COMPARING THE EFFECTS OF H2O, F, AND Cl ON NEAR-LIQUIDUS PHASE EQUILIBRIA OF A MODEL HIGH-Fe BASALT: IMPLICATIONS FOR VOLATILE INDUCED MANTLE

Introduction: Volatile species, such as H2O, CO2, F, and Cl, are important in the generation and differentiation of basaltic melts. On the Earth, dissolved H2O (or in certain circumstances CO2) is thought to be the dominant volatile in most basaltic magmas. Therefore, significant effort has been placed on understanding how water and CO2 affect basalt genesis and crystallization of terrestrial magmas [e.g., 1, 2]. However, water may not be the dominant volatile species in magmas on other planetary bodies [3-6]. Martian basalts are thought to be enriched in halogens (up to 0.2 wt% Cl and F) and depleted in water compared to terrestrial magmas [3-4, 6]. Lunar basalts contain small amounts of dissolved H2O but much less than terrestrial or Martian basalts [5, 7-9]. They also contain F and Cl but the exact amount is highly debated and is likely less than terrestrial or Martian magmas. Venusian magmas are thought to be water poor and CO2 rich, but the exact compositions (especially in terms of volatile contents) of these magmas are poorly constrained [10]. In order to understand how F and Cl affect liquidus depression, near liquidus basalt crystallization, and mantle melting, we compare experimental results of a high-Fe basalt composition with fluorine, and chlorine added independently. We also compare these results to experiments on the effect of water on olivine liquidus depression of basalts with varying bulk compositions. Finally, we perform preliminary calculations for the changes in basalt liquidus temperatures and Fe-Mg partitioning relevant for planetary magmas. Experimental technique: An Fe-rich model basalt (Martian Adirondack-class basalt Humphrey) was used as the starting composition. Experiments have been conducted nominally volatile free [11], with 0.7 wt% Cl [12], and with 1.75 wt% F [13] in graphite capsules at 0.5 2 GPa. Fluorine was added to the starting mix as AgF2 and Cl was added to the starting mix as AgCl. Hydrous (0.8 wt% bulk H2O) experiments [14] have been conducted on a similar bulk composition, which can be be compared with the F and Cl experimental results. Results: Figure 1 shows the P-T results for the volatile-free, fluorine bearing, chlorine bearing and water bearing experiments. From these experimental results we can calculate the change in temperature (∆T) from the anhydrous liquidus to the experimental temperature of the volatile bearing runs. T (C)* 115