HR: 1340h
AN: V33A-1172    [Abstracts]
TI: Hidden Records Of Critical Melt Fraction? Constant Intergranular Component In Mantle Xenoliths
AU: * Hiraga, T
EM: hiraga@eri.u-tokyo.ac.jp
AF: Univ of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032, Japan
AU: Kohlstedt, D L
EM: dlkohl@umn.edu
AF: Univ of Minnesota, Dept of Geology & Geophysics, Minneapolis, 55455, United States
AB: We recently discovered that the chemical composition of grain-grain interfaces (i.e., grain and interphase boundaries) relative to that of the grain interiors can be compared to element partitioning between melt and grains if the interface thickness is taken to be 2 mono-atomic layers. This analogy allows precise investigation of the composition of the intergranular component present in a rock based on a comparison of the concentrations of elements in a "whole rock" with those in a "clean rock" combined with the mineral mode, grain size, and temperature of equilibration. In this analysis, a clean rock is one reconstructed from the composition and mode of the minerals. In this paper, we have extracted information about intergranular components from data reported for 20 mantle xenoliths collected from 5 localities worldwide. We previously demonstrated that, at chemical equilibrium, interfaces are enriched in incompatible elements and that the enrichments are positively correlated with excess energies required for the solution of the elements into the crystal lattice [1]. The intergranular components in all the samples are consistently ~10³ larger than expected from equilibrium chemical segregation at grain-grain interfaces. We argue that this intergranular component, which is common to all mantle xenoliths, corresponds to an unrecognized ~0.2 vol% of melt, now possibly converted into micro-glass or micro- crystalline phases. Based on plausible upwelling rates for mantle xenoliths, it is unlikely that this melt formed during ascent. Instead, we conclude that this small amount of melt is a remnant of previous melting of the mantle. Consequently, it corresponds to the critical melt fraction below which melt is unable to escape from the rock by porous flow. [1] Hiraga T, Kohlstedt DL (2007) Geochimica et Cosmochimica Acta 71: 1266-1280
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1065 Major and trace element geochemistry
DE: 3619 Magma genesis and partial melting (1037)
DE: 3947 Surfaces and interfaces
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting