Hirochika SUMINO Geochemical Research Center (GCRC) University of Tokyo

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

2. Noble gas isotopes • 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 accumulatewith time. • Determinable with high sensitivity / precision using special mass spectrometric systems.

3. Noble gas componentsin 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→4He40K (E.C.) → 40Ar129I (β-) → 129Xe • Nucleogenic: Product of nuclear reactions induced by a-particles or neutrons. e.g., 6Li (n,a) → 3H (β-) → 3He18O (a,n) → 21Ne • FissiogenicFission products of 238U and 244Pu. • Cosmogenic: Product of spallation induced by cosmic-rays.

4. Helium isotope ratios of MORBs and OIBs 3He/4He (RA) RA = atmospheric 3He/4He = 1.4  10-6 4He/3He (Barfodet al., JGR 1999) low 3He/(U+Th) high 3He/(U+Th) less degassed degassed

5. 3He/4He of geochemical reservoirs Upwelling“Plume” Radiogenic (from U, Th) 3He/4He ~ 0.01 RA Solar (Primordial) 3He/4He > 120 RA + Lower mantle or core-mantle boundary ? Mid Ocean Ridge Basalts (MORB) 8 RA Hotspot 5~50 RA Atmosphere Atmosphere 3He/4He = 1 RA (1.410-6) Crust ~0.01 RA Crust MORB source 8 RA Mantle Plume source  50 RA

6. Neon isotopes of MORBs and OIBs MORB source Primordial Nucleogenic Crustal (Trieloffet al., EPSL 2002) high 22Ne/(U+Th) low 22Ne/(U+Th) degassed less degassed Atmosphere 3He/4He = 1 RA (1.410-6) 40Ar/36Ar = 296 MORB source 3He/4He ~ 8 RA 40Ar/36Ar ~ 40000 High 21Ne/22Ne OIB source (Plume) 3He/4He > 50 RA 40Ar/36Ar ~ 8000 Low 21Ne/22Ne 18O (a,n) → 21Ne

7. Where is the less degassed mantle domain? Convection mode A, B: two-layered C, D, E: whole mantle Less degassed reservoir A, B: lower mantle C: heterogeneities or deeper layers D: D” E: Core : high (3He, 20Ne)/(U+Th) (=more primitive, less degassed) (Porcelli & Ballentine, Rev. Mineral. Geochem. 2002)

8. He isotope evolution in the convecting mantle (Porcelli & Elliott, EPSL 2008)

9. Early separation of 3He-enriched hidden reservoir (Porcelli & Elliott, EPSL 2008) 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; Bouhifdet al., Nature Geosci. 2013) – D” layer with high 3He and U?(Tolstikhin & Hofmann, PEPI 2005)

10. Alternative model Different evolution resulted from different processing rate – several times for UM. – approx. once for LM. explains present-day 3He and 40Ar. (Gonnermann& Mukhopadhyay, Nature 2009)

11. When the two mantle domains separated? Correction for atmospheric contamination based on relationship with 20Ne/22Ne and primordial (= solar wind) 20Ne/22Ne value. (Mukhopadhyay, Nature 2012)

12. 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 MORB 70-99% for Iceland  (Almost) undegassed Iceland mantle source has been isolated since 4.45 Ga. (Mukhopadhyay, Nature 2012)

13. Where is the less degassed mantle domain? Convection mode A, B: two-layered C, D, E: whole mantle Less degassed reservoir A, B: lower mantle C: heterogeneities or deeper layers LLSVPs? D: D” E: Core : high (3He, 20Ne)/(U+Th) (=more primitive, less degassed) (Porcelli & Ballentine, Rev. Mineral. Geochem. 2002)

14. The undegassed mantle = LLSVPs ? If the undegassed mantle domains correspond to LLSVPs, – “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. (Paraiet al., EPSL 2009) (Bull et al., EPSL2009) “A low velocity anomaly beneath Iceland is confined to the upper mantle”. (Ritsemaet al., Science 1999)

15. 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?

16. Helium in subcontinental lithospheric mantle (SCLM) N= 154 Lherzolite, crush only Mean = 5.9 ± 2.2 RA Med. = 6.5 RA MORB Data: Africa (N=22; Aka et al., 2004; Barfodet al., 1999; Hilton et al., 2011; Hoppet al., 2004; 2007), Europe (N=51; Buikinet al., 2005; Correaleet al., 2012; Gautheronet al., 2005; Martelliet al., 2011; Sapienzaet 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; Czupponet al., 2009; 2010; Matsumoto et al., 1998; 2000; Hokeet al., 2000), South America (N = 11; Jalowitzki, unpublished data)

17. Closed system evolution of SCLM Metasomatic event(U/3He increase) Convecting mantle 3He/4He (RA) U/3He  30 6.0 RA U/3He  60 4.6 RA (KIM et al., Geochem. J. 2005) U/3He  3000 0.2 RA 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) 150 100 50 0 Time before present (Ma) U in metasomatized SCLM (for 6 RA): 90 ppbcf) 25 ppb (Archean) (Rudbucket al., Chem. Geol. 1998) 40 ppb (post-Archean) (McDonough, EPSL 1990)

18. Neon in SCLM Iceland source MORB source SCLM? 18O (a,n) → 21Ne Air (Jalowitzkiet al., in prep.) 22Ne/(U+Th): Iceland > MORB > Patagonian SCLM undegassed degassed enriched in U?

19. 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.