Igneous and metamorphic rocks that contain no minerals but have particularly high Rb:Sr ratios (see rubidium-strontium dating), or rocks that are too fine-grained for mineral separation, can be analysed as whole-rock samples. Whole-rock samples from different parts of the same body generally differ in rubidium content and the 87Sr:86Sr ratio of each can be plotted as a function of its 87Rb:86Sr ratio in an isochron diagram. At the time of the initial crystallization different parts of the sample, regardless of rubidium concentration, would have had the same 87Sr:86Sr ratio and hence plot as a horizontal line. With the passage of time 87Rb would be lost and corresponding amounts of radiogenic 87Sr gained. As the 87Sr:86Sr 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.
Subjects: Earth Sciences and Geography.