The Isotopic Composition of Strontium in Oceanic and Continental Basalts: Application to the Origin of Igneous Rocks

The isotopic compositions of strontium in 25 basalts and relatedvolcanic rocks from both oceanic and continental localitieshave been determined. The isotopic abundance of Sr⁸⁷, convenientlyexpressed as the Sr⁸⁷/Sr⁸⁶ ratio, was found to vary from 0?7047to 0?7101. The range of variation is outside experimental errorsand is considered to be significant. No consistent differencein the value of the Sr⁸⁷/Sr⁸⁶ ratio was observed for basaltsfrom continental and oceanic localities. The average Sr⁸⁷/Sr⁸⁶ratio for eleven oceanic basalts was found to be 0?7072?0?0003,whereas fourteen continental basalts and related volcanic rocksaveraged Sr⁸⁷/Sr⁸⁶=0?7082?0?0003. The errors are the standarddeviations of the mean. The average Sr⁸⁷/Sr⁸⁶ ratio for all25 basalts is 0?7078 with limits of variation of +0?002 and–0?003. The concentrations of rubidium and strontium in a representativenumber of the basalts were determined by isotope dilution. Fiveolivine basalts averaged Rb=17?6 p.p.m., Sr=390?55 p.p.m., Rb/Sr=0?044?0?018;whereas six tholeiitic basalts were found to contain 30?7 p.p.m.Rb, 504?167 p.p.m. Sr, Rb/Sr=0?060?0?024. On the basis of the most reliable rubidium and strontium analysesof igneous and sedimentary rocks available in the literaturethe Rb/Sr ratio of the near-surface part of the continentalcrust is estimated to be 0?25. If its average age is approximately2 billion years and its initial Sr⁸⁷/Sr⁸⁶ ratio was 0?704, thepresent average value would be 0?725. This estimate is compatiblewith results obtained for two composites of Palaeozoic shalewhich averaged Sr⁸⁷/Sr⁸⁶=0?7215?0?001. The hypothesis is advanced that there is sufficient enrichmentof Sr⁸⁷ in crustal materials so that the value of the Sr⁸⁷/Sr⁸⁶ratio of igneous rocks at the time of crystallization can beused as a criterion for the origin of the material. The initialSr⁸⁷/Sr⁸⁶ ratio of an igneous rock formed by assimilation, remelting,or granitization of old crustal material enriched in rubidiumis expected to be measurably higher than that of igneous rockswhich are products of fractional crystallization of basalt magmaor were derived from undifferentiated basaltic material at depthin the crust. The initial Sr⁸⁷/Sr⁸⁶ ratio, time of crystallization, and possibleco-magmatic relationships of differentiated intrusive igneousrocks or series of lava flows of different compositions canbe indicated by the convergence of the whole-rock strontiumdevelopment lines. This method is illustrated by the intrusivealkaline rocks of the Monteregian hills, Quebec.