Petrogenesis of basaltic shergottite Northwest Africa 8657: Implications for fO2 correlations and element redistribution during shock melting in shergottites

1Geoffrey H. Howarth,2Arya Udry,3James M. D. Day
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12999]
1Department of Geological Sciences, University of Cape Town, Rondebosch, South Africa
2Department of Geoscience, University of Nevada Las Vegas, Las Vegas, Nevada, USA
3Geosciences Research Division, Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, USA
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 8657 is an incompatible trace element-enriched, low-Al basaltic shergottite, similar in texture and chemistry to Shergotty, Zagami, and NWA 5298. It is composed of zoned pyroxene, maskelynite, merrillite, and Ti-oxide minerals with minor apatite, silica, and pyrrhotite. Pyroxene grains are characterized by patchy zoning, with pigeonite or augite cores zoned to Fe-rich pigeonite mantles. The cores have rounded morphologies and irregular margins. Combined with the low Ti/Al of the cores, the morphology and chemistry of the pyroxene grains are consistent with initial crystallization at depth (30–70 km) followed by partial resorption en route to the surface. Enriched rare earth element (REE) equilibrium melt compositions and calculated oxygen fugacities (fO2) conditions for pigeonite cores indicate that the original parent melts were enriched shergottite magmas that staged in chambers at depth within the Martian crust. NWA 8657 does not represent a liquid but rather entrained a proportion of pyroxene crystals from magma chambers where fractional crystallization was occurring at depth. Variation between fO2 and bulk-rock (La/Yb)N of the enriched and intermediate shergottites suggests that oxidation conditions and degree of incompatible element enrichment in the source may not be correlated, as thought previously. Shock melt pockets are characterized by an absence of phosphates and oxide minerals. It is likely that these phases were melted during shock. REEs were redistributed during this process into maskelynite and to a lesser extent the shock melt; however, the overall normalized REE profile of the shock melt is like that of the bulk-rock, but at lower absolute concentrations. Overall, shock melting has had a significant effect on the mineralogy of NWA 8657, especially the distribution of phosphates, which may be significant for geochronological applications of this meteorite and other Martian meteorites with extensive shock melt.

Dioctahedral phyllosilicates versus zeolites and carbonates versus zeolites competitions as constraints to understanding early Mars alteration conditions

1Jean-Cristophe Viennet,1Benjamin Bultel,2Lucie Riu,1Stephanie C. Werner
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005343]
1Centre for Earth Evolution and Dynamics, Department for Geosciences, University of Oslo, Norway
2Institut d’Astrophysique Spatiale, Université Paris-Sud, Orsay, France
Published by arrangement with John Wiley & Sons

Widespread occurrence of Fe,Mg-phyllosilicates have been observed on Noachian Martian terrains. Therefore, the study of Fe,Mg-phyllosilicates formation, in order to characterize early Martian environmental conditions, is of particular interest to the Martian community. Previous studies have shown that the investigation of Fe,Mg-smectite formation alone helps to describe early Mars environmental conditions, but there are still large uncertainties in terms of pH range, oxic/anoxic conditions, etc… Interestingly, carbonates and/or zeolites have also been observed on Noachian surfaces in association with the Fe,Mg-phyllosilicates.

Consequently, the present study focuses on the di/trioctahedral phyllosilicate/carbonate/zeolite formation as a function of various CO2 contents (100% N2, 10% CO2 / 90% N2, 100% CO2), from a combined approach including closed system laboratory experiments for 3 weeks at 120°C and geochemical simulations. The experimental results show that as the CO2 content decreases, the amount of dioctahedral clay minerals decreases in favour of trioctahedral minerals. Carbonates and dioctahedral clay minerals are formed during the experiments with CO2. When Ca-zeolites are formed, no carbonates and dioctahedral minerals are observed. Geochemical simulation aided in establishing pH as a key parameter in determining mineral formation patterns. Indeed, under acidic conditions dioctahedral clay minerals and carbonate minerals are formed, while trioctahedral clay minerals are formed in basic conditions with a neutral pH value of 5.98 at 120°C. Zeolites are favoured from pH >~7.2. The results obtained shed new light on the importance of dioctahedral clay minerals versus zeolites and carbonates versus zeolites competitions, to better define the aqueous alteration processes throughout early Mars history.

Impact ionisation mass spectrometry of platinum-coated olivine and magnesite-dominated cosmic dust analogues

1,2Jon K.Hillier, 3Z.Sternovsky,3S.Kempf, 2M.Trieloff, 2M.Guglielmino, 2F.Postberg, 1M.C.Price
Planetary and Space Science (in Press) Link to Article []
1Centre for Astrophysics and Planetary Science, University of Kent, Canterbury, Kent CT2 7NH, UK
2Klaus-Tschira-Labor für Kosmochemie, Institut für Geowissenschaften, Im Neuenheimer Feld 234-236, Universität Heidelberg, 69120 Heidelberg, Germany
3Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, CO 80303, USA

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Carbonate dissolution rates in high salinity brines: Implications for post-Noachian chemical weathering on Mars

1Charity M.Phillips-Lander, 2S.R.Parnell, 3L.E.McGraw, 4M.E.Elwood Madden
Icarus (in Press) Link to Article []
1School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd St., Norman, OK 73071, USA
Copyright Elsevier

A diverse suite of carbonate minerals including calcite (CaCO3) and magnesite (MgCO3) have been observed on the martian surface and in meteorites. Terrestrial carbonates usually form via aqueous processes and often record information about the environment in which they formed, including chemical and textural biosignatures. In addition, terrestrial carbonates are often found in association with evaporite deposits on Earth. Similar high salinity environments and processes were likely active on Mars and some areas may contain active high salinity brines today. In this study, we directly compare calcite and magnesite dissolution in ultrapure water, dilute sulfate and chloride solutions, as well as near-saturated sulfate and chloride brines with known activity of water (ɑH2O) to determine how dissolution rates vary with mineralogy and ɑH2O, as well as aqueous cation and anion chemistry to better understand how high salinity fluids may have altered carbonate deposits on Mars. We measured both calcite and magnesite initial dissolution rates at 298 K and near neutral pH (6–8) in unbuffered solutions containing ultrapure water (18 MΩ cm−1 UPW; ɑH2O = 1), dilute (0.1 mol kg−1; ɑH2O = 1) and near-saturated Na2SO4 (2.5 mol kg−1, ɑH2O = 0.92), dilute (0.1 mol kg−1, ɑH2O = 1) and near-saturated NaCl (5.7 mol kg−1, ɑH2O = 0.75). Calcite dissolution rates were also measured in dilute and near-saturated MgSO4 (0.1 mol kg−1, ɑH2O = 1 and 2.7 mol kg−1, ɑH2O = 0.92, respectively) and MgCl2 (0.1 mol kg−1, ɑH2O = 1 and 3 mol kg−1, ɑH2O = 0.73, respectively), while magnesite dissolution rates were measured in dilute and near-saturated CaCl2 (0.1 mol kg−1, ɑH2O = 1 and 9 mol kg−1, ɑH2O = 0.35).
Initial calcite dissolution rates were fastest in near-saturated MgCl2 brine, while magnesite dissolution rates were fastest in dilute (0.1 mol kg−1) NaCl and CaCl2 solutions. Calcite dissolution rates in near-saturated Na2SO4 were similar to those observed in the dilute solutions (−8.00 ± 0.12 log mol m−2 s−1), while dissolution slowed in both NaCl solutions (0.1 mol kg−1; −8.23 ± 0.10 log mol m−2 s−1 and (5.7 mol kg−1; −8.44 ± 0.11 log mol m−2 s−1), as well as near-saturated MgSO4 brine (2.7 mol kg−1; −8.35 ± 0.05 log mol m−2 s−1). The slowest calcite dissolution rates observed in the near-saturated NaCl brine. Magnesite dissolution rates were ∼5 times faster in the dilute salt solutions relative to UPW, but similar to UPW (−8.47 ± 0.06 log mol m−2 s−1) in near-saturated Na2SO4 brines (−8.41 ± 0.18 log mol m−2 s−1). Magnesite dissolution slowed significantly in near-saturated CaCl2 brine (−9.78 ± 0.10 log mol m−2 s−1), likely due to the significantly lower water activity in these experiments. Overall, magnesite dissolution rates are slower than calcite dissolution rates and follow the trend: All dilute salt solutions >2.5 mol kg−1 Na2SO4 ≈ UPW > 5.7 mol kg−1 NaCl >> 9 mol kg−1 CaCl2. Calcite rates follow the trend 3 mol kg−1 MgCl2 > 2.5 mol kg−1 Na2SO4 ≈ UPW ≈ all dilute salt solutions >2.7 mol kg−1 MgSO4 ≈ 5.7 mol kg−1 NaCl. Magnesite dissolution rates in salt solutions generally decrease with decreasing ɑH2O in both chloride and sulfate brines, which indicates water molecules act as ligands and participate in the rate-limiting magnesite dissolution step. However, there is no general trend associated with water activity observed in the calcite dissolution rates. Calcite dissolution accelerates in near-saturated MgCl2, but slows in near-saturated NaCl brine despite both brines having similar water activities (ɑH2O = 0.73 and 0.75, respectively). High Mg calcite was observed as a reaction product in the near-saturated MgCl2, indicating Mg2+ from solution likely substituted for Ca2+ in the initial calcite, releasing additional Ca2+ into solution and increasing the observed calcite dissolution rate. Calcite dissolution rates also increase slightly as Na2SO4 concentration increases, while calcite dissolution rates slow slightly with increasing concentration of MgSO4 and NaCl. However, all of the carbonate rates vary by less than 0.5 log units and are within or near the standard deviation observed for each set of replicate experiments.
Carbonate mineral lifetimes in high salinity brines indicate magnesite may be preferentially preserved compared to calcite on Mars. Therefore, Mg-carbonates that have experienced post-depositional aqueous alteration are more likely to preserve paleoenvironmental indicators and potential biosignatures. Rapid weathering of carbonates in circum-neutral pH sulfate brines may provide a potential source of cations for abundant sulfate minerals observed on Mars, Ceres, and other planetary bodies.

Radial mixing and Ru–Mo isotope systematics under different accretion scenarios

1,2,3Rebecca A. Fischer, 2Francis Nimmo, 4David P. O’Brien
Earth and Planetary Science Letters 482, 105-114 Link to Article []
1Smithsonian Institution, National Museum of Natural History, Department of Mineral Sciences, United States
2University of California Santa Cruz, Department of Earth and Planetary Sciences, United States
3Harvard University, Department of Earth and Planetary Sciences, United States
4Planetary Science Institute, United States
Copyright Elsevier

The Ru–Mo isotopic compositions of inner Solar System bodies may reflect the provenance of accreted material and how it evolved with time, both of which are controlled by the accretion scenario these bodies experienced. Here we use a total of 116 N-body simulations of terrestrial planet accretion, run in the Eccentric Jupiter and Saturn (EJS), Circular Jupiter and Saturn (CJS), and Grand Tack scenarios, to model the Ru–Mo anomalies of Earth, Mars, and Theia analogues. This model starts by applying an initial step function in Ru–Mo isotopic composition, with compositions reflecting those in meteorites, and traces compositional evolution as planets accrete. The mass-weighted provenance of the resulting planets reveals more radial mixing in Grand Tack simulations than in EJS/CJS simulations, and more efficient mixing among late-accreted material than during the main phase of accretion in EJS/CJS simulations. We find that an extensive homogeneous inner disk region is required to reproduce Earth’s observed Ru–Mo composition. EJS/CJS simulations require a homogeneous reservoir in the inner disk extending to ≥3–4 AU (≥74–98% of initial mass) to reproduce Earth’s composition, while Grand Tack simulations require a homogeneous reservoir extending to ≥3–10 AU (≥97–99% of initial mass), and likely to ≥6–10 AU. In the Grand Tack model, Jupiter’s initial location (the most likely location for a discontinuity in isotopic composition) is ∼3.5 AU; however, this step location has only a 33% likelihood of producing an Earth with the correct Ru–Mo isotopic signature for the most plausible model conditions. Our results give the testable predictions that Mars has zero Ru anomaly and small or zero Mo anomaly, and the Moon has zero Mo anomaly. These predictions are insensitive to wide variations in parameter choices.

Evidence for accretion of fine-grained rims in a turbulent nebula for CM Murchison

1Romy D. Hanna, 1Richard A. Ketcham
Earth and Planetary Science Letters 481, 201-211 Link to Article []
1Jackson School of Geosciences, University of Texas, Austin, TX 78712, USA
Copyright Elsevier

We use X-ray computed tomography (XCT) to examine the 3D morphology and spatial relationship of fine-grained rims (FGRs) of Type I chondrules in the CM carbonaceous chondrite Murchison to investigate the formation setting (nebular vs. parent body) of the FGRs. We quantify the sizes, shapes, and orientations of the chondrules and FGRs and develop a new algorithm to examine the 3D variation of FGR thickness around each chondrule. We find that the average proportion of chondrule volume contained in the rim for Murchison chondrules is 35.9%. The FGR volume in relation to the interior chondrule radius is well described by a power law function as proposed for accretion of FGRs in a weakly turbulent nebula by Cuzzi (2004). The power law exponent indicates that the rimmed chondrules behaved as Stokes number Stη>1 nebular particles in Kolmogorov η scale turbulence. FGR composition as inferred from XCT number appears essentially uniform across interior chondrule types and compositions, making formation by chondrule alteration unlikely. We determine that the FGRs were compressed by the impact event(s) that deformed Murchison ( Hanna et al., 2015), resulting in rims that are thicker in the plane of foliation but that still preserve their nebular morphological signature. Finally, we propose that the irregular shape of some chondrules in Murchison is a primary feature resulting from chondrule formation and that chondrules with a high degree of surface roughness accreted a relatively larger amount of nebular dust compared to smoother chondrules.

The selenium isotopic variations in chondrites are mass-dependent; Implications for sulfide formation in the early solar system

1J. Labidi, 1S. König, 1T. Kurzawa, 1A. Yierpan, 1R. Schoenberg
Earth and Planetary Science Letters 481, 212-222 Link to Article []
1Isotope Geochemistry, Department of Geosciences, Eberhard Karls Universität Tübingen, Wilhelmstraße 56, 72074 Tübingen, Germany
Copyright Elsevier

Element transfer from the solar nebular gas to solids occurred either through direct condensation or via heterogeneous reactions between gaseous molecules and previously condensed solid matter. The precursors of altered sulfides observed in chondrites are for example attributed to reactions between gaseous hydrogen sulfide and metallic iron grains. The transfer of selenium to solids likely occurred through a similar pathway, allowing the formation of iron selenides concomitantly with sulfides. The formation rate of sulfide however remains difficult to assess. Here we investigate whether the Se isotopic composition of meteorites contributes to constrain sulfide formation during condensation stages of our solar system. We present high precision Se concentration and δ82/78Se data for 23 chondrites as well as the first δ74/78Se, δ76/78Se and δ77/78Se data for a sub-set of seven chondrites. We combine our dataset with previously published sulfur isotopic data and discuss aspects of sulfide formation for various types of chondrites.

Our Se concentration data are within uncertainty to literature values and are consistent with sulfides being the dominant selenium host in chondrites. Our overall average δ82/78Se value for chondrites is −0.21±0.43‰ (n=23, 2 s.d.), or −0.14±0.21‰ after exclusion of three weathered chondrites (n=20, 2 s.d.). These average values are within uncertainty indistinguishable from a previously published estimate. For the first time however, we resolve distinct δ82/78Se between ordinary (−0.14±0.07‰, n=9, 2 s.d.), enstatite (−0.27±0.05‰, n=3, 2 s.d.) and CI carbonaceous chondrites (−0.01±0.06‰, n=2, 2 s.d.). We also resolve a Se isotopic variability among CM carbonaceous chondrites. In addition, we report on δ74/78Se, δ76/78Se and δ77/78Se values determined for 7 chondrites. Our data allow evaluating the mass dependency of the δ82/78Se variations. Mass-independent deficits ro excesses of 74Se, 76Se and 77Se are calculated relative to the observed 82Se/78Se ratios, and were observed negligible. This rules out poor mixing of nucleosynthetic components to account for the δ82/78Se variability and implies that the mass dependent Se isotopic variations were produced in a once-homogeneous disk.

The mass-dependent isotopic difference between enstatite and ordinary chondrites may reflect the contribution of a kinetic sulfidation process at anomalously high H2S–H2Se contents in the region of enstatite chondrite formation. Experimental studies showed that high H2S contents favor the formation of compact sulfide layers around metallic grains. This decreases the reactive surface, which tends to inhibit the continuation of the sulfidation reaction. Under these conditions sulfide growth likely occurs under isotopic disequilibrium and favors the trapping of light S and Se isotopes in solids; This hypothesis provides an explanation for our Se isotope as well as for previously published S isotope data. On the other hand, high δ82/78Se values in carbonaceous chondrites may result from sample heterogeneities generated by parent body aqueous alteration, or could reflect the contribution of ices carrying photo-processed Se from the outer solar system.