Abstract This paper presents a novel approach to calculate matrix-matched intensity limits for the spatially resolved detection of rare earth element (REE) enrichments in highly heterogeneous geological material from the Storkwitz carbonatite based on scanning Laser Induced Breakdown Spectroscopy (LIBS). A drill core from the Storkwitz carbonatite was mapped in detail with a LIBS drill core scanner. For reference purposes, μ-EDXRF was applied for qualitative REE detection, and a microprobe was used for quantitative REE analysis. Microprobe analysis of thin sections verified the existence of rare earth elements and revealed an accumulation of REEs around mineral rims. Microprobe measurements also revealed that the main carrier of rare earth elements in the analysed drill core are REE-carbonates with contents as high as 24.4 wt. % for Ce, 15.4 wt. % for La, and 9.2 wt. % for Nd. Apatite and pyrochlore are carrier for rare earth elements as well, however, with significantly lower concentrations of Ce (1.0 wt. %), La (0.3 wt. %), and Nd (0.3 wt. %). K-means clustering was applied on the LIBS mapping to separate classes that show similarities in chemical composition. The classes represent carbonates, silicates, and rock matrix, respectively. According to the microprobe results, most REE-carbonates were found to be related to elevated porosity or microfractures in the rock matrix, forming very thin rims of idiomorphic and hypidiomorphic minute crystals and rarely dense aggregates around exposed minerals and fractured rock fragments. Therefore, a buffer zone calculation was performed on the distinct mineral classes to extract pixel belonging to mineral rims only. Rims without any enrichment in rare earth elements were then used to calculate matrix-matched intensity limits for REE enrichments. Based on the observed similarity in chemical composition for rim pixel and rock matrix pixel, this intensity limit could be transferred to all pixel belonging to the rock matrix. Based on the calculated intensity limits for La, 1.4 area % of the sample was found to be enriched in REE-carbonates. This result was validated qualitatively by comparing it to a μ-EDXRF mapping of the same sample. Optical comparison showed good agreement and pixel counting confirmed similar zones of enrichment. The results reveal that La enrichments in REE-carbonates are reliably detected with LIBS. A mass balance calculation has shown that about 88 % of the REE enrichments in the investigated drill core seem to occur as pure REE-carbonates. The fast detection of drill core areas that are enriched in REEs makes LIBS a viable addition to the currently used methods in REE exploration.

中文翻译：

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使用 LIBS 和 k 均值聚类和空间栅格分析的组合检测 Storkwitz 异质钻芯中的富含 REE 区域

摘要 本文提出了一种计算矩阵匹配强度极限的新方法，用于基于扫描激光诱导击穿光谱 (LIBS) 的 Storkwitz 碳酸岩高异质地质材料中稀土元素 (REE) 富集的空间分辨检测。使用 LIBS 钻芯扫描仪详细绘制了来自 Storkwitz 碳酸岩的钻芯。作为参考，μ-EDXRF 用于 REE 定性检测，微探针用于 REE 定量分析。薄片的微探针分析证实了稀土元素的存在，并揭示了矿物边缘周围 REE 的积累。微探针测量还表明，分析的钻芯中稀土元素的主要载体是稀土碳酸盐，含量高达 24.4 重量％。Ce的百分比，15.4重量。% 为 La，和 9.2 重量。% 为 Nd。磷灰石和烧绿石也是稀土元素的载体，但是，Ce (1.0 wt. %)、La (0.3 wt. %) 和 Nd (0.3 wt. %) 的浓度要低得多。K-means 聚类应用于 LIBS 映射以分离显示化学成分相似性的类。这些类别分别代表碳酸盐、硅酸盐和岩石基质。根据微探针的结果，大多数稀土碳酸盐被发现与岩石基质中孔隙度升高或微裂缝有关，在裸露的矿物和破碎的岩石碎片周围形成非常薄的自形和双形微晶边缘和很少致密的聚集体。因此，对不同的矿物类别进行缓冲区计算以提取仅属于矿物边缘的像素。然后使用没有任何稀土元素富集的边缘来计算 REE 富集的基质匹配强度极限。基于观察到的边缘像素和岩石矩阵像素化学成分的相似性，可以将该强度限制转移到属于岩石矩阵的所有像素。根据计算的 La 强度极限，发现 1.4 面积百分比的样品富含 REE 碳酸盐。通过将其与同一样品的 μ-EDXRF 映射进行比较，该结果得到了定性验证。光学比较显示出良好的一致性，像素计数证实了相似的富集区。结果表明，使用 LIBS 可以可靠地检测到 REE 碳酸盐中的 La 富集。质量平衡计算表明，所研究的钻芯中约 88% 的 REE 富集似乎以纯 REE 碳酸盐形式出现。