Tl Isotope Compositions Research Articles

Thallium isotopes in Iceland and Azores lavas — Implications for the role of altered crust and mantle geochemistry

Mantle plumes are commonly perceived to have both a chemical and dynamic link with the subduction of ocean crust into the mantle. In principle, this should lead to the observation of chemical and isotopic signatures that are characteristic of ocean crust and marine sediments in ocean island basalts. This study investigates the thallium (Tl) isotope systematics of lavas from Iceland and the Azores archipelago, in order to determine if their compositions were affected by admixing of ferromanganese sediments or upper ocean crust altered at low temperature. Such materials are known to display strongly fractionated Tl isotope signatures relative to the ambient upper mantle. Two samples from the island of Terceira in the Azores archipelago have Tl isotope compositions significantly different from normal mantle, and this suggests the presence of Fe–Mn sediments. Combined Pb and Tl isotope modelling indicates that the Tl anomalies are not a feature of the Azores plume but produced by assimilation of modern Fe–Mn sediments during magma ascent through the ocean crust. Excluding these two anomalous lavas from Terceira, the Iceland and Azores samples have identical Tl isotope compositions, with an overall mean of ɛ 205Tl = − 1.5 ± 1.4 (2SD, n = 30) that is indistinguishable from the previously estimated upper mantle average ( ɛ 205Tl = − 2.0 ± 0.5). The near-constant Tl isotope compositions of the Iceland and Azores lavas may indicate that the respective mantle plume sources contain virtually no Fe–Mn sediments or altered upper ocean crust. Alternatively, it is possible that the lack of Tl isotope variation reflects quantitative removal of fractionated Tl from the slab during subduction and dehydration. A less straightforward explanation is that past marine environments produced sediments and altered marine basalts with nearly unfractionated Tl isotope compositions. All three scenarios have important implications and future Tl isotope studies will be able to identify the most feasible interpretation.

Thallium isotope constraints on the water fluxes of ridge flank hydrothermal systems

Hydrothermal activity in the oceans is not restricted to the high-temperature (T) vents that are found at mid-ocean ridge axes. Rather, ridge flanks have a total hydrothermal power output which exceeds the axial output by at least a factor of 3. As the temperatures are comparatively low at ridge flanks, the respective water flux must be several orders of magnitude larger than on-axis, to remove the inferred power output. Even small changes in the composition of the circulating fluids can therefore produce very significant chemical fluxes. Our understanding of the importance of these chemical fluxes with respect to global geochemical budgets is limited, however, because the partitioning of heat between warmer, more reactive fluids with T 40 °C and cooler fluids, which have T 20 °C, is only poorly constrained. In this study, we have used Tl isotope and concentration data for altered ocean crust and hydrothermal fluids to constrain the water fluxes and average fluid exit temperatures of ridge flank hydrothermal systems. The calculations are based on the observation that the upper ocean crust, which has been altered at low temperature, has fractionated Tl isotope compositions and elevated Tl concentrations. The observed systematics can be exploited to calculate the flux of low-T hydrothermal fluids (FLT) by mass balance: FLT×[Tl]sw×fupt=Fuoc×?[Tl]uoc Here [Tl]sw is the Tl concentration of seawater, fupt is the fraction of Tl removed from seawater by alteration, Fuoc is the mass flux of upper ocean crust affected by low-T hydrothermal alteration, and ?[Tl]uoc is the average change of Tl concentration observed for low-T altered ocean crust. The calculations indicate that the hydrothermal water flux of ridge flanks is (0.2–5.4) × 1017 kg/yr. This implies that the fluids have an average temperature anomaly of only about 0.1–3.6° relative to ambient seawater. Such low temperatures should severely restrict the effect of ridge flank hydrothermal systems on the marine budgets of 87Sr/86Sr and Mg. This conclusion is in accord with the results of previous rock alteration studies, which concluded that the fluxes of low-T hydrothermal systems are insufficient to balance the oceanic budgets of these elements.

Large thallium isotopic variations in iron meteorites and evidence for lead-205 in the early solar system

Lead-205 decays to 205Tl with a half-life of 15 Myr and should have been present in the early solar system according to astrophysical models. However, despite numerous attempts, Tl isotopic measurements of meteorites have been unable to demonstrate convincingly its former presence. Here, we report large (∼5‰) variations in Tl isotope composition in metal and troilite fragments from a range of iron meteorites that were determined at high precision using multiple collector inductively coupled plasma mass spectrometry. The Tl isotopic compositions of seven metal samples of the IAB iron meteorites Toluca and Canyon Diablo define a correlation with 204Pb/ 203Tl. When interpreted as an isochron, this corresponds to an initial 205Pb/ 204Pb ratio of (7.4 ± 1.0) × 10 −5. Alternative explanations for the correlation, such as mixing of variably mass-fractionated meteorite components or terrestrial contamination are harder to reconcile with independent constraints. However, troilite nodules from Toluca and Canyon Diablo contain Tl that is significantly less radiogenic than co-existing metal with isotope compositions that are variable and decoupled from 204Pb/ 203Tl. These effects are similar to those recently reported by others for Fe and Ni isotopes in iron meteorite sulfides and appear to be the result of kinetic stable isotope fractionation during diffusion. Though it cannot conclusively be shown that the metal fragments are unaffected by the secondary processes that disturbed the troilites, mass balance modeling indicates that the alteration of the troilites is unlikely to have significantly affected the Tl isotope compositions of the co-existing metals. It is therefore reasonable to conclude that the IAB metal isochron is a product of the in situ decay of 205Pb. If the I–Xe ages of IAB silicate inclusions record the same event as the 205Pb– 205Tl chronometer then crystallization of the IAB metal was probably completed between 10 and 20 Myr after the condensation of the first solids. This implies an initial solar system 205Pb/ 204Pb of (1.0–2.1) × 10 −4, which is in excellent agreement with recently published astrophysical predictions. Similar calculations yield an initial solar system Tl isotope composition of ε 205Tl = −2.8 ± 1.7. The Tl isotopic composition and concentration of the silicate Earth depends critically on the timing and mechanism of core formation and Earth’s volatile element depletion history. Modeling of the Earth’s accretion and core formation using the calculated initial solar system Tl isotope composition and 205Pb/ 204Pb, however, does not yield reasonable results for the silicate Earth unless either the Earth lost Tl and Pb late in its accretion history or the core contains much higher concentrations of Pb and Tl than are found in iron meteorites.

Thallium isotope composition of the upper continental crust and rivers—An investigation of the continental sources of dissolved marine thallium

The thallium (Tl) concentrations and isotope compositions of various river and estuarine waters, suspended riverine particulates and loess have been determined. These data are used to evaluate whether weathering reactions are associated with significant Tl isotope fractionation and to estimate the average Tl isotope composition of the upper continental crust as well as the mean Tl concentration and isotope composition of river water. Such parameters provide key constraints on the dissolved Tl fluxes to the oceans from rivers and mineral aerosols. The Tl isotope data for loess and suspended riverine detritus are relatively uniform with a mean of ε 205Tl = −2.0 ± 0.3 (ε 205Tl represents the deviation of the 205Tl/ 203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 10 4). For waters from four major and eight smaller rivers, the majority were found to have Tl concentrations between 1 and 7 ng/kg. Most have Tl isotope compositions very similar (within ±1.5 ε 205Tl) to that deduced for the upper continental crust, which indicates that no significant Tl isotope fractionation occurs during weathering. Based on these results, it is estimated that rivers have a mean natural Tl concentration and isotope composition of 6 ± 4 ng/kg and ε 205Tl = −2.5 ± 1.0, respectively. In the Amazon estuary, both additions and losses of Tl were observed, and these correlate with variations in Fe and Mn contents. The changes in Tl concentrations have much lower amplitudes, however, and are not associated with significant Tl isotope effects. In the Kalix estuary, the Tl concentrations and isotope compositions can be explained by two-component mixing between river water and a high-salinity end member that is enriched in Tl relative to seawater. These results indicate that Tl can display variable behavior in estuarine systems but large additions and losses of Tl were not observed in the present study.

Cenozoic marine geochemistry of thallium deduced from isotopic studies of ferromanganese crusts and pelagic sediments

Cenozoic records of Tl isotope compositions recorded by ferromanganese (Fe–Mn) crusts have been obtained. Such records are of interest because recent growth surfaces of Fe–Mn crusts display a nearly constant Tl isotope fractionation relative to seawater. The time-series data are complemented by results for bulk samples and leachates of various marine sediments. Oxic pelagic sediments and anoxic marine deposits can be distinguished by their Tl isotope compositions. Both pelagic clays and biogenic oozes are typically characterized by ϵ 205Tl greater than +2.5, whereas anoxic sediments have ϵ 205Tl of less than −1.5 ( ϵ 205Tl is the deviation of the 205Tl/ 203Tl isotope ratio of a sample from NIST SRM 997 Tl in parts per 10 4). Leaching experiments indicate that the high ϵ 205Tl values of oxic sediments probably reflect authigenic Fe–Mn oxyhydroxides. Time-resolved Tl isotope compositions were obtained from six Fe–Mn crusts from the Atlantic, Indian, and Pacific oceans and a number of observations indicate that these records were not biased by diagenetic alteration. Over the last 25 Myr, the data do not show isotopic variations that significantly exceed the range of Tl isotope compositions observed for surface layers of Fe–Mn crusts distributed globally ( ϵ 205Tl=+12.8±1.2). This indicates that variations in deep-ocean temperature were not recorded by Tl isotopes. The results most likely reflect a constant Tl isotope composition for seawater. The growth layers of three Fe–Mn crusts that are older than 25 Ma show a systematic increase of ϵ 205Tl with decreasing age, from about +6 at 60–50 Ma to about +12 at 25 Ma. These trends are thought to be due to variations in the Tl isotope composition of seawater, which requires that the oceans of the early Cenozoic either had smaller output fluxes or received larger input fluxes of Tl with low ϵ 205Tl. Larger inputs of isotopically light Tl may have been supplied by benthic fluxes from reducing sediments, rivers, and/or volcanic emanations. Alternatively, the Tl isotope trends may reflect the increasing importance of Tl fluxes to altered ocean crust through time.

The precise and accurate determination of thallium isotope compositions and concentrations for water samples by MC-ICPMS

We present optimised analytical protocols for the precise and accurate determination of Tl concentrations and isotope compositions (IC) for water samples by multiple collector inductively coupled plasma mass spectrometry (MC-ICPMS). The Tl abundance measurements utilise the isotope dilution (ID) technique. About 50 ml of seawater or river water, with ∼150 to 500 pg of Tl, are spiked with a 203Tl tracer and thallium is then separated from the sample matrix using a single anion-exchange column. The isotope compositions of spiked samples are measured by MC-ICPMS using either admixed Pb or Pt for mass bias correction. Replicate analyses of three natural water samples with Tl contents of 3 to 10 pg/g displayed external reproducibilities (1 S.D.) of 0.2% to 1.5%. The determination of Tl isotope compositions requires a minimum of 5 ng of Tl, equivalent to about 0.5 to 1.5 l of seawater or river water. Separation of Tl is achieved with a two-step anion-exchange procedure that generates Tl separates that are essentially Pb-free and that display very low overall matrix contents. The Tl isotope compositions are determined by MC-ICPMS using admixed NIST SRM 981 Pb for the mass discrimination correction. With these techniques, replicate analyses of a seawater and a river water sample displayed an external precision (2 S.D.) of about ±0.1‰. A detailed study was carried out to identify possible sources of systematic error. These investigations indicate that the Tl stable isotope data are not biased by blanks, chemical fractionation during laboratory handling, mass spectrometric errors, spectral interferences, matrix effects, or use of an unsuitable method of mass bias correction. Therefore, the Tl isotope data can be assumed to be accurate, to within the stated uncertainties. Analyses of the international seawater standard NASS-5 were carried out, to provide a reference for future Tl isotope studies. This sample was found to have a Tl concentration of 9.37±0.02 pg/g and a Tl isotope composition of ε 205Tl=−5.0±1.0 (ε 205Tl is the deviation of the 205Tl/ 203Tl isotope ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 10 4).

Thallium isotope variations in seawater and hydrogenetic, diagenetic, and hydrothermal ferromanganese deposits

Results are presented for the first in-depth investigation of Tl isotope variations in marine materials. The Tl isotopic measurements were conducted by multiple collector-inductively coupled plasma mass spectrometry for a comprehensive suite of hydrogenetic ferromanganese crusts, diagenetic Fe–Mn nodules, hydrothermal manganese deposits and seawater samples. The natural variability of Tl isotope compositions in these samples exceeds the analytical reproducibility (±0.05‰) by more than a factor of 40. Hydrogenetic Fe–Mn crusts have ϵ 205Tl of +10 to +14, whereas seawater is characterized by values as low as −8 ( ϵ 205Tl represents the deviation of the 205Tl/ 203Tl ratio of a sample from the NIST SRM 997 Tl isotope standard in parts per 10 4). This ∼2‰ difference in isotope composition is thought to result from the isotope fractionation that accompanies the adsorption of Tl onto ferromanganese particles. An equilibrium fractionation factor of α∼1.0021 is calculated for this process. Ferromanganese nodules and hydrothermal manganese deposits have variable Tl isotope compositions that range between the values obtained for seawater and hydrogenetic Fe–Mn crusts. The variability in ϵ 205Tl in diagenetic nodules appears to be caused by the adsorption of Tl from pore fluids, which act as a closed-system reservoir with a Tl isotope composition that is inferred to be similar to seawater. Nodules with ϵ 205Tl values similar to seawater are found if the scavenging of Tl is nearly quantitative. Hydrothermal manganese deposits display a positive correlation between ϵ 205Tl and Mn/Fe. This trend is thought to be due to the derivation of Tl from distinct hydrothermal sources. Deposits with low Mn/Fe ratios and low ϵ 205Tl are produced by the adsorption of Tl from fluids that are sampled close to hydrothermal sources. Such fluids have low Mn/Fe ratios and relatively high temperatures, such that only minor isotope fractionation occurs during adsorption. Hydrothermal manganese deposits with high Mn/Fe and high ϵ 205Tl are generated by scavenging of Tl from colder, more distal hydrothermal fluids. Under such conditions, adsorption is associated with significant isotope fractionation, and this produces deposits with higher ϵ 205Tl values coupled with high Mn/Fe.

High-precision analysis of Pb isotope ratios by multi-collector ICP-MS

We investigated high-precision Pb isotope ratio analysis by multi-collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS) using added thallium as an internal isotopic standard to correct for mass dependent isotopic fractionation. We compared MC-ICP-MS analysis of both an inter-laboratory standard, NBS 981, and geological samples to conventional thermal ionization mass spectrometry (TIMS). As expected, we found that analytical error in the latter was dominated by mass fractionation. In MC-ICP-MS, we found that fractionation appears to follow the exponential law, but that the fractionation coefficients, f, of Tl and Pb were not identical. This difference in fractionation coefficients cannot be compensated for by renormalizing to a different Tl isotopic composition as done in other studies. We found, however, that the f Pb/ f Tl ratio was constant over the course of an analytical session, allowing f Pb to be calculated from f Tl. An exponential law correction was then applied to the Pb isotope measurements which effectively eliminates errors associated with mass fractionation. Precision for the MC-ICP-MS analyses ranged from a factor of 2 to a factor of 6 better than for TIMS analyses for the 206 Pb/ 204 Pb and 208 Pb/ 206 Pb ratios respectively. Residual error in the MC-ICP-MS analyses was dominated by error in the analysis of 204 Pb , perhaps in part due to random errors introduced by correcting for a 204 Hg isobaric interference. We also found systematic errors in the MC-ICP-MS analyses compared to TIMS determinations that may be due to uneven background and collector biases in the instrument used. We found that these systematic errors were the same for both NBS 981 and the geological samples, so accurate correction factors could be generated from the standard analyses to correct the sample analyses. MC-ICP-MS has the additional advantages of requiring less preparative chemistry, less instrument time, and considerably less labor overall.

The precise measurement of Tl isotopic compositions by MC-ICPMS: Application to the analysis of geological materials and meteorites

The precision of Tl isotopic measurements by thermal ionization mass spectrometry (TIMS) is severely limited by the fact that Tl possesses only two naturally occurring isotopes, such that there is no invariant isotope ratio that can be used to correct for instrumental mass discrimination. In this paper we describe new chemical and mass spectrometric techniques for the determination of Tl isotopic compositions at a level of precision hitherto unattained. Thallium is first separated from the geological matrix using a two-stage anion-exchange procedure. Thallium isotopic compositions are then determined by multiple-collector inductively coupled plasma-mass spectrometry with correction for mass discrimination using the known isotopic composition of Pb that is admixed to the sample solutions. With these procedures we achieve a precision of 0.01–0.02% for Tl isotope ratio measurements in geological samples and this is a factor of ≥3–4 better than the best published results by TIMS. However, without adequate precautions, experimental artifacts can be generated that result in apparent Tl isotopic fractionations of up to one per mil. Analysis of five terrestrial samples indicate the existence of Tl isotopic variations related to natural fractionation processes on the Earth. Two of the three igneous rocks analyzed in this study display Tl isotopic compositions indistinguishable from our laboratory standard, the reference material NIST-997 Tl. A third sample, however, is characterized by ε Tl ≈ 2.5 ± 1.5, where ε Tl represents the deviation of the 205Tl/ 203Tl ratio of the sample relative to NIST-997 Tl in parts per 10 4. Even larger deviations were identified for two ferromanganese crusts from the Pacific Ocean, which display ε Tl-values of +5.0 ± 1.5 and +11.7 ± 1.3. We suggest that the large variability of Tl isotopic compositions in the latter samples are caused by low-temperature processes related to the formation of the Fe-Mn crusts by precipitation and scavenging from seawater. Thallium is the heaviest element for which such a natural isotopic fractionation has been verified. For a bulk sample of the C3V chondrite Allende we have determined a Tl isotopic composition of ε Tl = −2.1 ± 1.4, the lowest value measured in this study. By combining our data for Allende with a previously published Tl isotopic composition for the L3 chondrite Mezö-Madaras we obtain an upper limit of 9.2 × 10 −5 for the 205Pb/ 204Pb abundance ratio of the bulk solar system at the time of volatile element depletion. If the chemical depletion of volatile elements occurred early, during the condensation of the solar nebula, our result would indicate that the initial Solar System abundance of 205Pb is significantly lower than some current astrophysical estimates.

Accuracy and long-term reproducibility of lead isotopic measurements by multiple-collector inductively coupled plasma mass spectrometry using an external method for correction of mass discrimination

The precision of isotopic measurements of Pb by thermal ionization mass spectrometry (TIMS) is limited by the fact that this element does not possess an invariant isotope ratio that can be used for the correction of mass fractionation by internal normalization. Multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS) can overcome this limitation, because with plasma ionization, elements with overlapping mass ranges are thought to display identical mass discrimination. With respect to Pb, this can be exploited by the addition of Tl to the sample solutions; the mass discrimination factor obtained for Tl can then be used for the correction of the measured Pb isotope ratios. In this article we present the results of a detailed study that investigates the accuracy and precision of such an external correction technique for mass discrimination based upon the results of multiple analyses of a mixed standard solution of NIST SRM-981 Pb and SRM-997 Tl. Our data indicate that normalization of the Pb isotope ratios to the certified isotopic composition of SRM-997 Tl produces Pb isotopic results that are significantly lower than recently published reference values by TIMS. This systematic offset can be eliminated by renormalization of the Pb data to a different Tl isotopic composition to obtain an empirically determined mass discrimination factor for Pb that generates accurate results. It is furthermore shown that a linear law is least suited for the correction of mass discrimination, whereas a power or exponential law function provide significantly more accurate and precise results. In detail, it appears that a power law may provide the most appropriate correction procedure, because the corrected Pb isotope ratios display less residual correlations with mass discrimination compared to the exponentially corrected data. Using an exponential or power law correction our results, obtained over a period of over seven months, display a precision (2σ) of better than 60 parts per million (ppm) for 208Pb/ 206Pb and 207Pb/ 206Pb and of better than 350 ppm for 206Pb/ 204Pb, 207Pb/ 204Pb/ 204Pb, and 208Pb/ 204Pb. This represents a significant improvement compared to conventional TIMS techniques and demonstrates the potential of MC-ICPMS for routine, high-precision measurements of Pb isotopic compositions.

Distribution of lead and thallium in the matrix of the Allende meteorite and the extent of terrestrial lead contamination in chondrites

Selective chemical dissolution has been used to study the distribution of Pb and Tl in an ultrafine −20-μm matrix separate of Allende. The matrix was exposed to high-purity reagents ranging from H 2O, then HCl of increasing concentration and finally HF-HCl mixtures. A total of 17 extractions were obtained, each for a minimum period of 10 days. The isotopic compositions of the Pb released during the slow dissolution of the matrix fall into four distinct groups. The first, consisting of four extractions, released a component of terrestrial Pb isotopic composition with a total abundance of about 1 ppb. The next six extractions, which contained the bulk of the indigenous Pb and Tl corresponding to 96% and 94%, respectively, of the total matrix abundance, was of a reasonably homogeneous Pb isotopic composition with mean ratios of 206Pb 204Pb= 10.00and 207Pb 204Pb= 10.74 . In the final seven extractions, the released Pb falls into two higher isotopic groupings and probably results from the dissolution of debris from chondrules and inclusions. The apparent age of the internal matrix isochron is 4562 ± 14 My. The release of Pb and Tl shows a reasonable correlation with the matrix dissolution. This indicates that the Pb and Tl reside predominantly within the matrix phases rather than as a localised phase. The Tl isotopic composition of two matrix fractions and whole meteorite were measured and found to be indistinguishable from the terrestrial 205Tl/ 203Tl ratio. Measurement of a terrestrial reagent standard in the range 1–10 ng Tl gave, for 20 analyses, a mean 205Tl/ 203Tl ratio of 2.38907 ± 0.00102 (2σ). The estimate of terrestrial Pb contamination is considerably lower than the 6–300 ppb assumed in some recent studies in order to explain the phenomenon of apparent excess radiogenic Pb in chondrites. The problem of terrestrial Pb pollution and the evidence which argues against a relatively severe and homogeneous Pb contamination of meteorites, is briefly considered. The apparent initial isotopic composition of the bulk of the indigenous Pb in the Allende matrix was found to be 206Pb 204Pb= 9.57and 207Pb 204Pb= 10.47 . This is of a higher composition than the Pb in the Can˜on Diablo troilite phase and further indicates that the phenomenon of apparent excess radiogenic Pb in chondrites is real.