MINERAL PROBES OF MAGMATIC PROCESSES AT VALLES CALDERA, NORTHERN NEW MEXICO

The ~1 Ma Valles caldera and its volcanic products have played a central role in our understanding of catastrophic, caldera-forming eruptions. We have used mineral compositions determined on the UGA JEOL microprobe in attempts to understand processes in the magma chamber(s) responsible for the caldera-forming eruptions and ensuing post-caldera eruptions. Specifically we have examined the banding of the post-caldera South Mountain rhyolite to determine if the banding is due to magma mixing or is a textural feature. Secondly we are using apatite compositions (specifically S, F, Cl and by calculation, OH) contents to infer evolution of magmatic volatiles from before until after the caldera forming eruptions. In outcrop the South Mountain rhyolite shows extensive dark and light grey flow banding. Phenocrysts distinguishable in hand sample are rose quartz, sanidine, plagioclase, and biotite. In thin section, these phenocrysts plus hornblende are visible, along with groundmass glass. Preliminary analyses show that biotites within both both types of flow bands are compositionally similar (Mg # ~ 37 and wt% TiO₂ from 3.8 to 4.2%). These observations suggest that the flow banding is not due to magma mixing. Apatite in the post-caldera South Mountain rhyolite is fluorapatite (~3.5 wt% F, ~0.1 wt% Cl), consistent with eruption from a previously degassed magma chamber. An initial microprobe survey of the groundmass of the Tshirege member of the Bandelier tuff, the caldera-forming unit, indicates that apatite, if present, is rare and small. These findings are not unexpected, given the very low reported phosphate concentrations for this tuff.