^1,2Mario Trieloff,^1,2,3Ekaterina V. Korochantseva,^1,2,3Alexei I. Buikin,^1,2Jens Hopp,³Marina A. Ivanova,³Alexander V. Korochantsev
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13012]
¹Institut für Geowissenschaften, Universität Heidelberg, Heidelberg, Germany
²Klaus-Tschira-Labor für Kosmochemie, Universität Heidelberg, Heidelberg, Germany
³Vernadsky Institute of Geochemistry, Moscow, Russia
Published by arrangement with John Wiley & Sons

We studied three lithologies (light and dark chondritic and impact melt rock) differing in shock stage from the LL5 chondrite Chelyabinsk. Using the 40Ar-39Ar dating technique, we identified low- and high-temperature reservoirs within all samples, ascribed to K-bearing oligoclase feldspar and shock-induced jadeite–feldspar glass assemblages in melt veins, respectively. Trapped argon components had variable 40Ar/36Ar ratios even within low- and high-temperature reservoirs of individual samples. Correcting for trapped argon revealed a lithology-specific response of the K-Ar system to shock metamorphism, thereby defining two distinct impact events affecting the Chelyabinsk parent asteroid (1) an intense impact event ~1.7 ± 0.1 Ga ago formed the light–dark-structured and impact-veined Chelyabinsk breccia. Such a one-stage breccia formation is consistent with petrological observations and was recorded by the strongly shocked lithologies (dark and impact melt) where a significant fraction of oligoclase feldspar was transformed into jadeite and feldspathic glass; and (2) a young reset event ~30 Ma ago particularly affected the light lithology due to its low argon retentivity, while the more retentive shock-induced phases were more resistant against thermal reset. Trapped argon with 40Ar/36Ar ratios up to 1900 was likely incorporated during impact-induced events on the parent body, and mixed with terrestrial atmospheric argon contamination. Had it not been identified via isochrons based on high-resolution argon extraction, several geochronologically meaningless ages would have been deduced.

¹C. Bu,¹G. Rodriguez Lopez,¹C. A. Dukes,²O. Ruesch,²L. A. McFadden,³J.-Y. Li
Meteoritics & Planetary Science (in Press) Link to Article [DOI: 10.1111/maps.13024]
¹Laboratory for Astrophysics and Surface Physics, University of Virginia, Charlottesville, Virginia, USA
²NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
³Planetary Science Institute, Tucson, Arizona, USA
Published by arrangement with John Wiley & Sons

The formation of hydrated salts is an expected consequence of aqueous alteration of Main Belt objects, particularly for large, volatile-rich protoplanets like Ceres. Sulfates, present on water-bearing planetary bodies (e.g., Earth, Mars, and carbonaceous chondrite parent bodies) across the inner solar system, may contribute to Ceres’ UV and IR spectral signature along with phyllosilicates and carbonates. We investigate the presence and stability of hydrated sulfates under Ceres’ cryogenic, low-pressure environment and the consequent spectral effects, using UV–Vis–IR reflectance spectroscopy. H2O loss begins instantaneously with vacuum exposure, measured by the attenuation of spectral water absorption bands, and a phase transition from crystalline to amorphous is observed for MgSO4·6H2O by X-ray powder diffraction. Long-term (>40 h), continuous exposure of MgSO4·nH2O (n = 0, 6, 7) to low pressure (10−3–10−6 Torr) causes material decomposition and strong UV absorption below 0.5 μm. Our measurements suggest that MgSO4·6H2O grains (45–83 μm) dehydrate to 2% of the original 1.9 μm water band area over ~0.3 Ma at 200 K on Ceres and after ~42 Ma for 147 K. These rates, inferred from an Avrami dehydration model, preclude MgSO4·6H2O as a component of Ceres’ surface, although anhydrous and minimally hydrated sulfates may be present. A comparison between Ceres emissivity spectra and laboratory reflectance measurements over the infrared range (5–17 μm) suggests sulfates cannot be excluded from Ceres’ mineralogy.