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 hist ograms. For each histogram, the variability is calculated (V = s/m, wh ere m is the mean concentration and s its standard deviation). The stu dy of two basaltic series, each specific of a single igneous process ( fractional crystallization and partial melting), shows that trace elem ent variabilities are directly related to bulk partition coefficients. Considering different trace element behaviour models, quantitative re lationships are established between the variability and the descriptiv e parameters for the processes (bulk partition coefficients, mean degr ee of fractionation and related standard deviation). An important feat ure is that the variability is independent of the initial concentratio ns in the source. This method is applied for comparison to several bas alt suites on which case studies of magmatic processes have been condu cted. It is also applied to study the behaviour of trace elements in d ifferent geodynamical contexts (hotspot, mid-ocean ridge and arc volca nism). Finally, a set of numerical values of bulk partition coefficien ts is proposed that accounts for N-MORB genesis.