HR: 0800h
AN: T41D-1333    [Abstracts]
TI: Evidence for polybaric fractionation and disequilibrium crystallization in gabbroic cumulates recovered from the MARK area during ODP Leg 153
AU: * Meurer, W P
EM: wpmeurer@mail.uh.edu
AF: Dept. of Geosiciences, Univ. of Houston, 312 Sci. & Res. Bldg. 1, Houston, TX 77204-5007 United States
AB: Mineral major- and trace-element compositions were determined for samples from Hole 923A Section 12R1. Mineral trace-element concentrations show significant variability. For example, Zr in clinopyroxene varies from 7.7 to 119.4 ppm, Ce in plagioclase from 0.31 to 1.53 ppm, and Y in olivine from 0.12 to 0.52. Incompatible elements are strongly enriched in clinopyroxene rims with Zr increasing faster relative to other incompatible elements. Y in plagioclase shows little or no increase from core to rim but Ce shows a substantial increase. Of the compatible elements, Cr in clinopyroxene shows the largest range from 6000 to 250 ppm. Large variations in Cr contents do not correlate simply with incompatible elements. For example, in sample 923A 12R1 48, Cr-rich clinopyroxene (5000- 6000 ppm) have the lowest Y and Zr but as the Cr content decreases to 3500 ppm the Zr and Y increase by as much as 5 to 10 times. However, for Cr contents from 1000 - 2000 ppm (there is a gap in the Cr contents from 3500 to 2000 ppm) the Zr and Y contents show no systematic increase with decreasing Cr. The variation in Co and Ni in clinopyroxene is positively correlated and the trends defined by samples are sub parallel but shift along a plagioclase plus olivine control line. These relations suggest the Ni:Co ratio defined by each trend is indicative of how much fractionation of olivine ± plagioclase took place prior to clinopyroxene saturation (and therefore acts as barometer for the initial fractionation of the liquid). Previous paleomagnetic analysis indicates that the olivine gabbros in this core section cooled rapidly reaching their solidus temperatures in days to months. These thermal constraints eliminate any process requiring (1) volume diffusion in plagioclase or of +3/+4 cations in clinopyroxene, or (2) the introduction of highly fractionated liquids from nearby cumuluates. The mineral compositions indicate that polybaric fractionation and transport of crystals from depth took place. The over-enrichment of Zr relative to other incompatible elements in most crystals is interpreted to reflect disequilibrium crystallization and possibly dissolution/reprecipitation during the growth of clinopyroxene rims.
DE: 1527 Paleomagnetism applied to geologic processes
DE: 3035 Midocean ridge processes
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3618 Magma chamber processes (1036)
SC: Tectonophysics [T]
MN: Fall Meeting 2005