Plume–lithosphere interaction beneath Mt. Cameroon volcano, West Africa: Constraints from 238U– 230Th– 226Ra and Sr–Nd–Pb isotope systematics

Abstract

Precise measurements of 238U– 230Th– 226Ra disequilibria in lavas erupted within the last 100 yr on Mt. Cameroon are presented, together with major and trace elements, and Sr–Nd–Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess 238U– 230Th– 226Ra disequilibria with 230Th (18–24%) and 226Ra (9–21%) excesses, and there exists a positive correlation in a ( 226Ra/ 230Th)–( 230Th/ 238U) diagram. The extent of 238U– 230Th– 226Ra disequilibria is markedly different in lavas of individual eruption ages, although the ( 230Th/ 232Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the ( 230Th/ 238U)–( 206Pb/ 204Pb) and ( 226Ra/ 230Th)–( 206Pb/ 204Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high D U and D U/ D Th, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios ( 206Pb/ 204Pb > 20.47) due to late Mesozoic metasomatism.