Experimental insights into the formation of high-Mg basaltic andesites in the trans-Mexican volcanic belt |
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Authors: | Rachel?M?Weber Email author" target="_blank">Paul?J?WallaceEmail author A?Dana Johnston |
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Institution: | (1) Department of Geological Sciences, University of Oregon, Eugene, OR 97403-1272, USA; |
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Abstract: | 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 (1,080–1,150°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 Fo90, we numerically evaluated alternative equilibration conditions, using our starting bulk composition adjusted to be in equilibrium
with Fo92 olivine. This shifts equilibration conditions to higher temperatures (1,180–1,250°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. |
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