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

Hutchison, W., Finch, A. A., Borst, A. M., Marks, M. A.W., Upton, B. G.J., Zerkle, A. L., Stüeken, E. E. and Boyce, A. (2021) Mantle sources and magma evolution in Europe's largest rare earth element belt (Gardar Province, SW Greenland): New insights from sulfur isotopes. Earth and Planetary Science Letters, 568, 117034. (doi: 10.1016/j.epsl.2021.117034)

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 ( ) 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 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 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 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 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.

• Item Type: Articles
• Additional Information: This work is a contribution to the HiTech AlkCarb project and was funded by the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 689909. W.H. also acknowledges support from a UKRI Future Leaders Fellow-ship (MR/S033505/1). A.J.B. is funded by the NERC National En-vironment Isotope Facility award (NE/S011587/1) and the Scottish Universities Environmental Research Centre.
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Boyce, Professor Adrian

Creator Roles

Boyce, A.: Formal analysis, Methodology, Writing – review and editing

• Authors: Hutchison, W., Finch, A. A., Borst, A. M., Marks, M. A.W., Upton, B. G.J., Zerkle, A. L., Stüeken, E. E., and Boyce, A.
• College/School: College of Science and Engineering > Scottish Universities Environmental Research Centre
• Journal Name: Earth and Planetary Science Letters
• Publisher: Elsevier
• ISSN: 0012-821X
• ISSN (Online): 1385-013X
• Published Online: 17 June 2021
• Copyright Holders: Copyright © 2021 The Authors
• First Published: First published in Earth and Planetary Science Letters 568:117034
• Publisher Policy: Reproduced under a Creative Commons License