Research suggests more silicon in Earth's lower mantle than thought

May 04, 2012 by Bob Yirka report
Earth crust
Source: Wikipedia

For many years geophysicists have argued over the perplexing mystery regarding the amount of silicon in the Earth's mantle that is thought to have arrived there via impacts with asteroids.

The problem is that tests done to determine the composition of the mantel have found that there appears to be less in it proportionally, than there is in asteroids. Now new research by a Japanese team suggests that the lowest section of the Earth’s has more silicon in it than does the upper parts, perhaps solving the mystery. They have described their work in their paper published in the journal Nature.

To help clarify what lies far beneath our feet, geophysicists have subdivided the Earth’s mantle into three broad sections: the upper, middle and lower mantle. The upper mantle describes the crust and approximately 400 km below. The middle is about 250 km thick and the lower goes to about 2,900 km in depth.

The upper mantle is far easier to study of course, due to its proximity and thus the proportion of silicon in it is well understood. Not so well understood has been the composition of the middle and lower mantles. To study them, researches generally use seismic data recorded by sending shockwaves into the ground, but doing so thus far, has led more often to speculation than good science.

To get a better handle on what is happening so far beneath the ’s surface the Japanese team took a different approach; instead of trying to measure the lower mantle itself, they sought to recreate it in a lab where it could be measured much more easily. To do that, they mixed the ingredients (mainly silicate perovskite and ferropericlase) they believe exist in the lower mantel and placed them in a pressure chamber. There the sample was subjected to different pressure levels consistent with current theories describing the differing degrees of pressure at different levels of the mantle. They then applied the same seismic tests normally done on the real mantle. In so doing, they have come to believe that the lower mantle has a volume that is approximately 93% silicate perovskite, which when compared with data describing the upper crust gives an average amount of silicon for the entire mantle that is very nearly equal to that found in asteroids. Thus, the mystery, they say is solved.

Explore further: Marie strengthens to hurricane in Pacific off Mexico

More information: Nature 485, 90–94 (03 May 2012) doi:10.1038/nature11004

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Irukanji
5 / 5 (1) May 04, 2012
Is it really surprising when asteroids are composed of the same stuff as our planet?(and presumably Mars, Venus and Mercury).
simplicio
5 / 5 (1) May 04, 2012
Is it really surprising when asteroids are composed of the same stuff as our planet?(and presumably Mars, Venus and Mercury)

Not surprising, but it was surprising before because they said:

"The problem is that tests done to determine the composition of the mantel have found that there appears to be less silicon in it proportionally, than there is in asteroids".
Irukanji
not rated yet May 05, 2012
Is it really surprising when asteroids are composed of the same stuff as our planet?(and presumably Mars, Venus and Mercury)


I was thinking of the logic of asteroids being silicon, and since the current theory is that planets sort of accumulated bits and pieces from their vicinities, then it makes sense for the earth to be composed of the same stuff. Apparently there is a depletion of heavier metals like gold and platinum in the crust, but a higher amount in asteroids, so perhaps these metals are in higher abundance near the core?(I think an article was posted on here a few days ago about mining asteroids for precious metals, etc)
Torbjorn_Larsson_OM
not rated yet May 05, 2012
The whole field may be in a bit of upset right now, since models and observations show that differentiation (sorting out of the core leaving the mantle) may not have been as clearcut as believed. (Disclaimer: I'm interested in astrobiology, these bits and pieces are circumstantial - though I could give my refs if asked.)

Anyhow, the idea is that during differentiation iron, and the metals that like to form compounds with iron (siderophiles - nickel, gold, platinum - a few iron-like metals - see http://en.wikiped...fication ), went to the core and very little remained in the mantle. The mantle has the remaining elements - silicon and the metals that like to form compounds with silicon (lithophiles - most metals). The siderophiles we see today is believed to have backfilled the crust and mantle after solidification and differentiation respectively, delivered by impactors of which many were asteroids.
antialias_physorg
not rated yet May 06, 2012
Is it really surprising when asteroids are composed of the same stuff as our planet?(and presumably Mars, Venus and Mercury).

A planet has very different thermal dynamics than an asteroid. We're basically sitting on a very large smeltering oven. This means that different materials can settle at different depths at different concentrations (due to different densities). This is what makes it so hard to accurately predict what composition of the mantle (amd lower layers)