Construction of the oceanic lithosphere by magmatic intrusions: Petrological evidence from plutonic rocks formed along the fast-spreading East Pacific Rise

We report textural and compositional characteristics of plutonic rocks found in four sites exposed 5000 m below sea level along the southern East Pacific Rise system. Studied outcrops of plutonic rocks are located along escarpments in two transform faults (Garrett and Terevaka) and in two deep regions exposed by the action of propagating rifts (Pito Deep and Hess Deep). The transform faults have a large variety of heterogeneous plutonic rocks with mineral compositional trends extending from relatively primitive values in the olivine-bearing gabbros to very fractionated values in the ferrogabbros. An abundance of ferrogabbros in these transform faults suggests small and ephemeral magma chambers in which liquid and crystal mush evolved by crystal fractionation in small pockets and intrusive sills. Conversely, Pito Deep and Hess Deep have mainly homogeneous cumulates without the fractionated end members; instead, mineral compositions range to highly primitive values. The ubiquity of homogeneous olivine-bearing gabbros at Pito Deep and Hess Deep suggests a more robust magma chamber with frequent injection and mixing, involving larger volumes of crystal-liquid mush, which buffers compositions. We infer that the lithosphere created near ridge-transform intersections of fast-spreading centers is controlled by a colder mantle regime and will be lithologically and compositionally different from lithosphere created near hotter segment centers and away from such discontinuities. Our observations show that magmatic intrusion is a fundamental process in the construction of the lower oceanic crust.