Fortunately for geochronologists, such limestone-forming groundwater often also contains minute quantities of dissolved uranium, which gets incorporated into the limestone crystals as they form.

The most common form of uranium, U-238, is unstable. At a slow but steady rate, it sends off subatomic particles. The process transmutes U-238 first into another uranium isotope, U-234, and then into an isotope of the element thorium, Th-230.

Thorium, unlike uranium, does not dissolve in water. Therefore, scientists reason, all Th-230 found in limestone must be the product of radioactive decay of U-238. By making precise determinations of the amounts of all three isotopes (U-238, U-234 and Th-230), and knowing the rate at which the transmutations occur, researchers can estimate the age of a rock. Basically, the higher the ratio of Th-230 to U-238, the older the limestone.

To get these ratios, scientists previously measured the minute amounts of radioactivity emanating from limestone. With proper instruments, they can distinguish between the radiation coming from decaying U-238, U-234 and Th-230 atoms.

The theory is good, but implementing it is difficult. The amounts of radiation produced are minimal, and even small measurement errors can lead to big errors in estimating age.

To significantly reduce the measurement errors requires the use of a device known as a mass spectrometer. This device can actually measure quantities of U-238, U-234 and Th-230 atom by atom, instead of waiting for them to decay and measuring their radiation.

Previously, even a mass spectrometer was not capable of counting the extremely small amount of U-234 and Th-230 found in samples like the Zhoukoudian material. But by upgrading and calibration of instruments pioneered by scientists at Caltech, Ku and his Caltech colleagues demonstrated in 1987 the feasibility of using mass spectrometry to make the kind of measurements demanded by the Zhoukoudian da
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Contact: Eric Mankin
mankin@usc.edu
University of Southern California
13-May-1996