Degassing-induced chemical heterogeneity at the 2011-2012 Cordón Caulle eruption

Abstract

The mechanisms of hazardous rhyolitic eruptions such as Cordón Caulle, Chile, in 2011-2012 are controlled by poorly-understood shallow conduit processes. Here we characterise texturally and chemically heterogeneous domains (e.g. ash, breccias and tuffisites) generated via fracturing, gas fluxing and melt relaxation within the conduit, and preserved in glassy, oxidised and devitrified samples. Volatile trace element depletions (e.g. Zn, Pb) in relict degassing pathways record metal scavenging by fluxing gases. Diffusion modelling of preserved trace element concentration gradients (e.g. Li, Rb, Tl) at domain interfaces indicate deep-conduit degassing events were short-lived (-minutes) whereas late-stage venting from discrete locations was prolonged (-hours), corroborating visual observations of the eruption. Later-erupted vent deposits are volatile-depleted with respect to earlier-erupted bombs, indicating progressive syn-eruptive volatile loss. We show that a combination of in situ textural and volatile trace element analyses can provide new constraints on magmatic degassing in shallow systems depleted in H2O and CO2.