^1,2Antoine S. G. Roth,³Knut Metzler,⁴Lukas P. Baumgartner,⁵Beda A. Hofmann,⁶Ingo Leya
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12923]
¹Institute of Physics, University of Bern, Bern, Switzerland:
²Department of Earth Sciences, ETH Zurich, Zurich, Switzerland
³Institute for Planetology, University of Münster, Münster, Germany
⁴Institute of Earth Sciences, University of Lausanne, Lausanne, Switzerland
⁵Department of Earth Sciences, Natural History Museum Bern, Bern, Switzerland
⁶Institute of Physics, University of Bern, Bern, Switzerland
Published by arrangement with John Wiley & Sons

Renazzo-type carbonaceous (CR) chondrites are accretionary breccias that formed last. As such they are ideal samples to study precompaction exposures to cosmic rays. Here, we present noble gas data for 24 chondrules and 3 dark inclusion samples (DIs) from Shişr 033 (CR2). The meteorite was selected based on the absence of implanted solar wind noble gases and an anomalous oxygen isotopic composition of the DIs; the oxygen isotopes match those in CV3 and CO3 chondrites. Our samples contain variable mixtures of galactic cosmic ray (GCR)-produced cosmogenic noble gases and trapped noble gases of presolar origin. Remarkably, all chondrules have cosmogenic 3He and 21Ne concentrations up to 4.3 and 7.1 times higher than the DIs, respectively. We derived an average 3He-21Ne cosmic ray exposure (CRE) age for Shişr 033 of 2.03 ± 0.20 Ma (2 SD) and excesses in cosmogenic 3He and 21Ne in chondrules (relative to the DIs) in the range (in 10−8 cm3STP/g) 3.99–7.76 and 0.94–1.71, respectively. Assuming present-day GCR flux density, the excesses translate into average precompaction 3He-21Ne CRE ages of 3.1–27.3 Ma depending on the exposure geometry. The data can be interpreted assuming a protracted storage of a single chondrule generation prior to the final assembly of the Shişr 033 parent body in a region of the disk transparent to GCRs.

^1,2Jay Shah, ^1,2Helena C. Bates, ¹Adrian R. Muxworthy, ³Dominik C. Hezel, ²Sara S. Russell, ¹Matthew J. Genge
Earth and Planetary Science Letters 475, 106-118 Link to Article [https://doi.org/10.1016/j.epsl.2017.07.035]
¹Department of Earth Science and Engineering, Imperial College London, London, UK
²Department of Earth Sciences, Natural History Museum, London, UK
³Institute of Geology and Mineralogy, University of Cologne, Cologne, Germany
Copyright Elsevier

We present evidence for both early- and late-stage magnetic activity on the CV and L/LL parent bodies respectively from chondrules in Vigarano and Bjurböle. Using micro-CT scans to re-orientate chondrules to their in-situ positions, we present a new micron-scale protocol for the paleomagnetic conglomerate test. The paleomagnetic conglomerate test determines at 95% confidence, whether clasts within a conglomerate were magnetized before or after agglomeration, i.e., for a chondritic meteorite whether the chondrules carry a pre- or post-accretionary remanent magnetization. We found both meteorites passed the conglomerate test, i.e., the chondrules had randomly orientated magnetizations. Vigarano’s heterogeneous magnetization is likely of shock origin, due to the 10 to 20 GPa impacts that brecciated its precursor material on the parent body and transported it to re-accrete as the Vigarano breccia. The magnetization was likely acquired during the break-up of the original body, indicating a CV parent body dynamo was active ∼9 Ma after Solar System formation. Bjurböle’s magnetization is due to tetrataenite, which transformed from taenite as the parent body cooled to below 320 °C, when an ambient magnetic field imparted a remanence. We argue either the high intrinsic anisotropy of tetrataenite or brecciation on the parent body manifests as a randomly orientated distribution, and a L/LL parent body dynamo must have been active at least 80 to 140 Ma after peak metamorphism. Primitive chondrites did not originate from entirely primitive, never molten and/or differentiated parent bodies. Primitive chondrite parent bodies consisted of a differentiated interior sustaining a long-lived magnetic dynamo, encrusted by a layer of incrementally accreted primitive meteoritic material. The different ages of carbonaceous and ordinary chondrite parent bodies might indicate a general difference between carbonaceous and ordinary chondrite parent bodies, and/or formation location in the protoplanetary disk.

^1,2Steven Goderis, ³Roald Tagle, ⁴Jörg Fritz, ⁵Rainer Bartoschewitz, ^6,7Natalia Artemieva
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.013]
¹Earth System Science, Vrije Universiteit Brussel, Pleinlaan 2, BE-1050 Brussels, Belgium
²Department of Analytical Chemistry, Universiteit Gent, Krijgslaan 281-S12, BE-9000 Ghent, Belgium
³Bruker Nano GmbH, Am Studio 2D, 12489 Berlin, Germany
⁴Saalbau Weltraum Projekt, Liebigstrasse 6, 64646 Heppenheim, Germany
⁵Meteorite Laboratory, Weiland 37, D-38518 Gifhorn, Germany
⁶Planetary Science Institute, Tucson AZ85719, USA
⁷Institute for Dynamics of Geospheres RAS, 117334, Moscow, Russia
Copyright Elsevier

The Australasian tektite strewn field is exceptional, not only as the largest and most recent, but also as the only strewn field without an identified source impact crater. Therefore, scenarios without the formation of an impact crater, such as a low altitude cometary airburst, have proven hard to discard. Here, new geochemical evidence is presented for mixing of projectile and target material, which implies the formation of an Australasian tektite-related impact crater. First, ninety-two Australasian tektites were grouped according to their Cr, Co and Ni concentrations. Based on this data, Australasian tektites with the highest Ni contents (>200 μg/g) occur more than 1500 km south-southeast (SSE) of the northern Indochina region, with the highest concentration of Ni-rich tektites in South Vietnam, the islands of Borneo, Belitung, and Java, and reports in literature for Ni-rich tektites in central Australia. The tektites with the highest Cr and Ni abundances often also show highly siderophile element (HSE) enrichments of up to 4 ng/g Ir. The most Ni-rich samples exhibit broadly chondrite-relative HSE proportions. However, a chondritic impactor contribution appears to be inconsistent with the observed Ni/Cr, Ni/Co, and Cr/Co ratios. A previously suggested significant terrestrial mantle contribution can also not explain the siderophile element enrichments in combination with relatively low FeOtot (<7 wt%) and MgO (<4 wt%) contents. Elemental fractionation during impact cratering or tektite formation by an impactor with a chondritic signature may explain these observations. Alternatively, a projectile component from a primitive achondrite may be advocated, with contribution from a mafic to ultramafic extraterrestrial lithology with a relatively unfractionated HSE signature and Ni/Cr ratio distinctly higher than those of Earth’s mantle. Element distribution maps obtained from individual Australasian tektites document complex mingling processes of chemically distinct melt batches, each exhibiting variable contributions from distinct endmember compositions. These texturally recorded mingling processes are consistent with high-resolution numerical models of impact cratering processes that resolve the growth of Kelvin-Helmholtz instabilities at the projectile/target interface during impact, when both materials co-occur at high pressure. These numerical models indicate that Ni-rich tektite populations across the central part of the Australasian tektite strewn field could represent projectile-enriched material preferentially ejected downrange. Continued tracing of this Ni-rich component across the strewn field may help to constrain the location of the yet to be identified source crater of the Australasian (micro)tektites.

¹Arne Grumpe, ¹Natascha Mengewein, ¹Daniela Rommel, ²Urs Mall, ¹Christian Wöhler
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.07.008]
¹Image Analysis Group, TU Dortmund University, Otto-Hahn-Str. 4, Dortmund D–44227, Germany
²Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, Göttingen D–37077, Germany
Copyright Elsevier

It is well known that many common planetary minerals exhibit prominent absorption features. Consequently, the analysis of spectral reflectance measurements has become a major tool of remote sensing. Quantifying the mineral abundances, however, is not a trivial task. The interaction between the incident light rays and particulate surfaces, e.g., the lunar regolith, leads to a non-linear relationship between the reflectance spectra of the pure minerals, the so-called “endmembers”, and the surface’s reflectance spectrum. It is, however, possible to transform the non-linear reflectance mixture into a linear mixture of single-scattering albedos of the Hapke model.

The abundances obtained by inverting the linear single-scattering albedo mixture may be interpreted as volume fractions which are weighted by the endmember’s extinction coefficient. Commonly, identical extinction coefficients are assumed throughout all endmembers and the obtained volume fractions are converted to mass fractions using either measured or assumed densities. In theory, the proposed method may cover different grain sizes if each grain size range of a mineral is treated as a distinct endmember.

Here, we present a method to transform the mixing coefficients to mass fractions for arbitrary combinations of extinction coefficients and densities. The required parameters are computed from reflectance measurements of well defined endmember mixtures. Consequently, additional measurements, e.g., the endmember density, are no longer required. We evaluate the method based on laboratory measurements and various results presented in the literature, respectively. It is shown that the procedure transforms the mixing coefficients to mass fractions yielding an accuracy comparable to carefully calibrated laboratory measurements without additional knowledge. For our laboratory measurements, the square root of the mean squared error is less than 4.82  wt%. In addition, the method corrects for systematic effects originating from mixtures of endmembers showing a highly varying albedo, e.g., plagioclase and pyroxene.

¹Jeff A. Berger et al. (>10)
Journal of Geophysical Research, Planets (in Press) Link to Article [DOI: 10.1002/2017JE005290]
Department of Earth Sciences, University of Western Ontario, London, ON, Canada
Published by arrangement with John Wiley & Sons

Zinc and germanium enrichments have been discovered in sedimentary rocks in Gale Crater, Mars, by the Alpha Particle X-ray Spectrometer (APXS) on the rover Curiosity. Concentrations of Zn (910 ± 840 ppm) and Ge (65 ± 58 ppm) are 10s-100s of times greater than in Martian meteorites and estimates for average silicate Mars. Enrichments occur in diverse rocks including minimally to extensively altered basaltic and alkalic sediment. The magnitude of the enrichments indicates hydrothermal fluids, but Curiosity has not discovered unambiguous hydrothermal mineral assemblages. We propose that Zn- and Ge-rich hydrothermal deposits in the source region were dispersed in siliciclastic sediments during transport into the crater. Subsequent diagenetic mobilization and fractionation of Zn and Ge is evident in a Zn-rich sandstone (Windjana; Zn ~4000 ppm, Ge ~85 ppm) and associated Cl-rich vein (Stephen; Zn ~8000 ppm, Ge ~60 ppm), in Ge-rich veins (Garden City; Zn ~1300 ppm, Ge ~650 ppm), and in silica-rich alteration haloes leached of Zn (30-200 ppm). In moderately to highly altered silica-rich rocks, Ge remained immobile relative to leached elements (Fe, Mn, Mg, Ca), consistent with fluid interaction at pH << 7. In contrast, cross-cutting Ge-rich veins at Garden City suggest aqueous mobilization as Ge-F complexes at pH < 2.5. Multiple jarosite detections by the CheMin XRD and variable Zn concentrations indicate diagenesis of lower Mt. Sharp bedrock under acidic conditions. The enrichment and fractionation of Zn and Ge constrains fluid events affecting Gale sediments and can aid in unraveling fluid histories as Curiosity’s traverse continues.

¹Gavin G. Kenny, ^2,3Joseph A. Petrus, ⁴Martin J. Whitehouse, ⁵J. Stephen Daly, ¹Balz S. Kamber
Geochmica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.08.009]
¹Department of Geology, School of Natural Sciences, Trinity College, Dublin, Ireland
²School of Earth Sciences, University of Melbourne, Parkville, Australia
³Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada
⁴Department of Geosciences, Swedish Museum of Natural History, 104 05 Stockholm, Sweden
⁵UCD School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland
Copyright Elsevier

We report on the first zircon hafnium-oxygen isotope and trace element study of a transect through one of the largest terrestrial impact melt sheets. The differentiated melt sheet at the 1.85 Ga, originally ca. 200 km in diameter Sudbury impact crater, Ontario, Canada, yields a tight range of uniform zircon Hf isotope compositions (εHf(1850) of ca. -9 to -12). This is consistent with its well-established crustal origin and indicates differentiation from a single melt that was initially efficiently homogenised. We propose that the heterogeneity in other isotopic systems, such as Pb, in early-emplaced impact melt at Sudbury is associated with volatility-related depletion during the impact cratering process. This depletion leaves the isotopic systems of more volatile elements more susceptible to contamination during post-impact assimilation of country rock, whereas the systems of more refractory elements preserve initial homogeneities. Zircon oxygen isotope compositions in the melt sheet are also restricted in range relative to those in the impacted target rocks. However, they display a marked offset approximately one-third up the melt sheet stratigraphy that is interpreted to be a result of post-impact assimilation of 18O-enirched rocks into the base of the cooling impact melt.

Given that impact cratering was a more dominant process in the early history of the inner Solar System than it is today, and the possibility that impact melt sheets were sources of ex situ Hadean zircon grains, these findings may have significance for the interpretation of the early zircon Hf record. We speculate that apparent εHf-time arrays observed in the oldest terrestrial and lunar zircon datasets may be related to impact melting homogenising previously more diverse crust.

We also show that spatially restricted partial melting of rocks buried beneath the superheated impact melt at Sudbury provided a zircon crystallising environment distinct to the impact melt sheet itself.

^1,2Thomas S. Kruijer, ¹Thorsten Kleine
Earth and Planetary Science Letters 475, 15-24 Link to Article [https://doi.org/10.1016/j.epsl.2017.07.021]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA
Copyright Elsevier

The giant impact model of lunar origin predicts that the Moon mainly consists of impactor material. As a result, the Moon is expected to be isotopically distinct from the Earth, but it is not. To account for this unexpected isotopic similarity of the Earth and Moon, several solutions have been proposed, including (i) post-giant impact Earth–Moon equilibration, (ii) alternative models that make the Moon predominantly out of proto-Earth mantle, and (iii) formation of the Earth and Moon from an isotopically homogeneous disk reservoir. Here we use W isotope systematics of lunar samples to distinguish between these scenarios. We report high-precision 182W data for several low-Ti and high-Ti mare basalts, as well as for Mg-suite sample 77215, and lunar meteorite Kalahari 009, which complement data previously obtained for KREEP-rich samples. In addition, we utilize high-precision Hf isotope and Ta/W ratio measurements to empirically quantify the superimposed effects of secondary neutron capture on measured 182W compositions. Our results demonstrate that there are no resolvable radiogenic 182W variations within the Moon, implying that the Moon differentiated later than 70 Ma after Solar System formation. In addition, we find that samples derived from different lunar sources have indistinguishable 182W excesses, confirming that the Moon is characterized by a small, uniform ∼+26 parts-per-million excess in 182W over the present-day bulk silicate Earth. This 182W excess is most likely caused by disproportional late accretion to the Earth and Moon, and after considering this effect, the pre-late veneer bulk silicate Earth and the Moon have indistinguishable 182W compositions. Mixing calculations demonstrate that this Earth–Moon 182W similarity is an unlikely outcome of the giant impact, which regardless of the amount of impactor material incorporated into the Moon should have generated a significant 182W excess in the Moon. Consequently, our results imply that post-giant impact processes might have modified 182W, leading to the similar 182W compositions of the pre-late veneer Earth’s mantle and the Moon.

¹F.P. Leitzke, ¹R.O.C. Fonseca, ²P. Sprung, ³G. Mallmann, ¹M. Lagos, ¹L.T. Michely, ²C. Münker
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.07.009]
¹Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn, 53115 Bonn, Germany
²Institut für Geologie und Mineralogie, Universität zu Köln, 50674 Köln, Germany
³Research School of Earth Sciences, Australian National University, ACT 2601, Canberra, Australia
Copyright Elsevier

We present results of high-temperature olivine-melt, pyroxene-melt and plagioclase-melt partitioning experiments aimed at investigating the redox transition of Mo in silicate systems. Data for a series of other minor and trace elements (Sc, Ba, Sr, Cr, REE, Y, HFSE, U, Th and W) were also acquired to constrain the incorporation of Mo in silicate minerals. All experiments were carried out in vertical tube furnaces at 1 bar and temperatures ranging from ca. 1220 to 1300 °C. Oxygen fugacity was controlled via CO–CO2 gas mixtures and varied systematically from 5.5 log units below to 1.9 log units above the fayalite–magnetite–quartz (FMQ) redox buffer thereby covering the range in oxygen fugacities of terrestrial and lunar basalt genesis. Molybdenum is shown to be volatile at oxygen fugacities above FMQ and that its compatibility in pyroxene and olivine increases three orders of magnitude towards the more reducing conditions covered in this study. The partitioning results show that Mo is dominantly tetravalent at redox conditions below FMQ-4 and dominantly hexavalent at redox conditions above FMQ. Given the differences in oxidation states of the terrestrial (oxidized) and lunar (reduced) mantles, molybdenum will behave significantly differently during basalt genesis in the Earth (i.e. highly incompatible; average View the MathML source) and Moon (i.e. moderately incompatible/compatible; average View the MathML source). Thus, it is expected that Mo will strongly fractionate from W during partial melting in the lunar mantle, given that W is broadly incompatible at FMQ-5. Moreover, the depletion of Mo and the Mo/W range in lunar samples can be reproduced by simply assuming a primitive Earth-like Mo/W for the bulk silicate Moon. Such a lunar composition is in striking agreement with the Moon being derived from the primitive terrestrial mantle after core formation on Earth.

¹Claire-Marie Loudon et al. (>10)*
International journal of Astrobiology (in Press) Link to Article [DOI: https://doi.org/10.1017/S1473550417000234]
¹UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK
*Find the extensive, full author and affiliation list on the publishers website

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¹Thomas S.Kruijer, ¹Thorsten Kleine, ²Lars E.Borg, ¹Gregory A.Brennecka, ³Anthony J.Irving, ¹Addi Bischoff, ⁴Carl B.Agee
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2017.06.047]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Strasse 10, 48149, Münster, Germany
²Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, Livermore, 7000 East Avenue, CA 94550, USA
³Department of Earth and Space Sciences, University of Washington, Seattle, WA 98195, USA
⁴Institute of Meteoritics, University of New Mexico, Albuquerque, NM 87131, USA
Copyright Elsevier

Mars probably accreted within the first 10 million years of Solar System formation and likely underwent magma ocean crystallization and crust formation soon thereafter. To assess the nature and timescales of these large-scale mantle differentiation processes we applied the short-lived 182Hf–182W and 146Sm–142Nd chronometers to a comprehensive suite of martian meteorites, including several shergottites, augite basalt NWA 8159, orthopyroxenite ALH 84001 and polymict breccia NWA 7034. Compared to previous studies the 182W data are significantly more precise and have been obtained for a more diverse suite of martian meteorites, ranging from samples from highly depleted to highly enriched mantle and crustal sources. Our results show that martian meteorites exhibit widespread 182W/184W variations that are broadly correlated with 142Nd/144Nd, implying that silicate differentiation (and not core formation) is the main cause of the observed 182W/184W differences. The combined 182W–142Nd systematics are best explained by magma ocean crystallization on Mars within ∼20–25 million years after Solar System formation, followed by crust formation ∼15 million years later. These ages are indistinguishable from the I–Pu–Xe age for the formation of Mars’ atmosphere, indicating that the major differentiation of Mars into mantle, crust, and atmosphere occurred between 20 and 40 million years after Solar System formation and, hence, earlier than previously inferred based on Sm–Nd chronometry alone.