High precision delta O-17 isotope measurements of oxygen from silicates and other oxides: Method and applications

The use of infrared laser-assisted fluorination to release oxygen from mill igram quantities of silicates or other oxide mineral grains is a well-estab lished technique. However, relatively few studies have reported the optimis ation of this procedure for oxygen-17 isotope measurements. We describe her e details of an analytical system using infrared (10 mu m) laser-assisted f luorination, in conjunction with a dual inlet mass spectrometer of high res olving power (similar to 250) to provide delta(17)O and delta(18)O oxygen i sotope measurements from 0.5-2 mg of silicates or other oxide mineral grain s. Respective precisions (1 sigma) of typically 0.08 and 0.04 parts per tho usand are obtained for the complete analytical procedure. Departures from t he mass-dependent oxygen isotope fractionation line are quantified by Delta (17)O; our precision (1 sigma) of such measurements on individual samples i s shown to be +/-0.024 parts per thousand. In turn, this permits the offset between parallel, mass-dependent fractionation lines to be characterised t o substantially greater precision than has been possible hitherto, Applicat ion of this system to investigate the delta(17)O versus delta(18)O relation ship for numerous terrestrial whole-rock and mineral samples, of diverse ge ological origins and age, indicates that the complete data set may be descr ibed by a single, mass-dependent fractionation line of slope 0.5244+/-0.000 38 (standard error). Copyright (C) 1999 John Wiley & Sons, Ltd.