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The study of the geochemical composition of the oceanic crust of the Earth had important implications for our understanding of the composition, temperature and dynamics of the upper mantle as well as for the global mass transfer between different geochemical reservoirs. Early studies of the composition of rocks from the seafloor suggested a homogeneous basaltic composition compared to the highly variable continental crust. However, during the past 25 years increasingly detailed investigations of different tectonic structures of the oceanic lithosphere revealed significant compositional variations of the magmatic rocks forming the oceanic crust. As a consequence, early relatively simple petrogenetic and geodynamic models became more complex.
For example, incompatible element-depleted to ?enriched tholeiitic basalts have been found along the spreading axes and frequently there is no clear correlation of enrichment with proximity to well-defined deep mantle plume volcanism. Thus, the variations of incompatible element and radiogenic isotope compositions reflect heterogeneous mantle sources but the origin and distribution of these sources is debated. Alkaline basalts occur in oceanic intra-plate settings but also on some very slow-spreading segments as well as on propagating or failing oceanic rifts. Variable major element compositions of the basalts are on the one hand due to variable degrees and depths of partial melting but on the other hand major element variations in mantle source compositions have also been proposed. The average upper mantle potential temperature is estimated to be about 1300°C but variations exist, for example, close to ascending mantle plumes or due to increased cooling from the surface at very slow spreading rates.
Heterogeneities in the volatile (especially water) concentrations in the upper mantle probably contribute to the observed variations in magma compositions due to their effects on the mantle solidus. Although most of the oceanic crust consists of rocks of basaltic composition, more evolved magmas ranging from andesites to rhyolites are found occasionally and seem to be relatively frequent in plume-influenced spreading segments, possibly as a consequence of a thickened crust. These evolved magmas show evidence for significant assimilation of hydrothermally altered crustal material and signs of assimilation are also frequently observed in primitive magmas. Thus, assimilation-fractional crystallization (AFC) processes that are long known to be important in magmas ascending through the continental crust do also affect oceanic magmas.
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