THE RELATIONSHIP BETWEEN THE AGE OF THE LITHOSPHERE AND THE COMPOSITION OF OCEANIC MAGMAS: CONSTRAINTS ON PARTIAL MELTING, MANTLE SOURCES AND THE THERMAL STRUCTURE OF THE PLATES

Abstract

On the basis of different proportions and chemical compositions of shield and post-shield magmas, three types of oceanic intraplate volcanism appear to exist. The average SiO2 contents of primitive melts of most Pacific and Atlantic intraplate lavas show a regular decrease with increasing age of the lithosphere up to 70 Ma. The average pressures of melting of most magmas lie beneath the thermal boundary layer defined by the 1300°C isotherm, in accordance with geophysical models. The average melting pressures of shield tholeiites erupting at the largest hotspots on Earth suggest that erosion of the plate is restricted to strong plumes. Increasing average ratios of (Ce/Yb)N(=chondrite-normalized) and (Tb/Yb)N with increasing age of the lithosphere imply that residual garnet has an increasing influence on the melting of most magmas. An influence of MORB material in intraplate magmas is observed in volcanoes erupting on lithosphere younger than 15 Ma. Correlations between SiO2 and the rare earth element ratios suggest that the rare earth elements are more strongly influenced by the pressure of melting than by differences in source composition. Lavas with extremely low 143Nd/144Nd (e.g. Gough-Tristan da Cunha) have high (Nd/Sm)N for a given SiO2, in accordance with a long-term enriched mantle source. After a correction for the fractionation occurring at high melting pressures (a recalculation of all averages to 50% SiO2) the (Nd/Sm)N of most lavas can be modeled by 3–15% melting of depleted mantle sources.