⁴⁰Ar/³⁹Ar laser-probe dating of diamond inclusions from the Premier kimberlite

INCLUSIONS encapsulated by diamonds at the time of their formation provide a means for determining diamond crystallization ages and the chemistry of the surrounding upper mantle at that time. Sm-Nd studies of peridotitic inclusions, from Cretaceous-age kimberlites in southern Africa, suggest that the diamonds formed 3.3 Gyr ago. By contrast, eclogite-suite inclusions generally yield younger ages, sometimes approaching the time of kimberlite eruption. Here we report the results of ⁴⁰Ar/³⁹Ar laser-probe analyses of individual eclogitic clinopyroxene inclusions from Premier diamonds, which yield a mean age of 1,198 ± 14 Myr. This age agrees well with Sm-Nd (ref. 2) and ⁴⁰Ar/³⁹Ar (ref. 3) analyses on similar Premier inclusions, and is indistinguishable from the inferred time of emplacement of the host kimberlite (1,150-1,230 Myr)², which implies that diamond formation was essentially synchronous with kimberlite generation. The extrapolated non-radiogenic ⁴⁰Ar/³⁶Ar ratio of 334 ± 102 is similar to the present-day atmospheric composition. This value is inconsistent with Sr and Nd isotopic signatures from Premier eclogite inclusion, which suggest a depleted mantle source (⁴⁰Ar/³⁶Ar > 20,000). Pre-entrap-ment equilibration of the inclusions with an ³⁶Ar-rich fluid is the most probable explanation for the low non-radiogenic (⁴⁰Ar/³⁶Ar) composition.