Oxygen isotope and trace element mapping of growth and deformation of hematite single crystals from enriched banded iron-formations

Abstract Hematite is a resilient iron oxide that can persists on the surfaces of terrestrial planets for billions of years, preserving the isotopic signature of past water-rock interaction. Its oxygen isotopic composition can be determined by bulk and in-situ methods. However, in-situ oxygen isotope analysis of hematite by secondary ion mass spectrometry (SIMS) is complicated by instrument induced crystal orientation effects and crystallochemical zoning within the sample. In this study, we applied a multi-method approach to deconvolve these effects in naturally occurring single hematite crystals from recrystallized Precambrian banded iron-formations. Three well-formed hematite crystals, sourced from the Hamersley Province, Australia, and the Quadrilátero Ferrífero (QF), Brazil, were analyzed using a combination of electron back-scattered diffraction (EBSD), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) elemental mapping, laser fluorination (δ18OLF), and Sensitive High Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI) oxygen isotope analysis. Although these hematite crystals appeared macroscopically pristine and homogeneous, they exhibit complex internal textures that include growth zoning, twining, fracturing, and re-healing, which are not visible optically. Consequently, measured hematite isotopic composition varies widely across a single crystal, making it difficult to determine the isotopic composition of the solution from which the mineral precipitated. This study illustrates how a combination of high-spatial resolution tools provides an effective approach to characterize the oxygen isotope compositions of mineralizing solutions that precipitated or altered a hematite crystal. Our results, in a small number of samples, reveal that enrichment processes affecting Precambrian banded iron-formations in Australia and Brazil involved interaction with isotopically distinct fluids, despite similarities in starting materials and final products.