英文摘要 |
In the Rb-Sr dating system, ages are calculated from tie lines
connecting two minerals with different Rb/Sr ratios in slow cooling rocks.
However, not all of these ages have geological significance since complete
isotopic equilibrium is often not achieved in the cooling history. It may be
asked how we can obtain the exact or approximate closure ages for minerals in
the Rb-Sr dating system. Based on discussions from both graphical and
mathematical models, some criteria are suggested: 1. The exact closure
temperature age is obtained from the slope of the tie line between whole rock
and the first mineral to close. Moreover, if the closure temperature of the
first mineral to close is very near the emplacement temperature of rock, the
closure temperature age of the first mineral to close should be close to the
emplacement age of the plutonic rock. 2.The ages obtained connecting the
minerals and the whole rock should be regarded as the best estimations for the
closure temperature ages of these minerals. Moreover, the estimated ages shall
be considered as the upper limit of the age of closure temperature for that
mineral when (Rb/Sr) >(Rb/Sr) and the lower limit of the age for that mineral
when (Rb/Sr) < (Rb/Sr). 3. Some errors in the closure temperature age may
possibly occur if the age is from the tie line connecting the mineral and the
first mineral to close, especially, when the mineral of late closure has a much
higher Rb/Sr ratio than that of the first mineral to close. |