Keiichiro Kanehira

Metasomatic chloritization of gabbros in the Mid-Atlantic Ridge near 30°N

Abstract Metagabbros from the Mid-Atlantic Ridge near 30°N were originally formed as cumulates and then somewere metamorphosed with intense chemical migration, particularly with metasomatic introduction of Fe. The major mineral change during the metasomatism was chloritization, not only of mafic minerals but also of plagioclase. Ultimately, it resulted in the formation of almost monomineralic chlorite rocks, which resemble serpentinite in appearance. The chemical migration was probably caused by water (presumably sea water) percolating through rocks within the Ridge. Similar chlorite rocks were found in other dredges as well. Such rocks appear to be widespread in the Mid-Atlantic Ridge. In the past, they were probably mistaken for serpentinites. Such metasomatism would have an effect on the chemical composition of the ocean water.