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Primary Magmas at Mid-Ocean Ridges, 'Hotspots,' and Other Intraplate Settings: Constraints on Mantle Potential Temperature

Intraplate, “hotspot,” and mid-ocean ridge basalts (MORB) are characteristic of the modern Earth, and within the plate tectonics interpretation of the Earth’s behavior become the basis for a petrological model of the lithosphere and asthenosphere. This model is based on experimental studies of various primitive magmas and of peridotite + (C-H-O), including melting and phase relationships under variable oxygen fugacities ( fO 2 ). The deep mantle plume hypothesis requires a large potential temperature difference (� T p 200‐250 °C) between the upwelling plume and normal ambient mantle as sampled by mid-ocean ridge upwelling and by nonplume intraplate basalts. This core tenet of the deep mantle plume hypothesis is testable by comparison of primitive hotspot magmas with intraplate magmas and particularly with primitive MORB. We have examined picrites and olivine phenocryst compositions from Hawaiian volcanoes and a large new dataset of glass compositions from the Hawaiian Scientific Drilling Project, sampling deep into the main cone-building phase of Mauna Kea volcano. By incremental addition of olivine (reversing the crystallization of magmas saturated only in olivine + Cr-Al spinel) we infer the parental or primitive picrite magma compositions for distinctive compositional suites and individual volcanoes. We then calculate liquidus temperatures (1 atm) and infer conditions of melt separation