John Maclennan is accepting applications for PhD students.
John Maclennan is available for consultancy.
Research: Magmatic Processes
I have been trying to understand the processes that occur in the roots of Icelandic volcanoes, from mantle melting and melt transport through to mixing and crystallisation in convecting magma chambers. Much of this effort has centred on careful fieldwork and collection of suitable samples for geochemical analyses. In particular, I have made extensive use of high-precision ion probe data from melt inclusions to understand melting and mixing processes. Such geochemical data is then synthesised and combined with geophysical and geological observations to provide constraints on simple physical models of magmatic processes under Iceland.
Mid-ocean ridge processes
I have developed the first set of thermal models of generation of the oceanic crust at fast spreading ridges (such as the East Pacific Rise) to include petrological variation. Comparison of model results with a wide range of petrological, geophysical and geological observations from both active ridges and ophiolites has enabled me to constrain the distribution of crystallisation and hydrothermal heat loss at ridges. One surprising result is that hydrothermal circulation cools the lowermost ocean crust within a few kilometres of the ridge axis. This result has prompted a re-examination of the geochemical record of hydrothermal circulation in exposures of lower oceanic crust. This modelling work is complemented by my involvement in observational programmes in the East Pacific, one of which involves collection of a high-density grid of well-located samples of young lavas from the active ridge near 9 degrees N, and work in the Alvin submersible. The other was collection of the first in-situ intact continuous section of the upper oceanic crust and uppermost gabbros from the Integrated Ocean Drilling Programme Expedition 312.
Causes and consequences of North Atlantic uplift
A team of colleagues in the Department have been using clues from the sedimentary record in order to reconstruct the vertical motions of the areas surrounding the North Atlantic (particularly the British Isles). One important episode of uplift which occurred between 63 and 55 million years ago is associated with voluminous volcanic eruptions and the onset of plate spreading between Greenland and the Europe. I have linked petrological and geological observations from some of these eruptions to the observed uplift history via injection, cooling and solidification of magma near the base of the crust under the British Isles. Widespread uplift also coincides with the Paleocene-Eocene boundary. This uplift destabilised gas hydrate that was present in the basins fringing the North Atlantic. The release of hydrate may have played an important role in a period of rapid and extreme atmospheric warming that occurred at the Paleocene-Eocene boundary.