Rb-Sr and Sm-Nd isotopic variations in dissected crustal xenoliths

Abstract

Four crustal xenoliths from Scotland have been dissected in order to evaluate the effects of magma-xenolith interaction on Rb-Sr and Sm-Nd isotopic systematics, and hence the use of the xenoliths for accurate determination of source compositions. Each xenolith was sampled from the edge to the center. The Nd isotopic compositions are virtually uniform across each xenolith; however, there are significant variations in Rb, Sr, and REE concentration, as well as Rb Sr and Sm Nd ratios and Sr isotopic composition. Most of this variation appears to be inherited from the protolith. However, one sample shows systematic variations in Rb and Sr concentration and Sr isotopic composition with distance from the xenolith edges due to interaction with the host magma. This sample displays petrographic evidence for grain boundary melt infiltration. These data are consistent with theoretical considerations of transport times and diffusivities that place limits on the amount of modification that is possible via diffusion alone. The calculated effects of transport of even large amounts of crustal xenoliths on the compositions of xenoliths and host magmas will be small, provided transport times are short. Overall, the isotopic variations within the xenolith suite mirror the source rocks from which they were extracted and are inferred to reflect variations in the deep crust of Scotland. However, small samples, such as most crustal xenoliths, may be of little use in defining meaningful crustal residence ages because of modal variations in metamorphic minerals and isotopic re-equilibration. The one xenolith studied has extremely variable Sm Nd ratios due to the presence of garnet and yields a broad range of geologically meaningless Nd model ages, but yields an Sm-Nd isochron age of 396 ± 49 Ma. The Loch Roag xenolith has a large range of Rb Sr ratio, and the Sr isotopic data define an isochron age of 416±28 Ma. Both ages are taken to represent the timing of metamorphism and/or cooling from 600°C at the end of the Caledonian Orogeny in Scotland.