Quaternary International, Vol. 47/48, pp. 161—163, 1998. Q 1998 INQUA/Elsevier Science Ltd All rights reserved. Printed in Great Britain. PII: S1040 –6182(97)00083–9 1040— 6182/98 $19.00
PROBLEMS AND PERSPECTIVES OF RADIOMETRIC AGE DETERMINATIONS IN QUATERNARY SAMPLES Giorgio Ferrara Istituto di Geocronologia e Geochimica isotopica, CNR, Via Cardinale Maffi, 36, Pisa, Italy
The age determinations produced by the three categories of methods are subject to a certain degree of uncertainty; this uncertainty increases from Group 1 to Group 3, because the number of assumptions one must make to apply the methods correctly increases from Group 1 to Group 3. This means that the best radiometric data will be furnished by the methods of Group 1, which, however, are unable to cover all of the Quaternary.
The improvements made in high-sensitivity, highprecision mass spectrometers in the past 20 years have made it possible to carry radiometric age determination methods based on the radioactive decay of naturally occurring radionuclides to extreme limits. Among other things, it has become possible to date minute samples and measure very low ages (less than 106 yr). Indeed, the lower limit of radiometric age determination no longer depends upon the instrumental techniques, but rather upon the sample itself, i.e. upon the degree to which the initial conditions of the sample’s formation fulfil the theoretical conditions that allow the result of the measurement of the experimental parameters to represent a true age value. There are three basic categories of radiometric age determination methods, which are based on (1) long-lived radionuclides; (2) nuclides produced by cosmic rays; (3) the effects of radioactive decay processes on the surrounding materials. The methods of Group 1 employ radionuclides with long half-lives and are those used most commonly in geological studies that require radiometric age determinations of rocks and minerals. They can be applied to materials as old as the earth itself, a fact that makes it difficult to employ them for measuring very recent ages; only the K/Ar and, in some cases, the Rb/Sr methods can be used to determine the ages of samples less than a million years old. The radioactiveradiogenic nuclides of the U and Th chains can also be used (the 238U—234U—230Th—232Th systematics are thoroughly described in another paper in this volume). While the methods of Group 1 employ radionuclides of terrestrial origin, those of Group 2 employ stable and radioactive nuclides of cosmic origin, which are produced by interactions between cosmic radiation and the atmosphere or terrestrial rocks and which are present in minute amounts. The most important methods of Group 3 are fissiontracks (FT) and thermo-luminescence (TL). Unlike the methods mentioned above, these techniques do not measure the radioactive isotopes or their daughter products directly, but rather make indirect measurements based on numerous assumptions regarding the initial conditions of the system and, sometimes, its subsequent history.
K/Ar Potassium is one of the 10 most abundant elements in terrestrial rocks and occurs in many common igneous, metamorphic and sedimentary minerals. This fact, coupled with 40K’s relatively short half-life (1250 Ma), makes the K/Ar method the most commonly used method for dating recent and extremely recent rocks. With the 40Ar/39Ar variant method that employs the single crystal laser fusion technique, one can measure ages as recent as 35,000 yr (Deino et al., 1992), ages that practically overlap the range of 14C. The 40Ar/39Ar method offers two advantages: the age distribution of single grains allows the identification of contaminant crystals (Lo Bello et al., 1987; Feraud et al., 1990) and the effects of air contamination are considerably reduced with respect to the bulk fusion methods. Lower limits of 5000 yr have been reached with the ‘unspiked’ (Cassignol) method (Gillot and Cornette, 1986), even though one must allow for the possible presence of 40Ar in excess (40Ar ). Mineral 94 phases that crystallized at shallow depths during the upwelling of the magma should be used, rather than whole rocks and minerals that crystallized early, and which frequently contain 40Ar (Jaeger et al., 1985). 94 Good results have been obtained from sanidine, groundmass, and residual glass (Gillot et al., 1982). 40Ar can, however, also be present in sanidines and 94 leucites, as Villa (1992) demonstrated for the potassic volcanites of the Roman Magmatic Province. In this sort of situation the 39Ar/40Ar method must be used to determine the amount of 40Ar present, and then, in 94 most cases, a correct age can be obtained (Laurenzi and Villa, 1987). Deino et al. (1992) do not mention the presence of excess argon in their discussion of the dating of a 161
Campanian ignimbrite using single crystal laser fusion. This is not an a priori assumption and cannot be taken as a rule.
Rb/Sr In principle, the long half-life of 87Rb (4.8]109 a) makes the method unsuitable for the determination of very young ages. Only minerals with very high 87Rb/86Sr ratios ('100) can give results in the 100,000 yr range, as was demonstrated by Radicati di Brozolo et al. (1981) for the tufo di Villa Senni Formation in the Alban Hills (Italy). As Wendt (1985) demonstrated, in theory, ages between 20 and 40 thousand years can be measured under such conditions. To use the two minerals isochron, one must assume that the two minerals are in isotopic equilibrium, i.e. that the initial 87Sr/86Sr ratios of the two minerals were identical. This is frequently not the case, especially in anatectic crustal rocks (Ferrara et al., 1989). The Rb/Sr method is thus limited to very special cases in which minerals with very high Rb/Sr ratios are present, and Sr isotopic equilibrium can be verified.
COSMIC-RAYS-PRODUCED RADIONUCLIDES Among the radionuclides produced by cosmic rays, 14C is of fundamental importance for dating very recent activity when organic material is available. An excellent review of the new possibilities offered by using the 14C method with acceleration mass spectrometry has been recently published in this journal by Liunick et al. (1989). The recent discovery (Kurz et al., 1990; Craig and Poreda, 1986) of cosmogenic helium in terrestrial surface rocks represents a promising possibility for dating volcanic samples, because 3He is stable and can be used for ages up to a million years. Since it is one of the most frequently produced cosmogenic nuclides, the detec-
tion limits of modern rare gas mass spectrometers make it possible to use it to measure ages as low as a thousand years. Olivine and pyroxene have been used for these studies, and, at least until now, seem to be the only minerals suitable for these measurements. Only a few age determinations have been performed to date with this method because the production rate of cosmogenic 3He through time is insufficiently known (Kurz et al., 1990; Cerling and Craig, 1994). In 1990, Cerling used the method to date geomorphological surfaces, and more recently, Anthony and Poths (1992) obtained a series of 3He surface exposure dates ranging from 20 to 85 thousand years, with good reproducibility, from a Quaternary lava field. As they note, the 3He surface exposure technique ‘appears to hold substantial promise for the dating of young lavas’. Another important means by which Quaternary geochronology can be improved is to perform cross-calibration studies among the different methods, with the goal of discovering the limits of the various techniques. Gillot and Cornette (1986) compared the data obtained from the unspiked K/Ar method with those obtained from the 14C and TL methods. The K/Ar method yielded slightly older ages than the 14C method, a fact they interpret to mean that the 14C production rate fluctuates over time. Recently, Bard et al. (1990) compared very precise U/Th ages from Barbados corals with 14C ages for the same material, and observed the same discrepancy for the same time span (12—25,000 yr). Thus, the K/Ar and U/Th methods can be quite useful in calibrating the 14C geochronometer to cover periods '8,000 yr, which are beyond the limits of dendrochronology. For ages greater than 50,000 yr, Gillot and Cornette (1986) found differences between the K/Ar and TL ages of the samples; the TL ages were systematically lower than the K/Ar ones, probably as a consequence of a slight, anomalous fading of the TL age values. Hurford (1986) compared the results furnished by the different K/Ar techniques and the fission track technique in a study of the Pleistocene Bishop Tuff
TABLE 1 Method
K/Ar (spiked) K/Ar (unspiked) 39Ar94—40Ar (bulk) 39Ar94—40Ar (s. grain) Rb/Sr U/Th chains
Le, Sa, Grm, Bi, Phl, Wr, Pl Le, Sa, Grm, Pl Le, Sa, Grm (Bi, Phl, Wr) Sa, Pl Le, Sa, Gl Ca, corals, various minerals
5]104—AQt 2.5]103—AQt 5]104—AQt 3.5]104—AQt 4]104—AQt 2]102—5]105
C, wood, bones, palaeosols Ol, Px
Qz, Gl, Pl, Zr Zr, Sph, Ap, Gl
Do not resolve for 40Ar94 Providing no 40Ar94 Can resolve 40Ar94 Can resolve for contaminant crystals and 40Ar94 Inferior limit only with Rb/Sr'100 Inferior limit only using i.d. mass spectrometer (Edwards et al., 1986) Only 1 mg C using accelerator mass spectrometer Surface exposure. Still some uncertainties on 3He cosmic produced rates Possible fading effect for superior limit Strongly depending on U content of materials
Ap"Apatite; Bi"Biotite; Ca"Calcite; Gl"Glass; Grm"Groundmass; Le"Leucite; Ol"Olivine; Phl"Phlogopite; Pl"Plagioclase; Px"Pyroxene; Qz"Quartz; Sa"Sanidine; Sph"Sphene; Wr"Whole Rock; Zr"Zircon.
Problems and Perspectives of Radiometric Age Determinations in Quaternary Samples
Formation (Long Valley, California). The data obtained with the conventional (spiked) technique from sanidine, plagioclase, biotite, and glass (mean age 0.737 Ma) agree perfectly with those from 40Ar/39Ar studies of sanidine (0.734 Ma) and fission track studies of zircons (0.73—0.75 Ma). REFERENCES Anthony, E.Y. and Poths, J. (1992). 3He surface exposure dating and its implications for magma evolution in the Potrillo volcanic field, Rio Grande Rift, New Mexico (USA). Geochimica Cosmochimica Acta, 56, 4105—4108. Bard, E., Hamelin, B., Fairbanks, R.G. and Zuidler, A. (1990). Calibration of the 14C timescale over the past 30,000 years using mass spectrometric U/Th ages from Barbados cards. Nature, 345, 405—410. Cerling, T.E. (1990). Dating geomorphologic surfaces using cosmogenic 3He. Quaternary Research, 33, 148—156. Cerling, T.E. and Craig, H. (1994). Cosmogenic 3He production rates from 39°N to 46°N latitude, western USA and France. Geochimica Cosmochimica Acta, 58, 249—255. Craig, H. and Poreda, R.J. (1986). Cosmogenic 3He in terrestrial rocks: the summit lavas of Maui. Proceedings of National Academy of Sciences, ºSA, 83, 1970 —1974. Deino, A., Curtis, G. and Rosi, M. (1992). 40Ar/39Ar dating of the Campanian Ignimbrite, Campanian Region, Italy, 29th International Geological Congress, Kyoto, 24 August—3 September, 1992, Abstracts, p. 633. Edwards, R.L., Chen, J.H. and Wasserburg, G.J. (1986). 238U—234U— 230Th—232Th systematics and the precise measurement of time over the past 500.000 yr. Earth and Planetary Science ¸etters, 81, 175—192. Feraud, G., Lo Bello, Ph., Hall, C.M., Cantagrel, J.M., York, D. and Bernart, M. (1990). Direct dating of Plio Quaternary pumices by 40Ar/39Ar step-heating and sample grain laser fusion methods: the example of the Monts-Dore massif (Massif Central, France). I. »olcanol. Geoth. Res., 40, 39—53. Ferrara, G., Petrini, R., Serri, G. and Tonarini, S. (1989). Petrology and isotope geochemistry of San Vincenzo rhyolites (Tuscany, Italy). Bull. »olcanol, 51, 379—388. Gillot, P.Y. and Cornette, Y. (1986). The Cassignol technique for potassium argon dating, precision and accuracy: examples from the Late Pleistocene to recent volcanics from southern Italy. Chem. Geol., 59, 205—222.
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