Abstract:
Experimental melting studies indicate that secular variations in the geochemistry of many komatiites were determined by secular variations in the depth of melt segregation in plumes. Higher pressures stabilize garnet relative to olivine and pyroxenes, resulting in komatiites with lower Al2O3, and higher CaO2O3 andGd . The experimental work is consistent with the following pressures of melt segregation: 3-4 GPa for Tertiary and Cretaceous picrites and komatiites; 5-7 GPa for most komatiites with 2.7-Ga ages; and 9-14 GPa for komatiites with 3.5-Ga ages. Melting and melt segregation occurred deeper in the past largely because the Earth was hotter, and komatiites are a thermometer of this secular cooling. Depth of melting is critically dependent on internal plume temperature, and the geochemistry of most komatiites can be explained by plumes that were # 200°C hotter than the secular cooling Earth model of F.M. Richter. Hot Archean plumes that were 300-400°C above ambient mantle could have experienced melting in the transition zone and the top of the lower mantle. Geological evidence in support of hot plumes is rare, but the best examples can be found in the 2.7-Ga komatiites from the Boston Township of Ontario and the 3.3-Ga peridotite xenoliths from the Kaapvaal craton.