THE ORIGIN OF ABYSSAL PERIDOTITES - A NEW PERSPECTIVE

Abyssal peridotites have been interpreted to be residues of mantle mel ting beneath ocean ridges. Recent experimental data and models of mant le melting allow quantitative tests of this hypothesis. The tests show that abyssal peridotites are not purely melting residues. Their modal proportions and whole-rock compositions have far more olivine than wo uld be predicted from melting models. Nonetheless, the correlations be tween modal proportions of olivine and residual mineral chemistry, and the relationship between associated basalt and peridotite composition s, require an important role for melting. We suggest that abyssal peri dotite compositions result from a combination of melting and crystalli zation processes that are both a natural response to ascent of solid a nd melt beneath an ocean ridge. Different extents of melting create a range of residual peridotite and mantle melt compositions. The buoyant melts migrate upwards, where they encounter the surface thermal bound ary layer and crystallize olivine. The greater the ambient extent of m elting of the mantle, the higher the normative olivine contents of the melt, and the more melt is produced. Hence greater extents of melting lead to more olivine crystallization at shallow levels. This correlat ion between melting and crystallization within the mantle preserves th e observed relationships between peridotite modes and mineral composit ions. Significant implications of these results are: (1) the bulk comp osition of the oceanic crust differs from the primary melt composition s produced by partial melting of the mantle because of olivine crystal lization at the thermal boundary layer; (2) the actual thickness of ig neous crust may be variably thinner than would be calculated assuming total melt extraction; and (3) peridotite modes can be used to infer p olybaric mantle melting reactions only if the accumulated olivine is r emoved appropriately. (C) 1997 Elsevier Science B.V.