Mg-(Fe + Ti)-Al petrochemical diagram for the melting of mantle pyrolite: Implications for the derivation conditions of the parental magmas of major volcanic series

Abstract

The Mg-(Fe + Ti)-Al melting diagram for pyrolite based on experimental data from literature shows the composition of the liquid as a function of pressure and the degree of pyrolite melting. Three mechanisms of liquid separation from a mantle source material are discussed: (i) gravitational mechanism, which works at a degree of source material melting of 25%, (ii) filter pressing mechanism, which is efficient at degrees of melting lower than 2%, and (iii) nearly complete local melting of mantle material. Garnet in the solid residue is thought to play an important role by affecting the chemistries of mantle magmas. The comparison of petrochemical and experimental data in a Mg-(Fe + Ti)-Al ternary plot shows that picrite and ferropicrite alcaline primary magmas are segregated at depths of 120 and 210 km, respectively, in the garnet stability zone, at degrees of melting lower than 2%; and tholeiite basalt magmas are segregated above this zone. At degrees of melting of 25%, picrobasalt, komatiite-basalt, picrite, and ferropicrite primary magmas of the tholeiite series are derived at depths of 80, 130, 240, and 300 km, respectively. Ultrabasic komatiite magma is generated at high degrees of mantle source melting, with the solid residues devoid of garnet. The tholeiite basalt series can be produced by two parental melts: aluminous and magnesian basaltic, both separated from the mantle sources via the filter pressing mechanism: the former at depths shallower than 30 km in ocean spreading zones (TOR-2), and the latter at depths of 50–60 km in oceanic spreading zones (TOR-1) and in the subcontinental lithosphere. Primary magnesian basalt magmas of the calc-alkaline and tholeiite series are derived in the lithospheric mantle at the same depths and low degrees of melting. Different evolutionary trajectories of compositionally similar primary magmas are controlled by the conditions of their further fractional crystallization: in compressional environments and with fluids saturating the melts in subduction zones for the former and in extensional environments and free magma ascent to the surface for the latter. Ultrapotassic rock series, such as lamprophyres, leucitites, kamafugites, lamproites, and kimberlites, are most probably generated via the melting of the metasomatized subcratonic mantle.