HOTTER THAN YOU THOUGHT--KOMATIITES FROM THE BARBERTON GREENSTONE BELT

We report here evidence for the hottest, most oxidized komatiitic liquids found to have erupted onto Earth's surface. A sequence of three remarkably fresh, spinifex-bearing komatiites from within the 3.3 Ga Weltevreden Fm, at the top of the Onverwacht Gp, contains olivine (cores from Mg# 94 to 95.5), pyroxenes, which include cores of opx (Mg # 91), rims of pigeonite, and both rims and discrete crystals of augite (cores of Mg# 87), and chrome spinel (cores of Mg# 73 and Cr# 82). Chemical analyses of sixteen samples from the flows define an olivine accumulation line with incompatible element zero intercepts at MgO 54% suggesting equilibrium with Mg# 96 olivine. The finest-grained random spinifex sample contains MgO 35.1% and FeOT 8.6%, which would yield an Fe-Mg Kd of 0.30 if all iron is ferrous, or 0.37 if 20% of the iron is ferric. Several oriented spinifex samples contain MgO 30.6% and FeOT 9.3%, which yield reasonable Kds only at 20% oxidization of iron.

Previous studies have demonstrated that random spinifex zones are nearest to primary liquid compositions in texturally complex komatiitic flows. Apparent Kds found from the olivine accumulation trend or from microprobe analyses of the most magnesian olivines found in the flows are: Ni - 2.0, Co - 1.3, Mn - 0.50, Cr - 0.45, Sc - 0.10, Y - 0.03, V - 0.01, and Zr , Ti, and Al all less than 0.01. These values are very consistent with recent studies of very high temperature and/or highly oxidized magmas. V, Cr, and Fe are all at appropriate values for conditions as oxidizing as NNO+2.

The olivine-melt Fe-Mg Kd observed in this highly oxidized liquid, at 0.37, is higher than often reported, but consistent with other relatively fresh Barberton flows and consistent with some theoretical and experimental results. The temperature of eruption for such a liquid would be 1700 degrees Celsius, and even the much more conservative use of the oriented spinifex samples (MgO 30.6%) would yield a temperature of 1600 degrees. These high temperatures and oxidation states should have important implications for both Archean tectonism and mantle evolution. However, these komatiites are not boninites or kimberlites. Petrographic and field studies confirm that they represent aphyric liquids, erupted and transported long distances at the Earth's surface prior to the onset of any crystallization.