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Earth's Heavy Metal Allotment

September 19, 2011

As a materials scientist, I learned to value the useful properties of elements of high atomic number. At one time I had in my laboratory a quarter of a million dollars worth of platinum items that were used for preparing single crystals of oxides. Platinum's atomic number is 78. Those items not in use were dutifully locked in a safe that was so secure it would take me several tries to set the combination lock to open.

All my platinum items were dutifully inventoried each month as a precaution. One nearby manufacturer had several platinum crucibles stolen when the plant was closed because of a blizzard. As I remember, the theft was an "inside job," and the culprit was easily apprehended. It would be very hard to convert platinum to money undetected.

Unfortunately, the way that the universe creates elements via nucleosynthesis in stars favors low atomic numbers. That's why the Earth is mostly composed of minerals like silicates of lithium (atomic number 3), sodium (atomic number 11), magnesium (atomic number 12), and aluminum (atomic number 13), with a touch of iron (atomic number 26).

Figure captionAbundance of the chemical elements in Earth's upper continental crust as a function of atomic number.

(USGS).

Although the heavy elements may be rare in the Earth's crust, they should actually be rarer still. As the Earth formed about 4.5 billion years ago,[1] its iron content settled into the core, pulling most of the heavy elements with it. Platinum, for example, is very soluble in iron.[2] Our technological age would be a much more barren place if we were forced to live with Earth's initial crustal allotment of elements such as platinum, gold and tungsten.

As it turns out, the concentration of such elements in the crust is about four orders of magnitude higher than they should be. The working theory for this has always been that a meteor bombardment, called the Late Heavy Bombardment, seeded the crust with these useful elements. This was the same bombardment that cratered the moon. But how do you prove such a theory? You analyze rocks that are nearly four billion years old.[3-5]

Figure captionReminiscent of a scene from the film, This Island Earth.

The Lunar Late Heavy Bombardment.

(Drawing by Tim Wether,
via Wikimedia Commons).

Matthias Willbold and Tim Elliott of the University of Bristol did an isotopic analysis of elements in 3.8 billion-year-old rocks from Isua, Greenland. These rocks were collected by Stephen Moorbath of Oxford University.[5] The focus of the study was tungsten, which is present in these rocks at just ten parts per million.[5]

What they observed was a thirteen parts per million difference in the ratio of the isotopes, 182W/184W, between the 3.8 million year old rocks and present day rocks.[3] The old rocks had the higher ratio. As can be imagined, the analysis of such small quantities is exceedingly difficult.

Said Willbold,
"Extracting tungsten from the rock samples and analyzing its isotopic composition to the precision required was extremely demanding given the small amount of tungsten available in rocks. In fact, we are the first laboratory world-wide that has successfully made such high-quality measurements."[5]

The measurement is consistent with the late heavy bombardment being the source of the heavy elements, and it excludes other scenarios, such as partial remixing of a deep-mantle reservoir.[3]

This research was funded by the Natural Environment Research Council (NERC), the Science and Technology Facilities Council (STFC) and the German Research Foundation (Deutsche Forschungsgemeinschaft, DFG).[5]

References:

  1. "Age Of The Earth," US Geological Survey, July 9, 2007.
  2. Platinum-Iron Phase Diagram, PGM Database. See the PGM Database for other phase diagrams.
  3. Matthias Willbold, Tim Elliott and Stephen Moorbath, "The tungsten isotopic composition of the Earth's mantle before the terminal bombardment," Nature, vol. 477, no. 7363 (September 8, 2011), pp. 195-198.
  4. Thorsten Kleine, "Geoscience: Earth's patchy late veneer," Nature, vol. 477, no. 7363 (September 8, 2011), pp. 168-169
  5. "Where does all the gold come from?" Bristol University Press Release, September 7, 2011.
  6. This Island Earth (1955), Joseph M. Newman, Director, on the Internet Movie Database.