Complex Thermal History of Nakhla and Y000593

Introduction: In order to unravel the thermal history of the Martian meteorites Nakhla and Y000593 we applied several geo-thermometers including single and two pyroxene thermometry, the Fe, Ti-thermometer / oxybarometer and TEM microstructures. Application of the Fe,Ti-thermometer/oxyba-rometer: The presence of Fe,Ti-oxide phases enable the application of the Fe,Ti-thermometer /oxybaro-meter [1,2] to define fO 2 and closure temperature of the oxide pair. The method is based on the Fe and Ti exchange between coexisting rhombohedral oxide (il-menite solid solution) and spinel (titanomagnetite solid solution) pairs, which determines the closure temperature. The activities of magnetite in spinel and of hematite in ilmenite are used to constrain the fO 2 at the respective temperature. The most recent versions of this model include Ghiorso/Sack [2], which was developed to account for minor-element substitution in the oxides, and the Ca-QUILF model of Andersen [1], which considers equilibria between titanoferous mag-netite, ilmenite, augite, pigeonite, orthopyroxene, oli-vine and quartz in the system FeO-CaO-MgO-SiO 2-TiO 2. Typically, fO 2 determinations are made using the Fe 3+ /Fe 2+ ratios of these oxides at closure temperatures. The procedure requires oxide intergrowths large enough to allow clean electron microprobe analyses. The temperature – logfO 2 data for the Nakhlites Nak-hla are: Ghiorso/Sack: logfO 2 =-14.21 and T = 824 °C, ∆ FMQ =-0.20 and Ca-QUILF: logfO 2 =-14.49 and T = 795 °C, ∆ FMQ = 0.14 and for the new antarctic meteorite Y000593: Ghiorso/Sack: logfO 2 =-15.22 and T = 811 °C, ∆ FMQ =-0.94; Ca-QUILF: logfO 2 =-15.19 and T = 790 °C, ∆ FMQ =-0.46. The results indicate an fO 2 near the FMQ buffer relevant for most of the terrestrial rocks in the lower earth crust. The absolute error of the oxygen fugacity estimates is ± 0.5 log units. The results indicate slight differences in fO 2 between Nakhla and Y000593. The new Nakhlite may have crystallized under more reducing conditions compared with the Nakhla meteorite. Furthermore, there are large differences in the fO 2 , by 3 log units or more between Nakhla, Y000593 on the one hand, and other SNC meteorites on the other [3,4]. These differences may reflect variations in the degree of oxidation very early in the history of planet Mars. Clinopyroxene thermometry: Further details of the thermal history of nakhlites can be obtained by