Discussion
Rb-Sr age based on the individual minerals often yield younger age than that of the whole rock age. The reason for this is :
Answer: D
The reason given by the key is that individual minerals have a lower closure temperature and Sr diffusion is random at the smaller scale.
After a rock forms, a later heating event can make radiogenic \(^{87}\)Sr move out of Rb-rich minerals such as biotite into neighbouring minerals. The individual minerals close to Sr diffusion at relatively low temperatures, so their clocks are reset and record the later event, giving a younger age. But the Sr only moves over short distances, so a whole-rock sample of adequate size remains a closed system and still records the original age.
The option claiming a higher closure temperature for minerals reverses the cause. The size of the sample does not by itself change an age, and '87Kb' (for \(^{87}\)Rb) content explains nothing about the age difference.
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