Rare earth elements of ROV sample GeoB12338-2 from the Makran accretionary complex@en

Authigenic carbonates forming at an active methane-seep on the Makran accretionary prism mainly consist of aragonite in the form of microcrystalline, cryptocrystalline, and botryoidal phases. The d13Ccarbonate values are very negative (-49.0 to -44.0 per mill V-PDB), agreeing with microbial methane as dominant carbon source. The d18Ocarbonate values are exclusively positive (+ 3.0 to + 4.5 per mill V-PDB) and indicate precipitation in equilibrium with seawater at bottom water temperatures. The content of rare earth elements and yttrium (REE + Y) determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and solution ICP-MS varies for each aragonite variety, with early microcrystalline aragonite yielding the highest, cryptocrystalline aragonite intermediate, and later botryoidal aragonite the lowest REE + Y concentrations. Shale-normalised REE + Y patterns of different types of authigenic carbonate reflect distinct pore fluid compositions during precipitation: Microcrystalline aragonite shows high contents of middle rare earth elements (MREE), reflecting REE patterns ascribed to anoxic pore water. Cryptocrystalline aragonite exhibits a seawater-like REE + Y pattern at elevated total REE + Y concentrations, indicating higher concentrations of REEs in pore waters, which were influenced by seawater. Botryoidal aragonite is characterised by seawater-like REE + Y patterns at initial growth stages followed by an increase of light rare earth elements (LREE) with advancing crystal growth, reflecting changing pore fluid composition during precipitation of this cement. Conventional sample preparation involving micro-drilling of carbonate phases and subsequent solution ICP-MS does not allow to recognise such subtle changes in the REE + Y composition of individual carbonate phases. To be able to reconstruct the evolution of pore water composition during early diagenesis, an analytical approach is required that allows to track the changing elemental composition in a paragenetic sequence as well as in individual phases. High-resolution analysis of seep carbonates from the Makran accretionary prism by LA-ICP-MS reveals that pore fluid composition not only evolved in the course of the formation of different phases, but also changed during the precipitation of individual phases.发育于马克兰增生楔（Makran accretionary prism）活跃甲烷冷泉环境的自生碳酸盐岩，主要由微晶、隐晶及葡萄状三种相态的文石（aragonite）构成。其碳酸盐碳同位素（δ¹³C_carbonate）值极低，介于-49.0‰至-44.0‰（相对于维也纳佩尔迪多箭石标准V-PDB），与以微生物甲烷作为主要碳源的结论相符。碳酸盐氧同位素（δ¹⁸O_carbonate）值全部为正值，范围为+3.0‰至+4.5‰（V-PDB），指示其沉淀过程与底层水温下的海水达到同位素平衡。通过激光剥蚀-电感耦合等离子体质谱法（LA-ICP-MS）与溶液进样电感耦合等离子体质谱法（solution ICP-MS）测定的稀土元素与钇（REE+Y）含量，在不同文石相态中存在显著差异：早期微晶文石的REE+Y含量最高，隐晶文石居中，晚期葡萄状文石的REE+Y浓度最低。不同类型自生碳酸盐岩的页岩标准化REE+Y配分模式，反映了其沉淀时期孔隙流体的组成差异：微晶文石表现出较高的中稀土元素（MREE）含量，其REE配分模式对应缺氧孔隙水环境；隐晶文石在总REE+Y浓度升高时呈现类海水的REE+Y配分模式，表明其孔隙水中的REE浓度受海水影响而升高；葡萄状文石在生长初期表现为类海水的REE+Y配分模式，随着晶体生长进程逐渐出现轻稀土元素（LREE）富集，反映该胶结物沉淀过程中孔隙流体组成发生了变化。常规的样品前处理方法（包括碳酸盐相态的微钻取及后续溶液ICP-MS分析）无法识别单一碳酸盐相态中REE+Y组成的细微变化。为重建成岩早期孔隙流体组成的演化过程，需要一种能够追踪共生序列及单一相态中元素组成变化的分析方法。通过LA-ICP-MS对马克兰增生楔冷泉碳酸盐岩开展的高分辨率分析显示，孔隙流体组成不仅在不同碳酸盐相形成过程中发生演化，还在单一相的沉淀过程中产生变化。