PRESERVED INITIAL SR-87/S-86 IN APATITE FROM ALTERED FELSIC IGNEOUS ROCKS - A CASE-STUDY FROM THE MIDDLE PROTEROZOIC OF SOUTH-AUSTRALIA

Strontium isotope data from a suite of hydrothermally altered Middle Proterozoic granitoids and a related felsic volcanic rock show that the total-rock systems have been disturbed as a result of hydrothermal alteration. As such, unambiguous determination of initial Sr-87/Sr-86 is not possible using total-rock data, with consequent loss of valuable petrogenetic information. In contrast, initial Sr-87/Sr-86 values derived from apatite are uniform when compared with the discordant total-rock analyses. Evaluation of strontium diffusion kinetics in apatite, together with geological and isotopic evidence presented here, suggest that initial Sr-87/Sr-86 derived from apatite preserves the igneous initial Sr-87/Sr-86 and that Sr-87/Sr-86 in apatite is unlikely to have been modified as a result of igneous cooling or hydrothermal alteration. This study shows that under some conditions, apatite is resistant to Sr-87/Sr-86 modification and can yield petrogenetic information not attainable from total-rock analyses. Data presented here, together with data obtained in other studies, indicate that the strontium isotope system in apatite is more resistant to modification than feldspar and biotite. This approach may be of widespread application in determining initial Sr-87/Sr-86 of similarly altered ancient igneous rocks.