Uranium-series Crystal Ages

Abstract

As the collected papers in this volume aptly demonstrate, chemical and textural records contained within crystals have become increasingly useful tools for understanding the evolution of magmatic systems. An essential component of studies of the dynamics of magmatic systems is the ability to place the thermal, physical, and compositional evolution of magmas in a temporal context. Studies of the kinetics of crystal growth and reaction, and of diffusive re-equilibration of crystals and melts, can provide information about the duration of magmatic processes such as magma ascent (Rutherford 2008), crystal growth (Hammer 2008), and residence of crystals in melts other than the ones in which they crystallized (Costa 2008). However, the only method of extracting information from young crystals about the absolute age of magmatic processes is isotopic dating. The timescales of many sub-volcanic magmatic processes such as magma transport, differentiation, crystallization, and storage within the crust, appear to be commensurate with the half-lives of uranium-series (U-series) nuclides, and therefore this isotopic system offers the greatest potential to quantify timescales and rates of magmatic processes. We focus here on crystal dating using the decay products of 238U and 235U, including 238U-230Th-226Ra-210Pb disequilibria, 235U-231Pa disequilibria, and U-Pb dating of young crystals. In this chapter, we review the principles of U-series dating of crystals, the types of information that have been gained so far from U-series dating, and the potential for combining U-series crystal ages with other crystal-scale and magma-scale information to unravel the dynamics of magmatic systems. ### Principles of U-series dating U-Pb dating is based on the accumulation of 206Pb and 207Pb over time due to decay of 238U and 235U, respectively, and is conceptually similar to most isotopic dating techniques. U-Pb dating of zircon, both by TIMS and in …