Trace Element Analysis of Olivine: High Precision Analytical Method for Jeol Jxa-8230 Electron Probe Microanalyser

Olivine compositions provide critical information on the composition and origin of primary mantle-derived melts and their sources. The minor and trace elements (Ni, Mn, Ca, Al, Cr, Co, Ti, Zn, P, Na), which are present at concentrations over 10ppm, are especially informative and can be determined by electron probe microanalysis (EPMA). Here we report a new analytical method to analyse these elements with unprecedented precision for an electron microprobe.The method was developed using the new JEOL JXA 8230 EPMA at ISTerre, UJF, Grenoble, France. The facility has a tungsten source gun and is equipped with five wavelength-dispersive spectrometers (WDS) and one silicon drift detector energy-dispersive spectrometer (SDD, EDS). It is placed in an environment with closely controlled temperature (+/−0.5°C) and humidity (+/−4%).The analytical conditions are as follows: acceleration voltage 25kV, 900nA beam current; WDS detection for trace elements (Ni, Mn, Ca, Al, Cr, Co, Ti, Zn, P, Na) and EDS detection for Si, Mg and Fe; total counting time 12min; ZAF correction. The analysed volume is a half-sphere with a diameter of approximately 7μm for most elements or a cylinder with a diameter of 7μm and depth of approximately 2μm for Al and Na. Instrumental drift during analytical sessions was monitored by repeated measurements of olivine standards. For trace elements, the method yields detection limits from 4 to 10ppm and similar precision of individual analyses (2 standard errors). The detectable amount of an element is down to 2∗10−15 g and the precision for Fo in olivine is 200–300ppm (2 standard errors).Comparison of data obtained using the EPMA with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for a large range of olivine compositions confirms the accuracy of the EPMA which is similar to the precision noted above.