^1,2James Badro, ³John P. Brodholt, ^1,2Hélène Piet, ¹Julien Siebert, ⁴Frederick J. Ryerson
¹Institut de Physique du Globe de Paris, Sorbonne Paris Cité, UMR CNRS 7154, 75005 Paris, France
²Earth and Planetary Science Laboratory, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland;
³Department of Earth Sciences, University College London, London WC1E 6BT, United Kingdom;
⁴Lawrence Livermore National Laboratory, Livermore, CA 94550

We combine, for the first time to our knowledge, two approaches to study Earth’s core composition: a geochemical approach based on trace element depletion in the mantle and a geophysical approach based on a seismically lighter and faster (than pure iron−nickel) core. The joint approach allows making strong statements; first of all, as opposed to the current belief, Earth must have accreted material that is more oxidized than the present-day mantle, similar to that of planetesimals such as 4-Vesta, and got reduced to its present state during core formation. Secondly, core light-element concentrations in those conditions are 2.7% to 5% oxygen alongside 2% to 3.6% silicon; the oxygen concentrations in the core are higher than previously thought, and, conversely, silicon concentrations are lower than previous estimates.

Reference
Badro J, Brodholt JP, Piet H, Siebert J, Ryerson FR (2015) Core formation and core composition from coupled geochemical and geophysical constraints. Proceedings of the National Academy of Sciences 112, 12310-12314
Link to Article [doi:10.1073/pnas.1505672112]

^1,2Annemarie E. Pickersgill, ¹Roberta L. Flemming,^1,3Gordon R. Osinski
¹Department of Earth Sciences and Centre for Planetary Science and Exploration, University of Western Ontario, London, Ontario, Canada
²Department of Physics and Astronomy, The University of Western Ontario, London, Ontario, Canada
³School of Geographical & Earth Sciences, University of Glasgow, Lilybank Gardens, Glasgow, UK

Studies of shock metamorphism of feldspar typically rely on qualitative petrographic observations, which, while providing invaluable information, can be difficult to interpret. Shocked feldspars, therefore, are now being studied in greater detail by various groups using a variety of modern techniques. We apply in situ micro-X-ray diffraction (μXRD) to shocked lunar and terrestrial plagioclase feldspar to contribute to the development of a quantitative scale of shock deformation for the feldspar group. Andesine and labradorite from the Mistastin Lake impact structure, Labrador, Canada, and anorthite from Earth’s Moon, returned during the Apollo program, were examined using optical petrography and assigned to subgroups of the optical shock level classification system of Stöffler (1971). Two-dimensional μXRD patterns from the same samples revealed increased peak broadening in the chi dimension (χ), due to strain-related mosaicity, with increased optical signs of deformation. Measurement of the full width at half maximum along χ (FWHMχ) of these peaks provides a quantitative way to measure strain-related mosaicity in plagioclase feldspar as a proxy for shock level.

Reference
Pickersgill AE, Flemming RL, Osinski GR (2015) Toward quantification of strain-related mosaicity in shocked lunar and terrestrial plagioclase by in situ micro-X-ray diffraction. Meteoritics & Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12514]
Published by arrangement with John Wiley & Sons

^1,2Jennifer Buz,^1,3Benjamin P. Weiss,^3,4Sonia M. Tikoo,^3,4David L. Shuster,⁵Jérôme Gattacceca,¹Timothy L. Grove
¹Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA
²Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
³Department of Earth and Planetary Science, University of California, Berkeley, CA, USA
⁴Berkeley Geochronology Center, Berkeley, CA, USA
⁵CNRS, Aix-Marseille University, Aix-en-Provence, France

Recent paleomagnetic studies of Apollo samples have established that a core dynamo existed on the Moon from at least 4.2 to 3.56 billion years ago (Ga). Because there is no lunar dynamo today, a longstanding mystery has been the origin of magnetization in very young lunar samples [<~200 million years old (Ma)]. Possible sources of this magnetization include transient fields generated by meteoroid impacts, remanent fields from nearby rocks magnetized during an earlier dynamo epoch, a weak late dynamo, and spontaneous remanence formed in a near-zero field. To further understand the source of the magnetization in young lunar samples, we conducted paleomagnetic, petrographic, and 40Ar/39Ar geochronometry analyses on a young impact melt glass rind from the exterior of ~3.35 Ga mare basalt 12017. Cosmic ray track densities and our 40Ar/39Ar and cosmogenic 38Ar analyses constrain the glass formation age to be 10 μT) core dynamo field nor impact-generated fields.

Reference
Buz J, Weiss BP, Tikoo SM, Shuster DL, Gattacceca J, Grove TL (2015) Magnetism of a Very Young Lunar Glass. Journal of Geophysical Research (Planets)
Link to Article [DOI: 10.1002/2015JE004878]
Published by arrangement with John Wiley & Sons

¹Jerrod Lessel,¹Keith Putirka
¹Department of Earth and Environmental Sciences, California State University, 2576 East San Ramon Avenue, Mail Stop ST24, Fresno, California 93740, U.S.A.

Tests show that terrestrial mineral+liquid geothermobarometers are not well equipped for use on martian rocks, which tend to have much higher FeO and lower Al2O3. Here, we present new calibrations of thermometers and barometers using experimental data on martian samples from the literature. These new models recover P-T conditions with a greater accuracy compared to models calibrated using terrestrial compositions. We applied these new calibrations to primitive martian mantle-derived melts Yamato 980459 (Y98) and Northwest Africa (NWA) 6234 and several surface basalts (Gusev). Our new models yield similar P-T conditions for NWA and Y98 compositions of 1.4–1.7 GPa and 1500–1550 °C, which are close to estimates by most prior studies. Our models yield somewhat lower P estimates compared to Lee et al. (2009), apparently because our Si-activity model (from Beattie 1993) includes an Al2O3-correction (where lower Al2O3, as in martian samples, leads to lower P estimates). For Gusev basalt compositions, our new models yield P-T estimates of 1.0–1.3 GPa and 1340–1390 °C; furthermore, we also obtain P = 1.03 GPa and T = 1340 °C, for a Gusev composition from Monders et al. (2007), which comes very close to the Monders et al. (2007) estimate for multiple saturation, of 1.0 GPa and 1325 °C, derived from phase saturation relationships. Given the different ages of these meteorites, with Gusev at 3.65 Ga (Greeley et al. 2005) and Y98 at 4.3 Ga (Bouvier et al. 2005, 2008, 2009; Werner et al. 2014), their thermal contrasts may represent secular cooling of Mars. We estimate a mantle potential temperature difference of ~200 °C, with mantle potential temperatures of 1450 ±50 °C for Gusev and 1650 ±50 °C for Y98; this implies a cooling rate of 300 °C/Ga. This would appear to be a much more rapid rate of cooling compared to Earth, as may be expected by Mars’ higher surface/volume ratio.

Reference
Lessel J, Putirka K (2015) New thermobarometers for martian igneous rocks, and some implications for secular cooling on Mars. American Mineralogist 100, 2163-2171
Link to Article [doi:10.2138/am-2015-4732]
Copyright: The Mineralogical Society of America

¹Neva A. Fowler-Gerace, ^1,2Kimberly T. Tait
¹Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, Ontario M5S 3B1, Canada
²Department of Natural History, Mineralogy, Royal Ontario Museum, 100 Queens Park, Toronto, Ontario M5S 2C6, Canada

Phosphoran olivine (1–7 wt% P2O5) is a metastable phase known from fewer than a dozen meteoritic or terrestrial occurrences. We have thoroughly examined phosphoran olivine in the Springwater pallasite to characterize its distribution, textural relationships, and geochemistry. Phosphoran olivine is abundant in Springwater as randomly distributed millimeter-scale partial overgrowths on the P-free olivine crystals. Geochemical analyses support the substitution mechanism of P into the tetrahedral Si site with octahedral site vacancies for charge balance; observed trace element variations, on the other hand, are not related to P substitution. Element mapping reveals fine-scale oscillatory P zoning in unusual serrate patterns, indicating rapid crystal nucleation from a melt as proposed by Boesenberg and Hewins (2010) and a subsequently variable rate of crystallization. The timing of phosphoran olivine formation in Springwater is constrained to after the period of macroscopic olivine rounding but before the cooling of the metal matrix; because the phosphoran overgrowths overprint specific host grain boundary modifications, we suggest that the episode of extremely rapid cooling necessary to crystallize and preserve this rare phase may have been triggered by an additional impact to the parent body.

Reference
Fowler-Gerace NA, Tait KT (2015) Phosphoran olivine overgrowths: Implications for multiple impacts to the Main Group pallasite parent Body. American Mineralogist 100, 2043-2052
Link to Article [doi:10.2138/am-2015-5344]
Copyright: The Mineralogical Society of America

¹James J. Papike, ¹ Paul V. Burger, ¹Aaron S. Bell, ¹Charles K. Shearer, ²Loan Le, ³John Jones
¹Institute of Meteoritics, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, U.S.A.
²JSC Engineering, Technology and Science (JETS), NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
³NASA Johnson Space Center, Houston, Texas 77058, U.S.A.

Spinel is a very important rock-forming mineral that is found in basalts from Earth, Mars, the Earth’s Moon, and basaltic meteorites. Spinel can be used as a sensitive indicator of petrologic and geochemical processes that occur in its host rock. This paper highlights the role of increasing fO2 (from IW-1 to FMQ+2) in converting a >90% normal spinel to an ~25% magnetite (inverse) spinel, the trajectory of DVspinel/melt as it relates to the ratio of V3+/V4+ in the melt, and the crystal chemical attributes of the spinel that control the intrinsic compatibility of both V3+ and V4+. This work examines the nuances of the V partitioning and provides a crystal chemical basis for understanding Fe3+, Cr, and V substitution into the octahedral sites of spinel. Understanding this interplay is critical for using spinels as both indicators of planetary parentage and reconstructing the redox history of magmatic systems on the terrestrial planets. Three potential examples for this use are provided. In addition, this work helps explain the ubiquitous miscibility gap between spinels with changing ülvospinel contents.

Reference
Papike JJ, Burger PV, Bell AS, Shearer CK, Le L, Jones J (2015) Normal to inverse transition in martian spinel: Understanding the interplay between chromium, vanadium, and iron valence state partitioning through a crystal-chemical lens. American Mineralogist 100, 2018-2025
Link to Article [doi:10.2138/am-2015-5208]
Copyright: The Mineralogical Society of America

¹Michael T. Thorpe,¹A. Deanne Rogers,²Thomas F. Bristow,¹Cong Pan
¹Department of Geosciences, State University of New York at Stony Brook University, Stony Brook, NY, USA
²NASA Ames Research Center, Moffett Field, CA

Thermal emission spectroscopy is used to determine the mineralogy of sandstone and mudstone rocks as part of an investigation of linear spectral mixing between sedimentary constituent phases. With widespread occurrences of sedimentary rocks on the surface of Mars, critical examination of the accuracy associated with quantitative models of mineral abundances derived from thermal emission spectra of sedimentary materials is necessary. Though thermal emission spectroscopy has been previously proven to be a viable technique to obtain quantitative mineralogy from igneous and metamorphic materials, sedimentary rocks, with natural variation of composition, compaction, and grain size, have yet to be examined. In this work, we present an analysis of the thermal emission spectral (~270-1650 cm−1) characteristics of a suite of 13 sandstones and 14 mudstones. X-ray diffraction and traditional point counting procedures were all evaluated in comparison with thermal emission spectroscopy. Results from this work are consistent with previous thermal emission spectroscopy studies and indicate that bulk rock mineral abundances can be estimated within 11.2 % for detrital grains (i.e., quartz and feldspars) and 14.8 % for all other mineral phases present in both sandstones and mudstones, in comparison to common in situ techniques used for determining bulk rock composition. Clay-sized to fine silt-sized grained phase identification is less accurate, with differences from the known ranging from ~5-24% on average. Nevertheless, linear least squares modeling of thermal emission spectra is an advantageous technique for determining abundances of detrital grains and sedimentary matrix, and for providing a rapid classification of clastic rocks.

Reference
Thorpe MT, Rogers AD, Bristow TF, Pan C (2015) Quantitative Compositional Analysis of Sedimentary Materials Using Thermal Emission Spectroscopy: 1. Application to Sedimentary Rocks. Journal of Geophysical Research Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004863]
Published by arrangement with John Wiley & Sons

^1,2,3S. Goderis, ³A.D. Brandon, ⁴B. Mayer, ⁴M. Humayun
¹Earth System Science, Vrije Universiteit Brussel, Brussels, Belgium
²Department of Analytical Chemistry, Ghent University, Ghent, Belgium
³Department of Earth and Atmospheric Sciences, University of Houston, Houston, TX, USA
⁴National High Magnetic Field Laboratory and Department of Earth, Ocean & Atmospheric Science, Florida State University, Tallahassee, FL, USA

Ubiquitous nucleosynthetic isotope anomalies relative to the terrestrial isotopic composition in Mo, Ru, and other elements are known from both bulk chondrites and differentiated meteorites, but Os isotope ratios reported from such meteorites have been found to be indistinguishable from the terrestrial value. The carriers of s- and r-process Os must thus have been homogeneously distributed in the solar nebula. As large Os isotope anomalies are known from acid leachates and residues of primitive chondrites, the constant relative proportions of presolar s- and r-process carriers in such chondrites must have been maintained during nebular processes. It has long been assumed that partial melting of primitive chondrites would homogenize the isotopic heterogeneity carried by presolar grains. Here, ureilites, carbon-rich ultramafic achondrites dominantly composed of olivine and low-Ca pyroxene, are shown to be the first differentiated bulk Solar System materials for which nucleosynthetic Os isotope anomalies have been identified. These anomalies consist of enrichment in s-process Os heterogeneously distributed in different ureilites. Given the observed homogeneity of Os isotopes in all types of primitive chondrites, this Os isotope variability among ureilites must have been caused by selective removal of s-process-poor Os host phases, probably metal, during rapid localized melting on the ureilite parent body. While Mo and Ru isotope anomalies for all meteorites measured so far exhibit s-process deficits relative to the Earth, the opposite holds for the Os isotope anomalies in ureilites reported here. This might indicate that the Earth preferentially accreted olivine-rich restites and inherited a s-process excess relative to smaller meteorite bodies, consistent with Earth’s high Mg/Si ratio and enrichment of s-process nuclides in Mo, Ru, and Nd isotopes. Our new Os isotope results imply that caution must be used when applying nucleosynthetic isotope anomalies as provenance indicators between different classes of meteorites.

Reference
Goderis S, Brandon AD, Mayer B, Humayun M (2015) s-Process Os isotope enrichment in ureilites by planetary processing. Earth and Planetary Science Letters 431, 110–118
Link to Article [doi:10.1016/j.epsl.2015.09.021]
Copyright Elsevier

¹Evelyn Füri, ^2,3Peter H. Barry, ²Lawrence A. Taylor, ¹Bernard Marty
¹Centre de Recherches Pétrographiques et Géochimiques, CNRS-UL, 15 rue Notre Dame des Pauvres, BP 20, 54501 Vandoeuvre-lès-Nancy, France
²Planetary Geosciences Institute, Department of Earth Planetary Sciences, University of Tennessee, Knoxville, TN 37996-1410, USA
³Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK

Nitrogen and noble gas (Ne–Ar) abundances and isotope ratios, determined by step-wise CO2 laser-extraction, static-mass spectrometry analysis, are reported for bulk fragments and mineral separates of ten lunar mare basalts (10020, 10057, 12008, 14053, 15555, 70255, 71557, 71576, 74255, 74275), one highland breccia (14321), and one ferroan anorthosite (15414). The mare basalt sub-samples 10057,183 and 71576,12 contain a large amount of solar noble gases, whereas neon and argon in all other samples are purely cosmogenic, as shown by their 21Ne/22Ne ratios of ≈0.85 and 36Ar/38Ar ratios of ≈0.65. The solar-gas-free basalts contain a two-component mixture of cosmogenic 15N and indigenous nitrogen ((<0.5 ppm). Mare basalt 74255 and the olivine fraction of 15555,876 record the smallest proportion of ¹⁵N_cosm; therefore, their δ¹⁵Nδ15N values of −0.2 to +26.7‰+26.7‰ (observed at the low-temperature steps) are thought to well represent the isotopic composition of indigenous lunar nitrogen. However, δ¹⁵Nδ15N values ≤−30‰≤−30‰ are found in several basalts, overlapping with the isotopic signature of Earth’s primordial mantle or an enstatite chondrite-like impactor. While the lowest δ¹⁵Nδ15N values allow for nitrogen trapped in the Moon’s interior to be inherited from the proto-Earth and/or the impactor, the more ¹⁵N-enriched compositions require that carbonaceous chondrites provided nitrogen to the lunar magma ocean prior to the solidification of the crust. Since nitrogen can efficiently be incorporated into mafic minerals (olivine, pyroxene) under oxygen fugacities close to or below the iron-wustite buffer (Li et al., 2013), the mare basalt source region is likely characterized by a high nitrogen storage capacity. In contrast, anorthosite 15414 shows no traces of indigenous nitrogen, suggesting that nitrogen was not efficiently incorporated into the lunar crust during magma ocean Differentiation.

Reference
Füri E, Barry PH, Taylor LA, Marty B (2015) Indigenous nitrogen in the Moon: Constraints from coupled nitrogen–noble gas analyses of mare basalts. Earth and Planetary Science Letters 431, 195–205
Link to Article [doi:10.1016/j.epsl.2015.09.022]
Copyright Elsevier

Paul R. Renne^1,2, Courtney J. Sprain^1,2, Mark A. Richards², Stephen Self², Loÿc Vanderkluysen³, Kanchan Pande^4
11Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA.
²Department of Earth and Planetary Science, University of California–Berkeley, Berkeley, CA 94720, USA.
³Department of Biodiversity, Earth and Environmental Science, Drexel University, Philadelphia, PA 19104, USA.
⁴Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India.

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Reference
Renne PR, Sprain CJ, Richards MA, Self S, Vanderkluysen L, Pande K (2015) State shift in Deccan volcanism at the Cretaceous-Paleogene boundary, possibly induced by impact. Science 6256:76-78.
Link to Article [doi:10.1126/science.aac7549]