# Pb pb isochron dating

It depends on the accuracy of the measurements and the fit of the data to the line in each individual case.) For example, with Rb/Sr **isochron** **dating**, any age less than a few tens of millions of years is usually indistinguishable from zero.That encompasses the entire young-Earth timescale thousands of times over." in the decay equation.Age "uncertainty" When a "simple" *dating* method is performed, the result is a single number.There is no good way to tell how close the computed result is likely to be to the actual age.The available data for carbonates from 16 localities, with ages ranging from Archean to Mesozoic, show that except for one case (Mushandike Limestone) all model -value as would be expected from a closed-system isotopic homogenization in metamorphic carbonates has never been observed, suggesting: (1) in diagenetic or metamorphic conditions, Pb was homogenized in an open system containing both carbonate and extraneous Pb; and (2) the extraneous Pb had a signature of major terrestrial reservoir, which dominated the Pb budget in the renewed carbonate UPb isotopic systems.The published UPb and PbPb ages so far are not sufficiently precise to useful in the calibration of geologic time scale for the Phanerozoic.However, for metamorphic carbonates, PbPb **isochron** ages of marbles generally reflect faithfully the time of metamorphic recrystallisation.Additional information on the thermal histories and UPb fractionation of carbonates may be obtained usin the three-dimensional approach of Wendt (1984).

Carbonate UPb or PbPb ages alone do not yield conclusive geologic meanings.(Rocks which include several different minerals are excellent for this.) Each group of measurements is plotted as a data point on a graph.The X-axis of the graph is the ratio of in a closed system over time.The better the fit of the data to the line, the lower the uncertainty.For further information on fitting of lines to data (also known as regression analysis), see: Note that the methods used by isotope geologists (as described by York) are much more complicated than those described by Gonick.The simplest form of isotopic age computation involves substituting three measurements into an equation of four variables, and solving for the fourth.The equation is the one which describes radioactive decay: If one of these assumptions has been violated, the simple computation above yields an incorrect age.Consider some molten rock in which isotopes and elements are distributed in a reasonably homogeneous manner.Its composition would be represented as a single point on the **isochron** plot: Note that the above is somewhat simplified.It is not easily explained, in the general case, in any other way.The data points would be expected to start out on a line if certain initial conditions were met.

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