EPISODIC AGES OF GREENSTONES - A KEY TO MANTLE DYNAMICS

Two greenstone age minima at 2.5-2.2 Ga and 1.65-1.35 Ga may correlate with ages of supercontinents when a greater proportion of greenstones were recycled into the mantle by subduction. From the age distributio ns of greenstones and juvenile continental crust, earth history can be divided into three stages: I) >2.8 Ga, when greenstones and microcont inents formed and collided continuously, although probably not forming a supercontinent until about 3 Ga; II) 2.8-1.3 Ga, where a clear epis odisity is apparent in greenstone eruption and collision ages, and whe re two greenstone age minima coincide in age with supercontinents; and III) <1.3 Ga, when greenstones and continents appear to have formed a nd collided continuously, perhaps in response to significant overlap i n the times of dispersal and assembly of supercontinents. Greenstone a ge peaks 2.7, 1.9 and 1.3 Ga may reflect catastrophic overturn of the mantle initiated by episodes of sinking of cold lithospheric slabs thr ough the 660-km discontinuity to the D'' layer initiating an episode o f major mantle plume activity. Continuous production and preservation of greenstones in the last 1.3 Gy is consistent with seismic tomograph y data, which suggest a steady sinking of slabs into the lower mantle.