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Comment on:

Jackson, M.J., A. Mark Jellinek, Major and trace element composition of the high 3He/4He mantle: Implications for the composition of a non-chonditic Earth, Geochem. Geophys. Geosys., DOI 10.1002/ggge.20188

 

June 18, 2013, Don L. Anderson
It has long been taught in geophysics and planetary physics courses, but not in mantle geochemistry courses, that the Earth started hot and was extensively differentiated during accretion. This knowledge goes back to the extremely influential papers of Francis Birch, including his classic 1952 paper, his 1965 Presidential Address and his energetics of core formation papers. These papers form the foundations of modern geophysics but they are the antithesis of the Urey school of geochemistry (Urey, 1952) which produced Craig, Wasserburg and many other advocates of cold accretion, primordial Earth, crustal growth, continuous differentiation  and undegassed mantle. Thus, 1952 was a pivotal year for both mantle geophysics and mantle geochemistry.

The two sciences diverged from that point on. Schilling (Nature, 242, 1973, p. 565) states "Contrary to earlier views (Birch, 1965), the model implies that plumes are transporting to the Earth's surface a more primordial mantle material(s) than is present in the low velocity layer lying beneath those midocean ridge segments remote from plume." This was the nucleus of the geochemical version of the plume hypothesis. Tatsumoto (1978) and O'Hara (1973) almost immediately showed the flaws in Schilling's argument, and Tozer (1973 ) demonstrated the fluid dynamic implausibility of the geochemical model. Birch’s papers had already demonstrated that classical physics ruled out the assumptions in what became the canonical model of geochemistry.

The Birch ideas were extended to mantle geochemistry by Tatsumoto, Armstrong and Kay who developed top-down models of geochemistry. The mass balance associated with hot accretion and early differentiation was developed in many early papers and summarized in Chapter 8 of Theory of the Earth and Chapter 13 of New Theory of the Earth. The mass balance shows clearly that the whole mantle had to be processed to form the crust plus the kimberlites and carbonatites.

The atmosphere alone contains 77% of the argon produced over the whole age of the Earth. This does not mean that 23% of the mantle is primordial or undegassed but simply that the argon extraction process from the crust and upper mantle is 77% efficient, which is high. An implication is that some ancient 3He may also trapped in shallow mantle crystals such as olivine, thereby explaining high 3He/4He ratios without requiring any 3He-rich or undegassed primordial mantle, a basic tenet of the Urey school of geochemistry. The numerous paradoxes that occur in geochemical models are the result of unphysical and unnecessary assumptions that do not occur in physics based models.

The depleted parts of the mantle include the essentially barren perovskitiitic lower mantle (the residue of upward zone refining), the refractory infertile parts of the shallow mantle (the harzburgite-rich part of the boundary layer), and the so-called depleted but fertile MORB reservoir. The complements to the crust, enriched magmas, and kimberlites are mainly the lower mantle. The depleted but fertile eclogite transition region represents only about 6% of the mantle.

It is a mistake to think that the continental crust is the only enriched reservoir or that the MORB source is the only depleted part of the mantle. It represents part of the depleted fertile part, and resides mainly in the transition zone. The really depleted (barren) product of mantle differentiation is the mantle below the 650 km discontinuity.

Most of the mantle is subadiabatic, a result of both secular cooling and internal heating. The cooling core-mantle boundary also makes the lower mantle colder and less active than assumed in almost all mantle dynamic simulations.

There is also the important matter of the Second Law of Thermodynamics. Some mantle dynamic models are based on atmospheric and kitchen analogies, such as thermals, thunderheads and boiling water. Thermals and boiling occur when the temperature at the base of the system is held constant or increases with time due to an external energy source such as the Sun or a stove. When the stove is turned off, or when the Sun gets low in the sky, the boundary starts to cool and the thermals and boiling cease. All models of mantle dynamics that result in narrow thermals or updrafts use a constant temperature core-mantle boundary or a localized continuous source of heat or material. There is no equivalent in an isolated planet. Maxwell demons were an early attempt to get around the Second Law of Thermodynamics but they also required external sources of energy or information. Birch and others argued that upwellings are broad, due to compression and internal heating, that most of the U, Th and K are in the upper mantle, and that planets cool with time. This includes the core-mantle boundary. The core-mantle boundary has been cooling for at least 3 Gyr. This has produced the Earth’s magnetic field and the solid inner core, but it does not give narrow updrafts. Simulations of narrow thermals in the mantle all involve violations of the Second Law, compared to which the heatflow, helium, lead, thorium, and other paradoxes pale in comparison. The mantle is not a perpetual motion machine and the laws of physics must be obeyed.

References

June 13, 2013, Anders Meibom
I agree with Michele's comments. This game of departing from a chondritic starting composition is tempting but also very dangerous. One might solve a specific problem doing so, but it also "sets all loose". We have had several situations in cosmochemistry over the years where we have discussed this possibility but each time it was concluded that one would loose more than what could be gained.

If we depart in one case from the chondritic baseline, either for elemental ratios or for isotopes, then the first question is "how much is reasonable?" and the next question is "why then not for all the other problems as well"?

It then becomes a game of fitting the initial conditions to the problem instead of understanding the process.

Of course, when people have stared at the same problem for years without finding a real solution, it is tempting.

June 13, 2013, Michele Lustrino
This paper deals with an argument extremely important for the plumes debate. Much of the geochemistry applied to igneous petrology is based on the existence of a chondritic Earth. This means that the original 143Nd/144Nd isotopic composition of the Earth's mantle, before the crustal differentiation, is assumed to be equal to the CI ordinary chondrite (= 0.51264 or 0.51263). Rocks characterized by 143Nd/144Nd higher than this value indicate a derivation from a source with higher Sm/Nd elemental ratio compared with typical CI chondrite. One of the samarium isotopes (147Sm) decades into 143Nd, and, as consequence, an old mantle with high Sm/Nd would evolve with high (higher than typical chondrite) 143Nd/144Nd. Neodymium is more incompatible than Sm and, therefore, a mantle with high Sm/Nd (i.e., with low Nd/Sm) means a depleted composition.

One of the most puzzling features for plume advocates (not for plume-skeptics) is that high 3He/4He basalts, considered to represent the "purest" deep mantle melts, are characterized also by the highest 143Nd/144Nd. In other words, melts considered to be derived from an undegassed (and, therefore, 3He-rich) source share also evidence for a very depleted composition (in terms of REE abundances, with high Sm/Nd). Of course, plume-skeptics realise that high 3He/4He does not automatically mean high 3He, but can be simply related to a 4He-poor source.

The paper of Jackson and Jellinek try to overcome to this oddity whilst retaining the hypothesis that high 3He/4He means high 3He. They propose a non-chondritic Earth. These authors, indeed, propose that the "original" or "primitive" Earth's mantle 143Nd/144Nd value is not 0.51264 but much higher (0.513). Accepting this new reference value for the primitive mantle, the 143Nd/144Nd isotopic ratios of 3He/4He-rich OIBs can no longer be considered "depleted". In other words, high 143Nd/144Nd (around 0.513) should not derive from a depleted source (with high Sm/Nd) but should derive from a "primitive" mantle source, with unfractionated Sm/Nd elemental ratios.

My main critics concerns the MORB source. If we accept the hypothesis of a non-chondritic Earth, we have also to conclude that no MORBs derive from depleted mantle. This because the 143Nd/144Nd (and also the 87Sr/86Sr) isotopic range of 3He/4He-rich OIBs is indistinguishable from that of the typical N-MORBs (e.g., the N-Atlantic MORBs).

In conclusion, I believe that the model of Jackson, which follows on from many other papers suggesting a non-chondritic Earth, solves one problem but creates others in that it implies that all MORBs derive from an isotopically primitive mantle source.

last updated 14th June, 2013

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