^1,2,3J. Brian Balta, ^4,5Matthew E. Sanborn, ⁶Rhiannon G. Mayne, ⁴Meenakshi Wadhwa, ¹Harry Y. McSween Jr, ⁵Samuel D. Crossley
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12744]
¹Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee, USA
²University of Pittsburgh, Pittsburgh, Pennsylvania, USA
³Department of Geology and Geophysics, Texas A&M University, College Station, Texas, USA
⁴Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona, USA
⁵Department of Earth and Planetary Sciences, University of California at Davis, Davis, California, USA
⁶Monnig Meteorite Collection, School of Geology, Energy, and the Environment, Texas Christian University, Fort Worth, Texas 76109, USA
Published by arrangement with John Wiley&Sons

We present a geochemical study of recently discovered Martian meteorite Northwest Africa (NWA) 5790 and use our results to constrain its origin and relationship with the other nakhlites. This nakhlite is a clinopyroxene cumulate composed of phenocrysts of augite, olivine, and rare oxides surrounded by a mesostasis composed of vitrophyric glass, feldspars, oxides, phosphates, and fine-grained olivines and augite. Petrography, and major and trace element compositions of the phases present are consistent with derivation of NWA 5790 from a parental magma common to all the nakhlites. Olivine cores grew from a distinct, incompatible-element enriched magma and are surrounded by rims containing augite inclusions that grew from the nakhlite parental liquid, supporting previous arguments for xenocrystic olivine cores in nakhlites. Rare earth element microdistributions suggest derivation of NWA 5790 augites from an evolved, relatively oxidized magma, produced by augite fractionation from the common nakhlite parental liquid. Augite grain shapes and CSD patterns are consistent with rapid cooling and derivation near the top of the nakhlite cumulate pile, but patterns are distinct from other nakhlites thought to have formed near the stratigraphic top. The high mesostasis abundance (~44 vol%) indicates solidification near the top of the nakhlite pile close to locations suggested for nakhlites NWA 817 and Miller Range (MIL) 03346. However, the geochemical and petrographic characteristics of these three samples do not permit their placement in a simple stratigraphic order as would occur in a single lava flow. This lack of simple ordering suggests that the nakhlite lava flow split into multiple sections as would occur during breakouts from a single lava flow. Finally we note that NWA 5790 is unique among currently available nakhlites in having phenocryst abundances low enough to allow it to flow.

¹Camille Soulié,²Guy Libourel,¹Laurent Tissandier
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12792]
¹Centre de Recherches Pétrographiques et Géochimiques (UMR 7358, CNRS-Université de Lorraine), Vandoeuvre-lès-Nancy, France
²Observatoire de la Côte d’Azur, UMR 7293 Lagrange, Bd de l’Observatoire, Nice, France
Published by arrangement with John Wiley & Sons

Mg-rich olivine is a ubiquitous phase in type I porphyritic chondrules in various classes of chondritic meteorites. The anhedral shape of olivine grains, their size distribution, as well as their poikilitic textures within low-Ca pyroxene suggest that olivines suffer dissolution during chondrule formation. Owing to a set of high-temperature experiments (1450–1540 °C) we determined the kinetics of resorption of forsterite in molten silicates, using for the first time X-ray microtomography. Results indicate that forsterite dissolution in chondrule-like melts is a very fast process with rates that range from ~5 μm min−1 to ~22 μm min−1. Forsterite dissolution strongly depends on the melt composition, with rates decreasing with increasing the magnesium and/or the silica content of the melt. An empirical model based on forsterite saturation and viscosity of the starting melt composition successfully reproduces the forsteritic olivine dissolution rates as a function of temperature and composition for both our experiments and those of the literature. Application of our results to chondrules could explain the textures of zoned type I chondrules during their formation by gas-melt interaction. We show that the olivine/liquid ratio on one hand and the silica entrance from the gas phase (SiOg) into the chondrule melt on the other hand, have counteracting effects on the Mg-rich olivine dissolution behavior. Silica entrance would favor dissolution by maintaining disequilibrium between olivine and melt. Hence, this would explain the preferential dissolution of olivine as well as the preferential abundances of pyroxene at the margins of chondrules. Incipient dissolution would also occur in the silica-poorer melt of chondrule core but should be followed by crystallization of new olivine (overgrowth and/or newly grown crystals). While explaining textures and grain size distributions of olivines, as well as the centripetal distribution of low-Ca pyroxene in porphyritic chondrules, this scenario could also be consistent with the diverse chemical, isotopic, and thermal conditions recorded by olivines in a given chondrule.

¹Lucy F. Lim, ^1,2Richard D. Starr, ^1,3Larry G. Evans, ¹Ann M. Parsons, ⁴Michael E. Zolensky, ⁵William V.Boynton
Meteoritics&Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.12786]
¹NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
²Catholic University of America, Washington, District of Columbia, USA
³Computer Sciences Corporation, Lanham-Seabrook, Maryland, USA
⁴ARES, NASA Johnson Space Center, Houston, Texas, USA
⁵Lunar and Planetary Laboratory, University of Arizona, Tucson, Arizona, USA
Published by agreement with John Wiley & Sons

To evaluate the feasibility of measuring differences in bulk composition among carbonaceous meteorite parent bodies from an asteroid or comet orbiter, we present the results of a performance simulation of an orbital gamma-ray spectroscopy (GRS) experiment in a Dawn-like orbit around spherical model asteroids with a range of carbonaceous compositions. The orbital altitude was held equal to the asteroid radius for 4.5 months. Both the asteroid gamma-ray spectrum and the spacecraft background flux were calculated using the MCNPX Monte-Carlo code. GRS is sensitive to depths below the optical surface (to ≈20–50 cm depth depending on material density). This technique can therefore measure underlying compositions beneath a sulfur-depleted (e.g., Nittler et al. 2001) or desiccated surface layer. We find that 3σ uncertainties of under 1 wt% are achievable for H, C, O, Si, S, Fe, and Cl for five carbonaceous meteorite compositions using the heritage Mars Odyssey GRS design in a spacecraft-deck-mounted configuration at the Odyssey end-of-mission energy resolution, FWHM = 5.7 keV at 1332 keV. The calculated compositional uncertainties are smaller than the compositional differences between carbonaceous chondrite subclasses.