Mantle sources and magma evolution in Europe's largest rare earth element belt (Gardar Province, SW Greenland):new insights from sulfur isotopes

• Published in: Earth and Planetary Science Letters
• Main Authors: Hutchison, William, Finch, Adrian A., Borst, Anouk M., Marks, Michael A.W., Upton, Brian G.J., Zerkle, Aubrey L., Stüeken, Eva E., Boyce, Adrian J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2021
• Subjects: Rift; Magmatism; Sulfur; Geochemistry; REE; Volatiles; Greenland
• Online Access: https://risweb.st-andrews.ac.uk/portal/en/researchoutput/mantle-sources-and-magma-evolution-in-europes-largest-rare-earth-element-belt-gardar-province-sw-greenland(5ec9cae1-51e7-439e-bca6-7b9b898befdf).html; https://doi.org/10.1016/j.epsl.2021.117034; https://research-repository.st-andrews.ac.uk/bitstream/10023/23635/1/Hutchison_2021_EPSL_Mantle_sources_CC.pdf; https://www.sciencedirect.com/science/article/pii/S0012821X21002892?via%3Dihub#se0150

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 (δ 34 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 δ 34 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 δ 34 S have above the canonical mantle range (≤−1.4‰) and therefore require recycled surface S in their mantle source. Elevated values are explained by a period of Andean-style subduction ...