Immobile behaviour of PGE in highly altered ophiolitic mantle rocks from Outokumpu, Finland

摘要

1 IntroductionThe Outokumpu ophiolite assemblage consists of fragments and massifs of sub-oceanic lithospheric mantle peridotites (predominantly dunites and harzburgites now altered to serpentinite), mantled and enclosed in black shales and turbidites of the 1.9 Ga Svecofennian fold belt. The ophiolitic rocks have up to 40 m -wide selvages of carbonate, calc silicate and quartz rocks. These selvages represent alteration and metasomatic zones, as indicated by their high chromite, and low Zr, Th and other lithophile elements. Here, we present PGE data on the mantle rocks showing that the serpentinites and the altered rocks show virtually identical Cr-Ni-PGE distribution patterns, in all cases strongly resembling those of other oceanic and cratonic mantle rocks from elsewhere. This indicates that the PGE and Cr behaved extremely immobile during alteration and that the PGE are likely hosted by chromite, either in solid solution or in the form of included PGE rich phases.