¹N.Mangold et al. (>10)*
¹Laboratoire de Planétologie et Géodynamique de Nantes, CNRS, UMR6112, Université de Nantes, Nantes, France
*Find the extensive, full author and affiliation list on the publishers website

The Curiosity rover has analyzed various detrital sedimentary rocks at Gale crater, among which fluvial and lacustrine rocks are predominant [Grotzinger et al., 2014, 2015]. Conglomerates correspond both to the coarsest sediments analyzed and the least modified by chemical alteration, enabling us to link their chemistry to that of source rocks on the Gale crater rims. In this study, we report the results of 6 conglomerate targets analyzed by APXS and 40 analyzed by ChemCam. The bulk chemistry derived by both instruments suggests two distinct end-members for the conglomerate compositions. The first group (Darwin type) is typical of conglomerates analyzed before sol 540; it has a felsic alkali-rich composition, with a Na2O/K2O > 5. The second group (Kimberley type) is typical of conglomerates analyzed between sol 540 and 670 in the vicinity of the Kimberley waypoint; it has an alkali-rich potassic composition with Na2O/K2O < 2. The variety of chemistry and igneous textures (when identifiable) of individual clasts suggest that each conglomerate type is a mixture of multiple source rocks. Conglomerate compositions are in agreement with most of the felsic alkali-rich float rock compositions analyzed in the hummocky plains (as reported in Sautter et al., 2015). The average composition of conglomerates can be taken as a proxy of the average igneous crust composition at Gale crater. Differences between the composition of conglomerates and that of finer-grained detrital sediments analyzed by the rover suggest modifications by diagenetic processes (especially for Mg-enrichments in fine grained rocks), physical sorting and mixing with finer-grained material of different composition.

Reference
Mangold N et al. (2016) Composition of conglomerates analyzed by the Curiosity rover: Implications for Gale crater crust and sediment sources. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004977]
Published by arrangement with John Wiley & Sons

¹J. Lasue et al. (>10)*
¹IRAP, OMP, CNRS, Toulouse, France
*Find the extensive, full author and affiliation list on the publishers Website

Zinc-enriched targets have been detected at the Kimberley formation, Gale crater, Mars, using the Chemistry Camera (ChemCam) instrument. The Zn content is analyzed with a univariate calibration based on the 481.2 nm emission line. The limit of quantification for ZnO is 3 wt.% (at 95% confidence level) and 1 wt.% (at 68% confidence level). The limit of detection is shown to be around 0.5 wt.%. As of sol 950, 12 targets on Mars present high ZnO content ranging from 1.0 wt.% to 8.4 wt.% (Yarrada, sol 628). Those Zn-enriched targets are almost entirely located at the Dillinger member of the Kimberley formation, where high Mn and alkali contents were also detected, probably in different phases. Zn enrichment does not depend on the textures of the rocks (coarse-grained sandstones, pebbly conglomerates, resistant fins). The lack of sulfur enhancement suggests that Zn is not present in the sphalerite phase. Zn appears somewhat correlated with Na2O and the ChemCam hydration index, suggesting that it could be in an amorphous clay phase (such as sauconite). On Earth, such an enrichment would be consistent with a supergene alteration of a sphalerite gossan cap in a primary siliciclastic bedrock or a possible hypogene non-sulfide zinc deposition where Zn, Fe, Mn, would have been transported in a reduced sulfur-poor fluid and precipitated rapidly in the form of oxides.

Reference
Lasue J et al. (2016) Observation of >5 wt % zinc at the Kimberley outcrop, Gale crater, Mars. Journal of Geophysical Research, Planets (in Press)
Link to Article [DOI: 10.1002/2015JE004946]
Published by arrangement with John Wiley & Sons

^1,2Erin L. Walton, ³Thomas G. Sharp, ³Jinping Hu
¹MacEwan University, Department of Physical Sciences, Edmonton, AB, T5J 4S2, Canada
²University of Alberta, Department of Earth & Atmospheric Sciences, Edmonton, AB, T6G 2E3, Canada
³Arizona State University, School of Earth & Space Exploration, Tempe, AZ, 85287-1404, USA

Shock-produced melt within crystalline basement rocks of the Steen River impact structure (SRIS) are observed as thin (1 – 510 μm wide), interlocking networks of dark veins which cut across and displace host rock minerals. Solid-state phase transformations, such as ferro-pargasite to an almandine-andradite-majorite garnet and amorphization of quartz and feldspar, are observed in zones adjacent to comparatively wider (50─500 μm) sections of the shock veins. Shock pressure estimates based on the coupled substitution of Na+, Ti4+ and Si4+ for divalent cations, Al3+ and Cr3+ in garnet (14─19 GPa) and the pressure required for plagioclase (Ab62-83) amorphization at elevated temperature (14−20 GPa) are not appreciably different from those recorded by deformation effects observed in non-veined regions of the bulk rock (14─20 GPa). This spatial distribution is the result of an elevated temperature gradient experienced by host rock minerals in contact with larger volumes of impact-generated melt and large deviatoric stresses experienced by minerals along vein margins.
Micrometer-size equant crystals of almandine-pyrope-majorite garnet define the shock vein matrix, consistent with rapid quench (100─200 ms) at 7.5─10 GPa. Crystallization of the vein occurred during a 0.1─0.15 s shock pressure pulse. Majoritic garnet, formed during shock compression by solid state transformation of pargasite along shock vein margins, is observed in TEM bright field images as nanometer-size gouge particles produced at strain rates in the supersonic field (106─108). These crystals are embedded in vesiculated glass, and this texture is interpreted as continued movement and heating along slip planes during pressure release. The deformation of high-pressure minerals formed during shock compression may be the first evidence of oscillatory slip in natural shock veins, which accounts for the production of friction melt via shear when little or no appreciable displacement is observed. Our observations of the mineralogy, chemistry and microtextures of shock veins within crystalline rocks of the SRIS allow us to propose a model for shock vein formation by shear-induced friction melting during shock compression.

Reference
Walton EL, Sharp TG, Hu J (2016) Frictional melting processes and the generation of shock veins in terrestrial impact structures: evidence from the Steen River impact structure, Alberta, Canada. Geochimica et Cosmochimica Acta (in Press)
Link to Article [doi:10.1016/j.gca.2016.02.024]
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