Lithosphere evolution beneath the Kaapvaal Craton: Re–Os systematics of sulfides in mantle-derived peridotites

Abstract

In situ Re–Os analysis of sulfide phases in peridotite xenoliths from kimberlites in the Kaapvaal Craton has been used to analyse relationships between crustal events and the modification of the subcontinental lithospheric mantle (SCLM). Fifteen samples from the Western Terrane (Finsch, Kimberley and Jagersfontein) and 10 from the Southeastern Terrane (Northern Lesotho pipes) were serially sectioned to reveal the sulfide phases. The peridotites contain multiple generations of sulfides with widely varying Os contents, Re/Os and 187Os/ 188Os. Where ≥4 sulfides have been analysed in a sample, the T RD model age of whole-rock sample is younger than the maximum sulfide T RD, usually by 0.3–1.5 Ga. The Re–Os model ages of the whole-rock samples therefore represent mixtures, and are unlikely to date any specific geological event. Comparison of Re–Os systematics with peridotite compositions defines a clear relationship between sulfide model ages, whole-rock model ages and metasomatic modification of the SCLM. The oldest sulfide ages are preserved in the most depleted peridotites, typically at shallower levels of the SCLM. The proportion of sulfides with negative T RD ages increases, and the maximum sulfide T RD model ages tend to decrease, as the Mg# of olivine in the peridotites decreases. Interelement correlations indicate that sulfide introduction accompanies Fe-(±Al, Ca, Cr) metasomatism, attributed to asthenosphere-derived silicate melts and fluids. Fluids with high 187Os/ 188Os, inherited from the asthenosphere or scavenged from older silicates, tend to react with pre-existing sulfides, gradually obliterating older sulfides with high T RD. This process is most prominent toward the base of the lithospheric mantle. T MA ages of sulfides with 187Re/ 188Os<0.08, which are those least likely to have been disturbed by reaction with fluids, show peaks that correlate with the oldest crustal ages in each terrane (Western Terrane, 2.9–3.2 Ga; Southeastern Terrane, 3.0–3.6 Ga). The formation of the SCLM was earlier than, or contemporaneous with, the earliest crustal formation, and each terrane appears to have carried its own SCLM keel at the time of craton assembly. Other model-age peaks correlate with the timing of major crustal events, including the suturing of terranes (2.65, 2.75 Ga), and later rifting/collision (1.8–2.2 Ga, ca. 1 Ga) events. These correlations suggest that sulfides carrying asthenosphere-derived Os have been introduced into the SCLM during major tectonic events. Only the oldest 5% of available whole-rock data closely approach the age of lithosphere stabilisation suggested by the oldest sulfides in the Kaapvaal Craton. We infer that the bulk of the Kaapvaal SCLM had formed prior to 3 Ga.