GROWTH OF SUBCONTINENTAL LITHOSPHERIC MANTLE BENEATH ZIMBABWE STARTEDAT OR BEFORE 3.8 GA - RE-OS STUDY ON CHROMITES

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