(a) The number of parent atoms in a mineral grain decreases, and the number of daughter atoms increases, during radioactive decay. As a mineral grain ages, its representation on this plot moves continuously upward and to the left along the red line. The labeled points represent 0, 1, 2, and 3 half-lives. (b) A graph of the strontium-87/strontium-86 versus rubidium-87/strontium-86 ratios for the Juvinas meteorite. The data are obtained from mass spectrometer measurements of different minerals from the meteorite.