P.S. Goel

Total nitrogen in meteorites

Total N has been measured in a number of meteorites by neutron activation analysis using the reaction N14(n, p)C14. From each meteorite a number of chips have been analysed to investigate the variation of N contents in a sample. Many meteorites are found to contain a heterogeneous distribution of N. Eighteen chondrites, mostly of the classes C3, H4, H5, L4, L5, L6 and LL6, and six achondrites are found to have average N contents of 10–45 ppm. These do not show any clear-cut dependence of N on petrological group. However, the inherent heterogeneity or the fact that from most meteorite classes only single falls were studied might be responsible for this lack of correlation. In Cold Bokkeveld (C2) N is high (420 ppm). Unlike C, N content of ureilites is low (26 ppm). Nitrogen is enriched in the non-magnetic as compared to the magnetic fractions in H-group chondrites. Analyses of sieved Bjurbole phases show no enrichment of N in finer matrix material, nor any depletion in chondrules. In two gas-rich meteorites, Kapoeta and Assam, there is no excess N in the dark phases. Nine iron meteorites and three mesosiderites were analysed. Twenty analyses of Canyon Diablo and seven of Odessa establish a very heterogeneous N distribution in these meteorites.

Huge variations in the isotopic ratio 196Hg/202Hg in some acid-insoluble residues of Sikhote Alin and other iron meteorites

The insoluble materials obtained on dissolution of iron meteorites in 2 M H2SO4 have been analysed by radiochemical neutron activation to measure the isotopic ratio 196Hg/202Hg of Hg released at different temperatures. In many cases this ratio is found to be anomalous; its extreme values, relative to the reagent Hg, being: < 0.003 (2σ limit) and 1.26 ± 0.05. Often, in the same residue, both positive and negative deviations are found at different temperatures. n nThe anomalous, normal, and isotopically diluted Hg distillates have been re-irradiated to eliminate the influence of neutron self-shielding and interfering reactions from matrix elements. The isotopic anomaly persists in the distillates, proving that any artifacts during irradiation are essentially absent. n nThe presence of isotopically anomalous Hg in iron meteorites puts some severe constraints on the formation history of these objects. Either iron meteorites have been formed in a non-magmatic process entrapping presolar grains, or some foreign matter has been injected into their solidified parent bodies by collisions with objects of different isotopic composition.

Nitrogen and xenon in acid residues of iron meteorites

Abstract Total nitrogen, measured by neutron activation analysis, is highly enriched in residues from iron meteorites obtained by dissolution of the metal in dilute H 2 SO 4 , relative to the bulk value. On the average, the residues, representing 3% mass, contain 22% of total N. Group IA has more dissolved N than IIIA. Lithium and Ir show a distribution pattern parallel to N. Total Xe has been measured in several residues and its isotopic composition is, similar to atmospheric Xe for mass numbers 131 to 136 but not for 124 Xe and 126 Xe which are strongly depleted in the non-magnetic residues. It is suggested that iron meteorites have trapped in their micro-inclusions, some pre-solar nebular matter which is isotopically heterogeneous.

Simultaneous determination of nitrogen and lithium by thermal neutron activation analysis

Abstract The determination of lithium and nitrogen in a variety of materials by thermal neutron activation is described. The nuclear reactions used are 14 N(n,p) 14 C and 6 Li(n,α) 3 H. Radionuclides. 14 C and 3 H for counting are isolated by fusion of the irradiated sample in a vacuum system. Data are presented on lithium and nitrogen concentrations in several terrestrial standards. The new method allows reliable measurements on 10–50-mg samples.

Total nitrogen in iron meteorites

Total nitrogen contents have been measured by RNA of 30 iron and 4 stony-iron meteorites. Wide variations in N concentrations are noted (0.5 ppm to about 200 ppm). As a group, the IA irons have the highest nitrogen. This element is positively correlated with C, Ga, and Ge. Samples of meteorites that have been exposed to shock do not show any abnormal N distribution.

Total nitrogen in lunar soils, breccias and rocks

The total nitrogen contents of a number of lunar samples from Apollo 16 and 17 missions are reported. Solar wind is the main source for the observed excess nitrogen in most fines. Total nitrogen in the soils is found to be proportional to the solar wind rare gases Ar36 and Xe132. Linear correlations are also noted between the agglutinate contents of the soils and their carbon and nitrogen contents. Seventeen soils (Apollo 15, 16 and 17) have been sieved and nitrogen has been measured in various grain size fractions. An inverse correlation between the mean grain diameter and the nitrogen contents is seen, showing that a large fraction of the solar wind nitrogen is surface correlated. An apparent volume component, due to the presence of agglutinates, is found in most soils.

Lithium in tektites and natural glasses

Abstract Lithium concentrations in various tektites and natural glass samples have been determined using RNAA. For most tektites Li abundances range from 40 to 60 ppm, with the exception of bediasites (23 ppm). In other natural glasses of presumed impact origin, the Li values are considerably lower (6–11 ppm). It is suggested that the Li concentrations reflect mostly the original abundance of this element in parent materials. A terrestrial origin for tektites is supported by the Li range overlap between tektites and sedimentary rocks

Non-cosmogenic lithium-6 in iron meteorites

A neutron activation method is used to measure6Li via the reaction6Li(n, α)3H in iron meteorites. It is found that most6Li occurs in non-metallic inclusions which can be separated by dissolution of the metal in 4 M H2SO4. The non-magnetic portion of such a residue has as high as 0.9 ppm6Li, while in the bulk sample6Li ranges from 0.02 to 10 ppb. Average6Li contents (ppb) for various classes are IA, 1.8; IIAB, 1.6; and IIIA, 0.4. Cosmic-ray-produced6Li is generally much smaller than the native6Li.

Determination of nitrogen in iron meteorites

Nitrogen has been determined in Odessa and Canyon Diablo iron meteorites by three different methods: thermal neutron activation, Kjeldahl distillation, and alkali fusion. The nitrogen contents vary from 25 to 240 ppm. Large variations are observed even in adjacent pieces. Nitrogen minerals are apparently non-uniformly distributed in the metal.

Nitrogen and trace elements in tektites and impact glasses

Abstract Nitrogen and several trace elements have been studied in samples of one Muong Nong tektite, irghizites (tektite-like glass)/zhamanshinites (impactites) and several obsidian glasses. A detailed investigation of inter- and intra-layer samples from the Muong Nong tektite shows a chemical heterogeneity both within and between the layers. In general, the elemental variations seem to be due to a varying FeO content. Irghizites, both SiO2-rich and SiO2-poor, are compositionally more homogeneous than the zhamanshinites. The heterogeneous distribution of nitrogen, observed in the Muong Nong tektite and the zhamanshinites, is unlike that in any other tektite studied so far. This shows clearly that zhamanshinites have suffered lower peak temperatures than irghizites and Muong Nong tektites lower ones than normal “splash-form” tektites. Available chemical and petrological data on Muong Nong tektites suggest that they were formed by in situ melting of soil.