Subduction zones

Figure 1.1. Outline index map of the Japanese subduction zones. Thick lines with teeth are converging plate boundaries. Arrows indicate relative plate motions. Abbreviations su, Suruga trough sa, Sagami trough sf, South Fossa Magna triple junction och, Off Central Honshu triple junction ISTL, Itolgawa-Shizuoka Tectonic Line KSM, Kashima VLBl station (Uyeda, 1991).

These different sites of hydrothermal and ore-forming activity may have resulted from the mode of subduction of the Pacific Plate. Mariana-type subduction (characterized by a steep angle of subduction and back-arc basin formation Uyeda and Kanamori, 1979) during middle Miocene caused WNW-ESE extension, submarine hydrothermal activity, thick accumulation of bimodal (basaltic and dacitic) volcanic activity (Green tuff) and Kuroko-type formation (Shikazono and Shimizu, 1993). Plio-Pleistocene Chilean-type subduction (shallow-dipping subduction zone, E-W compression Uyeda and Kanamori, 1979) and oblique subduction of the Pacific Plate beneath the North American Plate led to uplift and expansion of land area, subaerial hydrothermal activity accompanied by meteoric water circulation, subaerial andesitic volcanic activity and formation of vein-type deposits. 

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. 

During the middle Miocene, Kuroko deposits, polymetallic vein-type deposits, gold-quartz vein-type deposits and Sb and Hg vein-type deposits were formed (see sections 1.3 and 1.6). Many vein-type deposits were formed not only in and nearby the Japanese Islands, but also at middle Miocene in northwest USA (Basin and Range Lipman, 1982), and elsewhere in the circum-Pacific regions (e.g., Peru). It is probable that large amounts of CO2 effused into the atmosphere from hydrothermal solution associated with this widespread mineralization and volcanic gas from subduction zones, causing an increase in temperature. 

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)-...

Barth MG, Foley SF, Horn 1 (2002) Partial melting in Archean subduction zones constraints from experimentally determined trace element partition coefficients between eclogitic minerals and tonahtic melts under upper mantle conditions. Precamb Res 113 323-340... 

Foley SF, Barth MG, Jenner GA (2000) Rutile/melt partition coefficients for trace elements and an assessment of the influence of ratile on the trace element characteristics of subduction zone magma. Geochim Cosmochim Acta 64 933-938... 

Pickett et al. 1997) in MORE samples played an important role, not only to address the problem of magma transfer times, but also in the development of dynamic melting models able to sustain such disequilibria over long periods. In subduction zone volcanoes, the combined study of and Ra- °Th disequilibria place unique constraints on the... 

The most important observations about U-series isotopes in arc lavas for this chapter are (1) the widespread excess of over °Th but deficit of with respect to Pa and (2) the extreme Ra enrichments in some arc lavas. We will explore the profound implications of these for magma genesis and transport at subduction zones. The conclusions apply most convincingly to the oceanic arcs where the observations are most extreme (the volcanic fronts of Tonga, Marianas, and eastern Sunda, and one or two volcanoes in some other arcs). Whether the conclusions apply elsewhere is harder to verify but there is no convincing reason with respect to U-series data to believe that they do not. 

First we compare the results for clinopyroxene which was the only phase common to both of these studies. The results from the pure H2O experiments are somewhat inconsistent. Keppler (1996) found that Th was more mobile than U, contrary to all observations, whereas Brenan et al. (1995) found the reverse. However, subduction zone fluids are almost certain to contain solutes like Na and Cl derived from seawater and Keppler and Wyllie (1990) showed that the solubility of U, but not Th, is enhanced by the presence of Cl, although the salinities used by Keppler (1996) were very high. Both Brenan et al. (1995) and Keppler (1996) found that U was an order of magnitude more fluid mobile than Th when NaCl was present, although, in the Brenan et al. (1995) experiments, the absolute D "opyroxene/flu.d lower in the presence of NaCl than... 

Several authors have suggested that the composition of subduction zone fluids is likely to change by chromatographic interaction during their passage through the mantle... 

Figure 14. (a) Plot of U-Pa versus U-Th disequilibria for arc lavas. The Tonga samples are those (23ipa/235u) (230xh/ 38 j < 1 as expected from U addition by subduction zone fluids. However, all... 

Ayers JC, Dittmer SK, Layne GD (1997) Partitioning of elements between peridotite and H2O at 2.0-3.0 GPa and 900-1000°C, and application to models of subduction zone processes. Earth Planet Sci Lett 150 381-398... 

Bourdon B, Turner S, Dosseto A (2003) Dehydration and partial melting in subduction zones constraints from U-series disequilibria. J Geophys Res (in press). 

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... 

Hall PS, Kincaid C (2001) Diapiric Flow at Subduction Zones A Reeipe for Rapid Transport. Science 292 2472-2475... 

Keppler H (1996) Constraints from partitioiung experiments on the composition of subduction-zone fluids. Nature 380 237-240... 

Kincaid C, Sacks IS (1997) Thermal and dynamic evolution of the upper mantle in subduction zones. J Geophys Res 102 12,295-12,315... 

McCulloch MT (1993) The role of subducted slabs in an evolving earth. Earth Planet Sci Lett 115 89-100 McCulloch MT, Gamble JA (1991) Geochemical and geodynamical constraints on subduction zone magmatism. Earth Planet Sci Lett 102 358-374... 

Newman S, Macdougall JD, Finkel RC (1986) Petrogenesis and °Th- U disequilibrium at Mt. Shasta, Califonua, and in the Cascades. Contrib Mineral Petrol 93 195-206 Nichols GT, Wylhe PJ Stem CR (1994) Subduction zone melting of pelagic sediments constrained by melting experiments. Nature 371 785-788... 

O Nions RK, McKenzie DP (1988) Melting and continent generation. Earth Planet Sci Lett 90 449-456 Parkinson IJ, Arculus RJ (1999) The redox state of subduction zones insights from arc-peridotites. Chem Geol 160 409-423... 

Peacock SM (1996) Thermal and petrologic stracture of subduction zones. In Subduction Top to Bottom. 

Plank T (1993) Mantle melting and crastal recychng in subduction zones. PhD Dissertation, Columbia Utuversity, New York City, New York... 

Plank T, Langmuir CH (1993) Tracing trace elements from sediment input to volcanic output at subduction zones. Nature 362 739-743... 

Poll S, Schmidt MW (1995) H2O transport and release in subduction zones experimental constraints on basaltic and andesitic systems. J Geophys Res 100 22,299-22,314 Pyle DM, Ivanovich M, Sparks RSJ (1988) Magma-cumulate mixing identified by U-Th disequilibrium dating. Nature 331 157-159... 

Reagan MK, Sims KW, Erich J, Thomas RB, Cheng H, Edwards RL, Layne G, Ball L (2003) Timescales of differentiation from mafic parents to rhyolite in North American continental arcs. J Petrol (in press) Regelous M, Collerson KD, Ewart A, Wendt JI (1997) Trace element transport rates in subduction zones evidence from Th, Sr and Pb isotope data for Tonga-Kermadec arc lavas. Earth Planet Sci Lett 150 291-302... 

Turner SP, George RMM, Evans PE, Hawkesworth CJ, Zellmer GF (2000b) Time-scales of magma formation, ascent and storage beneath subduction-zone volcanoes. Phil Trans R Soc Lond 358 1443-1464... 

Although the role of isotopieally light sediments cannot be disregarded (also discussed below), altered oceanic crust is likely to be a primary source of Li to most subduction zone fluids (Tatsumi et al. 1986). Thus, these fluids should be isotopieally heavier than MORE. The residue from this dehydration process may, therefore, be enriched in light Li relative to MORE. 

The first studies of Li isotopes in subduction zones concentrated on young convergent margin lavas. Moriguti and Nakamura (1998b) reported correlated Li isotope and fluid-mobile element (notably boron) concentration variations in the Izu arc, southeastern Japan (8 Li = +1.1 to +7.6), consistent with significant incorporation of Li from altered oceanic crust into arc lava sources (Fig. 6). A similar trend has been reported in samples of basalts and basaltic andesites from Mt. Shasta, California (5 Li = +2.5 to +6.5 Magna et al. 2003). 

Regardless of the ultimate sources of these compositions, these results clearly show that strongly isotopically fractionated Li from crustal sources plays a role in the mantle. Processes active in subduction zones appear to be cardinal in the control of the Li isotopic composition of different parts of the mantle. The results to date imply that both isotopically enriched (8 Li > MORE) and depleted (5T i < MORE) material are available for deep subduction, and that areas of the continental lithosphere may retain these records on long time scales. 

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