Helium Isotopic Composition of the Songduo Eclogites in the Lhasa Terrane, Tibet: Information from the Deep Mantle

Abstract

Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids. In this paper, we present the helium isotopic compositions of the Songduo eclogites in the Lhasa terrane, Tibet. We found that garnet and omphacite in the eclogites have different helium retention characteristics. The ⁴He content of most omphacite grains are about 10–20 times of that of garnet, suggesting that omphacite has a higher ability to capture ⁴He than garnet. Similarly, there is about 10–20 times difference in ⁴He content between omphacite and garnet in the same eclogite samples. The ³He/⁴He ratios of garnet and omphacite in these rocks range from 0.27 to 0.60 Ra (relative to the modern air ³He/⁴He ratio, 1.4×10⁻⁶). These ratios are within the range of both mantle- and crust-derived helium, suggesting mixed sources. The Songduo eclogites have much higher ³He/⁴He ratios than those observed in the Dabie eclogites of eastern China. Such high ratios are typically thought to be associated with deep mantle sources. We cautiously conclude that deep mantle materials might have been involved during the formation of the Songduo eclogites.

Keywords

eclogite / helium isotopic composition / HP/UHP metamorphic belt / Lhasa terrane / Qinghai-Tibet Plateau

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