Isochron-Plot of Long-Lived Decay Systems
Channel: Dominik Hezel
Date: 2023-04-12

Long-lived decay systems are those in which the half-life of the parent is significantly larger than 100 Ma. An example is the 87Rb to 87Sr system, with a long half-life of 48.1 Ga. For an isochron plot it is required that a number of components, e.g., various minerals, had variable 87Rb/86Sr ratios, but the same 87Sr/86Sr ratios. An example would be a melt, in which these minerals crystallise at about the same time. 87Rb then decays to 87Sr. Thereby, the 87Rb/86Sr ratio in the various minerals decrease, while their 87Sr/86Sr ratios increase. As a result, the compositions of the minerals move along lines with a negative slope. The connection of the endpoints of these lines define the isochron. The slope of this isochron can then be used to back-calculate when the various minerals crystallised, i.e., how old the rock is. The isochron plot itself is a parametric plot.