Experimental Petrology of the Basaltic Shergottite Yamato 980459: Implications for the Thermal Structure of the Martian Mantle

Introduction: Yamato 980459 (Y98) is an olivine-phyric basaltic shergottite [1] composed of 48% pyroxene, 26% olivine, 25% mesostasis, and 1% other minerals. Unlike the other Martian basalts, it contains no plagioclase. Olivine in Y98 is the most magnesian of all Martian meteorites. Thus Y98 is believed to be the most primitive [2] and its composition may be the closest to a primary or direct melt of the Martian mantle. As such, it provides a very useful probe of the mineralogy and depth of its mantle source region. Toward this end, we are conducting crystallization experiments on a synthetic Y98 composition [3,4] at Martian mantle pressures and temperatures. Experimental Methods: Experiments are being conducted using a Quickpress non-end-loaded piston cylinder apparatus with a pressure assembly comprised of a BaCO3 cell, MgO internal parts, and a graphite sample capsule. Temperature is monitored with a W5Re/W25Re (C-type) thermocouple. Pressures range from 4 to 16 Kbars and temperatures from 1380°C to 1625°C for experiments conducted so far. Friction correction to the pressure is 1.8±0.1 Kbar below gauge. The experimental starting composition (Table 1) is an average of reported whole-rock values [3,4]. Oxides and carbonates were ground together with an agate mortar and pestle, then melted completely in a muffle furnace and reground to ensure homogeneity. Oxygen fugacity is controlled using the method of [5] whereby starting material with a preset Fe2O3 content [6,7] is loaded into a graphite capsule which influences the fO2 by the oxidation of C to form CO2 [5]. The Fe2O3 content of the starting material was preset by placing it in a CO/CO2 gas mixing furnace at 800°C with log (fO2) maintained at -14.85 ±0.1 for 90 hrs with a resulting Fe2O3 concentration of 1.07 ± 0.06 wt% (corresponding to an Fe/Fe ratio of 0.062)[6,7]. The resulting fO2 for the piston cylinder runs is IW + 0.7 ± 0.1 log units [5]. Temperature and pressure ramps are done “hot piston out”. That is, the assembly is first brought up to 10% above the run pressure, then ramped up to run temperature over 30 minutes. Once the run temperature is reached, the pressure is brought down to run pressure. At the end of the run the assembly is quenched isobarically. Run conditions are shown in Table 2. In addition, a 1-atmosphere melting experiment has been performed in a gas mixing furnace with fO2 controlled at IW + 0.7±0.1 log units. Run products are analyzed using the Cameca SX-100 electron microprobe at NASA/JSC. Results: Experimental results are listed in Table 2 and plotted in Figure 1. The inferred experimental liquidus or olivine-in line, and the pyroxene-in line are plotted. For comparison the liquidus calculated from the MELTS program [8] using the average bulk composition of [3,4] is also plotted.