Primary fractionation and secondary alteration within an Archean ultramafic lava flow

Fifteen samples across a 4 m thick komatiite flow from the Val d'Or region in the southeast portion of the Abitibi greenstone belt have been analysed for major oxides and trace elements including the rare earth elements (REE). The flow has been subjected to low grade regional metamorphism: virtually all primary mineralogy (olivine and clinopyroxene) has been obliterated although primary textures are well preserved. Compositional differences between the upper, spinifex textured portion and the underlying massive portion of the flow are largely consistent with the primary fractionation of approximately 30% olivine with a composition close to Fo92. Variations in incompatible element ratios across the flow and in enrichments between the spinifex and massive units suggest that Si, Ce and possibly Lu have been lost to, and Ca, Sr and possibly Y have been gained from, the surroundings. Remobilization of other elements (e.g. Zr, Hf, and alkali metals and most of the REE) appears to have been confined within the flow. AI, Ti, V and Sc appear to have been immobile during alteration of the flow. For the flow as a whole values for many element ratios (e.g. Al/Zr, Al/Sc, Sc/Yb, Zr/Hf, K/Rb) are very similar to chondritic values. If the flow represents a 40% melt approximately, and if the residue was essentially dunitic then the source abundance for most elements was close to chondritic; exceptions are the REE (1.5 to 2×chondrite), Ti (enriched relative to chondrites), and V (depleted relative to chondrites).