Calorimetric study of the stability of high pressure phases in the systems CoOSiO2 and “FeO”SiO2, and calculation of phase diagrams in MOSiO2 systems

High temperature calorimetric measurements of the enthalpies of solution in molten if2 PbO · B₂O₃ of α- and γ-Fe₂SiO₄ and α-, β-, and γ-Co₂SiO₄ permit the calculation of phase relations at high pressure and temperature. The reported triple point involving α-, β-, and γ-Co₂SiO₄ is confirmed to represent stable equilibrium. The curvature in the α?β phase boundary in Co₂SiO₄ and of an α?γ boundary in Fe₂SiO₄ at high temperature is explained in part by the effects of compressibility and thermal expansion, but better agreement with the observed phase diagram is obtained when one considers the effect of small amounts of cation disorder in the spinel and/or modified spinel phases. The calculated ΔH⁰ and ΔS⁰ values for the α?β, α?γ, and β?γ transitions show that enthalpy and en changes both vary strongly in the series Mg, Fe, Co, and Ni, and are of equal importance in determining the stability relations. The disproportionation of Fe₂SiO₄ and Co₂SiO₄ spinel to rocksalt plus stishovite is calculated to occur in the 170–190 kbar region; cation disorder and/or changes in wüstite stoichiometry can affect the P?T slope. The calorimetric data for CoSiO₃ and FeSiO₃ are in good agreement with the observed phase boundary for pyroxene formation from olivine and quartz. The decomposition of pyroxene to spinel and stishovite at pressures near the coesite-stishovite transition is predicted in both iron and cobalt systems. The use of calorimetric data, obtained from small samples of high pressure phases, is very useful in predicting equilibrium phase diagrams in the 50–300 kbar range.     

Removal of230Th and231Pa from the open ocean

Concentrations of²³⁰Th and²³¹Pa were measured in particulate matter collected by sediment traps deployed in the Sargasso Sea (Site S₂), the north equatorial Atlantic (site E), and the north equatorial Pacific (Site P) as well as in particles collected by in situ filtration at Site E. Concentrations of dissolved Th and Pa were determined by extraction onto manganese dioxide adsorbers at Site P and at a second site in the Sargasso Sea (site D).Dissolved²³⁰Th/²³¹Pa activity ratios were 3–6 at Sites P and D. In contrast, for all sediment trap samples from greater than 2000 m, unsupported²³⁰Th/²³¹Pa ratios were 22–35 (average 29.7). Ratios were lower in particulate matter sampled at shallower depths. Particles filtered at 3600 m and 5000 m at Site E had ratios of 50 and 40. Results show that suspended particulate matter in the open ocean preferentially scavenges Th relative to Pa. Most of the²³⁰Th produced by decay of²³⁴U in the open ocean is removed by adsorption to settling particulate matter. In contrast, less than 50% of the²³¹Pa produced by decay of²³⁵U is removed from the water column by this mechanism. Mixing processes transport the remainder to other sinks.     

Fine chronology of volcanic processes using 238U-230Th systematics

The²³⁰Th-²³⁸U radioactive disequilibrium method was applied to the study of recent volcanic rocks from Costa Rica. Most samples are from the Irazu volcano. Some samples were dated by internal isochrons using the (²³⁰Th/²³²Th)-(²³⁸U/²³²Th) diagram, others were studied only by whole rock analyses. The evolution of the parent magma may be followed by the initial (²³⁰Th/²³²Th)₀ ratios of the rocks. A model involving a differentiating magma chamber that existed for 140,000 years under the Irazu volcano correlates well with the observations. Other volcanoes seem to be in earlier stages of their evolution. Continuing study may help to solve the tholeiitic to andesitic volcanism relationship.     

Neutron capture logging calibration and data analysis for environmental contaminant assessment

This paper describes a method to calibrate a neutron capture sonde equipped with a high resolution γ-ray detector, and analyze log data. The method utilizes the 1460.8-keV passive γ-ray of ⁴⁰K, the 770.3-keV capture γ-ray of ³⁹K, and a capture γ-ray from a target element. An equation containing the spectral line intensities for the two capture γ-rays, nuclear capture data, and the detector efficiency function expresses the concentration of the target element as a multiple of the ³⁹K concentration. The concentration of ³⁹K is easily deduced from the ⁴⁰K concentration, which is calculated directly from the line intensity for the 1460.8-keV γ-ray in a passive γ-ray spectrum.The calibration automatically adjusts to changes in the neutron transport properties of the logged medium that may result, for example, from variations in the H density and the concentrations of neutron poisons. Fluctuations in the neutron source output are similarly accommodated. The calibration utilizes U.S. Department of Energy (DOE) passive γ-ray calibration standards that contain well established concentrations of K, U, and Th. The passive γ-rays from K, U, and Th (and the U and Th decay progenies) provide data for the detector efficiency function determination.Data for proof-of-principle demonstrations of the method were acquired by logging boreholes penetrating the shallow subsurface at a DOE waste site with a simple, reliable neutron capture logging system. The system had a ²⁵²Cf source and a high purity germanium (HPGe) detector. Time gating could not be used to sort signals originating from capture and activation, but the excellent energy resolution permitted capture γ-ray identifications based solely on the γ-ray energies. Cl, H, and other elements were detected and assessed.A conventional calibration and data analysis method was also employed. The method was specific to Cl and was based on measurements in two Cl-impregnated concrete blocks. Cl concentrations inferred with this method were often consistent with the concentrations determined with the new method. When the two methods produced different Cl concentrations, the discrepancies could be explained by variations in formation parameters.     

Radiation-induced decomposition of U(VI) phases to nanocrystals of UO2

U⁶⁺-phases are common alteration products, under oxidizing conditions, of uraninite and the UO₂ in spent nuclear fuel. These U⁶⁺-phases are subjected to a radiation field caused by the α-decay of U, or in the case of spent nuclear fuel, incorporated actinides, such as ²³⁹Pu and ²³⁷Np. In order to evaluate the effects of α-decay events on the stability of the U⁶⁺-phases, we report, for the first time, the results of ion beam irradiations (1.0 MeV Kr²⁺) of U⁶⁺-phases. The heavy-particle irradiations are used to simulate the ballistic interactions of the recoil-nucleus of an α-decay event with the surrounding structure. The Kr²⁺-irradiation decomposed the U⁶⁺-phases to UO₂ nanocrystals at doses as low as 0.006 displacements per atom (dpa). U⁶⁺-phases accumulate substantial radiation doses (∼1.0 displacement per atom) within 100,000 yr if the concentration of incorporated ²³⁹Pu is as high as 1 wt.%. Similar nanocrystals of UO₂ were observed in samples from the natural fission reactors at Oklo, Gabon. Multiple cycles of radiation-induced decomposition to UO₂ followed by alteration to U⁶⁺-phases provide a mechanism for the remobilization of incorporated radionuclides.     

Naturally occurring radionuclides and rare earth elements in weathered Japanese soil samples

The activity concentrations of ²²⁶Ra and ²²⁸Ac in weathered Japanese soils from two selected prefectures have been measured using a γ-ray spectroscopy system with high purity germanium detector. The uranium, thorium, and rare earth elements (REEs) concentrations were determined from the same soil samples using inductively coupled plasma mass spectrometry (ICP-MS). For example, granitic rocks contain higher amounts of U, Th, and light REEs compared to other igneous rocks such as basalt and andesites. Therefore, it is necessary to understand the interaction between REEs and nature of soils since soils are complex heterogeneous mixture of organic and inorganic solids, water, and gases. In this paper, we will discuss about distribution pattern of ²³⁸U and ²³²Th along with REEs in soil samples of weathered acid rock (granite) collected from two prefectures of Japan: Hiroshima and Miyagi.     

Uranium and boron distributions in some oceanic ultramafic rocks

From mica fission-track maps the serpentinized and weathered portions of four ultramafic rocks from oceanic ridge systems contained 0.5 to 2.4 ppm U compared to only 2.5 ppb in clinopyroxene, 0.6 ppb in chromite and less than 7.0 ppb in olivine. Orthopyroxene grains contained 0.4 ppb U which is three orders of magnitude lower than had previously been reported.Long thin tracks from (n, α) reactions with boron were recorded in cellulose nitrate plastic and were counted like fission tracks. The track density from boron was 2×10⁴ times higher than that from uranium fission alone. Boron in serpentine was variable on a 50-μm scale attaining 155 ppm concentrations. Orthopyroxene grains, in contrast, had maximum concentrations of 0.8 ppm.Most of the uranium and boron in the rocks is believed to have been introduced during serpentinization. From known crystal-melt partitioning ratios the uranium and boron distributions are consistent with the ultramafic rocks being cumulates or residues from partial melting events.     

234Th/238U activity ratios in Pacific Ocean bottom waters

The²³⁴Th/²³⁸U activity ratios in the near-bottom waters at a station in the South Pacific have been measured. The activity ratios are close to the secular equilibrium value, ranging between 0.9 and 1.13 (± 8%), suggesting that the rate of removal of²³⁴Th by bottom-water scavenging processes at this station is slow compared to its rate of radioactive decay. The mean²³⁴U/²³⁸U activity ratios in these waters is 1.14 ± 0.02, the same as the reported values for the world oceans.     

40Ar-39Ar age determinations on the Owyhee basalt of the Columbia Plateau

⁴⁰Ar/³⁹Ar step-heating analyses have been performed on 11 samples of basalt from sites near Owyhee Reservoir of southeastern Oregon, U.S.A. These rocks were extruded during the great flood basalt episode of the Pacific Northwest. The whole-rock points are highly correlated on a plot of⁴⁰Ar/³⁶Ar versus³⁹Ar/³⁶Ar, corresponding to a common age of the samples of 14.3 ± 0.3 m.y. Inspite of this, individual “plateau” plots of the age versus fraction of³⁹Ar released do not give good plateaux. These age spectra exhibit to varying degrees a common structure in which lower age values are found at higher temperatures. This pattern may result from a closed-system redistribution of the argon isotopes. The usefulness of grinding the basalts in removing a loosely held atmospheric argon component is confirmed.     

The removal rates of234Th and228Th from waters of the New York Bight

The mean residence time (τ) of thorium with respect to non-radioactive removal from water was determined as a function of location in three traverses of the New York Bight using both²³⁴Th/²³⁸U and²²⁸Th/²²⁸Ra. τ correlates well with salinity increasing from about ten days near shore to 70 days at the shelf break. It shows a much weaker correlation with suspended matter concentrations both because suspended matter varies in its scavenging efficiency and because salinity is a longer-term integrator of scavenging potential.τ's computed from²²⁸Th/²²⁸Ra were often higher than those computed from²³⁴Th/²³⁸U either because of the detrital matter present as reflected in the²³²Th concentrations or because the water parcels had recently arrived from environments of lower scavenging efficiency.Anomalous isotopic concentrations found in three adjacent samples can best be explained as the result of an episodic release of²²⁸Ra from bottom sediments at a rate 200 times the normal one.     

226Ra and222Rn contents of Galapagos Rift hydrothermal waters —the importance of low-temperature interactions with crustal rocks

Hydrothermal waters collected by “Alvin” from the Galapagos Spreading Center are enriched in²²²Rn by factors of 50–200 over bottom waters. The²²⁶Ra in the same samples, however, is enriched by less than a factor of four over bottom waters. Enrichments of²²²Rn result primarily from α-recoil from rock surfaces while²²⁶Ra enrichments are dominantly produced by high-temperature alteration of cooling ridge volcanics. The abundances of both nuclides exhibit positive correlations with temperature. The data extrapolate to bottom water temperatures and compositions, demonstrating the importance of seawater mixing. Different vents, however, have different mixing lines, and vents with high²²²Rn have low²²⁶Ra. We propose these patterns result from variations in the extent of low-temperature crustal interaction with the hydrothermal fluids. Low-temperature crustal waters can maintain high steady state²²²Rn contents due to the α-recoil additions to the fluids. The²²⁶Ra, however, is strongly adsorbed at low-temperatures resulting in low concentrations of this nuclide in low-temperature crustal waters. Thus, physical mixing of a crustal water component with hydrothermal waters or variable crustal path lengths of the hydrothermal fluids can account for the variable mixing lines and²²²Rn/²²⁶Ra values of the hot springs.The²²²Rn/²²⁶Ra value appears to be a sensitive indicator of low-temperature crustal interaction. Values > 100 have experienced extensive crustal interaction and are indicative of diffuse hydrothermal flow. Values between 1 and 10 are indicative of primary hydrothermal fluids which have not experienced significant interaction with the crust. Values of²²²Rn/²²⁶Ra between 10³ and 10⁴ are indicative of interaction of the hydrothermal fluids with sediments. Such values are observed in water samples from the Galapagos hydrothermal mounds.     

Evolution of uranogenic and thorogenic lead, 2. Some differences in the variations of the206Pb/204Pb and208Pb/204Pb ratios

On the basis of a dynamic model of a continuous Pb isotope evolution, the variations in the isotopic ratios of uranogenic and thorogenic Pb in a number of young ore deposits, ocean sediments and volcanic rocks from mid-ocean ridges, oceanic islands, island arcs and continents are evaluated. The deviations in the Th/U ratio from its model values are calculated. A model parameterT which indicates a more ancient enrichment in U/Pb (crustal-type Pb sources) or younger enrichment in U/Pb (mantle-type Pb sources) is also calculated. The relative change in the Th/U ratio is close to 1 for mantle-type Pb sources (lowerT values) and decreases for crustal-type Pb sources (higherT values). An interpretation of these changes could be connected with the differences in the U and Th geochemical behaviours in metamorphic processes under the conditions of a deeper (mantle) or crustal source owing to the differences in their valence states. In some cases they have similar (U⁴⁺ and Th⁴⁺) migration abilities; in others (U⁶⁺ and Th⁴⁺), considerably different.     

High-pressure X-ray diffraction studies on β- and γ-Mg2SiO4

Pressure effects on the lattice parameters of β- and γ-Mg₂SiO₄ have been measured at room temperature and at pressures up to 100 kbar using a multi-anvil high-pressure X-ray diffraction apparatus. The volume changes (ΔV/V₀) at 90 kbar are 5.4 · 10^?2 and 4.2 · 10^?2 for β- and γ-Mg₂SiO₄, respectively. Isothermal bulk moduli at zero pressure have been calculated from least-square fits of the data to straight lines. They turn out to be 1.66 ± 0.4 and 2.13 ± 0.1 Mbar for β- and γ-Mg₂SiO₄, respectively. The α → γ transition obeys Wang's linear V_φ?ρ relation but the α → β transition does not.     

230Th-238U radioactive disequilibria in tholeiites from the FAMOUS zone (Mid-Atlantic Ridge, 36°50′N): Th and Sr isotopic geochemistry

We analyzed, U, Th and²³⁰Th/²³²Th activity ratios for a few tholeiites from the Mid-Atlantic Ridge FAMOUS zone at 36°50′N. The results show a fairly wide scatter for both Th/U and (²³⁰Th/²³²Th) ratios. Seawater contamination appears to be responsible for this scatter and, for the uranium, produces an increase in content yielding a (²³⁴U/²³⁸U) ratio greater than 1 and, for the Th, an increase of the (²³⁰Th/²³²Th) ratio which is a very sensitive indicator for contamination. Also, the latter often is selective: U, Th and Sr are not affected in the same manner.When discarding all data for contaminated samples, the FAMOUS zone appears to be very homogeneous with aTh/U ratio value of 3.05 and a (²³⁰Th/²³²Th) ratio value of 1.24. Comparison with other active volcanic areas reveals a negative correlation between (²³⁰Th/²³²Th) and⁸⁷Sr/⁸⁶Sr ratios for present lavas which is indicative of a consistency in Th-U and Rb-Sr fractionation in the source regions of these magmas. The Th isotopic geochemistry can thus provide useful information for the study of present volcanism, information as valuable as that from Sr, Pb or Nd isotopes.     

Uranium isotopes in the Greenland ice-sheet

The²³⁴U/²³⁸U activity ratio was measured in dated strata of the Dye-3 Greenland ice-sheet. Values were generally less than unity indicating a source in weathered crustal rock debris. Three levels showed elevated ratios which are attributed to entry of debris from the aborted Russian satellite Cosmos 954 following residence in the stratosphere.     

241Pu/239+240Pu ratios in polar glaciers

The analyses of plutonium isotopes in dated strata of polar ice sheets indicate that the²⁴¹Pu/^239+240Pu activity ratio produced in the U.S.-dominated nuclear tests during the 1950s was about 26, while that in the U.S.S.R.-dominated weapons tests in the early 1960s was between 12 and 14. This difference provides time horizons for sedimentary deposits. Further, the²³⁹Pu/²⁴⁰Pu ratio may show a similar difference in fallout values from these two periods of weapons testing and may provide an additional, and perhaps more sensitive, chronology for sediments.     

14C-39ArMe correlations in chondrites and their pre-atmospheric size

Cosmogenic¹⁴C has been measured in 12 chondrites and the stone phase of the mesosiderite Bondoc. For the chondrites analysed the activities vary between 44 and 72 dpm/kg; the low value of (4.5 ± 0.9) dpm/kg for Bondoc is essentially due to its large pre-atmospheric size and not to a terrestrial age of several half-lives of¹⁴C.In eight cases³⁹Ar in the metal phase from the same meteorite specimens had been measured previously. The results are combined to derive the pre-atmospheric radiiR₀ of the meteoroids and depth of burial of the samples investigated. Values ofR₀ between 35 and 82 cm are obtained; of 14 samples ten came from a depth of 10 cm or less. The preponderance of samples from shallow depths is ascribed to asymmetrical ablation losses of the meteoroids during their passage through the atmosphere.A compilation of all published¹⁴C concentrations in chondrites shows that the variations between different specimens from thesame meteorites are almost as large as those for samples fromdifferent meteorites. Thus, there is no need to invoke different orbits of the meteoroids and a strong spatial gradient in the primary cosmic-ray intensity to explain variations of low-energy-produced cosmogenic nuclides in different meteorites.     

The case for an unfractionated244Pu/238U ratio in high-temperature condensates

The coarse-grained, Ca-rich inclusions in the Allende meteorite are the highest-temperature condensates from the cooling solar nebula and, as such, the oldest solid objects in the solar system. All refractory elements with condensation points above the accretion temperature of the inclusions whose concentrations in them have been measured are seen to be present in the inclusions in unfractionated proportion to one another relative to C1 chondrites when data are averaged for a large number of inclusions. Observational data for U and theoretical data for both U and Pu suggest that these elements exhibited refractory behavior in the solar nebula. An experiment is proposed in which fissiogenic Xe and U contents are measured in a suite of these inclusions to obtain the²⁴⁴Pu/²³⁸U ratio of the solar system at the time of initial condensation with an uncertainty of ±15%.     

Removal of234Th from a coastal sea: Funka Bay,Japan

In Funka Bay of Hokkaido, Japan, seawater, suspended matter and settling matter were collected once every month in the summer of 1974. These samples were analyzed for²³⁴Th, a short-lived daughter of dissolved²³⁸U. A pronounced disequilibrium between²³⁴Th and²³⁸U, and a highly variable concentration of²³⁴Th were found. Positive correlation, however, exist among the deficiency of²³⁴Th relative to²³⁸U in seawater, the concentration of particulate²³⁴Th, the fraction of particulate²³⁴Th to total²³⁴Th in seawater, the total dry weight of suspended matter, and the primary productivity during the month previous to sampling. The specific activity of²³⁴Th for the settling particles (620 ± 170 dpm/g) was nearly equal to that for suspended particles (720 ± 600 dpm/g) but much greater than that for plankton (47 ± 24 dpm/g). These facts suggest that suspended particles are somehow closely related to the removal of heavy metals from seawater, in spite of the negligibly small settling flux of suspended matter. The residence time of thorium in Funka Bay (mean depth: 60 m) is found to be about 60 days, which is nearly equal to those of²¹⁰Pb and²¹⁰Po.     

Evidence for contamination of recent Hawaiian lavas from230Th-238U data

²³⁰Th-²³⁸U radioactive disequilibrium was studied in the historical lava flows of the Mauna Loa and Kilauea, Hawaii. Large variations of the (²³⁰Th/²³²Th) ratio among lavas of the same volcano that were erupted at a few years' interval are interpreted as due to contamination. The contamination probably occurs by assimilation of zeolitic minerals formed by seawater interaction while the magma resides in a superficial chamber.