Configurational entropy of basaltic melts in Earth's mantle.

Although geophysical observations of mantle regions that suggest the presence of partial melt have often been interpreted in light of the properties of basaltic liquids erupted at the surface, the seismic and rheological consequences of partial melting in the upper mantle depend instead on the properties of interstitial basaltic melt at elevated pressure. In particular, basaltic melts and glasses display anomalous mechanical softening upon compression up to several GPa, suggesting that the relevant properties of melt are strongly pressure-dependent. A full understanding of such a softening requires study, under compression, of the atomic structure of primitive small-degree basaltic melts at their formation depth, which has proven to be difficult. Here we report multiNMR spectra for a simplified basaltic glass quenched at pressures up to 5 GPa (corresponding to depths down to ∼150 km). These data allow quantification of short-range structural parameters such as the populations of coordination numbers of Al and Si cations and the cation pairs bonded to oxygen atoms. In the model basaltic glass, the fraction of [5,6]Al is ∼40% at 5 GPa and decreases to ∼3% at 1 atm. The estimated fraction of nonbridging oxygens at 5 GPa is ∼84% of that at ambient pressure. Together with data on variable glass compositions at 1 atm, these results allow us to quantify how such structural changes increase the configurational entropy of melts with increasing density. We explore how configurational entropy can be used to explain the anomalous mechanical softening of basaltic melts and glasses.

Hydrogen Incorporation in Plagioclase.

We conducted experiments at high pressure (P) and temperature (T) to measure hydrogen solubility in plagioclase (Pl) with a range of compositions (An15 to An94). Experiments were run at 700-850 °C, 0.5 GPa, and f O 2 close to either the Ni-NiO (NNO) or iron-wüstite (IW) oxygen buffers. Experiments at 700 °C on An15 (containing 0.03 wt% FeO) reveal no dependence of H solubility on f O 2 between IW and NNO, but experiments at 800-850 °C on other compositions (with 0.3-0.5 wt% FeO) demonstrate that H solubility is enhanced by a factor of ~2 to 3 at IW compared to NNO, consistent with previous experiments by Yang (2012a) on An58. By analogy with synthetic hydrogen feldspar (HAlSi3O8), we infer that the predominant mechanism for H incorporation in Pl is through bonding to O atoms adjacent to M-site vacancies, and we propose likely O sites for H incorporation based on M-O bond lengths in anhydrous Pl structures. Increased uptake of structurally bound H at low f O 2 is explained by the formation of defect associates resulting from the reduction of Fe3+ in tetrahedral sites to Fe2+, allowing additional H to be incorporated in adjacent M-site vacancies. This mechanism counteracts the expected effect of water fugacity on H solubility. We also speculate on possible substitutions of H on tetrahedral vacancies, as well as coupled H-F substitution. Enhanced incorporation of H in Pl at low f O 2 may have implications for estimating the water content of the lunar magma ocean. However, mechanisms unrelated to low f O 2 are needed to explain high H contents in terrestrial Pl xenocrysts, such as those found in basalts from the Basin and Range.