Measuring the degree of “nanotilization” of volcanic glasses: Understanding syn-eruptive processes recorded in melt inclusions

Abstract

Iron and water content substantially affect the physical properties of natural silicate melts and may, therefore, influence magmatic and volcanic processes such as crystallization, degassing, flow behaviour and eruptive style. Here we present Raman spectroscopic data for a set of synthetic and natural multicomponent glasses with varying iron oxidation state and water content. We systematically study the effect of different Raman excitation sources on the spectral response of both crystal free and magnetite nanolite bearing glasses spanning basaltic to calc- and per-alkaline rhyolitic compositions. Based on these data we document changes in Raman spectra resulting from the formation of nano-scale crystals. We show that the peak located at ~970 cm−1 is directly related to the presence of Fe2O3 dissolved in the quenched melt structure and that this feature is present regardless of the chemical composition of the sample and the Raman excitation source. We further show that a peak between 670 and 690 cm−1, which is not related to any other spectral feature of the glass structure, reveals the presence of nanolites. Based on systematic spectral investigations of this feature, we present a new index that allows to identify if iron is present in the nanocrystalline state and/or bound in the glass structure. Since the melt structural and physical effects of the formation of nanolites can heavily affect the flow behaviour of melts and the eruptive style of volcanoes, the results presented in this study significantly broaden the application of Raman spectroscopy for investigations of nano-heterogeneity in synthetic and natural glasses.

We apply this method to study both the degree of nanolitization as well as the H2O content and iron oxidation state of groundmass glasses as well as melt inclusions and glass embayments in explosive products from Pantelleria island (Italy). We observe that the process of nanotilization is not purely restricted to magnetite nanolites but that Raman spectroscopy may also identify the incipient crystalization of pyroxene and feldspar at sub-micron scale. The data document that nanolite formation correlates well with the observed intensity of the respective eruptions suggesting that structural changes in the melt, caused by incipient crystallization play an important role in defining the eruptive style of relatively low viscosity magmas.