Abstract
Magma crystallisation is a fundamental process driving eruptions and controlling the style of volcanic activity. Crystal nucleation delay, heterogeneous and homogeneous nucleation and crystal growth are all time-dependent processes, however, there is a paucity of real-time experimental data on crystal nucleation and growth kinetics, particularly at the beginning of crystallisation when conditions are far from equilibrium. Here, we reveal the first in situ 3D time-dependent observations of crystal nucleation and growth kinetics in a natural magma, reproducing the crystallisation occurring in real-time during a lava flow, by combining a bespoke high-temperature environmental cell with fast synchrotron X-ray microtomography. We find that both crystal nucleation and growth occur in pulses, with the first crystallisation wave producing a relatively low volume fraction of crystals and hence negligible influence on magma viscosity. This result explains why some lava flows cover kilometres in a few hours from eruption inception, highlighting the hazard posed by fast-moving lava flows. We use our observations to quantify disequilibrium crystallisation in basaltic magmas using an empirical model. Our results demonstrate the potential of in situ 3D time-dependent experiments and have fundamental implications for the rheological evolution of basaltic lava flows, aiding flow modelling, eruption forecasting and hazard management.
Highlights
The combined effects of crystal and vesicle textures on magma rheology and their relationship to eruptive style and intensity is a very active field of investigation
Experimental work on crystallisation kinetics has been conducted through the study of 2D textures; the texture of a volcanic rock is the final product of a dynamic process and it is difficult to quantify with snapshot experiments and 2D measurements
Oxide crystallisation kinetics are poorly experimentally investigated[3,30], they could have an important role in heterogeneous nucleation of pyroxene in basaltic melts promoting, for example, crystal aggregation[35]
Summary
The combined effects of crystal and vesicle textures on magma rheology and their relationship to eruptive style and intensity is a very active field of investigation. Earth scientists have shown that crystal shapes, besides their abundances, have a strong effect on magma rheological response[3,6] It is for this reason that accurate knowledge of the processes determining the formation (e.g., nucleation, growth, phase changes) of crystal phases with time is of critical importance to realistically describe dynamic processes occurring during magma transport and syn- and post-eruptive emplacement. Experimental work on crystallisation kinetics has been conducted through the study of 2D textures (e.g., refs7–10); the texture of a volcanic rock is the final product of a dynamic process and it is difficult to quantify with snapshot experiments and 2D measurements This is relevant when studying low-viscosity systems such as basalts where quench effects may alter the sample texture and chemistry. This allowed a full 3D picture with 3.2 micron[3] voxel size to be captured every three minutes of the growing crystals, whose individual number and volumes could be quantified using post-processing segmentation techniques (see Methods for details)
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