¹Lonia R.Friedlander, ¹Timothy D. Glotch, ²David L. Bish, ³M. Darby Dyar, ⁴Thomas G.Sharp, ¹Elizabeth C. Sklute, ⁵Joseph R. Michalski

¹Geosciences Department, Stony Brook University, Stony Brook, NY, 11794-2100 USA
¹Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington,
IN, 47405-1405 USA
³Department of Astronomy, Mount Holyoke College, 50 College Street, South Hadley, MA, 01075 USA
⁴School of Earth and Space Exploration, Arizona State University, PO Box 871404, Tempe, AZ,85287-1404 USA
⁵Planetary Science Institute, 1700 E. Fort Lowell, Tucson, AZ, 85719 USA

Many phyllosilicate deposits remotely detected on Mars occur within bombarded terrains. Shock metamorphism from meteor impacts alters mineral structures, producing changed mineral spectra. Thus, impacts have likely affected the spectra of remotely sensed martian phyllosilicates. We present spectral analysis results for a natural nontronite sample (NAu-1) before and after laboratory-generated impacts over five peak pressures between 10 – 40 GPa. We conducted a suite of spectroscopic analyses to characterize the sample’s impact-induced structural and spectral changes. Nontronite becomes increasingly disordered with increasing peak impact pressure. Every infrared spectroscopic technique used showed evidence of structural changes at shock pressures above ~25 GPa. Reflectance spectroscopy in the visible near-infrared (VNIR) region is primarily sensitive to the vibrations of metal-OH and interlayer H2O groups in the nontronite octahedral sheet. Mid-infrared (MIR) spectroscopic techniques are sensitive to the vibrations of silicon and oxygen in the nontronite tetrahedral sheet. Because the tetrahedral and octahedral sheets of nontronite deform differently, impact-driven structural deformation may contribute to differences in phyllosilicate detection between remote sensing techniques sensitive to different parts of the nontronite structure. Observed spectroscopic changes also indicated that the sample’s octahedral and tetrahedral sheets were structurally deformed, but not completely dehydroxylated. This finding is an important distinction from previous studies of thermally altered phyllosilicates in which dehydroxylation follows dehydration in a step-wise progression preceding structural deformation. Impact-alteration may thus complicate mineral-specific identifications based on the location of OH-group bands in remotely detected spectra. This is a key implication for martian remote sensing arising from our results.

Reference
Friedlander R, Glotch TD, Bish DL, Dyar MD, Sharp TG, Sklute EC, Michalski JR (2015) Structural and spectroscopic changes to natural nontronite induced by experimental impacts between 10 and 40 GPa. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2014JE004638]

Published by arrangement with John Wiley&Sons

^1,2,3T.J. Peter,^2,3J.I. Simon,²J.H. Jones,⁴T. Usui,⁴R. Moriwaki,⁵R.C. Economos,⁵A.K. Schmitt,⁵K.D. McKeegan
¹Lunar and Planetary Institute, Houston, TX 77058, USA
²Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, TX 77058, USA
³Center for Isotope Cosmochemistry and Geochronology, NASA Johnson Space Center, Houston, TX 77058, USA
⁴Department of Earth & Planetary Sciences, Tokyo Institute of Technology, Tokyo 152-8551, Japan
⁵Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, CA 90095, USA

The apparent lack of plate tectonics on all terrestrial planets other than Earth has been used to support the notion that for most planets, once a primitive crust forms, the crust and mantle evolve geochemically-independent through time. This view has had a particularly large impact on models for the evolution of Mars and its silicate interior. Recent data indicating a greater potential that there may have been exchange between the martian crust and mantle has led to a search for additional geochemical evidence to support the alternative hypothesis, that some mechanism of crustal recycling may have operated early in the history of Mars.
In order to study the most juvenile melts available to investigate martian mantle source(s) and melting processes, the trace element compositions of olivine-hosted melt inclusions for two incompatible-element-depleted olivine-phyric shergottites, Yamato 980459 (Y98) and Tissint, and the interstitial glass of Y98, have been measured by Secondary Ionization Mass Spectrometry (SIMS). Chondrite-normalized Rare Earth Element (REE) patterns for both Y98 and Tissint melt inclusions, and the Y98 interstitial glass, are characteristically light-REE depleted and parallel those of their host rock. For Y98, a clear flattening and upward inflection of La and Ce, relative to predictions based on middle and heavier REE, provides evidence for involvement of an enriched component early in their magmatic history; either inherited from a metasomatized mantle or crustal source, early on and prior to extensive host crystallization.
Comparing these melt inclusion and interstitial glass analyses to existing melt inclusion and whole-rock data sets for the shergottite meteorite suite, defines mixing relationships between depleted and enriched end members, analogous to mixing relationships between whole rock Sr and Nd isotopic measurements. When considered in light of their petrologic context, the origin of these trace element enriched and isotopically evolved signatures represents either (1) crustal assimilation during the final few km of melt ascent towards the martian surface, or (2) assimilation soon after melt segregation, through melt–rock interaction with a portion of the martian crust recycled back into the mantle.

Reference
Peters TJ, Simon JI, Jones JH, Usui T, Moriwaki R, Economos RC, Schmitt AK, McKeegan KD (2015) Tracking the source of the enriched martian meteorites in olivine-hosted melt inclusions of two depleted shergottites, Yamato 980459 and Tissint. Earth and Planetary Science Letters 418, 91–102.
Link to Article [doi:10.1016/j.epsl.2015.02.033]

Copyright Elsevier

¹Jennifer L. Claydon,¹Sarah A. Crowther,^1,2,3Vera A. Fernandes,¹Jamie D. Gilmour
¹School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Oxford Road, Manchester, M13 9PL, UK.
²Museum für Naturkunde- Berlin, Leibniz-Institut für Evolutions- und Biodiversitätsforschung, Invalidenstrasse 43, 10115 Berlin, Germany
³UNINOVA, Universidade Nova de Lisboa, Quinta da Torre, 2829-516 Monte de Caparica, Portugal
⁴The Centre for Earth Evolution and Dynamics, Univ. of Oslo, PO Box 1048 Blindern 0316 Oslo, Norway

The meteorite Graves Nunataks 06128/06129 is the only known example of felsic asteroidal crust. Knowledge of its history can help shed light on the evolution processes of planetesimals. The noble gases can be used to constrain both the chronology of meteorites and the processes that result in movements of volatile elements on asteroidal bodies. We have examined the I-Xe and Ar-Ar systems of the plagioclase-rich achondrite, Graves Nunataks 06129 by high-resolution laser step-heating of irradiated samples. Iodine and 129Xe∗ are both present but are released at different temperatures and do not show a correlation, therefore the I-Xe system in GRA 06129 has no chronological significance. We propose that radiogenic 129Xe∗ was lost from primary phases and parentless 129Xe∗ was later introduced into the rock by interaction with a fluid sourced from a reservoir that evolved with a high I/Xe ratio. This could have been the same halogen-rich fluid that induced the conversion of merrillite and pyroxene into chlorapatite. Inherited 40Ar (i.e. not generated by in situ decay of 40K) is also present in one of three fragments studied here and may have been introduced at the same time as parentless 129Xe∗.

Reference
Claydon CK, Crowther SA, Fernandes VA, Gilmour JD (2015) Noble gases and halogens in Graves Nunataks 06129: the complex thermal history of a felsic asteroid crust. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2015.03.01]

Copyright Elsevier