Abstract Gems & Gemology, Spring 2013, Vol. 49, No. 1

Aviat Diamonds: A Window into the Deep Lithospheric Mantle Beneath the Northern Churchill Province, Melville Peninsula, Canada

The Aviat kimberlites are located in Melville Peninsula in northeastern Nunavut. They are part of the Rae craton, which is composed of Archean granitoid rocks, supracrustal belts, and Paleoproterozoic metasedimentray rocks. The carbon isotopic composition, nitrogen concentration, and nitrogen aggregation state of 70 samples from these kimberlites were analyzed to help understand the composition of diamond-forming fluids, paragenesis of diamond source, mantle residence history, and thermal history of the subcratonic lithospheric mantle. The colors of samples were colorless, gray, brown, and green.

FTIR analysis revealed that the majority were type IaAB, including IaA and IaB. Total nitrogen content ranged from 13 to 1467 ppma. The Aviat diamonds were relatively rich in hydrogen. Absorption bands at 3107, 2786, and 1404 cm–1, caused by bending and stretching of the vinylidene group (C=CH2), were observed in most samples. An additional hydrogen-related band at 3237 cm–1 was also detected.

SIMS analysis revealed nitrogen content ranging from 1 to 1724 ppma. The δ13C isotopic analysis suggested that these diamonds were precipitated mainly from carbonate-bearing fluids. Large variations in nitrogen content and δ13C values, though they were not correlated, were observed in growth sectors, suggesting multiple events of fluid derivation from different sources, mainly ecologitic and also probably peridotitic. But the relationship between CL and carbon isotopic composition suggested that non-luminescent diamond may have been formed during one episode of growth. The Aviat diamonds were possibly formed from diamond-forming fluids or melts from several orogenic events, such as transportation of graphitized organic matter and marine carbonates into the deep mantle via subducting oceanic slabs.

Catastrophic platelet degradation was suggested by the positive correlation between platelets and B aggregates of nitrogen. This phenomenon was probably caused by transient heating events or deformation or both during mantle residence. Blue luminescence of nitrogen-rich samples may be related to intense degradation of platelets. The residence temperature of most samples was extrapolated to ~1050–1150°C, suggestive of a diamond source near the top of diamond stability field at ~150–170 km.

Abstracted by Kyaw Soe Moe