Generation of Mid-ocean Ridge Tholeiites

If a basaltic magma is separated from its mantle source regiononly after a large amount of fusion, as indicated for mid-oceanridge tholeiites by large ion lithophile (LIL) element studies,volatile-free phase relationships may be used to model closelythe fusion process at the site of origin. Volatile-free soliduscurves in pressure-temperature space display low-temperaturecusps where subsolidus phase transitions intersect the solidus,and these cusps are believed to be important in controllingthe depth of magma generation and the compositions of primarymagmas. In the system CaO-MgO-Al₂O₃-SiO₂, the solidus curvefor simplified plagioclase and spinel lherzolite has been determinedup to 20 kb, and a cusp has been found at 9 kb, 1300 °C,where the transition from simplified plagioclase to spinel lherzoliteintersects the solidus and forms an invariant point. Simplifiedbasaltic melts formed along the solidus curve change from quartz-normativecompositions at low pressures to olivine-normative compositionsat high pressures, and the composition of the first liquid producedat the 9 kb cusp resembles closely the composition of the least-fractionatedmid-ocean ridge tholeiites. It is proposed that these least-fractionatedbasalts have been modified very little by fractional crystallizationas they passed upward to the surface and are close to the compositionof primary basalt generated at the mantle equivalent of the9 kb cusp (30 km depth). Assuming a lherzolitic mantle source,the composition of this primary magma would be expected to remainfairly constant despite varying amounts of fusion as long asthese amounts are between about 2 and 35 per cent, and despitevariations in the proportions of minerals in the source region.For complex natural compositions in the mantle, the cusp wouldnot be a point but would be expected to appear as a low temperatureregion on the solidus extending over a small pressure rangeat about 9 kb and 1200–1250 °C. It is suggested thatmagma generation at the cusp causes a sharp change in the slopeof the geotherm beneath active ridges from approximately adiabaticat depths geater than about 30 km to strongly superadiabaticat depths less than 30 km. LIL element concentrations indicatethat a close approach to fractional fusion is not applicableto the generation of primary magmas at spreading centers. Instead,a model involving varying amounts of equilibrium fusion is preferred,with fresh mantle being continually supplied at the 9 kb cuspas mantle material convects upward along the rising limb ofa mantle convection cell. Primary magmas would thus be producedwith fairly uniform major element compositions but widely differentLIL element concentrations. Subsequent fractional crystallizationof the primary magmas would produce most of the observed majorelement variations in the erupted basalts but would have onlya second-order effect on the LIL element concentrations.