Anthony J. Hurford

The zeta age calibration of fission-track dating.

Uranium dosimeter glasses SRM 612, CN1 and CN2 have been calibrated against Co monitors in 79 irradiations in the Herald reactor over seven years. Mean values of the calibration factors B for each glass are B612 = (5.736 ± 0.055)·109 (74 irradiations), BCN1 = (1.883 ± 0.026)·109 (21 irradiations) and BCN2 = (2.014 ± 0.0 26)·109 (21 irradiations). Comparison of relative responses of the four dosimeters reveals that unaccounted errors exist in the response of the Co monitors and, to a lesser extent, in the response of SRM 612. The errors associated with the response of the natural uranium glasses CN1 and CN2 are represented by conventional (Poisson) “counting statistics”. These results show that attempts to calibrate a uranium glass against an activation monitor in only a small number of irradiations may produce results radically discrepant from the true value. The importance of systematic errors in neutron dosimetry for fission-track dating is also discussed. An alternative zeta (ζ) calibration approach is described, which circumvents absolute φ and λf evaluation: each dosimeter glass is calibrated repeatedly against zircon age standards from the Fish Canyon and Bishop tuffs, the Tardree rhyolite and Southern African kimberlites, to obtain empirical calibration factors ζ. The weighted mean ζ-values are 339 ± 5 for SRM 612, 113.0 ± 2.6 for CN1 and 121.0 ± 3.6 for CN2. Independent K/Ar, 40Ar/39Ar and Rb/Sr calibrating ages for the standards are discussed. For two of the three glasses, the presented ζ-values derived from each of the zircon standards are consistent within error. Compatibility of the kimberlite data with that of the other samples is discussed. Age calculation by direct comparison of track density ratios in sample and standard is rejected as grossly imprecise. Examination of the reproducibility of results from repeated measurements indicates the conventional calculation of error to be reasonable, but shows the approach of Johnson et al. (1979) to give a serious over-estimate of precision.

Standardization of fission track dating calibration: Recommendation by the Fission Track Working Group of the I.U.G.S. Subcommission on Geochronology

Problems of system calibration in fission track dating have complicated both the comparison of data from different workers and of fission track results with ages from other radioisotopic methods. Results of a Subcommission questionnaire prompted the formation of a Fission Track Working Group whose modified proposals were accepted at the 6th Fission Track Dating Workshop at Besancon France, in September 1988. Two alternative calibration approaches are recommended to all Fission Track workers: (1) for population method analyses of apatite only, an absolute calibration with selection of λf and measurement of neutron fluence; and (2) for all fission track techniques (including population methods) the zeta calibration constant approach. In both approaches, validity of calibration must be demonstrated by published analyses of age standards. A format for data presentation is recommended.

Improved modeling of fission-track annealing in apatite

Abstract Two abiding issues impact the reliability of apatite fission-track analysis and thermal history inversion in particular: reproducibility of track-length measurements and variability of annealing kinetics. In the companion to this paper, we addressed the first by demonstrating that using c-axis projection to normalize track lengths for crystallographic angle improves reproducibility among measurements acquired by Barbarand et al. (2003a, 2003b). We continue here by looking at the effect of c-axis projection on extrapolation of laboratory measurements to geological time scales. We find that c-axis projection improves agreement between predictions of empirical annealing models fit to measurements by Barbarand et al. (2003b) analysts 1 and 3, further corroborating its effectiveness in ameliorating observer bias. Furthermore, these annealing models closely match predictions from the Carlson et al. (1999) annealing data set, which was obtained by a different analyst using a different etching protocol. Normalizing for angle thus allows us to create a single annealing model that encompasses both data sets and etching techniques. By combining the different compositional varieties of apatite in the two data sets, the resulting model provides an improved basis for estimating apatite kinetic properties in unknowns using proxies such as composition, etch-figure dimension, and unit-cell parameters. Predictions from a fanning curvilinear model form accord with conventional expectations of annealing at high and low temperatures, and its use for thermal history inversion will not introduce spurious late cooling events.

A users' guide to fission track dating calibration

Abstract Misconceptions regarding the fission track dating method prompt the description of five handling procedures and the derivation of commonly used system calibrations. Problems of registration geometry, inhomogeneous uranium distribution, accumulated radiation damage and anisotropic etching largely invalidate all but the population and external detector methods, and impose strict limitations upon even their routine application. Complexities of unique, absolute thermal neutron dosimetry are illustrated by the calibration and use of the NBS pre-irradiated glass SRM 962. The 20% disparity in measured λ f decay constant values cannot be isolated from neutron fluence (φ) measurement and calibration. Frequently a calibration ratio (φ/λ f ) is evaluated against an age standard and then split into its component parts. To illustrate this interdependence of λ f and φ, zircon fission track ages, in agreement with independent K Ar ages, are obtained by calculating the same track count data with each of the preferred values of λ f (λ f = 7.03 × 10 −17 yr −1 and8.46 × 10 −17 yr −1 ) together with appropriate, selected neutron dosimetry schemes. An alternative approach is presented, formally relating unknown ages of samples to known ages of standards, either by direct comparison of standard and sample track densities, or by the repeated calibration of a glass against age standards. Practical recommendations are given for the reporting and critical assessment of fission track data.

Fission track dating of zircon: Improved etching techniques

A simple new technique for the etching of fission tracks in zircon has been developed which overcomes the practical difficulties of previously described techniques. The etchant is a low melting-point binary eutectic of KOH and NaOH which is used at 200–220°C. As for previous etchants the etching time is strongly dependent on the spontaneous track density. The etched tracks have very similar characteristics to those produced by the earlier 100N NaOH etchant.

Thermal evolution, rate of exhumation, and tectonic significance of metamorphic rocks from the floor of the Alboran extensional basin, western Mediterranean

High-grade metamorphic rocks drilled at Ocean Drilling Program Site 976 in the Alboran Sea show a PT path characterized by decompression from about 1050 MPa (40 km depth) to 350 MPa (13 km depth) accompanied by an increase in temperature from about 550°±50°C to 675°±25°C. The Ar/Ar dating on muscovite and apatite fission track analysis indicate that the final stage of exhumation and cooling occurred very rapidly in the interval 20.5–18 Ma, which coincides with the initiation of sedimentation in the Alboran Sea basin. The Alboran Sea formed by Miocene extension on the site of a Late Cretaceous? to Paleogene contractional orogen, and extension coincided with thrusting in the peripheral parts of the Betic-Rif arc, which surrounds the basin on three sides. Thermal modeling of the PT path was carried out with the aim of constraining geodynamic models for the formation of the basin. Variables considered in the modeling included (1) the thickness and thermal gradient of the postorogenic lithosphere; (2) the radiogenic heat production in the thickened crust; (3) the time gap (pause) between the end of contractional tectonics and the start of extension; (4) removal of lithospheric mantle below 125, 75, or 62.5 km; and (5) the rate of extension. The only combinations of variables that produce modeled PT paths with the observed characteristics involve high radiogenic heat production combined with a significant postcontractional pause (to produce high temperatures in rocks initially at 40 km depth), removal of lithosphere below 62.5 km (to produce further heating during decompression), extension by a factor of 3 in 6 m.y. (to delay the attainment of the maximum temperature until the rocks reached shallow depths), and final exhumation and cooling in about 3.3 m.y. (to satisfy radiometric and petrological constraints). This gives a maximum of about 9 m.y. for exhumation from 40 km depth to the surface. Lithospheric stretching in response to plate-boundary forces such as trench rollback, without removal of lithosphere, cannot explain the late onset of heating and the high temperatures reached by these rocks. Removal of lithosphere at depths significantly greater than 62.5 km cannot explain the combination of high temperatures reached by these rocks and the shallow depth at which they attained the maximum temperature. Only a combination of significant postcollisional radiogenic heating, then wholesale removal of lithospheric mantle below the orogenic crust, followed by rapid stretching can explain the observed PT path. These results appear to support some form of lithospheric delamination as the primary cause for the formation of the Alboran Sea basin.

Improved measurement of fission-track annealing in apatite using c-axis projection

Abstract Apatite fission-track length data are used extensively for thermal history inversion. However, several studies have documented instances of poor reproducibility of length data. We address this problem by using c-axis projection to normalize track lengths for crystallographic angle in the extensive laboratory annealing data set acquired by Barbarand et al. (2003a, 2003b). A new simplification reduces the c-axis projection model from six to four fitted parameters. Normalizing for track angle using c-axis projection improves every aspect of length measurement reproducibility examined. It accelerates convergence of mean length in single analyses; increases consistency among replicate measurements by a single analyst; enhances consistency of measurements of the same mounts by different analysts; and improves the match between analyses conducted with and without Cf-irradiation. C-axis projection is also shown to enhance the thermal sensitivity of length data. Based on these results, we assert that c-axis projection is a good means of compensating for observer bias, although it does not overcome differences caused by experimental error.

Resolution of the age structure of the detrital zircon populations of two Lower Cretaceous sandstones from the Weald of England by fission track dating

Modes in the frequency of distribution of fission track ages obtained from detrital zircon grains may prove characteristic of individual sandstone bodies, supporting the identification of the sources from which a particular flow of sedimentary detritus was derived and thus allowing new inferences to be made concerning palaeogeography. A computer program has been written and used to identify modes in the zircon fission track age distribution within two Lower Cretaceous sandstone samples from the Weald of southern England. Pronounced modes appear in one rock around 119 Ma, 160 Ma, 243 Ma and 309 Ma and in the other around 141 Ma, 175 Ma, 257 to 277 Ma and 394 to 453 Ma. The geological implications of these quite dissimilar zircon age spectra are discussed. It is concluded that they support the palaeogeographical models of Allen (1981) and indicate that the provenance of the first sample, from the Top Ashdown Sandstone member at Dallington in East Sussex, was almost entirely southerly, while that of the second, from the Netherside Sand member at Northchapel in West Sussex, was more varied, but predominantly westerly and northerly.

Natural long-term annealing of the zircon fission-track system in Vienna Basin deep borehole samples: constraints upon the partial annealing zone and closure temperature

Abstract Fission-track (FT) analysis of zircon from Cretaceous and Tertiary sandstone samples in Vienna Basin boreholes has provided constraints for the thermal stability of the zircon FT system over a geological timescale. Confined track lengths and ages were measured for samples from depths of ∼ 1.5-7.5 km, the deepest with a present environmental temperature of ∼ 200°C. Mean track lengths range from 10.3 to 10.8 μm, indistinguishable within error from the unannealed reference length of 10.7 ± 0.1 μm (± 1 standard error), thus showing no sign of systematic downhole reduction. Central zircon ages are consistently older than stratigraphic ages of parent rocks, with no single-grain ages significantly younger than their respective stratigraphic ages. Such FT age and length evidence strongly suggests that the tracks have not been significantly annealed since sediment deposition. Because the present geothermal regime as well as sample burial depths have been near-constant for at least the past 5 m.y. the lower temperature limit of the zircon FT partial annealing zone is > 200°C for a heating duration of the order of 5–10 m.y. Such evidence from long-term natural annealing is compatible with a zircon FT partial annealing zone cf ∼ 200–350°C derived by the extrapolation of laboratory annealing results based on onfined track length measurements, but cannot alone discriminate between different annealing models. For the zircon FT closure temperature, a perhaps over-simplified concept, these results are consistent with the oft-quoted temperature of ∼ 250°C for cooling rates of ∼ 10–100°C/m.y.

Constraints on the late thermotectonic evolution of the western Alps: EVidence for episodic rapid uplift

Forty-two new apatite and zircon fission track ages are presented for samples from the Western Alps in southern Switzerland, northern Italy, and southeastern France. Measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling, uplift, and exhumation of the Western Alps. Similar fission track zircon ages in the Penninic Gran Paradiso massif, Dent Blanche nappe, Sesia-Lanzo Zone, and Ivrea Zone indicate cooling of all four units to ∼225°C by 33 Ma. Differences in apatite ages reveal differential cooling of the four blocks between 33 Ma and the present. In the Sesia-Lanzo Zone, similarity of apatite ages regardless of elevation, together with near-volcanic confined fission track length patterns suggest rapid cooling and uplift at ∼25 Ma compared with slow cooling of other Western Alps units around 12 Ma. Uplift is thus not continuous but episodic, often over a short time interval beyond the resolution of other methods. Such episodes of uplift, as revealed here in the Sesia-Lanzo Zone, may be the rule rather than the exception.