HR: 0800h
AN: V41C-1398    [Abstracts]
TI: Partitioning of Trace Elements Between Clinopyroxene and Peridotitic Partial Melts: Implications for the Mechanism(s) by Which Piston-Cylinder Partial Melting Experiments Equilibrate
AU: * Johnston, A D
EM: adjohn@uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
AU: Schwab, B E
EM: bes21@humboldt.edu
AF: Department of Geology, Humboldt State University, Arcata, CA 95521 United States
AU: Mercer, C
EM: cmercer@darkwing.uoregon.edu
AF: Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403 United States
AB: Earlier piston-cylinder experiments in our laboratory produced a collection of mantle melting run products at 1.0 GPa that have now been analyzed by ion probe for selected REE, Ti, Cr, Rb, Sr, Y, Zr, and Nb. The glass phase in 18 run products, representing melt percentages of ~2-20 wt.%, yielded excellent data that were inverted to yield the first estimates ever of clinopyroxene/melt distribution coefficients, Ds, derived from direct peridotite partial melting experiments. Uncertainties were estimated with a Monte Carlo method. For the REE and Y, these Ds were then compared to Ds calculated with the widely-used model of Wood and Blundy (1997) and the two sets overlap at the 2sigma level in 123 of 128 cases (96%). This indicates to us that the experiments analyzed here are well-equilibrated with respect to major and trace element distributions and it suggests that the cpx/melt Ds derived here for other elements, not modeled by the Wood and Blundy formulation, are probably also correct for peridotite melting to within their 2sigma uncertainties. The degree to which our experiments appear to have equilibrated seems at odds with recent measurements of the diffusivities of REE in diopside which suggest that relatively small percentages of our starting mineral grains should have equilibrated diffusively. Instead, we suggest that equilibration occurs much more rapidly through the processes of recrystallization and grain coarsening, accomplished through dissolution and reprecipitation. This suggestion is supported by the observation that our final grain sizes are typically 5-10 times larger than the ~10-20 mm starting sizes, indicating that substantial mass transfer occurred in our experiments, probably mediated by the melt phase in which diffusion is faster. Preliminary results from a time series of experiments suggests that disequilibrium melting during the first day of long-duration (3-5 days) experiments results in excess melt which is subsequently reprecipitated as overgrowths on existing crystal substrates.
DE: 3630 Experimental mineralogy and petrology
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting