Geochronology: linking the isotopic record with petrology and textures — an introduction

Abstract

Abstract:One of the key aims of geochronology, and the subject of the papers in this Special Publication, is the linkage of isotopic ages to petrological and textural information. A close link between the two types of information greatly improves the constraints available from geochronology on the nature and rates of lithospheric processes such as metamorphism and deformation. There have been several key advances in this area over the past 10-20 years, relating to increased precision and accuracy of isotopic ages but also, and crucially, to the spatial resolution available to geochronologists. This resolution now approaches that on which petrological, chemical and textural information is obtained. We also, in this introduction, identify the barriers that have impeded further progress, which relate both to technical issues as well as to problems of understanding. Finally we set the papers in this volume in the context of the preceding discussion and outline the key ways in which these papers point towards further progress in the future.Time has always been recognized as a key variable in the Earth sciences, both in its own right and through the constraints that chronological information provides on the rates of, and hence the physical mechanisms for, Earth processes. In the study of the dynamics of the Earth's lithosphere during orogenesis and metamorphism, one of the key requirements of any time datum is that it is relatable in a straightforward manner to other geological information, such as data on temperature, pressure and deformation. It is only through this linkage that chronometric information can achieve its full potential in contributing to progress in understanding the dynamics of mountain belts and other metamorphic settings. This linkage is the focus of the set of papers in this volume, which emanate from a special symposium at the 2002 Goldschmidt Conference, held at Davos, Switzerland. The topic under discussion has been the subject of a review by one of us recently (Mtiller 2003). Inevitably, there is some overlap between this paper and Miiller (2003). However, the emphasis of the two papers is slightly different, with Mtiller (2003) concentrating largely on new in-situ techniques and their resulting prospects, whereas here we give a broader overview of the whole subject, with more emphasis on the dating of high temperature processes and the interrelated achievements of both modelling and experimental work in petrology. We also aim to highlight some general issues that are dealt with in specifics in the subsequent contributions. First, we give a brief account of the progress that has been made over the past two decades in our ability to extract the timing and conditions of deformation and metamorphism from minerals and in the interpretive framework in which these new data are analysed. Next we identify three key barriers that initially impeded further progress and which have been and continue to be the subjects of intensive research eflbrt. Finally, we discuss ways that are being developed to surm...

Disequilibrium textures versus equilibrium modelling: geochronology at the crossroads

“…This problem has been discussed extensively in the literature (e.g. Vance et al 2003 and references therein) and needs not be reiterated here. It is beneficial, however, to point out a few implications that apply to any isotope analysis.…”

Geosciences research in East Antarctica (0°E–60°E): present status and future perspectives

“…Accessory phase behaviour with regard to trace element geochemistry (including REE, P, Zr, Ti, U and Th) is a major topic in the microanalytical world and has potential in resolving many problems relating to the evolution of continents. In recent years, barriers have been broken in linking the isotopic record with petrology in complex and multiply metamorphosed and deformed terranes (Rubatto 2002;Müller 2003;Vance et al 2003). Within our reach is a new phase in accessory mineral research that will unravel complicated metamorphic and tectonic histories.…”

Chronostratigraphy of Monte Vulture volcano (southern Italy): secondary mineral microtextures and 39Ar-40Ar systematics

“…Such observations are common in metamorphic rocks (e.g. Vance et al 2003, and references therein); our work extends the database of such images also to volcanic rocks, and explicitly addresses the link between microtextures and isotopic record.…”

“…The realization that chemical disequilibrium textures are common and therefore influence the isotopic record has come of age in the recent geochronological literature on metamorphic rocks (e.g. Vance et al 2003), but had never been extended to volcanic rocks so far. The presumed "rapid quenching" of volcanic minerals was not thought to be compatible with deuteric chemical disequilibrium textures that we document here for the first time.…”