It has been known for more than 10 years that nominally anhydrous minerals may incite small amounts of melting in Earth's mantle. In recent years, there has been a great increase in experimental data on the H2O storage capacity of nominally anhydrous minerals. Yet, disagreement among models for the locus of dehydration melting in the upper mantle is growing rather than narrowing. Models span the gamut from requiring small amounts of hydrous melt throughout the upper mantle, to hydrous melting in a global low velocity zone layer at depths of ~80-200 km, to melting only beneath ridges and oceanic islands in a restricted interval a few 10s of km beneath the locus of dry melting. These disagreements persist because direct experimental investigations of the influence of small amounts of H2O on mantle melting are not feasible, and consequently understanding comes from parameterization of indirect experimental constraints. One key constraint on the possible locus of hydrous melting in the upper mantle is the range of feasible concentrations of H2O in near-solidus hydrous melts. For a mantle with a fixed amount of H2O, Cmantle, the maximum H2O concentration of an incipient partial melt is given by Cmantle/Dperid/melt, where Dperid/melt is the equilibrium bulk partition coefficient between the peridotite mineral residue and the partial melt. To address this issue, we conducted experimental determinations of Dmineral/melt for upper mantle minerals (garnet, cpx, opx) from 3 to 5 GPa using piston cylinder and multi-anvil devices. For minerals, concentrations of H2O were determined using low-blank SIMS techniques; for glasses, concentrations were determined by SIMS, FTIR, electron microprobe totals, and confocal Raman spectroscopy. Values for Dgarnet/melt span a wide range, and are apparently controlled by the concentration of minor elements in garnet, notably TiO2. Finally, values for Dpyx/melt depend strongly on the concentration of Al in pyroxene, and in particular on the abundance of tetrahedral Al in pyroxene. Combining experimental constraints on Dmineral/melt with the modal proportions and compositions of minerals near the peridotite solidus, it is possible to estimate the H2O concentration of incipient partial melts. For mantle with 100 ppm H2O, such partial melts have 1.2 wt.% H2O at 3 GPa and 2.3 wt.% at 7 GPa. Such modest concentrations of H2O provide only small stabilization of melt relative to a dry peridotite system, thereby supporting the inference that dehydration partial melting of normal mantle is feasible only near where the dry peridotite solidus is approached, meaning immediately below the locus of dry melting beneath ridges and mantle plumes.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 3619 Magma genesis and partial melting (1037);
- 3630 Experimental mineralogy and petrology