Revealing Redox Variability in the lithospheric mantle: Insights from Mg-chromite inclusions in diamonds from the Udachnaya kimberlite pipes

The study of mineral inclusions trapped within lithospheric diamonds stands as one of the fundamental approaches to directly study the Earth’s interior, enhancing our comprehension of its chemistry and deep geological processes. The inclusions provide crucial insights into the pressure (P), temperature (T), and redox conditions (fO2) occurring during the diamond nucleation and growth. Among the various mineral inclusions observed in lithospheric diamonds, those characterized by a peridotitic mineral assemblage (P-type), frequently exhibit the presence of Mg-chromite (Stachel et al., 2022) that suggests their involvement in redox-driven diamond formation from CO2-bearing melts. Since the current understanding of the Earth’s interior redox state primarily relies on peridotite samples from the shallow upper mantle based on the Fe3+ content of spinels (Ballhaus et al., 1991), investigating the chemistry of spinel mineral inclusions in diamonds would extend the knowledge of the mantle redox state to greater depths. In this study, we focused on a suite of nine diamonds extracted from the Udachnaya kimberlite pipes, located within the Siberian craton. These diamonds show multiple dark and transparent inclusions with sizes between 30 and 200 µm in diameter. The selected diamonds are polished to expose some of the inclusions. The chemical composition was determined by electron microprobe while textural features were observed by scanning electron microscopy. The Fe3+/ΣFe ratios of both encapsulated and polished inclusions were measured by in situ synchrotron Mössbauer spectroscopy at the ID18 beamline of the ESRF synchrotron (Grenoble, France), employing a 6 x 15 µm2 focused beam. The analyzed Mg-chromites exhibit FeO contents of 15-17 wt% and Cr# and Mg# of 0.85-0.92 and 0.54-0.61, respectively. The Mössbauer data collected on the Mg-chromites show a large variability with Fe3+/ΣFe ratios ranging from 0.07 to 0.28. Interestingly, variable chemical composition for the Mg-chromites among inclusions trapped in the same diamond along with fO2 ranging within 2 log units below the fayalite-magnetite-quartz buffer suggest possible changes in the chemistry of the diamond growth medium providing, therefore, evidence of redox heterogeneities in the lithospheric mantle underneath the Siberian platform.