Bridging the precision gap in zircon 238U-230Th and 238U-206Pb ages between ~100 and 350 ka: Development of a coupled ID-TIMS and ID-MC-ICPMS zircon dating technique

Abstract:

Zircon ²³⁸U-²³⁰Th (U-Th) geochronology is often used to date magmatic systems that crystallized over the past ~300 ka, while zircon ²³⁸U-²⁰⁶Pb (U-Pb) is often used to date systems > 1 Ma. In-situ U-Th techniques with high spatial resolution has been successful in measuring absolute timescales of magma reservoir processes. ID-TIMS U-Pb technique can determine higher-precision ages of the same processes in older rocks to understand long-term temporal relationships between tectonics and magmatism. Both techniques have contributed valuable temporal data about long-lived magmatic systems. The largest age uncertainties < 1 Ma exist for the age range of ~100-350 ka where neither technique can produce ages precise enough to distinguish magmatic processes that occur on < 10⁵ yr timescales.

Bridging this gap requires overcoming the major shortcomings of each dating technique: TIMS isotopic measurements on young zircon with low radiogenic Pb contents, in-situ U-Th dates on grains very near secular equilibrium, and sensitivity of young U-based chronometers to initial isotope disequilibrium. Small amounts of radiogenic Pb in young zircon can be accurately measured by ID-TIMS and the high ionization potential of MC-ICPMS allows for U-Th measurements near secular equilibrium. Our technique combines the U-Pb and U-Th data into an algorithm that describes intermediate daughter disequilibrium and incorporates appropriate common Pb corrections resulting in a single, higher-precision zircon age and thus improves temporal resolution of geologic events in this difficult to date age range (~150-300 ka).

We present preliminary results of our newly coupled ID-TIMS and ID-MC-ICPMS measurement and data reduction technique. To establish the accuracy of our algorithm and the dates it yields, we test our protocol using high-U (>500 ppm) zircons that have indistinguishable U-Th rim and interior ages at the age resolution possible with SHRIMP-RG so that coupled ID-TIMS and ID-MC-ICPMS whole-grain dates should match the SHRIMP-RG dates. Zircons from ~100 ka Coso Dome 36, 111 ka Sierra La Primavera, ~185 ka Long Canyon Rhyolite and ~300 ka post-Bishop Tuff rhyolites are used in our test. Ultimately, this technique can be applied to magmatic systems with complex zircon crystallization histories to understand pre-eruptive conditions.