Vesicular komatiites, 3.5-Ga Komati Formation, Barberton Greenstone Belt, South Africa: inflation of submarine lavas and origin of spinifex zones

Komatiites of the 3.5-Ga Komati Formation are ultramafic lavas (> 23% MgO) erupted in a submarine, lava plain environment. Newly discovered vesicular komatiites have vesicular upper crusts disrupted by syn-volcanic structures that are similar to inflation-related structures of modern lava flows. Detailed outcrop maps reveal flows with upper vesicular zones, 2-15 m thick. which were (1) rotated by differential inflation, (2) intruded by dikes from the interior of the flow, (3) extended, forming a flooded graben, and/or (4) entirely engulfed. The largest inflated structure is a tumulus with 20 m of surface relief., which was covered by a compound flow unit of spinifex flow lobes. The lava that inflated and rotated the upper vesicular crust did not vesiculate. but crystallized as a thick spinifex zone with fist-size skeletal olivine. Instead of representing rapidly cooled lava, the spinifex zone cooled slowly beneath an insulating upper crust during inflation. Overpressure of the inflating lava may have inhibited vesiculation. This work describes the oldest vesicular komatiites known, illustrates the first field evidence for inflated structures in komatiite flows., proposes a new factor in the development of spinifex zones, and concludes that the inflation model is useful for understanding the evolution of komatiite submarine flow fields.