Antigorite in deformed serpentinites from the Mid-Atlantic Ridge

Deformed, non-pseudomorphic serpentinites from fault zones in the Rainbow and Menez Horn areas, in the Mid-Atlantic Ridge, contain antigorite associated with variable amounts of chrysotile, while pseudomorphic or non-pseudomorphic lizardite + chrysotile serpentinites are the rule in this and other oceanic environments. A detailed TEM study of these deformed serpentinites shows that antigorite (polysomes m = 12 to 16) replaces chrysotile through dissolution-recrystallization, rather than through solid-state transition. This dissolution-recrystallization process is probably favoured by intense shear stress, the effects of which are preserved in the textures of these rocks. Oxygen isotope temperature estimates for these serpentinites fall well below 300 degrees C, confirming that Mid-Atlantic Ridge antigorite does not result from high-temperature prograde metamorphism, as it often does in other geological environments. Antigorite-bearing serpentinites, therefore, may occur locally in low-temperature, high-deformation settings, characterized by intense tectonic activity and major shear zones, as frequently found along the slow-spreading Mid-Atlantic Ridge. Technical difficulties may have limited the access to and sample recovery from important deformation settings, such as shear zones and fault scarps, thus explaining the relative scarcity of antigorite-bearing deformed serpentinites recovered from oceanic environments.