Assessment of crystallographic orientation effects on secondary ion mass spectrometry (SIMS) analysis of cassiterite

Abstract

Crystallographic orientation effects on ion microprobe analyses for U-Pb and O-isotopes have been reported for a number of oxide minerals, including rutile (TiO2) and baddeleyite (ZrO2). Here we evaluate the effects of crystal orientation on U-Pb and O-isotopic data measured by ion microprobe on cassiterite (SnO2), which is isostructural with rutile. The crystallographic orientations of mounted and polished grains of cassiterite were determined by electron backscatter diffraction (EBSD). Those grains were then analysed for U-Th-Pb isotopes and 18O/16O compositions using the SHRIMP RG and SHRIMP SI ion microprobes, respectively. Based on these data, cassiterite appears to show no dependence of key measurement parameters such as UO2/UO, Pb/UO, or 18O/16O ratios at the achieved precision with crystallographic orientation. The contrasting behaviour of isostructural cassiterite and rutile provides new insights into the mechanisms leading to crystallographic orientation effects during ion microprobe analyses with electronic structure proposed as being a significant factor.