2012

During the earliest history of the Earth, deep mantle melting and fractional crystallization played an important role.  Crystallization of the early mantle magma ocean(s) would likely result in dense, late-stage cumulate piles in the upper part of the transition zone (400-500 km depth) and/or near the core-mantle boundary (CMB), depending on the melt composition and the evolving melt-solid density relations. In the period shortly after the crystallisation of the magma ocean, hot rising plumes would partially melt through wide ranges of pressure (depth).  Melting at depths greater than a melt-solid neutral buoyancy level at about 300 km would lead to melt accumulation in a zone at 410 km or slightly deeper. Preliminary measurements of the Fe/Mg-partitioning between melt and solid residue indicate that the melts will be enriched in Fe throughout the mantle pressure range.  Therefore, solidified and thermally equilibrated melt material at 410 km or anywhere else would be denser than the ambient peridotitic mantle.  Such melt material, as well as dense cumulates in the transition zone, would likely sink to the CMB and potentially accrete into the presently observed antipodal Large Low Shear-Velocity Provinces (LLSVPs), stabilized by Earth's rotation.

The asthenosphere beneath the global network of spreading ridges is continuously sampled by partial melting, generating MORB.  In addition, the residual peridotites from the upper asthenosphere and lithosphere are locally exposed on the ocean floor by rifting-related and fracture zone fault tectonics.  Sampling of unaltered zero-age basalts and associated abyssal peridotites along the spreading axes by dredging and drilling is facilitated by shallow bathymetry and sparse sediment cover.  The participants on numerous oceanographic expeditions have provided a wealth of systematic along-along axis data sets of major and trace element abundances and isotope compositions. Although many studies have compiled and used such data to investigate aspects of mantle source heterogeneity and origin, as well as melting processes, new and innovative approaches by different individuals may provide additional insights