Oceanic Provinces Research Articles

New data on oceanic basalts, and their importance in general geology

The author points out that the associations of (continental) trap rocks and tholeiites of oceanic basins are very similar. Identical “mantle magmas” develop under continents and oceans and are largely independent of the nature of the overlying crust. Lavas of island arcs and geosynclines, distinctly different, have andesite-basalt associations in addition to tholeiitic ones, indicating geologic similarity of these kinds of features. This similarity is also indicated by the fact that island arcs are in some places along the trend of continental geosynclines; focal zones of deep earthquakes are present in both kinds of areas, have similar sedimentary sequences and have the same type of volcanic activity. The author uses these similarities and contrasts to postulate “that the principal structural types of the earth are represented by geosynclines and folded provinces on the one hand, and by (relatively) stable oceanic and continental provinces on the other.” Olivine basalt magma, at least in ocean basins, is...

Lead isotope compositions and uranium decay series disequilibrium in recent volcanic rocks

Concentrations of uranium, thorium, radium, total lead, lead-210 and lead isotopic composition are given for recent volcanic rocks from Tristan da Cunha, Faial, Azores and Mt. Vesuvius. Disequilibrium observed among uranium daughter products is interpreted in terms of chemical fractionations occurring during magmatic processes. In all cases, the fractionation pattern Ra > Th > U was observed. The fractionation indicated by the 230Th/238U ratio implies that removal of liquids from mantle sources lowers the Th/U ratio of the source region. This is consistent with the fractionation postulated to explain observed lead isotopic compositions in many oceanic volcanic provinces. The U-Pb fractionation pattern is more difficult to interpret than the Th-U and Ra-U fractionations.