Ground melting and ocellar komatiites: a lead isotopic study at Kambalda, Western Australia

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Abstract
Stratigraphically and geographically restricted ocellar komatiite flows at Kambalda, Western Australia, appear to represent the products of ground melting of sulphidic sediments by komatiites in lava channels that localized the Fe–Ni–Cu sulphide ores. An immiscible sulphide liquid formed and gravitationally separated from the melted sediment (xenomelt), the resultant buoyant silicate liquid being partly or wholly assimilated by the turbulently convecting komatiite magma. Rarely, the xenomelt gravitationally migrated to the top of flows, and overflowed into the less turbulent lava levees where it collected to form a separate layer overlying a komatiitic layer within a single flow. There was selective preservation of the hybrid felsic layer, as an upper ocellar unit within an ocellar komatiite flow, in lava levees flanking lava channels. The ocellar unit is enriched in elements previously concentrated in the sediments, and shows U–Th–Pb isotopic systematics akin to the underlying sediments. Moreover, the partitioning relationships of U and Pb between the immiscible xenomelt and sulphide liquid enhances the range of U/Pb ratios for components of the ocellar unit, thus allowing sufficient spread of modern uranogenic Pb isotopic ratios to form isochrons, albeit imprecise ones. The range and similarity of model Th/U data from these flows (2.8−3.9) and adjacent sulphidic sediments (2.3−4.4; mostly 2.8−3.9) contrasts with the generally invariable Th/U within Kambalda ultrabasic–basic flows (3.6−3.9), and further supports the ground-melting hypothesis.