¹My E.I. Riebe, ¹Henner Busemann, ¹Rainer Wieler, ¹Colin Maden
Geochimica et Cosmochimica Acta (in Press) Link to Article [http://dx.doi.org/10.1016/j.gca.2017.02.004]
¹ETH Zurich, Institute of Geochemistry and Petrology, Clausiusstrasse 25, 8092 Zurich, Switzerland
Copyright Elsevier

We analyzed all the noble gases in HF-soluble phases in the CI chondrite Ivuna by in-vacuum gas release using the “Closed System Step Etching” (CSSE) technique, which allows for direct noble gas measurements of acid-soluble phases. The main motivation was to investigate if there are primordial noble gases in HF-soluble phases in Ivuna, something that has not been done before in CI chondrites, as most primordial noble gases are known to reside in HF-resistant phases. The first steps under mild etching released He, Ne, and Ar with solar-like elemental and isotopic compositions, confirming that Ivuna contains implanted solar wind (SW) noble gases acquired in the parent body regolith. The SW component released in some etch steps was elementally unfractionated. This is unusual as trapped SW noble gases are elementally fractionated in most meteoritic material. In the intermediate etch steps under slightly harsher etching, cosmogenic noble gases were more prominent than SW noble gases. The HF-soluble portion of Ivuna contained primordial Ne and Xe, that was most visible in the last etch steps after all cosmogenic and most SW gases had been released. The primordial Ne and Xe in the HF-solubles have isotopic and elemental ratios readily explained as a mixture of the two most abundant primordial noble gas components in Ivuna bulk samples: HL and Q. Only small fractions of the total HL and Q in Ivuna were released during CSSE analysis; ∼3% of 20NeHL and ∼4% of 132XeQ. HL is known to reside in nanodiamond-rich separates and Q-gases are most likely carried by a carbonaceous phase known as phase Q. Q-gases were likely released from an HF-soluble portion of phase Q. However, nanodiamonds might not be the source of the HL-gases released upon etching, since nanodiamond-rich separates are very HF-resistant and the less tightly bound nanodiamond component P3 was not detected.

¹Steven W. Ruff, ²Victoria E. Hamilton
American Mineralogist 102, 235-251 Link to Article [https://doi.org/10.2138/am-2017-5618]
¹School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287-6305, U.S.A.
²Southwest Research Institute, Boulder, Colorado 80302, U.S.A.
Copyright: The Mineralogical Society of America

Previous observations by the Spirit rover in Gusev crater revealed a suite of rocks dubbed Wishstone and Watchtower Class in which the parent lithology and daughter products of a distinctive style of aqueous alteration are evident. Results from Spirit’s Miniature Thermal Emission Spectrometer (Mini-TES; ~2000–340 cm−1) were compromised by dust contamination of one of the instrument’s mirrors, for which a correction has since been developed. Now we have documented nearly 200 examples of rocks encompassing the span of alteration from Wishstone Class, which spectrally resemble minimally altered plagioclase-phyric basalt, to the most altered Watchtower Class. Among them is a rock dubbed Bruce that may be a previously unrecognized alteration spectral end-member. We employed factor analysis/target transformation and linear least-squares modeling to investigate the spectral characteristics and mineralogy of these rocks. Our results amplify those of a prior preliminary analysis showing that alteration produced a material resembling basaltic glass that masks the spectral features of plagioclase. The association of this amorphous silicate component with a ferric iron nanophase oxide phase identified via Spirit’s Mössbauer spectrometer is now clearly shown by our data, further characterizing the distinctive mineralogic expression of the alteration. These components and the absence of any recognizable secondary silicates or opaline silica may be an expression of alteration in the extreme aridity and cold of the martian environment. Similar mineralogic characteristics of soil measured with the CheMin X-ray diffraction instrument on the Curiosity rover in Gale crater may be an indication that this alteration process is widespread on Mars.