Volcanic front

Quaternary sulfur deposits are distributed along the present volcanic front. Intersections of transverse faults proposed by Carr et al. (1973) and the present volcanic front coincide with the locations of clusters of the sulfur deposits (Nishiwaki and Yasui, 1974). 

Several different hypotheses on the tectonic setting of the Kuroko mine area have been proposed. They include volcanic front of island arc (T. Sato, 1974 Horikoshi, 1975a), rifting of island arc (Cathles, 1983a), back-arc depression (Fujioka, 1983 Uyeda, 1983), and back-arc basin. 

Gold precipitations occur from hot springs at the Osorezan volcano which is located at the northern end of the volcanic front of the northeast Honshu (Fig. 2.16). 

Another interesting characteristic of the Osorezan hydrothermal system is that it is located at a volcanic front. This is different from low sulfidation epithermal Au-Ag veins... 

Fig. 2.23 shows the distributions of major geothermal systems and epithermal gold deposits of Japanese Islands. It is interesting to note that their distributions are similar and they are distributed close to the volcanic front. 

Recently, several submarine hydrothermal sites have been discovered from the seafloor of back-arc depression zones and volcanic fronts near the Japanese Islands (Okinawa Trough and Izu-Bonin) (Fig. 2.29). The studies on these areas are described below. 

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. 

One of the seemingly inescapable conclusions from the Ra- °Th disequilibrium data, at least for Tonga and the Mariana volcanic fronts, is that significantly less than 8000 years and arguably only a few half lives (ca. 1000-3000 yrs) can have elapsed since the generation of the Ra-excesses observed in the arc lavas plotted on Figure 10. 

Ewart A, Hawkesworth CJ (1987) The Pleistoeene-Reeent Tonga-Kermadee are lavas Interpretation of new isotopic and rare earth data in terms of a depleted mantle somce model. J Petrol 28 495-530 Fumkawa F (1993a) Magmatic processes under arcs and formation of the volcanic front. J Geophys Res 98 8309-8319... 

Hattori, K.H. and Guillot, S. (2003) Volcanic fronts form as a consequence of serpentinite dehydration in the forearc mantle wedge. Geology, 31(6), 525-28. 

Elliott T., Plank T., Zindler A., White W., and Bourdon B. (1997) Element transport from slab to volcanic front at the Mariana Arc. J. Geophys. Res. 102, 14991-15019. 

Mibe K., Fujii T., and Yasadu A. (1999) Control of the location of the volcanic front in island arcs by aqueous fluid connectivity in the mantle wedge. Nature 401, 259—262. 

This chapter focuses on subduction zone processes and their implications for mantle composition. It examines subduction contributions to the shallow mantle that may be left behind in the wedge following arc magma genesis, as well as the changing composition of the slab as it is processed beneath the fore-arc, volcanic front and rear arc on its way to the deep mantle. Much of this chapter uses boron and the beryllium isotopes as index tracers boron, because it appears to be completely recycled in volcanic arcs with little to none subducted into the deep mantle, and cosmogenic e, with a 1.5 Ma half-life, because it uniquely tracks the contribution from the subducted sediments. 

Returning to Figure 4, lead, like arsenic and antimony, shows a behavior similar to that of boron (Noll et al., 1993) indicating that it is extracted effectively from the slab through subsolidus metamorphic reactions and also beneath the volcanic front, presumably in a fluid However,... 

The concentration of elements such as potassium (Ryan et al., 1995, 1996a,b) and thorium increases across the volcanic arc, even when effects of partial melting are minimized by ratioing to other incompatible elements. A suggestion that the slab component is a fluid beneath the volcanic front and a sediment melt beneath the rear arc is consistent with lead and thorium... 

Figure 9 Subduction zone statistics histograms of depths of (a) the slab surface below the volcanic front and (b) the width of volcanic arcs. The vertical axis denotes arc lengths in km measured at the trench. This is our own compilation (unpublished) based on locations of quaternary volcanoes and slab surfaces from tomography...

Furukawa Y. (1993) Magmatic processes under arcs and formation of the volcanic front. J. Geophy. Res. 98, 8309-8319. 

Tatsumi Y. (1986) Formation of the volcanic front in subduction zones. Geophys. Res. Lett. 13, 717-720. 

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