THE EFFECTS OF MAGMA REPLENISHMENT PROCESSES ON 238U-230TH DISEQUILIBRIUM - EVIDENCE FROM ERUPTION HISTORIES OF POLYGENETIC VOLCANOES

Abstract

Short lived isotope data are increasingly used to investigate magma chamber residence times. Arc lavas have excellent potential as tracers of magma evolution because their magmas are often out of U-series isotope equilibrium, and some arc magmas have yielded surprisingly old pre-eruption U-series ages from mineral separates. A model is presented to examine the behaviour of U-Th isotopes for a range of conditions between closed system and steady-state magmatic systems. Magmas can be maintained out of secular equilibrium for long periods of time by periodic mixing events with new magma batches. The latter results in oscillations in (230Th/232Th) consistent with the relatively constant values observed in lavas of different ages from some centres. The effects of periodic reinjection of magma and mixing of old and young crystals on U-series mineral isochrons are also considered. Mixtures of crystals produced by either replenished or closed system processes can readily result in linear arrays. In many cases it may be valid to interpret these as minimum average residence times for the crystals, whereas the liquid residence times calculated from U-series data for periodically replenished systems are maximum values.