Chemical Characteristics of Oceanic Basalts and the Upper Mantle
Tholeiitic basalts (oceanic tholeiites) that form most of tne deeply submerged volcanic features in the oceans are characterized bv extremely low amounts of Ba, K, P, Pb, Sr, Th, U, and Zr as well as Fe2O3/FeO < 0.2 and Na/K > 10 in unaltered samples. Oceanic tholeiitles also have rare earth abundance-distribution patterns and ratios of K/Rb (1300) and Sr87/Sr86 (0.702) similar to or overlapping those of calcium-rich (basaltic) achondritic meteorites. The close compositional similarities between the oceanic tholciites and calcium-rich achondrltes indicates the relatively primitive nature of the oceanic tholeiites.
In.contrast, the alkali-rich basalts that cap submarine and island volcanoes arc relatively enrichcd in Ba, K, La, Nb, P, Pb, Pb206, Rb, Fe2O3, Sr, Sr87, Ti, Th, U, and Zr; i.e. in the same elements and isotopes that are concentrated ir. the sialic continental crusts by factors of 5 to 1000 more tljan the amounts readily inferred in the upper mantle.
These analytical data coupled with the field relationships indicate that the,alkali-rich basalt are derivative rocks, fractionated from the oceanic tholeiites by processes of magmatic differentiation, and that the bceanif tholeiites are the principal or \ only primary magma generated in the upper irtantic under the oceans.
Studies of the abundances and compositions of continental basalts show that essentially identical tholeiitic lavas, contaminated with Si, K, and the chemically coherent trace elements and radiogenic isotope from the sial, also have been the predominant or only magma generated in the mantle under the continents.
The chemical properties of oceanic tholeiites suggesjt that the upper mantle probably contains less than(in parts per million): Ba, 10; K, 100; Pb, 0.4; Rb, 10; Th, 0.2; and U, 0.1. The Sr87/Sr86 must be lesj than 0.7015; Th/U about 2; K/Rb about 1500-2000; and Fe2O3/FeO less than 0.1.
The integration of field and petrochemical data with seismic, density, and shock-wave studies suggests that the oceanic tholeiites are cither complete melts of the upper mantle or are generated from a mix of- this tholeiite and a magnesium-rich peridotite or dunik in proportions up to perhaps 1:4.
The Mhorovicic discontinuity under the oceans appears to mark the transition downward from a largely tholeiitic oceanic crust to either tholeiite reconstituted to blueschist or greenschist or to the ultramafic residue left after expulsion of oceanic tholeiite
Figures & Tables
Edited by Peter A. Roma and published in 1976, Mid-Atlantic Ridge contains a collection of related articles reprinted from other Geological Society of America publications as well as a brief review of exploration of the Mid-Atlantic Ridge from 1960 to 1975.