It is well known that the rare earth elements (REEs) incorporated in the crystal lattice of sedimentary apatite are commonly derived from ambient seawater. This study documents, for the first time, the possibility of mixed origin of apatite REEs present in the Egyptian Western Desert phosphorites, known as the Abu-Tartur phosphorites, one of the most important accumulations of sedimentary phosphorites in the Middle East and North Africa. Shale-like patterns of REEs, negative Ce anomalies, and a (La/Sm)N−(La/Yb)N binary diagram of the studied phosphorites indicate that the incorporation of REEs into apatite crystal lattices has occurred from the ambient seawater by substitution during late-stage diagenesis. The second origin is attributed to REE-bearing supergene fluids, which resulted in the occurrence of sedimentary britholite as green rims and patches containing about 14.93 wt% total REEs in places where the black phosphorites are gradually oxidized into the brown variety. For instance, the intensive chemical weathering induces the crystal lattice of carbonate fluorapatite to preferentially release Ca2+ and CO32− ion species to solution, resulting in the formation of a carbonate-depleted layer in which REEs, particularly heavy REEs, are incorporated into the preferential Ca2+ sites inside the altered apatite lattice, leading to britholite formation.

中文翻译：

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沉积海相磷灰石中稀土元素混合成因的可能性：埃及阿布-塔尔图高原白垩纪（Campanian-Maastrichtian）Duwi 组磷灰岩的案例研究

众所周知，沉积在磷灰石晶格中的稀土元素 (REE) 通常来自周围的海水。这项研究首次记录了埃及西部沙漠磷矿中存在的磷灰石 REE 混合成因的可能性，称为 Abu-Tartur 磷矿，这是中东和北非最重要的沉积磷矿堆积物之一。REE 的页岩样模式、Ce 负异常和所研究磷矿的 (La/Sm)N-(La/Yb)N 二元图表明，通过置换，REE 已从环境海水中掺入磷灰石晶格中在晚期成岩作用中。第二个起源归因于含稀土元素的表生流体，这导致在黑色磷矿逐渐氧化成棕色变种的地方，以绿色边缘和斑块的形式出现了含有约 14.93 wt% 总稀土元素的沉积硅铁矿。例如，强烈的化学风化诱导碳酸盐氟磷灰石的晶格优先向溶液中释放 Ca2+ 和 CO32- 离子物种，导致形成碳酸盐贫化层，其中 REE，特别是重 REE 并入优先 Ca2+改变的磷灰石晶格内的位点，导致硅铁矿形成。