LA-ICP-MS U-Pb ANALYSES ON TITANITE REFERENCE MATERIALS AT THE PARÁ-ISO LABORATORY (UFPA)

Titanite (CaTiSiO5) is a common accessory mineral in calcic-alkaline igneous, low- to high-grade metamorphic, hydrothermal and ore deposit-related rocks. It can accommodate variable amounts of REEs, U and Th, and typically incorporates significant content of initial/common (non-radiogenic) Pb.  The closure temperature for Pb diffusion in natural titanites ranges between 650-775 ºC, with inherited cores indicating temperatures >825 ºC. In addition, titanite demonstrates to be more susceptible to medium-high grade metamorphic and deformational processes than other petrochronometers (i.e. zircon and monazite). Thus, it is a widely used geochronometer for U-Pb dating of metamorphic, hydrothermal and igneous events. The experimental protocol for U-Pb dating of titanite using LA-Q-ICP-MS (laser ablation quadrupole inductively coupled plasma mass spectrometry) was implemented at the Pará-Iso Laboratory, Geoscience Institute, UFPA, based on analyses conducted on four reference titanites (Tory Hill, Khan, Mud Tank, and CHBK). The internal features of the most suitable crystals for dating (those darker and without inclusions/fractures) were assessed through backscattered electron images, to accurately position the spots in the best domains. The processing and reduction of raw analytical data were conducted using an Excel macro adapted to the specificities of Q-ICP-MS and the titanite U-Pb system. This is particularly crucial given the susceptibility of titanite to incorporate common Pb, which requires a careful evaluation and appropriate correction of this Pb content. The 207Pb-based correction is widely used in titanite U-Pb dating. This involves a linear regression on the common, uncorrected Pb data plotted on a Tera-Wasserburg diagram (Semitotal-Pb/U isochron), representing a mixture of the purely radiogenic component (238U/206Pb intercept age) and the initial Pb component (initial 207Pb/206Pb ratio). In this work, we employed both the 204Pb- and 207Pb-approaches, in order to compare the 204Pb-corrected 206Pb*/238U and 207Pb-corrected 238U/206Pb ages. With the exception of Tory Hill titanite, all other reference materials yielded identical 204Pb- and 207Pb-corrected ages. To validate the LA-Q-ICP-MS analytical protocol, the titanites Khan and CKHB were also analyzed using SHRIMP IIe at the University of São Paulo (USP). The Khan titanite proved more suitable as a primary reference material due to its higher analytical signal and lower common Pb content compared to other reference materials. The LA-Q-ICP-MS analyses on Tory Hill titanite yielded a mean 206Pb*/238U age of 1057.2 ± 2.5 Ma (n=79; MSWD=0.74) and a mean 207Pb-corrected 238U/206Pb age of 1043.8 ± 3.3 Ma (n=78; MSWD=0.60), both in agreement, respectively, with the mean 206Pb*/238U age of 1059.7 ± 1.2 Ma by ID-TIMS and 238U/206Pb lower intercept age of 1039 ± 23 Ma from the literature. The Khan titanite yielded a mean 206Pb*/238U age of 518.0 ± 4.9 Ma (n=26; MSDW=0.45), in accordance with the SHRIMP concordant age of 519.9 ± 1.8 Ma (n=18; MSWD=0.65) and the age range of 515-522 Ma (ID-TIMS) from the literature for the Khan titanite. The Mud Tank and CKHB titanites provided 206Pb*/238U ages of 318.4 ± 2.1 Ma (n=44; MSWD=0.30) and 93.9 ± 2.9 Ma (n=40; MSWD=0.05), respectively, identical to the literature ID-TIMS age of 319.20 ± 0.36 Ma for Mud Tank and the SHRIMP age of 93.8 ± 1.5 Ma (n=18; MSWD=0.10), along with the literature ID-TIMS age of 94.29 ± 0.15 Ma for CKHB titanite. The SHRIMP ages demonstrated good interlaboratory reproducibility, confirming the reliability of the implemented analytical protocol. Additionally, the results obtained by LA-Q-ICP-MS showed good accuracy and precision, with errors on ages at 2σ being less than 2%