Tracing changes to the oceanic crust
Alteration of the ocean crust, the outermost rocky layer of the Earth found under the oceans, is a fundamental ongoing geological process. High-temperature changes to the ocean crust occur at the oceanic ridges, the underwater mountain systems formed by plate tectonics. Diffusive low-temperature changes occur further away from the ridges (off-ridge). However, both processes produce secondary minerals and alter the chemistry of the oceanic crust. These changes can be introduced into the mantle layer within the Earth. This takes place in subduction zones, where one tectonic plate moves over another causing it to sink into the mantle. The 'Behavior of molybdenum and its isotopes during alteration of the oceanic crust' (MOLYALOC) project used Mo to study changes on- and off-ridge. The initiative also measured the Mo isotopic composition of the oceanic crust potentially entering the subduction zone. Mo is found in low levels in the mantle, but is extremely abundant in seawater where it occurs as a different isotope. These factors make Mo highly suitable for studying these geological processes. Samples of oceanic crust were taken from a drilling site situated at an oceanic ridge in the Pacific, off the coast of Central America. A profile of Mo isotopes through the altered oceanic crusts was constructed and the potential role of serpentine in carrying Mo into the mantle was examined. Research was also conducted into the effect of the subduction process in separating different Mo isotopes using eclogites, a type of metamorphic rock. The data were combined and used to gain an understanding of the composition of Mo isotopes entering the deep mantle and thus the composition of recycled components. MOLYALOC therefore showed that Mo could be used to study on- and off-ridge alterations to the oceanic plate.
Keywords
Ocean crust, oceanic ridge, on-ridge, off-ridge, molybdenum, isotope, subduction zone, eclogite
