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."
Deep answers
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.
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4 Comments
Add CommentWow pretty cool.
Reply | Report Abuse | Link to thisSo, before 3 billion years ago, the crust was just a thinner skin over a hotter, more fluid mantle? There's evidence of oceans over the planet almost 4 billion years ago, so maybe the ocean floor was a hotbed of hydrothermal activity until thicker continental plates formed at the theorized transition. This could be sort of like how a lake freezes over. At first, a thin crust forms on the surface but can be broken apart very easily. It's only when it gets thick enough where it can float out of the water a bit and exchange heat with the (generally) cooler air instead of the (probably) warmer water that the lake freezes over quickly.
Reply | Report Abuse | Link to thisAs the season progresses, the temperatures generally get lower and the ice locks together kind of like plates, and the heat transfer between the water below and the increasingly cold air above falls dramatically. The Earth is cooling slightly, but the comet/meteor impact rate is also dropping from the very high levels seen between 4.6 and 3.5 B years ago. As soon as the impacts and their associated heating stopped, proper continents could form, insulating the mantle ever more effectively, and eventually cut the heating off enough so that modern plate tectonics could get started.
This is purely speculation and not even a hypothesis. Any clarifying information is welcomed.
I've wondered if large impacts might have formed the first continental nuclei. These would have created vast pools of impact melt, which would have differentiated into an anorthosite upper layer and a denser lower layer. These thick anorthosite rafts would have been too buoyant to undergo subduction.
Reply | Report Abuse | Link to thisYeah, something like that from what I've read. Nobody really knows though exactly. There were a lot of things that were different back then. The mantle and outer core were probably more homogeneous back then, so whatever solidified in the beginning was not like what crust is made of now. Temperature differential was greater. It may well be that this heavier material, once it cooled and solidified simply sank right down the base of the mantle. lighter stuff slowly accumulates near the surface, bouyant enough to mostly stay at the top.
Reply | Report Abuse | Link to thisIt is always interesting. A time machine would be real fun.