Fluid flow and water-rock interaction across the active Nankai Trough subduction zone fore-arc revealed by boron isotope geochemistry

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doi: 10.1016/j.gca.2016.08.014
Author(s): Hüpers, Andre; Kasemann, Simone A.; Kopf, Achim J.; Meixner, Anette; Toki, Tomohiro; Shinjo, Ryuichi; Wheat, C. Geoffrey; You, Chen-Feng
Author Affiliation(s): Primary:
University of Bremen, MARUM-Center for Marine Environmental Sciences, Bremen, Germany
Other:
University of the Ryukyus, Japan
University of Alaska at Fairbanks, United States
National Cheng Kung University, Taiwan
Volume Title: Geochimica et Cosmochimica Acta
Source: Geochimica et Cosmochimica Acta, Vol.193, p.100-118. Publisher: Elsevier, New York, NY, International. ISSN: 0016-7037 CODEN: GCACAK
Note: In English. Includes appendices. 88 refs.; illus., incl. sketch map
Summary: Compositional changes, dehydration reactions and fluid flow in subducted sediments influence seismogenesis and arc magmatism in subduction zones. To identify fluid flow and water-rock interaction processes in the western Nankai Trough subduction zone (SW Japan) we analyzed boron concentration and boron isotope composition (δ11B) of pore fluids sampled across the subduction zone forearc from depths of up to ∼922 m below seafloor during four Integrated Ocean Drilling Program (IODP) Expeditions. The major structural regimes that were sampled by coring include: (1) sedimentary inputs, (2) the frontal thrust zone, (3) the megasplay fault zone, and (4) the forearc basin. From mass balance consideration we find that consumption of boron (B) by ash alteration and desorption of B from the solid phase, mediated by organic matter degradation, produces a net decrease in B concentrations with depth down to ∼120 µM and variable δ11B values in the range of ∼+20 ppm and +49 ppm. Interstitial water in sediments on the incoming oceanic plate are influenced by more efficient mobilization of exchangeable B from the solid phase due to higher temperatures and alteration of the oceanic crust that acts as a sink for 10B. At the tip of the megasplay fault zone, elevated B concentration and B isotopic composition suggest that underthrust coarse-grained slope sediments provide a pathway for fluids out of the upper (<2 km) accretionary prism. Silt and sand layers in the underthrust section of the downgoing plate favor fluid escape in seaward direction from depths equivalent to the temperature range of 60-150 °C. At both locations the δ11B signature evolves during updip migration through re-adsorption. Mass balance considerations suggest a shallower fluid source depth compared to pore fluids sampled previously near the decollement zone along the central portion of the Nankai margin.
Year of Publication: 2016
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 02 Geochemistry; B-11/B-10; Boron; Chemical composition; Dehydration; ICP mass spectra; IODP Site C0004; Integrated Ocean Drilling Program; Island arcs; Isotopes; Magmatism; Marine sediments; Mass spectra; Mobilization; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; Pacific Ocean; Pore water; Sediments; Seismicity; Spectra; Stable isotopes; Subduction zones; Water-rock interaction; West Pacific
Coordinates: N323000 N333000 E1373000 E1363000
Record ID: 2017009987
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands