ABSTRACT

Equations are derived giving the evolution of the ⁸⁷Sr/⁸⁶Sr ratio in mineral phases which crystallize from magmatic liquids with changing ⁸⁷Sr/⁸⁶Sr ratio, assuming continuous growth and no Sr diffusional exchange between crystals and residual liquid. The model shows, firstly, that separates of mineral phases which grow over the same range of residual liquid fraction from a magmatic liquid which crystallizes according to Rayleigh's distillation law, if characterized by different Rb/Sr ratios, define alignments in the ⁸⁷Rb/⁸⁶Sr vs. ⁸⁷Sr/⁸⁶Sr diagram. These alignments are not common isochrons: their slopes and y-intercepts change as crystallization proceeds and the slopes do not reflect the time-span elapsing from magma eruption or emplacement, but may give information about the time-span elapsing from the beginning of growth of the mineral phase., i.e., the time-scale of existence of the magma mass. Secondly, it shows that information may be inferred about the sequence of formation of mineral phases characterized by very low Rb/Sr ratios, simply by comparing the values of the ⁸⁷Sr/⁸⁶Sr ratio.