Diffusion-induced Lithium isotopic heterogeneity in olivines from peridotites of an oceanized mantle lithosphere at the Yunzhug ophiolite (central Tibet)
Description
The data are in situ Li elemental and isotopic compositions of olivines using a Cameca IMS- 1280HR SIMS at IGGCAS. The olivine grains are from the oceanized ancient subcontinental lithospheric mantle (SCLM) peridotities sampled from the Dingqing ophiolite within the Bangong-Nujiang suture zone in the cetral Tibet.The result shows that the systematic Li isotope changes with distance from the rim of olivine grains. δ7Li of olivine in dunites decrease from +10.46‰ to +1.33‰ while the distance to olivine grain rim increases from 26.15μm to 124.71μm. Li concentrations show a weak negative correlation with the distance to the rim. A negative correlation of δ7Li and Li content in olivine from dunite and harzburgite indicates recent diffusive ingress of Li into the peridotites. The extreme heavy Li isotopic composition requires the seawater or seawater alteration endmember in the mixing model, and reveals Li diffusion from seawater into olivine. As in dunites, olivines in a harzburgites sample also show large variations in δ7Li as a function of distance from the grain rim (e.g., 6.01 to 1.73 in sample 14YZ13). The data support that the behavior of Li in the oceanized SCLM peridotites may be controlled by Li diffusion from seawater, as Li activity in the liquid-state is higher than solid-state in transporting Li through the olivines in the peridotites. This study supports that seawater Li diffusion is one of the important factors for the heterogeneity of mantle Li isotopes in ophiolites.