The water that cycles among Earth’s oceans, lakes, and atmosphere is essential to life as we know it, but that’s just a portion of the water that makes its presence known on this planet. Oceans worth of water is dispersed, molecule by molecule, in the mantle, where it influences mineral material properties and thus geological dynamics (see the article by Marc Hirschmann and David Kohlstedt, Physics Today, March 2012, page 40). On time scales of millions to billions of years, the water cycle includes exchanges into and out of the mantle: Water is released from the mantle (or degassed) through volcanoes, and it’s returned (or regassed) through the subduction of hydrated minerals in tectonic plates. Other volatile species—including compounds of carbon, nitrogen, and sulfur—engage in their own deep cycling.
Now Rita Parai (Washington University in St. Louis) and Sujoy Mukhopadhyay (University of California, Davis) have established the first quantitative constraints on how that mantle–surface exchange evolved over time. They numerically modeled the degassing and regassing of xenon, which has advantages over other volatiles as a tracer of ancient dynamics. It has nine natural isotopes whose atmospheric abundances are known to have changed over time. Some of those isotopes are radiogenic, continuously produced in the decay and fission of radioactive nuclei in the mantle. And because Xe is a noble gas, its isotopes are fractionated only by physical processes, not chemical ones.
Parai and Mukhopadhyay tested numerous possible histories of degassing and regassing as a function of time, and they collected all the ones that gave Xe isotopic abundances consistent with the available data. The figure shows the net flux into the mantle for the successful trajectories. The researchers conclude that the mantle switched from net degassing to net regassing sometime between several hundred million years ago and 2 billion years ago. Furthermore, although it’s not evident from the figure, none of the successful histories had any significant Xe regassing before 2.5 billion years ago. Because water-bearing minerals also carry Xe, water regassing must have begun around the same time.
But evidence suggests that plate tectonics, and therefore subduction, has been going on for at least 3 billion years. For several hundred million years, then, rocky slabs were being returned from the crust to the mantle, but they carried no water at all—either because the water was somehow expelled in the process of subduction or because the ancient crust wasn’t hydrated to begin with. (R. Parai, S. Mukhopadhyay, Nature 560, 223, 2018.)
