It’s generally assumed that the Earth’s overall composition is similar to that of chondritic meteorites, the primitive, undifferentiated building blocks of the solar system. But a new study in Science Express led by Frederic Moynier, of the University of California at Davis, seems to suggest that Earth is a bit of an oddball.
Moynier and his colleagues analyzed the isotope signature of chromium in a variety of meteorites, and found that it differed from chromium’s signature in the mantle.
“We show through high-precision measurements of Cr stable isotopes in a range of meteorites, which deviate by up to ~0.4‰ from the bulk silicate Earth, that Cr depletion resulted from its partitioning into Earth’s core with a preferential enrichment in light isotopes,” the authors write. “Ab-initio calculations suggest that the isotopic signature was established at mid-mantle magma ocean depth as Earth accreted planetary embryos and progressively became more oxidized.”
The results point to a process known as “core partitioning,” rather than an alternative process involving the volatilization of certain chromium isotopes so that they would have escaped from the Earth’s mantle. Core partitioning took place early on Earth at high temperatures, when the core separated from the silicate earth, leaving the core with a distinct composition that is enriched with lighter chromium isotopes, notes William McDonough, from the University of Maryland at College Park, in an accompanying Perspective piece.
McDonough writes that chromium, Earth’s 10th most abundant element, is named for the Greek word for color and “adds green to emeralds, red to rubies, brilliance to plated metals, and corrosion-proof quality to stainless steels.” It is distributed roughly equally throughout the planet.
He says the new result “adds another investigative tool for understanding and documenting past and present planetary processes. For the cosmochemistry and meteoritics communities, the findings further bolster the view that the solar nebula was a heterogeneous mixture of different components.”
Source: Science. The McDonough paper will be published online today by the journal Science, at the Science Express website.
This article as written does not make intuitive sense. If heavier isotopes were separated from lighter ones then in theory they should have ended up in the core rather than the mantle and the lighter isotopes in the mantle. More information is needed about the mechanism of the separation process. It would also be helpful to know how the chromium isotope sampling from the core was done to obtain the evidence for me to make any sense of this. My guess would be through seismic data.
Separated, but not necessarily by gravity. The article do not mention how the separation happened, i can think of a few possibilities but all speculations, but as long as gravity is not the driving mechanism behind the separation, then there is no need for the heavier isotopes to end up closer to the core and the lighter above. Besides, the difference in atomic mass between the isotopes is so small that gravity would have problems separating them. A small turbulent activity would easily counter the separation effect and remix the constituents.
“More information is needed about the mechanism of the separation process.”
Heavier isotopes makes (slightly) stronger chemical bonds.
In mantle Cr occurs mainly in some chromates with bond energy much greater than metallic bonds in core, so after billions of years you could expect some mantle enrichment in heavier Cr.
Thanks to both. I realized that I could have looked up the answer, but it likely would have taken quite a while and maybe a chapter of textbook reading to find and understand it. As I take it now, the increased heat of the core would preferentially break down chromates containing lighter isotopes at the core-mantle boundary due to their lower chemical bond energy. This would then slowly over time enrich the core with lighter chromium isotopes and leave the mantle with heavier ones. In general as you get towards the center of the Earth chemical influences predominate over gravitational ones. After all the heat down there is being generated primarily by radioactive decay.
Another thing with the article that doesn’t make intuitive sense is the characterization of Earth as “oddball”, if the Cr isotopic separation is a result of core separation. The mantle flow driver may make it a bit more unique though. (Sitting where I can’t get access to the paper behind the pay-wall.)
Another factor that the paper itself should take into consideration but may not have is the rapid early crust formation that seems to have happened, pushing some siderophiles (iron loving elements) out and I presume giving some fractionation as well along the lines you guys mention. It seems to account for many differences Moon-Earth, so it may be vital depending on the element studied as I understand it. Cr is a lithophile I see, so the effect is secondary here.
Nitpick: not as I remember my astrobiology text book. Decay dominated the heat flow the first ~ 1 Gy, but after that it is mostly residual formation + decay heat that drives the over all flow.
But in general you are correct I believe, the gravitational potential drops towards the center of a mass so chemical (and pressure!) influences starts to dominate even more.
The topic gets more interesting, complex, and uncertain the deeper I look into it. Most texts consider the current contributions to Earth’s internal heat as follows: 20 percent is still thought to be residual gravitational binding energy (from accretion of planetismals), 45-80 percent from decay of radioactive elements (quite a bit of uncertainty there), 10 to 25 percent from nuclear fission for those who believe in the georeactor hypothesis (which I don’t). Small percentages may also come from heat released from heavy metals as they descended to the core, the effects of internal magnetic fields, and tidal forces acting internally.
Going back in time the only period where gravitational binding energy was the dominant process generating the internal heat was when planetismals were still smashing into Earth. As soon as the surface solidified it is thought that radioactive decay was already the dominant process. Both types of heat were greater then, but in roughly the same proportion. Radioactive element decay was greater at that time since short-lived isotopes were all decaying at that time. There are Archaean rocks with very high magnesium content (komatiites) which are unique to that era due to the higher internal heat then.
At, thanks! I completely misremembered the relevant figure and stupidly didn’t check. I’m sorry for any inconvenience and glad that you correct my missteps, I need to learn (and remember) this correctly.
So, I’m looking at fig 4.21 “Heat production vs time diagram (after Brown, 1986)” of Herve Martin’s chapter 4 in “Astrobiology”, and it shows the “All sources” asymptote merging with the “Long lived radioactive sources” curve ~ 3 Ga. Production, not flux (which is more complicated and depend on onset of convection and whatnot – the whole next chapter was about that).
Hmm. So your references are more distinctive on the data, I assume they aren’t general as the astrobiology text but specific on geothermal stuff. Mind if I ask about what references you have?
“komatiites” Komatiites and TTG composition both, actually. (And again I had forgotten about komatiites, but at least TTGs “stuck”. :-D)
We need to study these to unravel the mystery of Earth….
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