A new approach for constraining the magnitude of initial disequilibrium in Quaternary zircons by coupled uranium and thorium decay series dating

We have measured U-238-Pb-206, U-235-Pb-207 and Th-232-Pb-208 ages on Quaternary zircons by laser ablation, single-collector, magnetic sector inductively coupled plasma mass spectrometry (LA-ICP-MS). To obtain reliable ages for Quaternary zircons, corrections for initial disequilibrium associated with deficits and excesses of both Th-230 and Pa-231 relative to secular equilibrium resulting from differential partitioning during zircon crystallization or source melting must be made. In contrast, the Th-232-Pb-208 decay system is clearly advantageous for samples affected by disequilibrium because the Th-232 decay system lacks long-lived intermediate daughter isotopes. Conventionally, the initial disequilibrium for the U-238 and U-235 decay series has been determined by the distribution ratio between the melt and zircon (i.e.,f(Th/U) = (Th/U)(zircon)/(Th/U)(melt) and f(pa/U) = (Pa/U)(zircon)/(Pa/U)(melt)). In our study, these correction factors were determined from comparison of the measured U-238-Pb-206 and U-235-(207)pb ages with Th-232-Pb-208 ages obtained for three zircons of known eruption and, in some cases, zircon crystallization ages (Kirigamine Rhyolite, Bishop Tuff, and Toga Pumice). The resulting correction factors are f(Th/U) = 0.19 +/- 0.14 and F-Pa/U = 3.66 +/- 0.89 (Kirigamine Rhyolite), f(Th/U) = 0.24 +/- 0.20 and f(Pa/U) = 3.1 +/- 1.2 (Bishop Tuff), and f(Th/U) = 0.28 +/- 0.17 and f(Th/U) = 3.04 +/- 0.99 (Toga Pumice). Although the uncertainties of these f values are relatively large, our results support the adequacy of the conventional approach for correction of initial disequilibrium. A recent study published results that apparently show zircon crystallization ages are younger than the eruption age of Bishop Tuff. It seems to be difficult to eliminate these discrepancies, even if the Th/U partitioning and disequilibrium generated during partial melting are taken into account for recalculation of its zircon age. However, magma chamber process and history of Bishop Tuff are too complex to obtain accurate zircon ages by U-Pb method. To overcome this, therefore, the Th-Pb zircon dating method is a key technique for understanding complex, pre-eruptive magma processes, and further efforts to improve its precision and accuracy are desirable. (C) 2016 Elsevier B.V. All rights reserved.