U-series disequilibria in young (AD 1944) Vesuvius rocks: Preliminary implications for magma residence times and volatile addition

The results of a preliminary U-series study of the timescale of magmatic processes at Vesuvius are presented. Phonotephrites of the 1944 eruption of Vesuvius show 0-15% (230T)h-(238)U and 350-1150% (226)Ra-(230)Th disequilibria. Apparent U-Th internal isockrons for a lava and a ccmulate ilodule suggest crystal residence times of 12 and 39 ka, respectively. A tephra sample shows isotopic heterogeneity, possibly related to mixing of younger crystal-laden melt and older crystals giving apparent U-Tn ages of 0.4 and 18 ka, respectively. Mineral (226)Ra-(230)Th disequilibria on Ba-normalised internal isochron diagrams suggest Ra-Th ages of 1730-3300 years for the same rocks and phenocrysts. Minor (226)Ra/(230)Th heterogeneity be tween minerals and groundmass (or whole rock) is evidence of open-system Ra-Th behaviour. This heterogeneity suggests that there have been recent, post-crystallisation changes in melt composition that affected (226)Ra more than( 230)Th. Continued crystallisation in a Ra-enriched magma has subsequently resulted in Ra-Th disequilibria probably as a result of addition via a fluid-rich phase. Magma differentiation, residence time, transport, and pervasive gas addition at Vesuvius apparently occur over geologically short periods. (C) 1998 Elsevier Science B.V. All rights reserved.