The Action of Water Films at Å-Scales in the Earth: Implications for the Nankai Subduction System

Citation:
Brown, KM, Poeppe D, Josh M, Sample J, Even E, Saffer D, Tobin H, Hirose T, Kulongoski JT, Toczko S, Maeda L, shipboard party IODPL348.  2017.  The Action of Water Films at Å-Scales in the Earth: Implications for the Nankai Subduction System. EPSL. 463:266-276.

Date Published:

2017

Abstract:

Water properties change with confinement within nanofilms trapped between natural charged clay particles. We investigated nanofilm characteristics through high-stress laboratory compression tests in combination with analyses of expelled pore fluids. We utilized sediments obtained from deep drilling of the Nankai subduction zone at Site C0002 of the Integrated Ocean Drilling Program (IODP). We show that below 1–2 km, there should be widespread ultrafiltration of migrating fluids. Experiments to> 100 MPa normal compression collapse pores below a few ion monofilm thicknesses. A reduction towards a single condensing/dehydrating ion monofilm occurs as stresses rise >100–200 MPa and clay separations are reduced to <10–20 Å. Thus, porosity in high mineral surface area systems only consists of double and single monofilms at depths below a few km leaving little room for either bulk water or the deep biosphere. The resulting semipermeable properties result in variable segregation of ions and charged isotopes and water during active flow. The ultrafiltration and ion dehydration processes are coupled in that both require the partial immobilization of ions between the charged clay surfaces. The general effect is to increase salinities in residual pore fluids at depth and freshen fluids expelled during consolidation. Cessation of nanofilm collapse to a near constant ∼17 Å below 2 km depth at Nankai supports the contention for the onset of substantial geopressuring on the deeper seismogenic fault. The properties of monofilm water, thus, have considerable implications for the deep water properties of subduction zones generating major tremor and Mw 8+ earthquakes. Indeed, the combined effects of advective flow, ultrafiltration, diffusion, and diagenesis could provide a unifying explanation for the origins of overpressuring and pore water geochemical signals observed in many natural systems.

Keywords

nanofilms;
ultrafiltration;
ion dehydration;
pore fluid evolution;
porosity evolution;
overpressuring

DOI:

10.1016/j.epsl.2016.12.042

 (Left) Electric Double layer (EDL) development with nano-film compression of initially low salinity pore fluids. At low salinities EDL thickness are large and UF should initiate early during consolidation. At very low ion concentrations full contact of clay surfaces may occur. (Right) At elevated salinities EDL thicknesses are suppressed and only mono-films are attached to the mineral surfaces. UF initiates later during consolidation and may be associated with ion dehydration and the collapse from two to a single monofilm separating clay contacts.