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.

先前通过二次离子质谱法 (SIMS) 对赤铁矿样品的氧同位素组成进行的研究在单个样品中产生了较大的氧同位素变异性 (>3‰)。解卷积来自地质变化的仪器效应对于正确解释赤铁矿中记录的原位 SIMS δ ¹⁸ O 同位素组成至关重要。在这里，我们证明了 δ ¹⁸中的过度散射使用 SIMS 获得的 O 值来自真实样品变异性和晶体学诱导仪器质量分馏的组合。通过 SIMS、激光氟化、激光烧蚀电感耦合等离子体质谱 (LA-ICP-MS) 元素映射和电子背散射衍射 (EBSD) 的组合研究了赤铁矿的自然和晶体学诱导的同位素变异性。我们在来自巴西米纳斯吉拉斯州 Quadrilátero Ferrífero 的博物馆级赤铁矿标本上使用了敏感高分辨率离子微探针 - 稳定同位素 (SHRIMP-SI)，发现定向诱导的分析分馏对观察到的影响高达 5‰ SIMS δ ¹⁸O 同位素散射。晶体学上均匀的单一赤铁矿晶体的激光烧蚀 ICP-MS 元素映射揭示了不同的化学分带模式，表明晶体生长的几个离散阶段。晶体分带还与 δ ^{18 的}突变相吻合O测距约。11‰为单晶取向。这些结果突出了晶体生长机制与矿化流体同位素演化之间的密切联系，表明在赤铁矿的同位素组成可用于推断矿物沉淀和溶液组成的条件之前，需要进行高空间分辨率分析。结果还表明，需要了解晶体取向和晶体化学变异性，才能避免以前在通过离子微探针对赤铁矿进行氧同位素分析时遇到的问题。