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U-series disequilibria in young (A.D.1944) Vesuvius rocks : preliminary implications for magma residence times and volatile addition.

Research output: Contribution to Journal/MagazineJournal articlepeer-review

  • Stuart Black
  • Ray Macdonald
  • Benedetto Devivo
  • Christopher R. J. Kilburn
  • Guiseppe Rolandi
<mark>Journal publication date</mark>06/1998
<mark>Journal</mark>Journal of Volcanology and Geothermal Research
Issue number1-4
Number of pages15
Pages (from-to)97-111
Publication StatusPublished
<mark>Original language</mark>English


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% 230Th–238U and 350–1150% 226Ra–230Th disequilibria. Apparent U–Th internal isochrons for a lava and a cumulate nodule 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–Th ages of 0.4 and 18 ka, respectively. Mineral 226Ra–230Th disequilibria on Ba-normalised internal isochron diagrams suggest Ra–Th ages of 1730–3300 years for the same rocks and phenocrysts. Minor 226Ra/230Th heterogeneity between 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 226Ra more than 230Th. 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.