The effect of nonlinear decompression history on H 2 O/CO 2 vesiculation in rhyolitic magmas

Volcanic eruption style is modulated by the process of volatile (e.g., H 2 O, CO 2 ) exsolution from ascending magma. Volatile exsolution efficiency strongly depends on composition, namely the degree of melt polymerization (e.g., Cassidy et al., 2018;Mangan et al., 2004). Highly polymerized melts, such as rhyolites, can inhibit volatile exsolution, a phenomenon typically not observed in less polymerized melts, such as basalts. However, kinetic processes can impact volatile exsolution and impact eruption style. For example, fast decompression rates can promote Plinian basaltic eruptions (e.g., Lloyd et al., 2014;Sable et al., 2006), while slow decompression rates can result in effusive rhyolitic eruptions (e.g., Castro et al., 2013;

Section: Introductionmentioning

confidence: 99%

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

deGraffenried

Shea

2021

“…We assumed a linear decompression rate and equilibrium closed‐system degassing as an external boundary condition. Additional complexities include: Magma decompression is expected to be highly non‐linear (Barth et al., 2019; Gonnermann & Manga, 2007; Hajimirza et al., 2021; Su & Huber, 2017). Exsolution of water and subsequent nucleation and growth of bubbles at shallow depth increases the buoyancy of the magma and drives acceleration (Gonnermann & Manga, 2007).…”

Section: Discussionmentioning

confidence: 99%

3D Diffusion of Water in Melt Inclusion‐Bearing Olivine Phenocrysts

Mutch,

Newcombe,

Rudge

2024

Olivine‐hosted melt inclusions are an important archive of pre‐eruptive processes such as magma storage, mixing and subsequent ascent through the crust. However, this record can be modified by post‐entrapment diffusion of H+ through the olivine lattice. Existing studies often use spherical or 1D models to track melt inclusion dehydration that fail to account for complexities in geometry, diffusive anisotropy and sectioning effects. Here we develop a finite element 3D multiphase diffusion model for the dehydration of olivine‐hosted melt inclusions that includes natural crystal geometries and multiple melt inclusions. We use our 3D model to test the reliability of simplified analytical and numerical models (1D and 2D) using magma ascent conditions from the 1977 eruption of Seguam volcano, Alaska. We find that 1D models underestimate melt inclusion water loss, typically by ∼30%, and thus underestimate magma decompression rates, by up to a factor of 5, when compared to the 3D models. An anisotropic analytical solution that we present performs well and recovers decompression rates within a factor of 2, in the situations in which it is valid. 3D models that include multiple melt inclusions show that inclusions can shield each other and reduce the amount of water loss upon ascent. This shielding effect depends on decompression rate, melt inclusion size, and crystallographic direction. Our modeling approach shows that factors such as 3D crystal geometry and melt inclusion configuration can play an important role in constraining accurate decompression rates and recovering water contents in natural magmatic systems.

“…In this study we focused on confirming the fidelity of embayments to record constant‐rate decompression scenarios using a 1D numerical modeling approach. Not accounting for the nonlinearity in magma decompression in numerical modeling of embayment diffusion profiles may result in underestimated ascent times (Su & Huber, 2017). Additionally, stalling of viscous magma (e.g., Zimmer et al., 2007) may allow time for volatile re‐equilibration in embayments and, if unaccounted for, can lead to misleading decompression rate and ascent time estimates.…”

Section: Discussionmentioning

confidence: 99%

“…The embayment speedometer has been leveraged in explosive eruptions spanning a broad compositional range, with studies on rhyolite to rhyo‐dacite (Geshi et al., 2021; Humphreys et al., 2008; Liu et al., 2007; Myers et al., 2018, 2021; Saalfeld et al., 2022) as well as basaltic‐andesite to basalt (Ferguson et al., 2016; Lloyd et al., 2014; Moussallam et al., 2019; Newcombe et al., 2020; Zuccarello et al., 2022) providing insights into how magma decompression rate may (or may not) correlate with eruption composition and explosivity (Figure 1). However, attempts to model volatile concentration gradients in embayments have seen variable success, likely a result of complex magma decompression histories being met with simplifying model assumptions (deGraffenried & Shea, 2021; Su & Huber, 2017). For example, several studies have found that embayments often preserve interior H 2 O ± CO 2 concentrations and corresponding saturation pressures lower than those of co‐erupted melt inclusions, with the use of melt inclusion‐derived storage pressures therefore in some cases unsuccessful in producing good model fits to the observed concentration gradients (Lloyd et al., 2014; Myers et al., 2018, 2021; Saalfeld et al., 2022).…”

Section: Introductionmentioning

confidence: 99%

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

Hosseini

Myers

Watkins

et al. 2023

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.