Light Fe isotopes in arc magmas from cold subduction zones: Implications for serpentinite-derived fluids oxidized the sub-arc mantle. (1st February 2023)

• Record Type: Journal Article
• Authors: Chen, Zuxing; Chen, Jiubin; Tamehe, Landry Soh; Zhang, Yuxiang; Zeng, Zhigang; Zhang, Ting; Shuai, Wangcai; Yin, Xuebo
• Is Part Of: Geochimica et cosmochimica acta. Volume 342(2023)
• Page Start: 1
• Page End: 14
• Publication Date: 2023-02-01
• Subjects: Fe isotopes -- Arc magmas -- Serpentinite dehydration -- Oxygen fugacity -- Cold subduction zones; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2022.12.005
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Arc basalts typically have higher Fe 3+ /ΣFe ratios but lighter Fe isotopic compositions than mid-ocean ridge basalts (MORB); however, the reasons for these differences are unclear. Here, we report new Fe isotope data for fresh lavas from the Mariana subduction zone, along with available Fe-B-Sr-Nd isotope and B/Nb ratio data for global arc basalts, to constrain the role of serpentinite dehydration in sub-arc mantle oxidation. The basaltic lavas from the Mariana Arc, like other arc basalts from cold subduction zones in the west Pacific, have lighter Fe isotope compositions (δ 56 Fe = 0.02 ± 0.02‰; 1SD; n = 9) than the MORB-like δ 56 Fe values of the middle Mariana Trough basalts (δ 56 Fe = 0.09 ± 0.02‰; 1SD; n = 8). Notably, the significant negative correlations between the δ 56 Fe values and the proxy for the addition of serpentinite-derived fluids (i.e., B isotopes and B/Nb ratios) indicate that the serpentinite-derived fluid contribution is a first-order control on the across-arc Fe isotope variations. Dehydration of subducted slab serpentinites or dragged-down forearc serpentinites at sub-arc depths can release isotopically light Fe fluids in the form of Fe 2+ –SOx complexes. Sulfate-rich fluids containing adequate Fe to metasomatize the sub-arc mantle can cause light Fe isotope compositions and high Fe 3+ /ΣFe ratios in arc basalts. Alternatively, the low δ 56 Fe values and high Fe 3+ /ΣFe ratios of arc magmas may be generated by direct melting of the mélange, which includes dragged-down sulfate-rich forearc serpentinites. Comparatively, serpentinite-derived components are absent at back-arc depths because of the earlier breakdown of serpentine minerals. Accordingly, the back-arc basin basalts exhibit MORB-like δ 56 Fe-δ 11 B values and B/Nb–Fe 3+ /ΣFe ratios. Therefore, the across-arc Fe isotope variations and the decoupling of Fe 3+ /ΣFe and δ 56 Fe in global arc magmas can be explained by the different contributions of subducted serpentinites.