Serpentinization, oceanic crust

The geological process of the formation of serpentine from peridotite probably involves the synthesis of carbon compounds under FTT conditions (see Sect. 7.2.3). The hydrogen set free in the serpentinisation process can react with CO2 or CO in various ways. The process must be quite complex, as CO2 and CO flow through the system of clefts and chasms in the oceanic crust and must thus pass by various mineral surfaces, at which catalytic processes as well as adsorption and desorption could occur. 

Hess H. H. (1964) The oceanic crust, the upper mantle, and the Mayaguez serpentinized peridotite. In A Study ofSerpen-tinite, Natl. Research Council Publ., Natl. Acad. Sci., vol. 1188, pp. 169-175. 

Figure 2 Schematic representation of hydrothermal alteration of the igneous oceanic crust. The inset to the right gives H2O and CO2 contents in a depth profile of the oceanic crust, the two horizontal bars indicate typical H2O contents in troctolites and in intensively serpentinized harzburgite.

In a number of arcs (NE Jap an-Kurile s-Kamchatka, Aleutians, N. Chile), so-called double seismic zones are observed. Whereas the upper seismic zone correlates with the oceanic crust, it has been suggested (Seno and Yamanaka, 1996 Peacock, 2001) that the lower seismic zone corresponds to the limit of serpentine stability in the lower part of the oceanic lithosphere (see Figure 7). It has been argued that the lower seismic zone earthquakes are triggered by reactivation of ancient faults through fluid saturation, where the fluids derive from serpentine dehydration. 

Dehydration, or more generally, devolatilization of the oceanic crust is a process that combines continuous and discontinuous reactions in a variety of heterogeneous bulk compositions. In addition, within a vertical column—the sedimentary, mafic, and serpentinized peridotite layers— each experience a significant thermal gradient. The result is a continuous, but not constant, production of a fluid or melt, with the rate of mobile phase production generally decreasing with depth. Peaks in the volatile flux result from significant discontinuous reactions. However, despite the continuous fluid flux, trace elements may not necessarily be released continuously. 

Second, it is now apparent that continuous dehydration reactions involving hydrous phases in metasediment and upper oceanic crust with higher pressure stability than glaucophane plus extensive solid solution, such as lawsonite, chloritoid, phengite, and zoisite, provide a small but nearly continuous source of fluid from shallow depths to depths exceeding —250 km (review in Chapter 3. 17). In addition, hydrous phases such as serpentine and talc in the uppermost mantle of the subducting... 

The transition zone in the oceanic crust is represented by serpentine, the product of the Hess reaction of olivine with water. The serpentine forms in the lower oceanic crust, because of water percolation from the ocean through brittle -fissured basalts, and exhibits true plastic behavior, which prevents further migration of oceanic water into the peridotite mantle. Since the true plastic, that is, impermeable state of the serpentine is reached at pressures of 0.2-0.4 GPa and temperatures below 550°C, the thickness of the oceanic crust is about 11 km, if we account for the ocean water weight, Nikolaevskiy (1979), Lobkovsky (1988). 

---