Characterization of tephra from the northwest rift zone eruption, Newberry Volcano, Oregon

The northwest rift zone (NWRZ) eruption was the most recent episode of mafic activity to occur at Newberry Volcano, Oregon, USA. Vents of the NWRZ are aligned along a series of northwest trends extending ~32 km across the northwest and southwest flanks of the volcano. The total volume (DRE) of erupted material is 862 x 106 m3, with individual vents erupting volumes ranging from 0.1 x 106 to 381.9 x 106 m3. Eruption duration is not constrained, but tephra from NWRZ vents was deposited directly on top of 7700 ybp Mazama ash, leading to the interpretation that vents were active simultaneously and the eruptive period represents a large input of mafic magma into the Newberry system. Paleomagnetic work has suggested the eruption spanned a period of decades to a century. Individual vents produced a range of deposits, including large and small scoria cones, spatter cones and ramparts, tephra blankets, and extensive lava flows. These eruptive products suggest that individual eruptive episodes were characterized by eruption styles ranging from the passive effusion of lava to explosive activity. Compositional data for both tephra and lava show considerable variation (ranging from 51.3 to 58.4 wt. % SiO2). If the NWRZ eruption represents input of a single magma batch, some combination of fractionation, assimilation, and/or mixing must have occurred to produce the observed range in compositions. Here we present detailed analysis of tephra clasts collected from NWRZ vents, including whole rock, microphenocryst and glass compositions, and water contents derived from the plagioclase-liquid hygrometer described by Lange et al., 2009. We use these data to constrain the pre-eruptive storage conditions (pressure, temperature, and water content) of magma batches erupted at individual vents. Thermodynamic modeling of geochemical data is used to determine the genetic relationship between these magmas. In addition to petrological and geochemical constraints, we use spatial relationships and variations in deposit type to refine our understanding of the NWRZ eruption. The range in vent types and volumes of erupted materials make the NWRZ an ideal study area for constraining the effects of eruption volume, water content, and depth of magma storage on small mafic eruptions.