Cosmochmistry Papers will be on Summer Break for the next two weeks. We will return on August, 30th.
Enjoy your holiday!

¹Alex I. Sheen,¹Christopher D. K. Herd,¹Jarret Hamilton,^1,2Erin L. Walton
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13726]
¹Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alberta, T6G 2E3 Canada
²Department of Physical Sciences, MacEwan University, City Centre Campus, 10700 104 Ave, Edmonton, Alberta, T5J 4S2 Canada
Published by arrangement with John Wiley & Sons

Baddeleyite (ZrO2) is a common late-stage accessory mineral in basaltic shergottites and is a robust geochronometer for obtaining igneous crystallization ages via high-precision in situ SIMS U-Pb analysis. Amenability to SIMS U-Pb dating depends in large part on the size and abundance of baddeleyite grains, which are generally surveyed using microbeam methods. We examine the petrography, mineralogy, geochemistry, and baddeleyite distribution in eight basaltic shergottites to identify factors that may be used to predict baddeleyite distribution in unknown samples of Mars. Results suggest that fractional crystallization controls baddeleyite occurrence in shergottites to the first order; samples with pyroxene major element compositions extending beyond the 1-bar stability boundary generally have higher baddeleyite abundance compared with samples with pyroxene compositions terminating at or before the stability boundary. In samples which display two pyroxene composition trends (high-Ca and low-Ca), the largest baddeleyite grains tend to be associated with Fe-Ti oxides; in samples where pyroxene composition forms a continuous trend extending beyond the 1-bar stability boundary, the largest baddeleyite grains are typically associated with polymineralic late-stage pockets. Bulk HFSE content and fO2 do not appear to directly influence baddeleyite distribution. Based on our findings, we propose that pyroxene composition is a useful proxy for assessing baddeleyite abundance and distribution in shergottites and may aid in determining a sample’s feasibility for U-Pb geochronology prior to conducting detailed surveys for baddeleyite characterization.

¹Tianqi Xie,¹Gordon R. Osinski,^1,2Sean R. Shieh
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13728]
¹Department of Earth Sciences/Institute for Earth and Space Exploration, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7 Canada
²Department of Physics and Astronomy, University of Western Ontario, 1151 Richmond Street, London, ON, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

Earth’s moon is a primary exploration target for space agencies around the world. The Moon records and preserves key information about fundamental processes that shape planetary crusts such as impact cratering. Understanding shock effects in lunar anorthite (Ca-rich endmember of plagioclase feldspar), the principal component of anorthosite and the most common crustal mineral on the Moon, is key to the early evolution of the Moon and terrestrial planets in the solar system. However, there has not been a systematic study of shock effects in lunar anorthite using modern analytical techniques that could be used in future lunar surface exploration, such as Raman spectroscopy. This study examined 23 polished thin sections from all six Apollo missions using optical and Raman spectroscopy. We documented a variety of shock features recording low to moderate shock levels, including fractures, deformed twins, undulatory extinction, planar features, and partially isotropic plagioclase. A notable nonobservation was the absence of planar deformation features (PDFs) or completely isotropic (i.e., diaplectic glass) in this suite of samples. Raman spectroscopy results of the observed shock features show similar progressive changes in terrestrial samples: As shock level increases, band broadening, reduction of intensities, and peak loss were observed. Our Raman data are efficient in identifying shock levels and distinguishing planar features from PDFs and deformed twins, and differentiating amorphous areas from crystalline plagioclase, suggesting Raman spectroscopy as a useful tool for purposely selecting moderately to strongly shocked samples to return in future lunar missions. Our study can also help the interpretation of Raman data of impact materials from the past and future exploration missions and demonstrate the utility of Raman spectroscopy for documenting and selecting samples for future lunar missions.

^1,2,3Alan E. Rubin,^1,3Bastian Baecker
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13725]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, 90095–1567 USA
²Maine Mineral & Gem Museum, 99 Main Street, P.O. Box 500, Bethel, Maine, 04217 USA
³Baker Hughes, Baker-Hughes-Str. 1, Celle, 29229 Germany
Published by arrangement with John Wiley & Sons

Phosphorus X-ray maps of olivine phenocrysts in many type II (FeO-rich) porphyritic chondrules in LL3.00 Semarkona and CO3.05 Y 81020 reveal multiple sets of thin dark/bright (P-poor/P-rich) layers that resemble oscillatory zoning. Such discrete layers are generally not evident in BSE images or in Fe, Cr, Ca, Al, Mg, or Mn X-ray maps because rapid diffusion of these cations in olivine at high temperatures smoothed out their initial distributions, thereby mimicking normal igneous zoning. In contrast, the relatively slow diffusion of P in olivine preserves original dendritic or hopper morphologies of olivine crystals; these skeletal structures formed during quenching after initial chondrule melting. The skeletal olivine crystals were filled in with low-P olivine during cooling after one or more subsequent heating events, mainly involving the melting of mesostasis. Crystallization of mafic silicates depleted the mesostasis in FeO and MgO and enriched it in silico-feldspathic components. Sectioning of the olivine grains at particular orientations can produce apparent oscillatory zoning in P. Strong evidence of a secondary melting event is evident in Semarkona chondrule H5k. Phenocryst H5k-2 in this chondrule has a relict core (with rhythmic P zoning layers) that was fractured and severed; it is overlain by a set of differently oriented subparallel P-poor olivine layers. Chondrule C6f from Y 81020 contains a large multi-lobed olivine phenocryst that still preserves hopper cavities, partially outlined by P-poor/P-rich olivine layers. The thin P-rich rims surrounding many olivine phenocrysts could reflect a short period of rapid grain growth after a late-stage chondrule reheating event.

¹H. Palme,¹J. Zipfel
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13720]
¹Senckenberg Forschungsinstitut und Naturmuseum Frankfurt, Senckenberganlage 25, Frankfurt, 60325 Germany
Published by arrangement with John Wiley & Sons

Between 1973 and 1994, 15 samples of CI chondrites were analyzed by neutron activation analysis at the Max-Planck-Institute for Chemistry, Department of Cosmochemistry in Mainz, Germany. The analyses comprise nine Orgueil samples and three samples of Ivuna, two of Alais and one of Tonk. Samples came from various sources and had masses between 5 and 600 mg. Most data are published here for the first time. The results for the nine Orgueil samples demonstrate the essentially homogeneous chemical composition of Orgueil at a level of a few milligrams. The analytical results of Ivuna, Alais, and Tonk agree, with only few exceptions, with the results of Orgueil analyses. All samples agree within ±3% in their contents of Sc, Ir, Cr, Fe, Co, Zn, and Se. The elements Sc and Ir represent the refractory component; Cr, Fe, and Co the main component; and Zn and Se the volatile component. Thus, in all CI chondrites there are essentially the same fractions of the fundamental cosmochemical components. The essentially identical chemical composition of all samples shows that their water contents are constant at about 20 ± 5 wt%. There is excellent agreement between the data listed here with data reported in the relevant literature. There is no doubt that the CI composition is a well-defined entity, which is thought to represent the non-gaseous compositions of the solar nebula and the photosphere of the Sun. In addition, we conclude that the recently proposed new CI chondritic chlorine and Br values are too low, when compared to earlier measurements.