Earth's clock refined
13 Apr 2012 by Evoluted New Media
Some events in Earth’s history may have happened more recently than previously thought suggests new research from the British Geological Survey and the Massachusetts Institute of Technology (MIT).
Major geological events are usually dated by studying the uranium isotopic composition of minerals, but researchers have refined the data used to determine how much time has passed since rocks or minerals were formed, knocking up to 700,000 off their current predicted age.
As minerals form, they capture uranium, which undergoes radioactive decay to other elements, ending with lead. By more accurately measuring the relative amount of 238U and 235U, researchers now have a better understanding of how much time has passed since the mineral or rock was formed.
A 238U/235U ratio of 137.88 has been used to calculate uranium-lead (U-Pb) dates for the last 35 years, but when scientists evaluated the measurements used to reach the value, they came to a dead end and couldn’t trace it back to any standard units. The new study shows that many naturally occurring uranium-rich minerals like zircon actually have lower 238U/235U value with an average of 137.818 ± 0.045. Agreement between these values, other rocks, and meteorites indicate a new average 238U/235U value and uncertainty may also be representative of Earth’s bulk uranium isotopic composition.
One of the major effects of this work will be to decrease all previous U-Pb age determinations by up to 700,000 years for samples that are about 4.5 billion years old.
“This new determination will not only improve the accuracy of each U-Pb age but ultimately our understanding of events in Earth history,” said Blair Schoene, a geologist from Princeton University.
The new ratio will also allow geologists to place more accurate limits on the timing of a broad range of geological processes – from the initial formation of the planet to past evolutionary events and climate change.
