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Finally, pegmatite veins are important time-markers in orgens due to their cross-cutting relationships.It needs to be stressed that the daughter isotope 87Sr can be determined simultaneously with the parent isotope 87Rb (via 85Rb analysis, assuming a constant 87Rb/85Rb ratio).Pegmatites are ideal test cases as they often form late in an orogeny and close to the surface, therefore exhibit simple cooling histories.Furthermore, several U-bearing minerals (e.g., zircon, columbite-tantalite) allow comparison of Rb-Sr with U-Pb ages.In the reasoning that follows, the reader may recognize a sort of family resemblance to the reasoning behind step heating in the Ar-Ar method, although the two are not exactly alike. When an igneous rock is first formed, its minerals will contain varying concentrations of rubidium and strontium, with some minerals being high in rubidium and low in strontium, others being high in strontium and low in rubidium.We can expect these differences to be quite pronounced, because rubidium and strontium have different chemical affinities: as we have noted, rubidium substitutes for potassium, and strontium for calcium.They have different chemical compositions, and therefore have different Sr ratio, for reasons explained in the previous paragraph.
Strontium, on the other hand, does take part in chemical reactions, and can substitute chemically for such elements as calcium, which is commonly found in igneous rocks.
Whole-rock samples from different parts of the same body generally differ in rubidium content and the Sr ratio changes in each part of the rock, the slope of the isochron increases progressively, providing a measure of the age of the crystallization.
The intercept of the isochron at the ordinate indicates the isotopic composition of common strontium at the beginning of the process.
As rubidium easily substitutes chemically for potassium, it can be found doing so in small quantities in potassium-containing minerals such as biotite, potassium feldspar, and hornblende.
(The quantity will be small because there is much more potassium than rubidium in the Universe.) This means that if we wanted to date a rock, and if there was no Sr present initially.