HIGH-PRECISION U-PB GEOCHRONOLOGY OF LARGE IGNEOUS PROVINCES

A first-order problem in the study of large igneous provinces (LIPs) is their age and duration. Such constraints are necessary to evaluate their role in mass extinctions and as drivers of climate change. Crucial to evaluating the role of LIPs in earth history is high precision geochronology. Age precision at the 0.05% level or better is often required to fully evaluate the coincidence versus causality relationship between LIP eruption and extinction. This is currently hampered by relatively large uncertainties and systematic inaccuracies. Global correlations of LIPs and their use in continental reconstructions require careful integration of geochronological, paleomagnetic, and paleontological data sets.

In the past twenty years, U-Pb geochronology of basaltic LIPs has led to a revolution in our understanding of the age, distribution, and causes of their emplacement. U-Pb technique has centered on dating of baddeleyite and zircon, with precisions as low as 0.1% or better (e.g. ±250 ky for Siberian Traps). However, as we increase precision, inter-technique and interlaboratory biases (U-Pb vs. Ar-Ar or Re-Os) become increasingly important. Within the U-Pb system alone, systematic bias related to U decay constant uncertainties is significant. For example, many Precambrian baddeleyites and zircons are slightly discordant, making the weighted mean 207Pb/206Pb date more precise, but also older than 206Pb/238U dates.

The EARTHTIME initiative seeks to understand and minimize these biases through detailed inter-technique comparisons and the distribution of common calibration and tracer solutions for U-Pb. In addition, because of ultra-small zircon and/or baddeleyite grain size in many mafic rocks, in-situ SIMS analyses of baddeleyite (Chamberlain, this session) have yielded considerable success. Another underappreciated geochronological tool in Precambrian mafic rocks is apatite. Apatite is often abundant and has a closure temperature for Pb diffusion of approximately 400°C, which also allows evaluation of thermal resetting of magnetic signatures. In many rocks weighted mean 207Pb/206Pb dates for slightly discordant apatite data can yield uncertainties of 0.1-0.2%. Such advances in high-precision U-Pb geochronology combined with other dating methods, paleomagnetism and paleontology will lead to an increased understanding of the importance of LIPs on the planet.