HR: 16:15h
AN: T14B-02    [Abstracts]
TI: Crystal-melt separation and the development of isotopic heterogeneities in hybrid magmas.
AU: * Beard, J S
EM: jim.beard@vmnh.virginia.gov
AF: Virginia Museum of Natural History, 21 Starling Avenue, Martinsville, VA 24112, United States
AB: If a magma is a hybrid of two (or more) isotopically distinct end-members, at least one of which is partially crystalline, separation of melt and crystals after hybridization and homogenization will lead to the development of isotopic heterogeneities in the magma as long as some of the pre-existing crystalline material (antecrysts) retains any of its original isotopic composition. This holds true whether the hybridization event is magma mixing as traditionally construed, bulk assimilation, or melt assimilation. Once a magma-scale isotopic heterogeneity is formed by crystal-melt separation, it is essentially permanent, persisting regardless of subsequent crystallization, mixing, or equilibration events. The magnitude of the isotopic variability resulting from crystal-melt separation can be as large as that resulting from differential contamination/magma mixing or the presence of multiple isotopically distinct sources. In one model, a redistribution of one-third of the antecryst cargo yielded a crystal-enriched sample with 87Sr/86Sr of 0.7058, while the complementary crystal-poor sample has 87Sr/86Sr of 0.7068. In other models, crystal-rich samples are enriched in radiogenic Sr. Isotopic heterogeneities can be either continuous – controlled by the modal distribution of relict antecrysts – or discontinuous – when antecrysts are preserved as cores in isotopically zoned crystals. The first case may be exemplified by some isotopically zoned large volume rhyolites, formed by the eruptive inversion of an isotopically zoned magma chamber. In the latter case, the isotopic composition of the bulk crystal fraction will be distinct from that of any remnant or interstitial liquid. This may, for example, explain the presence of isotopically distinct late stage aplites in plutons. Crystal-melt separation provides an additional option for the interpretation of isotopically zoned/heterogeneous magmas. This option is particularly attractive for systems whose chemical variation is otherwise explicable by fractionation-dominated processes. It goes without saying that non-isotopic chemical heterogeneities can also develop in this fashion.
DE: 1036 Magma chamber processes (3618)
DE: 1040 Radiogenic isotope geochemistry
DE: 8104 Continental margins: convergent
DE: 8439 Physics and chemistry of magma bodies
SC: Tectonophysics [T]
MN: 2007 Fall Meeting