[M-TT44] Frontiers in Geochemistry

[MTT44-P02] Noble gas isotope ratios of CO₂ fluid inclusions in mantle-derived xenoliths by spot measurement -direct constraints on the origin of noble gas heterogeneity in subcontinental lithospheric mantle beneath back arc region

*Lena Yokokura¹, Hirochika Sumino², Takeshi Kuritani¹, Yuuki Hagiwara¹, Junji Yamamoto³ (1.Hokkaido University, 2.The University of Tokyo, 3.Kyusyu University)

Keywords:Noble gas isotope ratio , Laser spot analysis, Fluid inclusions

Isotope ratios of noble gases extracted from minerals or rocks are effective to investigate the origin of fluids within the samples. Longgang volcanos in northeastern China are expected to involve the influence of subducted slab-related component to subcontinental mantle beneath back-arc region of eastern Asia. We extracted noble gases by crushing 1-2 g olivine grains of five mantle xenoliths collected from the volcanos. The ³He/⁴He ranges from 6.45 Ra to 0.14 Ra where Ra is atmospheric ³He/⁴He. A possible cause of the significant variation of the ³He/⁴He is variable mixing ratio of CO₂ fluid inclusions and melt ones in minerals, which exhibit different ³He/⁴He. This is because the samples showing relatively high ³He/⁴He include abundant CO₂ fluid inclusions. To evaluate the assumption, we preferentially extracted noble gases from CO₂ fluid inclusions using laser ablation method. For the extraction, we made holes with 100-200 micrometer in diameter on polished olivine surfaces using an ultraviolet laser (UP-213, New Wave Research, Inc.). The ³He/⁴He of the CO₂fluid inclusions are higher than those obtained by crushing method. It implies that noble gas isotope ratios of the samples depend on the distribution of inclusions. In addition, there is a possibility that melt inclusions in minerals are the host of relatively low ³He/⁴He derived from subducted slab-related component.