DETRITAL ZIRCON U-TH-PB GEOCHRONOLOGY BY LA-MC-ICPMS AT THE ARIZONA LASERCHRON CENTER

The Arizona LaserChron Center (www.geo.arizona.edu/alc) utilizes a 193 nm excimer laser system (from Lambda Physik and New Wave Research) and a multicollector ICPMS (from GV Instruments) to conduct U-Th-Pb geochronologic analyses. This instrument is ideally suited for detrital zircon analyses because it has a rapid throughput (40 unknowns per hour), yields ages with a precision of 1-2% (2-sigma) for a single analysis and an accuracy of 1% for a set of 10 or more analyses, can measure 204 accurately, yields robust U/Th petrogenetic information, and has a spatial resolution down to 15 by 10 microns (pit width by depth). Our typical strategy for detrital samples is to analyze 100 zircon grains (selected at random) using a beam size appropriate for analysis of the smallest grains in the sample. Ages are calibrated relative to fragments of a Sri Lanka zircon grain that is calibrated by ID-TIMS and is cross-calibrated relative to R33, Temora, 91500, and FC-1. Standards are analyzed once every five unknowns, and each unknown is calibrated relative to 206/238, 208/232, and 206/207 of the closest six standard analyses. The resulting ages are plotted on relative age probability plots according to 206/238 ages (for <1.0 Ga grains) or 206/207 ages (for >1.0 Ga grains), and significance is attached only to age peaks that comprise three or more analyses. This is because clustering of ages is a more robust indicator of reliability than concordance or precision, but this strategy precludes attributing significance to an individual analysis and especially to the youngest grain in a sample.

The Arizona LaserChron Center is funded by the NSF/EAR Instrumentation and Facilities Program to operate as a multi-user facility, with ~50% subsidy of analytical costs for NSF-supported research. This yields a user cost of $400 for analysis of 100 unknowns. Additional support for student use is also provided by NSF. This style of operation is having a significant impact on provenance studies given that we currently generate ~40,000 detrital zircon ages for ~50 projects per year.