Uranium series analysis of 2006 augustine volcanics: An investigation into the timescales of magmatic processes.

摘要

The purpose of this study is to constrain the mode and tempo of the processes that generated lavas erupted in 2006 eruption from Augustine Volcano, Alaska. There have been 5 eruptions from Augustine over the last century that ejected andesitic to dacitic lava domes and pyroclastic flows. The short recurrence interval of Augustine, in conjunction with the variety of differentiated magma involved in each eruption makes Augustine an ideal location to study magmatic processes such as magma production, magma mixing, magma differentiation, and shallow degassing occurring over the decadal time-scales in explosive subduction-related volcanoes. Toward this end, 2006 Augustine ejecta were analyzed for abundances of short and long-lived U-series nuclides including 238 U, 230Th, 226Ra, and 210Pb as well as the abundances of a complete suite of major and trace elements.;Samples from Augustine have nearly constant Th/U rations, but excesses of (230Th) over (238U) that vary more than the analytical error. Based on documentation for basaltic parental magmas (Larsen et al., 2010) and a lack of evidence for garnet fractionation in differentiated magmas, variations in (230Th/232Th) values for Augustine magmas appear to result from mixing melts and crystals, which have average ages as high as 54 Ka.;Our most mafic sample (an enclave with 53 wt % SiO2) has ( 226Ra/230Th) = ∼1.2. With increasing concentrations of SiO2, the (226Ra/230Th) values decrease to ∼1 at SiO2 ∼ 56 wt%, increase to 1.45 at SiO 2 ∼ 59 wt%, then decrease to ∼1 in samples with >61 wt. % SiO 2. These data suggest that three magmas mingled and mixed during the 2006 eruption: basaltic andesites with Ra excesses generated by mantle melting; a 59 wt% SiO2 magma with 226Ra excesses generated from incongruent melting of young plutonic materials, and 61-62 wt % SiO 2 andesites with relatively long crustal residence times.;Excesses of (210Pb) over (226Ra) were measured in the most mafic and silicic samples at Augustine, whereas intermediate samples had either equilibrium (210Pb/226Ra) or small 210Pb deficits. The excesses in the basaltic andesites were likely generated during degassing within a compositional boundary layer. Excesses in high silica samples were likely produced by localized degassing through porous networks of vesicles.