¹Hikaru Yabuta et al. (>10)*
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.06.047]
¹Department of Earth and Planetary Systems Science, Hiroshima University, 1-3-1 Kagamiyama, Hiroshima 739-8526, Japan
*Find the extensive, full author and affiliation list on the publishers website
Copyright Elsevier

A comprehensive study of the organic chemistry and mineralogy of an ultracarbonaceous micrometeorite (UCAMM D05IB80) collected from near the Dome Fuji Station, Antarctica, was carried out to understand the genetic relationship among organic materials, silicates, and water. The micrometeorite is composed of a dense aggregate of ∼5 µm-sized hollow ellipsoidal organic material containing submicrometer-sized phases such as glass with embedded metal and sulfides (GEMS) and mineral grains. There is a wide area of organic material (∼15 × 15 μm) in its interior. Low-Ca pyroxene is much more abundant than olivine and shows various Mg/(Mg+Fe) ratios ranging from ∼1.0 to 0.78, which is common to previous works on UCAMMs. By contrast, GEMS grains in this UCAMM have unusual chemical compositions. They are depleted in both Mg and S, which suggests that these elements were leached out from the GEMS grains during very weak aqueous alteration, without the formation of phyllosilicates.

The organic materials have two textures—smooth and globular with an irregular outline—and these are composed of imine, nitrile and/or aromatic nitrogen heterocycles, and amide. The ratio of nitrogen to carbon (N/C) in the smooth region of the organics is ∼0.15, which is five times higher than that of insoluble organic macromolecules in types 1 and 2 chondritic meteorites. In addition, the UCAMM organic materials are soluble in epoxy and are thus hydrophilic; this polar nature indicates that they are very primitive. The surface of the material is coated with an inorganic layer, a few nanometers thick, that consists of C, O, Si, S, and Fe. Sulfur is also contained in the interior, implying the presence of organosulfur moieties. There are no isotopic anomalies of D, 13C, or 15N in the organic material.

Interstellar photochemistry alone would not be sufficient to explain the N/C ratio of the UCAMM organics; therefore, we suggest that a very small amount of fluid on a comet must have been necessary for the formation of the UCAMM. The GEMS grains depleted in Mg and S in the UCAMM prove a very weak degree of aqueous alteration; weaker than that of carbonaceous chondrites. Short-duration weak alteration probably caused by planetesimal shock locally melted cometary ice grains and released water that dissolved the organics; the fluid would likely have not mobilized because of the very low thermal conductivity of the porous icy body. This event allowed the formation of the large organic puddle of the UCAMM, as well as organic matter sulfurization, formation of thin membrane-like layers of minerals, and deformation of organic nanoglobules.

¹Nicole G. Lunning, ^1,2Kathryn G. Gardner-Vandy, ^1,3Emma S. Sosa, ¹Timothy J. Mccoya, ⁴Emma S. Bullock, ¹Catherine M. Corrigan
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2017.07.004]
¹Department of Mineral Sciences, Smithsonian Institution, National Museum of Natural History, Washington, DC 20560, USA
²Department of Geosciences, The University of Tulsa, Keplinger Hall L101 The University of Tulsa, 441 South Gary Avenue Tulsa, OK 74104
³Department of Geology and Environmental Geosciences, Lafayette College, 116 Van Wickle Hall, 4 South College Dr. Easton, PA 18042
⁴Geophysical Laboratory, Carnegie Institution for Science, 5251 Broad Branch Road NW Washington DC 20015
Copyright Elsevier

The meteorites Graves Nunataks (GRA) 06128 and 06129, however, are igneous meteorites dominated by oligoclase feldspar and have a basaltic trachyandesite-like whole rock composition. Formation of the GRA 06128/9 meteorites as primary melts on an oxidized planetesimal has been previously proposed (Day et al., 2009a; Day et al., 2012a; Gardner-Vandy et al., 2013 ; Wang et al., 2014). We show experimentally that anhydrous partial melting of an oxidized R chondrite at IW to IW+1 between 1120-1140°C produces melts of GRA 06128/9-like compositions: intermediate SiO2 and FeO concentrations that are enriched in volatile sodium. From a process perspective, GRA 06128/9-like magmas are complementary to partial melt residues such as olivine-rich brachinite and FeO-rich brachinite-like meteorites. Magmas of GRA 06128/9’s composition can be generated under equilibrium conditions, as demonstrated by MELTS modeling, but only at temperatures ∼1140°C. At lower degrees of partial melting liquids formed under equilibrium and non-equilibrium conditions follow distinct compositional pathways to reach GRA 06128/9-like melts. For lower degrees of melting, the non-equilibrium trend more closely resembles GRA 06128/9’s composition. Phase abundance modeling indicates that GRA 06128/9-composition magmas form by 14-22% silicate melting of an oxidized R-chondrite. We conclude that GRA 06128/9-composition magmas can be generated at ∼1140°C from partial melting of an oxidized chondritic precursor under both non-equilibrium and equilibrium conditions.

¹M. G. A. Lapotre,^1,2B. L. Ehlmann,³S. E. Minson,⁴R. E. Arvidson,¹F. Ayoub,¹A. A. Fraeman,⁵R. C. Ewing,⁶N. T. Bridges
Journal of Geophysical Research Planets (in Press) Link to Article [DOI: 10.1002/2016JE005133]
¹Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA
²Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
³USGS, Menlo Park, CA, USA
⁴Washington University in St. Louis, St. Louis, MO, USA
⁵Texas A&M University, College Station, TX, USA
⁶Applied Physics Laboratory, Johns Hopkins University, Laurel, MD, USA
Published by arrangement with John Wiley & Sons

During its ascent up Mount Sharp, the Mars Science Laboratory Curiosity rover traversed the Bagnold Dune Field. We model sand modal mineralogy and grain size at four locations near the rover traverse, using orbital shortwave infrared single scattering albedo spectra and a Markov-Chain Monte Carlo implementation of Hapke’s radiative transfer theory to fully constrain uncertainties and permitted solutions. These predictions, evaluated against in situ measurements at one site from the Curiosity rover, show that XRD-measured mineralogy of the basaltic sands is within the 95% confidence interval of model predictions. However, predictions are relatively insensitive to grain size and are non-unique, especially when modeling the composition of minerals with solid solutions. We find an overall basaltic mineralogy and show subtle spatial variations in composition in and around the Bagnold dunes, consistent with a mafic enrichment of sands with cumulative transport distance by sorting of olivine, pyroxene, and plagioclase grains during aeolian saltation. Furthermore, the large variations in Fe and Mg abundances (~20 wt%) at the Bagnold Dunes suggest that compositional variability induced by wind sorting may be enhanced by local mixing with proximal sand sources. Our estimates demonstrate a method for orbital quantification of composition with rigorous uncertainty determination and provide key constraints for interpreting in situ measurements of compositional variability within martian aeolian sandstones.