The lower lithosphere as a major source for continental flood basalts: a re-appraisal

Abstract

Abstract Continental flood basalt provinces (cfb) have isotopic ratios different from midocean ridge basalts, characteristics that may be due to (a) interaction of asthenospheric melts with old crust, (b) melting of enriched continental lower lithosphere, (c) upwelling of deep mantle plumes containing recycled components, (d) mixing of enriched and depleted mantle sources, or (e) combinations of these processes. It is apparent that several aspects of the chemistry of cfb are subcrustal in origin and therefore crustal contamination cannot be invoked as a generally applicable hypothesis. In addition wholesale extraction of cfb from the lower lithosphere is unlikely because the lithospheric mantle is believed to be a rather thin (< 150 km) reservoir of cold, anhydrous, granular peridotite. Dry melting of such peridotites would not produce a normal ‘basalt’ because they have already experienced a melting event. Metasomatism or enrichment processes can enhance the chemical budget of the lower lithosphere thus providing an adequate reservoir for small volume alkaline and potassic melts. However no conclusive evidence exists that such modification of the lower lithosphere is widespread enough to generate a laterally continuous, wet, enriched reservoir. It would appear that cfb require a dominant sub-lithospheric component (plume) to account for the large volumes of magma, in some cases produced over a short time period (e.g. Deccan). If so, the heterogeneity observed in mantle-derived magmas within flood basalt provinces reflects inhomogeneities within plumes rather than relatively shallow inhomogeneities in the lithosphere. The apparent concentration of depleted ( 87 Sr/ 86 Sr ≪ 0.7045) cfb in the northern hemisphere and enriched cfb ( 87 Sr/ 86 Sr ≫ 0.7045) in the southern hemisphere may be linked to the preponderance of enriched plumes in the Southern Oceans.