HR: 10:35h
AN: T32B-02    [Abstracts]
TI: Storage capacity of H$_{2}$O in the upper mantle and significance for melting at the 410 km discontinuity
AU: * Hirschmann, M M
EM: marc.m.hirschmann-1@umn.edu
AF: U. Minnesota, 108 Pillsbury Hall, Minneapolis, MN 55414 United States
AU: Aubaud, C
EM: aubau001@tc.umn.edu
AF: U. Minnesota, 108 Pillsbury Hall, Minneapolis, MN 55414 United States
AU: Withers, A C
EM: withe012@umn.edu
AF: U. Minnesota, 108 Pillsbury Hall, Minneapolis, MN 55414 United States
AB: Substantial data suggests that the transition zone has a large capacity to store H$_{2}$O in nominally anhydrous minerals, but the upper mantle is generally thought to have a much lower storage capacity for H$_{2}$O. Recent experimental and analytical advances suggest that the storage capacity of the upper mantle is also likely to be significant. First, improvements in analysis of H$_{2}$O in olivine [1,2] suggest that the solubility of H$_{2}$O in olivine is 3 times greater than previously believed [3], so the storage capacity of olivine reaches $\sim$0.5 wt.% at 410 km. Second, experimental measurements of coexisting mantle minerals at 1--2 GPa show that pyroxenes incorporate 10 times more H$_{2}$O than olivine [2,4]. If such strong pyroxene/olivine partitioning were to persist throughout the upper mantle, the storage capacity above 410 km could be greater than 1 wt.%, but more conservative assumptions still suggest that it is $>$0.4 wt.%. Thus, hydrous transition zone material advected above the 410 km discontinuity will not induce partial melting unless it has more than 0.4 wt.% H$_{2}$O and the H$_{2}$O retained in residual peridotite will be equivalent to the storage capacity of the upper mantle assemblage, i.e., a minimum of $\sim$0.4 wt.%. This is far in excess of the H2O of the dominant (MORB-source) upper mantle (100--200 ppm), so it is unlikely that dehydrated $"$wet$"$ transition zone material can be a volumetrically significant source of upper mantle material. In contrast to pyroxene, garnet appears to have a low H$_{2}$O storage capacity. Increasing modal garnet at the expense of pyroxene between 300 and 400 km depth likely results in diminished H$_{2}$O storage capacity with increasing depth and so there is a local storage capacity minimum just above the 410 km discontinuity. If this minimum were not present, then hydrous material melted just above the 410 km discontinuity would continue to melt as it is advected throughout the upper mantle. But owing to the minimum, the residues of any such melting at 410 km would cease melting as they upwell, though they would begin to melt again at depths of 250-300 km, where they again exceed the local H2O storage capacity. [1] Bell et al. JGR, 2003. [2] Koga et al. G3, 2003. [3] Kohlstedt et al. CMP, 1996 [4] Aubaud et al. GRL, submitted
DE: 8147 Planetary interiors (5430, 5724)
DE: 3630 Experimental mineralogy and petrology
DE: 1025 Composition of the mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting