Collaborative Research: Elucidating the Role of Titanomagnetite in Vesiculation of Silicic Magmas

The formation of bubbles (or volatile exsolution) from silicate melt is the most important phase transformation occurring in magmas en route to the surface during an explosive eruption. As dissolved magmatic species H2O and CO2 exsolve and produce a separate vapor phase, magma buoyancy and magma ascent rate increase. Bubble expansion driven by decompression accelerates vesiculation, further increasing magma ascent rate, and thereby the potential for increasing conduit overpressure. Thus, the processes governing volatile exsolution and outgassing, and particularly the timing of initial vapor phase nucleation with respect to depth in the conduit, control a magma's explosive potential. A key unresolved aspect of eruption of crystal-poor silicic magmas is whether bubble nucleation occurs homogeneously (in the bulk fluid) or heterogeneously (aided by a substrate). The distinction is important because homogeneous bubble nucleation requires substantively larger pressure overstepping, or supersaturation than heterogeneous bubble nucleation. Magnetite crystals are known to enhance the rate of bubble nucleation by providing energetically-favorable substrates, and yet failure to detect a sufficiently high abundance of these crystals in natural pumice has supported the inference that bubble nucleation occurs homogenously. However when numerical ascent models are supplied with measured bubble number densities and executed specifying homogeneous bubble nucleation, the predicted decompression and ascent rates are not only inconsistent with independent assessments but also present physical paradoxes. This study explores the possibility that nanometer sized magnetite particles are present in magmas prior to eruption, and that these crystals trigger bubble nucleation at much lower supersaturation values (i.e., at greater depth in the plumbing system) than would occur in their absence. This proposal provides educational opportunities for undergraduate and graduate students, and highlights participation of under-represented minorities at the University of Hawaii and Montclair State University (NJ). Professional development for Natural Science educators will be provided through a series of workshops focusing on local geology, volcanology, and Earth processes operating over short and long time scales.

This project addresses the question: do bubbles nucleate homogeneously or heterogeneously in explosively erupting rhyolite magma? The objectives of the work to be undertaken are to ascertain whether plinian rhyolite pumices contain nano-scale titanomagnetite crystals in numerical abundances that rival those of bubbles and to quantify the potential influence of titanomagnetite crystals in bubble nucleation. Addressing these issues is critical for understanding eruptive processes at a fundamental level and is of practical significance in the numerical modeling of conduit flow dynamics and thus assessment of volcanic hazard. The work plan centers around two tasks: (a) applying techniques of rock magnetism to characterize the number density and size distribution of titanomagnetite crystals in nominally aphyric pumice clasts spanning a range in bubble interconnectivity; and (b) performing novel laboratory crystallization experiments utilizing fO2 modulation to isolate the influence of titanomagnetite from other variables. Magnetic characterization (resolving the magnetic domain states, particle number densities, and size distributions of submicron titanomagnetite crystals) is promising because it lowers the detection limit of sparse Fe-Ti oxides by several orders of magnitude relative to traditional electron microprobe imaging. Pairwise measurement of clast permeability, to be obtained for a suite of natural rhyolites spanning a range in eruption parameters and textural attributes, will facilitate determination of magnetite-bubble nucleation chronology. In an experimental task, magnetite stability will be manipulated using ambient fO2 in order to generate rhyolite magma containing populations of magnetite crystals that span large ranges in number density and volume fraction. A subsequent vesiculation step will evaluate the quantitative control of magnetite nanoparticle number density on the bubble-formation process.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.