Distinguishing melting of heterogeneous mantle sources from crustal contamination: Insights from Sr isotopes at the Phenocryst Scale, Pisgah Crater, California

Compositionally heterogeneous basaltic centers from a variety of tectonic environments, including Pisgah Crater in the Mojave Desert region of California, exhibit secular changes in their chemistry that might be explained by the sequential melting of ultramafic to mafic mantle sources. We have analyzed phenocrysts from alkali basalts and hawaiites erupted at Pisgah Crater to investiqate the effects of open-system modifications imposed on basaltic systems. We present ⁸⁷Sr/ ⁸⁶Sr data for individual phenocrysts of amphibole and clinopyroxene and the first published results of single olivine grains, in addition to plagioclase. Each mineral phase exhibits a range in Sr isotope composition that may only partially overlap the isotopic composition of the other mineral phases, suggesting an interplay between two magmatic end-members that continued up to the time of eruption. Limited ⁸⁷Sr/ ⁸⁶Sr variability in minerals from early and intermediate lavas indicates only moderate syn-crystallization open-system modification, whereas minerals in late-erupted lavas have much higher ⁸⁷Sr/⁸⁶Sr, consistent with extensive open-system modification. Rimward increases in ⁸⁷Sr/⁸⁶Sr of plagioclase confirm that these changes occurred within the stability field of plagioclase and, therefore, at crustal or near-crustal depths. The major element compositions of olivine-hosted melt inclusions indicate that an Al-rich component of andesitic composition (⁸⁷Sr/⁸⁶Sr ≥0·7056), possibly derived from plagioclase-rich cumulates or pelites, was assimilated by magma generated from asthenosphere or young lithosphere with ⁸⁷Sr/⁸⁶Sr ≤0·7038. The results clearly demonstrate the utility of measuring the ⁸⁷Sr/⁸⁶Sr of individual minerals and indicate that Pisgah Crater basalts probably acquired isotopically enriched geochemical signatures from crustal contamination, rather than from mixing of heterogeneous mantle melts.