Transition region of the Earth's upper mantle

Abstract

The Earth's mantle is conventionally divided into three major subdivisions: the shallow mantle, the transition region and the lower mantle. High seismic velocity gradients extend from 400 km, the top of the transition region, to ~800 km. The relatively homogeneous part of the lower mantle starts near 800 km and extends to within several hundred kilometres of the core-mantle boundary. Major changes in mantle mineralogy occur near 400 and 650 km (refs 2-6). The question of whether these represent equilibrium phase boundaries in a homogeneous mantle or changes in chemistry is fundamental to many problems in earth sciences. There is abundant evidence that the Earth as a whole is a differentiated body and is inhomogeneous on many scales. For example, the extreme concentration of trace elements in the continental crust requires efficient extraction of these elements from all or most of the mantle. Nonetheless, the chemistry of the mantle remains a controversial issue which can be reduced to three basic questions: (1) is the mantle homogeneous in composition or is it chemically stratified? (2) Is the major-element chemistry of the mantle more similar to upper mantle peridotites or to chondrites? (3) What is the composition of the source region of basalts erupted at mid-ocean ridges? We address each of these questions using data from cosmochemistry, geochemistry, petrology, seismology and mineral physics.

Additional details