Subduction zones oceanic crust

The output of carbonate-derived CO2 (L) cannot be balanced with the amount of sedimentary carbonate CO2 being subducted except for the Central American and Sunda (Indonesia) volcanic arcs. This would suggest that, in both these localities, sediment-derived carbonate may be efficiently transported to the zones of magma generation. When carbonate CO2 from the subducting altered oceanic crust is taken into account, the output can be supplied solely from the slab indeed, at all localities, except Japan, only a fraction of the input CO2 is necessary to supply the output. These figures reinforce the notion that subduction zones act as conduits for the transfer of carbon into the mantle (Kerrick and Connolly 2001). 

During the early part of the Mesozoic Era a subduction zone in Fig. 15.3 stretched from the west coast of South America to the paleo-Pacific coast of East Antarctica. This subduction zone dipped under the Antarctic Peninsula, Thurston Island, Marie Byrd Land, Southern New Zealand, and Tasmania all of which were later moved into the positions they occupy at the present time (Elliot 1991). The subduction of oceanic crust caused the eruption of large amounts of... 

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). 

About 80% of magmatic activity in the ocean occurs at the MOR, with another 10% each at subduction zones and mantle hot spots. Hydrothermal interactions take place at convergent plate boundaries where oceanic crust is being subducted back into the... 

Seafloor spreading eventually pushes oceanic crust into subduction zones where the hydrothermal sediments and rock are recycled back into the mantle. A small fraction of these deposits is uplifted, or obducted, onto land. These rescued deposits are termed ophiolites. Because of their metal enrichments, they serve as major ore bodies and have been mined for various precious metals, such as copper, for thousands of years. 

Using average MORE or the range of compositions of oceanic basalts (e.g., Hofmann, 1988 Chapter 3.13 and http //petdb.ldeo.columbia.edu Lehnert et al., 2000), the fluxes derived here can be applied to determine the average compositions of oceanic crust that is subducted and recycled into the mantle. These compositions thus influence the composition of subduction zone magmas (see Chapter 3.18) and bear on the chemical mass balance of the mantle. 

Kimura G. and Ludden J. (1995) Peeling oceanic crust in subduction zones. Geology 23, 217-220. 

Subduction zones are the geotectonic settings where the Earth s mantle is refertilized. Whereas the various magmatic geotectonic settings produce oceanic and continental crust... 

A- -Vi = B- -V2 (where A, B are volatile free phases and Vi, V2 are hydrous phases or carbonates), involve hydrates and/or carbonates and change the mineralogy of a rock volume according to the stability fields of the minerals, but do not liberate a fluid. Prograde subduction zone metamorphism (as is true for any type of prograde metamorphism) generally reduces the amount of H2O that can be stored in hydrous minerals with depth. Thus, almost any part of the oceanic crust sooner or later becomes fluid saturated. In an equilibrium situation, the volatile content bound in hydrous phases and carbonates remains constant until fluid saturation occurs. Either continuous or discontinuous reactions may lead to fluid saturation in a rock. The point at which this occurs depends on initial water content, and pressure and temperature, and somewhat counter-intuitively, initial low water contents do not cause early complete dehydration, but delay the onset of fluid production to high pressures. 

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