Introduction and methods Carbonates replacing plagioclase glass in the Martian meteorite ALH84001 Macartney1, T. Tomkinson1, E.R.D. Scott2, M.R. Lee1, 1School of Geographical and Earth Sciences, University of Glasgow, UK, Email: a.macartney.1@research.gla.ac.uk 2Hawai’i Institute of Geophysics and Planetology, USA. f Introduction and methods The meteorite ALH84001 has been suggested to contain evidence for Martian bacteria [1]. These structures were found within secondary carbonates that can be divided into three types: Mg-Fe-Ca zoned discs, carbonate globules within glass and irregular carbonates in crush zones [2]. We have sought to test three models for carbonate formation: (i) glass and orthopyroxene replacement [3, 4]; (ii) pore space cementation [5] (iii) shock melt solidification [2]. Options (i) and (iii) preclude biosignatures. Using the thin section ALH84001, 173 (figure 1.A), an FEI DuoMill focused ion beam (FIB) instrument was used to make electron-transparent foils cutting across the plagioclase glass-carbonate interface (figure 1.B) for imaging and electron diffraction work using a FEI T20 TEM (figures 3.A.B.C) and scanning electron microscopy (figure 3.D). References: [1] McKay. D. and Gibson Jr. E. 1996. Science 273:924 [2] Scott. E. R. D. et al. 1997. Nature, 387: 377–379 [3] Treiman, A. 1995. Meteoritics, 30:294 –302. [4] Gleason. J. et al. 1997. Geochimica et Cosmochimica Acta 61:3503–3512. [5] Halevy. I. et al. 2011. PNAS, 108: 16895–16899 Results SEM imaging and elemental mapping has revealed carbonates in a variety of contexts from fractures to distinct clusters of carbonate-glass patches. These carbonate-glass patches, which are 0.1-0.2mm across, have carbonates inter-grown with plagioclase on a very fine scale. Each patch is composed of interconnected groups of zoned carbonate grains. In some areas the carbonate and the glass maintains neatly linear boundaries which suggests the glass intruded and abutted a pre-existing carbonate structure (figure 3.A and lower left of 3.C). Other glass-carbonate interfaces display irregular, non-linear, granular morphology highly suggestive of carbonates replacing the plagioclase glass (figure 3.B, 3.D. upper right of 3.C). The carbonate ‘bridge’ in figure 3.B providing an example of carbonates intruded and replacing plagioclase glass along a pre-existing line weakness. Chemical mapping (figure 2) shows Ca, Fe, Mg carbonate zonation. When observing multiple sites it appears the glass can directly interface with all chemical zone types. Carbonate zonation can repeat and overlay previous zonation areas (figure 2.A, lower centre). Figure 2.C highlights the Fe zone by artificially removing S. Conclusions and future research The morphological, visual and chemical evidence provided by this study supports the hypothesis that glass is being replaced by carbonate. However, the complex multiple shock history of ALH84001 is recognised by the authors. It is forwarded that an older generation of pre-existing carbonates are present, which were subsequently intruded by impact melt glass. This glass was then subsequently exposed to very low water-rock ratio fluids on the nano-scale, weathering the glass into a second generation of carbonates. As the water was consumed in the carbonation reaction the remaining fluid became increasingly saturated in cations, with this evolving fluid chemistry causing the carbonate zonation, beginning with Ca carbonates and progressing through Fe and Mg until finally the all water was consumed and carbonate formation ceases. This cyclical overlaying of glass being replaced by zoned carbonates, followed by further intrusion of shock melt glass may have been repeated many times. Such temporally spaced and chemically dynamic events may explain observed sharp boundary variations in the carbonates, as well as the heterogeneity in carbonate-glass interfaces. More work is needed to substantiate this hypothesis. A series of fluid-rock geochemistry experiments are planned for 2015, in collaboration with the British Geological Survey (Keith Bateman and Dr Chris Rochelle), to further assess carbonate formation under varying fluid and chemical controls using terrestrial Mars analogues and martian samples. Acknowledgements: We thank Dr Kevin Righter and NASA-JSC for loan of ALH84001, 173.