‘Time Capsule’ Rocks Provide Clues About Earth’s Mantle

SOPHIE BUSHWICK: This is Science Friday. I’m Sophie Bushwick. If you’re looking to really learn about the history of our planet, look to geology, like John McPhee’s stories chronicling the formation of our continent based on the rock layers in highway roadcuts.

But that record has its limits. Rocks and minerals get weathered, buried, heated, melted, and recycled, so geologists search out rare, super old geologic holdouts to tell about the earliest times. Ira’s here with the story of one such super old rock from the bottom of the sea.

IRA FLATOW: Writing in the journal Nature, researchers describe what they can learn about the chemical history of earth’s mantle. That is the layer right beneath the planet’s crust. What they can learn from studying rocks they think are at least 2.5 billion. Yes, that’s with a B, 2.5 billion years old.

Joining me now is Dr. Elizabeth Cottrell, Chair of the Department of Mineral Sciences at the Smithsonian National Museum of Natural History. She’s also curator of the National Rock Collection. Did you know we had one? And co-author of this study. Welcome back to Science Friday.

ELIZABETH COTTRELL: Thanks, it’s my pleasure to be here.

IRA FLATOW: Do we have a national rock collection?

ELIZABETH COTTRELL: We do. We do have a national rock collection. And it’s really an important part of US scientific infrastructure and international scientific infrastructure. We have rocks from all over the world and they are freely available for study to researchers around the globe.

If you think about it, we often spend a lot of money to go and get rocks from exotic locations, and it makes a lot of sense to curate them and make them available for use again, rather than having to go back and re-collect them.

IRA FLATOW: Yeah, I know you showed me some of these great old rocks. But I don’t want to get off the track here, because we could easily talk about rocks, one of my favorite subjects I want to get on the track to talk about where do you get a 2.5 billion year old rock?

ELIZABETH COTTRELL: Well, the most common locations for super old rocks are on the continents. But in this case, we think we have really old rocks that have been dredged from the sea floor. So the rocks in our study come from three different locations on the sea floor. One set of rocks was actually recovered by icebreakers under the North Pole.

IRA FLATOW: No kidding.

ELIZABETH COTTRELL: Yeah. Another set of rocks was recovered from the sea floor south of Africa, between Africa and Antarctica. And another set of rocks was recovered from the sea floor in the Pacific Ocean.

So we’ve gone to great lengths to acquire these rocks. They’re all from locations on the sea floor where the earth’s crust is spreading apart and new ocean floor, new ocean crust is being created.

IRA FLATOW: You mentioned the difficulty of getting these rocks. Are they rare? Are there a lot of them?

ELIZABETH COTTRELL: Well, this rock type in our study is not rare on the global scale. The mantle of our planet is about 70% of the volume of our planet. So by that metric, mantle rocks are not rare on the earth. But finding them at the surface is difficult.

And that is rare, because when the mantle melts, it creates the earth’s crust. And so the crust covers up the mantle. These few settings on the planet are places where the mantle is exposed, for example, in fractures. And we can send ships out, dredge the seafloor, literally drag a bucket along the seafloor and kind of go fishing for rocks. And in these rare locations, we find pieces of the mantle.

IRA FLATOW: Wow. And if you showed me one, what would it look like? I mean, does it look any different from an ordinary rock?

ELIZABETH COTTRELL: Well, no, probably not. They have abundant quantities of the mineral olivine. Now, you may know the mineral olivine by the trade name peridot. It’s August’s birthstone, a green mineral. And that’s the dominant mineral in this rock called peridotite, peridotite, named after this olivine.

But often these rocks on the seafloor, you know, they’ve rusted somewhat and so they can appear weathered and orangey. They also have the minerals orthopyroxene and spinel, which may or may not be minerals that you’ve heard of commonly.

And then deeper in the earth’s mantle, the mineral garnet replaces this mineral spinel. But peridotites with garnet are not recovered from the sea floor. So our rocks that we studied here are the minerals olivine, orthopyroxene, and spinel.

IRA FLATOW: So these mantle rocks are unusual, but are these the oldest rocks around?

ELIZABETH COTTRELL: No, by no means. These are not likely to be the oldest rocks around, but they are likely to be older than the average mantle that circulates and is recovered from these seafloor locations.

IRA FLATOW: And how do you– how do you date the rocks? How do you know just how old they are?

ELIZABETH COTTRELL: We don’t know how old these rocks are. What I can tell you about these rocks is that they have melted to extreme extents. If you think of these, these rocks are the residues of creating the earth’s crust.

In other words, these rocks have melted and given up their melt to create the earth’s crust. And so if you think of a wringing out a sponge and the water coming out, these rocks in our study have been squozed dry. The melt has been extracted.

And it’s been extracted to a large extent, an unusual extent, that we infer happened under really hot conditions deep in the earth. And those kinds of temperatures are not really available today, but would have been available in the Archean eon billions of years ago when the earth was hotter.

IRA FLATOW: So what do you seek to learn from them? What can they tell you?

ELIZABETH COTTRELL: Our team is interested in the history of oxygen in the deep earth. We are interested to understand how our planet evolves, how new crust forms, and the role of oxygen in that process. It’s all part of this big story about how earth has formed, how it’s evolved, and how our planet has become habitable.

One of the interesting things about the history of oxygen in the mantle is that the oxygen availability in these rocks governs things as basic as the gases that are emitted from volcanoes, the gases that would have formed Earth’s earliest atmosphere. And so when we’re thinking about planets and planetary formation and signatures of habitability, it all comes back to the rocks that make up the interiors of planets.

IRA FLATOW: So these rocks, as you say, they melted at a very high temperature, but without doing the equivalent of rusting?

ELIZABETH COTTRELL: Yeah, you are bringing up a prime example of oxidation in our everyday lives. Corrosion is an example of iron oxidation. And when metal rusts, an iron atom loses an electron. This is oxidation. You’re exactly right.

So that is exactly what we’re looking at in these samples. That’s what we’re analyzing. We’re looking at the amounts of oxidized iron and reduced iron to tell us something about how active oxygen was in the mantle in these ancient times. So it tells us something about how the composition of the earth’s mantle has evolved.

Or in this case, we’re suggesting that it hasn’t evolved so much and that this chemical signature is generated simply by the natural process of the planet cooling rather than by some other process that could have changed its chemical composition.

IRA FLATOW: It’s sort of the Earth just doesn’t make rocks like it used to.

ELIZABETH COTTRELL: That is the best way to say it.

[LAUGHS]

IRA FLATOW: How do you go further with this? Do you have to find more rocks or is this a thing you can follow up on in the lab?

ELIZABETH COTTRELL: This is something that we can follow up, both by analyzing more rocks. It’s always good in any science to reproduce results and test additional hypotheses. But we are definitely following up with laboratory work.

In our lab, we can create conditions such as are found in the deep earth. We can create very high pressures and very high temperatures and we can melt rocks in the lab and study the chemistry of that melting process under different conditions of oxygen availability, under different temperature conditions, and under different pressures. So my group is particularly interested in following up on laboratory experiments to help us understand the rock record.

IRA FLATOW: What would be the take home lesson that you learned from the discovery of these rare old rocks?

ELIZABETH COTTRELL: The take home message of our study is that we may be able to produce these really unusual chemistries in this type of rock by changing the temperature and pressure at which they melted, and not by changing the bulk composition of the rock.

And that is really important, because it really helps us as earth scientists to eliminate or to support entire classes of models about how the planet formed and has evolved for these billions of years, and how the atmosphere has evolved and how the interior of the planet has linked to planetary habitability.

IRA FLATOW: Well, Dr. Cottrell, I can talk about rocks forever because I love I love talking about them and looking at them. So I want to thank you for taking time to be with us today.

ELIZABETH COTTRELL: Thanks. It was so much fun. I’m thrilled to be here.

IRA FLATOW: Dr. Elizabeth Cottrell, Chair of the Department of Mineral Sciences at the famous Smithsonian’s National Museum of Natural History. She’s also curator of the National Rock Collection.

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