Metamorphism of deep‐seated rocks from the Garrett Ultrafast Transform (East Pacific Rise near 13°25′S)
Authors: Bideau Daniel , Hebert Rejean , Hekinian Roger , Cannat Mathilde 
Published: February 2008

In spite of 10 years of extensive exploration of the East Pacific Rise, the petrology of the fast slipping transforms remains poorly documented. In 1980, metagabbros and serpentinites were dredged from the deepest trough (4616–4820 m in depth) of the Garrett ultrafast transform which intersects the East Pacific Rise near 13°25′S (16 cm/yr total spreading rate). Previous works on their protolith concluded that the samples consist of serpentinized refractory upper mantle and lower crust gabbroic cumulates. In addition, several ultramafic rocks showing evidences of magmatic impregnation were believed to represent mantle‐crust transition zone as observed in ophiolite complexes. The present work focuses on the postigneous evolution of the rocks affected by hydrothermal circulation. An early stage of amphibolitization occurred at about 700°–400°C and probably down to 300°C in the most evolved gabbros (Ti‐Fe gabbros). The amphibole compositional variations suggest evolution of the metasomatic solution and temperature through time from the amphibolite facies down to the lower greenshist facies. High‐chlorine ferruginous hornblendes (3–5 wt % Cl) found in veins of the ferrogabbros suggests that pore fluids in this type of material evolved in situ to a ferruginous brine. The decreasing temperature (300°–200°C) and further fracturing favored more or less pervasive serpentinization of the olivine‐rich samples (ultramafics and olivine‐gabbros). The release of Ca++ in the solution contributed to the alteration of the plagioclase‐bearing rocks (troctolites and gabbros) which were invaded by calc‐silicates (hydrogrossular, prehnite, pumpellyite, and epidote), a process which is referred to as “rodingitization”. The occurrence of monomineralic veins of Al‐pumpellyite in some metagabbros suggests very low ƒO2 conditions in the lower ocean crust (PH2O<2 kbar). Low ƒO2 also inferred from the composition of the other calc‐silicates and indicates low permeability and limited fluid circulation during retrograde metamorphism. Further uplift and emplacement of the rocks on the seafloor of the transform domain were accompanied by low‐temperature crystallization of zeolite and aragonite (high μMg++ and μCO2) in monomineralic veins and clay minerals in veins and in external alteration rims. The Garrett's samples are, by many respects, comparable to typical ocean floor metabasites from various settings. The rocks recorded igneous and metamorphic transformations similar to those expected beneath spreading ridges unaffected by transform tectonics. The metamorphism is a consequence of the dynamics of spreading: propagation of the cracks and penetration of seawater deeper in the cooling crust. However, the extent of fissuring and the subsequent hydration probably increase drastically with proximity to the Garrett transform. Most of the observed metasomatic reactions have a limited extent and occurred at depth without significant deformation of the rocks. Rapid uplift of the rocks initiated by vertical components of the transform tectonics was probably induced by serpentinization of the ultramafics, buoyancy of serpentinite, and intense mass wasting at the surface of seafloor. However, emplacement by diapirism is not completely ruled out if the dredged samples are relatively undeformed, poorly altered rocks carried up by protusions of ductile serpentine which is now too fragile to be sampled by dredging.