Lary Ball

Chemical and isotopic constraints on the generation and transport of magma beneath the East Pacific Rise

Abstract 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.

An intercomparison of small- and large-volume techniques for thorium-234 in seawater

Abstract In this paper, an intercomparison of methods for the determination of 234Th in seawater is discussed. Samples were collected either from a shore-based 600 m water source, or from standard bottle casts in deep waters off Hawaii and the Southern Ocean. We compared large-volume techniques, which rely upon Mn cartridges for the collection of dissolved 234Th and its detection via gamma counting (>200-l samples), with small volume methods that employed either direct beta counting, or beta counting after radiochemical purification (2–20-l samples). Unique to this study is the presentation of small volume (2 and 5 l) 234Th methods. This new technique is an adaptation of 20-l methods that are based on the coprecipitation of thorium with Mn oxides followed by direct beta counting of the precipitate. The small volume Mn coprecipitation methods were found to be superior to other methods due to ease of sample collection, processing and low overall analytical uncertainties.

Rapid determination of 230Th and 231Pa in seawater by desolvated micro-nebulization Inductively Coupled Plasma magnetic sector mass spectrometry

Difficulties in determining the 230Th and 231Pa concentration of seawater have hindered rapid progress in the application of these unique natural tracers of particle scavenging and ocean circulation. In response, we have developed an ICP/MS analytical procedure combining a degree of sensitivity, precision and sample throughput that can facilitate the systematic measurement of basin-scale changes in 230Th and 231Pa seawater concentration, and provide important constraints on circulation and mixing rates in the deep ocean. Seawater samples are spiked with 229Th and 233Pa and equilibrated before pre-concentration using conventional methods of Fe oxyhydroxide co-precipitation and anion exchange. Isotopic ratios are measured using a Finnigan MAT Element magnetic sector Inductively Coupled Plasma mass spectrometer (ICP/MS) equipped with a desolvating micronebulizer. Measurements are done on 10–20 l seawater samples with an internal precision of ∼2% and a reproducibility of ∼5% (95% confidence intervals (CI)) in deep water. After correction for procedural blank, 232Th tailing, and 232Th1H interference, the detection limits are ∼3 fg for 230Th and ∼0.4 fg for 231Pa. Applied to 20 l volumes, these detection limits correspond to concentrations of 0.15 fg/kg for 230Th and 0.02 fg/kg for 231Pa, which are 5–15 times lower than typical concentrations in surface water. The capability of this method is illustrated by two seawater profiles from the Equatorial Atlantic region that show systematic variations in 230Th and 231Pa concentration consistent with patterns of deep water circulation.

Measurement of 234U/238U and 230Th/232Th in volcanic rocks using the Neptune MC-ICP-MS

We present a new multicollector inductively coupled plasma mass spectrometric (MC-ICP-MS) method for measuring isotopic ratios of 234U/238U and 230Th/232Th in volcanic rocks using the Thermo Fisher Neptune. Uranium isotope measurements using SRM U010 as bracketing standard produce agreement within several permil for NBL 112A, REMP-18, as well as for equilibrium rock standards. We also demonstrate that uranium standards are not appropriate for correcting thorium isotopic measurements due to differences between U and Th in both mass bias and mass-dependent ion transmission. Use of the thorium synthetic standard, UCSC ThA, as a bracketing standard produces data that agree well with ‘accepted values’ for other synthetic Th isotope standards and equilibrium rock standards.

Seasonal fluctuations of nitrite concentrations in the deep oligotrophic ocean

Abstract Concentrations of nitrite in the Sargasso Sea near Bermuda were measured monthly for 3 years using a chemiluminescent analysis capable of precise determination of concentrations as low as 1 nM. Ammonium and dissolved primary amine concentrations were also determined occasionally. The mean nitrite concentration over the entire period declined exponentially with depth between 300 and 1000 m ( r 2 > 0.99) and then declined slightly from 1.9 nM to 1.3 nM at 2600 m. From 150 to 250 m, the data had a bimodal distribution, with more than one-third of the observations comprising higher concentrations (mean ∼ 147 nM) during February-April that are characteristic of the “classic” primary nitrite maximum (PNM) and almost two-thirds comprising low concentration values (mean ∼ 17 nM) that would not have been detected by colorimetric analysis. A rapid oscillation between the two modes was observed. Combining the exponential [N0 2 − ]-depth relationship with Redfield ratio assumptions and apparent oxygen utilization (AOU) rates for the region (Jenkins, 1982), it was calculated that at steady state, nitrite turnover rates over the 150–1000 m depth interval range from 3 to 7 days. The depth integrated nitrite inventory reaches a maximum in the spring and is correlated with peaks in primary productivity and sediment flux. Ammonium concentrations were similar to, or higher than, nitrite concentrations and also increased dramatically during winter mixing with values of 50–100 nM in the 100–300 m depth interval before decreasing to 5–20 nM values at greater depths. During the remainder of the year, concentrations were relatively constant with depth compared to the nitrite concentration profile.