¹Ted L. Roush, ^1,2Janice L. Bishop, ^1,2Adrian J. Brown, ¹David F. Blake, ¹Thomas F. Bristow
¹NASA Ames Research Center, Moffett Field, CA 94035-0001
²SETI Institute, Mountain View, CA

Quantitative estimates of clay minerals on the martian surface, via remote sensing observations, provide constraints on activity, timing, duration, and extent of aqueous processes and the geochemical environment in martian history. We describe an analytical study to begin enabling quantitative estimates of phyllosilicates when mixed with martian analog materials. We characterize the chemistry, mineralogy, particle size distribution, and reflectance spectra of the end-member materials: saponite, montmorillonite, pyroxene, and palagonitic soil. Reflectance spectra were obtained for physical mixtures of saponite and montmorillonite with pyroxene, and saponite with palagonitic soil. We analyzed the diagnostic phyllosilicate spectral signatures in the 2.2-2.4 μm wavelength region in detail for the mixtures. This involved fitting the observed ∼2.3 or ∼2.2 μm band depth, associated with the presence of saponite and montmorillonite, respectively, as a function of the abundance of these materials in the mixtures. Based upon the band depth of the spectral features we find that 3-5 wt.% of the clay minerals in the mixture with pyroxene can be recognized and at 25 wt.% their presence is indisputable in the mixtures. When the saponite is mixed with the lower albedo palagonitic soil, its presence is clearly distinguishable via the 1.4 and 2.3 μm features at 25 wt.% abundance. These relationships, between abundance and band depth, provide an ability to quantitatively address the amount of these materials in mixtures. The trends described here provide guidance for estimating t

Reference
Roush TL, Bishop JL, Brown AJ, Blake DF, Bristow TF (2015) Laboratory reflectance spectra of clay minerals mixed with Mars analog materials: Toward enabling quantitative clay abundances from Mars spectra. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.06.035]
Copyright Elsevier

¹Scott Messenger, ¹Keiko Nakamura-Messenger, ¹Lindsay P. Keller, ^1,2Simon J. Clemett
¹Robert M. Walker Laboratory for Space Science, EIS Directorate, Astromaterials Research and Exploration Science Division, NASA Johnson Space Center, Houston, Texas, USA
²ESCG/ERC Inc., Houston, Texas, USA

We performed chemical, mineralogical, and isotopic studies of the first interplanetary dust particles (IDPs) collected in the stratosphere without the use of silicone oil. The collection substrate, polyurethane foam, effectively traps impacting particles, but the lack of an embedding medium results in significant particle fragmentation. Two dust particles found on the collector exhibit the typical compositional and mineralogical properties of chondritic porous interplanetary dust particles (CP-IDPs). Hydrogen and nitrogen isotopic imaging revealed isotopic anomalies of typical magnitude and spatial variability observed in previous CP-IDP studies. Oxygen isotopic imaging shows that individual mineral grains and glass with embedded metal and sulfide (GEMS) grains are dominated by solar system materials. No systematic differences are observed in element abundance patterns of GEMS grains from the dry collection versus silicone oil-collected IDPs. This initial study establishes the validity of a new IDP collection substrate that avoids the use of silicone oil as a collection medium, removing the need for this problematic contaminant and the organic solvents necessary to remove it. Additional silicone oil-free collections of this type are needed to determine more accurate bulk element abundances of IDPs and to examine the indigenous soluble organic components of IDPs.

Reference
Messenger S, Nakamura-Messenger K, Keller LP, Simon J. Clemett SJ (2015) Pristine stratospheric collection of interplanetary dust on an oil-free polyurethane foam Substrate. Meteoritics&Planetary Science (in Press)
Link to Article [DOI: 10.1111/maps.12473]
Copyright Elsevier

The most popular papers on Cosmochemistry Papers in May were:

1-Wasserburg GJ, Tripella O, Busso M (2015) Isotope Anomalies in the Fe-group Elements in Meteorites and Connections to Nucleosynthesis in AGB Stars. Astrophysical Journal 805, 7. Link to Article [doi:10.1088/0004-637X/805/1/7]

2-Burkhardt C, Schönbächler M (2015) Intrinsic W nucleosynthetic isotope variations in carbonaceous chondrites: Implications for W nucleosynthesis and nebular vs. parent body processing of presolar materials. Geochimica et Cosmochimica Acta (in Press) Link to Article [doi:10.1016/j.gca.2015.06.012]

3-Chen Y, Liu Y, Guan Y, Eiler JM, Ma C, Rossman GR, Taylor LC (2015) Evidence in Tissint for recent subsurface water on Mars. Earth and Planetary Science Letters 425, 55–63 Link to Article [doi:10.1016/j.epsl.2015.05.004]

4-McCubbin FM, Jones RH (2015) Extraterrestrial Apatite: Planetary Geochemistry to Astrobiology. Elements 11/3, 183-188 Link to Article [doi: 10.2113/gselements.11.3.183]

5-Xiong MY, Shelobolina ES, Roden EE (2015) Potential for Microbial Oxidation of Ferrous Iron in Basaltic Glass. Astrobiology 15(5), 331-340. Link to Article [doi:10.1089/ast.2014.1233]

¹O. Ruesch, ¹H. Hiesinger, ²E. Cloutis, ³L. Le Corre, ¹J. Kallisch, ²P. Mann, ^4,1K. Markus, ¹K. Metzler, ⁵A. Nathues, ³V. Reddy
¹Institut für Planetologie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany
²Department of Geography, University of Winnipeg, 515 Portage Avenue Winnipeg, Manitoba R3B 2E9, Canada
³Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719-2395, USA
⁴DLR, Institute of Planetary research, Berlin, Germany
⁵Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

The howardite, eucrite and diogenite (HED) meteorites are genetically related and represent the most voluminous group of achondrites. They are the closest analog materials to Vesta and V-type asteroids. Many of these meteorites were the focus of intense petrologic and visible to near infrared spectral studies. As ground-based and orbital observations of basaltic asteroids have increased, an improved understanding of HEDs is needed. For this study, we investigated 24 HED samples, mainly new finds from Northwest Africa (NWA). Visible to near infrared (up to 2.5 μm) spectral measurements under varying illumination and observation geometries were acquired for 4 samples. Phase reddening and bluing (i.e., increase and decrease in spectral slope) is observed for the visible slope as phase angle increase. Monotonic phase reddening can occur for the near infrared slope as phase angle increase. Non-systematic changes with phase angle are found for the band area ratio parameter. At phase angles higher than ∼60°, the decrease of reflectance and decrease of pyroxene bands depth are undistinguishable from admixture of low albedo material to HED samples. To assess the precision of empirical equations relating spectral properties and composition, the pyroxenes, feldspar, and olivine chemistry of the samples was determined. Using previous calibrations, systematic overestimations of the ferrosilite (Fs) and wollastonite (Wo) contents are found, especially in the 15-40 Fs range. To overcome such discrepancies, a new set of empirical equations is proposed. For an application of the new calibration, we selected two compositional end-member areas on Vesta on the basis of their iron content. For the iron-poor terrain we found an average pyroxene composition of Fs30Wo5 and for the iron-rich terrain an average of Fs47Wo14.

Reference
Ruesch O, Hiesinger H, Cloutis E, Le Corre L, Kallisch J, Mann P, Markus K, Metzler K, Nathues A, Reddy V (2015)
Near infrared spectroscopy of HED meteorites: Effects of viewing geometry and compositional variations. Icarus (in Press)
Link to Article [doi:10.1016/j.icarus.2015.06.034]

Copyright Elsevier

¹G.H. Howarth, ¹J.F. Pernet-Fisher, ²R.J. Bodnar, ¹L.A. Taylor
¹Planetary Geosciences Institute, Earth and Planetary Sciences Dept., The University of Tennessee, Knoxville, TN, 37996 USA.
²Department of Geosciences, Virginia Tech, Blacksburg, VA i24061, USA

Martian meteorite Northwest Africa 7755 is a new example of an enriched, lherzolitic shergottite, containing some of the coarsest-grained apatite yet identified in shergottite meteorites. Their size has permitted detailed observations of volatile distributions within single grains. We have demonstrated that some apatites have been invaded by shock melts, which act to devolatilize parts of grains, resulting in significant Cl-enrichment in the adjacent regions. The extent of chemical heterogeneity within single grains must be carefully considered in other shergottites, so that the effects of secondary modification of apatites are well-constrained, prior to interpreting the volatile contents and primary magmatic processes. Apatite grains unaffected by shock melts are OH-F enriched and Cl-poor (∼F50Cl14OH36), relative to interstitial apatites reported in other shergottites. The volatile compositions are similar to interstitial apatites reported in terrestrial mafic intrusions. Such apatites in terrestrial intrusions are argued to have formed after significant Cl-loss due to the exsolution and migration of Cl-rich brines. Calculated relative F2, Cl2, and H2O fugacities for NWA 7755 apatites show a trend of degassing rather than fractionation, noted in previous studies. Indeed, we interpret the volatile contents of apatites analyzed in the cumulate shergottite NWA 7755 to represent snapshots of the evolving late-stage residual liquid during exsolution of a Cl-rich brine. This fluid phase has subsequently been lost from an open magma system, migrating upward through the cumulate sequence enriching residual liquids in Cl. Alternatively, it formed a hydrothermal system in the martian crust surrounding the intrusion. Furthermore, by comparison with terrestrial examples, we suggest that the late-stage evolution of volatile-bearing phases in NWA 7755 is similar to that of comparable terrestrial mafic rocks. Primary cumulus apatites are F-rich, whereas interstitial apatites may be either Cl-rich, forming as a result of modification through the interaction with Cl-rich brines, or OH-F-rich forming from a magma having exsolved or degassed a Cl-rich fluid phase.

Reference
Howarth GH, Pernet-Fisher JF, Bodnar RJ, Taylor LA (2015) Evidence for the exsolution of Cl-rich fluids in martian magmas: Apatite petrogenesis in the enriched lherzolitic shergottite Northwest Africa 7755. Geochimica et Cosmochimica (in Press)
Link to Article [doi:10.1016/j.gca.2015.06.031]

Copyright Elsevier