Experimental insights into the formation of high-Mg basaltic andesites in the trans-Mexican volcanic belt

High-Mg basaltic andesites and andesites occur in the central trans-Mexican volcanic belt, and their primitive geochemical characteristics suggest equilibration with mantle peridotite. These lavas may represent slab melts that reequilibrated with overlying peridotite or hydrous partial melts of a peridotite source. Here, we experimentally map the liquidus mineralogy for a high-Mg basaltic andesite (9.6 wt% MgO, 54.4 wt% SiO2, Mg# = 75.3) as a function of temperature and H2O content over a range of mantle wedge pressures. Our results permit equilibration of this composition with a harzburgite residue at relatively high water contents (> 7 wt%) and low temperatures (1,080-1,150A degrees C) at 11-14 kbar. However, in contrast to the high Ni contents characteristic of olivine phenocrysts in many such samples from central Mexico, those of olivine phenocrysts in our sample are more typical of mantle melts that have fractionated a small amount of olivine. To account for this and the possibility that the refractory mantle source may have had olivine more Fo-rich than Fo(90), we numerically evaluated alternative equilibration conditions, using our starting bulk composition adjusted to be in equilibrium with Fo(92) olivine. This shifts equilibration conditions to higher temperatures (1,180-1,250A degrees C) at mantle wedge pressures (11-15 kbar) for H2O contents (> 3 wt%) comparable to those analyzed in olivine-hosted melt inclusions from this region. Comparison with geodynamic models shows that final equilibration occurred shallower than the peak temperature of the mantle wedge, suggesting that basaltic melts from the hottest part of the wedge reequilibrated with shallower mantle as they approached the Moho.