Journal Article

Insights Into Subduction‐Zone Fluid‐Rock Interactions and Carbon Cycling From Magnesium Isotopes of Subducted Ophiolitic Mélanges in the Arabian‐Nubian Shield

Gamaleldien, Hamed, Qu, Yuan‐Ru, Liu, Sheng‐Ao

2025

Overview

Fluid‐rock interactions play an important role in element mobilization, mass transfer, and formation of critical metals in subduction zones. However, tracking the multistage fluid‐rock interactions within subduction channels remains elusive. Here we conducted bulk‐rock major and trace element and magnesium (Mg) isotopic analyses on a suite of subducted ophiolitic mélange rocks from Wadi Al Barramiyah in the Arabian‐Nubian Shield of the Eastern Desert (ED) of Egypt. The rock suite includes serpentinites, talc rocks, talc‐dolomite rocks, tremolite‐dominated schists, and marbles. Talc rocks are characterized by low MgO contents and high δ26MgDSM‐3 values (0.03–0.13‰) relative to serpentinites (−0.18‰), indicating the release of isotopically light fluid during the metasomatic replacement of antigorite by talc. Tremolite‐dominated schists and talc‐dolomite rocks display higher CaO contents and lower δ26Mg (−0.25‰ to −0.03‰ and −1.04‰ to −0.18‰, respectively) than those of talc rocks and serpentinites. These signatures, along with high CaO/Al2O3 and low Rb/Sr ratios, indicate infiltration of low‐δ26Mg carbonate‐rich fluids, supported by extremely low δ26Mg (down to −2.38‰) observed in nearby marbles. Our findings demonstrate that antigorite dehydration liberates substantial numbers of H2O‐rich fluids, facilitating the dissolution of carbonate minerals in marbles. Subsequent carbonate metasomatism effectively sequesters carbon from aqueous carbon‐bearing fluids, transforming silicate minerals into carbonates. These new results highlight the significant role of mélange rocks in the multistage fluid‐rock interactions and carbon recycling in subduction zones, offering valuable insights into mantle Mg isotopic heterogeneity and crust‐mantle interactions. Plain Language Summary Subduction zones are geodynamic regions characterized by extensive fluid‐rock interactions that significantly influence the physics and chemistry of Earth's crust and mantle. We investigated Mg mobility and Mg isotope fractionation in subducted ophiolitic mélanges from Wadi Al Barramiyah, located in the Arabian‐Nubian Shield of the ED of Egypt, in order to constrain multistage fluid‐rock interactions in subduction zones. We found distinct Mg isotopic signatures across different rock types, reflecting diverse types and extents of fluid metasomatism. Such components, transported into the mantle via subduction, would cause Mg isotopic heterogeneity in the mantle and mantle‐derived magmas. These findings shed light on crust‐mantle interactions and carbon recycling in subduction zones, emphasizing the utility of Mg isotopes for tracing such processes. Key Points Talc rocks have δ26Mg higher than serpentinites, reflecting the release of 24Mg‐rich fluids Low δ26Mg of tremolite‐dominated schists and talc‐dolomite rocks document carbonate fluid infiltration Mg isotopes reveal multistage fluid‐rock interactions and carbon recycling in subduction zones

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Arabian‐Nubian Shield / Carbon / Carbon cycle / carbon recycling / Carbonate minerals / Carbonates / Dehydration / Dolomite / Dolostone / Earth crust / Fluids / fluid‐rock interaction / Fractionation / Heterogeneity / Isotope fractionation / Magnesium / Magnesium isotopes / Mass transfer / Minerals / Ophiolites / ophiolitic mélanges / Physics / Recycling / Rock / Rocks / Schists / Silicate minerals / Silicates / Subduction / Subduction zones / Trace elements