10.3 Laser probe dating

10.3.1 Method development

The application of the laser probe to K)Ar dating is now becoming an important technique, but surprisingly, the method was slow in development. Megrue (1967) pioneered the use of laser ablation for rare gas analysis, but did not apply the method to geochronology until six years later (Megrue, 1973). This study made use of the laser probe in order to date small clasts in a polymict lunar breccia. After activation, spots 100 :m in diameter were irradiated with single pulses from a ruby laser. Each pulse ablated a pit about 30 :m deep, equivalent to about 1 :g of rock, representing a miniature total-fusion analysis of the exposed surface. The aggregate gas fraction from several nearby spots was gettered and cryogenically trapped, before admission to the mass spectrometer for analysis. (Typical equipment is shown in Fig. 10.24). Analysis of ten different clasts revealed two arrays of data on a K)Ar isochron diagram with ages of approximately 3.7 and 2.9 Byr.

Fig. 10.24. Schematic illustration of laser ablation Ar)Ar dating equipment. After York et al. (1981).

York et al. (1981) developed the laser microprobe technique by showing that a de-focussed continuous-wave laser could be used to perform step heating analysis in a manner analogous to conventional 40)39 dating. The technique was demonstrated on a whole-rock sample of slate from the Kidd Creek mine, near Timmins, Ontario. The laser beam was focussed to generate a spot 0.6 mm in diameter, which caused progressive argon release from the surface after a few minutes, using a 1 watt power setting. The laser step heating analysis produced results consistent with conventional step heating of the same sample, representing the timing of a thermal event which opened the K)Ar system in the slate.

The low sensitivity of the MS-10 mass spectrometer used by York et al. (1981) limited application of the method, but in subsequent development a purpose-built continuous laser system was coupled to a high-sensitivity mass spectrometer. Layer et al. (1987) tested this system by analysing the hornblende standard Hb3GR. This is known from previous step heating analysis (Turner, 1971a) to yield a perfect plateau age (Fig. 10.25a). After activation, single grains up to 0.5 mm across were heated within the laser beam for 30 seconds at increasing power levels. After each heating episode, argon was gettered and then analysed. Excellent plateaus were generated (e.g. Fig. 10.25b), and the integrated release ages fell within error of the conventional step heating result.

Fig. 10.25. Step heating results for the Hb3GR standard. a) Conventional; b) laser single grain. Quoted ages are average (integrated release) ages. After Layer et al. (1987).

Laser probe dating was developed using continuous wave infra-red lasers, either de-focussed for step heating, or focussed for spot analysis. These lasers are effective for heating most samples, but cannot be focussed below a spot size of 50 :m, and are not effective for analysing pale coloured minerals such as feldspar. To overcome these problems, ultra-violet lasers have been introduced (e.g. Kelley et al., 1994). Ultra-violet laser light is obtained by frequency doubling, which is only possible using pulsed lasers. The power available with such a system is much lower, but it is effective for spot ablation because the energy is more efficiently concentrated in a 10 :m diameter spot. The ultra-violet laser ablation microprobe (UVLAMP) offers the opportunity for in situ analysis of thin sections with high spatial precision, both laterally and with depth in the sample. Kelley et al. demonstrated the depth resolution by using a rastered beam to ablate successive 2 :m deep steps into the surface of a K-feldspar grain. The resulting isotopic depth profile shows the diffusion of atmospheric argon into the surface of the grain (Fig. 10.26). The method shows great potential for detailed studies of argon diffusion in minerals (section 10.5.2).

Fig. 10.26. UVLAMP depth profiling measurements of total ⁴⁰Ar concentration and radiogenic ⁴⁰Ar in a K-feldspar grain. The data fit a model of radiogenic Ar diffusion out of the grain and atmospheric Ar diffusion into the grain, except for a high value at the surface probably due to adsorption. After Kelley et al. (1994).

10.3.2 Application of laser probe dating

In order to test the laser step heating method on a slowly-cooled geological system, Layer et al. (1987) analysed biotites from the Trout Lake batholith, NW Ontario. Laser step heating of a small (0.25 mm) biotite grain yielded an age of 2600 Myr which was identical to a conventional step heating analysis on 13 mg of biotite. However, laser step heating of a large (1 mm) biotite from the same hand-specimen yielded a significantly older age. The total release age for this grain (2654 " 5 Myr) was closer to the U)Pb intrusive age for the batholith of 2699 " 2 Myr. This shows that the larger biotites probably closed earlier during metamorphic cooling.

Wright et al. (1991) developed this study on the Trout Lake batholith by using the laser step heating method to date a range of single biotite grains of various sizes. Only grains having a regular shape similar to a thin cylinder were analysed. After measurement of the grain radii and activation in the reactor, each specimen was subjected to laser step heating analysis, during which the whole grain was bathed in the laser beam at increasing intensities. For samples displaying normal plateaus, their integrated ages were plotted against grain-size (Fig. 10.27). The results display a positive correlation between grain-size (cylindrical radius) and integrated age. This was attributed to diffusional argon loss during the original cooling history of the batholith. Wright et al. speculated that the scatter of data points to the right of the main array might represent large grains which were either damaged during sample crushing or consisted of natural aggregates of smaller sub-domains.

Fig. 10.27. Plot of integrated release ages against grain-size for single biotite grains from the Trout Lake batholith, Ontario. The plateau is attributed to Ar loss from sub-domains within large biotite grains. After Wright et al. (1991).

The positive correlation in Fig. 10.27 is explained by the larger surface area / volume ratio of smaller grains, resulting in a lower effective blocking temperature than for larger grains. Geological determinations of Ar diffusion in biotite (e.g. Onstott et al., 1989) can be used to calculate the size dependence of blocking temperature (0.1 mm = 275 ^oC; 0.23 mm = 295 ^oC). Hence, the array in Fig. 10.27 translates into a cooling curve for the batholith, of temperature against time. This yields a calculated cooling rate between 295 and 275 ^oC of about 0.33 ^oC/Myr. The high-temperature cooling curve of the pluton can be calculated between the older biotite ages and the U)Pb zircon age of 2700 Myr (with a blocking temperature estimated at around 750 ^oC). This segment of the cooling curve is much steeper, at around 5 ^oC/Myr.

Lee et al. (1990) tested the laser step heating method on biotite and hornblende grains which had suffered a thermal disturbance long after initial cooling. The sample consisted of baked Archean gneiss adjacent to an Early Proterozoic dyke, and both minerals were analysed by three methods: conventional step heating; single grain laser step heating; and laser spot dating. Biotite ages for the three methods clustered closely around 2050 Myr, interpreted as the time of dyke intrusion. On the other hand, hornblende produced very different results from the three techniques. Conventional step heating of a multi-grain population and laser spot dating generated very variable ages (Fig. 10.28), whereas laser step heating generated a good plateau with an apparent age of 2430 Myr. However, this does not correspond to a known geological event.

Fig. 10.28. Comparison of spot and step heating ages for a disturbed hornblende sample. a) profile of laser spot ages across a single grain; b) laser and conventional step heating profiles. After Lee et al. (1990; 1991).

Lee et al. (1991) speculated that the plateau could result from mixing of argon from different domains in the mineral before release. Heating experiments on the hornblende standard Mmhb-1 showed that argon was released in three principal pulses (Fig. 10.29). The first of these, at 930 ^oC, was correlated with the onset of structural breakdown at the margins of grains. However, the main phase of breakdown occurred at 1050 ^oC, forming a strong fabric parallel to cleavage and accompanied by the breakdown of titanite lamellae in the crystal. Finally, at 1130 ^oC the grains melted. The laser step heating plateau in Fig. 10.28 was formed by argon release between 960 and 1250 ^oC, suggesting that it may result from argon homogenisation in the grain during structural breakdown. Therefore, although laser step heating is a powerful technique, it is necessary to check data from disturbed systems by a second technique such as laser spot dating or laser depth profiling (e.g. Roberts et al., 2001).

Fig. 10.29. Argon release pattern observed in response to heating of the hornblende standard Mmhb)1. After Lee et al. (1991).

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