Abstract:
High-Mg basaltic andesites and andesites occur in the central
Trans-Mexican Volcanic Belt, primarily in the region 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. I have experimentally mapped the liquidus mineralogy
for a high-Mg andesite from the Pelagatos cinder cone as a function of
temperature and H20 content over a range of mantle wedge pressures.
The results,concur with a published thermo barometer for peridotite
melting and suggest that this composition could only be in equilibrium
with a harzburgite residue at relatively high water contents and low
pressures and temperatures. However, numerically adjusting the composition fo r equilibrium with more refractory mantle (Fo92) shifts
these conditions to lower water content s and higher pressures and
temperatures near where geodynamic models indicate peak mantle
wedge temperatures.