Résumé
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 Δlog
f
O
2
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
2
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
f
O
2
decrease with increasing depth, by a change in speciation from oxidized H
2
O to reduced H
2
in line with thermodynamic modelling of fluid-saturated and undersaturated peridotite. Still, despite our efforts we did not observe H
2
in the samples.