CONSTRAINING THE POTENTIAL TEMPERATURE OF THE ARCHEAN MANTLE - A REVIEW OF THE EVIDENCE FROM KOMATIITES

The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are signif!cantlji altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to FO93.6, implying eruption at around 1520 degrees C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to FO94.1, implying eruption at 1560 degrees C. However, inferences of erupted liquids containing 32-33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580 degrees C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18-20% MgO and erupted at circa 1400 degrees C. If 1580 degrees C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200 degrees C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900 degrees C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300 degrees C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200-300 degrees C temperature difference between present day jets and ambient mantle temperatures. An important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions.