Redox processes in subducting oceanic crust recorded by sulfide-bearing high-pressure rocks and veins (SW Tianshan, China)

Journal article


Publication Details

Author(s): Li J, Gao J, Klemd R, John T, Wang XS
Journal: Contributions To Mineralogy and Petrology
Publication year: 2016
Volume: 171
Journal issue: 8
Pages range: 1-24
ISSN: 0010-7999
Language: English


Abstract

The oxidized nature of the sub-arc mantle and hence arc magmas is generally interpreted as a result of the migration of subduction-related oxidizing fluids or melts from the descending slab into the mantle wedge. This is of particular importance seeing that the oxidization state of sub-arc magmas seems to play a key role in the formations of arc-related ore deposits. However, direct constraints on the redox state of subducted oceanic crust are sparse. Here, we provide a detailed petrological investigation on sulfide- and oxide-bearing eclogites, blueschists, micaschists, eclogite-facies and retrograde veins from the Akeyazi high-pressure (HP) terrane (NW China) in order to gain insight into the redox processes recorded in a subducting oceanic slab. Sulfides in these rocks are mainly pyrite and minor pyrrhotite, chalcopyrite, bornite, molybdenite, sphalerite and chalcocite, including exsolution textures of bornite–chalcopyrite intergrowth. Magnetite, ilmenite and pyrite occur as inclusions in garnet, whereas sulfides are dominant in the matrix. Large pyrite grains in the matrix contain inclusions of garnet, omphacite and other HP index minerals. However, magnetite replacing pyrite textures are commonly observed in the retrograded samples. The eclogite-facies and retrograde veins display two fluid events, which are characterized by an early sulfide-bearing and a later magnetite-bearing mineral assemblage, respectively. Textural and petrological evidences show that the sulfides were mainly formed during HP metamorphism. Mineral assemblage transitions reveal that the relative oxygen fugacity of subducted oceanic crust decreases slightly with increasing depths. However, according to oxygen mass balance calculations, based on the oxygen molar quantities (nO2), the redox conditions remain constant during HP metamorphism. At shallow levels (<60 km) in the subduction channel, interaction with oxidized fluid seems to have caused an increase of the oxygen fugacity and the oxidation state of exhuming HP rocks. This study suggests that oxygen components are not released in significant amounts during HP metamorphism of subducted oceanic crust and, thus, cannot be responsible for oxidizing the mantle wedge and increasing the oxidation state of sub-arc mantle melts.


FAU Authors / FAU Editors

Klemd, Reiner Prof. Dr.
Professur für Geochemie und Lagerstättenkunde
Li, Jilei
Professur für Geochemie und Lagerstättenkunde


External institutions with authors

Chinese Academy of Sciences (CAS) / 中国科学院
Freie Universität Berlin


How to cite

APA:
Li, J., Gao, J., Klemd, R., John, T., & Wang, X.-S. (2016). Redox processes in subducting oceanic crust recorded by sulfide-bearing high-pressure rocks and veins (SW Tianshan, China). Contributions To Mineralogy and Petrology, 171(8), 1-24. https://dx.doi.org/10.1007/s00410-016-1284-2

MLA:
Li, Jilei, et al. "Redox processes in subducting oceanic crust recorded by sulfide-bearing high-pressure rocks and veins (SW Tianshan, China)." Contributions To Mineralogy and Petrology 171.8 (2016): 1-24.

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Last updated on 2019-06-06 at 22:10