Matrix dependency of baddeleyite U–Pb geochronology by femtosecond-LA-ICP-MS and comparison with nanosecond-LA-ICP-MS

Abstract

Baddeleyite is a key mineral in geochronology of mafic rocks as it crystallizes in silica-undersaturated systems that do not grow zircon. It has been shown that nanosecond (ns-)LA-ICP-MS U–Pb analysis requires matrix-matched calibration due to significantly stronger element downhole fractionation in baddeleyite compared to zircon. Using zircon as external standard for downhole fractionation correction produces reverse discordant results with low precision intercept ages (≥5%). In contrast it has been shown that femtosecond (fs)-LA-ICP-MS can produce accurate and precise data for a variety of difficult matrices that require matrix-matching with ns-LA-ICP-MS. Here we compare U–Pb data obtained by ns-LA-ICP-MS and fs-LA-ICP-MS. We conducted spot as well as line scan analyses with both systems applying Plešovice zircon, Duluth zircon and Duluth baddeleyite as reference materials, and the well-characterized Phalaborwa baddeleyite as unknown sample. If the cause for previously observed reverse discordance is only downhole elemental fractionation, then raster analyses should remedy this even with ns-LA-ICP-MS. Our results show that elemental fractionation occurs in both fs- and ns-LA-ICP-MS and needs to be corrected for by application of baddeleyite as reference material. Although raster analyses are not affected by downhole fractionation, discordant ages result nevertheless. The underlying elemental fractionation process might be caused by ablation of material previously ablated and deposited along the raster path. Deposition of such pre-ablated material can undergo fractionation during condensation of which the material is incorporated later. In summary, spot analysis of matrix-matched calibration is the preferred method to obtain concordant high-precision U–Pb ages by both ns- and fs-LA-ICP-MS.