Lithium Isotope Ratios Research Articles

A Melt Inclusion Record of Volatiles, Trace Elements and Li–B Isotope Variations in a Single Magma System from the Plat Pays Volcanic Complex, Dominica, Lesser Antilles

Glass inclusions in plagioclase and orthopyroxene from dacitic pumice of the Cabrits Dome, Plat Pays Volcanic Complex in southern Dominica reveal a complexity of element behavior and Li–B isotope variations in a single volcanic center that would go unnoticed in a whole-rock study. Inclusions and matrix glasses are high-silica rhyolite with compositions consistent with about 50% fractional crystallization of the observed phenocrysts. Estimated crystallization conditions are 760–880 C, 200 MPa and oxygen fugacity of FMQ þ 1 to þ2 log units (where FMQ is the fayalite– magnetite–quartz buffer). Many inclusion glasses are volatile-rich (up to 6 wt % H2O and 2900 ppm Cl), but contents range down to 1 wt % H2O and 2000 ppm Cl as a result of shallow-level degassing. Sulfur contents are low throughout, with <350 ppm S. The trace element composition of inclusion glasses shows enrichment in light rare earth elements (LREE; (La/Sm)n 1⁄4 2 5–6 6) and elevated Ba, Th and K contents compared with whole rocks and similar or lower Nb and heavy REE (HREE; (Gd/Yb)n 1⁄4 0 5– 1 0). Lithium and boron concentrations and isotope ratios in melt inclusions are highly variable (20–60 ppm Li with dLi 1⁄4 þ4 to þ15 2‰; 60–100 ppm B with dB 1⁄4 þ6 to þ13 2‰) and imply trapping of isotopically heterogeneous, hybrid melts. Multiple sources and processes are required to explain these features. The mid-ocean ridge basalt (MORB)-like HREE, Nb and Y signature reflects the parental magma(s) derived from the mantle wedge. Positive Ba/Nb, B/Nb and Th/Nb correlations in inclusion glasses indicate coupled enrichment in strongly fluid-mobile (Ba, B) and less-mobile (Th, Nb) trace elements, which can be explained by fractional crystallization of plagioclase, orthopyroxene and Fe–Ti oxides. The dLi and dB values are at the high end of known ranges for other island arc magmas. We attribute the high values to a B and Li-enriched slab component derived from seafloor-altered oceanic crust and possibly further enriched in heavy isotopes by dehydration fractionation. The heterogeneity of isotope ratios in the evolved, trapped melts is attributed to shallow-level assimilation of older volcanic rocks of the Plat Pays Volcanic Complex.

Chromatographic Lithium Isotope Separation Using Cubic Antimonic Acid as Column‐Packing Material

Ion‐exchange chromatographic lithium isotope separation was performed by using granular cubic antimonic acid as column packing material at 50°C. While the single‐stage separation factor for the lithium isotopes was 1.0020, independent of the flow rate, HETP was a monotonously increasing function of flow rate both in breakthrough and reverse breakthrough experiments. After 442 cm chromatographic development, the 6Li atomic fraction increased to 0.256 in the reverse breakthrough experiment and decreased to 0.026 in the breakthrough experiment both from its original value of 0.074. The granular C‐SbA was kindly donated by Toagosei Chemical Industry Co. We acknowledge Professor M. Hosoe, the National Defense Academy, who measured the lithium isotopic ratios reported in this paper.

Behavior of lithium and its isotopes during weathering of Hawaiian basalt

We examined the pedogenic behavior of lithium (Li) and its isotopes in Hawaii by sampling same‐age lava flows under mean annual rainfall ranging from 18 to 300 cm. Lithium concentrations in these soils vary from 1 to 29 ppm. Whereas Na, K, and Ca are completely leached from the soil at the most humid and severely weathered site, Li, Mg, Si, and Al show significant retention due to their association with secondary clay minerals. In these soils, allochthonous Li delivered in marine and mineral aerosol mixes with basalt‐derived Li, modifying the isotopic composition of the Li pool. The ability of soil to retain Li is related to its effective cation exchange capacity, which in turn is governed by rainfall and leaching intensity and their resulting effect on mineralogy. Lithium isotope ratios have a large range (δ7Li of −0.4 to 14‰), well beyond the intershield or temporal variation of the lavas of Hawaiian volcanoes (2.5 to 5.7‰). The most unweathered samples have δ7Li (∼5–6‰) comparable to that of Mauna Kea and Mauna Loa lavas. The Li concentrations and isotope ratios together suggest that in arid to subhumid sites there is a net addition of isotopically heavier lithium from marine aerosol but that under greater rainfall there is a net loss of Li, and isotopically light Li is preferentially retained. Thus, during incongruent weathering of primary minerals, Li, especially 6Li, is adsorbed and sequestered by the clay mineral fraction of high cation exchange capacity soils in drier regions but lost from highly weathered, low cation exchange capacity acidic soils in the wetter regions.

Lithium Isotope Analyses of Inorganic Constituents from the Murchison Meteorite

Aqueous processes were important modifiers of solid matter during the early stages of solar system history. Lithium isotopes are sensitive indicators of such solid-liquid interactions because 7Li passes preferentially into solution and 6Li remains behind in the solid phase. Lithium isotope ratios of inorganic phases in the Murchison meteorite reveal that the value for the whole rock is simply the average of individual components with widely different isotopic compositions. 7Li content increases from chondrules to phyllosilicate-rich matrices to carbonates, as would be expected from the relative duration each component has spent during aqueous alteration on the parent asteroid.

Study of ion transport in Li2ZrO3 solid electrolytes with different lithium isotope ratios

Lithium ionic conductivity and spin-lattice relaxation rates were measured in Li2ZrO3 solid electrolytes with different 6Li and 7Li ratios. It is found that single-isotope electrolytes undergo a transition to the superionic state in the temperature range of 430–450 K, accompanied by an abrupt increase in conductivity. As a result of introduction of the other type of the isotope, the conductivity jump disappears in this temperature range. The transition to the superionic state is attributed to the redistribution of lithium ions over energetically nonequivalent lattice sites.

Temperature dependence of $delta;7Li, $delta;44Ca and Li/Ca during growth of calcium carbonate

Lithium and calcium isotope ratios, together with the Li/Ca element ratio, have been measured in calcites precipitated inorganically at temperatures ranging from 5 to 30 °C, and on seasonal growth bands from a coral. These measurements indicate the potential uses of these three environmental proxies. Carbonate Li isotopes are lighter than the solution from which they grow, probably due to equilibrium fractionation. This fractionation is not significantly influenced by temperature and, because Li is not involved in any known biological process, this suggests that carbonates will provide a recorder for the past Li-isotope composition of natural waters and hence for past weathering conditions. As has been observed previously for inorganic aragonite, the Ca-isotopes in inorganic calcite are lighter than the growth solution, and are only weakly dependent on temperature. This confirms that the temperature dependence observed in some species of foraminifera [Geochem. Geophys. Geosyst. 1 (2001)] reflects additional isotope fractionation due to biological processes during mineralization. Ca isotopes may therefore provide a powerful tool with which to investigate the processes of biomineralization. Finally, Li/Ca ratios in inorganic and biogenic carbonates increase as temperatures decrease with a sensitivity of ≈4% per °C. This temperature dependence of Li/Ca may prove useful to assess past temperatures in cold environments such as the deep ocean.

Temperature dependence of δ7Li, δ44Ca and Li/Ca during growth of calcium carbonate

Lithium and calcium isotope ratios, together with the Li/Ca element ratio, have been measured in calcites precipitated inorganically at temperatures ranging from 5 to 30 °C, and on seasonal growth bands from a coral. These measurements indicate the potential uses of these three environmental proxies. Carbonate Li isotopes are lighter than the solution from which they grow, probably due to equilibrium fractionation. This fractionation is not significantly influenced by temperature and, because Li is not involved in any known biological process, this suggests that carbonates will provide a recorder for the past Li-isotope composition of natural waters and hence for past weathering conditions. As has been observed previously for inorganic aragonite, the Ca-isotopes in inorganic calcite are lighter than the growth solution, and are only weakly dependent on temperature. This confirms that the temperature dependence observed in some species of foraminifera [Geochem. Geophys. Geosyst. 1 (2001)] reflects additional isotope fractionation due to biological processes during mineralization. Ca isotopes may therefore provide a powerful tool with which to investigate the processes of biomineralization. Finally, Li/Ca ratios in inorganic and biogenic carbonates increase as temperatures decrease with a sensitivity of ≈4% per °C. This temperature dependence of Li/Ca may prove useful to assess past temperatures in cold environments such as the deep ocean.

Seawater intrusion through the oceanic crust and carbonate sediment in the Equatorial Pacific: Lithium abundance and isotopic evidence

Large‐scale lateral advection of seawater within the oceanic crust is thought to be the cause of low heat flow in the Equatorial Pacific. Until now supportive evidence is limited to reversals of B and δ11B, Ca, Mg, SO4−2, Sr and 87Sr/86Sr in sediment pore waters. Here we report new evidence from a detailed study of Li and its isotopes in sediment cores from ODP Sites 844 and 851. Carbonates at these sites were significantly recrystallized leading to large variations in Li and δ7Li in the sediments and associated pore waters. In addition to diagenetic effects, distinct reversals in lithium concentration and isotopic ratio toward modern seawater composition are observed in waters near the basaltic basement, lending further support to the seawater intrusion hypothesis. Three endmembers are identified in the pore waters: seawater, a diagenetically altered component, and evolved crustal fluid. The new lithium isotopic results underscore the importance of diagenetic artifacts in carbonate sediments.

The lithium isotope ratio in metal-poor stars

Methods of deriving the Li-6/Li-7 isotope ratio from high resolution observations of the Li 16708 Angstrom line by means of spectral synthesis using both 1D hydrostatic and 3D hydrodynamical model atmospheres are discussed. Recent results are compared to

Measurement of lithium isotopic ratios as lithium tetraborate ions

Determination of 6Li/ 7Li ratios using Li 2BO 2 + ions analysed by thermal ionisation mass spectrometry with ion counting has been investigated by numerical simulation and experiment. It is possible to solve for one or more of the 6Li/ 7Li ratio, 10B/ 11B ratio and fractionation if one or more of the ratios of the 54, 55, 56, 57, 58 and 59 masses of Li 2BO 2 + are measured. Monte Carlo simulation of data sets suggests that within-sample precision on measurement of Li 2BO 2 + mass ratios is largely limited by counting statistics and should result in precision as good as 0.25‰ (1σ) on the 6Li/ 7Li ratio. However repeatability of 6Li/ 7Li and 10B/ 11B ratios gives precisions no better than approximately ±3‰ (1σ) and the probable cause is fractionation of the 6Li/ 7Li ratio on the filament due to the large Li + ion beam produced during analysis of Li 2BO 2 +. Plots of variation of sample fractionation with time confirms that fractionation is primarily controlled by varying 6Li/ 7Li ratio rather than varying fractionation of Li 2BO 2 + or 10B/ 11B ratio.

The Chemical Composition of Carbon Stars. II. The J‐Type Stars

Abundances of lithium, heavy elements, and carbon isotope ratios have been measured in 12 J-type Galactic carbon stars. The abundance analysis shows that in these stars the abundances of s-process elements with respect to the metallicity are nearly normal. Tc is not present in most of them, although upper limits have been derived for WZ Cas and WX Cyg, perhaps two SC-type rather than J-type carbon stars. The Rb abundances, obtained from the resonance λ7800 Rb I line, are surprisingly low, probably owing to strong non-LTE effects in the formation of this line in cool carbon-rich stars. Lithium and 13C are found to be enhanced in all the stars. These results together with the nitrogen abundances and oxygen isotope ratios measured by Lambert et al. and Harris et al. are used to discuss the origin of J stars. The luminosity and variability class of the stars studied would indicate that they are low-mass (M ≲ 2-3 M☉), less evolved objects than the normal carbon stars, although the presence of some luminous (Mbol < -5.5) J stars in our Galaxy (WZ Cas may be an example) and in other galaxies, suggests the existence of at least two types of J stars, with different formation scenarios depending upon the initial mass of the parent star. Standard evolutionary AGB models are difficult to reconcile with all the observed chemical characteristics. In fact, they suggest the existence of an extra mixing mechanism that transports material from the convective envelope down to hotter regions where some nuclear burning occurs. This mechanism would act preferably on the early-AGB phase in low-mass stars. Mixing at the He-core flash and the binary system hypothesis are also discussed as alternatives to the above scenario.

6Li from Solar Flares.

By introducing a hitherto ignored 6Li producing process, due to accelerated 3He reactions with 4He, we show that accelerated particle interactions in solar flares produce much more 6Li than 7Li. By normalizing our calculations to gamma-ray data, we demonstrate that the 6Li produced in solar flares, combined with photospheric 7Li, can account for the recently determined solar wind lithium isotopic ratio, obtained from measurements in lunar soil, provided that the bulk of the flare-produced lithium is evacuated by the solar wind. Further research in this area could provide unique information on a variety of problems, including solar atmospheric transport and mixing, solar convection and the lithium depletion issue, and solar wind and solar particle acceleration.

Analytical application of 2f-wavelength modulation for isotope selective diode laser graphite furnace atomic absorption spectroscopy.

Experiences in the analytical application of the 2f-wavelength modulation technique for isotope selective atomic absorption spectroscopy in a graphite furnace are reported. Experimental as well as calculated results are presented, mainly for the natural lithium isotopes. Sensitivity, linearity, and (isotope) selectivity are studied by intensity modulation and wavelength modulation. High selectivities can be attained, however, on the cost of detection power. It is shown that the method enables the measurement of lithium isotope ratios larger than 2000 by absorption in a low-pressure graphite tube atomizer.

Isotopic Composition of Lithium in Carbonaceous Chondrites Measured by a Modified Type Surface Ionization Mass Spectrometer.

The isotopic ratios of lithium, 7Li/6Li, have been measured by a modified type surface ionization mass spectrometer whose ion source was newly designed. It needs no chemical preparation prior to the ionization, and makes an analysis of small portions of sample possible. We mainly analyzed the Murchison meteorite and compared with terrestrial laboratory standard samples, spodumene and lepidolite. Many kinds of isotope anomalies for Murchison have been reported, and these anomalies are considered to be originated from presolar events. Spallation reactions are thought to be one of the processes that produced lithium isotopes, and to synthesize 7Li and 6Li isotopes as almost the same quantities. If lithium isotopes produced by such reactions in the presolar stage have been maintained in meteorites, there may be an isotopic anomaly of lithium whose ratio, 7Li/6Li, is extremely lower than that for terrestrial standards. The mean isotopic ratio, 7Li/6Li, for one laboratory standard, spodumene, is 10.519±0.0004, and that for another one, lepidolite, is 11.329±0.0009. The discrepancy between these lithium isotopic ratios is estimated to be due to the different fractionation processes of these minerals. The isotopic ratios for Murchison are within a range from 10.056 to 11.356. This range of the scattering for Murchison is within those for previous works. The lithium isotopic ratios for Murchison analyzed so far gave no significant difference from those for terrestrial standards.

Observations of 6Li in Metal Poor Stars

Methods and accuracies of determining the lithium isotope ratio in stellar atmospheres from high resolution observations of the profile of the λ6708 Li i doublet are discussed, and recent results for metal-poor disk stars and halo stars are reviewed. The data are compared to models for the Galactic evolution of the 6Li abundance. It is concluded that a much larger data set is needed to understand properly the formation of 6Li in the interstellar medium as well as the depletion in stars.

Application of the 2 f-wavelength modulation technique for the measurement of large lithium isotope ratios by diode laser graphite furnace atomic absorption spectroscopy

The measurement of large lithium isotope ratios by diode laser graphite furnace atomic absorption spectroscopy was investigated using a low pressure atomizer and 2f-wavelength modulation (WM) detection. The measurements were supported by computer simulations. Compared with a direct absorption measurement, the relative absorption sensitivity for 6Li is considerably reduced when 2f-WM is performed in the center of the 6Li D1 fine structure component, but it will be enhanced, when the center of modulation is tuned to the maximum in the red wing of the 2f-WM line profile. The results of calculated 2f-WM line strengths were used to deconvolute overlapping lines and enabled the measurement of 7Li/ 6Li isotope ratios as large as approximately 2000. The Li content of a sample with a strong sodium chloride matrix was determined by isotope dilution.

The evolution of [formula omitted] in standard cosmic-ray nucleosynthesis

We review the Galactic chemical evolution of 6 Li and compare these results with recent observational determinations of the lithium isotopic ratio. In particular, we concentrate on so-called standard Galactic cosmic-ray nucleosynthesis in which Li, Be, and B are produced (predominantly) by the inelastic scattering of accelerated protons and α off of CNO nuclei in the ambient interstellar medium. If O/Fe is constant at low metallicities, then the 6 Li versus Fe/H evolution – as well as Be and B versus Fe/H – has difficulty in matching the observations. However, recent determinations of Population II oxygen abundances, as measured via OH lines, indicate that O/Fe increases at lower metallicity; if this trend is confirmed, then the 6 Li evolution in a standard model of cosmic-ray nucleosynthesis is consistent with the data. We also show that another key indicator of 6 Li BeB origin is the 6 Li /Be ratio which also fits the available data if O/Fe is not constant at low metallicity. Finally we note that 6 Li evolution in this scenario can strongly constrain the degree to which 6 Li and 7 Li are depleted in halo stars.

Measurement of lithium isotope ratios by inductively coupled plasma mass spectrometry: application to geological materials

A method is described for the measurement of Li isotope ratios in minerals using ICP-MS. The technique is capable of achieving a relative precision of 0.8‰, and has successfully been applied to minerals containing less than 300 µg g–1 of Li. Sample material is dissolved in a combination of HCI and HF in sealed PTFE vessels. Separation of Li from the matrix is accomplished by column chromatography using Bio-Rad AG50W X-8 100–200 mesh resin. The use of isotopic reference material IRM-016 Li2 CO3 is suggested as a reference standard for calculating and correcting instrumental mass discrimination. The presence of excess amounts of concomitant ions in solution is shown to lead to matrix-induced mass discrimination. Lithium isotope ratios (7Li: 6Li) for the minerals studied range from 12.698 (–33.2‰ relative to the IRM-016) for an amblygonite sample from Tanco Mine, Manitoba, Canada to 12.250 (+3.5‰ relative to the IRM-016) for a lithiophorite sample from North Mountain Mine, AR, USA.

High precision isotopic measurement of lithium by thermal ionization mass spectrometry

A new procedure for the determination of lithium isotope ratios based on the use of lithium phosphate as the loading material by positive thermal ionization mass spectrometry has been investigated. The 6Li −/ 7Li + ion ratio was measured using a triple filament technique to minimize fractionation. A two-column ion exchange process was employed to separate lithium from sample matrix and convert to LiOH, which further reacts with phosphoric acid to form lithium phosphate. Optimal conditions for measurement were established. Lithium isotopes in reference material supplied by NIST as L-SVEC Li 2CO 3 were measured with a precision of 0.07%. A comparison was made by using two different types of thermal ionization mass spectrometers employing different detectors such as a Faraday cup assembly and secondary electron multiplier. The composition of lithium in several Li 2CO 3 reagents and in one meteorite was also determined by this procedure to examine possible variations in isotopic compositions.

Simultaneous measurement of lithium and boron isotopes as lithium tetraborate ion by thermal ionization mass spectrometry

A method for the measurement of Li2BO2+ ions by thermal ionization mass spectrometry employing a Daly ion-counting system was examined. By solving simultaneous quadratic equations of intensity ratios ranging between m/z 55 and 57, and between m/z 56 and 57, precise and accurate measurement of lithium and boron isotope ratios can be carried out simultaneously. The intensity ratio between m/z 54 and 57 can be used to check the internal consistency of the lithium and boron isotope ratios obtained. The isotopic fractionation rate with the species Li2BO2+ was low. Prior to measurement, a two-column ion-exchange process was used to separate lithium from the sample matrix and then to convert LiCl into LiOH, which subsequently reacts with H3BO3 to form lithium tetraborate. Lithium and boron isotope ratios were successfully measured simultaneously with precisions (relative standard deviations) of 0.05% for lithium and 0.01% for boron.