HR: 13:40h
AN: T13D-01    [Abstracts]
TI: Dramatic Strain Weakening in Wet and Dry Olivine Aggregates
AU: * Holyoke, C W
EM: Caleb_Holyoke_III@brown.edu
AF: Dept. of Geological Sciences Brown University, 324 Brook Street Box 1846, Providence, RI 02912 United States
AU: Tullis, J
EM: Jan_Tullis@brown.edu
AF: Dept. of Geological Sciences Brown University, 324 Brook Street Box 1846, Providence, RI 02912 United States
AB: Evidence of strain localization in olivine aggregates has been observed both in obducted slices of upper mantle and in high stress experiments (Post 1977). Most experimental studies of olivine aggregates have been done in gas apparatus at low P ($<$300 MPa) and thus at low flow stress, where deformation occurs by climb-accommodated dislocation creep (CADC) with little if any strain weakening. However, experimental studies of quartz and feldspar at high P ($>$1 GPa) have shown the existence of a low T, high stress dislocation creep regime where climb is limited and bulging recrystallization leads to extreme strain weakening and localization. Several recent papers have suggested that for olivine aggregates deforming by dislocation creep at high P ($>$1 GPa) at both high ($>$300 MPa) and low ($<$300 MPa) flow stresses, the addition of water causes a switch in the dominant slip system and thus the lattice preferred orientation (LPO), with important consequences for interpreting seismic anisotropy (e.g. Jung and Karato 2001). To explore high stress dislocation creep in olivine at both wet and dry conditions, we have deformed synthetic aggregates in general shear, using a modified molten salt assembly in a Griggs apparatus at P=1.65 GPa, T=1100$\deg$C, and shear strain rates of 10$^{-4}$/s and 5x10$^{-5}$/s. Aggregates were hot pressed from San Carlos olivine powders (10-25 $\mu$m) at 1.65 GPa for 12 hours. Some samples were dried for 48 hours at 900$\deg$C in a CO/CO$_{2}$ atmosphere, whereas for others 0.6 $\mu$l of water were added prior to weld sealing the sample capsule. Results: {\bf (1)} Wet samples undergo significantly more strain weakening (75-50%) before reaching steady state flow stresses than dry samples (40-25%) deformed at the same conditions. TEM microstructures indicate that the dominant deformation mechanism at the peak stress (400-800 MPa) in all samples is recrystallization-accommodated dislocation creep (RADC). As the samples strain weaken, dislocation climb becomes easier; in most samples RADC remains the principal deformation mechanism, although in those that weaken to $<$200 MPa there is a switch to CADC. {\bf (2)} The recrystallized grain size - flow stress data follow one piezometer relation for all conditions, regardless of deformation mechanism or water content. This result contrasts with the results of Jung and Karato (2001) who found that the recrystallized grain sizes increased with high water contents, and with results for quartz and feldspar which show a change in the slope of the recrystallized grain size/stress relationship for RADC. {\bf (3)} Sample LPOs are consistent with slip on (010)[0{\it kl}] (Type D) at high stresses ($>$300 MPa) and on (010)[100] (Type A) at low stresses, even though the amount of water added to the samples is greater than was found by Jung and Karato (2001) to cause a switch to (010)[001] and (100)[001], respectively. These results indicate that strain localization in olivine aggregates should occur at high stresses where deformation occurs by RADC, and that localization will be greater in wet aggregates.
DE: 8030 Microstructures
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
DE: 8162 Rheology--mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting