10.4 Timescale calibration

The high precision that is obtainable in K–Ar dating, particularly using the ⁴⁰Ar)³⁹Ar step heating method or the laser probe method, makes this method very useful for timescale calibration. This applies particularly to the Tertiary period, where K–Ar dating can exceed the precision of U–Pb zircon geochronology. The following sections will review some of the most important applications.

10.4.1 The magnetic reversal timescale

One of the most important applications of the K)Ar method has been to calibrate the magnetic reversal time-scale defined by sea floor magnetic anomaly ‘stripes’. The amount of ocean floor material recovered which is fresh (unaltered) enough for dating is limited, so most attention has been focussed on dating terrestrial sections (such as basic lavas) which yield a good magnetostratigraphy. The K)Ar method is really the only geochronometer capable of dating young basic rocks. Since its establishment, the reversal time-scale has been subject to almost continuous revision, and some landmarks are reviewed here.

Pioneering work was performed by Cox et al. (1963) on 0 ) 3 Myr-old lavas from California, and by McDougall and Tarling (1964) on 0 ) 3 Myr-old lavas from the Hawaiian islands. Cox et al. used K)Ar dates on sanidine, obsidian, biotite, and whole-rocks, while McDougall and Tarling worked on basalt whole-rocks. Good agreement between the two data sets confirmed that the reversal timescale is due to world-wide changes in the polarity of the Earth’s magnetic field, rather than post-crystallisation alteration phenomena, as had been suggested by some workers.

A comprehensive compilation of data for 354 terrestrial lavas (mostly from ocean islands) was used by Mankinen and Dalrymple (1979) to constrain the polarity timescale for the last 5 Myr more precisely, using the new K)Ar decay constants (Steiger and Jager, 1977). Not all of the available data were in perfect agreement; therefore Mankinen and Dalrymple used a statistical technique to calculate the most probable ages of the three most recent polarity epoch boundaries, such that the standard deviation of apparent dating inconsistencies was minimised (Fig. 10.30).

Fig. 10.30. Standard deviation of apparent dating inconsistencies as a function of ‘trial’ values for polarity boundaries. The best estimate of each boundary age is where error is a minimum. After Mankinen and Dalrymple (1979).

Unfortunately, the geological record of terrestrial lavas is too fragmented to extend the technique of detailed epoch-boundary dating back beyond 5 Myr. Therefore, Heirtzler et al. (1968) used sea-floor magnetic anomaly patterns to extend the terrestrial timescale to ca. 80 Myr by extrapolation. They took as a fixed point an age of 3.35 Myr for the epoch boundary between Gilbert (reversed) and Gauss (normal), based on the Sierra Nevada data of Doell et al. (1966). Heirtzler et al. extrapolated from the present day, through the Gilbert)Gauss point, to calibrate the older part of the time-scale, by assuming a constant spreading rate for the South Atlantic ridge over the last 80 Myr. This may seem a crude assumption, but Heirtzler et al. justified it on the basis of the good correlation between S Atlantic and N Pacific spreading rates.

During the next two decades, improvements to the reversal timescale were achieved by adding additional fixed points. LaBrecque et al. (1977) made the first major revision by using two fixed points to avoid the extreme extrapolation of Heirtzler et al. (1968). The younger point was again the Gilbert (R) ) Gauss (N) boundary, while the older (marine anomaly 29) was tied by magnetostratigraphy of Paleocene limestones near Gubbio, Italy, to the Cretaceous)Tertiary (K)T) boundary. Tying the reversal timescale to the geological column was advantageous in harmonising the timescales, but problematical in that it revealed the poor constraints on the K)T boundary itself. For example, sources cited by LaBrecque et al. (1977) for a K)T boundary age of 65 Myr were a poster published by Elsevier (van Eysinga, 1975) and a paper by van Hinte (1976). The latter was merely a citation of Berggren (1972), whose only constraints were a minimum of 57.1 " 3 Myr from Belgium and a maximum of 68.1 " 4 Myr from SE England. Fortunately the 65 Myr age for the K)T boundary has proved so robust in the long run that it has not even been affected by changes in the decay constants used! (This is a coincidence of course).

The next major step in calibrating the reversal timescale was made by Lowrie and Alvarez (1981), who used magnetostratigraphy in the 25 ) 130 Myr-old Gubbio limestones of Italy to interpolate between eleven fixed points. However, the large number of fixed points generated a kinked line, due to errors in the absolute age calibration of some points. Harland et al. (1982) ironed out some of these inflections to produce a useful ‘working timescale’ (Fig. 10.31). However, Harland’s approach had probably now reached its technical limit, since the absolute ages of the fixed points, based mainly on K)Ar glauconite ages, were not reliable enough for more accurate calibration.

Fig. 10.31. Diagram to show the fixed points which constrain the reversal time-scales of LaBrecque et al. (LKC)77, €); Lowrie and Alvarez (LA)81, Q ); and Harland et al. (H)82, 9). Inset shows calculated consequences for spreading rate on the South Atlantic Ridge. After Harland et al. (1982).

10.4.2 The astronomical timescale

Johnson (1982) proposed a completely new approach to the calibration of the geomagnetic polarity timescale, based on planetary mechanics (Milankovich forcing, section 12.4.2). According to this theory, small variations in the Earth’s orbit have led to variations in the intensity of solar radiation reaching the Earth, which were responsible for the glacial–interglacial cycles of the Quaternary period. These glacial cycles caused variations in the oxygen isotope composition of seawater which were recorded in fossil forams.

Because the Earth’s orbital variations can be projected back into the past very accurately, it is possible to ‘tune’ the oxygen isotope record in deep sea cores with a precision of better than 1%. Based on the coherent patterns in two high-quality cores from the western Pacific, Johnson was able to date the Brunhes–Matuyama magnetic reversal signature, which was preserved in sediment 7.25 m below the surface of core V28-239 (Fig. 10.32). He estimated a date of 790 kyr for the boundary, with a probable uncertainty of less than 5 kyr. However, this work did not receive much attention because of the large discrepancy between this date and the younger value of 730 kyr determined by Mankinen and Dalrymple (1979).

Fig. 10.32. Section of a DSDP core displaying the Brunhes–Matuyama reversal boundary. This is dated by tuning oxygen isotope variations (caused by glacial cycles) to the history of northern hemisphere insolation, calculated from orbital mechanics. Arrows show suggested correlation points. After Johnson (1982).

The publication of a new high-resolution oxygen isotope record for the Pleistocene (Shackleton et al., 1990) focussed attention back onto the discrepancy between the astronomical and K–Ar calibrations. A slight revision of the orbital tuning calculation placed the Brunhes– Matuyama boundary at 780 kyr and suggested that the date of Mankinen and Dalrymple (1979) might indeed be in error. Therefore, several high resolution K–Ar and ⁴⁰Ar)³⁹Ar studies were undertaken to test the age of this and other important reversal boundaries. The first of these studies were conducted simultaneously by Baksi et al. (1992), Spell and McDougall (1992) and Tauxe et al. (1992).

Baksi et al. (1992) calibrated the Brunhes–Matuyama boundary using ⁴⁰Ar)³⁹Ar ages of whole-rock samples of basalt lava, exposed in the caldera wall of Haleakala volcano on the island of Maui. The magnetic signature of the section had previously been studied, and gives such detailed coverage of the reversal boundary that several flows actually have magnetic signatures which are transitional between normal and reversed polarities. Samples were analysed in two different labs, yielding consistent results with an average age for the reversal boundary of 783 " 11 kyr, in excellent agreement with the astronomical calibration.

Tauxe et al. (1992) made a re-examination of the data analysis of Mankinen and Dalrymple (1979). The K–Ar ages used to constrain the Brunhes–Matuyama boundary exhibited significant internal disagreement (Fig. 10.33), as was shown in fact by the ‘error function’ plot of Mankinen and Dalrymple (Fig. 10.30). The failure of the error function to reach a zero value should have been a warning that the raw data were unreliable, but the scientific community (including the present author) was ‘bewitched’ by the statistical treatment of the data. In hindsight we can see that the low ages for the reversed (Matuyama) chron were probably caused by argon loss. In such cases, the appropriate remedy is to collect new high precision data, rather than to statistically filter old data. Therefore, Spell and McDougall (1992) performed a new dating study on the Valles Caldera, New Mexico, where the anomalous samples had come from. They used the ⁴⁰Ar–³⁹Ar laser fusion technique to date sanidine crystals from the critical section, and obtained older ages, confirming that the Brunhes–Matuyama boundary should be located at 780 " 10 kyr.

Fig. 10.33. Chart showing raw data used by Mankinen and Dalrymple to date the Brunhes–Matuyama reversal boundary. The best-fit age for the boundary was biassed downwards by some data (boxed) which was evidently affected by argon loss. Modified after Tauxe et al. (1992).

The use of the astronomical timescale reached a mature stage in 1994 when it was used to back calibrate the Fish Canyon sanidine standard, one of the primary ⁴⁰Ar/³⁹Ar dating standards (Renne et al., 1994). Sanidine crystals from Fish Canyon tuff had been used in several earlier studies to calibrate Ar–Ar ages for six important reversal boundaries. However, with the advent of a precise astronomical calibration of these boundaries, it was possible to reverse the calibration in order to determine an absolute geological age of 27.95 " 0.18 Myr for the eruption of the Fish Canyon tuff.

Absolute ages cannot be determined directly by the Ar–Ar method because a standard must always be used to calculate the neutron flux. However, high precision conventional K–Ar ages on undisturbed material allow absolute calibration of Ar–Ar standards such as Fish Canyon sanidine. Intercalibration studies by Baksi et al. (1996) and Renne et al. (1998b) were based on K–Ar dating of three older standards (SB3, GA-1550, and GHC-305). The resulting calibration ages for Fish Canyon tuff were 27.95, 28.02 and 28.15 Myr respectively, using the recommended ⁴⁰K half-life. All of these values fall within error of the astronomical calibration, but the median value is probably most reliable.

10.4.3 Intercalibration of decay constants

Despite the great advances made in K–Ar and Ar–Ar dating in the past few years, there remains one outstanding problem in the use of this method for high precision dating of older rocks. This is the observation that calibrated Ar–Ar ages are consistently slightly younger than U–Pb ages for the same rock unit. This has raised the question of whether the decay constant determination of Beckinsale and Gale (1969), adopted by Steiger and Jager (1977) as a recommended value, is significantly in error.

U–Pb dating is regarded as the ‘gold standard’ for geochronology (Lugmair, section 5.2.2), because the uranium decay constants are based on the best laboratory determinations, and because of the built-in check for disturbance. However, even U–Pb ages have non-negligible uncertainties arising from decay-constant errors, and these now exceed the analytical uncertainty of high-precision ages. Despite these difficulties, high-precision measurements have been made on a few ‘geologically ideal’ samples to test for concordance between U–Pb and Ar–Ar ages. Some of these results are presented in Table 10.1. All errors are quoted at the 2F level, and include uncertainties in the decay constants. Unfortunately, it can be seen that, although the Ar–Ar ages are about 1% less than the U–Pb age on each occasion, these two results are actually within error if the uncertainties in the decay constants are fully propagated into the age determination. This problem is complicated by the branched nature of ⁴⁰K decay, which means that two different decay constants have to be considered in any geological calibration.

Table 10.1 Comparison between U–Pb and Ar–Ar ages of ‘ideal’ samples

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Material U–Pb age +/– Ar–Ar age +/– Reference

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fish Canyon tuff 28.4 0.1 28.0 0.4 Renne et al. (1998b)

Siberian traps 251.3 1 250.0 4 Renne et al. (1998a)

Palisade rhyolite 1098 4 1088 15 Min et al. (2000)

Acapulco meteorite 4554 12 4507 50 Renne (2000)

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

As part of their study of this problem, Min et al. (2000) also reviewed the counting experiments of Beckinsale and Gale (1969) and compared them with two other determinations. These values are compared in Fig. 10.34 (after some minor corrections to the Beckinsale and Gale value adopted by Steiger and Jager, 1977). The results show that divergence between the three curves is not really significant for ages below 1 Byr, although it is quite large for very old rocks. Unfortunately, it remains unclear whether the young K–Ar age for the Acapulco meteorite is due to an error in the ⁴⁰K half-life or due to slow cooling of the meteorite from 4.55 to 4.50 Byr ago (Trieloff et al., 2001; Renne, 2001).

Fig. 10.34. Effect on calculated geological ages of using alternative experimentally-determined potassium decay constants. Deviations are very small below 1 Byr. After Min et al. (2000).

Recently, Schmitz and Bowring (2001) made a new U–Pb dating study on the Fish Canyon tuff, which confirmed the U–Pb age quoted in Table 10.1. However, the discrepancy between the U–Pb and K–Ar ages may not be due to an error in the decay constant. Hence, it is concluded that the ⁴⁰K decay constant of Steiger and Jager should probably still be used until more data are obtained to resolve this problem.

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