Helium and neon isotopic compositions in the ophiolites from the Yarlung Zangbo River, Southwestern China: The information from deep mantle

Abstract

The ophiolites from the Yarlung Zangbo River (Tibet), Southwestern China, were analysed for the contents of helium and neon and their isotopic compositions by stepwise heating. The serpentinites from Bainang showed a high 3He/4He value of 32.66R a (R a is referred to the 3He/4He ratio in the present air) in 700°C fraction. At lower temperature, all of the dolerites displayed as very high 3He/4He ratios as ones investigated for hotspots. It was clear that the high 3He/4He ratio was one of immanent characterics in the magma source formed the dolerites, suggesting that there was a large amount of deep mantle fluids in these rocks. In the three-isotope diagram of neon, the data points from the ophiolites of the Yarlung Zangbo River were arranged along the Loihi Line. This is in agreement with the characteristics of helium isotopes, revealing that the high-3He plume from deep mantle had played an important role in the formation of the Neo-Tethyan Ocean. The helium isotopic compositions in the basalts were far higher than atomospheric value but lower than the average value of MORB, although there were various degrees of alteration. The possible reasons were that basaltic magmas had been contaminated by crust-derived fluids.