Experimental determination of the H2O-undersaturated peridotite solidus

Abstract:

Knowledge of the H₂O-undersaturated lherzolite solidus places important constraints on the process of melt generation and mantle potential temperatures beneath oceanic spreading centers. The small concentration of H₂O (~50-200 µg/g) dissolved in the oceanic mantle is thought to exert a strong influence on the peridotite solidus, but this effect has not been directly determined. The utility of existing experimental data is limited by a lack of information on the concentration of H₂O dissolved in the peridotite and uncertainties involved with identifying small amounts of partial melt. We have developed an experimental approach for determining the peridotite solidus as a function of H₂O content that overcomes these difficulties. Our initial results demonstrate that the solidus temperature for spinel lherzolite containing 150 µg/g H₂O is higher than existing estimates for the anhydrous solidus. Our approach to determining the H₂O-undersaturated lherzolite solidus is as follows. First, a small proportion (~5 %) of San Carlos olivine spheres, ~300 μm in diameter, are added to a peridotite synthesized from high-purity oxides and carbonates. Melting experiments are then conducted in pre-conditioned Au₈₀Pd₂₀ capsules over a range of temperatures at a single pressure using a piston-cylinder device. Water diffuses rapidly in olivine resulting in thorough equilibration between the olivine spheres and the surrounding fine-grained peridotite, and allowing the spheres to be used as hygrometers. After the experiment, the concentration of H₂O dissolved in the olivine spheres is determined by secondary ion mass spectrometry.

Melting experiments, spaced 20°C apart, were performed from 1250 to 1430°C at 1.5 GPa. The starting material has the composition of the depleted MORB mantle of Workman and Hart (2005) containing 0.13 wt% Na₂O and 150 µg/g H₂O. The concentration of H₂O in the olivine spheres remains constant up to 1350°C, and then decreases systematically with increasing temperature. This indicates a solidus temperature of ~1360°C, which is ~25-50°C above the existing estimates for anhydrous solidus corrected for our starting composition. The H₂O-undersaturated solidus indicated by our experimental results suggests that potential temperatures for the oceanic mantle are higher than current estimates.