Noble gas isotopic evolution of the Earth’s mantle controlled by U and Th contents (just a review of noble gas reservoirs....) 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 elementisotope He 3 He 4 He Ne 20 Ne 21 Ne 22 Ne Ar 36 Ar 38 Ar 40 Ar Kr 78~86 Kr Xe 124~136 Xe

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.,  -decay of U, Th → 4 He 40 K (E.C.) → 40 Ar 129 I (β - ) → 129 Xe Nucleogenic: Product of nuclear reactions induced by a-particles or neutrons. e.g., 6 Li (n,  ) → 3 H (β - ) → 3 He 18 O ( ,n) → 21 Ne Fissiogenic Fission products of 238 U and 244 Pu. Cosmogenic: Product of spallation induced by cosmic-rays.

Helium isotope ratios of MORBs and OIBs degassedless degassed high 3 He/(U+Th) low 3 He/(U+Th) (Barfod et al., JGR 1999) R A = atmospheric 3 He/ 4 He = 1.4  He/ 4 He (R A ) 4 He/ 3 He

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

Neon isotopes of MORBs and OIBs MORB source 3 He/ 4 He ~ 8 R A 40 Ar/ 36 Ar ~ High 21 Ne/ 22 Ne OIB source (Plume) 3 He/ 4 He > 50 R A 40 Ar/ 36 Ar ~ 8000 Low 21 Ne/ 22 Ne Atmosphere 3 He/ 4 He = 1 R A (1.4  ) 40 Ar/ 36 Ar = 296 Primordial Radiogenic/ Nucleogenic 3 He 20 Ne, 22 Ne 36 Ar 4 He 21 Ne 40 Ar degassed less degassed (Trieloff et al., EPSL 2002) Nucleogenic MORB source Crustal Primordial 18 O ( ,n) → 21 Ne high 22 Ne/(U+Th) low 22 Ne/(U+Th)

Where is the less degassed mantle domain? (Porcelli & Ballentine, Rev. Mineral. Geochem. 2002) : high ( 3 He, 20 Ne)/(U+Th) (=more primitive, less degassed) 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

He isotope evolution in the convecting mantle (Porcelli & Elliott, EPSL 2008) Model inputs Initial 3 He/ 4 He120 or 330 R A Present 3 He/ 4 He8 R A Initial 3 He conc.(2.8 or 11)  atoms/g Present 3 He conc. 8.7  10 8 atoms/g Initial U conc.21 ppb Present U conc.3 ppb Initial U/Th3.8 Present U/Th2.5 Model results Factional melting rate 2.1–3.6  yr -1 Decrease in degassing rate 6.0–7.3  yr -1 3 He output from ridges490 – 2900 mol yr -1 obs.) 1000 mol yr -1

Early separation of 3 He-enriched hidden reservoir To maintain high 3 He/ 4 He as high as 50 R A, the plume source must have been isolated earlier or exhibit high 3 He/U. (Porcelli & Elliott, EPSL 2008) – Core with primordial He? (Porcelli & Halliday, EPSL 2001; Bouhifd et al., Nature Geosci. 2013) – D” layer with high 3 He 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 3 He and 40 Ar.

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

When the two mantle domains separated? 129 I (β - ) → 129 Xe (T 1/2 = 15.7 Ma) 244 Pu → 131 Xe, 132 Xe, 134 Xe, 136 Xe (T 1/2 = 80.0Ma) 238 U → 131 Xe, 132 Xe, 134 Xe, 136 Xe (T 1/2 = 4.47Ga) 244 Pu-derived 136 Xe: 1-40% for MORB 70-99% 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 ( 3 He, 20 Ne)/(U+Th) (=more primitive, less degassed) 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

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 3 He/ 4 He (primordial) and HIMU-low 3 He/ 4 He (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) N= 154 Lherzolite, crush only Mean = 5.9 ± 2.2 R A Med. = 6.5 R A MORB 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 3 He/ 4 He (R A ) Time before present (Ma) Convecting mantle 6.0 R A 4.6 R A 0.2 R A U/ 3 He  30 U/ 3 He  60 U/ 3 He  3000 Metasomatic event (U/ 3 He increase) (KIM et al., Geochem. J. 2005) Similar or higher radiogenic 4 He/ 40 Ar ratios (proxy for (U+Th)/K) than the MORB source suggest U/ 3 He increase mainly due to U (and Th, K) addition by slab-derived fluids rather than substantial loss of 3 He. (Yamamoto et al., Chem. Geol. 2004; Kim et al., Geochem. J. 2005) U in metasomatized SCLM (for 6 R A ): 90 ppb cf) 25 ppb (Archean) (Rudbuck et al., Chem. Geol. 1998) 40 ppb (post-Archean) (McDonough, EPSL 1990)

Neon in SCLM Air Iceland source MORB source SCLM? 22 Ne/(U+Th): Iceland > MORB > Patagonian SCLM undegasseddegassed enriched in U? (Jalowitzki et al., in prep.) 18 O ( ,n) → 21 Ne

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 3 He 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 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.