2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 306-10
Presentation Time: 9:00 AM-6:30 PM


BEBOUT, Gray E.1, TSUJIMORI, Tatsuki2, OTA, Tsutomu2, SHIMAKI, Yuri2, KUNIHIRO, Tak2, CARLSON, William D.3 and NAKAMURA, Eizo2, (1)Department of Earth and Environmental Sciences, Lehigh University, 1 West Packer Avenue, Bethlehem, PA 18015, (2)Pheasant Memorial Laboratory, Institute for Study of the Earth's Interior, Okayama University, Misasa, 682-0193, Japan, (3)Department of Geological Sciences, The University of Texas at Austin, Austin, TX 78712, geb0@lehigh.edu

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.