Uranium mineral dating

Edwards RL, Gallup CD, Cheng H (2003) Uranium-series dating of marine and lacustrine carbonates. Rev Mineral Geochem 52 363-405... 

Figure 1. Histogram of measurements of molar U/Ca ratio in a number of samples of reef-building corals and one giant clam sample (after Edwards 1988). Also indicated is the U/Ca ratio of seawater. This illustrates the point that corals do not fractionate U from Ca by large amounts when they make their skeletons. U/Ca ratios of corals are similar to values from inorganically precipitated marine aragonite. Mollusks along with most other biogenic minerals exclude uranium. Note that the horizontal axis is on a log scale and that the U/Ca ratio of the clam is almost 5 orders of magnitude lower than that of the corals. This difference is the fundamental reason why there are difficulties with uranium-series dating of mollusks.

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... 

Schmeide K, Pompe S, Buhner M, Heise KH, Bernhard G, Nitsche H (2000) Uranium (VI) sorption onto phyllite and selected minerals in the presence of humic acid. Radiochim Acta 88 723-728 Schwarcz HP, Latham AG (1989) Dirty calcites 1. Uranium series dating of contaminated calcite using leachate alone. Chem Geol 80 35-43... 

Eikenberg, X, Signer, P., Wieler, R. (1993) U-Xe, U-Kr, and U-Fb systematics for dating uranium minerals and investigations of the production of nucleogenic neon and argon. Geochim. Cosmochim. Acta, 57, 1053-69. 

Fairhall, A. W. Dating of Uranium Minerals by the Specific Radioactivity of Lead. J. chem. Educat. 40, 626 (1963). 

The laws of radioactive decay are the basis of chronology by nuclear methods. From the variation of the number of atoms with time due to radioactive decay, time differences can be calculated rather exactly. This possibility was realized quite soon after the elucidation of the natural decay series of uranium and thorium. Rutherford was the first to stress the possibility of determining the age of uranium minerals from the amount of helium formed by radioactive decay. Dating by nuclear methods is applied with great success in many fields of science, but mainly in archaeology, geology and mineralogy, and various kinds of chronometers are available. 

Shukolyukov YuA, Ashkinadze GS (1968) Determining the rate eonstant of spontaneous fission from the aeeumulation of xenon isotopes in uranium minerals (in Russian). Atomnaya Energiya 25 428-429 Shukolyukov YuA, Kirsten T, Jessberger EK (1974) The Xe-Xe speetnun teelmique, a new dating method. Earth Planet Sci Lett 24 271-281... 

The use of uranium, in its natural oxide form, dates back to at least 79 A.D., when it was used to add color to ceramic glazes.The German chemist Martin Klaproth is credited with discovering uranium in samples of the mineral pitchblende in 1789. It was first isolated as a metal in 1841 by Eugene-Melchior Peligot. Uranium was discovered to be radioactive by French physicist Henri Becquerel in 1896, who first discovered the process of radioactivity with uranium minerals. 

This series gives dates over the same time-range as the U-238 - Pb-208 analysis but can use mica, a mineral with a different distribution to uranium-containing minerals. This makes it applicable to artefacts not located adjacent to uranium minerals. 

As in the case of neptunium, the plutonium-239 isotope was found in uranium minerals, of course, in symbolic amounts. It is produced in uranium under the effect of natural neutrons. Thus, plutonium serves as a kind of the natural upper boundary of the periodic system and we can speak about two dates of its discovery. 

Radioisotopes may occur in the earth naturally as primordial radioisotopes, formed when the planet was created, or be produced by natural or artificial processes. Most fast decaying primordial radioisotopes have long disappeared from the planet since the earth originated about 4.5 billion years ago, such isotopes have decayed and reached a final, stable form. The relatively few primordial radioisotopes still extant in the earth today, therefore, decay very slowly. Among these are potassium-40 and some isotopes of uranium, such as uranium-235 and uranium-238, which are of use for dating archaeologically related minerals and rocks (see Textboxes 15 and 16). 

The best sealed-in minerals are zircons, zirconium silicate minerals which are formed when melted lava on the flanks of volcanoes solidifies. When the zircons crystallize out, they incorporate radioactive uranium (in particular 238U), which decays in several steps, leading Anally to the lead isotope 208Pb. The rate of decay is very low, as the half-life of uranium-238 is 4.5 x 109 years. Thus, the U-Pb-zircon method for age determination of Precambrian rock is very important. The fossils studied by Schopf were sandwiched between two lava layers (Schopf, 1999). The volcanic layers were dated to 3.458 0.0019 x 109 years and 3.471 0.005 x 109 years the age of the fossil layer (Apex chert) was thus determined to be about 3.465xlO9 years. 

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