MAGMA RECHARGE, CONTAMINATION AND RESIDENCE TIMES REVEALED BY IN SITU LASER ABLATION ISOTOPIC ANALYSIS OF FELDSPAR IN VOLCANIC ROCKS

Abstract

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 Chichon 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.