Browsing by Subject "geochemistry"

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    The oxygen, carbon & hydrogen isotope geochemistry of the Phalaborwa Complex, South Africa
    (2023) Munro, Joshua; Harris, Chris
    The 2060 ± 2 Ma Phalaborwa Complex is a pipe-like ultramafic to carbonatite intrusion formed from multiple magma pulses. This produced a main pipe of pyroxenite, carbonatite and foskorite (olivineapatite-magnetite-calcite) and many smaller syenite plugs around the pipe. The complex is the oldest known carbonatite in southern Africa and the only one known to host economic copper deposits. There are no known detailed studies on multi-mineral oxygen and hydrogen isotopes or multi-rock carbonate carbon isotope data for the Phalaborwa Complex. These isotope systems are useful for identifying mineral disequilibrium and alteration, analyzing contamination and constraining the mantle source. Previous work has shown that the nearby Bushveld Complex and Karoo lavas have higher-than-average-mantle oxygen isotope values, and constraining the Phalaborwa Complex magma δ18O values could help place it within the context of major magmatism events on the Kaapvaal Craton. The range in mineral δ18O values for all rock types in the Phalaborwa Complex is 2.24 to 18.3‰, but 90% of diopside and apatite have values within a 1.5‰ range. The δD values for Phalaborwa Complex phlogopite range from -112 to -52‰. The δ18O values of baddeleyite (2.99 ± 0.13‰, 1σ, n=2), olivine (6.12 ± 0.08‰, 1σ, n=2), diopside (7.37 ± 0.40‰, 1σ, n=24), magnetite (3.46 ± 0.75‰, 1σ, n=8), apatite (6.40 ± 0.59‰, 1σ, n=13) and aegirine (6.39 ± 0.65‰, 1σ, n=10) are thought to be magmatic, with most δ18O versus δ18O plots for different mineral pairs signifying equilibrium at high temperatures. Alkali feldspar (9.30 ± 1.15‰, 1σ, n=10) and micas (8.56 ± 3.16‰, 1σ, n=24) have more variable δ 18O values and phlogopite has probably undergone subsolidus exchange or alteration. Magma δ18O values estimated from constituent minerals for pyroxenites (~6.8 to 8.3‰), foskorite (~7.1 & 7.2‰) and carbonatite (~6.5 to 9.0‰) are generally higher than normal mantle magmas. The δ18O value of syenite magma estimated from aegirine is ~8.8 to 10.3‰, and in equilibrium with whole-rock syenite δ18O values (8.7 ± 0.4‰, 1σ, n=5). The phlogopite δD values are consistent with typical mantle magma water δD. Despite evidence for fluid-rock interaction in the carbonatite-foskorite rocks, carbonatite δ13C-δ18O range overlaps with the unaltered, primary igneous carbonatite field (δ18O 8.13 to 12.00‰, δ13C -3.19 to -5.60‰). Local basement rock has average bulk δ18O values of 8.6 ± 0.7‰ (1σ, n=14). This is too low to be a realistic contaminant capable of raising the δ18O values in the mafic rocks, since approximately 66% contamination would be required. The higher δ18O values in the whole-rock syenites are consistent with an origin by partial melting of metasomatized country rock. The high-δ18O values of Phalaborwa Complex pyroxenite-foskorite-carbonatite magmas are from a high-δ18O source, and are similar to those of the Rustenburg Layered Suite of the Bushveld Complex (2060 to 2055 Ma) and Karoo picrites (183 Ma). Hence, it is concluded that the stable isotope data is consistent with a geographically extensive and long-lived high-δ18O mantle source beneath southern Africa.