Stable isotope geochemistry (original) (raw)

1973, Earth-Science Reviews

This review is intended principally to be a reasonably comprehensive bibliography. It covers papers by workers in stable isotope geochemistry in the United States that have been published or presented at national meetings since the last quadrennial report. The titles of the papers in the bibliography are in most cases indicative of the general content, so only the highlights are described below. showed that with •13C •-7, the 'graphitic' carbon was enriched in 13C by 10-15%o relative to the rest of the carbon in the meteorite. The •13C values are highly variable, even within a single meteorite or in a single nodule within a meteorite. LUNAR ROCKS AND MINERALS REVIEWS A review of oxygen, carbon, and hydrogen isotope geochemistry in the marine environment was written by Garlick [1974]. Several reviews concerning the use of stable isotopes in studies of ore deposits and hydrothermal alteration were published in the October 1974 issue of Economic Geology. Sulfur and carbon isotopes were discussed by Rye and Ohmoto [1974], hydrogen and oxygen isotopes were discussed by Taylor [1974a], and a general review was given by White [1974]. METEORITES Without question, the most exciting recent development with regard to meteorites was the discovery by Clayton et al. [1973] and Clayton [1974] of an 160-enriched component in carbonaceous chondrites. This discovery was made by carefully measuring the 170/160 depletions in certain meteorites (notably in Allende) relative to ordinary chondrites. Instead of being half the ls0/160 depletions, as is normally the case and as is expected on the basis of chemical or physical isotopic fractionation effects, the data indicated that these carbonaceous chondrites represented a mixture of (1) a component having an 180/160 and 170/160 ratio similar to that of the earth, the moon, and ordinary chondrites and (2) an 160-enriched component that may be comprised of essentially pure 160; the latter component is identified as material that existed prior to condensation of the solar nebula 4.6 b.y. ago. These effects and their implications are presently under active investigation by R. N. Clayton and his associates. Onuma et al. [1972a, 1973] measured lsO/•60 ratios in a number of minerals from meteorites and showed that for most ordinary chondrites, equilibrium fractionations were 'frozen in' for various mineral pairs at T > 800øC. Onuma Stable isotope studies of returned lunar samples have served mainly to confirm that the observed isotopic effects are essentially independent of geographic location on the moon. Very similar relationships have now been observed in Apollo 11, 12, 14, 15, 16, and 17 samples and in the Luna 16 and 20 samples. An important result is that the 180/160 ratios of lunar igneous rocks are essentially identical to those of mafic and ultramafic igneous rocks on earth, as well as to the values in ordinary chondrites. All these rocks are distinctly enriched in lsO relative to the basaltic achondrites. The 180/160 fractionations among coexisting minerals are all indicative of very high temperatures of crystallization (T • 1000øC or higher), very similar to values observed in fresh terrestrial basalts [Clayton et al.,