Element fractionation in the petrogenesis of ocean island basalts

• Main Author: Elliott, Timothy Richard
• Published: Open University 1991
• Subjects: 552
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.541703

Analytical techniques have been developed to measure the very small [230ThP32Th] atomic ratios (3-8x10-6) typical of basaltic lavas using a single focussing mass spectrometer. Compared to conventional alpha-counting methods, the technique presented typically gives a 2-5 fold improvement in reproducibility of 230 Th/232Th ratios, and furthermore uses -5x less sample, has a simpler chemical prepartion and a data aquisition time -100x shorter. The above technique was used in investigating a suite of historic (< 1000a) basanites from La Palma, Canaries. The lavas have undergone significant post melt segregration processes, involving residence in deep (>4kbar) magma chambers, that are inferred to persist for at least 30ka. The basanites show geochemical variations related to differences in the degree of partial melting, that are estimated to be between 0.3-2%. Rare earth element (REE) modelling suggests clinopyroxene, rather than garnet control during melting, contrary to expectations from recent geophysical models (McKenzie & O'Nions, in press). The La Palma mantle source is inferred to be LREE enriched, K& Rb depleted, and shows many incompatible element ratios characteristic of 'himu' basalts (Weaver 1991). In contrast, the 206pb/204pb ratios of the La Palma basanites (19.6±0.05) are lower than the isotopically defined'high p' basalts (Zindler & Hart 1986) which suggests that the La Palma source is geochemically similar, but younger than a typical 'high t' source. A suite of young (< 10,000a) high MgO tholeiites from Iceland show order of magnitude variations in highly incompatible element concentrations that correlate with FeO contents. Variations within these lavas is primarily attributable to sampling of instantaneous melts from different depths within a dynamic melting column. However, additional correlations of radiogenic isotope ratios with major and trace element contents highlights mixing of various plume melts with some 20% mid-ocean ridge basalt, derived from melting of upper mantle material entrained during plume upwelling.