Magmatic sedimentology

Planetary differentiation is mediated by processes occurring during the transport, stalling and storage of magmas within the crust. We do not yet have a firm understanding of these processes, despite their critical control on determining the eruptibility of the magma, the extent of fractionation, and our ability to use the composition of erupted magmas as a probe of planetary interiors. Critically, we do not even have a firm concept of what stalled crustal bodies of magma look like and thus how they behave. A body dominated by liquid that solidifies by the progressive inwards propagation of crystal mushy zones from its margins is likely to undergo efficient mixing with replenishing magmas, with an ever-ready body of eruptible liquid potentially able to fractionate efficiently by crystal removal. In contrast, a magma body dominated by mush cannot be efficiently mixed with subsequent incoming magma batches, is unlikely to fractionate effectively, and may not ever erupt. This project is aimed at understanding how magmatic structures such as modal layering can be used to decide where on this spectrum any magma body may lie.

While the solid component of mush-dominated systems is likely to be relatively immobile, and therefore not play much part in the evolution of the magma body, this is not the case in liquid-dominated systems in which the solid component is likely to be highly mobile. Evidence of erosion and re-deposition of mineral grains is common in mafic intrusions, manifest by modal layering and layer truncation. Similar features in more silicic systems have also been attributed to sedimentological processes although some authors maintain they are caused by late-stage recrystallisation. This project is aimed at developing a rigorous understanding of the fluid dynamical behaviour of a low Reynolds number system in which cohesive particles are rapidly cemented during progressive solidification.

The student will develop a detailed understanding of magmatic sedimentology in a novel and challenging study involving the modification of what is already understood for clastic sediments, scaled to solidifying liquid-rich magma bodies ranging from dry basalt to more hydrous silicic compositions. Detailed outcrop-scale, microstructural and geochemical analysis will be made of modal layering in mafic and silicic bodies such as the Skaergaard Intrusion (East Greenland), the Cuillin of Skye, and the Tuolumne Intrusive Complex (Yosemite, USA), and comparison made with layering which is certainly related to recrystallisation (e.g. the doublets of the Stillwater Intrusion, USA). The Skaergaard examples provide the opportunity to link the source and rate of sediment supply to the geometry and composition of the modal layering. Scaled analogue experiments will be undertaken to determine the underlying physical controls of particle mobilisation and re-deposition.