Ultrabasic magmas and high-degree melting of the mantle

As the degree of melting of mantle peridotite increases, the liquids that are formed become more basic and less viscous, and the spacing between residual crystals increases. The settling velocities of residual crystals in partial melts consequently will increase by several orders of magnitude, from 9.4 × 10^–4 cm/s to 4.3 × 10^–1 cm/s for a 1 cm olivine grain, as the proportion of liquid increases from 15 to 60%.To produce an ultrabasic komatiitic magma from a source with commonly assumed mantle composition requires 50 to 80% melting. Before this degree of melting can be reached, a highly fluid picritic magma produced by 30 to 50% melting will segregate from the source. Ultrabasic magmas probably form by a sequential melting process and are derived from a residuum composed of refractory minerals and trapped liquid left by previous episodes of partial melting and magma extraction. Trace element concentrations in ultrabasic komatiite lavas are consistent with this theory.