More than a half century ago, Keith Bullen, pictured at left, discovered a zone of seismic “roughness”, called D′′, located between the core and the seismically-featureless lower mantle [1-4]. Before Herndon, seismic discontinuities within the Earth’s mantle, including at D′′, were ascribed to physical changes in a medium of uniform composition, i.e., pressure-induced changes in crystal structure, rather than boundaries between layers having different chemical compositions; in other words, physics without chemistry.
Only five chemical elements, iron, magnesium, silicon, oxygen, and sulfur, account for about 95% of the mass of any chondrite meteorite; 98%, if the four minor elements, nickel, calcium, aluminum, and sodium, are included . The figure at right shows the distribution of those elements between silicate and iron alloy for the Abee enstatite chondrite and, by inference the endo-Earth (lower mantle plus core). Note the occurrence of silicon, calcium and magnesium in the iron alloy portion. These high-oxygen-affinity elements occur in part in the iron alloy because of limited availability of oxygen in this highly reduced matter.
Generally, elements that have a high affinity for oxygen tend to be incompatible in an iron-based alloy. Upon cooling from a high temperature, these incompatible elements will precipitate as soon as thermodynamically feasible, as described by J. Marvin Herndon. Silicon combined with nickel and sank to form the Earth's nickel silicide inner core whereas calcium and magnesium combined with sulfur to form CaS and MgS which floated to the top of the core forming D′′[5-11] as illustrated at left.
Seismic investigations have revealed a 5-40 km thick layer within D′′, adjacent to the Earth’s core, called the Ultra-Low Velocity Zone, where earthquake S-wave velocity drops by about 30% and P-wave velocity reduces by about 10%. According to Herndon, CaS comprises the Ultra-Low Velocity Zone [10-11]. Qualitatively one might suspect lower earthquake wave velocities in CaS than in MgS because of the heavier atomic weight of calcium. This is an instance where experimental and/or theoretical investigations of wave velocities can be definitive.
The table at right shows the fundamental mass ratio relationships, discovered by Herndon, that identify the mineralogically determined parts of enstatite chondrites with seismically determined parts of the Earth .