Assessment of Natural Reference Materials for U-Pb Geochronology of Grossular-Andradite Garnet

Abstract

Garnet U-Pb dating by laser ablation-inductively coupled plasma-mass spectrometry requires the development of matrix-matched reference materials of variable chemistry and U mass fraction for accurate analysis. Additional calibration of existing primary reference materials is also justified based on the relatively poor calibration of some of the widely available primary reference materials that are currently utilised by the geoscience community. We present a micro sampling workflow combined with a refined ID-TIMS methodology for the generation of high precision (~ 0.1%) U-Pb dates from domains within garnet single crystals. Using this workflow, we calibrated two new natural andradite reference materials, the Jumbo andradite (And₉₉; 110.34 ± 0.03 (0.04) [0.13] Ma, n = 7, MSWD = 1.21) and the Tiptop andradite (And₈₇; 209.57 ± 0.11 (0.13) [0.26] Ma, n = 6, MSWD = 1.39). We also present additional calibration of the widely utilised Willsboro-Lewis andradite primary reference material (And₉₀; 1024.7 ± 9.5 (9.6) [9.6] Ma (2s; overdispersed), n = 6). Wafers of the Jumbo and Tiptop andradite reference materials are available from the authors upon request.