Strontium isotope disequilibrium in a porphyritic alkali basalt and its bearing on magmatic processes

Partly resorbed, strontium-rich phenocrysts of feldspar, apatite, kaersutite, and biotite in a 19 m.y.-old basaltic dike rock from Trans-Pecos, Texas, have initial Sr87/Sr86 ratios of about 0.7026. Untreated and acid-leached samples of the groundmass, which is alkalic and strontium rich, have initial Sr87/Sr86 ratios of about 0.7041. The magma from which the large phenocrysts grew evidently became contaminated with radiogenic strontium during the process of emplacement. Although the temperature of the assemblage must have been about 1000°C at time of intrusion, the magma cooled before the strontium in the phenocrysts and in the matrix could equilibrate isotopically. The results show that analyses of both groundmass and phenocrysts of some volcanic and hypabyssal igneous rocks can yield a more complete magmatic history for the rocks than whole-sample analyses alone.