^1,2,5Derek Sears,^1,2,5Daniel Ostrowski,³Heather Smith,^1,6Adonay Sissay,⁴Mihir Trivedi
Icarus (in Press) Link to Article [https://doi.org/10.1016/j.icarus.2021.114442]
¹Arkansas Center for Space and Planetary Sciences, University of Arkansas, Fayetteville, AR 72701, USA
²BAER Institute/NASA Ames Research Center, Moffett Field, CA 94035, USA
³USRA/NASA Ames Research Center, Moffett Field, CA 94035, USA
⁴NASA Ames Research Center, Moffett Field, CA 94035, USA
⁵Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701, USA
Copyright Elsevier

In order to find an additional quantitative way to estimate the petrographic type of unequilibrated ordinary chondrites (UOC), and one that can be used remotely in the study of asteroids, we have analyzed the near-infrared spectra of a suite of UOC observed falls. We obtained spectra from the RELAB database at Brown University and applied several methods for determining the amount of clinopyroxene (CPX) as a percentage of the total pyroxene in the meteorites. The presence of low-Ca CPX has long been known to be characteristic of little-metamorphosed ordinary chondrites. The methods we used were (1) naked-eye determination of the wavelength of the absorption features at ~1 μm and ~2 μm, (2) determination of the wavelengths of these features by fitting polynomial equations, and (3) determining the relative intensities of the CPX and OPX features after isolation by a curve fitting procedure. The measurements were then “calibrated” using data from the literature to obtain values for the amount of CPX in the total pyroxene. We find that there is an empirical relationship between the amount of CPX detected by these methods of spectrum analysis and the petrologic type.

Petrologic type = +4.402–0.019 × CPX%

We explain this empirical relationship (1) as evidence that in pyroxene bearing rocks the spectrum of pyroxene dominates (this has been known in the 1970s), (2) that low-Ca CPX is so abundant in these meteorites (up to 40 vol%) that it is easily detected by reflectance spectroscopy, and (3) compositional effects caused by Ca and Fe in the pyroxenes partially cancel out or are small. We thus have a new method of quantitatively measuring the level of metamorphic alteration experienced by these important meteorites and of assigning them a petrologic type of 3.0 to 3.9. More importantly, unlike existing methods, this can be applied remotely so that chondritic asteroid surfaces (i.e. those of Q and S asteroids) can also be characterized in terms of their metamorphic history. As an example, (433) Eros and (25143) Itokawa were found to be types ~3.5 and ~3.4, respectively. We briefly discuss the implications of this for understanding the history of meteorites and asteroids.

¹Maxime Piralla,²Romain Tartèse,¹Yves Marrocchi,²Katherine H. Joy
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13639]
¹CRPG, CNRS, UMR 7358, Université de Lorraine, Vandœuvre‐lès‐Nancy, F‐54500 France
²Department of Earth and Environmental Sciences, School of Natural Sciences, The University of Manchester, Oxford Road, Manchester, M13 9PL UK
Published by arrangement with John Wiley & Sons

Apatite has been widely used for assessing the volatile inventory and hydrothermal fluid compositions of asteroidal and planetary bodies. We report the OH, F, and Cl abundances, as well as the hydrogen isotope composition, of apatite in the CM1‐2 chondrite Boriskino and in the C1‐ungrouped Bench Crater meteorite. Apatite in both meteorites is halogen‐poor, close to the hydroxylapatite endmember composition, and characterized by average δDSMOW values of −226 ± 59% and 233 ± 92%, respectively. Compared to apatite, the matrix in Bench Crater is depleted in D with a δDSMOW value of −16 ± 119‰. Comparing apatite and water H isotope compositions yields similar apatite‐water D/H fractionation ΔDApatite‐Water of approximately 120–150‰ for both chondrites, suggesting that apatite formed at similar temperatures. Combining a lattice strain partitioning model with apatite F and Cl abundances in Boriskino and Bench Crater yields low F and Cl abundances <300 μg g−1 in apatite‐forming fluids, and fluid F/Cl ratios that are roughly consistent with the bulk F/Cl ratios of other CI and CM chondrites. This suggests that hydrothermal alteration on these meteorite parent bodies took place under closed‐system conditions. Based on the OH abundance estimates for the apatite‐forming fluids, we estimated the pH values of alteration fluids to be of approximately 10–13. Such alkaline fluid compositions are consistent with previous modeling and suggest that apatite formed late, toward the end of completion of hydrothermal alteration processes on the Boriskino and Bench Crater parent bodies.

¹Marc M. Hirschmann,²Edwin A. Bergin,³Geoff A. Blake,^4,5Fred J. Ciesla,⁶Jie Li
Proceedings of the National Academy of Sciences of the United States of America [PNAS] (in Press) Link to Article [https://doi.org/10.1073/pnas.2026779118]
¹Department of Earth and Environmental Sciences, University of Minnesota, Minneapolis, MN 55455;
²Department of Astronomy, University of Michigan, Ann Arbor, MI 48109;
³Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125;
⁴Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637;
⁵Chicago Center for Cosmochemistry, University of Chicago, Chicago, IL 60637;
⁶Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109

During the formation of terrestrial planets, volatile loss may occur through nebular processing, planetesimal differentiation, and planetary accretion. We investigate iron meteorites as an archive of volatile loss during planetesimal processing. The carbon contents of the parent bodies of magmatic iron meteorites are reconstructed by thermodynamic modeling. Calculated solid/molten alloy partitioning of C increases greatly with liquid S concentration, and inferred parent body C concentrations range from 0.0004 to 0.11 wt%. Parent bodies fall into two compositional clusters characterized by cores with medium and low C/S. Both of these require significant planetesimal degassing, as metamorphic devolatilization on chondrite-like precursors is insufficient to account for their C depletions. Planetesimal core formation models, ranging from closed-system extraction to degassing of a wholly molten body, show that significant open-system silicate melting and volatile loss are required to match medium and low C/S parent body core compositions. Greater depletion in C relative to S is the hallmark of silicate degassing, indicating that parent body core compositions record processes that affect composite silicate/iron planetesimals. Degassing of bare cores stripped of their silicate mantles would deplete S with negligible C loss and could not account for inferred parent body core compositions. Devolatilization during small-body differentiation is thus a key process in shaping the volatile inventory of terrestrial planets derived from planetesimals and planetary embryos.