HR: 1340h
AN: V23A-1228    [Abstracts]
TI: The Olivine Enigma: Why Olivine Does Not Affect Seismic Velocities in Oceanic Gabbros
AU: Miller, J
EM: miller@iodp.tamu.edu
AF: Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843- 3115, United States
AU: Miller, J
EM: miller@iodp.tamu.edu
AF: Integrated Ocean Drilling Program, 1000 Discovery Drive, College Station, TX 77843, United States
AU: Newman, J
EM: newman@geo.tamu.edu
AF: Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843- 3115, United States
AU: * Carlson, R L
EM: carlson@geo.tamu.edu
AF: Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843- 3115, United States
AB: Seismic velocities in oceanic gabbros are expected to increase with increasing olivine content because of the very high elastic moduli of Fo90, but this relationship has not been confirmed by laboratory studies of gabbros recovered by scientific ocean drilling. An inversion of modal analyses and measured elastic moduli based on a Voigt-Ruess-Hill (VRH) model shows that the effective elastic moduli of olivine in these rocks are anomalously low relative to olivine with a composition of Fo90. This anomaly might be explained by the low Mg numbers of the recovered oceanic gabbros, by a high density of open cracks in the olivine, by fine-scale alteration of olivine, or by a combination of these factors. The magnesium contents olivine grains in the gabbros have highly variable, but lower Mg numbers (median Fo73) and hence somewhat lower moduli than those of olivines that are typical of upper mantle rocks (~Fo90). SEM analyses indicate that cracks in the olivine grains are filled with talc/serpentine. This observation suggests that the cracks are filled in situ and hence not in themselves a cause of low moduli. However, cracks containing talc and/or serpentine make up 5-10% of the olivine grain volume; with additional alteration along the margins of some grains, total alteration ranges from 5 to 30%. More detailed electron microprobe analyses of selected gabbro samples from ODP Hole 923A indicate that olivine grains in these rocks are typically 10 to 20% altered to talc and serpentine, plus small amounts of tremolite and magnetite. Numerical (VRH) modeling indicates that the low Mg numbers and about 10% alteration of olivine to talc and/or serpentine in these rocks will account for the observed low effective olivine moduli and for the fact that there is no observed correlation between the olivine content and measured seismic velocity is oceanic gabbro samples. Thus, seismic velocities are not indicative of olivine content in a gabbroic lower crust owing to fine-scale in situ alteration of olivine to talc and/or serpentine.
DE: 1021 Composition of the oceanic crust
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 3036 Ocean drilling
DE: 3617 Alteration and weathering processes (1039)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting