Pyrolite, peridotite and basalt

The pyrolite (PYR-oxene + o-LI-vine) composition model was an artificially constructed aggregate, usually with ~1 part of basalt and ~3 part dunite (a type of peridotite, mostly olivine).

The origin of this model could be understood this way: in the mantle convection, the decompression of rising hot material can result in its partial melting: the melt is basalt, and the residue left behind is peridotite.

$\ce{hot material ->[melting][decompression] \underset{residue}{peridotite} + \underset{melt}{basalt}}$

The original material in mantle composition can therefore be recovered by putting peridotite and basalt back together.

In 1962, Ringwood (Ringwood, 1962 JGR) initially bring up the pyrolite as it being the upper mantle composition. Today has been mostly agreed upon. The transition zone has also been agreed to have a composition similar to pyrolite by the most. However, arguments about the composition of the lower mantle (which host most of the material of the Earth) remain.

If the lower mantle has the same composition as the upper mantle, the mantle would be of homogeneous composition and this how the pyrolytic model of the Earth gets its name. On the contrary, if the lower mantle has a chondritic composition, the mantle would have a heterogeneous composition, this was known as the chondritic model. The geophysical community focus on the difference in the Mg / Si abundance between the two models.

Reference

A. E. Ringwood, A model for the upper mantle. J. Geophys. Res. 67, 857–867 (1962).