Effect of olivine alteration on seismic velocities in oceanic gabbros

Author(s): Carlson, R. L.; Newman, J.; Miller, D. J.
Author Affiliation(s): Primary:
Texas A&M University, Department of Geology and Geophysics, College Station, TX, United States
Volume Title: AGU 2006 fall meeting
Source: Eos, Transactions, American Geophysical Union, 87( Suppl.); American Geophysical Union 2006 fall meeting, San Francisco, CA, Dec. 11-15, 2006. Publisher: American Geophysical Union, Washington, DC, United States. ISSN: 0096-3941 CODEN: EOSTAJ
Note: In English
Summary: Seismic velocities in oceanic gabbros are expected to increase with increasing olivine content, but this relationship is has not been observed in gabbros recovered by drilling, and inverse methods yield elastic moduli that are anomalously low. Two hypotheses have been advanced to explain these observations: one is that the olivine grains in these rocks may contain a high concentration of cracks that affect their elastic moduli, and the other is that the olivines in the gabbros have lower Mg numbers (median Fo73) and hence lower moduli than olivines that are typical of upper mantle rocks (≈Fo90). Our SEM analyses of gabbro samples show that cracks in the olivine grains are sealed by serpentine; because they are sealed in situ, cracks cannot explain the low moduli. The elastic moduli of Fo73 are higher than effective moduli of olivine in the samples. However, though very little alteration of olivine to serpentine was reported in the modal analyses of these rocks, our SEM analysis of gabbros from ODP Hole 923A shows significant alteration of olivine to serpentine and magnetite. To explore the effect of this alteration on the relationship between seismic velocities and modal mineral content, we have used the known properties of olivine (Fo73), serpentine, magnetite, plagioclase, pyroxene and amphibole to match VRH model velocities to measured velocities by adjusting the degree of alteration of olivine in the gabbro samples from Hole 923A. We assume that the alteration products are 95% serpentine and 5% magnetite. Our analysis of the 25 samples from Hole923A indicates that an average of 10-30% alteration of Fo73 to serpentine and magnetite is sufficient to explain the low elastic moduli, and suggest that the lack of correlation between seismic velocities and olivine contents arises from variable alteration that is not correlated with initial olivine content.
Year of Publication: 2006
Research Program: ODP Ocean Drilling Program
Key Words: 05 Petrology, Igneous and Metamorphic; 18 Geophysics, Solid-Earth; Alteration; Atlantic Ocean; Cracks; Crust; Elastic constants; Experimental studies; Fractured materials; Gabbros; Igneous rocks; Leg 153; Magnetite; Mid-Atlantic Ridge; Nesosilicates; ODP Site 923; Ocean Drilling Program; Oceanic crust; Olivine; Olivine group; Orthosilicates; Oxides; Plutonic rocks; Serpentine; Serpentine group; Sheet silicates; Silicates; Velocity structure
Coordinates: N233233 N233233 W0450154 W0450154
Record ID: 2007101306
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