An Archean peridotitic komatiite from the Barberton Mountain Land, South Africa, contains relict zoned skeletal olivine crystals with compositions as magnesian as 100 Mg/(Mg + Fe2+) = 93.6. This magnesian olivine is also relatively rich in nickel (0.51 percent NiO) and in chromium (0.18 percent Cr2O3). Experimental investigation of the near-liquidus phase relations of peridotitic komatiite as a function of pressure has shown that liquidus olivine for this bulk composition has a composition identical to the observed, most magnesian olivine in the rock. Experimental study of the effect of water on melting relations demonstrates a pronounced, pressure-dependent decrease of liquidus temperature for water-saturated conditions, but further demonstrates that the natural magma contained <0.2 percent H2O approximately at extrusion. The temperature of the completely liquid peridotitic magma at the Earth's surface was 1650° ± 20°C.