To understand the oxygen fugacity (fO(2)) during subduction initiation, we examined proto-arc boninites and associated mantle peridotites from the Troodos ophiolite, Cyprus. The Troodos ophiolite represents an exhumed piece of oceanic lithosphere that formed during subduction initiation in Neo-Tethys in Late Cretaceous. Dunites and surrounding harzburgites in the Kokkinorotsos podiform chromite deposit of the ophiolite were formed by reactions between peridotites and ascending boninitic magmas. The olivine-chromite oxybarometers show that the fO(2) values (Delta QFM -0.41 +/- 0.37) of the dunites are slightly higher than those (Delta QFM -0.85 +/- 0.52) of the harzburgites under mantle conditions, indicating that the mantle fO(2) was raised limitedly by percolation of the boninitic magmas. Meanwhile, the boninitic upper pillow lavas in the Margi area also recorded relatively reduced fO(2) values (Delta QFM -0.52 +/- 0.20) during olivine-chromite crystallization, comparable to the fO(2) of mid-ocean ridge basalts. Shallow-level processes (i.e., magma ascent, crystallization, crustal assimilation, and degassing) appear to have limited influence on the estimated fO(2) values in Troodos. The similarly low fO(2) values of the mantle peridotites and boninitic magmas suggest that the Troodos primary boninitic magmas were perhaps not as oxidized as the Izu-Bonin-Mariana (IBM) proto-arc boninitic magmas, though both mantle sources have captured the slab fluids released during subduction inception. This difference might be attributed to the different nature of the slab components (i.e., a more reduced subduction input for Troodos). Our study thus suggests that subducted slabs might not always release oxidized fluids, and the oxidation processes might be different for each nascent subduction zones.

Plain Language Summary Subduction zones have played a key role in mass exchange between the Earth's surface and its interior, and are one of the main factories for the generation of Earth's magmas. The redox state of arc lavas produced at subduction zones controls the formation of important mineral deposits. In fact, subduction zone lavas are believed to be generally more oxidized than mid-ocean ridge lavas, but the origin of their oxidized nature and how their redox state change during the birth of a subduction zone remains poorly understood. Boninites are special Mg- and Si-rich but Ti-poor lavas (SiO2 > 52 wt.%, MgO > 8 wt.%, and TiO2 < 0.5 wt.%), and are typical products of magmatism that formed during subduction initiation. Here, we show that the primary boninitic magmas of the Troodos ophiolite of Cyprus are in fact no more oxidized than mid-ocean ridge basalts, even though their mantle source interacted with the slab fluids released from the incipient subducted plate. Therefore, we propose that proto-arc magmas, that formed as the down-going slab began foundering, might not always be oxidized during the inception of a subduction zone.

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