Reduction of buried oxidized oceanic crust during subduction |
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| Institution: | 1. State Key Laboratory for Continental Tectonics and Dynamics, Institute of Geology, CAGS, Beijing 100037, China;2. School of Earth Sciences and Resources, China University of Geosciences Beijing, 20 Xueyuan Road, Beijing 100083, China;1. School of Earth Sciences, Zhejiang University, Hangzhou 310027, China;2. Key Laboratory of Crustal Dynamics, Institute of Crustal Dynamics, CEA, Beijing 100085, China;3. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;4. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;1. Institut des sciences de la Terre, University of Lausanne, 1015 Lausanne, Switzerland;2. ISTEP, UMR CNRS 7193, UPMC Sorbonne Universités, 75252 Cedex 05 Paris, France;3. Geosciences Rennes, UMR CNRS 6118, Université de Rennes 1, 35042 Rennes Cedex, France;4. Applied Geosciences, GUtech, PO Box 1816, Athaibah, PC 130, Sultanate of Oman;5. Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland |
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| Abstract: | The relationship among subducted oxidized oceanic crust and oxidation state of the subarc mantle, and arc magmas is one of the important aspects to evaluate convergent margin tectonics. However details of the oxidized mass transferred from buried oceanic crust to the overlying subarc mantle wedge remain obscure. Here we investigate the Songduo eclogites from south Tibet formed by the subduction of the paleo-Tethyan oceanic crust, and identify an abrupt decrease in pyrope and increase in almandine contents from the mantle to rim of garnet grains. This is coupled with a decrease in the Fe3 + content of epidote and Fe3 +/(Fe2 ++ Fe3 +) ratios from garnet core to rim domains, as well as speciation of calcite, a new mineral phase, in the rock matrix. Minor sulfates occur only as inclusions in garnet core domains, whereas sulfides are confined to the matrix as an accessory mineral phase. Aegirine augite occurs as relics or inclusions in garnet and omphacite. These features clearly suggest that oxidized components, Fe3 + and S6 +, were reduced as Fe2 + and S2 ?, respectively, at the subduction zone. Thermodynamic modeling in the P–T-log10fO2 space using updated Perplex_X programs further revealed that the Songduo eclogites experienced oxygen fugacity variation of up to 8 log10 units, with decreasing pressure. Petrological observations further suggest that the strong redox processes took place, after breaking of garnet, during the initial exhumation of the eclogites. CO2 and minor sulfur are subsequently transferred from the cold oceanic subduction zone to the overlying mantle wedge, partially released by arc volcanoes to atmosphere. Our study presents a case of C and S recycling between the Earth's exterior and interior. |
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