Are We Recording? Putting Embayment Speedometry to the Test Using High Pressure‐Temperature Decompression Experiments

Abstract

Despite its increasing application to estimate magma decompression rates for explosive eruptions, the embayment speedometer has long awaited critical experimental evaluation. We present the first experimental results on the fidelity of natural quartz‐hosted embayments in rhyolitic systems as recorders of magma decompression. We conducted two high pressure‐temperature isobaric equilibrium experiments and 13 constant‐rate, continuous isothermal decompression experiments in a cold‐seal pressure vessel where we imposed rates from 0.005 to 0.05 MPa s−1 in both H2O‐saturated and mixed‐volatile (H2O + CO2)‐saturated systems. In both equilibrium experiments, we successfully re‐equilibrated embayment melt to new fluid compositions at 780°C and 150 MPa, confirming the ability of embayments to respond to and record changing environmental conditions. Of the 32 glassy embayments recovered, seven met the criteria previously established for successful geospeedometry and were thus analyzed for their volatile (H2O ± CO2) concentrations, with each producing a good model fit and recovering close to the imposed decompression rate. In one H2O‐saturated experiment, modeling H2O concentration gradients in embayments from three separate crystals resulted in best‐fit decompression rates ranging from 0.012 to 0.021 MPa s−1, in close agreement with the imposed rate (0.015 MPa s−1) and attesting to the reproducibility of the technique. For mixed‐volatile experiments, we found that a slightly variable starting fluid composition (2.4–3.5 wt.% H2O at 150 MPa) resulted in good fits to both H2O + CO2 profiles. Overall our experiments provide confidence that the embayment is a robust recorder of constant‐rate, continuous decompression, with the model successfully extracting experimental conditions from profiles representing nearly an order of magnitude variation (0.008–0.05 MPa s−1) in decompression rate.

Deciphering Clues Regarding Magma Composition Encoded in Quartz‐Hosted Embayments and Melt Inclusions Through Direct Numerical Simulations

“…Because of the limited connection with surrounding melt, embayments may still preserve some volatiles and form a diffusion profile between the embayment interior and outlet, especially for volatiles with low diffusivity, such as CO 2 (Befus et al., 2023). This diffusion profile could constrain the magma ascent rate if the volatile concentrations in embayments prior to ascent is known (Befus et al., 2023; Ferguson et al., 2016; Geshi et al., 2021; Hosseini et al., 2023; Humphreys et al., 2008; Liu et al., 2007; Lloyd et al., 2014; Myers et al., 2016, 2018; Saalfeld et al., 2021).…”

“…Embayments have been increasingly used as a tool for estimating magma ascent rates (Befus et al, 2023;Elms et al, 2023;Ferguson et al, 2016;Geshi et al, 2021;Hosseini et al, 2023;Humphreys et al, 2008;Liu et al, 2007;Lloyd et al, 2014;Moussallam et al, 2019;Myers et al, 2016Myers et al, , 2018Myers et al, , 2021Newcombe et al, 2020;Saalfeld et al, 2021;Zuccarello et al, 2022). During magma ascent, the volatile concentrations of a melt decrease due to volatile saturation and exsolution during decompression.…”

“…When using embayments to constrain pre-and syn-eruptive processes, it is justified to assume uniform embayment compositions are representative of the average melt during magma storage only when the embayment compositions fully equilibrate. For example, this assumption is crucial when using volatile concentration profiles of embayments in erupted samples, where volatile concentrations are lower at the embayment outlet than in the interior, to estimate magma decompression rates (Befus et al, 2023;Ferguson et al, 2016;Geshi et al, 2021;Hosseini et al, 2023;Humphreys et al, 2008;Liu et al, 2007;Lloyd et al, 2014;Myers et al, 2016Myers et al, , 2018Saalfeld et al, 2021), as they depend on the difference between the observed concentration profile and an assumed flat initial profile. Based on the data from Befus et al (2023), Figure S6 in Supporting Information S1 provides an example of using embayment composition profile to estimate magma decompression rates assuming an flat initial profile.…”

“…In contrast, embayments are connected with the exterior melt for the duration of magmatic history following their formation. Therefore, embayment compositions may reflect volcanic processes that can partially or wholly overwrite the original composition of entrapped melt, such as magma ascent (Befus et al., 2023; Ferguson et al., 2016; Geshi et al., 2021; Hosseini et al., 2023; Humphreys et al., 2008; Liu et al., 2007; Lloyd et al., 2014; Myers et al., 2016, 2018; Saalfeld et al., 2021).…”

“…We choose to focus on the H 2 O and CO 2 in embayments and melt inclusions as a proof of concept because volatiles are used to infer pre‐eruptive and syn‐eruptive processes, such as crystallization pressure in magma storage (e.g., Métrich et al., 2010; Oppenheimer et al., 2011; Tollan et al., 2019; Wallace et al., 1999; Wei et al., 2022) and decompression rate in magma ascent (Befus et al., 2023; Ferguson et al., 2016; Geshi et al., 2021; Hosseini et al., 2023; Humphreys et al., 2008; Liu et al., 2007; Lloyd et al., 2014; Myers et al., 2016, 2018; Saalfeld et al., 2021). H 2 O and CO 2 also represent incompatible components with different properties in quartz‐hosted embayments and melt inclusions, with H 2 O having high initial abundance and diffusivity, while CO 2 generally has the opposite properties.…”