The formation of volcanic centers at the Colorado Plateau as a result of the passage of aqueous fluid through the oceanic lithosphere and the subcontinental mantle: New implications for the planetary water cycle in the western United States |
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| Institution: | 1. Department of Geology, University of Free State, PO Box 339, Bloemfontein 9301, South Africa;2. School of Earth and Environment, The University of Western Australia, Perth, WA 6009, Australia;3. CSIRO Exploration and Mining, PO Box 1130, Bentley, WA 6102, Australia;4. Karlsruhe Institute of Technology (KIT), Institute for Synchrotron Radiation/ANKA Light Source, PO Box 3640, 76021 Karlsruhe, Germany;5. School of Earth and Environmental Sciences, Seoul National University, 311 Ho, 25-1 dong, San 56-1, Sillim-dong, Gwanak-gu, Seoul 151-747, Republic of Korea;1. Research Scholar, Register number: 19113112132018, Department of Physics & Research Centre, Nesamony Memorial Christian College, Marthandam 629165, Tamilnadu, India;2. Assistant Professor, Department of Physics & Research Centre, Nesamony Memorial Christian College, Marthandam 629165, Tamilnadu, India;3. Manonmaniam Sundaranar University Abisekapatti, Tirunelveli 627012, Tamilnadu, India;1. Product Development Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India;2. Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India;3. Radiochemistry & Isotope Group, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India;1. Escuela de Geología, Universidad Industrial de Santander, Carrera 27 Calle 9, Bucaramanga, Colombia;2. Corporación Geológica Ares, Calle 44A, No. 53-96, Bogotá, Colombia;1. Centre for Crystallography & Computational Chemistry, CRD, PRIST University, Thanjavur 613403, India;2. Dept. of Physics, M.I.E.T. Engineering College, Trichy 620007, Tamil Nadu, India;3. Dept. of Physics, Annamalai University, Annamalai Nagar 608002, Tamil Nadu, India;4. PG and Research Dept. of Chemistry, Jamal Mohamad College (Autonomous), Trichy 620020, Tamil Nadu, India |
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| Abstract: | We provide new petrological evidence for the strong influence of water on the formation of the oceanic lithospheric mantle, the subcontinental mantle above, and the continental lithosphere. Our analysis throws new light on the hypothesis that new continental lithosphere was formed by the passage of silicate-rich aqueous fluid through the sub-continental mantle. In order to investigate this hypothesis, we analyzed a representative collection of lherzolite and harzburgite xenoliths from the sample volcano known as “The Thumb”, located in the center of the Colorado Plateau, western United States. The studied sample collection exhibits multi-stage water enrichment processes along point, line and planar defect structures in nominally anhydrous minerals and the subsequent formation of the serpentine polymorph antigorite along grain boundaries and in totally embedded annealed cracks. Planar defect structures act like monomineralic and interphase grain boundaries in the oceanic lithosphere and the subcontinental mantle beneath the North American plate, which was hydrated by the ancient oceanic Farallon plate during the Cenozoic and Mesozoic eras. We used microspectroscopical, petrological, and seismological techniques to confirm multi-stage hydration from a depth of ～150 km to just below the Moho depth. High-resolution mapping of the water distribution over homogeneous areas and fully embedded point, line and planar defects in olivine crystals of lherzolitic and harzburgitic origin by synchrotron infrared microspectroscopy enabled us to resolve local wet spots and thus reconstruct the hydration process occurring at a depth of ～150 km (T ≈ 1225 °C). These lherzolites originated from the middle part of the Farallon mantle slab; they were released during the break up of the Farallon mantle slab, caused by the instability of the dipping slab. The background hydration levels in homogeneous olivines reached ～138 ppm wt H2O, and the water concentration at the planar defects could reach up to ～1000 ppm wt H2O. However, the formation of antigorite in grain boundaries was found to be the primary hydration mechanism for harzburgitic samples originating from the subcontinental mantle (for hydration, T ≈ 600 °C). Additionally, the formation of antigorite in lherzolites could be found in annealed cracks. From these observations, we conclude that hydration induces multi-stage water enrichment of the mantle wedge by a process that is dominated by the growth and movement of ubiquitous cracks, which acts as planar defects. Cracks in the mantle seem to be the an important feature in both the water cycle of the subduction zone and the formation of the continental lithosphere. |
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