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Presents a brief discussion of the assumptions, accuracy and precision of 'geological' errors present in physical geochronometry. The five major physical methods of rock and mineral age determinations currently in use, are described.

The isotopic composition of lead in Red Sea chloride brine containing 0.5 ppm Pb is found to be similar to that of some Cenozoic ore leads such as galena at Rabigh in Saudi Arabia that may have formed during mineralization accompanying Tertiary rifting. Bir Ranga galena in Miocene sediments from United Arab Republic (Egypt) is also isotopically similar to lead in Red Sea brine. The chlorine brine must be considered a possible mineralizing fluid. Lead isotopes show promise for use in mineral prospect evaluation in that galena from Samrah is isotopically similar to that from Mahd adh Dhahab, which has been the only ore producer in Saudi Arabia since 1945. Drilling at Samrah does indicate a possible economic mineralization. The lead isotope data coupled with available geologic knowledge and geochronometry are used to tentatively divide the ore prospects of the Kingdom of Saudi Arabia into relative categories of mineralization age. Two Mesozoic and Cenozoic mineralizations are distinguished on the basis of a 207 Pb/ 204 Pb difference; an early Paleozoic mineralization grouping is outlined; and a late Precambrian mineralization period is suggested.

The potassium-argon method of geochronometry may be applied quantitatively to biotite, hornblende and sanidine if these minerals have been kept at low temperature since their formation. Fine-grained whole rocks, particularly of mafic varieties, may also have 100 % retentivity under suitable conditions. Other whole-rock samples may give 75 – 85 % of the true age. Young volcanic glasses may also be used for dating if they have not been devitrified but probably give somewhat low ages even in the best cases.

Samples from petrographic collections were analyzed with the potassium- argon method of geochronometry. The implicattons of the results with respect to the geological history of eastern Greenland are discussed. (D.J.C.)

Publisher Summary The large increase in the knowledge of nuclear physics and radiochemistry occasioned by the Manhattan Project during World War II has had important ramifications in most other fields of experimental science. Not only did the Project produce many new ideas and techniques, but also numerous young scientists who were interested in the application of these techniques to other areas of study. In earth science, geochemistry and nuclear geophysics have been profoundly affected by these developments. This is particularly true of the general area of age determination of rocks and minerals. More research in quantitative geochronometry has been conducted during the past decade than in the previous five years. In this chapter an attempt is made to review the status of the significant isotopic clocks. There has been no attempt to cover the literature exhaustively. The discussion is be concerned primarily with the developments of the past decade ending with the summer of 1954. This chapter starts with the discussion of the “lead methods” used to designate the age measurement. The other methods that have been discussed in this chapter are: the carbon-14 method, the strontium method, the potassium methods, the helium method, and the ionium method.

ARMSTRONG, D. A. and WINKLER, C. A. 1956. CompariDUSHMAN, SAUL. 1962. Scientific foundations of vacuum tive production of active nitrogen from nitrogen, nitric technique. J. M. Lafferty (Ed.), John Wiley and Sons, oxide and ammonia and from nitrogen at different New York, pp. 649673. discharge potentials. J. Phys. Chem., 60, pp. JAMES, L. H., LECK, J. H., and CARTER, G. 1964. Ion 1100-1 102. bombardment induced emission of sorbed gases from BAKSI, A. K. 1970. Whole-rock dating of young extrusives glass surfaces. Brit. J. Appl. Phys. 15, pp. 681689. by the potassium-argon method, unpubl. Ph.D. KIRSTEN, T. 1966. Determination of radiogenic argon in thesis, Univ. of Toronto, Toronto, Ontario, 121 p. potassium-argon dating. Schaeffer and Zahringer 1973. Quantitative unspiked argon runs in K-Ar (Eds.), Springer-Verlag, New York., pp. 7-39. dating. Can. J. Earth Sci., 10, pp. 1415-1419. KISTIAKOWSKY, G. B. and VOLPI, G. G. 1957. Reactions of 1974. Isotopic fractionation of a loosely held atnitrogen atoms; 1. Oxygen and oxides of nitrogen. J. mospheric argon component in the Picture Gorge Chem. Phys.,27, pp. 1141-1149. Basalts. Earth Planet. Sci. Lett., 21, pp. 431438. LIPSON, J. J. 1958. Potassium-argon dating of sedimenBAKSI, A. K. and WATKINS, N. D. 1973. Volcanic productary rocks. Bull. Geol. Soc. Am. 69, pp. 137-150. tion rates; comparison of oceanic ridges, islands and SCHAEFFER, 0.A. and HEYMANN, D. 1965. Comparisonof Columbia Plateau Basalts. Science, 180, pp. 493496. J6C1-J6Ar and 39Ar-3sAr cosmic ray exposure ages of BAKSI, A. K., YORK, D., and WATKINS, N. D. 1967. Ageof dated Fall Iron Meteorites. J. Geophys. Res., 70, pp. the Steens Mountain geomagnetic polarity transition. 215-224. J. Geophys. Res., 72, pp. 629945308. WATKINS, N. D. and BAKSI, A. K. 1974. MagnetostratigCARR, D. R. and KULP, J. L. 1957. Potassium-argon raphy and oroclinal folding of the Columbia River, method of geochronometry. Bull. Geol. Soc. Am. 68, Steens and Owyhee Basalts in Oregon, Washington pp. 763-784. and Idaho. Am. J. Sci., 274, pp. 148-189. Cox, A. V. and DALRYMPLE, G. B. 1967. Statistical treatWHEELDON, J. W. 1959. Absorption of sodium and argon ment of geomagnetic reversal data and the precision of by glass. Brit. J. Appl. Phys., 10, pp. 295-298. potassium-argon dating. J. Geophys. Res., 72, pp. 2603-2614.

Potassium-argon and rubidium-strontium age measurements have been made on mica separated from many rock types of the Manhattan Prong and New Jersey-New York Highlands (Reading Prong). Most of the rocks from the Manhattan Prong were dated within experimental error at 360 m.y. (million years). Since this age is identical with those obtained by the U-Pb method by other workers on intrusive pegmatites, it probably represents the last major metamorphic event in the area. The isotopic age of mica in the Highlands at least 10 miles from the zone where complete re-crystallization took place 360 m.y. ago is about 840 m.y. Apparent ages of mica in a transition zone range between these two. The ages in the transition zone are determined by the extent of re-crystallization which in turn is strongly dependent on the local structure. If the 1150-m.y. zircon age obtained by Tilton and others (1960) in the Highlands is the primary age of the basement in this region, major metamorphic events took place at 1150, 840, and 360 m.y. Relict ages as old as 480 m.y. in the Manhattan Prong suggest a fourth event if the rocks of the Prong were deposited in the Cambrian or later. This study illustrates both the difficulties and the effectiveness of isotopic geochronometry in areas of complex metamorphic history.

K-Ar dates for 123 mineral separates from 91 samples of granitic rocks, and 3 samples of Orocopia and Vitrefax Schist from Nevada, western Utah, the Mojave Desert, the Colorado Desert, and the northern and eastern Peninsular Ranges of southern California, range in age from 218 to 19 m.y.; however, most of the dates lie between 160 and 50 m.y. A majority of the hornblende and muscovite dates are nearly concordant with, but generally a few m.y. older than, coexisting biotite dates; some are highly discordant. The oldest plutons gave Late Triassic to Early Jurassic dates (El Paso and San Bernardino Mountains, 218 to 194 m.y.). Jurassic igneous activity reached a culmination slightly before 150 m.y. ago over a large area in northern Nevada and western Utah and supplied ash to the Morrison Formation of the Colorado Plateau. Several plutons of the northeastern Mojave Desert yielded similar dates. A similar major culmination of Mesozoic pluton emplacement in early and middle Late Cretaceous time is represented by scattered plutons in Nevada and probably by most of the plutons in the Peninsular Ranges and southern deserts of California. Earliest Tertiary dates were obtained from far-southeastern California (50 to 66 m.y.). Most dates in the Cordilleran batholith belt of the Southwest, which parallels the edge of the late Mesozoic continent, are between 160 and 75 m.y. and are essentially coeval with metamorphism in the parallel high-pressure, low-temperature Franciscan belt to the west (approximately 150 to 75 m.y.). About 75 m.y. ago, this pattern abruptly changed to a Laramide one with abundant 75 to 50 m.y. dates in a belt running through southern Arizona into southeastern California at a high angle to the edge of the continent. Only very sparse dates of this age are found in Nevada. Abundant Laramide dates are not encountered again until the Idaho, Boulder, and Coast Range batholiths. These important changes in Cordilleran tectonics may be effects of a change from dominantly dip-slip subduction during Franciscan time to dominantly north-northeast strike-slip subduction during Laramide time. Furthermore, the initiation of rifting of eastern North America in Triassic time, changes in spreading rate, and reorientation of spreading directions in the Atlantic Ocean appear to correlate with the beginning of widespread pluton emplacement in western North America, changes in the rate of magma generation in the Cordillera, and shifts of the Cordilleran tectonic pattern, respectively, during Mesozoic time.

Radiometric Dating for Geologists Edited by E. I. Hamilton and R. M. Farquhar. Pp. vii + 506. (Wiley (Interscience): London and New York, July 1968.) 147s.