by Mitchell Zimmer
When Desmond Moser of the Earth Sciences Department wanted to see if Western's Zircon & Accessory Phase Laboratory (ZAPLab) could be used to analyze Martian meteorites he didn’t suspect that the sample he had chosen would resolve an age dating conundrum. He chose the meteorite from the collection housed in the Royal Ontario Museum (ROM) because it was messy. “It was a beautiful meteorite that hadn’t been studied before and probably wasn’t highly sought after because it was so shocked,” says Moser. “It was one of the most shocked of that family of meteorites, but actually it has to be messed up before you can separate the signals and geochemistry.”
The shock was the result of the Martian rock being impacted by a meteor with enough force to eject it off of Mars and out of orbit until it made its way to Earth.
Drawing on a number of techniques from researchers assembled from ROM, the University of Wyoming, UCLA and the University of Portsmouth, the team began to analyze the specimen in a slightly different way. “We tweaked the techniques and added some new ones,” says Moser. Previously, samples would be crushed to release grains of crystals which would then be separated manually to yield clean geochemical signals. The new techniques take a thin slice of the meteorite and study the individual grains within by using beams of electrons and ions as narrow as one tenth of the width of a human hair. The data gathered from Western’s ZAPLab, one of the few electron nanobeam dating facilities in the world, revealed the growth histories of individual crystals. The team then combined this data with isotope dating methods (such as the measuring the decay of uranium to lead for example), but used a recently developed non-destructive technique that gently liberates atoms from the crystal surface using a focused beam of oxygen ions. The results from those tests were then compared to similar samples found here on Earth. “What we’ve done is use these different techniques on the rock so you can relate the value you get for that crystal to the value you get for that crystal in a context.”
The results turned out more like a detective story than Moser expected. The clues taken from different grains throughout the sample revealed that the meteorite started over 200 million years ago as a lava flow on Mars which picked up some other tiny crystals indicating deep hidden layer beneath the surface around four billion years old. Other measurements indicated that while the meteorite was launched from the surface of Mars, other crystals formed less than 20 million years ago and could have originated near the supervolcanoes at the Martian equator. “That’s what kind of blows me away about it,” adds Moser. “You can take a small piece of meteorite and reconstruct a major part of how its parent planet worked for most of its history. It’s pretty amazing how much information is archived in rocks.”
The ZAPLab takes advantage of a distinct trait of zircon. “The oldest known pieces of our planet are zircons,” explains Moser. “The ultimate survivor microminerals are zircons, that’s why it’s so important.” Geologists have detected early earth zircons, which are 4.3 billion years old, in ancient sediment. The chemical composition of these grains was used to determine when the Earth was cool enough to have oceans and therefore cool enough to have life. “All that information hinges on that mineral zircon.”
The published study served also as a test case. “We’re integrating material science techniques and surface science techniques on natural materials which is a growing field in all of geology and planetary science. We look specifically at the minerals, the phases , and the potential crystals you can get ages from. ... It is a bit of a niche but we’re in the front of groups doing that.” One of the benefits of working on such a complex meteorite, besides solving the Martian meteorite age conundrum, is that it served as a worst case scenario. As Moser says, “If we can figure it out in this rock, then we can figure it out in pretty much any rock.”
The plan is now to analyse a number of meteorites ranging from “early solar system condensates to pieces of the moon.” Moser’s group has preliminary results back from a number of those studies and are currently writing up papers for publication.
For more information about the study see "Western-led 'international beam team' solves Martian meteorite age puzzle"