Mineralogical controls on garnet composition in the ... · Mineralogical controls on garnet composition in the cratonic ... in diamond exploration (e.g., ... and Leonardos 1996)

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Contrib Mineral Petrol (2015) 169:13 DOI 10.1007/s00410-014-1102-7

ORIGINAL PAPER

Mineralogical controls on garnet composition in the cratonic mantle

P. J. A. Hill M. Kopylova J. K. Russell H. Cookenboo

Received: 4 May 2014 / Accepted: 24 December 2014 Springer-Verlag Berlin Heidelberg 2015

but requires the presence of spinel and reflects the thick-ness of the spinelgarnet transition zone. This requirement contradicts observations on natural occurrences of the trend and the thermobarometry of the host peridotites. In the pre-ferred model of a variably depleted mantle, the lherzolitic trend critically depends on the presence of clinopyroxene. The occurrence of lherzolitic garnet compositions in har-zburgite can be explained by exhaustion of clinopyroxene as a result of garnet buffering. The open system behavior of the peridotitic mantle also provides a better explana-tion for the harzburgitic trend in garnet compositions. In an isochemical mantle, the trend can be controlled by many possible reactions, and no single mineral is essential. In the variably depleted mantle, spinel is required to make the harzburgitic trend garnet.

Keywords Kimberlite concentrate Cratonic variably depleted mantle Harzburgitic garnet Lherzolitic garnet CaCr trend Mineral buffering

Introduction

Mantle petrologists have developed robust paradigms for gaining important geochemical insights into the continen-tal lithosphere from observed natural variations in garnet composition (e.g., Ryan et al. 1996; Griffin et al. 1999a; Schulze 2003). By contrast, the implications of the garnet compositions for mineralogical variations in the mantle are less well understood.

Most garnet available for mantle studies derives from individual grains in concentrates that have been disaggre-gated from their host rock; thus, the original parageneses are unknown. There have been several studies that inferred the mineralogy of the mantle from compositions of garnet

Abstract Garnet concentrates are a rich source of geo-chemical information on the mantle, but the mineralogi-cal implications of wide ranging garnet compositions are poorly understood. We model chemical reactions between mantle minerals that may buffer the CaCr lherzolitic garnet trend common in the lithospheric mantle. A har-zburgitic trend of garnet compositions featuring a lower increase in Cr with Ca relative to the conventional lher-zolitic trend is reported for the first time. Representation of garnet chemistry in terms of additive and exchange compo-nents in the Thompson space shows that the lherzolitic and harzburgitic trends are controlled by the cation exchanges MgFeAl Ca2Cr and MgFeAl4 Ca2Cr4, respectively. Various equilibrium reactions are presented to explain the trends assuming a closed or open system mantle. The com-positional variability of the natural garnets from the Canas-tra 8 kimberlite (Brazil) is modeled by a linear system of mass balance equations. The solution returns the reaction coefficients of products (positive values) and reactants (negative values), which are then evaluated against the observed mantle mineralogy. In the isochemical mantle, the lherzolitic trend can form in the absence of clinopyroxene,

Communicated by Timothy L. Grove.

Electronic supplementary material The online version of this article (doi:10.1007/s00410-014-1102-7) contains supplementary material, which is available to authorized users.

P. J. A. Hill M. Kopylova (*) J. K. Russell Department of Earth and Ocean Sciences, The University of British Columbia, Vancouver, BC V6T 1Z4, Canadae-mail: [email protected]

H. Cookenboo Watts, Griffis and McQuat Consulting Geologists and Engineers, #620 475 Howe St., Vancouver, BC V6C2B3, Canada

Contrib Mineral Petrol (2015) 169:13

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13 Page 2 of 20

in heavy mineral concentrates from kimberlite (Dawson and Stephens 1975; Schulze 2003). The garnets were divided into compositional clusters that imply coexistence with certain minerals and a certain parent rock. Among other types, the garnets were classified into lherzolitic (G9) and harzburgitic (G10) (Fig. 1). Garnets of the lherzolitic par-agenesis feature a strong positive lherzolitic CaCr trend explained by clinopyroxene buffering (Sobolev et al. 1973; Gurney 1984; Boyd et al. 1993). Specifically, the trend was explained as reflecting variations in the Cr content of the bulk mantle containing a Ca-rich mineral. Substitution of smaller Cr cations for the larger Al cations in the garnet lattice allows the X site to expand and accept more of the larger Ca ions (Smyth and Bish 1988; Smyth and McCor-mick 1995; Griffin et al. 1999a) from the clinopyroxene reservoir at the expense of smaller Mg and Fe ions.

However, the conventional explanation for the lherzolitic trend does not have a solid foundation in either experi-mental or theoretical data. Compilation of all experiments done on natural Cr- and Ca-bearing peridotites at 1,0751,600 C suggests that garnet compositions do not correlate with the absence or presence of clinopyroxene (Fig. 1b). Many garnets not equilibrated with clinopyroxene plot in the lherzolitic field, while garnets equilibrated with clino-pyroxene do not show the trend (Fig. 1b). The chaotic scatter of experimental garnets in the CaCr space cannot be ascribed to variations in the bulk compositions of the experimental systems, because in nature we also deal with a range of mantle protoliths. Furthermore, garnet fails to show the lherzolitic trend even in experiments employing a single fixed bulk composition (cf. Walter 1998; Takahashi 1986). Only one unpublished experimental study (Nickel 1983) produced a slope roughly paralleling the lherzolitic trend (dashed lines on Fig. 1b). The conventional explana-tion of the trend is further undermined by theoretical calcu-lations that can reproduce the trend only in the presence of

spinel (Ziberna et al. 2013). Moreover, the lherzolitic trend has been found in garnets from clinopyroxene-free harzbur-gites (Kopylova et al. 1999, MacKenzie and Canil 1999; Pearson et al. 1999; Kopylova and Caro 2004; Viljoen et al. 2009; van der Meer et al. 2013).

One of the goals for this work, therefore, is to calcu-late reactions with mantle phases that could result in the lherzolitic trend and to test the hypothesis that the trend requires buffering by clinopyroxene. For the calculations, we used garnet compositions from concentrate of the Canastra 8 kimberlite (Brazil). The Canastra 8 garnet com-positions show the common lherzolitic trend, but they also

2

4

6

8

01 2 3 4 5 6 7 8

CaO, wt%

Cr 2

O3,

wt%

GDC G10 G9

CCGE

a

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.1 0.2 0.3 0.4 0.5 0.6

Cr,

cpfu

Ca, cpfu

b

1.4

1.6

G10 G9

Not equilibrated with CpxEquilibrated with CpxEqulibrated with SplGar+Cpx+Opx+Gar+Spl (Brey et al. 1990)

Fig. 1 a Cr2O3 versus CaO plot for Canastra 8 concentrate garnet showing compositional boundaries for garnets in harzburgite (G10) and lherzolite (G9) fields (Gurney 1984). The CCGE trend from the Jericho kimberlite xenoliths (Kopylova et al. 2000) is also shown. The graphitediamond constraint (GDC) is from Grtter et al. (2006). b. Compositions of garnet (Ca vs. Cr, cations per formula units) in the presence or absence of clinopyroxene and spinel in experiments in natural Cr- and Ca-bearing peridotites. This compilation is based only on garnet compositions that contain more than 1 wt% Cr2O3 or CaO and uses experiments of Takahashi (1986), Brey et al. (1990), Canil and Wei (1992), Kinzler (1997), Andrew et al. (1998), Walter (1998) and Chepurov et al. (2013). Most experiments contain melt. In experiments by Brey et al. (1990), garnet grew from spinel seeds and assumed to be equilibrated with spinel. Dashed lines are slopes of the XCa/XCr correlation which vary with temperature (T = 1,0001,400 C) from 0.3 to 0.65 (Brenker and Brey 1997). A double-dashed red line is the average slope of XCa = 0.449 XCr (Brenker and Brey 1997) calculated from experiments of Nickel (1983). The slopes are shown from the plot origin. The yellow field covers data from Canastra 8 garnets from Fig. 1A


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demonstrate a unique correlation between CaO and Cr2O3 (Cookenboo 2005) featuring a more gentle slope in the conventional diagram (Fig. 1). Harzburgitic garnets do not show any trends normally, but certain linear relationships between Ca and Cr have been noticed in this compositional field. A correlation with a gentle slope in the CaOCr2O3 diagram (the so-called Cr-saturation array, Grtter et al. 2006) is suggested by the limit of the general composi-tional field exemplified by the graphitediamond constraint on Fig. 1 (line GDC). It had been proposed that the correla-tion reflects the presence of spinel and its capacity to buffer the Cr content of garnet (Grtter et al. 2006). Garnet in spi-nelgarnet peridotites collected from individual kimberlites plot on lines parallel to the GDC line (Grtter et al. 2006). Moreover, the slope of the CaCr correlation resembles the trend produced in garnet by equilibration with chromite and clinopyroxene [the chromiteclinopyroxenegarnet equilibrium (CCGE) line on Fig. 1]. We therefore test a working hypothesis that the harzburgitic trend requires the presence of spinel. Experimental data on garnet in equilib-rium with spinel in natural Ca- and Cr-bearing system are very scarce, and the garnet (Canil and Wei 1992; Kinzler 1997) falls in all fields on the CrCa plot (Fig. 1b). The only experimental work to report a significant number of spinel-equilibrated garnet compositions derived from lher-zolite (Brey et al. 1990) showed the trend with a lower slope passing through the lherzolitic trend but parallel to the harzburgitic trend (Fig. 1b).

Here, we constrain buffering minerals controlling for-mation of garnet trends through calculations of feasible mineral reactions. Our results can be used to reconstruct the mineralogy of a given mantle segment based on the concentrate garnets. The mineralogy, in turn, largely con-trols the diamond potential of the mantle and is a key factor in diamond exploration (e.g., Gurney and Zweistra 1995; Cookenboo and Grtter 2010).

Samples

Canastra kimberlites are located in the Brasilia Belt, a mobile belt comprised of thrust terranes that moved east over the Sao Francisco Craton during the Late Protero-zoic as the Amazonas Craton moved in from the west. The Sao Francisco Craton stabilized at approximately 2.72 Ga (Machado et al. 1992) extends west beneath the Brasilia Belt (Leonardos et al. 1993). The Parana Basin comprises sedimentary rocks and basalts, which accumulated dur-ing the Paleozoic to the west of the Canastra kimberlites (Fig. 2). Teleseismic surveys show that the lithosphere beneath the Brasilia Belt is more than 130 km thick (James and Assumpcao 1993). Mantle xenoliths from Tres Ran-chos diamondiferous kimberlites include spinel-free garnet

peridotite with coarse and sheared textures equilibrated at the 38 mW/m2 geotherm between 50 and 70 kb (Carvalho and Leonardos 1996). The xenoliths contain 212 vol% clinopyroxene and garnet from the lherzolitic trend. Few garnet grains from the harzburgite field have increased TiO2 (0.51.7 wt%) and are megacrystal (Carvalho and Leonardos 1996) or sourced from Ti-rich peridotites, which are commonly ilmenite-bearing and genetically related to megacrysts (Kopylova et al. 2009).

The diamondiferous Canastra 1 and Tres Ranchos kim-berlites were emplaced in the Brasilia Belt during the Early Cretaceous and Cenomanian (12095 Ma; Read et al. 2004). These kimberlites intruded the Neoproterozoic metasediments (quartzites and phyllites) of the Canastra and Araxs groups (Pimentel et al. 2011). Canastra 1 was the first of more than 30 kimberlitic bodies discovered in the region in a search for primary sources of unresorbed octahedral alluvial diamonds upstream along the upper San Francisco River. The Canastra 1 kimberlite is one of the four bodies in a linear NWSE trend stretching for more than 1 km. Its chrome pyrope garnet suite, like the Tres Ranchos kimberlite, is characterized by lherzolitic and slightly sub-calcic harzburgitic garnets extending to chrome-rich contents above the GDC (Carvalho and Leonardos 1996; Cookenboo 2005). Another NS linear emplacement trend 25 kilometers west of Canastra 1 com-prises eight kimberlite bodies (Fig. 2), the largest of which is Canastra 8 (21 ha) and the most northern is Canastra 18. The Canastra 8 trend kimberlites apparently sampled a very different mantle from the Canastra 1 kimberlites, which lack the harzburgitic trend in garnet compositions (Cooken-boo 2005).

Composition of mineral concentrates

Heavy mineral concentrates were extracted from Canastra kimberlites as part of the mineral resource evaluation pro-gram by De Beers and Brazilian Diamonds Ltd. Electron microprobe analyses of the concentrates were carried out in 1995 in the Anglo-American Research Labs (Johannesburg, South Africa). The electron microprobe Cameca SX-50 equipped with nine wavelength-dispersive spectrometers was used and operated at an acceleration potential of 20 kV and a beam current of 30 nA. Counts were collected for 10 s on the K- peak of all elements, and background count lev-els were estimated from long-term empirical trends. Stand-ards comprised pure oxides MgO (Mg), Cr2O3 (Cr), Al2O3 (Al), TiO2 (Ti) and Fe2O3 (Fe), as well as natural minerals rhodonite (Mn) and wollastonite (Ca, Si). Apparent con-centrations were corrected for matrix effects with an online computer program (Bence and Albee 1968). Lower limits of detection are calculated to be of the order of 0.06 wt%


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(at 2) for all elements. The calculated precision of CaO and Cr2O3 determinations is 2.5 % relative (at 2) and was confirmed to be of that order by reanalyzing the same spot 10, 25 and 100 times on an appropriate variety of garnet grains. A total of 4,760 analyses from Canastra 8 kimberlite include ~30 clinopyroxenes, 360 ilmenites, 2,360 spinels, 2,000 garnets, few rutile, magnetite and corundum grains. Canastra 18 analyses include 250 analyses (150 spinels, 70 garnets, few ilmenites).

A total of ~70 grains of Canastra 8 pyrope were re-analyzed in 2003 with the electron microprobe facil-ity in the Department of Earth and Ocean Sciences at the University of British Columbia as part of the quality

assurance protocol. The analysis was done on a fully auto-mated Cameca SX-50 operating in the wavelength-dis-persion mode with excitation voltage 15 kV, beam current 30 nA, peak count time 10 s (60 s for Na), background count time 5 s (30 s for Na), spot diameter 5 m. The standards X-ray lines, crystals and background positions (sin 105) were used for the elements listed: albite, NaK, TAP, 700/700; grossular, AlK, TAP, 800/800; diopside, MgK, TAP, 1,500/1,500; grossular, SiK, TAP, 700/700; grossular, CaK, PET, 750/750; rutile, TiK, PET, 650/600; synthetic magnesiochromite, CrK, LIF, 700/700; synthetic rhodonite, MnK, LIF, 300/200; and synthetic fayalite, FeK, LIF, 700/1,400. Data reduction

Fig. 2 Geologic location of Canastra kimberlites in the Brasilia Belt in relation to the Amazon Craton (to the west of the dashed outline) Paran Basin and the So Francisco Craton. The geological back-

ground is from Campos Neto et al. (2003), Canastra 8 and 1 clusters are from Cookenboo (2003, 2005)


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was done using the PAP (Z) method (Pouchou and Pichoir 1985). Electron microprobe analyses of garnet and spinel in Canastra 8 and 18 concentrates are reported in Supplementary material 1 and 2.

Garnet

Garnet from Canastra 8 concentrate shows two trends with strong positive correlations between Cr2O3 and CaO (Fig. 1): a steeper lherzolitic trend and a more gentle trend (i.e., harzburgitic) restricted predominately to the har-zburgitic field. Compositions of garnet from Canasta 18 kimberlite also display the two trends, but the trend of har-zburgitic garnet comprises fewer grains (Fig. 3a). Conse-quently, most of our work focused on the Canastra 8 kim-berlite where the harzburgitic trend is best developed.

To study the cation exchanges and the buffering reac-tions responsible for each trend, garnet compositions defining the two trends were separated. The samples were divided based on the G10/G9 fields of Gurney (1984) (Fig. 4). Interpretation of the compositional data was done in cation amounts to eliminate closure problems associ-ated with weight percent representation. The analyses were converted to cations per formula unit (cpfu) form based on 12 oxygens. The amount of Fe3+ was calculated on the premise of the charge balance. In the harzburgitic trend, the average amounts of Ti, Mn and Fe3+ were 0.007, 0.026 and 0.016 cpfu, respectively. These low-abundance cati-ons were combined with others thereby lowering the total number of components considered in the reactions. Based on expected cationic substitutions (Russell et al. 1999), we combined Ti with Si to make Si*, and Mn and Fe3+ with Fe2+ making Fe*.

Spinel

Spinel analyses from Canastra 8 and 18 concentrates (Fig. 3b, c) were converted to cations per formula unit based on four oxygens, and the Fe3+ content was deter-mined using the charge-balance distribution. The average cation amounts for Ti, Mn and Fe3+ were 0.012, 0.008 and 0.036 cpfu, respectively. As their quantities were insig-nificant, Ti was normalized out, while Mn and Fe3+ were added to Fe2+ making Fe*.

0

2

4

6

8

1 2 3 4 5 6 7 8

Cr 2

O3,

wt%

CaO, wt%

GDC

G10 G9Garnet Canastra 18a

8 10 12 14 16 18 2020

30

40

50

60

70Spinel Canastra 18

MgO, wt%

Cr 2

O3,

wt%

b

20

30

40

50

60

70

8 10 12 14 16 18 20

Spinel Canastra 8

Cr 2

O3,

wt%

MgO, wt%

c

Fig. 3 Compositions of garnet and spinel from kimberlites Canastra 18 and Canastra 8. a Plot of Cr2O3CaO (wt%) for Canastra 18 garnet analyses. b, c Cr2O3MgO (wt%) plots for spinels from Canastra 18 (b) and Canastra 8 (c)

0

0.1

0.2

0.3

0.4

0.5

0.1 0.2 0.3 0.4 0.5 0.6

Cr,

cpfu

Ca, cpfu

Lo-Cr Gar

Hi-Cr Gar

Hi-Cr,Hi-Ca Gar

Lo-Cr,Lo-Ca Gar

Fig. 4 CrCa (cpfu) plots for Canastra 8 garnet, with analyses from the lherzolitic and harzburgitic trend marked with different symbols. Also shown are end-members used for calculations of buffering reac-tions from Tables 1 and 2. Compositional boundaries for garnets in harzburgite (G10) and lherzolite (G9) fields are from Gurney (1984)


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13 Page 6 of 20

Tabl

e 1

End

-mem

bers

for m

antle

min

eral

s us

ed fo

r rea

ctio

ns m

odel

ing

Fe*

= F

e +

Mn

+ F

e3+

Si*

= S

i + T

i, G

ar g

arne

t, Sp

l s

pine

l, O

px

orth

opyr

oxen

e, C

px c

linop

yrox

ene,

Ol

oliv

ine,

Fo

fors

teri

te, F

a fa

yalit

e

Min

eral

Com

posi

tion

Nam

e of

the

end-

mem

ber

Roc

kL

ocat

ion

Ref

eren

ce

Gar

net f

rom

lh

erzo

litic

tren

dC

a 0.5

0Fe*

0.47

Mg 2

.05 A

l 1.5

5Cr 0

.43

Si* 3

O12

Hi-

Cr,

Hi-

Ca

Gar

Min

eral

con

cent

rate

Can

astr

a 8

Thi

s w

ork

Ca 0

.33F

e*0.

55M

g 2.1

4 Al 1

.91C

r 0.0

7 Si

* 3O

12L

o-C

r, L

o-C

a G

arM

iner

al c

once

ntra

teC

anas

tra

8T

his

wor

k

Gar

net f

rom

ha

rzbu

rgiti

c tr

end

Ca 0

.16

Fe* 0

.43M

g 2.3

9 Al 1

.73C

r 0.2

9 Si

3* O

12L

o-C

r Gar

Min

eral

con

cent

rate

Can

astr

a 8

Thi

s w

ork

Ca 0

.39

Fe* 0

.48M

g 2.1

5 Al 1

.63C

r 0.3

5 Si

* 3 O

12H

i-C

r Gar

Min

eral

con

cent

rate

Can

astr

a 8

Thi

s w

ork

Spin

elFe

* 0.2

9Mg 0

.7A

l 1.4

5Cr 0

.56O

4L

o-C

r Spl

Min

eral

con

cent

rate

Can

astr

a 8

Thi

s w

ork

Fe* 0

.41M

g 0.6

0Al 0

.61C

r 1.3

8O4

Hi-

Cr S

plM

iner

al c

once

ntra

teC

anas

tra

8T

his

wor

k

Ort

hopy

roxe

neC

a 0.0

3Fe*

0.18

Mg 1

.75A

l 0.0

3Cr 0

.01S

i*2O

6L

o-M

g O

pxG

ar-b

eari

ng c

rato

nic

peri

dotit

ew

orld

wid

e co

mpi

latio

nSe

e te

xt a

nd F

ig. 8

Ca 0

.01F

e*0.

13M

g 1.8

4Al 0

.01C

r 0.0

1Si*

2O6

Hi-

Mg

Opx

Gar

-bea

ring

cra

toni

c pe

rido

tite

wor

ldw

ide

com

pila

tion

See

text

and

Fig

. 8

Ca 0

.02F

e*0.

12M

g 1.7

3Al 0

.11C

r 0.0

2Si*

2O6

Hi-

Al O

pxG

arne

t-on

ly c

rato

nic

peri

dotit

eU

dach

naya

kim

berl

iteB

oyd

et a

l. (1

997)

Ca 0

.02F

e*0.

13M

g 1.8

0Al 0

.04C

r 0.0

1Si*

2O6

Lo-

Al O

pxSp

inel

gar

net c

rato

nic

peri

dotit

eU

dach

naya

kim

berl

iteB

oyd

et a

l. (1

997)

Clin

opyr

oxen

eC

a 0.9

2Fe*

0.05

1Mg 0

.98A

l 0.0

26C

r 0.0

26Si

* 2O

6SG

1 C

pxSp

l-G

ar p

erid

otite

Jeri

cho

kim

berl

iteK

opyl

ova

et a

l. (1

999)

Ca 0

.81F

e*0.

081M

g 0.9

5Al 0

.081

Cr 0

.081

Si* 2

O6

SG2

Cpx

Spl-

Gar

per

idot

iteJe

rich

o ki

mbe

rlite

Kop

ylov

a et

al.

(199

9)

Ca 0

.94F

e*0.

06M

g 0.8

3Al 0

.12C

r 0.0

5Si*

2O6

Hi-

Al C

pxG

ar-b

eari

ng c

rato

nic

peri

dotit

ew

orld

wid

e co

mpi

latio

nSe

e te

xt a

nd F

ig. 9

Ca 1

.00F

e*0.

08M

g 0.8

4Al 0

.04C

r 0.0

4Si*

2O6

Lo-

Al C

pxG

ar-b

eari

ng c

rato

nic

peri

dotit

ew

orld

wid

e co

mpi

latio

nSe

e te

xt a

nd F

ig. 9

Ca 0

.88F

e*0.

07M

g 0.9

3Al 0

.08C

r 0.0

4Si*

2O6

Hi-

Ca

Cpx

Gar

-bea

ring

cra

toni

c pe

rido

tite

wor

ldw

ide

com

pila

tion

See

text

and

Fig

. 9

Ca 0

.74F

e*0.

11M

g 1.0

2Al 0

.09C

r 0.0

4Si*

2O6

Lo-

Ca

Cpx

Gar

-bea

ring

cra

toni

c pe

rido

tite

wor

ldw

ide

com

pila

tion

See

text

and

Fig

. 9

Hyp

othe

tical

mel

tC

a 0.4

84Fe

* 0.2

86M

g 0.4

59A

l 0.6

7Si*

1.88

O6

MO

RB

mod

e of

mid

-oce

an ri

dge

basa

ltw

orld

wid

e co

mpi

latio

nA

reva

lo a

nd M

cDon

ough

(201

0)


1 3

Page 7 of 20 13

Calculation of buffering reactions

Principle

We solved a linear system of mass balance equations to model the possible buffering reactions consistent with the two observed compositional trends of garnet. The composi-tional variations in the garnet and other mantle minerals are the knowns, and the modes of the paragenetic minerals are the unknowns. This modeling has an advantage of being free of assumptions on the direction of the reaction and what minerals are reactants or products. The solution math-ematically distributes the reaction participants between the left and the right part of the balanced reaction. There is one equation for each element (see Supplementary material 3), and the number of elements used to capture the full compo-sitional variability of mineral solid solutions in the possible reaction participants was seven (six elements plus oxygen).

The maximum number of mineral components involved in the reaction is ten, as the compositional variabilities of the five common mantle phases (garnet, spinel, orthopyrox-ene, clinopyroxene and olivine) are each modeled by two end-member compositions. Since only six major element oxides vary significantly in the garnet and associated min-erals (see below), it is impossible to write ten equations for the mass balance and to calculate a single buffering reac-tion based on all ten end-member minerals. Instead, we calculated several possible reactions for various scenarios that excluded certain minerals. The sets of participating minerals are listed in Table 1 and the resulting reactions in Tables 2, 3, and 4 (see Supplementary material 4 for the corresponding matrices). The reaction coefficients are all normalized to 1,000 mol of the initial garnet for compara-tive purposes. We then converted the model mineral molar abundances (i.e., reaction coefficients) to volume equiva-lent ratios (see Supplementary material 3) in order to com-pare our model results against mineral modes for mantle peridotite.

The range of permissive reactions depends strongly on the compositional restrictions placed on the mantle. In an isochemical mantle, under the assumption of the closed system, the compositional variability of garnet is created entirely by buffering reactions with coexisting minerals. The reactions are driven by changing pressure or tem-perature. An alternative to this model is the open system behavior of the mantle. It has a range of bulk compositions because of the natural heterogeneity and varying degrees of depletion. Under an assumption of the variably depleted mantle, garnet trends reflect the level of Cr and Ca imposed by the bulk composition. The open system behavior of the mantle was modeled through extraction of Mid-Ocean Ridge Basalt (MORB).

Reaction participants

Garnet

The Thompson space transformation (Thomspon 1982) was used to determine the elemental exchanges respon-sible for the majority of the variance in garnet (see Sup-plementary material 3). CrAl1, FeMg1 and CaMg1 accounted for 89 % of the variance in the garnet samples in the lherzolitic trend and 86 % in the harzburgitic trend. For samples from the lherzolitic trend, plotting [CrAl] against [FeMgCa2] = [FeMg]2[CaMg] shows a 1:1 ratio (Fig. 5). Therefore, MgFeAl Ca2Cr exchange con-trols the lherzolitic trend in garnet composition. In contrast, the samples defining the harzburgitic trend show a 1:2 lin-ear relationship between [CaMg] and [CrAl] (Fig. 6). A linear relationship was also observed between [CaMg] and [FeMg] implying a linear dependence between the three exchanges [CrAl], [CaMg] and [FeMg]. Plotting [FeMgCa2] = [FeMg]2[CaMg] against [CrAl] shows a 4:1 relationship suggesting that a combined exchange MgFeAl4 Ca2Cr4 explains the harzburgitic trend.

Another difference between garnet compositions from the two trends is their average TiO2 content. The TiO2 con-tent is lower in garnet from the harzburgitic trend (mean ~0.06 0.05 wt%) than in the lherzolitic trend (mean ~0.19 0.12 wt%) as expected for the more depleted har-zburgitic mantle. Very strong cationic correlations (Fig. 3) between the compositional exchanges suggest that the vast majority of garnet analyses in both trends can be described as linear combinations of only two end-member composi-tions (Table 1; Fig. 4).

Spinel

It is possible that not all spinel in the Canastra 8 kimberlite is in equilibrium with garnet. In order to identify the subset of spinel equilibrated with harzburgitic garnet, the spinel compositions in the Canastra 8 kimberlite were compared with the spinel found in the Canastra 18 kimberlite (Fig. 7). Low-Cr spinel is missing in the Canastra 18 kimberlite where the harzburgitic trend is less pronounced; therefore, the low-Cr spinel is likely to be equilibrated with the har-zburgitic garnet. To account for this, all spinel analyses with Al


1 3

13 Page 8 of 20

used the same spinel end-members for modeling the lher-zolitic and harzburgitic trend.

Orthopyroxene

Orthopyroxene was selected as one of the reaction partici-pants due to its common presence in the harzburgitic and lherzolitic cratonic mantle. The absence of orthopyroxene in heavy mineral concentrate of Canastra 8 and the absence of published analyses of mantle pyroxenes from the Bra-silia Belt kimberlites forced us to choose orthopyroxene end-member compositions from elsewhere in the cratonic mantle. For this, we compiled orthopyroxene analyses from garnet peridotites from the Slave, Kaapvaal and Siberian cratons. The xenoliths included spinelgarnet and garnet-only coarse peridotites and sheared garnet peridotites from eight pipes (Nixon and Boyd 1973a, b; Cox et al. 1973; Kopylova et al. 1999; Kopylova and Caro 2004; Ionov et al. 2010). Analysis of the dataset of 150 cratonic orthopyrox-enes revealed two most important correlations, the negative MgOFeO and a weaker positive CaOAl2O3 correlations (Fig. 8). The general CaOAl2O3 trend visible in the world-wide data is complicated by negative CaOAl2O3 correla-tions observed in some rock types from specific locations (Fig. 8b). Cr2O3 varies independently of other elements. Thus, we employ two possible ranges of orthopyroxene compositions to accurately represent global heterogeneity of this phase in the cratonic mantle. The first range of com-positions shows a negative MgOFeO and a positive CaOAl2O3 correlations. We chose the end-members (labeled Hi-Mg and Lo-Mg on Fig. 8) from the CaOAl2O3 trend (green line on Fig. 8b) ignoring extremes with rare com-positions, for example, high-Al and high-Fe Lesotho sam-ples. The second possible range of orthopyroxenes is with a

negative CaOAl2O3 correlation at a constant FeO content (labeled Hi-Al and Lo-Al on Fig. 8). These analyses were taken from the Udachnaya garnet-only and spinelgarnet peridotites (Boyd et al. 1997). Orthopyroxenes from gar-net peridotites have lower Al2O3 content than orthopyrox-enes equilibrated with spinel (Boyd et al. 1997), and the balance of Al2O3 may have been used to make the garnet. The orthopyroxene compositions selected in such a way are listed in Table 1.

Clinopyroxene and olivine

Clinopyroxene may be involved in the buffering reactions of the lherzolitic trend, but it is not expected to be a critical phase for buffering harzburgitic garnets. The harzburgitic garnet trend, however, has a similar slope in the Cr2O3CaO space as garnet coexisting with clinopyroxene and chromite (the CCGE trend on Fig. 1). We therefore investi-gated a possible participation of clinopyroxene in both lher-zolitic and harzburgitic trends. There are no clinopyroxene grains in the Canastra 8 concentrate, and no clinopyroxene compositions are reported for the peridotitic mantle of the Brazilia Belt.

Three possible ranges of clinopyroxene were employed for the modeling. Because we tested the model that the trends may require spinel buffering, it was necessary to use compositions of clinopyroxene from a well character-ized suite of cratonic spinelgarnet peridotites, For this, we chose clinopyroxene in spinelgarnet peridotites in the Jericho kimberlite (Kopylova et al. 1999), i.e., SG1 and SG2 (Table 1). The other two alternative ranges of clino-pyroxene were chosen from the worldwide cratonic data-base discussed in the orthopyroxene section. A special care was taken to eliminate secondary Al- and Na-poor

Fig. 5 [CrAl] versus [FeMgCa2] plot for analyses of lherzolitic trend garnets. The correlation can be approximated by the 1:1 line


1 3

Page 9 of 20 13

clinopyroxene, replacing primary grains on margins and developing in interstices between primary phases (Boyd et al. 1997; Kopylova and Caro 2004). Figure 9a demon-strates that all clinopyroxene with Al2O3 22 wt%) clinopyroxenes of Jericho and subcalcic (CaO


1 3

13 Page 10 of 20

garnet involves production of forsterite and consumption of fayalite. In the orthopyroxene-free reaction [Reactions (2AC) in Table 2], the coefficients suggest that one mole of clinopyroxene can buffer 1.515 mol of garnet, depend-ing on the exact clinopyroxene composition. Garnet com-position changes as a result of concomitant evolution in the chemistry of spinel, clinopyroxene and olivine, and changing modal abundances of these phases. Reactions without olivine and spinel [Reactions (3AC) and (4AC)] are drastically different from the rest of the calculated reactions. These reactions predict very little or no changes in the abundance of garnet, spinel and olivine, a reduction in the orthopyroxene mode and production of clinopyrox-ene. These reactions are essentially exchange reactions with minor transformation of orthopyroxene to clinopy-roxene. The reactions without spinel [Reactions (4AC)] involve an extremely high amount of orthopyroxene com-pared to all other phases and especially garnet (Opx/Gar volume ratio = 2067, Table 2) and therefore are deemed unrealistic.

An analogous set of possible reactions was computed for the harzburgitic trend. Reactions without clinopyrox-ene (5A, B) or orthopyroxene (6AC) are feasible, as the

proportions of buffering phases are consistent with the observed modes in mantle peridotites (McDonough and Rudnick 1998; Pearson et al. 2003). The reactions that do not involve olivine [Reactions (7AC) or spinel Reactions (8AC)] resemble the corresponding scenarios for the lher-zolitic trend. In these, garnet evolves along the trend by cat-ion exchange involving spinel, olivine and pyroxenes; all minerals except pyroxenes stay at almost constant modal abundances. The side effect of the buffering is a transfor-mation of one type of pyroxene to another, mainly from orthopyroxene to clinopyroxene.

The above reactions can be subdivided into two types (Tables 2, 3) by simplifying the equilibria and calculat-ing the differences in the modes of respective minerals (i.e., orthopyroxenes, clinopyroxenes, garnets, spinels and olivines) on the left and right sides of the reaction (cf. Kopylova et al. 2000). In the absence of olivine and spinel, the garnet trends result from exchange reactions accompa-nied by minor pyroxene transformation. If the involvement of olivine and spinel is postulated, both lherzolitic and harzburgitic trends can be produced by a single net trans-fer reaction, which makes garnet and olivine out of spinel and 2 mol of pyroxenes. The composition of the new garnet

Fig. 7 CrAl (cpfu) plots for kimberlite spinels from Canastra 18 pipe (a), Canastra 8 (b) and Canastra 8 truncated (c). Also shown are end-members used for calculations of buffering reactions from Table 1


1 3

Page 11 of 20 13

varies, depending on the initial spinel and pyroxene com-positions, but the general pattern of the reaction is prede-termined by the balance of Si. Garnet can be made from spinel only if olivine is also produced, and pyroxene is consumed to provide the source of Si for new nesosilicates. The reaction increases Cr in garnet by breaking spinel and using its Cr. This reaction is responsible for transforma-tion of peridotite from the spinel to garnet depth facies and thus will be controlled by changing pressure (ONeil 1981; Klemme 2004; Ziberna et al. 2013). If garnet is produced at the expense of spinel and pyroxenes, each garnet composi-tion in the trend would be equilibrated with a specific spi-nel composition and correspond to a certain pressure.

One can estimate an approximate change in pressure driving garnet compositions along the Canastra 8 trends using spinelgarnet barometers. The barometer based on the spinel composition equilibrated with garnet (ONeil 1981) predicts that decompression drives garnet composi-tion up the lherzolitic trend and to the right along the harzburgitic trend. Calculated equilibration of Hi-Cr, Hi-Ca Gar with Hi-Al Spl (Reactions 13) and Hi-Ca Gar with Hi-Al Spl (Reactions 57) in the presence of Fo93 occurs at 23 kb (ONeil 1981), whereas the right part of the reac-tion, which assumes equilibration of Lo-Cr, Lo-Ca Gar and Lo-Ca Gar with Hi-Cr Spl, requires 35 kb. Another barometer based on the Cr content of garnet buffered by spinel (Grtter et al. 2006) predicts the opposite evolution of the lherzolitic garnet composition with changing pres-sure. Lherzolitic trend stretches from 31 kb at its high-Ca, high-Cr end to 19 kb at the low-Ca, low-Cr end (Fig. 10). The harzburgitic trend forms at almost constant pressures between the 31 and 35 kbar (Fig. 10).

The results of the modeling in Tables 2 and 3 can be summarized as follows. The lherzolitic trend can be pro-duced by many types of buffering reactions, but all of the reactions require the presence of spinel. Spinel-free Reac-tions (4AC) are non-feasible, as they involve unrealis-tically high orthopyroxene/garnet ratios. Interestingly, lherzolitic trend can be modeled even in the absence of clinopyroxene (Reactions 12) if spinel participates in the buffer. As for the harzburgitic trend, it can be buffered by a larger number of possible reactions, including spinel-free Reactions 8AC. Chromium content in garnet from the harzburgitic trend changes insignificantly, so the presence of spinel is not mandatory to balance the evolving garnet compositions. The major change in the Ca content in the harzburgitic trend is achieved by changing the clinopyrox-ene mode.

Isochemical vs variably depleted mantle

Our modeling dictates that in the closed system mantle, garnet in the lherzolitic trend can only be generated in

the presence of spinel and records variations in equilib-rium pressure. This is supported by thermodynamic mod-eling and calculated phase diagrams for relatively fertile mantle compositions. Specifically, Ziberna et al. (2013) demonstrate that, in spinel-free assemblages, garnet does not vary in Cr content over the range of 9001,500 C and 25100 kb confirming the earlier conclusion by Brey et al. (1990) that only the presence of spinel can cause

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 0.4 0.8 1.2 1.6

CaO

, wt%

Al2O

3, wt%

b

Hi-Mg

Lo-Mg

Hi-Al

Lo-Al

32

33

34

35

36

37

38

3.5 4 4.5 5 5.5 6 6.5 7 7.5

Jericho

Gahcho KueLesotho pipes

Udachnaya

MgO

, wt%

FeO, wt%

aHi-Mg

Lo-MgLo-AlHi-Al

Fig. 8 Compositions of orthopyroxenes from cratonic garnet peri-dotites. a FeOMgO (wt%), b Al2O3CaO (wt%). The analyses were taken from spinelgarnet, garnet-only coarse peridotites and sheared garnet peridotites from Lesotho pipes (Nixon and Boyd 1973a, b; Cox et al. 1973), Jericho (Kopylova et al. 1999), Gahcho Kue (Kopy-lova and Caro 2004) and Udachnaya (Ionov et al. 2010) kimberlites. Trends of anticorrelated FeMg content and correlated Al and Ca contents are marked at thick yellow lines. Orthopyroxene composi-tions marked as solid green circles and labeled as Hi-Mg, Lo-Mg, Hi-Al and Lo-Al (Table 1) were chosen as end-members for mode-ling. Rare compositions of highly Ca- and Al-rich orthopyroxenes of Lesotho were not used for the modelling as non-representative


1 3

13 Page 12 of 20

variations in the Cr content of garnet. Within the restric-tions of Ziberna et al. (2013) modeling, trends parallel to the lherzolitic trend can be produced by pressuretempera-ture conditions found on a mantle geotherm (Fig. 10). The extremely depleted, clinopyroxene-free mantle composition shifts the garnet trend to the harzburgitic field and changes its slope from positive to negative, but the trend still does not resemble the harzburgitic trend (Fig. 10).

The mandatory involvement of spinel in generation of the lherzolitic trend, postulated here and in Ziberna et al. (2013), suggests that the Cr content of the lherzolitic garnet is a perfect measure of its depth of origin. Garnets defin-ing extended lherzolitic trends may reflect sampling by the kimberlite over depths of 70170 kms (P = 2050 kb), cor-responding to the range of Cr2O3 contents in garnet from 1 to 12 wt%. If garnets from the lherzolitic trend have, indeed,

equilibrated with spinel, then the garnet Cr2O3 contents are indicative of formation pressures and temperatures. Moreo-ver, single-crystal garnet barometers that assume equilibra-tion of garnet with spinel (Ryan et al. 1996; Grtter et al. 2006) would work perfectly for all grains from the lher-zolitic trends. All these consequences are testable.

We searched for reported correlations between equilib-rium pressures and temperatures and the Cr2O3 contents in garnets of the lherzolitic trend. A correlation between Ni-in-Gar temperature below 1,100 C and the maximal and average Cr2O3 garnet contents is observed in the worldwide cratonic database (Fig. 2 of Griffin et al. 1999a). Some local datasets on mantle xenoliths (Kopylova et al. 1999; Ionov et al. 2010) also show more chromian lherzolitic gar-nets being derived from deeper peridotites.

The empirical evidence on the absence of spinel buffer-ing on the lherzolitic garnet trend is more abundant. Firstly, in many localities worldwide, spinel is absent in garnet peri-dotites in which garnet analyses plot on the lherzolitic trend (e.g., Gurney and Switzer 1973; Shee et al. 1982; Skinner 1989; Carvalho and Leonardos 1996; Schmidberger and Francis 1999; Kopylova et al. 2000; Kopylova and Caro 2004; Ionov et al. 2010; Howarth et al. 2014). Secondly, the assumption of spinel buffering forces aluminous spinel in equilibrium with chromian garnet (Reactions 13) and there-fore yields lower pressure estimates (ONeil 1981) for the lat-ter, contradicting the presence of more chromian lherzolitic garnets in deeper peridotites (Kopylova et al. 1999; Ionov et al. 2010). Finally, there is abundant indirect evidence for the absence of Cr saturation and spinel buffering for many garnet concentrates with the lherzolitic trend. When the empirical garnet barometer of Grtter et al. (2006) is applied, the garnet compositions yield lower pressure estimates than the pressure

16

18

20

22

24

0 0.5 1 1.5 2 2.5 3 3.5

Lesotho pipes

Gahcho Kue

Udachnaya

Jericho

Secondary Gahcho Kue

Secondary JerichoC

aO, w

t%

Al2O

3, wt%

Hi-Al

Hi-Ca

Lo-Ca

Lo-Al

a

SG1

SG2

16

18

20

22

24

0 0.5 1 1.5 2 2.5 3 3.5Hi-Al

Lo-Al

Hi-Ca

Lo-Ca

CaO

, wt%

b

SG1

SG2

Cr O , wt%2 3

Fig. 9 Compositions of clinopyroxenes from cratonic garnet perido-tites. a Al2O3CaO (wt%), b Cr2O3CaO (wt%). The analyses were taken from spinelgarnet, garnet-only coarse peridotites and sheared garnet peridotites from Lesotho pipes (Nixon and Boyd 1973a, b; Cox et al. 1973), Jericho (Kopylova et al. 1999), Gahcho Kue (Kopy-lova and Caro 2004) and Udachnaya (Ionov et al. 2010) kimberlites. Secondary Al- and Na-poor clinopyroxenes, replacing primary grains on margins and developing in interstices between primary phases (Kopylova and Caro 2004), are shown separately from primary clino-pyroxenes. On Fig. 1a, black lines show thresholds for the analyses not considered representative for clinopyroxene equilibrated with gar-net in the cratonic mantle, i.e., secondary (Al2O3 22 wt%) clinopyroxenes, and very low-Ca (CaO


1 3

Page 13 of 20 13

Tabl

e 2

Buf

feri

ng re

actio

ns fo

r the

lher

zolit

ic tr

end

in th

e is

oche

mic

al m

antle

1 R

eact

ion

type

was

det

erm

ined

by

calc

ulat

ing

the

diff

eren

ce in

com

posi

tions

of s

olid

sol

utio

n m

iner

als

on th

e le

ft a

nd ri

ght s

ides

of t

he re

actio

n, a

s de

scri

bed

in K

opyl

ova

et a

l. (2

000)

. Vol

ume

ratio

s of

reac

tant

s w

ere

dete

rmin

ed b

ased

of m

iner

al d

ensi

ty a

nd re

actio

n co

effic

ient

s (S

uppl

emen

tary

mat

eria

l 3)

2 W

here

ort

hopy

roxe

ne is

not

pre

sent

, the

gar

net/s

pine

l rat

io o

f rea

ctan

ts is

repo

rted

No

Feat

ures

Rea

ctan

tsPr

oduc

tsR

eact

ion

type

1O

px/G

ar v

olum

e ra

tios

of re

acta

nts2

No

clin

opyr

oxen

e

(1A

)O

px w

ith a

rang

e of

Mg

cont

ents

1,00

0 H

i-C

r, H

i-C

a G

ar +

1,1

05 H

i-A

l Sp

l + 2

,954

Hi-

Mg

Opx

+ 1

35 F

a1,

411

Lo-

Cr,

Lo-

Ca

Gar

+ 6

95 H

i-C

r Sp

l + 2

,132

Lo-

Mg

Opx

+ 5

46 F

oSp

l + 2

Opx

= G

ar +

Ol

Opx

/Gar

= 1

.6

(1B

)O

px w

ith c

onst

ant M

g1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 1

,380

Hi-

Al

Spl +

2,3

20 L

o-A

l Opx

+ 8

0 Fa

1,59

0 L

o-C

r, L

o-C

a G

ar +

790

Hi-

Cr

Spl +

1,1

50 H

i-A

l Opx

+ 6

70 F

oSp

l + 2

Opx

= G

ar +

Ol

Opx

/Gar

= 1

.3

No

orth

opyr

oxen

e

(2A

)C

px fr

om S

pl-G

ar

peri

dotit

es1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 2

66 H

i-A

l Spl

+ 7

4 SG

2 C

px +

56

Fo +

53

Fa89

4 L

o-C

r, L

o-C

a G

ar +

373

Hi-

Cr S

pl +

288

SG

1 C

px +

56

Fo +

53

FaSp

l + 2

Cpx

= G

ar +

Ol

Gar

/Spl

= 1

0.6

(2B

)C

px w

ith c

orre

late

d C

aA

l1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 2

75 H

i-A

l Sp

l + 1

23 H

i-A

l Cpx

+ 3

9 Fo

+ 5

9 Fa

903

Lo-

Cr,

Lo-

Ca

Gar

+ 3

73 H

i-C

r Spl

+ 3

18

Lo-

Al C

pxSp

l + 2

Cpx

= G

ar +

Ol

Gar

/Spl

= 1

0.3

(2C

)C

px w

ith c

onst

ant A

l1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 3

47 H

i-A

l Sp

l + 6

61 L

o-C

a C

px +

7 F

o +

48

Fa94

6 L

o-C

r, L

o-C

a G

ar +

401

Hi-

Cr S

pl +

769

H

i-C

a C

pxSp

l + 2

Cpx

= G

ar +

Ol

Gar

/Spl

= 8

.1

No

oliv

ine

(3A

)C

px fr

om S

pl-G

ar

peri

dotit

es1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 4

10 H

i-A

l Sp

l + 1

22 S

G1

Cpx

+ 2

,379

Lo-

Mg

Opx

1,00

3 L

o-C

r, L

o-C

a G

ar +

407

Hi-

Cr

Spl +

409

SG

2 C

px +

2,0

86 H

i-M

g O

pxE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

1.3

(3B

)C

px w

ith c

orre

late

d C

aA

l1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 4

25 H

i-A

l Sp

l + 1

82 L

o-A

l Cpx

+ 2

,208

Lo-

Mg

Opx

1,00

0 L

o-C

r, L

o-C

a G

ar +

425

Hi-

Cr

Spl +

424

Hi-

Al C

px +

1,9

66 H

i-M

g O

pxE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

1.2

(3C

)C

px w

ith c

onst

ant A

l1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 4

18 H

i-A

l Sp

l + 1

,315

Hi-

Ca

Cpx

+ 3

,423

Lo-

Mg

Opx

1,00

0 L

o-C

r, L

o-C

a G

ar +

418

Hi-

Cr

Spl +

1,8

94 L

o-C

a C

px +

2,8

44 H

i-M

g O

pxE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

1.9

No

spin

el

(4A

)C

px fr

om S

pl-G

ar

peri

dotit

es1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 3

6,47

4 L

o-M

g O

px +

4,3

23 S

G1

Cpx

+ 8

63 F

o1,

005

Lo-

Cr,

Lo-

Ca

Gar

+ 3

4,75

7 H

i-M

g O

px +

6,0

40 S

G2

Cpx

+ 8

48 F

aE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

20.

3

(4B

)C

px w

ith c

orre

late

d C

aA

l1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 1

19,6

20 L

o-M

g O

px +

19,

855

Lo-

Al C

px +

3,2

90 F

o1,

000

Lo-

Cr,

Lo-

Ca

Gar

+ 1

15,5

83 H

i-M

g O

px +

23,

891

Hi-

Al C

px +

3,2

90 F

aE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

66.

7

(4C

)C

px w

ith c

onst

ant A

l1,

000

Hi-

Ca,

Hi-

Cr G

ar +

90,

233

Lo-

Mg

Opx

+ 4

8,46

7 H

i-C

a C

px +

1,3

67 F

o1,

000

Lo-

Cr,

Lo-

Ca

Gar

+ 7

8,23

3 H

i-M

g O

px +

60,

467

Lo-

Ca

Cpx

+ 1

,367

Fa

Exc

hang

e re

actio

n an

d O

px

Cpx

Opx

/Gar

= 5

0.3


1 3

13 Page 14 of 20

Tabl

e 3

Buf

feri

ng re

actio

ns fo

r the

har

zbur

gitic

gar

net t

rend

for t

he is

oche

mic

al m

antle

1 W

here

ort

hopy

roxe

ne is

abs

ent,

the

garn

et/s

pine

l rat

io is

repo

rted

No

Feat

ures

Rea

ctan

tsPr

oduc

tsR

eact

ion

type

Opx

/Gar

vol

ume

ratio

s of

reac

tant

s1

No

clin

opyr

oxen

e

(5A

)O

px w

ith a

rang

e of

Mg

cont

ents

1,00

0 H

i-C

a G

ar +

2,1

26 H

i-A

l Spl

+ 3

,917

H

i-M

g O

px2,

517

Lo-

Ca

Gar

+ 6

09 H

i-C

r Spl

+ 8

84

Lo-

Mg

Opx

+ 1

,459

Fo

+ 5

7 Fa

Spl +

2 O

px =

Gar

+ O

lO

px/G

ar =

2.2

(5B

)O

px w

ith c

onst

ant M

g1,

000

Hi-

Ca

Gar

+ 2

,650

Hi-

Al S

pl +

5,6

70

Lo-

Al O

px2,

920

Lo-

Ca

Gar

+ 7

30 H

i-C

r Spl

+ 1

,840

H

i-A

l Opx

+ 1

,810

Fo

+ 1

00 F

aSp

l + 2

Opx

= G

ar +

Ol

Opx

/Gar

= 3

.2

No

orth

opyr

oxen

e

(6A

)C

px fr

om S

pl-G

ar p

erid

otite

s1,

000

Hi-

Ca

Gar

+ 4

56 S

G2

Cpx

+ 1

39 F

o89

4 L

o-C

a G

ar +

45

Hi-

Al S

pl +

61

Hi-

Cr

Spl +

670

SG

1 C

px +

30

FaSp

l + 2

Cpx

= G

ar +

Ol

Gar

/Spl

pro

duct

= 2

1.6

(6B

)C

px w

ith c

orre

late

d C

aA

l1,

000

Hi-

Ca

Gar

+ 6

12 H

i-A

l Cpx

+ 1

21 F

o89

5 L

o-C

a G

ar +

69

Hi-

Al S

pl +

36

Hi-

Cr

Spl +

822

Lo-

Al C

px +

16

FaSp

l + 2

Cpx

= G

ar +

Ol

Gar

/Spl

pro

duct

= 2

2.7

(6C

)C

px w

ith c

onst

ant A

l1,

000

Hi-

Ca

Gar

+ 1

63 H

i-A

l Spl

+ 2

,359

L

o-C

a C

px +

4 F

o1,

063

Lo-

Ca

Gar

+ 1

00 H

i-C

r Spl

+ 2

,234

H

i-C

a C

px +

66

FaSp

l + 2

Cpx

= G

ar +

Ol

Gar

/Spl

= 1

7.3

No

oliv

ine

(7A

)C

px fr

om S

pl-G

ar p

erid

otite

s1,

000

Hi-

Ca

Gar

+ 1

15 H

i-A

l Spl

+ 6

37 S

G2

Cpx

+ 1

,489

Hi-

Mg

Opx

1,00

2 L

o-C

a G

ar +

113

Hi-

Cr S

pl +

783

SG

1 C

px +

1,3

40 L

o-M

g O

pxE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

0.8

(7B

)C

px w

ith c

orre

late

d C

aA

l1,

000

Hi-

Ca

Gar

+ 7

5 H

i-A

l Spl

+ 7

03 H

i-A

l C

px +

888

Hi-

Mg

Opx

1,00

0 L

o-C

a G

ar +

75

Hi-

Cr S

pl +

878

Lo-

Al

Cpx

+ 7

13 L

o-M

g O

pxE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

0.5

(7C

)C

px w

ith c

onst

ant A

l1,

000

Hi-

Ca

Gar

+ 9

8 H

i-A

l Spl

+ 5

,314

L

o-C

a C

px +

3,5

04 H

i-M

g O

px1,

000

Lo-

Ca

Gar

+ 9

8 H

i-C

r Spl

+ 4

,627

H

i-C

a C

px +

4,1

91 L

o-M

g O

pxE

xcha

nge

reac

tion

and

Cpx

O

pxO

px/G

ar =

2.0

No

spin

el

(8A

)C

px fr

om S

pl-G

ar p

erid

otite

s1,

000

Hi-

Ca

Gar

+ 8

,117

Lo-

Mg

Opx

+ 3

82

SG1

Cpx

+ 2

40 F

o1,

002

Lo-

Ca

Gar

+ 7

,575

Hi-

Mg

Opx

+ 9

24

SG2

Cpx

+ 2

35 F

aE

xcha

nge

reac

tion

and

Cpx

O

pxO

px/G

ar =

4.5

(8B

)C

px w

ith c

orre

late

d C

aA

l1,

000

Hi-

Ca

Gar

+ 2

0,09

8 L

o-M

g

Opx

+ 2

,609

Lo-

Al C

px +

583

Fo

1,00

0 L

o-C

a G

ar +

19,

250

Hi-

Mg

O

px +

3,4

57 H

i-A

l Cpx

+ 5

83 F

aE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

11.

2

(8C

)C

px w

ith c

onst

ant A

l1,

000

Hi-

Ca

Gar

+ 1

6,23

3 L

o-M

g

Opx

+ 6

,467

Hi-

Ca

Cpx

+ 3

22 F

o1,

000

Lo-

Ca

Gar

+ 1

4,23

3 H

i-M

g

Opx

+ 8

,467

Lo-

Ca

Cpx

+ 3

22 F

aE

xcha

nge

reac

tion

and

Opx

C

pxO

px/G

ar =

9.1


1 3

Page 15 of 20 13

estimates from traditional poly-mineral barometers (Grtter et al. 2006). This can be exemplified by the Jericho garnets in spinel-free peridotites (Kopylova et al. 1999), whose pres-sure of formation (the combination of orthopyroxene-garnet pressure and two-pyroxene temperature of Brey et al. 1990) is 1540 kb higher than the corresponding pressures evaluated from the Cr content of garnet. It can be explained only by the fact that the Cr content of the garnet is lower than the theo-retical Cr content that would be imposed onto the garnet by the coexisting spinel (Ryan et al. 1996; Grtter et al. 2006). Only a very minor proportion of garnets from each locality demonstrates the Cr saturation for a given Ca content; it is these grains that define the realistic, maximal pressure on the cluster of points in a pressuretemperature space (Ryan et al. 1996) or a CaCr space (Grtter et al. 2006).

We conclude that although a minor part of the observed variability of garnet composition in the lherzolitic trend can relate to its buffering by spinel in the isochemical man-tle, this explanation alone cannot satisfactory explain all observations on the lherzolitic trend. This suggests that the assumption of an isochemical (i.e., closed) mantle compo-sition used for the modeling was unrealistic and the open system behavior of the mantle should be considered.

The present view on the origin of the CaCr correlation in the lherzolitic trend envisions two processes. Firstly, the varying bulk composition of the mantle constrains the level of Cr enrichment in garnet. The Cr in garnet is controlled

by the bulk Cr content of the system (Brey et al. 1990). The range of Cr contents in garnets is the result of the nat-ural heterogeneity of the mantle and degrees of depletion (e.g., Griffin et al. 1999a, b; Klemme 2004). One can trace how the average Cr contents of garnet decrease from the Archean to Proterozoic and to Phanerozoic mantle (Fig. 11) reflecting either higher depletion of the older mantle (Her-zberg and Zhang 1996; Griffin et al. 1999b) or the absence of later re-fertilization (Griffin et al. 2009). Indeed, the Archean cratonic mantle with an average olivine Mg num-ber of 92.6 was estimated to have formed at the degree of melt extraction between 37 and 47 %, depending on the pressure of melting (Pearson and Wittig 2008). At such high degrees of melting (>20 %, Kushiro 1994), clinopy-roxene will be exhausted from the residue (Kushiro 1994) and garnet compositions should depart from the lherzolitic trend and drastically lower CaO contents shifting to the harzburgitic field. The common occurrence of harzburgitic garnet in the cratonic mantle suggests its generation at the degrees of melt extraction exceeding 20 %.

In the younger, Proterozoic mantle, garnet would have lower Cr2O3 contents, which would decrease together with CaO along the lherzolitic trend as progressively less melt is extracted from the younger lherzolite residue. The degree of depletion of the residues would decrease through time (Griffin et al. 1999b). Typical Phanerozoic subcontinental mantle experienced


1 3

13 Page 16 of 20

Tabl

e 4

Buf

feri

ng re

actio

ns fo

r the

lher

zolit

ic a

nd h

arzb

urgi

tic g

arne

t tre

nds

for t

he v

aria

bly

depl

eted

man

tle

1 C

alcu

late

d as

F (v

ol%

) = V

mel

t/(V

mel

t + V

solid

s in

the

depl

eted

man

tle)

2 C

alcu

late

d as

F (v

ol%

) = V

mel

t/Vre

acta

nts

3 Su

bstit

utin

g SG

1 C

px fo

r Lo-

Ca

Cpx

wou

ld c

hang

e re

actio

n co

effic

ient

s up

to 1

8 %

for C

px, 2

8 %

for O

px, 1

5 %

for O

l, 8

% fo

r Spl

, 1 %

for G

ar a

nd 7

% fo

r MO

RB

No

Feat

ures

Fert

ile m

antle

Dep

lete

d m

antle

Deg

ree

of m

elt e

xtra

ctio

nV

olum

e ra

tios

of p

rodu

cts

Lher

zolit

ic tr

end

(9)

No

Cpx

, no

Spl

2,14

1 L

o-C

r, L

o-C

a G

ar +

183

,062

Lo-

M

g O

px +

2,1

89 F

o1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 1

75,6

31

Hi-

Mg

Opx

+ 4

,093

Fa

+ 8

,480

MO

RB

2.51

2.4

2 O

px/G

ar =

96.

4

(10)

No

Opx

, no

Spl

1,52

3 L

o-C

r, L

o-C

a G

ar +

5,4

73 S

G2

C

px +

54

Fa1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 3

,474

SG

1

Cpx

+ 9

17 F

o +

2,4

74 M

OR

B17

.81

15.4

2 %

Cpx

/Gar

= 2

.0

(11)

No

Ol,

no S

pl1,

079

Lo-

Cr,

Lo-

Ca

Gar

+ 6

,217

SG

2

Cpx

+ 1

,404

Lo-

Mg

Opx

1,00

0 H

i-C

r, H

i-C

a G

ar +

4,6

26 S

G1

C

px +

1,88

1 H

i-M

g O

px +

1,27

8 M

OR

B7.

416

.92

%C

px/G

ar =

2.7

(12)

No

Spl,

2 C

px7,

081

Lo-

Cr,

Lo-

Ca

Gar

+ 1

2,18

7 SG

1

Cpx

+ 6

,896

Hi-

Mg

Opx

+ 2

96 F

a1,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 5

,574

SG

2

Cpx

+ 1

2,14

1 Fo

+17

,775

MO

RB

37.0

1 29

.02

%C

px/G

ar =

3.3

(13)

No

Spl,

1 C

px, 1

Opx

4,33

8 L

o-C

r, L

o-C

a G

ar +

4,3

52 S

G1

C

px +

3,4

97 H

i-M

g O

px +

176

Fa

1,00

0 H

i-C

r, H

i-C

a G

ar +

6,6

09

Fo +

10,

238

MO

RB

46.0

1 34

.12

%C

px/G

ar R

eact

ant =

0.6

(14)

All

phas

es p

rese

nt85

9 L

o-C

r, L

o-C

a G

ar +

259

Hi-

Cr

Spl +

382

Hi-

Mg

Opx

+ 4

38 S

G1

Cpx

31,

000

Hi-

Cr,

Hi-

Ca

Gar

+ 4

81

Fo93

+ 3

89 M

OR

B8.

818

.12

%C

px/G

ar R

eact

ant =

0.3

Har

zbur

gitic

tren

d

(15)

No

Spl,

no C

px, 2

Ol

924

Lo-

Ca

Gar

+ 2

2,60

2 L

o-M

g

Opx

+ 6

22 F

o1,

000

Hi-

Ca

Gar

+ 2

2,06

2 H

i-M

g

Opx

+ 4

70 F

a +

496

MO

RB

1.11

,2 %

Opx

/Gar

= 1

2.1

(16)

1 Sp

l, no

Cpx

, low

-Al O

px1,

000

Hi-

Ca

Gar

+ 2

21 L

o-A

l Opx

679

Lo-

Ca

Gar

+ 1

13 H

i-C

r Spl

+ 2

93

Fo +

0.9

Fa

17.3

1 15

.62

%O

px/G

ar R

eact

ant =

0.1

(17)

2 Sp

l, no

Cpx

, hig

h-A

l Opx

1,00

0 H

i-C

a G

ar +

73

Hi-

Al S

pl +

327

H

i-A

l Opx

759

Lo-

Ca

Gar

+ 1

28 H

i-C

r Spl

+ 3

07

Fo +

567

MO

RB

15.2

1 13

.92

%O

px/G

ar R

eact

ant =

0.2

(18)

All

phas

es p

rese

nt, H

i-C

r Spl

1,00

0 H

i-C

a G

ar +

189

Hi-

Mg

Opx

764

Lo-

Ca

Gar

+ 9

4 H

i-C

r Spl

+ 9

6

SG1

Cpx

3 +

188

Fo9

3 +

377

MO

RB

10.7

1 10

.12

%C

px/G

ar =

0.0

7

(19)

All

phas

es p

rese

nt, L

o-C

r Spl

1,00

0 H

i-C

a G

ar +

15

Fo93

416

Lo-

Ca

Gar

+ 3

94 H

i-A

l Spl

+ 3

19

Hi-

Mg

Opx

+ 1

02 S

G2

Cpx

+ 4

95

MO

RB

15.7

1 14

.52

%O

px/G

ar =

0.4


1 3

Page 17 of 20 13

was recast in mole proportions (Table 1) and added as a phase to the model assemblage of mantle minerals.

Similar to our calculations for the isochemical mantle, we calculated several possible reactions that excluded cer-tain minerals (Table 4). The tested mineralogical scenarios were with clinopyroxene (Reactions 1014, 1819) or without it (Reactions 9, 1517); the latter were calculated

without spinel (Reactions 9, 15), using a fixed spinel com-position (Reaction 16) or a solid solution spinel (Reac-tion 17). We also varied the composition of orthopyroxene, from high-Mg (Reactions 13, 1819) to low-Mg (Reac-tion 15) and from low-Al (Reaction 16) to high-Al (Reac-tion 17), and represented olivine either as two end-mem-bers (Reactions 9, 10, 13, 1516) or as the most common cratonic olivine Fo93 (Reactions 14, 1819).

The compositions of garnet defining the lherzolitic trend can be ascribed to a variety of buffering equilibria with the degree of melt extraction between 2 and 46 vol% [Reac-tions (914) in Table 4]. The reaction that does not involve clinopyroxene (Reaction 9) is deemed infeasible because the observed variations in garnet composition would require enormous quantities of model orthopyroxene (e.g., volume Opx/Gar ratio of 97/1; Table 4). Additionally, the clinopy-roxene-free reaction places forsterite in the fertile mantle and fayalitein the depleted mantle. All other buffering reac-tions with clinopyroxene (Reactions 1014) seem feasible, as they predict the existence of low-Ca garnet and fayalite in the fertile mantle, the mineral proportions observed in mantle peridotites and the realistic degree of mantle depletion. The trend can form with (Reaction 14) or without spinel (Reac-tions 913), so the presence of spinel is not mandatory for the trend development. Reactions (13) and (14) envision disap-pearance of clinopyroxene and its replacement by orthopy-roxene resulting in garnet buffering. We conclude that the presence of clinopyroxene is essential for buffering the lher-zolitic trend, and under some circumstances, the initially pre-sent clinopyroxene can be completely used up by the reaction.

Many possible reactions (Reactions 1519) can con-trol the harzburgitic trend. Reactions without spinel (Reac-tion 15) appear unrealistic because the predicted garnet com-positions are at odds with the bulk composition of mantle modeled by the melt extraction. Low-Ca garnet occurs in the more depleted mantle (Fig. 11), yet Reaction (15) places this garnet in fertile mantle. This reaction is also considered unlikely due to an overly high amount of orthopyroxene that needs to be involved (Opx/Gar ratio of 12/1). The reactions with spinel (Reactions 1619) look more feasible because the reaction coefficients are of the same order for all par-ticipating minerals and do not require an excessive amount of one phase. The reactions also correctly place the high-Ca garnet in the fertile mantle and predict that garnet becomes less calcic by reaction with orthopyroxene and olivine. For-mation of the harzburgitic trend does not critically depend on the presence or absence of clinopyroxene.

Concluding remarks

Our modeling reproduced lherzolitic and harzburgitic trends in garnet chemistry assuming the constant or varied

5

Cr2

O3,

wt% 10

5

CaO, wt%

Phanerozoic

Cr2

O3,

wt%

10

5

Cr2

O3,

wt% 10

5

Proterozoic

Archeana

b

c

10

Fig. 11 Temporal evolution of the composition of mantle garnet from the Archean (a) to Proterozoic (b) and Phanerozoic (c). Open fields are garnet compositions from the North China craton (Griffin et al. 1998), observed in the mantle below the crust of the correspond-ing ages. Gray dots are individual garnet analyses from the perido-titic (Cr2O3 >1 wt%) garnet database, which includes 13,000 analyses from 226 localities from most continents (Griffin et al. 1999a, b). The boundary separating harzburgitic (G10) and lherzolitic fields is from Gurney (1984)


1 3

13 Page 18 of 20

bulk composition of the mantle. In the isochemical mantle, the lherzolitic trend can form without clinopyroxene, but cannot form without spinel. In the variably depleted man-tle, the lherzolitic trend critically depends on the presence of clinopyroxene rather than the buffering by spinel. Obser-vations on natural occurrences of the lherzolitic trend bet-ter match the modeling that assumes the variably depleted mantle. A wide range in the Cr2O3 contents in garnet and the parallel more restricted range in CaO contents mirrors wide variations in the composition of the mantle and its open system behavior as opposed to reflecting the thickness of the spinelgarnet transition in a closed system model. The preferred model of the variably depleted mantle also fits abundant evidence on the changing bulk composition of the mantle with the tectonic setting (Pearson and Wit-tig 2008), the time of formation (Griffin et al. 1998, 1999b) and good correlations of the garnet and mantle composi-tions (e.g., Brey et al. 1990; Griffin et al. 1999a, b). The reaction in the variably depleted mantle relies on the par-ticipation of clinopyroxene and is supported by observa-tions on the presence of lherzolitic trend garnet in natural lherzolites (Sobolev et al. 1973; Gurney 1984; Dawson and Stephens 1975). Furthermore, the proposed reaction allows us to explain a puzzling occurrence of lherzolitic trends in garnets hosted by harzburgite (Kopylova et al. 1999; Mac-Kenzie and Canil 1999; Pearson et al. 1999; Kopylova and Caro 2004; van der Meer et al. 2013, Viljoen et al. 2009). Because clinopyroxene is present only on one side of the buffering reaction, the clinopyroxene can be exhausted after forming garnet. In this situation, garnet from the lher-zolitic trend will be left in the harzburgitic mantle, where the absence of visible clinopyroxene relates to its ultimate

exhaustion in the completed reaction. The trends little variation in Ca content with widely varying Cr is linked to substitution MgFeAl Ca2Cr that develops when Ca can be freely sourced by garnet from the reservoir of Ca in clinopyroxene.

The open system behavior of the peridotitic mantle also provides a better explanation to the harzburgitic trend in garnet. In the isochemical mantle, the trend can be buffered by many possible reactions, and none of the minerals is essential for the trend development. In the variably depleted mantle, on the contrary, spinel is required to make the harz-burgitic trend garnet, as its formation critically depends on Cr provided by spinel. Our modeling supports the explana-tion of Cr-saturation arrays (Grtter et al. 2006) as resulting from equilibration with spinel. These lines of maximum Cr in garnet for its Ca content were argued to reflect the maxi-mum Cr contents in a population of garnet imposed by spi-nel buffering in few Cr-rich samples (Grtter et al. 2006). The slope of the CaOCr2O3 correlation in the harzburgitic trend resemble the slope of the Cr-saturation arrays and the slope of another observed trend in garnet equilibrated with spinel (Kopylova et al. 2000) because the gentle increase in Cr2O3 content in garnet with a large increase in CaO indi-cates the reservoir of Cr available to garnet. The Cr content in garnet is almost constant, defined by the distinct ratio of the CrCa substitution Ca2Fe5Cr3 Mg7Al3, because Cr is drawn from the coexisting spinel.

Worldwide, lherzolitic trend is present in all peridotitic garnet from Proterozoic and Archean terranes (Fig. 12; Grtter et al. 1999; Griffin et al. 1999a, b), whereas the harzburgitic trend so far has been found only in kimber-lites in the Canastra 8 cluster. The rarity of the harzburgitic

2 4 6 8 10CaO, wt%

G10 G9

cN=245, Baffin Island

GDC

2

4

6

8

10

12

14

0 2 4 6 8 10

Cr 2O

3, w

t%

CaO, wt%

G10 G9

aN=30436, Sample bhp83313

GDC

2 4 6 8 10

CaO, wt%

G10 G9

bN=2084, Victoria Island

GDC

Fig. 12 Typical compositions of peridotitic garnet on Archean and Proterozoic cratons as exemplified by samples from the online Indica-tor Mineral Chemistry Database (Northwest Territories Geosciences Office, Canada). a Sample bhp83769, b sample 084395, c sample

084673. The thick green line is the position of the harzburgitic trend. Also shown are GDC line (Grtter et al. 2006), harzburgitic (G10) and lherzolitic fields (1984) and the boundary between the G9 field and the wehrlitic field on the right (Sobolev et al. 1973)


1 3

Page 19 of 20 13

trend may be linked to the generally low mode of spinel in the moderately depleted mantle. The Canastra 8 mantle may then be unique in the high-Cr and low-Si bulk com-position that allowed crystallization of abundant spinel. In such depleted mantle, garnet and spinel coexist at a wider range of pressures (1655 kb for the 35 mW/m2 geotherm, see Fig. 2f) than in the relatively fertile mantle (1138 kb, Fig. 2a, both in Ziberna et al. 2013). Low geotherms also contribute to the wider pressuretemperature field of the spinelgarnet peridotite (Ziberna et al. 2013).

Acknowledgments This research was made possible by an NSERC grant to MGK. The authors thank the Brazilian Diamonds Ltd. for the data and the permission to publish, H. Grtter and L. Ziberna for their insights on formation of cratonic garnet, D. Klimentieva for help with the thermodynamic calculations and S. Cairns and B. Elliott for help with the NWT KIMM database. The manuscript benefited from reviews of D. Canil and an anonymous reviewer.

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Mineralogical controls on garnet composition in the ... · Mineralogical controls on garnet composition in the cratonic ... in diamond exploration (e.g., ... and Leonardos 1996)