The Galápagos Islands’ extraordinary biodiversity famously helped inspire Charles Darwin to formulate his theory of the evolution of life on Earth. But the volcanic islands also offer a window into our planet’s even deeper past. The volcanoes, including the one in figure 1, sit atop a mantle plume that channels deep-mantle material to Earth’s surface. And the portion of the mantle tapped by the plume has been unusually stagnant over the planet’s 4.5-billion-year history.
Fernandina volcano in the Galápagos Islands is one of several sites around the world where geoscientists can discern Earth’s original composition. (Photo by tomowen/Shutterstock.com.)
Fernandina volcano in the Galápagos Islands is one of several sites around the world where geoscientists can discern Earth’s original composition. (Photo by tomowen/Shutterstock.com.)
Over geologic time, Earth’s crust and much of its mantle are in constant, albeit slow, motion, as tectonic plates are recycled from the crust to the mantle and back again. Like the churning of butter, the churning of the planet’s thickest layer serves not to homogenize its components but to separate them based on their density, volatility, and chemical properties. As a result, almost nothing we encounter on Earth’s surface bears any relation to the planet’s average composition.
But some pockets of the mantle seem to have been immune to that mixing and have instead remained undisturbed by geological processes since at least the first 100 million years of the planet’s history. (For more on the analysis that makes that conclusion possible, see Physics Today, October 2010, page 16.) When bits of those primitive materials make their way to the surface—as they do in the Galápagos, Iceland, and a few other volcanic regions—they provide scientists with a valuable look back in time to reveal what the infant planet was originally made of.
Now Sandrine Péron (a postdoc at the University of California, Davis, at the time she did the work, now at ETH Zürich) and colleagues have used a newly developed technique to analyze some primordial mantle samples for their krypton, an element present only at the parts-per-trillion level.1 The findings paint a picture not only of krypton itself but of carbon, hydrogen, nitrogen, and oxygen—all the building blocks of life.
Mantle fingerprints
Early Earth was a hot place, as the young planet was frequently enduring energetic collisions with the planetesimals that it hadn’t yet cleared out of its orbit. The life-giving elements, on the other hand, tend to form compounds with low boiling points, such as methane, ammonia, and water. It wouldn’t seem that those volatile substances would stick around long in such an environment—if they could even condense in the first place.
Clearly, Earth does have an abundance of volatile elements, and they had to have come from somewhere. They’re not created in earthly nuclear reactions in appreciable amounts, so they must have either been part of the planet’s original composition or been delivered later, perhaps by a comet. Knowing how Earth got its volatiles could help researchers understand how usual or unusual our planet’s circumstances are—and perhaps how likely they are to have been replicated elsewhere in the universe.
That’s where krypton comes in. As a gaseous element, it’s physically similar to other volatiles, so it’s likely to have condensed and degassed along with them. And its six naturally occurring isotopes are present in different ratios in different possible sources. If a particular sample of krypton has an isotopic composition matching what’s found in the solar wind, for example, that’s at least circumstantial evidence that the krypton came from the Sun.
The challenge in working with krypton is that there’s so little of it. The element itself is rare enough, but its least abundant isotopes are two to three orders of magnitude rarer still: A typical 4 g sample of mantle material could have just a few hundred thousand atoms of 78Kr. Moreover, it’s subject to contamination. The gas entrained in volcanic rock could be bubbles of the original mantle gas, or it could be air that made its way into the lava after an eruption. Any given sample likely contains unknown amounts of both, and previous efforts to analyze mantle krypton have been stymied by the atmospheric contamination.
Péron developed a way around that problem.2 She’d crush the sample a bit at a time, and she’d separately analyze the gas released at each step. If it showed signs of having been contaminated by air, she’d exclude it from the measurement. Otherwise, she’d keep it.
But Péron couldn’t make that determination by looking at the krypton alone. Too little is released at each stage to separately analyze, and without knowing the mantle krypton composition, it’s not possible to know how much, if any, atmospheric krypton is present. Instead, she turned to the more abundant neon. With only three natural isotopes, neon is less effective at distinguishing among many possible sources. But the isotopic composition of mantle neon and atmospheric neon are both well known, and they’re significantly different. Only if the neon released at a crushing step matched the expected mantle composition would Péron keep the krypton released in the same step.
Volatile two-step
The stepwise analysis takes a long time—about a week for a single sample—and requires throwing away half to two-thirds of the data. But it gives a better estimate of the primordial mantle’s krypton composition than has been possible before.
The results of Péron and colleagues’ measurements are shown in figure 2. Their Galápagos and Iceland measurements reassuringly match one another. On the other hand, the mantle krypton is distinctly different from that of air. The difference means two things. First, the krypton in the mantle definitely isn’t just recycled atmospheric krypton. Second and conversely, the krypton in the air—and by extension, other volatiles—can’t have come solely from the degassing of the primordial mantle. Later in its history, Earth must have received another delivery of volatiles from somewhere else.
The isotopic makeup of krypton can be used to trace its source. The krypton found in mantle samples from Iceland and the Galápagos Islands differs from that of air. But the levels of five of the six isotopes closely match those of phase Q, a carbonaceous material found in meteorites. The deficiency of 86Kr in the mantle follows a pattern of similar anomalies observed in other elements; it could be a sign that the protoplanetary disk that formed the solar system was not well mixed. (Adapted from ref. 1.)
The isotopic makeup of krypton can be used to trace its source. The krypton found in mantle samples from Iceland and the Galápagos Islands differs from that of air. But the levels of five of the six isotopes closely match those of phase Q, a carbonaceous material found in meteorites. The deficiency of 86Kr in the mantle follows a pattern of similar anomalies observed in other elements; it could be a sign that the protoplanetary disk that formed the solar system was not well mixed. (Adapted from ref. 1.)
In five of the six isotopes—all but 86Kr—the new mantle measurements are a reasonable match to a class of meteorites called chondrites, which are thought to represent the original building blocks of the solar system. Figure 2 shows the krypton isotopic composition of two chondritic references: average carbonaceous chondrites (chondrites that originated from the outer solar system, where volatiles could more easily condense) and phase Q, a poorly characterized material that carries most of the heavy noble gases found in many types of meteorites.
But the krypton in Earth’s mantle couldn’t have come directly from either of those sources because the 86Kr levels don’t match. The discrepancy is a bit of a puzzle, but it’s also a clue. Similar anomalies—specifically, a deficit in the mantle of an element’s most neutron-rich isotope—have been observed for at least eight other elements, including calcium, titanium, and nickel. Krypton, however, is the first volatile element found to follow the pattern.
Neutron-rich isotopes form in stars mostly through the r-process, or rapid neutron capture, as opposed to the s-process, or slow neutron capture. The protoplanetary disk that formed the solar system likely drew on the remains of a few different stars—some with more vigorous r-processes than others. If the stars’ contributions weren’t well mixed, the part of the disk that formed into Earth could have been relatively enriched in s-process matter. Moreover, the fact that krypton shows the same anomaly as other elements seems to indicate that Earth acquired its early dose of krypton and other volatiles at the same time, and from the same source, as it accreted its nonvolatile elements.
If that explanation is right, then there should be some meteorites—remnants of the same part of the protoplanetary disk that formed Earth—that show the same 86Kr deficit as the mantle does. None have yet been found, but data on krypton isotopes in meteorites are sparse. Péron’s next plan is to turn her analysis technique to different types of meteorites to see whether she can find any with a krypton composition that matches Earth’s in all six isotopes.
