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

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 3Ra and 6 Ra (Ra = 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 lowhelium isotope ratios measured.

Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland

“…For this reason, our xenolith localities have focused on sites north of the GGF. Xenolith suites from the Northern Highland terrane were entrained by Permo-Carboniferous intraplate silica-poor mafic dykes (basanite, melanephelinite and monchiquites), formed by tectonic decompression during continental extension (Kirstein et al, 2006). These dykes have ocean island basalt (OIB)-like mantle source compositions (Upton et al, 2004;Kirstein et al, 2006).…”

“…Xenolith suites from the Northern Highland terrane were entrained by Permo-Carboniferous intraplate silica-poor mafic dykes (basanite, melanephelinite and monchiquites), formed by tectonic decompression during continental extension (Kirstein et al, 2006). These dykes have ocean island basalt (OIB)-like mantle source compositions (Upton et al, 2004;Kirstein et al, 2006). While debate continues about the involvement of a mantle plume during Permo-Carboniferous rifting, He isotope data from the Scottish mantle lithosphere (including Streap Com'laidh xenoliths) do not support the hypothesis of involving a mantle plume (Kirstein et al, 2004) and instead suggests that magma was generated as a complex response to tectonic collapse of the Variscan Orogen (Kirstein et al, 2004 and references therein).…”

“…These dykes have ocean island basalt (OIB)-like mantle source compositions (Upton et al, 2004;Kirstein et al, 2006). While debate continues about the involvement of a mantle plume during Permo-Carboniferous rifting, He isotope data from the Scottish mantle lithosphere (including Streap Com'laidh xenoliths) do not support the hypothesis of involving a mantle plume (Kirstein et al, 2004) and instead suggests that magma was generated as a complex response to tectonic collapse of the Variscan Orogen (Kirstein et al, 2004 and references therein). An overview of this period of magmatic activity in Scotland can be found in Upton et al (2004).…”

The tectonomagmatic evolution of Scotland

“…Rapid ascent of the magmas through the crust is indicated by the preservation of mantle xenoliths and xenocrysts and by high Al contents and Al iv /Al vi ratios in clinopyroxene phenocrysts. Upton et al (2004) evaluated the evidence that one or more mantle plumes promoted the Carboniferous-Permian magmatism in Scotland and found it unconvincing, a conclusion also reached by Kirstein et al (2004) on the basis of the helium isotope signatures of lithospheric mantle xenoliths. They considered it more likely that all the alkaline magmatism was due to adiabatic decompression melting of the mantle during various transtensional regimes (Smedley 1986;Read et al 2002).…”

Carboniferous and Permian magmatism in Scotland

“…Helium isotope data (Kirstein et al 2004) provide no reason to invoke plume involvement. It is more likely that the Dinantian, together with all of the Silesian and Permian, alkali basaltic magmatism was solely due to adiabatic decompression melting of the underlying mantle during successive transtensional regimes (Smedley 1986a, b;Read et al 2002).…”