The Redox State of Incipient Oceanic Subduction Zones: An Example From the Troodos Ophiolite (Cyprus)

• Published in: Journal of geophysical research. Solid earth 2023-04, Vol.128 (4), p.n/a
• Main Authors: Hu, Wen‐Jun, Zhou, Mei‐Fu, Ribeiro, Julia M., Malpas, John, Wu, Ya‐Dong, Bai, Zhong‐Jie
• Format: Article
• Language: English
• Subjects: Basalt; boninites; Chromite; Cretaceous; Crystallization; Degassing; Earth mantle; Earth surface; Fluids; Fugacity; Geophysics; Lava; Lithosphere; Magma; mantle peridotites; Mineral deposits; Oceans; Olivine; Oxidation; Oxygen; oxygen fugacity; Paleoceanography; Percolation; Redox properties; Silica; Silicon dioxide; Slabs; Subduction; Subduction (geology); subduction initiation; Subduction zones; Titanium dioxide; Troodos ophiolite
• ISSN: 2169-9313; 2169-9356
• DOI: 10.1029/2022JB025008

To understand the oxygen fugacity (fO2) 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 fO2 values (∆QFM −0.41 ± 0.37) of the dunites are slightly higher than those (∆QFM −0.85 ± 0.52) of the harzburgites under mantle conditions, indicating that the mantle fO2 was raised limitedly by percolation of the boninitic magmas. Meanwhile, the boninitic upper pillow lavas in the Margi area also recorded relatively reduced fO2 values (∆QFM −0.52 ± 0.20) during olivine‐chromite crystallization, comparable to the fO2 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 fO2 values in Troodos. The similarly low fO2 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