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

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. W .W. Rubey's (1) conclusion that most of the terrestrial inventory of volatile species (particularly H 2 O, CO 2 , and Cl) results from to volcanic degassing was widely accepted for decades. However, Rubey's work predated the development of plate-tectonic theory, and it has since been shown that the amount of water subducted (900 teragrams͞year) (2) is significantly larger than the amount of water released by midocean ridge volcanism (200 teragrams͞year) (2) and arc volcanoes (perhaps tens of teragrams͞year) (2, 3) combined. This apparent discrepancy between terrestrial sinks and sources of water (Ϸ700 teragrams͞year), combined with the recent recognition that quantities of water equivalent to several ocean volumes may be stored in the uppermost mantle in dense hydrous aluminum silicates (4), poses serious problems for the hypothesis of terrestrial origin for Earth's oceans and has helped to prompt increased interest in the possible ongoing accretion of extraterrestrial volatiles (5, 6).The apparent deficit between outgassing and subduction (2, 3, 7) poses a challenge for hypotheses of rapid volatile accretion on a young Earth (8), as well as for the older hypothesis of terrestrial origin for oceans (1). This fundamental challenge to both theories may be mitigated by recognition of another potential terrestrial degassing pathway. Previous work ignores diffuse transport of volatiles from subducted lithosphere through the metamorphic fluid system in tectonically active continental and island arc crust at convergent margins. Based on a determination of the large-scale permeability of the active continental crust (9, 10), we show that the potential for diffuse degassing of continental and arc crust at convergent margins exceeds the water subduction rate.Permeability is a measure of the relative ease of fluid flow under unequal pressure and is independent of fluid properties. I...

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