Time steps of depletion and enrichment in the Kaapvaal craton as recorded by subcalcic garnets from Finsch (SA)

Abstract

Twenty four subcalcic garnets from the Finsch mine (SA) were analysed for major and trace elements and Lu–Hf and Sm–Nd-isotopes. Subcalcic garnets are common in heavy mineral concentrates from kimberlites where they stem from disrupted (cpx-free) harzburgites and dunites and they occur as inclusions in diamonds. They control the abundance of most trace elements in such rock types. Thus, the investigation of the trace elements and isotopic composition of subcalcic garnets should directly provide information on the origin, genesis and the age of their host rock. We were able to distinguish two groups of garnets: group-1 with low Cr# (< 11.4), which correlates negatively with CaO and positively with (Lu/Er) N and group-2 displaying high and variable Cr# (10.9–34.3) but constantly low (Lu/Er) N. These findings imply that group-1 garnets were mainly generated by depletion in the garnet and group-2 garnets by partial melting in the spinel stability field. Group-1 garnets yield a well defined Hf isochron with an age of 2.52 ± 0.06 Ga, likely representing the final depletion of the subcratonic mantle. The high initial of this isochron ( εHf ~ + 25) indicates that the mantle was already significantly depleted prior to this last depletion event. Group-2 garnets scatter around this isochron at very low Lu/Hf ratios. Sm–Nd does not yield a well defined isochron for either garnet group. Group-1 garnets form an errorchron corresponding to about 1.3 ± 0.39 Ga, while most of group-2 garnets form an errorchron corresponding to 0.4 ± 0.12 Ga. These findings imply that the present day mantle underneath Finsch (and by inference of the Kaapvaal craton) is the residue of multiple, in situ depletions interfaced with subducted residues of partial melting at low pressures in ocean ridge like settings. The first depletion may have been at around 3.6 Ga for group-1 garnets and occurred mainly in the garnet stability field. For group-2 garnets, the first depletion event took place at low pressures in the spinel or plagioclase stability field prior to 2.5 Ga. The second and final depletion event took place at around 2.5 Ga in the garnet stability field underneath an existing continental crust. This depleted mantle was subsequently metasomatized at least twice. Re-enrichment occurred in two stages, one at around 1.3 ± 0.39 Ga by a subduction related fluid phase and a second at around 0.4 ± 0.12 Ga with melt as metasomatic agent.