^1,2Ryota Moriwaki,^3,4Tomohiro Usui,²Minato Tobita,²Tetsuya Yokoyama
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.014]
¹Planetary Exploration Research Center, Chiba Institute of Technology, Address: 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan
²Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Address: 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
³Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Address: 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan
⁴Earth-Life Science Institute, Tokyo Institute of Technology, Address: 2-12-1 Ookayama, Meguro, Tokyo 152-8551, Japan
Copyright Elsevier

Radiogenic isotopic compositions of shergottite meteorites suggest that early planetary differentiation processes, which are related to the crystallization of the Martian Magma Ocean (MMO), resulted in the geochemically heterogeneous Martian mantle. In order to understand the early geochemical evolution of Mars, we investigated the Pb isotope systematics in the depleted Martian mantle on the basis of the analyses of two geochemically depleted shergottites, Dar al Gani (DaG) 476 and Yamato 980459 (Y-980459). Their initial Pb isotopic compositions were estimated from geochemical analyses of highly leached acid residues and age-correction calculations using reference crystallization ages. This yielded μ-values (²³⁸U/²⁰⁴Pb) for the DaG 476 and Y-980459 source reservoirs of 2.33 ± 0.07 and 2.32 ± 0.06, respectively. These μ-values are distinct from those of other depleted shergottite source reservoirs (e.g., 1.4 ± 0.1 for the Tissint meteorite) and show a negative correlation with corresponding ¹⁴⁷Sm/¹⁴⁴Nd, ¹⁷⁶Lu/¹⁷⁷Hf, ɛ¹⁸²W, and ε¹⁴²Nd compositions. Such correlations between long- and short-lived isotopic signatures suggest that a geochemically heterogeneous depleted shergottite source mantle was formed on the early Mars. This geochemical heterogeneity would have been formed by variable mixing of depleted and enriched end-member components that originally formed by fractional crystallization in the MMO. Local remelting in the geochemically depleted Martian mantle after the crystallization of the MMO is another possible explanation for the formation of a geochemically heterogeneous depleted shergottite source mantle.

^1,2F.Jourdan,^1,2T.Kennedy,²G.K.Benedix,³E.Eroglu,¹C.Mayer
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.036]
¹Western Australian Argon Isotope Facility, John de Laeter Centre, TIGeR, Curtin University, Australia
²Space Science and Technology Centre, School of Earth and Planetary Sciences, Curtin University, Australia
³Discipline of Chemical Engineering, WA School of Mines: Minerals, Energy and Chemical Engineering, Curtin University, Perth, WA 6845, Australia
Copyright Elsevier

Eucrites are extraterrestrial basalts and cumulate gabbros formed, and subsequently more or less metamorphosed, at the crustal level of the HED (Howardite-Eucrite-Diogenite) parent body, thought to be the asteroid 4 Vesta. Unbrecciated eucrites offer the best way to understand the igneous, metamorphic and cooling processes occurring in the crust of Vesta following accretion since they were not substantially affected/altered by secondary impact processes. The ⁴⁰Ar/³⁹Ar system of unbrecciated eucrites should be in a relatively pristine state, and thus can inform us on the early volcanic and thermal history of the HED parent body, and, in particular, the cooling history of various crustal parts below the ∼300 °C isotherm, which represents the average closure temperature of the Ar diffusion in plagioclase.

We analysed plagioclase and pyroxene (± groundmass) separates of two cumulate (Moore County and Moama), and five (Caldera, BTN 00300, EET 90020, GRA 98098, QUE 97053) equilibrated basaltic eucrites with the ⁴⁰Ar/³⁹Ar technique using a Thermo© ARGUS VI multi-collection mass spectrometer. The two cumulate unequilibrated gabbros also gave cooling ages of 4531 ± 11 Ma and 4533 ± 12 Ma and combined with a fast cooling rate estimated from lamella thicknesses, suggest that magmatic activity persisted up to 4533 ± 11 and 4535 ± 12 Ma and that the plutons were intruded in a relatively shallow part of the crust, above the metamorphosed regions. Four equilibrated eucrites yielded a well-defined cluster of ages between 4523 ± 8 Ma to 4514 ± 6 Ma. Those ages indicate when the part of the upper crust, where those eucrites probably resided (∼10-15 km deep), cooled below ∼300°C at a rate of 17.3 ± 3.6°C/Ma (2σ). Such a slow cooling rate combined with available global thermal models, supports the hypothesis of a global crustal metamorphism by burial and reheating of lava flows. Finally, an age of 4531 ± 5 Ma was obtained for metamorphosed eucrite EET 90020 and, combined with petrographic observations, indicates the age of a major crustal excavation event by impact. ⁴⁰Ar diffusion models suggest that it is possible to differentiate impact vs crustal cooling provided that a sufficient quantity of pyroxene is measured by ⁴⁰Ar/³⁹Ar.

¹K. Metzler,²D. C. Hezel,³J. Nellesen
The Astrophysical Journal 887, 230 Link to Article [DOI
https://doi.org/10.3847/1538-4357/ab58d0]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany
²Institut für Geologie und Mineralogie, University of Cologne, Zülpicher Straße 49a, D-50674 Köln, Germany
³Fakultät Maschinenbau, Technische Universität Dortmund, Leonhard-Euler-Str. 2, D-44227 Dortmund, Germany

Chondrules are approximately millimeter-sized beads of crystallized silicate melt. They formed mainly in the first ~3 Ma of the Sun’s protoplanetary disk and are the main constituents of chondritic asteroids. Here we report on the size–frequency distributions (2D and 3D) of chondrules in the brecciated ordinary chondrite (OC) Northwest Africa (NWA) 5205. We investigated three large (centimeter- to decimeter-sized) chondritic lithic clasts of a particular textural type (“cluster chondrite”) with eye-catching different chondrule sizes. One clast shows the largest mean chondrule size (~1.5 mm) ever measured in a chondrite. As in the other OCs, we find a positive correlation between the minimum and mean chondrule size, which we consider as an argument for chondrule size sorting. Chondrule size–frequency distributions in the clasts are distinctly more symmetric than the about log-normal distributions in other OCs. Furthermore, we find a co-enrichment of chondrule types with a priori small mean sizes (type I, porphyritic) in clasts with overall small mean chondrule sizes. We consider this as the fingerprint of an additional/second size-sorting process, which acted later on these chondrule populations. This process possibly subdivided a typical LL-type chondrule population into several subpopulations with different mean chondrule sizes. We speculate that this second sorting occurred in a unidirectional gas stream or headwind, e.g., by settling of chondrules through an asteroidal atmosphere or interaction with an expanding impact plume. Possibly, fine-grained matrix was almost completely removed by this, and the size-sorted chondrule subpopulations accreted in a hot state separately in different regions of the asteroid.

^1,2Guillaume Florin,¹Béatrice Luais,² Tracy Rushmer,^2,3Olivier Alard
Geochimica et Cosmochimica Acta 269, 270-291 Link to Article [https://doi.org/10.1016/j.gca.2019.10.038]
¹Centre de Recherches Pétrographiques et Géochimiques, CRPG-CNRS – UMR 7358, Université de Lorraine, 15 Rue Notre Dame des Pauvres, 54500 Vandœuvre-lès-Nancy, France
²Department of Earth and Planetary Sciences, Macquarie University, NSW 2109, Australia
³Géosciences Montpellier, UMR 5243, CNRS & Université Montpellier, 34095 Montpellier, France
Copyright Elsevier

Ordinary chondrites (OCs) are classified into three groups, according to their oxidation state, which increases from the H to L to LL groups. This is demonstrated by the decrease in metal content (H = ∼8 vol%, L = ∼4 vol%, and LL = ∼2 vol%), and by a positive correlation between Δ¹⁷O and %Fa through the OC sequence. Compared to other chondrites, OCs exhibit the largest variation in oxidation state, but there is an ongoing debate on the processes that control this variation. To constrain the causes of the variations in the oxidation state with respect to the associated nebular versus parent bodies processes, we investigated the elemental and isotopic variations of germanium (moderately siderophile and volatile) in the bulk sample, as well as in the metal, silicate and sulfide phases, over a range of petrographic types for the H, L, and LL ordinary chondrites.

We found that δ^74/70Ge_metal is a proxy for the δ^74/70Ge_bulk composition and that each OC group is distinguishable by their δ^74/70Ge_metal, which increases from −0.51 ± 0.09‰ for H chondrites, −0.31 ± 0.06‰ for L chondrites, and, finally, to −0.26 ± 0.09‰ for LL chondrites (2σ SD). Additionally, the OC sequence exhibited a positive correlation, from H to L to LL, between δ^74/70Ge_metal and %Fa, as well as oxygen isotopes (δ¹⁷O, δ¹⁸O and Δ¹⁷O), that was not a consequence of a “size sorting effect” on chondrules (i.e., chondrule mixing) or metamorphic processes in the parent bodies but, rather, was the result of nebular processes. We propose that the correlation between the δ^74/70Ge values and %Fa, Δ¹⁷O, δ¹⁸O can be explained by an increasing proportion of accreted hydrated phyllosilicates, from the H, L to LL groups, with high δ^74/70Ge and Δ¹⁷O. We found that 10 to 15% of phyllosilicates, with a composition of [Ge] = 4–7 ppm and δ^74/70Ge = 3–2.5‰, is needed to change the δ^74/70Ge from H to LL, which corresponds to a Δ¹⁷O ≈ 8–7‰. This value agrees with the Δ¹⁷O ≈ 7‰ composition of the accreted nebular component reported by Choi et al. (1998). During thermal metamorphism, phyllosilicates destabilize, liberating germanium that will be incorporated in the metal, then leading to its high δ^74/70Ge signature.

High-temperature metamorphism can explain the lack of δ^74/70Ge_metal variation with the petrologic type in the OC, even for the type 3 chondrites (T ≈ 675 °C), implying a complete reaction even at low petrologic types. In addition, metal-silicate re-equilibration in response to thermal metamorphism results in a decrease in Δ^74/70Ge_{metal-silicate} from 0.33‰ to 0.06‰, within the H chondrite group, which is interpreted as the result of δ^74/70Ge_silicate variation. The mean positive Δ^74/70Ge_{metal-silicate} fractionation factor of +0.22 ± 0.36‰ (error propagation on individual error) also displays a remarkable similarity to the direction of isotopic fractionation with other germanium isotopic metal-silicate datasets, such as the magmatic iron meteorites, the Earth silicate reservoirs. We propose that the Δ^74/70Ge_{metal-silicate} and the negative δ^74/70Ge values of OCs are inherited from metal-silicate melting and partial exchange before planetesimal accretion in a light isotope-enriched gas. Finally, the δ^74/70Ge_metal-Δ¹⁷O_silicate correlation between the IIE iron meteorites and OCs, provides new evidence for the existence of a highly reduced HH group.

^1,2E.B.Rampe et al. (>10)
Geochemistry (in Press) Link to Article [https://doi.org/10.1016/j.chemer.2020.125605]
¹NASA Johnson Space Center, Houston, TX, USA
²Chesapeake Energy, Oklahoma City, OK, USA
Copyright Elsevier

The Mars Science Laboratory Curiosity rover arrived at Mars in August 2012 with a primary goal of characterizing the habitability of ancient and modern environments. Curiosity was sent to Gale crater to study a sequence of ∼3.5 Ga old sedimentary rocks that, based on orbital visible and near- to short-wave infrared reflectance spectra, contain secondary minerals that suggest deposition and/or alteration in liquid water. The sedimentary sequence in the lower slopes of Mount Sharp in Gale crater preserves a dramatic shift on early Mars from a relatively warm and wet climate to a cold and dry climate, based on a transition from smectite-bearing strata to sulfate-bearing strata. The rover is equipped with instruments to examine the sedimentology and identify compositional changes in the stratigraphy. The Chemistry and Mineralogy (CheMin) instrument is one of two internal laboratories on Curiosity and includes a transmission X-ray diffractometer (XRD) and X-ray fluorescence (XRF) spectrometer. CheMin measures loose sediment samples scooped from the surface and drilled rock powders, and the XRD provides quantitative mineralogy to a detection limit of ∼1 wt.% for crystalline phases. Curiosity has traversed >20 km since landing and has primarily been exploring an ancient lake environment fed by streams and groundwater. Of the 19 drilled rock samples analyzed by CheMin as of sol 2300 (January 2019), 15 are from fluvio-lacustrine deposits that comprise the Bradbury and Murray formations. Most of these samples were drilled from units that did not have a clear mineralogical signature from orbit. Results from CheMin demonstrate an astounding diversity in the mineralogy of these rocks that signifies geochemical variations in source rocks, transportation mechanisms, and depositional and diagenetic fluids. Most detrital igneous minerals are basaltic, but the discovery in a few samples of abundant silicate minerals that usually crystallize from evolved magmas on Earth remains enigmatic. Trioctahedral smectite and magnetite at the base of the section may have formed from low-salinity pore waters with a circumneutral pH in lake sediments. A transition to dioctahedral smectite, hematite, and Ca-sulfate going up section suggests a change to more saline and oxidative aqueous conditions in the lake waters themselves and/or in diagenetic fluids. Perhaps one of the biggest mysteries revealed by CheMin is the high abundance of X-ray amorphous materials (15 to 73 wt.%) in all samples drilled or scooped to date. CheMin has analyzed three modern eolian sands, which have helped constrain sediment transport and mineral segregation across the active Bagnold Dune Field. Ancient eolian sandstones drilled from the Stimson formation differ from modern eolian sands in that they contain abundant magnetite but no olivine, suggesting that diagenetic processes led to the alteration of olivine to release Fe(II) and precipitate magnetite. Fracture-associated halos in the Stimson and the Murray formations are evidence for complex aqueous processes long after the streams and lakes vanished from Gale crater. The sedimentology and composition of the rocks analyzed by Curiosity demonstrate that habitable environments persisted intermittently on the surface or in the subsurface of Gale crater for perhaps more than a billion years.

¹Takashi Yoshizaki,^1,2,3William F.McDonough
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.011]
¹Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, Miyagi 980-8578, Japan
²Department of Geology, University of Maryland, College Park, MD 20742, USA
³Research Center of Neutrino Sciences, Tohoku University, Sendai, Miyagi 980-8578, Japan
Copyright Elsevier

Comparing compositional models of the terrestrial planets provides insights into physicochemical processes that produced planet-scale similarities and differences. The widely accepted compositional model for Mars assumes Mn and more refractory elements are in CI chondrite proportions in the planet, including Fe, Mg, and Si, which along with O make up >90% of the mass of Mars. However, recent improvements in our understandings on the composition of the solar photosphere and meteorites challenge the use of CI chondrite as an analog of Mars. Here we present an alternative model composition for Mars that avoids such an assumption and is based on data from Martian meteorites and spacecraft observations. Our modeling method was previously applied to predict the Earth’s composition. The model establishes the absolute abundances of refractory lithophile elements in the bulk silicate Mars (BSM) at 2.26 times higher than that in CI carbonaceous chondrites. Relative to this chondritic composition, Mars has a systematic depletion in moderately volatile lithophile elements as a function of their condensation temperatures. Given this finding, we constrain the abundances of siderophile and chalcophile elements in the bulk Mars and its core. The Martian volatility trend is consistent with ⩽7 wt% S in its core, which is significantly lower than that assumed in most core models (i.e., >10 wt% S). Furthermore, the occurrence of ringwoodite at the Martian core-mantle boundary might have contributed to the partitioning of O and H into the Martian core.

^1,2A.E.Pickersgill,^2,3D.F.Mark,¹M.R.Lee,^4,5G.R.Osinski
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.01.025]
¹School of Geographical & Earth Sciences, University of Glasgow, Gregory Building, Lilybank Gardens, Glasgow G12 8QQ, UK
²NERC Argon Isotope Facility, Scottish Universities Environmental Research Centre (SUERC), Rankine Avenue, East Kilbride G75 0QF, UK
³Department of Earth & Environmental Science, University of St Andrews, St Andrews, KY16 9AJ, UK
⁴Institute for Earth and Space Exploration, University of Western Ontario, 1151 Richmond Street, London, ON, Canada
⁵Department of Earth Sciences, University of Western Ontario, 1151 Richmond Street, London, ON, Canada
Copyright Elsevier

The age of the Gow Lake impact structure (Saskatchewan, Canada) is poorly constrained, with previous estimates ranging from 100 to 250 Ma. Using a combination of step-heating and UV laser in situ ⁴⁰Ar/³⁹Ar analyses we have sought to understand the ⁴⁰Ar/³⁹Ar systematics of this small impact crater and obtain a more precise and accurate age. This structure is challenging for ⁴⁰Ar/³⁹Ar geochronology due to its small size (∼5 km diameter), the silicic composition of the target rock, and the large difference in age between the impact event and the target rock (∼1.2 Ga). These factors can serve to inhibit argon mobility in impact melts, leading to retention of ‘extraneous’ ⁴⁰Ar and anomalously older measured ages. We mitigated the undesirable effects of extraneous ⁴⁰Ar retention by analysing small volume aliquots of impact glass using step-heating and even smaller volumes via the UV laser in situ ⁴⁰Ar/³⁹Ar technique. Although primary hydration of impact-generated glasses enhanced the diffusivity of ⁴⁰Ar inherited from silica-rich melts, data still had to be corrected for extraneous ⁴⁰Ar by using isotope correlation plots to define the initial trapped ⁴⁰Ar/³⁶Ar components. Our inverse isochron age of 196.8 ± 9.6/9.9 Ma (2σ, analytical/external precision) demonstrates that the Gow Lake event occurred within uncertainty of the Triassic-Jurassic boundary, but there is no evidence that it was part of an impact cluster.

^1,2Janice L.Bishop,²Christoph Gross,^1,3 Jacob Danielsen,⁴Mario Parente,⁵Scott L.Murchie,⁶ Briony Horgan,⁷James J.Wray,⁶Christina Viviano,⁶Frank P.Seelos
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2020.113634]
¹SETI Institute, Mountain View, CA, United States of America
²Freie Universität Berlin, Berlin, Germany
³San Jose State University, San Jose, CA, United States of America
⁴University of Massachusetts at Amherst, Amherst, MA, United States of America
⁵Johns Hopkins University Applied Physics Lab, Laurel, MD, United States of America
⁶Purdue University, West Lafayette, IN, United States of America
⁷Georgia Institute of Technology, Atlanta, GA, United States of America
Copyright Elsevier

Refined calibrations of CRISM images are enabling identification of smaller deposits of unique aqueous materials on Mars that reveal changing environmental conditions at the region surrounding Mawrth Vallis. Through characterization of these clay-sulfate assemblages and their association with the layered, phyllosilicate units of this region, more details of the aqueous geochemical history can be gleaned. A stratigraphy including five distinct mineral horizons is mapped using compositional data from CRISM over CTX and HRSC imagery across 100s of km and from CRISM over HiRISE imagery across 100s of meters. Transitions in mineralogic units were characterized using visible/near-infrared (VNIR) spectral properties and surface morphology. We identified and characterized complex “doublet” type spectral signatures with two bands between 2.2 and 2.3 μm at one stratigraphic horizon. Based on comparisons with terrestrial sites, the spectral “doublet” unit described here may reflect the remnants of a salty, evaporative period that existed on Mars during the transition from formation of Fe-rich phyllosilicates to Al-rich phyllosilicates. Layered outcrops observed at Mawrth Vallis are thicker than in other altered regions of Mars, but may represent processes that were more widespread in wet regions of the planet during its early history. The aqueous geochemical environments supporting the outcrops observed here include: (i) the formation of Fe³⁺-rich smectites in a warm and wet environment, (ii) overlain by a thin ferrous-bearing clay unit that could be associated with heating or reducing conditions, (iii) followed by a transition to salty and/or acidic alteration phases and sulfates (characterized by the spectral “doublet” shape) in an evaporative setting, (iv) formation of Al-rich phyllosilicates through pedogenesis or acid leaching, and (v) finally persistence of poorly crystalline aluminosilicates marking the end of the warm climate on early Mars. The “doublet” type units described here are likely composed of clay-sulfate assemblages formed in saline, acidic evaporative environments similar to those found in Western Australia and the Atacama desert. Despite the chemically extreme and variable waters present at these terrestrial, saline lake environments, active ecosystems are present; thus, these “doublet” type units may mark exciting areas for continued exploration important to astrobiology on Mars.

¹Ebihara, M.,¹Hayano,¹K.,¹Shirai, N.
Analytica Chimica Acta (in Press) Link to Article [DOI: 10.1016/j.aca.2019.12.031]
¹Department of Chemistry, Graduate School of Science, Tokyo Metropolitan University, 1-1 Minami-osawa, Hachioji, Tokyo, 192-0397, Japan

We currently do not have a copyright agreement with this publisher and cannot display the abstract here

¹Pincelli M. Hull et al. (>10)
Science 367, 266-272 Link to Article [DOI: 10.1126/science.aay5055]
¹Department of Geology and Geophysics, Yale University, New Haven, CT 06511, USA.
Reprinted with Permission of AAAS

The cause of the end-Cretaceous mass extinction is vigorously debated, owing to the occurrence of a very large bolide impact and flood basalt volcanism near the boundary. Disentangling their relative importance is complicated by uncertainty regarding kill mechanisms and the relative timing of volcanogenic outgassing, impact, and extinction. We used carbon cycle modeling and paleotemperature records to constrain the timing of volcanogenic outgassing. We found support for major outgassing beginning and ending distinctly before the impact, with only the impact coinciding with mass extinction and biologically amplified carbon cycle change. Our models show that these extinction-related carbon cycle changes would have allowed the ocean to absorb massive amounts of carbon dioxide, thus limiting the global warming otherwise expected from postextinction volcanism.