Browsing by Author "Myers, Madison L."

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Are We Recording? Putting Embayment Speedometry to the Test Using High Pressure‐Temperature Decompression Experiments
(American Geophysical Union, 2023-06) Hosseini, Behnaz; Myers, Madison L.; Watkins, James M.; Harris, Megan A.
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.
Evolution of magma decompression and discharge during a Plinian event (Late Bronze-Age eruption, Santorini) from multiple eruption-intensity proxies
(Springer Science and Business Media LLC, 2021-02) Myers, Madison L.; Druitt, Timothy H.; Schiavi, Federica; Gurioli, Lucia; Flaherty, Taya
We have coupled three independent methods to investigate the time evolution of eruptive intensity during the sub-Plinian and Plinian phases of the 3600-year BP Late Bronze-Age eruption of Santorini Volcano: (1) mass eruption rate based on new lithic isopleth maps for multiple layers of the fall deposit, (2) magma decompression rate calculated from vesicle number densities, and (3) magma decompression rate calculated from H2O gradients in melt reentrants, with methods 2 and 3 measured on the same suite of pyroclasts. Mass eruption rate increased by two orders of magnitude, reaching 210 × 106 kg s−1 at the peak of the Plinian phase (plume height 28.4 ± 1.0 km); it then declined in the final stage of fallout emplacement following the first generation of pyroclastic surges. Decompression rates from melt reentrants (0.008 to 0.25 MPa s−1) are two to three orders of magnitude lower than those from vesicle number densities, assuming heterogeneous vesicle nucleation (2 to 19 MPa s−1). Melt reentrants are thought to record slow decompression in the deep feeder conduit, whereas vesicles record much higher rates of decompression in the shallow conduit due to the steep, nonlinear pressure gradients associated with magma vesiculation and fragmentation. Upwardly converging flow from a dike-like, deep conduit to a more cylindrical, shallow conduit may also have played a role in causing upwardly accelerating flow. Variations in deep decompression rate recorded by melt reentrants are decoupled from mass eruption rate, whereas those recorded by vesicles lie in between. Taken with the transition from unsteady to steady Plinian eruption, this may reflect the existence of transient flow conditions in the conduit system due to widening and lengthening of a deep feeder dike as Plinian eruption progressed. As the mass eruption rate rose to its peak value, the fragmentation level fell in the conduit due to increasing rates of magma strain and decompression.
Pre-eruptive rhyolite magma ascent rate is rapid and independent of eruption size: a case study from Ōkataina Volcanic Centre, Aotearoa New Zealand
(Springer Science and Business Media LLC, 2023-03) Elms, Hannah C.; Myers, Madison L.; Nichols, Alexander R. L.; Wallace, Paul J.; Wilson, Colin J. N.; Barker, Simon J.; Charlier, Bruce L. A.
Volatile measurements in mineral-hosted sealed melt inclusions, and open-ended embayments, have previously been used to study magma ascent dynamics in large rhyolitic eruptions. However, despite occurring more frequently, smaller-volume explosive events remain under-studied. We present magmatic volatile data from quartz-hosted melt inclusions and embayments for eight post-25.4 ka rhyolitic eruptions at Ōkataina Volcanic Centre, Aotearoa New Zealand. Seven originated from within the main caldera, and the other erupted from the associated Ōkareka Structural Embayment. Melt inclusions preserve volatile contents of 2.92–5.82 wt% H2O and 13–126 ppm CO2, indicating pre-eruptive storage depths of 4.5–7.4 km, with younger eruptions being more shallow. The lack of correlation between H2O, CO2, inclusion size or distance to the crystal rim suggests magma bodies experienced variable degrees of degassing during magma storage, with some amount of post-entrapment volatile modification prior to and concurrent with final magma ascent. Diffusion modelling of measured H2O gradients in melt embayments indicates ascent rates of 0.10–1.67 m.s−1 over time spans of 20–230 min for the intra-caldera events. In contrast, ascent rates for the eruption from the Ōkareka Structural Embayment may be more rapid, at 1.59–4.4 m.s−1 over a time span of 22–34 min. Our findings imply that the final, pre-eruptive magma movement towards the surface could be less than a few hours. Comparisons with published data for caldera-forming explosive events reveal no clear relationships between final ascent rate, eruption size or initial volatile content, implying that other factors besides eruption volume control rhyolite magma ascent.