SPINEL FROM ARCHEAN IMPACT SPHERULES

Large meteorite impacts are recorded in distal fall deposits preserved in two early Archean greenstone belt sequences (3.2-3.5 Ga). Mapping, stratigraphic, and geochemistry studies have been used to identify at least four impact layers in the Barber-ton greenstone belt of southern Africa and one in the Pilbara greenstone belt of Western Australia. Spherules that represent impact melt droplets distinguish these impact layers from associated tuffs. Spinel with unusual compositional traits occurs within spherules in two of these impact layers. Spinel is the only preserved primary phase, and is found within up to 10% of the spherules in the best preserved layers. Some spherules contain up to 40% spinel, which is often the only primary phase that crystallized within the spherules. Spinel morphologies include dendrites and skeletal octahedra, typical of those found in high temperature melts. Spinel compositions are similar to those of Cenozoic impacts, including very high Ni and ferric iron contents, but the Archean spinel is distinctive in being composed dominantly of the chalcophile elements Fe, Ni, Cr, and V. This spinel is not metamorphic or metasomatic in origin. Closely associated komatiites in the underlying Onverwacht Group, both fresh and highly altered, have unaltered spinel with igneous compositions and morphologies quite distinct from the impact spinel. Characteristics of spinel preserved in these Archean impact spherules suggest a rather complex series of steps following a large bolide impact. Initial high temperature reduction produced melt droplets, some of which were immiscible silicate-metal/sulfide. Density differences allowed separation of some of these immiscible melts, which subsequently evolved independently and formed spherules with highly variable chalcophile element contents. Oxidation during atmospheric reentry increased Ni/Fe and Fe+3/Fe+2 in the chalcophile-siderophile melts. Archean impact spinel has higher Ni/Fe than Cenozoic impact spinel, though distinctly lower Fe+3/Fe+2, which probably reflects a substantially less oxidizing atmosphere during the Archean. The Archean atmospheric oxidation may in fact be a transient phenomenon associated with a large impact into the terrestrial ocean resulting in very high atmospheric H2O/H-2. Bolide size is estimated to be 24 km in diameter based on the average diameter of 0.85 mm for the spinel-bearing spherules. This estimate is consistent with the observed Ir fluence in spinel-bearing impact layers of approximately 5.3 mg/cm2, which would be produced by a 30 km diameter chondritic bolide.