Crystal shapes in volcanic rocks and what they can tell us about magmatic conditions


Crystal morphologies are the result of a competition between thermodynamic crystal properties and the kinetics of crystal growth. Decades of crystallisation experiments have led to the view that crystal shapes reflect magmatic undercoolings, with crystal growth at temperatures far below the magmas liquidus producing elongated, skeletal crystal shapes, and crystal growth closer to the liquidus resulting in more equant shapes. However, plagioclase microlite shapes in arc volcanic rocks are often euhedral and stubby, despite having grown in severely undercooled conditions in the conduit, contradicting the experimental view on the relationship between undercooling and crystal shape. Here, we show that plagioclase crystal morphologies systematically change from prismatic to tabular during microlite growth, both in natural volcanic rocks and in basaltic and silicic crystallisation experiments. We use this crystal size-shape relationship to constrain nucleus shapes and relative growth rates of plagioclase in silicate melts, and we show that even euhedral crystal shapes are controlled by melt diffusivities. This surprising finding offers an opportunity to explore melt properties in the conduit through the study of microlite shapes.


Dr Martin Mangler is a postdoctoral research assistant at Durham University (UK). After his undergraduate studies in mineralogy at Tuebingen University (Germany), Martin studied magma dynamics at Popocatepetl volcano (Mexico) for his PhD at Imperial College London. Martin uses mapping, bulk-rock and microanalytical techniques, experiments and modelling to explore pre- and syn-eruptive magmatic processes and their timescales.

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This seminar will be streamed using the video conferencing software, Zoom.