In whole-rock Rb-Sr geochronology, cogenetic rocks with a common initial 87Sr/86Sr ratio evolve to different present-day ratios if they have different Rb/Sr ratios, because 87Sr accumulates from 87Rb decay over time.
87Rb decays to 87Sr with a long half-life (~48.8 billion years), so over 1 billion years, rocks with higher Rb/Sr ratios accumulate proportionally more radiogenic...
Full option-by-option analysis available to enrolled students.