MICROANALYSIS OF OXYGEN ISOTOPES IN INSULATORS BY SECONDARY ION MASS-SPECTROMETRY

Microanalyses for oxygen isotopes of insulating samples in polished thin sections have been achieved using a commercially available secondary ion mass spectrometer equipped with an auxiliary high voltage electron flood gun and modified to allow the detection of secondary ions sputtered with initial kinetic energies up to 4500 eV. A Cs+ primary ion beam enhanced the negative secondary ion signal. Sample charging was controlled by use of the electron gun (impact energy almost-equal-to 3.4 keV). Secondary ions with > 300 eV initial energy sputtered from a range of silicate minerals do not show detectable variations in instrumental fractionation over a range of tens of electronvolts, providing immunity to variations in sample charging. In addition, the interference of (OH-)-O-16 on O-17- is effectively eliminated from the mass spectrum at these energies, allowing the measurement of all three oxygen isotopes at low mass resolution. Analytical precision is limited by counting statistics and the depth of the sputtered crater to almost-equal-to +/- 1 parts per thousand (1sigma) for 18/16 ratios. If 17/16 ratios are also desired, it is possible to select count times to achieve almost-equal-to +/- 2 parts per thousand precision for both.