But, says Platt, "3 billion years ago, our planet might have been fundamentally different, and we simply don't have enough data or imagination" to conjure it up yet. He points out that "you could still have had processes that go on on an active planet even if they don't fit in with the processes we have today." He argues that traces of eclogite cropping up in diamonds about 3 billion years ago do not necessarily mean that a more modern supercontinent cycle (also known as the Wilson Cycle) had to have started at that time. He points out that eclogite has been known to crystallize some 80 kilometers below the surface, not having necessarily needed the deeper subduction implicated in the Wilson Cycle.
Don Anderson, a professor emeritus of California Institute of Technology's Division of Geological and Planetary Sciences agrees that finding eclogite in itself might not be enough to indicate the emergence of full-blown continental plates. "True plate tectonics may have started later," he says.
Nevertheless, Platt says, Shirey and Richardson's methods look sound: "It's still really interesting data, and it clearly does mean something." And even if Platt is not ready to fully accept the premise put forth in the new paper, for now, he says, "I can't come up with a better one."
Shirey is now turning his focus to hunting down diamonds from other areas of the planet to see how their inclusions compare to those he and Richardson have already found. One ancient formation, the Zimbabwe craton, is of interest because "it looks like it was formed in a different way" from others, such as that in Australia (Pilbara) or South Africa (Kaapvaal).
And another frontier is deep Earth itself. Nonetheless, "it's very hard to look deep into the mantle," where clues about early geologic dynamics might linger, Shirey says. "We think diamonds are forming in the mantle all the time. They just never make it up because there are reactions going on, and they get reabsorbed."
So-called deep-mantle geodynamics is "a whole new area of research," Shirey says. Below continents lurks the lithosphere, some 220 to 225 kilometers below the surface, which is separated from the mantle by the transition zone (400-700 km deep). Even the upper portions of the mantle are some 700 to 1,000 km below the surface—a generally solid but conductive part of the planet. Any intact rocks from that depth would be a proverbial goldmine for geologists. "If we can get minerals—like we can get diamonds—that haven't reacted, all the better," Shirey says. "Whatever they carry with them in their lattice is going to be frozen information." Information that could help advance scientific understanding of Earth's earliest continents—as well as, Platt points out, the geology of exoplanets.