Terrestrial Specimens Research Articles

Abundances of the fourteen rare-earth elements, scandium, and yttrium in meteoritic and terrestrial matter

Abundances and isotopic ratios have been determined for the fourteen rare-earth elements (REE) (Pm excluded) plus Sc and Y by neutron-activation analysis in nineteen meteorites and in three terrestrial specimens. The meteorites investigated included eleven chondrites (1 carbonaceous, 2 pigeonitic, 2 hypersthenic, 3 bronzitic and 3 enstatitic), two calcium-rich and three calcium-poor achondrites, one mesosiderite and two iron meteorites; the terrestrial specimens included a fresh volcanic basalt, and a deep-seated peridotitic and an eclogitic specimen. The absolute abundance of a typical REE (La) in bronzitic and hypersthenic chondrites was found to be 0.34 p.p.m. Relative REE, Sc and Y distributions were identical in the five different chondrite categories. On an absolute basis, the REE, Sc, and Y per 10 6 Si atoms varied monotonically over the chondrite spectrum i.e. from carbonaceous through enstatitic meteorites. Meteoritic isotopic abundances of the sensitive Yb 168/Yb 174 ratio agreed to within ±5 per cent of the terrestrial abundance ratios. Absolute REE abundances in the two calcium-rich achondrites, Nuevo Laredo and Pasamonte, were from 10 to 12 times greater than the average chondritic values. No REE fractionation was found in these achondrites; however, Sc was fractionated by a factor of ~2 relative to the REE. Fractionation of the REE, Sc and Y was found in all three calcium-poor achondrites, Johnstown, Shalka and Norton County, relative to the chondritic meteorites. Absolute REE, Sc and Y abundances varied from ~0.05 to ~0.5 times the comparable values in chondrites. Eu has been depleted in Johnstown and Norton County by ~3—4 times. Th has been depleted in Johnstown to the same degree as La. No REE and Y fractionation was found in the mesosiderite, Estherville; however, Sc has been fractionated. Absolute REE abundances in the two Fe meteorites were ~10 −4 times the REE abundances in chondrites. In one iron, the REE may have been fractionated. Absolute REE. Sc and Y abundances in the uneroded Kilauea Iki-22 basalt, peridotitic and eclogitic specimens were from 30 to 10 times greater than the abundances in chondrites. Compared with chondrites, the light REE have been enriched by factors of up to 5, which agrees with the enrichments of the light REE in terrestrial sediments and igneous rocks, etc. Theoretical calculations of expected REE abundances by Clayton et al. agree well with the present chondritic REE values. For convenience, a description of the detailed procedure for the REE, Sc and Y determinations (now in current use at this Laboratory) has been relegated to the Appendix.

The Determination of the Age of Iron and Stony Meteorites from their Radioactivity*

AbstractObserved meteor and meteorite velocities (Öpik, Boothroyd) indicate that a small fraction, perhaps in the neighborhood of 15 percent, of all meteorites striking the earth are of extra‐solar origin. Helium‐radium age measurements (Paneth, Urry, & Koeck) on 23 iron meteorites, none of which was of known heliocentric velocity, disclose a uniform distribution of last‐solidification ages with no ages clearly greater than the age of the earth. Thus, either the specimens are all from the solar system, as Paneth proposed, or the galactic system is of the same age as the earth. Two critical experiments are in progress, based on the following evidence. The abundance‐ratio of the C, O, Si, Cl, Fe, Co, and Ni isotopes in meteorites has been studied by various workers, and is found to be the same as in the terrestrial elements. Moreover, the ratio of the two independent isotopes of uranium, 92U235 and 92U238, is the same in all terrestrial specimens, regardless of the geologic age of the mineral in which they are found. It is therefore taken as a reasonable premise, but regarded as a working assumption only, that the original isotopic ratio of any element is a constant of nature and is independent of its place of origin in the galactic system. Measurements of the actinouranium vs. uranium I activity‐ratio, and of the specific activity of potassium, are in progress on stony meteorites selected for their known heliocentric velocities. Some of these are of extra‐solar origin while others are solar‐system meteorites. The final results of the analyses give the difference between the age of the atoms in the meteorites and the same type of atoms in the earth. The method is independent of all influence from varying chemical and physical conditions during the history of the meteorite, as it deals entirely with isotopic ratios. The study of both the AcU, UI ratio and the K40, K41 ratio provides an independent check on the basic assumption of the method. The apparatus, technique, and mathematical background for these experiments are fully described. The announcement of the numerical results of these analyses is deferred until all the observations and control runs have been repeated many times, in order to assure their complete reliability.