Discovery of coesite from the martian shergottite Northwest Africa 8657 Author links open overlay panel

1,2,3Sen Hu,4Yang Li,1Lixin Gu,1Xu Tang,1Ting Zhang,5Akira Yamaguchi,1,2,3 Yangting Lin,1Hitesh Changela
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.021]
1Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2Innovation Academy for Earth Science, Chinese Academy of Sciences, Beijing 100029, China
3College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
4Center for Lunar and Planetary Sciences, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
5National Institute of Polar Research, Tokyo 190-8518, Japan
Copyright Elsevier

We report occurrences of coesite in a martian meteorite, expending previously-reported silica polymorphs such as stishovite (El Goresy et al., 2000), seifertite (Goresy et al., 2008; Sharp et al., 1999), and post-stishovite (El Goresy et al., 2000). The coesite was found in the shock-induced melt regions of NWA 8657, usually coexisting with deformed quartz and silica glass. Three morphological types of coesite have been identified: (I) in a silica-maskelynite assemblage, (II) needle grains, and (III) granular grains embedded in maskelynite. Transmission Electron Microscopy (TEM) shows that all types of coesite appear distributed in silica glass and/or nano-phase maskelynite. The stishovite-like morphology of Type II coesite and the presence of deformed quartz suggest coesite to have inverted from stishovite during decompression. The impact-induced peak pressures and temperatures are estimated at ∼ 18-30 GPa and ∼ 2000 ℃ respectively, based on static high pressure experiments (Langenhorst and Deutsch, 2012; Zhang et al., 1996). The polymorphs aggregates of silica in NWA 8657 indicate that the shock-induced melts in this meteorite cooled slower than those in other stishovite-bearing martian meteorites, but fast enough to preserve coesite.

A small S-MIF signal in Martian regolith pyrite: Implications for the atmosphere

1Andrew G.Tomkins,1Sarah L.Alkemade,1Sophie E.Nutku,2Natasha R.Stephen,1Melanie A.Finch,3Heejin Jeon
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.022]
1School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria 3800, Australia
2Plymouth Electron Microscopy Centre, University of Plymouth, Drake Circus, Plymouth, Devon, PL4 8AA, United Kingdom
3Centre for Microscopy, Characterisation and Analysis, University of Western Australia, 35 Stirling Highway, Perth, Western Australia 6009, Australia
Copyright Elsevier

The past Martian atmosphere is often compared to the Archean Earth’s as both were dominated by CO2-rich and O2-poor chemistries. Archean Earth rocks preserve mass-independently fractionated sulfur isotopes (S-MIF; non-zero Δ33S and Δ36S), originating from photochemistry in an anoxic atmosphere. Thus, Martian crustal rocks might also be expected to preserve a S-MIF signature, providing insights into past atmospheric chemistry. We have used secondary ion mass spectrometry (SIMS) to investigate in situ, the sulfur isotope systematics of NWA 8171 (paired to NWA 7034), a Martian polymict breccia containing pyrite that formed through hydrothermal sulfur addition in a near-surface regolith setting. In this meteorite, pyrite grains have a weighted mean of Δ33S of -0.14 ± 0.08 ‰ and Δ36S = -0.70 ± 0.40 ‰ (2 s.e.m.), so the S-MIF signature is subtle. Sulfur isotope data for four additional shergottites yield Δ33S values that are not resolvable from zero, as in previous studies of shergottites. At first glance the result for the polymict breccia might seem surprising, but no Martian meteorite yet has yielded a S-MIF signature akin to the large deviations seen on Earth. We suggest that S-MIF-bearing aerosols (H2SO4 and S8) were produced when volcanic activity pushed a typically oxidising Martian atmosphere into a reduced state. After rain-out of these aerosols, S8 would tend to be oxidised by chlorate, dampening the S-MIF signal, which might be somewhat retained in the more abundant photolytic sulfate. Then in the regolith, mixing of aqueous surface-derived sulfate with igneous sulfide (the latter with zero MIF), to form the abundant pyrite seen in NWA 8171, would further dampen the S-MIF signal. Nonetheless, the small negative Δ33S anomalies seen in Martian meteorites imply that volcanic activity was sufficient to produce a reducing atmosphere at times. This volcanically-driven atmospheric evolution would tend to produce high levels of carbonyl sulfide (OCS). Given that OCS is a relatively long-lived strong greenhouse gas, the S-MIF signal implies that volcanism periodically generated warmer conditions, perhaps offering an evidence-based solution to the young wet Mars paradox.

Effects of oxidation on pyroxene visible-near infrared and mid-infrared spectra

1Molly C.McCanta,2M. Darby Dyar
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113978]
1Department of Earth and Planetary Sciences, University of Tennessee, 1621 Cumberland Ave, Knoxville, TN 37996, United States of America
2Department of Astronomy, Mount Holyoke College, 50 College St, South Hadley, MA 01075, United States of America
Copyright Elsevier

Pyroxene spectral features in the visible near-infrared (VNIR) and mid-infrared (MIR) wavelengths are affected by oxidation resulting from traditional metamorphic processes as well as impact metamorphism. The observed effects are due to modifications in the crystal arising from changes in crystallization temperature or pressure or from substituting Fe3+ for Fe2+. Highly oxidized pyroxenes from terrestrial mantle xenoliths and shock experiments indicate that the spectral effects of oxidation are greater in clinopyroxene than orthopyroxene because clinopyroxene can accommodate more Fe3+ structurally. Changes in clinopyroxene VNIR related to increasing oxidation include a shift in the 0.8 μm absorption band to shorter wavelengths and a strengthening of the Fe2+↔Fe3+ intervalence charge transfer (IVCT) band, which reduces the band depth of the 1.0 μm feature by ~20%. Although shocked clinopyroxenes are oxidized to similar levels to that seen in the mantle xenoliths, the effects of shock overprint those of oxidation in the VNIR. These include a decrease of ~76% intensity of the 2.35 μm feature and a decrease of ~70% intensity of the 1.0 μm feature. In the MIR, the effects of oxidation and shock are minimal, resulting in a 5% overall decrease in band depth. These shifts and changes can be interpreted as a result of changes in the polyhedra surrounding the Fe cations which reduce crystal field splitting and the order of the crystal structure. Determination of planetary surface composition through VNIR remote sensing methods requires careful consideration of potential changes induced via shock and/or oxidation processes.

Effect of Sulfur Speciation on Chemical and Physical Properties of Very Reduced Mercurian Melts

1Brendan A.Anzures,1Stephen W.Parman,1Ralph E.Milliken,2Olivier Namur,3Camille Cartier,4Sicheng Wang
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.024]
1Brown University, Department of Earth and Planetary Sciences, USA
2KU Leuven, Department of Earth and Environmental Sciences, Belgium
3CRPG/CNRS, University of Lorraine, France
4Centre for Mined Land Rehabilitation, Sustainable Minerals Institute, The University of Queensland, Brisbane 4072, Australia
Copyright Elsevier

The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5-4 wt.%) compared with other terrestrial planets (<0.1 wt.%), a result of high S solubility under its very low oxygen fugacity (estimated ƒO2 between IW-3 and IW-7). Due to decreasing O availability at these low ƒO2conditions, and an abundance of S2-, the latter acts as an important anion. This changes the partitioning behaviour of many elements (e.g. Fe, Mg, and Ca) and modifies the physical properties of silicate melts. To further understand S solubility and speciation in reduced magmas, we have analysed 11 high pressure experiments run at 1 GPa in a piston cylinder at temperatures of 1250 to 1475 °C and ƒO2 between IW-2.5 to IW-7.5. S K-Edge XANES is used to determine coordination chemistry and oxidation state of S species in highly reduced quenched silicate melts. As ƒO2 decreases from IW-2 to IW-7, S speciation goes through two major changes. At ∼IW-2, FeS, FeCr2S4, Na2S, and MnS species are destabilized, CaS (with minor Na2S) becomes the dominant S species. At ∼ IW-4, Na2S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low ƒO2affect the activities of SiO2, MgO and CaO in the melt, stabilizing enstatite at the expense of forsterite, and destabilizing plagioclase and clinopyroxene. These shifts cause the initial layering of Mercury’s solidified magma ocean to be enstatite-rich and plagioclase poor. Our results on S speciation at low ƒO2 are also applicable to the petrologic evolution of enstatite chondrite parent bodies and perhaps early Earth.

Photon Stimulated Desorption of MgS as a Potential Source of Sulfur in Mercury’s Exosphere

1,2Micah J. Schaible,3Menelaos Sarantos,4Brendan A. Anzures,4Stephen W. Parman,1,2,5Thomas M. Orlando
Journal of Geophysical Research (Planets)(in Press) Link to Article [https://doi.org/10.1029/2020JE006479]
1School of Chemistry and Biochemistry, Georgia Institute of Technology
2Center for Space Technology and Research, Georgia Institute of Technology
3Heliophysics Science Division, NASA Goddard Space Flight Center
4Department of Earth, Environmental and Planetary Sciences, Brown University
5School of Physics, Georgia Institute of Technology
Published by arrangement with John Wiley & Sons

Mercury has a relatively high sulfur content on its surface, and a signal consistent with ionized atomic sulfur (S+) was observed by the fast ion plasma spectrometer (FIPS) instrument on the MESSENGER spacecraft. To help confirm this assignment and to better constrain the sources of exospheric sulfur at Mercury, 193 nm photon stimulated desorption (PSD) of neutral sulfur atoms (S0) from MgS substrates was studied using resonance enhanced multiphoton ionization (REMPI) and time‐of‐flight (TOF) mass spectrometry. Though the PSD process is inherently non‐thermal, the measured velocities of ejected S0 were fit using flux weighted Maxwellian distributions with translation energies ˂E> expressed as translational “temperatures” = ˂E>/μkB. A bi‐modal distribution consisting of both thermal (= 300 ) and supra‐thermal (>1000 ) components in roughly a 2:1 ratio was found to best fit the data. The PSD cross‐section was measured to be approximately 4×10‐22 cm and, together with the velocity distributions, was used to calculate the PSD source rate of S0 into the exosphere of Mercury. Exosphere simulations using the calculated rates demonstrate that PSD is likely the primary source of S0 in Mercury’s exosphere at low (<1000 km ) altitudes.

The Aguas Zarcas (CM2) meteorite: New insights into early solar system organic chemistry

1Sandra Pizzarello,2Christopher T. Yarnes,3George Cooper
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13532]
1School of Molecular Sciences, Arizona State University, Tempe, Arizona, 85287‐1604 USA
2Stable Isotope Facility, University of California, One Shields Ave. MS 1, Davis, California, 95616 USA
3NASA‐Ames Research Center, Moffett Field, California, 94035 USA
Published by arrangement with John Wiley & Sons

To date, the CM2 class of carbonaceous chondrites has provided the most detailed view of organic synthesis in the early solar system. Organic‐rich chondrites actually observed falling to Earth (“Falls”), for example, the Murchison meteorite in 1969, are even more rare. The April 23, 2019 fall of the Aguas Zarcas meteorite is therefore the most significant CM2 fall since Murchison. Samples collected immediately following the fall provide the rare opportunity to analyze its bulk mineralogy and organic inventory relatively free of terrestrial contamination. According to the Meteoritical Bulletin, Aguas Zarcas (“AZ” or “Zarcas”) is dominated by serpentine, similar to other CM2 chondrites. Likewise, our initial analyses of AZ were meant to give a broad view of its soluble organic inventory relative to other carbonaceous chondrites. We observe that while it is rich in hydrocarbons, carboxylic acids, dicarboxylic acids, sugar alcohols, and sugar acids, some of these classes may be of lesser abundance than in the more well known carbonaceous chondrites such as Murchison. Compared generally with other CM2 meteorites, the most significant finding is the absence, or relatively low levels, of three otherwise common constituents: ammonia, amino, acids, and amines. Overall, this meteorite adds to the building database of prebiotic compounds available to the ancient Earth.

Absolute dating of the L-chondrite parent body breakup with high-precision U–Pb zircon geochronology from Ordovician limestone

1,2,3Shi Yong Liao,4Magdalena H.Huyskens,4Qing-Zhu Yin,1Birger Schmitz
Earth and Planetary Science Letters 547, 116442 Link to Article [https://doi.org/10.1016/j.epsl.2020.116442]
1Astrogeobiology Laboratory, Department of Physics, Lund University, Lund, Sweden
2Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
3CAS Center for Excellence in Comparative Planetology, Hefei, China
4Department of Earth and Planetary Sciences, University of California, Davis, CA 95616, USA
Copyright Elsevier

The breakup of the L-chondrite parent body (LCPB) in the mid-Ordovician is the largest documented asteroid breakup event during the past 3 Gyr. It affected Earth by a dramatic increase in the flux of L-chondritic material and left prominent traces in both meteorite and sedimentary records. A precise constraint on the timing of the LCPB breakup is important when evaluating the terrestrial biotic and climatic effects of the event, as well as for global stratigraphic correlations. Direct dating using heavily shocked L chondrites is hampered by both incomplete initial K-Ar degassing and isotopic resetting by later impact events. In order to better constrain the absolute age of this event we carried out high-precision U–Pb dating of zircons from three limestone beds recording discrete volcanic ash fallouts within mid-Ordovician strata in southern Sweden. These strata are rich in fossilized L-chondritic meteorites (1-20 cm large) that arrived on Earth shortly after the breakup event. Zircons from the ash-bearing layers provide stratigraphically consistent depositional ages that range from 464.22 ± 0.37 Ma to 465.01 ± 0.26 Ma. Combined with recently published 3He profiles that pinpoint the arrival on Earth of the first dust from the breakup, and sedimentation rates constrained by cosmogenic 21Ne in the fossil meteorites, the LCPB breakup is estimated to have occurred at 465.76 ± 0.30 Ma. This provides the presently most precise absolute dating of the LCPB breakup, enabling a robust global stratigraphic correlation of bounding strata. Based on our new U–Pb data for the ash-bearing beds, the absolute ages for the boundaries of biozones and Dapingian–Floian stages overlap within error with those given by the 2012 Geological Timescale and require no modification.

Identifying primitive noble gas components in lunar ferroan anorthosites

1J.F.Pernet-Fisher,1K.H.Joy,1J.D.Gilmour
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113977]
1Department of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
Copyright Elsevier

Lunar ferroan anorthosites are the ideal samples for investigating primitive volatile systematics. Not only are these lithologies thought to be direct crystallization products of the Lunar Magma Ocean (LMO), but many samples display short (T38 < 5 Myr) cosmic ray exposure (CRE) ages, minimizing the effects of cosmic ray spallation reactions. Here we report noble gas (He, Ne, Ar, Kr, Xe) abundances and isotope systematics for nine ferroan anorthosites (FAN) collected during the Apollo 16 mission and one anorthosite sample collected during the Apollo 15 mission. The CRE ages calculated for these samples range from T38 ~ 0.13 to ~226 Myr, indicating that not all anorthosites were emplaced at the lunar surface at the same time.

In general, He-Ne-Ar-Kr-Xe isotope systematics can be accounted for by variable contributions from cosmogenic spallation reactions and solar-wind implantation. The Xe isotope systematics of lunar anorthosites offer our best chance of resolving primitive Xe components on the Moon. Three of the samples investigated here (60,515, 65,325, 60,025) display a Xe isotope signature within error of terrestrial air. These samples have likely been comprised by anomalously adsorbed terrestrial air, as was also recognized by early Xe isotope studies of lunar anorthosites (e.g., Niemeyer and Leich, 1976). The three samples that have the shortest CRE ages (69,955, 60,135, 60,015) display ratios of heavy Xe isotopes (134Xe and 136Xe) over lighter isotopes (130Xe and 132Xe) that are lower than air and solar wind. Mixing modeling for these three samples suggests that such signatures can be accounted for by the addition of up to ~30% cometary Xe (based on the reported Xe isotope composition of comet 67P/Churyumov-Gerasimenko; Marty et al., 2017) to mixtures of adsorbed terrestrial air and Solar Wind. One sample (60135) displays lower than solar 136Xe/132Xe from gases extracted in an intermediate temperature heating step, indicating that such a component may have only been superficially implanted. However, two other samples (69,955, 60,015) display heavy Xe isotope ratios deficits only in the highest temperature gas extraction steps, indicating that this component is hosted within the plagioclase crystal structure. It is not clear how a cometary component was introduced into the lunar crust. In one scenario, cometary Xe was mixed directly into the LMO during periods of high impact bombardment (such as the Late Veneer) prior to the formation of the lunar crust before ~4.2 Ga. Alternatively, cometary Xe may have been directly implanted into plagioclase crystals via diffusion as a result of micrometeorite impacts over geological time in the near surface lunar environment.

Characterization of the Ryugu surface by means of the variability of the near-infrared spectral slope in NIRS3 data

1A. Galiano et al. (>10)
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113959]
1INAF-IAPS, Rome, Italy
Copyright Elsevier

The Near-Earth Asteroid 162,173 Ryugu (1999 JU3) was investigated by the JAXA Hayabusa2 mission from June 2018 to November 2019. The data acquired by NIRS3 spectrometer revealed a dark surface with a positive near-infrared spectral slope. In this work we investigated the spectral slope variations across the Ryugu surface, providing information about physical/chemical properties of the surface.

We analysed the calibrated, thermally and photometrically corrected NIRS3 data, and we evaluated the spectral slope between 1.9 μm and 2.5 μm, whose values extend from 0.11 to 0.28 and the mean value corresponds to 0.163±0.022. Starting from the mean value of slope and moving in step of 1 standard deviation (0.022), we defined 9 “slope families”, the Low-Red-Slope families (LR1, LR2 and LR3) and the High-Red-Sloped families (HR1, HR2, HR3, HR4, HR5, HR6). The mean values of some spectral parameters were estimated for each family, such as the reflectance factor at 1.9 μm, the spectral slope, the depth of bands at 2.7 μm and at 2.8 μm. A progressive spectral reddening, darkening and weakening/narrowing of OH bands is observed moving from the LR families to the HR families.

We concluded that the spectral variability observed among families is the result of the thermal metamorphism experienced by Ryugu after the catastrophic disruption of its parent body and space weathering processes that occurred on airless bodies as Ryugu, such as impact cratering and solar wind irradiation. As a consequence, the HR1, LR1, LR2 and LR3 families, corresponding to equatorial ridge and crater rims, are the less altered regions on Ryugu surface, which experienced the minor alteration and OH devolatilization; the HR2, HR3, HR4, HR5 families, coincident with floors and walls of impact craters, are the most altered areas, result of the three processes occurring on Ryugu. The strong reddening of the HR6 family (coincident with Ejima Saxum) is likely due to the fine-sized material covering the large boulder.

A short-lived 26Al induced hydrothermal alteration event in the outer solar system: Constraints from Mn/Cr ages of carbonates

1Robbin Visser,1Timm John,2Martin J.Whitehouse,3Markus Patzek,3Addi Bischoff
Earth and Planetary Science Letters 547, 116440 Link to Article [https://doi.org/10.1016/j.epsl.2020.116440]
1Freie Universität Berlin, Institut für Geologische Wissenschaften, Berlin, Germany
2Swedish Museum of Natural History, Stockholm, Sweden
3Institut für Planetologie, University of Münster, Münster, Germany
Copyright Elsevier

A key process in the early solar system that significantly affects the further evolution and transport of highly volatile elements throughout the solar system hydrothermal parent body alteration. To determine whether hydrothermal alteration in outer solar system parent bodies occurred more or less simultaneously or due to a sequence of multiple different events, we investigated low-temperature hydrothermally altered CM and CI chondrites along with volatile-rich CM-like clasts and C1 clasts with abundant mineral phases that contain volatiles. In this respect, C1 clasts are particularly important as they closely resemble the CI chondrites but originate from isotopically different parent bodies. Specifically, we applied the SIMS-based Mn/Cr in situ dating technique to carbonates, a common hydrothermally formed phase in low-temperature hydrothermally altered meteorites. The Mn/Cr ages of dolomites in CI chondrites and C1 clasts as well as calcites in CM chondrites and CM-like clasts reveal that nearly all carbonates in low-temperature hydrothermally altered clasts and chondrites were formed within a brief period between 2-6 Ma after CAI formation. Given this sharp separation, and that hardly any material contains carbonates formed later than ∼6 Ma after CAI formation, hydrothermal alteration likely occurred near-contemporaneously among different parent bodies in the outer solar system. Further, the timing of hydrothermal alteration matches peak heating of 26Al decay that ceased at ∼5 Ma after CAI formation. Hereby, these results are consistent with a model in which the carbonates in low-temperature hydrothermally altered parent bodies precipitated from the fluid produced by melting ice. The results also show that other potential heating events (e.g., impacts) only negligibly contributed to creating environments where fluid-mediated dissolution and precipitation of carbonates was possible. Additionally, the isotopic (H, O, Cr, and S) differences between C1 clasts and CI chondrites are most likely not caused by differences in timing of hydrothermal aqueous alteration and, thus, are best explained by spatially different isotopic reservoirs.