ANOMALOUS COMPOSITION OF REE ORTHOPHOSPHATE STANDARDS: WHY ARE THEY ANOMALOUS, AND DO THEY HAVE ANY ROLE IN TRACE ELEMENT ANALYSES?

In an effort to obtain accurate analysis by electron probe microanalysis, it is imperative that high-quality analyses of standards are obtained. Any uncertainty in the composition of the standard can be propagated through the analysis of unknowns. Fourteen REE(+Sc,Y)-phosphate standards have been examined for their Pb contents. The orthophosphate standards, grown at Oak Ridge National Laboratory and distributed by the Smithsonian Institution's Department of Mineral Sciences, are widely used as electron microprobe standards (Jarosewich and Boatner, 1991, Geost. News. 15, 397). We have found that some of these standards contain significant quantities of Pb. This is presumably due to the flux, PbHPO4, used to dissolve the oxide starting materials prior to the crystal synthesis, and to promote large crystal size. The grains selected for this analysis were random, and the locations of the analyzed surface to original crystal faces could not be determined in most cases. The LREE/MREE-PO4 standards contain significant and variable amounts of Pb (in order of decreasing average REE [average PbO(wt%); range of PbO]): Ce (1.90: 1.77-2.04); Pr (0.92: 0.85-1.00); La (0.90: 0.26-1.54); Sm (0.86: 0.59-1.13); Nd (0.86: 0.50-1.22); Eu (0.64: 0.31-0.98); and Gd (0.39: 0.07-0.71). The remaining REE-PO4, and Sc-/Y-PO4 contain Pb in concentrations near or below detection. What is even more striking is that the Pb content varies considerably not only within each grain but even more so from grain to grain. Pb is only present in significant amounts in the monoclinic, monazite structure (LaPO4-GdPO4), orthophosphates, and is absent, or nearly so, in the tetragonal, xenotime structure (TbPO4-LuPO4). This is consistent with a distinct structural change from monoclinic to tetragonal in the high-temperature forms of these orthophosphates that occurs between GdPO4 and TbPO4. These results suggest that accurate determination of Pb concentrations of REE-PO4 standards is imperative if accurate determination of REE concentrations in LREE-rich phases is necessary (e.g. monazite, apatite). Furthermore, the Pb in the orthophosphate standards has only a minimal effect on the measured trace element concentrations in minerals (e.g. U, Th and Pb in monazite dating) when compared to using stoichiometric REE-phosphate standards.