In-situ LA-ICPMS U-Pb dating of Sulfates: Applicability of carbonate reference materials as matrix-matched standards
Abstract. Recent developments on analytical capabilities in the field of in situ laser ablation mass spectrometry (LA-ICPMS) have expanded the applications of U-Pb geochronometers in low-U minerals such as carbonates or garnets. Although the rapid evolution of the technique relies on well characterized matrix-matched reference materials, the use of non-matrix-matched standards has been evaluated given the unavailability of standards for some minerals. In this article, we explore the suitability of using carbonate as reference materials for in situ U-Pb dating of sulfates. We have used the astrochronologically dated gypsum and anhydrite samples deposited during the Messinian Salinity Crisis (5.97–5.33 Ma) and compared these dates with the U-Pb ages obtained by LA-ICPMS. Although the majority of the samples failed due to the elevated common-Pb content and low 238U / 204Pb ratios, five of the samples showed a higher dispersion on U / Pb ratios. The obtained dates in four of these samples are comparable with the expected ages while another gave an unexpected younger age, each of them with 6–11 % of uncertainty. The pit depth of the spots showed that the sulfates ablate faster than carbonates, but the offset due to the crater geometry mismatch or downhole fractionation is not noticeable. To sum up, the bias between the U-Pb and expected cyclostratigraphic ages, if any, is included in the uncertainty and thus, the results obtained here suggest that carbonate reference materials are reliable for in situ U-Pb dating of sulfates.
