AGE DIAGNOSTIC ISOTOPES

Written By Aubrey Whymark 2013-2017

Argon-Argon (40Ar-39Ar) method

What does it measure?

The age the rock/tektite was last melted.

The fundamentals:

Since the rock was last melted, which theoretically expelled all pre-existing argon, the 40K (half life 1,250 Ma) decays to 40Ar in the sample. The sample is irradiated with fast neutrons by the researcher and the 39K, which is the stable form of potassium, forms 39Ar. 39Ar has a very short half-life of 269 years and is therefore naturally absent in samples. The 39Ar produced is used as an indicator of potassium content. Measurements of four Ar isotopes (36Ar, 37Ar, 39Ar, 40Ar) are taken and the ratios of argon are used to calculate the date, after applying corrections and with reference to known age standards. (Alden, 2011, and Bottomley et al., 1990).

What can be concluded?

Dating Tektites

Australasian tektites have been dated between 0.690 ±0.028 Ma and 0.887 ±0.034 Ma (Storzer et al., 1984). Many studies have come up with ages in the region of  ±0.785 Ma, although Yamei et al. (2000) considered the best representative age to be 0.803 Ma. The date will likely be refined as the decay constant for 40K is refined. In Chapter 4 the author concluded the impact may have occurred between 783 and 818 ka, with 788 ka being the favoured date, for reasons discussed elsewhere on this web site. A list of dates obtained can be found below and summarised in the figure to the right:
Using isotopic ratios to date the tektite sample directly is just one method of dating the event. Astrochronology tied into oxygen isotopes from foraminifera in deep sea cores indicates which decay constant is more likely. Layers of volcanic ash above and below the tektite event can be dated in deep sea cores with sedimentation rate estimates used to more accurately define the age. Micropalaeontology can also be used to determine a broad age. By utilising a number of methods a more accurate date can be established as everything comes into agreement (and where not in agreement then the source of error - maybe the decay constant - can be investigated).
 
RIGHT: Ar-Ar age of tektites. Dot represents calculated age. Line represents error bar. Details can be found below.
Storzer et al. (1984) recorded an 40Ar-39Ar age of 0.887 ±0.034 Ma for a single australite and 0.690 ±0.028 Ma for a single indochinite. The difference in age is likely a consequence of the different thermal history of australites and indochinites.

Izett and Obradovich (1992) obtain a mean age for Australasian tektites of 0.77 ±0.02 Ma. This date was recalculated to 0.783 ±0.021 Ma by Yamei et al. (2000) based on new standards by P. R. Renne et al., Chem. Geol. 145, 117 (1998).

Kunz et al. (1995) analysed four australites and four indochinites and yielded a mean age of 0.786 ±0.012 Ma

Yamei et al. (2000) calculated ages ranging from 0.761 ±0.017 Ma to 0.816 ±0.007 Ma, with the best representative age considered to be 0.803 ±0.003 Ma

Trieloff et al. (2007) determined the mean plateau ages of four Asian tektites at 0.784 ±0.009 Ma, four australites at 0.790 ±0.016 Ma and inferred a single tektite forming event at 0.786 ±0.013 Ma. This work is related to Kunz et al. (1995).

Discussion:

The 40Ar-39Ar method is the standard modern dating method for tektites. It improves on the K-Ar method as a realistic age can be obtained by using argon isotope ratios whereas the conventional K-Ar method requires the extraction of all of the Argon from a sample to obtain a realistic age (Izett and Obradovich, 1992). This method can also be used to date volcanic rocks, such as ash deposits or lava flows. The relative stratigraphic positions enhance the control on the age. The biggest uncertainty appears to derive from uncertainty in the decay constant of 40K.

Potassium-Argon (40K-40Ar) method

What does it measure?

Resetting of the clock

The age the rock/tektite was last melted.

The fundamentals:

One must be aware that heating up a tektite will potentially partially or fully reset the atomic clock. This may occur due to natural forest fires or from human activity. It will result in a younger apparent age.
 
(Simplified). Since the rock was last melted, which theoretically expelled all pre-existing argon, the 40K (half life 1,250 Ma) decays to 40Ar in the sample. The rock is melted by the researcher and a precise amount of 38Ar is added to the gas for calibration purposes. Measurements of three Argon isotopes are made (36Ar, 38Ar & 40Ar). The amount 36Ar determines the amount of atmospheric argon present and an air correction is made to the 38Ar and 40Ar values. The amount of added 38Ar is known and therefore the amount of 40Ar can be determined by comparison to it (Alden, 2011). Meanwhile, the amount of 40K is calculated. This is usually done indirectly by measuring the amount of 39K and using the accepted ratio of 40K/39K. The amount of parent isotope (40K) that has decayed to the daughter isotope (40Ar) is established and, as the half life is known, the age of the sample can be calculated.

What can be concluded?

Australasian tektites formed 0.86 ±0.06 million years before present (McDoughgall and Lovering, 1969).

Discussion:

The potassium-argon (K-Ar or 40K-40Ar) method is a favourable, although superseded, method to date tektites. This is because the 40K decays to 40Ar, which is a gas. When the tektite formed, the rock was melted and the previously accumulated argon was outgassed or lost. This effectively reset the clock at the time the tektite formed. The K-Ar date, assuming experimental procedure was correct and all the argon was extracted from the sample, should be used as a maximum age in case not all the argon was outgassed when the tektite formed.

K-Ar method requires the extraction of all of the Argon from a sample to obtain a realistic age (Izett and Obradovich, 1992). Failure to extract all the argon, which requires sufficient temperatures and time-spans, results in an under-estimate of the age. This was a problem when this method was first used. The K-Ar method has now largely been superseded by the Ar-Ar method that relies on ratios rather than absolute values and is therefore more reliable.

Fission tracks of 238U

What does it measure?

Fission tracks of uranium isotope 238U measure the solidification age of the tektite (Fleischer & Price, 1964). During re-entry australites are ablated and form a flange. The flange can be dated using this method and compared with the body of the australite to establish whether there is a significant time gap between formation and entry through the atmosphere (Fleischer & Price, 1964). This was an important step in establishing whether tektites had spent any significant time in space. The clock may be reset or partially reset by subsequent thermal events, such as a bush fire in the Australian outback. Therefore any age estimate will be a minimum age and, assuming correct experimental procedure, will not be an over-estimate of age.

The fundamentals:

Two thin polished slices of the tektite are made. Firstly the 238U content of the sample must be calculated. This is derived indirectly by measuring the amount of 235U in the sample. One of the tektite slices and a standard glass of known uranium content are irradiated with thermal neutrons, which induce fission of the 235U. The two tektite glass and the standard glass surfaces are etched for around 30 seconds in hydrofluoric acid to enhance the fission tracks. The fission tracks are counted under an optical microscope.

The irradiated tektite comprises spontaneous 238U and induced 235U fission tracks. The untouched tektite yields only spontaneous 238U fission tracks. The standard glass of known uranium content yields induced 235U tracks. The number of induced 235U tracks in the tektite glass can be determined by deducting the number of spontaneous 238U tracks, as seen in the untouched tektite slice. The 235U content can then be determined by comparison with the standard glass. The 238U content can then be calculated based on known 238U/235U ratios. With a knowledge of the number of expected tracks produced per unit of time at the calculated 238U concentration the age of the tektite can be calculated. (Walker, 2005; Fleischer & Price, 1964; Yagi, Kuroda and Koshimizu, 1982).

What can be concluded?

Gentner et al. (1969) calculates fission track ages of australites to be between 0.11 to 0.71 Ma. Fleischer and Price (1964) with the decay constant corrected (Gentner et al., 1969) dated 8 australites between 0.09 and 0.65 Ma. It is probable that the more recent ages are from thermally altered tektites. These thermally lowered fission track ages can be corrected because the mean diameter of the etched fission tracks is reduced in these thermally altered specimens. The mean diameter reduction of the fission track is plotted against the reduction degree of track density for various glasses. A correction can then be applied which indicates a mean age of 0.7 Ma for australites (Gentner et al., 1969).

Calculated fission track ages can be visualised to the right and are detailed below.
RIGHT: Fission track ages of tektites. Dot represents calculated age. Line represents error bar.
Gentner et al. (1969) calculated the age of Muong Nong-type impact glasses as between 0.61 and 0.72 Ma, with no age correction factor applied.

Fleischer & Price (1964) concluded there was no significant track density difference between an australite body and its flange. Therefore the formation age and ablation age were essentially the same.

Gentner et al. (1970) dated deep sea Australasian tektites to 0.71 ±0.01 Ma. 

Storzer and Wagner (1980) calculated the age of Australites as 0.82 ±0.05 Ma and the age of Indochinites and Philippinites as 0.74 ±0.05 Ma, suggesting two separate tektite forming events.

Yagi et al. (1982) reported an average fission track age of six Muong Nong-type impact glasses from Laos, Vietnam and Thailand plus on splashform indochinite from Vietnam as 0.67 Ma. Ages ranged from 0.65 to 0.70 Ma.

Kashkarov et al. (1985) studied 31 tektites from Vietnam and concluded three different age groups of 0.47 ±0.02 Ma, 0.62 ±0.02 Ma and 0.81 ±0.04 Ma.

Yan et al. (1988) analysed four splashform tektites from Hainan Island, China and determined fission track ages of 0.68 ±0.06 Ma, 0.72 ±0.06 Ma, 0.70 ±0.06 Ma and 0.73 ±0.06 Ma.

Bigazzi and de Michele (1996) determined the age of a Muong Nong-type impact glass from Guang-Dong as 0.61 ±0.05 Ma.

The age of Australasian tektites, as determined by the fission track method, therefore appears to be between 0.61 and 0.81 Ma. The probability is that fission track dating may under-estimate age and, so long as experimental data is correct, should not over-estimate age. This would certainly lean towards an age of 0.70 to 0.81 Ma.

References