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 varietyof tectonic environments, including Pisgah Crater in the MojaveDesert region of California, exhibit secular changes in theirchemistry that might be explained by the sequential meltingof ultramafic to mafic mantle sources. We have analyzed phenocrystsfrom alkali basalts and hawaiites erupted at Pisgah Crater toinvestigate the effects of open-system modifications imposedon basaltic systems. We present ⁸⁷Sr/⁸⁶Sr data for individualphenocrysts of amphibole and clinopyroxene and the first publishedresults of single olivine grains, in addition to plagioclase.Each mineral phase exhibits a range in Sr isotope compositionthat may only partially overlap the isotopic composition ofthe other mineral phases, suggesting an interplay between twomagmatic end-members that continued up to the time of eruption.Limited ⁸⁷Sr/⁸⁶Sr variability in minerals from early and intermediatelavas indicates only moderate syn-crystallization open-systemmodification, whereas minerals in late-erupted lavas have muchhigher ⁸⁷Sr/⁸⁶Sr, consistent with extensive open-system modification.Rimward increases in ⁸⁷Sr/⁸⁶Sr of plagioclase confirm that thesechanges occurred within the stability field of plagioclase and,therefore, at crustal or near-crustal depths. The major elementcompositions of olivine-hosted melt inclusions indicate thatan 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 younglithosphere with ⁸⁷Sr/⁸⁶Sr 0·7038. The results clearlydemonstrate the utility of measuring the ⁸⁷Sr/⁸⁶Sr of individualminerals and indicate that Pisgah Crater basalts probably acquiredisotopically enriched geochemical signatures from crustal contamination,rather than from mixing of heterogeneous mantle melts. KEY WORDS: assimilation; basalts; melt inclusions; minerals; Sr isotopes