Petrogenetic Significance of Isotope and Trace Element Variations in Volcanic Rocks From the Mid-Atlantic

High precision ⁸⁷Sr/⁸⁶Sr analyses, together with determinationsof Rb, Sr, K₂O, Na₂O and, in a few cases, other elements, arereported for about fifty volcanic rocks (mainly basaltic) fromthe Atlantic Ocean basin. Results for dredged basalts from theReykjanes Ridge and Charlie Gibbs Fracture Zone, and an enstatite-forsteritebasalt from Kolbeinsey islet, support the general observationthat ocean-ridge tholeiites have uniformly low ⁸⁷Sr/⁸⁶Sr ratios(0.70294±4) and lithophile element contents comparedwith the most primitive basalts on ocean islands, includingthe Neovolcanic zones of Iceland, although progressive decreasein these quantities away from Iceland has not been confirmed.In contrast, the ocean island alkali basalts generally havehigher ⁸⁷Sr/⁸⁶Sr ratios (0.70334±5 for the Snaefellsnespeninsula of Iceland, 0.70343±4 for Jan Mayen, 0.70509±4for Tristan da Cunha and 0.70369±3 for Bouvetøya).The chief exception is Ascension Island, where volcanic rocksranging from alkali-olivine basalt to trachyte give a mean valueof 0.70284±4. The constancy of this ratio throughouteruptive sequences on any single island indicates that Sr-isotopecharacteristics are primary features. These variations, which are far outside analytical errors, areconsidered in the light of the geochemistry and isotope systematicsof ocean basalts in general. The implied isotopic (and lithophileelement) heterogeneities of the source regions have to be interpretedaccording to either equilibrium or disequilibrium melting models.The former, which is normally assumed, requires large-scale(domain) isotopic inhomogeneities within the mantle, which musthave existed over thousands of m.y. unless the Rb/Sr ratio ofextracted liquids is lower than that of the bulk source (aswould be the case if phlogopite were a residual phase). In thecase of disequilibrium melting, the inhomogeneities are reducedto the mineral scale, as observed in some studies of ultramaficnodules. It is shown that disequilibrium melting models couldgenerally account for the observed isotopic variations of oceanicrocks, although difficulties are again encountered unless phlogopiteis a stable residual phase. Evaluation of the relative importanceof these melting processes cannot be made at the present time.