Magma cooling and differentiation - uranium-series isotopes

Timescales of magma cooling and differentiation can be quantified by measuring and modelling the composition of uranium-series (U-series) isotopes in crystals and whole rock samples. The U-series isotope composition of crystals yields a wide range of timescales: from a few tens of years to several hundred thousand years. Magma differentiation is inferred to be generally rapid (a few 1,000 years) at mid-ocean ridges and oceanic islands compared to island, continental arcs and intra-continental volcanic centres (1,000 to several 100,000 years). For large volume, highly silicic rocks (dacites, rhyolites), timescales of differentiation 100,000 years are generally inferred, reflecting the time required to heat up and assimilate the crust. U-series timescale information can be used to distinguish between different models for magma differentiation. For example, a model of frequent magma recharge combined with crustal assimilation seems to best explain U-series isotope data in many instances (and is probably the most realistic physical model). It is shown that, once the thermal regime that allows melting of the country rock is met (often requiring hundreds of thousands years), it takes only a few 1,000 years to produce highly silicic compositions.