Helium isotope signature of lithospheric mantle xenoliths from the Permo-Carboniferous magmatic province in Scotland — no evidence for a lower-mantle plume

Abstract

Abstract Noble gas studies of well-characterized spinel-peridotite-facies lithospheric mantle xenoliths and garnet megacrysts from Scottish Permo-Carboniferous dykes, sills and vents demonstrate that the mantle beneath Scotland during the late Palaeozoic was more radiogenic than the source of mid-ocean ridge basalts (MORB). The samples were collected from the Northern Highland Terrane and the Midland Valley Terrane, which vary from Archaean-Proterozoic to Proterozoic-Palaeozoic in age. Helium isotope ratios of between 3 R a and 6 R a ( R a = atmospheric ratio) indicate that there has been time-integrated U-Th enrichment of the subcontinental mantle. This enriched mantle was preferentially melted following the transition from early Palaeozoic compression to late Palaeozoic extensional tectonics. Helium isotope ratios provide no clear evidence for the presence of undegassed plume-type mantle beneath this part of Scotland during the Permo-Carboniferous. The measured helium ratios do not discount the presence of a low-helium plume similar to those of the European Cenozoic volcanic province. A passive origin, however, is preferred for the Permo-Carboniferous magmatism due to the protracted activity, relatively small-extruded volumes of mafic magma and the low-helium isotope ratios measured.