[S-GC32] Volatiles in the Earth - from Surface to Deep Mantle

[SGC32-06] Hydrogen isotopes of high-³He/⁴He picrites from West Greenland: implications for the primordial water signature in the mantle

★Invited Papers

*Jia Liu¹、Jingjun Zhou¹、Qunke Xia¹、Kenji Shimizu²、Lotte Larsen³、Takayuki Ushikubo²、Eero Hanski⁴ (1.Zhejiang University、2.Kochi Institute for Core Sample Research、3.Geological Survey of Denmark and Greenland、4.Oulu Mining School)

キーワード：primordial earth, hydrogen isotopes, melt inclusion, West Greenland

The hydrogen isotope composition of water in mantle-derived rocks provides critical clues to the origin of our planet’s water. Based on analysis of both water contents and D/H ratios of melt inclusions in high-³He/⁴He lavas of the Icelandic mantle plume (e.g., Baffin Island picrites), which have usually been thought to be least degassed or disturbed by long-term mantle convection, it has been suggested that primordial water inherited from the proto-solar nebula is preserved in deep mantle. While it has been in doubt given that the D/H ratios of melt inclusions could be easily kinetically fractionated during inward/outward diffusion of hydrogen, there is no direct evidence to argue for or against this possibility. Using secondary iron mass spectrometry, we measured water contents and D/H ratios of olivine-hosted glassy melt inclusions and quenched glass from pillow rims in high-³He/⁴He picrites in West Greenland. Water contents range from 850 to 1050 ppm, whereas δD ranges from -75±8 ‰ to -140±8 ‰, which forms the trends modeled by kinetic fractionation driven by inward water diffusion into the melt inclusions. The primary δD value of these picrites is inferred to be around -72 ‰, which is consistent with recent results from the high-³He/⁴He submarine basaltic glasses from Loihi, Hawaii. Thus, our work supports the existence of globally consistent primordial water likely originated from chondritic material.