134 FEATURE © Blackwell Publishing Ltd, The Geologists’ Association & The Geological Society of London, Geology Today, Vol. 28, No. 4, July–August 2012 Feature Snapshots of ancient oceanic mantle captured in British and Irish ophiolites B. O’Driscoll, E.J. Derbyshire & R. Gertisser School of Physical and Geographical Sciences, Keele University, Keele, Staffordshire, ST5 5BG, UK b.o’driscoll@esci.keele.ac.uk The mantle is not commonly exposed at Earth’s surface. Hands-on investigation is necessarily restricted to mantle xenoliths that have been transported upwards by deeply sourced volcanic activity, or to dredged samples of abyssal peridotites. But it is ophiolites, which represent partially- to-wholly preserved slivers of ‘obducted’ oceanic lithosphere, which are particularly valuable resources. Ophiolites allow an assessment of the timing, causes and extent of processes that operate in the mantle, facilitating the coupling of field-based investigations with geochemical analysis of these otherwise inaccessible rocks. Furthermore, ophiolites may preserve a range of oceanic mantle lithologies (e.g. harzburgite, lherzolite and dunite) and such variation allows a detailed evaluation of the distribution and relative timing of events acting upon the convecting upper mantle. A distinctive feature of some types of ophiolites is the presence of lenticular seams of so-called podiform chromitite, a rock that contains ≥ 60 per cent Cr-spi- nel. Podiform chromitites occur in the vicinity of the crust-mantle transition (petrological Moho) and are typical of oceanic mantle that has undergone high degrees of partial melting in subduction zone environ- ments. In addition to providing important information about the melting processes that are capable of locally concentrating so much Cr-spinel, podiform chromitite also exhibits significantly enhanced concentrations of the platinum-group elements (PGE: Os, Ir, Ru, Rh, Pd and Pt), so that there are significant economic advan- tages to understanding the processes involved in their formation as well. Several of the classic ophiolites of Great Britain and Ireland contain a mantle portion, each of which reveals a complex and fascinating story of partial mantle melting reflecting cycles of oceanic lithosphere creation and destruction over hundreds of millions of years. The British and Irish geological records contain a number of ophiolite slivers, attesting to the pro- tracted and complex tectonic history preserved in their respective rock records (Fig. 1). An older suite of ophiolites preserves lithosphere derived from the Iapetus Ocean basin, and is associated with an Or- dovician (~470 Ma) island arc collision (Grampian orogenesis) that preceded final Iapetus closure during the Caledonian Orogeny. These ophiolites are exposed as far north as the Shetland Islands, in the Grampian Highlands and Midland Valley regions of Scotland and in north-west and western Ireland (Fig. 1). A younger ophiolite, exposed on the Lizard peninsula on the southern coast of Great Britain, was probably emplaced in the Devonian period. The Lizard ophiolite is possibly a fragment of Rheic Ocean lithosphere, ob- ducted during Variscan orogenesis and the collision of the Laurentian and Gondwanan continents. Recognition of ophiolites in the field is based on the presence of a characteristic lithological sequence (Fig. 2a). However, the destructive nature of the col- lision zones in which ophiolite complexes form means that such sequences may be only partially complete. In an idealized section, the base of the ophiolite is marked by the obduction thrust, the structure that allows emplacement of ophiolite sequences over con- tinental crust. Underlying the obduction thrust is a metamorphic sole, which ideally preserves a zone of inverse metamorphic grade (i.e. decreasing intensity of metamorphism with depth). The mantle sequence, usually several kilometres thick, comprises varying proportions of lithologically distinct types of perido- tite (see Fig. 2a,b). The relative abundances of these lithologies indicate the style and degree of partial