Rheology of nanocrystal-bearing andesite magma and its roles in explosive volcanism

Abstract

Recent petrological and experimental studies have proposed that explosive volcanism may originate from the formation of nanoscale crystals in magma and the resultant ductile–brittle transition. However, the rheology of magma with quantified volume fractions of nanoscale crystals has not been investigated before, and thus, the formation of nanoscale crystals causing magma fragmentation that explains the origin of explosive eruptions is not conclusive. Here, we investigate the rheology of andesite magma with nanoscale crystals (magnetite). For this, a glass fibre elongation experimental apparatus with a heating furnace was developed at the synchrotron radiation X-ray system (SPring-8). During melt elongation, we observed the formation of crystals using small-angle X-ray scattering and wide-angle X-ray diffraction. Our experimental data demonstrate that magma viscosity increases with the formation of nanoscale crystals, but the degree of the increase is much lower than that predicted from analogue materials. Finally, we conclude that nanocrystal formation in intermediate composition magmas cannot explain rheological transition and other mechanisms such as nanocrystal agglomeration (not observed in our experiments) and/or heterogeneous nucleation of gas bubbles on nanocrystals are required to induce mafic to intermediate explosive volcanisms.