Remnant of an early Earth

Earth’s mantle is a convecting solid. It is hot enough that the material in the mantle flows viscously on a geologic timescale; over millions of years it churns and mixes. How effective is it at mixing? Over the 4.5 billion year history of the Earth, the mantle should have overturned completely several hundred times.

Imagine taking a tube of Aquafresh (the tri-colored toothpaste), emptying it into a jar, and then stirring it a hundred times. Would you be able to find any part of the toothpaste’s original colors? This article tells the story of a group of scientists basically doing exactly that for the Earth.

When the solar system formed, it received elements that were recently made in supernovas. Some of these elements are still around (heavy atoms like the Rare Earth Elements make it possible for me to type this story) but some atoms don’t live that long. Radioactive elements are made in supernovas, but they gradually decay over time. Some radioactive elements like Uranium are still around today, but others decay faster, with much shorter half-lives.

Geologists call these isotopes “short-lived” even if their half-lives are millions of years because they’ve long since decayed away. There is no trace of those isotopes left in the Earth, except for the daughter isotopes – the atoms that the short-lived nuclei decayed into. The only way we know about these short-lived isotopes is to find places with extra daughter isotopes. If you find such a place, with more of a daughter isotope than the rest of the planet, you know it must have somehow formed when that isotope was alive.

For example, we find some meteorites with extra of the isotope Magnesium 26 (magnesium with 14 neutrons). 26-Mg is made by decay of a short-lived isotope of aluminum, 26-Al. That means those meteorites must have formed within the first few million years of the solar system, since the half-life of 26-Al is only 730,000 years. If a rock formed 10 million years after the solar system formed, there’d be almost no 26-Al left and that rock would get no extra 26-Mg.

Some short-lived isotopes can be used to tell really interesting geologic stories. For example, an isotope of the element Hafnium (mass 182) decays to Tungsten with a half-life of 8.9 million years. These elements are rare but scientists can measure them today using modern instruments, and they hold information about an important geologic story.

Hafnium and Tungsten behave different chemically. Hafnium fits into minerals with oxygen backbones, like those of our mantle and crust, while Tungsten goes into metals like those that made the core. If the Earth’s core formed after all the Hafnium 182 had decayed, there would be almost no Tungsten 182 in the mantle because it all would have gone into the core.

The Earth’s mantle has some Tungsten 182 in it and geologists use that amount to estimate that the core formed something like 50 million years after the planet formed. Beyond that measurement, if you found two rocks with different amounts of Tungsten 182, that would imply they must have formed before Earth’s core formed. However, the whole planet has churned like that tube of Aquafresh since then, so the whole planet should have about the same ratio of Tungsten 182 to other isotopes of Tungsten. It would be stunning to find a rock from Earth that differed in its Tungsten isotope ratio.

That, of course, is the measurement that this group, led by Dr. Hanika Rizo (now at the University of Quebec), pulled off. They found rocks that shouldn’t exist based on what we know about the mantle.

Baffin Island, in northeastern Canada, contains a flood basalt province. The rocks in this photo are pillow basalts from that island; they were erupted about 60 million years ago as the plume that is found today under Iceland began to erupt. This is one of two sites this research group found to contain an anomalous Tungsten isotope signal; the other was the Ontong Java Plateau, a gigantic platform in the Pacific Ocean (https://tmblr.co/Zyv2Js1oYgyfZ).

Both of these sites are large igneous provinces. They seem to reflect a pulse of hot mantle coming up from deep, reaching the crust, and melting. The fact that both of them have tungsten isotopes that differ from the rest of the planet means that something in the source of these rocks, something deep in the mantle, formed in the first 50 million years of this planet’s lifetime and somehow hasn’t been mixed back in since.

Geophysical models tell us that a normal, convecting mantle shouldn’t be able to keep parts isolated for 4.5 billion years. That’s like stirring the toothpaste and finding a part that clearly shows the three colors. If everything stirred, the individual colors would be long gone. The only way for this to happen is if some properly kept part of the mantle from being stirred.

Deep in the mantle there are two large areas that are distinct seismically – we call them LLSVP or “superplumes” (more here: https://tmblr.co/Zyv2Js1pESuuK). We don’t know exactly what those things are, but some geologists have proposed they could be sources for plumes. If they’re denser than the surrounding mantle, they could be made of stuff stuck near the core mantle boundary for billions of years and if part of it got too hot, it could rise up to the surface and melt.

Something like the LLSVP being stuck in the mantle for >4 billion years is required by this measurement. Somehow, the Earth’s mantle took material in it 4.5 billion years ago, when the planet was in its infancy, and stored it until its release in a large igneous province a wink of an eye ago (geologically speaking). We can’t say exactly how the Earth did this yet, but knowing that these rocks exist will give us a fundamentally new insight into the mantle below.

-JBB

Image credit: Don Francis of McGill University. Press release: https://carnegiescience.edu/node/2031

Original paper: http://science.sciencemag.org/content/352/6287/809.full