James A. Van Orman

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 (CO2, H2O, 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.

High Pre-Eruptive Water Contents Preserved in Lunar Melt Inclusions

Primitive magmatic melt inclusions from the Moon contain as much water as some terrestrial mid-ocean ridge magmas. 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.

Hydrogen Isotopes in Lunar Volcanic Glasses and Melt Inclusions Reveal a Carbonaceous Chondrite Heritage

Common Water The Moon has been traditionally considered bone-dry, but in recent years a number of studies have shown that during mantle melting, the lunar mantle had as much water as Earths upper mantle. Saal et al. (p. 1317, published online 9 May; see the cover) measured the isotopic composition of hydrogen dissolved in volcanic glass and olivine-hosted melt inclusions recovered from the Moon by the Apollo 15 and 17 missions. Lunar magmatic water was indistinguishable from the bulk water in carbonaceous chondrites and similar to terrestrial water, implying a common origin for the water contained in the interiors of Earth and the Moon. Hydrogen isotope ratios in lunar samples imply a common origin for Earth’s and the Moon’s water. 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.

Re-examination of the lunar magma ocean cumulate overturn hypothesis: melting or mixing is required

Abstract There is a long-standing hypothesis that the last fraction of the lunar magma ocean crystallized into a layer of dense, Ti-rich cumulate minerals at shallow depths (∼100 km) early in the moon’s history. Many questions remain about the stability of these high-Ti cumulates. It has been suggested that the cumulates subsequently sank deep into the moon because of gravitational instability, but high-Ti material is required at shallower depths by 3.5 Ga to create the high-Ti mare basalts and picritic glasses. The high-Ti material may have re-erupted from depth, or some or all of it may have remained at shallow depths throughout lunar history. Data on phase stabilities, bulk compositions, densities, and temperatures of melting and crystallizing in addition to results from numerical modeling suggest that the high-Ti cumulates would sink only under highly specific conditions. Five scenarios for sinking high-Ti cumulate materials are examined, and only two are found plausible. In particular, it is found that simple sinking of solidified high-Ti cumulates is unlikely because the temperature at which the cumulates solidify is low, and viscosity under these conditions is very high. It is, however, possible that high-Ti cumulates mixed with a substantial fraction of olivine would have viscosity low enough to allow them to sink as solids. Further, because clinopyroxene and ilmenite melt in a ratio of 2:1, remelted high-Ti cumulates would be negatively buoyant and sink as liquids, percolating downward through the underlying mantle and beginning to recrystallize ilmenite at 200 km depth, making a hybrid, heterogeneous mantle.

Diffusive fractionation of trace elements during production and transport of melt in Earth’s upper mantle

We have developed a numerical model to investigate the importance of diffusive chemical fractionation during production and transport of melt in Earth’s upper mantle. The model incorporates new experimental data on the diffusion rates of rare earth elements (REE) in high-Ca pyroxene [Van Orman et al., Contrib. Mineral. Petrol. 141 (2001) 687^703] and pyrope garnet [Van Orman et al., Contrib. Mineral. Petrol., in press], including the dependence of diffusivity on temperature, pressure and ionic radius. We find that diffusion exerts an important control on REE fractionation under conditions typical of melting beneath slow spreading centers, provided that grain radii are V0.5 mm or greater. When partitioning is diffusion-limited, REE are fractionated less efficiently than under equilibrium conditions, and this effect becomes more pronounced as the melting rate, grain size, and efficiency of melt segregation increase. Data for the light REE in abyssal peridotite clinopyroxene (cpx) grains from the slow spreading America^ Antarctic and Southwest Indian ridge systems are better explained by melting models that allow for diffusive exchange than by models that assume complete solid^melt equilibration. The data are best fit by models in which the initial cpx grain radii are V2^3 mm and melt extraction is very efficient (near fractional). Diffusion is likely to play a strong role in REE fractionation during intergranular melt transport in the upper mantle. Complete equilibrium between solid and melt requires very sluggish melt transport, with ascent rates on the order of a few centimeters per year. Complete disequilibrium, on the other hand, requires rapid transport (s 30 m yr 31 ) through a permeable network with channel spacing considerably larger than the grain size. Diffusive fractionation during percolation through depleted spinel peridotite can lead to a wide variety of REE patterns in the melt, depending on the porosity and channel spacing of the melt network. These include ultra-depleted patterns when the porosity is very small (6 0.005 for an intergranular network with 2 mm grain radii) and patterns of relative LREE enrichment at moderate porosity (0.02^0.03). fl 2002 Elsevier Science B.V. All rights reserved.

Uranium and thorium diffusion in diopside

This paper presents new experimental data on the tracer diffusion rates of U and Th in diopside at 1 atm and 1150‐ 1300oC. Diffusion couples were prepared by depositing a thin layer of U‐Th oxide onto the polished surface of a natural diopside single crystal, and diffusion profiles were measured by ion microprobe depth profiling. For diffusion parallel to [001] the following Arrhenius relations were obtained: log 10 DU D . 5:75 0:98/ .418 28 kJ=mol/=2:303RT log10 DTh D . 7:77 0:92/ .356 26 kJ=mol/=2:303RT. The diffusion data are used to assess the extent to which equilibrium is obtained during near fractional melting of a high-Ca pyroxene bearing mantle peridotite. We find that the diffusion rates for both elements are slow and that disequilibrium between solid and melt will occur under certain melting conditions. For near-fractional adiabatic decompression melting at ascent rates > 3c m=yr, high-Ca pyroxene will exhibit disequilibrium effects. High-Ca pyroxene will become zoned in U and Th and the melts extracted will be depleted in these incompatible elements relative to melts produced by equilibrium fractional melting. U and Th diffusivities in high-Ca pyroxene are similar, and diffusive fractionation of these elements will be limited. Numerical solutions to a dynamic melting model with diffusion-controlled chemical equilibration indicate that the activity ratio [ 230 Th= 238 U] in a partial melt of spinel peridotite will be slightly less than 1 for a broad range of melting parameters. This result reinforces the already widely accepted conclusion that melting of spinel peridotite cannot account for 230 Th excesses in mid-ocean ridge and ocean island basalts, and that garnet must therefore be present over part of the melting column.

Distribution of shortening between the Indian and Australian plates in the central Indian Ocean

We analyze a single-channel seismic (SCS) reflection profile that completely crosses the zone of deformed oceanic lithosphere in the central Indian Ocean at 78.8” E. By summing the apparent shortening on all seismically resolvable faults (throws > _ 10 m), we find that 11.2 + 2 km of shortening has occurred at this longitude during the past 7.5 m.y. This estimate, together with the 27 f 5 km of shortening previously estimated from a multichannel seismic (MCS) profile farther east at 81.5” E, are consistent with the west-to-east increase in shortening predicted by Euler poles which treat the Indian and Australian plates as separate tectonic units. Our result therefore provides direct evidence from the deformation itself that the compression of oceanic lithosphere in the central Indian Ocean, originally regarded as ‘intraplate’, is better described as constituting part of a broad boundary zone between distinct Indian and Australian plates. We also examine the size statistics of faults revealed in SCS and MCS profiles running nearly normal to the deformation trends in the longitude band 78.8” E-81.5” E. The N-S extent of the deformation does not change appreciably over these longitudes. We find that the average fault spacing remains constant at about 7 km, whereas the mean throw increases systematically from west to east ( ff 74 m to N 177 m). Basically the contribution of ‘small’ faults (those with throws of lo-50 m) decreases systematically across the deforming region (i.e., with increasing amount of shortening). This suggests that the deformation occurs by reactivation of a select fault population, that these faults continue to add displacement with time and that relatively few new faults are initiated. We also infer from the fault size statistics that the contribution to the deformation of faults below the resolution of the seismic methods (- 10 m for SCS and - 50 m for MCS) is likely to be quite limited.

Diffusive relaxation of carbon and nitrogen isotope heterogeneity in diamond: a new thermochronometer

The spatial distribution of carbon and nitrogen isotopes in diamonds provides information on mantle residence time. Diamonds with long residence at high temperature will gradually lose their initial zoning patterns due to diffusion. Using experimentally determined carbon self-diffusion coefficients and nitrogen diffusion coefficients derived from aggregation experiments, we have modeled the diffusive relaxation of zoning profiles with a spectrum of wavelengths. Carbon isotope heterogeneity will be preserved on wavelengths greater than ∼1m after 1 billion years’ residence at 1400 K, and on wavelengths greater than ∼200m after 1 million years’ residence at 2000 K. Nitrogen isotope zoning is relaxed much more slowly, with 0.1m zoning preserved over the age of the Earth at 1400 K and 1m zoning preserved after 1 million years at 2000 K. The large difference in diffusive relaxation times between carbon and nitrogen isotopes means that initially correlated carbon and nitrogen profiles will lose their correlation after sufficient diffusion of carbon has taken place. This could be used as a thermochronometer. Carbon isotope heterogeneity in diamonds associated with lower mantle mineral assemblages has significantly smaller amplitude than nitrogen isotope heterogeneity, consistent with diffusive relaxation at high temperatures in the lower mantle.

Effective radii of noble gas atoms in silicates from first-principles molecular simulation

Abstract An understanding of how noble gas atoms are dissolved in mantle minerals and melts is necessary to infer geological information from the constraints provided by noble gas geochemistry. For this purpose, first-principles molecular simulations are carried out on liquid and crystalline (stishovite) silica systems with dissolved noble gas atoms (He, Ne, Ar, Kr, and Xe). The first principles nature of the simulations, which do not involve empirical force field parameters, enables the determination of the effective radii and structural environments of the noble gas atoms. The noble gas atoms are shown to be highly compressible, so that their effective radii depend strongly on the molar volume of the system (which in turn depends on pressure). Due to the continuous nature of interatomic forces, the effective radii also depend on the extent to which the surrounding atoms can relax in response to the presence of the noble gas atom. In this regard, different definitions of effective radii are relevant in different situations: “equilibrium radii” that correspond to the optimal interatomic distances at the molar volume of the system, and “repulsive wall” radii that correspond to the interatomic distances where the interatomic potentials of mean force change from attractive to repulsive at that molar volume. The equilibrium radii determine the interatomic distances in a melt, and the repulsive wall radii determine the interatomic distances for interstitial sites in a crystal. Based on these effective radii, the structural environment surrounding the noble gas atoms at high pressure is shown to correspond to a close packing of O atoms around the central noble gas atom. Compression of the noble gas atoms is shown to correspond closely to the compression of the porosity within the silicate melt structure.

Isotope fractionation in silicate melts

Arising from G. Dominguez, G. Wilkins & M. H. Thiemens 473, 70–73 (2011)10.1038/nature09911