A COMPARISON OF IN SITU U-PB DATING TECHNIQUES FOR CA. 1-GA TITANITE

As in situ U-Pb dating of titanite becomes more widespread, it is worthwhile to compare the two available in situ analytical techniques – secondary ion mass spectrometry (SIMS) and laser-ablation inductively coupled plasma mass spectrometry (LAICPMS). We measured U-Pb dates by SIMS and LAICPMS in ca. 1-Ga titanite grains from the Adirondack Mountains (New York). SIMS analyses (~20 min) sputtered pits 30-40 µm diameter x 3-4 µm deep. LAICPMS analyses (~30 sec) ablated pits 35-40 µm diameter x 15 µm deep. SIMS U-Pb ratios (0.7-4.7% 1σ) are more precise than LAICPMS U-Pb ratios (1.0-16.3% 1σ) for this suite of low-Pb, moderate-U titanite grains. Specifically, ²³⁸U/²⁰⁶Pb errors (~2.3% 1σ) are greater than ²⁰⁷Pb/²⁰⁶Pb errors (~1.0% 1σ) for SIMS measurements, suggesting that the U/Pb calibration dominates SIMS analytical uncertainties. In contrast, ²⁰⁷Pb/²⁰⁶Pb errors (~5.1% 1σ) are greater than ²³⁸U/²⁰⁶Pb (~2.6% 1σ) errors for LAICPMS measurements, indicating that Pb sensitivity dominates LAICPMS analytical uncertainties. Without common-Pb corrections, both SIMS and LAICPMS data define linear trends on Tera-Wasserburg diagrams. However, model regressions of the aggregate datasets have large age uncertainties, large MSWDs, and/or poor probabilities-of-fit, suggesting scatter in the data beyond analytical uncertainty.

Four generations (Types) of Adirondack titanite, previously distinguished on the basis of microstructure, composition, and oxygen isotopes (Bonamici et al. 2014; 2015), yield different U-Pb data patterns. The spread of ²³⁸U/²⁰⁶Pb ratios in Type 2 analyses is consistent with diffusive Pb loss previously documented for these grains. The scatter in Type 3 and Type 4 U-Pb ratios is consistent with variable recrystallization inferred from chemical zoning. Greater uncertainties on LAICPMS analyses result in overlap of Type 2, 3, and 4 data, but the larger LAICPMS dataset (n = 280) provides fuller characterization of variability within the Types. On the other hand, the more precise, but less numerous, SIMS data (n = 100) clearly separate the different Types. These results demonstrate that the choice of in situ analytical technique affects the interpretation of intracrystalline processes (e.g., Pb diffusion, recrystallization), and, thus, the geologic interpretation of U-Pb titanite dates.