Picture a geologist. She is probably a hardcore hiker, hammering rocks and climbing mountains. She wears rugged pants and sturdy boots. Granted, geology involves this, but its ultimate goal is to reconstruct the story of our planet.
As geologists, we describe how mountains are built and where tectonic plates were in the past. By looking at rocks, we decipher the inner structure of Earth and map extraterrestrial planets millions of kilometers away from us. It is hard to imagine that a lot of this work is not done by hiking and hammering rocks in the field. In fact, some geologists would pass out at the sole thought of dirt on their hands! Having gone to the farthest ends of the planet with a scientific question in mind, we come back with hundreds of kilograms of rocks in our backpacks, and we take them to the lab, far from the very features that we are trying to understand – and this is where the fun work starts. In the lab, we dissect rock samples to extract information about their story. How did these rocks form? What happened to them over the past hundreds of millions of years? To answer these questions and more, we need to use some tools that will help us reconstruct the story of a rock, and from this, tell the story of our planet and other planets.
To understand phenomena at the largest scale imaginable, like the formation of a planet, we often have to observe nature at the tiniest scale possible: atoms, their atomic structures and their atomic masses. Rocks are made of minerals, and minerals are made of the orderly arrangement of atoms. How old a rock is, in which environment it formed when it was altered by fluids, and what type of fluids altered it are questions that are answered by a discipline known as isotope geochemistry.
Isotopes are atoms of the same chemical element differing by their atomic structure and their atomic mass (given by a varying number of neutrons and a fixed number of protons in their nuclei), and they are mainly formed in stars during their lives and deaths. The creativity of stars in shaping chemical elements in different structures includes the formation of stable isotopes (which do not break down over time) such as those of oxygen (O) and hydrogen (H). Because different atomic structures and masses determine different behaviors between isotopes of the same elements, stable isotopes have allowed us to learn that water is transported from the surface of Earth into the deep mantle and returned to the surface hundreds of millions of years later. We now know that the journey of water in the deep Earth is intricate and that, on its journey back towards the surface, it can trigger chemical reactions involved in the sustainment of microbial life.
But even majestic entities like stars get distracted, old and tired, and they make mistakes. The (apparent!) mistakes made by stars are called unstable or radioactive isotopes, which break down into stable ones at a predictable rate over time. As scary as radioactivity may sound, it is the very reason why we know how old Earth is and when mountains formed, and as such it should be cherished and respected. Just like we should cherish stars for producing diversity in chemical elements and beautiful imperfections in atoms, as isotopes are among the most fascinating tools to decipher science.
The radioactive decay of the uranium (U) isotopes 235U and 238U (with masses 235 and 238 respectively) into some of the stars’ grandchildren – that is, respectively, the lead (Pb) isotopes 207Pb and 206Pb – at a known rate allows me to date the migration of deep energy sources like H2 and CH4, which sustain microbial life in the deep subsurface of our Planet. This is possible because some minerals trap 235U and 238U, and H2– and CH4-bearing fluids when forming. The same dating technique also allows me to obtain the Pb isotope composition of these H2– and CH4-bearing fluids, making it possible for me to trace the sources and pathways of these fluids. The stable isotope ratios of the element vanadium (V) reflect the physicochemical characteristics of these H2– and CH4-bearing fluids, and those of the element zinc (Zn) may reveal other chemical species dissolved in these fluids. Stars’ creativity and mistakes allow me to date the undatable and trace the untraceable.
These measurements are carried out using a technique known as mass spectrometry, which is based on the different deflection of atoms along different trajectories based on their atomic masses. Hence, as an example, U with mass 235 (235U) can be separated from U with mass 238 (238U) and from Pb with masses 206 and 207 (206Pb and 207Pb, respectively), and each mass is measured by a detector at the end of each mass-specific trajectory. The application of equations allows me to calculate ages and isotope ratios from raw measurements.
Isotopes are evidence that nature is incredibly vain and wants to be investigated. We often find that our scientific questions are directed by nature herself, which enjoys luring the scientific mind by posing riddles that lead us to learn more about her. Just like a good detective with a witness, all we have to do as scientists is to trick nature into giving hints. Isotopes are the most primordial storytellers of the universe. They recorded the story of nature from its very beginning, and this story has become part of our bodies thanks to this geochemical cycle taking place in the universe since the beginning of time.
Veronica Peverelli
(This article was written during a Deep Carbon Lab collaboration with Durham University, UK.)
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