ERUPTION RATES AND ISOTOPIC SYSTEMATICS OF OCEAN ISLANDS - FURTHER EVIDENCE FOR SMALL-SCALE HETEROGENEITY IN THE UPPER MANTLE

Estimated eruption rates for a representative sampling of young volcanic ocean islands, combined with isotopic compositional ranges of erupted lavas, provide evidence for small-scale geochemical heterogeneity in the Earth's upper mantle. Mid-ocean spreading ridge basalts constitute approximately 90% of the present-day mantle-derived volcanic products, and their averaged isotopic compositions approximate the mean isotopic composition of the upper mantle. Isotopic compositions which depart from this average reflect other mantle components possibly derived from recycled materials. These other components are most apparent in lavas from ocean islands with low eruption rates, and by inference, low degrees of melting of their mantle sources. Lavas from islands with relatively high eruption rates are characterized by limited ranges of less extreme isotopic compositions. The appearance of isotopically distinct components at lower eruption rates indicates that these components, which have survived complete homogenization by mechanical mixing and chemical diffusion, are minor in volume, are preferentially melted, and exert an important control on the isotopic compositions of erupted lavas of ocean islands. The isotopic variation of ocean island lavas may be thus explained by variable degrees of melting by thermal plumes of the upper mantle which is heterogeneous on a small scale, or variable degrees of melting within compositionally heterogeneous plumes. Total ranges of intra-island isotopic variation in ocean islands are not necessarily greater at low eruption rates, providing evidence for an irregular distribution of small-scale heterogeneities in the upper mantle, or variable degrees of internal heterogeneity of plumes. © 1990.