from Wikipedia
« »  
One of the major drawbacks ( and, conversely, the most important use ) of utilizing Rb and Sr to derive a radiometric date is their relative mobility, especially in hydrothermal fluids.
Rb and Sr are relatively mobile alkaline elements and as such are relatively easily moved around by the hot, often carbonated hydrothermal fluids present during metamorphism or magmatism.

1.752 seconds.