Mantle lherzolite xenolith in basalt - closeup of hand sample

Click on image to enlarge.          Photo © Daniel R. Snyder

Closeup of hand sample of mantle peridotite shown in previous post. Basalt has enclosed a small block of peridotite (bottom center) as well as the larger block (top), of which only a portion is included in the hand sample. Basalt is gray, olivine is light green, orthopyroxene is olive green, clinopyroxene is emeral green, spinel is dark gray-brown. Note that there is almost no reaction zone between the peridotite and the basalt. San Carlos Indian Reservation, Arizona. Scale in centimeters.

In the 1960's, amid speculation as to the composition of the mantle, Ringwood (1966)* developed the concept of a non-specific olivine-pyroxene rock which he named "pyrolite". To avoid the ongoing debate about mineral composition, pyrolite was to be defined in chemical terms, such that a material with the agreed-upon bulk chemistry would meet the requirements of density (as inferred from seismic data) and of minor elements necessary to conform to the composition of magmas that were thought to be derived from partial melting of the mantle. Different pyrolite models were proposed by several workers, and synthetic pyrolite was produced. Ringwood (1986)* produced a diagram of the mineral assemblages, densities, and phase transformations displayed by pyrolite from a depth of 100 kilometers to 850 kilometers.

Although Ringwood was widely respected and had a number of colleagues and followers, he also had detractors. One of these was Don Anderson, who refuted the concept of pyrolite on several grounds. His main objections were that the pyrolite model did not  adequately address trace elements and isotopes, or evidence for mantle heterogeneity. He also pointed out that materials of made up of different proportions of major elements could satisfy the density requirements, and criticized supporters of the pyrolite model for making ad hoc changes to the definitions as they went along. Anderson favored a dominant role for eclogite in the mantle. His main concessions were to acknowledge that "the mantle between about 800 km and 2600 km appears to be relatively homogeneous" and that "it appears that MgO and SiO2 in approximately equal molar proportions are implied for the lower mantle" (Anderson, 1989)*.

A.E. Ringwood died in 1993. Don L. Anderson published The New Theory of the Earth in 2007**. It repeats verbatim much of the of the earlier edition, but without most of the invective against the pyrolite advocates.


*Anderson, Don L., (1989), Theory of the Earth, Blackwell, 353 p.
**Anderson, Don L., (2007), New Theory of the Earth, Cambridge University Press, 384 p.
*Ringwood, A.E., (1966), Mineralogy of the mantle, in P. M. Hurley (ed.), Advances in the Earth Sciences, MIT Press.
*Ringwood, A.E., (1986), Constitution and evolution of the mantle, Proceedings of the 4th International Kimberlite Conference, v. 2.  Geological Society of Australia.