Rare Earth Element and Incompatible Trace Element Abundances in Emeralds Reveal Their Formation Environments

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.

In Situ U–Th–Pb Dating of Parisite: Implication for the Age of Mineralization of Colombian Emeralds

“…Emerald, parisite and calcite formed at the same time. In contrast to parisite, as a pure REE-F-carbonate, emeralds (mean data from [26]) and the wall rock (this study) show relatively low REE concentrations. However, parisite reflects the negative Eu anomaly of the wall rock (Figure 3).…”

“…The two large emerald districts in Colombia, especially that of Muzo, are famous for their high-quality crystals, and for their unique genesis. In contrast to other great emerald deposits worldwide, such as in Brazil [26], which are due to metasomatic interaction between acid magmatism and basic-ultrabasic rocks, the Colombian ones are formed by low-temperature hydrothermal brines that provide the necessary elements from the wall rocks that are of sedimentary origin. Flat crustal normalized REE and incompatible trace element patterns of emeralds [26], as well as the REE leaching of the wall rock (this study), support the theory of a non-magmatic controlled environment to form parisite.…”

“…Alonso-Perez et al [26] compare the rare-earth element and incompatible trace element abundances of emeralds that have formed in different environments. They confirmed that the emerald deposits in Colombia or "type IIb emeralds" are generated in a sedimentary environment characterized by flat, crustal-normalized, REE patterns very similar to shales and loess (Figure 3).…”

“…Figure 3 shows upper crust normalized REE pattern of the parisite compared to emeralds [26], calcite and the wall rock composition 1m and 1cm away from the veins (Table 3). Emerald, parisite and calcite formed at the same time.…”