R.D. Russell

ISOTOPIC CONSTITUTION OF RADIOGENIC LEADS AND THE MEASUREMENT OF GEOLOGICAL TIME

Isotopic abundances are reported for lead extracted from 1 galena and 96 uranium minerals. The radiogenic leads are mostly from the Athabasca province of the Canadian Shield. Chemical analyses are given for 9 of the uranium-bearing minerals. Ages have been calculated from the radiogenic Pb 207 /Pb 206 ratios for all samples with sufficiently low common lead contamination, and from the Pb 206 /U 238 ratios for the chemically analyzed samples. These have been used to establish that the Athabasca province was being actively mountain built from 1860 to 1630 million years ago, but not subsequently. It is suggested that the many younger ages determined for the pitchblende deposits in that region are correct and indicate solution and redeposition of the deposits at a later time. The method of extracting and preparing small quantities of lead as lead tetramethyl for analysis in a mass spectrometer is described.

Dating galenas by means of their isotopic constitutions—II

Abstract Recently a rather large number of analyses of common lead samples has become available, and these in turn have led to a better understanding of the precise nature of the physical and geological processes resulting in the observed variations of lead isotope ratios. Specifically, there now appear to be three classes of leads, each of which has lead isotope ratios that vary in a remarkably simple pattern. These are meteoritic leads, anomalous leads, and leads from certain conformable lead ore deposits. The meteoritic leads and conformable leads both appear to have developed in surroundings where thorium and uranium had very similar properties, and in both cases the thorium to uranium ratio corresponds to a present value of 3 · 73 ± 0·03. This is taken to be evidence that both classes developed under predominantly reducing conditions. In contrast, the presence of thorium to uranium ratio present during the production of anomalous leads is extremely variable, suggesting an oxidizing environment. It has been suggested that all vein leads may be anomalous to some degree and therefore should be interpreted with this possibility in mind. The methods for dating galenas proposed previously by the present writers are re-examined in view of these developments and it is shown that they may lead to more explicit information on the age and history of lead ores than has usually been obtained.

Gas source mass spectrometry of trace leads from Sudbury, Ontario

Abstract The measurement of lead isotope abundances with a gas source mass spectrometer has been limited to lead ores and minerals. A free radical technique for the preparation of lead tetramethyl is described which has reduced by a substantial amount the sample size necessary for a precise gas source analysis. In this way, the range of minerals which may be studied by means of a gas source mass spectrometer has been greatly extended. The free radical method has been applied to the study of lead isotope abundance variations in the mining district of Sudbury, Ontario. The analysis of lead from various sulphides has yielded lead isotope ratios which are linearly related on a plot of Pb207/Pb204 vs. Pb206/Pb204 with a slope of 0.131 ± 0.003; the standard deviation of points from their best straight line is 0.37 per cent of the average Pb207/Pb204 value. (The intercomparison technique previously used by us to determine the ratios of ordinary leads was not used for these analyses.) It is concluded that the linear relationship is the result of just two geological events, which fact simplifies the possible interpretations. The maximum times of these two events are 21.30 ± 50 m.y. and 1280 ± 50 m.y. respectively. Two interpretations of the results are suggested. The first gives ages of 1950 and 350 m.y. for the primary and secondary events. The two events in the second interpretation are considered to have occurred 1600 and 950 m.y. ag. The second interpretation is more easily defended on geological grounds.

The argon-40 content of the atmosphere and the age of the earth

Abstract Applying the theory that the continents havo been formed by the addition of volcanic material from depth during geologic time, and assuming that this material released all of its radiogenic argon-40 while it was in the molten state, an age of the earth has been calculated from the argon-40 content of the earths atmosphere. Using current estimates of the potassium content of the crustal rocks, volume of the continents, and the argon-40 content of the atmosphere, we find that all the argon-40 in the atmosphere could have been released in this way if the earth were 5,300 million years old. This value is in reasonable agreement with other estimates of the age of the earth and of the universe. It is therefore concluded that this is a reasonable explanation for the derivation of the earths atmospheric argon.

Additional regularities among discordant lead-uranium ages

Abstract The ratios N 206 N 238 and N 207 N 235 are linearly related in twelve samples of pitchblende from the west ore body of the Ace Mine, Goldfields, Saskatchewan. A similar relationship has been noted before ( Ahrens , 1955b) in monazites and uraninites mainly from southern Africa. Wetherill (1956a and b) has shown that such linearity could be explained by a chemical removal of lead from the mineral at some single time subsequent to its formation. It is pointed out that such an explanation leads to an apparent relationship between the time of chemical removal of lead and the age of the mineral. There appears to be a preferential deficiency (9%) of Pb207 over Pb206 in specimens participating in the linear relationships. This suggests physical control of lead escape rather than chemical. One possible process involves recoil of atoms following alpha-particle emission. Whatever the correct explanation, we find a convergent age of 1800 m.y. for the west ore body of the Ace Mine.

Discrimination in solid source lead isotope abundance measurement

Abstract Solid source isotope analysis of lead is subject to errors resulting from isotopic fractionation and other discriminations. Simple theories predict the pattern of fractionation and, in particular, predict that replicate analyses of a sample should lie along lines of slope1.5 y/x, for they−x plot and2.0 z/x for thez−x plot (wherex = 206 Pb/ 204 Pb, y = 207 Pb/ 204 Pb andz = 208 Pb/ 204 Pb). Because we were beginning to use double spiking for lead isotope analyses, which technique assumes the above fractionation pattern as the only source of discrimination, we studied the discrimination in our mass spectrometer. New single rhenium filament analyses are presented for which the numerical values of the slopes are(1.32 ± 0.04)y/x and(1.53 ± 0.05)z/x respectively. The new data include single and triple filament analyses of the same sample. The uncertainties (standard deviation) of the slopes are small enough to demonstrate the significance of the differences between observed and theoretical values. This precluded the use of the theoretical fractionation slopes for double spiking with rhenium filaments. On the other hand, it appears that the theoretical slopes serve adequately as the basis for double spike correction of lead isotope data produced with tantalum filaments. It is found difficult to postulate a discrimination process that will adequately explain the discrepancy between the observed and predicted slopes. Moreover, it is difficult to know whether the observed discrepancies can be generalized from mass spectrometer to mass spectrometer.

Further applications of concordia plots to rock lead isotope abundances

Abstract This paper pursues the application of concordia-type plots to the interpretation of the isotopic abundances of lead in rocks and minerals, as proposed by Ulrych [1]. Additional examples are examined, including leads from Balmat, New York, and from the Vesuvius, Italy. In all cases the data are obtained from the published literature. It is pointed out that the procedures can be extended to evaluate the 238 U/ 204 Pb ratios for the first stage of the two stage model assumed. The values thus obtained for the data available are in surprisingly good agreement and, when normalized to the same value for the age of the earth, give a mean value for the eight suites of samples equal to 8.70±0.12. This value is compared with values found by Ostic, Russell and Stanton [2] and it is concluded that any systematic difference might be accounted for by discrimination in the mass spectrometer techniques.

Isotopic abundances of lead from a “frequently-mixed” source

Abstract Isotope ratios have been calculated for hypothetical leads as they would have developed in a succession of systems of random composition, each closed with respect to transfer of uranium, thorium and lead. The resulting ratios cannot be approximated by a single-stage model, but are reasonably well described by a two-stage anomalous lead model. This observation leads to important consequences in lead abundance interpretations. In order to relate the study to an existing geological environment, the numerical values chosen were based on the anomalous leads from Broken Hill, Australia. The accepted interpretation for this suite of leads, which is well supported by independent argon-potassium and strontium-rubidium measurements, incorporates a two-stage model with the stages occupying the periods 4500 m.y. to 1600 m.y., and 1600 m.y. to 510 m.y. This model was simulated with a digital computer and compared with more complex models in which additional events were introduced between 1600 m.y. and 510 m.y. The greatest number of events considered was ten. In every stage but the first, the238U/204Pb ratio for each sample was drawn at random from a lognormal distribution with mean value corresponding to 8.98 and 67% of the values lying between one-third and three times this value. In none of the cases did the calculated isotope ratios lie along primary isochrons, but they all did lie approximately along anomalous lead lines (secondary isochrons). Among other conclusions, this result implies that the assumption of frequent mixing of their source materials through sequential geologic processes is not sufficient to justify the interpretation of lead isotope ratios of individual samples on the basis of a single-stage model.

Analysis of Canadian long shot data

Abstract A pilot analysis of the Project Long Shot nuclear explosion data received by the Canadian seismic stations indicates a compressional first arrival as expected from an impulsive explosion source except at one station. At all stations the P phase arrived earlier than predicted with the largest residuals at the most distant sites. A comparison earthquake in the Rat Islands area indicates a similar bias trend. The average unified magnitude and standard deviation for all Canadian stations is shown to be 6.01 ± 0.40 which agrees well with the worldwide average determination of 5.99 ± 0.52. Surface waves, recorded at 4 stations, gave magnitudes from 4.9 to 5.2. The P wave amplitudes appear to be anomalously low in central British Columbia and high in eastern Canada. Spectral investigations show both common and individual characteristics among the stations.

A nomograph for the interpretation of anomalous lead isotope abundances

This paper describes the construction of a simple nomograph for finding solutions for t1 and t2 in the formula, R = (eλ′t1 − eλ′t2)α(eλt1 − eglt2) which is useful for interpreting anomalous lead isotope abundances. The nomograph does not require interpolation. Formulae are given for the slope of the best line through the measured isotope ratios and its standard deviation. These assume equal errors in the Pb206Pb204 and Pb207Pb204 ratios.