Constraining Alteration Processes Along the Siccar Point Group Unconformity, Gale Crater, Mars: Results from the Sample Analysis at Mars Instrument

1B.Sutter et al. (>10)
Journal of Geophysical Research (Planets) Link to Article []
1Jacobs Technology, NASA Johnson Space Center, Houston, TX, USA
Publishes by arrangement with JohnWiley & Sons

Results from the Sample Analysis at Mars (SAM)-evolved gas analyzer (EGA) on board the Mars Science Laboratory Curiosity rover constrained the alteration history and habitability potential of rocks sampled across the Siccar Point unconformity in Gale crater.
The Glasgow member (Gm) mudstone just below the unconformity had evidence of acid sulfate or Si-poor brine alteration of Fe-smectite to Fe amorphous phases, leaching loss of Fe-Mg-sulfate and exchange of unfractionated sulfur 34S (δ34S=2±7‰) with enriched 34S (20±5‰, V-CDT). Carbon abundances did not significantly change (322-661 μgC/g) consistent with carbon stabilization by amorphous Al- and Fe-hydroxide phases. The Gm mudstone had no detectable oxychlorine and extremely low nitrate. Nitrate (0.06 wt.% NO3), oxychlorine (0.13 wt% ClO4), high C (1472 μg C/g), and low Fe/Mg-sulfate concentration (0.24 wt.% SO3) depleted in 34S (δ34S = -27‰ ± 7), were detected in the Stimson formation (Sf) eolian sandstone above the unconformity. Redox disequilibrium through the detections of iron sulfide and sulfate supported limited aqueous processes in the Sf sandstone. Si-poor brines or acidic fluids altered the Gm mudstone just below the unconformity but did not alter underlying Gm mudstones further from the contact. Chemical differences between the Sf and Gm rocks suggested that fluid interaction was minimal between the Sf and Gm rocks. These results suggested that the Gm rocks were altered by subsurface fluids after the Sf placement. Aqueous processes along the unconformity could have provided habitable conditions and in some cases, C and N levels could have supported heterotrophic microbial populations.

Thermochemical evolution of the acapulcoite–lodranite parent body: Evidence for fragmentation-disrupted partial differentiation

1Michael P. Lucas,1Nicholas Dygert,2Jialong Ren,2Marc A. Hesse,2Nathaniel R. Miller,1Harry Y. McSween
Meteoritics & Planetary Science (in Press) Link to Article []
1Department of Earth & Planetary Sciences, University of Tennessee, 1621 Cumberland Ave., 602 Strong Hall, Knoxville, Tennessee, 37996 USA
2Department of Geological Sciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, Texas, 78712 USA
Published by arrangement with John Wiley & Sons

Primitive achondrites of the acapulcoite–lodranite clan (ALC) are residues of partial melting that displays a continuum of thermal metamorphism and partial melting most likely set by burial depth within an internally heated, primordial acapulcoite–lodranite parent body (ALPB). New major and trace element data from eight ALC meteorites and the application of several thermometric methods suggest that the ALPB was affected by partial differentiation disrupted by rapid cooling from peak, magmatic temperatures. Application of rare earth element-in-two-pyroxene thermometry recovers temperatures of 1125–1250 °C for lodranites, while two-pyroxene solvus and Ca-in-olivine thermometry recover lower temperatures for ALC meteorites (941–1114 °C and 686–850 °C, respectively). Major and trace element disequilibrium in acapulcoite and transitional groups provides evidence for cryptic melt infiltration and melt rock reaction within these layers of the ALPB. From lodranites, we determined rapid cooling rates of ~1 to ~26 °C yr−1 from peak temperatures, consistent with collisional fragmentation of the parent body during differentiation. After this initial period of rapid cooling, cooling rates decreased by two to four orders of magnitude through Ca-in-olivine closure temperatures (~750 °C). We hypothesize that the primordial ALPB possessed an onion shell-type layered structure that was disrupted by collisional breakup during partial differentiation. Thermal modeling suggests that ALC samples originate from ~300 m to ~10 km radius collisional fragments that cooled rapidly over time scales of several to ~20,000 yr, then reaccreted to form a slower cooling, second-generation rubble-pile asteroid. The source of ALC meteorites is a second-generation (or later) rubble-pile body of S-type spectral class located near the Jupiter 3:1 mean motion resonance in the Main Belt of asteroids.

Abundances of siderophile elements in H-chondrite metal grains: implications for the origin of metal in unequilibrated ordinary chondrites

1,2Guillaume Florin,1,3Olivier Alard,2Béatrice Luais,1Tracy Rushmer
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia
2Université de Lorraine, CNRS, CRPG, F-54000 Nancy, France
3Géosciences Montpellier, UMR 5243, CNRS & Université Montpellier, 34095 Montpellier, France
Copsright Elsevier

Understanding the evolution of metal in the protoplanetary disk is necessary to constrain the first steps of metal-silicate formation and the early stages of the evolution of the protoplanetary disk. We measured the siderophile elemental compositions (PGE, Ni, Co, Fe, Cu, Ga, Ge) of individual metal grains in H ordinary chondrites by laser ablation inductively coupled plasma mass spectrometry to investigate their formation. We analyzed unequilibrated ordinary chondrites (H3) to constrain processes affecting the metal before accretion, and inferred the effects of metamorphism by comparing their elemental compositions to those of equilibrated chondrites (H4–H6). Our results highlight large variations of refractory (Re, Os, W, Ir, Ru, Mo, Pt) and moderately volatile siderophile element (Pd, Au, Ga, Ge) concentrations among metal grains in H3 samples that permit to classify them according to their Ge/Ir ratios and HSE contents. These intergrain variations are progressively homogenized in H4–H6 samples due to their increasing degrees of metamorphism. To constrain the origin of the metal, we modeled its evolution during melting and crystallization. Our melting model of a single metallic precursor containing 1.5 wt.% C and up to 12 wt.% S reproduces well the observed range of siderophile element compositions in the metal. Metal grains show a range of W, Mo, and Ga compositions that we interpret to reflect various local (grain-scale) oxidation states during the melting event(s) due to the heterogeneous distribution of various oxidizing components within the precursors. The very similar HSE compositions of H and L/LL metal grains suggests that the variations of bulk metal abundance and HSE concentrations observed among the different classes of ordinary chondrites (H, L, LL) result from the heterogeneous physical distribution of a relatively chemically homogeneous metal component among OC parent bodies, and not from a chemical (sensu lato) gradient between H and LL chondrites.

Nd isotope variation between the Earth–Moon system and enstatite chondrites

1Shelby Johnston,1Alan Brandon,2Claire McLeod,3Kai Rankenburg,4Harry Becker,1Peter Copeland
Nature 611, 501–506 Link to Article [DOI]
1Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA
2Department of Geology and Environmental Earth Science, Miami University, Oxford, OH, USA
3John De Laeter Centre, Curtin University, Bentley, Western Australia, Australia
4Freie Universitat, Berlin, Germany

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Malotas (b), a new eucrite from an old fall

1Marcela E. Saavedra,2Julia Roszjar,3My E. I. Riebe,1María E. Varela,4Shuying Yang,4Munir Humayun,5Ryoji Tanaka,3H. Busemann
Meteoritics & Planetary Science (in Press) Link to Article []
1ICATE-CONICET, Av. España 1512 Sur, San Juan, J5402DSP Argentina
2Department of Mineralogy and Petrography, Natural History Museum Vienna, Burgring 7, 1010 Vienna, Austria
3Department of Earth Sciences, Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland
4National High Magnetic Field Laboratory and Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, Florida, 32310 USA
5Institute for Planetary Materials, Okayama University, 827 Yamada, Misasa, Tottori, 682-0193 Japan
Published by arrangement with John Wiley & Sons

On the night of June 22, 1931 at 4 h 30 min, a fireball was seen in the vicinity of Malotas, Argentina. During the atmospheric trajectory (southwest to northeast), it experienced several fragmentation events. After the fall, a piece was given to Professor Juan A. Olsacher (National University of Córdoba City, Argentina), who collected some further pieces. One of those samples was officially classified as an H5 ordinary chondrite termed Malotas. The present work focuses on the study of another two pieces rediscovered recently in the Museo de Mineralogía y Geología Dr. Alfred Stelzner in Cordoba City, Argentina. The first piece turned out to be an achondritic meteorite termed Malotas (b). Petrographic features, chemical composition, and oxygen isotopes point to a monomict basaltic eucrite belonging to the Stannern-trend chemical subgroup of eucrites. The occurrence of anorthitic plagioclase veins in clinopyroxene, veinlet apatite, irregular-shaped pockets of silica and troilite and porous silica signal metasomatism and thermal annealing before a late thermal event took place after brecciation. The latter was possibly recorded in the nominal U/Th-4He ages of 1.2–3.4 Ga detected in this work, whereas nominal K-Ar gas retention ages are within the range 3.5–4.2 Ga and may have escaped late thermal modifications. The second piece is classified as an L5 chondrite. The different cosmic ray exposure ages of 3, ~50, and 27 Ma determined for the H5 and L5 chondrites and the eucrite samples, respectively, might signal a common fall as a result of the breakup of a polymict meteoroid.

Genetics, Age, and Crystallization History of Group IC Iron Meteorites

1,2Hope A.Tornabene,2Richard D.Ash,2Richard J.Walker,1Katherine R.Bermingham
Geochimica e Cosmochimica Acta (in Press) Link to Article []
1Department of Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA
2Department of Geology, University of Maryland, College Park, Maryland, 20742, USA
Copyright Elsevier

The IC iron meteorite group is characterized utilizing nucleosynthetic mass-independent isotopic compositions and 182W age constraints, coupled with siderophile element concentration measurements and modeling of crystal-liquid fractionation processes. The six IC irons analyzed, Arispe, Bendego, Chihuahua City, Nocoleche, NWA 2743, and Winburg have indistinguishable Mo and W genetic isotopic compositions and are consistent with derivation from the same parent body, which formed in the non-carbonaceous (NC) nebular reservoir. A pre-exposure µ182W value (parts-per-million deviations in isotopic ratios from terrestrial standards) for the six IC irons of -337 ± 5 corresponds to a metal-silicate segregation age of 1.0 ± 0.4 Myr after calcium-aluminum-rich inclusion (CAI) formation. This age is similar to those determined for other NC iron groups. Siderophile element abundances of the IC irons are generally similar and characterized by minor depletions in the more volatile siderophile elements. Highly siderophile element (HSE) distributions among the IC group suggest that the initial parent body core was S-rich, with preferred model results indicating an initial melt composition with ∼18 wt% S, 2 wt% P and 0.03 wt% C. Processes in addition to fractional crystallization, such as late-stage parent body modification, possibly as a result of impacts, and subsequent metal-melt mixing, are required within the first 100 Myr of Solar System history to explain the range of HSE abundances.

Controls on determining the bulk water content of the Moon

1Ananya Mallik,2Sabrina Schwinger,1Arkadeep Roy,1Pranabendu Moitra
Meteoritics & Planetary Science (in Press) Link to Article []
1Department of Geosciences, University of Arizona, Tucson, Arizona, 85721 USA
2Institute of Planetary Research, German Aerospace Center, Berlin, 12489 Germany
Published by arrangement with John Wiley & Sons

The Moon is much wetter than previously thought. The estimated bulk H2O concentrations based on the analyses of H2O in lunar materials show a wide range from 5 to 1650 ppm. To better constrain bulk H2O in the lunar magma ocean (LMO), we model LMO crystallization and vary DH (concentration of H2O in LMO mineral/concentration of H2O in melt), interstitial melt fraction, and initial LMO depth. We take the highest and lowest values of DH reported in the literature for the LMO minerals. We assess the bulk H2O content required in the initial magma ocean to satisfy two observational constraints: (1) H2O measured in plagioclase grains from ferroan anorthosites and (2) crustal mass from GRAIL. We find that the initial bulk LMO H2O that best explains the H2O content in crustal plagioclase is strongly dependent on DH rather than interstitial melt fractions or initial LMO depths, with a drier magma ocean (10 ppm H2O) being favored with higher DH and a wetter magma ocean (100–1000 ppm H2O) with lower DH. This underscores the importance of constraining DH specific to lunar conditions in future studies. We also demonstrate that crustal mass is not an effective hygrometer.

Rapid characterisation of Mars’ mantle reservoirs by in situ laser ablation 87Sr/86Sr analysis of shocked feldspar (maskelynite

1Daniel L.Burgin,1James M.Scott,1Petrus J.le Roux,2Geoffrey Howarth,1Marshall C.Palmer,1Thomas A.Czertowicz,1Marianne Negrini,1,3Malcolm R.Reid,1,3Claudine H.Stirling
Geochimica et Cosmochimica Acta (in Press) Link to Article []
1Department of Geology, University of Otago, Dunedin 9054, New Zealand
2University of Cape Town, Rondebosch, South Africa
3Centre for Trace Element Analysis, University of Otago, Dunedin 9054, New Zealand
Copyright Elsevier

The 87Sr/86Sr isotopic properties of martian meteorites, measured by isolation and purification of Rb and Sr fractions, have long been used to partially characterise Mars’ mantle. However, this method is time-consuming, destructive, and subject to incorporation of terrestrial contamination due to precipitation of phases in fractures and/or grain boundaries prior to meteorite collection. In this study, we test the effectiveness of in situ acquisition of 87Sr/86Sr by laser ablation in maskelynite – a typically low Rb/Sr (< 0.1) feldspar-composition glass formed during high-P shock metamorphism – as a method of rapid characterisation of Mars’ mantle Sr isotope ratios. Element concentration maps and the results of unwashed and gently surface-leached bulk rock analyses indicate that terrestrial Sr has infiltrated the studied meteorites and affected bulk rock trace element budgets. However, maskelynite trace element concentrations are largely unaffected and their martian igneous Rb/Sr is preserved. In situ 87Sr/86Sr analyses of maskelynite, checked against a newly developed Sr feldspar reference material (KAN, presented here and available on demand), accurately and precisely distinguish different shergottite mantle reservoirs. The measurements reproduce published data, have uncertainties on the 4th to 5th decimal place, and yield statistically indistinguishable results in analytical sessions separated by long periods of time. The data from 11 enriched shergottites reveal there to be subtle Sr isotope variation within the enriched shergottite mantle reservoir, or that there was crustal assimilation of radiogenic components into the shergottites, or that the shergottite liquids were extracted over a time period during which the mantle reservoir 87Sr/86Sr evolved. The limited published age range of the enriched shergottites, coupled with the absence of a correlation between in situ 87Sr/86Sr and REE, Sr or Pb concentration in maskelynite, suggests that the isotope range is best explained by small variations in the enriched mantle reservoir. Given the presence of maskelynite (or plagioclase) in many meteorites, the in situ method will be useful when there are only very small volumes of material available and/or where terrestrial contamination is suspected.

The effect of collisional erosion on the composition of Earth-analog planets in Grand Tack models: Implications for the formation of the Earth

Icarus (in Press) Link to Article []
1Institut de Physique du Globe de Paris, Université de Paris, 1 Rue Jussieu, Paris, France
2Museum für Naturkunde, Invalidenstrasse 43, Berlin, Germany
3Michigan State University, Earth and Environmental Sciences, 288 Farm Ln, East Lansing, MI 48824, USA
4Laboratoire d’Astrophysique de Bordeaux, Allée Geoffroy St Hilaire, Bordeaux, France
5Institut Universitaire de, France
Copyright Elsevier

Impact-induced erosion of the Earth’s early crust during accretion of terrestrial bodies can significantly modify the primordial chemical composition of the Bulk Silicate Earth (BSE, that is, the composition of the crust added to the present-day mantle). In particular, it can be particularly efficient in altering the abundances of elements having a strong affinity for silicate melts (i.e. incompatible elements) as the early differentiated crust was preferentially enriched in those. Here, we further develop an erosion model (EROD) to quantify the effects of collisional erosion on the final composition of the BSE. Results are compared to the present-day BSE composition models and constraints on Earth’s accretion processes are provided. The evolution of the BSE chemical composition resulting from crustal stripping is computed for entire accretion histories of about 50 Earth analogs in the context of the Grand Tack model. The chosen chemical elements span a wide range of incompatibility degrees. We find that a maximum loss of 40wt% can be expected for the most incompatible lithophile elements such as Rb, Th or U in the BSE when the crust is formed from low partial melting rates. Accordingly, depending on both the exact nature of the crust-forming processes during accretion and the accretion history itself, Refractory Lithophile Elements (RLE) may not be in chondritic relative proportions in the BSE. In that case, current BSE estimates may need to be corrected as a function of the geochemical incompatibility of these elements. Alternatively, if RLE are indeed in chondritic relative proportions in the BSE, accretion scenarios that are efficient in affecting the BSE chemical composition should be questioned.

Visible and near-infrared reflectance spectra of igneous rocks and their powders

1Yan Zhuang,1,2Hao Zhang,1Pei Ma,1Te Jiang,3Yazhou Yang,4Ralph E.Milliken,5,2Weibiao Hsu
Icarus (in Press) Link to Article []
1School of Earth Sciences, China University of Geosciences, Wuhan, China
2CAS Center for Excellence in Comparative Planetology, Hefei, China
3Key Laboratory of Space Weather, National Space Science Center, Chinese Academy of Sciences, Beijing, China
4Department of Earth, Environmental, and Planetary Sciences, Brown University, Providence, RI, USA
5Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, China
Copyright Elsevier

Most solid planetary bodies in the solar system are covered by a layer of fine particles and the topic of light scattering by small particles has been thoroughly studied in the past decades. In contrast, light reflection from intact rocks has received much less attention, though the spectral features of fresh rocks are more diagnostic than that of highly space-weathered regolith grains. As high spatial-resolution spectral images obtained by modern space-borne and in-situ sensors have become available, it is important to understand the spectral feature links between rocks and powders made by crushing the rocks. In this work, we selected 13 terrestrial igneous rocks with a 1 μm absorption feature and measured the visible and near-infrared reflectance spectra of their slabs and powders in three size fractions, 0-45 μm, 90-125 μm, and 450-900 μm. We have found that the spectral characteristics of these samples can be divided into two groups. For slabs with reflectance lower than 0.1 at 0.5 μm, they have less pronounced 1 μm absorption feature. For slabs with reflectance higher than 0.1, they have pronounced 1 μm feature, consistent with that of their powdered counterparts. By using the equivalent-slab and the Hapke model, we obtained the optical constants and single scattering albedo values of the samples. The dependence of single scattering albedo on effective absorption thickness indicates that the differences between the spectral characteristics of rock slabs and powdered samples are likely controlled by the degree of weak surface scattering contributions. We reconstructed the spectrum of a powdered lunar meteorite which best matches the Chang’E-4 rock and found that the reconstructed rock spectra are very close to the rock spectrum observed in suit by Chang’E-4.