Crust degassing

Sulfur in the sediments and oceanic crust which is derived from seawater subducts to deeper parts. This subduction flux is estimated to be ca. 4 x lO mol/m.y. (Shikazono, 1997). Therefore, degassing S flux from back-arc and island arc ((2.3-8.2) x lO mol/m.y.) seems to be not different from the subduction flux, although uncertainty of estimated degassing and subduction flux is large. 

The oceanic crust is also a sink for seawater as its porosity increases with age because cooling and chemical weathering lead to the continuing development of fractures and fissures. Most of this trapped water is degassed back into ocean when the crust and its overlying sediment undergo subduction. 

Schwartzmann, D. W. (1973b) Argon degassing and the origin of the sialic crust. Geochim. Cosmochim. Acta, 37, 2479-95. 

With the exception of Davies, who favored whole-mantle convection all along, the above authors concluded that it was only the upper mantle above the 660 km seismic discontinuity that was needed to balance the continental crust. The corollary conclusion was that the deeper mantle must be in an essentially primitive, nearly undepleted state, and consequently convection in the mantle had to occur in two layers with only little exchange between these layers. These conclusions were strongly reinforced by noble gas data, especially He/ He ratios and, more recently, neon isotope data. These indicated that hotspots such as Hawaii are derived from a deep-mantle source with a more primordial, high He/" He ratio, whereas MORBs are derived from a more degassed, upper-mantle reservoir with lower He/ He ratios. The noble-gas aspects are treated in Chapter 2.06. In the present context, two points must be mentioned. Essentially all quantitative evolution models dealing with the noble gas evidence concluded that, although plumes carry... 

Torgersen T. (1989) Terrestrial helium degassing fluxes and the atmospheric helium budget implications with respect to the degassing processes of continental crust. Chem. Geol. 79, 1-14. 

In the broadest terms, Matuso (1960) recognized that the composition of volcanic gas reflects the balance of contributions from magmatic degassing and from the crust. An oft-quoted sequence of volatile release with decreasing pressure, based largely on experimental observations of solubility in the melt, is... 

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