The Influence of crustal contamination on REE-enrichment in carbonatites of the Kalkfeld group (Namibia)

碳酸盐岩中稀土元素的高含量通常是由地球化学富集的地幔源的低程度熔融、晶体分馏、碳酸盐-硅酸盐熔体不混溶、熔体-盐水不混溶和热液蚀变等因素共同作用的结果。然而，地壳污染通常被认为在碳酸盐岩岩浆活动中起不了重要作用。然而，最近申请者的一项研究表明，碳酸盐岩岩浆与硅酸盐围岩的相互作用可以通过涉及Si的耦合取代机制导致磷灰石中强烈的REE富集。在凯泽斯图尔证明了这一点之后，申请人认为这种过程在碳酸盐岩系统中可能具有普遍的重要性。本项目将研究碳酸盐岩-围岩相互作用对碳酸盐岩稀土富集的影响。该研究将使用达马拉兰省（纳米比亚）Kalkfeld组的野外样本进行，因为这些碳酸盐岩显示出不同的稀土富集和稀土矿物学，并含有不同数量的硅酸盐矿物（角闪洞、斜辉石、石英、长石），可能表明地壳污染。为了验证这一假设，将收集不同碳酸盐的结构、矿物化学和地质年代数据（显微镜、EMPA、LA-ICP-MS）以及流体包裹体数据（显微测温、拉曼光谱）并相互比较。预期结果将允许(1)更详细地表征碳酸盐岩的岩浆-热液演化；(2)重建稀土矿化的成因，包括围岩相互作用和热液叠印的潜在影响。

High concentrations of REE in carbonatites are generally explained by a combination of factors, including low-degree melting of geochemically enriched mantle sources, crystal fractionation, carbonate-silicate melt immiscibility, melt-aqueous brine immiscibility and hydrothermal alteration. Crustal contamination, however, is a process that is typically not considered to play an important role during carbonatite magmatism. Nevertheless, a recent study of the applicants demonstrated that the interaction of carbonatite magma with silicate wall rocks can cause strong REE enrichment in apatite via a coupled substitution mechanism involving Si. Having shown this for the Kaiserstuhl, the applicants suggest that such processes may be of general importance in carbonatitic systems.This project will investigate the impact of carbonatite-wall rock interaction on REE enrichment in carbonatites. The study will be performed using field examples from the Kalkfeld group in the Damaraland Province (Namibia), because these carbonatites show variable REE enrichment and REE mineralogy and contain variable amounts of silicate minerals (amphibole, clinopyroxene, quartz, feldspar) that may indicate crustal contamination.To test this hypothesis, textural, mineral chemical and geochronological data (microscopy, EMPA, LA-ICP-MS) as well as fluid inclusion data (microthermometry, Raman spectroscopy) for the different carbonatites will be gathered and compared with each other. The expected results will allow for (1) characterizing the magmatic to hydrothermal evolution of the carbonatites in great detail and (2) reconstructing the genesis of the REE mineralizations including the potential influence of wall-rock interaction and hydrothermal overprint.