Ever wondered how the solar system formed and why the planets and asteroids are the way they are? I do all the time. That’s why I’m in the second year of my MSc in Geology; the research year. I thought I’d share a few shots of the meteorite samples I’m doing my research on, and a brief run-down on what I’m doing, what I’m aiming to achieve and why any of this counts as geology.

At the risk of you, my beloved reader, experiencing a fit of explosive eye-glazing, I will first present a picture of my very first mounted meteorite sample (not as dirty as it sounds);

The white bit in this picture is 4.5672 billion years old. It’s one of the oldest solids in the Solar System and dates back to a time when the Sun was just kicking off its fire-juggling party. The minute concentrations of iron in this rock and the iron in your blood are from the same star-derived reservoir. But I digress.

This is a sample of a Calcium-Aluminium rich Inclusion (or CAI) from a carbonaceous chondrite (stoney-iron meteorites). These things formed in a very hot environment, and the minerals within have gone through between one and three stages of melting. The heat inherent in the environment was not due to the sun, but radioactive decay of unstable isotopes such as 26 Aluminium (Half life of ~703 Ka).

What I’m doing with these tiny inclusions (which are all less than 10mm in diameter), is determining the major mineral constituents, of each of those I’m looking at the minor or trace element concentrations and finally dating them by determining comparative ²⁶Mg deficits (if any). So what involved in each step?

Step 1: Mineralogy: Using an Electron Micro Probe, I am able to determine the major elemental weight percentages of each mineral “phase” of the targeted CAI;

Each shade of grey in the above image is a different mineral. In this case, the lightest phase (Phase 1, points of sampling are orange) is melilite, the second lightest (Phase 2, coloured blue) is pyroxene, phase 3 (in green) is anorthite, and the nearly black phase 4 (in red) is spinel. The above image is an electron back-scatter image of a Type B1 CAI from the carboneceous chondrite NWA 2364.

Step 2: Trace Elements: Using Laser Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS), I am determining the concentrations of 34 chemical elements that are present to an accuracy of a few parts per million (ppm) or less. The elements I’m looking for range through Uranium, Thorium, Lead, Titanium, the Rare Earth Elements (REE) and more. By comparing the concentrations of these elements in different CAIs, whole meteorites and planets, you can get an idea of what was around when each formed and how the areas of formation differed in their respective elemental composition. Did the Earth and these CAIs form in the same place in the solar nebula? The answer at the moment is no, so why did the solar nebula cloud have differing concentrations of elements from one point to another? That’s the big questions relating to how these various bodies formed.

I like to think of this step as a real life game of Asteroids, with a scientific slant.

Step 3: Magnesium Deficits and Dating: Using another kind of ICPMS, namely a Multi-Collector, I will be determining the concentrations of the isotopes of magnesium (Mg) in each of my collected samples. As I mentioned previously, these samples were heated by the decay of ²⁶Al. This particular isotope decays to ²⁶Mg. So the more ²⁶Mg in a sample, the older it is. Any deficit in ²⁶Mg compared to that of the maximum found in CAIs can be correlated to the time between CAI formation and the formation of whatever you’re looking at. So by getting the ²⁶Mg/²⁴Mg ratio from these samples, I can determine their relative ages from oldest to youngest. This is of interest because knowing over what time span CAIs were forming can help you determine whether it all happened at once in a very short time span (and was thus stopped by some process of the sun’s formation), whether there were several exclusive periods of CAI formation (perhaps by injection of ²⁶Al from nearby supernovae), or whether it happened slowly and steadily over 6 half lives of ²⁶Al (most likely).

Why does this count as geology? By knowing what was around when the Earth formed and thus what it is made of (i.e, how the chemical composition of the solar nebula changed over time) and how old it is in comparison to other bodies in the solar system, you can build more accurate models of the chemical composition of the materials that make up the crust, mantle and the core of the Earth. This helps in the understanding of how and why things are the way they are. It also makes up the underpinning of mantle geochemistry, volcanic petrology and chemistry, and environmental and atmospheric evolution (which links to the formation of life (abiogenesis) and the like.

So that’s what I’m spending most of my time doing. Any questions, just post a comment and I’ll endeavour to answer.