An Experimental Study of an Olivine Ugandite Magma and Mechanisms for the Formation of its K-Enriched Derivatives

Abstract

Phase relations in an olivine ugandite from south-west Uganda have been determined between 10 and 40 kb mainly under volatile present conditions, with 5, 15, 25 and 40 wt. per cent added H₂O, and H₂O + CO₂ under oxygen fugacities > NNO buffer. For volatile absent conditions and under H₂O saturated to undersaturated conditions, olivine is the only liquidus phase up to 30 kb with clinopyroxene and minor chromite occurring closer to the liquidus with increasing pressure. Phlogopite occurs up to 20 kb in the hydrous runs about 200 °C below the liquidus. Clinopyroxene is the only liquidus phase at pressures greater than 30 kb under H₂O undersaturated conditions. The absence of garnet and orthopyroxene suggests that it is highly unlikely that the olivine ugandite magma can be derived by partial melting of a peridotitic mantle source in which H₂O is the only volatile. In experiments with CO₂/(CO₂ + H₂O) ∼ 0.75 (mole fraction), garnet + orthopyroxene + clinopyroxene occur near the liquidus at 30–35 kb. Garnet is absent at 20 kb and olivine does not occur near the liquidus. These results suggest that the olivine ugandite magma may be derived by small degrees of partial melting of a peridotitic mantle source containing both H₂O and CO₂ at 2O and CO₂ and the results of the low pressure experiments with H₂O only, a model involving early fractionation of olivine and olivine + clinopyroxene is proposed to explain the K-enrichment in the series from olivine ugandite ↑ugandite ↑ olivine leucitite. The late-stage K-enrichment in this series by which leucitites are derived from olivine leucitites probably involves further fractionation of olivine + clinopyroxene and the addition of phlogopite. More extreme K-enrichment in other lava series of south-west Uganda and elsewhere may involve both a magmatic process and the incorporation of K from xenolithic sources.