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 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; [email protected] * Correspondence: [email protected]; 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/YbCC = ~2), consistent with a sedimentary source origin. Type IA emerald REE patterns have upturns in the heavy REE (La/YbCC = ~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 Citation: Alonso-Perez, R.; Day, links emerald formation with continental suture zones. Diamonds, rubies, and sapphires have been J.M.D. Rare Earth Element and considered as ‘plate tectonic gemstones’ based on mineral inclusions within them, or associations Incompatible Trace Element Abundances in Emeralds Reveal with plate tectonic indicators. Emeralds are distinct plate tectonic gemstones, recording geochemical Their Formation Environments. evidence for origin within their mineral structure, and indicating that plate tectonic processes have Minerals 2021, 11, 513. https:// led to emerald deposit formation since at least the Archaean. doi.org/10.3390/min11050513 Keywords: emeralds; gemstones; trace elements; rare earth elements; plate tectonics; sediments; Academic Editors: Paul Sylvester, s-type granite Alexander R. Cruden, Sytle M. Antao, Huifang Xu, Nigel J. Cook, Theodore J. Bornhorst, Hanumantha Rao Kota and Anna H. Kaksonen 1. Introduction Emeralds are a green variety of beryl (Be Al Si O ) and have been prized gems Received: 29 April 2021 3 2 6 18 since antiquity [1]. Beryl is a cyclosilicate composed of hexagonal rings (Si O ) connected Accepted: 11 May 2021 6 18 Published: 13 May 2021 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 Publisher’s Note: MDPI stays neutral color of emerald [2]. Emeralds are scarce commodities requiring the interaction of fluids with regard to jurisdictional claims in from aluminous quartzo-feldspathic Be-rich sources with Cr–V-Fe-rich mafic–ultramafic published maps and institutional affil- igneous/metamorphic (tectonic–magmatic–metamorphic-related), or Cr–V-rich sedimen- iations. 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) (Figure1)[1,3]. Deposits containing emeralds are typically described individually in the literature and Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. emphasize significant complexity in the geological setting (e.g., [4–11] and Supplementary This article is an open access article Information (SI) for a detail list of bibliography). Attempts to classify emeralds in a distributed under the terms and coherent manner, and to enable use of this mineral variety more widely to understand conditions of the Creative Commons geological processes have linked their formation to broadly magmatic (tectonic–magmatic- Attribution (CC BY) license (https:// related), metamorphic (tectonic–metamorphic-related), and sedimentary processes [10] creativecommons.org/licenses/by/ (Figure1). These studies have demonstrated variability in the major element chemical 4.0/). 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 Minerals 2021, 11, x FOR PEER REVIEW 2 of 20 Minerals 2021, 11, 513 2 of 21 magmatic-related), metamorphic (tectonic–metamorphic-related), and sedimentary pro- cesses [10] (Figure 1). These studies have demonstrated variability in the major element chemical characteristics of emeralds, especially V, Cr and Fe contents, with limited quan- availabletitative data for av traceailable element for trace abundances element abundances [10,12,13 [10,12,13]]. While. While some some emerald emerald deposits de- are hosted inposits sedimentary are hosted in rocks sedimentary (including rocks Colombian (including Colombian emeralds, emeralds, Type IIB) Type or IIB) granitoids or gran- (types IB, IC, IIC),itoids the (types preponderance IB, IC, IIC), the (~80%) preponderance of worldwide (~80%) emeraldof worldwide deposits emerald are deposits related toare magmatically related to magmatically derived granitic fluids that have interacted with mafic–ultramafic derived granitic fluids that have interacted with mafic–ultramafic host rocks (types IA, IID). host rocks (types IA, IID). FigureFigure 1. 1. SummarySummary of total of total age distribution age distribution of emerald of emeraldhost rock deposits host rock classified deposits by type. classified Type by type. Type classifications (e.g., IC, IA, IB, IIA, IID, IIC and IIB) are based on host rock (abbreviations: M–UM = classificationsmafic–ultramafic; (e.g., Sed = IC, sedimentary; IA, IB, IIA, Meta IID, = metamorphic; IIC and IIB) Sed are– basedMig = sedimentary on host rock–migmatite) (abbreviations: M–UM = mafic–ultramafic;and environment of Sedformation = sedimentary; [10]. Compilation Meta of = metamorphic;ages are in Tables Sed–Mig 1 and S4; =relative sedimentary–migmatite) distribu- and environmenttion is shown (wider of formation regions of [ 10indicator]. Compilation bars indicate of agesa larger are population in Table1 of and deposits Table of S4; that relative age, distribution and times can be discontinuous). is shown (wider regions of indicator bars indicate a larger population of deposits of that age, and timesT canhe most be discontinuous). refined model for emerald genesis comes from studies of Colombian sedi- ment-hosted deposits, which although tectonically and temporally distinct [14], were formedThe by mostthe interaction refined of model evaporitic for brines emerald and black genesis shale comes[15,16]. The from thermal studies reduc- of Colombian sediment-hostedtion of sulphate, at 300 deposits,–330 °C by which brine, althoughin the presence tectonically of organic andmatter temporally in the black distinct shale [14], were formedled to albitization, by the interaction the formation of of evaporitic pyrite (FeS brines2), and andBe-bearing black mineralization shale [15,16]. with The the thermal reduc- tionformation of sulphate, of virtually at Fe 300–330-free, V and◦C Cr by-rich brine, emeralds. in the Despite presence their of sedimentary organic matter origin, in the black the Colombian emerald deposits are ultimately related to subduction and orogeny, with shale led to albitization, the formation of pyrite (FeS2), and Be-bearing mineralization withthe formation the formation of a sedimentary of virtually back Fe-free,-arc basin V andand Cr-richthe sourcing emeralds. of sediments Despite from their the sedimentary Jurassic to Cretaceous Andean arc [17]. In contrast, most of the world’s emeralds are re- origin,lated to magmatica the Colombianlly derived emerald fluids depositsthat have interacted are ultimately with mafic related–ultramafic to subduction host rocks and orogeny, with(types the IA, formationIID, [10], and of formation a sedimentary models for back-arc these types basin of emerald and the are sourcing generally related of sediments from theto individual Jurassic todeposits Cretaceous. For example, Andean whether arc [ 17there]. In is contrast,a link as to most how ofmagmatic, the world’s meta- emeralds are relatedmorphic toand magmatically sedimentary-hosted derived emeralds fluids are thatformed have is, hitherto interacted, unknown with. mafic–ultramafic host rocksTo (types examine IA, the IID, formation [10], and of emeralds, formation we modelspresent the for first these comprehensive types of emerald trace ele- are generally relatedment abundance to individual analyses deposits. of inclusion For-free example, emeralds whether from ten representative there is a link global as to loca- how magmatic, tions spanning 13 different economically viable deposits, covering the major classification metamorphic and sedimentary-hosted emeralds are formed is, hitherto, unknown. types of emerald [10] (Figure 1). Unlike previous studies using microbeam techniques [12,13,18]To examine, we present the solution formation trace element of emeralds, abundance we analyses present from the the first dissolution comprehensive of trace elementsignificant abundance quantities of emerald analyses (>10 of mg). inclusion-free This method emeraldshas the advantage from tenof enabling representative