HR: 12:05h
AN: DI42A-08    [Abstracts]
TI: The Infidelity of Melt Inclusions?
AU: * Baker, D R
EM: donb@eps.mcgill.ca
AF: Earth and Planetary Sciences, McGill University, 3450 rue University, Montreal, QC H3A 2A7, Canada
AU: * Baker, D R
EM: donb@eps.mcgill.ca
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, 00143, Italy
AU: Freda, C
EM: freda@ingv.it
AF: Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, Rome, 00143, Italy
AB: Melt inclusions are routinely used as evidence of magmatic compositions prior to volcanic eruptions. However, it has long been known that kinetic processes can modify melt inclusion compositions during trapping. We investigated the fidelity of melt inclusions as records of magmatic compositions by artificially creating melt inclusions through crystallization of plagioclase and clinopyroxene from a hawaiitic basalt bulk composition at 1.0 GPa, 1150 °C, or 75 °C undercooling. We compared melt inclusion compositions to those of melts 100's of μm away from the crystals and found measurable differences in the compositions. We modeled the concentration profiles of Al, Fe, P, S, and Cl in front of the crystals using classical impurity rejection theory during growth at a constant rate followed by a growth hiatus during which diffusive relaxation occurred. The values of the growth rates and times were constrained by measured crystal sizes and the experimental duration. The diffusion coefficients for the elements investigated were either calculated from transition state theory (Al, Fe) or measured in the same bulk composition as that used for this study (S and Cl from the literature, new measurements for P). An envelope of models bracket the observed compositional profiles of the elements studied and predict enrichments, or depletions, at the crystal-melt interface that are quantitatively similar to those seen in the melt inclusions. The differences between the melt far from the crystals and that at the interface are only 20% (relative) for the major elements, Al, Fe, but are about 50% for S and Cl and can exceed 100% for P. These differences correlate with the relative chemical diffusion coefficients of these ions in the melt. Based upon these experiments and models we advise caution in the use of melt inclusions as indicators of pre-eruptive melt compositions.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1036 Magma chamber processes (3618)
DE: 1043 Fluid and melt inclusion geochemistry
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8439 Physics and chemistry of magma bodies
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting