¹Max W. Schmidt,¹Giuliano Kraettli
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2022JE007187]
¹ETH, Zuerich, Switzerland
Published by arrangement with John Wiley & Sons

Eleven isobaric experimental series simulate the fractional crystallization of a 1150 km deep lunar magma ocean. Crystallization begins at 1850 ^oC with olivine (to 32 per cent solidified, pcs), followed at 1600 ^oC by olivine+opx±Cr-spinel (to 62 pcs), at 1210 ^oC cpx+plagioclase±olivine±Ti-spinel (to 97 pcs) and at 1060 ^oC quartz+cpx+plagioclase+Ti-spinel, leaving 1.8 wt% residual magma that crystallizes minor K-feldspar and apatite in addition. Melt compositions remain near 45 wt% SiO₂, while FeO increases from 11 to 26 wt%, TiO₂ peaks at 4 wt% at Ti-spinel saturation.

The available experimental liquid lines of descent yields an overall fractional crystallization sequence of olivine→opx→cpx+plagioclase→quartz→FeTi-oxide. Plagioclase appears concomitantly with cpx, a result of the low magma ocean floor pressures (≤ 1 GPa) after 66-76 % of olivine+opx-fractionation. A few wt% of FeTi-oxides form mostly once the quartz+plagioclase+cpx-cotectic is reached, cumulates densities remain ≤3740 kg/m³. Scaled to a full magma ocean, plagioclase appears at 210-120 km depth, mainly as a function of bulk Al₂O₃. As buoyancy driven plagioclase-cpx separation is likely limited, these depths may correspond to the primordial lunar crustal thickness. Allowing for complete plagioclase flotation to the quartz+plagioclase+cpx+FeTi-oxide±olivine cotectic yields 95-70 km primordial crust of anorthosite and quartz-gabbro, far in excess of the 35-50 km observed. This supports an overturn of primordial layers, re-melting of dense gabbroic cumulates in the harzburgitic cumulate mantle leading to further mixing and differentiation. We posit that such complex density induced convection led to a lunar marble cake mantle with primitive and fairly evolved reprocessed cumulates next to each other.

¹R.J.Smith et al. (>10)
Journal of Geopyhsical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2021JE007128]
¹Department of Geosciences, SUNY Stony Brook, Stony Brook, NY, 11794 USA
Published by arrangement with John Wiley & Sons

The Curiosity rover in Gale crater is investigating a mineral transition observed from orbit – an older “clay unit” to a younger “sulfate unit” – hypothesized to reflect the aridification of Mars’ climate. Below this transition, the rover detected crystalline Ca-sulfates with minor Fe-sulfates but also found that some fraction of a rock’s bulk SO3 is often in the poorly constrained X-ray amorphous component. Here, we characterize the abundances and compositions of the X-ray amorphous sulfur-bearing phases in 19 drilled samples using a mass balance approach, and in a subset of 5 samples using evolved SO2 gas measured by the SAM instrument. We find that ∼20-90 wt% of a sample’s bulk SO3 is in the X-ray amorphous state and that X-ray amorphous sulfur-bearing phase compositions are consistent with mixtures of Mg-S, Fe-S, and possibly Ca-S phases, likely sulfates or sulfites. These phases reside in the bedrock, perhaps as cementing agents deposited with detrital sediments or during early diagenesis, and in diagenetic alteration halos deposited after lithification during late diagenesis. The likely presence of highly soluble Mg-sulfates in the rocks suggests negligible fluid flow through the bedrock post-Mg-sulfate deposition. The X-ray amorphous sulfur-bearing phases probably became amorphous through dehydration in the current Martian atmosphere or inside the CheMin instrument. X-ray amorphous sulfur-bearing materials likely contribute to orbital spectral detections of sulfates, and so our results help form multiple hypotheses to be tested in the sulfate unit and are important for understanding the evolution of the Martian surface environment at Gale crater.

¹E.Rader,¹S.Ackiss,²A.Sehlke,³J.Bishop,⁴B.Orrill,¹K.Odegaard,¹M.Meier,^1,5A.Doloughan
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115084]
¹University of Idaho, Department of Geological Sciences, Moscow, ID 83844, USA
²NASA Ames Research Center/ Bay Area Environmental Research Institute, Moffett Field, CA 94035, USA
³SETI Institute, Moutain View, CA, USA
⁴Arizona State University, Tempe, AZ, USA
⁵Terracon, Olathe, KS, USA
Copyright Elsevier

The microcrystalline texture in basaltic lava, scoria, and spatter can vary widely from pure glass to holocrystalline due to complex cooling histories after eruption. How quickly a molten rock cools is a function of the environmental surroundings, including water, ice, sustained heat source, and atmospheric conditions. Thus, petrologic texture serves as an indicator of cooling history captured in the rock record. As basalt is a common component of terrestrial bodies across the solar system, relating the abundance of crystalline components to spectral character would allow for a more thorough understanding of the cooling history and emplacement conditions on planetary surfaces. Visible/near-infrared (VNIR) reflectance spectroscopy has been used to examine the absorptions associated with volcanic glass, however, the non-linearity of absorption features in this spectral region requires complex spectral unmixing modeling to achieve modal percentages of minerals. Here we present evidence that average reflectance from 500 to 1000 nm (referred to as R_500–1000) of solid surface samples is indicative of the crystal texture and degree of glassiness of basaltic rocks. Several factors, such as sample surface roughness, lichen cover, coatings, weathering, and chemical composition can affect the R_500–1000 of a sample. However, our data indicate that these factors can be sufficiently controlled during sample selection to attribute relative glassiness values to basaltic surfaces. This quick and straightforward method requires no sample preparation or modeling and is demonstrated with training data from sixteen rocks from five basaltic flow fields with differing mineralogy, surface qualities, and geochemistry across Idaho and Oregon, USA. We further test our relationship with two published datasets of synthetic and natural basalts, as well as a subset of our own data collected with our methods to examine the sensitivities of the correlation. This method has the potential to broadly identify glassier basaltic lavas across planetary surfaces. This could be applied toward understanding lava eruption temperatures, cooling rates, magma petrogenesis, paleoclimate reconstruction, and astrobiology due to the involvement of water in quenching of lava.