Redox evolution of western Tianshan subduction zone and its effect on deep carbon cycle

Knowing the phase relations of carbon-bearing phases at high-pressure (HP) and high-temperature (HT) condition is essential for understanding the deep carbon cycle in the subduction zones. In particular, the phase relation of carbon-bearing phases is also strongly influenced by redox condition of subduction zones, which is poorly explored. Here we summarized the phase relations of carbon-bearing phases (calcite, aragonite, dolomite, magnesite, graphite, hydrocarbon) in HP metamorphic rocks (marble, metapelite, eclogite) from the Western Tianshan subduction zone and high-pressure experiments. During prograde progress of subduction, carbonates in altered oceanic crust change from Ca-carbonate (calcite) to Ca,Mg-carbonate (dolomite), then finally to Mg-carbonate (magnesite) via Mg–Ca cation exchange reaction between silicate and carbonate, while calcite in sedimentary calcareous ooze on oceanic crust directly transfers to high-pressure aragonite in marble or amorphous CaCO3 in subduction zones. Redox evolution also plays a significant effect on the carbon speciation in the Western Tianshan subduction zone. The prograde oxygen fugacity of the Western Tianshan subduction zone was constrained by mineral assemblage of garnet-omphacite from FMQ − 1.9 to FMQ − 2.5 ​at its metamorphic peak (maximum P-T) conditions. In comparison with redox conditions of other subduction zones, Western Tianshan has the lowest oxygen fugacity. Graphite and light hydrocarbon inclusions were ubiqutously identified in Western Tianshan HP metamorphic rocks and speculated to be formed from reduction of Fe-carbonate at low redox condition, which is also confirmed by high-pressure experimental simulation. Based on petrological observation and high-pressure simulation, a polarized redox model of reducing slab but oxidizing mantle wedge in subduction zone is proposed, and its effect on deep carbon cycle in subduction zones is further discussed. Keywords: Oxygen fugacity, Western Tianshan, Subduction zone, Deep carbon cycle, Polarized redox model