^1,2Thomas Smith,¹Ingo Leya
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13827]
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.

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.

¹Addi Bischoff et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13821]
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.

^1,2,3Zoë E. Wilbur et al. (>10)
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13823]
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.

¹H.Teffeteller,²J.Filiberto,¹M.C.McCanta,²A.H.Treiman,³L.Keller,⁴D.Cherniak,⁵M.Rutherford,⁵R.F.Cooper
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115085]
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.

¹Rachel Y.Sheppard,²Ralph E.Milliken,²Kevin M.Robertson
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2022.115083]
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.

^1,2J.A.Cartwright,²K.V.Hodges,²M.Wadhwa
Earth and Planetary Science Letters 590, 117576 Link to Article [https://doi.org/10.1016/j.epsl.2022.117576]
¹Department of Geological Sciences, University of Alabama, Tuscaloosa, AL 35487-0338, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-6004, USA
Copyright Elsevier

Impact events on planetary surfaces can leave significant volumes of melt, archived in planetary regoliths, which provide important information regarding the timing and nature of these events. For example, an observed ca. 3.9-4.1 Ga age cluster within lunar samples has been interpreted as indicative of a significant Solar System-wide event: the so-called “Late Heavy Bombardment”. Here, we report data from a laser ablation microprobe 40Ar/39Ar study of clasts within two unpaired howardite meteorites (NWA 1929 and Dho 485) to explore the impact history of their asteroid parent body – (4)Vesta. Laser microprobe dates for the howardites varied broadly between 3.5 to 4.5 Ga (NWA 1929) and 2.5 to 4.5 Ga (Dho 485), but show no clear cluster in ages at ca. 3.9-4.1 Ga. Consistent with previously reported U-Pb dates for HED meteorites, our data suggest an extended impact bombardment period on (4)Vesta as compared to the distribution of 40Ar/39Ar impactite dates for available samples from the Apollo and Luna sample archives. The impact history of Vesta revealed here highlights that current models of the impact flux in the inner Solar System based on the Late Heavy Bombardment hypothesis require refinement.

¹Edward D.Young,²Catherine A.Macris,¹Haolan Tang,²Arielle A.Hogan,^1,3Quinn R.Shollenberger
Earth and Planetary Science Letters 589, 117575 Link to Article [https://doi.org/10.1016/j.epsl.2022.117575]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, United States of America
²Earth Sciences, Indiana University–Purdue University Indianapolis, United States of America
³Lawrence Livermore National Laboratory, United States of America
Copyright Elsevier

We use new experiments and a theoretical analysis of the results to show that the isotopic fractionation associated with laser-heating aerodynamic levitation experiments is consistent with the velocity of flowing gas as the primary control on the fractionation. The new Fe and Mg isotope data are well explained where the gas is treated as a low-viscosity fluid that flows around the molten spheres with high Reynolds numbers and minimal drag. A relationship between the ratio of headwind velocity to thermal velocity and saturation is obtained on the basis of this analysis. The recognition that it is the ratio of flow velocity to thermal velocity that controls fractionation allows for extrapolation to other environments in which molten rock encounters gas with appreciable headwinds. In this way, in some circumstances, the degree of isotope fractionation attending evaporation is as much a velocimeter as it is a barometer.