U–Pb dating of andradite-rich garnet by SIMS

Abstract

Andradite-rich garnet, a common mineral in various rocks such as alkaline and skarn rocks, could have high U/Pb ratios and is suitable for U–Pb dating. With the advantage of higher spatial resolution, ion probes can provide a new perspective to interpret garnets that have experienced a complex evolutionary history. However, the potential matrix effects have not been evaluated, which hindered its application. This study revealed seven grandite samples with the andradite composition ranging from 52 to 95 mol%, including six various composition samples with known ages and one unknown sample dated from a skarn deposit. Based on measurements by using a CAMECA IMS 1280-HR secondary ion mass spectrometer (SIMS), all the samples yielded U–Pb ages consistent with the corresponding recommended values, indicating the absence of the matrix effect among the compositions of the studied garnet samples. A negative correlation between measured UO₂⁺/UO⁺ and the Al₂O₃ content was observed. Therefore, we suggest that a reference material with a similar Al₂O₃ content to unknown samples would be better. In addition, a positive correlation among Fe, REE, and U content exists in each sample, implying that U in garnet lattices is mainly controlled by a multi-element coupled substitution mechanism. As an application case, garnet from Zhibula skarn deposit was dated to demonstrate the relationship with the unexposed acidic pluton.