Total nitrogen content of deep sea basalts

Abstract

Various models have been suggested concerning the origin and evolution of the earth's atmosphere. An estimate of the nitrogen content of the mantle could further constrain atmospheric models. Total nitrogen content was determined by thermal neutron activation analysis via 14N(n,p) 14C. The 14C was converted to carbon dioxide and counted in miniature low level proportional counters. The total nitrogen content of U.S.G.S. standards BCR-1 and G-2 as determined by different laboratories is variable, probably due to atmospheric adsorption by the finely ground samples. Total nitrogen content was determined in deep sea basalt glasses from three regions: East Pacific Rise ( 15 ± 4, 18 ± 4, and 7 ± ppm2 N ), Mid-Atlantic Rift (FAMOUS Region: 22 ± 5, 18 ± 3, and 10 ± 2 ppm N ) and the Juan de Fuca Ridge ( 17 ± 4 ppm N ). Matrix material from the same samples as the glasses was available from the East Pacific Rise ( 37 ± 6, 26 ± 4, and 34 ± 6 ppm N ) and the Mid-Atlantic Rift ( 39 ± 4 ppm N ) which are about 50 to 100% greater than the associated glasses. The increased matrix abundance may be due to incorporation of chemically bound nitrogen from sea water rather than dissolved molecular nitrogen. The nitrogen content of the FAMOUS samples are inconsistent with the model of Langmuir et al. (1977) for petrogenesis based on trace element data. Factors which can affect the observed nitrogen content in the basalts and the interpretation in terms of the mantle nitrogen abundance are discussed ( e.g. partial melting and degassing of the basalts). A lower limit of about 2 ppm N in the mantle can be estimated.