Hydrogen contents in clinopyroxene from Martian meteorites (nakhlites) using Elastic Recoil Detection Analysis.

Water contents in Nominally Anhydrous Minerals (NAMs) have been recently quantified in some Martian meteorites [1, 2, 3, 4]. These data were obtained with different methods (mainly SIMS and FTIR) and on a restricted number of SNC (Shergottite, Nakhlite, Chassinite) meteorites. We present here new, high precision data on total hydrogen in clinopyroxene crystals from three nakhlites, obtained by Elastic Recoil Detection Analysis (ERDA) technique. ERDA has been recently greatly improved [5, 6] and is now a very powerful tool to analyse hydrogen in NAMs. We concentrated our study on nakhlites because, i) they are the least affected by shock metamorphism among the SNC meteorites and hence the most likely to preserve their original water contents; [7] and ii) they mainly consist of clinopyroxene, whose crystalline structure can accommodate measurable amounts of hydrogen [8]. We chose MIL 03346, Nakhla and NWA998 specimens because they are representative of the whole spectrum of textural, mineralogical, petrological and geochemical features and cooling history reported for nahklites [7, 9, 10]. A careful selection of clinopyroxene single grains was done and samples were prepared for ERDA following the specific protocol [6]. Electron microprobe analysis was carried out on the same single crystals to check for chemical homogeneity. Clinopyroxene from the three investigated meteorites show similar chemical composition (En37Fs23Wo40).and similar H contents. Our results strictly refer to the crystal core region. Slightly increasing water contents are observed from MIL03346 (170 to 276 ppm H2O) to Nakhla (214 to 301 ppm H2O) to NWA998 (301 to 383 ppm H2O). The relative mean uncertainty of H measure is 15%. These values fall within the wide range of water contents previously reported for martian clinopyroxene and for clinopyroxene from various geological environments of the Earth (e.g., mantle xenoliths, basalts and eclogite) obtained by different methods. Based on modal compositions of the three nakhlites [7], the bulk water content of the three naklites calculated using our new H data ranges from 131 to 278 ppm, assuming water content in olivine and orthopyroxene from literature [3] and no water contribution from the mesostasis. These data are consistent with the low (50-150 ppm) bulk H2O content reported in [11]. The restricted and almost identical water content of clinopyroxene from the three nakhlites supports their crystallization from the same parent magma or, more unlikely, from magmas with identical water content. References. [1] Boctor, N.Z., D’Alexander, C.M.O., Wang, J., Hauri, E. (2003): GCA, 67, 3971–3989; [2] Dyar, M.D., Mackwell, S.J, Seaman, S.J., Marchand, G.J. (2004): LPS XXXV, Abstr. #1348.; [3] Boctor, N.Z., Wang, J., D’Alexander, C.M.O., Hauri, E., Irving, A.J. (2005): LPS XXXVI, Abstr. #1751. [4] Peslier, A.H. (2009): J. Volcanol. Geotherm. Res., in press; [5] Raepsaet, C., Bureau, H., Khodja, H., Aubaud, C., Carraro, A. (2008): Nucl. Instr. Meth. Phys. Res. B, 266, 1333–1337; [6] Bureau, H., Raepsaet, C., Khodja, H., Carraro, A., Aubaud, C. (2009): GCA, 73, 3311–3322; [7] Treiman, A.H. (2005): Chemie der Erde-Geochem., 65, 203-270; [8] Ingrin, J. & Skogby, H. (2000): Eur. J. Min., 12, 543-570. [9] Day, J.M.D., Taylor, L.A., Floss, C., McSween, H.Y.JR. (2006): Met. Planet. Sci., 41, 581–606; [10] Treiman, A.H. & Irving, A.J. (2008): Met. Planet. Sci., 43, 829–854; [11] Filiberto, J. & Treiman, A.H. (2009): Geology, 37;1087-1090.