Abstract Despite Nb and Ta being considered as ‘geochemical twins’, most Ta mineral deposits are associated with muscovite-bearing peraluminous granites and related pegmatites (MPG), whereas significant Nb deposits are related to peralkaline to metaluminous A1-type granites and syenites as well as nepheline syenites. Metaluminous to peraluminous A2-type granites and syenites have a lower potential for rare-metal mineralization. The Nb/Ta ratios are highly variable within each type of rare-metal-enriched felsic igneous suite and causes of the Nb-Ta geochemical fractionation remain poorly understood. Our compilation of whole-rock geochemical data indicates that Nb/Ta ratios generally anticorrelate with Ta, and at given Ta contents the Nb/Ta ratios increase from MPG through A2-type to A1-type igneous suites. However, the Nb-Ta compositions of A1-type and silica-undersaturated felsic magmatic rocks are indistinguishable. New regression models to estimate the mineral/melt partition coefficients DNb and DTa indicate that fractional crystallization of biotite and ilmenite significantly decreases Nb/Ta, but an extreme degree of fractional crystallization is needed to explain the observed range of Nb-Ta variations in rare-metal-enriched igneous suites. Importantly, metasomatic rocks formed by the alteration of igneous suites and their country-rocks are commonly highly enriched in Ta and Nb. Their low Nb/Ta ratios suggest that magmatic-hydrothermal processes involving fluids and hydrosaline melts also play a critical role in Nb-Ta geochemical fractionation and HFSE enrichment in general. We show that the Nb-Ta compositions of the primitive end-members of rare-metal-enriched felsic igneous suites fingerprint the nature and evolution of their sources including metasomatic processes in the Earth's crust and upper mantle.

中文翻译：

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从 Nb-Ta 地球化学分馏看长英质火成岩的岩浆和岩浆-热液演化，对稀有金属成矿成因的意义

摘要 尽管 Nb 和 Ta 被认为是“地球化学双胞胎”，但大多数 Ta 矿床都与含白云母的过铝质花岗岩和相关伟晶岩 (MPG) 相关，而重要的 Nb 矿床也与过碱性至金属铝质 A1 型花岗岩和正长岩有关。作为霞石正长岩。金属铝至过铝 A2 型花岗岩和正长岩的稀有金属矿化潜力较低。在每种富含稀有金属的长英质火成岩岩套中，Nb/Ta 比率变化很大，而且 Nb-Ta 地球化学分馏的原因仍然知之甚少。我们对全岩地球化学数据的汇编表明，Nb/Ta 比值通常与 Ta 反相关，并且在给定的 Ta 含量下，Nb/Ta 比值从 MPG 增加到 A2 型到 A1 型火成岩套件。然而，A1 型和二氧化硅不饱和长英质岩浆岩的 Nb-Ta 成分无法区分。用于估计矿物/熔体分配系数 DNb 和 DTa 的新回归模型表明，黑云母和钛铁矿的分级结晶显着降低了 Nb/Ta，但需要极端程度的分级结晶来解释所观察到的稀有 Nb-Ta 变化范围。富含金属的火成岩套房。重要的是，由火成岩套及其围岩蚀变形成的交代岩通常富含 Ta 和 Nb。它们的低 Nb/Ta 比率表明，涉及流体和盐碱熔体的岩浆-热液过程通常在 Nb-Ta 地球化学分馏和 HFSE 富集中也起着关键作用。