¹Ronny Schoenberg, ¹Alexandra Merdian, ²Chris Holmden, ¹Ilka C. Kleinhanns, ¹Kathrin Haßler, ¹Martin Wille, ¹Elmar Reitter
¹Department of Geosciences, University of Tuebingen, Germany
²Department of Geological Sciences, University of Saskatchewan, Canada

The depletion of chromium in Earth’s mantle (∼2,700 ppm) in comparison to chondrites (∼4,400 ppm) indicates significant incorporation of chromium into the core during our planet’s metal-silicate differentiation, assuming that there was no significant escape of the moderately volatile element chromium during the accretionary phase of Earth. Stable Cr isotope compositions – expressed as the ‰-difference in 53Cr/52Cr from the terrestrial reference material SRM979 (δ53/52CrSRM979 values) – of planetary silicate reservoirs might thus yield information about the conditions of planetary metal segregation processes when compared to chondrites. The stable Cr isotopic compositions of 7 carbonaceous chondrites, 11 ordinary chondrites, 5 HED achondrites and 2 martian meteorites determined by a double spike MC-ICP-MS method are within uncertainties indistinguishable from each other and from the previously determined δ53/52CrSRM979 value of –0.124 ± 0.101 ‰ for the igneous silicate Earth. Extensive quality tests support the accuracy of the stable Cr isotope determinations of various meteorites and terrestrial silicates reported here. The uniformity in stable Cr isotope compositions of samples from planetary silicate mantles and undifferentiated meteorites indicates that metal-silicate differentiation of Earth, Mars and the HED parent body did not cause measurable stable Cr isotope fractionation between these two reservoirs. Our results also imply that the accretionary disc, at least in the inner solar system, was homogeneous in its stable Cr isotopic composition and that potential volatility loss of chromium during accretion of the terrestrial planets was not accompanied by measurable stable isotopic fractionation. Small but reproducible variations in δ53/52CrSRM979 values of terrestrial magmatic rocks point to natural stable Cr isotope variations within Earth’s silicate reservoirs. Further and more detailed studies are required to investigate whether silicate differentiation processes, such partial mantle melting and crystal fractionation, can cause stable Cr isotopic fractionation on Earth and other planetary bodies.

Reference
Schoenberg R, Merdian A, Holmden C, Kleinhanns IC, Haßler K, Wille M, Reitter E (2016) The stable Cr isotopic compositions of chondrites and silicate planetary reservoirs. Geochmica et Cosmochmica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.03.013]
Copyright Elsevier

^1,2Geoffrey H. Howarth, ^3,4Yang Liu, ^3,4Yang Chen, ¹John F. Pernet-Fisher, ¹Lawrence A. Taylor
¹Earth and Planetary Sciences Department, Planetary Geosciences Institute, University of Tennessee, Knoxville, Tennessee, USA
²Department of Geological Sciences, University of Cape Town, Rondebosch, South Africa
³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
⁴Division of Geology and Planetary Science, California Institute of Technology, Pasadena, California, USA

Apatite is the major volatile-bearing phase in Martian meteorites, containing structurally bound fluorine, chlorine, and hydroxyl ions. In apatite, F is more compatible than Cl, which in turn is more compatible than OH. During degassing, Cl strongly partitions into the exsolved phase, whereas F remains in the melt. For these reasons, the volatile concentrations within apatite are predictable during magmatic differentiation and degassing. Here, we present compositional data for apatite and merrillite in the paired enriched, olivine-phyric shergottites LAR 12011 and LAR 06319. In addition, we calculate the relative volatile fugacities of the parental melts at the time of apatite formation. The apatites are dominantly OH-rich (calculated by stoichiometry) with variable yet high Cl contents. Although several other studies have found evidence for degassing in the late-stage mineral assemblage of LAR 06319, the apatite evolutionary trends cannot be reconciled with this interpretation. The variable Cl contents and high OH contents measured in apatites are not consistent with fractionation either. Volatile fugacity calculations indicate that water and fluorine activities remain relatively constant, whereas there is a large variation in the chlorine activity. The Martian crust is Cl-rich indicating that changes in Cl contents in the apatites may be related to an external crustal source. We suggest that the high and variable Cl contents and high OH contents of the apatite are the results of postcrystallization interaction with Cl-rich, and possibly water-rich, crustal fluids circulating in the Martian crust.

Reference
Howarth GH, Liu Y, Chen Y, Pernet-Fisher JF, Taylor LA (2016) Postcrystallization metasomatism in shergottites: Evidence from the paired meteorites LAR 06319 and LAR 12011. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12576]
Published by arrangement with John Wiley & Sons

¹N. G. Rudraswami, ¹M. Shyam Prasad, ²E. V. S. S. K. Babu,²T. Vijaya Kumar
¹National Institute of Oceanography (Council of Scientific and Industrial Research), Goa, India
²National Geophysical Research Institute (Council of Scientific and Industrial Research), Hyderabad, India

Micrometeorites that pass through the Earth’s atmosphere undergo changes in their chemical compositions, thereby making it difficult to understand if they are sourced from the matrix, chondrules, or calcium–aluminum-rich inclusions (CAIs). These components have the potential to provide evidence toward the understanding of the early solar nebular evolution. The variations in the major element and trace element compositions of 155 different type (scoriaceous, relict bearing, porphyritic, barred, cryptocrystalline, and glass) of S-type cosmic spherules are investigated with the intent to decipher the parent sources using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry. The S-type cosmic spherules appear to show a systematic depletion in volatile element contents, but have preserved their refractory trace elements. The trends in their chemical compositions suggest that the S-type spherules comprise of components from similar parent bodies, that is, carbonaceous chondrites. Large fosteritic relict grains observed in this investigation appear to be related to the fragments of chondrules from carbonaceous chondrites. Furthermore, four spherules (two of these spherules enclose spinels and one comprised entirely of a Ca-Al-rich plagioclase) show enhanced trace element enrichment patterns that are drastically different from all the other 151 cosmic spherules. The information on the chemical composition and rare earth elements (REEs) on cosmic spherules suggest that the partially to fully melted ones can preserve evidences related to their parent bodies. The Ce, Eu, and Tm anomalies found in the cosmic spherules have similar behavior as that of chondrites. Distinct correlations observed between different REEs and types of cosmic spherules reflect the inherited properties of the precursors.

Reference
Rudraswami NG, Prasad MS, Babu EVSSK, Kumar TV (2016) Major and trace element geochemistry of S-type cosmic spherules. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12618]
Published by arrangement with John Wiley & Sons

^1,2Matthias M.M. Meier, ¹Christophe Cloquet,¹Bernard
¹Centre de Recherches Pétrographiques et Géochimiques (CRPG), UMR 7358, Université de Lorraine, CNRS, 54500 Vandœuvre-lès-Nancy, France
²Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse 25, 8092 Zurich, Switzerland

We have measured the concentration, isotopic composition and thermal release profiles of Mercury (Hg) in a suite of meteorites, including both chondrites and achondrites. We find large variations in Hg concentration between different meteorites (ca. 10 ppb to 14’000 ppb), with the highest concentration orders of magnitude above the expected bulk solar system silicates value. From the presence of several different Hg carrier phases in thermal release profiles (150 – 650 °C), we argue that these variations are unlikely to be mainly due to terrestrial contamination. The Hg abundance of meteorites shows no correlation with petrographic type, or mass-dependent fractionation of Hg isotopes. Most carbonaceous chondrites show mass-independent enrichments in the odd-numbered isotopes 199Hg and 201Hg. We show that the enrichments are not nucleosynthetic, as we do not find corresponding nucleosynthetic deficits of 196Hg. Instead, they can partially be explained by Hg evaporation and redeposition during heating of asteroids from primordial radionuclides and late-stage impact heating. Non-carbonaceous chondrites, most achondrites and the Earth do not show these enrichments in vapor-phase Hg. All meteorites studied here have however isotopically light Hg (δ202Hg = ∼-7 to -1) relative to the Earth’s average crustal values, which could suggest that the Earth has lost a significant fraction of its primordial Hg. However, the late accretion of carbonaceous chondritic material on the order of ∼2%, which has been suggested to account for the water, carbon, nitrogen and noble gas inventories of the Earth, can also contribute most or all of the Earth’s current Hg budget. In this case, the isotopically heavy Hg of the Earth’s crust would have to be the result of isotopic fractionation between surface and deep-Earth reservoirs.

Reference
Meier MMM, Cloquet C, Marty B (2016) Mercury (Hg) in meteorites: variations in abundance, thermal release profile, mass-dependent and mass-independent isotopic fractionation. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.03.007]
Copyright Elsevier

^1,2Emmanuel Jacquet, ^3,4Jean-Alix Barrat, ^5,6Pierre Beck, ¹Florent Caste, ⁷Jérôme Gattacceca, ⁷Corinne Sonzogni,^1,8Matthieu Gounelle
¹Institut de Minéralogie de Physique des Matériaux et de Cosmochimie, CNRS & Muséum National d’Histoire Naturelle, UMR 7202, Paris, France
²Canadian Institute for Theoretical Astrophysics, Toronto, Ontario, Canada
³Laboratoire Domaines Océaniques, UMR 6538, Université Européenne de Bretagne, Bretagne, France
⁴CNRS UMR 6538 (Domaines Océaniques), U.B.O.-I.U.E.M., Plouzané Cedex, France
⁵1 Univ. Grenoble Alpes, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Grenoble, France
⁶CNRS, IPAG, Grenoble, France
⁷CEREGE UM 34, CNRS/Université d’Aix-Marseille 3, Aix-en-Provence, France
⁸Institut Universitaire de France, Paris, France

Northwest Africa (NWA) 5958 is a carbonaceous chondrite found in Morocco in 2009. Preliminary chemical and isotopic data leading to its initial classification as C3.0 ungrouped have prompted us to conduct a multitechnique study of this meteorite and present a general description here. The petrography and chemistry of NWA 5958 is most similar to a CM chondrite, with a low degree of aqueous alteration, apparently under oxidizing conditions, and evidence of a second, limited alteration episode manifested by alteration fronts. The oxygen isotopic composition, with ∆’17O = −4.3‰, is more 16O-rich than all CM chondrites, indicating, along with other compositional arguments, a separate parent body of origin. We suggest that NWA 5958 be reclassified as an ungrouped carbonaceous chondrite related to the CM group.

Reference
Jacquet E, Barrat J-A, Beck P, Caste F, Gattacceca J, Sonzogni C, Gounelle M (2016) Northwest Africa 5958: A weakly altered CM-related ungrouped chondrite, not a CI3. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12628]
Published by arrangement with John Wiley & Sons

^1,2Melissa A. Morris, ³Stuart J. Weidenschilling,²Steven J. Desch
¹State University of New York at Cortland, Cortland, New York, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
³Planetary Science Institute, Tucson, Arizona, USA

Chondrules represent one of the best probes of the physical conditions and processes acting in the early solar nebula. Proposed chondrule formation models are assessed based on their ability to match the meteoritic evidence, especially experimental constraints on their thermal histories. The model most consistent with chondrule thermal histories is passage through shock waves in the solar nebula. Existing models of heating by shocks generally yield a good first-order approximation to inferred chondrule cooling rates. However, they predict prolonged heating in the preshock region, which would cause volatile loss and isotopic fractionation, which are not observed. These models have typically included particles of a single (large) size, i.e., chondrule precursors, or at most, large particles accompanied by micron-sized grains. The size distribution of solids present during chondrule formation controls the opacity of the affected region, and significantly affects the thermal histories of chondrules. Micron-sized grains evaporate too quickly to prevent excessive heating of chondrule precursors. However, isolated grains in chondrule-forming regions would rapidly coagulate into fractal aggregates. Preshock heating by infrared radiation from the shock front would cause these aggregates to melt and collapse into intermediate-sized (tens of microns) particles. We show that inclusion of such particles yields chondrule cooling rates consistent with petrologic and isotopic constraints.

Reference
Morris MA, Weidenschilling SJ, Desch SJ (2016) The effect of multiple particle sizes on cooling rates of chondrules produced in large-scale shocks in the solar nebula. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12631]
Published by arrangement with John Wiley & Sons

¹Justin Filiberto, ^2,3Juliane Gross,⁴Francis M. McCubbin
¹Department of Geology, Southern Illinois University, Carbondale, Illinois, USA
²Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
³Department of Earth and Planetary Sciences, The American Museum of Natural History, New York, New York, USA
⁴NASA Johnson Space Center, Houston, Texas, USA

Previous estimates of the volatile contents of Martian basalts, and hence their source regions, ranged from nearly volatile-free through estimates similar to those found in terrestrial subduction zones. Here, we use the bulk chemistry of Martian meteorites, along with Martian apatite and amphibole chemistry, to constrain the volatile contents of the Martian interior. Our estimates show that the volatile content of the source region for the Martian meteorites is similar to the terrestrial Mid-Ocean-Ridge Mantle source. Chlorine is enriched compared with the depleted terrestrial mantle but is similar to the terrestrial enriched source region; fluorine is similar to the terrestrial primitive mantle; and water is consistent with the terrestrial mantle. Our results show that Martian magmas were not volatile saturated; had water/chlorine and water/fluorine ratios ~0.4–18; and are most similar, in terms of volatiles, to terrestrial MORBs. Presumably, there are variations in volatile content in the Martian interior as suggested by apatite compositions, but more bulk chemical data, especially for fluorine and water, are required to investigate these variations. Finally, the Noachian Martian interior, as exemplified by surface basalts and NWA 7034, may have had higher volatile contents.

Reference
Filiberto J, Gross J, McCubbin FM (2016) Constraints on the water, chlorine, and fluorine content of the Martian mantle. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12624]
Published by arrangement with John Wiley & Sons

^1,2,3Hideaki Miyamoto, ^1,4Takafumi Niihara, ⁵Takeshi Kuritani, ¹Peng K. Hong, ¹James M. Dohm, ²Seiji Sugita
¹University Museum, University of Tokyo, Tokyo, 113-0033, Japan
²Department of Earth and Planetary Sciences, University of Tokyo, Tokyo, 113-0033, Japan
³Planetary Science Institute, Tucson, Arizona, USA
⁴Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas, USA
⁵Department of Natural History Sciences, Hokkaido University, Sapporo, Japan

Remote sensing observations by recent successful missions to small bodies have revealed the difficulty in classifying the materials which cover their surfaces into a conventional classification of meteorites. Although reflectance spectroscopy is a powerful tool for this purpose, it is influenced by many factors, such as space weathering, lighting conditions, and surface physical conditions (e.g., particle size and style of mixing). Thus, complementary information, such as elemental compositions, which can be obtained by X-ray fluorescence (XRF) and gamma-ray spectrometers (GRS), have been considered very important. However, classifying planetary materials solely based on elemental compositions has not been investigated extensively. In this study, we perform principal component and cluster analyses on 12 major and minor elements of the bulk compositions of 500 meteorites reported in the National Institute of Polar Research (NIPR), Japan database. Our unique approach, which includes using hierarchical cluster analysis, indicates that meteorites can be classified into about 10 groups purely by their bulk elemental compositions. We suggest that Si, Fe, Mg, Ca, and Na are the optimal set of elements, as this set has been used successfully to classify meteorites of the NIPR database with more than 94% accuracy. Principal components analysis indicates that elemental compositions of meteorites form eight clusters in the three-dimensional space of the components. The three major principal components (PC1, PC2, and PC3) are interpreted as (1) degree of differentiations of the source body (i.e., primitive versus differentiated), (2) degree of thermal effects, and (3) degree of chemical fractionation, respectively.

Reference
Miyamoto H, Niihara T, Kuritani T, Hong PK, Dohm JM, Sugita S (2016) Cluster analysis on the bulk elemental compositions of Antarctic stony meteorites. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12634]
Published by arrangement with John Wiley & Sons

¹Alexei V. Fedkin,²Lawrence Grossman
¹Department of the Geophysical Sciences, The University of Chicago, Chicago, Illinois, USA
²Enrico Fermi Institute, The University of Chicago, Chicago, Illinois, USA

The degree to which dust enrichment enhances the oxygen fugacity (fO2) of a system otherwise solar in composition depends on the dust composition. Equilibrium calculations were performed at 10−3 bar in systems enriched by a factor of 104 in two fundamentally different types of dust to investigate the iron oxidation state in both cases. One type of dust, called SC for solar condensate, stopped equilibrating with solar gas at too high a temperature for FeO or condensed water to be stabilized in any form, and thus has the composition expected of a nebular condensate. The other has CI chondrite composition, appropriate for a parent body that accreted from SC dust and low-temperature ice. Upon total vaporization at 2300 K, both systems have high fO2, >IW. In the SC dust-enriched system, FeO of the bulk silicate reaches ~10 wt% at 1970 K but decreases to <1 wt% below 1500 K. The FeO undergoes reduction because consumption of gaseous oxygen by silicate recondensation causes a precipitous drop in fO2. Thus, enrichment in dust having the composition of likely nebular condensates cannot yield a sufficiently oxidizing environment to account for the FeO contents of chondrules. The fO2 of the system enriched in water-rich, CI dust, however, remains high throughout condensation, as gaseous water remains uncondensed until very low temperatures. This allows silicate condensates to achieve and maintain FeO contents of 27–35 wt%. Water-rich parent bodies are thus excellent candidate sources of chondrule precursors. Impacts on such bodies may have created the combination of high dust enrichment, total pressure, and fO2 necessary for chondrule formation.

Reference
Fedkin AV, Grossman L (2016) Effects of dust enrichment on oxygen fugacity of cosmic gases. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12627]
Published by arrangement with John Wiley & Sons

¹Kelsey B. Williams, ²Colin R.M. Jackson, ³Leah C. Cheek, ⁴Kerri L. Donaldson-Hanna, ⁵Stephen W. Parman, ⁵Carle M. Pieters, ⁶M. Darby Dyar, ⁵Tabb C. Prissel
¹Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, Missouri 63130, U.S.A.
²Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015, U.S.A.
³Department of Earth and Environment, Boston University, 685 Commonwealth Avenue, Boston, Massachusetts 02215, U.S.A.
⁴Atmospheric, Oceanic and Planetary Physics, Oxford University, Clarendon Laboratory, Parks Road, Oxford, Oxfordshire OX1 3PU, U.K.
⁵Department of Earth, Environmental, and Planetary Sciences, Brown University, 324 Brook Street, Providence, Rhode Island 02912, U.S.A.
⁶Department of Astronomy, Mount Holyoke College, 217 Kendade Hall, 50 College Street, South Hadley, Massachusetts 01002, U.S.A.

Visible to near-infrared (V-NIR) remote sensing observations have identified spinel in various locations and lithologies on the Moon. Experimental studies have quantified the FeO content of these spinels (Jackson et al. 2014), however the chromite component is not well constrained. Here we present compositional and spectral analyses of spinel synthesized with varying chromium contents at lunar-like oxygen fugacity (fO2). Reflectance spectra of the chromium-bearing synthetic spinels (Cr# 1–29) have a narrow (~130 nm wide) absorption feature centered at ~550 nm. The 550 nm feature, attributed to octahedral Cr3+, is present over a wide range in iron content (Fe# 8–30) and its strength positively correlates with spinel chromium content [ln(reflectancemin) = −0.0295 Cr# – 0.3708]. Our results provide laboratory characterization for the V-NIR and mid-infrared (mid-IR) spectral properties of spinel synthesized at lunar-like fO2. The experimentally determined calibration constrains the Cr# of spinels in the lunar pink spinel anorthosites to low values, potentially Cr# < 1. Furthermore, the results suggest the absence of a 550 nm feature in remote spectra of the Dark Mantle Deposits at Sinus Aestuum precludes the presence of a significant chromite component. Combined, the observation of low chromium spinels across the lunar surface argues for large contributions of anorthositic materials in both plutonic and volcanic rocks on the Moon.

Reference
Williams KB, Jackson CRM, Cheek LC, Donaldson-Hanna KL, Parman SW, Pieters CM, Dyar MD, Prissel TC (2016) Reflectance spectroscopy of chromium-bearing spinel with application to recent orbital data from the Moon. American Mineralogist 101, 678-689
Link to Article [doi:10.2138/am-2016-5408CCBYNCND]
Copyright: The Mineralogical Society of America