Sideways views of Scottish garnets: insights into metamorphic processes (lecture)

Dr Tim Dempster, University of Glasgow

Clough Medal and Joint Celebrity Lecture with the Geological Society of Edinburgh

Garnets are capable of recording original compositions during growth and hence allow determination of pressure-temperature paths and durations of metamorphic events. However, studies of garnet from the Scottish Highlands have questioned some key concepts of metamorphic equilibrium. Surface and internal zoning patterns reveal a failure to equilibrate at both staurolite and sillimanite isograds, and question assumptions about fluid availability and rates of intergranular transport. This emphasises the importance of kinetic controls on metamorphic reactions. Individual porphyroblasts may have a unique response to prograde metamorphism and control both subsequent reaction pathways and index mineral distribution.

Background reading: Dempster, T. J., La Piazza, J., Taylor, A. G., Beaudoin, N. and Chung, P. (2017) Chemical and textural equilibration of garnet during amphibolitefacies metamorphism: The influence of coupled dissolution-reprecipitation. Journal of Metamorphic Geology, 35, 1111-1130.

Tim is currently a senior lecturer at the University of Glasgow. His research interests are focused on the use of minerals to reveal information on crustal processes, including using zircon as an indicator of low and medium grade metamorphic processes and garnet zoning to assess the role of diffusion processes in controlling metamorphic reactions. Other interests include the study of grain boundaries and the movement of fluids through the crust and the morphology of mineral surfaces as a guide to reaction history, palaeoporosity and deformation in crustal rocks. In addition, he researches chemical zoning in accessory minerals as a monitor of crystallisation history to better understand the initial phases of orogenic evolution and the links between surface processes and tectonics.