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<p>Kimberlites allow exceptional insights deep into cratons by rapidly bringing to the surface well-preserved mantle fragments from a broad depth range. Water, mainly dissolved as hydroxyl, OH, is a key parameter for craton long-term stability as it affects viscosity and melting properties. We report here a multi-disciplinary (FTIR, Mössbauer, XANES, EPMA, LA-ICPMS) study of 17 harzburgite and dunite xenoliths from the Jagersfontein mine (Kaapvaal, South Africa) that are typical residues of high-degree melting (Mg# 92–94) and display minimum metasomatic interaction with the kimberlite as evidenced using trace elements. Thermobarometry yields 742–887&#xa0;°C and 30-37.5 kbar for the equilibrium conditions of garnet free peridotites and 674–1084&#xa0;°C and 27–51 kbar for garnet bearing peridotites. Oxygen fugacity, expressed as Δlog<i>f</i>O<sub>2</sub> relative to the FMQ buffer, varies between − 1.32 and − 0.29 ± 0.5 in the Cr-spinel bearing peridotites and between − 2.87 and − 0.57 ± 0.5 in garnet peridotites. OH content ([OH]) in the bulk rock varies between 29 and 99 ppm wt. H<sub>2</sub>O. Considering the low evidences of metasomatic interactions within the peridotites and the OH equilibrium between minerals, we suggest that the hydrogen content within the xenoliths remained pristine for billions of years. The [OH] decrease with depth can be explained by melting or, because bulk [OH] and Δlog<i>f</i>O<sub>2</sub> decrease with increasing depth, by a change in speciation from oxidized H<sub>2</sub>O to reduced H<sub>2</sub> in line with thermodynamic modelling of fluid-saturated and undersaturated peridotite. Still, despite our efforts we did not observe H<sub>2</sub> in the samples.</p>

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How dry is the Archean mantle lithosphere beneath the Kaapvaal craton, southern Africa?

  • 期刊论文

  • 发表日期：

    2026年5月13日

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  • Jessy Dominique,
  • Bertrand N. Moine,
  • Ioana B. Radu,
  • Jean-Luc Devidal,
  • Ana E. Pradas del Real,
  • Gelu Costin,
  • Dmitri A. Ionov,
  • Nathalie Bolfan-Casanova

摘要

Kimberlites allow exceptional insights deep into cratons by rapidly bringing to the surface well-preserved mantle fragments from a broad depth range. Water, mainly dissolved as hydroxyl, OH, is a key parameter for craton long-term stability as it affects viscosity and melting properties. We report here a multi-disciplinary (FTIR, Mössbauer, XANES, EPMA, LA-ICPMS) study of 17 harzburgite and dunite xenoliths from the Jagersfontein mine (Kaapvaal, South Africa) that are typical residues of high-degree melting (Mg# 92–94) and display minimum metasomatic interaction with the kimberlite as evidenced using trace elements. Thermobarometry yields 742–887 °C and 30-37.5 kbar for the equilibrium conditions of garnet free peridotites and 674–1084 °C and 27–51 kbar for garnet bearing peridotites. Oxygen fugacity, expressed as ΔlogfO2 relative to the FMQ buffer, varies between − 1.32 and − 0.29 ± 0.5 in the Cr-spinel bearing peridotites and between − 2.87 and − 0.57 ± 0.5 in garnet peridotites. OH content ([OH]) in the bulk rock varies between 29 and 99 ppm wt. H2O. Considering the low evidences of metasomatic interactions within the peridotites and the OH equilibrium between minerals, we suggest that the hydrogen content within the xenoliths remained pristine for billions of years. The [OH] decrease with depth can be explained by melting or, because bulk [OH] and ΔlogfO2 decrease with increasing depth, by a change in speciation from oxidized H2O to reduced H2 in line with thermodynamic modelling of fluid-saturated and undersaturated peridotite. Still, despite our efforts we did not observe H2 in the samples.