Trace element variations in Atlantic Ocean basalts and Proterozoic dykes from northwest Scotland: Their bearing upon the nature and geochemical evolution of the upper mantle

Abstract

Trace element data on ocean floor and ocean island basalts from the Atlantic and from early Proterozoic dyke swarms are considered in relation to the nature and causes of upper mantle heterogeneity. Studies of dredged and drilled basalts from the North Atlantic have enabled the compositions of erupted ridge basalts to be monitored in space and time and have revealed the following features: (a) that while melting processes may give rise to appreciable REE variation at a particular site, ratios of the more incompatible elements remain remarkably consistent in basalts erupted at a particular ridge segment for tens of millions of years and (b) that there are major differences in incompatible element ratios in basalts erupted at different ridge segments. The relationships suggest that the volumes of mantle with these characteristic incompatible element ratios must be large, but that the processes associated with basalt generation are not capable of significantly changing incompatible element ratios. Nevertheless, the processes which have produced these incompatible element variations must be incorporated into any model for mantle evolution. Our preferred model invokes both crustal extraction and incipient mantle melting as the major mechanisms for changing incompatible element mantle source regions: both represent very small degrees of mantle melting. A depleted upper mantle variably veined with incompatible element enriched liquids or fluids (derived through incipient melting at greater depths) is most consistent with the trace element and isotopic compositions of North Atlantic basalts, many of which are not normal mid-ocean ridge basalts (N-type MORB). Ocean island basalts may represent more intensely veined and, in some cases at least, more refractory sources sampled at lower degrees of partial melting. Mantle from which incipient melts have been removed probably represents the N-type MORB source. Extraction of crustal material, having high contents of the large ion lithophile elements but low contents of the high field strength ions (ie., Ta and Nb), from a veined/metasomatized mantle source, leaves the upper mantle residue enriched in Ta and Nb relative to the large ion lithophile elements. This might explain the enrichment of Ta and Nb in most mantle sources of continental and ocean island basic lavas. Mantle sources enriched by incipient melting processes are continually dissipated by mantle convection, but may be preserved for a significant time beneath stable continents. Trace element data on early Proterozoic tholeiitic to picritic dyke swarms from Scotland suggest that short-term mantle heterogeneities may have existed beneath the region at that time and could have been related to the processes of crustal generation 600 m.y. earlier.