HR: 16:35h
AN: V24A-03    [Abstracts]
TI: The Effect of Melt Pressure on the Rheology of Compacting, Partially Molten Peridotite
AU: * deMartin, B
EM: bjd@mit.edu
AF: MIT/WHOI Joint Program, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AU: Hirth, G
EM: ghirth@whoi.edu
AF: WHOI, MS #8, Woods Hole, MA 02543 United States
AU: Evans, B
EM: brievans@mit.edu
AF: MIT, 77 Massachusetts Ave., Cambridge, MA 02139 United States
AB: The rheology of partially molten rock controls rock strength beneath spreading centers, deformation of the mantle wedge under subduction zones, and migration of melt to hot spots and volcanic arcs. Our understanding of these regions has been predominately shaped by chemical analyses of rocks and by remotely collected geophysical data. To interpret these data, however, requires knowledge of the relationships among deformation, melt topology, and melt migration. Most previous experimental studies of these relationships in partially molten rocks were conducted using undrained experiments, i.e., where melt cannot leave the matrix during deformation. For this configuration, melt pressure is inferred to roughly equal the minimum principle stress, but is actually unknown. By contrast, we have performed drained tests in which both melt pressure and compaction rates were measured independently. First, samples were synthesized by hot-isostatic pressing (HIP) fine-grained olivine power (10-38 $\mu$m) with a prescribed amount of mid-ocean ridge basalt (MORB) powder ($<$ 15 $\mu$m) in a gas-medium apparatus at $1200\deg$C and 300 MPa for 10 hours. Melt fractions (MORB contents) ranged from 0-30 $%$. Subsequently, samples were reinserted into the apparatus and deformed in the standard triaxial configuration. Melt flow out of the sample was accommodated by a glassy carbon bead reservoir (grain size 80-200 $\mu$m) located above the sample. A small alumina cylinder centered within the reservoir transferred the load from the pistons to the sample. Melt pressure was controlled by regulating the pressure of argon gas in contact with the melt in the reservoir. Sample compaction was measured by recording the position of a piston with the pore pressure generator. Variations of melt fraction on the strength of drained samples at P$_{m}$ = 30 or 50 MPa, where P$_{m}$ is the melt pressure, affect strength in the same way as previously observed under undrained conditions. Within the uncertainty of our measurements, changing P$_{m}$ from 30 to 50 MPa did not affect the strength of the rock. However, undrained rocks (P$_{m}$ = 300 MPa) are approximately 5 times weaker than those with the same initial melt fraction, but deformed at P$_{m}$ =30 MPa. Finally, compaction rates of drained triaxially loaded samples are significantly greater than those under isostatic compressive loads (i.e., hydrostatic melt extraction). This last result suggests that shear-enhanced compaction may play in important role in deformation and melt extraction at oceanic spreading centers and the mantle wedge of subduction zones.
DE: 8434 Magma migration
DE: 8162 Rheology--mantle
DE: 5114 Permeability and porosity
DE: 5120 Plasticity, diffusion, and creep
DE: 5139 Transport properties
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting