Herndon's Interior Earth Composition

After Inge Lehmann discovered Earth's inner core in 1936 [1], Frances Birch in 1940 (wrongly) pronounced its composition to be partially crystallized nickel-iron metal [2] believing that the Earth's mantle was of uniform composition. But, as early as the 1930s, the seismologist, Keith Bullen, pictured at left, recognized that the Earth's mantle is not uniform as previously thought. He discovered that there is a major boundary which separates the mantle into two parts. The lower mantle is featureless, while the upper mantle appears like layers of veneer, as illustrated at right by earthquake wave speeds.

In the illustration at right, the seismic speeds of the shear waves are indicated by a dashed line, the compression waves by a solid line. Believing as Birch did that the Earth's mantle is of uniform composition, geophysicists since have attempted to explained these layers as "phase changes", different crystal structures of the same chemical composition produced by pressure caused by the overburden weight of the mantle above. Bullen also discovered a narrow band between the lower mantle and the core where seismic "irregularities" occur.

When J. Marvin Herndon first conceived of the inner core being fully crystallized nickel silicide [3], it was like parting the curtains a bit and glimpsing an entirely new realm of Earth science, a realm where discoveries could be made in a step-by-step logical progression of understanding based upon the properties and behavior of matter.

After an inspiring conversation with Inge Lehmann, Herndon, pictured at left, progressed through the following logical exercise: If the inner core is in fact the compound nickel silicide, as he had suggested (click here for pdf), then the Earth's core must be like the alloy portion of an enstatite chondrite meteorite. If the Earth's core is in fact like the alloy portion of an enstatite chondrite, then the Earth's core must be surrounded by a silicate-rock shell, like the silicate-rock portion of an enstatite chondrite. But the enstatite chondrite type of silicate-rock is essentially devoid of oxidized iron (FeO), unlike the silicate-rock of the upper part of the upper mantle, which contains appreciable FeO. Thus, the Earth's enstatite-chondrite-like silicate-rock shell, if it exists, should be bounded by a seismic "discontinuity", the boundary where earthquake waves change speed and direction because of the different compositions.

Using only the mass of the Earth's core, and the silicate-rock to alloy ratio of the Abee enstatite chondrite, Herndon calculated the mass of the Earth's enstatite-chondrite-like silicate-rock shell and found it virtually identical to the mass of the Earth's lower mantle. He calculated the boundary to within about 1.2% of the radius of the seismic discontinuity that separates the lower mantle from the upper mantle. The table at left shows the comparison between the fundamental mass ratios of the inner 82% of the Earth, the region below the seismic boundary that separates the upper and lower mantle. What this means is that the Endo-Earth, the inner 82% of the Earth, is like the Abee enstatite chondrite and the chemical compositions of Endo-Earth parts can be estimated from compositions of corresponding Abee parts [4-10].

In 1980, J. Marvin Herndon first published the discovery of that identity in the Proceedings of the Royal Society of London in an article entitled "The Chemical

Composition of of the Interior Shells of the Earth" (click here for pdf). This also means that the seismic discontinuity which separates the lower mantle from the upper mantle is a boundary between regions of different chemical compositions, not simply different crystal structures having the same composition.

Both ordinary chondrites and enstatite chondrites consist mainly of iron metal, iron sulfide, and silicate minerals. Imagine heating one of those meteorites to a high temperature in a gravitational field. What would happen is this: The iron sulfide minerals would alloy with the iron metal and that mass, being denser, would settle to the bottom like steel on a steel-hearth. The Earth is like a spherical steel-hearth.

The vertical axis of the figure at left shows the weight percent alloy of each of 157 ordinary chondrites (shown as open circles) and 10 enstatite chondrites (shown as filled circles). The dashed lines show the points on the vertical axis which correspond to the Earth's core, expressed as weight percent of the whole Earth and as weight percent of the Endo-Earth (lower mantle plus total core). This figure shows that the alloy weight percent of the Earth's core is consistent with that for enstatite chondrites, like the Abee meteorite. It also shows that the Earth cannot be like an ordinary chondrite, as was assumed by Francis Birch and many who followed him.

Note the horizontal axis, which show's the meteorites' oxygen content. See, the enstatite chondrites are relative low in oxygen compared to ordinary chondrites The low oxygen content of enstatite chondrites and the Earth, established during formation, has important consequences on Earth-core composition. If the Earth were like an ordinary chondrite, the only major and minor elements in the core would be iron (Fe), nickel (Ni), and sulfur (S). But, being like an enstatite chondrite, the Earth's core contains in addition to those elements, some silicon (Si), some magnesium (Mg), and some calcium (Ca), as shown in the graph to the right.

When the Earth formed, and when the matter of certain enstatite chondrites formed, the availability of oxygen was so limited that some normally lithophile "rock-loving" elements could not find oxygen and were thus constrained to reside in the alloy, in the core. The trace element uranium (U) was among these. Generally lithophile elements are incompatible in an iron alloy and tend to precipitate out. How these elements precipitate out, as described by J. Marvin Herndon, determines the structures within the Earth's core.

The figure at left is a schematic representation of Herndon's Earth interior. Note that the composition of the upper mantle is shown as unknown. Why? Because the upper mantle has several seismic discontinuities, appearing like layers of veneer. At present, the compositions of those layers are unknown. Indeed, the upper mantle may consist of two or more components, one being, for example, ordinary chondrite matter.

The lower mantle consists of mainly of enstatite (MgSiO₃), like the silicate of the Abee enstatite chondrite, but in a different (perovskite) crystal structure because of the pressure by the weight above. Like the Abee-silicate, the lower mantle has essentially no oxidized iron (FeO).

Now consider the Earth's fluid core. Imagine, for sake of discussion, that at some point in the past the Earth's core was so hot that all of its elements were dissolved in the liquid iron alloy. As the Earth's core began to cool, the incompatible elements would begin to precipitate out.

At a high temperature, calcium (Ca) and magnesium (Mg) would grab sulfur to precipitate as CaS and MgS and, being less dense, would float to the top of the core, causing the seismic "irregularities" observed there. Other highly incompatible trace elements, such as uranium, would find a thermodynamically feasible way to precipitate.

At a somewhat lower temperature, under appropriate conditions, silicon (Si) and nickel (Ni) would combine and precipitate, settling downward, forming the Earth's nickel silicide inner core.

In the Birchian view of the Earth being like an ordinary chondrite, on the other hand, the parts of the core are inexplicable without making ad hoc assumptions for which there is no corroborating evidence.