Diffuse fluid flux through orogenic belts: Implications for the world ocean

doi:10.1073/pnas.132275699

Diffuse fluid flux through orogenic belts: Implications for the world ocean

^*U.S. Geological Survey, Menlo Park, CA 94025; and ^‡Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095

Communicated by Mary Lou Zoback, U.S. Geological Survey, Menlo Park, CA (received for review November 7, 2001)

Abstract

Fifty years ago a classic paper by W. W. Rubey [(1951) Geol. Soc. Am. Bull. 62, 1111–1148] examined various hypotheses regarding the origin of sea water and concluded that the most likely hypothesis was volcanic outgassing, a view that was generally accepted by earth scientists for the next several decades. More recent work suggests that the rate of subduction of water is much larger than the volcanic outgassing rate, lending support to hypotheses that either ocean volume has decreased with time, or that the imbalance is offset by continuous replenishment of water by cometary impacts. These alternatives are required in the absence of additional mechanisms for the return of water from subducting lithosphere to the Earth's surface. Our recent work on crustal permeability suggests a large capacity for water upflow through tectonically active continental crust, resulting in a heretofore unrecognized degassing pathway that can accommodate the water subduction rate. Escape of recycled water via delivery from the mantle through zones of active metamorphism eliminates the mass-balance argument for the loss of ocean volume or extraterrestrial sources.

Footnotes

↵ † To whom reprint requests should be addressed. E-mail: seingebr{at}usgs.gov.
↵ § The global mass flux is q _zρA, where q _z is the (volumetric) vertical fluid flux, ρ is the fluid density, and A is the “tectonically active” area of the continents (≈1.3 × 10¹³ m²). To calculate the volumetric flux, we assume a constant vertical permeability k _z of 10^−18.3 m² below 12.5-km depth (Fig. 1 B) and apply Darcy's Law (q _z = (k _z/μ)(−δ[P + ρgz]/δz), where μ is fluid viscosity, ρ is fluid density, g is gravitational acceleration, and z is elevation above a datum. Near the base of the crust μ ≈ 1 × 10⁻⁴ kg/(m⋅s) and ρ ≈900 kg/m³. Lithostatic conditions imply that the driving force gradient (−δ[P + ρgz]/δz) is ≈ 20 MPa/km.
↵ ¶ The devolatilization flux of 1.4 × 10⁻⁸ kg/(m²⋅s) is the arithmetic mean of the flux data compiled by Manning and Ingebritsen (9) for crustal depths greater than 10 km. This calculation also assumes μ ≈1 × 10⁻⁴ kg/(m⋅s) and ρ ≈900 kg/m³.
↵ ‖ Transit times are calculated from fluid particle velocities defined by q _z/n, where n is effective or connected porosity.