Abstract

Measurements of OH and Cl growth zoning in apatite phenocrysts from the Cerro Galan ignimbrite describe the tumultuous volatile history of a magma chamber in the days to months prior to a supereruption. The chemical stratigraphy in these crystals provides a unique relative time scale not otherwise available when studying pre-eruptive volatiles in ancient magma chambers, and it allows us to temporally correlate the data within each crystal, resulting in a “volatile history.” The five crystals studied show broadly similar volatile histories, but distinctive differences between the various crystals suggest that each recorded a specific perspective on the magma chamber prior to eruption, perhaps correlating to different depths or laterally separated regions within the >1000 km3 magma chamber. All crystals are consistent with the interpretation of a multistage magmatic history, including a period of residence in a quiescent magma, and a period of rapid growth under strongly varying conditions. Several apparent “events” during this period of growth, consistent with two competing processes of H2O degassing and recharge, are recorded by nearly all of these crystals. Such zoning could not have survived for more than ∼400 d prior to eruption, suggesting that during the days to months leading up to a supereruption, apatite records key aspects of the processes that directly lead to and influence these catastrophic events.

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