HR: 1340h
AN: T23A-0532    [Abstracts]
TI: Grain Growth Kinetics of Orthopyroxene: Implications for the Deformation of Deep Continents
AU: * Skemer, P
EM: philip.skemer@yale.edu
AF: Yale University, 210 Whitney Ave, New Haven, CT 06511 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, 210 Whitney Ave, New Haven, CT 06511 United States
AB: Orthopyroxene grain-growth kinetics are a critical factor that control the rheological behavior of orthopyroxene in peridotites. It has been observed in so-called S-P mylonitic peridotites that orthopyroxene has a much smaller recrystallized grain-size than coexisting olivine (Boullier and Gueguen, 1975). Furthermore, we have found that this fine-grained orthopyroxene has been deformed by a grain-size sensitive mechanism, in contrast to olivine, which has been deformed by dislocation creep (Skemer and Karato, in prep). If this fine-grained orthopyroxene microstructure can persist for a sufficient period, then the rheological properties of the upper mantle will be considerably different from those predicted by olivine-dominated models. To explore the importance of this observation, we have conducted a series of experiments on the grain growth kinetics of synthetic orthopyroxene (Mg_0.9Fe_0.1SiO₃) at upper mantle temperatures and pressures. Fine-grained samples were statically annealed at 5 GPa and 1200-1400°C, for 0.5-3 hours, under water-rich conditions, in a Kawai- type multi-anvil apparatus. Both the starting materials and run-products were examined optically and with an SEM; grain-sizes were determined using the mean-intercept technique. Water contents were determined using infrared spectroscopy. All run products contain both fine-grained and coarse-grained regions. The area of the coarse-grained regions increases with temperature and the duration of the experiment. We interpret this microstructure to indicate that the fine-grained regions experienced ``normal'' growth-growth while the coarse-grained regions experienced ``abnormal'' grain-growth. The grain-boundary mobility calculated from normal growth regions is approximately 3*10^-14 (m/s)/(N/m²) at 1300°C. The grain-boundary mobility calculated from the abnormal growth regions is somewhat larger (by a factor of 2-6), which is not seen in comparable experiments with olivine. A possible cause for this discrepancy is the influence of impurities or secondary phases on grain- boundary migration: when these effects play in important role, the effective grain-boundary mobility for abnormal grain growth will be higher than the grain-boundary mobility for normal grain- growth (e.g. Yan et al., 1977). Generally, the grain-boundary mobility for water-rich orthopyroxene is very similar to the grain boundary mobility calculated for dry olivine at 300 MPa (Karato, 1989). Thus, we expect that for comparable water contents, orthopyroxene grain-growth is significantly more sluggish than olivine grain-growth. In order to explain the fine-grained orthopyroxene in S-P mylonites, we hypothesize that water-poor conditions or impurities have drastically reduced the kinetics of grain growth. More experiments are needed to confirm this hypothesis.
DE: 3902 Creep and deformation
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005