Contrasting behavior for Li-Mg isotopes during subduction: Insights from garnet in the Yardoi schists, Tibet

It has long been recognized that garnet has the capacity to preserve the trace element and isotopic signature of distinct metamorphic growth zones because of its high closure temperature. Combined with the large size of certain garnet porphyroblast, this allows investigating variations in metamorphic conditions such as pressure, temperature, deviatoric stress, and fluid composition, which occur during subduction-related metamorphism. Here, one garnet porphyroblast of 6 cm diameter was sampled from the Yardoi schists of Tibet, and the major-, trace-, and Li-Mg isotopic compositions of distinct growth zones were determined in situ. The δ7Li values range from +6.0 ‰ to +4.1 ‰ and follow ‘S-shape’ patterns on both sides of the garnet’s core, revealing a two-stage growth process corresponding to the fluid-assisted sequential recrystallization of chlorite and micas during prograde metamorphism. By contrast, once corrected for the overprinting by retrograde metamorphism, the δ26Mg values vary monotonously from -1.73 ‰ in the core to -1.32 ‰ in the outer rim, reflecting a single-step process interpreted to result from increasing temperature and the solid-state recrystallization of chlorite-biotite during prograde metamorphism. This different behavior of Li and Mg isotopes is interpreted to result from the fact that Li is more fluid-mobile than the major element Mg.