For a long time, geologists have used microscopic crystals called zircons as “time capsules” for dating rock strata. The tiny crystals are so durable it was believed they were virtually impermeable. Now, however, inclusions inside the zircons appear to be vastly different in age. This could have drastic effects on how certain formations are dated.
According to Science Magazine News, Earth’s time capsules “may be flawed.” An Australian team took a look at about 7,000 zircons from the Jack Hills of western Australia, a conglomerate formation containing pebbles that have undergone heavy bouts of metamorphism. The zircons were thought to be between 2.65 and 3.05 billion years old. A few had inclusions, and some of the inclusions that were dated using radiometric means came out as young as 800 million years – the assumed date of the surrounding metamorphic rock.
This means that zircons are not as protected from outside influence as thought. The scientists could find no way in for younger radioactive material to get inside some of the “young” inclusions – no hairline fractures, for instance. If carried in by fluids, “the fluids may have traveled along defects in the zircon’s crystal structure caused by radioactive decay or along pathways that are either too small to see or oriented such that they’re invisible.”
Reporter Sid Perkins described how this finding may “stir people up”:
In recent years, some researchers have used analyses of zircons and their inclusions—and in particular, the temperatures and pressures they’ve been exposed to since their formation—to infer the presence of oceans or of modern-style plate tectonics on Earth more than 4 billion years ago, well before previously suspected, Rasmussen says. But based on the team’s new findings, which will be reported next month in Geology, those conclusions are suspect, he notes.
Another geologist was even more worried. “The results ‘suggest that analyses of zircon inclusions can’t be trusted much at all,’ adds Jonathan Patchett, an isotope geochemist at the University of Arizona in Tucson.” Another geologist was not so pessimistic but warned that use of zircon dating information will have to be done more carefully from now on….
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