Lithium Behavior during Growth of Metasedimentary Garnets from the Cignana UHP Locality, Italy

Abstract:

We investigated major and trace element concentrations and δ⁷Li in garnets in Lago di Cignana metasedimentary rocks (peak conditions ~550˚C, 2.5-3.0 GPa), following the EPMA-SIMS approach of Tsujimori et al. (2014; IMA conference abstract). Previous work on the devolatilization history of these rocks (Bebout et al., 2013; Cook-Kollars et al., 2014; both in Chemical Geology) provides a petrologic and geochemical context for this study. Lithium is of interest as a tracer of fluid-rock interactions and because of its potential to isotopically fractionate during diffusional processes.

All garnets are almandine-rich with strongly decreasing MnO and increasing MgO toward rims. HREEs, Y, and Li also show strong zoning, with elevated concentrations in cores (15-50 ppm Li) and marked high-concentration anomalies (up to 117 ppm Li, 5500 ppm Y), with little or no major element shift, as growth annuli at which some garnets have elevated δ⁷Li. In all garnets, rutile inclusions appear abruptly at annuli and outward toward rims, accompanied by inclusions of a Ca- and LREE-rich phase and decreased Nb concentrations in garnet.

These relationships appear to reflect prograde garnet-forming reaction(s) that in part involved titanite breakdown to stabilize rutile, which resulted in delivery of more abundant Y and HREEs at surfaces of growing garnets to produce growth annuli. The co-enrichment of Li and Y+REEs is attributed to their mutual incorporation via a charge-coupled substitution (Carlson et al., 2014; American Mineralogist); thus the increased Li uptake is a passive consequence of the elevated concentrations of Y+REEs. Distributions of δ⁷Li are complex, with most garnets showing only subtle core-to-rim variation other than at Y+REE annuli. At annuli, some garnets display elevated δ⁷Li (by up to 8‰), while others in the same rock do not. Small-scale fluctuations in δ⁷Li may correlate with abrupt shifts in major and trace element concentrations, suggesting that changes in the minerals involved as reactants exert some control on the evolution of δ⁷Li.