Subduction zones fluxes

At the convergent plate boundaries, CO2 degasses not only from back-arc basins by hydrothermal solutions but also from terrestrial subduction zones by volcanic gases and hydrothermal solutions. However, the studies on CO2 degassing from terrestrial subduction zones are not many. Seward and Kerrich (1996) have shown that hydrothermal CO2 flux from terrestrial geothermal system (such as Taupo volcanic zone in New Zealand) exceeds lO mol/year which is comparable to that of midoceanic ridges (Table 3.4). 

Sano and Williams (1996) calculated present-day volcanic carbon flux from subduction zones to be 3.1 x 10 mol/year based on He and C isotopes and C02/ He ratios of volcanic gases and fumaroles in circum-Pacific volcanic regions. Williams et al. (1992) and Brantley and Koepenich (1995) reported that the global CO2 flux by subaerial volcanoes is (0.5-2.0) x lO mol/m.y. and (2-3) x 10 mol/m.y. (maximum value), respectively. Le Guern (1982) has compiled several measurements from terrestrial individual volcanoes to derive a CO2 flux of ca. 2 x 10 mol/m.y. Le Cloarec and Marty (1991) and Marty and Jambon (1987) estimated a volcanic gas carbon flux of 3.3 X 10 mol/m.y. based on C/S ratio of volcanic gas and sulfur flux. Gerlach (1991) estimated about 1.8 x 10 mol/m.y. based on an extrapolation of measured flux. Thus, from previous estimates it is considered that the volcanic gas carbon flux from subduction zones is similar to or lower than that of hydrothermal solution from back-arc basins. 

Fig. 4.11. Atmospheric CO2 variation estimated by modified GEOCARB II model including volcanic eruption rate of circum-Pacific region by Kennett et al. (1977) (Kashiwagi et al., 2000). y represents the contribution of the flux from back arc basin to that from subduction zones at present. Rco = PcOi/PcOi 02 Pfesent-day PC02)-...

Davies JH, Biekle MJ (1991) A physical model for the volume and composition of melt produeed by hydrous fluxing above subduction zones. Phil Trans R Soc Lond 335 355-364 Davies JH, Stevenson DJ (1992) Physical model of source region of subduction zone voleanies. J Geophys Res 97 2037-2070... 

Zimmer MM, Fischer TP, Hilton DR, Alvaredo GE, Sharp ZD, Walker JA (2004) Nitrogen systematics and gas fluxes of subduction zones insights from Costa Rica arc volatiles. Geochem Geophys Geosys 5 Q05J11, doi 10.1029/2003GC000651... 

Ryan, J., Mooris, J Behout, G. and Leeman, B. (1996) Describing chemical fluxes in subduction zones Insights from depth-profiling studies of arc and forearc rocks, in Subduction Top to Bottom (eds G.E. Bebout, D.W. Scholl, S.H. Kirby and J.P. Platt), American Geophysical Union, Geophysical Monograph, Vol. 96, pp. 263-68. 

Becker H, Jochum KP, Carlson RW (2000) Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones. Chem Geol 163 65-99... 

Fischer T. P., Giggenbach W. F., Sano Y., and Williams S. N. (1998) Fluxes and sources of volatiles discharged from Kudryavy, a subduction zone volcano Kurile Islands. Earth Planet. Sci Lett. 160, 81-96. 

As noted earlier, quantitative element fluxes in subduction zones are tricky. They depend on magma production rates, which are often poorly known. They also depend on specific fluid-fluxing or sediment melting models, which implicitly or explicitly invoke a specific thermal structure and mineralogy for the slab, a subject of much debate. 

Kastner M., Morris J., Chan L. H., Saether O., and Luckge A. (2000) Three distinct fluid systems at the Costa Rica subduction zone chemistry, hydrology, and fluxes, Goldschmidt 2000. J. Conf. Abstr. 5, 572. 

Rea D. K. and Ruff L. J. (1996) Composition and mass flux of sediment entering the worlds subduction zones implications for global sediment budgets, great earthquakes and volcan-ism. Earth Planet. Sci. Lett. 140, 1-12. 

Using average MORE or the range of compositions of oceanic basalts (e.g., Hofmann, 1988 Chapter 3.13 and http //petdb.ldeo.columbia.edu Lehnert et al., 2000), the fluxes derived here can be applied to determine the average compositions of oceanic crust that is subducted and recycled into the mantle. These compositions thus influence the composition of subduction zone magmas (see Chapter 3.18) and bear on the chemical mass balance of the mantle. 

---