The effects of magma replenishment processes on U-238-Th-230 disequilibrium

Short lived isotope data are increasingly used to investigate magma chamber residence times. Are lavas have excellent potential as tracers of magma ev olution because their magmas are often out of U-series isotope equilibrium, and some are magmas have yielded surprisingly;old pre-eruption U-series ag es 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-s tate 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 (Th-230/Th-232) consistent with the r elatively constant values observed in lavas of different ages from some cen tres. The effects of periodic reinjection of magma and mixing of old and yo ung crystals on U-series mineral isochrons are also considered. Mixtures of crystals produced by either replenished or dosed system processes can read ily result in linear arrays. In many cases it may be valid to interpret the se 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. Copyright (C) 1999 Elsevier Science Ltd.