¹H. C. Bates,¹A. J. King,²K. S. Shirley,³E. Bonsall,³C. Schröder,⁴F. Wombacher,⁵T. Fockenberg,¹R. J. Curtis,¹N. E. Bowles
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14043]
¹Planetary Materials Group, Natural History Museum, London, UK
²Atmospheric, Oceanic and Planetary Physics, University of Oxford, Oxford, UK
³Biological and Environmental Sciences, Faculty of Natural Sciences, University of Stirling, Stirling, UK
⁴Institut für Geologie und Mineralogie, Universität zu Köln, Köln, Germany
⁵Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Bochum, Germany
Published by arrangement with John Wiley & Sons

On the microscale, the Winchcombe CM carbonaceous chondrite contains a number of lithological units with a variety of degrees of aqueous alteration. However, an understanding of the average hydration state is useful when comparing to other meteorites and remote observations of airless bodies. We report correlated bulk analyses on multiple subsamples of the Winchcombe meteorite, determining an average phyllosilicate fraction petrologic type of 1.2 and an average water content of 11.9 wt%. We show the elemental composition and distribution of iron and iron oxidation state are consistent with measurements from other CM chondrites; however, Winchcombe shows a low Hg concentration of 58.1 ± 0.5 ng g⁻¹. We demonstrate that infrared reflectance spectra of Winchcombe are consistent with its bulk modal mineralogy, and comparable to other CM chondrites with similar average petrologic types. Finally, we also evaluate whether spectral parameters can estimate H/Si ratios and water abundances, finding generally spectral parameters underestimate water abundance compared to measured values.

^1,2Alan E. Rubin,^2,3Tasha L. Dunn,³Kyla Garner,³Malena Cecchi,³Mitchel Hernandez
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.14046]
¹Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California, USA
²Maine Mineral & Gem Museum, Bethel, Maine, USA
³Department of Geology, Colby College, Waterville, Maine, USA
Published by arrangement with John Wiley & Sons

In general, barred olivine (BO) chondrules formed from completely melted precursors. Among BO chondrules in unequilibrated ordinary chondrites, there are significant positive correlations among chondrule diameter, bar thickness, and rim thickness. In the nebula, smaller BO precursor droplets cooled faster than larger droplets (due to their higher surface area/volume ratios) and grew thinner bars and rims. There is a bimodal distribution in the olivine FeO content in BO chondrules, with a hiatus between 11 and 19 wt% FeO. The ratio of (FeO rich)/(FeO poor) BO chondrules decreases from 12.0 in H to 1.6 in L to 1.3 in LL. This is the opposite of the case for porphyritic chondrules: the mean (FeO rich)/(FeO poor) modal ratio increases from 0.8 in H to 1.8 in L to 2.8 in LL. During H chondrite agglomeration, most precursor dustballs were small with low bulk FeO/(FeO + MgO) ratios and moderately high melting temperatures. The energy available for chondrule melting from flash heating was relatively low, capable of completely melting many ferroan dusty precursors (to form FeO-rich BO chondrules), but incapable of completely melting many magnesian dusty precursors (to form FeO-poor BO chondrules). When L and LL chondrites agglomerated somewhat later, significant proportions of precursor dustballs were relatively large and had moderately high bulk FeO/(FeO + MgO) ratios. The energy available from flash heating was higher, capable of completely melting higher proportions of magnesian dusty precursors to form FeO-poor BO chondrules. These differences may have resulted from an increase in the amplitude of lightning discharges in the nebula caused by enhanced charge separation.