Variable Oxidation States in the Winchcombe CM Chondrite Revealed by Synchrotron Mössbauer Source Spectroscopy

Introduction: The study of CM (Mighei) carbonaceous chondrite meteorites is important for understanding the delivery of water and organic materials to the terrestrial planets [1]. However, CM meteorites have been altered by water driven reactions on their parent body that modified their mineralogy and their water and organic content [2]. CM meteorites have also been disrupted by impacts forming regolith breccias, further modifying their composition and juxtaposing materials with different geological histories [3]. It has recently been proposed that CMs may have been disaggregated and reaccreted not only at the macro-scale but also at the grain scale several times during their history [4]. Superimposed on this complexity is the possibility that a diversity of fluids were active on CM parent body(ies) that may have altered discrete parts of CM parent asteroid(s) over time. One way to track the evolution of these fluids is to measure the oxidation state of secondary minerals that form during different stages of aqueous alteration. Here we present Synchrotron Mössbauer Source (SMS) Spectroscopy to measure Fe2+ and Fe3+ charge state of secondary minerals produced by progressive aqueous alteration within the Winchcombe CM meteorite [5]. Sample preparation: SMS is a transmissive technique. CM meteorite samples needed to be thin (<100 µm) to permit X-ray transmission through the sample but thick enough to produce a spectrum from sample-beam interactions. The micrometer scale intergrowths of secondary alteration minerals within the Winchcombe meteorite [5] provides additional complexity as thicker samples >100 µm will have a larger interaction volume that will include the bulk material alongside the mineral of interest while thinner <1 µm samples will only contain the mineral of interest within the transmission volume but may be too thin to produce a reasonable spectrum. Two types of samples were prepared for this experiment. Two sections of the Winchcombe meteorite (P31497 and P31499) were embedded in resin. The sample was cut to 5×5 mm width and then polished from both sides using thin section preparation techniques until it was 100 µm thick to produce 5×5×0.1 mm samples. Seven samples from the Winchcombe meteorite (two from P31503 and five from P31495) were prepared using the Helios Plasma Focused Ion Beam Scanning Electron Microscope (PFIB-SEM) at the Kelvin Nanocharactersation Centre at the University of Glasgow (UofG). The samples were extracted using a bulk milling lift out approach to prepare samples with dimensions 20×10×5 µm. These lift outs were welded to a Cu post using Pt. Methods: All samples were characterized using large area SEM–Energy Dispersive X-ray Spectroscopy mapping using a Zeiss Sigma SEM at the Geoanalytical Electron Microscopy and Spectroscopy Lab at UofG. The generated Backscatter Electron images and false colour element maps were used to identify regions of interest and for navigation during the SMS experiment. All samples were measured on the ID14 beamline at the European Synchrotron Radiation Facility (ESRF). Over 150 spectra were acquired in multi-bunch mode using SMS in transmission mode. Each spectrum was acquired for at least 60 minutes. Some specific regions of interest were acquired for 3-4 hours to achieve better statistics. A standard sample was measured every four hours to verify source stability. Results were then processed using the SyncMoss software to identify Fe species present and calculate Fe3+/Fe2+ ratios. Results and Discussion: Except for a metal grain, no P-FIB prepared samples produced SMS spectrum in a reasonable time frame, due to the samples being too thin. The 100 µm thick samples all produced quantifiable spectra. The SMS spectra reveal that there is variation in Fe3+/Fe2+ ratios across the samples. This variability in Fe3+/Fe2+ reflects the underlying mineral which have consistent Fe3+/Fe2+ ratios across the sample. This suggests that fluid driven alteration within these samples resulted from the evolution of a single fluid. However, some SMS spectra from the periphery of sample P31497 had different Fe3+/Fe2+ ratios for the same minerals. We interpret this variation in oxidation state to reflect that these objects are contained in a different lithology within the Winchcombe meteorite, likely the cataclastic matrix [6]. These results suggest that each individual lithology was affected by a different fluid with a different redox state. Therefore, this implies that multiple discrete fluids were active on the CM parent body(ies).