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isochron A line joining points of equal time intervals or ages. In geochronology the slope of the isochron may be used to determine the age of a suite of rocks. For example, if it is assumed that all the rocks formed from one magma had the same initial 87Sr:86Sr ratio (see initial strontium ratio), then there is a simple equation to describe the growth of radiogenic 87Sr:87Sr = 87Sro + 87Rb(eλt− 1) where 87Sro is the number of atoms of the 87Sr isotopes incorporated into the rock at the time of formation, 87Sr and 87Rb are the numbers of these isotopes after time t, and λ is the decay constant. Because the number of 86Sr is constant, we can derive an equation: 87Sr:86Sr = 87Sro:86Sr + (87Rb:86Sr)(eλt− 1) which would give a family of straight lines when plotted on a graph of 87Sr:86Sr (on the y axis) against 87Rb:86Sr (on the x axis). All rock specimens belonging to a co-magmatic suite will plot as points on a straight line called an ‘isochron’ because all points on the line represent systems having the same age (t) and the same initial 87Sr: 86Sr ratio. In order to date co-magmatic igneous rocks by the whole-rock isochron method, a suite of rocks must be collected which span as wide a range of Rb/Sr ratios as possible, so that the slope of the isochron will be well defined. The age of the suite of rocks is obtained from the slope (m) using the equation m = eλt− 1. Isochrons can also be determined in lead—lead dating by plotting a series of growth curves. See also ISOCHRON MAP.

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