High-pressure elastic properties of dolomite melt supporting carbonate-induced melting in deep upper mantle.

Deeply subducted carbonates likely cause low-degree melting of the upper mantle and thus play an important role in the deep carbon cycle. However, direct seismic detection of carbonate-induced partial melts in the Earth's interior is hindered by our poor knowledge on the elastic properties of carbonate melts. Here we report the first experimentally determined sound velocity and density data on dolomite melt up to 5.9 GPa and 2046 K by in-situ ultrasonic and sink-float techniques, respectively, as well as first-principles molecular dynamics simulations of dolomite melt up to 16 GPa and 3000 K. Using our new elasticity data, the calculated VP/VS ratio of the deep upper mantle (∼180-330 km) with a small amount of carbonate-rich melt provides a natural explanation for the elevated VP/VS ratio of the upper mantle from global seismic observations, supporting the pervasive presence of a low-degree carbonate-rich partial melt (∼0.05%) that is consistent with the volatile-induced or redox-regulated initial melting in the upper mantle as argued by petrologic studies. This carbonate-rich partial melt region implies a global average carbon (C) concentration of 80-140 ppm. by weight in the deep upper mantle source region, consistent with the mantle carbon content determined from geochemical studies.

The interplay of structure and dynamics at grain boundaries.

Molecular simulations are carried out to address the structure and atomic diffusion at grain boundaries. We use an inherent structure approach, which maps each configuration in a molecular dynamics trajectory to the potential energy minimum ("inherent structure") it would reach by a steepest descent quench. Dynamics are then decomposed into a combination of displacements within an inherent structure and transitions between inherent structures. The inherent structure approach reveals a simple structural picture of the grain boundary that is normally obscured by the thermal motion. We apply our methodology to polycrystalline MgO. Grain boundary atoms are identified as atoms that are undercoordinated in the inherent structure, relative to those in the perfect crystal. Our method enables the calculation of grain boundary diffusion coefficients without arbitrary assumptions about which atoms or spatial regions belong to the grain boundary, and the results are shown to be consistent with estimates from experiments. The inherent structure approach also enables the elementary steps in the diffusion process to be elucidated. We show that the process in MgO grain boundaries primarily involves vacancy hops, but that there is also significant motion of other nearby atoms during such a hop.

Volatile content of lunar volcanic glasses and the presence of water in the Moon's interior.

The Moon is generally thought to have formed and evolved through a single or a series of catastrophic heating events, during which most of the highly volatile elements were lost. Hydrogen, being the lightest element, is believed to have been completely lost during this period. Here we make use of considerable advances in secondary ion mass spectrometry to obtain improved limits on the indigenous volatile (CO(2), H(2)O, F, S and Cl) contents of the most primitive basalts in the Moon-the lunar volcanic glasses. Although the pre-eruptive water content of the lunar volcanic glasses cannot be precisely constrained, numerical modelling of diffusive degassing of the very-low-Ti glasses provides a best estimate of 745 p.p.m. water, with a minimum of 260 p.p.m. at the 95 per cent confidence level. Our results indicate that, contrary to prevailing ideas, the bulk Moon might not be entirely depleted in highly volatile elements, including water. Thus, the presence of water must be considered in models constraining the Moon's formation and its thermal and chemical evolution.

Isotope fractionation by thermal diffusion in silicate melts.

Isotopes fractionate in thermal gradients, but there is little quantitative understanding of this effect in complex fluids. Here we present results of experiments and molecular dynamics simulations on silicate melts. We show that isotope fractionation arises from classical mechanical effects, and that a scaling relation based on Chapman-Enskog theory predicts the behavior seen in complex fluids without arbitrary fitting parameters. The scaling analysis reveals that network forming elements (Si and O) fractionate significantly less than network modifiers (e.g., Mg, Ca, Fe, Sr, Hf, and U).

Room-temperature compression and equation of state of body-centered cubic zirconium.

Zirconium (Zr) has properties conducive to nuclear applications and exhibits complex behavior at high pressure with respect to the effects of impurities, deviatoric stress, kinetics, and grain growth which makes it scientifically interesting. Here, we present experimental results on the 300 K equation of state of ultra-high purity Zr obtained using the diamond-anvil cell coupled with synchrotron-based x-ray diffraction and electrical resistance measurements. Based on quasi-hydrostatic room-temperature compression in helium to pressure P = 69.4(2) GPa, we constrain the bulk modulus and its pressure derivative of body-centered cubic (bcc) ß-Zr to be K = 224(2) GPa and K' = 2.6(1) at P = 37.0(1) GPa. A Monte Carlo approach was developed to accurately quantify the uncertainties in K and K'. In the Monte Carlo simulations, both the unit-cell volume and pressure vary according to their experimental uncertainty. Our high-pressure studies do not indicate additional isostructural volume collapse in the bcc phase of Zr in the 56-58 GPa pressure range.

The role of diffusion-driven pure climb creep on the rheology of bridgmanite under lower mantle conditions.

The viscosity of Earth's lower mantle is poorly constrained due to the lack of knowledge on some fundamental variables that affect the deformation behaviour of its main mineral phases. This study focuses on bridgmanite, the main lower mantle constituent, and assesses its rheology by developing an approach based on mineral physics. Following and revising the recent advances in this field, pure climb creep controlled by diffusion is identified as the key mechanism driving deformation in bridgmanite. The strain rates of this phase under lower mantle pressures, temperatures and stresses are thus calculated by constraining diffusion and implementing a creep theoretical model. The viscosity of MgSiO3 bridgmanite resulting from pure climb creep is consequently evaluated and compared with the viscosity profiles available from the literature. We show that the inferred variability of viscosity in these profiles can be fully accounted for with the chosen variables of our calculation, e.g., diffusion coefficients, vacancy concentrations and applied stresses. A refinement of these variables is advocated in order to further constrain viscosity and match the observables.

Hydrogen isotopes in lunar volcanic glasses and melt inclusions reveal a carbonaceous chondrite heritage.

Water is perhaps the most important molecule in the solar system, and determining its origin and distribution in planetary interiors has important implications for understanding the evolution of planetary bodies. Here we report in situ measurements of the isotopic composition of hydrogen dissolved in primitive volcanic glass and olivine-hosted melt inclusions recovered from the Moon by the Apollo 15 and 17 missions. After consideration of cosmic-ray spallation and degassing processes, our results demonstrate that lunar magmatic water has an isotopic composition that is indistinguishable from that of the bulk water in carbonaceous chondrites and similar to that of terrestrial water, implying a common origin for the water contained in the interiors of Earth and the Moon.

Transport coefficients in silicate melts from structural data via a structure-thermodynamics-dynamics relationship.

The viscosity and diffusivities of silicate melts under high-pressure, high-temperature conditions are difficult to obtain experimentally. Estimation and extrapolation of transport coefficients are further complicated by their extreme sensitivity to melt composition. Our molecular-dynamics simulations show that, over a broad range of melt composition, temperature, and pressure, the diffusivities correlate with the excess entropy; approximations to the latter can be obtained from the knowledge of the radial distribution function. Using this structure-thermodynamics-dynamics relationship, we show that transport properties of silicate melts can be estimated quantitatively using static structure factor data from experiments.

High pre-eruptive water contents preserved in lunar melt inclusions.

The Moon has long been thought to be highly depleted in volatiles such as water, and indeed published direct measurements of water in lunar volcanic glasses have never exceeded 50 parts per million (ppm). Here, we report in situ measurements of water in lunar melt inclusions; these samples of primitive lunar magma, by virtue of being trapped within olivine crystals before volcanic eruption, did not experience posteruptive degassing. The lunar melt inclusions contain 615 to 1410 ppm water and high correlated amounts of fluorine (50 to 78 ppm), sulfur (612 to 877 ppm), and chlorine (1.5 to 3.0 ppm). These volatile contents are very similar to primitive terrestrial mid-ocean ridge basalts and indicate that some parts of the lunar interior contain as much water as Earth's upper mantle.