Petrology and Geochemistry of 3.3 Ga Komatiites– Weltevreden Formation, Barberton Greenstone Belt

Introduction: Komatiites provide several ways of assessing the chemical and thermal evolution of the mantle. Komatiitic liquids are produced from very high degrees of partial melting of the deep mantle, and erupted as extremely fluid lavas. Komatiites are important in the study of the early Earth because they are a significant constituent of the Archean crust. It has been proposed that komatiitic liquids are also an important component of volcanism on Io. On Earth they are found almost entirely within the first two billion years of the geologic record -the Archean. In this study we present petrographic and geochemical data on an unusually fresh sequence of these rocks, as well as a predicted fractional crystallization model which accounts for several critical observed traits of these flows. Background: The Barberton Greenstone Belt (BGB) was formed during the period ~3.6 – 3.2 Ga. It consists of three stratigraphic groups, the 3.6-3.3 Ga Onverwacht Group which contains a number of komatiitic sequences, the 3.3-3.2 Ga Fig Tree Group of arc-related volcanics and sediments, and the 3.2 Ga Moodies Group [1]. The 3.3 Ga Weltevreden Formation is located in the northern facies of the BGB (north of the Inyoka Fault), is about 180 Ma younger than the Komati Formation, and correlative with the Mendon Formation, both in the southern facies. The Weltevreden and Mendon Formations lie conformably beneath basal rocks of the Fig Tree Group. The thickness of the Weltevreden is unknown because the base is not exposed, but many hundreds and possibly a few thousand meters of section are present, with individual flow units being 10-500m [1]. Petrography: Thinner komatiitic flows of the Weltevreden Formation, the subject of this report, are compositionally layered into random spinifex, oriented spinifex, and peridotite lithologies. The spinifex layers consist of acicular olivine, with chromite, orthopyroxene, pigeonite, and augite (Figure 1a). Pyroxene consists of fine acicular orthopyroxene cores with pigeonite and augite rims. Pyroxene also displays dendritic-barred textures. Augite also occurs as fine discrete crystals. Chromite occurs as equant and cruciform morphologies. The peridotite layers consist of fresh equant olivine phenocrysts with serpentinized rims and veins, discrete augite, fine acicular orthopyroxene crystals rimmed with clinopyroxene and equant and cruciform chromite (Figure 1b). Figure 1. (a) Random spinifex composed of olivine, now serpentine, and fresh orthopyroxene rimmed by augite. (b). Peridotite composed of olivine phenocrysts and intersertal orthopyroxene. Both photomicrographs taken in crossed-polarized light.