Geochemical variations in a pyroclastic succession on St. Kitts,west Indies

Major and trace element analyses are presented for a series of fifty successive pyroclast-fall units in the Mansion formation, St. Kitts. These deposits are thought to have been erupted from Mt. Misery Volcano between 3,000 and 10,000 years ago. Six groups are recognized within the succession on the basis of chemical composition or patterns of chemical variation. Large scale fluctuations are superimposed on the finer oscillations. Trends to increasingly basic compositions in successive beds are thought to reflect tapping of progressively lower levels in a differentiated magma column. Sudden changes to a more acid composition probably follow more prolonged periods of quiescence and differentiation. A basaltic andesite composition which is relatively rare for the lava flows and domes of St. Kitts proved to be the most common composition encountered in the Mansion pyroclastics. A relationship is noted between the chemical composition of a unit and some of its physical properties. Recognition of these compositional patterns may have an application in predicting the likely course of any future eruptions.