洋壳物质再循环与稀土成矿作用：多元同位素体系制约

Carbonatite-related rare-earth element (REE) deposits are the most important REE resources in the world. However, the metallogenesis of these REE deposits is still not clear. A hypothesis proposed recently is that the recycling of marine sediments enriched into the source is a key factor for the formation of the carbonatite-related giant rare-earth-element deposits. This is a rather important advance but the hypothesis still needs to be tested. Here we plan to trace the source of late mesozoic carbonatite and related alkaline and mafic rocks in a“profile”of North China Craton from the edge to the inner, utilizing Sr-Nd-Pb-Hf-Mg-Ca-Zn isotopes, to understand the variation of recycling materials added to the mantle across the “profile”, and then to compare the difference in volumn and component of recycling crust meterials that added to the mantle between REE mineralized carbonatite and non-mineralized carbonatite. Typical REE mineralized and non-mineralized carbonatites from the world and China will be additionally studied on their nature of sources using Sr-Nd-Pb-Hf-Mg-Ca-Zn isotopes, to further test the relationship between the REE mineralization and addition of recycling oceanic materials to the source. Finally, the source of Bayan Obo deposit will be further constrained through 1) additional Sr-Nd-Pb-Hf-Mg-Ca-Zn isotope studies and 2) comparison of its source characters with those of contemporary mafic rocks and carbonatite occurred in region from north to southeast in North China Craton, testing the “source-controlled-REE-mineralization”hypothesis.

与碳酸岩相关的稀土矿床是世界上最重要的稀土来源，然而其稀土成矿的控制因素仍不清楚。源区再循环洋壳物质加入被认为可能是碳酸岩稀土巨量富集的关键控制因素。但这一认识还有待进一步检验。本项目拟选取华北板块陆缘-陆内的晚中生代碳酸岩“剖面”，综合运用Sr-Nd-Pb-Hf-Mg-Ca-Zn同位素体系进行源区示踪，查明板块不同位置地幔源区再循环物质加入的空间变化规律，对比成矿与不成矿碳酸岩源区再循环物质加入成分和比例的差别。在此基础上，补充开展国内外其他代表性成矿/不成矿碳酸岩的同位素研究、示踪源区性质，进一步查明源区再循环物质加入与碳酸岩成矿关系。最后，示踪并对比白云鄂博矿床与板块不同位置的相关基性岩及碳酸岩的源区性质，进一步制约白云鄂博矿床源区再循环物质加入情况，检验白云鄂博矿床是否符合上述成矿认识。

为检验火成碳酸岩的源区是否有再循环的沉积碳酸盐，开展了：（1）Ca同位素分析测试方法研究；（2）碳酸岩岩浆演化过程Ca、Fe同位素地球化学行为研究；（3）包括华北地区在内的全球代表性火成碳酸岩源区示踪研究。研究成果包括：（1）查明碳酸岩-碱性岩岩浆流体出溶过程Fe同位素分馏行为，为应用Fe同位素示踪碳酸岩源区性质奠定基础；（2）发现Ca同位素MC-ICPMS测试的基质效应“陷阱”，改进和完善了Ca同位素的MC-ICPMS质谱高精度测试方法；（3）精确测定了火成碳酸岩Ca同位素组成的基本特征，并论证了碳酸质岩浆演化过程中Ca同位素的地球化学行为，从Ca元素的角度揭示了碳酸盐组分来源于地幔物质本身，而无需再循环沉积碳酸盐加入。研究成果推翻了前人在《Science Advance》发表的相关数据结果和认识，为Ca同位素在火成碳酸岩研究中的应用奠定了坚实基础。成果于2021年发表后成为GPL期刊最受关注的文章之一（年度点击阅读量排前3）并被Jochen Hoefs教授编入2021年出版的经典教材《Stable Isotope Geochemistry(第九版)》。