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

Minerals ◽  
2021 ◽  
Vol 11 (5) ◽  
pp. 513
Author(s):  
Raquel Alonso-Perez ◽  
James M. D. Day

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.

Minerals ◽  
2019 ◽  
Vol 9 (2) ◽  
pp. 93
Author(s):  
Avhatakali Raphalalani ◽  
Georges-Ivo Ekosse ◽  
John Odiyo ◽  
Jason Ogola ◽  
Nenita Bukalo

The aim of the present study was the paleoenvironmental reconstruction of the prevailing environment under which the Lwamondo and Zebediela kaolin deposits were formed. Hence, this study reports deuterium and oxygen stable isotope values and trace and rare earth element concentrations for two samples of kaolin. Upper continental crust-normalised trace-element patterns reveal that large ion lithophile elements and high-field-strength elements are generally depleted in Lwamondo and Zebediela kaolins, whereas transition trace elements are generally enriched in these kaolins. Upper continental crust-normalised rare earth element (REE) patterns show that there is a slight enrichment of heavy REEs (HREEs) compared to light REEs (LREEs) in these kaolins. The δ18O and δD stable isotope values for kaolinite from Lwamondo ranged from 17.4‰ to 19.1‰ and from −54‰ to 84‰, respectively, whereas those values for kaolinite from Zebediela varied from 15.6‰ to 17.7‰ and from −61‰ to –68‰ for δ18O and δD, respectively. The REE patterns and the content of other trace elements indicate ongoing kaolinitisation in the Lwamondo and Zebediela kaolins with minimum mineral sorting. The sources of the kaolins varied from basic to acidic and these were derived from an active margin tectonic setting. Lwamondo kaolin was deposited in an oxic environment whereas Zebediela kaolin was deposited under suboxic/anoxic conditions. Based on the δ18O and δD values of the kaolinite, they formed in a supergene environment at temperatures generally below 40 °C.


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