GEOCHEMICAL AND PETROLOGIC INSIGHTS INTO MAGMATIC PROCESSES AT MOUNT ADAGDAK AND MOUNT MOFFETT, ADAK ISLAND, AK

Mount Moffett and Mount Adagdak are neighboring volcanoes on Adak Island in the western Aleutians with sparsely-studied eruptive histories, despite significant Holocene eruptive activity evident as prominent tephra deposits. The compositions of recently sampled lavas and tephras from both volcanoes range from near-primary basalt to dacite. Basaltic samples contain olivine, clinopyroxene and plagioclase phenocrysts with complex zoning. Basaltic andesites to low-SiO₂ andesites (<60 wt% SiO₂), contain phenocrysts of orthopyroxene, clinopyroxene and plagioclase. High-SiO₂ andesites (>60 wt% SiO₂) and dacites are highly porphyritic with zoned and rimmed plagioclase and amphibole phenocrysts. Major element trends reveal calc-alkaline, medium-K compositions that largely overlap for the two volcanoes, although Moffett compositions are generally higher K₂O at similar SiO₂ than Adagdak compositions. Trace element compositions are more distinctive with Rb, Ba, Ce, Zr, and Nb all higher in the Moffett compositions than those from Adagdak with Ba showing the largest contrast (~200 ppm). Normalized REE patterns are characterized by a negative slope in the light to middle REE range and flat to concave up in the middle to heavy REE range. No samples display Eu anomalies despite the observed plagioclase phenocrysts in nearly all of the lavas and tephras. Based on phenocryst assemblages, ASI (Alumina Saturation Index) vs. SiO₂ trends, and Dy/Dy* vs. Dy/Yb trends, mafic magmas from Adagdak and Moffett experienced early crystallization dominated by clinopyroxene, which drove both the ASI and Dy/Dy* values to ~0.8 at 53 wt% SiO₂. Crystallization of intermediate (55-60 wt% SiO₂) magmas was likely dominated by plagioclase with subordinate pyroxene as indicated by flattening of the ASI vs. SiO₂ trend within this compositional interval. Subsequent crystallization of the more silicic compositions (>60 wt% SiO₂) included plagioclase and amphibole, as observed in the eruptive products and in the increase in ASI and decrease in Dy/Dy* over this interval. Geochemistry along with ongoing melt inclusion and Fe-Ti-oxide analyses will elucidate conditions of magma storage and evolution for Adagdak and Moffett, and facilitate correlation of Holocene tephras to their eruptive center.