HR: 1340h
AN: V43B-1575    [Abstracts]
TI: Towards a comprehensive and internally consistent database for partition coefficients of REEs in ultramafic minerals
AU: * Harbert, A
EM: artemish@rice.edu
AF: Department of Earth Science, MS-126, Rice University, 6100 Main St., Houston, TX 77005
AU: Lee, C A
EM: ctlee@rice.edu
AF: Department of Earth Science, MS-126, Rice University, 6100 Main St., Houston, TX 77005
AB: Despite a good understanding of partitioning behaviors of highly incompatible elements in clinopyroxenes, their partitioning behaviors in olivine and orthopyroxene are currently not as well constrained. Creating an internally consistent database of mineral-melt partition coefficients in these minerals should be a priority, due to their relative importance in systems where clinopyroxene has been preferentially melted out. However, unlike in clinopyroxenes, concentrations of highly incompatible elements, such as the rare earths, in olivine and orthopyroxene are often too low to be analyzed by in-situ techniques. An alternative approach is to physically separate and dissolve natural phenocryst-magma pairs, allowing for more accurate measurement of incompatible elements. However, partition coefficients (D) measured in this manner can vary by orders of magnitude, raising the issue of equilibration and contamination problems. To work around these problems, we measured mineral-mineral subsolidus partition coefficients in peridotite xenoliths using physical separation and dissolution of minerals and analyzing them using inductively coupled plasma mass spectrometry. We find that olivine/cpx and opx/cpx partition coefficients correlate with ionic radius. The heavy to middle REEs are consistent with a version of Blundy and Wood's Brice formulation modified for subsolidus partitioning and pinned to measured Lu partition coefficients. The light REE Ds deviate from this trend due to either grain boundary partitioning or small amounts of contamination. However, extrapolation of the modified subsolidus Brice equation to high ionic radii allows us to infer LREE partition coefficients. Temperature effects are currently being considered, but ignoring for now any temperature effect, we can use our new empirical Dmineral/cpx values combined with experimentally calibrated Dcpx/melt to create a comprehensive and internally consistent database for mineral-melt partition coefficients in ultramafic systems.
DE: 3699 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005