Controls of melt genesis and evolution at Buldir Volcano in the western Aleutian Island Arc

The geochemistry of western Aleutian seafloor volcanoes indicates that primitive dacitic lavas (63-71% SiO2, Mg/Mg+Fe > 0.65) containing low abundances of middle and heavy rare-earth elements (<5x chondrites Yb) were probably produced by melting of an isotopically MORB-like (e.g., 87Sr/86Sr < 0.7027) pyroxenite or subducted basaltic source. Andesites and dacites from emergent volcanoes throughout the Aleutians often also have low abundances of middle and heavy rare-earth elements (REE's), but these characteristics are widely attributed to fractional crystallization and the removal of amphibole from mafic melts in crustal magma chambers. For this study, geochemical data were collected for 50 lavas from Buldir Volcano, the westernmost emergent volcano in the Aleutian island arc, to investigate the role of amphibole fractionation versus source variability in the genesis of andesites and dacites from Buldir. Buldir lavas range from basalt to dacite and define a strongly calc-alkaline or low-Fe igneous series. Andesites and dacites at Buldir have low abundances of middle and heavy REE's (<8x chondrite Yb), and relatively unradiogenic Sr isotopic compositions (0.7028 < 87Sr/86Sr < 0.7032). Modeling results indicate that fractional crystallization involving up to 50% amphibole of a water-rich magma chamber played a role in the genesis of many Buldir lavas, but isotopic variability indicates that the Buldir lavas must also be the products of one or more distinct sources. Mixtures of 20-40% western Aleutian seafloor dacite combined with 60-80% Buldir andesite can produce the more unradiogenic lavas at Buldir (87Sr/86Sr < 0.7030). Combined petrographic and geochemical observations indicate that high-water contents are primarily responsible for the distinctive major and trace element evolutionary pathways observed at Buldir compared to the more common transition calcalkaline (medium-Fe) volcanic systems of the eastern Aleutian arc.