Constraints on the emplacement of Martian nakhlite igneous rocks and their source volcano from advanced micro-petrofabric analysis

1S.Griffin et al. (>10)
Journal of Geophysical Research (Planets)(in Press) Open Access Link to Article []
1School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK
Published by arrangement with John Wiley & Sons

The Martian nakhlite meteorites, which represent multiple events that belong to a single magma source region represent a key opportunity to study the evolution of Martian petrogenesis. Here 16 of the 26 identified nakhlite specimens are studied using coupled electron backscatter diffraction (EBSD) and emplacement end-member calculations. EBSD was used to determine shape preferred orientation (SPO) of contained augite (high Ca-clinopyroxene) phenocrysts by considering their crystallographic preferred orientation (CPO). Parameters derived from EBSD, and energy dispersive X-ray spectroscopy (EDS) data were used in basic emplacement models to assess their dominant mechanism against three end-member scenarios: thermal diffusion, crystal settling, and crystal convection. Results from CPO analyses indicate low intensity weak-moderate CPO. In all samples, a consistent foliation within the <001> axes of augite are observed typically coupled with a weaker lineation CPO in one of the other crystallographic axes. These CPO results agree best with crystal settling being the dominant emplacement mechanism for the nakhlites. Modelled crystal settling results identify two distinguishable groups outside of the model’s resolution indicating the presence of secondary emplacement mechanisms. Comparison of the two identified groups against petrofabric, geochemical, and age parameters indicate random variability between individual meteorites. Therefore, coupled petrofabric and emplacement modelling results identify an overarching characteristic of a dominant crystal settling emplacement mechanism for the nakhlite source volcano despite exhibiting random variation with each discharge through time.

Oxidized and reduced sulfur observed by the Sample Analysis at Mars (SAM) instrument suite on the Curiosity rover within the Glen Torridon region at Gale crater, Mars

1G.M.Wong et al. (>10)
Journal of Geophysical Research (Planets)(In Press) Open Access Link to Article []
1Department of Geosciences, The Pennsylvania State University, University Park, PA, USA
Published by arrangement with John Wiley & Sons

The Mars Science Laboratory (MSL) Curiosity rover has been assessing the habitability and geologic history of Gale crater, Mars since landing in 2012. One of the primary objectives of the mission was to investigate a clay-bearing unit identified using orbital spectral data, designated the Glen Torridon (GT) region. This region was of particular interest because of its elevated abundance of clay minerals that may have preserved geochemical evidence of ancient habitable environments. The Curiosity rover explored the GT region for ∼750 sols and analyzed eight drilled samples with the Sample Analysis at Mars (SAM) instrument suite using evolved gas analysis-mass spectrometry. Evolved sulfur-bearing gases provided insight about the composition of sulfur-containing compounds in Martian samples. Evolved gases were analyzed by three methods to understand the oxidation state of sulfur in the samples: (1) SO2 evolution temperature, (2) quadratic discriminant analysis comparing SAM data to SAM-like laboratory investigations, and (3) sulfur isotope values from evolved 34SO2/32SO2. The results of these three methods were consistent with the majority of sulfur in the GT region being in an oxidized state, but two of the eight samples analyzed by SAM were consistent with the presence of small amounts of reduced sulfur. The oxidized and reduced sulfur could have a variety of sources and represents a nonequilibrium assemblage that could have supported putative ancient chemolithotrophic metabolisms.

The noble gas inventory in metal samples and troilite inclusions from IIIAB iron meteorites: Reinvestigating the live 129I-129Xe dating method

1,2Thomas Smith,1Ingo Leya
Meteoritics & Planetary Science (in Press) Open Access Link to Article []
1Physics Institute, University of Bern, Sidlerstrasse 5, Bern, CH-3012 Switzerland
2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, 19 Beitucheng Western Road, Chaoyang District, Box 9825, Beijing, 100029 China
Published by arrangement with John Wiley & Sons

The live 129I-129Xe dating technique, which internally corrects for shielding, is particularly well suited for large iron meteorites, for which shielding corrections might be difficult to obtain. In addition, the half-life of 129I of 16 Ma would allow the study of the important question of the constancy—or variability—of the galactic cosmic rays over a time scale not covered by other cosmogenic nuclides. Here, we present the results of a noble gas study of metal samples and adjacent troilite inclusions from the four IIIAB iron meteorites Cape York (including Agpalilik), Casas Grandes, Trenton, and Grant. The major result is that we can directly determine 129Xenc concentrations caused by (live) 129I decay for Cape York, Casas Grandes, Trenton, and Grant. The 129Xenc concentrations can be used, if combined to 129I activity concentrations (not measured by us), to calculate cosmic ray exposure (CRE) ages using the (live) 129I-129Xe chronometer. For the light noble gases measured in metal and troilite samples, the new data confirm the earlier estimates of a production rate ratio 21Necos(troilite)/21Necos(metal) in the range 3.08–3.53. Surprisingly, 38Arcos in troilite from Agpalilik and Casas Grandes is only, respectively, ~36% and ~44% relative to that in the respective metal phases. Considering that 38Arcos is only produced from iron but not from sulfur, the 38Arcos concentration measured in troilite is expected to be ~64% relative to that in adjacent metal, that is, some 38Arcos is missing. Considering krypton in troilite, only 80Kr, 82Kr, and 83Kr are higher than the blank, likely indicating a spallogenic contribution. On average, ~4.2% of measured 80Kr, ~2.5% of 82Kr, and ~11.6% of 83Kr are spallogenic.

Experimental crystallization of the lunar magma ocean, initial selenotherm and density stratification, and implications for crust formation, overturn and the bulk silicate Moon composition

1Max W. Schmidt,1Giuliano Kraettli
Journal of Geophysical Research (Planets) (in Press) Link to Article []
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.

X-ray amorphous sulfur-bearing phases in sedimentary rocks of Gale crater, Mars

1R.J.Smith et al. (>10)
Journal of Geopyhsical Research (Planets) (in Press) Link to Article []
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.

Average VNIR reflectance: A rapid, sample-free method to estimate glass content and crystallinity of fresh basaltic lava

Icarus (in Press) Link to Article []
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 R500–1000) 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 R500–1000 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.

Asteroid 2008 TC3, not a polymict ureilitic but a polymict C1 chondrite parent body? Survey of 249 Almahata Sitta fragments

1Addi Bischoff et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article []
1Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Published by arrangement with John Wiley & Sons

On October 7, 2008, the asteroid 2008 TC3 exploded as it entered the Earth’s atmosphere, producing significant dust (in the atmosphere) and delivering thousands of stones in a strewn field in Sudan, collectively known as the Almahata Sitta (AhS) stones. About 600 fragments were officially recovered in 2008 and 2009. Further rocks were collected since the fall event by local people. From these stones, 249 were classified at the Institut für Planetologie in Münster (MS) known as MS-xxx or MS-MU-xxx AhS subsamples. Most of these rocks are ureilitic in origin (168; 67%): 87 coarse-grained ureilites, 60 fine-grained ureilites, 15 ureilites with variable texture/mineralogy, four trachyandesites, and two polymict breccias. We identified 81 non-ureilitic fragments, corresponding to 33% of the recovered samples studied in Münster. These included chondrites, namely 65 enstatite chondrites (43 EL; 22 EH), 11 ordinary chondrites (OC), one carbonaceous chondrite, and one unique R-like chondrite. Furthermore, three samples represent a unique type of enstatite achondrite. Since all AhS stones must be regarded as individual specimens independent from each other, the number of fresh ureilite and enstatite chondrite falls in our meteorite collections has been increased by several hundred percent. Overall, the samples weigh between <1 and 250 g and have a mean mass of ~15 g. If we consider—almost 15 years after the fall—the mass calculations, observations during and after the asteroid entered the atmosphere, the mineralogy of the C1 stones AhS 91A and AhS 671, and the experimental work on fitting the asteroid spectrum (e.g., Goodrich et al., 2019; Jenniskens et al., 2010; Shaddad et al., 2010), the main portion of the meteoroid was likely made of the fine-grained (carbonaceous) dust and was mostly lost in the atmosphere. In particular, the fact that C1 materials were found has important implications for interpreting asteroid 2008 TC3’s early spectroscopic results. Goodrich et al. (2019) correctly suggested that if scientists had not recovered the “water-free” samples (e.g., ureilites, enstatites, and OC) from the AhS strewn field, 2008 TC3 would have been assumed to be a carbonaceous chondrite meteoroid. Considering that the dominating mass of the exploding meteoroid consisted of carbonaceous materials, asteroid 2008 TC3 cannot be classified as a polymict ureilite; consequently, we state that the asteroid was a polymict carbonaceous chondrite breccia, specifically a polymict C1 object that may have formed by late accretion at least 50–100 Ma after calcium–aluminum-rich inclusions.

The effects of highly reduced magmatism revealed through aubrites

1,2,3Zoë E. Wilbur et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article []
1Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, 85721 USA
2University of Nevada, Las Vegas, Las Vegas, Nevada, 89154 USA
3Jacobs, NASA Johnson Space Center, Mail Code XI3, Houston, Texas, 77058 USA
Published by arrangement with John Wiley & Sons

Enstatite-rich meteorites, including the aubrites, formed under conditions of very low oxygen fugacity (ƒO2: iron-wüstite buffer −2 to −6) and thus offer the ability to study reduced magmatism present on multiple bodies in our solar system. Elemental partitioning among metals, sulfides, and silicates is poorly constrained at low ƒO2; however, studies of enstatite-rich meteorites may yield empirical evidence of the effects of low ƒO2 on elemental behavior. This work presents comprehensive petrologic and oxygen isotopic studies of 14 aubrites, including four meteorites that have not been previously investigated in detail. The aubrites exhibit a variety of textures and mineralogy, and their elemental zoning patterns point to slow cooling histories for all 14 samples. Oxygen isotope analyses suggest that the aubrite parent bodies may be more heterogeneous than originally reported or may have experienced incomplete magmatic differentiation. Contrary to the other classified aubrites and based on textural and mineralogical observations, we suggest that the Northwest Africa 8396 meteorite shows an affinity for an enstatite chondrite parentage. By measuring major elemental compositions of silicates, sulfides, and metals, we calculate new metal–silicate, sulfide–silicate, and sulfide–metal partition coefficients for aubrites that are applicable to igneous systems at low ƒO2. The geochemical behavior of elements in aubrites, as determined using partition coefficients, is similar to the geochemical behavior of elements determined experimentally for magmatic systems on Mercury. Enstatite-rich meteorites, including aubrites, represent valuable natural petrologic analogues to Mercury and their study could further our understanding of reduced magmatism in our solar system.

An experimental study of the alteration of basalt on the surface of Venus

Icarus (in Press) Link to Article []
1Department of Earth and Planetary Sciences, University of Tennessee at Knoxville, 1621 Cumberland Avenue, 602 Strong Hall, Knoxville, TN 37996, United States of America
2Lunar and Planetary Institute, 3600 Bay Area Blvd, Houston, TX 77058, United States of America
3NASA Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, United States of America
4Rensselaer Polytechnic Institute, Troy, NY, United States of America
5Dept. Earth, Environmental, & Planetary Sciences, Brown University, Providence, RI 02912, United States of America
Copyright Elsevier

Characterizing the surface of Venus has been complicated by its thick atmosphere and caustic surface conditions (~470 °C, 90 bars). Several approaches, including the collection of spectral data, thermodynamic modelling, lander missions, and surface weathering laboratory experiments have progressed our view of what lies at the surface. However, surface-atmosphere interactions remain somewhat unconstrained and interpretations of the spectral data rely on an understanding of the surface-atmosphere alteration. We used a cold-seal pressure vessel apparatus pressurized with pure CO2 gas and both synthetic and natural glassy basalts specimens to simulate chemical weathering on the surface of Venus for a duration of two weeks. The extent of alteration was described from the surface of samples to depth using Rutherford Backscatter Spectroscopy (RBS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) techniques. We describe the alteration zones of reacted basalt specimens and report an enrichment of divalent cation species at the near surfaces of basalts; Ca2+ was enriched by ~5 wt% and Fe2+ was enriched by ~1–2 wt%. The enrichment at the near surface favors the production of iron (Fe) oxide(s) and carbonates on the surfaces, which form discontinuous coatings on all reacted samples in two weeks duration. Our results aid in the interpretation of radar emissivity data by constraining which alteration products should be present on the surface and suggesting timeframes necessary for their detection. Assuming that radar emissivity is able to discern weathered basalt, especially those dominated by carbonates and/or semiconducting minerals Fe oxide(s), our results suggest that basalts at Idunn Mons in Imdr Regio previously thought to be anywhere from 2.5 million to a few years old (Smrekar et al., 2010; D’Incecco et al., 2017; Filiberto et al., 2020; Cutler et al., 2020) could instead be as young as 65,000 years to 110,000 years depending on basalt type.

Presence of clay minerals can obscure spectral evidence of Mg sulfates: implications for orbital observations of Mars

1Rachel Y.Sheppard,2Ralph E.Milliken,2Kevin M.Robertson
Icarus (in Press) Link to Article []
1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America
2Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, United States of America
Copyright Elsevier

The martian crust is often viewed through the lens of its dominant secondary minerals, Noachian phyllosilicates and Hesperian sulfates, based on orbital spectral observations. However, the effects of surface exposure on the spectra of these hydrous minerals are not fully understood. We use an environmental chamber to measure changes in near-infrared (NIR) spectral absorptions related to H2O in smectite (montmorillonite) and Mg-sulfate under different temperature, pressure, and relative humidity conditions with relevance to the surface of Mars. Observed spectral differences are attributed to changes in water content (hydration state), mineral phase, and degree of crystallinity. It is observed that even minor changes in hydration state and phase (for Mg sulfate) cause perceptible changes in NIR H2O absorption features when measured in a controlled laboratory setting under dry Mars-like conditions. Based on these results and the known ability of smectite to rehydrate under increased RH, smectites exposed at the surface of Mars are expected to exchange water with the martian atmosphere under specific conditions, making them active participants in the present-day hydrological cycle of Mars, and in theory these hydration-dehydration processes should be detectable using NIR reflectance spectroscopy. However, some of the spectral changes associated with these hydration changes are subtle and may not be detectable with orbital or landed VNIR spectrometers. Furthermore, we find that the presence of clay minerals can spectrally mask the presence of Mg sulfates under a range of hydration states if the clay minerals are above ∼10 wt% abundance. Random noise was added to the laboratory spectral data to simulate orbital-quality reflectance data, and it is observed that expected changes related to hydration state and crystallinity are likely difficult to detect in current orbital VNIR data such as CRISM and OMEGA. This highlights the importance of future in situ NIR reflectance observations to accurately determine the extent to which hydrous minerals exposed as the surface cycle water with the martian atmosphere under present-day environmental conditions and to properly assess the role of hydrous minerals in the martian water budget.