Magnesium-rich mane to ultramafic extrusions were most common in the Archean and pose interesting petrological problems. The high Mg content of komatiites (>18 wt%, for example, is usually interpreted as indicating an origin at higher temperatures than exist in mantle melting zones in the modern Earth. Current contrasting models for the origin of komatiites in the mantle require either high degrees of melting or lower degrees of melting at great depth. A potential complementary mechanism for Mg enrichment in magmas involves the melting of magnesite-bearing garnet Iherxolite. In this model, the ascending primary mafic or ultramafic magma is enriched in MgO by the loss of some off the CO2 to the adjacent mantle at pressures of ∼2.2 GPa, where the magma becomes saturated with CO2. To generate komatiite in this way from a picritelike parent, for example, requires that the primary magma lose some of its major and trace element components to the adjacent mantle concurrently with the CO2. Production of magnesian magmas by magnesite breakdown may not have required the heat or depth of those produced by other means; this mechanism may help to explain some apparently low Archean geothermal gradients, as well as the contemporaneity of Archean diamonds and komatites. The mantle magnesite could have formed by direct reaction of primordial CO2 or CO with hot, protomantle material during Earth's accretionary period.