Growth of subcontinental lithospheric mantle beneath Zimbabwe started at or before 3.8 Ga: Re-Os study on chromites

Re-Os isotope results on chromites separated from age-constrained Archean (2.7 to 3.5 Ga) ultramafic intrusions in the Zimbabwe craton show that chromites average a present-day ¹⁸⁷Os/¹⁸⁸Os ratio of 0.1066, significantly below the bulk silicate Earth value. The source reservoir is interpreted to reflect the Zimbabwean subcontinental lithospheric mantle (SCLM). Archean Re depletion of the SCLM has previously been postulated on the basis of xenolith studies from southern Africa, Siberia, and the Wyoming craton. Because our samples are age constrained, however, the Re-Os data also shed light on the evolution of the Archean SCLM through time. They indicate that the Zimbabwean SCLM began to be separated from asthenospheric mantle before 3.8 Ga and grew quasi-continuously throughout the Archean. This result is reconciled with present-day SCLM data in forward modeling.