Thisis based on the assumption that H2O/Ce ratios in the meltdo not change during mantle partial melting or crystal frac-tionation, leading to the same H2O/Ce ratios in lunar MIscompared to their mantle source. In addition, the La/Yband Sm/Dy ratios could vary by a factor of 4 in individuallunar samples (Fig. S5), possibly indicating the mixing ofmagma produced by different degrees of partial melting(Shimizu, 1998; Slater et al., 2001). Therefore, it is impor-tant to test whether our assumption of constant H2O/Ceratio during mantle partial melting and crystal fractiona-tion is valid.

This assumption is well supported by terrestrial sub-marine MORB MI data, which show constant H2O/Ceratios (variations within a factor of 2) over a wide rangeof major element concentrations (5 to 13 wt% MgO,Fig. S6). Such a wide range of MgO concentrations repre-sents variable degrees of upper mantle partial melting andcrystal fractionation. The constant H2O/Ce ratio for allthese submarine MORB MIs strongly supports correlatedH2O and Ce behaviors during these magma processes.When applied to lunar conditions, the H2O/Ce ratio isexpected to behave the same way unless compatible mineralhosts for H2O or Ce are involved in the process. This isunlikely the case because, most mineral phases discoveredon the Moon are incompatible with respect to H2O orCe, except for rare minerals such as apatite, which onlyoccur at late stages of magma crystallization (Heikenet al., 1991).

Other evidence supporting the assumption of constantH2O/Ce ratio during magma evolution is based on multiplemeasurements in two specific lunar samples: (1) In lunarsample 10020, the H2O/Ce ratio in homogenized MIs with6.5 wt% MgO and 18.1 wt% FeO is about the same as thatin the glassy parts (1.7–2.3 wt% MgO and 5.6–7.4 wt%FeO) of partially glassy MIs (Fig. 6a, Chen et al., 2015and this study); and (2) In lunar sample 74235, the highestH2O/Ce ratio in homogenized MIs with 6.5–7.2 wt% MgOand 20.2–22.2 wt% FeO is about the same as that in the par-tially glassy MIs (2.1–3.2 wt% MgO and 8.1–15.5 wt%FeO), as well as the glassy matrix (1.8 wt% MgO and12.4 wt% FeO) (Fig. 6a, this study). Consistent H2O/Ceratios in less evolved and more evolved MIs from the abovetwo samples indicate similar behavior of H2O and Ce dur-ing crystal fractionation.

Both of the above evidences strongly support relativelyconstant H2O/Ce ratios during mantle partial melting andcrystal fractionation processes. To further assess the effectof partial melting, H2O/Ce ratios are plotted versus MgOand FeO concentrations for all lunar MIs in Fig. 11. If

H2O/Ce variations in lunar samples are dominated by theextent of partial melting, a strong dependence betweenH2O/Ce ratios and the major elements (MgO and FeO usedin here) is expected. Such a dependence, however, is absentas can be seen in Fig. 11. In Fig. 11, all MIs with less than3.5 wt% MgO and 15 wt% FeO are natural glassy MIs in10020 and 74235, which experienced significant post-entrapment crystallization. Homogenized MIs in the samesamples contain much higher MgO and FeO, while preserv-ing the same H2O/Ce ratios compared to the natural glassyMIs. The effect of homogenization unambiguously showsthat the low MgO and FeO concentrations in the naturalMIs from 10020 and 74235 are the result of post-entrapment crystallization, not differences in the degree ofpartial melting.

With the above considerations, we can exclude partialmelting or magma evolution as factors controlling theobserved H2O/Ce ratio variations in lunar samples. Furtherstudies on the behavior of H2O/Ce ratio during lunarigneous processes will be helpful to verify our conclusion.

4.3.5. Degassing loss of H2O during magma ocean

crystallization

The Moon is thought to have experienced a globalmagma ocean stage, which produced a plagioclase flotationcrust as it solidified, and might have caused an overturn ofthe mantle due to a gravitationally unstable stratify (e.g.Elkins-Tanton et al., 2011). Degassing loss of volatiles dur-ing lunar magma ocean crystallization could be quite signif-icant, especially before the formation of a plagioclase crust.When the plagioclase crust is thick enough, degassing lossof volatiles would be sufficiently prohibited. During the lateoverturn, however, part of the solidified lunar mantle couldhave been exposed to the surface, resulting in further degas-sing. Due to the complicated crystallization history of thelunar magma ocean, it is possible for different parts of thelunar mantle to have experienced varying degrees of degas-sing. An intuitive expectation is that the deeper part of thelunar mantle could have experienced less degassing com-pared to the shallower mantle. Experimental petrologystudies (e.g. Longhi, 1992; Lee et al., 2009) indicate thatpyroclastic glass deposits originate from a deeper source(1.6 to 3.0 GPa, 300 to 600 km) compared to the marebasalts (0.7–2.0 GPa, 150–300 km). If the H2O/Ce varia-tions in lunar MIs are truly representative of their mantlesources, it is possible that the pyroclastic glasses originatefrom a less degassed part of the lunar mantle that betterpreserved the primitive H2O/Ce signal of the lunar magmaocean.

With all the above considerations, we think the H2O/Ceratio of 74220 (�50) better represents the pre-eruptivemagma and the primitive lunar mantle. Based on this, weestimate the primitive lunar mantle to have contained84 ppm H2O.

4.4. Volatile depletion trend for the Moon based on melt

inclusion data

Based on previous discussions on volatile/refractoryratios, abundances of volatile elements (H2O, F, Cl, S)

Fig. 11. Testing the effect of partial melting on lunar H2O/Ce ratios by plotting H2O/Ce versus (a) MgO and (b) FeO content for lunar MIs.In general, H2O/Ce ratios show no clear dependence on MgO and FeO content. MIs with low MgO and FeO concentrations are all partiallyglassy ones from 10020 and 74235 that were significantly affected by post-entrapment crystallization. Homogenized MIs in 10020 and 74235have much higher MgO and FeO concentrations, but essentially the same H2O/Ce ratios compared to the natural MIs. In the legend, ‘‘HMI”and ‘‘NMI” stand for homogenized and natural melt inclusions, respectively. *One homogenized MI and one natural MI data in 10020 andone MI data in 12008 are from Chen et al. (2015); MI data in 74220 are from Ni et al. (2017a); glass beads data are from Hauri et al. (2015).

Table 3Estimated volatile abundances in the primitive lunar mantle based on lunar MI data. Chlorine abundance is italicized because of theuncertainty in our approach using Cl/K ratios (discussed in Section 4.2.3).

Element H2O F Cl S Li Na K

Abundance (ppm) 84 5 0.39 73 1.1 398 45.5Element Cu Zn Pb Rb Cs GaAbundance (ppm) 11 2.9 0.018 0.57 0.0041 0.57

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and moderately volatile elements (Na, K, Li, Cu, Zn, Pb,Cs, Rb, and Ga) for the primitive lunar mantle can be esti-mated based on lunar MI data (Table 3). In Table 3, theH2O/Ce ratio for 74220 is used to estimate H2O abundancein the lunar mantle. F/Nd ratios for 12040 are excludedwhen estimating F abundance. Sulfur abundance is esti-mated using the geometric mean of maximum S/Dy ratios

for all lunar sample (S/Dy = 109 1.4), ignoring the poten-

tial difference between low- and high-Ti basalts.If the estimated volatile abundances are compared to

those for the BSE, a volatile depletion trend for the Mooncan be obtained (Fig. 12). From Fig. 12, the inferred degreeof volatile depletion in the primitive lunar mantle relative tothe primitive terrestrial mantle can be ranked as followsbased on the limited available data:

Zn>Pb�Rb>Cu�Ga�Na�Cs>K�F>H2O�S>Li.

The degree of depletion is more than a factor of 10 forZn, and is smaller for other elements. This order of volatiledepletion is not correlated with the half condensation tem-perature (Lodders, 2003), the experimentally determinedvolatility trend by Norris and Wood (2017), or the orderof volatile depletion during volcanic eruptions on the lunarsurface (Section 4.1). Our data also do not follow the lunarvolatile depletion trend suggested by Albarede et al. (2015),especially for elements with 50% Tc lower than 800 K. Thelack of correlation between lunar volatile depletion and50% Tc temperatures can be partly explained by the 50%

Tc temperatures being calculated at 10�4 bar total pressure,which might not be applicable to the condensation of vola-tiles on the Moon. Wang and Jacobsen (2016) argued thattheir K isotope data, for example, are best explained by thecondensation of the Moon under a pressure higher than10 bar after the Giant Impact, which is dramatically differ-ent from the pressure used to calculate the 50% Tc temper-atures (10�4 bar H2, Lodders, 2003). Experimental work byNorris and Wood (2017) also shows that volatilities of ele-ments during vapor-melt reaction at one atmosphere pres-sure and 1300 �C could be significantly different from thatin a nebular gas at 10�4 bar. Hence, the trend for thedegrees of volatile element depletion may be used to con-strain the physical conditions during Moon formation.

Despite the uncertainties in predicting behavior of vola-tile elements during Moon formation based on their halfcondensation temperatures, one general feature in Fig. 12is that the lunar mantle is not very depleted in highly vola-tile elements (e.g. 50% Tc < 800 K). This observation is inaccordance with recent estimation of the bulk silicate Mooncomposition by Hauri et al. (2015), except that our estima-tion is mostly based on lunar MI data. Several models havebeen proposed to reconcile the existence of highly volatileelements (especially H2O) in the lunar mantle since the dis-covery of high concentrations of intrinsic H2O in lunarsamples. These models might also be applicable in explain-ing the lunar volatile depletion trend estimated in thisstudy. One possibility is that the Moon formed in a rela-tively low-energy impact (Hauri et al., 2015), in which only

Fig. 12. Volatile depletion trend for the lunar mantle on a melt inclusion perspective. Half condensation temperatures are from Lodders(2003). The composition of the bulk silicate earth is from McDonough and Sun (1995), except for H2O, which comes from H2O/Ce = 184 inChen et al. (2015). Abundances of refractory elements used for estimation of volatile abundances in this study are assumed to be ‘‘BSE-like” inthe lunar mantle. Error bars are 1r geometric standard deviation considering data variations in lunar MIs. For H2O, our preferred values forthe Moon are plotted in solid symbols, while the related complications are plotted in open symbols and discussed in the text. The volatiledepletion trend suggested in Albarede et al. (2015) is plotted in dashed line as a reference.

38 P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41

part of the Moon-forming materials was volatile depleted.The most significant weakness of this model is that it can-not explain the indistinguishable stable isotopic composi-tions between the Earth and Moon (Pahlevan, 2014). Asecond possibility is the incomplete loss of highly volatileelements during an energetic giant impact (e.g. Pahlevanet al., 2016; Nakajima and Stevenson, 2018). Pahlevanet al. (2016) provided a solution by formation of a gas disksurrounding the lunar magma ocean after the giant impactthat would be capable of maintaining hundreds of ppm dis-solved H species in the magma ocean. Nakajima andStevenson (2018) postulated that, if the upper parts of theMoon-forming disk were dominated by heavy species, thenthe loss of H2O would be limited by diffusion, causing inef-ficient depletion of H2O in the disk. Another way to explainthe abundances of highly volatile elements in the Moon isby asteroidal/cometary bombardment in the lunar magmaocean before it fully solidified (Bottke et al., 2010; Elkins-Tanton and Grove, 2011; Hauri et al., 2015; Barnes et al.,2016; Hauri et al., 2017). The last model is capable ofexplaining the abundances of highly volatile elements onthe Moon, and also to reconcile isotopic compositions ofsulfur, chlorine, hydrogen, and nitrogen in lunar samples(Hauri et al., 2017). In general, explaining the observedvolatile depletion trend for the Moon requires multi-disciplinary work to better understand the behavior of vola-tile elements during the giant impact, existence of the lunarmagma ocean, as well as magma generation and eruptionon the Moon.

5. CONCLUSIONS

In this paper we report volatile, major and trace ele-ment, and transition metal data in olivine-hosted meltinclusions from 5 mare basalt samples, including bothhigh-Ti (10020 and 74235) and low-Ti basalts (12040,15016, and 15647), and moderately volatile element data

for one melt inclusion, two glassy embayments, and twoglass beads from 74220 and 15421. Using our new datacombined with literature data, H2O/Ce ratios are foundto be correlated with the cooling rate of different lunar sam-ples, indicating an important role of degassing in causinglarge H2O/Ce variations among lunar samples. After rulingout other possibilities such as loss of H2O during homoge-nization, mantle partial melting, magma evolution, andingassing during eruption, we conclude that the H2O/Ceratio of �50 for 74220 is deemed more representative ofthe primitive lunar mantle. In addition, in situ analyses onMIs from 74220 show similar S/Dy, Zn/FeO, Pb/Ce,F/Nd, Cs/Rb, Rb/Ba, Cl/K, Na/Sr, Ga/Lu, K/Ba, andLi/Yb ratios compared to other lunar basalts, making itunlikely to be sourced from a volatile-enriched localheterogeneity in the lunar mantle. Previous studies regard-ing 74220 as an anomalous lunar sample are based on bulkanalyses, which were significantly affected by surface-correlated volatiles.

A volatility trend for volcanic eruption on the lunarsurface is obtained by comparing post-entrapment-crystallization corrected volatile abundances in melt inclu-sions, glassy embayments, and glass beads from 74220: H2

O > Cl > F � Cu � Zn > S � Ga > Cs > Rb � Pb > Na >K � Li. The estimated volatility trend is significantly differ-ent from the trend of half condensation temperatures ofthese elements and indicates that melt-vapor equilibriumexperiments applicable to lunar conditions are importantto understand volatile behaviors during lunar volcanism.

Combining lunar melt inclusion data obtained in thisstudy and from the literature, a volatile depletion trendcan be obtained for the Moon. This lunar volatile depletiontrend does not show obvious dependence on half condensa-tion temperatures of the volatile elements and is also differ-ent from the volatility trend for volcanic eruptions on thelunar surface. The lack of correlation indicates that theenvironment for volatile depletion for the Moon differs

P. Ni et al. /Geochimica et Cosmochimica Acta 249 (2019) 17–41 39

from that for nebular condensation. The depletion trendinferred for the primitive lunar mantle will provide vitalconstraints on the Moon formation model, requiring vola-tiles to be partially preserved during the Giant Impact, orreplenished to the Moon before fully crystallization of thelunar magma ocean.

ACKNOWLEDGMENT

This research is supported by NASA grant NNX15AH37G. Wewould like to thank NASA CAPTEM for providing the lunar sam-ples, K. P. Jochum for providing the MPI-DING glass standards,Yunbin Guan for his assistance in SIMS analyses, J. C. Barrette forthe assistance in LA-ICP-MS analyses, and Miki Nakajima forhelpful discussions. We thank Francis Albarede and an anonymousreviewer for their constructive reviews that greatly improved themanuscript. And we thank Francis for his courtesy of sharing anExcel file on lunar basalt data from Wiesmann (1975). The electronmicroprobe Cameca SX100 used in this study was purchased usingNSF grant EAR-9911352.

APPENDIX A. SUPPLEMENTARY MATERIAL

Supplementary data to this article can be found online athttps://doi.org/10.1016/j.gca.2018.12.034.

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Associate editor: Maud Boyet