[S-GC35] Volatiles in the Earth - from Surface to Deep Mantle

[SGC35-11] The possibility of the depleted mantle being a high ³He/⁴He reservoir

*藤田 遼¹、石川 晃^1,2、角野 浩史³、Michael Garcia⁴ (1.東京工業大学理学院地球惑星科学系、2.海洋研究開発機構海底資源センター、3.東京大学大学院総合文化研究科広域科学専攻、4.Department of Earth Sciences, The University of Hawaii)

キーワード：ハワイ、マントル捕獲岩、ヘリウム同位体

The origin and composition of the high-³He/⁴He reservoir in the Earth's deep interior have long been debated, and ocean island basalts (OIB) and Baffin Island picrites with high ³He/⁴He ratios (>20 Ra) remain significant research focus. Incompatible trace element ratio and Sr, Nd, and Pb isotopes in high-³He/⁴He OIB and Baffin picrites are known not to be primordial and commonly show the depleted nature compared to chondrites and estimated bulk silicate earth, suggesting that incompatible element depleted mantle is a discrete high ³He/⁴He reservoir [1]. Although several experiments have indicated that He might be more compatible than U or Th during partial melting of the mantle [2], it remains unclear whether residual peridotites can accommodate high ³He/⁴He ratios within the mantle. In order to verify the possibility that the ancient depleted mantle is a high ³He/⁴He reservoir, we performed He isotope analyses of olivines extracted from peridotite xenoliths from Oahu Island (Salt Lake Crater) and Kaula Island, Hawaiian Islands. These peridotite xenoliths show unradiogenic Os and radiogenic Hf isotopic compositions. It suggests being recycled lithospheric mantle depleted by ancient (>1 Ga) melting, independent of modern igneous activity in the East Pacific Rise [3]. The result demonstrated that the ³He/⁴He ratio of the mantle xenoliths shows a wide variation from 2.4 Ra to 9.5 Ra, indicating the radiogenic ingrowth of ⁴He from U and Th. The ³He/⁴He ratio is significantly correlated with the depletion indices of peridotites such as whole-rock Al₂O₃ and Yb contents, and the more depleted peridotite shows a lower ³He/⁴He ratio. This implies that He is more incompatible than U and Th during mantle melting, inconsistent with the prediction of laboratory experiments [2, 4]. In contrast, the correlation line between ³He/⁴He ratio and whole-rock Al₂O₃ deviates from the value of depleted MORB mantle, and the olivine from fertile peridotite xenoliths tend to have higher ³He/⁴He ratios than that of MORB. This suggests that unradiogenic He has survived from homogenization processes and can be retained in the ancient depleted mantle.

[1] Stuart, F. M., Lass-Evans, S., Godfrey Fitton, J., & Ellam, R. M. (2003). Nature, 424(6944), 57-59. [2] Parman, S. W., Kurz, M. D., Hart, S. R., & Grove, T. L. (2005). Nature, 437(7062), 1140-1143. [3] Bizimis, M., Griselin, M., Lassiter, J. C., Salters, V. J., & Sen, G. (2007). EPSL, 257(1-2), 259-273. [4] Heber, V. S., Brooker, R. A., Kelley, S. P., & Wood, B. J. (2007). GCA, 71(4), 1041-1061.