Hydrous phase equilibria of a Mexican high-silica andesite:A candidate for a mantle origin?

Abstract

A group of Pleistocene-recent lavas, with 62 to 64% SiO2 and 5.6 to 3.5% MgO, have erupted along a linear fault system near the volcanic front of central Mexico. Not only is the composition of this group of lavas distinctive in comparison to andesites from other parts of the Mexican Volcanic Belt, but their typical pyroxene phenocryst assemblages may include hornblende and rarely plagioclase. Consequently, the experimental (water-saturated) P-T conditions, up to 3 kbar, necessary to reproduce the various phenocryst assemblages have been explored for the most Mg-rich andesite. The phenocryst assemblages in this group of lavas can be reproduced over a range of water concentrations (∼6 to ∼3 wt%) at temperatures consistent with those derived from phenocryst mineral geothermometers (910 to 1055°C). The phase fields of the phenocryst assemblages are ∼200°C below the water-saturated liquidus (>1100°C), which is defined by orthopyroxene, followed by olivine, ∼30°C below. However, the orthopyroxene and olivine assemblage has never been found in the Valle de Bravo lavas. Three observations are consistent with a mantle source for this andesitic lava that contains up to 176 ppm Ni and 257 ppm Cr. Both experimental liquidus olivine (Fo90) and orthopyroxene (En88) have compositions appropriate to a lherzolite source; the composition of a water-saturated partial melt of lherzolite at 1000°C and 10 kbar is comparable to these andesites. Last, a contemporaneous andesite lava that erupted along the same fault system contains nodules of hornblende-spinel-lherzolite, representative of the Mexican subarc mantle.