The Earth I: The Mantle Lecture 45

Structure of the Earth h (km) V 1012 km3 Mean ρ kg/m3 Mass 1024 kg %   h (km) V 1012 km3 Mean ρ kg/m3 Mass 1024 kg % Atmo-sphere 0.000005 0.00009 Hydro-sphere 3.80 0.00137 1026 0.00141 0.024 Crust 17 0.0087 2750 0.4 Mantle 2883 0.899 4476 4.018 67.3 Core 3471 0.177 10915 1.932 32.3 Whole Earth 6371 1.083 5515 5.974 100.00

The Mantle Most of the mass of the Earth consists of its mantle, so we’ll begin with that. We have three kinds of constraints on its composition: Geophysical: including seismic velocities, density, and moment of inertia (the latter constrains density distribution) Cosmochemical: the Earth must have a composition that can plausibly be derived from the nebular raw material (i.e., chondritic) Geochemical: direct analyses of mantle samples (xenoliiths, alpine peridotites); Indirect: basalts produced by melting of the mantle. Experimental mineral physics helps us translate composition into mineralogy and physical properties.

Peridotite The composition that meets these constraints is peridotite by definition a rock composed of 50% or more of olivine Xenoliths in basalts are dominantly peridotite, (specifically lherzolite) Many are harzburgitic, an Ol-Opx-rock which can be derived from by melting out a basaltic component. Peridotite has appropriate seismic velocities and densities and can be derived from an approximately chondritic composition by remove of metal (which forms the core). Peridotite

Silicates go from tetrahedral to octahedral structure at 660 km. Phase Changes In the upper mantle, the aluminous phase changes from plagioclase to spinel to garnet Silicates go from tetrahedral to octahedral structure at 660 km.

Lower Mantle The transition to octahedral structure results in a sharp increase in seismic velocity at 660 km. In the deepest mantle, another significant phase change occurs, which may partly account for the seismic properties of the lowermost ~200 km, called D’’. At present, seismic data are consistent with a radially uniform mantle composition with variation in seismic velocity resulting from self-compression and phase changes. Compositional zonation, however, cannot be entirely ruled out. In the lower mantle, the dominant mineral is Mg-perovskite: MgSiO3 with the structure of CaTiO3. Next most abundant mineral is Mg,FeO (magnesiowüstite or ferripericlase)

Estimating Earth’s Composition A goal of geochemistry since at least Goldschmidt and F. W. Clark has been to determine the composition of the Earth. Since the mantle is most of the Earth, this effort focuses on the compositions of peridotites. These, however, have been variously modified from a ‘primitive mantle’ composition, by addition or removal of melts. Hart & Zindler estimated Mg/Si and Al/Si ratios by taking the intersection of the peridotite trend with the chondrite trend.

Estimating Earth’s Composition Palme and O’Neill (2003) began by adopting an FeO concentration of 8.1% (the least modified peridotites have a MgO/(MgO+FeO) molar ratio (Mg#) of 0.89. Using that they calculated an MgO concentration for the mantle of 36.8%. They tend deduced CaO, Al2O3, and SiO2 from the intersection of the peridotite trend with MgO = 36.8%.

Composition of the Silicate Earth

Estimating Trace Element Concentrations Lithophile elements with 50% condensation temperatures above that of Mg (1340K) are generally present in constant proportions in chondritic meteorites. Among major elements, the refractory lithophile elements (RLE’s) include Ca, Al, and Ti. Lithophile trace element concentrations can be estimated from the chondritic ratios of these elements (e.g., Nd/Al). Non-refractory and non-lithophile element concentrations can be estimated from their ratios to RLE’s or from isotope ratios. For example, the K/U ratio in both the mantle and crust is about 13,000. Knowing U, we can estimate K. Also, the 40Ar in the atmosphere provides a minimum estimate of K in the Earth as 120 ppm. The Rb/Sr ratio can be estimated from 87Sr/86Sr ratios and the intersection of the mantle array with εNd = 0 (assuming the Sm/Nd ratio of the Earth is chondritic). Using this approach, concentrations of most elements in the periodic table can be build up. Estimating ‘atmophile’ element concentrations such as H, N, C, and the rare gases is more problematic, although for some of these elements, most of the inventory is in the atmosphere. Following convention, we’ll call this composition Bulk Silicate Earth (BSE) or Primitive Mantle.

The 142Nd Conundrum The non-chondritic 142Nd/144Nd of the Earth presents a challenge to the conventional approach to estimating Earth’s composition based on the chondritic RLE assumption. The Earth’s 142Nd/144Nd ratio is ~0.2 epsilon units (20 ppm) higher than chondritic. Given the short half-life of 146Sm (68 Ma), this implies a Sm/Nd ~6% greater than chondritic (only ~3% variation among all chondrites). This opens the possibility that the RLE’s are not present in the Earth in chondritic proportions. Sm and Nd are extremely similar chemically. If their ratio varies this much, other ratios might vary even more. Three possible explanations have been proposed.

Nebular Isotopic Heterogeneity Small variations in the isotopic compositions of chondrites have been observed and can be related to incomplete mixing of s- and r+p process nuclides. 142Nd is s-process, 144Nd is s- and r-process. 146Sm is p-only. Incomplete mixing of red giant or supernova debris could cause either or both the 142Nd/144Nd or 146Sm/144Nd ratio to vary. But Earth plots of the mixing line. Also, recent studies with complete sample dissolutions suggests chondrites are homogeneous.

Early Enriched Reservoir Boyet and Carlson suggested that as the Earth’s magma ocean cooled, an incompatible element-enriched crust formed - the Early Enriched Reservoir - analogous to lunar highlands, but not plag enriched). This eventually became unstable, sank the the base of the mantle, where it has remained ever since. The rest of the Earth, including the crust and all recognized mantle reservoirs have differentiated from the complimentary Early Depleted Reservoir. EER, a.k.a. D’’

Collisional Erosion Caro et al. (2008) as well as others, have suggested that as planets evolve from the planetesimal size to planet size, collisions are energetic enough to partially melt the bodies, producing a magma ocean that crystallizes an incompatible element-enriched crust (essentially the same idea as Boyet and Carlson). Collisions are also energetic enough to intermittently blast away a fraction of this crust and the incompatible elements in it. At the end of the process, the planet is depleted in incompatible trace elements (is not a significant effect for major elements like Si).

Do Geoneutrinos have the Answer? Still debated, but the consequences are important. Among the highly incompatible elements are K, U, and Th - radioactive elements that produce the energy to drive geologic processes. In the collisional erosion hypothesis, K, U, and Th could be ~40% lower than the ‘standard model’. In the EER hypothesis, this amount of K, U, and Th is concentrated in the deep mantle. One way to try to resolve this is to measure the amount of neutrinos being produced by radioactive decay. A very small fraction of these neutrinos induce the so-called inverse beta reaction, creating a neutron and positron from a proton. The positron can then be detected when it is annihilated..

Results so far: Ambiguous