Primary sulphide melt inclusions in mantle-derived megacrysts and pyroxenites

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    Abstract

    Inclusions of sulphides are common in clinopyroxene megacrysts and Al-augite pyroxenite xenoliths in undersaturated continental basalts. The sulphides are typically FeS with 2-4 wt.% Ni and minor Co and Cu. The morphology of the inclusions and their relations to growth planes in the pyroxenes show that the sulphides were trapped as drops of immiscible melt. These nucleated on the surfaces of crystals growing from magmas, and are thus primary inclusions. Compound CO2-sulphide inclusions are evidence for the coexistence of three immiscible fluids: silicate melt, sulphide melt and supercritical CO2. Hollow tabular to spherical sulphide inclusions result from the trapping of sulphide melts with = or <10 wt.% CO2 in solution. Primary CO2 inclusions have densities of 1.07-1.189 g/cm3; this corresponds with trapping P of 10-15 kbar at 1000-1200oC and confirms the origin of the megacrysts in the uppermost mantle. A wide variety of secondary inclusions formed by the decrepitation of the sulphide-CO2 inclusions during entrainment and transport by the host basalt. Clinopyroxene megacrysts containing sulphide inclusions show a very narrow compositional range, compared to the worldwide population of Al-augite megacrysts. This suggests that S-saturation was reached at a similar stage of mantle fractionation in similar magmas. Dumping of S (and Ni, Co, Cu) in the uppermost mantle is an important part of the overall process of mantle metasomatism. (Authors' abstract)-G.R. Mineralogisk-Geologisk Museum, Sars Gate 1, 0562 Oslo 5, Norway.

    Original languageEnglish
    Pages (from-to)279-294
    Number of pages16
    JournalLithos
    Volume20
    Issue number4
    Publication statusPublished - 1987

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