Ocean crust carbonate content

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. 

In an analogous fashion to water, the CO2 concentrations in sediments come from an arc-by-arc compilation of carbonate contents of various drill core lithologies (Plank and Langmuir 1998). The amount of reduced (organic) carbon in sediments is taken as 1% based on average contents of metamorphosed sediments of the Catalina Schist (Bebout 1995). A value of 0.214 wt % is used for the amount of CO2 stored in altered oceanic crust (Alt and Teagle 1999). 

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