Abstract We present protocols for high-spatial resolution measurement of oxygen isotope ratios of goethite (α-FeOOH) with the Sensitive High Mass Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI) and propose a natural sample as a potential goethite reference material (RM) for ion microprobe analysis. We assess the effects of goethite chemical composition, crystallographic orientation, and texture on the accuracy and repeatability of SHRIMP-SI δ18O (δ18OSIMS) results. Synthetic goethites evaluated as potential δ18OSIMS RM are powdery, porous, and finely crystalline; they do not yield repeatable results. A dense colloform goethite from the Capao topaz mine, Minas Gerais, Brazil, fulfills major prerequisites: it is stoichiometrically relatively pure, yields repeatable oxygen isotope results, and occurs in abundance to produce a RM for long-term use. We use an average laser fluorination δ18OLF-VSMOW value of −17.3 ± 0.3‰ (1SD) obtained for five aliquots of this RM to normalize all δ18OSIMS measurements. Multiple δ18OSIMS analyses of a large fragment of the Capao L4 (CL4) RM analyzed in three different runs yield an overall repeatability of −17.3 ± 0.5‰ (2SD, n = 294) for all three runs combined. Natural variability and crystal orientation effects are the main reasons for the excess spread of the δ18OSIMS results compared to the spot internal precision (ca. 0.2‰). All δ18OSIMS analyses (n = 1027) in various aliquots of CL4, randomly oriented and analyzed in 26 sessions during eight distinct runs, yield an overall repeatability of −17.3 ± 0.7‰ (2SD), confirming that CL4 is a suitable SIMS RM. After ascertaining its suitability as a RM, we used CL4 to standardize analyses of other natural goethite samples with the SHRIMP-SI and compared δ18OSIMS and laser fluorination results to test the relationship between natural properties (e.g., porosity, minor elements substituting for Fe), preparation procedures (e.g., polish and relief), instrument conditions, and the overall precision and accuracy of the SIMS analyses. Samples containing minor elements substituting for Fe (e.g., Al, Mn, Cu, Zn, etc.) or as contaminants (e.g., Si, P) require significant matrix corrections. Because we could not find homogenous natural goethite samples showing a large range in metal concentrations, we extrapolate our calibration curves beyond the composition of our calibration goethite samples. δ18OSIMS results corrected for instrument mass fractionation (using CL4) and compositionally dependent matrix effects (using several calibration goethites of known elemental composition) are less precise but statistically indistinguishable from their laser fluorination results. However, porous samples are unsuitable for SHRIMP-SI δ18O analysis. Dense colloform samples yield repeatable results for individual growth bands, showing that the high spatial resolution, moderate precision, and speed of analysis of the SHRIMP-SI can resolve variations in oxygen isotope composition acquired during sample growth. (U Th)/He geochronology of equivalent aliquots from the same goethite samples reveal that the combination of the two methods permits the extraction of temporal variation in the isotopic compositions of meteoric solutions in the geological past.

中文翻译：

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离子微探针原位测量针铁矿中氧同位素的方案

摘要 我们提出了高空间分辨率测量针铁矿 (α-FeO​​OH) 的氧同位素比值的方案，使用灵敏的高质量分辨率离子微探针 - 稳定同位素 (SHRIMP-SI)，并提出了一种天然样品作为潜在的针铁矿参考材料 (RM ) 用于离子微探针分析。我们评估针铁矿化学成分、晶体取向和纹理对 SHRIMP-SI δ18O (δ18OSIMS) 结果的准确性和可重复性的影响。被评估为潜在 δ18OSIMS RM 的合成针铁矿是粉状、多孔和细晶；它们不会产生可重复的结果。来自巴西米纳斯吉拉斯州 Capao 黄玉矿的致密胶体针铁矿满足主要先决条件：它在化学计量上相对纯净，产生可重复的氧同位素结果，并大量出现以产生长期使用的 RM。我们使用从该 RM 的五个等分试样中获得的 -17.3 ± 0.3‰ (1SD) 的平均激光氟化 δ18OLF-VSMOW 值来标准化所有 δ18OSIMS 测量值。在三个不同的运行中分析的 Capao L4 (CL4) RM 的一个大片段的多次 δ18OSIMS 分析产生了 -17.3 ± 0.5‰ (2SD, n = 294) 的所有三个运行组合的总体重复性。与光斑内部精度（约 0.2‰）相比，自然变异性和晶体取向效应是 δ18OSIMS 结果过度扩散的主要原因。所有 δ18OSIMS 分析（n = 1027）在 CL4 的各种等分试样中，随机定向并在 8 个不同的运行期间在 26 个会话中进行分析，产生 -17.3 ± 0.7‰ (2SD) 的总体重复性，证实 CL4 是合适的 SIMS RM。在确定其适合作为 RM 后，我们使用 CL4 用 SHRIMP-SI 对其他天然针铁矿样品进行标准化分析，并比较 δ18OSIMS 和激光氟化结果，以测试天然特性（例如孔隙率、微量元素替代 Fe）、制备程序（例如抛光和浮雕）之间的关系)、仪器条件以及 SIMS 分析的整体精度和准确度。含有替代 Fe（例如，Al、Mn、Cu、Zn 等）或作为污染物（例如，Si、P）的微量元素的样品需要显着的基质校正。由于我们无法找到显示出大范围金属浓度的均质天然针铁矿样品，因此我们将校准曲线外推到校准针铁矿样品的成分之外。针对仪器质量分馏（使用 CL4）和成分相关的基质效应（使用几种已知元素组成的校准针铁矿）校正的 δ18OSIMS 结果不太精确，但在统计上与激光氟化结果无法区分。然而，多孔样品不适合 SHRIMP-SI δ18O 分析。致密胶体样品对单个生长带产生可重复的结果，表明 SHRIMP-SI 的高空间分辨率、中等精度和分析速度可以解决样品生长过程中获得的氧同位素组成的变化。(U Th)/He 对来自相同针铁矿样品的等分试样进行年代学研究表明，这两种方法的结合可以提取地质过去大气溶液同位素组成的时间变化。