Frank Wombacher

Stable isotope compositions of cadmium in geological materials and meteorites determined by multiple-collector ICPMS

A new technique for the precise and accurate determination of Cd stable isotope compositions has been developed and applied to geological materials and meteorites. The Cd isotope analyses are performed by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS) using external normalization to Ag for mass bias correction. The accuracy of the new procedure was ascertained by the comparison of data for meteorites with published results acquired by thermal ionization mass spectrometry and double spiking. Some results were also confirmed by measurements using external normalization to Sb on a different MC-ICPMS instrument. A long-term reproducibility of ± 1.1 eCd/amu (2 sd) was obtained for separate dissolutions and multiple analyses of several rock and meteorite samples (eCd/amu represents the deviation of a Cd isotope ratio of a sample relative to the JMC Cd standard in parts per 104, normalized to a mass difference of 1 amu). As little as 5–20 ng of Cd are sufficient for the acquisition of precise and accurate data. Terrestrial rock and mineral samples display little variations in Cd isotope compositions (eCd/amu between −1 and +1.2), except for a tektite sample that was found to be enriched in the heavy Cd isotopes by +7.6 eCd/amu. The carbonaceous chondrites Orgueil, Murchison and Allende have Cd isotope ratios that are unfractionated relative to the JMC Cd standard and terrestrial rocks. The ordinary chondrites analyzed in this study and a Rumuruti chondrite display Cd isotope fractionations, ranging from −19 to +36 eCd/amu. These results suggest that substantial (inorganic) natural Cd isotope fractionations are generated only by evaporation and/or condensation processes. The lack of resolvable Cd isotope variations between the different carbonaceous chondrites, despite large differences in Cd concentrations, implies that the primary depletion of Cd in the early solar system did not involve Rayleigh evaporation. The Cd isotope fractionation in ordinary and Rumuruti chondrites is probably due to the redistribution of Cd by evaporation and condensation processes during thermal metamorphism on the parent bodies. Models that explain the enrichments of highly volatile elements in unequilibrated ordinary chondrites by primary equilibrium condensation appear to be inconsistent with the Cd isotope data.

Separation of Mg, Ca and Fe from geological reference materials for stable isotope ratio analyses by MC-ICP-MS and double-spike TIMS

Near quantitative separation of analyte elements from the sample matrix is commonly required to obtain precise and accurate stable isotope data, especially if MC-ICP-MS is used in conjunction with the standard-sample bracketing (SSB) technique for mass bias drift correction. Here, we report a robust procedure that allows for the combined chemical separation of Mg, Ca and Fe, using ion-exchange columns that contain 1 ml AG50W-X8 (200–400 mesh) cation exchange resin. Magnesium was separated for isotope ratio analyses from many geological sample types by a single pass through this column. For Mg purification, Be, Ti, Mn, Fe and Al are selectively eluted using dilute HF and an acetone–HCl mixture. The separation of Mg from Ca-dominated carbonate samples and/or a combination of Mg, Ca and Fe separation from the same sample aliquot, is achieved by the adsorption of Ca and Fe onto the ion-exchange resin from 10 M HCl. Following purification, geological reference materials, including water, bone, carbonate and sediment samples, igneous and sedimentary rocks and a chondritic meteorite were analysed by MC-ICP-MS (Mg and Fe isotopes) and double-spike TIMS (Ca isotopes). Average external repeatabilities were ±0.16‰ for 26Mg/24Mg, ±0.26‰ for 44Ca/40Ca and ±0.05‰ for 56Fe/54Fe (2sd; n ≥ 5). Comparison with published data documents the accuracy of the results. For Mg isotope analyses using SSB-MC-ICP-MS, matrix-induced mass bias effects were studied using element additions. The artificial matrices left a memory in subsequent standard analyses, likely due to depositions on the cones. This observation allowed for the detection of matrix effects in unknown samples. Finally, the current status of Mg and Ca zero-delta reference materials is briefly discussed.

Investigation of the mass discrimination of multiple collector ICP-MS using neodymium isotopes and the generalised power law

Neodymium isotopes have been used to show that the large instrumental mass discrimination of plasma source mass spectrometers cannot be accurately corrected with the common mass fractionation laws. The present study investigates the application of the generalised power law (GPL) for mass bias correction. To this end, the Nd isotope ratio data acquired with two Nu Plasma multiple collector ICP-MS instruments were compared with published Nd reference values determined by thermal ionisation mass spectrometry. The GPL applies a variable, n, such that it can be used to describe different mass-dependencies. In particular, the GPL is equivalent to the power law for n = 1 and approaches the exponential law for n → 0. The exponential and power law corrected data displayed significant (>100 ppm) deviations from the reference values for some Nd isotope ratios, whereas the instrumental mass discrimination was accurately described by the GPL. For three datasets acquired under optimal operating conditions this was achieved with n ≈ −0.23, but during routine operation of the instrument the n-values were observed to vary between −0.2 and −0.4. This shows that the GPL is a valuable tool for data evaluation in isotope ratio mass spectrometry and for the characterisation of instrumental mass-dependencies. The observed mass-dependencies are in accord with isotope fractionation by kinetic transport processes, if some of the mass discrimination is governed by reduced masses.

Origin and geochemical evolution of the Madeira-Tore Rise (eastern North Atlantic)

The Madeira-Tore Rise, located ∼700 km off the NW African coast, forms a prominent ridge in the east Atlantic. The age and origin of the rise are controversial. This study presents major and trace element, Sr, Nd, Pb, Hf isotope and 40Ar/39Ar age determinations from volcanic rocks dredged from different sites along the rise. In addition, isotopic compositions of rock samples from Great Meteor Seamount in the central Atlantic are presented. The new radiometric and paleontologically constrained ages identify two major episodes of volcanism: The first is the base of the rise (circa 80 to >95 Ma) and the second is seamounts on the rise (0.5–16 Ma). It is proposed that interaction of the Canary hot spot with the Mid-Atlantic spreading center formed the deep basement of the Madeira-Tore Rise and the J-Anomaly Ridge west of the Atlantic spreading center in the Mid-Cretaceous. Age and geochemical data and plate tectonic reconstructions suggest, however, that the recovered Late Cretaceous volcanic rocks represent late stage volcanism from the time when the Madeira-Tore Rise was still close to the Canary hot spot. Long after moving away from the influence of the Canary hot spot, the Madeira-Tore Rise was overprinted by late Cenozoic volcanism. Miocene to Pleistocene volcanism at the northern end of the rise can be best explained by decompression mantle melting beneath extensional sectors of the Azores-Gibraltar Fracture Zone (African-Eurasian plate boundary). The geochemical compositions of these volcanic rocks suggest that the magmas were variably contaminated by enriched material within or derived by melting of enriched material underplated at the base of the lithosphere, possibly originating from the Cretaceous Canary plume. Alternatively, these late Cenozoic volcanic rocks may have derived from decompression melting of enriched pyroxenitic/eclogitic material in the upper mantle. Isotopically more depleted Pliocene to Pleistocene volcanism at the southern end of the Madeira-Tore Rise may be related to the nearby Madeira hot spot.

Natural and Anthropogenic Cd Isotope Variations

Cadmium is a transition metal with eight naturally occurring isotopes that have atomic mass numbers of between 106 and 116. The large Cd isotope anomalies of meteorites have been subject to investigation since the 1970s, but improvements in instrumentation and techniques have more recently enabled routine studies of the smaller stable Cd isotope fractionations that characterize various natural and anthropogenic terrestrial materials. Whilst the current database is still comparatively small, pilot studies have identified two predominant mechanisms that routinely generate Cd isotope effects – partial evaporation/condensation and biological utilization. Processes that involve evaporation and condensation appear to be largely responsible for the Cd isotope fractionations of up to 1‰ (for 114Cd/110Cd) that have been determined for industrial Cd emissions, for example from ore refineries. Cadmium isotope measurements hence hold significant promise for tracing anthropogenic sources of this highly toxic metal in the environment. The even larger Cd isotope fractionations that have been identified in the oceans (up to 4‰ for 114Cd/110Cd) are due to biological uptake and utilization of dissolved seawater Cd. This finding confirms previous work, which identified Cd as an essential marine micronutrient that exhibits a phosphate-like distribution in the oceans. The marine Cd isotope fractionations are of particular interest, as they can be used to study micronutrient cycling and its impact on ocean productivity. In addition, they may also inform on past changes in marine nutrient utilization and how these are linked to global climate, if suitable archives of seawater Cd isotope compositions can be identified.

Pb, Nd, and Sr Isotopes and REE Systematics of Cambrian Sediments from New Zealand: Implications for the Reconstruction of the Early Paleozoic Gondwana Margin along Australia and Antarctica

The Takaka Terrane of New Zealand’s South Island contains a Middle to early Late Cambrian intraoceanic island arc assemblage that consists of the terrigeneous to volcanogenic Haupiri Group sediments and the predominantly mafic Devil River Volcanics. Pb, Nd, and Sr isotopes, and major and trace element data are presented here for the Haupiri Group sediments. These data have implications for (1) the stratigraphy of the Takaka Terrane, (2) the reconstruction of the Cambrian Gondwana margin along Australia/Antarctica, and (3) geochemical and isotope provenance studies in general. Nd isotopes, Th/Sc, and rare earth elements (REE) show that the volcaniclastic components in the Haupiri Group sediments reflect the concomitant island arc and back‐arc volcanism. These sediments can probably be linked with the Early Cambrian Kanmantoo Group and Ordovician Lachlan Fold Belt sediments in SE Australia. To explain the Nd isotope compositions, published detrital zircon populations, and paleocurrent data, an Antarctic source with Grenvillian and Ross‐Delamerian age granitoids, as well as a significant portion of reprocessed Paleoproterozoic and/or Archean crust, is required. We adopt a bipolar subduction model for the Cambrian Gondwana margin along Australia/Antarctica and support the previous suggestion that the Cambrian arc assemblages of the Takaka Terrane of New Zealand and the Antarctic Bowers Terrane belong to the same arc system with a paleogeographical position offshore the Antarctic continent.

Stable Isotope Analysis by Multiple Collector ICP-MS

Publisher Summary This chapter summarizes the important aspects of the new field of stable isotope research by Multiple Collector Inductively-Coupled Plasma Mass Spectrometers (MC-ICPMS). It provides an introduction to MC-ICPMS instrumentation, analytical capabilities and measurement techniques. A summary of stable isotope work conducted by MC-ICPMS until August 2001 is also provided. MC-ICPMS instruments were designed specifically to overcome the limitations of other ICP-MS techniques for the measurement of isotope ratios. MC-ICPMS offers a number of key advantages over existing methods for stable isotope ratio measurements. The ionization efficiency of the plasma source permits the study of a wide range of elements. The high sensitivity allows the analysis of small samples and the precision achievable by MC-ICPMS is suitable for the resolution of the small isotopic differences that occur in geological samples. In-situ stable isotope studies are also possible by MC-ICPMS with a laser-ablation sample introduction system. MC-ICPMS is opening up new avenues of stable isotope research in both geo- and cosmochemistry. It requires further developments to improve the instrumental sensitivity and to provide a more robust physical framework for accurate and precise data acquisition.

Tantalum isotope ratio measurements and isotope abundances determined by MC-ICP-MS using amplifiers equipped with 1010, 1012 and 1013 Ohm resistors

Due to analytical difficulties related to the low abundance of 180Ta (about 0.012%), the absolute isotope composition of tantalum is not well known and possible natural variations in 180Ta/181Ta are so far unconstrained. Improved precision is required in order to evaluate the homogeneity of Ta isotope distributions among solar system materials and whether natural Ta stable isotope variations exist on Earth. Using a Neptune™ multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) system and different resistors in the Faraday cup amplifier feedback loops (a 1010 Ω for 181Ta; 1012 or newly developed 1013 Ω resistors for 180Ta and Hf interference monitor isotopes) now allows relative analyses of 180Ta/181Ta with an intermediate precision of ca. ±4e (e refers to one part in 10 000) using 25 to 100 ng Ta and thus even for sample sizes available from meteorites (e.g., 1 g). The 1013 Ω amplifier resistors proved to be of paramount importance for high-precision Ta isotope ratio measurements of low amounts of material. Tailing effects from the large 181Ta beam have previously been underestimated. A thorough assessment of this effect revealed a tailing contribution of ∼2.5% on the currently recommended IUPAC ratio. Potential systematic biases in the mass discrimination correction are assumed being of minor importance compared to an uncertainty of ∼0.4% achieved for the estimate of the “true” 180Ta/181Ta ratio. We propose a new 180Ta/181Ta isotope ratio of 0.00011705(41), equivalent to 181Ta/180Ta = 8543(30), yielding isotope abundances of 0.011704(41) % for 180Ta and 99.988296(41) % for 181Ta, and an absolute atomic weight for tantalum of 180.9478787(38) u (all uncertainties with k = 2).

Tracing copper derived from pig manure in calcareous soils and soil leachates by 65Cu labeling.

Copper is used as a growth promoter in animal husbandry, resulting in high Cu concentrations in animal manure. We tested whether Cu would be mobilized in soils receiving excessive loads of manure, both from recently added and from aged fractions. To discriminate between these Cu sources, manure was labeled with (65)Cu. After soil application of 0, 15, and 30 Mg manure ha(-1), leachate was collected in free-draining lysimeters (40 cm depth) under undisturbed soil over a 53 day period. Determining the total amounts of Cu and the fractions of (65)Cu in leachate and the soil profile enabled us to trace the translocation of Cu derived from labeled manure. More than 84% of the applied Cu was retained in the top 2 cm of soil. Less than 0.01% of the applied Cu was detected overall in the leachate. Of this amount, however, 38% (± 8.9 SE) was leached within 8 days after application. The total Cu concentration in leachates (32-164 μg L(-1)) frequently exceeded the Chinese groundwater quality standard of 50 μg L(-1). The added (65)Cu, however, accounted for less than 3.6% of the total Cu leaching load, suggesting that Cu from older sources and/or geological background controls contamination, regardless of current land management.

High Temperature Geochemistry and Cosmochemistry

In this chapter, we summarize the evidence for high temperature Ca isotope fractionation in natural and experimental samples including minerals, igneous and metamorphic rocks and silicate melts and discuss the underlying isotope fractionation mechanisms. Furthermore, we outline the evidence for primordial nucleosynthetic variability of Ca isotopes in meteorites and their components, suggesting a diverse stellar origin of the Ca isotopes that make up our Solar System. We also present a brief synopsis of observed mass-dependent Ca stable isotope fractionation in meteoritic materials, which allow insights into the local conditions during the formative stage of our Solar System. Lastly, we provide an overview of the 40K–40Ca decay system and its use to track the evolution of Earth’s mantle, continental and oceanic crust over time as well as its potential to date igneous and sedimentary rocks.