Celeste G. Engel
R.G. Havens
A.E.J. Engel
1965
<p><span>Tholeiitic </span>basalts<span> (</span>oceanic<span> tholeiites) that form most of the deeply submerged volcanic features in the oceans are characterized by extremely low amounts of Ba, K, P, Pb, Sr, Th, U, and Zr as well as Fe</span><sub>2</sub><span>O</span><sub>3</sub><span>/FeO < 0.2 and Na/K > 10 in unaltered samples. </span>Oceanic<span> tholeiites also have rare earth abundance-distribution patterns and ratios of K/Rb (1300) and Sr</span><sup>87</sup><span>/Sr</span><sup>86</sup><span> (0.702) similar to or overlapping those of calcium-rich (basaltic) achondritic meteorites. The close compositional similarities between the </span>oceanic<span> tholeiites and calcium-rich achondrites indicates the relatively primitive nature of the </span>oceanic<span> tholeiites. In contrast, the alkali-rich </span>basalts<span> that cap submarine and island volcanoes are relatively enriched in Ba, K, La, Nb, P, Pb, Pb</span><sup>206</sup><span>, Rb, Fe</span><sub>2</sub><span>O</span><sub>3</sub><span>, Sr, Sr</span><sup>87</sup><span>, Ti, Th, U, and Zr; i.e. in the same elements and isotopes that are concentrated in the sialic continental crusts by factors of 5 to 1000 more than the amounts readily inferred in the </span>upper<span> </span>mantle<span>. These analytical data coupled with the field relationships indicate that the alkali-rich </span>basalts<span> are derivative rocks, fractionated from the </span>oceanic<span> tholeiites by processes of magmatic differentiation, and that the </span>oceanic<span> tholeiites are the principal or only primary magma generated in the </span>upper<span> </span>mantle<span> under the oceans. Studies of the abundances and compositions of continental </span>basalts<span> show that essentially identical tholeiitic lavas, contaminated with Si, K, and the chemically coherent trace elements and radiogenic isotopes from the sial, also have been the predominant or only magma generated in the </span>mantle<span> under the continents. The </span>chemical<span> properties of </span>oceanic<span> tholeiites suggest that the </span>upper<span> </span>mantle<span> probably contains less than (in parts per million): Ba, 10; K, 1000; Pb, 0.4; Rb, 10; Th, 0.2; and U, 0.1. The Sr</span><sup>87</sup><span>/Sr</span><sup>86</sup><span> must be less than 0.7015; Th/U about 2; K/Rb about 1500-2000; and Fe</span><sub>2</sub><span>O</span><sub>3</sub><span>/FeO less than 0.1. The integration of field and petrochemical data with seismic, density, and shock-wave studies suggests that the </span>oceanic<span> tholeiites are either complete melts of the </span>upper<span> </span>mantle<span> or are generated from a mix of this tholeiite and a magnesium-rich peridotite or dunite in proportions up to perhaps 1:4. The Mohorovičić discontinuity under the oceans appears to mark the transition downward from a largely tholeiitic </span>oceanic<span> crust to either tholeiite reconstituted to blueschist or greenschist or to the ultramafic residue left after expulsion of </span>oceanic<span> tholeiite.</span></p>
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10.1130/0016-7606(1965)76[719:CCOOBA]2.0.CO;2
en
Geological Society of America
Chemical characteristics of oceanic basalts and the upper mantle
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