Preliminary study on noble gas isotopes of mantle-derived peridotite xenoliths along the Tan-Lu Fault belt, Eastern China

Abstract

In order to understand the nature of noble gas reservoirs beneath continents, elemental and isotopic compositions of noble gases were analyzed from spinel-lherzolites with anhydrous mineral assemblages (i.e., olivine+clinopyroxene+orthopyroxene +spinel±apatite±garnet); these are considered to be typical of the upper lithospheric mantle. Isotopic compositions of the noble gases were measured for peridotite xenoliths enclosed in Cenozoic alkaline basalts in eastern-central China to provide insight on mantle characteristics. The results show that the noble gas abundances and isotopic compositions in mantle-derived xenoliths from eastern-central China vary widely:4He abundances from 0.0024×10−6 to 25.0×10−6 (cm3stp/g);3He/4He ratios range between 0.33RA and 4.62 RA;40Ar/36Ar ratios from 250.56 to 1233.86;40Ar abundances from 0.97×10−6 to 34.18×10−6(cm3/g). Only three measured Ne isotopes show a degree of homogeneity:20Ne from 0.3 to 0.7 (cm3stp/g);20Ne/22Ne ratios from 10.5 to 11.0; and20Ne abundances are also rather uniform, from 35.8×10−13 to 47.8×10−13 (cm3stp/g), and129Xe/130Xe ratios from 6.50 to 6.70. It can be seen that3He/4He ratios vary largely, and40Ar/36Ar ratios are significantly lower than MORB and Proterozoic crust, but close to that of atmospheric values. On the basis of the noble gas abundances and isotope compositions, it can be concluded that Ne, Kr and Xe in mantle-derived xenoliths from NE China have atmospheric origin. The results also show the heterogeneity of noble gases in peridotite from the upper mantle in eastern China, which accords with the results of previous geochemical studies.