Phlogopite in the generation of olivine-melilitites from Namaqualand, South Africa and implications for element fractionation processes in the upper mantle

Abstract

Major and trace element and Sr, Nd and Pb isotope analyses are presented for thirteen olivine-melilitites from Namaqualand, South Africa. Major element variations are consistent with derivation from carbonated garnet-peridotite at depths of at least 100 km and trace element abundances indicate melt fractions of ⩽ 4%. Ubiquitous negative K anomalies and low, buffered K 2O concentrations are interpreted to reflect the effect of residual phlogopite during melting. It is suggested that phlogopite stability and low melt potassium saturation concentrations are enhanced by high CO 2/(CO 2 + H 2O) conditions. Residual phlogopite can also account for low measured Rb/Sr, Ba/Sr and Th/U ratios in the melilitites. REE abundances are controlled by residual garnet and hence Sm/Nd ratios are low (0.13–0.18). U/Pb ratios vary from 0.05 to 5 and are a function of Pb concentration which is in turn controlled by residual Pb-rich phase (probably sulphide). Nd and Sr isotopes are comparable with OIB from St. Helena, although two samples extend to higher 87Sr/ 86Sr ratios. Present day Pb isotopes are much more variable and partly reflect radiogenic growth since emplacement as a result of the highly variable U/Pb ratios. Many of the trace element characteristics of the melilitites are distinct from those of within-plate potassic magmas despite both being derived from phlogopite-bearing, enriched mantle source regions. This can be attributed to the depth at which source enrichment occurred and the subsequent control exerted by phlogopite and carbonate during melting. In contrast to melilitites, potassic magmas are derived from shallower depths under low CO 2/(CO 2 + H 2O) conditions and at higher temperatures at which phlogopite melts more readily. The incompatible element ratios of the melilitites are also similar to those both observed in HIMU ocean island basalts (OIB) and inferred for HIMU OIB source regions from isotope variations (viz, low Sm/Nd, Rb/Sr, K/Nb, Th/U and high U/Pb and Ce/Pb). It is suggested that HIMU OIB's may be derived from sources that have been subject to enrichment by a melt generated in the presence of residual phlogopite.