Charge neutralization and oxygen isotopic analysis of insulators with the ion microprobe

The Cameca IMS 3f ion microprobe at Arizona State University has been modified to allow the analysis of ¹⁸O/¹⁶O ratios on 10- to 20-micrometer spots in insulating minerals in thin section. A primary beam of K⁺ or Cs⁺ is used, and charge is neutralized by an electron flood gun operating at -7.5 kilovolts. Negative secondary ions are accelerated to -4.5 kiloelectronvolts into the mass spectrometer, where ¹⁸O^- is detected on the electron multiplier by pulse counting, and ¹⁶O^- is detected on the electron multiplier or directly measured by an electrometer. In either configuration, internal precisions of 1.2 to 1.7 per mil (1 standard error of the mean) in ¹⁸O/¹⁶O ratios can be achieved. Preliminary analyses, showing quartz to be 10 to 15 per mil lighter than hydrothermally altered plagioclase, suggest that matrix effects are important. -Authors