The lithospheric mantle underneath the Gibeon Kimberlite field (Namibia): A mix of old and young components—Evidence from Lu–Hf and Sm–Nd isotope systematics

Abstract

We have determined major and trace elements and the Sm–Nd and Lu–Hf isotope systematics in minerals of 19 garnet peridotites from the Gibeon Townlands and Hanaus kimberlite pipes. These are situated within the mixed age Proterozoic to Archean (?) Rehoboth crustal province in Namibia. Our geothermobarometric results confirm previous results on the similarities of the conductive geothermal gradients underneath the Rehoboth province and the Kaapvaal craton and also that the Rehoboth mantle must have lost a significant proportion of its lithosphere. We distinguish peridotites with sinusoidal REE patterns similar as they commonly occur in the Archean Kaapvaal subcratonic mantle (named by us as σ type peridotites) and peridotites with LREE depleted patterns typical for the Proterozoic (N type). Both types are reenriched restites of partial melting processes. Time constraints for these processes can be found from the Sm–Nd and Lu–Hf isotope systems and from published Re–Os data. The Lu–Hf isotope system dates an enrichment event of a highly depleted mantle (ɛHf=+29.4) for the σ-peridotites at around 1.9Ga. Enrichment for the N-peridotites occurred between 850 and 900Ma ago in a slightly depleted mantle (ɛHf=+ 3.3). The last enrichment event at around 460Ma may be recorded by the Sm–Nd system in the σ-peridotites. A published Re–Os isotope data set from the neighbouring Louwrensia pipe reports Re depletion ages up to 2.1Ga (chondritic Re–Os Earth model) and Re model ages of 2.4Ga. The latter ages agree with estimates for the timing of first partial melting for the σ peridotites as deduced from the Lu–Hf isotope system combined with model calculations for partial melting.