Helium and argon isotopic composition of cenozoic alkali basalts and Mantle-Derived xenoliths from Kuandian, liaoning province, China

Abstract

The noble gas isotopic composition and content data of 2 alkali basalts, 3 lherzolite xenoliths and one clinopyroxene megacryst from the Kuandian region have confirmed the occurrence of a fractionation of noble gases during magmatism. Light noble gases such as He and Ne are high in mobility and appear to be incompatible as compared with heavy ones (such as Kr and Xe). Therefore, light noble gases are abundant in volcanics, especially in the volcanics with bubbles; lherzolite xenoliths have relatively high heavy noble gases. The clinopyroxene megacryst has the lowest abundance of noble gases, probably due to its highP-T origin. Noble gas isotopic composition of the clinopyroxene megacryst reveals that the mantle source beneath the Kuandian area has an MORB-like reservoir with3He/4He ratio of ≈ 10 Ra (Ra: atmospheric3He/4He ratio) and40Ar/36 Ar ratio of 345.6. The lherzolite xenoliths possess moderate3He/4He ratios of 2.59–4.53 Ra, reflecting the loss of primary helium during rock deformation or metasomatism caused by enriched mantle fluids during the up-lifting. The alkali volcanics have very low3He/4He ratios (0.47–0.61 Ra), indicating a contribution of radiogenic4He, probably having resulted from crust contamination. Most of the samples have excess21Ne and22Ne as compared with atmospheric neon, but Kr and Xe isotopic compositions are indistinguishable from atmospheric values within uncertainties with only individual samples having excess129Xe,134Xe and136Xe.