(LA,Q)-ICPMS trace-element analyses of Durango and McClure Mountain apatite and implications for making natural LA-ICPMS mineral standards

Abstract

Apatite, the most abundant phosphate mineral in the Earth's crust and uppermost mantle, is able to accept a wide variety of trace elements into its crystal structure. Many of these trace element substitutions are below the detection limit of Electron Microprobe Analysis, but can be determined by laser-ablation inductively coupled plasma mass spectrometry (LA-ICPMS). LA-ICPMS elemental abundance determinations typically employ sample-standard bracketing using either standard glasses or an appropriate matrix-matched reference material. In this study we have undertaken laser ablation (>3000 analyses) and low-blank solution Q(quadrupole)-ICPMS trace-element analyses on crushed 150–300μm aliquots of Durango and McClure Mountain apatite to assess the accuracy of apatite elemental abundance determinations when using NIST 612 standard glass as the primary LA-ICPMS trace element standard. An accuracy (relative to the solution data) and precision of <5% can be obtained for most trace elements (Y, the REE, Sr, Mn, V, Th and U) in LA-ICPMS analyses of crushed Durango separates; the McClure Mountain data are similarly accurate for most trace elements but yield larger intra-crystal variability. Durango raster and image mapping experiments demonstrate some Durango crystals are more homogenous than others; the raster experiments also show that Durango typically exhibits less zoning parallel to the C-axis compared to perpendicular to the C-axis. A protocol for developing a homogenous Durango apatite trace-element reference material is suggested, and involves slicing the interior portions of several Durango crystals parallel to the C-axis, undertaking rapid LA-ICPMS raster experiments to characterize trace-element zoning, crushing the most homogenous crystal to 150–300μm and determining its trace-element contents by low-blank solution ICPMS. This generic approach can easily be modified and applied to characterize other natural LA-ICPMS mineral standards.