Refertilization in a Convecting Mantle: Effects on Primary MORB Compositions

Abstract

The widely held view is that mid-ocean ridge basalts (MORBs) are produced by polybaric fractional melting of the sub-oceanic mantle in which the composite primary MORBs are derived from a relatively fertile mantle by 10-25% total melting (McKenzie, 1985; Johnson et al., 1990). The differences in primary MORB compositions are inferred to result principally from differences in the pressure and extent of melting (Klein and Langrnuir, 1987; Kinzler and Grove, 1992). The compositional variations between abyssal peridorites, in this scenario, reflect different degrees of fractional melting in which the least depleted peridotites represent the residues of ~10% melting and the most depleted peridotites represent the residues of ~25% melting (Dick and Fisher, 1984; Michael and Bonatti, 1985). It has been suggested, however, that the abundances of moderately incompatible elements (e.g., Na) in abyssal peridotites are not compatible with these rocks having been formed as residues of variable degrees of partial melting (Elthon, 1992). It was suggested instead that abyssal peridotites are a mixture of two distinct components. One component is a depleted harzburgite that has either no clinopyroxene (CPX) or that has < 2% CPX with very low abundances of Na20 (0.01 wt. %), Ti (300 ppm), Zr (0.1 ppm), and Ce (0.02 • chondrites). The other component is a basaltic liquid, which is potentially highly variable in composition. If the chemical variations in abyssal peridotites reflect some type of melt addition (or refertilization) process into depleted harzburgite mantle, there are profound implications for how the source regions of MORBs have evolved. Most importantly, if the mantle melts to produce a harzburgite (< 2% CPX) on a routine basis, the differences in primary composite MORBs will reflect most strongly the differences in the local mantle composition as the mantle enters the midocean ridge melting regime. In light of the variable compositions of mantle xenoliths and peridotite massifs as well as the strong evidence for melt addition processes in these samples, it might also be expected that the sub-oceanic mantle is both variable in composition and is influenced by meltaddition (refertilization) processes. It is proposed here that the MORB source region has been through the mid-ocean ridge melting regime at least 1 to 5 times during normal mantle evolution and consists of a highly depleted harzburgite mantle (similar to that found in peridotite massifs) mixed with basaltic/komatiitic material that has refertilized the mantle. In this scenario, the ascending mantle beneath midocean ridges varies significantly in composition, depending on the amount and type of basaltic/ komatiitic mantle added. Within the melting regime, the mantle melts to whatever extent is necessary to deplete the mantle to harzburgite (+ a few percent clinopyroxene) where thermal considerations stop further melting. It is proposed, therefore, that differences in primary MORB compositions are principally due to differences in the amount and composition of basaltic/komatiitic material added into the source region.