238U-230Th radioactive disequilibria in historic lavas from the Canary Islands and genetic implications

Abstract

Abstract Lava samples from 10 historic or recent eruptions on La Palma, Tenerife and Lanzarote (Canary Islands) have been analyzed for U and Th contents, and Th, Sr and O isotopes. The sample suite ranges from mantle-derived tholeiites and basanites to evolved phonolites. A phonolite from La Palma has identical Th isotope ratio as basanite from the same 1949 eruption, and was most probably formed by crystal fractionation of a basanitic magma. In contrast, the Th isotope ratios in the Teide phonolites (Tenerife) slightly decrease with differentiation. These latter were produced either by crystal fractionation in magma pockets isolated for about 35,000 y, or by an assimilation-fractional crystallization (AFC) process involving old syenites as contaminants. The Sr and O isotope ratios show very small variations from 0.7030 to 0.7032 and 5.2 to 5.9‰, respectively, in the analyzed samples (except in the most evolved phonolite from Tenerife), but Th isotope ratios are significantly different between the three islands. The 230 Th 232 Th ratios vary from 1.1 in La Palma, to 0.94-1.01 in Lanzarote and to 0.9 or lower in Tenerife. A possible interpretation of this variation is mixing of melts derived from an undepleted mantle source, similar in composition to the Tristan da Cunha mantle plume, with melts of the suboceanic lithosphere. The proportion of the plume-derived magmas would be higher below the center of the archipelago. A variably metasomatized mantle source beneath each island is an alternative explanation. The few available 3 He 4 He ratios are correlated with 230 Th 232 Th ratios and are compatible with both these models. They indicate the existence of a mantle source (or a metasomatic component) with a high Th/U and (Th + U) He ratios below the Canary Islands.