Submarine metamorphism of gabbros from the Mid-Cayman Rise: Petrographic and mineralogic constraints on hydrothermal processes at slow-spreading ridges

The coarse-grained, submersible-collected rocks from the Mid-Cayman Rise in our study span a vertical section of about 700 m and range from amphibolite to gabbro. Petrographic and mineralogic studies showed that: (1) some deformation is present in all rocks; (2) the distribution of deformation is not uniform within a specimen; and (3) alteration has taken place preferentially in the deformed zones. The abundance of amphibole decreases with sample depth and argues that seawater flux into the oceanic crust decreased with depth. The compositional changes in rocks indicate that partially exchanged seawater was the metamorphic fluid that supplied Na, K, and H₂O to and removed Ca from the rock. In all, about 15% of the original rock has been transformed to amphibolite in the 700 m vertical section. Our study indicates that deformation of oceanic crust is necessary for providing pathways for seawater penetration which in turn is necessary for the submarine metamorphism to occur. If deformation continues to take place during the spreading of seafloor, alteration may also continue to occur along newly formed fractures and cracks.