Meteorite/Planetary Science Quiz

Question #1

In our existing inventory of meteorites are there any that, in bulk elemental composition, distinguish themselves as being a perfect likeness of the Earth?

The answer is no.

The Earth, with all terrestrial planets, is assumed to have chondritic proportions of the refractory elements, although the absolute concentrations of elements in the Earth is established by data from the Primitive Mantle. While overall, the Earth has a volatile depletion pattern similar to that of chondrites, it has a Fe/Al value at ~ 20+/-2, which preclude some chondrite types from being the analog composition for the Earth.

Aluminum, a refractory lithophile element, is considered least likely of the major lithophile elements to be incorporated in the Earth's core. Therefore, the combined Fe content of core and silicate Earth (crust plus mantle) and the Al content of silicate Earth establishes the Earth's bulk value of Fe/Al at ~ 20+/-2, given a core with 85% Fe. Thus, this Fe/Al constraint for the Earth provides a measure of evaluating which of the different groups of chondritic meteorites are the best candidates as analogs to the bulk Earth's composition.

Chondritic meteorites, including carbonaceous, enstatite and ordinary chondrites, display a range of Fe/Al ratios, with many having a value that approaches 20. However, the EH enstatite chondrites have high Fe/Al values (35) and the CV and CK carbonaceous chondrites have low Fe/Al values (13-15).

So while the composition of Earth, integrated from core to atmosphere, is comparable to that of undifferentiated meteorites (chondrites) none of these chondrite groups possess the major element characteristics that make them suitable analogs for the bulk Earth.

Although some chondrite types come close, we have not found any meteorites that are a perfect likeness, in bulk elemental composition, to Earth.

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