Subduction relics in the subcontinental lithospheric mantle evidence from variation in the δ18O value of eclogite xenoliths from the Kaapvaal craton

Mantle eclogites are commonly accepted as evidence for ancient altered subducted oceanic crust preserved in the subcontinental lithospheric mantle (SCLM), yet the mechanism and extent of crustal recycling in the Archaean remains poorly constrained. In this study, we focus on the petrological and geochemical characteristics of 58 eclogite xenoliths from the Roberts Victor and Jagersfontein kimberlites, South Africa. Non-metasomatized samples preserved in the cratonic root have variable textures and comprise bimineralic (garnet (gt)–omphacite (cpx)), as well as kyanite (ky)- and corundum (cor)-bearing eclogites. The bimineralic samples were derived from a high-Mg variety, corresponding to depths of ~ 100–180 km, and a low-Mg variety corresponding to depths of ~ 180–250 km. The high-Al (ky-, cor-bearing) eclogites originated from the lowermost part of the cratonic root, and have the lowest REE abundances, and the most pronounced positive Eu and Sr anomalies. On the basis of the strong positive correlation between gt and cpx δ 18 O values ( r 2 = 0.98), we argue that δ 18 O values are unaffected by mantle processes or exhumation. The cpx and gt are in oxygen isotope equilibrium over a wide range in δ 18 O values (e.g., 1.1–7.6‰ in garnet) with a bi-modal distribution (peaks at ~ 3.6 and ~ 6.4‰) with respect to mantle garnet values (5.1 ± 0.3‰). Reconstructed whole-rock major and trace element compositions (e.g., MgO variation with respect to Mg#, Al 2 O 3 , LREE/HREE) of bimineralic eclogites are consistent with their protolith being oceanic crust that crystallized from a picritic liquid, marked by variable degrees of partial melt extraction. Kyanite and corundum-bearing eclogites, however, have compositions consistent with a gabbroic and pyroxene-dominated protolith, respectively. The wide range in reconstructed whole-rock δ 18 O values is consistent with a broadly picritic to pyroxene-rich cumulative sequence of depleted oceanic crust, which underwent hydrothermal alteration at variable temperatures. The range in δ 18 O values extends significantly lower than that of present day oceanic crust and Cretaceous ophiolites, and this might be due to a combination of lower δ 18 O values of seawater in the Archaean or a higher temperature of seawater–oceanic crust interaction.