Dynamics of diamond growth: evidence from isotope and FTIR trends

The inhomogeneous nature of many diamonds from kimberlites reflects evolution of their growth conditions. Six peridotitic and 8 eclogitic Yakutian macrodiamonds displaying changing internal growth morphology have been studied for carbon (C) and nitrogen (N) isotopes and N content and aggregation. Using cathodoluminescence (CL) images of central polished sections of diamonds as maps, C and N isotopes and N content were measured by secondary ion mass spectrometry (SIMS) in traverses from core to rim of the crystals. N concentration and aggregation were also determined in the same traverses by Fourier transform infrared spectroscopy (FTIR). In the studied diamonds the N content and degree of aggregation decrease from the core to the rim zones. In general, both SIMS and FTIR N content measurements are in agreement, showing similar trends for the same diamonds. The SIMS results are more variable because of their higher resolution of sampling (a few microns from the surface of the single growth zones), while FTIR analyses represent averages of a few diamond zones (Harte et al., 1999; Hauri et al., 1999, Bulanova et al., 2002). All peridotitic diamonds measured have heavy C isotope composition (-8.9 to -2 per mil). No common overall isotopic trend between the stones was found. The total internal variation in C isotope compositions within studied individual peridotitic diamonds was only 4 %o, consistent with levels of C isotope fractionation expected from simple fractionation of a fluid phase in the mantle (Deines, 1980). Typically, peridotitic diamonds show very little internal variation in δC. Significant internal isotopic variations are observed in three peridotitic diamonds with complex internal structures. Within these diamonds, δC increases and δN decreases from the core to the rim growth zones. All 8 eclogitic diamonds studied also have heavy carbon isotope compositions (-8.8 to +1.7 per mil). Eclogitic diamonds show larger magnitude internal isotopic variations compared to peridotitic diamonds. For instance, δC varies by 4 to 6 %o within individual crystals. A smooth decrease in δC from the core to the rim zones was identified in eclogitic diamonds showing simple, concentric growth banding in the octahedral form. However, variation does exist. One eclogitic diamond from the Mir pipe displayed variable δC in the core zone, accompanied by variable N content and N isotopic composition. The rim zone of the same diamond was very homogenous in all these parameters. The range of N isotope concentration in the core of all studied diamonds is wide (-8.8 to +9.5 %o), but rim values vary only from – 6.7 to 0.37 %o. The peridotitic diamonds show a different scale of decrease of δN from their cores to rims, while eclogitic diamonds display both decreases and increases in δN A wider range of isotopic values in the cores may reflect either varying source, or, the likelihood that conditions of growth during diamond nucleation were far from equilibrium compared to conditions prevailing during the latter stages of diamond growth, reflected by the narrower compositions of the rims. The trends evident in C and N isotopic composition within single diamonds enable interpretation of the type of processes responsible for diamond formation. Based on the results of thermodynamic models (Deines, 1980; Deines et al., 1989) our data are consistent with peridotitic diamonds forming by precipitation from CO2-rich fluid. In the frame of the same model the eclogitic diamonds were more likely grown from CH4-rich fluids that underwent isotopic fractionation.