A high-δ18O mantle source for the 2.06 Ga Phalaborwa Igneous Complex, South Africa?

Abstract

The 2060 ± 2 Ma Phalaborwa Complex is a pipe-like, ultramafic to carbonatite intrusion formed from multiple magma pulses. The complex is made up of a main pipe consisting of clinopyroxenites, ultramafic pegmatoids, carbonatites and foskorite (olivine-apatite-magnetite-calcite assemblage), surrounded by many smaller syenite plugs. The range in mineral δ18O values for all rock types and minerals analysed in the Phalaborwa Complex is 2.24 to 18.3‰. However, 24 analyses of the most abundant and robust mineral, diopside, all have δ18O values between 6.2 and 7.7‰. The δ18O values of baddeleyite, olivine, diopside, magnetite, apatite and aegirine are thought to be magmatic. Most mineral pairs have differences in δ18O value that are consistent with magmatic equilibrium at high temperatures down to closure temperature. Alkali feldspar and phlogopite have more variable δ18O values, and both minerals may have undergone subsolidus O-exchange. The δD values for petrographically fresh Phalaborwa Complex phlogopite range from -77 to -48‰ with a mean of -64 ± 9‰ (1σ, n=19). The phlogopite δD values are consistent with subduction-related magmatic water. Despite petrographic evidence for fluid-rock interaction in the carbonatite-foskorite rocks, the carbonatite δ13C and δ18O range (δ18O, 8.13 to 12.00‰; δ13C, -3.19 to -5.60‰) overlaps with the unaltered, primary igneous carbonatite field.

Magma δ18O values estimated from silicate and oxide minerals are mostly higher than normal mantle magmas (pyroxenites, ~7.6‰; foskorite, 7.2‰). The δ18O value of syenite magma estimated from aegirine is 7.8 ± 0.9‰ (1σ, n=8), in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n=5). Local basement rocks have average bulk δ18O values of 8.6‰, and realistic proportions of assimilation by a mantle-derived magma (δ18O, 5.7‰) could not have produced the δ18O values in the pyroxenites or foskorites. Instead, it is proposed that the high-δ18O values of Phalaborwa Complex magmas reflect that of the mantle source. High δ18O values are also a feature of the Rustenburg Layered Suite of the Bushveld Complex (2060 to 2055 Ma), which may have had a similar high-δ18O mantle source. The higher δ18O values of the satellite syenites are consistent with an origin by partial melting of metasomatised country rock.