Crystallographic and crystallochemical controls on oxygen isotope analysis of hematite by SIMS

Abstract

Previous work on oxygen isotopic compositions of hematite samples by secondary ion mass spectrometry (SIMS) has yielded large oxygen isotopic variability (>3 ‰) within individual samples. Deconvolving instrumental effects from geological variation is of vital importance in correctly interpreting in situ SIMS δ18O isotope compositions recorded in hematite. Here, we demonstrate that the excess scatter in δ18O values acquired with SIMS results from a combination of true sample variability and crystallographically-induced instrument mass fractionation. The natural and crystallographically-induced isotopic variability of hematite was investigated by a combination of SIMS, laser fluorination, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) elemental mapping, and electron back-scattered diffraction (EBSD). We used the Sensitive High Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI) on a museum-quality hematite specimen from the Quadrilátero Ferrífero, Minas Gerais, Brazil, and orientation-induced analytical fractionation was found to contribute up to 5 ‰ to the observed SIMS δ18O isotopic scatter. Laser ablation ICP-MS elemental mapping of crystallographically uniform single hematite crystals revealed distinct chemical zonation patterns that indicate several discrete stages of crystal growth. Crystal zoning additionally coincides with abrupt shifts in δ18O ranging ca. 11 ‰ for a single crystallographic orientation. These results highlight an intimate link between crystal growth mechanism and isotopic evolution of the mineralizing fluid, showing the need for high spatial resolution analysis before the isotopic composition of hematite can be used to infer conditions of mineral precipitation and solution compositions. The results also demonstrate that an understanding of both crystallographic orientation and crystallochemical variability are needed to obviate problems previously encountered in oxygen isotope analysis of hematite by ion microprobe. Show more