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 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.
Highlights
Emeralds are a green variety of beryl (Be3 Al2 Si6 O18 ) and have been prized gems since antiquity [1]
The first comprehensive solution trace element abundance dataset for emeralds, spanning all major deposit type: type IA (Brazil, Madagascar, South Africa, Zambia), IB (Australia), IC (Nigeria), IIA (Austria), IIB (Colombia), IIC (United States of America) and IID
While the pure emeralds major element data are good approximations of compositions, these analyses are likely to be less accurate than electron microprobe analyses of emeralds, especially for SiO2, which is calculated by difference, combined with the calculation of H2 O in the mineral structure [10], meaning that we use these data as advisory values only, to filter samples for the trace element geochemical compositions
Summary
Emeralds are a green variety of beryl (Be3 Al2 Si6 O18 ) and have been prized gems since antiquity [1]. Beryl is a cyclosilicate composed of hexagonal rings (Si6 O18 ) 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]. 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
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