Isotopes crash course!

When people think of isotopes and radiation, images of nuclear plants and catastrophes sometimes go through their minds. Those are just one part of the story, however. Isotopes are actually very important to us geochronologists and geochemists when it comes to getting information out of rocks we have collected—such as the ones we’ve gotten on this science cruise. This piece will give a general overview of isotopes and how we use them to get meaningful data!

To start off – an element on the periodic table contains protons, neutrons, and electrons, and is defined by the number of protons it has. This value is called its “atomic number” and is often abbreviated as “Z”. An isotope of an element has the same number of protons, but can vary in its number of neutrons. For example, the element oxygen has 8 protons, but can have 8, 9, or 10 neutrons; therefore, oxygen has 3 isotopes. When we describe oxygen isotopes, we refer to them as oxygen 16, 17, and 18 (8 protons plus however many neutrons). Oxygen 16 and 18 are used most frequently in practical applications because there’s only a small amount of oxygen 17 in the world.

Now that we know how an isotope is defined, how do we use them? It depends on what information we want. Isotopes can be broken into two primary categories: stable and radiogenic. Stable isotopes are just as they sound - very stable! They do not undergo any type of decay to other elements. Radiogenic isotopes DO undergo decay, and this is where you may have heard of terms like half-lives and radioactivity.

Now back to the original question – what the heck do we use these for? Well, stable isotopes can be used in a variety of ways. Most commonly, stable isotopes are used as a “tracer” to figure out the history of a rock or the paleoclimate in which the rock originally formed (disclaimer: we aren’t using stable isotopes in this study). No dating is involved here because nothing is decaying at any known rate. This “tracer” works because these stable isotopes are mass dependent. Think back to our oxygen example… an isotope of oxygen 18 is a smidge heavier than the isotope of oxygen 16. Why? Because those two extra neutrons add weight (mass). See the cartoon of the kitty to help visualize this concept…

Radiogenic isotopes can be used for 2 things. As a tracer, and for dating—we are doing both on our samples from the Rio Grande Rise. Let’s discuss the dating component first because some of us may have heard of half-lives and techniques like carbon-dating without any context. Radioactivity is the result of an inherent instability in the number of protons and neutrons in an element which causes it to turn into other elements and/or isotopes over time. A half-life is how long it takes for half of an isotope of an element to decay away. Imagine you have an isotope that has a half-life of 100 years. In 100 years, only 50% of that isotope is left, and after 200 years only 25% of that isotope is left. It has experienced two half-lives, and a half of a half is a quarter. Below is a table to help!

One of the most well-known applications of this technique is carbon dating. Our technique is similar; we cannot actually use carbon dating because, 1. The element being dated needs to be present in the rock and carbon is not present in many volcanic rocks, and 2. The half-life of carbon is too short for the rocks we are interested in. The half-life of carbon-14 is only 5,715 years. After ~7 half-lives our current instrumentation cannot detect such small amounts of material. You just divided something in half 7 times and have only ~0.78% left of the total! It would sort of be like trying to measure the distance between Seattle to Portland with a ruler; the unit of measurement is the wrong scale for the distance you’re measuring.

We use the Ar-Ar dating technique, which is based off of potassium (K) decaying to argon (Ar). This is an ideal system for us because of two reasons: we have K in our rocks and the half-life of the system is in the millions of years! The rocks we are looking to date are 30-130 million years old, so Ar-Ar makes sense for this context. The age-dating work will be done at Oregon State University and is critical to understanding the timing of volcanism in this region.

What about using radiogenic isotopes as tracers? The objective then is to say something about the composition of an original magma body because the composition is always changing. The chemical composition of a magma chamber evolves as it begins to crystallize; some elements prefer to go into crystal structures while other elements prefer to stay in the liquid portion of the magma. Those radiogenic isotopes can tell us how “enriched” the original magma body was in different elements. They allow us to see what composition a magma had before it began to crystallize—something we can’t do using normal elemental techniques. This data can be combined with other types to give insight on where the magma was originally sourced — from (potentially) the outer core to the upper mantle. These questions of deep or shallow mantle sources help us understand magma plumes, which are ultimately thought to be the sources of supervolcanoes!

By Emily Cahoon