Rare-earth analysis by a mass spectrometric stable isotope dilution technique
Mass spectrometric stable isotope dilution technique for analysis of rare earth element abundance
MASS SPECTROMETRY↗
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Rare gases in the past and present solar wind
The elemental abundances, fluxes and isotopic compositions of rare gases in the past and present-day solar wind are discussed on the basis of direct measurements and fossil evidence in lunar and meteorite samples. Spacecraft measurements of the abundances of He, O, Si, Fe, Ne, and Ar with respect to H in the solar wind are shown to agree with relative abundance measurements of these elements in the solar photosphere and corona, while the lack of evidence of systematic depletion with increasing ionic mass suggests solar abundances of the much less abundant Kr and Xe in the solar wind. Relative He, Ne and Ar abundances in the ancient solar wind as derived from lunar ilmenite and the Pesyanoe meteorite are similar to present-day values, except for a factor of two underabundance of He, while the heavy rare gases appear to be enriched by up to a factor of five. The isotopic compositions of solar wind Ar, Kr and Xe have apparently been fairly constant over most of the solar lifetime, whereas He and possibly Ne show evidence of an increasing abundance of their heavy isotopes with increasing solar wind age.
Pepin, R. O.↗
Acquisition and Early Losses of Rare Gases from the Deep Earth
Direct observations show that the deep Earth contains rare gases of solar composition distinct from those in the atmosphere. We examine the implications of mantle rare gas characteristics on acquisition of rare gases from the solar nebula and subsequent losses due to a large impact. Deep mantle rare gas concentrations and isotopic compositions can be obtained from a model of transport and distribution of mantle rare gases. This model assumes the lower mantle closed early, while the upper mantle is open to subduction from the atmosphere and mass transfer from the lower mantle. Constraints are derived that can be incorporated into models for terrestrial volatile acquisition: (1) Calculated lower-mantle Xe-isotopic ratios indicate that the fraction of radiogenic Xe produced by I-129 and Pu-244 during the first about 10(exp 8) yr was lost, a conclusion also drawn for atmospheric Xe. Thus, either the Earth was made from materials that had lost >99% of rare gases about (0.7-2) x 10(exp 8) yr after the solar system formed, or gases were then lost from the fully formed Earth. (2) Concentrations of 3He and 20Ne in the lower mantle were established after these losses. (3) Neon-isotopic data indicates that mantle Ne has solar composition. The model allows for solar Ar/Ne and Xe/Ne in the lower mantle if a dominant fraction of upper mantle Ar and Xe are subduction-derived. If Earth formed in the presence of the solar nebula, it could have been melted by accretional energy and the blanketing effect of a massive, nebula-derived atmosphere. Gases from this atmosphere would have been sequestered within the molten Earth by dissolution at the surface and downward mixing. It was found that too much Ne would be dissolved in the Earth unless the atmosphere began to escape when the Earth was only partially assembled. Here we consider conditions required to initially dissolve sufficient rare gases to account for the present lower mantle concentrations after subsequent losses at 10(exp 8) yr. It is assumed that equilibration of the atmosphere with a thoroughly molten mantle was rapid, so that initial abundances of gases retained in any mantle layer reflected surface conditions when the layer solidified. For subsequent gas loss of 99.5% and typical solubility coefficients, a total pressure of 100 atm was required for an atmosphere of solar composition. Calculations of the pressure at the base of a primordial atmosphere indicate that this value might be exceeded by an order of magnitude or more for an atmosphere supported by accretional energy. Surface temperatures of about 4000 K would have been produced, probably high enough to melt the deep mantle. Initial distributions of retained rare gases would then be determined by the history of surface pressure and temperature during mantle cooling and solidification, i.e., the coupled cooling of Earth and atmosphere. The Earth's thermal state was determined by its surface temperature and the efficiency of convection in the molten mantle, estimated to be sufficient to maintain an adiabatic gradient. Because the melting curve is steeper than the adiabat, solidification of the mantle proceeded outward from the interior. Incorporation of atmospheric gases in the mantle therefore occurred over a range in surface temperature of a few thousand degrees Kelvin. The thermal state of the atmosphere was controlled by total luminosity of the Earth (energy) released by accreting planetesimals and the cooling Earth), nebular temperature and pressure, and atmospheric opacity. The energy released by accretion declined with time as did nebular pressure. Analytical solutions for an idealized (constant opacity radiative atmosphere show that declining energy sources under constant nebular conditions result in slowly diminishing surface temperature but dramatically increasing surface pressure. For such an atmosphere with declining nebular pressure but constant total luminosity, surface pressure decreases gradually with decreasing temperaure. A decline in accretion luminosity might be compensated by energy released as the mantle cools for about 10(exp 5) year, after which luminosity must decline. The total complement of dissolved rare gases will depend on the particular evolutionary path determined by the declining accretional luminosity, the Earth thermal history, removal of the nebula, and opacity variations of the atmosphere. Models for these coupled evolutionary histories for Earth's acquisition of nebular-derived noble gases are in progress. The later losses required at about 10(exp 8) yr (depleting the interior concentrations of the sequestered solar gases by a factor of > 100) were presumably related to the major impact in which the Moon formed.
Porcelli, D.↗
Space exploration and the history of solar-system volatiles
The thermochemical history of volatile substances in all solar-system planets, satellites, and planetoids is discussed extensively. The volatiles are viewed as an interface between the abiotic and biotic worlds and as a key to the history of bodies of the solar system. A flowsheet of processes and states is exhibited. Differences in bulk volatiles distribution between the planetary bodies and between the interior, surface, and atmosphere of each body are considered, as well as sinks for volatiles in degassing. The volatiles-rich Jovian and Saturnian satellites, the effect of large-planet magnetosphere sweeps on nearby satellites, volatiles of asteroids and comets, and the crucial importance of seismic, gravity, and libration data are treated. A research program encompassing analysis of the elemental and isotopic composition of rare gas in atmospheres, assay of volatiles-containing phases in regoliths, and examination of present or past atmospheric escape/accretion processes is recommended.
Fanale, F. P.↗
The Apollo 16 deep drill core
Numerous investigations have been undertaken on samples from the Apollo 16 deep drill core. These studies are diverse in character and range from grain size analyses, through chemical and mineralogical studies to investigations of nuclear particle tracks, rare gases, and isotopic abundances. In order to comprehend the significance of the studies of the mineral chemistry of the clasts below 1 mm in size in several samples with respect to other studies, it became obvious that a review of all previous works was desireable. After reviewing the available literature it can be concluded that only four major stratigraphic divisions exist in the core section. Whether these represent four single events or multi-stage events within one unit is uncertain; however, it appears that accumulation of the material in the core has taken place during a period of 1 billion years, and that the material is predominantly of locally derived Highlands origin.
Meyer, H. O. A.↗
Origin and history of the adcumulate eucrite, Moama as inferred from REE abundances, Sm-Nd and U-Pb systematics
The abundances of Sm-Nd and U-Pb isotopes and of rare earth elements (REE) in Moama cumulate eucrite were investigated. The Sm-Nd data plotted on a Nd-143/Nd-144 vs Sm-147/Nd-144 evolution diagram define a line with a slope corresponding to an age of 4.58 plus or minus 0.05 billion years and an initial Nd-143/Nd-144 ratio of about 0.50684. Moama was found to contain the lowest overall trivalent REE abundances, the most light REE-depleted abundance pattern, and largest positive Eu anomaly of any eucrite yet studied. The REE data indicate that Moama could have been derived by about 1 to 5% fractional crystallization from a liquid with REE contents similar to Juvinas, or about 20 to 30% fractional crystallization from a Sioux County-like liquid. The data are consistent with the hypothesis that the Moama, Serra de Mage, and Moore county cumulate meteorites could have been derived from the same parent liquid. Results of the U-Th-Pb study indicate that the majority of the Pb in Moama is of terrestrial origin, and suggest that the meteorite has undergone a minor thermal event causing re-equilibration of the U-Pb system.
Hamet, J.↗
Ca isotopes in refractory inclusions
The present absolute isotope abundance measurements of Ca in Ca-Al-rich inclusions from the Allende and Leoville meteorites show the extreme rarity of nonlinear isotope effects in Ca, in sharp contrast to the endemic effects in Ti. The Ca in the Ca-Al-rich inclusions shows mass-dependent isotope fractionation effects whose range is a factor of 20 wider than the range previously established for bulk meteorites and for terrestrial and lunar samples. A correlation is found between Ca and Mg isotope fractionation effects and inclusion type, and a possible correlation between isotope fractionation and rare earth element abundance patterns is discussed.
Niederer, F. R.↗
Resonance ionization mass spectrometry for isotopic abundance measurements
Resonance ionization mass spectrometry (RIMS) is a relatively new laser-based technique for the determination of isotopic abundances. The resonance ionization process depends upon the stepwise absorption of photons from the laser, promoting atoms of the element of interest through progressively higher electronic states until an ion is formed. Sensitivity arises from the efficiency of the resonant absorption process when coupled with the power available from commercial laser sources. Selectivity derives naturally from the distinct electronic structure of different elements. This isobaric discrimination has provided the major impetus for development of the technique. Resonance ionization mass spectrometry was used for analysis of the isotopic abundances of the rare earth lutetium. Isobaric interferences from ytterbium severely effect the ability to measure small amounts of the neutron-deficient Lu isotopes by conventional mass spectrometric techniques. Resonance ionization for lutetium is performed using a continuous-wave laser operating at 452 nm, through a sequential two-photon process, with one photon exciting the intermediate resonance and the second photon causing ionization. Ion yields for microgram-sized quantities of lutetium lie between 10(6) and 10(7) ions per second, at overall ionization efficiencies approaching 10(-4). Discrimination factors against ytterbium greater than 10(6) have been measured. Resonance ionization for technetium is also being explored, again in response to an isobaric interference, molybdenum. Because of the relatively high ionization potential for Tc, three-photon, two-color RIMS processes are being developed.
Miller, C. M.↗
Isotopic and REE studies of lunar basalt 12038 - Implications for petrogenesis of aluminous mare basalts
Sr, Nd, and Sm isotopic studies of lunar basalt 12038, one of the so-called aluminous mare basalts, are reported. The evolution of the Sr and Nd isotopic compositions and the rare earth element (REE) abundances is successfully modeled within the framework of the model developed by Nyquist et al. (1977, 1979) for Apollo 12 olivine-pigeonite and ilmenite basalts. It is pointed out that the isotopic and trace element features of 12038 can by modeled as produced by partial melting of a cumulate mantle source which crystallized from a lunar magma ocean with a chondrite-normalized REE pattern of constant negative slope. Chondrite-normalized La/Yb is equal to 2.2 for this hypothetical magma ocean pattern.
Nyquist, L. E.↗
Chronology and isotopic geochemistry of apollo 14 basalts and Skaergard Gabbro, Eastern Greenland
Work completed on Apollo 14 basalts has been published. The two dates obtained from these rocks comprised the oldest and two of the three oldest ages (4.1 and 4.3 billion years) known for lunar maria basalts; thus their ages are important in understanding the moon's earliest history. Owing to the antiquity of these rocks, two more fragments have been dated as part of a second ASEE/NASA SFF program. The new ages are 3.95 and 4.12 billion years, thus further establishing and amplifying the earlier results. This work, although perhaps more interesting for its chronologic information, was begun as a test of chemical and petrographic models. Fragments of Apollo basalt were placed into five categories, based on petrologic and chemical, especially rare-earth element, composition. Isotopic studies were begun in an attempt to determine if the five groups of basalts were related by age or initial isotopic composition (isotopic composition of lava at time of extrusion). Although a few of the representatives of the five groups have the same age and/or initial strontium-isotopic composition, within the analtytical uncertainties, most apparently are unrelated. Petrologic implications of these data will be published in an appropriate journal.
Dasch, E. J.↗
Boundary conditions for the paleoenvironment: Chemical and physical processes in the pre-solar nebula
Detailed study of the first interstellar hydrocarbon ring, cyclopropenylidene (C3H2), is continuing. The singly deuterated isotope of this molecule, C3HD, was observed in several cold interstellar clouds. The results of a large survey for C3H2 in galactic sources of various types will soon be completed. It appears that cyclopropenylidene is present in virtually all interstellar clouds of at least moderate density. In order to make the first determinations of the CO2/CO abundance ratio in interstellar sources, observations of protonated CO2 were pursued. The spectrum from 18.5 to 22 GHz for several interstellar clouds is being systematically measured. Particular attention is being given to the cold, dark clouds TMC-1 and L124N, which may be formation sites for solar mass stars. The phenomena of maser emission from molecules of methanol is being studied in certain interstellar clouds. A comparison of 1 millimeter continuum emission from dust with the column density of carbon monoxide as determined from the rare C(18)O isotope for 4 molecular clouds in the Galaxy is nearing completion. Papers published during the period of this report are listed.
Irvine, William M.↗
The irradiation history of lunar samples
Apollo 11 and 12 lunar samples history of irradiation exposure to galactic cosmic rays and solar wind, using rare gas and Gd isotope measurements
Burnett, D. S.↗
Cosmic ray exposure of SNC meteorites and constraints on their derivation from Mars
There is chemical, mineralogical, and isotopic evidence that several rare meteorites originated from the planet Mars. The eight specimens (4 sphergottites, 3 nakhlites, and 1 chassignite) possess a variety of characteristics that suggest a mutual relationship. It can be predicted with reasonable confidence that these SNC meteorites were exposed to cosmic rays at relatively shallow depths for three distinct periods of time during the past approximately 10 million years. Within the context of a Martian origin, this implies that either relatively large objects were ejected from Mars at a common time and underwent three disruptive events in space or that three ejection events occurred within the past approximately 10 million years on restricted segments of Martian terrain with 1300 million year-old rocks. If mechanisms can be identified which satisfy the geochemical properties of SNC meteorites, as well as their cosmic ray exposure, and dynamical problems associated with ejection out of a large gravity field, then the case for the SNC meteorites being samples of Mars can become stronger.
Bogard, D. D.↗
Rare gas analysis of size fractions from the Fayetteville meteorite
Eight size separates of grains from the Fayetteville meteorite ranging from less than 20 microns to greater than 1 millimeter are being analyzed for their rare gas elemental and isotopic composition. Measurements on five samples were performed. All five reveal a mixture of solar, planetary, cosmic ray produced and radiogenic gases. The solar component is of particular interest since it suggest that the meteorite may represent a fragment of an ancient protoplanetary regolith which was exposed to the solar wind. Solar wind elements are implanted in the outer few hundred Angstroms of exposed grains and are therefore expected to be surface correlated. At present the data do not suggest that such a correlation exists, but the final conclusion must await further analyses and data reduction.
Jordan, Jim L.↗
The collapse of white dwarfs to neutron stars
The observable consequences of an accreting white dwarf collapsing directly to a neutron star are considered. The outcome depends critically upon the nature of the wind that is driven by neutrino absorption in the surface layers as the dwarf collapses. Unlike previous calculations which either ignored mass loss or employed inadequate zoning to resolve it, a characteristic mass-loss rate of about 0.005 solar mass/s and an energy input of 5 x 10 exp 50 ergs/s is found. Such a large mass-loss rate almost completely obscures any prompt electromagnetic display and certainly rules out the production by this model of gamma-ray bursts situated at cosmological distances. The occurrence of such collapses with the Milky Way Galaxy might, however, be detected and limited by their nucleosynthesis and gamma-ray line emission. To avoid the overproduction of rare neutron-rich isotopes heavier than iron, such events must be very infrequent, probably happening no more than once every thousand years.
Woosley, S. E.↗
Technical Considerations for Advanced Propulsion Systems
This presentation reviews concerns involving advanced propulsion systems. The problems involved with the use of Am-242m, is that it has a high "eta" plus an order of magnitude larger fission cross section than other fissionable materials, and that it is extremely rare. However other americium isotopes are much more common, but extremely effective isotopic separation is required. Deuterium-Tritium fusion is also not attractive for space propulsion applications. Because the pulsed systems cannot breed adequate amounts of tritium and it is difficult and expensive to bring tritium from Earth. The systems that do breed tritium have severely limited performance. However, other fusion processes should still be evaluated. Another problem with advanced propellants is that inefficiencies in converting the total energy generated into propellant energy can lead to tremendous heat rejection requirements. Therefore Many. advanced propulsion concepts benefit greatly from low-mass radiators.
Houts, Michael G.↗
Heart Imaging System
Johnson Space Flight Center's device to test astronauts' heart function in microgravity has led to the MultiWire Gamma Camera, which images heart conditions six times faster than conventional devices. Dr. Jeffrey Lacy, who developed the technology as a NASA researcher, later formed Proportional Technologies, Inc. to develop a commercially viable process that would enable use of Tantalum-178 (Ta-178), a radio-pharmaceutical. His company supplies the generator for the radioactive Ta-178 to Xenos Medical Systems, which markets the camera. Ta-178 can only be optimally imaged with the camera. Because the body is subjected to it for only nine minutes, the radiation dose is significantly reduced and the technique can be used more frequently. Ta-178 also enables the camera to be used on pediatric patients who are rarely studied with conventional isotopes because of the high radiation dosage.
Source record↗
Detection of Abiotic Methane in Terrestrial Continental Hydrothermal Systems: Implications for Methane on Mars
The recent detection of methane in the Martian atmosphere and the possibility that its origin could be attributed to biological activity, have highlighted the importance of understanding the mechanisms of methane formation and its usefulness as a biomarker. Much debate has centered on the source of the methane in hydrothermal fluids, whether it is formed biologically by microorganisms, diagenetically through the decomposition of sedimentary organic matter, or inorganically via reduction of CO2 at high temperatures. Ongoing research has now shown that much of the methane present in sea-floor hydrothermal systems is probably formed through inorganic CO2 reduction processes at very high temperatures (greater than 400 C). Experimental results have indicated that methane might form inorganically at temperatures lower still, however these results remain controversial. Currently, methane in continental hydrothermal systems is thought to be formed mainly through the breakdown of sedimentary organic matter and carbon isotope equilibrium between CO2 and CH4 is thought to be rarely present if at all. Based on isotopic measurements of CO2 and CH4 in two continental hydrothermal systems, we suggest that carbon isotope equilibration exists at temperatures as low as 155 C. This would indicate that methane is forming through abiotic CO2 reduction at lower temperatures than previously thought and could bolster arguments for an abiotic origin of the methane detected in the martian atmosphere.
Socki, Richard A.↗