Chemical Characteristics of Oceanic Basalts and the Upper Mantle

Abstract

Research Article| July 01, 1965 Chemical Characteristics of Oceanic Basalts and the Upper Mantle A. E. J ENGEL; A. E. J ENGEL Dept. Earth Sciences, University of California, La Jolla, Calif Search for other works by this author on: GSW Google Scholar CELESTE G ENGEL; CELESTE G ENGEL U. S. Geological Survey, La Jolla, Calif Search for other works by this author on: GSW Google Scholar R. G HAVENS R. G HAVENS U. S. Geological Survey, Denver, Colo Search for other works by this author on: GSW Google Scholar Author and Article Information A. E. J ENGEL Dept. Earth Sciences, University of California, La Jolla, Calif CELESTE G ENGEL U. S. Geological Survey, La Jolla, Calif R. G HAVENS U. S. Geological Survey, Denver, Colo Publisher: Geological Society of America Received: 21 Dec 1964 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1965, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1965) 76 (7): 719–734. https://doi.org/10.1130/0016-7606(1965)76[719:CCOOBA]2.0.CO;2 Article history Received: 21 Dec 1964 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. E. J ENGEL, CELESTE G ENGEL, R. G HAVENS; Chemical Characteristics of Oceanic Basalts and the Upper Mantle. GSA Bulletin 1965;; 76 (7): 719–734. doi: https://doi.org/10.1130/0016-7606(1965)76[719:CCOOBA]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Tholeiitic basalts (oceanic 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 Fe2O3/FeO < 0.2 and Na/K > 10 in unaltered samples. Oceanic tholeiites 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 tholeiites and calcium-rich achondrites indicates the relatively primitive nature of the oceanic tholeiites.In contrast, the alkali-rich basalts that cap submarine and island volcanoes are relatively enriched 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 in the sialic continental crusts by factors of 5 to 1000 more than the amounts readily inferred in the upper mantle.These analytical data coupled with the field relationships indicate that the alkali-rich basalts are derivative rocks, fractionated from the oceanic tholeiites by processes of magmatic differentiation, and that the oceanic tholeiites are the principal or only primary magma generated in the upper mantle 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 isotopes from the sial, also have been the predominant or only magma generated in the mantle under the continents.The chemical properties of oceanic tholeiites suggest that the upper mantle 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 Sr87/Sr86 must be less 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 either complete melts of the upper mantle 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 oceanic crust to either tholeiite reconstituted to blueschist or greenschist or to the ultramafic residue left after expulsion of oceanic tholeiite. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.