Rare-earth elements and isotope ratios at DSDP Leg 82 Holes

We report 48 analyses of rare-earth elements (REE) and 15 143Nd/144Nd and 87Sr/86Sr analyses for basalts from the eight holes drilled during Leg 82. Discrete and distinct REE patterns and 143Nd/144Nd 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 143Nd/144Nd 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 143Nd/144Nd 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.

Geochemistry at DSDP Leg 92 Holes

The major-element and most of the trace-element data from the different laboratories that contributed to the study of samples recovered during Leg 82 are presented in the following tables. The different basalt groups, identified on the basis of their chemical properties (major and trace elements), were defined from the data available on board the Glomar Challenger as the cruise progressed (see site chapters, all sites, this volume). Most of the data obtained since the end of the cruise and presented in these tables confirm the classification that was proposed by the shipboard party (see site chapters, all sites, this volume). Nevertheless, special mention should be made about Site 564. The shipboard party proposed a single chemical group at this site but noticed significant variations down the hole, mainly in trace-element data. However, the range of variation was small compared to the precision of the measurements. These variations were confirmed by the onshore studies (see papers in Part IV of this volume, especially Brannon's paper, partly devoted to this topic).

(Table 2) Geochemistry and petrograhy of DSDP Leg 82 Holes and MAPCO cruise of Jean Charcot in the Mid-Atlantic Ridge area

The compositions of abyssal glasses obtained on Leg 82 of the awGlomar Challenger and the MAPCO cruise of Jean Charcot have been investigated. Two main compositional groups of Atlantic glasses (A1 and A2) that are separated in space and time were identified. The distribution of these groups in the studied area allowed mapping of the transition zone from A1 to A2 between 30-35°N MAR. We infer that the compositional groups of abyssal glasses of the Atlantic and other oceans reflect the depth of separation of primary melts from the oceanic mantle. Specifically, the primary melt of Group A1 separates from the mantle at a depth of 30-60 km (spinel-peridotite facies) and those for Group A2 from a depth of 15-30 km (plagioclase-peridotite facies). Modifications of dynamic models of the ocean lithosphere are discussed.