Tl Isotopes Research Articles

High-spin states and isomers in the one-proton-hole and three-neutron-hole204Tl isotope

The high-spin structure of the neutron-rich ${}^{204}$Tl isotope has been studied up to a 11.2-MeV excitation energy and a $I=30$ spin range using the deep-inelastic heavy-ion $\ensuremath{\gamma}$-spectroscopy method with reactions of ${}^{48}$Ca on thick ${}^{208}$Pb and ${}^{238}$U targets. The established structure of yrast levels involves four isomeric states up to ${I}^{\ensuremath{\pi}}={22}^{\ensuremath{-}}$, the highest spin state available for the maximally aligned four valence holes. The observations are interpreted and quantitatively confirmed by shell-model calculations. The rates of the identified M2 and E3 isomeric decays are discussed and a striking analogy is found for the yrast level structures and $\ensuremath{\gamma}$ decays observed in the ${18}^{+}$ to ${22}^{\ensuremath{-}}$ and $45/{2}^{\ensuremath{-}}$ to $53/{2}^{+}$ spin ranges in ${}^{204}$Tl and ${}^{203}$Hg, respectively. In the highest spin part of the scheme, two prominently populated yrast states are tentatively identified as the ${3}^{\ensuremath{-}}$ ${}^{208}$Pb core excitation built on the ${22}^{\ensuremath{-}}$ and ${20}^{+}$ maximally aligned four-hole states. Their energies are reproduced well by using energy shifts observed in experiments for the ${}^{208}$Pb core octupole excitation coupled to simpler intrinsic structures.

New age for ferromanganese crust 109D-C and implications for isotopic records of lead, neodymium, hafnium, and thallium in the Pliocene Indian Ocean

[1] This study presents a high-resolution record of osmium and thallium isotopes in a ferro-manganese (Fe-Mn) crust from the Indian Ocean, Antipode 109D-C. These results, when combined with additional new Os isotope data from ODP Hole 756B in the southeast Indian Ocean, define a new best estimate for the age at the base of this crust of ∼6.5 Ma, which is significantly different from a previous estimate of ∼15 Ma based on Co-flux modeling. The Tl isotope record obtained for the Indian Ocean resembles that for the Pacific Ocean with a small but well-defined increase occurring over the last ∼5 Myr. This contrasts with two records from the Atlantic Ocean which do not have resolvable variations. Ocean basin–scale Tl isotope variation may be inconsistent with the inferred modern marine residence time for Tl of ∼20 kyr but could be explained by an increase in ocean crust production rates in the Pacific and Indian oceans since ∼10 Ma. The improved age model for 109D-C reveals that the Hf isotope composition of Indian Ocean bottom waters has remained homogenous over the last ∼6 Myr. Thus, this isotope system does not bear any evidence that the influence of North Atlantic Deep Water in the formation of Indian Ocean bottom waters has changed during that time. However, because of the lack of knowledge about Hf isotopes as a tracer of ocean circulation, we cannot conclude that export of NADW decreased over the last 6 Myr.

Low-lying states of valence-hole nuclei in the 208Pb region

Systematic calculations of low-lying states for Ir, Pt, Au, Hg and Tl isotopes with neutron numbers between 120 and 125 have been performed within the framework of the SDG-pair approximation of the shell model. We employ a monopole and quadrupole pairing plus quadrupole–quadrupole-type interaction with optimized parameters, which are assumed to be constants for nuclei with the same proton number or neutron number. We calculate binding energies of the ground states, low energy level schemes, electric quadrupole and magnetic dipole moments, and E2 transition rates. Our results are reasonably consistent with the available experimental data as well as previous theoretical studies, in particular, for low-lying yrast states. We also demonstrate that low-lying states of nuclei studied here are usually well represented by very simple configurations in collective nucleon-pair basis.

Deexcitation Energy of Superdeformed Secondary Minima as a Probe to Density Dependence of Symmetry Energy

Deexcitation energies of superdeformed secondary minima of odd-odd Au and Tl isotopes are investigated with the relativistic mean field (RMF) model where the isoscalar-isovector coupling is included to change the symmetry energy. It is verified by the theoretical analysis and numerical results that the deexcitation energies of superdeformed secondary minima relative to the ground states in these heavy nuclei are sensitive to differences in the symmetry energy. In particular, the linear correlation between the deexcitation energies of odd-odd Au and Tl isotopes and the neutron skin thickness in 208Pb is established. Moreover, explorations are extended to superdeformed candidates of other mass regions. It is found that the linear correlation can even be established between the deexcitation energies and the symmetry pressure at subsaturation density. These indicate that deexcitation energies can serve as a probe to the density dependence of the symmetry energy.

Thallium isotope variations in an ore-bearing continental igneous setting: Collahuasi Formation, northern Chile

Thallium is a highly incompatible element and a large fraction of the bulk silicate Earth Tl budget is, therefore, expected to reside in the continental crust. Nonetheless, the Tl isotope systematics of continental rocks are essentially unexplored at present. Here, we present new Tl isotope composition and concentration data for a suite of 36 intrusive and extrusive igneous rocks from the vicinity of porphyry Cu deposits in the Collahuasi Formation of the Central Andes in northern Chile. The igneous lithologies of the rocks are variably affected by the hydrothermal alteration that accompanied the formation of the Cu deposits. The samples display Tl concentrations that vary by more than an order of magnitude, from 0.1 to 3.2 μg/g, whilst ε 205Tl ranges between −5.1 and +0.1 (ε 205Tl is the deviation of the 205Tl/ 203Tl isotope ratio of a sample from a standard in parts per 10 4). These variations are primarily thought to be a consequence of hydrothermal alteration processes, including metasomatic transport of Tl, and formation/breakdown of Tl-bearing minerals, which are associated with small but significant Tl isotope effects. The Tl abundances show excellent correlations with both K and Rb concentrations but no co-variation with Cu. This demonstrates that Tl displays only limited chalcophile affinity in the continental crust of the Collahuasi Formation, but behaves as a lithophile element with a distribution that is primarily governed by partitioning of Tl + into K +-bearing phases. Collahuasi samples with propylitic alteration features, which are derived from the marginal parts of the hydrothermal systems, have, on average, slightly lighter Tl isotope compositions than rocks from the more central sericitic and argillic alteration zones. This small but statistically significant difference most likely reflects preferential retention of isotopically heavy Tl in alteration phases, such as white micas and clays, which formed during sericitic and argillic alteration.

The thallium isotope composition of carbonaceous chondrites — New evidence for live 205Pb in the early solar system

The extinct radionuclide 205Pb, which decays to 205Tl with a half-life of 15 Ma, is of considerable cosmochemical interest, as it is the only short-lived isotope that is produced exclusively by s-process nucleosynthesis. Evidence for the existence of 205Pb in the early solar system has only recently been obtained from analyses of IAB iron meteorites, but significant uncertainties remain about the initial 205Pb abundance and Tl isotope composition of the solar system. In an attempt to better constrain these values, a comprehensive 205Pb– 205Tl isochron study was carried out on ten carbonaceous chondrites of groups CI, CM, CV, CO and CR. The Pb and Cd isotope compositions of the meteorites were also determined, to correct for terrestrial Pb contamination and eliminate samples that exhibit fractionated Tl isotope compositions from thermal processing. The analyses revealed only limited variation in ε 205Tl, with values of between − 4.0 and + 1.2, but nonetheless the Tl isotope compositions correlate with Pb/Tl ratios. This correlation is unlikely to be due to stable isotope fractionation from terrestrial weathering or early solar system processes, and is most readily explained by in situ decay of 205Pb to 205Tl. Previous 53Mn– 53Cr and 107Pd– 107Ag studies of bulk carbonaceous chondrites provide evidence that the Pb–Tl isochron records volatile fractionation in the solar nebula at close to 4567 Ma. The isochron thus yields the initial 205Pb abundance and Tl isotope composition of the solar system, with values of 205Pb/ 204Pb SS,0 = (1.0 ± 0.4) × 10 − 3 and ε 205Tl SS,0 = − 7.6 ± 2.1, respectively. These results confirm the previous Pb–Tl data for IAB iron meteorites, which provided the first clear evidence for the existence of live 205Pb in the early solar system. The initial 205Pb SS,0 abundance inferred from carbonaceous chondrites demonstrates that the 205Pb– 205Tl decay system is well suited for chronological studies of early solar system processes that produce fractionations in Pb/Tl ratios, including core crystallization and the mobilization of volatiles during thermal processing. The 205Pb SS,0 abundance is close to the upper limit of nucleosynthetic production estimates for AGB stars and thus in accord with contributions of such stars to the early solar system budget of freshly synthesized radioisotopes.

Ability of 201Tl and 123I-BMIPP mismatch to diagnose myocardial ischemia in patients with suspected coronary artery disease

A mismatch defect between (201)TL and (123)I-BMIPP dual isotope SPECT (d-SPECT) is useful to detect myocardial ischemia in patients with acute coronary syndrome. However, whether mismatched d-SPECT findings reflect actual myocardial ischemia in stable patients with suspected, but unknown ischemic heart disease is unclear. The present study assesses the significance of a d-SPECT mismatch among such patients. Forty-nine patients with suspected stable coronary heart disease who had been referred for chest pain, ECG abnormalities or multiple risk factors (66 +/- 11 years old, 34 males) with a d-SPECT mismatch participated in this study. All of them underwent coronary angiography (CAG) to assess coronary artery disease. The entire myocardial area on d-SPECT images was divided into 17 segments, each of which was scored from 0 (normal) to 4 (defect). The d-SPECT mismatch score (MS) was defined as the summed BMIPP defect score (BM-TDS) minus the summed defect score (TL-TDS). The inclusion criterion was MS >or= 1, and the mismatch was defined as true positive if the mismatched area was concordant with the territories supplied by significant coronary stenotic arteries by CAG. Ischemic heart disease was judged by coronary angiography in 31 (63%) patients (IHD group), of which 24 (49.0%) were true positives. Of the remaining 18 (37%) patients without no significant coronary stenosis (non-IHD group), 12 (24%) had some types of organic heart disease. If MS >or= 4 was defined as the threshold for an ischemic positive mismatch, then the sensitivity and specificity were 80% and 63%, respectively. However, mismatch scores did not significantly differ between the groups with true positive-IHD and organic heart disease in non-IHD group (6.6 +/- 4.4 vs. 6.4 +/- 3.7). A d-SPECT mismatch score of >or=4 was an appropriate cutoff at which diagnosis of myocardial ischemia in patients who were screened for ischemic heart disease. However, since patients with non-ischemic but organic heart disease can also present with abnormal mismatch findings, coronary angiography or CT might be warranted to differentiate IHD from non-IHD.

Thallium isotopes as a potential tracer for the origin of cratonic eclogites

Cratonic eclogites are inferred to originate either from subducted ocean crust or mantle melts accreted onto the roots of continents. These models have different implications for the growth of continents, but it is currently difficult to determine the origin of individual eclogite suites. Upper ocean crust altered at low temperatures and marine sediments both display high thallium (Tl) concentrations and strongly fractionated Tl isotope signatures relative to the ambient upper mantle. In this study we carry out the first examination of the suitability of Tl isotopes as a tracer for an ocean-crust origin of cratonic eclogites. We have analysed the Tl isotope composition of clinopyroxene and garnet in six eclogites from the Kaalvallei and Bellsbank kimberlite pipes in South Africa. Minerals were pre-cleaned with an HCl leaching technique and the leachates display variably light Tl isotope ratios. These most likely reflect low-temperature hydrothermal alteration occurring after eruption of the kimberlite that carried the eclogites to the surface. The leached mineral pairs all display identical Tl isotope ratios, strongly suggesting that the source of the analysed Tl is identical for each mineral pair. It is, however, not possible to exclude the possibility that the analysed Tl originates from kimberlitic material that was not removed by the cleaning procedure. Only one of the six samples exhibits a Tl isotope composition different from ambient mantle. Assuming that the Tl isotope signatures indeed represent the eclogite minerals and not any form of contamination, the Tl isotope composition in this sample is consistent with containing a minor component (<3%) of ocean crust altered at low temperatures. Thallium isotopes may become one of the most sensitive indicators for the presence of low-T altered ocean crust because of the stark contrast in Tl concentration and isotopic composition between the mantle and altered ocean crust. In fact, no other chemical or isotopic tracer could have provided an indication that any of the samples studied here had a subduction origin. However, much work is still required before it becomes clear if Tl isotope measurements are a viable means to establish the origin of cratonic eclogites.

Preparing a journey to the east of 208Pb with ISOLTRAP: Isobaric purification at A = 209 and new masses for 211-213Fr and 211Ra

With the Penning trap mass spectrometer ISOLTRAP, located at ISOLDE/CERN, preparatory work has been performed towards mass and decay studies on neutron-rich Hg and Tl isotopes beyond N = 126 . The properties of these isotopes are not well known because of large isobaric contamination coming mainly from surface-ionised Fr. Within the studies, production tests using several target-ion source combinations were performed. It was furthermore demonstrated around mass number A = 209 that the resolving power required to purify Fr is achievable with ISOLTRAP. In addition, masses of several isobaric contaminants, 211-213Fr and 211Ra , were determined with a three-fold improved precision. The results influence masses of more than 20 other nuclides in the 208Pb region.

Investigation of thallium fluxes from subaerial volcanism—Implications for the present and past mass balance of thallium in the oceans

A suite of 34 volcanic gas condensates and particulates from Kilauea (Hawaii), Mt. Etna and Vulcano (Italy), Mt. Merapi (Indonesia), White Island and Mt. Nguaruhoe (New Zealand) were analysed for both Tl isotope compositions and Tl/Pb ratios. When considered together with published Tl–Pb abundance data, the measurements provide globally representative best estimates of Tl/Pb = 0.46 ± 0.25 and ε 205Tl = −1.7 ± 2.0 for the emissions of subaerial volcanism to the atmosphere and oceans (ε 205Tl is the deviation of the 205Tl/ 203Tl isotope ratio from NIST SRM 997 isotope standard in parts per 10,000). Compared to igneous rocks of the crust and mantle, volcanic gases were found to have (i) Tl/Pb ratios that are typically about an order of magnitude higher, and (ii) significantly more variable Tl isotope compositions but a mean ε 205Tl value that is indistinguishable from estimates for the Earth’s mantle and continental crust. The first observation can be explained by the more volatile nature of Tl compared to Pb during the production of volcanic gases, whilst the second reflects the contrasting and approximately balanced isotope fractionation effects that are generated by partial evaporation of Tl during magma degassing and partial Tl condensation as a result of the cooling and differentiation of volcanic gases. Mass balance calculations, based on results from this and other recent Tl isotope studies, were carried out to investigate whether temporal changes in the volcanic Tl fluxes could be responsible for the dramatic shift in the ε 205Tl value of the oceans at ∼55 Ma, which has been inferred from Tl isotope time series data for ferromanganese crusts. The calculations demonstrate that even large changes in the marine Tl input fluxes from volcanism and other sources are unable to significantly alter the Tl isotope composition of the oceans. Based on modelling, it is shown that the large inferred change in the ε 205Tl value of seawater is best explained if the oceans of the early Cenozoic featured significantly larger Tl output fluxes to oxic pelagic sediments, whilst the sink fluxes to altered ocean crust remained approximately constant.

Cross sections of multiparticle photonuclear reactions with 203, 205Tl isotopes

An experiment on the irradiation of the natural mixture of 203,205Tl isotopes by the bremsstrahlung beam of pulsed racetrack microtron RTM-70 (Skobeltsyn Institute of Nuclear Physics, Moscow State University) at an electron energy of 67.7 MeV was performed. The yields and integral cross sections of multiparticle photonuclear reactions with 203,205Tl isotopes were measured.

Calculations of the neutron skin and its effect in atomic parity violation

We perform calculations for the neutron skin of nuclei and its contribution to atomic parity nonconservation (PNC) in many isotopes of Cs, Ba, Sm, Dy, Yb, Tl, Pb, Bi, Fr, and Ra. Three problems are addressed: (i) neutron-skin-induced errors to single-isotope PNC, (ii) the possibility of measuring neutron skin using atomic PNC, and (iii) neutron-skin-induced errors for ratios of PNC effects in different isotopes. In the latter case the correlations in the neutron skin values for different isotopes lead to cancellations of the errors; this makes the isotopic ratio method a competitive tool in a search for new physics beyond the standard model.

Thallium isotope evidence for a permanent increase in marine organic carbon export in the early Eocene

The first high resolution thallium (Tl) isotope records in two ferromanganese crusts (Fe–Mn crusts), CD29 and D11 from the Pacific Ocean are presented. The crusts record pronounced but systematic changes in 205Tl/ 203Tl that are unlikely to reflect diagenetic overprinting or changes in isotope fractionation between seawater and Fe–Mn crusts. It appears more likely that the Fe–Mn crusts track the Tl isotope composition of seawater over time. The present-day oceanic residence time of Tl is estimated to be about 20,000 yr, such that the isotopic composition should reflect ocean-wide events. New and published Os isotope data are used to construct age models for these crusts that are consistent with each other and significantly different from previous age models. Application of these age models reveals that the Tl isotope composition of seawater changed systematically between ~ 55 Ma and ~ 45 Ma. Using a simple box model it is shown that the present day Tl isotope composition of seawater depends almost exclusively on the ratio between the two principal output fluxes of marine Tl. These fluxes are the rate of removal of Tl from seawater via scavenging by authigenic Fe–Mn oxyhydroxide precipitation and the uptake rate of Tl during low temperature alteration of oceanic crust. It is highly unlikely that the latter has changed greatly. Therefore, assuming that the marine Tl budget has also not changed significantly during the Cenozoic, the low 205Tl/ 203Tl during the Paleocene is best explained by a more than four-fold higher sequestration of Tl by Fe–Mn oxyhydroxides compared with at the present day. The calculated Cenozoic Tl isotopic seawater curve displays a striking similarity to that of S, providing evidence that both systems may have responded to the same change in the marine environment. A plausible explanation is a marked and permanent increase in organic carbon export from ~ 55 Ma to ~ 45 Ma, which led to higher pyrite burial rates and a significantly reduced flux of Fe–Mn oxide removal as a result of increased biological uptake of Fe and Mn.

New levels and a lifetime measurement inTl204

The {sup 205}Tl(n,2n{gamma}) reaction was used to populate excited states in {sup 204}Tl. The {gamma}-ray detection was accomplished with the GEANIE spectrometer, a Compton suppressed array of 26 Ge detectors. An energetic beam of neutrons was provided by the pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility. The time-of-flight technique was used to determine the incident neutron energies. {gamma}-ray excitation functions were determined from incident neutron energy of 1 MeV up to E{sub n}=25 MeV. The level scheme of {sup 204}Tl was enriched and the partial level scheme and nuclear structure above the previously known 7{sup +} isomer at 1104-keV excitation energy were established for the first time up to E{sub x}{approx}2.3 MeV. The high-spin part of the level scheme exhibits striking similarities to that of the neighboring {sup 202}Tl isotope, suggesting similarities in the underlying nuclear structure. The half-life of the 7{sup +} isomer was measured with a more precise result (T{sub 1/2}=60.7{+-}1.2 {mu}s), in agreement with literature values. A lower limit for the excitation energy of the {pi}h{sub 11/2}{nu}i{sub 13/2} structure with J{sup {pi}}=12{sup -} is proposed.

The effects of core formation on the Pb- and Tl- isotopic composition of the silicate Earth

We have performed metal-silicate partitioning experiments at 2 GPa and 1650–2180 °C to investigate the behaviour of Pb and Tl during terrestrial core formation. The aim was to test the hypothesis that metal core formation followed by late sulphide addition to the core resulted in the concentrations and isotopic compositions of Pb and Tl in the silicate Earth. We investigated D Pb met/sil and D TL met/sil as functions of the sulphur content of the metal and measured the equilibrium Tl isotope fractionation between the coexisting phases. Lead is moderately siderophile under the likely conditions (initially reducing [Wade, J., Wood, B.J., Core formation and the oxidation state of the Earth, Earth Planet. Sci. Lett. 236(2005) 78–95.]) of core segregation on Earth so that the μ( 238U/ 204Pb) of the bulk silicate Earth should have increased by a factor of 6.5 ( D Pb ∼ 13) as the core separated. In the case of Tl, core segregation should have reduced the Tl concentration of the BSE by about 50%. Neither the Pb nor Tl isotopic compositions of the bulk silicate Earth can, however, be completely explained by S-free iron core formation. Thallium isotopes were found not to be significantly fractionated by metal or sulphide separation from silicate. Addition of sulphur to the metal greatly increases metal-silicate partition coefficients for both Pb and Tl. D Pb met/sil increases by a factor of 15 and D TL met/sil by a factor of 45 as S increases from 0 to 35% in the metal phase. This means that extraction of sulphide from a molten mantle would result in D Pb sulph/sil of ∼ 40 and D TL sulph/sil of ∼ 60. We used the latter results to calculate the effects of late sulphide extraction on the Pb and Tl isotopic compositions of the silicate Earth. For a bulk Earth with μ of 0.7 addition of 1.6% sulphide to the core 100–140 Myr after the beginning of the solar system is sufficient to displace the Pb-isotopic composition of the silicate Earth into the region indicated by estimates in the literature. The Tl concentration and ɛ 205Tl of bulk silicate Earth are also consistent with this extent of sulphide addition. Raising μ of the Earth reduces the amount of sulphide addition to the core needed to satisfy the Pb and Tl isotopic constraints but for values of μ above 0.9 (< 0.65% sulphide) the sulphide has insufficient effect. Separation of sulphide from a mixture of silicate crystals and melt would, however, because of the incompatible nature of Pb and Tl amplify the sulphide effect. We show that, for initial μ of 0.7, as little as 0.1% sulphide extraction (at 150–200 Myr) from a mixture of 91% crystals and 9% melt would be sufficient to displace the isotopic compositions of Pb and Tl of the BSE to the required values. Late volatile loss of Pb and Tl is, therefore, not required by the partitioning and currently available isotopic data.

Natural fractionation of 238U/235U

The isotopic composition of U in nature is generally assumed to be invariant. Here, we report variations of the 238U/235U isotope ratio in natural samples (basalts, granites, seawater, corals, black shales, suboxic sediments, ferromanganese crusts/nodules and BIFs) of ∼1.3‰, exceeding by far the analytical precision of our method (≈0.06‰, 2SD). U isotopes were analyzed with MC-ICP-MS using a mixed 236U–233U isotopic tracer (double spike) to correct for isotope fractionation during sample purification and instrumental mass bias. The largest isotope variations found in our survey are between oxidized and reduced depositional environments, with seawater and suboxic sediments falling in between. Light U isotope compositions (relative to SRM-950a) were observed for manganese crusts from the Atlantic and Pacific oceans, which display δ238U of −0.54‰ to −0.62‰ and for three of four analyzed Banded Iron Formations, which have δ238U of −0.89‰, −0.72‰ and −0.70‰, respectively. High δ238U values are observed for black shales from the Black Sea (unit-I and unit-II) and three Kupferschiefer samples (Germany), which display δ238U of −0.06‰ to +0.43‰. Also, suboxic sediments have slightly elevated δ238U (−0.41‰ to −0.16‰) compared to seawater, which has δ238U of −0.41±0.03‰. Granites define a range of δ238U between −0.20‰ and −0.46‰, but all analyzed basalts are identical within uncertainties and slightly lighter than seawater (δ238U=−0.29‰).Our findings imply that U isotope fractionation occurs in both oxic (manganese crusts) and suboxic to euxinic environments with opposite directions. In the first case, we hypothesize that this fractionation results from adsorption of U to ferromanganese oxides, as is the case for Mo and possibly Tl isotopes. In the second case, reduction of soluble UVI to insoluble UIV probably results in fractionation toward heavy U isotope compositions relative to seawater. These findings imply that variable ocean redox conditions through geological time should result in variations of the seawater U isotope compositions, which may be recorded in sediments or fossils. Thus, U isotopes might be a promising novel geochemical tracer for paleo-redox conditions and the redox evolution on Earth. The discovery that 238U/235U varies in nature also has implications for the precision and accuracy of U–Pb dating. The total observed range in U isotope compositions would produce variations in 207Pb/206Pb ages of young U-bearing minerals of up to 3Ma, and up to 2Ma for minerals that are 3 billion years old.

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.

Development of rapid and precise Pb isotope analytical techniques using MC-ICP-MS and new results for GSJ rock reference samples

A rapid and precise routine Pb isotope analytical technique for rock samples was developed using multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The mass discrimination of the Pb isotope ratios was corrected with the 205Tl/203Tl ratio of a Tl isotope reference material (NIST SRM 997) added to samples. The optimization of the analytical parameters and the minimization of the memory and matrix effects enabled to acquire the Pb isotope data rapidly (three samples per hour) with a reproducibility of 0.01% by consumption of 150 ng Pb per analysis. Repeated analyses of a Pb isotope reference material, NIST SRM 981, gave the average values of 206Pb/204Pb = 16.9308 ± 0.0010, 207Pb/204Pb = 15.4839 ± 0.0011, and 208Pb/204b = 36.6743 ± 0.0030 (2σ).Pb isotope ratios of eleven Geological Survey of Japan rock reference samples were determined using this technique. In anion-exchange chemical separation, complete removal of Hg and Tl from the Pb fraction was confirmed. It was revealed that the Tl-normalization minimized the matrix effects caused by remaining impurities and medium, and the reproducibility of seven independent analyses of JB-2 was comparable to that of NIST SRM 981. Pb isotope ratios obtained for the rock reference samples showed a good agreement with those given previously.