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Apatite Thermochronology in Modern Geology

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Apatite Thermochronology in Modern Geology Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 Apatite thermochronology in modern geology F. LISKER1*, B. VENTURA1 & U. A. GLASMACHER2 1Fachbereich Geowissenschaften, Universita¨t Bremen, PF 330440, 28334 Bremen, Germany 2Institut fu¨r Geowissenschaften, Ruprecht-Karls-Universita¨t Heidelberg, Im Neuenheimer Feld 234, 69120 Heidelberg, Germany *Corresponding author (e-mail: fl[email protected]) Abstract: Fission-track and (U–Th–Sm)/He thermochronology on apatites are radiometric dating methods that refer to thermal histories of rocks within the temperature range of 408–125 8C. Their introduction into geological research contributed to the development of new concepts to interpreting time-temperature constraints and substantially improved the understanding of cooling processes within the uppermost crust. Present geological applications of apatite thermochronological methods include absolute dating of rocks and tectonic processes, investigation of denudation histories and long-term landscape evolution of various geological settings, and basin analysis. Thermochronology may be described as the the analysis of radiation damage trails (‘fission quantitative study of the thermal histories of rocks tracks’) in uranium-bearing, non-conductive using temperature-sensitive radiometric dating minerals and glasses. It is routinely applied on the methods such as 40Ar/39Ar and K–Ar, fission minerals apatite, zircon and titanite. Fission tracks track, and (U–Th)/He (Berger & York 1981). are produced continuously through geological time Amongst these different methods, apatite fission as a result of the spontaneous fission of 238U track (AFT) and apatite (U–Th–Sm)/He (AHe) atoms. They are submicroscopic features with an are now, perhaps, the most widely used thermo- initial width of approximately 10 nm and a length chronometers as they are the most sensitive to low of up to 20 mm (Paul & Fitzgerald 1992) that can temperatures (typically between 40 and c. 125 8C be revealed by chemical etching. Crucially, fission for durations of heating and cooling in excess of tracks are semi-stable features that can self-repair 106 years), ideal for investigating the tectonic and (shorten and eventually disappear) by a process climate-driven surficial interactions that take place known as annealing at a rate that is a function of within the top few (,5 km) kilometres of the both time and temperature. The extent of any track Earth’s crust. These processes govern landscape evo- shortening (exposure to elevated temperatures) in lution, influence climate and generate the natural a sample can be quantified by examining the distri- resources essential to the wellbeing of mankind. bution of fission-track lengths. This introductory chapter provides a brief The determination of a fission-track age (a number overview of apatite thermochronology and its appli- that relates to the observable track density) depends cation to geological studies. We focus on three on the same general equation as any radioactive topics: (1) methodological developments; (2) con- decay scheme: it requires an estimate of the relative cepts and strategies for the interpretation of thermo- abundance of the parent isotope and of the daughter chronological data; and (3) applications to various product. However, unlike most methods of radio- geodynamic settings. For more detailed insights metric dating, it measures the effect, rather than the on apatite thermochronology the reader is referred product, of a radioactive decay scheme, that is it to published reviews by Green et al. (1986, refers to the number of 238U atoms and the number 1989b), Laslett et al. (1987), Duddy et al. (1988), of spontaneous fission tracks per unit volume. This Wagner & Van den Haute (1992), R. W. Brown fission-track density is obtained by counting the et al. (1994), Gallagher et al. (1998), Gleadow number of spontaneous tracks intersecting a polished et al. (2002), Ehlers & Farley (2003) and Reiners internal surface of a mineral grain viewed under high & Brandon (2006). magnification (1000–1250Â) using an optical microscope. Depending on the sample and aims of the study, a typical fission-track sample age consists Fission-track thermochronology of a weighted mean of 20–100 single-grain ages. Basics of the method Further details and background information on prac- tical aspects of fission-track age determination are Fission-track thermochronology/-chronometry (for provided by, for example, Fleischer et al. (1975), differentiation cf. Reiners et al. 2005) is based on Naeser (1979) and Donelick et al. (2005). From:LISKER, F., VENTURA,B.&GLASMACHER, U. A. (eds) Thermochronological Methods: From Palaeotemperature Constraints to Landscape Evolution Models. Geological Society, London, Special Publications, 324, 1–23. DOI: 10.1144/SP324.1 0305-8719/09/$15.00 # Geological Society of London 2009. Downloaded from http://sp.lyellcollection.org/ by guest on September 25, 2021 2 F. LISKER ET AL. An important dimension to fission-track thermo- there is significant (or over-) dispersion within the chronology is the semi-stable nature of tracks, population of single-grain ages. Green pointed out whereby annealing can change the significance of that where there is evidence for heterogeneity a measured age. The observable density of spon- (extra Poissonian variation) within a dataset, taneous tracks in a sample (age) is a function of detected by statistical tests such as x2 (Galbraith track length (probability of intersecting the plane 1981), the conventional pooled age, based on the of observation). All newly formed tracks in apatite ratio of the number of spontaneous and induced have a length of approximately 16 mm(c. 11 mm tracks (Ns/Ni), with its Poisson standard error, in zircon). If a sample (e.g. a volcanic apatite) was becomes meaningless. An alternative approach created at 10 Ma and then resided at low tempera- based on the mean of ratios of individual track den- tures (,40 8C), the population of tracks reduces in sities (rs/ri) was used to give a larger estimate of the length to a mean value of approximately 15 mm error to allow for an extra-Poisson component in the causing an insignificant (not resolvable) reduction dispersion of single-grain ages, but this approach in track density and a measured age within an implied a sample should have a single age. In error of 10 Ma. However, if, during its history, the reality, there are a number of different causes of het- same sample experienced elevated temperatures erogeneity within a dataset (beyond a bad exper- (but not sufficient to cause total resetting) in its iment), such as variable responses to partial history there will be significant track shortening to resetting due to variations in apatite grain compo- a level defined by the maximum heating. This will sition (see later) and/or a range of provenance cause a reduction in observable track density and, ages. Thus, it is important to assess to what the over- therefore, measurable age. dispersion is due rather than make to allowance for it with larger errors. In 1984 Hurford et al. proposed the use of prob- Some milestones in the evolution of the ability density diagrams, a type of continuous histo- fission-track method gram that plots each grain-age error as a Gaussian density function, as a way of visualizing a mixed Despite early recognition of fission tracks age dataset. However, this type of approach can (Baumhauer 1894; Silk & Barnes 1959), it was not obscure useful information by inappropriately until the early 1960s that their application to geo- weighting it with poor information (i.e. an overlap logical dating was first proposed (Price & Walker effect associated with broad, imprecise, peaks). To 1963) and subsequently developed (Wagner 1966; overcome this problem Galbraith developed the Gentner et al. 1967; Naeser 1967). Early dating radial plot (Galbraith 1988, 1990) (Fig. 1), which studies were tasked with finding practical ways to is now routinely used across the chronological com- etch tracks, measure uranium contents that mirror munity. Coupled to this development Galbraith & the dated grain and define the time–temperature Laslett (1993) also produced the widely adopted stability fields of fission tracks in different uranium- random effects model that gives a central age esti- bearing minerals, tektites and glasses. Studies mate of the population of grains ages with a relative conducted between 1970 and 1983 highlighted the standard deviation of the population of ages known fundamental issues that needed to be resolved in as the age dispersion (normally expressed as a per- order to enable routine and accurate age determi- centage variation). nation. Foremost was a lack of consensus on the Having established protocols to measure fission- value of the spontaneous fission decay constant for track ages and assess data quality and structure, the 238U, to be used in the equation for age calculation. next major developments were related to under- In order to circumvent this and other fundamental standing the significance of the determined ages. problems associated with the measurement of Whilst track-length measurement had been used to neutron fluence, Hurford & Green (1983) advanced detect track annealing more or less since the a suggestion made by Fleischer and Hart at a methods inception, it was not until the mid 1980s meeting in Austria in 1971 for a comparative that studies demonstrated the utility of such data. approach to AFT dating through the use of a propor- Advances included moving away from using semi- tionality constant. The resultant ‘Zeta’ calibration tracks (projected track) to length measurement method (Hurford & Green 1983) has become the based on surface-parallel confined tracks. Although standard approach to fission-track age determination more numerous, semi-tracks contain less infor- (Hurford 1990). mation and have significant sources of bias, particu- Until 1980 most fission-track ages were calcu- larly towards longer lengths (Laslett et al. 1994). A lated using a pooled age, based on the ratio of total key paper by Gleadow et al. (1986b) demonstrated counts of spontaneous and induced tracks.
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