Rapid cooling and cold storage in a silicic magma reservoir recorded in individual crystals

Abstract

Silicic volcanic eruptions pose considerable hazards, yet the processes leading to these eruptions remain poorly known. A missing link is knowledge of the thermal history of magma feeding such eruptions, which largely controls crystallinity and therefore eruptability. We have determined the thermal history of individual zircon crystals from an eruption of the Taupo Volcanic Zone, New Zealand. Results show that although zircons resided in the magmatic system for 10 to 10 years, they experienced temperatures >650° to 750°C for only years to centuries. This implies near-solidus long-term crystal storage, punctuated by rapid heating and cooling. Reconciling these data with existing models of magma storage requires considering multiple small intrusions and multiple spatial scales, and our approach can help to quantify heat input to and output from magma reservoirs.

Repeated Rhyolite Eruption From Heterogeneous Hot Zones Embedded Within a Cool, Shallow Magma Reservoir

“…The timescales of storage under potentially eruptible conditions are typically constrained using the kinetics of solid‐state cation diffusion. The closure temperature of this process varies depending on the mineral‐trace element pair considered but generally requires near‐ to subsolidus temperatures (Barboni et al, ; Cooper & Kent, ; Rubin et al, ). Whereas the term mush is applied broadly to magma with a high‐crystal content, it is important to recognize the spectrum of dynamics operative over a range of crystallinity.…”

“…Hood, which featured prominently in the initial formulation of the cold storage model (Cooper & Kent, ). Yet large‐ and small‐volume eruptions associated with larger, long‐lived magma systems produce zircon thermometry that records high‐temperature excursions (e.g., Kent & Cooper, ; Szymanowski et al, ), including Kaharoa, which has been proposed as an exemplar of cold magma storage (Rubin et al, , ). However, a method by which to resolve the durations of these reheating events has proven elusive (Barboni et al, ; Kent & Cooper, ; Rubin et al, ; Szymanowski et al, ; Wilson et al, ).…”

“…Among several key issues that confront the use of the geologic record to inform interpretations of volcano monitoring data, the length of the eruptibility window, that is, the duration that mobile magma is stored prior to eruption, has become controversial (Barboni et al, ; Biggs & Pritchard, ; Cooper & Kent, ; Sparks & Cahsman, ; Wilson, ). On one hand, comparison among isotopic ages of multiple accessory phases, trace element diffusion timescales, and thermodynamic considerations suggests eruptible magma is a transient feature within a storage regime dominated by cool, crystalline, and perhaps subsolidus conditions that require an external energy source to mobilize into an eruption (Cooper & Kent, ; Rubin et al, ; Szymanowski et al, ). Conversely, elevated temperatures recorded by zircon crystals suggest eruptible magma may be stored for 10 3 –10 4 years (Barboni et al, ; Edmonds et al, ; Tierney et al, ).…”

Seismic evidence for significant melt beneath the Long Valley Caldera, California, USA

“…Although controversial, lithium diffusion experiments argue that these reservoirs only approach a melt fraction required for eruptability (>35%, i.e., above rheological lock-up) (Cashman et al, 2017) for, at most, centuries after rapid heating from new intrusions (i.e., "cold storage"; Rubin et al, 2017). So, while the reactivation necessary to produce a www.gsapubs.org | Volume 46 | Number 9 | GEOLOGY contemporary 23% melt fraction at Long Valley likely occurred no later than 90 ka, these "cold storage" hypotheses could suggest significantly more-recent changes to the reservoir.…”

Magma Reservoir Below Laguna del Maule Volcanic Field, Chile, Imaged With Surface‐Wave Tomography

“…Large silicic magma reservoirs, which are capable of producing both repeated modest‐volume and large caldera‐forming eruptions, are generally thought to comprise a crystal mush—a crystalline framework containing greater than ~40–50% crystals—from which less crystalline and thus eruptible magma batches may be extracted (Bachmann & Bergantz, ; Hildreth, ). A growing body of evidence indicates that shallow, eruptible (generally requiring >60% melt) magma bodies are ephemeral and that extensive crystal‐rich reservoirs from which they are assembled may be destabilized rapidly when intercepted by ascending melt (e.g., Andersen, Jicha, et al, ; Andersen et al, ; Cooper & Kent, ; Druitt et al, ; Parks et al, ; Rubin et al, ; Singer et al, ; Szymanowski et al, ). Moreover, these eruptible magma chambers and the upper reaches of their surrounding crystal‐rich reservoirs reflect transcrustal magma systems that extend vertically into the lower crust (Cashman et al, ).…”