HR: 10:35h
AN: V52A-02    [Abstracts]
TI: Generating variable Mg-numbers during melt segregation of TTG arc magmas
AU: * Rushmer, T
EM: Tracy.Rushmer@uvm.edu
AF: University of Vermont, Dept. of Geology, Burlington, VT 05405, United States
AU: Getsinger, A
EM: agetsing@uvm.edu
AF: University of Vermont, Dept. of Geology, Burlington, VT 05405, United States
AU: Jackson, M D
EM: m.d.jackson@imperial.ac.uk
AF: Dept. of Earth Science and Engineering, Royal School of Mines Building Imperial College, London, SW7 2BP, United Kingdom
AB: Major, trace, and REE compositions of both Archean TTGs and modern adakite-like magmas have been used in conjunction with batch melting experiments and models to infer source rock compositions, depths of melting, and tectonic setting. However, the impact of physical melt segregation processes on the magma geochemistry has not been considered to date. When melt initially migrates, it can interact with its partially molten host and this process may have a profound impact on the composition of the segregated melt as it leaves the source region. In this study, we are investigating through numerical modeling and experimental testing, the hypothesis that TTG arc crust formation is not only a function of partial melting of a mafic source region, but the time and length scales for melting and extraction, and melt segregation mechanisms themselves. In the experimental investigation we have designed melt segregation equilibrium (MSE) experiments to reproduce the local changes in bulk composition that are predicted to occur in response to buoyancy-driven melt segregation along grain edges and associated compaction of the solid residue. We conducted the experiments between 925-1000°C at 1.4 GPa; the same conditions as an earlier set of direct partial melting experiments on the same mafic amphibolitic starting material. The MSE experimental results show distinct differences in the melt and solid phase compositions and solid phase stability when compared with the results from the earlier direct partial melting experiments. Overall, the resulting melt compositions in the MSE experiments are lower in the An component and have higher Mg-numbers when compared with the direct partial melting results. Modally, the charges have changed too, with a reduction in hornblende and plagioclase and an increase in garnet and clinopyroxene as a function of increasing temperature. One interesting aspect is that hornblende is stabilized and is a new phase in a number of the experiments at the mid-temperature range. The results suggest that if dynamic melt segregation and equilibrium processes are active, they may modify the normally robust geochemical indicators, such as Mg-numbers, which are typically used to develop models of TTG petrogenesis.
DE: 8145 Physics of magma and magma bodies
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly