The oxygen isotope composition of Karoo and Etendeka picrites: High δ18O mantle or crustal contamination?

Abstract

Explanations for the high δ 18 O values in Tuli/Mwenezi picrites are limited to (1) alteration, (2) crustal contamination, and (3) derivation from mantle with an abnormally high δ 18 O. Previously, a variety of models that range from crustal contamination to derivation from the 'enriched' mantle lithosphere have been suggested to explain high concentrations of incompatible elements such as K, and average εNd and εSr values of −8 and +16 in Mwenezi (Nuanetsi) picrites. However, the primitive character of the magmas (Mg# 73), combined with the lack of correlation between δ 18 O values and radiogenic isotopic compositions, MgO content, or Mg# is inconsistent with crustal contamination. Thus, an 18 O-enriched mantle source having high incompatible trace element concentration and enriched radiogenic isotope composition is indicated. High δ 18 O values are accompanied by negative Nb and Ta anomalies, consistent with the involvement of the mantle lithosphere, whereas the high δ 18 O themselves are consistent with an eclogitic source. Magma δ 18 O values about 1 ‰ higher than expected for mantle-derived magma are also a feature of the Bushveld mafic and ultramafic magmas, and the possibility exists that a long-lived 18 O-enriched mantle source has existed beneath southern Africa. A mixed eclogite peridotite source could have developed by emplacement of oceanic lithosphere into the cratonic keel during Archaean subduction.

Petrogenesis of lamprophyres synchronous to kimberlites from the Wajrakarur kimberlite field: Implications for contrasting lithospheric mantle sources and geodynamic evolution of the eastern Dharwar Craton of southern India

“…143 Nd/ 144 Nd and 87 Sr/ 86 Sr isotopic compositions were measured using a NuPlasma HR Multi collector‐ICP‐MS at University of Cape Town. Sample preparation and equipment conditions for the analyses are detailed in Harris et al ().…”

Accelerated Contribution of the Paleo–Congo River to Global Seawater 87Sr/86Sr Change Following Eocene‐Oligocene Collapse of the African Surface

“…The sequential separations of Sr and Nd were done following the stacked‐column method approach of Míková and Denková (), after Pin et al (). Separated Sr and Nd fractions were analyzed using AEON's NuPlasma high‐resolution multicollector ICP‐MS, fitted with a DSN‐100 desolvating nebulizer at UCT (Harris et al, for details). Sr isotope analyses were corrected for Rb interference using the measured 85 Rb signal and the natural 85 Rb/ 87 Rb ratio, while instrumental mass fractionation was addressed using the exponential law and a 86 Sr/ 88 Sr value of 0.1194.…”

Stable isotope constraints on the fluid source of hydrothermal breccia pipes in the Tankwa Karoo depocentre, South Africa: Breakdown of authigenic minerals during sill intrusion

“…The powdered samples were digested in hot HF/HNO 3 using screw-top Savilex beakers. Further details of methods employed are given in Harris et al (2015). The mineral content of the samples in Table 1 was determined by X-ray diffraction (XRD), using a Phillips PW1130/90 X -ray diffractometer employing Ng -filtered Cu-Kα radiation.…”