U-Th/He systematics of fluid-rich 'fibrous' diamonds – evidence for pre- and syn-kimberlite eruption ages

The physical characteristics and impermeability of diamonds allow them to retain radiogenic ⁴ He produced in-situ from radioactive decay of U, Th and Sm. This study investigates the U-Th/He systematics of fibrous diamonds and provides a first step in quantification of the uncertainties associated with determining the in-situ produced radiogenic ⁴ He concentration. Factors determining the total amount of measured helium in a diamond are the initial trapped ⁴ He, the in-situ produced radiogenic ⁴ He, α-implantation, α-ejection, diffusion, and cosmogenic ³ He production. Alpha implantation is negligible, and diffusion is slow, but the cosmogenic ³ He component can be significant for alluvial diamonds as the recovery depth is unknown. Therefore, samples were grouped based on similar major and trace element compositions to determine possible genetically related samples. A correlation between the ⁴ He and U-Th concentrations approximates the initial ⁴ He concentration at the axis-intersect and age as the slope. In this study, the corrections were applied to eight fibrous cubic diamonds from the Democratic Republic of the Congo and two diamonds from the Jwaneng kimberlite in Botswana. A correlation exists between the ⁴ He and U-Th concentrations of the group ZRC2, 3, and 6, and of the group CNG2, 3, and 4 and both correlations deviate significantly from a 71 Ma kimberlite eruption isochron. The U-Th/He dating method appears a promising new approach to date metasomatic fluid events that result in fibrous diamond formation and this is the first evidence that some fibrous diamonds can be formed 10s to 100s Myr before the kimberlite eruption.