VARIABILITY OF TRACE-ELEMENTS IN BASALTIC SUITES

This paper discusses the distribution of trace element concentrations in cogenetic suites of lavas, based on the study of trace element histograms. For each histogram, the variability is calculated (V = s/m, where m is the mean concentration and s its standard deviation). The study of two basaltic series, each specific of a single igneous process (fractional crystallization and partial melting), shows that trace element variabilities are directly related to bulk partition coefficients. Considering different trace element behaviour models, quantitative relationships are established between the variability and the descriptive parameters for the processes (bulk partition coefficients, mean degree of fractionation and related standard deviation). An important feature is that the variability is independent of the initial concentrations in the source. This method is applied for comparison to several basalt suites on which case studies of magmatic processes have been conducted. It is also applied to study the behaviour of trace elements in different geodynamical contexts (hotspot, mid-ocean ridge and arc volcanism). Finally, a set of numerical values of bulk partition coefficients is proposed that accounts for N-MORB genesis.