Matrix effects and improved calibration procedures for SIMS titanite U-Pb dating

Titanite is a U-bearing accessory mineral that can serve as a geochronometer. Compared with zircon, titanite is more likely to contain multi-stage fluid-induced growth. Thus, individual titanite grains may yield complex U-Pb ages that reveal the entire geological history of the rock. Secondary ion mass spectrometry (SIMS), which has high sensitivity and spatial resolution, is a good choice for U-Pb titanite analyses. A titanite U-Pb standard named YQ82 was developed with Pb-206/U-238 age of 1837.6 +/- 1.0 Ma (2 standard error (SE), n = 4) and Pb-207/Pb-206 age of 1845. 0 +/- 1.1 Ma (2SE, n = 4), as determined herein using isotope dilution isotope ratio mass spectrometry (ID-IRMS). However, U-Pb dating of YQ82 via SIMS yielded a bias of up to 14% compared to the ID-IRMS value, indicating a matrix effect. To correct for this matrix effect, six U-Pb titanite dating reference materials (RMs) were analyzed. Based on the correlations between the age biases and the Fe contents of the six RMs, a calibration protocol was established using the index of the Fe content (FeO+/IP, intensity of primary beam) and a power-law relationship between the calibrated Pb+/U+ and UO+/U+. Using this calibration protocol, the U-Pb ages decreased the age bias from approximately +/- 14% to +/- 1.4%. To test the protocol, a titanite sample (BMB108) with a low Fe content (Fe2O3, 0.4 wt%) was calibrated using MKED1 (Fe2O3, 1.5 wt%) as the RM. Without the calibration, the apparent SIMS Pb-206/U-238 age was 10% younger than the thermal ionization mass spectrometry (TIMS) Pb-206/U-238 age (1890 +/- 4 Ma). Using the calibration method, the apparent SIMS U-Pb age was 1898 +/- 14 Ma (2SE, n = 19), which is consistent with the TIMS age. This study shows that a considerable matrix effect related to Fe content is present in the SIMS titanite U-Pb dating results. The calibration protocol developed herein improved the SIMS titanite U-Pb dating accuracy, yielding a more accurate thermal history of the samples.