Principles of isochrons

Consider the case of an intermediate magma fractionating to produce a series of subsequently more evolved liquids as it crystallises. This will have the effect of changing the parent–daughter ratios of various radioactive decay schemes due to differences in compatibility of the parent and the daughter in the crystallising minerals. For example, since Rb is likely to be enriched in more evolved magmas with respect to Sr, the parental andesite will have comparatively low Rb/Sr, whereas more evolved magmas from the same system (e.g. granites) will contain progressively more Rb with respect to Sr (Figure 7.1).

However, because the crystallisation takes place over a relatively short period of time with respect to the half life of 87Rb, all minerals crystallised (and any remaining liquid) will have the same 87Sr/86Sr. This is the key point – there is a spread in Rb/Sr between the different minerals in the rock, or between the different magmas in the fractionating series in Figure 7.1. Therefore, there is a spread in 87Rb/86Sr between these minerals or magmas. However, when they crystallised, all of the minerals or comagmatic magmas would have incorporated Sr which had the same 87Sr/86Sr ratio, since crystallisation is a chemical process, and will not significantly affect the isotopes of the elements.

So too, for a single rock, such as a granite composed of the minerals quartz, feldspar and biotite (for argument's sake), each mineral will have differing amounts of trace elements but the same daughter isotopic ratio when it crystallises. For example, feldspars have huge amounts of both Rb and Sr. In contrast, micas, such as biotite, typically do not have as much Rb and Sr; however, they have a lot of Rb compared with Sr, i.e. they have a high Rb/Sr ratio, whereas feldspars will have relatively lower Rb/Sr ratios. Quartz has virtually no Rb or Sr, but typically has very low Rb/Sr. However, if we ground a representative portion of the whole rock up, we would have a mixture which is an average of both the feldspar and the biotite (and quartz), and so will have a Rb/Sr ratio somewhere in between.