Magmatic degassing histories from apatite volatile stratigraphy

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.