A new appraisal of ilmenite U-Pb dating method by LA-SF-ICP-MS

Titanium (Ti) is a crucial metal with wide-ranging applications in various industries. Ilmenite minerals are the main source of Ti metal. As a valuable petrogenetic indicator and geochronometer, ilmenite mineral occurs commonly in igneous and metamorphic rocks and different types of deposits (e.g., Fe-Ti oxide, diamonds, and placer deposits). Ilmenite U-Pb geochronology started by isotope dilution-thermal ionization mass spectrometry thirty years ago and by in situ methods recently due to low U and Pb concentrations. A constrained calibration method has been established using rutile TB-1 due to the lack of reference material (RM). However, rutile (e.g., RMJG) cannot be a primary standard in all our analyses. Thus, a new primary standard and accurate calibration method for further applications is urgently needed. By laser ablation sector field inductively coupled plasma mass spectrometry (LA-SF-ICP-MS), optimizing ablation settings and adding N2 (similar to 3.0 mL min-1) have been employed to minimize the matrix effects between zircon 91500 and ilmenite. Thus, zircon 91500 can be utilized as a RM to measure the Pb/U ratio in ilmenite. Zircon 91500 has a similar U-Pb fractionation and average normalized Pb/U ratio with ilmenite BC269, instead of rutile RMJG, probably owing to the different ablation settings used by us compared to the two previous studies. More accurate ages with much smaller age offsets of <= 1.6% were obtained for the five ilmenite samples calibrated by zircon 91500 compared to those calibrated by Ti- and Fe-bearing minerals, including rutile RMJG, garnet PL-57, and wolframite YGX. These results demonstrate that the direct calibration method used here with 91500 as an external standard is effective for in situ U-Pb dating of ilmenite samples under a wide range of ablation settings, i.e., spot sizes varying from 44 to 120 mu m, repetition rate from 5 to 10 Hz, and laser fluences from 3 to 4 J cm-2. HG79, XL32333, and BC269 have high U and Pb contents and can be utilized as potential RMs for ilmenite minerals. This approach offers new insights into understanding the diagenetic and ore-forming processes related to ilmenite minerals. Zircon has a similar U-Pb fractionation and average normalized Pb/U ratio with ilmenite; our direct calibration method using zircon 91500 as an external standard is effective for in situ U-Pb dating of ilmenite under wide ablation settings.