^1,2Luca Bindi, ^3,4Ming Chen, ^4,5Xiande Xie
Scientific Reports 7, 42674 Link to Article [doi:10.1038/srep42674]
¹Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, I-50121 Florence, Italy
²CNR-Istituto di Geoscienze e Georisorse, Via La Pira 4, I-50121 Florence, Italy
³State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
⁴Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou 510640, China
⁵Key Laboratory of Mineralogy and Metallogeny, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China

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^1,2Chenguang Sun,¹Michelle Graff,¹Yan Liang
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.003]
¹Department of Earth, Environmental and Planetary Sciences, Brown University, Box 1846, Providence, RI 02912, USA
²Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
Copyright Elsevier

Trace element partition coefficients between anorthitic plagioclase and basaltic melts (D) have been determined experimentally at 0.6 GPa and 1350-1400 °C in a lunar high-Ti picritic glass and a mid-ocean ridge basalt (MORB). Plagioclases with 98 mol% and 86 mol% anorthite were produced in the lunar picritic melt and MORB melt, respectively. Based on the new experimental partitioning data and those selected from the literature, we developed parameterized lattice strain models for the partitioning of monovalent (Na, K, Li), divalent (Ca, Mg, Ba, Sr, Ra) and trivalent (REE and Y) cations between plagioclase and silicate melt. Through the new models we showed that the partitioning of these trace elements in plagioclase depends on temperature, pressure, and the abundances of Ca and Na in plagioclase. Particularly, Na content in plagioclase primarily controls divalent element partitioning, while temperature and Ca content in plagioclase are the dominant factors for REE partitioning in plagioclase. From these models, we also derived a new expression for DRa/DBa that can be used for Ra-Th dating on volcanic plagioclase phenocrysts, and a new model for plagioclase-melt noble gas partitioning. Applications of these partitioning models to fractional crystallization of MORB and lunar magma ocean (LMO) indicate that (1) the competing effect of temperature and plagioclase composition leads to small variations of plagioclase-melt DREE during MORB differentiation, but (2) the temperature effect is especially significant and can vary anorthite-melt DREE by over one order of magnitude during LMO solidification. Temperature and plagioclase composition have to be considered when modeling the chemical differentiation of mafic to felsic magmas involving plagioclase.

^1,2Martin David Suttle, ^1,2Matthew J. Genge, ³Luigi Folco, ²Sara S. Russell
Geochmica et Cosmochmica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.03.002]
¹Imperial College London, South Kensington, London, SW72AZ, UK
²The Natural History Museum, Cromwell Rd, London SW7 5BD, UK
³Dipartimento di Scienze della Terra, Università di Pisa, 56126 Pisa, Italy
Copyright Elsevier

We analysed 44 fine-grained and scoriaceous micrometeorites. A bulk mid-IR spectrum (8-13μm) for each grain was collected and the entire micrometeorite population classified into 5 spectral groups, based on the positions of their absorption bands. Corresponding carbonaceous Raman spectra, textural observations from SEM-BSE and bulk geochemical data via EMPA were collected to aid in the interpretation of mid-IR spectra. The 5 spectral groups identified correspond to progressive thermal decomposition. Unheated hydrated chondritic matrix, composed predominantly of phyllosilicates, exhibit smooth, asymmetric spectra with a peak at ∼10μm. Thermal decomposition of sheet silicates evolves through dehydration, dehydroxylation, annealing and finally by the onset of partial melting. Both CI-like and CM-like micrometeorites are shown to pass through the same decomposition stages and produce similar mid-IR spectra. Using known temperature thresholds for each decomposition stage it is possible to assign a peak temperature range to a given micrometeorite. Since the temperature thresholds for decomposition reactions are defined by the phyllosilicate species and the cation composition and that these variables are markedly different between CM and CI classes, atmospheric entry should bias the dust flux to favour the survival of CI-like grains, whilst preferentially melting most CM-like dust. However, this hypothesis is inconsistent with empirical observations and instead requires that the source ratio of CI:CM dust is heavily skewed in favour of CM material. In addition, a small population of anomalous grains are identified whose carbonaceous and petrographic characteristics suggest in-space heating and dehydroxylation have occurred. These grains may therefore represent regolith micrometeorites derived from the surface of C-type asteroids. Since the spectroscopic signatures of dehydroxylates are distinctive, i.e. characterised by a reflectance peak at 9.0-9.5μm, and since the surfaces of C-type asteroids are expected to be heated via impact gardening, we suggest that future spectroscopic investigations should attempt to identify dehydroxylate signatures in the reflectance spectra of young carbonaceous asteroid families.

C.T.Adcock et al. (>10)*
Nature Communications 8, 14667 Link to Article [doi:10.1038/ncomms14667]
¹Department of Geoscience, University of Nevada, Las Vegas, 4505 South Maryland Parkway, Las Vegas, Nevada 89154, USA
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¹Samuel M. Clegg et al. (>10)*
Spectrochimica Acta Part B: Atomic Spectroscopy 129, 64-85 Link to Article [http://dx.doi.org/10.1016/j.sab.2016.12.003]
¹Los Alamos National Laboratory, USA
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¹Rebecca J. Thomas, ^1,2Brian M. Hynek, ¹Mikki M. Osterloo, ³Kathryn S. Kierein-Young
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005183]
¹Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA
²Department of Geological Sciences, University of Colorado, Boulder, CO, USA
³Sapphire LLC, Boulder, CO, USA
Published by arrangement with John Wiley & Sons

The best locations at which to detect evidence for early life on Mars are in material formed in near-surface aqueous environments, particularly where this resulted in the deposition of minerals such as clays that are favorable to preservation of organics. The geological history of the Margaritifer region has resulted in exceptional potential to preserve such deposits and to render them discoverable. Due to its topographic setting at the interface between highlands and lowlands, Margaritifer was a major sink for water and sediments in the early, Noachian, period, potentially creating environments that were habitable and conducive to clay-formation. Subsequently, during the Late Hesperian to Amazonian, the ancient surface was extensively disrupted in association with the formation of multiple chaos regions. This activity had the potential to expose any astrobiological evidence from the earlier period. We used orbital image, spectral and topographic data to investigate the extent and means of exposure of Noachian clay-bearing deposits across the region. We find that they are indeed exposed over a very wide area in Margaritifer, and that their mineralogy is most consistent with clay-formation in a low energy near-neutral pH groundwater environment. We additionally find that evidence for subsequent acidic groundwater activity is absent, indicating that biosignature preservation in these units is favored, perhaps to a greater degree than for similar deposits in the surrounding region. Further, due to the intense Hesperian-Amazonian geologic activity here, early clay-bearing units are exposed to a greater degree than achievable in regions with more localized erosive mechanisms.

¹Leos Pohl, ¹Daniel T. Britt
Advances in Space Research 59, 1473-1485 Link to Article [http://dx.doi.org/10.1016/j.asr.2016.12.028]
¹Department of Physics, Physical Sciences Bldg. 430, University of Central Florida, 4111 Libra Drive, Orlando, FL 32816-2385, USA

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¹Kai-tuo Wang, ¹Patrick N Lemougna, ¹Qing Tang, ¹Wei Li, ¹Xue-min Cui
Gondwana Research 44, 1-6 Link to Article [http://dx.doi.org/10.1016/j.gr.2016.11.001]
¹School of Chemistry and Chemical Engineering and Guangxi Key Lab of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China

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¹H. E. Maynard-Casely
Crystallography Reviews 23, 74-117 Link to Article [http://dx.doi.org/10.1080/0889311X.2016.1242127]
¹Australian Nuclear Science and Technology Organisation (ANSTO)

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A.El Goresy et al. (>10)*
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12832]
¹Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany
Published by arrangement with John Wiley & Sons
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Mineral inventories of enstatite chondrites; (EH and EL) are strictly dictated by combined parameters mainly very low dual oxygen (fO2) and sulfur (fS2) fugacities. They are best preserved in the Almahata Sitta MS-17, MS-177 fragments, and the ALHA 77295 and MAC 88136 Antarctic meteorites. These conditions induce a stark change of the geochemical behavior of nominally lithophile elements to chalcophile or even siderophile and changes in the elemental partitioning thus leading to formation of unusual mineral assemblages with high abundance of exotic sulfide species and enrichment in the metallic alloys, for example, silicides and phosphides. Origin and mode of formation of these exotic chondrites, and their parental source regions could be best scrutinized by multitask research experiments of the most primitive members covering mineralogical, petrological, cosmochemical, and indispensably short-lived isotopic chronology. The magnitude of temperature and pressure prevailed during their formation in their source regions could eventually be reasonably estimated: pre- and postaccretionary could eventually be deduced. The dual low fugacities are regulated by the carbon to oxygen ratios estimated to be >0.83 and <1.03. These parameters not only induce unusual geochemical behavior of the elements inverting many nominally lithophile elements to chalcophile or even siderophile or anthracophile. Structure and mineral inventories in EL3 and EH3 chondrites are fundamentally different. Yet EH3 and EL3 members store crucial information relevant to eventual source regions and importantly possible variation in C/O ratio in the course of their evolution. EL3 and EH3 chondrites contain trichotomous lithologies (1) chondrules and their fragments, (2) polygonal enstatite-dominated objects, and (3) multiphase metal-rich nodules. Mineralogical and cosmochemical inventories of lithologies in the same EL3 indicate not only similarities (REE inventory and anomalies in oldhamite) but also distinct differences (sinoite-enstatite-graphite relationship). Oldhamite in chondrules and polygonal fragments in EL3 depict negative Eu anomaly attesting a common cosmochemical source. Metal-dominated nodules in both EL3 and EH3 are conglomerates of metal clasts and sulfide fragments in EH3 and concentrically zoned C-bearing metal micropebbles (≥25 μm ≤50 μm) in EL3 thus manifesting a frozen in unique primordial accretionary metal texture and composition. Sinoite-enstatite-diopside-graphite textures reveal a nucleation and growth strongly suggestive of fluctuating C/O ratio during their nucleation and growth in the source regions. Mineral inventories, sulfide phase relations, sinoite-enstatite-graphite intergrowth, carbon and nitrogen isotopic compositions of graphite, spatial nitrogen abundance in graphite in metal nodules, and last but not least ¹²⁹I/¹²⁹Xe and ⁵³Mn/⁵³Cr systematics negate any previously suggested melting episode, pre-accretionary or dynamic, in parental asteroids.