Along the Mid-Atlantic Ridge near Iceland and the Azores, major element indices of the degree of mantle melting (Na/Mg in lavas and pyroxene content in peridotites) suggest an unusually high degree of melting, if one assumes constant source composition. In contrast, trace element indices (high La/Sm and K/Ti) from the same regions, interpreted in the same way, indicate a small degree of melting. This apparent paradox is easily resolved; the mantle source composition is not constant along the ridge (e.g., Schilling, 1973). This is borne out by radiogenic isotope ratios, which indicate a long-term enrichment in incompatible elements (such as La and K) in the mantle source where the degree of melting is large (e.g., Hart et al., 1973). Enriched areas with apparent high degrees of melting areas have been interpreted as "hotspots" in accord with the notion that high temperature and chemical enrichment are correlated in the mantle. However, because this correlation between temperature and enrichment is poorly understood and may vary from place to place, there is debate over their relative importance in controlling igneous crustal thickness, crustal composition, axial depth, and geoid height.
Work in the 14° to 16°N region provides constraints for deconvolving the effects of temperature and composition on mantle melting. There is a substantial gradient over 150 km along the ridge, from geochemically "normal" MORB in the north (moderately high Na/Mg and low La/Sm) to strongly "enriched" MORB in the south (low Na/Mg and high La/Sm) (Fig. F3), and there appears to be a large gradient in crustal thickness, based on interpretation of gravity data, increasing away from the fracture zone. One hypothesis holds that "enriched" basalts are derived by partial melting of veins that compose a few percent of the volume of the source region. The volumetric proportion of veins in peridotite drill core, and future isotope measurements on these veins, will place constraints on the original proportion and composition of these veins prior to decompression melting.
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