An Early Garnet Redox‐Filter as an Additive Oxidizer in Lower Continental Arc Crust Traced Through Fe Isotopes

Li, Qiwei; Nebel-Jacobsen, Yona; Zhao, Jun‐Hong; Nebel, Oliver; Richter, Marianne; Cawood, Peter A.; Wang, Qiang

doi:10.1029/2020jb021217

Cited by 5 publications

(3 citation statements)

References 127 publications

(276 reference statements)

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“…It has been proposed that magma crystallization could significantly modify the f O 2 of magmas. For instance, garnet fractionation can trigger oxidation in continental arc lavas (Cao et al., 2021; Q. W. Li et al., 2021; Tang et al., 2018). However, garnet commonly crystallizes at high pressure, and its fractionation is thus favored in subduction zones with thick arc crust (>45 km thick) (Lee & Tang, 2020), whereas the crust of the Troodos ophiolite is only about 6 km thick (Abelson et al., 2001).…”

Section: Discussionmentioning

confidence: 99%

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

Zhou

Ribeiro

et al. 2023

JGR Solid Earth

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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.

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Section: Discussionmentioning

confidence: 99%

Section: Limited Effects Of Shallow-level Processes On Fomentioning

confidence: 99%

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

Zhou

Ribeiro

et al. 2023

JGR Solid Earth

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“…One recent hypothesis suggests that garnet fracDonaDon preferenDally depletes Fe 2+ over Fe 3+ in the melt, thus driving oxidaDon in arcs built on thick crusts . This hypothesis is supported by the extremely low Fe 3+ /∑Fe in magmaDc garnets in deep arc cumulates, the correlaDon between sulfur oxidaDon and crustal thickness [Tang et al, 2020], and arc magma Fe isotopes [Du et al, 2022;Li et al, 2021]. The garnet hypothesis also offers an explanaDon for the preferenDal occurrence of calcalkaline magmas in arcs built on thick crusts [Chen et al, 2023;Tang et al, 2023; and has implicaDons for the role of orogenesis in modulaDng magmaDc volaDle degassing [Sun and Lee, 2022].…”

Section: Introducdonmentioning

confidence: 96%

Reevaluating the oxidation effect of garnet crystallization

Tang,

Wang,

Lee

2023

Preprint

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Garnet has been proposed to be an important fractionating phase during magmatic differentiation in thick volcanic arcs because garnet fractionation can reproduce the hallmarks of continental crust by driving residual magmas to higher silica, lower total iron, and higher iron oxidation states. Recently authors have measured the partition coefficients for Fe2+ and Fe3+ between garnet and melt and concluded that garnet crystallization has little influence on arc magma oxidation. However, these conclusions are based on conceptual errors in modeling the effects of garnet. First, their models intentionally limited the extent of fractionation to minimize the effect of garnet fractionation, inconsistent with existing experiments, including their own experiments. Second, Fe2+ partitioning was indirectly and incorrectly parameterized using Fe-Mg exchange coefficients, inconsistent with experimental partition coefficients. Application of these recent models leads to unrealistic Fe2+ partitioning behavior with progressive fractionation. When these models are corrected with proper mass balance and partitioning relationships, garnet as a driver of oxidation remains a viable and testable hypothesis.

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Amphibole fractionation as a key driver for oxidation of magmas in convergent margins

Luo,

Wang,

Nebel

et al. 2024

Earth and Planetary Science Letters

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An Early Garnet Redox‐Filter as an Additive Oxidizer in Lower Continental Arc Crust Traced Through Fe Isotopes

Cited by 5 publications

References 127 publications

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

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

Reevaluating the oxidation effect of garnet crystallization

Amphibole fractionation as a key driver for oxidation of magmas in convergent margins

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