Estimates of possible diffusional effects in trace element separations

Igneous material dredged from the Rio Grande rise, South Atlantic Ocean, includes basaltic rocks, some having mafic nodules and megacrysts, and volcanic breccias composed largely of basaltic fragments. These samples represent the only volcanic rocks recovered from this aseismic rise. Bulk compositions show alkalic basalt, trachybasalt, and trachyandesite; the rock types are similar to those of nearby Tristan da Cunha, Gough, and the Walvis ridge. Microprobe analyses show basaltic groundmass to have olivine, Fo₈₅, pyroxene, Fs₁₃Wo₄₆, feldspar, An₇₁, plus interstitial alkali feldspar. Mafic nodules and megacrysts have olivine, Fo_86–90 and pyroxene Fs_6–7.5Wo_45–46; Al₂O₃ 2.5–4 wt.%.The Rio Grande rise rocks have compositional characteristics of an alkalic basaltic suite, and not of mid-ocean ridge tholeiite. Based on mineral compositions, nodules and megacrysts in basalt are interpreted as cognate inclusions. Because oceanic alkalic basaltic rocks are almost invariably associated with islands and seamounts, the Rio Grande rise probably represents a series of alkalic-basalt islands that formed and eventually subsided during rifting of the South Atlantic; the dredged volcanic breccias are probably slump deposits from those volcanoes. This interpretation lends support to the Rio Grande rise having formed at a hot spot, but the possibility of alkalic rocks having formed along fracture zones should not be discounted.