[1] Deformed rocks sampled from a corrugated detachment fault surface near the Mid-Atlantic Ridge (15°45 0 N) constrain the conditions of deformation and strain localization. Samples recovered in situ record deformation restricted to the cold (shallow) lithosphere (greenschist facies), with no evidence for significant high-temperature deformation either at the fault zone or in the footwall near it. High-temperature deformation (720-750°C) is observed only at two sites, and cannot be directly linked to the detachment. Detachment faulting was coeval with dyke intrusions that cross cut it, as demonstrated by the presence of undeformed and highly deformed diabase found in shear zones, and by the presence of chill margins in diabase against fault rock. Basalts are very scarce and restricted to clasts in breccias, with no evidence of pillows or extrusive structures. Gabbros crop out along mass-wasted and fault scarps structurally below the detachment. Footwall rocks show little or no deformation, due to strain localization along a narrow shear zone (<200 m) with fluid flow, as required to form talc-and amphibole schists after an ultramafic protolith. We speculate that the alteration front in a heterogeneous lithosphere may be a rheological boundary that may localize deformation during long periods of time. Our observations and other geological evidence elsewhere suggest that this detachment model limited to the cold (shallow) lithosphere is applicable to other corrugated surfaces along slow-and intermediate-spreading ridges. These observations preclude detachment models rooting in melt-rich zones (i.e., Atlantis Bank, Southwest Indian Ridge) or recording high-temperature deformation. We infer that oceanic detachment faults (1) localize strain at T < 500-300°C, (2) persist during active magmatism, and (3) root at shallow rheological boundaries, such as a melt-rich zone or magma chamber (''hot'' detachments) or an alteration front (''cold'' detachments).
Serpentinites are an important component of the oceanic crust generated in slow to ultraslow spreading settings. In this context, the MOHO likely corresponds to a hydration boundary, which could match the 500 • C isotherm beneath the ridge axis. Textures from serpentinites sampled in ridge environments demonstrate that most of the serpentinization occurs under static conditions. The typical mineralogical association consists of lizardite ± chrysotile + magnetite ± tremolite ± talc. Despite the widespread occurrence of lizardite, considered as the low temperature serpentine variety, oxygen isotope fractionation suggests that serpentinization starts at high temperature, in the range of 300-500 • C. The fluid responsible for serpentinization is seawater, possibly evolved by interaction with the crust. Compared with fresh peridotites, serpentinites are strongly hydrated (10-15% H 2 O) and oxidized. Serpentinization, however, does not seem to be accompanied by massive leaching of major elements, implying that it requires a volume increase. It results in an increase in chlorine, boron, fluorine, and sulfur, but its effect on other trace elements remains poorly detailed. The presence of serpentinites in the oceanic crust affects its physical properties, in particular by lowering its density and seismic velocities, and modifying its magnetic and rheological properties. Serpentinization may activate hydrothermal cells and generate methane and hydrogen anomalies which can sustain microbial communities. Two types of hydrothermal field have been identified: the Rainbow type, with high temperature (360 • C) black smokers requiring magmatic heat; the low temperature (40-75 • C) Lost City type, by contrast, can be activated by serpenintization reactions.
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