Using Volatile Element Concentration Profiles in Crystal‐Hosted Melt Embayments to Estimate Magma Decompression Rate: Assumptions and Inherited Errors

Abstract

Abstract Magma decompression rate has profound impacts on volcanic eruption style as it determines the time available for most kinetic processes (e.g., volatile exsolution, crystal nucleation and growth) that influence the explosive‐effusive eruption transition. Thus, accurately quantifying decompression rate is a critical goal for understanding volcanic eruption dynamics. A recently developed technique uses crystal‐hosted pockets of melt that remain open to the host magma (melt embayments) to calculate an average decompression rate. Diffusion of volatile elements (e.g., H2O, CO2) out of the embayment during decompression creates “frozen” concentration gradients upon quenching that can be modeled to calculate the time needed to create the gradients. This geospeedometer is increasingly used, but inherent assumptions associated with the modeling and their impact on calculated decompression rates are poorly quantified. Therefore, we have conducted a numerical investigation to assess the impact of three common model simplifications pertaining to rhyolitic magmas: 1D diffusion models, equilibrium degassing, and isothermal decompression. We find that the greatest deviation between imposed and calculated decompression rates occur when 1D models are applied to “necked” embayments that have a constriction where the embayment joins with the far field melt. Simplifying to equilibrium degassing can also introduce modeling errors when disequilibrium conditions exist, though the prevalence of one or the other condition in nature is currently under debate. Assuming isothermal conditions introduces little error into modeled timescales. All of our modeling results are summarized into a list of best practices to minimize error in modeled timescales due to modeling assumptions.

Keywords

• diffusion modeling
• melt embayments
• decompression rate