2:30 PM - 2:45 PM
[MIS18-03] Vapor phase crystallization from a hydrous silicate melt: an experimental simulation of diktytaxitic texture
Keywords:crystallization experiment, evaporation–deposition, silica polymorph, diktytaxitic texture, lava dome
The microscopic morphology found in volcanic rocks records a series of processes: magma ascent, crystallization, and eruption. Dome-forming lavas and volcanic ash from Vulcanian eruptions sometimes exhibit diktytaxitic textures, in which feldspar microlites are arranged in a network pattern and the microscopic voids are not filled by glass but remain angular pores (e.g. Gaunt et al. 2016; Kushnir et al. 2016). This network of microporosity has the potential to control the explosivity of eruptions by allowing pervasive degassing during the final stages of magma ascent and eruption (Kushnir et al. 2016). On the other hand, diktytaxitic textures are almost always associated with high-temperature polymorphs of silica such as cristobalite and tridymite, which may reduce gas permeability (Horwell et al. 2013; Boudon et al. 2015). However, the mechanisms, conditions, and formation times of these combinations of high-temperature silica polymorphs and diktytaxitic textures are not yet known. Therefore, we attempted to simulate the diktytaxitic texture through hydrothermal experiments.
We used pumice clasts from the Taisho eruption of Sakurajima volcano which has glassy groundmass (SiO2 = 72.5 wt.%) as a starting material. The pumice was placed in Au capsules with water sufficient to saturate the capsules under the experimental conditions, and heated at 2.5–40 MPa and 750–950°C for 6–384 hours. Scanning electron microscopy (JEOL JSM-7100F, JCM-7000) and energy-dispersive X-ray analysis (JEOL JED-2300F) were used for microscopic morphological observation and compositional analysis of the starting materials and heated samples. Micro-Raman spectroscopy (JASCO NRS-5100) was used to identify the crystallized phases. Phases in equilibrium under experimental conditions were calculated using Rhyolite-MELTS.
Diktytaxitic textures were observed in samples heated at 10–20 MPa and 850°C for 4–8 days. These P-T conditions are nearly identical to the solidus conditions (850°C at 19 MPa and 870°C at 10 MPa) calculated using Rhyolite-MELTS for the glass composition of the starting material. In these experiments, cristobalite and alkali feldspar were deposited on the inner wall of the Au capsule away from the pumice sample. This is evidence of the vapor phase growth of these minerals during the experiments, indicating efficient evaporation of silicate melts near their solidus. Under conditions far from the solidus, diktytaxitic textures do not form because the equilibrium melt fraction increases at higher temperatures and the crystallization of fine-grained nanolites or ultrananolites predominates in the melt due to the high degree of supercooling at lower temperatures. The following mechanisms have been proposed for the formation of diktytaxitic texture: (1) halogen-induced corrosion and (2) melt segregation caused by gas-driven filter presses. Although these mechanisms can occur simultaneously, it was found that diktytaxitic texture can be formed by melt evaporation and condensation alone. These findings constrain the outgassing of lava domes and shallow intruded magma and provide new insights into the final stages of hydrous magma crystallization.
In this presentation, we also present an example of diktytaxitic texture in the Ogurayama lava dome from Towada Volcano, Japan, for comparison with the experimental samples. This sample is characterized by a diktytaxitic groundmass composed of feldspar microlites (10–50 µm) and euhedral tridymite precipitated in cavities. Interestingly, the tridymite surface shows a spiral growth pattern with a step spacing of a few microns. No such pattern is observed in the experimental samples. This suggests a low degree of silica supersaturation in the volcanic gas at the time of formation of the Ogurayama lava dome, and that evaporation and condensation may have proceeded much more slowly than in the experiment.
We used pumice clasts from the Taisho eruption of Sakurajima volcano which has glassy groundmass (SiO2 = 72.5 wt.%) as a starting material. The pumice was placed in Au capsules with water sufficient to saturate the capsules under the experimental conditions, and heated at 2.5–40 MPa and 750–950°C for 6–384 hours. Scanning electron microscopy (JEOL JSM-7100F, JCM-7000) and energy-dispersive X-ray analysis (JEOL JED-2300F) were used for microscopic morphological observation and compositional analysis of the starting materials and heated samples. Micro-Raman spectroscopy (JASCO NRS-5100) was used to identify the crystallized phases. Phases in equilibrium under experimental conditions were calculated using Rhyolite-MELTS.
Diktytaxitic textures were observed in samples heated at 10–20 MPa and 850°C for 4–8 days. These P-T conditions are nearly identical to the solidus conditions (850°C at 19 MPa and 870°C at 10 MPa) calculated using Rhyolite-MELTS for the glass composition of the starting material. In these experiments, cristobalite and alkali feldspar were deposited on the inner wall of the Au capsule away from the pumice sample. This is evidence of the vapor phase growth of these minerals during the experiments, indicating efficient evaporation of silicate melts near their solidus. Under conditions far from the solidus, diktytaxitic textures do not form because the equilibrium melt fraction increases at higher temperatures and the crystallization of fine-grained nanolites or ultrananolites predominates in the melt due to the high degree of supercooling at lower temperatures. The following mechanisms have been proposed for the formation of diktytaxitic texture: (1) halogen-induced corrosion and (2) melt segregation caused by gas-driven filter presses. Although these mechanisms can occur simultaneously, it was found that diktytaxitic texture can be formed by melt evaporation and condensation alone. These findings constrain the outgassing of lava domes and shallow intruded magma and provide new insights into the final stages of hydrous magma crystallization.
In this presentation, we also present an example of diktytaxitic texture in the Ogurayama lava dome from Towada Volcano, Japan, for comparison with the experimental samples. This sample is characterized by a diktytaxitic groundmass composed of feldspar microlites (10–50 µm) and euhedral tridymite precipitated in cavities. Interestingly, the tridymite surface shows a spiral growth pattern with a step spacing of a few microns. No such pattern is observed in the experimental samples. This suggests a low degree of silica supersaturation in the volcanic gas at the time of formation of the Ogurayama lava dome, and that evaporation and condensation may have proceeded much more slowly than in the experiment.