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 and 40Ar/39Ar analyses on similar Premier inclusions, and is indistinguishable from the inferred time of emplacement of the host kimberlite (1,150-1,230 Myr), 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 inclusions, which suggest a depleted mantle source (40Ar/36Ar>20,000). Pre-entrapment equilibration of the inclusions with an 36Ar-rich fluid is the most probable explanation for the low non-radiogenic (40Ar/36Ar) composition. (author)