[en] We are using the short- and long-lived members of the U- and Th-decay series as natural analogues of contaminant transport at INEEL. We have thus far analyzed 32 groundwater samples for U isotopics and concentration, and 12 large-volume samples (∼ 1000L) for long- and short-lived isotopes of Th, Ra and Pa. Here we report on general trends in concentration and isotope data. Uranium concentrations range from 0.3 to 3.6 ppb and 234U/238U activity ratios range between 1.6 and 3.1. Groundwater nearest the local recharge supplied by Birch Creek and Little Lost River has high 234U/238U ratios, in contrast to the low to moderate values of groundwater dominated by the regional southwesterly flow of the Snake River Plain Aquifer. Contours of high 234U/238U ratios delineate preferential flow-paths extending southward from the Birch Creek and Little Lost River recharge areas. 'Low-isotope ratio zones' separate preferential flow zones. These are interpreted to be due to greater extent of reaction with the host basalt which, in turn, probably indicates longer groundwater residence times. Thorium concentrations are between 0.1 and 0.6 ppt and 230Th/232Th activities range from 0.87 to 1.42. Protactinium concentrations are between 0.14 and 0.01 fg/L. Protactinium-231 and uranium concentrations are positively correlated. Radium-226 concentrations range from 0.65 to 7.8 fg/L. Radium-226 concentrations are negatively correlated with both uranium concentrations and 234U/238U activities along one flow path toward a 'low-isotope ratio zone'. This also suggest greater extent of water/rock interaction in accordance with the uranium isotopic data. In general, waters entering the aquifer from the north show highest degrees of disequilibrium. Decreases in activity ratios away from the local recharge areas reflect interaction with the host basalt and mixing with regional groundwaters flowing from them the northeast

[en] Current models using U- and Th-series disequilibria to study radioisotope transport in groundwater systems mostly consider a steady-state situation. These models have limited applicability to the vadose zone (UZ) where the concentration and migratory behavior of radioisotopes in fluid are often transitory. We present here, as a first attempt of its kind, a model simulating the non-steady state, intermittent fluid transport in vadose layers. It provides quantitative constraints on in-situ migration of dissolved and colloidal radioisotopes in terms of retardation factor and rock-water interaction (or water transit) time. For uranium, the simulation predicts that intermittent flushing in the UZ leads to a linear relationship between reciprocal U concentration and ²³⁴U/²³⁸U ratio in percolating waters, with the intercept and slope bearing information on the rates of dissolution and α-recoil of U isotopes, respectively. The general validity of the model appears to be borne out by the measurement of uranium isotopes in UZ waters collected at various times over a period during 1995-2006 from a site in the Pena Blanca mining district, Mexico, where the Nopal I uranium deposit is located. Enhanced ²³⁴U/²³⁸U ratios in vadose-zone waters resulting from lengthened non-flushing time as prescribed by the model provide an interpretative basis for using ²³⁴U/²³⁸U in cave calcites to reconstruct the regional changes in hydrology and climate. We also provide a theoretical account of the model's potential applications using radium isotopes.

[en] Short Communication

[en] We have developed a mass spectrometric technique for the measurement of ²³⁰Th/²³²Th ratios in young volcanic rocks. We show that we can measure ²³⁰Th/²³²Th ratios on MORB samples of ≅ 1 gram with an accuracy and reproducibility of 0.5-1.0%. This represents an improvement of at least a factor of 5-10 in sample size and precision over conventional alpha spectrometry methods. Using this technique, we have measured distinct excesses of ²³⁰Th activity relative to ²³⁸U activity for axial samples from the Juan de Fuca (JDF) and Gorda Ridges. These enrichments are 13-15% but range up to 40% in one sample. Low boron concentrations and ²³⁴U/²³⁸U ratios corresponding to secular equilibrium verify the absence of detectable seawater contamination. From this we infer that primary magmatic processes are the source of the measured ²³⁰Th activity excesses, and that Th is indeed more incompatible than U during partial melting of MORB sources. ²³⁰Th/²³²Th activity ratios for the axial samples from JDF appear to be relatively high for uncontaminated MORB, ranging from 1.35 to 1.41. From this we infer that the JDF basalts formed from a uniform source highly depleted in Th relative to U (Th/U_wt=2.15-2.25). One Gorda axial sample has a somewhat lower ²³⁰Th/²³²Th activity ratio of 1.31, suggesting formation from a source only slightly less depleted (Th/U=2.32±0.02). Based on the small range in ³⁰Th/²³²Th ratios for axial basalts from JDF, dating off-axis JDF basalts may be feasible. (orig.)

[en] We present our latest results in the development of resonance ionization mass spectrometry (RIMS) using cw lasers to measure isotope ratios of importance in geochemistry and geochronology. The RIMS method provides a higher ionization efficiency than thermal ionization allowing analysis of smaller sample sizes, while maintaining equivalent internal precision. The bias introduced in ^230/232Th ratio measurements by the RIMS method when using narrow-band and broad-band lasers is discussed. Spectra of ²³⁹Th, which can be used as an internal standard, are also presented

[en] Studies of U-series disequilibria near uranium ore deposits can provide valuable information on the mobility of actinides and their daughters over the range of timescales needed to assess the stability of proposed waste repositories. We have applied highly sensitive TIMS methods to obtain 238U-234U-230Th dates for three whole rock samples within a ∼30 in long fracture emanating into surrounding tuff from the deposit at Pena Blanca, Mexico. The 238U-234U-230Th data lie on a whole-rock isochron that requires closed-system behavior for the last 380 ka. Preliminary 231Pa-235U data for the U-rich vein also indicates closed system behavior for at least the last 100 ka. In contrast, 226Ra/230Th activity ratios range from 0.76-0.99 which indicates more recent Ra mobility within the fracture most likely due to surface water infiltration. Our results require uranium, thorium and protactinium stability despite recent radium mobility and provide important constraints on repository stability over ∼100 ka timescales

[en] New data on the U, Pu, and P distributions in less metamorphosed H-chondrites coupled with literature results, permit a provisional picture to be assembled of the chemistry of these elements and for the rare earth elements in ordinary chondrites and the changes brought about by chondritic metamorphism. Preferential associations of phosphates with metals and/or sulfides in all chondrites strongly indicate an 'initially' siderophile or conceivably chalcophile character for P in ordinary chondrite precursor materials with phosphate subsequently formed by oxidation. This oxidation occurred prior to or during chondritic metal-silicate fractionation. Uranium is initially concentrated in chondrule glass at approx. 100 ppb levels with phosphates in H-3 chondrites being essentially U-free (<20 ppb). As chondrule glass devitrified during metamorphism, U migrated into phosphates reaching approx. 50 ppb in Nadiabondi (H-5) merrillite and 200 to 300 ppb in merrillite from equilibrated chondrites but 'froze out' before total concentration in phosphates occurred. Relative ²⁴⁴Pu fission track densities in the outer 5 μm of olivine and pyroxene grains in contact with merrillite and with chondrule mesostasis in Bremervoerde (H-3) give Pu(mesostasis)/Pu(merrillite) < 0.01, implying total concentration of Pu in phosphates. Results are reported and discussed. (author)

[en] The high energy secondary neutron flux produced by 500-650 MeV protons at the beam stop of the Los Alamos Meson Physics Facility (LAMPF) is evaluated for use in measuring the microdistribution of thorium in geologic materials, e.g. in individual mineral grains, approx. >= 50 μm in size. The relative Th/U fission rate is found to be 0.5 with a Th track production rate of 1.2 x 10⁴ tracks/cm² x ppm x mAh. The background of long interaction tracks (<= 10 μm) limits the sensitivity of this method to samples of 3-5 ppm Th. We find a similar background of long interaction tracks in a LAMPF proton irradiation. (orig.)

[en] The microscale distribution of U and Th has been studied by fission track radiography in a set of Ca-Al-rich inclusions from the Allende meteorite. In the Type B inclusions, the major phases melilite and fassaite are important actinide host phases with similar Th/U and roughly three times higher concentrations in melilite than fassaite. Significant enrichments are also found on grain boundaries and in some regions of alteration. On the rims of Type B inclusions and throughout all other inclusions studied, perovskite is the dominant actinide host phase. However, perovskite does not have a characteristic Th/U value. Overall, neither alteration nor loss or gain of an actinide-rich phase appears to have been an important Th/U fractionation mechanism; volatility differences may be the dominant factor. The Th/U and REE abundance patterns for the spinel plus perovskite rim suggest derivation of this rim by volatilization of interior material. The time scale for this heating is required to be brief (<10⁴ sec), but within this constraint spinel-perovskite rim formation by several mechanisms is possible. (author)

[en] Two meteorites were analyzed by PIXE with the Los Alamos Nuclear Microprobe. The enstatite achondrite Pena Blanca Spring and the ordinary chondrite St. Severin were chosen as likely candidates for use in ²⁴⁴Pu (t/sub 1/2/ = 82 my) cosmochronology and geochronology. These applications require the meteoritic minerals to have unfractionated actinides and lanthanides relative to cosmic elemental abundance ratios. The PIXE analyses produced evidence of actinide-lanthanide fractionation in Pena Blanca Spring oldhamite (CaS) whereas the St Severin phosphates, whitlockite and chlorapatite, do not exhibit this fractionation