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Some of these rocks are sedimentary, and include minerals which are themselves as old as 4.1 to 4.2 billion years.
Rocks of this age are relatively rare, however rocks that are at least 3.5 billion years in age have been found on North America, Greenland, Australia, Africa, and Asia.
A plot is constructed of Pb-206/Pb-204 versus Pb-207/Pb-204.
If the solar system formed from a common pool of matter, which was uniformly distributed in terms of Pb isotope ratios, then the initial plots for all objects from that pool of matter would fall on a single point.
Enoto and his co-authors' results confirm for the first time speculation dating back to 1925 about this phenomenon.
Back then, scientists suggested that energized, radioactive particles might zip through the booms and flashes of a thunderstorm.
Unfortunately, the age cannot be computed directly from material that is solely from the Earth.
Enoto's results show that those beams of invisible energy — especially the gamma-rays — excite ambient nitrogen and oxygen in the atmosphere, knocking out neutrons from the molecules the gamma rays encounter. But take one of those neutrons away, and you're left with nitrogen-13, an unstable, radioactive isotope. In this case, that signature is a gamma-ray with an energy of 0.511 megaelectron volts.The actual underlying assumption is that, if those requirements have not been met, there is no reason for the data points to fall on a line.The resulting plot has data points for each of five meteorites that contain varying levels of uranium, a single data point for all meteorites that do not, and one (solid circle) data point for modern terrestrial sediments.The most direct means for calculating the Earth's age is a Pb/Pb isochron age, derived from samples of the Earth and meteorites.This involves measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204).
And from the slope of the line we can compute the amount of time which has passed since the pool of matter became separated into individual objects.