PROTRACTED RECHARGE ASSOCIATED WITH INDEPENDENT RHYOLITE ERUPTIONS FROM SOUTH SISTER VOLCANO, OREGON CASCADES, USA

South Sister (SS) volcano, Oregon Cascades, USA, repeatedly erupted rhyolite during the last 50 ka, including the late-Holocene Rock Mesa (rrm) and Devils Chain (rdc) rhyolites, erupted ~300 years apart from two multi-vent complexes 3-6 km from each other. Despite this spatial-temporal proximity and broadly similar compositions and mineralogy, previous work (Stelton & Cooper, 2012, EPSL) on zircon dates and compositions indicates that these rhyolites accumulated over >50 k.y. in physically distinct, mushy reservoirs. We present a study of compositional zoning in orthopyroxene crystals to examine the magmatic processes leading up to the rrm and rdc eruptions. Orthopyroxene crystals in both rhyolites span similar compositional ranges, Mg# = 48-76, and commonly have reversely zoned interior and rim domains and less common, high-gradient normal zones. Taken together, the observed compositional zonation likely reflects several processes associated with recharge of a shallow magma reservoir including reheating and movement of crystals within a heterogeneous magma body. Often the Fe-Mg gradients of these zones are relaxed compared to those of less mobile minor elements (Al, Ti, Ca) indicating reequilibration has occurred. Fe-Mg interdiffusion models of the reverse and normal zoned domains from both rhyolites produce timescales ranging from several years to centuries. Particularly for rrm, the longest timescales are dominated by the normally zoned domains yielding a wider range of timescales than observed in rdc. Notably, however, the dominant population of reversely zoned timescales from rdc overlaps with the best estimate of the repose period between these eruptions derived from carbon dates, 305±262 yr (Scott, 1987, GSA Special Paper 212). Rather than discrete eruption “triggers,” we suggest these timescales reflect a period of frequent shallow magma intrusion, and likely unrest, from a common deep source. The unrest lasted several centuries or more, affected an area at least 6-km wide, and disturbed the physically distinct rrm and rdc magma bodies. Similar intrusions likely continued following the rdc eruption as evidenced by subsequent, nearby mafic eruptions, the anomalous temperatures and magmatic C and He isotope ratios of springs near SS, and ground deformation ongoing since the mid-1990s.