Primary melt compositions in the Earth's mantle

Primary melts are the melt compositions in equilibrium with their source at the time of extraction from that source. Many mantle-derived melts are derived chiefly from peridotite, but have additional components in the source that originate as recycled crust and sediments, or as ultramafic rocks formed by solidification of migrating mantle melts. Most melts originate between 40 and 120 km depth, with exceptions up to 250 km, particularly around and beneath cratons, where fluxed by volatile components. Melts of dry peridotite are varieties of basalt with increasing MgO and alkalies toward high pressures: they range from picrite and komatiite at high degrees of melting to basanite and nephelinite at small melt fractions. More alkaline compositions, including melilitite, kimberlite, lamprophyre, and carbonatite, require H₂O and CO₂ in the source. Water promotes melts richer in silica than basalt, whereas CO₂ causes SiO₂-poor melts. At high pressures, primary melts may be transitional between silicate and carbonate melt compositions. Volatile components depress the melting point of peridotite by 150–400°C relative to dry melting conditions, and melting in oxidized conditions occurs at lower temperatures than in reduced conditions or with H₂O alone. Incipient melts are widespread, universal precursors to more voluminous melt production and cause modification and evolution of the lithosphere.