Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 Geochronology: linking the isotopic record with petrology and textures - an introduction DEREK VANCE 1'2, WOLFGANG MULLER 3 & IGOR M. VILLA 4 1Department of Geology, Royal Holloway, University of London, Egham, Surrey, TW20 OEX, UK (e-mail: [email protected]) 2present address: Department of Earth Sciences, University of Bristol, Wills Memorial Building, Bristol BS8 1R J, UK 3Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia (e-mail: wolfgang.mueller@ anu. edu. au) 4 Isotopengeologte, Erlachstrasse 9a, 3012 Bern, Switzerland and Dipartimento di Scienze Geologiche e Geotecnologie, Universitgt di Milano-Bicocca, 20126 Milano, Italy (e-mail: [email protected]) 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 the emphasis of the two papers is slightly differ- in the Earth sciences, both in its own right and ent, with Mtiller (2003) concentrating largely on through the constraints that chronological infor- new in-situ techniques and their resulting pro- mation provides on the rates of, and hence spects, whereas here we give a broader overview the physical mechanisms for, Earth processes. In of the whole subject, with more emphasis on the the study of the dynamics of the Earth's litho- dating of high temperature processes and the sphere during orogenesis and metamorphism, one interrelated achievements of both modelling and of the key requirements of any time datum is that experimental work in petrology. We also aim to it is relatable in a straightforward manner to other highlight some general issues that are dealt with geological information, such as data on tempera- in specifics in the subsequent contributions. First, ture, pressure and deformation. It is only through we give a brief account of the progress that has this linkage that chronometric information can been made over the past two decades in our ability achieve its full potential in contributing to progress to extract the timing and conditions of deformation in understanding the dynamics of mountain belts and metamorphism from minerals and in the and other metamorphic settings. This linkage interpretive framework in which these new data are is the focus of the set of papers in this volume, analysed. Next we identify three key barriers that which emanate from a special symposium at the initially impeded further progress and which have 2002 Goldschmidt Conference, held at Davos, been and continue to be the subjects of intensive Switzerland. The topic under discussion has been research eflbrt. Finally, we discuss ways that are the subject of a review by one of us recently being developed to surmount these barriers, with (Mtiller 2003). Inevitably, there is some overlap particular reference to the contributions in this between this paper and Miiller (2003). However, volume. From: VANCE, D., MIDLLER,W., (~c VILLA, I. M. (eds) 2003. Geochronology: Linking the Isotopic Record with Petrology and Textures. Geological Society, London, Special Publications, 220, 1-24. 0305-8719/03/$15 9 The Geological Societyof London 2003. Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 2 D. VANCE ETAL Progress in quantifying rates of thermal, largely been driven by technology with the advent, baric and structural evolution of first, of multi-collector mass spectrometers (e.g. Esat 1984; for an early geochronological use see the lithosphere Vance & O'Nions 1990) and techniques for in-situ The last three decades have seen enormous pro- dating of metamorphic mineral phases, such as ion gress in our understanding of metamorphic belts microprobe (e.g. Compston et al. 1984; Harrison in general. This progress has its origins in et al. 1995) and laser ablation methods (Horn et al. three main fields of activity. Firstly, there has 2000; Ballard et al. 2001; Kosler et al. 2001; Foster been a dramatic expansion in the number and et al. 2002). the quality of experimental constraints on the The two steps forward described above have pressure and temperature conditions of formation largely been powered by technology and data. of key metamorphic minerals, as evidenced by These newly available databases provided the the increasing variety of mineral end-members initial impetus for developments in the third main incorporated in databases of mineral thermody- field which has led progress in understanding namic properties (e.g. Holland & Powell 1998). orogenic belts: the numerical simulation of the The acquisition of this new experimental mechanical and thermal adjustments that are the database has gone hand-in-hand with advances consequences of orogenesis and the causes of in our ability to simulate the phase equilibria of metamorphism. The application of analytical metamorphic rocks using computer models (e.g. solutions of heat transport equations to one- Spear 1988; Berman 1991; Powell et al. 1998). dimensional models of metamorphic belts in The pioneering predictive modelling of mineral the 1970s (Bickle et al. 1975), was followed by phase equilibria in the 1970s was restricted to more general one-dimensional models involving simple chemical systems like KFMASH and was numerical solutions (England & Thompson largely semi-quantitative (e.g. Hensen 1971; 1984), and has culminated more recently in the Thompson et al. 1977). In the last 10-15 years, development of two-dimensional models that on the other hand, the quantitative analysis of more fully simulate the temperature and defor- chemical systems that more realistically match mation fields in orogenic belts (Huerta et al. real silicate rocks has been made possible by an 1999; Jamieson et al. 2002). These models were improved knowledge of basic thermodynamic initially designed as an interpretive framework quantities and activity models for many more for thermobarometric, chronometric and struc- phases and mineral chemical end-members (e.g. tural data, but they also make predicitions about Holland & Powell 1998). It is also more common the pressure-temperature-deformation-time to view metamorphic events in terms of P-T-X histories of deeply buried rocks (and the surface relations (Berman 1991; Kerrick 1991) in development of orogenic belts) that the newly chemically open systems rather than as simple available databases can be used to test. heating of a closed system (J~iger et al. 1967). One of the key requirements that the numeri- These advances have greatly improved our cal models make of pressure-temperature- ability to model the temperature and depths of deformation-time data is that the time constraint burial of exhumed packets of lithosphere presently is not only precise and accurate but that it can be exposed at the surface of the Earth. Significantly, easily related to the other three variables (e.g. they have also allowed segments of pressure- Thompson & England 1984). In other words, temperature paths to be extracted from rocks. These chronological constraints can add immeasurably latter are key inputs to thermal and mechanical more to our understanding of metamorphism if models that seek to clarify the plate tectonic they can be linked unequivocally to P-T-X-d settings of orogenesis (e.g. England & Thompson data. Moreover, one of the key outputs of 1984; Huerta et al. 1999; Jamieson et al. 2002). A numerical models is P-T-X-d evolution. That second key input to such models is time, and the being the case, a deep understanding of the constraints that chronometric information provides portion of a P-T-X-d history when different on the rates at which burial, heating and deforma- minerals, or parts of them, record time, pressure, tion occur. This is where the second major advance temperature and deformation, is clearly essential in this field has occurred. The source of if these types of data are to test such models. The information on the chronology of metamorphism recent attempts at achieving such an under- is, of course, radiogenic isotope geology and the standing are the subject of many of the contri- last three decades have also seen unprecedented butions in this book. As these articles make advances in our ability to measure the isotope abundantly clear, however, the linkage of a time composition of radiogenic elements, both in the constraint obtained from radiogenic isotopes in precision available and in the quantity of material minerals to other types of information, such as required for the analyses. This leap forward has petrological and structural data, is not always Downloaded from http://sp.lyellcollection.org/ by guest on September 26, 2021 PETROLOGY AND TEXTURES - AN INTRODUCTION 3 straightforward. The reasons why are discussed structural evolution. This difficulty arises because in the next section. It is important to note that the datable minerals are present as accessory while faster analytical protocols have increased phases whose relationship to the main rock the sample throughput, this does not per se texture is often equivocal but, more importantly, guarantee an improved understanding of isotopic because their chemistry, involving silicates or ages.