The redox state of the heterogeneous mantle: insights from deeply recycled C-S-bearing crustal materials

Fossil subduction zones are critical for studying the deep geochemical cycles of carbon (C), oxygen (O), and sulfur (S). This study focuses on graphite-sulfides-magnetite-bearing garnet clinopyroxenites from the External Ligurian region (Northern Apennines, Italy) as indicators of deep recycling processes of subducted crust. These rocks crystallized from eclogite-derived melts (P ≥ 3 GPa and 1100 °C) after undergoing prolonged recycling in the mantle. Their unique composition provides valuable insights into the redox state and partitioning of Fe³⁺/Fe²⁺ associated with carbon and sulfur during subduction and subsequent mantle processes.

Using TEM-EELS and Synchrotron micro-Mössbauer analyses, we observed significant heterogeneities in Fe³⁺/Fe²⁺ distribution and its partitioning among mineral phases. Clinopyroxenites exhibit three generations of clinopyroxenes: unexsolved crystals in garnet cores with Fe³⁺/ΣFe = 0.16–0.38, clinoenstatite-exsolution-bearing grains with Fe³⁺/ΣFe = 0.03–0.10, and Al-poorer rims devoid of Fe³⁺. In contrast, garnets show Fe³⁺/ΣFe-poor cores (0–0.03) and slightly higher ratios in the rims (0.04–0.07). These variations indicate a progressive redistribution of Fe³⁺ between garnets and clinopyroxenes in response to temperature decreases from 1100 to 950 °C.

Calculated oxygen fugacities (fO₂) reveal notable variations. At 3 GPa, the samples range from oxidized (ΔFMQ = -1.25 to 0) to reduced (ΔFMQ = -4.2 to -1.6). At 1.5 GPa, values span from -1.2 to -0.6 to below -5, suggesting that graphite likely formed through the reduction of previously oxidized carbon phases. This redox evolution is attributed to sub-solidus decompression in a closed system, with no significant fluid or melt-rock interaction.

The findings highlight the potential of these clinopyroxenites to record the intricate interplay of redox conditions, temperature, and pressure during subduction. The results also underscore the importance of mantle recycling processes in governing the fate of carbon and sulfur in the Earth's interior. By shedding light on these processes, this study opens new perspectives on the geochemical cycles of volatile elements within the convective mantle.