A MAJOR AND TRACE ELEMENT APPROACH TO BASALTS

Study of the major and trace element contents of ocean basalts and theoretical investigations of mineral/melt equilibria have been combined in an approach to the petrogenesis of basalts.;Calculated liquid lines of descent can be used to model the chemical evolution of periodically refilled magma chambers. Periodically refilled magma chambers reach a steady state composition which invariably lies on potential liquid lines of descent of the parental magma or on mixing lines between the liquid line of descent and the parental magma. These numerical results are confirmed by mathematical analysis. Idealized periodically refilled magma chambers do not lead to enrichment of incompatible trace elements without corresponding changes in at least some of the major elements.;These theoretical results can be applied to new data for basalts from the French-American Mid-Ocean Undersea Study (FAMOUS) area on the mid-Atlantic ridge. Twenty-six rare-earth element analyses, and the compositions of minerals and their glass inclusions, have been determined. The variation in rare-earth element abundances for samples with very similar major element abundances demonstrates that the basalts cannot have been derived from a single parental magma. Some samples can be related to one another by crystallization at low pressure, but many of the least differentiated samples appear to have undergone a history of polybaric crystallization, evidence of which has been preserved in the minerals. The dominant control on the chemistry of the samples seems to result from dynamic melting of a homogeneous mantle source.;The analysis of periodically refiled magma chambers and the data from the FAMOUS area suggest that some least differentiated oceanic basalts still bear the chemical imprint of their primary magmas. Theoretical analysis shows that the abundance of FeO in a primary magma from the mantle reflects the FeO/MgO ratio of the mantle source, the degree of melting and the pressure at which melting begins. Relative pressures and degrees of melting can often be constrained so that FeO contents of basalts that have undergone crystallization primarily of olivine can give information about variations in the FeO/MgO ratio of mantle sources.;Theoretical analysis demonstrates that the distribution of major and trace elements during melting and crystallization can be calculated using single element distribution coefficients (Kd's). Such calculations require an empirical understanding of the variations of Kd's with temperature, pressure, and composition. Evaluation of the Kd's for anorthite and albite components between plagioclase and liquid in dry, complex systems at one atmosphere allows the construction of a plagioclase saturation surface from which the temperature of appearance and composition of plagioclase crystallizing from a melt can be predicted in compositions ranging from tholeiitic basalt to dacite. The nature of the variations of the Kd's provide constraints on models of silicate melts. These new results for plagioclase can be coupled with previous results for olivine to calculate liquid lines of descent for olivine-plagioclase-liquid equilibria. Such calculations are in close agreement with experimental data of Bender et al. (1978) on an ocean ridge basalt.;Comparison of FeO contents with trace element and isotopic contents of basalts reveals some striking correlations and leads to the following conclusions. In local areas different degrees of melting of sources homogeneous in FeO/MgO often account for the data. Over larger regions, FeO/MgO in the mantle varies significantly, sometimes in a way opposite to what would be expected from either trace elements or isotopes. Thus major elements, trace elements and isotopes may each give a different perspective important to the understanding of the evolution of the mantle.