Serpentinization Rates at Slow-Spreading Mid-Ocean Ridges: From Sample Scale to Plate-Boundary

Abstract:

About 25% of the crust formed at slow spreading mid-ocean ridges includes a component of tectonically exhumed and partially serpentinized mantle-derived peridotites. Exhumation occurs along large offset normal faults (detachment faults). In this presentation we outline a conceptual and testable model for serpentinization at slow-spreading ridges, based on petrological observations of samples of variably serpentinized peridotites from the Mid Atlantic and Southwest Indian ridges, on tectonic and geophysical data and on current interpretations of plate-boundary processes at slow spreading ridges. Serpentinization at mid-ocean ridges is a heterogeneous and multistage process occurring along fractures, in the footwall of axial detachments. The initial and most pervasive stage of serpentinization results in the typical serpentine mesh texture. We propose that it occurs when hydrothermal fluids reach extensively microfractured fresh peridotite. Subsequent stages of serpentinization appear to involve higher fluid-rock ratio along larger fractures and veins that are spaced by at least a few decimeters. Experimental data indicate that each stage of serpentinization identified in a given sample may have occurred at very high rates relative to the rates of tectonic exhumation. However, serpentinization (initial and later stages) may be a protracted process at the km-scale, associated with complex fracturation of the detachment's footwall due to combined tectonic and reaction-induced stresses. We outline possible consequences of this conceptual model in terms of crustal structure, hydrogen production and of the relations between serpentinization and black smoker-type hydrothermal circulation at slow-spreading ridges.