Nanometer Scale Titanomagnetite Assisted Bubble Nucleation: A Joint Magnetic and Textural Characterization Approach

Abstract:

Vesiculation of silicate magma begins with bubble nucleation, which can occur heterogeneously or homogenously. The presence of a nucleation substrate (such as magnetite crystals, facilitating heterogenous nucleation) reduces the supersaturation pressure required for bubble formation in silicic melts to <5 MPa. Without a substrate, homogenous nucleation requires supersaturation >100 MPa, a pressure that nears or exceeds the storage conditions preceding many explosive eruptions. The high supersaturation pressure and storage depth required for homogenous bubble nucleation is not only inconsistent with shallow crustal storage, but the conundrum is enhanced by the apparent lack of magnetite particles in ‘aphyric’ rhyolite tephra that could serve as heterogeneous nucleation substrates. This logical disconnect would be resolved if magnetite particles exist at high number density but at sizes below the detection limit of standard petrographic methods, i.e., at sub-micron diameter. This research uses magnetic characterization methods to investigate whether nm scale titanomagnetite particles exists in rhyolite pumice, and, if so, to resolve the timing of its formation relative to eruption.

Pumice for this work erupted from Glass Mountain (GM), USA, (73.5 % SiO2, 1100 AD, Subplinian), and from the caldera-forming Pudahuel Ignimbrite (PI), Chile (74.3% SiO2, 0.45 Ma, Plinian). Both tephra sets span similar average values of total vesicularity (73.5% and 72.8%) and connected vesicularity (62.8% and 65%) for GM and PI respectively. Magnetic analyses indicate the presence of 0.0011-0.0041 vol% (GM) and 0.0064-0.0228 vol% (PI) titanomagnetite. Both sites exhibit room temperature magnetic behaviors consistent with, but not diagnostic of, a mixed assemblage of stable single domain titanomagnetite and finer superparamagnetic particles. Titanomagnetite abundance is not correlated with vesicularity or connected porosity, which we interpret as evidence that titanomagnetite formed prior to magma ascent and vesicle deformation. Our preliminary assessment indicates that the number densities of sub-micron titanomagnetite particles are sufficient to have promoted heterogeneous bubble nucleation in the GM and PI eruptions.