The Volume and Composition of Melt Generated by Extension of the Lithosphere |
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| Authors: | McKENZIE D; BICKLE M J |
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| Institution: | Department of Earth Sciences, Bullard Laboratories Madingley Road, Cambridge CB3 0EZ |
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| Abstract: | Calculation of the volume and composition of magma generatedby lithospheric extension requires an accurate mitial geotherm,and knowledge of the variation and composition of the melt fractionas a function of pressure and temperature. The relevant geophysicalobservations are outlined, and geotherms then obtained fromparameterized convective models. Experimental observations whichconstrain the solidus and liquidus at various pressures aredescribed by simple empirical functions. The variation in meltfraction is then parameterized by requiring a variation from0 on the solidus to 1 on the liquidus. The composition of the melts is principally controlled by themelt fraction, though those of FeO, MgO, and SiO2 in additionvary with pressure. Another simple parameterization allows theobserved compositions of major elements in 91 experiments tobe calculated with a mean error of 1.1%, and those of TiO2 andNa2O to 0.3%. These expressions are then used to calculate theexpected compositions of magma produced by adiabatic upwelling.The mean major element composition of the most magnesium-richMORB glasses resemble the mean composition calculated for amantle potential temperature Tp of 1280?C. Adiabatic meltingduring upwelling of mantle of this temperature generates a meltthickness of 7 km. The observed variations of the MgO and TiO2concentrations in a large collection of MORB glass compositionssuggest that extensive low pressure fractional crystallizationoccurs, but that its effect on the concentrations of SiO2, Al2O3,and CaO is small. There is no evidence that normal oceanic crustis produced from magmas containing more than 11% MgO. The mantlepotential temperature within hot rising jets is about 1480?Cand can generate 27 km of magma containing 17% MgO. Extension of the continental lithosphere generates little meltunless ß> 2 and Tp> 1380?C. The melts generatedby larger values of ß or of Tp are alkali basalts,and change to tholeiites as the amount of melting increases.Large quantities of melt can be generated, especially at continentalmargins, where estimates of ß obtained from changesin crustal thickness will in general be too small. |
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