Subduction zones ratios

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. 

Ransom B, Spivack AJ, Kastner M (1995) Stable Cl isotopes in subduction-zone pore waters implications for fluid-rock reactions and the cycling of chlorine. Geology 23 715-718 Rau GH, Sweeney RE, Kaplan IR (1982) Plankton ratio changes with latitude differences... 

In the case of arc volcanism, the critical observation is that the C02/ He ratio of arc-related volatiles is significantly greater than that of MORBs (Marty et ai, 1989 Sano and Marty, 1995 Sano and Williams, 1996 Varekamp et ai, 1992). In a compilation of subduction zone type gases, Marty and Tolstikhin (1998) report a median value of 11.0 3.3 X 10, or 5 times that of MORBs. Consequently, an estimate of the proportion of carbon from nonmantle sources (—80%), presumably the subducted slab, can be inferred by scaling to the (upper mantle) C02/ He value. 

Fig. 1. Recycling of volatile elements in the Earth. The x-axis is the ratio of the surface (atmosphere, oceans, crust, sediments) inventory divided by the present-day flux at ridges. The y-axis is the amount of volatile elements carried by the oceanic crust and sediments to subduction zones by the volcanic flux at arcs. Neon is not represented because its volcanic flux is not known. Data sources Craig el al. (1975), Matsuo et al. (1978), Le Guern (1982), Staudacher Allegre (1988), Staudigel ct a/. (1989), Allard (1992), Marty (1992,1995), Rea Ruff (1996), Sano Williams (1996), Marty Tolstikhin (1998), Marty Zimmermann (1999), and references therein.

However, within the cold environment of subduction zones, the temperature dependence of phase relations in olivine near depths of 410 km does exhibit particular sensitivity to bulk Mg/(Mg + Fe) ratios. Indeed, the effect on olivine phase relations of iron enrichment is largely analogous to that of lower temperatures noted above. Forpyrolitic values (Mg/(Mg- -Fe) = 0.90), equilibrium phase relations (Fei et al., 1991) predict uplift and broadening of the sharp j3 transition in the cold slab, replacement of the sharp 13 transition by a more diffuse... 

Samoa, like Iceland, has a moderate plume flux, but it is located on a fast moving plate near the Tonga Trench. It shows high, but variable, He/ He ratios (11-24 Ra) during its alkalic shield-building stage (Farley et al. 1992 Poreda and Farley 1992). These variations have been attributed to mixing between a plume source and recycled components derived from the nearby subduction zone (Farley 1995). 

Overview. There does not seem to be any direct relationship between plate speed or plume flux and the value of He/" He measured at ocean islands. It does seem possible that somewhat more variable He/" He ratios may be found on old oceanic lithosphere or near continental margins/subduction zones, but this cannot be firmly concluded based on the available evidence (Fig. 5). Buoyancy flux, plate speed and lithosphere age may serve to modulate the He/" He signal at ocean islands, but it is clear that they do not control it. 

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