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 Sr -87/Sr-86 across Single plagioclase feldspar crystals using laser ablation interfaced to a multicollector inductively coupled plasma source mass spect rometer. The speed of analysis proves to be a particular advantage permitti ng representative populations of crystals to be characterized, or enabling reconnaissance analysis of rocks to test for mineral-scale isotopic heterog eneity. Our results from magmatic crystals in lavas from El Chichon volcano confirm trends previously obtained by the more time-consuming microdrillin g method. For the first time with confidence, the laser data allow us to as sociate abrupt changes in Sr-87/Sr-86 with distinct petrographic discontinu ities such as dissolution surfaces in the crystal. Because the isotopic var iations within crystals would be expected to equilibrate diffusively with t ime, the width over which abrupt changes in Sr-87/Sr-86 occur can also be u sed to constrain timescales at which the crystal has been held at magmatic temperatures. (C) 2001 Elsevier Science B.V. All rights reserved.