14.6     In situ cosmogenic isotopes

 

14.6.1  Al-26 meteorite exposure ages

 

Atmospheric production of 26Al is much lower than 14C or 10Be because the progenitor, 40Ar, constitutes only about 1% of atmospheric gases. In situ production at the Earth’s surface is also low, due to atmospheric attenuation of cosmic rays. Consequently, the first measurements of cosmogenic 26Al were in studies of the Moon and meteorites. After the break-up of their parent bodies, meteorite fragments are exposed to intense cosmic-ray bombardment during their travel through space, causing substantial 26Al production. If these fragments have cosmic exposure ages of at least a few Myr, their surfaces will reach saturation in 26Al (t1/2 = 0.7 Myr).

 

            After falling to Earth, atmospheric shielding protects meteorite fragments from significant further 26Al production, and decay of this inventory can then be used to determine a terrestrial residence age. Because the abundances of 26Al in meteorites are comparatively high they do not demand accelerator mass spectrometry. Instead, they are measured using non-destructive ( counting, by putting the whole meteorite fragment in a large shielded detector. Attenuation of ( particles by the sample itself is corrected by empirical modelling (e.g. Evans et al., 1979).

 

            In the first large-scale survey of Antarctic meteorites, Evans et al. (1979) compared the 26Al activities in these samples with those in ‘falls’ (observed at impact), which have a zero terrestrial residence. The falls had a moderately well-defined range of 26Al activities for a given compositional class of meteorites (Fig. 14.48). The outlier from this class can be attributed to a failure to reach 26Al saturation, due to the short cosmic-ray exposure history of the fragment. In contrast, Antarctic meteorites ranged to substantially lower activities, indicating significant terrestrial residence ages in several cases. Unfortunately, these values are only semi-quantitative, due the relatively long half-life of 26Al, and due to uncertainties in production. The latter problem arises because of the poor penetrative capacity of the low-energy cosmic rays which generate 26Al, making the cosmic production rate susceptible to depth within the fragment.

Fig. 14.48. Histogram of 26Al activities in Antarctic meteorites, compared with American ‘falls’. After Evans et al. (1979).

 

            Although 26Al represents a good reconnaissance tool for terrestrial age determination (e.g. Evans and Reeves, 1987), 36Cl provides a more precise method (Nishiizumi et al., 1979). This arises from its shorter (0.3 Myr) half-life, and from more accurately known saturation values, due to its generation by penetrative high-energy cosmic rays. However, the analysis is more technically demanding, and must be performed by accelerator mass spectrometry. Results of a large 36Cl study of Antarctic meteorite ages are shown in Fig. 14.49 (Nishiizumi et al., 1989a).

Fig. 14.49. Histograms of calculated terrestrial residence age for Antarctic meteorites from the Allan Hills and Yamato Mountains areas, based on the decay of cosmogenic 36Cl. After Nishiizumi et al. (1989a).

 

            The high quality of terrestrial 36Cl ages for Antarctic meteorites has led to their use to determine the half-life of another cosmogenic nuclide, 41Ca. With a half-life of only 0.1 Myr, this shows promise as a precise dating tool, but an efficient AMS analysis method has only recently been developed (Fink et al. 1990). Another problem limiting the application of 41Ca has been uncertainty in the half-life. To solve this problem, Klein et al. (1991) performed 41Ca analyses on aliquots of Antarctic iron meteorites which had already been dated by 36Cl. The results revealed a strong linear correlation between the abundance of the two species, whose slope corresponds to the ratio of the half-lives. Taking a 36Cl half-life of 301 " 4 kyr yields a precise 41Ca half-life of 103 " 7 kyr.

 

 

14.6.2  Al–Be terrestrial exposure ages

 

Because of atmospheric attenuation of cosmic rays, most terrestrial materials have 26Al/27Al ratios less than 10!14. However, in some aluminium-poor minerals such a quartz, the content of (non-cosmogenic) 27Al may be as low as few ppm, so that after a few thousand years of exposure to cosmic rays, 26Al/27Al ratios of 10!11 to 10!13 may be generated, within the measurement range of AMS. These data can then be used to calculate exposure ages of terrestrial rock surfaces.

 

            The principal obstacle to AMS analysis of 26Al is the formation of sufficient negative Al ions during sputtering, which is only about 25% efficient (Middleton and Klein, 1987). The metal species must be used rather than the oxide because the latter suffers a severe interference from MgO. However, Mg does not form negative metal ions at all, so there are no isobaric interferences on the Al metal-ion signal.

 

            The principal application of 26Al as a geochronometer is in the measurement of rock exposure ages. It would be possible to use this nuclide alone for this purpose, but in view of the many possible permutations of exposure and erosion history, the use of two nuclides with different half-lives provides a more powerful constraint on these models. The normal choice is to combine 26Al measurements (t1/2 = 0.705 Myr) with 10Be (t1/2 = 1.51 Myr).

 

            The atmospheric 26Al/10Be production ratio has been determined as about 4 H 10!3 by sampling from high-flying aircraft, but the in situ production ratio in quartz has been measured as 6 (Nishiizumi et al., 1989b). Because the atmospheric 10Be production rate is comparatively high, great care must be taken to ensure that rock samples to be used for exposure dating are not contaminated by the atmospheric or so-called ‘garden variety’ of 10Be (Nishiizumi et al., 1986). Because of its resistance to chemical weathering, quartz is comparatively resistant to contamination by ‘garden variety’ 10Be. This, along with its low 27Al content makes it an excellent material for exposure dating. In quartz, 10Be is derived from spallation of 16O, while 26Al is produced by spallation of 28Si and mu-meson capture by the same species. Most of this production occurs in the top half-metre of the rock surface, but limited  26Al production can occur at depths of up to 10 m (Middleton and Klein, 1987).

 

            There are two limiting models for the interpretation of surface exposure data (e.g. Nishiizumi et al., 1991a). These are illustrated on a plot of 26Al/10Be ratio against absolute 10Be abundance (Fig. 14.50). The upper curve shows the effect of increasing exposure age, for the case where the erosion rate is zero. The lower curve shows the effect of different steady-state erosion rates, for the case where exposure age is infinite (relative to the half-lives of 26Al and 10Be). At the point of saturation (after a few half-lives), the 26Al/10Be ratio is 2.88. Unfortunately, the limited (factor of two) difference between the half-lives of 26Al and 10Be causes only a relatively small separation between the curves for the two end-member models. This places limits on the resolving power of the Al–Be method, given the relatively large analytical errors on the AMS measurements.

Fig. 14.50. Plot of analysed 26Al/10Be against 10Be abundance (corrected to production at sea-level) for Allan Hills quartz samples, used as a measure of minimum exposure age and/or maximum erosion rate. After Nishiizumi et al. (1991a).

 

            Application of the Al)Be exposure method is demonstrated in Fig. 14.50 using data from nunataks of the Allan Hills area of Antarctica (Nishiizumi et al., 1991a). The results display a range of Al/Be ratios close to the steady-state erosion curve. However, the zero-erosion-rate (variable exposure age) model cannot be ruled out. The lowest 26Al/10Be yields the strongest constraint, representing a minimum exposure age of 1.4 Myr or a maximum erosion rate of 0.24 mm/kyr. Samples to the left of the erosion line in Fig. 14.50 may be explained by burial under ice for some period in the past. During times of burial, points move downwards to the left, due to the greater rate of decay of 26Al relative to 10Be.

 

            The combination of these two cosmogenic isotopes can also be used to date sediment burial, as demonstrated by Granger et al. (2001) in a dating study on cave sediments from Mammoth Cave, Kentucky. Sediments entered this cave system from the Green River, a tributary of the Ohio River, which cut down into a karst landscape. There is a very strong likelihood that sediments were exposed on the land surface prior to their transport and deposition in the cave system. Therefore, quartz grains in the sediment are expected to have reached steady state abundances of cosmogenic Al and Be, with a 26Al/10Be saturation ratio of 6 (Fig. 14.51).

 

            After burial in the cave system, radioactive decay begins, and the samples follow a trend to the lower left in Fig. 14.51. This allows the dating of sediments at different levels in the cave system, so that the down-cutting of the river can be traced against time. Projection of the decay trend back to the steady state line also allows the erosion rate of the surficial sediment source to be estimated at around 3 m / Myr. The combination of 26Al and 10Be is particularly suitable for burial dating because the pre-burial cosmogenic production of the two nuclides occurs under similar conditions. Accurate modelling of isotope production during the surface exposure history of the sediment is a prerequisite to obtaining accurate burial ages (Granger and Muzikar, 2001).

Fig. 14.51. Plot of analysed 26Al/10Be against 10Be in sediments from different levels in Mammoth Cave, Kentucky. Vertical displacements allow the dating of cave deposits from their burial ages, while the intersection with the exposure curve indicates an erosion rate of 3 mm/kyr for the sediment source. After Granger et al. (2001).

 

 

14.6.3  Chlorine-36 exposure ages

 

The concept of exposure dating using in situ produced cosmogenic 36Cl was suggested by Davis and Schaeffer (1955), but could not be effectively applied until the advent of AMS analysis (Phillips et al., 1986). Although developed after the 10Be and 26Al methods, 36Cl offers several advantages. It is applicable to a variety of rock types because it is generated from three parents (K, Ca, and Cl) with different chemistry, while most rocks have low background levels of chlorine. Furthermore, interferences by nucleogenic 36Cl are minimal, and contamination from atmospheric 36Cl is only a problem in severely weathered material.

 

            The three main sources of in situ cosmogenic 36Cl are neutron activation of 35Cl and neutron-induced spallation of 40K and 40Ca. A subordinate source is from negative muon capture by 40Ca. The relative importance of these production routes depends on the relative K: Ca: Cl abundances in the target and the degree of shielding by overlying rock. Zreda et al. (1991) made an empirical study of spallation production, while Liu et al. (1994) studied the depth dependence of the neutron activation reaction. Spallation reactions are caused by fast neutrons, which decrease exponentially with depth. However, activation reactions require (slow) thermal neutrons, which are produced when fast neutrons undergo glancing collisions with substrate atoms. Hence, slow neutrons reach a peak intensity at about 15 cm depth in rocks (Fig. 14.52). The occurrence of multiple routes makes the calculation of total production rates more complex, but it also offers the possibility of greater age control, as will be shown below.

Fig. 14.52. Calculated production profiles for 36Cl by spallation and neutron activation (normalised to equal values at the surface) as a function of rock mass per unit area. After Liu et al. (1994).

 

            The simplest scenario for cosmogenic dating is the instantaneous transport of rock from below the cosmic-ray penetration depth to the surface, followed by exposure without shielding by other material (such as snow cover) and without significant erosion. In this case, all production routes for 36Cl can be summed. However, corrections must be made for the latitude-dependence of cosmic-ray intensity and the altitude-dependence of atmospheric shielding.

 

            A relatively simple scenario is provided by surface-exposure dating of rocks at Meteor Crater, Arizona (Nishiizumi et al., 1991b). Samples were taken from the upper few cm of large blocks in the ejecta blanket of the impact. The lithology of these blocks shows that they were from strata buried at depths of over 10 m before the impact. On the other hand, their large size suggests that they were unroofed of any overlying ash blanket soon after the impact event. The surfaces of the sampled ejecta blocks were found to be coated with ‘rock varnish’, which takes thousands of years to develop. Therefore, erosion was probably negligible in the arid climate of Arizona, so that 36Cl abundances translate directly into an exposure age. Good agreement was found between 36Cl ages and AlBe ages, with a consensus of ages around 50 kyr. A few younger ages (e.g. Monument rock) may indicate more recent exposure of these blocks above the ash blanket.

 

            Another type of simple scenario is achieved in relatively young lava flows, which are instantaneously exposed at the surface and have not yet suffered significant weathering. However, in many geological environments, erosion is a significant factor. In this case, a single cosmogenic isotope determination only allows the solution of exposure age at known erosion rate or erosion rate at known exposure age. For example, using a lava flow from Nevada of known age, Shepard et al. (1995) were able to estimate the weathering rate based on 36Cl/35Cl ratios.

 

            A more complex scenario arises when both the exposure age and the erosion rate of a deposit are unknown. This applies particularly to rapidly eroding sedimentary deposits. To investigate the constraints which may be applied to such a system, Zreda et al. (1994) considered a theoretical model for the erosional exposure of a deposit with buried clasts. These clasts, initially buried within the deposit, are gradually exposed at the surface by erosion of the fine- grained matrix (Fig. 14.53). This model applies to the problem of dating glacial moraines based on the exposure ages of boulders on the moraine surface, and also to dating meteorite impacts based on blocks exposed on the ejecta blanket. Zreda et al. argued that if 36Cl dates are available from several different boulders on a moraine or ejecta blanket, the spread of ages (outside analytical error) could be used to model the exposure history of the deposit.

Fig. 14.53. Schematic illustration of the exhumation of clasts from a heterogeneous deposit by erosion of the matrix. After Zreda et al. (1994).

 

            To demonstrate these effects, Zreda and Phillips (1995) modelled 36Cl/Cl ratios for several boulders buried at depths from zero to 300 g/cm2 (approximately 1.5 m depth in soil of density equal to 2). Total 36Cl production was attributed 50% to spallation reactions and 50% to neutron activation. The erosion rate was set so that the deepest boulder just reaches the surface at the present day. The modelling results show a wide range of 36Cl/Cl ratios, both below and slightly above the growth curve for zero depth (Fig 14.54). The growth curves for deeply buried boulders start with slow production, due to the shielding effect of the overlying matrix. As the boulder approaches the surface, the rate of production accelerates, but the total 36Cl inventory remains well below that of a surface sample. On the other hand, boulders which were initially subject to shallow burial can actually show greater total 36Cl production than at the surface, due to the peak of neutron activation production at a depth of 50 g/cm2.

Fig. 14.54. Modelled 36Cl/Cl inventories for rock boulders buried as various depths in an eroding deposit, showing peak cosmogenic production just below the surface. After Zreda and Phillips (1995).

 

            Zreda and Phillips (1995) modelled the erosion history of ejecta blocks from Meteor Crater, Arizona, in a similar way. However, in this case the spread of apparent exposure ages was based on 1000 model points randomly buried from zero to a chosen maximum depth (Fig. 14.55). The model results were compared with the actual spread of ages in four blocks at Meteor Crater, with a mean age of 49.7 " 0.9 kyr (Monument Rock was excluded). The best fit was obtained by assuming burial to a maximum depth of 300 g/cm2 (ca. 1.5 m), removed at 30 g/cm2/kyr (0.15 mm/yr), so that all boulders reached the surface within 10 kyr. Such modelling cannot yield a unique solution to the erosion process, and the model results should ideally be compared with a larger set of measured ages. Nevertheless, the modelling does suggest that the four blocks yield exposure ages close to the estimated time of impact.

Fig. 14.55. Histogram of model 36Cl exposure ages for 1000 rock clasts exhumed from depths up to ca. 1.5 m in a deposit with a formation age of 50 kyr. A few samples over-estimate the real age due to greater cosmogenic production just below the surface. After Zreda and Phillips (1995).

 

            In principal, the use of two spallogenic nuclides (e.g. 10Be and 26Al) allows the deconvolution of exposure ages and erosion rates. However, as noted above, the closely spaced trajectories of the constant erosion and the constant exposure saturation lines make the deconvolution weak (e.g. Fig. 14.39). A more powerful application of the 36Cl technique makes use of the neutron activation route to 36Cl, in comparison with the purely spallation nuclides (10Be or 26Al). Because spallation and neutron activation yield peak nuclide production at different depths in a geological surface, they should yield a clear resolution of exposure histories involving different rates of erosion (Liu et al., 1994). With this objective in mind, Bierman et al. (1995) described a method for isolating the neutron activation component of 36Cl, by releasing chlorine from fluid inclusions within rock samples. However, the method has not been demonstrated in practice. Alternatively, a crude assessment of the rate of erosion can be obtained by simply ignoring spallogenic 36Cl production.

 

            This approach was taken by Phillips et al. (1997) in a study on glacial deposits of the Wind River Range, Wyoming. Ten boulders from glacial moraines were analysed for both 36Cl and 10Be, and are plotted on the ClBe plot in Fig. 14.56. The limits for zero erosion and for infinite age (erosional equilibrium) are given, as in the AlBe plot. However, they are now much further apart, allowing easy resolution between the two models. Most of the data lie below the line corresponding to 0.5 mm of erosion per kyr, indicating that the surfaces of these boulders are not undergoing significant loss by erosion. The boulders were sampled from six different moraines, and their ages indicate that one moraine dates from the last glaciation (ca. 20 kyr), three from the penultimate glaciation (ca. 130 100 kyr) and two from older glaciations, assuming rapid exposure of the boulders on the moraine surface.

Fig. 14.56. Plot of analysed 36Cl/10Be against 36Cl abundance for boulders from glacial moraines of the Wind River Range, Wyoming. Solid lines indicate erosions rates; dashed lines are exposure age isochrons. See text for discussion. After Phillips et al. (1997).

 

            A different application of the 36Cl method was proposed by Stone et al. (1996), using 40Ca spallation as a tool for exposure dating of calcite. The abundance of the target nuclide, along with the relatively low abundance of chlorine in calcite, makes this an analytically favourable method which shows great promise for exposure dating of karst landscapes. For example, two recent studies (Zreda and Noller, 1998; Mitchell et al., 2001) used 36Cl abundances in fault scarps to date the displacement of fault systems in carbonate rocks. In both cases, 36Cl measurements were made at several points down the face of the fault scarp. 36Cl accumulation was then used to determine the date when each sample was first exposed at the surface, allowing a reconstruction of past movement on the fault.

 

 

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