EMPLACEMENT CONDITIONS OF KOMATIITE MAGMAS FROM THE 3.49 GA KOMATI FORMATION, BARBERTON GREENSTONE-BELT, SOUTH-AFRICA

This paper provides new constraints on the crystallization conditions of the 3.49 Ga Barberton komatiites. The compositional evidence from i gneous pyroxene in the olivine spinifex komatiite units indicates that the magma contained significant quantities of dissolved H2O. Estimate s are made from comparisons of the compositions of pyroxene preserved in Barberton komatiites with pyroxene produced in laboratory experimen ts at 0.1 MPa (1 bar) under anhydrous conditions and at 100 and 200 MP a (1 and 2 kbar) under H2O-saturated conditions on an analog Barberton composition. Pyroxene thermobarometry on high-Ca clinopyroxene compos itions from ten samples requires a range of minimum magmatic water con tents of 6 wt.% or greater at the time of pyroxene crystallization and minimum emplacement pressures of 190 MPa (6 km depth). Since high-Ca pyroxene appears after 30% crystallization of olivine and spinel, the liquidus H2O contents could be 4 to 6 wt.% H2O. The liquidus temperatu re of the Barberton komatiite composition studied is between 1370 and 1400 degrees C at 200 MPa under H2O-saturated conditions. When compare d to the temperature-depth regime of modern melt generation environmen ts, the komatiite mantle source temperatures are 200 degrees C higher than the hydrous mantle melting temperatures inferred in modem subduct ion zone environments and 100 degrees C higher than mean mantle meltin g temperatures estimated at mid-ocean ridges. When compared to previou s estimates of komatiite liquidus temperatures, melting under hydrous conditions occurs at temperatures that are similar to 250 degrees C lo wer than previous estimates for anhydrous komatiite. Mantle melting by near-fractional, adiabatic decompression takes place in a melting col umn that spans similar to 38 km depth range under hydrous conditions. This depth interval for melting is only slightly greater than that obs erved in modem mid-ocean ridge environments. In contrast, anhydrous fr actional melting models of komatiite occur over a larger depth range ( similar to 130 km) and place the base of the melting column into the t ransition zone. (C) 1997 Elsevier Science B.V.