EXPERIMENTAL CONSTRAINTS ON THE ROLE OF GARNET PYROXENITE IN THE GENESIS OF HIGH-FE MANTLE PLUME DERIVED MELTS

Abstract

The anhydrous phase relations of an uncontaminated (primitive), ferropicrite lava from the base of the Early Cretaceous Paraná-Etendeka continental flood basalt province have been determined between 1 atm and 7 GPa. The sample has high contents of MgO (~14·9 wt %), FeO* (14·9 wt %) and Ni (660 ppm). Olivine phenocrysts have maximum Fo contents of 85 and are in equilibrium with the bulk rock, assuming a KDFe-MgOl-liquid of 0·32. A comparison of our results with previous experimental studies of high-Mg rocks shows that the high FeO content of the ferropicrite causes an expansion of the liquidus crystallization field of garnet and clinopyroxene relative to olivine; orthopyroxene was not observed in any of our experiments. The high FeO content also decreases solidus temperatures. Phase relations indicate that the ferropicrite melt last equilibrated either at ~2·2 GPa with an olivine clinopyroxene residue, or at ≥5 GPa with a garnet-clinopyroxene residue. The low bulk-rock Al2O3 content (9 wt %) and high [Gd/Yb]n ratio (3·1) are consistent with the presence of residual garnet in the ferropicrite melt source and favour high-pressure melting of a garnet pyroxenite source. The garnet pyroxenite may represent subducted oceanic lithosphere entrained by the upwelling Tristan starting mantle plume head. During adiabatic decompression, intersection of the garnet pyroxenite solidus at ~5 GPa would occur at a mantle potential temperature of ~1550°C and yield a ferropicrite primary magma. Subsequent melting of the surrounding peridotite at ~4·5 GPa may be restricted by the thickness of the overlying sub-continental lithosphere, such that dilution of the garnet pyroxenite melt component would be significantly less than in intra-oceanic plate settings (where the lithosphere is thinner). This model may explain the limited occurrence of ferropicrites at the base of continental flood basalt sequences and their apparent absence in ocean-island basalt successions. © The Author 2005. Published by Oxford University Press. All rights reserved.