Rapid magma ascent and generation of 230Th excesses in the lower crust at Puyehue-Cordón Caulle, Southern Volcanic Zone, Chile

Determining the time involved for magma transport, storage, and crystallization provides an important link between volcanic output and magma dynamics in the mantle and crust. Relatively few U-Th isotope studies have focused on extended eruptive periods of individual volcanoes (i.e. 104 to 105 yrs), yet this approach is important for understanding the tempo of magma storage, mixing, and fractional crystallization over the lifetime of an arc volcano. New U-series, trace-element, and Sr isotope data from basaltic to rhyolitic lavas and tephras erupted over the last 70 kyrs at the Puyehue-Cordón Caulle volcanic complex in the Andean Southern Volcanic Zone (SVZ) constrain the timescales of intracrustal magmatic processes and identify the subducted and crustal components involved in magma genesis. Internal U-Th mineral isochrons from five lavas and three airfall tephra deposits are indistinguishable from their eruption ages, indicating a short period of crystal residence in the magma prior to eruption. The (230Th/232Th) ratios define a narrow range compared to that of all SVZ lavas, suggesting that Puyehue basalt derives from a relatively uniform mantle source. Dacites and rhyolites have the largest U excesses and likely evolved via fractional crystallization of a plagioclase-dominated mineral assemblage. In contrast, basalts and basaltic andesites have 230Th excesses, a characteristic not previously observed in frontal arc stratovolcanoes of the Andean SVZ. The 230Th excesses are interpreted to reflect a relatively small degree of fluid flux melting coupled with assimilation and melting of the lower crust. Lower crustal processes, therefore, have dampened the 238U excesses that were generated during fluid addition to the mantle wedge. Although prior 238U-230Th-^{226}Ra studies of lavas from other southern SVZ stratovolcanoes (36 to 41 °S) have inferred that slab additions and the extent of mantle melting were nearly constant along strike of the arc, our results suggest that MASH processes like those envisioned by Hildreth and Moorbath (1988) in the northern SVZ also occur in the thinner crust part of the southern SVZ.