Abstract
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 (δ34S) 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 δ34S 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 δ34S above the canonical mantle range (≤−1.4‰) and therefore require recycled surface S in their mantle source. Elevated δ34S 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 δ34S 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.
Highlights
Alkaline igneous rocks are important natural sources of rare earth elements (REE) and high field-strength elements (HFSE) (Kogarko, 1990)
We have demonstrated that mineralized alkaline complexes originate from mantle sources enriched in REE, HFSE, alkalis and volatiles
We suggest the co-location of REE, HFSE, alkalis and volatiles in high concentrations in the mantle source is critical to the formation of these mineralized alkaline complexes; these elements are incompatible during mantle melting (Schilling et al, 1980), and during transport through the crust, volatiles and alkalis play a critical role in amplifying REE and HFSE contents of the residual melt/fluid
Summary
Alkaline igneous rocks are important natural sources of rare earth elements (REE) and high field-strength elements (HFSE) (Kogarko, 1990). Notable are Motzfeldt and Ilímaussaq which are among the world’s largest alkaline ore deposits (Marks and Markl, 2015; Finch et al, 2019). The primitive melts that fed these complexes have been linked to an enriched lithospheric mantle source, but it is unclear how heterogeneous this mantle was and whether enrichment originated from prior subduction (Bartels et al, 2015) or plume processes (Halama et al, 2003). A greater understanding of Gardar mantle sources is critical to elucidating the origins of Motzfeldt and Ilímaussaq (and large alkaline ore deposits generally) because it is unclear whether their REE and HFSE enrichment reflects similar magmatic processes (i.e. degree of partial melting and fractionation) or similar mantle sources
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