How does the rubidium-strontium isochron method date a meteorite?

The rubidium-strontium isochron method dates a meteorite by measuring the decay of the radioactive parent isotope rubidium-87 (${}^{87}$Rb) into the stable daughter isotope strontium-87 (${}^{87}$Sr).12

When a meteorite or its components form, minerals within the rock typically share the same initial strontium isotopic composition (${}^{87}$Sr/${}^{86}$Sr ratio) but possess varying amounts of rubidium relative to strontium. Over time, ${}^{87}$Rb decays into ${}^{87}$Sr, causing the ${}^{87}$Sr/${}^{86}$Sr ratio to increase at different rates depending on the initial concentration of rubidium in each mineral.2

To determine the age, scientists plot the current ${}^{87}$Sr/${}^{86}$Sr ratios of these minerals against their current ${}^{87}$Rb/${}^{86}$Sr ratios on a graph. If the system has remained closed since formation, the data points form a straight line known as an isochron. The slope of this line is proportional to the time elapsed since the minerals formed or were last thermally reset, while the intercept on the vertical axis reveals the initial strontium ratio. If the data points do not align, it indicates the system was disturbed, often by shock.23