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Host: Benjamin Thompson
First up on this week’s show, we’ll be learning about the ground underneath our feet, a long way underneath our feet. The Earth’s mantle is sandwiched between the planet’s outer core and the thin crust we walk about on. Although the mantle makes up a huge 84% of the Earth’s volume, this enigmatic zone is difficult to study. But thanks to an unusual type of explosive eruption bringing deep rock samples to the surface, we can get important clues as to its composition. Jon Woodhead from the University of Melbourne in Australia has been studying new and old samples of these rocks known as kimberlites. Geoff Marsh got him on the phone to uncover their secrets.
Interviewee: Jon Woodhead
So, kimberlites are very rare volcanic rocks but they’re not really like most other volcanic rocks. In fact, they actually come from the deepest depths of any samples that we have.
Interviewer: Geoff Marsh
And they get to the Earth’s crust quite violently, don’t they? Paint a picture for us of a kimberlite eruption.
Interviewee: Jon Woodhead
So, when most people think of volcanic eruptions they think of typical volcanic cones, such as Mount Etna or Stromboli – relatively gentle volcanic eruptions. Kimberlites are quite different. They’re extremely energetic, volatile, rich eruptions and they just blast holes through the crust. They don’t form any sort of organic edifice. They just leave a huge crater in the ground.
Interviewer: Geoff Marsh
Given that we’ve never been able to directly observe the mantle, how do you know how deep these kimberlite rocks actually come from?
Interviewee: Jon Woodhead
It’s really because they bring up diamonds and the diamonds have trapped within them high-pressure minerals that we know must have formed at great temperatures and pressures, great depths of the Earth.
Interviewer: Geoff Marsh
So, tell me about your kimberlite dataset then and exactly what you did to unravel their secrets.
Interviewee: Jon Woodhead
So, we very carefully gathered together kimberlites from around the globe. There are samples that cover the past 2.5 billion years of Earth history. We dissolved them. We extracted different elements from them. In particular, two elements – neodymium and hafnium – and we measured their isotopic composition and to our great surprise, they appear to all derive from a single deep mantle source. Their isotopic compositions remain constant through time, essentially.
Interviewer: Geoff Marsh
And you suggest in your paper that their isotopic composition reflects a sort of primitive chemistry of the Earth’s mantle. How do you come to that conclusion?
Interviewee: Jon Woodhead
Well, their composition is very similar to a class of meteorites called the chondritic meteorites and they have a composition that we think best reflects that of the early Earth, just after the core formed. So, the fact that these kimberlites have that composition and it’s remained constant through time leads us to believe that there are still remnants of that very primitive early Earth still surviving in the deep mantle.
Interviewer: Geoff Marsh
Presumably that’s quite surprising, given how much we know things move around over time at the crust, that part of the mantle might have remained the same over more than half of the Earth’s history.
Interviewee: Jon Woodhead
Yeah, that’s right. Yeah, it’s actually quite surprising because we know that tectonic plates are continually being recycled back into the Earth’s mantle at subjection zones, and so there’s inevitably quite a mixture of components down there. But it still seems that some parts of the deeper mantle have remained preserved, and this is important to know from the perspective of understanding the planet’s geochemical evolution.
Interviewer: Geoff Marsh
So, where exactly do you think this pristine reservoir is in the mantle because the mantle is pretty big, isn’t it?
Interviewee: Jon Woodhead
Well, we know that it can’t be in the upper mantle because we do have other rock types that source the upper mantle and they don’t show these compositions. It really has to be somewhere in the deeper mantle, and we know that kimberlites, at least some kimberlites, come from at least 800 kilometres’ depth and so that gives us our best estimate of where these materials may lie, at least at that depth.
Interviewer: Geoff Marsh
Now, you mentioned in your paper that some of the more recent kimberlites are slightly different in their composition. Does that muddy the story somewhat?
Interviewee: Jon Woodhead
Yeah, there is some evidence that the very youngest kimberlites – those just a few hundred million years old – have been perturbed slightly. But while there are kimberlites that show that feature, there are still some young kimberlites that show this primordial signature, so we believe that the bulk of the source is still intact, but that small regions of the source may have been perturbed relatively recently.
Interviewer: Geoff Marsh
So, what’s next then for you?
Interviewee: Jon Woodhead
I guess, so far, we’ve only really found kimberlites that go back 2.5 billion years. It will be interesting now to try and find some older materials to try and extend this story a little bit, but kimberlites are quite rare rocks, so I’m not sure what our chances are of finding any older materials really. We rely on the diamond industry to find those rocks for us, I guess.
Interviewer: Geoff Marsh
I suppose a modern-day kimberlite explosion would be pretty interesting.
Interviewee: Jon Woodhead
Yeah, that will be great to see that, but from a distance, I guess.
Host: Benjamin Thompson
That was Jon Woodhead from the University of Melbourne talking to Geoff Marsh. You can read Jon’s paper and a News and Views article over at nature.com.
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