INTEGRATING GEOCHEMISTRY, THERMOMETRY AND ISOTOPE RATIOS TO IMPROVE AGE INTERPRETATION THROUGH TANDEM QUADRUPOLE LA-ICPMS AND CA-TIMS ON THE SAME ZIRCON CRYSTALS

Analytical innovations of the past decade have significantly improved the temporal resolution of the U-Pb chronometer, but with greater resolution 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 present a novel tandem in situ and isotope dilution analytical approach maximizing both spatial and analytical resolution. Our technique involves: imaging of zircon grains via cathodoluminescence; combined spot U-Th-Pb and trace element analysis of zircons using a UP213 laser coupled to a X-Series 2 quadrupole ICPMS; chemical abrasion isotope dilution analysis of the same crystals using EARTHTIME mixed tracers, which yield individual analysis at the 0.03% level of precision and accuracy.

As a proof of concept, we placed a variable number—from zero to 20—of 25 µm ablation spots on several grains of TEMORA zircon that had been previously annealed and chemically abraded prior to mounting in epoxy. Following ablation, the mount was polished with 0.3 µm alumina and cleaned in an ultrasonic bath. The grains were extracted from the epoxy with tweezers, and split into two groups: one group was subjected to a second round of chemical abrasion before dissolution, while the other group was immediately dissolved with the ET2535 spike.

Our LA-ICPMS results illustrate a 3 to 5-fold variation in trace element concentrations and ratios over >150° of cooling as estimated from Ti-in-zircon thermometry. Some geochemical parameters are bimodal, suggesting mixing of crystal/magma batches prior to final solidification. Our CA-TIMS results illustrate that all samplings are analytically indistinguishable at the 0.03% resolution of ID-TIMS, regardless of the presence or number of laser ablation pits, or treatment prior to dissolution. The results are also indistinguishable from earlier analyses using the ET535 spike. A minority of grains contain inherited cores of slightly older age, compared to the main growth phase of igneous zircon at 416.9 Ma. The entire geochemical and temperature range is captured within this dated igneous population, thus we can confidently assign petrological significance to the ca 0.1 Ma errors of the CA-TIMS analysis.