Decoding granite microstructures

Dr Brendan Dyck
Research Fellow 2016

Dr Brendan Dyck
University of Cambridge

Microscopic structures preserved in rocks contain a unique record of the processes that occur deep in the Earth’s interior.  Using powerful new techniques to interpret this record Brendan is constructing a model of granite migration and solidification that will reveal insights into the formation and evolution of the continents.

Convective motions of the solid rock deep in the Earth’s interior cause localised melting to form magma. When partial melting occurs in the Earth’s continents, the magmas formed have a granitic composition. Little is known about how granitic magma migrates upwards through the crust, despite it comprising more than 25% of the Earth’s surface. This gap in our knowledge not only limits our understanding of how our planet’s crust has evolved but, since granite intrusions in the shallow crust can host quantities of copper, tin and zinc, it also limits our ability to exploit these economically valuable resources. The core objective of Brendan’s research is to develop a model of granitic magma migration and solidification. The size, shape, and relative orientation of mineral grains, together with the way they fit together (the rock microstructure) record the processes that acted to form the Earth’s crust. Over the last decade, significant progress has been made on developing our understanding of microstructures so they can now be used to quantify solidification and cooling history, and to quantify the spatial distribution of small quantities of solidified magma.

"The size, shape, and relative orientation of mineral grains, together with the way they fit together (the rock microstructure) record the processes that acted to form the Earth’s crust."

However, these new techniques were developed for rocks of basaltic composition and have not yet been applied to granitic magma bodies and the problem of the evolution of the continents. Brendan is constructing a database of key microstructural attributes from a range of granites, which will enable him to isolate and identify the parameters that control granite microstructure, permitting him to use microstructures to decode cooling rates and solidification history. Using a combination of microstructural interpretation and detailed geochemical analysis he will analyse samples from ancient magma conduits to decode mechanisms of magma migration. By examining granites emplaced in the shallow crust Brendan can use this method to decode their solidification history and the mechanisms by which economically important elements are concentrated to form ore deposits. This pioneering project will deliver fundamental insights into the formation of Earth’s continents and produce a framework with which the next generation of granite mineral deposit models can be derived.