Experimental evidence for the pressure dependence of fission track annealing in apatite

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Abstract

Fission track analysis has seen a major expansion in application to general geological problems reflecting its advances in understanding the temperature dependence of track annealing and track length distributions. However, considerable uncertainties still persist, in particular concerning the stability of fission tracks subjected to the interaction of environmental physical parameters (e.g. pressure, temperature, stress) and in extrapolation of laboratory data to geological time scales. In this work, we studied the fading behavior of spontaneous fission tracks in basic apatite [hexagonal Ca5(PO4)3(OH, F, Cl)] when exposed simultaneously to laboratory pressures, temperatures and stress over varying time spans. The experiments showed that track fading is a complex recovery mechanism, which is extremely sensitive to the coupling of these three parameters. In particular, a strong decrease in the fission track fading rate was observed as a function of increasing pressure. And a nearly temperature-independent dramatic increase in fission track recovery was observed as a function of stress. Consequently, (1) the stability field of fission tracks in apatite increases towards temperatures higher than 110°C depending on the absolute pressure; (2) closure ages in apatite are underestimated (>100% for an ideal geothermobarometric gradient); (3) related exhumation and erosion rates are overestimated above the closure temperature and underestimated below the closure temperature; and (4) since the widely applied statistical description of thermally induced fading kinetics does not account for the influences of either pressure or stress and is based on fission track annealing data produced at ambient pressure, the accuracy in extrapolating fission track data to geological time scales and in their application to dynamical systems must be cast into doubt.

Introduction

The constant and known rates of radioactive nuclear decay mean that the age of a mineral can be determined from ratios of the concentrations of parent to daughter elements. In track dating there is direct optical counting of chemically etched fission fragment tracks. Prior to etching latent tracks are generally complex linear trails of radiation damage which may be intermittent, lying on fragment trajectories, and consisting of point and extended defects, and/or new phases (e.g. amorphous material) in which a fragment’s dissipated kinetic energy is stored [1], [2], [3], [4].

Concerns about dating methods in general come from such factors as the possible gain or loss of the daughter element at some point in the history of the mineral (e.g. stability of the daughter product with respect to the geological environment). Other important factors include chemical composition and stoichiometry, crystal structure, and detailed technical mineral specifications [5], [6], [7], [8], [9], [10]. By far the most important experimentally verified influential factor for the case of fission tracks from 238U, however, has been temperature, and this especially for minerals used most commonly in geological applications. The general concept of a closure temperature – a specific temperature below which a daughter product (or track length) becomes ‘frozen’ in a crystal – was adopted for many radiometric systems [11]. Yet despite the recognition of temperature as a crucial parameter other potentially influential important physical parameters (e.g. pressure and stress) were taken to be negligible. The fact that the basic theory of atomic diffusion requires there to be an exponential decrease in the intrinsic diffusion coefficient with increasing pressure [8], [12] was ignored.

In materials science, experiments on self diffusion in metals were conducted before the beginning of the 1970s, confirming basic theoretical concepts for lead [13], [14] and zinc [15]. Unclear results for other metals (e.g. silver and uranium [16]) were attributed to their polycrystalline character. Diffusion in the alkali and noble metals [17] seemed to be independent of pressure. In the late 1960s and the 1970s, the annealing of fission tracks in minerals and glass (e.g. zircon and tectite [18], [19]) at that time poorly understood as a complicated diffusion-enhanced process, was qualitatively investigated under pressure and stress. The results again indicated that temperature was the main driving force for fission track annealing. This early work was followed, e.g., by the description of a minor pressure dependence of argon diffusion in phlogopite [20], and more recently a non-linear pressure dependency of the diffusion of argon was discovered in rhyolitic glasses/melts [21]. Moreover, diffusional mechanisms under pressure were constrained in order to define the closure temperature of geothermometers based on Fe–Mg exchanges in olivine [22]. However, no systematic investigation of the influence of pressure and stress on diffusional mechanisms and recovery-rates in minerals relevant to geological dating systems has yet been reported.

This clear absence of reliable data on the pressure and stress dependencies of basic recovery processes governing those dating systems which involve crystalline annealing, or which are based on gaseous daughter products, prompted us to establish an experimental program for the quantitative study of the annealing systematics of fission tracks in minerals. We describe here results obtained for fission track annealing in apatite – one of the most commonly used minerals in geochronology – under pressure, temperature and stress.

Spontaneous fission tracks are nearly exclusively formed by the natural decay of 238U present in crystalline or amorphous structures. When the tracks form, they are oriented randomly in space. Age determinations are based on a two-dimensional sampling of tracks intersecting an internal surface of the material, (e.g. by determining the density of fission tracks per cm2 compared to the absolute amount of 238U in the crystal). The probability of a shortened track intersecting a randomly selected surface is lower than the probability of a long track intersecting the same surface. Thus, since the fission track age is determined by counting the number of tracks intersecting a surface, an older age is expected for a sample with longer tracks [5]. Based on this straightforward relationship of track density with track length, many experimental determinations of track stability were carried out over the past 20 years. A major purpose of studies of the evolution of track lengths as a function primarily of temperature and, secondarily, of time, was of course the development of realistic models leading to reliable extrapolation of the laboratory-derived length evolution data to geological time spans [23], [24], [25], [26].

Here we measured the lengths and the density of etched spontaneous fission tracks in several apatite specimens, of different provenance and of different chemical composition. The samples were subjected (1) to various pressures P at constant temperature T and (2) to various temperatures and evolving compressional stress σ at constant pressure. In any of the PT experiments, only one parameter was systematically changed in order to isolate the effect of each variable. Morphologically, no differences were observed between naturally annealed fission tracks and those fission tracks annealed under P, T and σ (Fig. 1, Fig. 7), so that fission track analyses were performed using standard procedures.

We preferred, for two reasons, to investigate the annealing behavior of spontaneous tracks instead of using induced tracks as usually measured for most experimental T annealing studies [5], [9], [10], [25], [26]. The calculation for an exhumation path plus the calculation of exhumation and erosions rates are based on the analyses of spontaneous tracks in natural samples. This in turn might well introduce errors due to differences in the nature of the activated defects in induced and spontaneous tracks and their corresponding mobilities and lifetimes. Additionally, it is very probable that the self-annealing of freshly induced tracks is more active before final trapping of the defects than it is the case for ancient spontaneous tracks. Experiments on induced tracks might therefore not reflect the ‘true’ annealing behavior of spontaneous tracks. We are currently performing a series of P–T experiments on induced tracks in order to quantify the basic differences in their annealing behavior compared to the annealing of spontaneous tracks.

Section snippets

Experimental materials: description and preparation

Four types of apatite single crystals (length ≈30 mm, diameter ≈15 mm) of different composition were used for the experiments: a dark blue apatite from Siberia (Sludjanka), a light blue apatite from Canada, a dark green apatite from Madagascar and a light yellow–green apatite from Mexico (Durango). The relative fluorine/chlorine composition of the samples varied as follows: Siberia – 1:0.29, Canada – 1:0.08, Mexico – 1:0.27, Madagascar – 1:0.12. The mean initial FT length in all four types of

Experimental arrangement

Nearly 50 successful experiments were performed in studying the following PT couplings: (1) 0.1 MPa, 250°C; (2) 0.1 MPa, 500°C; (3) 100 MPa, 250°C; (4) 200 MPa, 250°C; (5) 300 MPa, 250°C; (6) 600 MPa, 500°C; (7) 800 MPa, 500°C; and (8) 2000 MPa, 500°C. The experimental run times were fixed individually between 17 and 2200 h (Table 1). Depending on the experimental conditions, different apparatuses (furnaces, externally heated pressure vessels, internally heated pressures vessels, solid-medium

Sample preparation after treatment

The samples were sometimes broken after the experiments due to decompression, but did not change color. Both initial and final apatites (e.g. run products) were embedded in epoxy resin and ground on 1000 mesh SiC papers to expose internal crystal surfaces. These were then polished on a Struers Rotopol-22 apparatus equipped with a Rotoforce-4 sample holder, using 3 and 1/4 μm diamond pastes. Approximately 200 μm of material were removed from the crystal surface in order to avoid blurring of the

Experimental results

Fig. 1 shows two optical photomicrographs of pressure-annealed fission tracks compared to the unannealed sample. Morphologically, no differences could be observed between the naturally annealed sample, the T-, the PT-, and the P–T–σ-annealed samples, so that in all cases, fission track analyses were performed using the standard procedure already described.

Discussion

Pressure experiments similar to those described here were earlier conducted on crystals of zircon [27]. In these experiments, pressure-dependent annealing was not observed and the resulting recovery behavior was similar to annealing at atmospheric pressure [28]. This observation on zircon, together with our results for apatites of different compositions, seems to establish experimentally that fission track recovery kinetics are dependent upon the specific material properties of each type of

Conclusion

In this work, we have demonstrated that the annealing of fission tracks in apatite is extremely pressure and stress sensitive. In their present form applications of current annealing models to geological problems are limited and introduce significant errors in the derivation of closure ages, exhumation paths, and of exhumation and erosion rates. This in turn underlines the need for a deeper understanding of the basic physics of diffusion kinetics in geological applications generally, including

Acknowledgements

We thank B. Goffé for providing the experimental apparatuses for performing experiments at pressure and temperature, and E. Rutter for providing the Heard rig to carry out experiments at pressure, temperature and stress. The supervision of the deformation experiments by R. Holloway is gratefully acknowledged. We also thank M. Brunel, F. Brunet, A. Carter, K. Gallagher, A. Hurford, R. Jonkheere, D. Mainprice, J.-F. Ritz and G. Wagner for interesting and extended discussions and technical

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    Present address: LGCA, Université J. Fourier, 1381 rue de la Piscine, 38041 Grenoble, France.

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