Problems with isotope dating

Here, a single uranium–lead isotopic analysis can provide an age more precise than can be obtained by the whole rock isochron method involving many analyses.When single minerals are analyzed, each grain can be studied under a microscope under intense side light so that alterations or imperfections can be revealed and excluded.An ideal mineral is one that has sufficient parent and daughter isotopes to measure precisely, is chemically inert, contains little or no significant initial daughter isotopes, and retains daughter products at the highest possible temperatures.A specific datable mineral like lead of mass 206 relative to that of mass 204 has changed from an initial value of about 10 present when the Earth was formed to an average value of about 19 in rocks at the terrestrial surface today.In this case, the slope of the line in potassium–argon (K–Ar) dating, for example, because most minerals do not take argon into their structures initially.In rubidium–strontium dating, micas exclude strontium when they form, but accept much rubidium.If the Earth’s interior were a simple and homogeneous reservoir with respect to the ratio of uranium to lead, a single sample extracted by a volcano would provide the time of extraction. No parent–daughter value for a closed system is involved, rather just a single isotopic measurement of lead viewed with respect to the expected evolution of lead in the Earth.

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A lead-rich mineral formed and isolated early in Earth history would have a low lead-206 to lead-204 ratio because it did not receive subsequent additions by the radioactive decay of uranium.Similar studies have shown that the samarium–neodymium (Sm–Nd) parent–daughter pair is more resistant to secondary migration but that, in this instance, sufficient initial spread in the abundance of the parent isotope is difficult to achieve.trace minerals may form, each concentrating certain elements and radioactive trace elements within the rock.By careful selection, certain minerals that contain little or no daughter element but abundant parent element can be analyzed.In some cases the discovery of a rare trace mineral results in a major breakthrough as it allows precise ages to be determined in formerly undatable units.For example, the mineral those composed primarily of one or more ferromagnesian, dark-coloured minerals).

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