SHIFTING MANTLE SOURCES BENEATH SOUTH SISTER VOLCANO (OR)?

Since the late Miocene, volcanism in the Cascade Range in central Oregon consists of two distinct volcanic assemblages. The earliest of the two was a calc-alkaline andesite belt located in the Western Cascades, and the youngest is the High Cascades, which consists of eruptions of mafic magmas along with the creation of silicic centers. The High Cascades bimodal volcanics was followed by localized crustal extension and declining subduction rates, adding complexity to causes of mantle melting in the region. Mirroring broad regional volcanic trends, South Sister volcano features an intermediate period of volcanism followed by eruptions of rhyolite and basalt. These young mafic lavas provide an opportunity to evaluate the driving cause of volcanism beneath the volcano: extension or subduction? We conducted a detailed petrologic study on six mafic lavas that erupted from South Sister and its volcanic field (from ~22 to 8kya) to determine if pre-eruptive conditions, and, therefore, the mantle sources are changing with time. Spinel (chromian-spinels) and olivine compositions, measured using the microprobe at the University of California, Davis, were incorporated into model geo-thermometers, hygrometers, and oxygen barometers. Mineral compositions recorded variable pre-eruptive temperatures (1066°C-1197°C), H₂O contents (0.2 wt% to 4.0 wt%), and oxygen fugacities (∆QFM +0.6 to +1.87). These results combined with a detailed geologic mapping study from the literature show that, as eruptive age decreases, the pre-eruptive temperatures increase and H₂O contents decrease. Oxygen fugacities are highest in the oldest flow and become more reducing in the youngest flows. Modeled oxygen fugacities for all the basalts are elevated compared to established values for mid-ocean ridge basalts from the literature (~QFM), suggesting that there is still a component of subduction modified mantle involved in South Sister volcanism, despite the low water contents in the most recent eruptions.