George W. Wetherill

Lead metabolism in the normal human: stable isotope studies.

Kinetic and metabolic balance studies in a healthy man fed a diet normal in lead content and labeled with lead-204 indicated that approximately two-thirds of his assimilated lead was dietary in origin; the remainder was inhaled. Kinetic analysis shows that the isotopic data can be interpreted by a three-compartment model.

Stone Meteorites: Time of Fall and Origin

The fact that twice as many chondritic meteorites are observed falling in the afternoon as in the morning is not believed to be primarily of social origin, but to be a dynamic effect. Monte Carlo calculations show that the observed afternoon excess is not compatible with a lunar or Apollo asteroidal origin. Compatibility appears to require a source having an aphelion near Jupiter, such as could be provided conceivably by the Hilda or Trojan families of asteroids, or by short-period comets.

Asteroidal Source of Meteorites

The evolution of asteroidal orbits initially near the Kirkwood Gap at the 1:2 commensurability with Jupiters period provides a mechanism for the production of meteorites from the asteroid belt without excessive velocity change. The resulting yield (∼ 109 grams per year) and the orbital elements of Earth-crossing objects are in agreement with observational data on meteorites.

Chondrites: Initial Strontium-87/Strontium-86 Ratios and the Early History of the Solar System

A sodium-poor, calcium-rich inclusion in the carbonaceous chondrite Allende had a 87Sr/86Sr ratio at the time of its formation of 0.69880, as low a value as that found in any other meteorite. The higher 87Sr/86Sr ratios found in ordinary chondrites indicate that their formation or isotopic equilibration occurred tens of millions of years later.

RbSr studies of CI and CM chondrites

Abstract RbSr whole rock analyses have been performed on 2 CI and 3 CM chondrites. Four of these stones (Ivuna, Orgueil, Cold Bokkeveld and Erakot) were previously studied in this laboratory and were shown to be discordant from a 4.6 Gyr isochron. The fifth, Murchison, was not previously studied. The new data support the discordance of the first four stones, and indicate that Murchison is also discordant. Studies of Sr isotope ratios in unspiked Orgueil show that the discordance is not due to inhomogeneities in the Sr 84 Sr 86 ratio caused by incomplete mixing of nucleosynthesis products. In order to gauge the effects of weathering, two leaching experiments were performed on fresh, interior samples of Murchison; one for a period of 1.5 hr and the other for 117 hr. The results indicate that the relative solubility of nonradiogenic Sr is approximately twice that of Rb and radiogenic Sr is more soluble than the nonradiogenic Sr. This gives the residue a lower model age than the whole rock both by increasing the Rb Sr ratio and by decreasing the Sr 87 Sr 86 ratio. This result is in contrast to that expected from studies of ordinary chondrite finds, which generally show higher model ages than falls. The constancy of K Rb and K Sr ratios between the two leaching experiments, and their difference from the unaltered whole rock ratios suggest that the bulk ratios are produced by dissolution of a single phase, and the higher radiogenic Sr content by selective leaching of other phases.

Delayed appearance of tracer lead in facial hair.

Three adult men were fed 204 Pb - a rare, stable isotope of lead - daily for about 100 days. Simultaneous blood and facial hair measurements of this tracer and of total lead concentrations were made by mass spectrometric isotope dilution analysis. Although the blood showed an immediate response to the intake of the tracer, the facial hair showed a more gradual response and a delay of approximately 35 days. Since the pattern of appearance of lead in hair does not appear to represent a simple time delay of blood lead concentration, the existence of a physiological pool of lead fed by the blood and giving rise to the content in hair is suggested. Hair lead values should, therefore, be interpreted as the integral of the blood lead values over the mean life of this intermediate pool - about 100 days.

Dynamical Studies of Asteroidal and Cometary Orbits and their Relation to the Origin of Meteorites

A quantitative evaluation of the possibilities of meteorites being derived from various objects in the solar system has been made. Meteorites derived from Apollo asteroids have eccentricities in agreement with observed values; exposure ages are somewhat too long and the mass yield too low. Except for an unobserved excess of very young exposure ages, the exposure ages of meteorites derived from lunar impacts are similar to observed chondrites ages, but the eccentricities are too low. Meteorites derived from comets appear to have excessively long exposure ages, but the present methods of calculation are probably not valid for orbits of this kind.

Of Time and the Moon

Considerable information concerning lunar chronology has been obtained by the study of rocks and soil returned by the Apollo 11 and Apollo 12 missions. It has been shown that at the time the moon, earth, and solar system were formed,∼4.6 ∼ 109 years ago, a severe chemical fractionation took place, resulting in depletion of relatively volatile elements such as Rb and Pb from the sources of the lunar rocks studied. It is very likely that much of this material was lost to interplanetary space, although some of the loss may be associated with internal chemical differentiation of the moon. It has also been shown that igneous processes have enriched some regions of the moon in lithophile elements such as Rb, U, and Ba, very early in lunar history, within 100 million years of its formation. Subsequent igneous and metamorphic activity occurred over a long period of time; mare volcanism of the Apollo 11 and Apollo 12 sites occurred at distinctly different times, 3.6 ∼ 109 and 3.3 ∼ 109 years ago, respectively. Consequently, lunar magmatism and remanent magnetism cannot be explained in terms of a unique event, such as a close approach to the earth at a time of lunar capture. It is likely that these phenomena will require explanation in terms of internal lunar processes, operative to a considerable depth in the moon, over a long period of time. These data, together with the low present internal temperatures of the moon, inferred from measurements of lunar electrical conductivity, impose severe constraints on acceptable thermal histories of the moon. Progress is being made toward understanding lunar surface properties by use of the effects of particle bombardment of the lunar surface (solar wind, solar flare particles, galactic cosmic rays). It has been shown that the rate of micrometeorite erosion is very low (angstroms per year) and that lunar rocks and soil have been within approximately a meter of the lunar surface for hundreds of millions of years. Future work will require sampling distinctly different regions of the moon in order to provide data concerning other important lunar events, such as the time of formation of the highland regions and of the mare basins, and of the extent to which lunar volcanism has persisted subsequent to the first third of lunar history. This work will require a sufficient number of Apollo landings, and any further cancellation of Apollo missions will jeopardize this unique opportunity to study the development of a planetary body from its beginning. Such a study is fundamental to our understanding of the earth and other planets.

The distribution of trace quantities of germanium between metal, silicate and sulflde phases

Abstract The distribution of trace quantities of Ge between metal, silicate and sulfide phases has been studied hydrothermally at controlled PO2 by means of oxygen buffer techniques. Radioactive Ge68-labeled olivine of varying Mg and Fe composition and Ge68-labeled iron were synthesized for this study. At 900°C and 500 bars, Ge is strongly siderophile in the iron-olivine system even at the equilibrium PO2 of iron-wustite. Similar strongly siderophilic behavior of Ge was also observed in the Fe-FeS system at this temperature and pressure. Thermodynamic data indicate that the free energy of formation of germanium dioxide lies between that of the iron oxide and nickel oxide at all temperatures. In the presence of metallic iron, the reduction of GeO2 to Ge and the formation of Fe-Ge alloy are spontaneous reactions. If nickel is added to the system, the metal phase (Fe-Ni alloy) can exist at a higher PO2 than pure iron. At the low Ni content of about 10 per cent, as found in most iron meteorites, the change in PO2 Of metal-wustite from iron-wustite is small and is unlikely to affect the siderophilic behavior of Ge. A discussion is given to indicate that the diffusion of Ge out of the metal phase is possible if the Ni content of the metal is very high. From these results, it seems unlikely that a simple one-stage separation of metal from silicate would cause the extremely low Ge content observed in some iron meteorites e.g. iron meteorites of chemical groups IVA and IVB. Other possible fractionation processes involving Ge are also discussed.

The Beginning of Continental Evolution

Abstract Lunar studies carried out in connection with the Apollo program permit considerable understanding of the early history of this planetary body. The resulting thermal history involves primary fractionation of the outer half of the lunar mass by accretional energy followed by subsequent formation of the mare basalts by radioactive heating at the boundary between the fractionated and unfractionated regions. Application of this same model to the earth implies the entire earth was initially melted and geochemically fractionated. Review of the geological and geochronological evidence for the most ancient rocks indicates that not until 3400 m.y. ago did the earth cool sufficiently to permit the formation of extensive areas of stable crust, and that this is the reason more ancient rocks have not been found more abundantly. This same line of reasoning implies that there is a class of planetary bodies intermediate in mass between the earth and the moon, possibly including Mars, which are less magmatically active than either the earth or moon, whereas the tectonic activity of others, possibly Venus, resembles that of the early earth.