Highly siderophile element systematics of the 3.3 Ga Weltevreden komatiites, South Africa: Implications for early Earth history

Highly siderophile element (HSE: Os, Ir, Ru, Pt, Pd, and Re) abundances, Re–Os isotopic systematics, and major-, minor-, and lithophile trace element abundances are determined for a suite of remarkably fresh komatiites from the Weltevreden Formation (Fm.) of the Barberton Greenstone Belt, South Africa. The komatiite lavas examined are calculated to have contained ca. 31% MgO upon emplacement, and are among the most MgO-rich lavas ever erupted onto Earth's surface. Based on the partitioning behavior of V, indicating reduced conditions of magma formation, as well as the low calculated H2O abundance of ~ 0.02% in the mantle source, anhydrous melting for the Weltevreden Fm. komatiites is inferred. The high calculated liquidus temperatures of the emplaced komatiite lavas of ca. 1600 °C require an unusually hot (~ 1800 °C) mantle source, > 100 °C hotter than the ambient contemporaneous mantle. These observations are most consistent with formation of the Weltevreden komatiites via deep (~ 530 km) melting in a mantle plume. The Re–Os isotopic data for twelve whole-rock samples and an olivine separate define a precise Re–Os isochron age of 3266 ± 8 Ma (2σmean) and an initial 187Os/188Os of 0.10440 ± 5 (γ187Os = − 0.13 ± 0.05). The Re–Os data, coupled with the relatively high calculated HSE abundances in the mantle source totaling ~ 80% of those in modern Primitive Mantle (PM) estimates, indicate that at least some portions of the deep, early Archean mantle evolved with essentially chondritic Re/Os and absolute HSE abundances that are typical of the sources of late Archean komatiites. If the HSE budget of the terrestrial mantle was established via late accretion of several large planetesimals following the cessation of major chemical interaction between the core and the mantle, our data indicate that by 3.3 Ga, these materials were well homogenized within the mantle domains sampled by the Weltevreden komatiites and, by inference, within the mantle as a whole.

► Data indicate dry conditions of magma generation for Weltevreden komatiites.
► The komatiites formed in a mantle plume, at T = ~ 1830 °C and at a depth of 530 km.
► Re–Os data define an isochron with an age of 3266 ± 8 Ma (γ187Os = − 0.13±0.05).
► Deep mantle sources of Archean komatiites evolved with no changes in their Re/Os.
► The total HSE abundances in the Archean mantle were ~ 80% of PUM.