Harrison S. Brown

ISOTOPIC COMPOSITION AND DISTRIBUTION OF LEAD, URANIUM, AND THORIUM IN A PRECAMBRIAN GRANITE

The isotopic compositions and concentrations of lead and uranium have been determined in some separated minerals and the composite of a granite from Monmouth township, Haliburton County, Ontario. The chemical and mass spectrometric methods that were used are described. The age of the zircon from the granite is 1050 million years. Much of the lead, uranium, and thorium exists in chemically unstable and presumably interstitial phases of the granite. A comparison of the observed amounts of uranium, thorium, and lead in the various minerals with those amounts that should have been present, had these three elements existed within the minerals as closed systems, shows a non-balance of these elements in every case. It appears that the granite as a whole has closely approximated a closed system since it was formed with respect to uranium and its decay products, but has been an open system with respect to thorium and its decay products. Interpretations concerning the relationship of these data to lead ores are discussed.

The Composition of Meteoritic Matter: III. Phase Equilibria, Genetic Relationships and Planet Structure

An attempt has been made to study the composition of meteorites on a more quantitative basis than has been attempted heretofore. In particular, investigations have been made of experimental and theoretical approaches that might lead to more rigid comparisons between terrestrial and meteoritic matter. Stress has been placed particularly upon the following studies: (a) the distribution of elements between meteoritic phases; (b) average composition as a function of metal-phase content; (c) correlation between element distribution, thermochemical data, and general thermodynamic considerations. It is demonstrated that if one assumes that the observed distributions of elements represent equilibrium distributions, then equilibrium must have been established at temperatures of the order of 3000 C. and pressures of the order of

The Construction of a Mass Spectrometer for Isotope Analysis

10^{5}-10^{6} atm

An Experimental Method for the Estimation of the Age of the Elements

. Similarly, it is demonstrated that the conditions at which equilibrium was achieved varied from meteorite to meteorite in such a way that the greater the metal-phase content, the greater the temperature and/or the pressure. The data indicate strongly that meteorites had their origin in a planet similar to the earth in general physicochemical characteristics.

The Branching Ratio ofK40Radioactive Decay

A statistical study has been made of the analyses of the silicate phases of 107 selected stony meteorites. Frequency-distribution curves have been plotted, and arithmetic means, together with standard deviations and precisions, have been calculated for the major constituents.

The Isotopic Composition of Meteoritic Copper

The complete constructional details for a 180° Dempster type mass spectrometer are given. The ions have a radius of curvature of 15 cm, and are accelerated through a potential of 2000 volts. Circuit diagrams for the voltage stabilizers and magnet current stabilizer are included.