The role of lithospheric mantle in continental flood volcanism: Thermal and geochemical constraints

Abstract

Continental flood basalts (CFB) are commonly said to form by direct melting of metasomatized lithospheric mantle, either during major lithospheric extension or when a mantle plume impinges on the base of the lithosphere. We tested these ideas in a thermomechanical model that combines lithospheric dynamics and mantle convection. Dry melting was assumed, and the proportions of melt from different source regions were monitored. In all cases, >96% of melt was found to come from asthenosphere or plume, with minimal amounts from continental lithosphere. During passive lithosphere extension the total amount of melt is small, and the proportion from the lithosphere is <3%. During plume interaction and concurrent extension, magmas formed during the first half of a magmatic episode contain virtually no lithospheric melt, and during the second half the proportion reaches only ∼2%. The proportion of lithosphere heated above its solidus is greater but never exceeds 3–4% of the total source. These results were used to test the concept that the distinctive chemical signature of CFB is a result of melting of lithospheric mantle. Our calculations show that if CFB are hybrid magmas and the proportion of lithospheric melt is less than 10–30%, the composition required for the lithospheric source is unrealistic (e.g., εNd < −40; Nb/U negative). Crustal contamination of asthenospherederived magmas can explain many of the characteristics of CFB if the primary magmas were produced by melting in plumes at sublithosphere depths (>100 km) and have high concentrations of both MgO (>20%) and incompatible trace elements (K2O ∼ 1%). The very low Nb and Ta concentrations in certain CFB cannot, however, be explained by this process. Ratios of Nb to elements such as La or U are lower in many flood basalts and picrites than in all likely source materials: they are almost as low as in most rocks from the continental crust, and they are far lower than in peridotites from the lithospheric or asthenospheric mantle. Another process must therefore fractionate Nb and Ta. We suggest that this takes place during the passage of magma through the lithospheric mantle, perhaps because of differences in the reaction rates of minerals in metasomatized peridotite. The probability that CFB are hybrid magmas containing material from aesthenospheric and lithospheric mantle and in many cases from continental crust, as well as the possibility that some elemental ratios change during magma‐lithosphere interaction, casts serious doubt on the reliability of such rocks as probes of the lithospheric mantle.