Low-δ18O tephra from a compositionally zoned magma body: Fisher Caldera, Unimak Island, Aleutians

We present the results of an oxygen isotope study of phenocrysts in pumice clasts and ash layers produced by the 9100 yr BP composite dacite-basaltic andesite climactic eruption that formed Fisher Caldera in the eastern Aleutians. Products of the eruption represent a low-δ18O magma with δ18O plagioclase (+4.79±0.24‰) and clinopyroxene (3.81±0.23‰) corresponding to equilibrium at magmatic temperatures. Dacitic and overlying basaltic–andesitic tephra of the climactic eruption, subsequent intracaldera basaltic to andesitic lavas, and a cumulate inclusion, are similarly low in δ18O. Other analyzed lavas and pyroclastics of Unimak island and the lower Alaska peninsula, as well as precaldera Fisher basalt, have normal δ18O magmatic values (>+5.5‰). We propose a model in which prior to 9100 yr BP, normal mantle-derived basaltic magma coalesced in a large shallow precaldera magma chamber during Late Wisconsin glaciation. Lowering of magmatic δ18O resulted then from long-term assimilation of ∼5–10% of syn-glacial hydrothermally-altered country rocks. Differentiation of basaltic magma was concurrent with this assimilation and produced low-δ18O Fisher dacites, cumulates, and post-caldera crystal-richer lavas. We propose the use of δ18O values of phenocrysts (especially alteration-resistant pyroxene) in tephra as a tool for tephrochronological and tephrostratigraphic correlation. Distinctly low-δ18O values are useful in identification of the Fisher ash in the eastern Aleutians and in the lower Alaska Peninsula.