Alan M. Volpe

238U-230Th-226Ra disequilibrium in young Mt. Shasta andesites and dacites

The paper describes 238U-series nuclides and 230Th/232Th ratios measured by mass spectrometry in mineral separates of young Mt. Shasta andesites and dacites. The results constrain the timing of recent calc-alkaline magma fractionation at this volcano. Hotlum, Misery Hill and Black Butte rocks show small, < 13% 230Th-238U and < 6% 226Ra-230Th, disequilibria. Plagioclase have 7–26% 226Ra excesses, magnetite and groundmass have 4–5% 226Ra deficits, and pyroxenes have equilibrium (226Ra/230Th) activity ratios. Internal (230Th)-(238U)and Ba-normalized (226Ra)-(230Th) isotope diagrams for Hotlum and Black Butte dacites suggest that closed-system Th-U and Ra-Th fractionation occurred less than 10,000 years ago. Significant 226Ra-230Th disequilibria in the Black Butte dacite strongly suggests that this rock erupted more recently than 9400 years ago. Results for Hotlum andesites suggest a longer pre-eruption crystal residence time compared to the dacites. There may also have been recent open Ra-Th system changes in the melt composition. Initial Th/U ratios for the rocks are low (2.43–2.57), similar to those in mid-ocean ridge basalts (MORB), and preclude significant assimilation of crust with markedly different Th-U composition.

At-sea high-resolution chemical mapping: extreme barium depletion in North Pacific surface water

We have modified an inductively coupled plasma mass spectrometer (ICPMS) for use at sea, which permits mapping of dissolved Ba at high temporal and spatial resolution. While evaluating this system off the Pacific coast of Baja California, we discovered Ba depletion greater than 60% in surface waters over tens of kilometers. Observed Ba abundance is among the lowest reported in any ocean water. No corresponding depletion was seen in Sr, which strongly suggests that acantharians are not the primary source of biogenic Ba in these surface waters.

Radiocesium in North San Francisco Bay and Baja California coastal surface waters

Radiocesium, 137Cs, and rare earth elements (REEs) were determined in suspended material and dissolved fractions of waters across the salinity gradient in North San Francisco Bay (estuary). We describe the variation of this conservative isotope tracer with salinity and sediment load. REE data are used to differentiate marine and terrigenous source terrains for suspended material and dissolved fractions. We estimate that about 1-4 x 10(10) Bq of 137Cs migrates annually on suspended material through the North Bay. In addition, 137Cs concentrations were measured in surface waters off Baja California. Combined in situ water density (sigma(t)) and 137Cs data distinguish between California Current and Gulf of California water, and delineate areas of upwelling, where nutrient-rich, deep Pacific Intermediate water, with little or no 137Cs, is brought to the surface off promontories along Baja California.

Real-time ocean chemistry for improved biogeochemical observation in dynamic coastal environments

We describe a new ocean observation system that integrates mass spectroscopy, hydrographic instrumentation, and satellite imagery (SeaWIFS). We used a quadrupole ICP mass spectrometer at sea to acquire continuous trace element data during separate surveys of the Baja California coastal margin, and the San Diego Bay and coastal environment. There is evidence for extreme Ba depletion in surface waters off the Baja coast, which is the result of biological productivity and marine barite precipitation. The synoptic data are used to elucidate the biochemical mechanism of barium removal; to constrain the spatial and temporal boundaries over which the phenomenon occurs; and to quantify surface flux to sediments. Further systems application led to mapping of the distribution of a suite of biologically active trace metals (Mn, Ni, Zn, Cu, and Cd) in San Diego Bay, and the Bay signature was tracked in tidal plumes into the coastal ocean. The continuous data provided chemical gradients within the Bay, with which we estimate the contaminant metal flux that is discharged from the Bay into the coastal ocean during tidal pumping.

Pre-concentration and measurement of low levels of gamma-ray emitting radioisotopes in coastal waters

We describe extensive testing of a large-volume, high-speed water sampler for the concentration and measurement of radionuclides using high-resolution gamma ray spectrometry. The sampler processed hundreds to thousands of liters of natural waters with variable suspended sediment and salinity loads at flow rates of 10-201/min. Extraction of most radionuclides in the water column was accomplished through the combination of physical filtration down to 0.1 microm particle size and chemical separation of dissolved species on cellulose-based inorganic sorbent beds without recourse to complex, or hazardous chemistry. Performance and extraction efficiencies for suites of radioisotopes were determined in the laboratory and in the field with river and coastal ocean water samples. Extraction and recovery efficiencies are better than 90% for most fission and activation product radioisotopes. This methodology has broad application to the study of the distribution and fate of radioisotopes in coastal waterways.

Forensic Analyses of Suspect Illicit Nuclear Material

A small metal sample, alleged to be a substance that could substitute for highly enriched uranium in a nuclear weapon, was subjected to qualitative and quantitative forensic analyses using methods of materials science, radioisotopic chemistry, inorganic chemistry, and organic chemistry. The specimen was determined to be moderately pure Sc, likely derived from a uranium refining operation. Although no fissionable species or weaponization signatures were detected, the sample did exhibit some unusual properties. These anomalies included lanthanide fractionation, with concentrations of Dy, Ho, and Er elevated by factors greater than 100 over normal levels, and the presence of long, odd-chain fatty acids.

Real-time ocean chemical measurement: at-sea ICP-MS experiments

We describe the first at-sea deployment and operation of an inductively coupled plasma mass spectrometer (ICP-MS) for continuous measurement of isotopes and trace element concentrations in sea-water. The purpose of these experiments was to demonstrate that an ICP-MS can be operated in a harsh environment with no degradation in performance, and that accurate trace element data can be acquired on rapid analytical time scales. Evaluation at sea involved performance testing, characterization and calibration of a real-time sea-water analytical methodology, and continuous sea-water profiling over an extended three day transect from the Gulf of California to San Diego. We show that mass spectrometers can rapidly, precisely and accurately determine trace element concentrations in sea-water, thus allowing high-resolution mapping of large areas of surface sea-water. This analytical capability represents a significant advance toward real-time observation and characterization of water mass chemistry in dynamic coastal environments. While evaluating the ICP-MS off the coast of Baja California, we discovered barium depletion of 65% in surface waters over tens of kilometres.

Stable isotopic characterization of ammonium metavanadate (NH4VO3).

This paper describes hydrogen ((2)H/(1)H), nitrogen ((15)N/(14)N), and oxygen ((18)O/(16)O) isotopic characterization of ammonium metavanadate (NH(4)VO(3)), a toxic industrial chemical (TIC). We analyzed nineteen high purity compounds obtained from nine suppliers, which show large ranges in trivariate stable isotope compositions, nearly 100-fold greater than analytical uncertainty. Covariation between δ(2)H and δ(15)N values indicates these ratios can be used to trace ammonia compounds, which are critical for the industrial purification of vanadyl ions and precipitation of ammonium metavanadate crystals. δ(2)H and δ(18)O plot far from the Meteoric Water Line (MWL), and suggest materials and industrial processing may lead to decoupling of H and O isotopes. We show how stable isotope characterization is a valuable forensic tool that discriminates between NH(4)VO(3) samples due to differences in source materials, modes of production, and facility location.