Oceanic crust hydrated

Chauvel C., Goldstein S. L., and Hofmann A. W. (1995) Hydration and dehydration of oceanic crust controls Pb evolution in the mantle. Chem. Geol. 126, 65-75. 

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. 

These geochemical arguments imply that TTGs have been derived from the mantle in two stages. First the mantle melted to form a basalt and then that basalt became hydrated and remelted to form a TTG magma. However, the fact that TTGs have mantle-like initial Sr and Nd ratios means that the basaltic TTG precursor had a short crustal residence time (i.e. there was a short time interval between the creation and destruction of the basaltic crust), the normal case for oceanic crust. 

The salinities of vent fluids have been shown to range from values of about 30% (176 mM/kg Von Damm and Bischoff 1987) to 200% (1090 mM/kg Massoth et al. 1989) seawater values (546 mM/kg). These variations are important because Cl is the major complexing anion in the hydrothermal fluids. The observed salinities cannot be accounted for by hydration of the oceanic crust (Cathles 1983) or by precipitation and dissolution of Cl-bearing mineral phases (Edmond et al. 1979b Seyfried et al. 1986) but are interpreted to be a result of phase separation at the top of the magma chamber followed by mixing of the brines and more dilute... 

Aqueous chemistry is one of the oldest forces of change in the solar system. It started less than 20 million years after the gases of the solar nebula began to coalesce into solid objects.2 Water is also the most abundant volatile molecule in comets. On the earth, the oceans alone contain about 1.4 x 1021 kilograms or 320,000,000 cubic miles of water. Another 0.8 x 1021 kilogram is held within the rocks of the earth s crust, existing in the form of water of hydration. The human... 

Hydrogen is the commonest element found in the Universe and is a major constituent of stars. It is relatively much less common on Earth but nevertheless it forms nearly 1 per cent by mass of the crust and oceans, mainly as water and in hydrates and hydroxide minerals of the crust. It is ubiquitous in biology. 

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