Syneruptive Pressure-Temperature-Time Paths of Basaltic Magma
Abstract
We have constrained syneruptive pressure-temperature-time (P-T-t) paths of mafic magmas using a combination of short-timescale cooling and decompression chronometers. Decompression histories of basaltic magma during syneruptive ascent can be constrained using concentration gradients of volatiles in olivine-hosted melt embayments [2, 3]. Thermal histories of olivine phenocrysts in the last few seconds to hours of eruption can be constrained using concentration gradients of MgO inside olivine-hosted melt inclusions (MIs), produced in response to syneruptive cooling and crystallization of olivine on the MI walls [1]. We have applied these techniques in concert to determine P-T-t paths of magma during explosive eruptions of Fuego, Seguam and Kilauea volcanoes.
Syneruptive cooling rates recorded by MgO zonation in MIs from Kilauea and Seguam are consistent with air quenching over 10s of seconds upon fragmentation and eruption. The interpretation of zonation in MIs from arc magmas containing ~4 wt% H2O hinges on the relationship between H2O content and the diffusivity of MgO (DMgO) in the melt: if DMgO is only weakly dependent on H2O, we find evidence for 10s of °C of syneruptive cooling during the last ~1-7 min of magma ascent at Fuego, consistent with models of cooling driven by adiabatic vapor expansion [4]; however, if the dependence of DMgO on H2O is high (as inferred by [5]), our observations of MgO zonation in MIs from Fuego and Seguam are suggestive of a delicate balance between heat production (e.g., latent heat of crystallization) and adiabatic cooling, resulting in ~isothermal magma ascent followed by rapid quenching on eruption. Concentration gradients of S, H2O and CO2 in melt embayments from Seguam provide evidence for magma 'pausing' over a range of pressures from ~60-310 MPa prior to final ascent and eruption. The amount of adiabatic cooling experienced by a decompressing magma is dependent on vapor fraction, which is in turn controlled by magma volatile content and the extent of melt-vapor segregation during ascent. Pausing of magma at shallow depths likely encourages melt-vapor segregation, thereby reducing the vapor fraction of the magma available to drive adiabatic cooling. [1] Newcombe et al. 2014; [2] Lloyd et al. 2014; [3] Ferguson et al. 2016; [4] Mastin 2002 [5] González-García et al. 2018- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43A..04N
- Keywords:
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- 1038 Mantle processes;
- GEOCHEMISTRY;
- 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGY;
- 3651 Thermobarometry;
- MINERALOGY AND PETROLOGY;
- 3652 Pressure-temperature-time paths;
- MINERALOGY AND PETROLOGY