The effect of inclusion-fluid fractionation on different isotopic systems used to date diamonds

Abstract

Diamond formation ages are crucial in understanding carbon mobility and diamond growth/destruction processes in the mantle linked to large tectono-magmatic events. Different dating methods have yielded an increasingly varied array of ages, creating discussion around what events the ages are recording. We report Sm-Nd, U-Pb, and Rb-Sr isotope systematics of lherzolitic garnet inclusions from Victor mine diamonds. Sm-Nd isochrons reflect diamond formation ages of 818 ± 251 and 624 ± 248 Ma, within uncertainty of a previously determined Re-Os isochron age from sulfide inclusions, potentially linking diamond growth to the Franklin Large Igneous Province (LIP) or rifting of Rodinia. The U-Pb (no systematic age relations) and Rb-Sr isotope systematics (182 ± 56 Ma) of the same garnet inclusions record significantly younger parent-daughter isotope fractionation events, within error of the kimberlite eruption age. Parent/daughter isotope fractionation models can reproduce these contrasting age results for single inclusions through garnet-fluid element fractionation at temperatures of 700-800 °C during kimberlite eruption. Fluid-mobile elements like Rb-Sr and Pb are highly incompatible in garnet, fractionating Rb/Sr and U/Pb between garnet and fluid during eruption, and therefore, these systems record ages approaching kimberlite eruption. In contrast, even in the presence of a fluid phase, garnet accommodates both Sm and Nd at 700-800 °C. Re-distribution of trace elements can be achieved through diffusion from the fluid film into the garnet rim during initial cooling. Once Sm-Nd are fully re-incorporated into the garnet, they record diamond formation ages, even if their concentrations are not homogeneous from rim to core.