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July 24, 2013 — A University of Wyoming research professor has helped solve the question of how old Mars meteorites are and when volcanism actually occurred on the red planet.
Kevin Chamberlain, a research professor in UW’s Department of Geology and Geophysics, used newly developed mineral-dating techniques he created to determine the age of volcanism on Mars at 200 million years ago, as well as the timing of a large-impact event 22 million years ago that launched rocks off the surface of Mars. The rocks eventually fell to Earth as meteorites.
Chamberlain is one of seven co-writers of a research paper, titled “Solving the Martian Meteorite Age Conundrum Using Micro-Baddeleyite and ‘Launch-Generated Zircon’,” that was published in today’s issue of Nature, an international weekly journal of science that publishes peer-reviewed research in all fields of science and technology.
“The combination of techniques allowed us to determine the magmatic age of the lava on Mars as well as the time that the sample was launched into space by a bolide impact,” Chamberlain says. “Our results also solved an ongoing debate about the age of magmatism that most Martian meteorites appear to have sampled.”
Chamberlain developed the new dating technique at UW with Norbert Swoboda-Colberg, a lab technician in the UW Department of Geology and Geophysics, and Susan Swapp, a senior research scientist, also in geology and geophysics.
The dating technique required the use of a specialized instrument called a secondary ionization mass spectrometer or SIMS, of which only three exist in North America, Chamberlain says. The instrument analyzes a mineral sample by excavating microscopic pits (about 1 micron deep by 20 microns in diameter) in the rock sample and analyzing the isotopic compositions of the excavated material. For scale, the diameter of a human hair is roughly 100 microns, Chamberlain says.
Using a SIMS instrument at UCLA, Chamberlain analyzed 18 different crystals of the minerals baddeleyite and zircon. All 18 were found within a 20-millimeter square (roughly three-fourths of an inch) region of a polished surface of the meteorite. Both minerals are major reservoirs for uranium in meteorites.
The large crystals are each less than 15 microns in length, too small to separate physically from the rock. The new dating technique locates the grains using electron beam imaging instruments, and then analyzes them in-situ without needing to break the rock apart.
Using the mass spectrometer, Chamberlain measured the ratio of lead to uranium, which allowed him to calculate the age of the meteorite grains.
“We solve the (age) conundrum by determining the degree of shock-induced strain within the crystals, using in-situ electron nanobeam instruments at the University of Western Ontario,” he says. “By combining microstructural analysis with uranium/lead isotopic measurements in the same crystals, we established both the age that the rock formed and the time it was launched off the surface of Mars.”
Chamberlain says both techniques are relatively non-destructive, which made their use ideal for meteorite samples. Outside of the excavated pit, the rest of the sample remained intact.
The project was partially funded through a faculty research grant Chamberlain obtained from the Wyoming NASA Space Grant Consortium. Meteorite samples were loaned by the Royal Ontario Museum in Toronto. Scientists from UW, the University of Western Ontario, UCLA and the Royal Ontario Museum collaborated on the project.
To date, 65 samples of Martian meteorites have been discovered on Earth. Many were found in either Antarctica or the Sahara Desert, Chamberlain says. In those two places, there are broad plains with no mountains above the ice or sand -- which means that, if any rock is found on those surfaces, it had to come from space, Chamberlain explains. There are many different types of meteorites, but these 65 have bits of Martian atmosphere trapped within them, he says.
“The 65 samples are basaltic compositions,” he says, noting it’s the same material found on the ocean floor or on the surface of the moon. “The fact that they’re (meteorites) all so similar in composition begs the question, because a lot of Mars is not basaltic. NASA’s rovers are looking at sandstones for evidence of water and streams, for example. The surface of Mars has a lot of variety.”
However, the large volcano on Mars is basaltic in nature. Other researchers have speculated that many of the Martian meteorites were a result of a few large bolide impacts on a lava flow on the flanks of that volcano.
The 200-million-year age of volcanism from these newest research findings challenges previous research methods that interpret the ages of ejected igneous crust from Mars to be as old as 4 billion years, which would mean that the planet’s volcano would have been extinct for a long time, Chamberlain says.
“The eruption formed the lava. Rocks crystallized and formed during the volcanic eruption,” he says. “If we can determine when the rocks crystallized from the lava, we are dating one of the eruptions of the volcano. Having evidence that Mars was geologically active fairly recently is a pretty big deal.”
Chamberlain says the timing of the large-impact event is between 22 million and 2 million years ago.
“It (the meteorite) hit Mars hard enough that part of the planet’s surface escaped and entered interplanetary space,” he says. “During millions of years, some of it’s been caught by the Earth’s gravity.”
Chamberlain presented results of the group’s research at an invited talk at the Goldschmidt 2012 International Geochemistry Conference in Montreal last summer, and at UW’s Department of Physics and Astronomy colloquium this past spring.
“We plan to apply these techniques to additional meteorites from Mars, the moon and several asteroids to gain a better understanding of the evolution of the solar system,” he says.
For a look at a video of the research -- provided by Desmond Moser from the University of Western Ontario, the paper’s lead author -- go to http://communications.uwo.ca/media/agepuzzle/.
Kevin Chamberlain, a research professor in UW’s Department of Geology and Geophysics, is a co-author of a paper that appeared in Nature. Chamberlain used newly developed mineral-dating techniques he created to determine the age of volcanism on Mars at 200 million years ago, as well as the timing of a large-impact event 22 million years ago that launched rocks off the surface of Mars.