Contrasting timescales of crystallization and magma storage beneath the Aleutian Island arc

Geologic, chronologic, and U–Th isotope data from Pleistocene–Recent basaltic to rhyolitic lavas and their phenocrysts constrain the long-term evolution of magmatic processes at Seguam Island, Aleutian Island arc, and suggest a model in which erupted magmas were derived from a single, deep seated reservoir for ∼130 kyrs of its eruptive history. The monotonic evolution in (230Th / 232Th)0 ratios is consistent with radiogenic ingrowth of 230Th in a long-lived magma reservoir between 142 and 9 ka. Internal U–Th mineral isochrons from seven lavas and one ignimbrite are indistinguishable from their eruption ages as constrained by 40Ar / 39Ar dating, which implies a short period (i.e., 103 yrs or less) of crystal residence in the magma prior to eruption. These results can be reconciled if small batches of magma are repeatedly extracted from a deep, thermally buffered, basaltic reservoir, followed by decompression-driven crystallization and differentiation in small chambers or conduits in the upper crust immediately prior to eruption. Stratovolcano collapse at 9 ka produced a 4 km diameter caldera and a 70 m thick dacitic ignimbrite covering ∼12 km2. The ignimbrite, as well as subsequent rhyolitic and basaltic lava flows, signal a decrease in (230Th / 232Th)0 and 87Sr / 86Sr ratios. The abrupt change in magma composition at 9 ka most likely reflects disruption of the long-lived reservoir by rapid ascent of basaltic magma with low (230Th / 232Th) and 87Sr / 86Sr ratios and excess 226Ra. While the physiochemical connection between cone collapse and rise of new magma into the shallow crust remains to be explored, the shift in isotopic composition and evidence for a protracted > 100 kyr period of magma storage, would have been missed had the focus been on only the most recent historical eruptions. Our results suggest that: not all arc basalt produced in the mantle wedge separates and ascends rapidly, magma ascent rates and storage times may vary significantly over periods of 104 to 105 yrs at a single arc volcano, and U–Th isotopes offer a means of better understanding relationships between explosive eruptions and changes in magma reservoir dynamics in island arcs, provided a sufficiently long period of volcanic activity is examined.