Mineral reveals chemistry of young Earth
The forces involved in moving the ground beneath our feet are massive and often very destructive. And the destruction of earth through tectonic forces can present unusual problems. And what are you supposed to do if you want to look back billions of years into the past to discover what Earth’s chemical conditions might have been at the very start of life, because there’s a good chance that the land you want to study has already been destroyed. Well that was the challenge faced by Dustin Trail and his team at the University of Rochester, New York, until they came across a very special and very hardy mineral called a ‘zircon’.
Dustin - We don't know how life started on Earth, despite our planet being essentially right beneath our feet. And part of what makes this problem so hard is that we live on this remarkably dynamic planet. So rocks are being created and destroyed at plate tectonic boundaries. And one can imagine that if you are thinking about a planet that's been active for 4.5 billion years, that the available resources, both in terms of rocks and minerals, are going to become more and more limited the further we travel back in time. So we have to find a way to transport ourselves back billions of years. And that's one of the major goals of this work. Charles Darwin put it in a letter to his contemporary Joseph Hooker back in the early 1870s, something along the lines of life could have emerged in a warm little pond. And so in some respects, this is what we are studying as part of this work. What was that warm little pond like?
Will - It sounds unfortunate like tectonics were getting in the way of this study.
Dustin - That is correct. Tectonics both creates and destroys rocks. So what we need first is a remarkably durable mineral. And it turns out that we have such a mineral. Zircon, which is a zirconium silicate. It's a physically and both chemically durable mineral. So it is able to withstand the tests of time. It also has a couple of other key properties. It incorporates radioactive uranium into its structure. So when zircon is crystallizing from a high temperature fluid or from a magma, it incorporates uranium which decays to lead. And so by measuring uranium and lead within the crystal, we can obtain an absolute age, and that's an important component if we want to connect the age with chemistry. And so the zircons that we were interested in studying as part of this study approach 4 billion years old.
Will - What is it about zircons that make them so durable, that allow them to survive this long?
Dustin - There are two ways to think about this. So zircon is a remarkably hard mineral, which makes it physically durable. So it is capable of being transported by wind and water without physically breaking down. The second part is that it is chemically durable. Once a zircon forms and locks in its chemistry from the time of formation, any later metamorphic event or in some cases a magma that interacts with that zircon in most cases does not modify its original chemistry.
Will - These zircons then when they form, do they create some kind of snapshot of earth's chemistry all that time ago?
Dustin - That is exactly right. The important thing to keep in mind is that the mere presence of the mineral itself is not diagnostic of a particular rock type or a tectonic environment. It is the chemistry, the trace constituents within the zircon structure that provide us with clues as to what its formation environment may have been like. So it's sort of like decoding that chemistry, getting that chemistry to tell a story about the physical and chemical conditions of our planet 4 billion years ago.
Will - So it would be to chemists what amber would be to biologists?
Dustin - Exactly right. It's a remarkable mineral that preserves chemistry from the time of its formation.
Will - And what did the ones that dated back all this time tell you about the chemistry of earth?
Dustin - They've told us a remarkable amount about our planet during its first 500 million years. For example, they have told us that there was likely water rock interaction on our planet as early as 4.3 billion years ago, including the alteration of preexisting rock to form sediments. And they've also told us that the volcanic emanations that were coming out of our planet at that time were dominated by CO2 and water and nitrogen, actually very similar to today, rather than methane or ammonia. So they have already provided us with clues about what the surface of our planet was like at that time.
Will - And so how do you fancy the odds then that that could be where life started forming?
Dustin - Well, I am certainly an optimist. I <laugh> we know for instance what the output was. We know that life started on our planet right now. We are struggling with what the input was. We don't know what the planetary conditions of our planet were like a billion years ago. And so that is really what we are after as part of this study is can we better constrain the inputs that resulted in this amazing output, life on our planet?
Will - If these zircons then can help us understand the chemical pathways that led to life starting on our own planet, is there perhaps scope for it to help us find life on others?
Dustin - I think we're at this amazing time in which humankind is searching for life on other planets and we still don't have an idea of how or when life started on our own planet. And I think this is work that will factor up prominently in the search for life outside of our planet.
Will - A remarkable time capsule then.
Dustin - Exactly.
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