Latter-day origin of diamonds of eclogitic paragenesis

Abstract

Syngenetic mineral inclusions in diamonds provide the best means of determining their origin. Among such inclusions, the peridotitic paragenesis of olivine, orthopyroxene and chrome-pyrope garnet is on average far more abundant than the eclogitic paragenesis of pyrope–almandine garnet and omphacitic clinopyroxene1–3. Diamonds of peridotitic paragenesis from the ∼ 100-Myr-old Kimberley and Finsch kimberlites in southern Africa were previously shown to have originated in 150–200-km-thick lithosphere beneath the Kaapvaal craton ∼3,300 Myr ago4,5. At a few localities, such as the Premier kimberlite in southern Africa and the Argyle lamproite in northwestern Australia, diamonds of eclogitic paragenesis predominate6–8, allowing recovery of sufficient material for complementary analysis, the results of which are reported here. Eclogitic garnet and clinopyroxene inclusions in Premier and Argyle diamonds yield Sm–Nd isochron ages of 1,150 and 1,580 Myr respectively, compared with host diatreme emplacement ages9,10 of 1,100–1,200 Myr. Eclogitic inclusion ages and precursor isotopic signatures indicate a second genetically distinct origin of diamonds, apparently related in time and space to kimberlite or lamproite magmatism.