X-ray fluorescence spectrometry was used on board the Glomar Challenger during Leg 82 for analysis of "hygromagmaphile" trace elements that are useful to identify so-called "depleted" or "enriched" basalts; the results were used in choosing the new sites to be drilled on the basis of the "enriched" or "depleted" character of previous sites. This chapter describes a hitherto unpublished method for deducing concentrations from intensity measurements; the method yielded the precision (better than 1 ppm for Nb) necessary for purposes of the cruise.The procedure for the matrix-effect correction is described. The case of Nb is discussed in detail because Nb concentrations in basalts are the lowest among the elements investigated (Nb, Zr, Ti, Y, V) and because it is the key element for deducing the enriched or depleted character of the different basalt units. Ta neutron-activation measurements made on shore after the cruise demonstrate, on the basis of the well-known constant Nb/Ta ratio in basalts, that the accuracy of the shipboard measurements of Nb concentrations in basalt units is about 0.5 ppm.

We report 48 analyses of rare-earth elements (REE) and 15 143 Nd/ 144 Nd and 87 Sr/ 86 Sr analyses for basalts from the eight holes drilled during Leg 82. Discrete and distinct REE patterns and 143 Nd/ 144 Nd ratios characterize the eight holes, with little variation observed downhole except in Holes 561 and 558, thus suggesting dominantly long-term temporal and large-scale spatial variations in the mantle source of these basalts beneath the Mid-Atlantic Ridge over the last 35 Ma of its spreading activity. There is a good inverse correlation between 143 Nd/ 144 Nd and (La/Sm) EF with one exception in Hole 558 (approximately 35 Ma), the latter suggesting a recent (35 Ma) light REE depletion event, perhaps caused by dynamic or fractional melting. Short-term temporal and small-scale spatial mantle source variability is also evident in Hole 561 (approximately 18 Ma), which has rapid fluctuations in REE patterns and 143 Nd/ 144 Nd ratios (suggesting rapid transfer of magma from the time of melting) and is evidence contrary to the presence of a well-mixed magma chamber at this particular site and time. The mantle source variations noted can be interpreted within two extreme models. The first model invokes a convecting mantle depleted in large ion lithophile elements (LILE) and containing lumps (or veins) of LILE-enriched material of various shapes and sizes, passively and randomly distributed throughout. A second more restrictive model considers the interaction of fixed mantle plumes and the LILE-depleted asthenosphere flowing towards a migrating Mid-Atlantic Ridge (MAR) axis. With the exception of Hole 558 and the uncertainties of reconstructions of absolute plate movements in the region, the observed variations can be explained by two hot spots; the nearly ridge-centered Azores hot spot (plume) and another hot spot located beneath the African plate that may be affecting the source of basalts currently erupting at the MAR axis at 35°N and which, in the past, would have produced the New England chain of seamounts on the North American plate and (later) the Atlantis-Great Meteor chain on the African plate. Basalts erupted south of the Hayes Fracture Zone have not been affected by either of these two hot spots over the last 35 Ma and appear to have been continuously derived from the LILE-depleted source. Subaxial flow downridge from the Azores plume appears to have started 9 Ma, on the basis of the southward converging V-shaped time-transgressive ridges branching from the Pico and Corves Island, or not earlier than 16 Ma, on the basis of the geochemical results. Variations within Hole 558 remains unexplained by the latter model, unless we hypothesize a third hot spot.

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