thorium-230-deficiency method . The calculation of an age in years for fossil coral or shell from 10,000 to 250,000 years old, based on the departure of ionium (thorium-230) from equilibrium with uranium-238 and uranium-234 as a result of carbonate recrystallization shortly after its formation or burial. The age depends on the growth toward the equilibrium ratio, which is directly related to the passage of time. Cf: uranium-series age method; thorium-230/protactinium-231 deficiency method. Syn: ionium-deficiency method.