¹M. Melwani Daswani,¹E. S. Kite
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2017JE005319]
¹Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois, USA
Published by arrangement wit John Wiley & Sons

Chloride-bearing deposits on Mars record high-elevation lakes during the waning stages of Mars’ wet era (mid-Noachian to late Hesperian). The water source pathways, seasonality, salinity, depth, lifetime, and paleoclimatic drivers of these widespread lakes are all unknown. Here we combine reaction-transport modeling, orbital spectroscopy, and new volume estimates from high-resolution digital terrain models, in order to constrain the hydrologic boundary conditions for forming the chlorides. Considering a T = 0 °C system, we find: (1) individual lakes were >100 m deep and lasted decades or longer; (2) if volcanic degassing was the source of chlorine, then the water-to-rock ratio or the total water volume were probably low, consistent with brief excursions above the melting point and/or arid climate; (3) if the chlorine source was igneous chlorapatite, then Cl-leaching events would require a (cumulative) time of >10 yr at the melting point; (4) Cl masses, divided by catchment area, give column densities 0.1 – 50 kg Cl/m2, and these column densities bracket the expected chlorapatite-Cl content for a seasonally-warm active layer. Deep groundwater was not required. Taken together, our results are consistent with Mars having a usually cold, horizontally segregated hydrosphere by the time chlorides formed.

¹F.Tosi et al.
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2017.08.012]
¹INAF-IAPS Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere, 100, I-00133 Rome, Italy
Copyright Elsevier

Ac-H-6 ‘Haulani’ is one of five quadrangles that cover the equatorial region of the dwarf planet Ceres. This quadrangle is notable for the broad, spectrally distinct ejecta that originate from the crater Haulani, which gives the name to the quadrangle. These ejecta exhibit one of the most negative (‘bluest’) visible to near infrared spectral slope observed across the entire body and have distinct color properties as seen in multispectral composite images. Besides Haulani, here we investigate a broader area that includes other surface features of interest, with an emphasis on mineralogy as inferred from data obtained by Dawn’s Visible InfraRed mapping spectrometer (VIR), combined with multispectral image products from the Dawn Framing Camera (FC) so as to enable a clear correlation with specific geologic features.

Our analysis shows that crater Haulani stands out compared to other surface features of the quadrangle. Albedo maps obtained in the near infrared range at 1.2 μm and 1.9 μm reveal that the floor and ejecta of Haulani are indeed a patchwork of bright and dark material units. Visible to near-infrared spectral slopes display negative values in crater Haulani’s floor and ejecta, which are indicative of a younger age. Spectral features centered at ∼2.7 μm and ∼3.1 μm, respectively diagnostic of magnesium-bearing phyllosilicates and ammoniated phyllosilicates, show a substantial decrease in band depth in crater Haulani’s floor and bright ejecta. Similar, but less prominent, spectral behavior is observed in other small craters of this quadrangle. There is a general trend in quadrangle Ac-H-6 for the two 2.7-μm and 3.1-μm band depths to increase from the northwest to the southeast. However, it is worth noting that the correlation between these two spectral parameters is generally strong in the Haulani crater’s area, but much weaker elsewhere, which indicates a variable degree of mixing between these two major mineral phases.

^1,2Agata M. Krzesińska
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12933]
¹Department of Earth Sciences, Natural History Museum, London, UK
²Institute of Geological Sciences, Polish Academy of Sciences, Wrocław, Poland
Published by arrangement with John Wiley & Sons

Three-dimensional X-ray tomographic reconstructions and petrologic studies reveal voluminous accumulations of metal in Pułtusk H chondrite. At the contact of these accumulations, the chondritic rock is enriched in troilite. The rock contains plagioclase-rich bands, with textures suggesting crystallization from melt. Unusually large phosphates are associated with the plagioclase and consist of assemblages of merrillite, and fluorapatite and chlorapatite. The metal accumulations were formed by impact melting, rapid segregation of metal-sulfide melt and the incorporation of this melt into the fractured crater basement. The impact most likely occurred in the early evolution of the H chondrite parent body, when post-impact heat overlapped with radiogenic heat. This enabled slow cooling and separation of the metallic melt into metal-rich and sulfide-rich fractions. This led to recrystallization of chondritic rock in contact with the metal accumulations and the crystallization of shock melts. Phosphorus was liberated from the metal and subsumed by the silicate shock melt, owing to oxidative conditions upon slow cooling. The melt was also a host for volatiles. Upon further cooling, phosphorus reacted with silicates leading to the formation of merrillite, while volatiles partitioned into the residual halogen-rich, dry fluid. In the late stages, the fluid altered merrillite to patchy Cl/F-apatite. The above sequence of alterations demonstrates that impact during the early evolution of chondritic parent bodies might have contributed to local metal segregation and silicate melting. In addition, postshock conditions supported secondary processes: compositional/textural equilibration, redistribution of volatiles, and fluid alterations.

¹Shu-Zhou Wang,^1,2Ai-Cheng Zhang,¹Run-Lian Pang,¹Jia-Ni Chen,³Li-Xin Gu,¹Ru-Cheng Wang
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12931]
¹State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing, China
²Lunar and Planetary Science Institute, Nanjing University, Nanjing, China
³Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China
Published by arrangement with John Wiley & Sons

Northwest Africa (NWA) 7755 is a newly found enriched lherzolitic shergottite. Here, we report its detailed petrography and mineralogy. NWA 7755 contains both poikilitic and non-poikilitic lithologies. Olivine has different compositional ranges in the poikilitic and non-poikilitic lithologies, Fa30–39 and Fa37–40, respectively. Pyroxene in the non-poikilitic lithology is systematically Fe-richer than that in the poikilitic lithology. The chromite grains in non-poikilitic lithology are highly Ti-richer than those in the poikilitic lithology. The chemical variations of olivine, pyroxene, and chromite between the poikilitic and non-poikilitic lithologies support a two-stage formation model of lherzolitic shergottites. Besides planar fractures and strong mosaicism in olivine and pyroxene, shock-induced melt veins and pockets are observed in NWA 7755. Olivine grains within and adjacent to melt veins and/or pockets have either transformed to ringwoodite, amorphous phase, or dissociated to bridgmanite plus magnesiowüstite. Merrillite in melt veins has completely transformed to tuite; however, apatite only has partially transformed to tuite, indicating a relatively sluggish transformation rate. The partial transformation from apatite to tuite resulted in fractional devolatilization of Cl and F in apatite. The fine-grained mineral assemblage in melt veins consists mainly of bridgmanite, minor magnesiowüstite, Fe-sulfide, Fe-phosphide, and Ca-phosphate minerals. The coexistence of bridgmanite and magnesiowüstite in these veins indicates a shock pressure of >~24 GPa and a temperature of 1800–2000 °C. Coesite and seifertite are probably present in NWA 7755. The presence of these high-pressure minerals indicates that NWA 7755 has experienced a more intense shock metamorphism than other enriched lherzolitic shergottites.

^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.