HR: 10:35h
AN: DI42A-02    [Abstracts]
TI: Extending the Wet Mantle Solidus: Implications for H2O Transport and Subduction Zone Melting Processes
AU: * Till, C B
EM: ctill@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States
AU: Grove, T L
EM: tlgrove@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States
AU: Withers, A C
EM: withe012@umn.edu
AF: University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AU: Hirschmann, M M
EM: Marc.M.Hirschmann-1@umn.edu
AF: University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, MN 55455, United States
AU: Medard, E
EM: emedard@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States
AU: Chatterjee, N
EM: nchat@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, United States
AB: Recent experimental studies (Grove et al., EPSL 249, 74-89, 2006) determine the solidus for primitive undepleted peridotite at H₂O -saturated conditions to 3.2 GPa and reveal the presence of hydrous phases on the solidus above 2 GPa. We present new data from piston cylinder and multi-anvil experiments that extend the peridotite solidus to 5 GPa at H₂O -saturated conditions. The H₂O -saturated solidus extends from 800°C at 3.2 GPa to 840°C at 5 GPa. Olivine, orthopyroxene, clinopyroxene, garnet ± chlorite ± Ti-clinohumite are stable on the solidus from 3.2 to 5 GPa. The hydrous phase chlorite (12 wt% H₂O) is stable on the H₂O-saturated solidus from 2 GPa to 3.6 GPa. Above 3.6 GPa, the H₂O- saturated solidus and chlorite stability fields diverge. The presence of chlorite on the H₂O-saturated solidus has important implications for melting processes at subduction zones and H₂O storage in the mantle wedge and subducted lithosphere. Our findings suggest chlorite is stable in the mantle wedge. This provides a new mechanism for transporting hydrous fluids released from the slab in the forearc to the base of the mantle wedge. In addition, chlorite stable within the subducted lithospheric mantle may be an important courier of H₂O to melting zones beneath arcs. The location of the breakdown of chlorite and the 10Å phase (both >10 wt% H₂O) will control the maximum depth attained by the majority of H₂O in the subducted lithosphere. This H₂O-out reaction in the slab follows the chlorite-out boundary between 2 - 4.5 GPa and the 10Å-out boundary at pressures greater than 4.5 GPa. For the 10Å phase to transport H₂O greater than 4.5 GPa, the mantle lithosphere must remain below 650°C up to 6 GPa, a value colder than those predicted by most thermal models. Therefore, all hydrous minerals with >10 wt% H₂O in the subducted lithosphere become unstable at depths of 120 - 150 km in most arcs and elucidate the maximum depth of H₂O-saturated melting at subduction zones. Ti-clinohumite (~3 wt% H₂O) is stable over a wide P-T range in the subducted lithosphere and may transport small amounts of H₂O deep into the mantle.
DE: 3612 Reactions and phase equilibria (1012, 8412)
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3619 Magma genesis and partial melting (1037)
DE: 3621 Mantle processes (1038)
DE: 3630 Experimental mineralogy and petrology
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting