Ferric Iron in Eclogitic Garnet and Clinopyroxene from the V. Grib Kimberlite Pipe (NW Russia): Evidence of a Highly Oxidized Subducted Slab

• Published in: Journal of Petrology
• Main Authors: Marras, Giulia, Mikhailenko, Denis, Mccammon, Catherine A., Agasheva, Elena, Stagno, Vincenzo
• Format: Report
• Language: English
• Published: 2024
• Subjects: Geochemistry & Geophysics; Arkhangelsk Diamondiferous Province; V. Grib kimberlite; eclogites; diamond; metasomatism; redox; Mossbauer spectroscopy; Arkhangelsk; Siberia
• Online Access: http://ir.gig.ac.cn/handle/344008/78423; http://ir.gig.ac.cn/handle/344008/78424; https://doi.org/10.1093/petrology/egae054

Estimates of oxygen fugacity of eclogitic rocks are linked to the redox evolution of the oceanic protolith during subduction and its residence in the lithospheric mantle, and, based on knowledge of pressures and temperatures, allow modelling of the speciation of volatile elements and diamond (or graphite) versus carbonate stability. To date, the oxygen fugacity of mantle eclogites has been shown to vary between -6 (Kasai, Congo and Udachnaya, Siberia) and -0.1 (Udachnaya, Siberia) log units (relative to the fayalite-magnetite-quartz buffer, FMQ), linked to the low Fe3+ contents of garnets. In this study, we investigated the Fe oxidation state of coexisting garnet and clinopyroxene hand-picked out of 17 diamond-free high-MgO and low-MgO mantle eclogites (dated at 2.84 Ga) from the Grib kimberlite pipe (East-European platform). Measured Fe3+/& sum;Fe values range between 0.03 and 0.19 for garnet and 0.18-0.38 for clinopyroxene, the former being higher than what was measured previously in garnets equilibrated at mantle conditions. The Fe3+/& sum;Fe of the reconstructed bulk rock ranges between 0.10 and 0.15 for high-MgO eclogites and 0.10 and 0.24 for low-MgO eclogites (with uncertainties of +/- 0.02 and +/- 0.03 in both cases). Thermobarometric calculations result in equilibration pressures and temperatures of 3.0-5.2 (+/- 0.4) GPa and 720-1050 (+/- 60) degrees C for both high-MgO and low-MgO eclogites, slightly lower than previous P-T estimates of mantle eclogites from the Udachnaya kimberlite pipe (Siberian craton). At these conditions, triangle logfo(2) (FMQ) calculated using the available oxythermobarometric model varies from -1.7 to -0.6 log units for high-MgO eclogites and from -2.9 to 0.9 log units for low-MgO eclogites. Samples recording triangle logfo(2) (FMQ) <= -1 log units overlap with North Slave, West Africa and Udachnaya eclogites, with no difference among eclogite types. The average values of -1.2 (+/- 0.4) log units for high-MgO and -0.6 (+/- 1.1) log units for low-MgO eclogites suggest ...