Geochemical constraints on fluid-rock reactions, fluid sources, and flow pathways along the CRISP transect; IODP Expedition 334

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Author(s): Solomon, E. A.; Torres, M. E.; Harris, R. N.; Formolo, Michael; Nuzzo, Marianne
Integrated Ocean Drilling Program, Expedition 334 Scientists
Author Affiliation(s): Primary:
University of Washington, School of Oceanography, Seattle, WA, United States
Oregon State University, Corvallis, OR, United States
University of Tulsa, Tulsa, OK, United States
Geological Survey of Portugal, Portugal
Volume Title: AGU 2011 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2011; American Geophysical Union 2011 fall meeting, San Francisco, CA, Dec. 5-9, 2011. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: Fluid flow in subduction zones influences the shallow thermal structure and fluid content of the subducting and upper plates, fault zone stability and seismogenesis, and the transfer of elements and isotopes to the oceans, volcanic arc, and mantle. Most of our knowledge of subduction zone hydrogeology results from 3 decades of studying accretionary margins. More recently, focused studies have started examining the tectonics and hydrogeology at erosive margins. The CRISP project aims to understand the processes that control the nucleation and seismic rupture of large earthquakes at erosional subduction zones, focusing on the region offshore the Osa Peninsula of Costa Rica. IODP Expedition 334 penetrated the slope sediments at Sites U1378 and U1380, the entire sequence of the slope sediments and the upper part of the basement at Site U1379, and input sediments and oceanic crust at Site U1381. Typical of erosive margins is the upper plate provenance of the material in the subduction channel. To date, the nature and progressive changes of properties of this upper plate material down the subduction zone is unconstrained. Fluids advected within fault zones in the upper plate may record fluid-mineral reactions occurring at depths marking the onset seismogenesis. Fluid chemistry is predictably altered with increasing temperature and pressure, and can facilitate estimation of the depth and type of fluid sources. Three zones of fluid flow have been identified along the CRISP transect from pore fluid solute and isotope ratio profiles. The flow at each site overprints the general geochemical profiles influenced by in situ diagenetic reactions such as ash alteration, ongoing microbial metabolic reactions, and carbonate precipitation/dissolution. At Site U1379, a zone from 600-800 mbsf contains a fluid with low Cl concentrations and peaks in the concentrations of thermogenic hydrocarbons. The geothermal gradient at this site is too low to support the in situ production of thermogenic hydrocarbons or for extensive clay dehydration, suggesting a deeper source for this fluid and migration along permeable sand horizons and fault zones. At Site U1378, there is a monotonic decrease in Cl, Mg, and K concentrations and increase in Ca concentrations with depth, suggesting diffusional communication with fluids below the base of the hole. It is likely that this fluid resides in the fault zone within the basement imaged in the seismic reflection profile at this site. The in situ temperature at the sediment/basement contact is too low to support in situ clay dehydration or thermogenic hydrocarbon formation, thus this fluid must also be transported from greater depths. Collectively, the fluids collected at Sites U1379 and U1378 carry a signature of clay dehydration and reaction with mafic rocks from the eroding Osa basement at elevated temperatures. At Site U1381, the geochemical profiles below ≈50 mbsf reflect diffusional communication with a fluid with seawater-like chemistry in the igneous basement, consistent with rapid local seawater advection in the Cocos crust.
Year of Publication: 2011
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 18 Geophysics, Solid-Earth; Alteration; CRISP transect; Chemical reactions; Continental margin; Costa Rica Seismogenesis Project; Crust; Depositional environment; East Pacific; Expedition 334; Fault zones; Faults; Fluid flow; IODP Site U1378; IODP Site U1379; IODP Site U1380; Integrated Ocean Drilling Program; North Pacific; Northeast Pacific; Oceanic crust; Pacific Ocean; Provenance; Sediments; Seismicity; Subduction zones; Water-rock interaction
Coordinates: N083600 N083600 W0840424 W0840424
N084051 N084051 W0840201 W0840202
N083532 N083532 W0840438 W0840438
Record ID: 2015077175
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