¹Tetsuya Yokoyama, ¹Yusuke Fukami, ¹Wataru Okuia, ¹Nobuaki Ito, ¹Hiroshi Yamazaki
¹Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Ookayama, Tokyo 152-8851, Japan

Precise Sr isotopic compositions in samples from sequential acid leaching experiments have been determined for three carbonaceous chondrites, Allende, Murchison, and Tagish Lake, together with those in the bulk aliquots of these meteorites. The chondritic acid leachates and residues were characterized by Sr isotope anomalies with variable μ84Sr values (106 relative deviation from a standard material) ranging from +120 to −4700 ppm−4700 ppm, documenting multiple nucleosynthetic sources within a single meteorite. In addition, the μ84Sr patterns across leaching samples for individual chondrites differed from one another. The highest μ84Sr values were observed for leaching Step 3 (HCl+H2O, 75 °C) for Allende and Murchison likely because of the incorporation of calcium and aluminum-rich inclusions (CAIs). In contrast, extremely low μ84Sr values were observed in the later fractions (Steps 6 and 7) for Murchison and Tagish Lake, suggesting the existence of s-process-enriched presolar SiC grains derived from AGB stars.
A μ84Sr–ϵ54Cr diagram was prepared with the CAIs and bulk aliquots of carbonaceous chondrites and other meteorites (noncarbonaceous) that were plotted separately; however, they still formed a global positive correlation. CAIs presented the highest μ84Sr and ϵ54Cr values, whereas carbonaceous chondrites and noncarbonaceous meteorites had intermediate and the lowest μ84Sr and ϵ54Cr values, respectively. The positive trend was interpreted as resulting from global thermal processing in which sublimation of high μ84Sr and ϵ54Cr carriers generated the excess μ84Sr and ϵ54Cr signatures in CAIs, while noncarbonaceous planetesimals accreted from materials that underwent significant thermal processing and thus had relatively low μ84Sr and ϵ54Cr values. Apart from the global trend, the carbonaceous chondrites and noncarbonaceous meteorites both exhibited intrinsic variations that highlight an isotopic dichotomy similar to that observed in other isotope combinations (e.g., ϵ54Cr–ϵ50Ti, ϵ54Cr–Δ17O). A plausible scenario for creation of the intrinsic variations involves local thermal processing (e.g., flash heating for chondrule formation) caused by additional selective destruction of presolar grains differently than that caused by global thermal processing. The existence of such a global positive trend and local variations for two meteorite groups suggests a complicated dynamic history for the dust grains with respect to thermal processing, material transportation, and mixing in the protoplanetary disk prior to planetesimal formation.

Reference
Yokoyama T, Fukami Y, Okui W, Ito N, Yamazaki H (2015) Nucleosynthetic strontium isotope anomalies in carbonaceous chondrites. Earth and Planetary Science Letters 416, 46–55
Link to Article [doi:10.1016/j.epsl.2015.01.040]

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^1,2Sarah J. Day, ¹Stephen P. Thompson, ²Aneurin Evans, ¹Julia E. Parker
¹Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire OX11 0DE, UK
²Astrophysics Group, Keele University, Keele, Staffordshire, ST5 5BG, UK

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Day SJ, Thompson SP, Evans A, Parker JE (2015) In situ apparatus for the study of clathrate hydrates relevant to solar system bodies using synchrotron X-ray diffraction and Raman spectroscopy. Astronomy&Astrophysics 574, A91

¹Mathieu Roskosz, ¹Hugues Leroux
¹Unité Matériaux et Transformations, Université Lille 1, CNRS, UMR 8207, F-59655 Villeneuve d’Ascq, France

Crystalline silica (SiO2) is recurrently identified at the percent level in the infrared spectra of protoplanetary disks. By contrast, reports of crystalline silica in primitive meteorites are very unusual. This dichotomy illustrates the typical gap existing between astrophysical observations and meteoritical records of the first solids formed around young stars. The cometary samples returned by the Stardust mission in 2006 offer an opportunity to have a closer look at a silicate dust that experienced a very limited reprocessing since the accretion of the dust. Here, we provide the first extended study of silica materials in a large range of Stardust samples. We show that cristobalite is the dominant form. It was detected in 5 out of 25 samples. Crystalline silica is thus a common minor phase in Stardust samples. Furthermore, olivine is generally associated with this cristobalite, which put constraints on possible formation mechanisms. A low-temperature subsolidus solid–solid transformation of an amorphous precursor is most likely. This crystallization route favors the formation of olivine (at the expense of pyroxenes), and crystalline silica is the natural byproduct of this transformation. Conversely, direct condensation and partial melting are not expected to produce the observed mineral assemblages. Silica is preserved in cometary materials because they were less affected by thermal and aqueous alterations than their chondritic counterparts. The common occurrence of crystalline silica therefore makes the cometary material an important bridge between the IR-based mineralogy of distant protoplanetary disks and the mineralogy of the early solar system.

Reference
Roskosz M, Leroux H (2015) A Significant Amount of Crystalline Silica in Returned Cometary Samples: Bridging the Gap between Astrophysical and Meteoritical Observations. Astrophysical Journal Letters, 801 L7.
Link to Article [doi:10.1088/2041-8205/801/1/L7]

¹Fred J. Ciesla
¹Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, IL 60637, USA

One explanation for the enhanced ratio of volatiles to hydrogen in Jupiter’s atmosphere compared to a a gas of solar composition is that the planet accreted volatile-bearing clathrates during its formation. Models, however, suggest that S would be over abundant if clathrates were the primary carrier of Jupiter’s volatiles. This led to the suggestion that S was depleted in the outer nebula due to the formation troilite (FeS). Here, this depletion is quantitatively explored by modeling the coupled dynamical and chemical evolution of Fe grains in the solar nebula. It is found that disks that undergo rapid radial expansion from an initially compact state may allow sufficient production of FeS and carry H2S-depleted gas outward where ices would form, providing the conditions needed for S-depleted clathrates to form. However, this expansion would also carry FeS grains to this region, which could also be incorporated into planetesimals. Thus for clathrates to be a viable source of volatiles, models must account for the presence of both H2S in FeS in the outer solar nebula.

Reference
Ciesla FJ (2015) Sulfidization of Iron in the Dynamic Solar Nebula and Implications for Planetary Compositions. Astrophysical Journal Letters 800 L6.
Link to Article [doi:10.1088/2041-8205/800/1/L6]

¹A. Geminale et al. (>10)*
¹IAPS Istituto di Astrofisica e Planetologia Spaziali, INAF Istituto Nazionale di AstroFisica, Via del Fosso del Cavaliere, 100-00133 Rome, Italy
*Find the extensive, full author and affiliation list on the publishers Website

The aim of this work is to extract the surface contribution in the Martian visible/near-infrared spectra removing the atmospheric components by means of Principal Component Analysis (PCA) and target transformation (TT). The developed technique is suitable for separating spectral components in a data set large enough to enable an effective usage of statistical methods, in support to the more common approaches to remove the gaseous component. In this context, a key role is played by the estimation, from the spectral population, of the covariance matrix that describes the statistical correlation of the signal among different points in the spectrum. As a general rule, the covariance matrix becomes more and more meaningful increasing the size of initial population, justifying therefore the importance of sizable datasets. Data collected by imaging spectrometers, such as the OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) instrument on board the ESA mission Mars Express (MEx), are particularly suitable for this purpose since it includes in the same session of observation a large number of spectra with different content of aerosols, gases and mineralogy. The methodology presented in this work has been first validated using a simulated dataset of spectra to evaluate its accuracy. Then, it has been applied to the analysis of OMEGA sessions over Nili Fossae and Mawrth Vallis regions, which have been already widely studied because of the presence of hydrated minerals. These minerals are key components of the surface to investigate the presence of liquid water flowing on the Martian surface in the Noachian period. Moreover, since a correction for the atmospheric aerosols (dust) component is also applied to these observations, the present work is able to completely remove the atmospheric contribution from the analysed spectra. Once the surface reflectance, free from atmospheric contributions, has been obtained, the Modified Gaussian Model (MGM) has been applied to spectra showing the hydrated phase. Silicates and iron-bearing hydrated minerals have been identified by means of the electronic transitions of Fe2+ between 0.8-1.2 μm, while at longer wavelengths the hydrated mineralogy is identified by overtones of the OH group. Surface reflectance spectra, as derived through the method discussed in this paper, clearly show a lower level of the atmospheric residuals in the 1.9 hydration band, thus resulting in a better match with the MGM deconvolution parameters found for the laboratory spectra of Martian hydrated mineral analogues and allowing a deeper investigation of this spectral range.

Reference
Geminale A et al. (2015) Removal of atmospheric features in near infrared spectra by means of principal component analysis and target transformation for the study of hydrated minerals on Mars. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.02.012]

Copyright Elsevier

^1,2Alison R. Santos, ^1,2Carl B. Agee, ^1,2Francis M. McCubbin, ^1,2Charles K. Shearer, ¹Paul V. Burger, ³Romain Tartèse, ^3,4Mahesh Anand
¹Institute of Meteoritics, University of New Mexico, 200 Yale Blvd SE, Albuquerque, NM, 87131, USA
²Department of Earth and Planetary Sciences, University of New Mexico, 200 Yale Blvd SE, Albuquerque, NM, 87131, USA
³Planetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
⁴Department of Earth Sciences, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK

The martian meteorite Northwest Africa (NWA) 7034 was examined both petrographically and geochemically using several micro-beam techniques including electron probe microanalysis and secondary ion mass spectrometry. We have identified various clast types of igneous, sedimentary, and impact origin that occur within the breccia, and we define a classification scheme for these materials based on our observations, although our primary focus here is on the petrology of the igneous clasts. A number of different igneous clasts are present in this meteorite, and our study revealed the presence of at least four different igneous lithologies (basalt, basaltic andesite, trachyandesite, and an Fe, Ti, and P (FTP) rich lithology). These lithologies do not appear to be related by simple igneous processes such as fractional crystallization, indicating NWA 7034 is a polymict breccia that contains samples from several different igneous sources. The basalt lithologies are a good match for measured rock compositions from the martian surface, however more exotic lithologies (e.g., trachyandesite and FTP lithologies) show this meteorite contains previously unsampled rock types from Mars. These new rock types provide evidence for a much greater variety of igneous rocks within the martian crust than previously revealed by martian meteorites, and supports recent rover observations of lithologic diversity across the martian surface. Furthermore, the ancient ages for the lithologic components in NWA 7034 indicate Mars developed this lithologic diversity in the early stages of crust formation.

Reference
Santos AR, Agee CB, McCubbin FM, Shearer CK, Burger PV, Tartèse R, Anand M (2015) Petrology of igneous clasts in Northwest Africa 7034: Implications for the petrologic diversity of the martian crust. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.023]

Copyright Elsevier

^1,2W. Akram, ^1,2M. Schönbächler, ³S. Bisterzo, ³R. Gallino
¹School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL, UK
²Institute for Geochemistry and Petrology, ETH, Clausiusstrasse 25, 8092 Zürich, Switzerland
³Dipartimento di Fisica, Università di Torino, Via P. Giura 1, I–10125 Torino, Italy

A growing number of elements show well–resolved nucleosynthetic isotope anomalies in bulk–rock samples of solar system materials. In order to establish the occurrence and extent of such isotopic heterogeneities in Zr, and to investigate the origin of the widespread heterogeneities in our solar system, new high–precision Zr isotope data are reported for a range of primitive and differentiated meteorites. The majority of the carbonaceous chondrites (CV, CM, CO, CK) display variable ε96Zr values (⩽ 1) relative to the Earth. The data indicate the heterogeneous distribution of 96Zr–rich CAIs in these meteorites, which sampled supernova (SN) material that was potentially synthesized by charged–particle reactions or neutron-captures. Other carbonaceous chondrites (CI, CB, CR), ordinary chondrites and eucrites display variable excesses (ε96Zr ⩽ 1) correlated with small depletions in 91Zr (ε91Zr ⩽ 0.2) relative to the Earth and enstatite chondrites. In contrast to the CAI–related heterogeneity, this correlation provides evidence for variable contributions of average solar system s–process material to different regions of the solar system, with the Earth representing the most s–process enriched material. New s–process model calculations indicate that this s–process component was produced in both low and intermediate mass asymptotic giant branch (AGB) stars. The bulk rock heterogeneity is different to the s–process signature resolved in a previous Zr leaching experiment, which was attributed to low mass AGB stars. The bulk rock heterogeneity requires several nucleosynthetic sources, and therefore opposes the theory of the injection of material from a single source (e.g., supernova, AGB star) and argues for a selective dust–sorting mechanism within the solar nebula. Thermal processing of labile carrier phases is considered and, if correct, necessitates the destruction and removal of non–s–process material from the innermost solar system. New Zr isotope data on mineral separates and a fusion crust sample from chondrites indicate that this non–s–process material could be silicates.

Reference
Akram W, Schönbächler M, Bisterzo S, Gallino R (2015) Zirconium isotope evidence for the heterogeneous distribution of s–process materials in the solar System. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.013]

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¹C.Pan,¹A. D. Rogers,^2,3J. R. Michalski
¹Department of Geosciences, Stony Brook University, Stony Brook, NY, USA
²Planetary Science Institute, Tucson, Arizona, USA
³Department of Earth Sciences, Natural History Museum, London, UK

Central peaks of impact craters contain materials exhumed from depth and therefore, investigation of these materials provide clues to subsurface geology and mineralogy. A global spectral survey of central peaks of Martian impact craters between 10–200 km diameter was completed using Mars Odyssey Thermal Emission Imaging System (THEMIS) data. Twenty-six central peaks with distinctive spectral signatures from surrounding plains were identified and characterized with thermal infrared and visible/near-infrared data. The distribution of spectrally distinct central peaks (SDCPs) shows some degree of regional clustering, with most craters found in western Noachis Terra, Tyrrhena Terra, within the northern rim of Hellas Basin, and fewer in the northern lowlands. With the exception of four craters in western Noachis Terra, SDCPs contain only one spectrally distinct unit at THEMIS resolution (100 m/pixel). The maximum number of spectrally distinct units observed was three, in Jones and Ostrov craters. The western Noachis Terra SDCPs may expose crustal stratigraphies of multiple igneous compositions or impact materials from Argyre. In the highlands, most SDCP units are consistent with enrichments in olivine or pyroxene relative to surrounding plains, suggesting olivine- and pyroxene-basaltic lithologies; few are olivine- and pyroxene-poor. No spatial trend in spectrally-derived compositions of SDCPs was observed. Three SDCPs contain THEMIS signatures consistent with high abundances of phyllosilicates, which may contain the most phyllosilicate-rich lithologies found in central peak-associated materials globally.

Reference
Pan C, Rogers AD, Michalski JR (2015) Thermal and Near-Infrared Analyses of Central Peaks of Martian Impact Craters: Evidence for a Heterogeneous Martian Crust. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004676]

Published by arrangement with John Wiley&Sons

¹Laurette Piani, ¹François Robert, ¹Laurent Remusat
¹Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie, Sorbonne Universités, UMR CNRS 7590, Université Pierre et Marie Curie, IRD, Muséum National d’Histoire Naturelle, 57 rue Cuvier, Case 52, 75231 Paris Cedex 5, France

Organic matter and hydrous silicates are intimately mixed in the matrix of chondrites and in-situ determination of their individual D/H ratios is therefore challenging. Nevertheless, the D/H ratio of each pure component in this mixture should yield a comprehensible signature of the origin and evolution of water and organic matter in our solar system.
We measured hydrogen isotope ratios of organic and hydrous silicates in the matrices of two carbonaceous chondrites (Orgueil CI1 and Renazzo CR2) and one unequilibrated ordinary chondrite (Semarkona, LL3.0). A novel protocol was adopted, involving NanoSIMS imaging of H isotopes of monoatomatic (H−) and molecular (OH−) secondary ions collected at the same location. This allowed the most enriched component with respect to D to be identified in the mixture. Using this protocol, we found that in carbonaceous chondrites the isotopically homogeneous hydrous silicates are mixed with D-rich organic matter. The opposite was observed in Semarkona. Hydrous silicates in Semarkona display highly heterogeneous D/H ratios, ranging from 150 to 1800×10−61800×10−6 (δDSMOW=−40δDSMOW=−40 to 10 600‰). Organic matter in Semarkona does not show such large isotopic variations. This suggests limited isotopic exchange between the two phases during aqueous alteration. Our study greatly expands the range of water isotopic values measured so far in solar system objects. This D-rich water reservoir was sampled by the LL ordinary chondrite parent body and an estimate (≤9%) of its relative contribution to the D/H ratio of water in Oort cloud family comets is proposed.

Reference
Piani L, Robert F, Remusat L (2015) Micron-scale D/H heterogeneity in chondrite matrices: A signature of the pristine solar system water?
Earth and Planetary Acience Letters, 415, 154–164
Link to Article [doi:10.1016/j.epsl.2015.01.039]

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¹Mathieu Touboul, ²Peter Sprung, ^1,3Sarah A. Aciego, ^1,4Bernard Bourdon, ^1,2Thorsten Kleine
¹Institute for Geochemistry and Petrology, ETH Zürich, Clausiusstrasse 25, 8092 Zurich, Switzerland.
²Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm Klemm-Str. 10, 48149 Münster, Germany
³Department of Earth and Environmental Sciences, 2534 C.C. Little Building, 1100 N. University, Ann Arbor, MI, 48109, USA
⁴Laboratoire de Géologie des Lyon, Ecole Normale Supérieure de Lyon, CNRS and UCBL, 46, Allée d’Italie, 69364 Lyon cedex 7, France

The 182Hf-182W systematics of 12 whole-rock eucrites, including basaltic and cumulate samples, have been investigated. The Hf-W isotope systematics of both basaltic and cumulate eucrites are consistent with derivation from a single mantle source characterized by a strongly suprachondritic Hf/W ratio (180Hf/184W of ∼19). The elevated Hf/W of this mantle source was established by core formation within ∼1 Ma after CAI formation or, alternatively, represents that of the residual melt of a magma ocean from which the eucrites ultimately formed. In the latter case the time of core formation is more uncertain and core formation may have occurred slightly later than ∼1 Ma. The investigated basaltic eucrites fall into three distinct age groups with Hf-W ages of ∼4 Ma (Stannern), ∼11 Ma (Bereba, Bouvante) and ∼22 Ma (Camel Donga, Juvinas) after CAI formation and provide evidence for a protracted history of magmatism and crustal metamorphism on the eucrite parent body, lasting for at least ∼20 Ma. Evidence for even later activity is provided by the cumulate eucrites, which exhibit only small if any variations in 182W/184W in spite of variable 180Hf/184W, indicating cooling below the Hf-W closure temperature when 182Hf was nearly extinct. On the basis of three cumulate eucrites, a Hf-W age of 38±21 Ma is inferred, but even younger ages would also be consistent with the Hf-W data.

Reference
Touboul M, Sprung P, Aciego SA, Bourdon B, Kleine T (2015) Hf-W chronology of the eucrite parent Body. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.02.018]

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