HR: 13:55h
AN: DI43A-02    [Abstracts]
TI: Is Garnet Stronger Than Olivine?
AU: * Katayama, I
EM: ikuo.katayama@yale.edu
AF: Dept Geology and Geophysics, Yale Univ, 210 Whitney Ave, New Haven, CT 06520 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Dept Geology and Geophysics, Yale Univ, 210 Whitney Ave, New Haven, CT 06520 United States
AB: Garnet and olivine are two important minerals in two geochemically distinct rock types, peridotite and eclogite. Consequently, the relative strength between garnet and olivine is a key factor in understanding the geodynamic behavior of two distinct geochemical "reservoirs". Here we investigated rheology of garnet by shear deformation experiments using the Griggs-type solid medium apparatus. Garnet and olivine samples, whose compositions are Alm46Prp52Grs2 and Fo90 respectively, were together sandwiched between alumina pistons in simple shear geometry. Because of uncertainty in reading differential stress from an external load-cell, we used dislocation density of deformed olivine crystals to infer stress level during deformation. Oxygen fugacity was buffered by the Ni/NiO reaction, and the water content was relatively low (< 200 ppm H/Si) based on infrared spectra and olivine fabric. We performed two experiments at 1GPa and 1473K using different strain-rates, approximately 1x10^-3 and 1x10^-4 s^-1. In both cases, garnet has larger strain than olivine, which indicates that garnet is weaker than olivine at the given conditions. Dislocation creep is likely the dominant deformation mechanism according to the stress exponent of ~3 based on the stress and strain-rate relation and dynamic recrystallization in deformed garnet. The difference in strain between the garnet and olivine samples is small when the samples are deformed at a higher strain-rate. This may be due to significant contribution of Peierls mechanism in olivine at higher stress because of lower Peierls stress for olivine (9GPa) than garnet (~16GPa). Although our experiments indicated that garnet is weaker than olivine, the creep strength of garnet is likely depending on its chemical composition. If we calculate strength of pyrope based on homogenous temperature normalization (Karato et al. 1995), pyrope is more resistant than olivine. This is consistent with the well-known observation that Mg-rich garnet in mantle peridotite is usually less deformed than olivine. In contrast, garnet in subducted oceanic crust contains higher Fe content, suggesting that garnet-rich layer is weaker than the surrounding peridotitic upper mantle. Our results provide new insights into the geodynamic behavior of garnet-rich regions including the processes of separation of a garnetite layer at around the 660 km discontinuity.
DE: 1030 Geochemical cycles (0330)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 1236 Rheology of the lithosphere and mantle (7218, 8160)
SC: Study of Earth's Deep Interior [DI]
MN: Fall Meeting 2005