|
Uranium–lead (U–Pb) dating is one of the oldest〔Boltwood, B.B., 1907, On the ultimate disintegration products of the radio-active elements. Part II. The disintegration products of uranium: American Journal of Science 23: 77-88.〕 and most refined of the radiometric dating schemes. It can be used to date rocks that formed from about 1 million years to over 4.5 billion years ago with routine precisions in the 0.1–1 percent range.〔Parrish, Randall R.; Noble, Stephen R., 2003. Zircon U-Th-Pb Geochronology by Isotope Dilution – Thermal Ionization Mass Spectrometry (ID-TIMS). In Zircon (eds. J. Hanchar and P. Hoskin). Reviews in Mineralogy and Geochemistry, Mineralogical Society of America. 183-213.〕 The uranium–lead dating method relies on two separate decay chains, the uranium series from 238U to 206Pb, with a half-life of 4.47 billion years and the actinium series from 235U to 207Pb, with a half-life of 704 million years. These uranium to lead decay routes occur via a series of alpha (and beta) decays, in which 238U with daughter nuclides undergo eight total alpha and six beta decays whereas 235U with daughters only experience seven alpha and four beta decays.〔Romer, R.L. 2003. Alpha-recoil in U-Pb geochronology: Effective sample size matters. Contributions to Mineralogy and Petrology 145, (4): 481-491.〕 The existence of two 'parallel' uranium-lead decay routes (238U to 206Pb and 235U to 207Pb) leads to multiple dating techniques within the overall U–Pb system. The term ''U–Pb dating'' normally implies the coupled use of both decay schemes in the 'concordia diagram' (see below). However, use of a single decay scheme (usually 238U to 206Pb) leads to the U–Pb isochron dating method, analogous to the rubidium-strontium dating method. Finally, ages can also be determined from the U–Pb system by analysis of Pb isotope ratios alone. This is termed the lead-lead dating method. Clair Cameron Patterson, an American geochemist who pioneered studies of uranium–lead radiometric dating methods, is famous for having used it to obtain one of the earliest accurate estimates of the age of the Earth. ==Mineralogy== Uranium-lead dating is usually performed on the mineral zircon (ZrSiO4), though it can be used on other minerals such as monazite, titanite, and baddeleyite. The zircon mineral incorporates uranium and thorium atoms into its crystalline structure, but strongly rejects lead. Therefore, we can assume that the entire lead content of the zircon is radiogenic. Where crystals such as zircon with uranium and thorium inclusions do not occur, a better, more inclusive, model of the data must be applied. Uranium-lead dating techniques have also been applied to other minerals such as calcite/aragonite and other carbonate minerals. These types of minerals often produce lower precision ages than igneous and metamorphic minerals traditionally used for age dating, but are more common in the geologic record. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Uranium–lead dating」の詳細全文を読む スポンサード リンク
|