Development of in situ analysis of δ7Li in garnets to decipher metamorphic processes

Variations in bulk rock Li concentrations and isotopes have been documented in metamorphic rocks and related to the infiltration of fluids during metamorphism, however details about variability in fluid infiltration over the prolonged history of metamorphism are obscured by the lack of spatially highly resolved in-situ analysis. For example, growth zones within garnets preserve a wealth of information about the history experienced by rocks during prograde metamorphism, and partial relaxation of these profiles are known to be speedometers. In situ analysis by SIMS measurements of Li isotopes and concentrations of garnets have the potential to reveal the detailed evolution of fluid infiltration and diffusion recorded in these garnets. Before garnet can be analyzed with confidence, standards must be developed, which cover a wide range of major-element compositions, so that unknowns can be analyzed by matrix matching. The standards need to be homogeneous within measuring accuracy. Accuracy in the ‰ range for Li-isotopes requires > 20ppm, a concentration rarely found in natural garnets. Hence we synthesized glasses made from garnet powders doped with additional Li. In addition, we compared the measured Li isotope composition of non-doped garnet glass to those of the parent garnets to determine whether they can be used as proxies - and hence standards - for natural garnets. Garnets were melted in closed Pt-tubes, to avoid Li loss or gain, which would result in a change in Li isotopic composition. SIMS measurements of these garnet-glass pairs compared favorably. SIMS measurements of glass created from melting the spessartine-rich garnets ( 20 ppm Li) show d7Li that varies by only 1‰and that are only 0.7‰ different from the original garnets. SIMS measurements of almandine-rich garnet (UWG) which contains less Li ( 4ppm) shows variation of 4‰. The measured glass and garnet values fall within our estimates of measurement uncertainty for these concentrations. With these encouraging results we have now embarked on measuring enriched glasses ( 110ppm) of garnet composition to establish the composition dependence of Li-isotope instrumental fractionation.