Thermo-ionization Mass Spectrometry Research Articles

Zr precipitation kinetics in irradiated fuel dissolution solution by TIMS and ICP-MS: a combined study

This study presents the combined analysis of leftover zirconium in irradiated fuel dissolution solution through time. Thermo-ionisation mass spectrometry was used to analyse the kinetics start and final aliquots with uncertainties lower than 3%, while inductively coupled plasma-mass spectrometry (ICP-MS) was used for intermediary aliquots. In order to obtain low uncertainties using ICP-MS, dissolution solutions were spiked with lutetium, used as a process tracer, in order to perform a relative analysis. The analytical parameters were optimised and uncertainties were lower than 3% for real nuclear samples. This method shows the input of ICP-MS for concentration evolutions determination.

Systematics of U–Th disequilibrium in calcrete profiles: Lessons from southwest India

Pedogenic carbonates (calcretes) have often been proposed as possible markers of the pedogenetic processes in soils under semi-arid climate. However, precise chronological constraints on their formation are required in order to investigate the climatic and paleo-environmental conditions that prevailed during and after their formation, and to improve our understanding of the physical and chemical conditions that promoted their development and preservation. Moreover, these authigenic calcium carbonate precipitates provide us with an interesting test of the U–Th radioactive disequilibrium dating method, the reliability of which has been demonstrated extensively in aragonitic corals and calcitic continental speleothems, but which remains much more questionable in highly porous, chemically and mineralogically complex media such as soil profiles. We report here 238U–234U–230Th radioactive disequilibrium analyses measured by Thermo-Ionization Mass Spectrometry in calcretes from two sites in southwest India. Since calcretes are impure carbonates mixed with various amounts of parent rock and weathered minerals, all ages must be computed using the isochron technique, involving the extraction of several coeval subsamples from the same soil horizon and U–Th analyses after total dissolution (TSD) method, in order to correct for the detrital component. 232Th is usually used as index of the detrital contamination, and the isochron age is derived from the isotopic composition (230Th/238U; 234U/238U) of the pure authigenic carbonate end-member of the mixing line.Our results show that each set of samples taken from decimetric blocks is characterized by a well-defined isochron line in a 3-D Osmond diagram (232Th/238U; 230Th/238U; 234U/238U). The regression lines obtained for most of the samples show probabilities of fit satisfying the mathematical assumptions of the isochron model (in most cases ≫0.05), and mean square weighted deviations (MSWD) values around 1. The compositions of the end-members are highly variable between the different isochrons, both for the carbonate phase, indicating various ages of cristallisation, and for the detrital phase, indicating various origins and compositions. Notably, our data demonstrate that the detrital component is in all cases out of secular equilibrium, by contrast with the standard paradigm of “dirty carbonate” dating, which is based on the assumption of average continental crust Th/U elemental ratio, and secular equilibrium values of the (230Th/238U) and (234U/238U) ratios. At the first site (Coimbatore), we obtain U–Th ages between 304±38ka and 44±2.9ka (2σ) from the base to the top of the profile. These suggest discontinuous precipitation of the different facies in good agreement with micromorphological observations. At the second study site (Gundlupet), the carbonate nodules sampled from two different profiles, yield consistent ages of 200ka, which suggests that precipitation of nodular carbonate occurred across the entire pediment toposequence during a single event. Surprisingly, in mere contradiction with the standard model of impure carbonate dating, two puzzling samples display “vertical isochrons”, i.e., variable 232Th/238U without any variation of the 230Th/238U and 234U/238U ratios. Such patterns require extremely high 232Th/238U ratios, in an as yet unidentified detrital phase.Taken at face value, these results suggest that no clear correlation can be inferred between the precipitation of these pedogenic carbonates and major orbitally-driven climate variations recorded in other archives during the last glacial/interglacial cycles.

Uranium-series dating of carbonate formations overlying Paleolithic art : interest and limitations

Abstract : The Uranium-Thorium (U/ Th) series dating method, developed 50 years ago, has proven its usefulness and reliability for the dating of marine (corals) and continental (speleothems) secondary carbonates deposits. Recently, improvements of the analytical techniques (TIMS – Thermo-Ionization Mass Spectrometry, and then MC-ICPMS – Multicollector-Inductively Coupled Plasma Mass Spectroscopy and laser ablation) allow the dating of very small samples and increase the potential of this method. Given the difficulties of dating cave art (other than drawings created with charcoal, which can be directly dated by 14C), indirect dating methods have been sought. During the last decade, several publications have reported the dating by the U/ Th method of thin layers of calcite overlying Paleolithic paintings and engravings or the support of these representations. In these cases, the age of calcite formation is assumed to provide a minimum age (terminus ante quem) for the underlying paintings or engravings or a maximum age (terminus post quem) when it is the support that is dated. The current article describes the relevance and potential of this method when applied to the dating of calcitic layers deposited above or below prehistoric drawings, together with the specific difficulties encountered in U/ Th dating of such thin deposits. An initial difficulty is that thorium may be present in the calcite from the beginning (detritic thorium), making age corrections necessary. Another difficulty is that in the humid conditions prevalent in caves, the walls may have been subject to runoff over time. In this case, thin calcite layers covering paintings or engravings may have been altered, with possible chemical exchange between the water and the calcite. The most probable effect of this ‘open system’ behavior is the leaching of uranium, leading to an overestimation of the age of the calcite. Recent applications of the U/ Th method to the dating of rock art have shown that this phenomenon, if not correctly identified by means of independent methods, may become a significant source of error. For this reason, it is important to know the concentrations of uranium in each calcitic sample, as this makes it possible to detect local anomalies that have led to a substantial loss of this element. In a recent paper concerning the U/ Th dating of eleven Paleolithic decorated caves in the Cantabrian Region (Spain), extremely early dates were determined (more than 41,000 years in one case) and the authors speculated that certain representations could have been produced by Neanderthals. However as detailed analytical data (uranium content) have not been published one cannot appreciate the reliability of the ages obtained. Then, in the absence of confirmation by an independent dating method, it is premature to base an archaeological reasoning on these dates. This article emphasizes the necessity of carrying out several analyses on the same sample, and when possible on several layers from its thickness. Moreover it is important to perform cross dating using U/ Th and 14C (or even using other elements such as 226Ra or 231Pa) in order to verify the consistency of the results. Several recent examples will illustrate this necessity. It should be also recalled that the calibration curves used to correct radiocarbon ages are largely based on the simultaneous dating by U/ Th and 14C of the same samples of speleothems and corals. Finally, it has to be mentioned that when the deposits underlying paintings or engravings are studied, the data obtained could be distant in time from the creative act. In fact, the growth of calcite is controlled by environmental factors and is favored during temperate and humid periods. Thus, a large number of calcitic layers overlying paintings could have been deposited during the Holocene. They could also have grown during a brief warming of the last glacial period, or represent a mean age between several growing periods. Nevertheless, these chronological data could bring relevant information, once their validity has been verified.

Application of Nanosecond-UV Laser Ablation–Inductively Coupled Plasma Mass Spectrometry for the Isotopic Analysis of Single Submicrometer-Size Uranium Particles

For the first time, laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) was used to carry out isotopic measurement on single submicrometer-size uranium particles. The analytical procedure was applied on two particle-containing samples already analyzed in the same laboratory by established techniques for particle analysis: combination of the fission track technique with thermo-ionization mass spectrometry (FT-TIMS) and secondary ion mass spectrometry (SIMS). Particles were extracted from their initial matrix with ethanol and deposited on a polycarbonate disk where they were fixed in a layer of an organic compound (collodion). Prior to the isotopic analysis, particles were precisely located on the disk's surface by scanning electron microscopy (SEM) for one sample and using the fission track technique for the other sample. Most of the particles were smaller than 1 μm, and their (235)U content was in the femtogram range. (235)U/(238)U ratios were successfully analyzed for all located particles using a nanosecond-UV laser (Cetac LSX 213 nm) coupled to a quadrupole-based ICPMS (Thermo "X-Series II"). LA-ICPMS results, although less precise and accurate (typically 10%) than the ones obtained by FT-TIMS and SIMS due to short (20-40 s), transient, and noisy signals, are in good agreement with the certified values or with the results obtained with other techniques. Thanks to good measurement efficiency (~6 × 10(-4)) and high signal/noise ratio during the analysis, LA-ICPMS can be considered a very promising technique for fast particle analysis, provided that uranium-bearing particles are fixed on the sample holder and located prior to isotope measurement.

Combined application of alpha-track and fission-track techniques for detection of plutonium particles in environmental samples prior to isotopic measurement using thermo-ionization mass spectrometry

The fission track technique is a sensitive detection method for particles which contain radio-nuclides like 235U or 239Pu. However, when the sample is a mixture of plutonium and uranium, discrimination between uranium particles and plutonium particles is difficult using this technique. In this study, we developed a method for detecting plutonium particles in a sample mixture of plutonium and uranium particles using alpha track and fission track techniques. The specific radioactivity (Bq/g) for alpha decay of plutonium is several orders of magnitude higher than that of uranium, indicating that the formation of the alpha track due to alpha decay of uranium can be disregarded under suitable conditions. While alpha tracks in addition to fission tracks were detected in a plutonium particle, only fission tracks were detected in a uranium particle, thereby making the alpha tracks an indicator for detecting particles containing plutonium. In addition, it was confirmed that there is a linear relationship between the numbers of alpha tracks produced by plutonium particles made of plutonium certified standard material and the ion intensities of the various plutonium isotopes measured by thermo-ionization mass spectrometry. Using this correlation, the accuracy in isotope ratios, signal intensity and measurement errors is presumable from the number of alpha tracks prior to the isotope ratio measurements by thermal ionization mass spectrometry. It is expected that this method will become an effective tool for plutonium particle analysis. The particles used in this study had sizes between 0.3 and 2.0μm.

U–Pb systematics of zircon and titanite from the Gardnos impact structure, Norway: Evidence for impact at 546 Ma?

Zircon and titanite were investigated in impactites of the Gardnos structure, a crater formed in Sveconorwegian (ca. 1 Ga) crust, which was then overridden in the Devonian by Caledonian nappes. Observed deformation features in zircons are granular texture, planar microstructures, and likely the incorporation of organic carbon during impact causing black staining of the zircon grains. The grains were studied by scanning electron microscopy (SEM) and cathode luminescence (CL) and dated by U–Pb isotope dilution - thermo-ionization mass spectrometry (ID-TIMS). Zircon grains without impact related features have U–Pb data showing moderate discordance (5–13%) and indicating formation ages mostly in the range of 1600–1000 Ma, except detrital zircon ages as old as >2481 Ma, reflecting the diversity of target rocks in the area. Titanite with concordant ages of 995–999 Ma dates metamorphism during final juxtaposition of the Telemarkia on the Idefjorden terrane to the east. Zircon grains with demonstrated or presumed shock features yield highly discordant (14–40%) U–Pb data, with a majority of them plotting along an array with a lower intercept of about 340 Ma reflecting the influence of the Caledonian orogeny and recent Pb-loss. One zircon grain was totally reset at 379 Ma during late Caledonian metamorphism, which also caused local growth of new titanite. A specific group of zircon grains yields data with relatively high discordance for moderate U contents, and five of these analyses, including that of a grain with proven granular or aggregate texture, fit a discordia line with an upper intercept of 546 ± 5 Ma. These features are interpreted as indicating zircon break-down to an amorphous state during impact, with subsequent recrystallization into microcrystalline aggregates causing extensive to complete Pb loss. We further suggest that their crystallinity prevented Pb loss during the Caledonian orogeny, while the small subgrain size and increasing metamictisation allowed more recent disturbances. We thus interpret the 546 Ma age as the approximate time of impact.

Deposition and remobilization of uranium in the North Baikal region: Evidence from the U-Pb isotopic systems of uranium ores

Based on the study of local volumes of minerals, including their microsampling and subsequent analysis of Pb/Pb and U/Pb isotope ratios with the classic methods of isotopic dilution and thermoionization mass spectrometry (TIMS), U-Pb and Pb-Pb isotopic datings of minerals were carried out in uranium ores from deposits in the Akitkan and Nechera-Nichatka ore districts (North Baikal region). Reliable evidence in favor of the Middle Devonian (384 ± 8 Ma) remobilization of Paleoproterozoic primary uranium concentrations and the redeposition of uranium as pitchblende 2 has been obtained for the first time for ores of the Akitkan district. The Paleoproterozoic (1832 ± 13 Ma) age of uraninite mineralization and the timing of the latest (377 ± 5 Ma) transformation of uranium ores at the Chepok deposit (Nechera-Nichatka district) are substantiated.

Sources of material for massive sulfide deposits in the Urals: Evidence from the high-precision MC-ICP-MS isotope analysis of Pb in galena

This work reports the results of one of the pioneer investigation of the Pb isotope geochemistry based on modern high-precision multichannel mass spectrometry with inductively coupled plasma (MC-ICP-MS). We studied the main mineral and age groups of massive sulfide deposits in the Urals: superlarge Gai deposit (~9 Mt); large deposits, such as Sibai (Cu + Zn, ~3 Mt), Uchaly (~7 Mt), and Saf’yanov (~2 Mt); and eight medium and small deposits. The lead isotopic compositions of these deposits were studied in 54 galena samples. According to Albarede [1], the MC-ICP-MS method has opened “a new era in isotope geochemistry,” particularly in the study of the Pb isotopic composition [2, 3]. Until recently, the method of thermoionization mass spectrometry (TIMS) dominated in this field. The overwhelming majority of Pb isotope data reported in the world during the last 50 years is based on the TIMS measurement of 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb ratios with an error of 0.1‐0.2% or more. Application of the MC-ICP-MS method decreased the error by 5‐ 10 times. Therefore, it has become possible to investigate minor local and regional variations in the Pb isotopic composition, decrease significantly (from ± 35 to ± 4 Ma) the analytical error of the model Pb‐Pb age calculation, and unravel “short” correlation trends of the isotopic characteristics of Pb. We have developed and applied a special MC-ICPMS version based on the NEPTUNE (ThermoFinnigan) device at the Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (Moscow). The method consists in the high-precision isotopic analysis of Pb from Tl-doped solutions and the normalization of all current Pb isotope data relative to the

Strontium and Geolocation, the Pathway to Identification for Deceased Undocumented Mexican Border‐Crossers: A Preliminary Report*

Much of the difficulty associated with identifying and repatriating deceased undocumented border-crossers stems from an inability to narrow down the search area to more probable options. Analysis of the isotopic variation in the tooth enamel of modern Mexican populations is currently underway at the University of California Santa Cruz. Using Thermo Ionization Mass Spectrometry, the pilot research analyzed strontium isotopes located in the donated permanent teeth of Mexican-born individuals of known origin from four states. The preliminary results reveal the formation of three distinctly significant regions in the data set. Using the technology outlined here, a map documenting the isotopic variation in modern Mexican tooth enamel is being complied to use for cross comparison with deceased border crossers of unknown origin.

Speleothem rupture in karst: tectonic or climatic origin? U-Th dating of rupture events in Salamandre Cave (Gard, southeastern France)

Abstract Caves are relatively protected from the main external erosional factors. Therefore, they constitute potentially reliable places for long-term conservation of continental history. Moreover their secondary carbonated deposits, the speleothems, can be dated precisely on the 0-500 ka time-scale using U-series isotopes measured by thermo-ionisation mass spectrometry (TIMS). Tectonic events (tectonic displacements and earthquakes) may change cave morphology and induce speleothem breaking or displacement as has been shown by previous studies performed mainly in Italy [Forti et Postpischl, 1984; Postpischl et al., 1991 for example]. Nevertheless, collapses of speleothems observed today in caves are difficult to interpret as their origin may be linked to several other natural processes. We studied the Aven de la Salamandre cave located in southeastern France (Gard), an area between the Cevennes fault and the Nîmes fault, where evidence of Quaternary vertical movements was previously described. However, this region is not considered to be a seismic active zone. The Aven de la Salamandre cave is characterized by numerous broken speleothems. Some of them are very large and a lot of them are covered by growth of new calcite or new speleothem generation. We report here 13 TIMS U/Th analysis performed on two broken speleothems recovered by second generation calcite growth. Dating results are discussed through time corrections due to detrital content of calcite. In the first sequence, a 7 ± 0.35 ka fracture event is identified. In the second sequence, the age of the breakdown is between 1.1 ± 0.1 and 6.3 ± 2 ka. These events could thus be contemporary. Hypotheses for the origin of this fracture event are presented and discussed: (i) karstic catastrophic event due to intense climatic changes or to cavity collapse (break down of hanging wall, gravity, landslide…); (ii) co-seismic ruptures. The first conclusion of this study is that these collapse episodes in the Aven de la Salamandre cave cannot be due to the direct effect of an earthquake or climatic event. Only indirect effects of flooding (by mobilization of the argillaceous components of the floor and consecutive destabilization of the speleothems growing upon it) or earthquake effects (more likely by local effects than by wave front passage) are privileged. By comparing our dating with regional destructive known events (in other karsts, in cliffs and scarps), dated by relative chronology, we are lead to propose a regional generalized event precisely dated here at 7 ka. This second conclusion doesn’t contradict the presence of unbroken speleothems older than 100 ka found in other caves in the neighborhood as local effects is one of the predominant factors relative to speleothem stability. As a final conclusion, this paper promotes the use of speleothems as reliable datable tools for assessing regional stability problems (sensitivity to seismic hazards, to destructive intense climatic events…) as is done for paleoclimatic reconstruction.

207Pb/206Pb and 238U/230Th dating of uranium migration in carbonate fractures from the Palmottu uranium ore (southern Finland)

Precambrian U ore deposits, such as Oklo, Cigar-Lake or Palmottu, constitute invaluable analogues of nuclear waste repositories that provide direct evidence of U mobility or sequestration, over very large time intervals in geological formations. In this study, pervasive millimeter thick calcite veins filling microfractures in gneiss and granite surrounding the Palmottu U ore (Finland) were analysed, as fingerprints of past fluid circulation through the site. Stable Pb isotopes and short halflife 234U-230Th isotopes, all products of the U decay chain, have been chosen to investigate long term (over a Ga) and recent (within the last 500 ka) U migrations, respectively. Lead, U and Th isotopes have been analysed by thermo-ionisation mass spectrometry on mg-size bulk carbonate samples, and ion probe analyses of Pb isotopes performed on microinclusions of pyrite, monazite and coffinite. A striking contrast was found between the preservation of a well defined Pb–Pb isochron (1925±70 Ma, MSDW=2.47, N=17) on the one hand, and fractionated 230Th/238U ratios (230Th/238U<1) on the other hand. This shows that although the veins were formed simultaneously to the ore itself, during the Fennoscandian orogeny, and behaved mainly as a closed system for several billion years, U migrations did occur in the last 0.5 Ma. The recent U mobility is probably related to changes in the groundwater circulation through the bedrock. Nowadays, the dispersion of the U is, however, restricted to within a few meters around the U ore. It is shown that complex processes of dissolution and re-adsorption (or re-crystallisation) took place within these tiny carbonate fracture fillings. Relatively unradiogenic 206Pb/204Pb ratios measured by SIMS in coffinite micro-inclusions demonstrate recent precipitation of this mineral. Coffinite being the main U-bearing phase in the carbonate fracture fillings, its crystallization probably played a major role in U redistribution within the fractures, and in the relatively restricted dispersion of U through the bedrock.

Cross dating (Th/U- 14C) of calcite covering prehistoric paintings in Borneo

We present the first application of cross-dating (Th/U measured by thermo-ionization mass spectrometry (TIMS) and 14C measured by accelerator mass spectrometry (AMS)) of calcite covering prehistoric paintings. Th/U age estimates of cave drapery range from 9800 to 27,300 yr B.P. while conventional 14C age is estimated between 9900 and 7610 yr B.P. depending on the dead carbon correction. The age discrepancy is attributed to a disturbance of Th/U and/or 14C geochemical systems, showing the limits of the geochronological approach applied to this kind of material. For the Th/U system, the poor consistency of U data (U content, 234U/ 238U activity ratios) and apparent ages argue for open system conditions. For 14C system, variation of the dead carbon fraction (dcf) and a possible mixing of successive generations of calcite could account for age discrepancy. Nevertheless, one sample shows concordant ages for the two methods. Compatible ages through corrections for open system conditions are assumed for other samples. Then, the cross-dating suggests 9900 yr as the minimum age of the piece of drapery; the underlying painting must be older. This study of rock art demonstrates the presence of a Pleistocene population before 9900 yr in the southeast of Borneo, whereas previously the only population in evidence in this area was of Austronesian type from ∼5000 to 6000 yrs ago.

Electrodeposition of241Am and244Cm in Spent Nuclear Fuel Samples for Alpha-Spectrometry

Determination of fission products and actinide elements produced during irradiation in a nuclear power reactor is of importance for the chemical characterization of the spent nuclear fuels. Especially, the exact amount of transuranium elements, such as Np, Pu, Am, Cm and their isotopes, are useful to understand the burn-up behavior of the fuels because the isotopes of these elements are produced and decay by various pathways during irradiation and cooling. The radioactive material samples containing alpha-emitting nuclides should be handled in a shielded facility. Consequently, the analytical method for these materials, including sample pretreatment, is limited to some extent compared with that for non-radioactive samples. Generally, the determination of the actinides in neutron irradiated samples is widely performed by alpha-spectrometry after separation by ion exchange or solvent extraction. Thermoionization mass spectrometry is also widely used to measure isotope ratios after separation of the elements by ion exchange chromatography. Several methods have been reported for the electrodeposition of actinides in electrolyte buffer solutions, such as mixed oxalate-chloride matrix, 1 NaHSO4-H2SO4-NH4 buffer system, 2 ammonium oxalate-ammonium sulfate, 3 isopropanol medium 4 and NaHSO4-Na2SO4 matrix, 5 since Talvitie's method 6 using ammonium sulfate was introduced in 1972. However, most of these methods were applied for the electrodeposition of actinides in environmental samples containing trace amount of nuclides. Of these methods, the matrix of NaHSO4-Na2SO4 was selected as an electrolyte because this system has an advantage over pH adjustment, which is controlled only by the concentrations of the NaHSO4 and Na2SO4 solutions, and also has not been applied for Am and Cm in spent nuclear fuel samples. This method is especially convenient to handle the radioactive samples in a shielded space because no additional pH adjustment with acid or base is required. We tried the electrodeposition of 241 Am and 244 Cm in spent

Episodic and short-lived granitic pulses in a post-collisional setting: evidence from precise U–Pb zircon dating through a crustal cross-section in Corsica

In western Corsica, a large granitoid batholith was emplaced during Carboniferous and Permian times. These rocks display an evolution through time from high Mg–K calc-alkaline, to calc-alkaline, then alkaline affinities. Moreover, the magmatic processes can be observed at different crustal levels. New and precise Isotope Dilution and Thermo-Ionisation Mass Spectrometry (ID-TIMS) U–Pb dating suggests that granite magmatism in Corsica did not simply reflect the continuous thermal evolution of a thickened crust. A precise timing for the post-collisional magmatism is documented with four distinct short-lived episodes, respectively at 345, 338, 305 and 280 Ma, separated by periods of inactivity. These four magma pulses display variable Nd isotopic signatures related to mantle–crust interactions. Granite magma generation and emplacement is strongly connected to the thermal and tectonic activity of the lithosphere. An increasing contribution of a depleted mantle to the magmatic processes is documented for the last igneous event. This is interpreted to reflect the emplacement of depleted mantle materials in the lower crust, which enhanced heat flow and triggered partial melting of previously underplated mafic rocks. This increasing contribution of mantle materials marks the onset of a new geodynamic cycle characterised by an extensional tectonic regime, preluding to the opening of the Alpine orogenic tract.

Behavior of Li and its isotopes during serpentinization of oceanic peridotites

Analyses of Li and Li isotopes in serpentinized peridotites have been performed using Thermo‐Ionisation Mass Spectrometry (TIMS) and Secondary Ion Mass Spectrometry (SIMS) techniques on samples collected from the southwest Indian Ridge (SWIR). In the bulk samples, Li concentrations range from 0.6 to 8.2 ppm, while whole rock δ6Li values range from −2.9 to −14‰. In situ analyses display a greater range in both Li concentration (0.1–19.5 ppm) and Li isotopic composition (−27 to +19‰), with the serpentinized portions having higher Li concentrations than the associated relict phases. These variations may reflect changes in Li partitioning and isotopic fractionation between serpentine and fluid with temperature and water/rock ratio. They may also be explained by changes in the composition of the serpentinizing fluid over the course of serpentinization. As the serpentine forms by interaction with a circulating fluid, it preferentially removes 6Li, causing the Li in the fluid to become isotopically heavier. The isotopic composition of the initial hydrothermal fluid is dominated by basalt‐derived Li, which easily overwhelms the very low Li content originally present in seawater. As this fluid circulates through ultramafic rocks, it induces the formation of serpentine that incorporates this mantle‐derived Li. Hence, Li in serpentine is mainly derived from oceanic crust rather than from seawater and serpentinization involves Li recycling within this crust. Consequently, Li isotopes are good tracers of the hydrothermal contribution in serpentinizing fluid. These results imply that serpentinized peridotites are probably only a minor sink of oceanic Li.

(234U/238U) Disequilibria and Sr Isotopic Variations in a Laterite: Dating or Tracing U Mobility?

Isotopic dating of laterites remains problematic and controversial. Comparison of the available data shows contradictory ages. The Rb-Sr, K-Ar and 4~ chronometers provide ages of several tens of millions years ([1], [2], [3]), while (234U/238U) disequilibria ([4], [5]) and l~ analyses ([6]) lead to recent ages. In fact, the different chronometers were not applied strictly to the same soil fractions (whole rock, clay fraction, pure mineral phase), which could partly explain the contradictory results. Actually, any analysed fraction may consist of inherited, altered and weathering mineral phases. This study was thought to analyse the same soil fractions with two different chronometers, the Rb-Sr and the (234U/238U) disequilibria methods. They were applied on < 0.2 gm fraction from the Goyoum laterite (Cameroon) consisting of three petrologic units: a saprolite at the base, a intermediate nodular zone, and a soft topsoil. The < 0.2 gm fraction was extracted from two profiles, it contains essentially kaolinite and Feoxyhydroxides. The Rb and Sr concentrations and the Sr and U isotope ratios were determined by thermoionisation mass spectrometry. The data reveal complex and wide variations of the 87Sr/86Sr and 87Rb/86Sr ratios with depth, with different patterns in each profile. No isochron line can be drawn in an 878r/86Sr v s S7Rb/s6sr diagram. The data scatters in the mixing and in the isochron diagrams suggest a Sr isotopic heterogeneity which could represent either an initial Sr isotopic variability of the analysed fractions, or a secondary process disturbing a previously homogeneous system. The (234U/238U) activity ratio varies from 0.85 to 1.20 in each profile (see Fig. 1). No correlation appears with the 87Sr/86Sr ratio. The variations of the (234U/238U) activity ratio with depth are similar in both profiles. An important disequilibrium is visible at the base of the saprolite with a (234U/238U) ratio significantly below unity. Above, the values tend progressively towards equilibrium. A second important (234U/23Su) disequilibrium occurs at the base of the nodular zone with values below and above unity. Like in the saprolite, the values tend towards equilibrium in the nodular zone. A third similar sequence could possibly exist in the topsoil. The fact that the (234U/238U) variations are similar in both studied profiles inclines us to believe that these variations are characteristic of the whole lateritic profile. The lateritic formation would then contain two horizons of U fractionation with progressive return to secular equilibrium above them. The lower horizon at the base of the saprolite, shows an enrichment of 23Su which could result from leaching of the bedrock initially in secular equilibrium, inducing preferential loss of 234U and crystallisation of residual minerals depleted in 234U. The progressive trend to secular equilibrium suggests no further fractionation in the saprolite The intermediate horizon at the base of the nodular zone could record perturbation of the kaolinite + Feoxyhydroxides mixture that could be either a recrystallisation of the pre-existing minerals or an addition of one or several new mineral phases. The fact that (234U/238U) activity ratios are again below unity suggest another episode of mineral leaching with formation of 234U-enriched fluids. However, migration of these fluids over only short distances could explain crystallisation of minerals enriched in 234U. Return to the secular equilibrium of the mineral fractions suggests a confinement of the process in the transition zone. For each fractionation front, a migration rate of 6 to 8 m/Ma may be calculated. The occurrence of (234U/238U) disequilibria in the Goyoum laterite emphasises that this laterite is presently affected by geochemical fractionations. The location of the fractionation zones at the lower parts of the petrographical units, allows to consider that the fractionation of U mirrors the pedological history of the laterite. It may also be considered that the

Thin-Film Reactive Interdiffusion in The TI-AL System

ABSTRACTWe have studied the thermodynamics and kinetics of growth of Al-Ti phases in thin-film interdiffusion couples, A lOOnm thick aluminum layer is vapor-deposited on a poly- or single-crystal titanium substrate, previously etched by ion bombardment. The sample is then annealed during a time ranging from a few minutes to a few hours, at temperatures from 300 to 600°C. The Al atomic fraction vs depth is determined using a sputtering technique, associated with thermo-ionization mass spectrometry (STIMS). At low temperature (300–450°C), titanium diffuses along the grain boundaries of the micro-crystalline aluminum layer, to form an Al-rich phase. At higher temperature (450–500°C), the T1AI3 phase grows rapidly up to the surface. Above 460°C, an intermediate Ti-72at/Al phase grows between TiAl3 and Ti‥ At 600°C, the aluminum layer becomes morphologically unstable.

Copper Isotope Separation by Ion Exchange Electromigration and Copper Isotope Analysis by Thermo-Ionization Mass Spectrometry

The separation coefficient of copper isotopes, 63Cu and 65Cu, in the electromigration process through cation exchange m em brane was experimentally determined as 3.2 x l0- 4. The appropriate heating condition of low ionizing filament current for thermo-ionization mass spectrometry was developed and this method was applied to the copper isotope analysis in the present work.

Calcium Isotope Effects in Ion Exchange Electromigration and Calcium Isotope Analysis by Thermo-Ionization Mass Spectrometry

Calcium ions were made to electromigrate along a cation exchange membrane. The abundance ratios of the calcium isotopes (Ca-40, 42, 43, 44, 48) in the migrated bands were measured by thermo-ionization mass spectrometry. The lighter isotopes were enriched in the front part of the migrated band. The increments in the isotope abundance ratios were found to be proportional to the mass differences of the isotopes. The observed e-values per unit mass difference (ε/ΔM) were 1.26 x 10-4 (at 20 °C), 1.85 x 10-4 (at 25 °C) and 2.4 x 10-4 (at 40 °C). The mass spectrometry was improved by using a low temperature for the evaporation of CaI2.

872 CALCIUM DYNAMICS IN INFANCY

Calcium (Ca) disorders and therapeutic effectiveness can be assessed by measurements of bone dynamics using stable tracer methods. A pulse of IV Ca-46 is followed by a 9-15 day continuous feeding of Ca-48. Ca isotopic abundances in serum, urine, and feces are determined by thermoionization mass spectrometry (0.5% precision). Kinetic parameters (mg/kg/d) are calculated by nonlinear, weighted least squares analysis and growth is compensated for by using expanding pool sizes. Two infants (Inf.) had Ca turnover rates 12 times that of a control adult (Ad); bone Ca deposition (vo+) and resorption (vo-) were determined with 2 to 5% precision. Two infants with osteogenesis imperfecta (OI) were studied before (B) and while taking ascorbic acid (C). Inf show excessive endogenous fecal (vf) to urinary (vu) Ca excretion as compared to adults, suggesting a major role for vf in regulating Ca balance in Inf. An Inf with OI had similar Ca dynamics except for a decrease in bone Ca fluxes. Ascorbic acid, as suggested for clinical use, had no effect on bone calcium balance.