¹R.C. Greenwood et al. (>10)
Meteoritics & Planetary Science (in Press) Open Access Link to Article [https://doi.org/10.1111/maps.13968]
¹Planetary and Space Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA UK
Published by arrangement with John Wiley & Sons

As part of an integrated consortium study, we have undertaken O, Cd, Cr, Si, Te, Ti, and Zn whole rock isotopic measurements of the Winchcombe CM2 meteorite. δ66Zn values determined for two Winchcombe aliquots are +0.29 ± 0.05‰ (2SD) and +0.45 ± 0.05‰ (2SD). The difference between these analyses likely reflects sample heterogeneity. Zn isotope compositions for Winchcombe show excellent agreement with published CM2 data. δ114Cd for a single Winchcombe aliquot is +0.29 ± 0.04‰ (2SD), which is close to a previous result for Murchison. δ130Te values for three aliquots gave indistinguishable results, with a mean value of +0.62 ± 0.01‰ (2SD) and are essentially identical to published values for CM2s. ε53Cr and ε54Cr for Winchcombe are 0.319 ± 0.029 (2SE) and 0.775 ± 0.067 (2SE), respectively. Based on its Cr isotopic composition, Winchcombe plots close to other CM2 chondrites. ε50Ti and ε46Ti values for Winchcombe are 3.21 ± 0.09 (2SE) and 0.46 ± 0.08 (2SE), respectively, and are in line with recently published data for CM2s. The δ30Si composition of Winchcombe is −0.50 ± 0.06‰ (2SD, n = 11) and is essentially indistinguishable from measurements obtained on other CM2 chondrites. In conformity with petrographic observations, oxygen isotope analyses of both bulk and micromilled fractions from Winchcombe clearly demonstrate that its parent body experienced extensive aqueous alteration. The style of alteration exhibited by Winchcombe is consistent with relatively closed system processes. Analysis of different fractions within Winchcombe broadly support the view that, while different lithologies within an individual CM2 meteorite can be highly variable, each meteorite is characterized by a predominant alteration type. Mixing of different lithologies within a regolith environment to form cataclastic matrix is supported by oxygen isotope analysis of micromilled fractions from Winchcombe. Previously unpublished bulk oxygen isotope data for 12 CM2 chondrites, when combined with published data, define a well-constrained regression line with a slope of 0.77. Winchcombe analyses define a more limited linear trend at the isotopically heavy, more aqueously altered, end of the slope 0.77 CM2 array. The CM2 slope 0.77 array intersects the oxygen isotope field of CO3 falls, indicating that the unaltered precursor material to the CMs was essentially identical in oxygen isotope composition to the CO3 falls. Our data are consistent with earlier suggestions that the main differences between the CO3s and CM2s reflect differing amounts of water ice that co-accreted into their respective parent bodies, being high in the case of CM2s and low in the case of CO3s. The small difference in Si isotope compositions between the CM and CO meteorites can be explained by different proportions of matrix versus refractory silicates. CMs and COs may also be indistinguishable with respect to Ti and Cr isotopes; however, further analysis is required to test this possibility. The close relationship between CO3 and CM2 chondrites revealed by our data supports the emerging view that the snow line within protoplanetary disks marks an important zone of planetesimal accretion.

¹A.В. VERCHOVSKY,¹F. A. J. ABERNETHY,¹M. ANAND,¹S. J. BARBER,¹R. FINDLAY,¹I. A. FRANCHI,¹R. C. GREENWOOD,¹M. M. GRADY
Meteoritics & Planetary Science (in Press) Open Access Link to Article [doi: 10.1111/maps.13983]
¹School of Physical Sciences, The Open University, Milton Keynes, UK
Published by arrangement with John Wiley & Sons

Two bulk Winchcombe along with six other CM2 meteorite samples weresubjected to quantitative evolved gas analysis. The observed release patterns for almost allvolatile species demonstrate close similarity for all the samples and especially between thosefor Winchcombe. This can be considered as a fingerprint for this petrological type ofmeteorites. We identified several gases including H2,H2O, O2, CO, CO2, and SO2releasedin different temperature ranges. The sources and mechanisms of their release were alsoestablished. Some of the gases, H2, CO, and CO2, are released as a result of oxidation ofmacromolecular organic material from oxygen derived from oxygen-bearing minerals (a partof CO2is also released as a result of decomposition of carbonates). The others, O2andH2O, are associated with the phase transformation/decomposition of phyllosilicates and(oxy)hydrates, while a high-temperature release of SO2is associated mostly with thedecomposition of sulfides and in few cases also with sulfates. A low-temperature release ofSO2is due to evaporation and oxidation of elemental sulfur from the meteoritic matrix andorganic material. The total concentrations of H (mostly represented by H2O), C, and S,calculated according to calibration of the quadrupole mass spectrometer with referencegases and decomposition of solid samples (CaSO42H2O and NaHCO3) are in reasonableagreement with those determined by independent methods. Variations in the ratio of thecarbon amounts released as CO2and CO (CCO2/CCO) between the samples could be anindicator of their terrestrial weathering.

¹Gordon R. OSINSKI,¹Adam B. COULTER,¹Roberta L. FLEMMING,¹Alexandra OZARUK,¹Annemarie E. PICKERSGILL,¹Alaura C. SINGLETON
Meteoritics & Planetary Science (in Press) Open Sccess Link to Article [doi: 10.1111/maps.13986]
¹Department of Earth Sciences, University of Western Ontario, London, Ontario, Canada
Published by arrangement with John Wiley & Sons

The~5 km diameter Gow Lake impact structure formed in the Canadian Shield ofnorthern Saskatchewan approximately 197 Myr ago. This structure has not been studied indetail since its discovery during a regional gravity survey in the early 1970s. We report hereon field observations from a 2011 expedition that, when combined with subsequentlaboratory studies, have revealed a wealth of new information about this poorly studiedCanadian impact structure. Initially considered to be a prototypical central peak (i.e., acomplex) impact structure, our observations demonstrate that Gow Lake is actually atransitional impact structure, making it one of only two identified on Earth. Despite its age,a well-preserved sequence of crater-fill impactites is preserved on Calder Island in themiddle of Gow Lake. From the base upward, this stratigraphy is parautochthonous targetrock, lithic impact breccia, clast-rich impact melt rock, red clast-poor impact melt rock, andgreen clast-poor impact melt rocks. Discontinuous lenses of impact melt-bearing brecciaalso occur near the top of the red impact melt rocks and in the uppermost green impactmelt rocks. The vitric particles in these breccias display irregular and contorted outlines.This, together with their setting within crater-fill melt rocks, is indicative of an origin asflows within the transient cavity and not an airborne mode of origin. Following impact, ahydrothermal system was initiated, which resulted in alteration of the crater-fill impactites.Major alteration phases are nontronite clay, K-feldspar, and quartz.