Metasomatism within the ocean crust

Abstract

From ridge to trench, the ocean crust undergoes extensive chemical exchange with seawater, which is critical in setting the chemical and isotopic composition of the oceans and their rocky foundation. Although the overall exchange fluxes are great, the first-order metasomatic changes of crustal rocks are generally minor (usually <10% relative change in major element concentrations). Drastic fluid-induced metasomatic mass transfers are limited to areas of very high fluid flux such as hydrothermal upflow zones. Epidotization, chloritization, and serizitization are common in these upflow zones, and they often feature replacive sulfide mineralization, forming significant metal accumulations below hydrothermal vent areas. Diffusional metasomatism is subordinate in layered (gabbroic-doleritic-basaltic) crust, because the chemical potential differences between the different lithologies are minor. In heterogeneous crust (mixed mafic-ultramafic lithologies), however, diffusional mass transfers between basaltic lithologies and peridotite are very common. These processes include rodingitization of gabbroic dikes in the lithospheric mantle and steatitization of serpentinites in contact to gabbroic intrusions. Drivers of these metasomatic changes are strong across-contact differences in the activities of major solutes in the intergranular fluids. Most of these processes take place under greenschist-facies conditions, where the differences in silica and proton activities in the fluids are most pronounced. Simple geochemical reaction path models provide a powerful tool for investigating these processes. Because the oceanic crust is hydrologically active throughout much of its lifetime, the diffusional metasomatic zones are commonly also affected by fluid flow, so that a clear distinction between fluid-induced and lithology-driven metasomatism is not always possible. Heterogeneous crust is common along slow and ultraslow spreading ridges, were much of the extension is accommodated by faulting (normal faults and detachment faults). Mafic-ultramafic contacts hydrate to greater extents and at higher temperatures than uniform mafic or ultramafic masses of rock. Hence, these lithologic contacts turn mechanically weak at great lithopheric depth and are prone to capture much of the strain during exhumation and uplift of oceanic core complexes. Metasomatism therefore plays a critical role in setting rheological properties of oceanic lithosphere along slow oceanic spreading centers, which – by length – comprise half of the global mid-ocean ridge system.