Geochronological tools

The abundance of a trace element is often too small to be accurately quantihed using conventional analytical methods such as ion chromatography or mass spectrometry. It is possible, however, to precisely determine very low concentrations of a constituent by measuring its radioactive decay properties. In order to understand how U-Th series radionuclides can provide such low-level tracer information, a brief review of the basic principles of radioactive decay and the application of these radionuclides as geochronological tools is useful. " The U-Th decay series together consist of 36 radionuclides that are isotopes (same atomic number, Z, different atomic mass, M) of 10 distinct elements (Figure 1). Some of these are very short-lived (tj j 1 -nd are thus not directly useful as marine tracers. It is the other radioisotopes with half-lives greater than 1 day that are most useful and are the focus of this chapter. 

The majority of published °Pb reports address the utility of °Pb as a geochronological tool rather than as an element that is involved in complex biogeochemical cycles. Nonetheless, some of these studies do provide insight into the geochemical behavior of °Pb and °Po. Nearly all of the lead in the world s surface oceans is believed to be of anthropogenic origin—derived from combustion... 

Short-lived isotopes of Pb such as Pb (ti/2 = 22.26 year), Pb (tj/2 = 10.6 h), " Pb (ti/2 = 26.8 min) are useful tracers of transient signals for studying sediment fluxes in estuaries [62], deposition of atmospheric particles [63,64], scavenging of trace metals by aquatic particles [65], and oceanic circulation [66]. Lead-210 in particular, has been extensively used as a geochronological tool for studying the accumulation rates of sediments, peat, and glacial ice during the past two centuries [67,68]. 

Expert group 3 geochronology data Geologists use a wide variety of geochronological tools and methods to estimate the age of rocks and sediments. Using measurements of radioactive isotopes, geologists are able to date all geological materials, from billions of years to the present time. 

Obviating such difficulties may he possible using Os/Os, that is to say Os/ Os and Os/ Os, dating (Yin et al. 1993). The approach is a high-precision one, and its use with a molybdenite sample imphed that it can become a geochronological tool even capable... 

Pb dating Nuisance or powerful tool in geochronology. 6th V.M. Goldchmidt Conference, Heidelberg, Germany. J Conf Abstr 1 439... 

It has been recognized that the Lu-Hf isotopic system in zircon is a powerful tool for deciphering the evolution of the earth s crust and mantle. " Zircon normally contains 0.5-2 wt % Hf, which results in an extremely low Lu/Hf ratio (" Lu/" Hf < 0.002) and consequently a negligible radiogenic growth of Hf due to the decay of Lu. Therefore, the Hf/" Hf ratio of zircon can be regarded as the initial value at the time when it crystaUized. LA-ICP-MS with a multiple ion collector system has also been employed to study the hafnium isotopic composition of zircon and baddeleyite standards in U-Pb geochronology. °°... 

Modem tectonics research is heavily dependent on the tools of geochronology and thermochronology and, as a consequence, has become an important driver of analytical innovation. Understanding the pace of thermal, deformational, and erosional processes requires the development of chronometers that are both accurate and precise, and it must be possible to apply these tools selectively to features formed during specific tectonic events. These three needs—accuracy, specificity, and precision— help define the frontiers of geochronology and thermochronology research. 

Hurtado J. M., Chatterjee N., Ramezani J., Hodges K. V., and Bowring S. A. (xxxx) Electron microprobe chemical dating of uraninite as a reconaissance tool for leucogranite geochronology. Earth Planet. Sci. Lett, (in preparation). 

Our understanding of the evolution of the earth and the solar system had its beginning in radioactivity. Radioactivity provided a tool for absolute geochronology, it was found to be an important heat source within the planets and it produced small changes in the isotopic composition of some elements, which enabled us to trace geochemical processes back to the earth s past. Moreover, radioactive processes are a source of energy for human beings. 

The availability of separated isotopes in the late 1940 s made possible analyses by isotope dilution mass spectrometry. Geologists were the first to capitalize on the new tool, and by the mid-1950 s the field of geochronology was well established. Extremely precise and accurate concentration determinations down to the sub-ppm level are required for meaningful age calculations. 

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