COMPOSITION OF THE EARTH UPPER-MANTLE

Modeling of N-MORB generation has been performed using PARMEL and COMA GMAT computer codes. Primary magma composition was calculated as mixtu re of individual melt fractions arising during decompression dynamic m elting of peridotitic material. This composition has been further modi fied by fractional crystallization until MgO content of the derivative melt reaches the value of the average N-MORB. This modeling requires larger contribution from the deeper parts of the melting volume. The s eparation of the most incompatible elements (U and Th) during this pro cess becomes possible due to the participation of small fraction volat ile-rich melt coming from the garnet lherzolite zone situated much dee per than the main part of the melting volume. The estimated parameters governing dynamic processes of magma generation were used to evaluate abundances in N-MORB and OIB sources for a number of incompatible ele ments. Their primitive mantle normalized values exhibit the same featu res as those discovered by Hofmann [1] by comparison of oceanic and co ntinental crusts. The estimated abundances of incompatible elements in N-MORB and OIB sources show that for the most incompatible elements c ontribution from the OIB-source to their budget in the bulk upper mant le should not be neglected. The simulation of trace-element behavior d uring the generation of highly enriched primitive magmatic melts (kimb erlites and lamproites) shows that the main source of incompatible ele ments for them lies in the depleted mantle (N-MORB-source).