RHYOLITE ERUPTION ON A MID-OCEAN RIDGE: ALARCON RISE, GULF OF CALIFORNIA

Discovery of a rhyolite dome on the ~50-km-long Alarcon Rise mid-ocean ridge in the mouth of the Gulf of California offers the unique opportunity to study the eruption, fragmentation, and dispersal of highly viscous, volatile-charged magma on the deep seafloor (2200-2300 mbsl). Extensive ROV-based sampling around the rhyolite dome included push cores, clast-scoop bags and rock samples, all guided by 1-m-resolution autonomous underwater vehicle (AUV) maps. Fault-scarp analysis and geochemical mapping along the rise indicates that tectonic deformation and crustal stretching is more pronounced in the northern end of the rise where evolved volcanism occurred. Extensive fracturing, blue altered-glass and red hydrothermal staining along steep linear ridges of andesitic pillow lavas that extend north and south of the central rhyolite dome may indicate that they formed from vertical deformation above a viscous dike associated with rhyolite eruption. Extrusion of the rhyolite dome at the center of the structural lineament produced 0.01 km³ of coherent rhyolite lava, autoclastic breccia, pumiceous breccia, talus breccia and pelagic ash. Most rhyolite vitriclasts are coarser grained and concentrated around the lava dome, but rare fine-grained fluidal rhyolite ash occurs in pelagic sediment at least 150 m from the dome. Vitriclasts in pelagic sediment west of the dome are predominantly intermediate in composition, whereas MORB vitriclasts are the primary glass shard component in sediment deposited on the rhyolite dome and on basalt flows to the east. Pumiceous breccia was only observed in a localized area suggesting that it may be an older or spatially confined lithofacies. Textural, FTIR and componentry analyses indicate that flow banded pumiceous clasts occurs with green vitriclasts, both of which have a large range in H₂O contents from 2.0 up to 8.0 wt%. Such hydrated glasses are in stark contrast to less vesicular rhyolite ash and lava breccia clasts (1.6-2.0 wt% H₂O), which are saturated in volatiles for the water depth/pressure of eruption. We argue that the pumiceous deposits represent fragmented and hydrated fracture networks that served as degassing pathways during protracted lava dome effusion and degradation.