Astrid H. Holzheid

Geochemical Evidence for Magmatic Water Within Mars from Pyroxenes in the Shergotty Meteorite

Observations of martian surface morphology have been used to argue that an ancient ocean once existed on Mars. It has been thought that significant quantities of such water could have been supplied to the martian surface through volcanic outgassing, but this suggestion is contradicted by the low magmatic water content that is generally inferred from chemical analyses of igneous martian meteorites. Here, however, we report the distributions of trace elements within pyroxenes of the Shergotty meteorite—a basalt body ejected 175 million years ago from Mars—as well as hydrous and anhydrous crystallization experiments that, together, imply that water contents of pre-eruptive magma on Mars could have been up to 1.8%. We found that in the Shergotty meteorite, the inner cores of pyroxene minerals (which formed at depth in the martian crust) are enriched in soluble trace elements when compared to the outer rims (which crystallized on or near to the martian surface). This implies that water was present in pyroxenes at depth but was largely lost as pyroxenes were carried to the surface during magma ascent. We conclude that ascending magmas possibly delivered significant quantities of water to the martian surface in recent times, reconciling geologic and petrologic constraints on the outgassing history of Mars.

Sulfur Saturation Limits in Silicate Melts and their Implications for Core Formation Scenarios for Terrestrial Planets

Abstract This study explores the controls of temperature, pressure, and silicate melt composition on S solubility in silicate liquids. The solubility of S in FeO-containing silicate melts in equilibrium with metal sulfide increases significantly with increasing temperature but decreases with increasing pressure. The silicate melt structure also exercises a control on S solubility. Increasing the degree of polymerization of the silicate melt structure lowers the S solubility in the silicate liquid. The new set of experimental data is used to expand the model of Mavrogenes and O’Neill (1999) for S solubility in silicate liquids by incorporating the influence of the silicate melt structure. The expected S solubility in the ascending magma is calculated using the expanded model. Because the negative pressure dependence of S solubility is more influential than the positive temperature dependence, decompression and adiabatic ascent of a formerly S-saturated silicate magma will lead to S undersaturation. A primitive magma that is S-saturated in its source region will, therefore, become S-undersaturated as it ascends to shallower depth. In order to precipitate magmatic sulfides, the magma must first cool and undergo fractional crystallization to reach S saturation. The S content in a metallic liquid that is in equilibrium with a magma ocean that contains ~200 ppm S (i.e., Earth’s bulk mantle S content) ranges from 5.5 to 12 wt% S. This range of S values encompasses the amount of S (9 to 12 wt%) that would be present in the outer core if S is the light element. Thus, the Earth’s proto-mantle could be in equilibrium (in terms of the preserved S abundance) with a core-forming metallic phase.

Textural equilibria of iron sulfide liquids in partly molten silicate aggregates and their relevance to core formation scenarios

The influence of the coexistence of silicate and iron sulfide melts on the wetting of mantle olivine by iron sulfide melt is investigated. The dihedral angle between iron alloy and olivine is unaffected by the presence of silicate melt at 1.5 GPa and 1370–1410°C and is much greater than the maximum allowable dihedral angle for melt connectivity at low melt fractions. In experimental charges where silicate and sulfide melt occupy the same melt pocket, the sulfide melt adheres to olivine grain faces rather than forming discrete droplets. The relative interfacial energy between iron sulfide and basaltic silicate melt is lower than that of iron sulfide with olivine, but the contrast is not sufficient for basaltic silicate melt to wet FeS-olivine grain boundaries. This result implies that very large silicate melt fractions are necessary to mobilize iron sulfide melts in partially molten silicate phase assemblages. Therefore segregation of Fe-rich metallic melts by porous flow at low melt fraction is not a likely process for metal separation. The experiments also investigated the influence of the presence of a thermal gradient on sulfide melt migration. Although the sulfide melt can not migrate through the olivine network by porous flow, evidence in the experiments indicates mobility of sulfide through the interconnected silicate melt network. The sulfide melt pools in contact with silicate melt at the hot, top part of the experimental charges grow over time at the expense of the silicate melt pools in the cooler part of the charge. This points to a diffusive transport mechanism for local sulfide redistribution. We review literature data on metal-olivine wetting angles and find that the metal composition has a dominant influence on the wetting behavior of a metallic liquid on an olivine matrix. Iron-rich metallic liquid compositions are characterized by large dihedral angles (>90°). The metal-olivine dihedral angle decreases with increasing amounts of dissolved light element (S, C, O). For light element contents in excess of ∼59 at %, the dihedral angle approaches 60°, and the wetting behavior of a metallic liquid is similar to that of a silicate liquid.

Iron sulfide stoichiometry as a monitor of sulfur fugacity in gas-mixing experiments

Abstract We explore a method to utilize the stoichiometry of iron sulfide to determine the sulfur fugacity in experiments containing CO-CO2-SO2 gas mixtures. The Fe-S phase diagram shows that the stoichiometry of iron sulfide melts is closely related to the sulfur fugacity ƒs2 at a given temperature and ambient pressure. We derive equations that relate the sulfur fugacity to the mole fraction of sulfur Xs in the iron sulfide from available literature data, and a solution model using the Redlich-Kister approximation for the excess Gibbs energy of mixing. We test the method by exposing iron sulfide to CO-CO2-SO2 gas mixtures and subsequently analyzing the “Fe-S monitor” for its stoichiometry. Most sulfur fugacities calculated from the equilibrium gas composition agree within 5-10% (1200 °C) and 1-4% (1400 °C) with those derived from the sulfur mole fraction in the monitor and literature data calibration, which is consistent with the spread observed in literature data. There were no suitable literature data for a full calibration at 1300 °C, so we combined the available literature and our data to find the sulfur fugacity as a function of mole fraction sulfur. Overall the Fe-S monitor technique is a convenient method to determine the sulfur fugacity in high-temperature experiments containing CO-CO2-SO2 gas mixtures as long as oxygen fugacities remain below that of the iron-wüstite or iron-magnetite buffer.