From: Duluth, Minnesota, U.S. (West end of Lake Superior)
Member Rating: 3.4
Message 1 of 10 (519654)
08-15-2009 9:19 PM
Back in my geology school days in the mid to late 70's, my understanding was that basaltic rocks were unsuitable for radiometric dating. This is because concentrations of elements such as Uranium (U), Rubidium (Rb), and Potassium (K) are too low in such mafic rocks.
But I now see members referring to radiometric datings of oceanic crust rocks that I must presume to be basaltic. I have done searches to try to determine how these datings are done, but have never been able to find anything.
So, the question is, how are these basaltic rock datings done?
Professor, geology, Whatsamatta U
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Member Rating: 2.6
Well,I'mmore on the amateur thoretical side. But Radiogenic Isotope Geology (2nd Ed. is a great resource. Combined with Google Site Search I come up with:
The first accurate measurement of meteorite initial ratios was made by Papanastassiou et al. (1969) on basaltic achondrites. These differ from chondritic meteorites in showing evidence of differentiation after their accretion from the solar nebula. However, they may not have participated in the full planetary differentiation process which generated iron meteorites. Their low Rb/Sr ratios have resulted in only limited radiogenic Sr production since differentiation, so an accurate initial ratio determination is possible.
In order to make this determination, Papanastassiou et al. analysed whole-rock samples from seven different basaltic achondrites, yielding an isochron (Fig. 3.10) without any excess scatter over analytical error. An age of 4.39 " 0.26 Byr was calculated using the old decay constant (8 = 1.39 H 10!11 yr!1). The initial ratio of 0.69899 " 5 was referred to by Papanastassiou et al. as the ‘Basaltic Achondrite Best Initial’ or BABI. This value represents a bench-mark to which other meteorite initial ratios may be compared. Birck and Allegre (1978) repeated this study with the addition of separated minerals from Juvinas and Ibitira, yielding an identical initial ratio, but an improved age determination of 4.57 " 0.13 Byr (same decay constant). However, Rb)Sr mineral isochrons are not possible for other achondrites due to later disturbance of the Rb—Sr system.
Chondritic meteorites have been readily dated by the Rb)Sr method, but achondrites are more problematical. Bulk samples usually have low Rb/Sr ratios, yielding ages of low precision, while separated minerals in many achondrites yield Rb)Sr ages below 4.5 Byr, indicative of disturbance. The Sm)Nd system in separated minerals from achondrites is more resistant to re-setting, yielding better age estimates. The first Sm)Nd dating study was performed by Notsu et al. (1973) on the achondrite Juvinas, but with low analytical precision. Lugmair et al. (1975) obtained much more precise results on minerals from the same meteorite (Fig. 4.1) yielding an age of 4560 " 80 Myr (2F).
Numerous other basaltic achondrites have been dated by Sm–Nd, and with the exception of Stannern (Lugmair and Scheinin, 1975), all yield ages in the range 4550 ) 4600 Myr. These age determinations have since been superseded by Pb–Pb dating studies (section 5.3). However, the good agreement between the Sm–Nd and Pb–Pb dates has served the important function of confirming the 147Sm half-life of 106 Byr.
Fig. 4.1. Sm)Nd isochron for whole-rocks and minerals from the basaltic achondrite Juvinas. Nd isotope ratios are affected by the choice of normalising factor for mass fractionation. Data from Lugmair et al. (1975).
In contrast to these difficulties, several recent studies have generated good isochrons from sulphide-poor material. One example is a dating study on the Deccan basalts (Allegre et al., 1999). A suite of ten whole-rock basalt samples formed an excellent Re–Os isochron with a good spread of data points. Using a decay constant of 1.663 H 10!11 yr!1, the isochron gave a precise age of 65.6 " 0.3 Myr, in excellent agreement with previous K–Ar and Ar–Ar ages averaging 64.5 " 1.5 Myr. This study shows that young whole-rock suites are capable of generating Re–Os isochrons, although older material may be more problematical.
Age spectrum results from the better of two basaltic komatiite samples are shown in Fig. 10.19. In contrast to the komatiites, these samples display significant excess argon in the low- and high-temperature gas releases, with an integrated age of 3778 Myr. Nevertheless, the best plateau age of 3447 Myr is in close agreement with the best komatiite results. The saddle-shaped form is well known for samples containing excess argon.
The Ar)Ar dating technique was found to be particularly useful for dating small whole-rock samples of lunar material, especially fine-grained mare basalts. The dashed profile in Fig. 10.20 shows a typical release pattern (Turner and Cadogan, 1974), attributed to 8% radiogenic Ar loss from K-rich sites with low Ar retentivity. However, other samples showed either a sharp decrease in apparent age in the high-temperature fractions, or, particularly in fine-grained rocks, a progressive decrease in apparent age over most of the gas release. The latter examples led workers to suspect that Ar redistribution was occurring within the sample, possibly during the irradiation process.
Fig. 10.19. Age and Ca/K spectrum for two runs on a basaltic komatiite showing evidence of inherited Ar during low- and high-temperature emission from high Ca/K domains. Arrows separate successive gas releases with identical ages. After Lopez Martinez et al. (1984).
Fig. 10.20. The effect of fine crushing on a 40)39 age spectrum, due to 39Ar recoil. Dashed profile = analysed rock chip of a lunar mare basalt. Solid profile = similar sample activated after fine powdering. After Turner and Cadogan (1974).
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.
Allegre, C. J., Birck, J.-L., Capmas, F. and Courtillot, V. (1999). Age of the Deccan traps using 187Re–187Os systematics. Earth Planet. Sci. Lett. 170, 197–204.
Birck, J. L. and Allegre, C. J. (1978). Chronology and chemical history of the parent body of basaltic achondrites studied by the 87Rb)87Sr method. Earth Planet. Sci. Lett. 39, 37)51.
Dalrymple, G. B. and Moore, J. G. (1968). Argon 40: excess in submarine pillow basalts from Kilauea Volcano, Hawaii. Science 161, 1132)5.
Goldstein, S. J., Murrell, M. T. and Williams, R. W. (1993). 231Pa and 230Th chronology of mid-ocean ridge basalts. Earth Planet. Sci. Lett. 115, 151)9
Lopez Martinez, M., York, D., Hall, C. M. and Hanes, J. A. (1984). Oldest reliable 40Ar/39Ar ages for terrestrial rocks: Barberton Mountain komatiites. Nature 307, 352)4.
Lugmair, G. W. and Scheinin, N. B. (1975). Sm)Nd systematics of the Stannern meteorite. Meteoritics 10, 447)8 (abstract).
Lugmair, G. W., Scheinin, N. B., and Marti, K. (1975). Search for extinct 146Sm, I. The isotopic abundance of 142Nd in the Juvinas meteorite. Earth Planet. Sci. Lett. 27, 79)84
McDougall, I., Polach, H. A. and Stipp, J. J. (1969). Excess radiogenic argon in young subaerial basalts from the Auckland volcanic field, New Zealand. Geochim. Cosmochim. Acta 33, 1485)1520.
McDougall, I. and Tarling, D. H. (1964). Dating geomagnetic polarity zones. Nature 202, 171)2
Notsu, K., Mabuchi, H., Yoshioka, O., Matsuda, J. and Ozima, M. (1973). Evidence of the extinct nuclide 146Sm in ‘Juvinas’ achondrite. Earth Planet. Sci. Lett. 19, 29)36.
Papanastassiou, D. A., Wasserburg, G. J. and Burnett, D. S. (1969). Initial strontium isotopic abundances and the resolution of small time differences in the formation of planetary objects. Earth Planet. Sci. Lett. 5, 361)76
Turner, G. and Cadogan, P. H. (1974). Possible effects of 39Ar recoil in 40Ar/39Ar dating. Proc. 5th Lunar Sci. Conf. Pergamon, pp. 1601)15.
Williams, R. W. and Gill, J. B. (1992). Th isotope and U-series disequilibria in some alkali basalts. Geophys. Res. Lett. 19, 139)42.
Also a Google Scholar Search turns up bunches of hits.
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