Effects of crystallization and bubble nucleation on the seismic properties of magmas

Abstract

Seismic tomography of potentially hazardous volcanoes is a prime tool to assess the location and dimensions of magmatic reservoirs. Seismic velocities are strongly affected by processes occurring within the conduit or in the magma chamber, such as crystallization and bubble exsolution. However, the limited number of constrained measurements does not allow yet to link seismic tomography and the textural state of a particular volcanic system. In this study, we investigated a chemically simplified melt in the system CaO‐Na2O‐Al2O3‐SiO2‐H2O‐CO2, which undergoes plagioclase crystallization and bubble exsolution. A Paterson‐type internally heated gas pressure apparatus was employed to measure ultrasonic velocities at a constant pressure of 250 MPa and at temperature from 850 to 700°C. Magmatic processes such as crystallization, bubble nucleation, and coalescence have been recognized throughout the measurements of seismic velocities in the laboratory. Compression and shear wave velocities increase nonlinearly during crystallization. At a crystal fraction exceeding 0.45, the formation of a crystal network favors the propagation of seismic waves through magmatic liquids. However, bubble nucleation induced by crystallization leads to an increase of magma compressibility resulting in a lowering of the wave propagation velocities. These two processes occur simultaneously and have a competing influence on the seismic properties of magmas. In addition, as already observed by previous authors, when the bubble fraction is less than 0.10, the decrease in seismic velocities is more pronounced than for higher bubble fractions. The effect of bubble coalescence on elastic properties is thus lower than the effect of bubble nucleation.

Physical State and Structure of the Crust Beneath the Western‐Central United States From Multiobservable Probabilistic Inversion

“…The V p / V s ratio for common rock types varies from ∼1.63 for felsic rocks to ∼2.1 for mafic and ultra mafic rocks or uncosolidated sediments (Christensen, ; Fountain et al, ; Wang et al, ; Zandt & Ammon, ) and is commonly considered a useful proxy for bulk crustal composition (Chevrot & Van Der Hilst, ; Christensen, ; Gercek, ; Liu & Gao, ; Nair et al, ; Shillington et al, ; Tarkov & Vavakin, ; Zandt & Ammon, ). However, it also known that the presence of melts/fluids can significantly affect the V p / V s ratio and mask the bulk compositional signature (Caricchi et al, ; Chatterjee et al, ; Hacker et al, ; Johnson & Poland, ; Nakajima et al, ; Owens & Zandt, ; Takei, ; Tripoli et al, ; Ueki & Iwamori, ; Zhang et al, ). With this caveat in mind, we will make use of our Vp/Vs model (Figures ) to provide some first‐order constraints into possible compositional structures for the crust in the study region.…”

The crustal structure of the Arizona Transition Zone and southern Colorado Plateau from multiobservable probabilistic inversion

“…in small planar cracks [e.g., Nakajima et al ., ]. Experimental evidence also suggests that the presence of gas bubbles in melt can produce low values of all three V s , V p , and V p ∕ V s , as bubbles tend to affect V p more strongly [e.g., Caricchi et al ., ; Tripoli et al ., ]. However, the nucleation of bubbles in magmas at these depths is extremely unlikely.…”

Quiescent‐explosive transitions during dome‐forming volcanic eruptions: Using seismicity to probe the volcanic processes leading to the 29 July 2008 vulcanian explosion of Soufrière Hills Volcano, Montserrat

“…The spectral content of the resulting LF wave is dependent on conduit properties such as the acoustic velocity of the magma, conduit length, and interface properties such as impedance contrast [ Neuberg and O'Gorman , ; Jousset et al , ; Sturton and Neuberg , , ]. A decrease in the acoustic velocity of the magma, e.g., due to bubble nucleation and growth [ Tripoli et al , ], could result in a slower propagation of interface waves and a lower frequency waveform [ Neuberg and O'Gorman , ]. Decreasing the acoustic velocity of the magma can also reduce the amount of inverse dispersion of the wave and may result in a delay in the higher‐frequency part of the wave [ Sturton and Neuberg , ].…”

“…Increasing pressure in the magma conduit would both increase seismic velocity of the magma (due to reduced bubble content) and cause the bubble exsolution level to shallow, resulting in the observed higher seismic waveform frequencies [ Neuberg and O'Gorman , ; Jousset et al , ]. Jousset et al [] modeled frequency increases at SHV of several hertz by rapidly reducing conduit lengths from 1000 m to 900 m. Small changes in bubble nucleation density can produce significant changes to seismic velocities [ Tripoli et al , ], and therefore seismic frequencies.…”