Geochronology: Linking the isotopic record with Petrology and Textures

W. J. Davis
Geological Survey of Canada

1 In determining the age of minerals and rocks it is often possible to derive a precise number that can be interpreted as a date, it is another matter to accurately interpret the number in terms of the timing of a discrete geological event or process. This is the challenge addressed by the contributors to an edited volume in the Geological Society Special Publication series entitled Geochronology: Linking the isotopic record with petrology and textures. The volume consists of fourteen papers derived from a special symposium held at the 2002 Goldschmidt Conference in Davos, Switzerland.

2 Advances in analytical capabilities over the past 10-15 years allow geochronological studies to be carried out at a scale similar to petrological observation and within the realm of mineral processes. A variety of techniques is now available for high spatial resolution isotopic analyses (ion probe, laser ablation; microsampling). For example, reduction in sample size using careful grain selection or in situ methods permit isotopic analysis at the sub-50 µm scale. The power of these technical advances, and the importance of understanding the context in which they are applied, is highlighted by the papers in this volume.

3 The topic is very broad and this is reflected in the range of papers from detailed technical notes on analytical methods to a review paper on metamorphic reaction rates. As a single collection, the book is not a definitive representation of the current state of knowledge within any one area as only a handful of papers are of a review nature. This is alleviated somewhat by a very useful introductory chapter by the editors that outlines the current state of knowledge, ties the disparate papers together, and provides a solid reference list.

4 The volume is divided into four sections of unequal length: 1) Improving the link between accessory phase chronometers and petrological information; 2) Advances in the chronometry of major minerals – pro-grade histories; 3) Texturally controlled (‘ in situ') chronometry; and 4) Understanding transport processes in rocks.

5 The three papers in the first section focus on petrological observations combined with trace element and isotopic data of accessory minerals to link their formation with major mineral phases, and ultimately to the pressure-temperature-time path of metamorphic rocks. The paper by Foster and Parrish on metamorphic monazite is a useful review that also contains some new ideas on establishing monazite within a quantitative metamorphic framework. Papers by Whitehouse, and Moller et al. are good examples of the type of information that can be extracted through integrated trace element and isotopic analyses of zircon

6 The second section contains four papers on dating major rock forming minerals. The paper by Anczkiewicz and Thirlwall is a technical paper dealing with improved analytical methods for preparing garnet for Sm-Nd analyses. The paper by Stowell and Tinkham provides a case study integrating geobarometric data with Sm-Nd garnet age dating in the Cordillera of western USA. Zheng et al. use stable isotope data to demonstrate the importance of evaluating chemical and isotopic equilibrium in the interpretation of isotopic mineral isochrons. Romer and Rotzler demonstrate the importance of understanding the reaction history of metamorphic rocks for accurate interpretation of UPb data in metamorphic titanite.

7 Cliff and Meffan-Main's paper in the section on texturally controlled ‘ in situ' chronometry further highlights the importance of considering isotopic disequilibrium. Careful in situ microsampling of petrologically defined domains is used to isolate material that attained equilibrium at different times. VanHaecke et al.'s paper on using ICPMS for Rb-Sr analyses, although an interesting technical investigation, seems at odds with the overall theme of increasing accuracy through increased analytical and contextual precision.

8 The last section includes five papers focused on transport processes in rocks and their role in interpreting geochronological data. As the editors point out, our generally poor understanding of these processes remains a major impediment. Baxter presents a useful review of natural metamorphic reaction rates and suggests the discrepancy between lab based and natural estimates for regional metamorphic reactions reflects the very important role fluids play in rock reactions. Kriegsman and Nystrom present a review of melt segregation rates in migmatites along with a companion paper outlining a detailed case study. Papers by Wartho and Kelly and Kramar deal with in situ argon geochronology and the interpretation of diffusion profiles. The important role of fluids and deformation to the interpretation of thermal histories of minerals is highlighted.

9 In general, the papers in this volume are well presented and illustrated. The index is particularly useful. The volume will be of general interest to geochronologists and metamorphic petrologists. Its message of employing multiple analytical techniques to fully interpret geochronological data within petrological context represents the current benchmark for accurate geochronological calibration of polymetamorphosed rocks in orogenic belts.