¹Jibamitra Ganguly, ²Massimiliano Tirone, ³Kenneth Domanik
Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.07.030]
¹Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA
²Institut für Geologie, Mineralogie, Geophysik, Rühr-Universität, D-44780 Bochum, Germany
³Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
Copyright Elsevier

We have carried out detailed thermometric and cooling history studies of several LL-, L- and H-chondrites of petrologic types 5 and 6. Among the selected samples, the low-temperature cooling of St. Séverin (LL6) has been constrained in an earlier study by thermochronological data to an average rate of ∼2.6 °C/My below 500 °C. However, numerical simulations of the development of Fe-Mg profiles in Opx-Cpx pairs using this cooling rate grossly misfit the measured compositional profiles. Satisfactory simulation of the latter and low temperature thermochronological constraints requires a two-stage cooling model with a cooling rate of ∼50-200 °C/ky from the peak metamorphic temperature of ∼875 °C down to 450 °C, and then transitioning to very slow cooling with an average rate of ∼2.6 °C/My. Similar rapid high temperature cooling rates (200-600 °C/ky) are also required to successfully model the compositional profiles in the Opx-Cpx pairs in the other samples of L5, L6 chondrites. For the H-chondrite samples, the low temperature cooling rates were determined earlier to be 10-20 °C/My by metallographic method. As in St. Séverin, these cooling rates grossly misfit the compositional profiles in the Opx-Cpx pairs. Modeling of these profiles requires very rapid cooling, ∼200-400 °C/ky, from the peak temperatures (∼810-830 °C), transitioning to the metallographic rates at ∼450 – 500 °C. We interpret the rapid high temperature cooling rates to the exposure of the samples to surface or near surface conditions as a result of fragmentation of the parent body by asteroidal impacts. Using the thermochronological data, the timing of the presumed impact is constrained to be ∼4555 – 4560 My before present for St. Séverin (Fig. 3). We also deduced similar two stage cooling models in earlier studies of H-chondrites and mesosiderites that could be explained, using the available geochronological data, by impact induced fragmentation at around the same time. Diffusion kinetic analysis shows that if a lower petrological type got transformed by the thermal effect of shock impacts to reflect higher metamorphic temperature, as has been suggested as a possibility, then the peak temperatures would have had to be sustained for at least 10 ky and 80 ky, respectively, for transformation to the petrologic types 6 and 4. Finally, we present a model that reconciles textural data supporting an onion-shell parent body of H-chondrites with rapid cooling rate at high temperature caused by impact induced disturbance, and also discuss alternatives to the onion shell parent body model.

^1,2Bruce Fegley Jr., ³Nathan S. Jacobson, ²K. B. Williams, ⁴J. M. C. Plane, ⁵L. Schaefer, ^1,2Katharina Lodders
The Astrophysical Journal 824, 103 Link to Article [http://dx.doi.org/10.3847/0004-637X/824/2/103]
¹Planetary Chemistry Laboratory, McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA
²Department of Earth & Planetary Sciences, Washington University, St. Louis, MO 63130, USA
³Materials Division, NASA Glenn Research Center, MS106-1, 21000 Brookpark Road, Cleveland, OH 44135, USA
⁴School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
⁵Harvard—Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Extensive experimental studies show that all major rock-forming elements (e.g., Si, Mg, Fe, Ca, Al, Na, K) dissolve in steam to a greater or lesser extent. We use these results to compute chemical equilibrium abundances of rocky-element-bearing gases in steam atmospheres equilibrated with silicate magma oceans. Rocky elements partition into steam atmospheres as volatile hydroxide gases (e.g., Si(OH)4, Mg(OH)2, Fe(OH)2, Ni(OH)2, Al(OH)3, Ca(OH)2, NaOH, KOH) and via reaction with HF and HCl as volatile halide gases (e.g., NaCl, KCl, CaFOH, CaClOH, FAl(OH)2) in much larger amounts than expected from their vapor pressures over volatile-free solid or molten rock at high temperatures expected for steam atmospheres on the early Earth and hot rocky exoplanets. We quantitatively compute the extent of fractional vaporization by defining gas/magma distribution coefficients and show that Earth’s subsolar Si/Mg ratio may be due to loss of a primordial steam atmosphere. We conclude that hot rocky exoplanets that are undergoing or have undergone escape of steam-bearing atmospheres may experience fractional vaporization and loss of Si, Mg, Fe, Ni, Al, Ca, Na, and K. This loss can modify their bulk composition, density, heat balance, and interior structure.

¹V. Clesi, ¹M.A. Bouhifd, ¹N. Bolfan-Casanova, ¹G. Manthilake, ¹A. Fabbrizio, ¹D. Andrault
Geochimica et Cosmochmica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.07.029]
¹Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS UMR 6524, OPGC-IRD, Campus Universitaire des Cézeaux, 6 Avenue Blaise Pascal, 63178 Aubie‘re Cedex, France
Copyright Elsevier

This study investigates the metal-silicate partitioning of Ni, Co, V, Cr, Mn and Fe during core mantle differentiation of terrestrial planets under hydrous conditions. For this, we equilibrated a molten hydrous CI chondrite model composition with various Fe-rich alloys in the system Fe-C-Ni-Co-Si-S in a multi-anvil over a range of P, T, fO2fO2 and water content (5 – 20 GPa, 2073 – 2500 K, from 1 to 5 log units below the iron-wüstite (IW) buffer and for XH2OXH2O varying from 500 ppm to 1.5 wt%). By comparing the present experiments with the available data sets on dry systems, we observes that the effect of water on the partition coefficients of moderately siderophile elements is only moderate. For example, for iron we observed a decrease in the partition coefficient of Fe (View the MathML sourceDmet/silFe) from 9.5 to 4.3, with increasing water content of the silicate melt, from 0 to 1.44 wt%, respectively. The evolution of metal-silicate partition coefficients of Ni, Co, V, Cr, Mn and Fe are modelled based on sets of empirical parameters. These empirical models are then used to refine the process of core segregation during accretion of Mars and the Earth. It appears that the likely presence of 3.5 wt% water on Mars during the core-mantle segregation could account for ∼∼ 74% of the FeO content of the Martian mantle. In contrast, water does not play such an important role for the Earth; only 4 to 6% of the FeO content of its mantle could be due to the water-induced Fe-oxidation, for a likely initial water concentration of 1.8 wt%. Thus, in order to reproduce the present-day FeO content of 8 wt% in the mantle, the Earth could initially have been accreted from a large fraction (between 85 to 90%) of reducing bodies (similar to EH chondrites), with 10 to 15% of the Earth’s mass likely made of more oxidized components that introduced the major part of water and FeO to the Earth. This high proportion of enstatite chondrites in the original constitution of the Earth is consistent with the 17O17O, 48Ca48Ca, 50Ti50Ti, 62Ni62Ni and 90Mo90Mo isotopic study by Dauphas2014. If we assume that the CI-chondrite was oxidized during accretion, its intrinsically high water content suggests a maximum initial water concentration in the range of 1.2 to 1.8 wt% for the Earth, and 2.5 to 3.5 wt% on Mars.

¹Tasha L. Dunn,^2,3Juliane Gross,⁴Marina A. Ivanova,⁵Simone E. Runyon,⁶Andrea M. BruckMeteoritics & Planetary Sciences (in Press) Link to Article [DOI: 10.1111/maps.12691]
¹Department of Geology, Colby College, Waterville, Maine, USA
²Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
⁴Vernadsky Institute of Geochemistry, Moscow, Russia
⁵Department of Geosciences, University of Arizona, Tucson, Arizona, USA
⁶Department of Chemistry, SUNY Stony Brook, Stony Brook, New York, USA
Published by arrangement with John Wiley & Sons

Bulk isotopic and elemental compositions of CV and CK chondrites have led to the suggestion that both originate from the same asteroid. It has been argued that magnetite compositions also support this model; however, magnetite has been studied almost exclusively in the equilibrated (type 4-6) CKs. Magnetite in seven unequilibrated CKs analyzed here is enriched in MgO, TiO2, and Al2O3 relative to the equilibrated CKs, suggesting that magnetite compositions are affected by metamorphism. Magnetite in CKs is compositionally distinct from CVs, particularly in abundances of Cr2O3, NiO, and TiO2. Although there are minor similarities between CV and equilibrated CK chondrite magnetite, this is contrary to what we would expect if the CVs and CKs represent a single metamorphic sequence. CV magnetite should resemble CK3 magnetite, as both were metamorphosed to type 3 conditions. Oxygen fugacities and temperatures of CVox and CK chondrites are also difficult to reconcile using existing CV-CK parent body models. Mineral chemistries, which eliminate issues of bulk sample heterogeneity, provide a reliable alternative to techniques that involve a small amount of sample material. CV and CK chondrite magnetite has distinct compositional differences that cannot be explained by metamorphism.

¹Blanca Rincón-Tomás, ¹Bahar Khonsari, ¹Dominik Mühlen, ¹Christian Wickbold, ²Nadine Schäfer, ²Dorothea Hause-Reitner, ^1,3Michael Hoppert, ^2,3Joachim Reitner
International Journal of Astrobiology 15, 219-229 Link to Article [DOI: http://dx.doi.org/10.1017/S1473550416000264]
¹Georg-August-University Göttingen, Institute of Microbiology and Genetics, Grisebachstraße 8, 37077 Göttingen, Germany
²Georg-August-University Göttingen, Göttingen Centre of Geosciences, Goldschmidtstraße 3, 37077 Göttingen, Germany
³Göttingen Academy of Sciences and Humanities, Theaterstraße 7, 37073 Göttingen, Germany

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^1,2,3Nicola J. Potts,^1,4,5Romain Tartèse,^1,6Mahesh Anand,²Wim van Westrenen,^1,7Alexandra A. Griffiths,¹Thomas J. Barrett,¹Ian A. Franchi
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12681]
¹Planetary and Space Sciences, The Open University, Milton Keynes, UK
²Faculty of Earth and Life Sciences, VU University Amsterdam, 1081 HV Amsterdam, the Netherlands
³School of GeoSciences, University of Edinburgh, Edinburgh, UK
⁴Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, ⁵Muséum National d’Histoire Naturelle, Sorbonne Universités, CNRS, UPMC & IRD, Paris, France
⁶Department of Earth Sciences, The Natural History Museum, London, SW7 5BD, UK
⁷School of Earth, Atmospheric and Environmental Sciences, University of Manchester, UK
Published by arrangement with John Wiley & Sons

Recent studies geared toward understanding the volatile abundances of the lunar interior have focused on the volatile-bearing accessory mineral apatite. Translating measurements of volatile abundances in lunar apatite into the volatile inventory of the silicate melts from which they crystallized, and ultimately of the mantle source regions of lunar magmas, however, has proved more difficult than initially thought. In this contribution, we report a detailed characterization of mesostasis regions in four Apollo mare basalts (10044, 12064, 15058, and 70035) in order to ascertain the compositions of the melts from which apatite crystallized. The texture, modal mineralogy, and reconstructed bulk composition of these mesostasis regions vary greatly within and between samples. There is no clear relationship between bulk-rock basaltic composition and that of bulk-mesostasis regions, indicating that bulk-rock composition may have little influence on mesostasis compositions. The development of individual melt pockets, combined with the occurrence of silicate liquid immiscibility, exerts greater control on the composition and texture of mesostasis regions. In general, the reconstructed late-stage lunar melts have roughly andesitic to dacitic compositions with low alkali contents, displaying much higher SiO2 abundances than the bulk compositions of their host magmatic rocks. Relevant partition coefficients for apatite-melt volatile partitioning under lunar conditions should, therefore, be derived from experiments conducted using intermediate compositions instead of compositions representing mare basalts.

¹Paul A. Giesting,²Justin Filiberto
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12675]
¹Department of Earth and Planetary Science, University of Tennessee, Knoxville, Tennessee, USA
²Department of Geology, Southern Illinois University, Carbondale, Illinois, USA
Published by arrangement with John Wiley & Sons

Potassic-chloro-hastingsite has been found in melt inclusions in MIL 03346, its paired stones, and NWA 5790. It is some of the most chlorine-rich amphibole ever analyzed. In this article, we evaluate what crystal chemistry, terrestrial analogs, and experiments have shown about how chlorine-dominant amphibole (chloro-amphibole) forms and apply these insights to the nakhlites. Chloro-amphibole is rare, with about a dozen identified localities on Earth. It is always rich in potassium and iron and poor in titanium. In terrestrial settings, its presence has been interpreted to result from medium to high-grade alteration (>400 °C) of a protolith by an alkali and/or iron chloride-rich aqueous fluid. Ferrous chloride fluids exsolved from mafic magmas can cause such alteration, as can crustal fluids that have reacted with rock and lost H2O in preference to chloride, resulting in concentrated alkali chloride fluids. In the case of the nakhlites, an aqueous alkali-ferrous chloride fluid was exsolved from the parental melt as it crystallized. This aqueous chloride fluid itself likely unmixed into chloride-dominant and water-dominant fluids. Chloride-dominant fluid was trapped in some melt inclusions and reacted with the silicate contents of the inclusion to form potassic-chloro-hastingsite.

^1,2D. Turrini, ³V. Svetsov, ⁴G. Consolmagno, ⁵S. Sirono, ⁶S. Pirani
Icarus (in Press) Link to Article [doi:10.1016/j.icarus.2016.07.009]
¹Istituto di Astrofisica e Planetologia Spaziali INAF-IAPS, Via Fosso del Cavaliere 100, 00133 Rome, Italy
²Departamento de Fisica, Universidad de Atacama, Copayapu 485, Copiapó, Chile
³Institute for Dynamics of Geospheres, Leninskiy Prospekt 38-1, Moscow 119334, Russia
⁴Specola Vaticana, V-00120, Vatican City State
⁵Graduate School of Earth and Environmental Sciences, Nagoya University, Tikusa-ku, Nagoya 464-8601, Japan
⁶Lund Observatory, Department of Astronomy and Theoretical Physics, Lund University, Box 43, SE-221 00 Lund, Sweden
Copyright Elsevier

The survival of asteroid Vesta during the violent early history of the Solar System is a pivotal constraint on theories of planetary formation. Particularly important from this perspective is the amount of olivine excavated from the vestan mantle by impacts, as this constrains both the interior structure of Vesta and the number of major impacts the asteroid suffered during its life. The NASA Dawn mission revealed that olivine is present on Vesta’s surface in limited quantities, concentrated in small patches at a handful of sites not associated with the two large impact basins Rheasilvia and Veneneia. The first detections were interpreted as the result of the excavation of endogenous olivine, even if the depth at which the detected olivine originated was a matter of debate. Later works raised instead the possibility that the olivine had an exogenous origin, based on the geologic and spectral features of the deposits. In this work we quantitatively explore the proposed scenario of a exogenous origin for the detected vestan olivine to investigate whether its presence on Vesta can be explained as a natural outcome of the collisional history of the asteroid over the last one or more billion years. To perform this study we took advantage of the impact contamination model previously developed to study the origin and amount of dark and hydrated materials observed by Dawn on Vesta, a model we updated by performing dedicated hydrocode impact simulations. We show that the exogenous delivery of olivine by the same impacts that shaped the vestan surface can offer a viable explanation for the currently identified olivine-rich sites without violating the constraint posed by the lack of global olivine signatures on Vesta. Our results indicate that no mantle excavation is in principle required to explain the observations of the Dawn mission and support the idea that the vestan crust could be thicker than indicated by simple geochemical models based on the Howardite-Eucrite-Diogenite family of meteorites.

^1,2Christopher Hamann, ^1,3Dieter Stöffler, ^1,3Wolf Uwe Reimold
Geochimica et Cosmochmica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.07.018]
¹Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstraße 43, 10115 Berlin, Germany
²Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstraße 74–100, 12249 Berlin, Germany
³Humboldt-Universität zu Berlin, Unter den Linden 6, 10099 Berlin, Germany
Copyright Elsevier

We analyzed the interaction of spherical, 6.36-mm-diameter, Cu-bearing aluminum projectiles with quartz sand targets in hypervelocity impact experiments performed at NASA Ames Vertical Gun Range. Impact velocities and inferred peak shock pressures varied between 5.9–6.5 km/s and ∼41–48 GPa, respectively. Shocked particles (“impact melt particles”) coated with thin crusts of molten projectile material were recovered from the floors of the ca. 33-cm-diameter craters and the respective ejecta blankets. Through petrographic and chemical analyses (optical microscopy, FE-EMPA, SEM-EDX, and XRF analysis) we show that these particles have a layered structure manifested in distinct layers of decreasing shock metamorphism. These can be characterized by the following physical and chemical reactions and alteration products: (i) complete melting and subsequent recrystallization of the projectile, forming a distinct crystallization texture in the fused metal crust; (ii) projectile–target mixing, involving a redox reaction between Cu-bearing Al alloy und SiO2, leading to formation of khatyrkite (CuAl2), Al2O3 melt, euhedral silicon crystals, and spherical droplets of silicon; (iii) melting of quartz to lechatelierite and formation of planar deformation features in relic quartz grains; and (iv) shock lithification of quartz grains with fracturing of grains, grain-boundary melting, planar deformation features, and complete loss of porosity. To our knowledge, this is the first report of khatyrkite formed experimentally in hypervelocity impact experiments. These results have implications for the understanding of a similar redox reaction between Al–Cu metal and siliceous impact melt recently postulated for the Khatyrka CV3 carbonaceous chondrite. Moreover, these results bear on the processes that lead to layers of regolith on the surfaces of planetary bodies without atmospheres, such as asteroids in the main belt (e.g., 4 Vesta), and on the Moon. Specifically, impacts of mm-sized projectiles at velocities between 4–6 km/s into regolith-covered, asteroidal surfaces in the main belt should yield similar impact melt particles that feature a continuum of shock effects, i.e., partially to completely molten projectile remnants adhering to impact-melted regolith agglomerates, as well as projectile-contaminated impact melts and local shock melting along grain boundaries.

^1,2 Alan E. Rubin
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12679]
¹Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, California, USA
²Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, USA
Published by arrangement with John Wiley & Sons

Al Haggounia 001 and paired specimens (including Northwest Africa [NWA] 2828 and 7401) are part of a vesicular, incompletely melted, EL chondrite impact melt rock with a mass of ~3 metric tons. The meteorite exhibits numerous shock effects including (1) development of undulose to weak mosaic extinction in low-Ca pyroxene; (2) dispersion of metal-sulfide blebs within silicates causing “darkening”; (3) incomplete impact melting wherein some relict chondrules survived; (4) vaporization of troilite, resulting in S2 bubbles that infused the melt; (5) formation of immiscible silicate and metal-sulfide melts; (6) shock-induced transportation of the metal-sulfide melt to distances >10 cm; (7) partial resorption of relict chondrules and coarse silicate grains by the surrounding silicate melt; (8) crystallization of enstatite in the matrix and as overgrowths on relict silicate grains and relict chondrules; (9) crystallization of plagioclase from the melt; and (10) quenching of the vesicular silicate melt. The vesicular samples lost almost all of their metal during the shock event and were less susceptible to terrestrial weathering; in contrast, the samples in which the metal melt accumulated became severely weathered. Literature data indicate the meteorite fell ~23,000 yr ago; numerous secondary phases formed during weathering. Both impact melting and weathering altered the meteorite’s bulk chemical composition: e.g., impact melting and loss of a metal-sulfide melt from NWA 2828 is responsible for bulk depletions in common siderophile elements and in Mn (from alabandite); weathering of oldhamite caused depletions in many rare earth elements; the growth of secondary phases caused enrichments in alkalis, Ga, As, Se, and Au.