Carbon-Free Melting of Fertile Garnet Peridotite in Water-Undersaturated Systems

The presence of small amounts of volatile species has been shown to have a profound effect on the generation of magmas in the Earth's upper mantle. The addition of volatiles, most notably H2O and CO2, depresses the solidus temperature of mantle peridotite and thus causes the initiation of small degrees of melting at much greater depths than would be expected for volatile-free peridotites (Asimow and Langmuir, 2003). This process is complicated by the fact that there are two separate major volatile species, each of which can have distinct effects on melting properties. To fully understand the changes in melting processes driven by volatiles, it is therefore necessary to conduct experiments across a wide range of volatile concentrations and under the pressure, temperature, and bulk compositions of interest in melting. Previous experiments have yet to reach this goal for the following reasons: 1.) most have been conducted at pressures of 2 GPa or less, which is too low for natural volatile-driven melting, 2.) water contents have been either much too high or too low to be representative, particularly of melting beneath mid-ocean ridges, and 3.) it has proven difficult to produce experiments in carbon-free systems. Using natural KLB-1 peridotite powder as a starting material, we have produced a fertile garnet peridotite by recrystallizing it sub-solidus at 3 GPa. We then used this product as starting material for a series of modified iterative sandwich experiments as described in Hirschmann and Dasgupta (2007). We have adapted the double-capsule design of Kagi et al. (2005) as a method of controlling carbon contamination of piston- cylinder experiments. These steps have allowed us to produce experiments containing a melt with approximately 1 weight percent water in equilibrium with the minerals of a fertile garnet peridotite under carbon-free conditions, and we are approaching a melt composition that is representative of the melt that would be generated by hydrous melting of this peridotite in the Earth's upper mantle. 1. Asimow, P. D. and C. H. Langmuir (2003). "The importance of water to oceanic mantle melting regimes." Nature 421(6925): 815-820.

2. Hirschmann, M. M. and R. Dasgupta (2007). "A modified iterative sandwich method for determination of near-solidus partial melt compositions. I. Theoretical considerations." Contributions to Mineralogy and Petrology 154: 635-645.

3. Kagi, R., O. Muntener, et al. (2005). "Piston-cylinder experiments on H2O undersaturated Fe-bearing systems: An experimental setup approaching f(O2) conditions of natural calc-alkaline magmas." American Mineralogist 90(4): 708-717.