High Pressure Phase Equilibrium Investigation of the Home Plate Pyroclastic Basalt Fastball and Application to Melting in the Martian

Introduction: Until recently, the SNC meteorites represented the only source of information about martian igneous processes [1]. This changed with the Mars Exploration Rovers which have analyzed basalts on the surface of Mars in both Gusev Crater and Meridiani Planum. These basalts are thought to be much older than the basaltic SNC meteorites [2] and have significantly different bulk chemistry [3-5]. Recent experimental works have explored some of these compositions to determine if they could represent mantle derived melts [6-7] and have attempted to connect these surface basalt compositions with the cumulate SNC meteorites through crystallization [8-9]. To complement these models, we have experimentally investigated the near-liquidus phase equilibria of the Fastball basalt, analyzed at Home Plate (Gusev Crater), to test if it represents a primitive mantle derived melt. Home Plate: Home Plate is a plateau in the Columbia Hills of Gusev Crater [10]. It is a layered sequence of clastic rocks with alkali basaltic composition, and has experienced some aqueous alteration [1011]. The outcrop is mostly basaltic glass with lesser pyroxene, olivine, plagioclase, nanophase Fe-oxide, and magnetite (from Mössbauer and Miniature Thermal Emission Spectroscopy) [10, 12]. Based on stratigraphy, structure, sedimentology, mineralogy, and bulk chemistry it is thought to represent a pyroclastic deposit [10, 13]. Pyroclastic deposits on the Moon typically represent magmatic liquids [14-15], therefore we have assumed that Fastball is a liquid composition and experimentally investigated the melting phase relations of the bulk composition at pressure. We chose the Fastball composition because 1) it has the highest Mg# of all of the Homeplate rocks suggesting it is the most likely to represent a primitive magma [10] and 2) it has relatively low abundances of Cl and SO3 suggesting that it is not extensively altered [10]. Inverse Experiments: Near-liquidus phase relations can determine whether or not a basalt is a mantle derived liquid. If a magma is co-saturated at a single pressure (P) and temperature (T) with expected mantle minerals (olivine + orthopyroxene ± cpx ± plagioclase/spinel/garnet) of appropriate chemical compositions, that magma could reasonably represent a mantlederived melt [16]. This approach has been previously applied to martian samples: meteorites Yamato 980459 [17] and NWA 1068 [18] and to the Gusev Crater Adirondack-class basalts [6-7]. We apply the same approach here to determine whether the Homeplate-class pyroclastic basalts are mantle derived liquids. Experimental technique: The starting Fastball composition was made from a mixture of oxides and carbonates, fired at 1400° C at 1 atm to ensure homogeneity and to drive-off volatiles, and stored in a desiccator. Experiments were conducted in an endloaded piston-cylinder apparatus using graphite capsules, BaCO3 sleeves, and crushable MgO spacers. All the assembly parts were dried at 300-1000 °C to minimize water contamination. Temperature was measured with a W5%Re/W26%Re thermocouple. Samples were pressurized and then rapidly heated to temperature where they remained for 19-24 hours. Experimental run products were analyzed using a Cameca SX100 electron microprobe at NASA JSC for major element abundances. Results: Figure 1 shows the liquidus P-T results for the Fastball composition. The liquidus T increases from 1425-1450 °C at 1.1 GPa to 1450-1470 °C at 1.5 GPa. Experiments at ≤1.3 GPa have olivine (Fo76) on the liquidus whereas at 1.5 GPa, opx (En77Wo2) is the liquidus phase. This suggests that near ~1.4 GPa there is a multiple saturation with both olivine and orthopyroxene on the liquidus.