Subduction ridge

Several hydrothermal sites have been discovered on the seafloor of Izu-Bonin arc that is located at the eastern margin of the Philippine Sea plate (Fig. 2.32). This arc has been formed, related to the westward subduction of the Pacific plate (Fig. 2.32). Hydrothermal mineralization occurs both in back-arc depression and volcanic chain (Shichito-Iwojima Ridge). Hydrothermal venting and mineralizations are found... 

As mentioned already in Chapter 2, submarine volcanism occurs not only at midoceanic ridges but also at subduction-related tectonic settings such as the Shikoku and Daito Basins, Farce Vela Basins, and Mariana Trough, Okinawa Trough and Izu Bonin Arc (e.g.. Wood et al., 1980 Dick, 1982 Delaney and Boyle, 1986). 

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). 

We speculate from the above argument that primordial sulfur degasses from midoceanic ridges even at present time as well as He, because subduction flux to mantle seems to be small. However, we need more detailed study on long-term S cycle including hydrothermal S flux to evaluate this speculation. 

Keywords Porphyry Cu-Au, Philippines, Ridge subduction, adakites... 

Hydrate Ridge is located 80 km west of Newport, Oregon on the second accretionary ridge of the Cascadia subduction of the Juan de Fuca Plate and the North American Plate. The northern summit is in 600 m of water depth with an area of... 

Parameter Mid-Ocean Ridge and Oceanic Inlraplate Trailing Plate-Continental Margin Subduct) on (Trench) Continent- Continent Collision Rift Basins Intra-Plate (Craion)... 

Marine sediments cover the ocean floor to a thickness averaging 500 m. The deposition rates vary with topography. The rate may be several millimetres per year in nearshore shelf regions, but is only from 0.2 to 7.5 mm per 1000 years on the abyssal plains. Oceanic crustal material is formed along spreading ridges and moves outwards eventually to be lost in subduction zones, the major trenches in the ocean. Because of this continual movement, the sediments on the seafloor are no older than Jurassic in age, about 166 million years. 

Figure 12 Variability of trace element concentrations in MORE, expressed as 100 standard deviation/mean concentration. The data for Global MORE are from the PETDB compilation of (Su, 2002). All segments refers to 250 ridge segments from all oceans. Normal segments refers to 62 ridge segments that are considered not to represent any sort of anomalous ridges, because those might be affected by such factors as vicinity to mantle plumes or subduction of sediments (e.g., back-arc basins and the Southern Chile Ridge). The Atlantic MORE, 40-55° S, from which samples with less than 5% MgO have been removed (source le Roux et al., 2002).

With realistic estimates of the volatile output from the mantle, particularly the mantle wedge, it is possible to assess the state of volatile mass balance for the mantle—comparing inputs via subduction zones with outputs via arc, mid-ocean ridge, and (possibly) plume-related magmatism. Understanding the volatile systematics of the mantle is a key component in defining its structure and evolutionary history. 

Kamenetsky V. S., Crawford A. J., Eggins S., and Miihe R. (1997) Phenocryst and melt inclusion chemistry of near-axis seamounts, Valu Fa Ridge, Lau Basin insight into mantle wedge melting and the addition of subduction components. Earth Planet. Sci. Lett. 151, 205-223. 

Sobolev A. V. and Chaussidon M. (1996) H2O concentrations in primary melts from supra-subduction zones and midocean ridges imphcations for H2O storage and recycling in the mantle. Earth Planet. Set Lett. 137, 45—55. 

Ono S. (1998) Stability limits of hydrous minerals in sediment and mid-ocean ridge basalt compositions implications for water transport in subduction zones. J. Geophys. Res. 103, 18253-18267. 

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