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Wasserburg, G. J.

Publications and source records attributed to Wasserburg, G. J..

At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

A comparative petrologic study of iron meteorites with radiogenic Ag-107 anomalies

The characteristics of the sulfide and metal phases of Derrick Peak, Cape York, Grant, Gibeon, and Santa Clara iron meteorite samples are examined. The textures and mineralogies of the samples are described. The effects of thermal and shock events on the distribution of Ag, and the relationship between Ag-107 and Pd are investigated. The correlation between the thermal histories and metal and sulfide nodules of the Gibeon and Santa Clara specimens is analyzed. It is noted that the Pd-107-Ag107 systematics involve the isotopic chemistry and cooling history of iron meteorites, the intensity of late extraterrestrial shock occurrences, and thermal processing.

Teshima, J.↗
NASA NTRS1986

Formation ages and evolution of Shergotty and its parent planet from U-Th-Pb systematics

The isotopic composition of Pb from Shergotty, Zagami, and EETA 79001 meteorites was determined for different phases. Using phases with low U-238/Pb-204 ratio, the initial leads of these meteorites were defined. Samples from all three meteorites were shown to have distinct initial leads, and, thus to have evolved from different reservoirs over most of solar system history in a low U-238/Pb-204 environment. It follows that the parent planet of the shergottites has a high Pb-204 concentration relative to U and must also be high in other volatiles. The possibility of the Martian origin of the SNC-type meteorites is discussed.

Chen, J. H.↗
NASA NTRS1986

Trends of Sr and Nd isotopes through time near the Japan Sea in northeastern Japan

Nd and Sr isotopic analyses were performed on selected volcanic rocks of 27-6 Ma age in the Green Tuff region of northeastern Japan, in adjacent areas, and in two ultramafic inclusions from Ichinomegata. The isotopic data plot generally along the region of the mantle array, and the volcanic rocks vary in epsilon(Nd) from -0.9 to 5.7, and in epsilon(Sr) from 14.1 to -16.3. It is found that the volcanic rocks are part of an evolutionary pattern over a time period of 27 Ma, and that the pattern goes from an initially enriched source to that of a very depleted source. A greater-than-15-Ma preopening stage, characterized by an enriched isotopic signature, and a less-than-15-Ma postopening stage, characterized by a depleted signature, are identified. It is suggested that the degree of continental crust involvement is high during the preopening stages of a back-arc basin when it is associated with a rifted continental mass.

Nohda, S.↗
NASA NTRS1985

Extinct nuclides - Much ado about nothing

Attention is given to the status of research on short lived nuclei, whose importance is considerable despite their essential absence (outside cosmic ray products) in virtue of their place in the theoretical understanding of broader cosmological problems. It is presently suggested that they are a key to the earliest processes in solar system formation, and may furnish a link with presolar processes in the interstellar medium or in the intense activity of the early sun.

Wasserburg, G. J.↗
NASA NTRS1985

Self-shielding in O2 - A possible explanation for oxygen isotopic anomalies in meteorites?

The production of isotopic effects in oxygen due to self-shielding of photolysing radiation by the Schumann-Runge bands of (O-16)2 is investigated. The model studied is the simple case of incoming radiation falling on a gaseous medium of uniform composition. The enhancement of O-17 and O-18 production upon photodissociation of O2 is calculated for various column densities, and its dependence on temperature and the presence of other molecules is examined. The issue of whether the products of photolysis can be trapped and thus preserve a peculiar isotopic composition is discussed.

Navon, O.↗
NASA NTRS1985

Willy: A prize noble Ur-Fremdling - Its history and implications for the formation of Fremdlinge and CAI

The structure and composition of Willy, a 150-micron-diameter Fremdling in CAI 5241 from the Allende meteorite, are investigated using optical, secondary-electron, and electron-backscatter microscopy and electron-microprobe analysis. The results are presented in diagrams, maps, tables, graphs, and micrographs and compared with those for other Allende Fremdlinge. Willy is found to have a concentric-zone structure comprising a complex porous core of magnetite, metal, sulfide, scheelite, and other minor phases; a compact magnetite-apatite mantle; a thin (20 microns or less) reaction-assemblage zone; and a dense outer rim of fassaite with minor spinel. A multistage formation sequence involving changes in T and fO2 and preceding the introduction of Willy into the CAI (which itself preceded CAI spinel and silicate formation) is postulated, and it is inferred from the apparent lack of post-capture recrystallization that Willy has not been subjected to temperatures in excess of 600 C and may represent the precursor material for many other Fremdlinge.

Armstrong, J. T.↗
NASA NTRS1985

Strontium and neodymium isotopes in hot springs on the East Pacific Rise and Guaymas Basin

Solutions collected from 21 deg N, East Pacific Rise (Epr) and Guaymas Basin, Gulf of California, are analyzed for Nd isotopic composition and Sm and Nd concentrations. The results indicate extensive but not complete isotopic exchange with Sr in the depleted oceanic crust and that Sr concentrations in these solutions are buffered. In contrast, the Nd data exhibit a wide range in isotopic composition and concentration between vents. Many samples show substantial contributions from MORB, but all have isotopic compositions below MORB, in spite of enrichments in Nd up to 100 times seawater. It is shown that the fluids must exchange Nd with a sedimentary reservoir having an isotopic composition less than Pacific seawater. Low-temperature reactions with metalliferous sediments on the flanks of the EPR may provide such a source. Using a simple box method, estimates of the hydrothermal fluxes of Nd are compared to fluxes which are necessary to maintain a radiogenic isotopic composition of about -3 in the Pacific against the influx of Antarctic waters. It is concluded that erosion from island arcs is the main source of radiogenic Nd in the Pacific.

Piepgras, D. J.↗
NASA NTRS1985

Absolute isotopic abundances of Ti in meteorites

A general formalism is presented for determining possible isotope compositions of exotic Ti, and illustrated with results from Ti isotope abundances measured in Ca-Al-rich inclusions of the Allende and Leoville meteorites and samples of other meteorites. Previously identified correlations between Ti-50 and Ca-48 in FUN (isotope fractionation effects and unknown nucleosynthetic effects) inclusions is shown to hold true, thereby supporting contributions from a neutron-rich equilibrium or nucleosynthetic equilibrium during formation of the meteorites. Extension of the study to carbonaceous meteorites indicated that condensation, volatilization and recondensation occurred during formation, although the isotope fractionation observed cannot yet be explained.

Niederer, F. R.↗
NASA NTRS1985

Sm-Nd in marine carbonates and phosphates - Implications for Nd isotopes in seawater and crustal ages

The possibility of establishing a record of variations in the isotopic composition of Nd in seawater over geologic time is explored. To construct such a record, a phase must be identified which incorporated Nd with the same isotopic composition as seawater at the time of its formation, preserves that composition, and which is relatively common in sediments. To evaluate the suitability of carbonates and phosphates, the Rb, Sr, Sm, and Nd concentrations and the Nd and Sr isotopic composition of a variety of modern and ancient marine calcite, aragonite, and apatite samples have been measured and the results are presented and discussed.

Shaw, H. F.↗
NASA NTRS1985

Short-lived nuclei in the early solar system

An overview is given of the existence and abundance of short-lived nuclei that were present in the early solar system. It is shown that there are excesses of certain isotopes that may be assigned to the decay of short-lived radioactive nuclei corresponding to an abundance of about 0.0001 relative to a neighboring stable isotope. This abundance is roughly similar to the level of general nonlinear isotopic anomalies present in meteoritic material (excluding oxygen) which are not attributable to short-lived nuclei. If these results are representative of the bulk solar system, they indicate that about 0.0001 solar mass of freshly synthesized material of peculiar composition was added to the protosolar nebula as it was formed from the interstellar medium. This material must have been synthesized about three million yr before the formation of the solar system.

Wasserburg, G. J.↗
NASA NTRS1984

A Nd and Sr isotopic study of the Trinity peridotite Implications for mantle evolution

Field evidence is reported which indicates that the Trinity peridotite in Northern California was partially melted during its rise as part of the upwelling convecting mantle at a spreading center. A Sm-Nd mineral isochron for a plagioclase Iherzolite yielded an age of about 427 Ma which is significantly higher than that expected for depleted mantle during this period. The age is interpreted as the time of crystallization of trapped melt in the plagioclase Iherzolite P-T field, and probably represents the time when the massif was incorporated as a part of the oceanic lithosphere. The Sm-Nd model age of the plagioclase Iherzolite total rock is 3.4 AE. This suggests that the peridotite was derived from a mantle that was depleted early in earth history. Although most available data indicate that the depleted upper mantle has been relatively well stirred through time, the Trinity data suggest that very ancient Nd isotropic values are preserved and chemical and physical heterogeneities are sometimes preserved in the depleted source of midocean ridge basalts as well as the oceanic lithosphere which they intrude.

Jacobsen, S. B.↗
NASA NTRS1984

Eruption age of an approximately 100,000-year-old basalt from Ar-40/Ar-39 analysis of partially degassed xenoliths

Eleven samples from five partially degassed Mesozoic granitic xenoliths and two late Pleistocene host basalt samples from Sawmill Canyon in Sierra Nevada, California, have been analyzed using stepwise Ar-40/Ar-39 dating techniques. The petrography and analytic techniques are described and the results are presented. Isotopic compositions of Ar released from the xenoliths in several extraction steps at temperatures below about 900 C were colinear in Ar-36/Ar-40 versus Ar-39/Ar-40 diagrams and defined isochrons, giving a mean age of degassing of 119,000 + or - 7000 yr. Ar-40 extracted at higher temperatures included ancient radiogenic Ar-40 that never diffused from the xenoliths during immersion in the magma. This Ar-40 caused an increase in the apparent age for the high-temperature extractions. The high precision of the eruption age determined by this method is comparable to that obtained elsewhere by conventional K/Ar dating of sanidine.

Gillespie, A. R.↗
NASA NTRS1983

The isotopic composition of silver and lead in two iron meteorites - Cape York and Grant

Anomalies in silver isotope composition in the metal phases of the Cape York (IIIA) and Grant (IIIB) iron meteorites are studied together with the lead isotopic composition of both the metal and sulfide phases of Cape York. Following extensive surface cleaning, the Ag-107/Ag-109 ratio in the metal phases of the meteorites is found to be in excess of the terrestrial ratio, and of that found in the sulfide phases. A definite correlation between the Ag-107/Ag-109 and Pd-108/Ag-109 ratios is observed for these meteorites, indicating the in situ decay of Pd-107 and supporting the widespread presence of Pd in the early universe. Lead, determined after cleaning and with chemical separations using low blank levels, is found to exist in variable proportions in the metal and sulfide phases. The sulfides appear to be dominated by radiogenic modern lead, which may be explained by terrestrial contamination or by late metamorphism in the meteorite parent body.

Chen, J. H.↗
NASA NTRS1983

Chemical and isotopic study of extraterrestrial particles from the ocean floor

An Rb-Sr isotopic analysis of deep-sea spherules (DSS) from the Pacific, vesicular iridium-rich glassy objects (VIRGO) from an Antarctic-Ocean diatomaceous-ooze core, and equigranular objects (EGO) associated with or included in VIRGO, is reported. The techniques of sample collection, preparation, chemical analysis, and isotopic analysis (by mass spectrometry) are described in detail, and the results are presented in tables. The Sr-87/Sr-86 determination has a precision of 1-2 percent in microsamples of 2-8 x 10 to the 11th Sr atoms. The DSS are found to have Sr concentrations and Sr-87/Sr-86 (0.730-0.757) typical of chondritic meteorites. Extreme Rb depletion is attributed to volatization during meteoroid atmospheric heating. The VIRGO, while having major-element compositions like that of meteorites and high Ir levels suggesting extraterrestrial origin, have Sr-87/Sr-86 = 0.703-0.705; and the associated EGO has Sr-87/Sr-86 = 0.701 + or - 0.001, typical of MORB or basaltic achondrites. Hence their extraterrestrial origin is not proved, and some lithic component other than the EGO must be the source of the high Ir concentration.

Papanastassiou, D. A.↗
NASA NTRS1983

Influence of the Mediterranean outflow on the isotopic composition of neodymium in waters of the North Atlantic

Measurements of the Nd-143/Nd-144 ratio, expressed as epsilon-Nd(0), made in sea water samples obtained at depths from 0 to 4850 m at stations 95, 101, and 30 of North-Atlantic cruise 109-1 of the RV Atlantis II are reported and analyzed. In the 1000-m sample at station 95, determined by salinity, O2, PO4, NO3, SiO2, potential-temperature, and pressure profiles to correspond to the core of the Mediterranean outflow, an epsilon-Nd(0) value of -9.8 + or - 0.6 was found, a significant increase over the layers above and below, where epsilon-Nd(0) is about -12. Mixing estimations give a value for pure Mediterranean waters of approximately -6; continental drainage or volcanic deep-sea sediments are considered possible sources for this more radiogenic Nd. The epsilon-Nd(0) peak in the western North Atlantic (station 30) is found at the surface, and the deep waters are less radiogenic than in the eastern North Atlantic, suggesting multiple Nd sources. Possible origins and distribution mechanisms are discussed.

Piepgras, D. J.↗
NASA NTRS1983

Ar-39 recoil losses and presolar ages in Allende inclusions

Argon analyses were performed for five coarse-grained, Ca-Al-rich inclusions from the Allende meteorite. The samples were neutron-irradiated in evacuated ampuoles, and the Ar in the ampuoles and in the samples was analyzed. Up to 60 percent of Ar-39 was lost from the samples into the ampuoles due to recoil during neutron-irradiation; this loss resulted in substantial increases in the apparent Ar-40-Ar-39 ages of the samples. Substantial amounts of trapped Ar-36 of unknown origin were found in the inclusions, and the presence of trapped Ar-40 could not be ruled out. It is inferred that the interpretation of high Ar-40/K-40 ratios as very high (presolar) ages may be subject to question. It is noted that the degrees of Ar-39 recoil loss reported is comparable to recoil loss in terrestrial rocks and cannot be attributed to KCl contamination of the samples.

Villa, I. M.↗