Superchondritic Nb/Ta and Zr/Hf ratios in peridotites and eclogites of the subcontinental lithospheric mantle: data from mantle xenoliths

Abstract

The analysis of author’s and literary data on concentrations of Nb, Ta, Zr and Hf and on Nb/ Ta and Zr/Hf relations in eclogite and peridotite xenoliths from a subcontinental lithospheric mantle (SCLM) beneath Pcm cratons allowed us to establish the following: (1) Xenoliths of mantle peridotites from the kimberlites of Kaapvaal, North American and Siberian cratons (temperature from 900–1000 to 1350–1400 °C, pressure from 1.5–2.0 to 5–6 GPa, Δ log f O2 FMQ from –5 to –3) are enriched with Nb and Ta relative to chondrite CI and primitive mantle PM, whereas peridotites from the mantle of the North China craton are close to CI in their content or depleted in comparison to it; (2) Despite significant fluctuations, superhondritic values of Nb/Ta (> 17.4) and Zr/Hf (> 36.0) are characteristic of most peridotite xenoliths from the subcratonic mantle, with the exception of North China craton mantle xenoliths, where they are close to or below CI; (3) In the xenoliths of eclogites from the mantle of the Kasai, Man, Slave cratons (temperature from 900–1000 to 1400–1500 °C, pressure from 3 to 5–6 GPa), the content of Nb is 2–3 orders of magnitude higher than those in CI and PM, and the ratio of Nb/Ta ranges from chondritic to superchondritic values. The results of the analysis thus indicate that the SCLM zones, corresponding to the area of diamond stability, are one of the tanks in BSE with superchondritic Nb/Ta and Zr/Hf values.