CONDITIONS THAT GIVE RISE TO EFFICIENT SEGREGATION AND ERUPTION OF RHYOLITE-OBSIDIANS IN THE CASCADES: A PETROLOGIC STUDY OF RHYOLITES FROM MEDICINE LAKE, SOUTH SISTER, AND LASSEN (Invited Presentation)

I present new pre-eruptive temperatures and volatile contents of effusively erupted, crystal-poor, rhyolite-obsidians from South Sister, Medicine Lake and Lassen volcanic fields in order to determine common conditions that give rise to efficient segregation and eruption of rhyolite melts in the Cascades. Application of a geo-thermometer and hygrometer reveal that five crystal-poor (< 5%) rhyolite-obsidians from Medicine Lake have pre-eruptive temperatures that range from 813-932°C and H₂O contents that range from 2.4 to 6.2 wt%. Sixteen crystal-poor (<6%) obsidians sampled from the South Sister have temperatures that range from 825-865°C and with 3.2-4.5 wt% H₂O. Five crystal-poor (<8%) rhyolites from Lassen have temperatures that range from 884-936°C and H₂O contents of 2.6-5.3 wt%, based on previously reported phenocryst compositions from the literature. The intensive variables recorded by phenocrysts in the obsidians are consistent with crystallization in the upper crust; however, low phenocryst modes in the obsidians suggest that crystallization is kinetically hindered. Decompression rates associated with effusive eruptions are not sufficient enough to cause a kinetic lag in crystallization, and, therefore, the rhyolite-obsidians must have been super-heated prior to eruption so as to purge the melts of nucleation sites, preventing extensive crystal growth. Superheating can be achieved through heating or fluid-undersaturated decompression. Given the temperatures and petrology of the rhyolites, superheating was likely generated via fluid under-saturated decompression, which also requires magmatic sources of the rhyolites to be fluid under-saturated. Moreover, the magmatic systems beneath South Sister and Medicine Lake must be routinely fluid under-saturated as crystal-poor andesites and dacites frequently erupt in these volcanic fields and are subject to same kinetic processes that kinetically hinder crystal growth in the rhyolites. Fluid under-saturated magmatic systems are uncommon in volcanic arcs, given the hydrous nature of arc volcanics, the infrequency of rhyolite eruptions, and the paucity of discrete rhyolitic bodies in arc batholiths, making the magmatic plumbing systems beneath South Sister, Medicine Lake and Lassen unique among others in the Cascades.