Christiane Wagner

Trace element analysis with the electron microprobe: New data and perspectives

Abstract This work presents a procedure developed for trace element analysis using the electron microprobe (EMP). The method is demonstrated by analysis of seven glasses prepared from reference rock powders, with compositions of granite, granodiorite, andesite, diabase, and basalt. The melting process was adapted to prevent the loss of volatile elements and to obtain homogeneous samples. A routine procedure for analysis of major and minor (above 1000 ppm) elements yielded results in fairly good agreement with published values. The methods presented in this study produced detection limits as low as: (1) 6-8 ppm when the Kα peaks of transition metals of the first row (Cr and Ni) were used; (2) 23 ppm with soft Lα peaks (Y, Zr, and Sr); (3) 15 ppm with high-energy Lα peaks (rare earth elements); and (4) 35 ppm with the Ma peaks of heavy elements (Pb and Th). These limits were achieved after total counting times (peak + two background measurements) of about 15 min with a beam of 35 kV and 500 nA. Precision (± 2σ) on the weight percent concentrations was below 50% for concentrations above 13 ppm for group 1 elements, 35 ppm for group 2, 25 ppm for group 3, and 55 ppm for group 4. In the present work, a commercial software package was adapted for quantitative trace element analysis. One modification addresses beam-sensitive materials for which the long counting times required for measurements need to be divided into subsets (10-20 s count), each acquired from different sites of the sample surface, to minimize damage. This analytical mode is referred to as ‘‘multi-site’’ mode.

Sector-zoned phlogopites in igneous rocks

Sector-zoned micas are described in three occurrences of igneous rocks. Basal sectors (001) and lateral sectors (010) are well defined. Analyses of probably isochronous growth points indicate that there are consistent chemical composition differences between the sectors, the (010) sector being richer in Si and poorer in Ti, Fe, Al and Ba relative to the (001) sector. Within each sector type Fe/ (Fe + Mg) increases from core to rim. These differences vary in amplitude from one occurrence to the other, but are systematic for micas with extremely different tetrahedral cations. Possible factors influencing this type of crystallization are briefly reviewed: growth rate, geometry of protosites and bulk composition of the liquid.