HR: 16:00h
AN: V12H-01 INVITED     [PDF]
TI: Volatile Emissions from Subduction-related Volcanoes: Major and Trace Elements
AU: * Fischer, T P
EM: fischer@unm.edu
AF: Dept. of Earth & Planet. Sci., University of New Mexico, Albuquerque, NM 87131-1116 United States
AU: Hilton, D R
EM: drhilton@ucsd.edu
AF: Geosciences Research Division, Scripps Institution of Oceanography, La Jolla, CA 92093-0244 United States
AB: Present-day volatile emissions associated with subduction zone volcanism can be estimated in two ways. One approach is to assume magma production rate at arcs is 20% that of MOR and scale to the MOR $^{3}$He flux (1000 mol/yr) to obtain a mantle-derived arc He-3 flux of 200$\pm$40 mol/yr. This flux and measured gas ratios (x$_{I}$/$^{3}$He where x$_{I}$ is the gas species of interest) obtained from volcanic and hydrothermal samples is then used to calculate volatile emissions. A global arc CO$_{2}$ flux of 0.3 to 3.1 x 10$^{12}$ mol/yr has been obtained in this way. Another approach is to use individual arc volcano SO$_{2}$ fluxes (determined by remote sensing) in combination with CO$_{2}$/SO$_{2}$ ratios of high temperature fumaroles to calculate volcanic CO$_{2}$ fluxes. Integrating over an individual arc, and using a power-law distribution to include non-measured volcanoes, it is possible to produce a volatile flux estimate for a particular arc. Summing over all arcs allows a global estimate (e.g. $\sim$ 1.6 x10$^{12}$ mol/yr for arc CO$_{2}$). \\ There are caveats with both methods. In the former case, it is assumed that the mantle wedge is characterized by a similar $^{3}$He content to MORB-source. In the latter case, the distribution of SO$_{2}$ fluxes is decidedly uneven necessitating poorly-justified extrapolations. For example, there is little data available from the I-B-M, Lesser Antilles and Philippines whereas Central American volcanoes have numerous published SO$_{2}$ fluxes. A further issue (in addition to geographical bias), is the absence of volatile fluxes from submarine arcs. Despite these problems, global estimates of SO$_{2}$ and CO$_{2}$ fluxes by both methods vary by only one order of magnitude [1]. It is emphasized that these are present-day estimates as paleo-degassing rates of arc magmas are poorly constrained and depend entirely on estimates of magma intrusion and extrusion rates [2]. \\ The same approach has been used for other species although the flux of magmatic N$_{2}$, H$_{2}$O, HCl, HF from arcs remains poorly constrained (N$_{2}$: $\sim$ 6 x10$^{8}$ to 2 x10$^{10}$ mol/yr; H$_{2}$O: $\sim$ 8 x10$^{12}$ mol/yr; HCl $\sim$1 x10$^{10}$ to 4x10$^{11}$ mol/yr, HF: $\sim$3 x10$^{9}$ to 3 x10$^{11}$ mol/yr)[1,3]. Due to the preferential partitioning of HCl and HF into volcano hosted hydrothermal systems, fluxes from magma bodies are probably much larger than what is emitted into the atmosphere. \\ Trace element emissions from subduction related volcanoes are also poorly constrained but are potentially significant. High temperature ($>$700C) volcanic gas samples show that concentrations of Be, Rb, Sr, Ru, Rh, Pd, Cd, W, Re, Pt, Pb, Bi, Se, Sc are in the 5 to 1300 ug/L range and up to 25000 ug/L of B have been measured [4,5]. Using these concentrations and the global arc SO$_{2}$ flux gives estimates of trace element fluxes on the order of 3 x10$^{4}$ to 8 x10$^{6}$ mol/yr (and 3 x10$^{9}$ mol/yr of B). These flux estimates are certainly upper limits because low temperature ($<$200C) gases that make up the majority of emissions have much lower trace element concentrations. Further work is needed to better constrain volatile contributions of volcanoes to the atmosphere, and to improve global geochemical models which assess the impact of volcanic gases on the atmosphere.\\ {[1]} Hilton, Fischer \& Marty (2002) Rev. Min. vol 47 for review [2] Kerrick Rev. Geophys. (2001) vol. 39 \#4. [3] Symnods, Rose \& Reed (1988) nature vol 334, p. 415 [4] Fischer, Shuttleworth \& O'Day (1998) Fres. J. Anal. Chem. vol 362, p. 457 [5] Taran et al., (1995) Gochim. Cosmochim. Acta vol. 59, p. 1749
DE: 0330 Geochemical cycles
DE: 0370 Volcanic effects (8409)
DE: 1065 Trace elements (3670)
DE: 8409 Atmospheric effects (0370)
DE: 8499 General or miscellaneous
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting