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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.

The objectives of this report are: (1) Stratigraphic study part of integrated research project examining migration of radionuclides at Pena Blanca; (2) Immediate objectives of study include: locate stratigraphic contacts in third dimension; measure rock properties (matrix permeability, porosity, mineralogy, cation exchange capacity) of cored section; determine fracture frequency and orientation; (3) Results of study serve as primary inputs to flow and transport models.

We present new uranium-series and cosmogenic nuclide age data obtained by sensitive thermal ionization and accelerator mass spectrometric techniques for samples of dusty polar ice from Allan Hills, Antarctica. These data are used to further evaluate the applicability of these nuclides for direct radiometric dating of polar ice. In order to minimize artifacts during sample processing we developed an improved method utilizing ethylenediaminetetraacetic acid (EDTA) to stabilize uranium-series nuclides dissolved in polar ice during ice melting and filtration. Our measurements revealed low uranium-series daughter abundances and large uranium-series ( 234 U/ 238 U, 230 Th/ 234 U, 226 Ra/ 230 Th, 231 Pa/ 235 U) disequilibria in the filtered 10 Be/ 36 Cl ratios are most likely recently inherited from precipitation. The short-lived daughter 226 Ra is the exception and is enhanced in the 238 U– 234 U– 230 Th– 226 Ra disequilibria in the volcanic dust bands also suggests that ice ages are greater than 10 ka, consistent with recoil-based ages. Our relatively young ice ages can be compared with previously published relatively old terrestrial ages of meteorites from this area and suggests that meteorites at Allan Hills have experienced a multi-stage history of ice accumulation and/or ice flow. Our results also suggest that large uranium-series disequilibria can be inherited from precipitation, which may provide a method for dating clear ice samples.

We present new uranium-series plagioclase and glass data for the 1996 eruption of the North Gorda Ridge. The glass data provide a more accurate estimate of ‘zero-age’ disequilibria for use in external isochron dating than was previously available. Furthermore, plagioclase–glass 226Ra–230Th disequilibria delimit the degree of initial fractionation of radium from barium during crystal growth, with effective DRa/DBa ∼0.25–0.5. These data are inconsistent with the common assumption that DRa=DBa but are qualitatively consistent with theoretical model predictions that radium and barium should be fractionated during crystallization, with DRa/DBa∼0.2. In more detail, differences between model predictions and data could be explained by an extreme combination of model and data uncertainties, but more likely suggest suppression of efficient fractionation during rapid crystallization. We also assess the extent to which use of barium as an analog for radium would result in underestimating 226Ra–230Th disequilibria produced during melting. Effects of a lower value of DRa on most melting models are small, with a slight increase in the porosity or melt fraction for a given value of 226Ra/230Th. The new plagioclase data also indicate that plagioclase accumulation and assimilation in the crustal reservoir would have only a negligible effect on mantle-derived 226Ra–230Th disequilibria. External isochron ages from U–Th, U–Pa, and Th–Ra data calculated using initial disequilibria from the 1996 sample are concordant, with one exception. Anomalously young ages for off-axis samples most likely reflect volcanism up to 1 km off-axis. Radium ages for near-axis dredge samples for other areas of the North Gorda Ridge generally range from 2000 to 4000 yr, similar to calculated steady-state eruptive periodicity of 3000 yr for this ridge segment. However, comparison of radium excesses for the 1996 eruption and a nearby older lava suggests that recent volcanism at this site occurs with relatively short-lived (

Interpretation of U-series disequilibria in midocean ridge basalts is highly dependent on the bulk partition coefficients for U and Th and therefore the mineralogy of the mantle source. Distinguishing between the effect of melting processes and variable source compositions on measured disequilibria (238U-230Th-226Ra and 235U-231Pa) requires measurement of the radiogenic isotopes Hf, Nd, Sr, and Pb. Here, we report measurements of 238U-230Th-226Ra and 235U-231Pa disequilibria; Hf, Nd, Sr, and Pb isotopic; and major and trace element compositions for a suite of 20 young midocean ridge basalts from the East Pacific Rise axis between 9°28′ and 9°52′N. All of the samples were collected within the axial summit trough using the submersible Alvin. The geological setting and observational data collected during sampling operations indicate that all the rocks are likely to have been erupted from 1991 to 1992 or within a few decades of that time. In these samples, 230Th excesses and 226Ra excesses are variable and inversely correlated. Because the eruption ages of the samples are much less than the half-life of 226Ra, this inverse correlation between 230Th and 226Ra excesses can be considered a primary feature of these lavas. For the lava suite analyzed in this study, 226Ra and 230Th excesses also vary with lava composition: 226Ra excesses are negatively correlated with Na8 and La/Yb and positively correlated with Mg#. Conversely, 230Th excesses are positively correlated with Na8 and La/Yb and negatively correlated with Mg#. Th/U, 230Th/232Th, and 230Th excesses are also variable and correlated to one another. 231Pa excesses are large but relatively constant and independent of Mg#, La/Yb, Th/U, and Na8. The isotope ratios 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, and 208Pb/206Pb are constant within analytical uncertainty, indicating that they were derived from a common source. The source is homogeneous with respect to parent/daughter ratios Lu/Hf, Sm/Nd, Rb/Sr, and Th/U; therefore, the measured variations of Th/U, 230Th, and 226Ra excesses and major and trace element compositions in these samples are best explained by polybaric melting of a homogeneous source, not by mixing of compositionally distinct sources.

RECENT detailed surveys of the East Pacific Rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast-spreading ocean ridge crests1–7. In parallel with geological and geochemical investigations, it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium-series disequilibria methods8–11. Here we use 238U-230Th and235U-231Pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9°31′ N on the East Pacific Rise. Most of the ages are younger than would be expected based on off-axis distance and spreading rate. We infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0.5–2 km outside the axial summit caldera, with some volcanism occurring as far as 4 km off-axis. Melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas.

We present 238 U- 230 Th - 226 Ra disequilibria and cosmogenic 3 He and 36 Cl data for the Bluewater flow of the Zuni-Bandera volcanic field in western New Mexico. The 238 U- 230 Th disequilibria measured on separated groundmass phases yield an internal isochron age of 68 ka (+24/–20 ka; 2σ). This value cannot be directly compared with surface exposure ages unless erosion rates are known. The apparent (zero erosion) ages determined from both the 3 He concentration (47.5 ± 5 ka; 2σ) and the 36 Cl concentration (41.2 ± 8.8 ka; 2σ) are significantly younger than the U-Th isochron age. When minimum estimates of surface erosion based on flow morphology are considered, the 3 He concentrations indicate a minimum exposure age of 60 ka, in good agreement with the U-Th isochron age, with a minimum erosion rate of 1.7 mm/k.y. and an erosion rate as high as 5 mm/k.y. in other locations. Correcting for erosion has little effect on the model 36 Cl age and, as a result, the 36 Cl age is significantly younger than the U-Th isochron age and erosion-corrected 3 He ages; this discordance is attributed to a lack of closed-system behavior in the 36 Cl system. These new ages have local significance for the geochronology of the Zuni-Bandera volcanic field; however, their larger significance is in their applicability to dating Quaternary basalts and quantifying erosion rates.