ELECTRON-MICROPROBE DETERMINATION OF MINOR AND TRACE TRANSITION-ELEMENTS IN SILICATE MINERALS - A METHOD AND ITS APPLICATION TO MINERAL ZONING IN THE PERIDOTITE NODULE PHN1611

We present a WDS analytical technique for the determination by electron microprobe of low concentrations of the transition elements Sc, Ti, V, Cr, Mn, Co and Ni in ferro-magnesian silicate minerals. The quality of measurements is improved by: 1. (1) Optimization of counting times, taking into account the anticipated analytical precision. 2. (2) Monitoring instrumental and sample stabilities, by a statistical analysis of peak and background intensity values sampled at intervals of 1 s. Simultaneous measurement of major elements is essential in evaluating bias related to instrumental drift. 3. (3) Precise determination of background, achieved by a least-squares fitting of the measured background to a calculated one. The latter is calculated by using a modified form of the Kramer's equation. This approach is also effective in correcting peak overlap interferences. The efficiency of this method was checked by performing core-to-rim analysis of garnet, clinopyroxene and orthopyroxene prophyroclasts in the peridotite nodule PHN 1611. This led to precise determination of trace-element chemical zoning in these minerals. © 1990.