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Solid Earth An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 22 Mar 2019

Research article | 22 Mar 2019

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This discussion paper is a preprint. A revision of the manuscript was accepted for the journal Solid Earth (SE).

Fluid–rock interactions in the shallow Mariana forearc: carbon cycling and redox conditions

Elmar Albers1, Wolfgang Bach1, Frieder Klein2, Catriona D. Menzies3,a, Friedrich Lucassen1, and Damon A. H. Teagle3 Elmar Albers et al.
  • 1Department of Geosciences and MARUM–Centerfor Marine Environmental Sciences, University of Bremen, Bremen, 28359, Germany
  • 2Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole (MA), 02543, USA
  • 3Ocean and Earth Science, National Oceanography Centre, University of Southampton, Southampton, SO16 7PP, UK
  • anow at: Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen, AB24 3FX, UK

Abstract. Few data exist that provide insight into processes affecting the long-term carbon cycle at shallow forearc depths. To better understand the mobilization of carbon in sediments and crust of the subducting slab, we investigated carbonate materials that originate from the subduction channel at the Mariana forearc (< 20 km) and were recovered during International Ocean Discovery Program Expedition 366. The calcium carbonates exist as vein precipitates within metavolcanic and metasedimentary clasts. The clasts represent portions of the subducting lithosphere, including ocean island basalt, that were altered at lower blueschist facies conditions and were subsequently transported to the forearc seafloor by serpentinite mud volcanism. Euhedral aragonite and calcite and the lack of deformation within the veins suggest carbonate formation in a strain-free environment, that is after peak metamorphism affected their hosts. Intergrowth with barite and marked negative Ce anomalies in carbonate attest the precipitation within a generally oxic environment that was not controlled by serpentinization. Strontium and O isotopic compositions in carbonate (87Sr/86Sr = 0.7052 to 0.7054, δ18OVSMOW = 20 to 24 ‰) imply precipitation from slab-derived fluids at temperatures between ~ 130 and 300 °C. These temperature estimates are consistent with the presence of blueschist facies phases, such as lawsonite coexisting with the carbonates in some veins. Incorporated carbon is inorganic (δ13CVPDB = −1 to +4 ‰) and likely derived from the decarbonation of carbonaceous sediment and/or oceanic crust. These findings provide evidence for the mobilization of carbon in the downgoing slab at depths of < 20 km. Our study for the first time shows in detail that a portion of this carbon forms carbonate precipitates in the subduction channel of an active convergent margin. This process may be an important asset in understanding the deep carbon cycle since it highlights that C is lost from the subducting lithosphere before reaching greater depths.

Elmar Albers et al.
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Elmar Albers et al.
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Short summary
To understand the fate of carbon in subducted oceanic sediments and crust, we studied carbonate phases in rocks from the Mariana subduction zone. These show that carbon is liberated from the downgoing plate at depths less than 20 km. Some of the carbon is subsequently trapped in minerals and likely subducts to greater depths, whereas fluids carry the other part back into the ocean. Our findings imply that shallow subduction zone processes may play an important role in the deep carbon cycle.
To understand the fate of carbon in subducted oceanic sediments and crust, we studied carbonate...