Experimental Insights Into the Formation of High-Mg Andesites in the Trans-Mexican Volcanic Belt

High-Mg basaltic andesites and andesites occur in the central Trans-Mexican Volcanic Belt, primarily in the Chichinautzin Volcanic Field south of Mexico City, and their primitive chemical characteristics suggest equilibration with mantle peridotite. These lavas may represent either slab melts that re-equilibrated with peridotite during ascent or hydrous partial melts of a peridotite source. We have experimentally mapped the liquidus mineralogy for a high-Mg basaltic andesite (9.6 wt% MgO, 54.4 wt% SiO2) from the Pelagatos cinder cone as a function of temperature and water content over a range of mantle wedge pressures. We chose this primitive composition rather than a true andesite because samples from the Chichinautzin region with >55 wt% SiO2 and relatively high MgO and Mg# contain textural evidence for contamination in the form of quartz xenocrysts or reaction products. Our experimental results show that the Pelagatos composition could be in equilibrium with a harzburgite residue (with Fo90 olivine) at relatively high water contents (>7 wt%) and low temperatures (1080-1150°C) at pressures ranging from 11 to 14 kbar. These results agree well with a published thermobarometer for peridotite melting, so we use this thermobarometer to estimate equilibration conditions for other primitive magmas in the region. In contrast to the high Ni contents found in olivine phenocrysts in many high-Mg basaltic andesites and andesites produced in Chichinautzin, the olivine phenocrysts in the Pelagatos lava contain Ni contents typical of lherzolite or harzburgite melts that have subsequently fractionated a small amount of olivine after segregating from their mantle source. Because the refractory mantle source for Pelagatos may have had Fo>90 olivine, we estimate formation conditions for the composition recalculated to be in equilibrium with Fo92 olivine. This calculation shifts equilibration conditions to higher temperatures (1190-1270°C) at mantle wedge pressures (11-14 kbar) for water contents (3-7 wt%) closer to analyzed values for melt inclusions from Chichinautzin. For other high-Mg compositions from this region, these calculations indicate similar equilibration conditions in the mantle wedge. Based on seismic tomography and current geodynamic models of this subduction zone, final equilibration of these melts occurred at lower pressures than the hottest part of the mantle wedge.