The better our instruments are, the more things we can see. This is an old truism in science—think of telescopes for the most well-known example—and it continues to be true today. Here's a look at what we can do with two of Earth's rarest elements, rhenium and osmium. Until the 1990s, our instruments couldn't even measure these elements reliably. Now that we can measure them in the parts-per-trillion range, we're finding that rhenium and osmium are a unique team.
How Rhenium and Osmium Behave
Actually, these elements are rare only at the Earth's surface. Rhenium and osmium are part of the platinum group of metals, and like the rest of that group they are strong siderophiles—they love iron. When the Earth first formed, they dissolved in the molten iron, along with cobalt, nickel, phosphorus, and several other elements. Today almost all of the planet's rhenium and osmium are in the iron core, leaving the silicate mantle with just a whisper, less than one part per billion.
The Rhenium-Osmium Clock
Rhenium, which occurs at about 200 parts per trillion outside the core, comes in two isotopes, of atomic weight 185 and 187. Rhenium-187 is very slightly radioactive, turning into osmium-187 with a half-life of 42.3 billion years. (This happens when a neutron decays to a proton, leaving the atomic weight unchanged.) Therefore osmium very, very slowly collects more of the 187 isotope.
With its pitifully slow radioactive "clock" and extreme rarity, rhenium would seem like a marginal candidate for study. But rhenium and osmium, practically twins on the periodic table, behave differently when rocks melt and recrystallize.
If there's no iron around, osmium has no preference between melts and solid minerals, but rhenium strongly favors the melt. In geochemist's lingo, osmium is a compatible element while rhenium is strongly incompatible. Therefore, a body of rock in the mantle that undergoes any degree of melting stops its rhenium-osmium clock. And over time this extraction of melt, through volcanism, also concentrates rhenium from the mantle into the crust and continents, which are the incompatible dregs of billions of years of mantle activity.
Rhenium-Osmium: Deep Clues to Mantle Evolution
The extra rhenium entering the crust changes the apparent date on the rhenium-osmium clock in crustal rocks. If we tried to use the rhenium-osmium clock as a dating method, these variations would be noise in the data. But because we can already tell the ages of rocks by other means, we can turn that noise into a signal—information about changes related to melting in the mantle and crust.
So a careful study of rhenium and osmium can shed light on some deep geologic problems. For one thing, there's more of these metals in the crust and upper mantle than there should be. The reason is that after the core formed some 4.5 billion years ago, more iron rained down upon the Earth in the form of asteroids during the "late bombardment." So in effect, Earth got a final frosting of rhenium and osmium on the crust and upper mantle. Through osmium isotope studies we can examine how this frosting has mixed with the rest of the mantle. We can also look for signs of mixing with the osmium-poor lower mantle and with the osmium-rich core.
This is a simplified picture, of course. For a glimpse of some of the complexities, visit this short article by geochemist Richard Walker on his mantle studies.
Osmium, Rhenium and the Cosmos
As meteorites continue to rain down on Earth, they add osmium to the sedimentary record. Meteorite osmium hasn't gone through 4 billion years of mantle evolution, so its balance of isotopes is different from Earth's.
There are at least two ways to use this information. Karl Turekian is one scientists who uses osmium to study the continuing flux of dust from space onto Earth. And larger pulses of osmium in the record of seafloor sediments point to asteroid impacts, giving researchers a new technique for estimating how big an impactor was.
For a truly cosmic look at rhenium-187, see this PDF article from the Darmstadt Heavy-Ion Laboratory that explores its behavior as a deep probe of the age of the universe.
Other Rhenium and Osmium Facts
Rhenium is exhaled in the sulfide gases of a single volcano, Kudriavy, out in the Kurile Islands in far east Russia. (See more details on the Kudriavy page.)
Osmium is the densest element of them all (22.57 grams per cubic centimeter). Osmium tetroxide smells so bad that the element got its name from the scientific Greek osme, odor. High-end pen nibs have osmium tips.