The mantle source for MORBs is both one the best understood and least understood mantle domains. To first order, it appears to be the depleted complement to the LIL-enriched continental crust, and, compared to the mantle source for OIBs, it is relatively homogeneous. On the other hand, the sustained effort of the community to discern the vagaries of MORB melting and the evolutionary history of the mantle that melts has led us to ask important questions about the nature and origin of chemical variations in MORBs for which there are no simple answers.
Mantle disciples of old were convinced that the mantle was "homogeneous". Pioneers of the isotope trade subsequently proved beyond a shadow of a doubt that the mantle was chemically and isotopically heterogeneous, and that at least some component of that heterogeneity was ancient. But still, the MORB mantle was considered to be by-and-large homogeneous (after all, MORBs are geochemically rather boring...). Numerous compelling observations argue that the principal source of MORB heterogeneity is the mixing of plume-derived material into otherwise "normal" MORB mantle. This effect is very clearly seen as the Mid-Atlantic Ridge approaches the Icelandic plume as well as at various locales including the Galapagos Islands, Easter Island, the Azores, and the southern MAR near St. Helena.
Detailed investigations of small seamounts that represent barely-off-ridge eruptives, have further shown that the MORB mantle is markedly heterogeneous, even where it is far removed from the nearest plume. These heterogeneities, best explained as being blobs or veins of enriched material in a peridotite matrix, have geochemical affinities with various types of plumes. It is reasonable to suppose, then, that the day-to-day operation of the MORB mantle leads to the inclusion of bits and pieces of relatively enriched, perhaps recycled material.
However, in terms of major element chemistry, MORBs and abyssal peridotites show systematic variations with distance from hotspots and ridge depth (Dick et al., 1984; Klein and Langmuir, 1987). Shallow ridges are thought to represent a high degree of melting with a relatively deep intergrated depth of melting. Deep ridges segements
are thought to represent small degrees of relatively shallow level melts. Salters (1995) has argued that ridge-segment-averaged d(Lu/Hf) and d(Sm/Nd) correlate with ridge depth, but suggests, in contrast to the Fe8.0 data, that larger proportions of melt at deep ridges are generated in the garnet stability field. Bourdon et al. (1995) present a similar argument, but with contradictory conclusion, based on their identification of a global, negative correlation between MORB (230Th/238U) and ridge depth, which they interpret
as reflecting greater depths for the initiation of melting beneath shallow ridges, due to the greater potential for Th-U fractionation by garnet.
The validity of all of these arguments hinges on the assumption of compositional similarity or regularity between all MORB sources. The correlations themselves are "rough", and therefore, strict chemical homogeneity is not a requirement, but systematic compositional variation with ridge depth (or with Na8.0, Fe8.0, d(Sm/Nd), or 230Th/238U) undermines the basis for the interpretations of the correlations. If the extent to which the "normal" MORB source is polluted with plume components is a function of any of these parameters, then source heterogeneity may play a very important role in producing the observed correlations. In this case, it becomes difficult to draw simple inferences concerning the depth or extent of melting. We have examined data for MORBs from the literature and find that global correlations between MORB isotope and trace element ratios, and between these parameters and ridge depth, do indeed exist. Our study suggests that an important component of variation is due to variable pollution of the MORB source by plume-like source materials with a range of compositions (as observed in OIBs). This is not to say that the extent and depth of melting do not play a role in producing chemical variations in MORBs, but that these must be considered in the context of systematic source variations.
Bourdon, B., Zindler, A., Elliot, T. & Langmuir, C.H., Nature, submitted (1995).
Dick & Fisher, In Kimberlites II, (Kornprobst, J., ed.) 295-308 (1984).
Klein & Langmuir, J. Geophys. Res. 92, 8089-8115 (1987).
Salters, Earth Planet. Sci. Lett., submitted (1995).