Strontium isotopic equilibration: a solution to a paradox

Previous work has shown that the granitic Murrumbidgee Batholith, Australia, is probably wholly derived from preexisting crustal material and has a maximum emplacement age of 426 m.y. New data indicate that four of six Rb-Sr total rock isochrons for major phases of the batholith yield anomalous ages of about 490 m.y. Total rock isochrons for two plutons are in agreement with the intrusive age. The approximate preservation of Rb-Sr systematics during a thermal event may be explained in terms of two parameters: (1) the volume throughout which strontium isotopic equilibration is effective during the thermal event, and (2) the minimum subvolumes into which the body can be divided such that each subvolume has the same mean Rb/Sr (and thus 87Sr/86Sr) as the mean Rb/Sr (and 87Sr/88Sr) for the entire body. Provided that the Rb/Sr of the volumes are not changed significantly in the thermal event, then if volume (1) < volume (2) the original pre-event isochron relationships will be approximately preserved. If (1) > (2), the thermal event will effectively reset Rb-Sr whole rock isotopic systems throughout the body. Complete homogenization of stontium isotopes over the entire body appears to have taken place but in fact homogenization need only have taken place on the much smaller scale represented by (1). (Auth.)