45: As Above, So Below

Last episode, we met the oldest slices of Earth’s mantle, the hidden layer below our familiar surface world. Let’s recap by answering a few basic questions.

A peridotite from Isua, 3.7 billion years old. Not today’s location, and slightly younger, but close enough. From Nutman et al., 2021, Precambrian Research.

Where are these rocks? Sitting on a lonely tundra in southwest Greenland, almost on the Arctic Circle. These rocks are part of the Isua region, a U-shaped complex of stone stretching miles across, which holds the most pristine rocks of their age.

Speaking of age, how old are these rocks? The region is 3.8 to 3.7 billion years old. In Earth history, consider this the early springtime: March 3-11 on the great Earth calendar. The mantle rocks we met last episode are the oldest in this spread.

What are these rocks? If you held one in your hand, you would see small, glittering green crystals scattered with flecks of black and brown. This green rock is called peridotite. Peridotite is the most common rock on Earth, or I should say inside the Earth. It’s very rare on the surface, but very common in the mantle, over 25 miles below our feet.

If these strange green rocks formed deep in the mantle, how did they get to the surface? There are a few ways for the mantle to reach daylight. These peridotites likely formed just below the seafloor and were scraped onto the surface as islands crashed into each other, 3.8 billion years ago.

Now that we know the basics, the wheres, whens, and whats of these weird rocks, we can ask more hard-hitting questions. Remember, these are the oldest tangible slices of the mantle, a hidden realm we can’t even visit today.

In this episode, we ask: What was the Earth’s mantle like 3.8 billion years ago? If I take the oldest piece of Earth’s underworld and compare it to a modern piece, would they look similar or completely different? And if they are different, why should surface dwellers like us care? To answer that question, let’s rewind to the very, very beginning of Earth’s history.

Part 1: The Parting of the Ways

A polished pallasite meteorite, with mixed iron and olivine.

At the very start of our journey in Season 1, we met two lonely particles floating in space: a crystal of green olivine, and a speck of metallic iron. Over time, more pieces assembled and the Earth grew from dust-sized to asteroid-sized. Here, olivine and iron lived side-by-side: green and grey mixed together in a jumbled chaos. These early days of the Solar System are still preserved in a few rare meteorites: they’re called pallasites. When you cut open a pallasite, you’ll see beautiful windows of green olivine crystals surrounded by shiny gray iron. Pallasites are by far my favorite type of meteorite, go check them out if you haven’t already. If you want to learn more, check out Episode 5.

 Inside a small, fist-sized asteroid, olivine and iron are all jumbled together randomly. But if you add more and more material to these asteroids, growing them into planet-size, they become less stable. Iron is twice as dense as olivine, and slowly, inexorably, gravity pulls iron inward to the core. Lighter crystals like quartz rise to the surface and form a thin crust. Meanwhile, olivine is stuck in the middle: too dense to rise up, too light to sink down. This is how the mantle formed: it’s a story we learned in Episodes 5, 6, and 7, and one we’ve revisited every so often: it’s a bedrock idea in modern geology.

With that recap over, let’s revisit today’s question: has the Earth’s mantle changed since it first formed from asteroids way back in Season 1? In other words, has the underworld remained a stable, consistent realm, or has it changed as radically as the surface has over 4 billion years?

For the surface world, we can see these changes with your eyes: rocks like tonalite and banded iron formations are extremely common in Season 2, but very rare today. But if I placed our ancient Greenland peridotite, 3.8 billion years old, next to a modern-day sample of mantle, they would basically look the same. Two dark green lumps of rock would sit side by side on your table, defying you to spot any major differences.

In cases like these, where the visible world fails us, we must use a different set of tools: smaller, invisible, microscopic. We need to turn to chemistry. If the mantle hasn’t visibly changed over 4 billion years, has it changed chemically? In 1923, a young man was asking the same question. As he searched for an answer, he would become the father of modern geochemistry.

A young Victor Goldschmidt

Victor Moritz Goldschmidt was born in 1888 in Zurich, Switzerland. Goldschmidt was born into science: his father was a chemist, and named young Victor after one of his colleagues. Eventually the family settled in Oslo, Norway, where Victor would spend most of his life. Goldschmidt took his father’s love of chemistry and applied it to the geologic world. He made many advances in geology that we’ve discussed on the show, even if we didn’t mention his name. For example: how metamorphic rocks change with temperature and pressure, and how magmas partially freeze back into solid rock. Victor was also interested in practical applications for his research, ideas that would help society. For example, he examined how to extract phosphorus from rocks as a source of fertilizer. In short, he was constantly on the hunt for elements, searching for the periodic table in the natural world.

Victor’s most famous achievement came in 1923, at age 35, when he compared his academic research with his practical, applied science. Imagine Victor sitting in a Norwegian laboratory, a winter snowstorm raging outside. In one hand, he holds a meteorite from outer space. In the other, he holds a piece of slag, waste material from a mine. In each piece, Victor notices a pattern.

In the meteorite, he sees two phases: steely iron and glittering crystal, just like we discussed. When he looks at the industrial slag, he sees similar phases of metal and crystal. In both cases, the samples formed when molten rock cooled down and separated into different materials. The meteorite cooled in outer space, the slag cooled inside a factory. Victor looks up from his rocks and sees the Periodic Table of Elements hanging on his wall. He looks back down at his rocks and has a revelation.

Victor could clearly see certain elements in his samples: metal iron and silicon crystals. But he knew there were traces of other elements as well, ones he couldn’t see like phosphorus or gold. He guessed that some elements preferred to hang out with metallic iron, and others inside crystals like olivine. In short, Victor realized that you could point to any element on the periodic table, and it would either sink down into the iron core or rise up into the silicon-rich mantle and crust when the Earth formed billions of years ago.

Victor couldn’t reach Earth’s iron core for a sample, but he could test this idea on his meteorite and his factory slag. He worked his way through all the known elements, seeing if they were on Team Iron or Team Silicon. Sure enough: elements like phosphorus and zirconium preferred to hang with silicon crystals. Other elements like nickel and gold preferred to hang out inside iron. This was true inside meteorites, inside slag, and inside many, many other rocks. Eventually, Victor published his findings: a color-coded version of the periodic table split into Team Iron, Team Silicon, and other teams we’ll meet later. This fancy, colorful periodic table is called the Goldschmidt Classification in Victor’s honor. Check out our website if you want to see where your favorite element lies.

Obviously, I’m simplifying this story a lot. The world is a messy, imperfect place. If all Earth’s iron sank into the core, we wouldn’t have any steel, any rust on the surface. But still, it’s a good set of guidelines. For example, if you’re a miner looking for nickel, you can look at the Goldschmidt chart and see that it likes iron, so look for iron-rich zones.

Before we bring this idea back to the ancient mantle and core, I want to quickly wrap up Victor’s biography. Warning: there’s some heavy stuff ahead. In 1940, the Nazis invaded Norway. As a Jewish scientist, Victor was forced inside the Berg concentration camp and was soon slated to be sent to Auschwitz. He was waiting at the harbor with over 500 other Norwegian Jews, men, women, and children, waiting for the boat that would take them to the most notorious concentration camp. Suddenly, a shout came from the Gestapo, asking for Victor Goldschmidt. Perhaps he paused before answering yes, but when he did, he was freed and sent back to his house. Victor’s university had successfully convinced the Nazis that his work was critical to Norway. Victor was incredibly lucky. Over 500 people were taken on that ship to Germany. Only 9 would survive the concentration camps.

Victor did not waste his opportunity. He fled to Sweden, then England, but always longed to return to Norway. After the war ended, he finally got that chance in 1946, but time was not on his side. Victor was never the healthiest man, and his time in concentration camps and the stress of flight had made matters worse. Friends who met him on his return found a changed man with a hunched back, baggy eyes and hollow cheeks, unrecognizable from his vibrant younger days. He died a year later at the early age of 59.

Despite his tragic later years, Victor Goldschmidt had the last laugh after death. His Goldschmidt chart alone secured him as the father of modern geochemistry, but he made even more discoveries that will have to wait for another day. The Geochemical Society’s highest honor is the V. M. Goldschmidt Award, and its annual meeting is called the Goldschmidt Conference, or “Goldschmidt” for short. I was attending Goldschmidt 2022 in Honolulu, Hawaii, when I accepted my current position in Michigan.

Finally, if you ever take a geochemistry class, you will learn about the Goldschmidt chart and Teams Iron and Silicon. It’s an essential point of a modern geology education. With that in mind, let’s see how these teams fared in the early mantle, and return to Greenland.

Part 2: The Muffin Paradox

Let’s regroup after that biography. We want to know how Earth’s mantle behaved 3.8 billion years ago. We’re examining pieces of strange green rock known as peridotite, rare slices of the mantle from deep below our feet. Every so often, pieces of peridotite will get shoved onto Earth’s surface, rare glimpses of the world below.

Visually, the world’s oldest peridotites don’t tell us that much. They look strikingly like modern peridotites: glittering green with flecks of black and brown. But the chemistry of these rocks, their microscopic atoms, can tell us stories that color and texture alone can’t. Thanks to Victor Goldschmidt, we have an entire periodic table of elements at our disposal. According to Victor, some of these atoms should have been dragged down to the core, along with heavy metal iron. Other atoms should prefer to stay in the mantle with olivine.

Don’t worry, I’m not going to go over every single element. Instead, I’m just going to tell the story of one special metal: osmium.

Osmium is one of those overlooked elements in the middle of the periodic table, overshadowed by its close neighbors of platinum and gold. Most folks haven’t heard of it, but it has some unique properties. If you held a chunk of osmium in your hand, you would see a shiny, hard, blue-gray metal. It would feel extremely heavy- it is the densest stable element in the universe. A piece the size of a grape would feel as heavy as a pineapple. A piece the size of your fist would be as heavy as an average adult. A chunk of osmium by itself is harmless, but grinding it into a power is an incredibly bad idea. Osmium powder is highly volatile, foul-smelling, and toxic, causing permanent blindness and even death. The most common place you’ll find it is in the tips of fountain pens, which need a very hard, durable metal. Again, as long as you don’t powder the pen, you’re fine.

 So, according to Victor Goldschmidt, do you think Osmium is on Team Iron or Team Silicon? In other words, when the early Earth formed, did osmium sink to the Earth’s core, or hang around the mantle and surface? You’ve probably guessed the answer already: osmium is extremely dense. Osmium is absolutely a member of Team Iron, Team Core.

If you picked up a meteorite like Victor Goldschmidt and looked at the osmium inside, you’d detect a decent amount scattered throughout, evenly mixed between the iron and olivine crystals. This is because the meteorite hasn’t split into separate layers, it isn’t big enough. Keep that point in mind, it’s going to be very important in a minute: meteorites have decent amounts of osmium. As meteorites crashed together in Season 1 to form the Earth, dense osmium should separate, sinking down into the core alongside its buddy iron.

At least, that’s what’s supposed to happen in theory. The reality is much cooler.

When scientists looked at peridotites, slices of Earth’s mantle, they expected to find very low amounts of osmium. Remember, it should have sunk down into the core. However, they found normal amounts of osmium, the same amount as you’d find inside small meteorites, not a large, layered planet.

Here, we have a paradox. If you only look at osmium, you would assume that planet Earth never separated into different layers, that it was all mixed together like a giant asteroid. But we know that these layers do exist! Earth’s magnetic field tells us there’s a big ball of iron at the core, and earthquakes tell us exactly where that core is. So why is osmium different? You might say, well, it’s just one weird element, an anomaly. But there are around eight similar elements that behave this way- they should have all sunk to the core, but there’s more hanging around up in the mantle than you’d expect.

So what gives, and honestly why do we care? Humans can’t visit the mantle, what does it matter if a few weird elements aren’t behaving properly? We’re biased from living on the surface, but Earth is mostly mantle- it’s the largest, thickest layer. The slow, putty-like motion of the mantle is a driver in plate tectonics, the process that governs surface geology. Consider the mantle like the engine in a car. Just because you’re not looking at it all the time doesn’t mean it’s not important, and if you see something even a little weird, you want to check it out, even if it’s ultimately nothing.   

But the osmium is telling us something. There is a fairly well-accepted explanation. I’m going to start with a kitchen analogy, then explain it scientifically. I feel I’ve said the words mantle and core and osmium so many times now they’re losing meaning. So let’s switch to muffins and blueberries.

Let’s say it’s your first time making blueberry muffins, that you’re an inexperienced baker. You mix the batter all together quickly, plop the blueberries in, and shove it in the oven. As the batter is beginning to bake, you surf on your phone, looking for more tips on social media. To your horror, you realize you’ve made a big mistake. Blueberries like to sink to the bottom of batter. If you let your muffins finish as is, they’ll be unbalanced, with all the blueberries crammed at the bottom.

In a panic, you pull out your half-baked muffins, shove some new blueberries in the top, and re-close the oven. The result isn’t pretty, but isn’t a disaster. The muffins are still loaded with blueberries at the bottom, but there are some floating around near the top- the ones you added later.

Before I explain this in terms of the Earth, a quick tip. If you want to avoid this scenario, cover your fruit in flour first- it will help them float instead of sink. There’s no geology analogy for that, but I thought I’d give aspiring bakers a head-start. Anyway!

Let’s replace our muffin with the Earth, and our blueberries with osmium and its’ siblings. The beginning of our baking fiasco is how osmium acted when Earth first formed from asteroids: osmium sank into the core like blueberries in batter, leaving the upper mantle blueberry-free, I mean osmium-free. The only way to replace that osmium is to add new asteroids later, from outer space. In other words, your late, panicked sprinkling of blueberries on a half-baked muffin is simulating a titanic meteor shower.

At some point in Earth’s early history, sometime in February on the Earth Calendar, Earth was pelted by a freak asteroid swarm, potentially for millions of years. Keen listeners might be thinking “Wait, we’ve discussed this idea before!”, and indeed we have. In Episode 34, we gave this catastrophic event a name: the Late Heavy Bombardment that pelted the Solar System with asteroids. It’s called late, because it happened after the chaotic first days of the Solar System- things should have calmed down already. It’s called a heavy bombardment because you’re getting pelted with asteroids. The reason why is still debated, but it’s thought that as giant planets like Jupiter drifted away from the Sun, they flung asteroids out of their way, flinging them towards us like a cranky child.

A model of early mantle evolution. In stage two, later meteorites change the chemistry of the mantle, adding a “late veneer” of elements.

When we finished Episode 34, we left the Bombardment on an ambiguous note. Some astronomers argued for more impact craters on the Moon and Mars from around 4 billion years ago, including the huge craters making the Man in the Moon. Others argued there wasn’t enough evidence for a Bombardment, and I even finished that episode saying the idea might be falling out of favor.

Ironically, while the evidence from outer space might be debatable, the evidence from Earth’s mantle is fairly well accepted. The best way to explain osmium and other very dense elements in the mantle is by adding them later, by intense, protracted meteor showers. This idea was first proposed when looking at modern slices of Earth’s mantle, glittering green peridotites that had more osmium than expected. But now we can test this idea using Earth’s oldest peridotites from Greenland, 3.8 billion years ago- March 3 on the Calendar.

If a Bombardment of meteors happened in way back in February, then the Greenland rocks should also have a decent amount of osmium. In short, the blueberries should already have been sprinkled in by then. And when researchers like Vickie Bennett and Julia van de Locht investigated, that’s exactly what they found. I’d like to do a brief biography of Vickie Bennett one day, like we’ve done for other Greenland researchers. She’s an expert in the early mantle, but there will be time in a future episode. Today’s been a dense session as is- I’ve tried to keep it manageable, but let’s wind down, back up, and process what we’ve learned.  

Summary:  

Earth’s mantle separated from the core early in the planet’s history, January on the Earth Calendar. Dense iron sank down to the core, while lighter crystals like olivine rose up into the mantle. Researchers like Victor Goldschmidt realized that other elements would choose sides in this great division, like picking teams on a schoolyard. The reality is a bit more complicated. When scientists first got their hands on real pieces of mantle, glittering green peridotite rocks, they noticed that some elements weren’t behaving as expected. There were more heavy elements like osmium sticking around, elements that should have sunk into the core. As strange as it sounds, this anomaly is best explained by a massive meteor shower lasting millenia, adding new elements that would otherwise be trapped in Earth’s core. This same meteor shower gave us the massive craters of the Man in the Moon.

To sum up that summary: our mantle has an unusual chemistry because it was pelted by asteroids in February on the Earth Calendar. This evidence comes from the oldest slices of Earth’s mantle in the Isua region of Greenland. Next episode, we move onto new rocks, forward in time and back on Earth’s surface. It’s time to see how volcanos were erupting 3.8 billion years ago.