Professor Marian Holness

Research: Textural Development in High-Temperature Rocks

My research is concentrated on understanding the processes which occur during the melting and solidification of rocks – these include the formation and segregation of crustal melts, and the evolution of the crystal mush forming at the margins of cooling magma chambers. I approach these problems using petrographic techniques coupled with geochemical analysis to decode rock history.

Current research includes:

Layered Mafic Intrusions

I am particularly interested in the microstructural record of layered mafic intrusions. Using the geometry of three-grain junctions as a proxy for cooling rates I have demonstrated that cumulates record step-wise changes in cooling rate linked to the internal thermal buffering by the release of latent heat, and that these step-wise changes can be used to constrain the thickness of the mushy layer on the chamber margins. With my research group I am currently using electron back-scatter diffraction (EBSD) to quantify the extent of deformation in cumulates, to test the extent to which they deform under their own weight to expel any residual liquid. This work involves study of the Skaergaard, Rum, Bushveld, Sept Iles and Baima intrusions.

The Microstructural Record of Fluid Dynamical Regimes During Solidification

Accumulations of cargo crystals on the floors of sills record the strength of magma convection during crystal settling. While fining-upwards sequences can be interrogated to discern whether the crystals settled from static or convecting magma, some accumulations coarsen upwards: such coarsening upwards can only occur if the crystals grew as suspended grains in a convecting magma. With my research group I am exploring the extent to which magma convected using a detailed grain size examination of olivine-phyric sills on the Isle of Skye, together with analogue experiments (in collaboration of Dr Jerome Neufeld (BPI) and Dr. Rob Farr (Unilever)). In conjunction with this work, I am developing my work on mafic dykes, in which the shape and size of plagioclase grains provide an indication of whether crystallisation occurred predominantly in marginal solidification fronts (with an essentially crystal-free bulk magma) or whether grains grew as isolated crystals of clusters suspended in a vigorously convecting magma. Convection can only occur once the dyke has stopped acting as a conduit, and if the magma viscosity remains sufficiently low. I am investigating laterally propagating dykes associated with the North Atlantic Paleogene Igneous Province to discern the distance from the source at which the crystal load on the cessation of lateral flow is sufficient to damp convection.

Granites

Most recently I have begun to extend my quantitative microstructural work from gabbros into more silicic systems. The main driver for this is the ongoing paradigm shift from the concept of large vats of magma to the idea that most silicic crustal magma-bearing systems comprise crystal-rich mushy regions. The microstructural evolution of such a crystal-rich system will have profound effects on its physical properties, including rheology and permeability.