Expansion and Contraction of the Patagonian Ice Sheet and Its Influence on Magma Storage Beneath Mocho-Choshuenco Volcano, Chile

Abstract

Glacial loading and unloading could impact eruptive outputs at mid- to high-latitude arc volcanoes, yet the influence on magma storage conditions remains poorly understood. Mocho-Choshuenco volcano in the Andean Southern Volcanic Zone has been impacted by the advance and retreat of the Patagonian ice sheet (PIS). We present stratigraphic observations, conceptual and numerical models of ice (un)loading during the most recent glacial-interglacial cycle, and use mineral-based thermobarometry and thermodynamic modeling to explore the evolution of the magma plumbing system. Crustal assimilation was evident during glaciation when incompatible element-rich basaltic andesitic lavas at 26 to 18 ka crystallized at depths of 11–15 km. During this period a >5 km3 silicic reservoir began to grow at 10–15 km, similar to the depth of maximum mean stress change by ∼5.5 MPa caused by PIS loading. Volatile exsolution within the reservoir might have generated overpressure that initiated diking and propelled two caldera-forming, amphibole-bearing rhyolites at 13.5 and 11.5 ka. We propose that increased compressional stresses associated with ice loading during the Last Glacial Maximum inhibited eruption and promoted growth of the deep silicic reservoir. Explosive eruptions from this reservoir 3–5 kyr following rapid ice unloading likely reflect the interplay of viscous relaxation of crustal wallrocks and delayed, but abrupt, volatile exsolution from rhyolitic melt to generate overpressure and dike formation. This interdisciplinary approach can be applied to other glaciated volcanoes and arcs to examine how ice loading and unloading modulate upper crustal magma fluxes that are governed at greater depths by rates of mantle melting.