EAGER: U-PB DATING OF BISHOP TUFF ZIRCON BY SERIAL SECTIONING FOR GEOCHRONOMETER CALIBRATION AND MAGMATIC INSIGHTS

Northern Arizona University, Flagstaff AZ

Investigators

Abstract

Understanding the rates, durations, and interplay between geologic processes relies on accurate dating of minerals. Under favorable circumstances, it is possible to confirm the accuracy of these dates by applying different dating techniques to the same, or related, minerals. An increasingly important benchmark in mineral dating as well as a valuable source of insights into the evolution of voluminous explosive eruptions is the >600 km3 Bishop Tuff of eastern California, USA. This project will test the hypothesis that U-Pb and Ar/Ar dates for the minerals zircon and sanidine, respectively, are essentially identical in the Bishop Tuff. It will determine whether there are U-Pb age heterogeneities within Bishop Tuff zircon and, if present, to more rigorously quantify them. If the age distribution with zircon is homogeneous, this study will validate use of the Bishop Tuff as a key intercalibration point between the U-Pb and Ar/Ar radiometric clocks. If older zircon domains are present, their ages and their population distributions will validate previous evidence that dates for zircon cores can be considerably older than the sanidine dates. Rapid confirmation or rejection, as enabled by EAGER funding, of the hypothesis that the U-Pb and Ar/Ar dates are identical will have an immediate impact on efforts to improve the accuracy of Ar/Ar dating, a technique capable of providing precise dates for events in the Cenozoic Era, and for geologic materials that may be difficult to date by other methods. The outcome will inform models for how magmas responsible for supervolcanoes accumulate beneath Earth?s surface. The project will explore novel approaches to acquiring spatially sensitive and analytically more robust ion microprobe dates for Bishop Tuff zircons. Analyses will be performed on zircon rims and on oriented cross-sections exposed during optical interferometry-calibrated serial sectioning, resulting in a high spatial sensitivity with respect to crystal growth. Grains will be mounted sequentially so that rim and interior analyses can be performed in tandem, reducing the reliance on a standard for confirming age differences within grains, if present. Sputtering will entail a 100 nA ion beam instead of the normally employed 10-12 nA beam, resulting in enhanced radiogenic Pb yields and reduced analytical uncertainties. Once the age distribution within the BT zircons has been quantified, mass balance analyses will be performed in order to assess whether the zircons crystallized in a few ka and, if older zircon domains are present, the extent to which they could skew the ages obtained by single crystal TIMS analyses. The new technical approaches to be explored in this study could advance the precision of situ U-Pb dating and be adopted by other in situ geochronologists seeking to gain more insight into age heterogeneities and their distributions within crystals for timescales of magma chamber processes. The proposed study will also provide research training in spatially sensitive U-Pb zircon dating for a female graduate student.