Alkaline igneous complexes are often rich in rare earth elements (REE) and other metals essential for modern technologies. Although a variety of magmatic and hydrothermal processes explain the occurrence of individual deposits, one common feature identified in almost all studies, is a REE-enriched parental melt sourced from the lithospheric mantle. Fundamental questions remain about the origin and importance of the mantle source in the genesis of REE-rich magmas. In particular, it is often unclear whether localized enrichments within an alkaline province reflect heterogeneity in the mantle source lithology (caused by prior subduction or plume activity) or variations in the degree of partial melting and differentiation of a largely homogeneous source. Sulfur isotopes offer a means of testing these hypotheses because they are unaffected by high temperature partial melting processes and can fingerprint different mantle sources. Although one must be careful to rule out subsequent isotope fractionation during magma ascent, degassing and crustal interactions.

Here, we present new S concentration and isotope (${\delta }^{34}\mathrm{S}$) measurements, as well as a compilation of major and trace element data, for a suite of alkaline magmatic units and crustal lithologies from the Mesoproterozoic Gardar Province. Samples span all phases of Gardar magmatism (1330–1140 Ma) and include regional dykes, rift lavas and the alkaline complexes Motzfeldt and Ilímaussaq, which represent two of Europe's largest REE deposits. We show that the vast majority of our 115 samples have S contents >100 ppm and ${\delta }^{34}\mathrm{S}$ of −1 to 5‰. Only 8 samples (with low S contents, <100 ppm) show evidence for crustal interactions, implying that the vast majority of Gardar melts preserve the S isotopic signature of their magma source. Importantly, samples from across the Gardar Province have ${\delta }^{34}\mathrm{S}$ above the canonical mantle range (≤−1.4‰) and therefore require recycled surface S in their mantle source. Elevated ${\delta }^{34}\mathrm{S}$ values are explained by a period of Andean-style subduction and mantle metasomatism which took place ∼500 Ma before rift onset and are also supported by trace elements signatures (e.g. Ba/La) which match modern subduction zones.

Comparing the various generations of Gardar magmas, we find that ${\delta }^{34}\mathrm{S}$ values, large ion lithophile elements (K, Ba, P) and selective incompatible elements (Nb and light REE) are particularly enriched in the Late Gardar dykes, alkaline complexes and clusters of silica-undersaturated dykes spatially associated with the alkaline complexes. These data indicate that subduction-related metasomatism of the Gardar mantle was spatially heterogeneous, and that alkaline complexes are sourced from localized mantle domains highly enriched in ³⁴S, REE, alkalis and volatiles (particularly, F). Since alkalis and volatiles play an essential role in driving extreme differentiation of alkaline melts and fluids, we suggest the co-location of these species plus incompatible metals at high concentrations in the lithospheric mantle is a critical first-step in the genesis of a world-class alkaline REE deposit. S isotopes are powerful tools for identifying enriched mantle domains and the sources of mineralized alkaline igneous bodies.

碱性火成岩复合物通常富含稀土元素 (REE) 和其他现代技术必不可少的金属。尽管各种岩浆和热液过程可以解释单个矿床的发生，但几乎所有研究中都确定的一个共同特征是来自岩石圈地幔的富含稀土元素的母体熔体。地幔源在富含稀土元素的岩浆成因中的起源和重要性仍然存在基本问题。特别是，通常不清楚碱性区域内的局部富集是否反映了地幔源岩性的异质性（由先前的俯冲或羽流活动引起）或大部分均质源的部分熔融和分化程度的变化。硫同位素提供了一种检验这些假设的方法，因为它们不受高温部分熔融过程的影响，并且可以指纹不同的地幔来源。尽管在岩浆上升、脱气和地壳相互作用过程中必须小心排除随后的同位素分馏。

在这里，我们提出了新的 S 浓度和同位素（${\delta }^{34}\mathrm{秒}$) 测量，以及主要和微量元素数据的汇编，用于中元古代加达尔省的一组碱性岩浆单元和地壳岩性。样本跨越 Gardar 岩浆活动的所有阶段（1330-1140 Ma），包括区域岩脉、裂谷熔岩和碱性复合体 Motzfeldt 和 Ilímaussaq，它们代表了欧洲最大的两个稀土矿床。我们表明，我们的 115 个样品中绝大多数的 S 含量 >100 ppm，并且${\delta }^{34}\mathrm{秒}$-1 到 5‰。只有 8 个样品（低 S 含量，<100 ppm）显示地壳相互作用的证据，这意味着绝大多数 Gardar 熔体保留了其岩浆源的 S 同位素特征。重要的是，来自 Gardar 省各地的样本有${\delta }^{34}\mathrm{秒}$高于标准地幔范围 (≤-1.4‰)，因此需要在地幔源中回收表面 S。高架${\delta }^{34}\mathrm{秒}$ 值是由一段安第斯式俯冲和地幔交代作用来解释的，这发生在裂谷开始前约 500 Ma，并且还得到与现代俯冲带相匹配的微量元素特征（例如 Ba/La）的支持。

比较各代 Gardar 岩浆，我们发现 ${\delta }^{34}\mathrm{秒}$值，大离子亲石元素（K、Ba、P）和选择性不相容元素（Nb 和轻稀土）在晚加尔达岩脉、碱性复合物和与碱性复合物空间相关的二氧化硅不饱和岩脉簇中特别富集。这些数据表明，Gardar 地幔与俯冲相关的交代作用在空间上是异质的，碱性复合物来自局部地幔域，富含³⁴S、REE、碱和挥发物（特别是 F）。由于碱金属和挥发物在推动碱性熔体和流体的极端分化方面起着至关重要的作用，我们认为这些物种与岩石圈地幔中高浓度不相容金属的共存是世界形成的关键第一步——类碱性稀土矿床。S同位素是识别富集地幔域和矿化碱性火成体来源的有力工具。