Release of oxygen (oxidized fluids) by growth of ferromanganese garnets and oxygen circulation in subduction zone

Subduction zones are important for oxygen cycling between deep and surficial Earth reservoirs. Highly oxidizing materials from Earth's surface may be transported to the mantle along with the subducting slabs and alter the redox states of the mantle wedge and the 410-660 km transition zone. The circulation of oxygen or oxidizing fluids in subduction zones and how subducted materials influence the deep mantle are poorly constrained. Here we report growth of Fe-Mn garnet from highly oxidized pelagic ferromanganese chert within a high-pressure metamorphic complex in the Qilian Orogen, North West (NW) China. Two types of ferric-iron-rich garnets were identified. Type I is spessartine garnet that has clear compositional zonation of Al₂O₃-FeOt-MnO oscillatory growth. The Fe³⁺/∑Fe ratio in the core is as high as 1.0, but decreases to 0.1 in the outer rim, along with an increase of Fe²⁺ content and a significant decrease in Mn content. Type II is andradite-calderite garnet that contains extremely high Fe³⁺. Our results have two implications: (1) continuous reduction of Mn⁴⁺ (to Mn²⁺) and Fe³⁺ (to Fe²⁺) from the ferromanganese chert as garnet growth must be accompanied by releasing large quantity of oxygen or highly oxidized fluids during prograde high-pressure metamorphism in subduction zones, and (2) Fe³⁺-rich garnets together with hematite in the subducting slab can remain stable over a large pressure range, and provide an alternative interpretation for the high content of Fe³⁺ in the deep mantle. Our results are significant for understanding the oxygen recycling in oceanic cold subduction zones, and the initiation of the Great Oxygen Event (GOE) as well.