Thorium-230 dating

Hall BL, Henderson GM (2001) Use of TIMS uranium-thorium dating to determine past reservoir effects in lakes Two examples from Antarctica. Earth Planet Sci Lett 193 565-577 Hamelin B, Bard E, Zindler A, Fairbanks RG (1991) " U/ U mass spectrometry of corals How accurate is the U-Th age of the last interglacial period. Earth Planet Sci Lett 106 169-180 Hanebuth T, Stattegger K Grootes PM (2000) Rapid flooding of the Sunda Shelf a late-glacial sea-level record. Science 288 1033-1035... 

This is exploited in the technique of uranium-thorium dating, which involves measuring the amount of thorium-230 that has accumulated in a substance by decay of uranium. If the object contained no thorium at all when it was formed, the ratio of remaining U to accumulated °Th is a measure of the age. The object being dated must not have had access to sources of fresh uranium that could reset the clock. This is true, for example, of coral left stranded on fossil beaches when sea levels recede, or of... 

The half-life of U is four orders of magnitude greater than any of the intermediate daughters. Therefore, any closed system, regardless of the initial state, will approach a state of secular equilibrium such that the activities of all the intermediate daughters are the same and are equal to the activity (Bateman, 1910). External processes that fractionate nuclides within the decay chain disrupt the state of secular equilibrium. The subsequent growth or decay of the intermediate nuclides back towards equilibrium with can be used to date the fractionation event (see Ivanovich et al., 1992). Uranium-thorium dating of speleothems is possible because of the extreme... 

Thorium isotope concentrations and ratios, as well as parent and daughter isotope concentrations, are used to date and study the formation and metamorphosis of rocks and sediments. For example, has been used to date coral reef terraces (4). / Th disequiUbria and Th/ Th... 

Figure 1. Schematic diagram showing a TRU-spec extraction chromatography method for separation of uranium, thorium, protactinium, and radium from a single rock aliquot. Further purification for each element is normally necessary for mass spectrometric analysis. Analysis of a single aliquot reduces sample size requirements and facilitates evaluation of uranium-series dating concordance for volcanic rocks and carbonates. For TIMS work where ionization is negatively influenced by the presence of residual extractant, inert beads are used to help remove dissolved extractant from the eluant.

In the past three years, MC-ICPMS has emerged as an alternative to TIMS for precise measurement of the U-series isotopes with comparable or better precision. U-Th isotopes can now be routinely measured at the sub-permil level. Previously, this had only been demonstrated using charge-collection TIMS applied to thorium isotope measurement. Data collection efficiency, sample size requirements, and detection limits can also be greatly improved over TIMS. For the U- U- Th system applied to carbonate samples, this has extended the dating range beyond 600,000 years, and °Th-age uncertainties of 2000 years are now attainable on 300,000 year-old samples (e g., Stirling et al. 2001). 

However, the Slowey et al. (1995) work on shallower carbonate bank sediments (off of the Bahamas) was successful and led to the dating of a number of important events in the marine oxygen isotope record (including the timing of the Last Interglacial (Slowey et al. 1996), Termination II (Henderson and Slowey 2000) and portions of the Penultimate Interglacial (Slowey et al. 1996, Robinson et al. 2002). The shallower depths obviate the most serious of the initial thorium problems encountered in the Nicaragua Rise work. 

Cheng H, Edwards RL, Murrell MT, Goldstein S (1998) The systematics of uranium-thorium-protactinium dating. Geochim Cosmochim Acta 62(21) 3437-3452... 

Cheng H, Edwards RL, Mttrrell MT, Benjamin TM (1998) Uranium-thorittm-protactinium dating systematics. Geochim Cosmochim Acta 62 3437-3452 Cherdyntsev W, Kazachevskii IV, Kttz mina YA (1965) Age of carbonate determined from the isotopes of thorium and uranium. Geochem Int 2 749-756... 

Edmonds HN, Moran SB, Hoff JA, Smith JN, Edwards RL (1998) Protactinium-231 and Thorium-230 abundances and high scavenging rates in the Western Arctic ocean. Science 280 405-407 Edwards RL, Gallup CD, Cheng H (2003) Uranium-series dating of marine and lacustrine carbonates. Rev Mineral Geochem 52 363-405... 

Cheng H, Lawrence Edwards R, Mnrrell MT, Benjamin TM (1998) Uraninm-thorium-protactininm dating systematics. Geochim Cosmochim Acta 62 3437-3452... 

Because there is no natural mechanism for producing isotopically pure Th, any measurable Th in a sample for °Th/U dating implies the presence of at least some °Th when the sample (mineral or mineral assemblage) was formed. This initial °Th must then be subtracted from the total measured °Th before a valid °Th/U age can be calculated. In addition, a correction for initial U isotopes is required when the material being dated is a mechanical mixture of an isotopically homogeneous detrital material with an initially thorium-free mineral such as chemically precipitated carbonate. In these cases we are usually interested only in the time of formation of the Th-free material, and so must also subtract the detrital and before calculating the °ThAJ age. 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

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