Siderophile and lithophile trace-element evolution of the Archaean mantle

Progressive depletion of several Archaean volcanic sequences in siderophile (Fe, Ti, Mn, V), lithophile (Zr, Sr, P, Y), and chalcophile (Zn, Cu) elements with higher stratigraphic level are reported for the Barberton Mountain Land (Tjakastad Subgroup) in South Africa, and are also observed in the eastern Pilbara Block (Western Australia), a number of sequences in the Yilgarn Block (Western Australia) and the Superior Province of Canada. These trends are shown by both high-Mg basalts and tholeiitic basalts, and are not dependent on the Mg number of the rocks. Because these variations are contrasted to the Skaergaard-type iron enrichment trend, and as they are unlikely to have been produced by secondary alteration, it is suggested they reflect secular depletion of source mantle regions following repeated extraction of basic partial-melt fractions. High-Mg basalts and peridotitic komatiites of the Tjakastad Subgroup are separated by compositional gaps for MgO, AI2O3, CaO, Ni and Co, as well as on the Ol-Op-Cp-Qz diagram, which militate against their relation by continuous crystal fractionation. Archaean high-Mg basalts are typically Qz normative, as distinct from picrites. Using the FeO-MgO (mol%) diagram, liquidus temperatures are estimated for the high-Mg basalts and komatiitic peridotites. The degree of partial melting (F) is deduced by mass balance calculation of FeO + MgO. Assuming olivine-dominated residues of partial melting, and using temperature-dependent Kd^MgO and Kd^FeO coefficients after Roeder and Emslie (1970), the mantle Mg number can be estimated. For the Archaean data, source Mg numbers fall mainly in the range 80-90, suggesting a relatively ferroan mantle. Computations of mantle trace-element levels are attempted, assuming equilibrium batch melting, normative residual mineral assemblages, and apply cited mineral/melt partion coefficients (D). The possibility of an Archaean mantle rich in iron, and possibly other siderophile elements, is supported by comparisons between Archaean and modern oceanic tholeiites. The distribution of mantle-melting events in space and time must have resulted in significant major- and trace-element heterogeneities. Subsidence of dense refractory mantle residues into undepleted regions may have triggered mantle diapirism, probably constituting a major factor underlying Archaean tectonic activity.