HR: 09:30h
AN: V51A-07    [PDF]
TI: K/Li/Yb Fractionation in Micas
AU: * Feineman, M D
EM: feineman@uclink.berkeley.edu
AF: University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720-4767 United States
AU: * Feineman, M D
EM: feineman@uclink.berkeley.edu
AF: Lawrence Livermore National Laboratory, Dept. of Earth Sciences, L-206, Livermore, CA 94551 United States
AU: Ryerson, F J
EM: ryerson@llnl.gov
AF: Lawrence Livermore National Laboratory, Dept. of Earth Sciences, L-206, Livermore, CA 94551 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Dept. of Earth and Planetary Science, Berkeley, CA 94720-4767 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Lawrence Berkeley National Laboratory, Earth Sciences Division MS 90R-1116, Berkeley, CA 94720 United States
AB: Island arc basalts (IAB) have been shown to have high Li/Yb relative to mid-ocean ridge basalts (MORB) and ocean island basalts (OIB). In addition, IAB have high K/Li relative to rare earth element (REE) fractionation. A positive correlation between Ba/Ti and Li/Yb has also been observed. Therefore, the generation of IAB must involve interaction with a component having the elemental abundances K, Ba$>$Li$>$Yb. Although some subducted sediments or altered oceanic crust may have this characteristic, the control of mica over K mobility in arcs is such that the affect of mica stability and breakdown on all of the above elements must be considered. We have performed mineral/fluid trace element partitioning experiments for two K-bearing micas, phlogopite and phengite, at 2.0 GPa, 900$\deg$C. Both K and Ba are strongly compatible in phlogopite (D$_{K}$=$\sim$44, D$_{Ba}$=$\sim$50) and phengite (D$_{K}$=$\sim$32, D$_{Ba}$=$\sim$21), Li is moderately compatible in phengite (D$_{Li}$=$\sim$2), and Yb is somewhat incompatible in phlogopite (D$_{Yb}$=$\sim$0.16). Although we do not have Yb partitioning data for phengite, biotite/muscovite partitioning data from natural rocks indicate that REEs are not significantly fractionated between the micas. Previous studies indicate that K mobility in arcs is controlled by mica stability in the subducting slab, and perhaps the overlying mantle wedge. Sediment dehydration and melting experiments have shown that K, Ba, and Li are released following mica breakdown with progressive heating. However, bulk sediment addition cannot explain Li/REE ratios or the extreme K/Li in some arc lavas. Our results indicate that mica (when present) will be the primary host of K, Ba, and Li in the eclogitic assemblage resulting from reactions in the crust and/or sediment layer as subduction progresses. Gradual breakdown of the micas with increasing pressure and temperature will contribute K, Ba, and to a lesser extent Li to the arc lava source, while contributing very little REE. The concentrations and relative fractionation of the REE are primarily determined by the degree and depth of melting in the mantle. The K/Li ratio may be useful as an indicator of relative mica contribution to the slab component, while the Li/Yb ratio may be useful as an indicator of slab vs. wedge contribution to arc lavas.
DE: 1065 Trace elements (3670)
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 3660 Metamorphic petrology
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting