^1,2Alan E.Rubin,^1,3,4YeLia
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.009]
¹Department of Earth, Planetary & Space Sciences, University of California, Los Angeles, CA, 90095-1567, USA
²Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, ME, 04217, USA
³Key Laboratory of Planetary Sciences, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing, 210034, China
⁴Chinese Academy of Sciences Center for Excellence in Comparative Planetology, China
Copyright Elsevier

Primitive CO3.00–3.1 chondrites contain ˜2-8 vol.% magnetite, minor troilite and accessory carbide and chromite; some CO3.1 chondrites have fayalite-rich veins, chondrule rims and euhedral matrix grains. All CO3.00–3.1 chondrites contain little metallic Fe-Ni (0.4–1.2 vol.%). CO3.2–3.7 chondrites contain 1–5 vol.% metallic Fe-Ni, minor troilite, accessory chromite and 0-0.6 vol.% magnetite. Magnetite is formed in primitive CO3 chondrites from metallic Fe by parent-body aqueous alteration, resulting in decreased metallic Fe-Ni and an increase in the proportion of high-Ni metal grains. The paucity or absence of magnetite in CO chondrites of subtype ≥3.2 suggests that magnetite is destroyed during thermal metamorphism; thermochemical calculations from the literature suggest that magnetite is reduced by H₂ and reacts with SiO₂ to form fayalite and secondary kamacite. Analogous processes of magnetite formation and destruction occur in other chondrite groups: (1) Primitive type-3 OC have opaque assemblages containing magnetite, carbide, Ni-rich metal and Ni-rich sulfide, but OC of subtype >3.4 contain little or no magnetite. (2) Primitive R3 chondrites and clasts (subtype ≲3.5) contain up to 6 vol.% magnetite, but most R chondrites contain no magnetite. The principal exception is magnetite with 9–20 wt.% Cr₂O₃ in a few R4-6 chondrites. Magnetite grains with high Cr₂O₃ behave like chromite and are more stable under reducing conditions. (3) CK chondrites average ˜4 vol.% magnetite with substantial Cr₂O₃ (up to ˜15 wt.%); these magnetite grains also are stable against reduction during metamorphism. (4) The modal abundance of magnetite decreases with metamorphic grade in CV3 chondrites. (5) Chromite occurs instead of magnetite in those rare samples classified CR6, CR7 and CV7.

¹Addi Bischoff et al. (>10)
Geochemistry (Chemie der Erde) (In Press) Link to Article [https://doi.org/10.1016/j.chemer.2019.07.007]
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm Str. 10, D-48149, Münster, Germany
Copyright Elsevier

On July 10, 2018 at 21:29 UT extended areas of South-Western Germany were illuminated by a very bright bolide. This fireball was recorded by instruments of the European Fireball Network (EN). The records enabled complex and precise description of this event including the prediction of the impact area. So far six meteorites totaling about 1.23 kg have been found in the predicted location for a given mass during dedicated searches. The first piece of about 12 g was recovered on July 24 close to the village of Renchen (Baden-Württemberg) followed by the largest fragment of 955 g on July 31 about five km north-west of Renchen.
Renchen is a moderately-shocked (S4) breccia consisting of abundant highly recrystallized rock fragments as well as impact melt rock clasts. The texture, the large grain size of plagioclase, and the homogeneous compositions of olivine (˜Fa26) and pyroxene (˜Fs22) clearly indicate that Renchen is composed of metamorphosed rock fragments (L5-6). An L-group (and ordinary chondrite) heritage is consistent with the data on the model abundance of metal, the density, the magnetic susceptibility as well as on O-, Ti-, and Cr-isotope characteristics. Renchen does not contain solar wind implanted noble gases and is a fragmental breccia. An unusually large mm-sized merrillite-apatite aggregate shows trace element characteristics like other phosphates from ordinary chondrites.
Data on the bulk chemistry, IR-spectroscopy, cosmogenic nuclides, and organic components also indicate similarities to other metamorphosed L chondrites. Noble gas studies reveal that the meteorite has a cosmic ray exposure (CRE) age of 42 Ma and that most of the cosmogenic gases were produced in a meteoroid with a radius of at max. 20 cm based on the radionuclide 26Al and 10-150 cm based on cosmogenic 22Ne/21Ne. K-Ar and U/Th-He gas retention ages are both in the range ˜3.0 to 3.2 Ga. Both systems do not show evidence for a complete reset 470 Ma ago, and may instead have recorded the same resetting event 3.0 Ga ago.

^1,2,3E.S.Steenstra,²V.T.Trautner,³J.Berndt,³S.Klemme,²W.van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113408]
¹The Geophysical Laboratory, Carnegie Institution of Science, Washington, DC, United States of America
²Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
³Institute of Mineralogy, University of Münster, Germany
Copyright Elsevier

Chalcophile and siderophile element abundances are used to provide important constraints on the interior compositions of planetary bodies as well as the pressure (P) – temperature (T) conditions that prevailed during core formation. The oxygen fugacity (fO₂) during core formation varied considerably between the various terrestrial planets and asteroidal bodies in our solar system. Mercury, the aubrite parent body (AuPB) and some terrestrial precursor bodies may have differentiated at highly reduced conditions.

At present knowledge about how the metal liquid-silicate melt and sulfide liquid-silicate melt partitioning behavior of major and trace elements are affected by high S concentrations in the silicate melt at highly reducing conditions is incomplete. Here, we experimentally study the metal-silicate and sulfide-silicate partitioning behavior of trace elements in reduced silicate melts over a wide range of S contents as a function of redox state at 1 GPa and 1833–1883 K. Silicate melt S contents ranged between ~0.5 and ~20 wt%, with a corresponding silicate FeO range of ~0.4 to ~17.5 wt%, in a fO₂ range between 1 and 9 log units below the iron-wüstite buffer. Our results reproduce the decrease of the S concentration at sulfide saturation (SCSS) with decreasing FeO contents down to ~3 wt%, as well as its strong increase at <3 wt% FeO. At S contents exceeding >6–9 wt% S, the FeO contents increase again.

Results show that most elements (Mg, Ti, V, Cr, Mn, Cu, Zn, Se, Nb, Cd, Sb, Te, Ta, Tl, Pb and Bi) are more chalcophile than siderophile at reducing conditions, whereas Si, Co, Ni, Ga, Ge, Mo and W preferentially partition into Fe-rich melts instead of sulfide liquids. Silicon, Ti, Se, and Te preferentially partition into FeS over (Fe,Mg,Ca)-S liquids, whereas Mn, Zn and Cd are more compatible in the latter. As proposed by Wood and Kiseeva (2015), chalcophile elements such as Cu, Se and Te behave less chalcophile with increasing S concentrations of the silicate melt, whereas the opposite is observed for nominally lithophile elements such as Mg, Ca and Ti.

The results can be used to improve interpretations of the observed trace element systematics of aubrites and other reduced achondrites. All of the volatile elements considered here behave chalcophile at the reducing conditions inferred for differentiation of the AuPB. A significant degree of the observed volatile element depletions in aubrites may therefore reflect their preferential partitioning into sulfide liquids, rather than degassing during or after differentiation of the AuPB. These results suggest that, depending of the extent of core merging, precursor body differentiation and the efficiency of sulfide liquid segregation, reduced precursor bodies that were incorporated in the early Earth were likely more rich in volatile elements than currently assumed.

¹A.Stephant et al. (>10)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2019.07.045]
¹School of Physical Sciences, The Open University, Milton Keynes, UK
Copyright Elsevier

Northwest Africa (NWA) 10989 is a recently found lunar meteorite, we used to elucidate the history of volatiles (H and Cl) in the Moon through analysis of its phosphates. The petrology, bulk geochemistry and mineralogy of NWA 10989 are consistent with it being a lunar meteorite with intermediate-iron bulk composition, composed of 40% of mare basaltic material and ∼60% non-mare material, but with no obvious KREEP-rich basaltic components. It is probable that the source region for this meteorite resides near a mare–highlands boundary, possibly on the farside of the Moon. Analyses of chlorine and hydrogen abundances and isotopic composition in apatite and merrillite grains from NWA 10989 indicate sampling of at least two distinct reservoirs of volatiles, one being similar to those for known mare basalts from the Apollo collections, while the other potentially represents a yet unrecognized reservoir. In situ Th-U-Pb dating of phosphates reveal two distinct age clusters with one ranging from 3.98±0.04 to 4.20±0.02 Ga, similar to the ages of cryptomare material, and the other ranging from 3.32±0.01 to 3.96±0.03 Ga, closer to the ages of mare basalts known from the Apollo collections. This lunar breccia features mixing of material, among which a basaltic D-poor volatile reservoir which doesn’t appear to have been recorded by Apollo samples.

¹Andreas Morlok,^2,3Christopher Hamann,⁴Dayl Martin,¹Iris Weber,⁴Katherine H.Joy,¹Harald Hiesinger,⁴Roy Wogelius,¹Aleksandra Stojic,⁵Joern Helbert
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113410]
¹Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Museum für Naturkunde, 10115 Berlin, Germany
³Bundesanstalt für Materialforschung und –prüfung, 12489 Berlin, Germany
⁴School of Earth and Environmental Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
⁵Institute for Planetary Research, DLR, Rutherfordstrasse 2, 12489 Berlin, Germany
Copyright Elsevier

We obtained mid-infrared spectra and major-element analyses of glasses produced in pulsed laser experiments of basalt. Materials from pits excavated in a basalt slab, as well as of a larger, separated melt droplet were studied. The results of this study show that these glasses exhibits spectral features clearly distinguishable from the unprocessed starting material. Spectra and chemistry show changes, which could be the result of not only melting but also vaporization.

Christiansen Features (CF) for the melt glass in the laser-excavated pits are at 8.3–8.5 μm, and a dominating Reststrahlen Band (RB) at 10.1–10.5 μm in wavelength. The spectra of the powdered glass droplet has a CF at 8.8–8.9 μm and a RB at 10.3–10.5 μm. The spectra are clearly different from the spectra of the surrounding starting material, which shows CF between 8.0 and 8.3 μm, and ample RBs between 9.3 μm and 14.7 μm, typical olivine, plagioclase and pyroxene features.

The results reflect the chemical composition, which shows significant losses of volatiles like K2O and Na₂O, as well as of moderate volatiles like FeO, SiO₂, and MgO. Refractories TiO₂, Al₂O₃, and CaO tend to be enriched compared to the bulk starting composition. This indicates loss of material through evaporation.

While the spectra of size fractions of the powdered bulk melt glass droplet follow this trend in general, but, because of contamination by the experimental set-up, CaO was found to be strongly enriched in contrast to the other refractories TiO₂ and Al₂O₃.

At least the composition of the glasses in the laser-excavated pits could serve as an ‘endmember’ for the sequence of glassy materials expected to be produced in high energy impact processes involving a basaltic target.

Correlation of CF with SiO₂ contents and the SCFM (SiO₂/(SiO₂ + CaO + FeO + MgO)) index show similar behaviour of the pit melts like found in earlier studies. However, when the position of the RB in the pit glass is correlated with the SiO₂ content, the result shows a different trend compared with earlier studies. Consequently, the data presented in this study could help distinguishing between surface regions formed by volcanic processes and such modified by high-velocity impacts, where evaporation could play a central role.

This is of high interest for remote sensing studies of Mercury, which, because of its proximity to the Sun, was probably affected by high-velocity impacts to a very high degree.

¹Mikhail Yu.Zolotov
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113404]
¹School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287-1404, USA
Copyright Icarus

Results of Ceres’ exploration with the Dawn spacecraft are modeled and discussed in terms of rock/organic/elemental composition, density and porosity in the interior, and formation, migration and geological evolution of the body. Carbon-rich surface composition is used to assess phase and elemental composition of the interior. The consistent bulk density and surface composition suggest an abundant organic matter within the body. Ceres is modeled as a chemically uniform mixture of CI-type carbonaceous chondritic rocks and 12–29 vol% of macromolecular organic matter. Water ice, gas hydrates or high porosity (>10%) are not required to explain bulk density. Ceres may not have a partially differentiated interior structure because gravity and shape could be explained by compaction of chemically uniform materials. Gravity data suggest a two-layer structure with an abrupt density change. Gravity may not reflect the current global density distribution in the interior because the implied bulk porosity >9% and grain density > 2380 kg m⁻³ disagree with organic-rich compositions. In contrast, Ceres’ polar flattening indicates mild density gradients that could be explained by two-layer and gradual compaction models. The flattening implies grain density of 2200–2350 kg m⁻³ that is consistent with the organic-rich interior. Viscosity of warmed rock-organic mixtures at depth could account for the observed relaxation of long wavelength topography. The organic-rich composition together with abundant surface carbonates, NH₄-bearing phases suggests Ceres’ formation at larger heliocentric distances and later than CI chondrites. Ceres-forming materials could have been more water-rich than parent bodies of CI chondrites and excessive water could have been lost from the body. A majority of Ceres’ surface compounds could have formed through water-rock-organic reactions in a middle interior followed by collisional stripping of an upper interior.

^1,2,3Meng‐Hua Zhu,²Kai Wünnemann,⁴Ross W.K. Potter,⁵Thorsten Kleine,⁶Alessandro Morbidelli
Journal of Geophysical Research, Planets (in Press) Link to Article [https://doi.org/10.1029/2018JE005826]
¹Space Science Institute, Macau University of Science and Technology, Taipa, Macau
²Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
³CAS Center for Excellence in Comparative Planetology, China
⁴Department of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI, USA
⁵Institut für Planetologie, University of Münster, Münster, Germany
⁶Département Lagrange, University of Nice–Sophia Antipolis, Nice, France
Published by arrangement with John Wiley & Sons

The Moon exhibits striking geological asymmetries in elevation, crustal thickness, and composition between its nearside and farside. Although several scenarios have been proposed to explain these asymmetries, their origin remains debated. Recent remote sensing observations suggest that (1) the crust on the farside highlands consists of two layers: a primary anorthositic layer with thickness of ~30‐50 km and on top a more mafic‐rich layer ~ 10 km thick; and (2) the nearside exhibits a large area of low‐Ca pyroxene that has been interpreted to have an impact origin. These observations support the idea that the lunar nearside‐farside asymmetries may be the result of a giant impact. Here, using quantitative numerical modeling, we test the hypothesis that a giant impact on the early Moon can explain the striking differences in elevation, crustal thickness, and composition between the nearside and farside of the Moon. We find that a large impactor, impacting the current nearside with a low velocity, can form a mega‐basin and reproduce the characteristics of the crustal asymmetry and structures comparable to those observed on the current Moon, including the nearside lowlands and the farside’s mafic‐rich layer on top of a primordial anorthositic crust. Our model shows that the excavated deep‐seated KREEP (potassium, rare‐earth elements, and phosphorus) material, deposited close to the basin rim, slumps back into the basin and covers the entire basin floor; subsequent large impacts can transport the shallow KREEP material to the surface, resulting in its observed distribution. In addition, our model suggests that prior to the asymmetry‐forming impact, the Moon may have had an 182W anomaly compared to the immediate post‐giant impact Earth’s mantle, as predicted if the Moon was created through a giant collision with the proto‐Earth.

¹D.S.Vogt,¹S.Schröder,¹ K.Rammelkamp,¹P.B.Hansen,¹S.Kubitza,^1,2H.-W.Hübers
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113393]
¹Deutsches Zentrum für Luft- und Raumfahrt e.V. (DLR), Institut für Optische Sensorsysteme, Berlin, Germany
²Humboldt-Universität zu Berlin, Institut für Physik, Berlin, Germany
Copyright Elsevier

Chlorine and fluorine play an important role in the geological history of Mars due to their high concentration in Martian magmas and their influence on the generation and evolution of Martian basalts. Chlorine-bearing salts could also facilitate the formation of eutectic brines that could be important for the fluvial history of Mars. The LIBS instruments of ChemCam and SuperCam can detect emission lines of Cl and F, but the intensity of these emission lines is comparatively low, making it difficult to quantify them correctly. A promising alternative is the quantification by molecular emission of diatomic molecules like CaCl and CaF, which can be observed as intense molecular bands in LIBS spectra if Ca is also present. However, the nonlinear dependence of the band intensity on the concentrations of both elements needs to be considered. In this study, we expand upon our previous analysis of molecular bands by investigating samples which produce CaCl bands, CaF bands, or both. We find that the highest CaCl band intensities are found in samples containing more Ca than Cl, while the strongest CaF bands are found in samples with roughly equal concentrations of Ca and F. Both observations can be described by the model that we present here. We also find that the CaCl band is significantly stronger for a sample containing CaCl₂ than it is for a sample containing the same concentrations of Ca and Cl in separate bonds. The opposite is true for the CaF band, which is significantly weaker for the sample containing CaF₂ bonds than it is for the sample that does not contain CaF₂ bonds. These matrix effects are partially attributed to fragmentation during the ablation process and differences in the dissociation energies. Furthermore, we observe that CaF formation is not affected by competing CaCl formation, while CaCl is strongly affected by competing CaF formation. All measurements are done in simulated Martian atmospheric conditions in order to assist the analysis of Martian LIBS data.

^1,3E.S.Steenstra,²A.X.Seegers,² R.Putter,³ J.Berndt,³S.Klemme,⁴S.Matveev,¹ E.Bullock,²W.van Westrenen
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2019.113391]
¹The Geophysical Laboratory, Carnegie Institution of Science, Washington D.C., the United States of America
²Faculty of Science, Vrije Universiteit Amsterdam, the Netherlands
³Institute of Mineralogy, University of Münster, Germany
⁴Department of Geosciences, Utrecht University, the Netherlands
Copyright Elsevier

The differences in FeO mantle contents and core masses between the terrestrial planets suggest the oxygen fugacity (fO₂) during their differentiation likely varied significantly. The metal-silicate partitioning of siderophile (iron-loving) elements is a function of fO₂ and of their valence state(s) in silicate melts. Silicon (Si) is known to partition into metal at low fO₂ and has been proposed as a possible light element in the cores of Mercury and the aubrite parent body (AuPB).

To systematically study the metal-silicate partitioning behavior of siderophile elements into Si-bearing metal, 69 high pressure (P) – temperature (T) metal-silicate partitioning experiments were performed under moderately to highly reducing conditions. Oxygen fugacities ranged between 1 and 7 units below the iron-wüstite buffer (ΔIW). Experimental pressures and temperatures ranged between 1 and 5 GPa and 1883 to 2273 K, respectively. A comparison of the ΔIW values and the fO₂ based on the Si-SiO₂ buffer (ΔSi-SiO₂) indicates that the activity coefficient of FeO in silicate melts decreases significantly from reducing to highly reducing conditions under C-saturated conditions. It was found that at conditions more reducing than ΔIW = −3 to −4, the metal-silicate partitioning behavior of the majority of the siderophile elements deviates significantly from values corresponding to their expected valence state(s). These results indicate that the activity in metal of the elements considered, including that of Si itself, is decreased as a function of Si metal content, and a thermodynamic approach was used to quantify these effects. Interaction coefficients of trace elements in Si-bearing, Fe-rich alloys (ε_M^Si) derived from the new experiments are in good agreement with previously proposed values at similar pressures below 5 GPa. However, ε_M^Si values obtained for C-free systems decrease within the 1 to 11 GPa range, suggesting extrapolation of lower-pressure parameters may yield erroneous results at much higher pressures. Altogether, the new results provide an extensive experimental foundation for future studies of planetary differentiation under (highly) reduced conditions.

¹Edwin S.Kite,¹Mohit Melwani Daswani
Earth and Planetary Science Letters 524, 1157118 Link to Article [https://doi.org/10.1016/j.epsl.2019.115718]
¹University of Chicago, Chicago, IL, USA
Copyright Elsevier

Ancient hydrology is recorded by sedimentary rocks on Mars. The most voluminous sedimentary rocks that formed during Mars’ Hesperian period are sulfate-rich rocks, explored by the Opportunity rover from 2004–2012 and soon to be investigated by the Curiosity rover at Gale crater. A leading hypothesis for the origin of these sulfates is that the cations were derived from evaporation of deep-sourced groundwater, as part of a global circulation of groundwater. Global groundwater circulation would imply sustained warm Earthlike conditions on Early Mars. Global circulation of groundwater including infiltration of water initially in equilibrium with Mars’ CO₂ atmosphere implies subsurface formation of carbonate. We find that the CO₂ sequestration implied by the global groundwater hypothesis for the origin of sulfate-rich rocks on Mars is 30–5000 bars if the Opportunity data are representative of Hesperian sulfate-rich rocks, which is so large that (even accounting for volcanic outgassing) it would bury the atmosphere. This disfavors the hypothesis that the cations for Mars’ Hesperian sulfates were derived from upwelling of deep-sourced groundwater. If, instead, Hesperian sulfate-rich rocks are approximated as pure Mg-sulfate (no Fe), then the CO₂ sequestration is 0.3–400 bars. The low end of this range is consistent with the hypothesis that the cations for Mars’ Hesperian sulfates were derived from upwelling of deep-sourced groundwater. In both cases, carbon sequestration by global groundwater circulation actively works to terminate surface habitability, rather than being a passive marker of warm Earthlike conditions. Curiosity will soon be in a position to discriminate between these two hypotheses. Our work links Mars sulfate cation composition, carbon isotopes, and climate change.