Oceanic lithosphere - 3. The origin and evolution of oceanic lithosphere: The geochemistry and origin of oceanic lavas

Oceanic lavas are the most voluminous volcanic products on Earth. Between 1 5 and 20 km(3) of new oceanic crust is created every year by eruption of mi docean ridge basalt (MORB) along seafloor spreading axes. MORB lavas are ol ivine- and hypersthene-normative tholeiites highly depleted in incompatible trace elements such as Rb, Sr, Ba, K, Zr, Y and the light rare earth eleme nts. Isotopically, they are characterised by low radiogenic Sr and Pb and h igh radiogenic Nd. They are formed by decompressional melting of shallow ma ntle material as it rises beneath spreading axes. This upper mantle source was previously depleted in incompatible elements by extraction of continent al crustal material early in Earth history. On a global scale MORB lavas ar e remarkably uniform, but relatively enriched varieties (E-MORB) are known from all ridge segments. These enriched lavas are transitional in compositi on to ocean island basalts (OIB) which have much higher contents of incompa tible elements and much more varied isotopic compositions than typical MORB lavas. Ocean island basalts are believed to form by partial melting of man tle plumes that rise from near the core-mantle boundary. These plumes entra in mantle material that has been relatively enriched by recycled crustal ma terials in subduction zones. Mixing of plume mantle and depleted MORB mantl e on a variety of scales can explain the occurrence of: E-MORB lavas. Large -scale superplumes are believed to be responsible for the formation of ocea nic plateaus, which are built by voluminous eruptions of ocean island basal ts.