[S-GC37] Volatiles in the Earth - from Surface to Deep Mantle

[SGC37-11] Identification of multiple components of noble gas isotopes in backarc lithospheric mantle

*Lena Yokokura¹, Hirochika Sumino², Takeshi Kuritani¹, Yuuki Hagiwara³, Junji Yamamoto⁴ (1.Hokkaido University, 2.The University of Tokyo, 3.Japan Agency for Marine-Earth Science and Technology, 4.Kyusyu University)

Keywords:mantle xenolith, noble gas isotope ratio, fluid inclusion

I investigated isotope ratios of noble gases (³He/⁴He and ⁴⁰Ar/³⁶Ar) extracted by laser microprobe methods for upper mantle-derived xenoliths collected from northeast China. The heterogeneous noble gas isotope ratios (ex. 6.45–0.14 Ra) obtained from the mantle xenoliths by crushing method may reflect local metasomatism induced by multi-components with various origins. Observation of inclusions would be effective in investigating the cause of the heterogeneity of noble gas isotope ratios extracted from the xenoliths by crushing method. On the basis of shape and chemical composition, the inclusions were classified into three types. I performed quantitative verification of the correlation between ³He/⁴He and area ratio of three types of inclusions on the polished surface of ten olivine grains in each sample. As a result, it is calculated that type (3) inclusions with negative crystal shape have a relatively high ³He/⁴He, and types (1) and (2) inclusions may have a relatively low ³He/⁴He. In order to verify the speculation and to determine the ⁴⁰Ar/³⁶Ar for each type of the inclusions, I applied laser spot analysis of noble gas isotopes for type (2) and type (3) inclusions. The type (2) inclusions showed relatively low ³He/⁴He and atmospheric ⁴⁰Ar/³⁶Ar. In contrast, type (3) inclusions showed relatively high ³He/⁴He and ⁴⁰Ar/³⁶Ar, which were almost typical of SCLM values. The atmospheric ⁴⁰Ar/³⁶Ar and relatively low ³He/⁴He. The type (2) inclusions are composed of melt inclusions. Considering the growth stage of inclusion form, type (2) inclusions would be vestiges of melt infiltrated to the upper mantle beneath northeast China later than type (3) inclusions, whose shape represents a matured stage. Taking the tectonic setting of the studied area into account, the coexistence of the inclusions with different noble gas isotopic compositions indicates that the upper mantle with SCLM-like noble gas isotopic compositions was infiltrated by the melt associated with slab-related components.