Oxy-thermobarometry of grospydite and high-Ca eclogite xenoliths from Zagadochnaya kimberlite pipe (Yakutia, Siberian craton)

Abstract

Grospydites are a rare variety of eclogites composed of grossular–pyroxene – disthene (kyanite) occurring only in a few kimberlite pipes worldwide. The pressures, temperatures and redox conditions of formation for grospydites have been poorly investigated, even though such parameters are needed to better understand their origin and the scarcity of elemental carbon (graphite/diamond), a common feature of these rocks. In this study, we determined the chemical composition and Fe³⁺/∑Fe of coexisting garnet and clinopyroxene from nine C-free kyanite-bearing eclogite xenoliths from Zagadochnaya pipe (Yakutia). Five samples show garnet with Ca# >45 mol% [Ca# = Ca/(Ca + Mg + Fe + Mn)] and are classified as grospydites, while the remaining samples are high-Ca eclogites with Ca# of 23–38 mol%. Fe³⁺/ΣFe of garnet ranges between 0.03 and 0.09 (± 0.02), overlapping with values measured for eclogitic garnets equilibrated at mantle conditions. Fe³⁺/ΣFe in clinopyroxene spans from 0.21 to 0.46 (± 0.03), slightly higher than values measured in eclogites from other localities worldwide. The calculated equilibration pressures (3.7–6.7 GPa for grospydites and 4.3–5.6 GPa for high-Ca eclogites) and temperatures (920–1220 °C for grospydites and 1010–1060 °C for high-Ca eclogites) result in fo₂ values ranging from − 2.0 to − 0.2 log units for grospydites and − 1.9 to −1.7 for high-Ca eclogites relative to the fayalite-magnetite-quartz buffer. The δ¹⁸O values measured in two garnet-clinopyroxene pairs confirm that samples equilibrated at mantle conditions. Our study extends the knowledge of grospydites in terms of Fe^3+/ΣFe and P-T-fo₂ within the global eclogite database. Despite the finding that equilibration P-T-fo₂ values are compatible with the stability of graphite/diamond, the lack of elemental C in the investigated samples can be explained as either (1) these rocks formed from a slab that lost C during subduction, or (2) these rocks underwent metasomatism with C-poor fluids/melts or (3) a carbon-free layer was sampled by the ascending kimberlitic melts.