ERUPTIVE HISTORY AND MAGMATIC EVOLUTION PROCESSES OF LONG-LIVED AKUTAN VOLCANO, EASTERN ALEUTIAN ISLANDS

We use 56 new ⁴⁰Ar/³⁹Ar ages, whole-rock chemistry, and geologic mapping to develop an eruptive chronology and upper crustal magmatic model for Akutan volcano, Akutan Island, in the eastern Aleutian island arc. Akutan Island is the site of long-lived volcanism, and the entire island exposes volcanic rocks as old as 3.3 Ma. Our current focus is on the 225 km² western half of the island, where our results show that the focus of volcanism has shifted over the last ~700 ka, and that on occasion, multiple volcanic centers that produce disparate magma types were active contemporaneously, including within the Holocene. This suggests that that closely spaced vents within volcanic clusters may be supplied by trans-crustal magma systems that can remain discrete from one another even when just a few km apart. Durations for individual centers include ~10 k.y (Lava Peak), ~22 k.y. (Cascade Bight), ~40 k.y. (Flat Top), >160 k.y. (Long Valley), and >280 ka (Akutan proper).

Earliest Akutan lavas are basaltic and have high and variable La/Yb ratios consistent with low degrees of partial melting in the mantle wedge. A subset of Pleistocene lavas are calc-alkaline andesites and likely formed by mixing of mafic and evolved magmas, not through crystallization-derived differentiation. In contrast, Holocene lavas, erupted mostly from Akutan proper, plot along low La/Yb, discrete tholeiitic, basalt-to-dacite evolutionary trends that differ only subtly in initial TiO₂ contents. Holocene basalts and basaltic andesites have diverse phenocryst contents, whereas the tholeiitic andesite and dacites are phenocryst poor and lack pervasive evidence for magma mixing. Plutonic-textured blocks, spanning a wide range in compositions, are present in the deposits of the volcano’s 1.6-ka caldera-forming eruption. They are interpreted to represent (1) completely crystallized injections of basaltic andesitic liquid and (2) crystal cumulates from tholeiitic fractional crystallization of the active Akutan magma system. The mineralogy and chemical compositions of Pleistocene to recent Akutan lavas, coupled with existing geodetic and seismic interpretations from the island, collectively support a conceptual model for the active magmatic system wherein hot, relatively dry magmas stall and differentiate shallowly (5–10 km depth) prior to eruption.