zircons dating

Nora Holcomb, 37 years old


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How old are you? How old are your grandparents? Do you know how old Canada is? Or the pyramids in Egypt? Do you know how old Earth is?

At present, Chemostrat can determine U-Pb ages for zircon and apatite crystals. Zircon is a robust mineral and so the crystals preserve the age at which they formed or underwent high grade metamorphism. Consequently, U-Pb zircon geochronology can be employed to constrain the age of the basement rocks and in turn can help to identify sediment dispersal patterns and to correlate sandstones. If the analysed zircon crystal has not suffered either Pb loss or U gain, it will plot on the concordia line from which its age can be deduced. Sandstones frequently contain detrital zircon grains and if these grains are undisturbed zircons dating concordant, their ages provide some clue as to their provenance. Generally at least fifty grains from each sandstone sample need to be analysed in order to obtain reliable data. The age of apatite grains can be calculated by plotting their U-Pb isotopic composition to form a discordia line.

The nitty gritty on radioisotopic dating Radioisotopic dating is a key tool for studying the timing of both Earth's and life's history. Radioactive decay Radioisotopic dating relies on the process of radioactive decay, in which the nuclei of radioactive atoms emit particles. This releases energy in the form of radiation and often transforms one element into another. For example, over time, uranium atoms lose alpha particles each made up of two protons and two neutrons and decay, via a chain of unstable daughters, into stable lead. Although it is impossible to predict when a particular unstable atom will decay, the decay rate is predictable for a very large number of atoms.
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Uranium—lead dating , abbreviated U—Pb dating , is one of the oldest [1] and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised [2] from about 1 million years to over 4. The dating method is usually performed on the mineral zircon. The mineral incorporates uranium and thorium atoms into its crystal structure , but strongly rejects lead. Therefore, one can assume that the entire lead content of the zircon is radiogenic , i. Thus the current ratio of lead to uranium in the mineral can be used to determine its age [ citation needed ]. The method relies on two separate decay chains , the uranium series from U to Pb, with a half-life of 4. The above uranium to lead decay routes occur via a series of alpha and beta decays, in which U with daughter nuclides undergo total eight alpha and six beta decays whereas U with daughters only experience seven alpha and four beta decays. The existence of two 'parallel' uranium—lead decay routes U to Pb and U to Pb 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.

The mineral zircon adds three more fundamental advantages to uranium—lead dating. First, its crystal structure allows a small amount of tetravalent uranium to substitute for zirconium but excludes with great efficiency the incorporation of lead. It might be said that one begins with an empty box. Second, zircon, once formed, is highly resistant to change and has the highest blocking temperature ever observed. Finally, with few predictable exceptions, zircon grows or regrows only in liquid rock or in solid rock reheated to approach its melting point. Combining all of these attributes, it is often possible to measure both the time of crystallization and the time of second melting in different parts of the same grain or in different selected grains from the same rock.

Zircons are a gold standard for dating. They can yield ages that are statistically significant, but geologically meaningless. How confident are geologists in the ages of formations they study? The part of the story not told, though, is that many sources of bias can creep in.