A new estimation method for Os isotopic composition of convecting upper mantle and its application to Tethyan oceanic upper mantle
Huang Qishuai1,2, Liu Deliang1,2, Gong Xiaohan1,2, Zhang Xiaoran3, Shi Rendeng1
1. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing 100101;
2. University of Chinese Academy of Sciences, Beijing 100049;
3. The University of Hong Kong, Hong Kong
It is essential to obtain precise Os isotopic composition of convecting upper mantle before the application of Re-Os isotope system.However,published estimates for convecting upper mantle,especially calculation from mantle peridotites,are hindered by ubiquitous mantle heterogeneity.Given the ancient refractory domains contribute little to MORB formation,early-crystallizing dunite which have the same initial 187Os/188Os ratios with mantle source,can be used to estimate the Os isotopic composition of convecting upper mantle based on zircon U-Pb ages.A carbonaceous chondritic mantle source for the Nagqu ophiolite are indicated by Cr-spinel Os isotopes and zircon U-Pb ages from cumulate dunite and gabbro in the same cumulate section.According to this model,ancient mantle events are recorded in chromitites and host rocks in Luobusa and Dongqiao,which may related to ancient subcontinental lithospheric mantle.In contrast,the chromitite-free peridotites(Zedang lherzolite) show carbonaceous chondritic Os isotopic composition without ancient mantle events information.