EXPLORING THE PETROCHRONOLOGY OF ACCESSORY MINERALS THROUGH TANDEM QUADRUPOLE LA-ICPMS AND ISOTOPE DILUTION TIMS ON THE SAME CRYSTALS

Analytical innovations of the past decade have significantly promoted the application of the U-Pb chronometer, but with greater analytical scrutiny has come increased awareness of the geological complexity of the target minerals. It becomes critical to the application of radioisotope geochronology to establish the petrologic context of the ages extracted from individual crystals. To this end, we can now pursue petrochronology with a tandem in situ and isotope dilution analytical approach maximizing both spatial and temporal resolution. Laser ablation ICPMS using a high-sensitivity quadrupole analyzer is particularly well-suited to petrochronology given its ability to rapidly quantify the entire mass range (Si through U) of major and trace elements in accessory minerals, simultaneous with 3-5% RSD resolution U-Th-Pb ages on individual 25 to 40 micron spot analyses. Our analytical workflow involves: imaging of accessory mineral grains via cathodoluminescence or back-scattered electrons; combined spot U-Th-Pb and trace element analysis using a UP213 laser coupled to a X-Series 2 quadrupole ICPMS; and isotope dilution analysis (including chemical abrasion for zircons) of the same crystals using EARTHTIME mixed tracers, which yield individual analysis at the 0.05% level of precision and accuracy.

Several applications of this tandem approach will be presented, including zircon and titanite petrochronology of silicic rhyolites and neoblastic metamorphic titanite petrochronology in ductile shear zones. These examples will illustrate the frontier efforts in petrochronology, including more sophisticated modelling of the averaging effects of different sampling techniques, more selective methods of intra-crystal ID-TIMS sampling, and innovative crystal surface and serial depth profiling in situ ion probe and LA-ICPMS analysis.