Noble gas isotopic evolution of the Earth’s mantle controlled by U and Th contents

(just a review of noble gas reservoirs....)

2013. 10. 30@Workshop on Particle Geophysics, Sendai

Hirochika SUMINO

Geochemical Research Center (GCRC)University of Tokyo

Cover a wide mass range. Insensitive to chemical processes.

– because of chemical inertness. Sensitive to mixing of several reservoirs.

– vary by several orders of magnitude depending on the origin.

Provide temporal information.– because some isotopes accumulate with time.

Determinable with high sensitivity / precision using special mass spectrometric systems.

Noble gas isotopes element isotope

He3He

4He

Ne

20Ne

21Ne

22Ne

Ar

36Ar38Ar40Ar

Kr 78~86Kr

Xe 124~136Xe

Noble gas components in the solar system Solar / Primordial:

Original composition of material from which the solar system or the Earth formed.

Radiogenic: Produced by decay of radioactive nuclides.e.g., a-decay of U, Th → 4He

40K (E.C.) → 40Ar129I (β-) → 129Xe

Nucleogenic: Product of nuclear reactions induced by a-particles or neutrons.e.g., 6Li (n,a) → 3H (β-) → 3He

18O (a,n) → 21Ne Fissiogenic

Fission products of 238U and 244Pu. Cosmogenic:

Product of spallation induced by cosmic-rays.

Helium isotope ratios of MORBs and OIBs

degassedless degassedhigh 3He/(U+Th) low 3He/(U+Th)

(Barfod et al., JGR 1999)

RA = atmospheric 3He/4He = 1.4 10-6

3He/4He (RA)

4He/3He

Plume source 50 RA

Hotspot5~50 RA

3He/4He of geochemical reservoirs Solar (Primordial)

3He/4He > 120 RA

Radiogenic (from U, Th) 3He/4He ~ 0.01 RA +

Mid Ocean Ridge Basalts (MORB)

8 RA

Atmosphere

Crust

Mantle

Atmosphere3He/4He = 1 RA (1.410-6)

MORB source8 RA

Upwelling“Plume”

Lower mantle or core-mantle boundary ?

Crust~0.01 RA

Neon isotopes of MORBs and OIBs

MORB source3He/4He ~ 8 RA40Ar/36Ar ~ 40000High 21Ne/22Ne

OIB source (Plume)3He/4He > 50 RA40Ar/36Ar ~ 8000Low 21Ne/22Ne

Atmosphere3He/4He = 1 RA (1.410-6)40Ar/36Ar = 296

Primordial Radiogenic/Nucleogenic

3He 20Ne, 22Ne

36Ar

4He21Ne40Ar

degassedless degassed

(Trieloff et al., EPSL 2002)

Nucleogenic

MORB source

Crustal

Primordial

18O (a,n) → 21Ne

high 22Ne/(U+Th) low 22Ne/(U+Th)

Where is the less degassed mantle domain?

(Porcelli & Ballentine, Rev. Mineral. Geochem. 2002)

: high (3He, 20Ne)/(U+Th) (=more primitive, less degassed)

Convection modeA, B: two-layeredC, D, E: whole mantle

Less degassed reservoirA, B: lower mantle C: heterogeneities or

deeper layers D: D” E: Core

He isotope evolution in the convecting mantle

(Porcelli & Elliott, EPSL 2008)

Model inputsInitial 3He/4He 120 or 330 RA

Present 3He/4He 8 RA

Initial 3He conc. (2.8 or 11) 1010 atoms/g

Present 3He conc. 8.7 108 atoms/g

Initial U conc. 21 ppb

Present U conc. 3 ppb

Initial U/Th 3.8

Present U/Th 2.5

Model resultsFactional melting rate 2.1–3.6 10-9 yr-1

Decrease in degassing rate

6.0–7.3 10-10 yr-1

3He output from ridges 490 – 2900 mol yr-1

obs.) 1000 mol yr-1

Early separation of 3He-enriched hidden reservoir

To maintain high 3He/4He as high as 50 RA, the plume source must have been isolated earlier or exhibit high 3He/U. (Porcelli & Elliott, EPSL 2008)– Core with primordial He? (Porcelli & Halliday, EPSL 2001; Bouhifd et al., Nature Geosci. 2013)– D” layer with high 3He and U? (Tolstikhin & Hofmann, PEPI 2005)

(Porcelli & Elliott, EPSL 2008)

Alternative model

(Gonnermann & Mukhopadhyay, Nature 2009)

Different evolution resulted from different processing rate– several times for UM.– approx. once for LM.explains present-day 3He and 40Ar.

When the two mantle domains separated?

(Mukhopadhyay, Nature 2012)

Correction for atmospheric contamination based on relationship with 20Ne/22Ne and primordial (= solar wind) 20Ne/22Ne value.

When the two mantle domains separated?

129I (β-) → 129Xe (T1/2 = 15.7 Ma)

244Pu → 131Xe, 132Xe, 134Xe, 136Xe (T1/2 = 80.0Ma)

238U → 131Xe, 132Xe, 134Xe, 136Xe (T1/2 = 4.47Ga)

244Pu-derived 136Xe: 1-40% for MORB70-99% for Iceland

(Almost) undegassed Iceland mantle source has been isolated since 4.45 Ga.

(Mukhopadhyay, Nature 2012)

Where is the less degassed mantle domain?

(Porcelli & Ballentine, Rev. Mineral. Geochem. 2002)

: high (3He, 20Ne)/(U+Th) (=more primitive, less degassed)

Convection modeA, B: two-layeredC, D, E: whole mantle

Less degassed reservoirA, B: lower mantleC: heterogeneities or

deeper layers LLSVPs?

D: D”E: Core

The undegassed mantle = LLSVPs ?

(Bull et al., EPSL 2009)

– “LLSVPs are features that have existed since the formation of the Earth and cannot exclusively be composed of subducted slabs”. (Mukhopadhyay, Nature 2012).

– Consistent with EM-high 3He/4He (primordial) and HIMU-low 3He/4He (recycle) components in Polynesian OIBs. (Parai et al., EPSL 2009)

If the undegassed mantle domains correspond to LLSVPs,

“A low velocity anomaly beneath Iceland is confined to the upper mantle”. (Ritsema et al., Science 1999)

Possible primordial noble gas reservoirs and their U estimations

LLSVPs – a mixture of undegassed mantle and subducting materials (Mukhopadhyay, Nature 2012)

~20 ppb (BSE value) or more U. ~40% or more of total U in the mantle.

D” layer – a mixture of early-formed crust and chondritic debris (Tolstikhin & Hofmann, PEPI

2005) ~70 ppb U

~30% of total U in the mantle.

Can be discriminated via geoneutrino?

Helium in subcontinental lithospheric mantle (SCLM)

0 2 4 6 8 10 120

10

20

30

40

Cou

nt

3He/4He (RA)

N= 154Lherzolite, crush onlyMean = 5.9 ± 2.2 RA

Med. = 6.5 RA

MO

RB

Data: Africa (N=22; Aka et al., 2004; Barfod et al., 1999; Hilton et al., 2011; Hopp et al., 2004; 2007), Europe (N=51; Buikin et al., 2005; Correale et al., 2012; Gautheron et al., 2005; Martelli et al., 2011; Sapienza et al., 2005), Siberia (N=18; Yamamoto et al., 2004; Barry et al., 2007), Eastern Asia (N = 28; Sumino, unpublished data; Kim et al., 2005; Chen et al., 2007; He et al., 2011; Sun, unpublished data), Australia (N = 24; Czuppon et al., 2009; 2010; Matsumoto et al., 1998; 2000; Hoke et al., 2000), South America (N = 11; Jalowitzki, unpublished data)

Closed system evolution of SCLM

0

2

4

6

8

10

12

14

-150 -100 -50 0

3 He/

4 He

(RA)

Time before present (Ma)150 100 50 0

Convecting mantle

6.0 RA

4.6 RA

0.2 RA

U/3He 30

U/3He 60

U/3He 3000

Metasomatic event(U/3He increase)

(KIM et al., Geochem. J. 2005)

Similar or higher radiogenic 4He/40Ar ratios (proxy for (U+Th)/K) than the MORB source suggest U/3He increase mainly due to U (and Th, K) addition by slab-derived fluids rather than substantial loss of 3He.

(Yamamoto et al., Chem. Geol. 2004; Kim et al., Geochem. J. 2005)

U in metasomatized SCLM (for 6 RA): 90 ppbcf) 25 ppb (Archean) (Rudbuck et al., Chem. Geol. 1998)

40 ppb (post-Archean) (McDonough, EPSL 1990)

Neon in SCLM

0.03 0.04 0.05 0.06 0.079

10

11

12

13

20Ne

/22 Ne

21Ne/ 22Ne

Popping rock Iceland Patagonian SCLM

Air

Iceland source MORB source

SCLM?

22Ne/(U+Th): Iceland > MORB > Patagonian SCLMundegassed degassed enriched in U?

(Jalowitzki et al., in prep.)

18O (a,n) → 21Ne

Summary

Noble gas (especially He) isotopic evolution in the mantle is directly related to U and Th contents in their reservoirs.

As the deep mantle plume source associated with primordial noble gases, the strongest candidates are LLSVPs and D” layer possibly enriched in 3He and U+Th. They contain 30-40% of total U and Th in the mantle, thus would be detectable via future geoneutrino observation.

SCLM enriched in U and Th is another reservoir of noble gases in the mantle. Although it contains 10-30 times as much of U than the convecting mantle, its small volume fraction (ca. 1.5% ) results in insignificant contribution to global geoneutrino flux. However, it may be significant for a detector located in continental margin.