Michael E. Wieser

Isotopic compositions of the elements 2009 (IUPAC Technical Report)

The Commission on Isotopic Abundances and Atomic Weights (CIAAW) of the International Union of Pure and Applied Chemistry (IUPAC) completed its last update of the isotopic compositions of the elements as determined by isotope-ratio mass spectrometry in 2009. That update involved a critical evaluation of the published literature and forms the basis of the table of the isotopic compositions of the elements (TICE) presented here. For each element, TICE includes evaluated data from the “best measurement” of the isotope abundances in a single sample, along with a set of representative isotope abundances and uncertainties that accommodate known variations in normal terrestrial materials. The representative isotope abundances and uncertainties generally are consistent with the standard atomic weight of the element Ar(E) and its uncertainty U[Ar(E)] recommended by CIAAW in 2007.

Atomic weights of the elements 2005 (IUPAC Technical Report)

The latest evaluation of atomic weight determinations and other cognate data has warranted 16 changes for the standard atomic weights of the elements, Ar(E), from those published previously in the 2001 Table of Atomic Weights. The revised standard atomic weights are as follows: Ar(Al) = 26.981 5386(8), Ar(Bi) = 208.980 40(1), Ar(Cs) = 132.905 4519(2), Ar(Co) = 58.933 195(5), Ar(Au) = 196.966 569(4), Ar(La) = 138.905 47(7), Ar(Mn) = 54.938 045(5), Ar(Nd) = 144.242(3), Ar(P) = 30.973 762(2), Ar(Pt) = 195.084(9), Ar(Sm) = 150.36(2), Ar(Sc) = 44.955 912(6), Ar(Na) = 22.989 769 28(2), Ar(Ta) = 180.947 88(2), Ar(Tb) = 158.925 35(2), Ar(Th) = 232.038 06(2). A recommendation is made that δ13C values of all carbon-bearing materials be measured and expressed relative to Vienna Pee Dee Belemnite (VPDB) on a scale normalized by assigning consensus values of -46.6 ‰ to L-SVEC lithium carbonate and +1.95 ‰ to NBS 19 calcium carbonate.

High precision calcium isotope ratio measurements using a magnetic sector multiple collector inductively coupled plasma mass spectrometer

Calcium isotope abundances were measured using a Finnigan Neptune magnetic sector multiple collector inductively coupled plasma mass spectrometer capable of resolving all molecular isobaric interferences from 42Ca, 43Ca, 44Ca, 46Ca and 48Ca. Scattering events caused by the intense 40Ar+ ion beam did not contribute to the uncertainty in the baseline of the calcium mass spectrum. Quantitative separation of the calcium from the sample matrix was carried out to ensure that the measurements were independent of the sample type. In addition, thorough desolvation of the aerosol was found to have a significant effect on the stability and sensitivity of the method. The stable mass bias of the instrument enabled normalization of the measured isotope abundance ratios relative to standard reference materials. Delta values including δ44Ca/43Ca, δ44Ca/42Ca and δ48Ca/42Ca were measured with external reproducibilities better than ±0.2‰ (2s) from 10 ppm solutions of calcium-containing materials, including sea-water and biogenic and non-biogenic marine carbonates.

The influence of organic matter on the boron isotope geochemistry of the gulf coast sedimentary basin, USA

Large variations in the boron isotopic composition of sedimentary environments make boron an attractive monitor of fluid/rock interactions during diagenesis. Studies of B in marine sediments have shown that preferential adsorption of 10B on clay minerals leaves pore waters enriched in 11B. During diagenesis, clay minerals recrystallize and incorporate 10B into the mineral structure (Spivack, A.J., Palmer, M.R., Edmond, J.M., 1987. The sedimentary cycle of the boron isotopes. Geochim. Cosmochim. Acta 51, 1939–1949). This process should cause a depletion of B in the pore water with an increase in the δ11B. In the Gulf Coast sedimentary basin (USA), however, there is a general increase in B-content of formation waters (Land, L.S., Macpherson, G.L., 1992. Origin of saline formation waters. Cenozoic Section, Gulf of Mexico Sedimentary Basin. Geochim. Cosmochim. Acta 76, 1344–1362; Moldovanyi, E.P., Walter, L.M., 1992. Regional trends in water chemistry, Smackover Formation, Southwest Arkansas: Geochemical and physical controls. AAPG Bull. 76, 864–894.) and a decrease in δ11B with depth. This suggests that another source of 10B exists in deep basinal environments. We know that oil reservoir brines are commonly enriched in boron (Collins, A.G., 1975. Geochemistry of Oilfield Waters. Elsevier, New York, p. 496.), therefore this study examines organic matter as a possible source of boron during thermal maturation. Samples of water, oil, and cored sediments were collected from three stacked hydrocarbon reservoirs in the Gulf of Mexico sedimentary basin at a depth of 3500–4350 m. Extraction of boron from organic matter (oil and kerogen) was done by Parr Bomb volatilization, with mannitol used as a B-complexing agent. The isotope ratios were measured using negative thermal ionization and compared to in situ analyses using secondary ion mass spectrometry. The δ11B values of pore filling clays in sandstone reservoirs is −2±2‰. The B-content of the clay averages 144 ppm. Oil field waters show a range in B-content from 8–85 ppm and δ11B values from +28 to +37‰, increasing from the lowest to the uppermost reservoir. There is an apparent 11B-enrichment of fluids with progressive migration through clay-rich sediments. Very little B (ppb) was found in the oil, but kerogen extracted from the oil source rock (Sassen, R., 1990. Lower Tertiary and Upper Cretaceous source rocks in Lousiana and Mississippi: implications to Gulf of Mexico crude oil. AAPG Bull. 74, 857–878.) contains significant B (140 ppm) with a δ11B of −2±2‰, similar to the pore filling clay minerals in the sandstones. While kerogen comprises only ∼2% of the sedimentary basin, its influence can be significant if B with distinctly low δ11B is released over a specific temperature interval during thermal maturation. The release of B from organic compounds could cause the observed regional 10B enrichment in waters deep in the Gulf Coast basin.

Molybdenum isotope variations in molybdenite: Vapor transport and Rayleigh fractionation of Mo

Molybdenum isotopes in 20 molybdenite samples, dated by the Re-Os method and representing a range of geologic settings, show mass-dependent fractionation spanning 0.63‰ per atomic mass unit (amu). Previous Mo isotope data for molybdenite reveal variations in fractionation of <0.5‰/amu. Interpretation of these data is hampered, however, by limited sample numbers in each study, lack of a common standard for interlaboratory comparison, and limited range of geologic settings. Here we show that Mo isotope compositions of molybdenites do not correlate with crystallization temperature, age, geographic distribution, or geologic conditions. Rather, Rayleigh distillation may explain variations of as much as 0.34‰/amu in a single molybdenite occurrence, exceeding the proposed variability in average continental crust. Vapor transport and rapid precipitation of Mo in propagating fractures may account for isotope fractionation of Mo (and perhaps other metals) at very small scales. If so, the average isotopic composition of Mo at each molybdenite occurrence may be representative of bulk crust. Our results suggest that the isotopic composition of Mo delivered to the oceans is uniform geographically and through geologic time.

Variations in the boron isotope composition of Coffea arabica beans

Abstract Significant B isotope abundance variations were found for a variety of Coffea arabica beans from a number of coffee-growing regions around the world. This may be attributed to the influence of local sources of B (including soil, water and fertilizer) each having a characteristic B isotope abundance ratio of its own. The results of this preliminary study indicate that B isotope abundance data can be used to study the biogeochemical cycle of B, an important micronutrient. In addition, the isotopic data can be employed in quality assurance programs of commercial coffee as the quality depends to a large extent on the genetic and geographic origin of the coffee.

Molybdenum isotopic evidence for oxic marine conditions during the latest Permian extinction

The latest Permian extinction (LPE), ca. 252 Ma, represents the most severe extinction event in Earths history. The cause is still debated, but widespread marine anoxic to euxinic (H2S rich) conditions, from deep to shallow water environments, are commonly suggested. As a proxy for marine oxygen levels, we analyzed δ98/95Mo of two LPE sections that represent a gradient in water depth on the northwest margin of Pangea. Results from deep-water slope environments show a large shift in δ98/95Mo values from −2.02・to +2.23・at the extinction horizon, consistent with onset of euxinic conditions. In contrast, sub-storm wave base shelf environments show little change in the molybdenum isotopic composition (−1.34・to +0.05・, indicating ongoing oxic conditions across the LPE. These results indicate that areas of the continental shelf of northwest Pangea underwent mass extinction under oxic conditions throughout the LPE event, and that shallow-water anoxia was therefore not a global phenomenon.

A laser extraction/combustion technique for in situ δ13C analysis of organic and inorganic materials

A CO(2) laser extraction system is described for in situ delta(13)C analysis of organic and inorganic materials. Carbonaceous compounds volatilized by the laser are quantitatively converted to CO(2) gas by a combustion furnace mounted after the sample chamber. Gases produced by the laser and combustion processes are swept by helium carrier gas and separated by a packed gas chromatography column prior to their introduction to an isotope ratio monitoring mass spectrometer. A sample of lentil bean was analyzed at a spatial resolution of 200 µm and yielded delta(13)C values with precision of +/- 0.3 per thousand. The accuracy of delta(13)C measurements was better than +/- 0.5 per thousand from NBS 22 (mineral oil), USGS 24 (graphite), and IAEA CO-1 (calcium carbonate). Copyright 1999 John Wiley & Sons, Ltd.

The absolute isotopic composition and atomic weight of molybdenum in SRM 3134 using an isotopic double-spike

Analytical techniques have been developed to perform a fully calibrated measurement of the isotopic composition of molybdenum reference materials using multiple collector inductively coupled plasma mass spectrometry. A correction for instrumental mass bias was performed using a molybdenum double-spike prepared from gravimetric mixtures of 92Mo and 98Mo isotope spikes. The absolute isotopic composition of the NIST SRM 3134 molybdenum reference material with 2 s.d. uncertainty was determined to be 92Mo = 14.65(4), 94Mo = 9.187(12), 95Mo = 15.873(10), 96Mo = 16.673(3), 97Mo = 9.582(6), 98Mo = 24.29(3), and 100Mo = 9.74(2). The atomic weight of molybdenum in SRM 3134 was calculated as Ar = 95.949(3). Delta values (reported as δ98/95Mo relative to SRM 3134) were determined for SCP Science – PlasmaCal molybdenum as −0.42(10)‰ and for Johnson-Matthey pure molybdenum rod as −0.45(12)‰. The total natural variation of molybdenum of −1.5‰ to +3‰ results in a calculated atomic weight interval of Ar = [95.944, 95.956].

Fission yields of molybdenum in the Oklo natural reactor

The isotopic compositions of molybdenum in six uranium-rich samples from the Oklo Zone 9 natural reactor were accurately measured by thermal ionization mass spectrometry. The samples were subjected to an ion exchange separation process that removed the isobaric elements zirconium and ruthenium, with high efficiency and a low blank. Molybdenum possesses seven isotopes of which 92,94,96Mo are unaffected by the fission process, enabling the raw data to be corrected for isotope fractionation by normalising to 92Mo/96Mo, and to use 94Mo to correct for the primordial component in each of the fission-produced isotopes. This enables the relative fission yields of Mo to be calculated from the isotopic composition measurements, to give cumulative fission yields of 1:0.941:0.936:1.025 for 95,97,98,100Mo, respectively. These data demonstrate that the most important nuclear process involved in reactor Zone 9 was the thermal neutron fission of 235U. The consistency of the relative cumulative fission yields of all six samples from different locations in the reactor, implies that Mo is a mobile element in the uraninite comprising Zone 9, and that a significant fraction of molybdenum was mobilized within the reactor zone and probably escaped from Zone 9, a conclusion in agreement with earlier published work.