Calibration of the Eu Oxybarometer for Nakhlites

Introduction: Martian meteorites have various characteristics, which are direct clues to understanding the petrogenesis of Mars rocks. The variation in oxidation state among the Martian meteorites must have important implications for redox conditions of the Martian crust/mantle and overall differentiation on Mars. Wadhwa [1] and Herd et al. [2] reported that Martian basalts were formed under a range of oxidation states, suggesting complex petrogenesis processes. The nakhlites, which have rather different characteristics from basaltic shergottites, may give us additional clues to Martian petrogenesis. The oxidation states of meteorites are usually described by the oxygen fugacity (fO2) under which the meteorites crystallized. One of the methods to estimate the oxygen fugacity is the depth of Eu anomaly. Eu/Eu is determined by the oxygen fugacity and partitioning is different for Eu and Eu. Therefore, the depth of Eu anomaly in a mineral is a function of the oxygen fugacity and the Eu/Eu in the melt from which the mineral crystallized. This method has some advantages over another major method, the two-oxide oxybarometer [3], which can more easily be affected by subsolidus processes. The Eu oxybarometer can analyze the cores of the earliest formed crystals in Martian meteorites, which means it can give us a better indication of the oxygen fugacity of the parent melt. The calibration of the Eu oxybarometer has been done with the basaltic shergottites before [4]. However, it has never been applied to nakhites (Oe et al. [5] measured the depth of Eu anomaly in the synthetic pyroxene only at QFM). Partition coefficients are strongly affected by phase compositions, especially pyroxene Ca content and melt Al content [e.g., 5,6]. The composition of nakhlite pyroxene is rather different from basaltic shergottite pyroxene. Thus, there may be problems in applying the Eu oxybarometer calibration for the basaltic shergottites [7] to nakhlites. Thus, we report in this abstract preliminary results of our experimental calibration of the depth of Eu anomaly in pyroxene vs. oxygen fugacity for nakhlites. Experimental Methods: Starting material consisted of glass powders of the NJ4 composition that our group has studied previously [8,9]. This composition was doped with 1.0 wt% SrCO3, 0.7 wt% Sm2O3, 1.0 wt% Eu2O3 and 0.7 wt% Gd2O3. ~125mg pellets of these mixtures were put on Pt loops and homogenized in CO-CO2 gas mixing DelTec furnaces at 1300 oC for 48 hours, and then quenched. Homogenized charges were placed back into the furnaces slightly below the liquidus and held at constant temperature or cooled down at 0.5 oC/hr to various temperatures, growing pyroxene crystals. Oxygen fugacities of experiments were IW, IW+1.5, and QFM (IW+3.5). We analyzed quenched charges with the Cameca SX-100 Electron Microprobe at JSC using carefully selected background positions. We also reanalyzed charges from [5] under the same conditions. Results and Discussion: To date we have had 3 successful experiments. Experimental conditions for these are shown in Table 1, and pyroxene compositions are compared with Nakhla pyroxene cores in Fig. 1. There is general agreement between experimental pyroxene compositions and Nakhla pyroxene core compositions.