Electron probe determination of trace elements in olivine: thermometry of depleted peridotites

A method for calculation of the maximum permissible (for thermometry) error of analyte determination by trace elements is presented based on the example of the De Hoog-Gall Al-in-olivine thermometer. Analytical conditions ensuring an acceptable analytical error have been established, and the possibility to reach them has been estimated. A technique of JEOL JXA-8100 analysis of olivines for trace elements has been elaborated: Accelerating voltage is 20 kV, probe current is 400 nA, and counting time per line and background measurement is 10 s. The number of measurements per analysis is 25. Metrological characteristics of determination of Na2O, Al2O3, CaO, Cr2O3, and MnO have been calculated. Their detection limit (with confidence probability of 0.842) does not exceed 9 ppm. For an error of 10 rel.%, the quantitation limit of these oxides is within 45-100 ppm. Compositional peculiarities of olivines from depleted megacrystalline peridotites of the Udachnaya pipe have been revealed. The content of Mn significantly decreases with Mg# increase. The contents of Na, Al, Ca, and Cr show a strong mutual positive correlation. It is shown that electron probe microanalysis can provide an error sufficient for evaluating temperature by trace elements. The results of temperature determination by the Al-in-olivine thermometer agree with the thermometry data by H. O'Neil and B. Wood and differ considerably from the lower values estimated by D. Canil and C. Ryan with Ni thermometers.