minerals Article Rare Earth Element and Incompatible Trace Element Abundances in Emeralds Reveal Their Formation Environments Raquel Alonso-Perez 1, * and James M. D. Day 2   Citation: Alonso-Perez, R.; Day, J.M.D. Rare Earth Element and Incompatible Trace Element Abundances in Emeralds Reveal Their Formation Environments. Minerals 2021, 11, 513. https:// doi.org/10.3390/min11050513 Academic Editors: Paul Sylvester, Alexander R. Cruden, Sytle M. Antao, Huifang Xu, Nigel J. Cook, Theodore J. Bornhorst, Hanumantha Rao Kota and Anna H. Kaksonen Received: 29 April 2021 Accepted: 11 May 2021 Published: 13 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA 2 Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093, USA; jmdday@ucsd.edu * Correspondence: ralonso@fas.harvard.edu; Tel.: +1-617-301-2175 Abstract: Emeralds require the unusual association of typically compatible elements (Cr, V), with incompatible Be to form, and occur in complex tectonic settings associated with sediments (type IIB; Colombia) or, more commonly, with magmatism and regional metamorphism (IA). Precise rare earth element (REE) and incompatible trace element abundances are reported for a global suite of emeralds, enabling the identification of the environments in which they formed. Type IIB emeralds have nearly flat continental crust normalized REE patterns (La/Yb CC = ~2), consistent with a sedimentary source origin. Type IA emerald REE patterns have upturns in the heavy REE (La/Yb CC = ~0.3), a feature also shared with South African emeralds occurring in Archaean host rocks. Modeling of type IA emerald compositions indicates that they form from magmatic fluids of sedimentary (S)-type granite melts interacting with Cr, V-rich mafic–ultramafic crustal protoliths. This geochemical signature links emerald formation with continental suture zones. Diamonds, rubies, and sapphires have been considered as ‘plate tectonic gemstones’ based on mineral inclusions within them, or associations with plate tectonic indicators. Emeralds are distinct plate tectonic gemstones, recording geochemical evidence for origin within their mineral structure, and indicating that plate tectonic processes have led to emerald deposit formation since at least the Archaean. Keywords: emeralds; gemstones; trace elements; rare earth elements; plate tectonics; sediments; s-type granite 1. Introduction Emeralds are a green variety of beryl (Be 3 Al 2 Si 6 O 18 ) and have been prized gems since antiquity [1]. Beryl is a cyclosilicate composed of hexagonal rings (Si 6 O 18 ) connected by Be atoms on tetrahedral sites, and Al atoms on octahedral sites [1]. It is substitution on the octahedral site, primarily by Cr and/or V, that is responsible for the vivid green color of emerald [2]. Emeralds are scarce commodities requiring the interaction of fluids from aluminous quartzo-feldspathic Be-rich sources with Cr–V-Fe-rich mafic–ultramafic igneous/metamorphic (tectonic–magmatic–metamorphic-related), or Cr–V-rich sedimen- tary crustal protoliths. Despite the limiting formation requirements, there are nearly fifty recognized emerald deposits globally, occurring across North and South America, Europe, Asia, Africa and Australasia, and they range in inferred formation age from the Archaean (~3 Ga) to the Cenozoic (~9 Ma) (Figure 1)[1,3]. Deposits containing emeralds are typically described individually in the literature and emphasize significant complexity in the geological setting (e.g., [4–11] and Supplementary Information (SI) for a detail list of bibliography). Attempts to classify emeralds in a coherent manner, and to enable use of this mineral variety more widely to understand geological processes have linked their formation to broadly magmatic (tectonic–magmatic- related), metamorphic (tectonic–metamorphic-related), and sedimentary processes [10] (Figure 1). These studies have demonstrated variability in the major element chemical characteristics of emeralds, especially V, Cr and Fe contents, with limited quantitative data Minerals 2021, 11, 513. https://doi.org/10.3390/min11050513 https://www.mdpi.com/journal/minerals