Abstract

Isotopic fingerprinting of mineral phases in volcanic rocks has been successfully employed recently to track magma evolution and to identify populations from different sources. Integration of this approach with textural characterisation allows chemical evolution to be integrated with physical changes (growth, nucleation, mixing). In plutonic rocks this approach has been shown to be valid, despite the potential for isotopic re-equilibration during more protracted cooling than volcanic rocks. In fact, the degree of isotopic reequilibration can be used to constrain the cooling rate of the rock, which, in turn, relates to the emplacement history. At the Rum layered mafic intrusion, NW Scotland, isotopically distinct plagioclase cores and rims suggest relatively rapid cooling (at the scale of an individual layer) of the order 0.1°C per year, consistent with sill-like emplacement. The origin of isotopic variation is consistent with growth from a progressively contaminated magma prior to transport and deposition. The Basement Sill of the Dry Valleys Complex, Antarctica, contains an opx-rich tongue claimed to be emplaced as a crystal mush into a crystal-poor magmatic envelope. Given the broadly similar dimensions and compositions of the Rum Intrusion and Basement Sill we expect to be able to use micro-isotopic analyses of cumulus plagioclase crystals in the opx tongue to a) determine whether the magmatic source is the same as the rest of the sill, b) constrain the effects of contamination during crystal growth and emplacement and c) constrain cooling pathways