THROUGH A GLASS DARKLY; AN INVESTIGATION INTO THE VC-1 RHYOLITE, SW VALLES CALDERA, NM

A previously unknown 58ft thick rhyolite unit was discovered during the first Continental Scientific Drilling Program (CSDP) of southwest Valles Caldera in New Mexico in 1984. A single unpublished K/Ar age suggest the unit is younger than the underlying Bandelier tuff and older than the overlying Battleship Rock tuff, which is consistent with the initial core description characterizing the unit as a flow. Our goal is to constrain the emplacement mechanism of this unit using physico-chemical analysis. Assuming emplacement as a single lava flow, we would predict a water content of 0.1 wt% H₂O at the top of the unit (the equilibrium H₂O content at atmospheric pressure) and continuously increasing water content towards the center of the unit. The top 5ft of the unit consists of brecciated ash, pumice, and glass. The rest of the unit consists of massive glass showing flow banding, changing from sub-vertical to sub-horizontal and increasing in intensity with depth. Preliminary water content calculated from mass loss during the DSC measurements show the top of the unit having 0.13 wt% H₂O with increasing water content through to the middle of the unit with values from 0.28 wt% to 0.39 wt%. Viscometry data from a sample of 13ft depth suggests about 0.5 wt% H₂O. Cooling rates have been modeled for three samples using calorimetric geospeedometry and curiously, we found a cooling rate of 6 deg/year at the top of the unit, while the middle of the unit’s cooling rate is 9 deg/year. The inferred water contents by mass loss are consistent with emplacement as a flow, although the apparent lack of variation of cooling rate is surprising. Ongoing work includes precise determination of water content using Fourier-transform infrared spectroscopy (FTIR), additional DSC measurements of samples ranging from the middle to bottom of the unit, compositional information from microprobe and creep viscometry.