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Noble gas isotopic evolution of the Earth’s mantle controlled by U and Th contents (just a review of noble gas reservoirs....). Hirochika SUMINO Geochemical Research Center (GCRC) University of Tokyo. 2013. 10. 30 @Workshop on Particle Geophysics, Sendai. Noble gas isotopes. - PowerPoint PPT Presentation
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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.