1. School of the Environment, Geography and Geosciences, University of PortsmouthBurnaby Road, Portsmouth PO1 3QL, UK
2. Department of Geology and Geophysics, School of Geosciences, Kings College, University of Aberdeen, Aberdeen, AB24 3UE, UK
3. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology77, Massachusetts Avenue, Cambridge, MA 02139-4307, USA
4. 6, Mackenzie Crescent, Bettyhill, By Thurso, Sutherland KW14 7SY, UK
5. Department of Earth Sciences, The UniversitySouth Road, Durham DH1 3LE, UK