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Abstract Plagioclase microlites in a magma nucleate and grow in response to melt supersaturation (Δϕplag). The resultant frozen plagioclase crystal size distribution (CSD) preserves the history of decompression pathways (dP/dt). SNGPlag is a numerical model that calculates the equilibrium composition of a decompressing magma and nucleates and grows plagioclase in response to an imposed Δϕplag. Here, we test a new version of SNGPlag calibrated for use with basaltic andesite magmas and modeldP/dtfor the ca. 12.6 ka Curacautín eruption of Llaima volcano, Chile. Instantaneous nucleation (Nplag) and growth (Gplag) rates of plagioclase were computed using the experimental results of Shea and Hammer (J Volcanol Geotherm Res 260:127–145, 10.1016/j.jvolgeores.2013.04.018, 2013) and used for SNGPlag modeling of basaltic andesite composition. MaximumNplagof 6.1 × 105 cm h−1is achieved at a Δϕplagof 44% and the maximumGplagof 27.4 μm h−1is achieved at a Δϕplagof 29%. Our modeled logdP/dtavgrange from 2.69 ± 0.09 to 6.89 ± 0.96 MPa h−1(1σ) with an average duration of decompression from 0.87 ± 0.25 to 16.13 ± 0.29 h assuming a starting pressurePiof 110–150 MPa. These rates are similar to those derived from mafic decompression experiments for other explosive eruptions. Using assumptions for lithostatic pressure gradients (dP/dz), we calculate ascent rates of < 1–6 m s−1. We conducted a second set of Monte Carlo simulations usingPiof 15–30 MPa to investigate the influence of shallower decompression, resulting in logdP/dtavgfrom 2.86 ± 0.49 to 6.00 ± 0.86 MPa h−1. ThedP/dtmodeled here is two orders of magnitude lower than those calculated by Valdivia et al. (Bull Volcanol, 10.1007/s00445-021-01514-8, 2022) for the same eruption using a bubble number density meter, and suggests homogeneous nucleation raisesdP/dtby orders of magnitude in the shallow conduit. Our modeling further supports the rapid-ascent hypothesis for driving highly explosive mafic eruptions.
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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−1in 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 + CO2profiles. 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.
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- Award ID(s):
- 1922513
- PAR ID:
- 10418415
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 24
- Issue:
- 6
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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