Lithium isotope systematics of the Sumdo Eclogite, Tibet: Tracing fluid/rock interaction of subducted low-T altered oceanic crust

Abstract

Abstract The Sumdo eclogite, located in the central Lhasa Terrane, Tibet, has received much attention as it has important implications for the tectonic evolution of the Tibet orogenic belt. Aiming to constrain the Lithium (Li) isotope characteristics of rocks in subduction zones, we investigated whole-rock major and trace elements, Li isotopes as well as oxygen (O) and radiogenic (Sr, Nd, and Pb) isotopes on 20 Sumdo eclogite samples. The investigated samples display REE patterns similar to N-MORB, with relatively homogeneous bulk eNd(t=260Ma) values ranging from +4.0 to +7.8. These data suggest that protoliths of the Sumdo eclogite were N-MORB-type oceanic basalts. Yet, initial 87Sr/86Sr and δ18O are heterogeneous and range from 0.703703 to 0.706645 and from +4.9 to +8.9‰, respectively. A positive correlation between Sr and oxygen isotopic compositions suggests that the MORB protoliths had experienced low temperature seafloor alteration prior to subduction. The Li contents and δ7Li values of Sumdo eclogite vary from 3.64 to 29.7 µg/g and −1.1 to +4.0‰, respectively, significantly lower than those of low T altered MORBs (AOC) that usually have high Li contents (up to 33 µg/g) and heavy Li isotopic compositions (δ7Li up to +14‰). Modelling results suggests limited Li isotopic fractionation during dehydration. After eliminating the effect of diffusion, we suggest that a rehydration stage during exhumation may be responsible for the Li isotope systematics of the Sumdo eclogite. The fluids (