The genesis of continental basic alkaline magmas—an interpretation in terms of redox melting

Redox melting has recently been proposed as a major process for the generation of partial melts in the upper mantle by interaction of upwelling CH₄-rich fluids from a reduced deep asthenosphere with relatively oxidized (f_O2≈ IW + 2 - 4 log units) overburden. Oxidation of CH₄ produces H₂O and solid carbon, and melting is caused by the increase in α_H2Q, which depresses the mantle solidus temperature. An assessment is made here of the ability of the redox melting model to account for the great variety of alkaline basic magmatism observed in continental regions. The hypothetical cases of a stable continental cratonic region and a developing continental rift are treated for the cases of variable degrees of fluid introduction and melting. Events caused by reduced fluid introduction into continental regions are considered to form important contrasts to those outlined in general accounts of oceanic redox melting processes, mainly due to the discontinuous and incipient nature of melting beneath the continents, and the absence of a fluid-free melt layer. Important conclusions are: (1) In regions with heat flow too low to cause partial melting, extensive amphibole and/or mica crystallization may occur. (2) Initial melts will be CO₂-rich, produced by either buffered CO₂ + H₂O-rich fluids or by disequilibrium redox melting, corresponding to kimberlitic, melilititic, or ultramafic lamprophyric composition depending on pressure and tectonic setting. (3) More extensive melting will always be in H₂O-rich environments, leading to the volumetric predominance of basanitic-nephelinitic and alkali-basaltic compositions in alkaline volcanic fields. (4) Episodic reactivation of melted regions will produce more reduced and ‘depleted’ low A1₂O₃, Na₂O, and CaO melt types without the initial CO₂- rich melting phase; in cratonic regions lamproites, rather than kimberlites, should result. (5) Tensional faulting in rift environments promotes enrichment at shallow levels by both solidification of melts and by metasomatic introduction of carbonates produced by oxidation of reduced fluids around fractures. (6) Partial melting of these enriched areas at a later stage of rift development occurs preferentially due to the presence of water in: (a) minerals of the enriched zones; and (b) produced by the oxidation of methane. Resulting melts may be highly alkaline (ultrapotassic) to high melt fractions, but this alkalinity will be diluted by interaction with surrounding peridotitic rock prior to melt escape, so that alkalinity should be broadly proportional to the density of enrichment. (7) The direct influence of redox melting wanes with progressive rifting, and in the oceanic stages of rifting it is subordinate to decompression melting. Further assessment of the redox melting processes, particularly as regards mantle enrichment events, awaits clarification of enrichment types from studies of nodules and alkaline volcanic rocks.