Differences in the lithology and chemistry of mid-ocean ridge basalts generated at Pacific and Atlantic spreading axes reflect a simple correlation to the spreading rate. On the basis of experimental data and thermal models, these differences are attributed to variation in the nature of magma supply and fractionation processes as a function of spreading rate, rather than to processess in the upper mantle. Thermal and kinematic models for ocean ridges predict stable steady-state magma reservoirs at faster-spreading axes (>5 cm/yr) and more restricted, transient systems at slower-spreading (<5 cm/yr) axes. The chemical and lithological character of basalts confirms this pattern and suggests that slow-spreading axes(e.g. the Mid-Atlantic Ridge) are characterized by polybaric magma fractionation in complex reservoir systems, reflected by the widespread accumulation of calcic plagioclase in low viscosity melt, phenocryst reaction morphologies and pyroxene-dominated fractionation ‘extracts’. Fast-spreading axes (e.g. East Pacific Rise) show low-pressure basaltic fractionation trends to iron-rich compositions with little evidence for plagioclase accumulation or crystal-liquid reaction, in comparatively stable shallow-level chambers. Spreading rates of 5–6 cm/yr in the Gulf of California may be reflected by ‘intermediate’ fractionation regimes influenced by some magma crystallization at depth with incipient accumulation of plagioclase followed by further fractionation at low pressure. The major effect of spreading rate differences appears to be on the lithological and chemical patterns of basalts induced after partial melting, with minimal influence on the composition of primary melts.

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