Compositional inhomogeneities in a single Icelandic tholeiite flow

Abstract

New trace element analyses are reported for 25 samples of basalt taken from a vertical traverse in an 11 m thick single flow of Icelandic tholeiite. Compositional variations among the samples substantially exceed those expected from analytical uncertainties and are random with respect to height in the flow. These variations in an undifferentiated single flow suggest a short-range segregation model in which the proportions of phenocrysts. groundmass minerals and residual liquid vary randomly among different samples of the flow. A least-squares mixing model is used to determine whether the compositional variations reflect different proportions of crystallizing phases and residual liquid. Most elements (alkalis and alkaline earths excepted) are fit to within their analytical uncertainties, supporting the short-range segregation model. A Monte-Carlo calculation is used to model the phase modes and compositions of various samples of a hypothetical basalt. Most compositional and interelement variations for the Icelandic basalt resemble those of the hypothetical basalt. The calculations show that short-range segregation produces inhomogeneity as large as interflow compositional differences and results in incoherence among elements with different geochemical behaviors while preserving coherence among elements of similar behavior.