LITHIUM IN UHP METASEDIMENTARY GARNETS FROM LAGO DI CIGNANA, ITALY: COUPLED SUBSTITUTIONS AND THE ROLE OF ACCESSORY PHASES

We investigated major/trace element concentrations and δ⁷Li in garnets in Cignana metasedimentary rocks (peak conditions 550˚C, 3.0 GPa), relating these observations to reconstructed prograde devolatilization history (Bebout et al., 2013; Chem. Geol.) and trace element budgets as related to accessory phases. 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. HREE, 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; 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 highly-zoned Ca- and REE-rich phase and decreased Nb concentrations in garnet.

These relationships appear to reflect prograde garnet-forming reaction(s) in part involving titanite breakdown to stabilize rutile, which resulted in delivery of more abundant Y and HREE at surfaces of growing garnets to produce growth annuli. Calculations using Theriak-Domino (de Capitani and Petrakakis, 2010; Amer. Mineral.) demonstrate titanite breakdown to rutile during early-stage garnet growth. The co-enrichment of Li and Y+REEs is attributed to their mutual incorporation via a charge-coupled substitution (Carlson et al., 2014; Amer. Mineral.); thus increased Li uptake is a passive consequence of 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 fluctuation in δ⁷Li may correlate with abrupt shift in major/trace element concentrations, suggesting that changes in the minerals involved as reactants exert some control on the evolution of δ⁷Li. For one garnet, late-stage growth following partial resorption produced deviation in major/trace element composition, including Li ppm, accompanied by a 10-15‰ negative shift in δ⁷Li perhaps reflecting change in the mechanism of incorporation or source of Li.