Composition of Earth's Mantle

Abstract

Mantle rocks, peridotites, are exceedingly rare on the surface. They are only exposed in special tectonic settings on the continents and the ocean floor. In addition to studying mantle peridotites, investigating the melting products of Earth's mantle, the basalts of the oceanic crust, has therefore supplied invaluable information about the nature and timing of the processes that regulate the mantle's compositional evolution. The chemical and isotopic data of peridotites and oceanic basalts show that Earth's mantle has evolved for >4.5 billion years by convectively driven partial melting and recycling of the transient oceanic and long-lived continental crust back into the mantle. Although the underlying mechanisms may have changed during Earth's history, the current highly processed state of Earth's mantle reflects the time-integrated balance between the outgoing flux by partial melting, which generates the oceanic and eventually the continental crust, and the rate of the crustal return flux back into the mantle. The convective cycling of Earth's mantle through melting regions in the shallow mantle, the mantle processing rate, depends on the vigor of thermo-chemical convection and is the ultimate motor that drives the compositional evolution of Earth's mantle and crust. The present compositional range and structure of Earth's mantle are thus intrinsically related to its dynamics. Understanding the processes that govern the compositional evolution of Earth's mantle is therefore a first-order geochemical and geodynamic problem, at the root of understanding our planet's principal mode of operation.