Experimental conditions for major, minor, and trace element analysis of olivine and low-Ca pyroxene are described and analytical accuracy tested using suites of natural samples spanning a wide range of Mg/Fe.

Special attention was given to sample cleanliness to avoid contamination, instrumental vacuum to eliminate hydrides in the secondary ion spectrum, and sample preparation to give good precision in measured intensities. Especially bothersome were molecular interferences in the secondary ion spectrum which were separated from analytical peaks using mass resolution (M/ΔM) over 3000. Careful analysis of the secondary ion spectrum allowed choice of either high or low mass resolution depending on the presence of interferences. Lithium (0.005), F (1?), Na (0.01), Mg, Al (0.1), Si, P (5), K (0.05), Mn (0.1), Fe, and Co (1) were analyzed at low resolution with detection limits (ppm) in parentheses. Elements requiring high resolution include Ca (1), Sc (1), Ti (1), V (1), Cr (1) and Ni (5).

The secondary-ion intensities for Mg and Si do not correlate linearly with composition whereas Fe is nearly linear. The simple relation of the Mg/(Mg + Fe) ratio to the measured secondary ion ratio Mg+/(Mg+ + Fe+) enabled major element determination to within ±1 mol% of forsterite or enstatite content. The yield of Mg and Fe secondary ions is a complex function of composition, and Mg+/Si+ and Fe+/Si+ are not simply related to atomic ratios in the target.

To test accuracy of minor element determination as a function of major element variation, secondary-ion intensities were compared with compositions based on electron probe measurements. Some elements (Al, Cr, Ti, Mn) give a linear relationship with no obvious matrix effect, but Ni, and possibly Ca and Co, definitely depend on the matrix. The linear relationship allows composition determination to within ±10% of the amount present by reference to known standards. A major effort is required to calibrate Li, K, Na, F, V and Sc for which few reliable standards exist.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.