Mineral-scale Sr isotope variation in plutonic rocks – a tool for unravelling the evolution of magma systems

Isotope ratios of elements such as Sr, Nd, Pb and Hf can be used as tracers of magmatic sources and processes. Analytical capabilities have evolved so that we can now analyse isotope ratios in situ, and can therefore use isotopic tracers within single minerals to track the changing magmatic environment in which a given mineral grew. This contribution shows that Sr isotope ratios in feldspars that make up plutonic rocks will typically preserve initial isotopic variations provided precise and accurate age corrections can be applied. Variations in initial isotope ratio can give a core-to-rim record of magmatic evolution and can be used to diagnose open system events such as contamination and magma recharge and mixing. New single grain Sr isotope data are presented from the Dais Intrusion, Antarctica, which reflect an open system origin for the crystals. The crystal cargo appears to be aggregated and assembled during transport and emplacement. This model, as opposed to a magma body crystallising post emplacement, may be more applicable to plutonic rocks in general, and is testable using the in situ isotopic determination methods described here

Magma recharge, contamination and residence times revealed by in situ laser ablation isotopic analysis of feldspar in volcanic rocks.

We illustrate a rapid and effective method for determining variations in 87Sr/86Sr across single plagioclase feldspar crystals using laser ablation interfaced to a multicollector inductively coupled plasma source mass spectrometer. The speed of analysis proves to be a particular advantage permitting representative populations of crystals to be characterized, or enabling reconnaissance analysis of rocks to test for mineral-scale isotopic heterogeneity. Our results from magmatic crystals in lavas from El Chichón volcano confirm trends previously obtained by the more time-consuming microdrilling method. For the first time with confidence, the laser data allow us to associate abrupt changes in 87Sr/86Sr with distinct petrographic discontinuities such as dissolution surfaces in the crystal. Because the isotopic variations within crystals would be expected to equilibrate diffusively with time, the width over which abrupt changes in 87Sr/86Sr occur can also be used to constrain timescales at which the crystal has been held at magmatic temperatures