The Unique Nature of Argyle Fancy Diamonds: Internal Structure, Paragenesis, and Reasons for Color

Abstract

The Argyle lamproite mine is world famous for fancy colored diamonds, ranging from cognac and champagne shades of brown through Cape yellows to rare pink to intense red and even rarer blue stones. The majority of diamonds are type IaAB, displaying platelet degradation and having plastic and brittle deformation, mainly after diamond formation, indicative of high pressure/temperature conditions. The deformation produced optical defects, causing different Argyle diamond colors.White and brown Argyle diamonds belong to both eclogitic and peridotitic parageneses, but yellow-and pinkcolored ones studied here are eclogitic. Nitrogen contents, N aggregation states, and formation temperatures of yellow diamonds are correlated with their internal structure. The strongly deformed and internally brecciated yellow diamonds have low to moderate contents of highly aggregated N and high temperatures of formation. The undeformed yellow diamonds are richer in moderately aggregated N, and their formation temperatures are lower. The intensity of N3, H3, and H4 bands of the photoluminescence spectra of these diamonds is higher in the more deformed crystals.The pink eclogitic diamonds contain low to moderate N, are highly aggregated, and have high temperatures of formation, but differ from each other in the deformation level recorded by their internal structures. All identified photoluminescence peaks in these pink diamonds are higher in intensity in the strongly deformed crystals.Geothermometry, based on N contents, aggregation state, age, and temperature relationships of the diamonds, shows that most eclogitic diamonds resided at 1,250° to 1,300°C near the base of the lithospheric mantle, slightly deeper than the peridotitic diamonds. One eclogitic pink diamond contains a two-phase garnetomphacite inclusion reequilibrated from precursor majorite with composition indicative of formation pressure of 9.5 to 10 GPa, equivalent to ~300-km depth, the deepest identified for Argyle diamond formation to date. Internal structures of eclogitic diamonds reflect evidence of their formation under stress in a subduction zone setting.