Abstract

Komatiitic flows (ultramafic lavas with MgO > 18%) from the small Tipasjärvi Archean greenstone belt (eastern Finland) are of two types: (1) thin lava flows with microspinifex textures showing no chemical variation along a vertical cross section; (2) a thicker flow with important chemical variation in which macroscopic spinifex textures are found within small veins in the upper part of this flow. Two different trends of fractional crystallization can be demonstrated by major- and some trace-element variations in the upper and the lower parts of the flow: olivine crystallizes and settles to the base of the flow; in the upper part, the major phase that crystallizes seems to be clinopyroxene.Such double-trend fractionation has also been shown in some other komatiitic flows of similar ages from Australia and Canada. However, some komatiitic flows indicate a simpler fractionation process involving olivine only.Rare-earth-element (REE) distribution patterns in the thick Finnish flow show two quite different patterns. The upper part of the flow has strongly LREE-depleted patterns, whilst those in the lower part of the flow are flat. It is suggested that the flow is composite and that two source regions are involved.

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