Mantle Structure

The mantle is the cream filling between a silicon-rich crustal meringue and an iron-rich core base. Unlike a common pie, however, it’s composed of layers detected by seismic wave tests and is not some separate ingredient like lemon or coconut, but a transition from silicon to iron in mixtures that includes oxygen, magnesium, aluminum, calcium, sodium, potassium and hydrogen in combinations that change by layer.

Starting at the top, the beginning depths, temperatures and distinguishing characteristics of these layers are:

Crust
Moho - 7 km - 500 C - increase in seismic velocity
Lithosphere - 50 km - 900 C - low viscosity, rigid
Asthenosphere - 200 km - 1100 C - flowing, low velocity
Transition Zone - 410 km - 1800 C - distinctive seismic patterns
Lower Mantle - 660 km - no agreement - high viscosity
Anomalies - 1700 km - no agreement - lateral velocity anomalies
D” Layer - 2891 km - 4000 C - ultralow velocity, distinctive seismic patterns
Core

Olivine, a compound of iron, silicon, magnesium and oxygen [(Mg,Fe)₂SiO₄], is identified as the most important component in the asthenosphere. In the lithosphere, it combines with elements like calcium and aluminum to produce pyroxene. Above that, in the Moho, pyroxene replaces the calcium and aluminum with potassium to become amphibole. That, in turn, replaces the potassium with calcium, sodium and hydrogen (water) to create biotite.

The ratio of iron and magnesium in olivine can vary, which is why they are represented by a comma, rather than a more specific number in the formula. The permutations of both olivine and pyroxene produce many of the silicon rocks found on the surface that have created the perception that the mantle is predominately composed of silicon.

Below the asthenosphere, in the transition zone, the forsterite-fayalite form of olivine changes its crystalline structure into wadsleyite [(Mg,Fe²⁺)₂(SiO₄)], which in turn becomes ringwoodite [(Mg,Fe²⁺)₂(SiO₄)]. The important feature of their structures is that changes in oxygen create spaces for water molecules which are being released as parts of the crust sink into the transition zone. Eli Ohtani thinks the presence of water trapped in these two forms may explain the layer’s seismic properties.

In the lower mantle, the crystalline structure changes again, this time to magnesium silicate perovskite [(Mg,Fe)SiO₃]. Water is lost and iron becomes more important. With an atomic weight of 26, it has a partially filled third orbit. In its ferrous form [F²⁺], it has two electrons available to bond with other atoms; in its ferric form [F³⁺] it has three.

As pressure increases, the spin speed within the atom changes from high to low. Ferric iron changes its spin at 70 gigapascals which is about 1700 kilometers down in the area where seismic anomalies have been detected. Ferrous iron changes its spin pattern at 120 gigapascals which is about 2600 kilometers down, just above the D” layer where the crystalline structure changes again to one called post-perovskite.

The two valences can appear in the same compound in different ratios, while the amount of iron relative to magnesium also varies. Such differences explain why detectable seismic patterns are hard to replicate with current technology: the deeper one goes into the mantle, the greater the amount of iron that has changed its spin cycle.

Neither of the transition zone compounds are known from samples from the mantle. Wadsleyite was found in the Peace River meteorite from Canada. Ringwoodite was first identified in the Tenham meteorite in Australia and has since been seen in other meteorites from other parts of the world. It’s the laboratory conditions under which the two were synthesized that suggests they are the primary components of the transition zone.

A team headed by James Brado put samples of magnesium silicate perovskite through laboratory tests to produce evidence of spin changes and the deep mantle conditions that could produce them. They didn’t indicate where they obtained their raw material.

Notes:
Brado, James, Guillaume Fiquet, and François Guyot. Thermochemical State of the Lower Mantle: New Insights from Mineral Physics.

Ohtani, Eli. Recent Progress in Experimental Mineral Physics: Phase Relations of Hydrous Systems and the Role of Water in Slab Dynamics.

Both appear in Earth’s Deep Mantle: Structure, Composition and Evolution, 2005, edited by Robert D. van der Hilst, Jay D. Bass, Jan Matas and Jean Trampert for the American Geophysical Union.