Shock-recovered maskelynite indicates low-pressure ejection of shergottites from Mars

Diaplectic feldspathic glass, commonly known as maskelynite, is a widely used impact indicator, notably for shergottites, whose shock conditions are keys to their geochemistry and launch mechanism. However, classic reverberating shock recovery experiments show maskelynitization at higher shock pressures (>30 gigapascals) than the stability field of the high-pressure minerals found in many shergottites (15 to 25 gigapascals). Most likely, differences between experimental loading paths and those appropriate for martian impacts have created this ambiguity in shergottite shock histories. Shock reverberation yields lower temperature and deviatoric stress than single-shock planetary impacts at equivalent pressure. We report the Hugoniot equation of state of a martian analog basalt and single-shock recovery experiments, indicating partial-to-complete maskelynitization at 17 to 22 gigapascals, consistent with the high-pressure minerals in maskelynitized shergottites. This pressure explains the presence of intact magmatic accessory minerals, used for geochronology in shergottites, and offers a new pressure-time profile for modeling shergottite launch, likely requiring greater origin depth.

The Critical Point and the Supercritical State of Alkali Feldspars: Implications for the Behavior of the Crust During Impacts

The position of the vapor-liquid dome and of the critical point determine the evolution of the outermost parts of the protolunar disk during cooling and condensation after the Giant Impact. The parts of the disk in supercritical or liquid state evolve as a single thermodynamic phase; when the thermal trajectory of the disk reaches the liquid-vapor dome, gas and melt separate leading to heterogeneous convection and phase separation due to friction. Different layers of the proto-Earth behaved differently during the Giant Impact depending on their constituent materials and initial thermodynamic conditions. Here we use first-principles molecular dynamics to determine the position of the critical point for NaAlSi3O8 and KAlSi3O8 feldspars, major minerals of the Earth and Moon crusts. The variations of the pressure calculated at various volumes along isotherms yield the position of the critical points: 0.5-0.8 g cm-3 and 5500-6000 K range for the Na-feldspar, 0.5-0.9 g cm-3 and 5000-5500 K range for the K-feldspar. The simulations suggest that the vaporization is incongruent, with a degassing of O2 starting at 4000 K and gas component made mostly of free Na and K cations, O2, SiO and SiO2 species for densities below 1.5 g cm-3. The Hugoniot equations of state imply that low-velocity impactors (<8.3 km s-1) would at most melt a cold feldspathic crust, whereas large impacts in molten crust would see temperatures raise up to 30000 K.

Petrologic evidence for low-temperature, possibly flood evaporitic origin of carbonates in the ALH84001 meteorite

High-temperature models for origin of the carbonates in Martian meteorite ALH84001 are implausible. The impact metasomatism model, invoking reaction between CO2 rich fluid and the host orthopyroxenite, requires conversion of olivine into orthopyroxene, yet olivine in ALH84001 shows no depletion in carbonate-rich areas; or else conversion of orthopyroxene into silica, which should have yielded a higher silica/carbonate ratio. The impact melt model implies that the fracture-linked carbonates, as products of melt injection, should appear as continuous planar veins, but in many areas they do not. Both vapor deposition and impact melting seem inconsistent with the zoned poikilotopic texture of many large carbonates. The popular hydrothermal model is inconsistent with the virtual absence of secondary hydrated silicates in ALH84001. Prior brecciation should have facilitated alteration. Hydrothermal fluids would be warm, and rate of hydration of mafic silicates obeys an Arrhenius law, at least up to approximately 100 degrees C. Most important, hydrothermal episodes tend to last for many years. Many areas of the ancient Martian crust show evidence for massive flooding. I propose that the carbonates formed as evaporite deposits from floodwaters that percolated through the fractures of ALH84001, but only briefly, as evaporation and groundwater flow caused the water table to quickly recede beneath the level of this rock during the later stages of the flood episode. The setting might have been a layer of megaregolith beneath a surface catchment of pooled floodwater, analogous to a playa lake. Carbonate precipitation would occur in response to evaporative concentration of the water. To explain the scarcity of sulfates in ALH84001, the water table must be assumed to recede quickly relative to the rate of evaporation. During the period when ALH84001 was above the water table, evaporation would have slowed, as the evaporation front passed beneath the surface of the debris layer, and possibly earlier, if the shrinking pool of surface water developed a porous sulfate crust. Alternatively, ALH84001 may have developed as a Martian form of calcrete, i.e., the evaporating flood(s) may have been entirely below ground as it (they) passed slowly through ALH84001. The greatest advantage of the flood evaporite model is that it exposes ALH84001 to carbonate precipitation without prolonged exposure to aqueous alteration. The model also seems consistent with the heavy and extremely heterogeneous oxygen isotopic compositions of the carbonates. However, this hypothesis seems no more than marginally consistent with the suggestion of McKay et al. [1996] that the carbonates are biogenic.

B1-B2 Transition in Calcium Oxide from Shock-Wave and Diamond-Cell Experiments

Volume and structural data obtained by shock-wave and diamond-cell techniques demonstrate that calcium oxide transforms from the B1 (sodium chloride type) to the B2 (cesium chloride type) structure at 60 to 70 gigapascals (0.6 to 0.7 megabar) with a volume decrease of 11 percent. The agreement between the shockwave and diamond-cell results independently confirms the ruby-fluorescence pressure scale to about 65 gigapascals. The shock-wave data agree closely with ultrasonic measurements on the B1 phase and also agree satisfactorily with equations of state derived from ab initio calculations. The discovery of this B1-B2 transition is significant in that it allows considerable enrichment of calcium components in the earth's lower mantle, which is consistent with inhomogeneous accretion theories.