リチウムの地球・宇宙化学 Ｌｉ同位体のトレーサーとしての可能性と課題

First Lithium Isotope Data from Rivers and Subsurface Water in the Pamirs

We report on the measurement of the 2s-3s transition frequencies in the stable lithium isotopes and 2s-2p isotope shift measurements of 7,9,10,11Be, using a femtosecond frequency comb. For the beryllium isotopes, we extract the changes in the mean-square nuclear charge radius along the isotope chain by comparison with high-precision atomic mass shift calculations. The 2s-3s transition frequency is compared with theoretical calculations, and possibilities to extract an absolute value for the nuclear charge radius of lithium isotopes are discussed.

Geochemical behavior of stable strontium isotopes during continental weathering process: A review

Experimental calibration of the vapour–liquid phase relations and lithium isotope fractionation in the system H₂O–LiCl at 400°C/20–28 MPa

Large zinc isotope variations in eastern Pacific seamount basalts as a result of mantle metasomatism

Equilibrium lithium isotope fractionation in Li-bearing minerals

Goethite (a-FeOOH), the most common product of weathering of Fe-bearing minerals, is an attractive target for (U-Th)/He geochronology because of its abundance and relative stability in the surficial environment. Combined (U-Th)/He4 He/3 He geochronology of 24 goethite samples representative of different source rocks, precipitation mechanisms, solid-solution compositions, purity, and porosity provides information on the influence of these factors on U-Th-He contents, distribution, and retentivity in natural goethites. Measured activation energies (E) for He diffusion from goethites range from 11.2 ± 0.5 to 74.5 ± 3.2 kcal.mol-1 , with an average value at 36.2 ± 13.5 (1s) kcal.mol-1 ; and frequency factors (lnD0/r0 2) range from -4.1 ± 0.5 to 62.8 ± 1.9 s-1 , with an average value of 24.1 ± 13 (1s) s-1. The average activation energy of ~ 35 kcal.mol-1 indicates that He retentivity in goethite is comparable to other phases dated by the (U-Th)/He method. A major distinction between goethite and other phases used in (U-Th)/He geochronology is its large frequency factor, a possible consequence of the fact that supergene goethite is composed of masses of cryptocrystalline aggregates and not single crystals.Armed with an improved understanding of goethite’s He retentivity, and its suitability to (U-Th)/He and cosmogenic 3He analyses, two field sites were investigated in detail to unravel the history of mineral precipitation, exhumation and erosion that created the characteristic plateau landscapes at the Quadrilatero Ferrifero and Carajas, Brazil. (U-Th)/He geochronology of goethite and cosmogenic 3He in goethite and hematite reveal mineral precipitation ages as old as 55 Ma and apparent exposure ages > 5 Ma for canga-cemented plateaus at the Quadrilatero Ferrifero. Pebbles of hypogene hematite-magnetite from colluvia or shallow creeks draining the canga-cemented plateaus record a much longer exposure history than in situ canga blocks, showing that even older duricrusts, now eroded, once blanketed these plateaus. The long-term erosion history obtained from cosmogenic 3He on BIF plateaus confirms that relic surfaces persist in the landscape for millions of years. Combined 3He and (U-Th)/He dating shows that cangas are preferentially goethite cemented by biogeochemical reactions in the subsurface. Physically stable but biogeochemically dynamic, cangas armor the landscape by pervasive and recurrent iron cycling and cementation, slowing erosion of weathered BIF and friable hematite-magnetite ore. At Carajas, (U-Th)/He geochronology reveals a longer history of weathering, spanning from ca. 80 Ma to the present. Cosmogenic 3He measurements for hematite blocks cemented onto the plateaus yield apparent exposure ages > 7 Ma and erosion rates as low as 0.08 m.Ma-1 , confirming that plateau surfaces are virtually immune to physical erosion for tens of millions of years. In contrast, (U-Th)/He geochronology of ferruginous duricrusts blanketing low elevation (250-100 m) plains surrounding the Carajas plateaus reveal that the low elevation plain is diachronous and range from ~ 10 Ma away from the plateau to ~1 Ma next to it, having evolved by scarp retreat. The virtual cessation of scarp retreat at some sites suggests that metamorphosed banded iron-formations provide effective barriers to retreating escarpments, helping to preserve some of the oldest continuously exposed landsurfaces on Earth.To derive paleoenvironmental information about these landsurfaces, new protocols were developed for high-spatial resolution δ18O analysis of goethite with the Sensitive High Mass Resolution Ion Microprobe – Stable Isotopes (SHRIMP-SI). A natural sample from the Capao Mine, Minas Gerais, Brazil, was extensively characterized and now provides a working goethite reference material (RM) for ion microprobe analyses. A laser fluorination value of -17.22 ± 0.03 ‰ (1 σ) was obtained for an aliquot of this RM. The repeatability of SHRIMP-SI measurements of the Capao L4 RM was often better than 1.5‰ (2 s). Using Capao L4 as a RM, several natural goethite samples were analysed to test the relationship between natural properties, preparation procedures, instrument conditions, and the overall reliability of the δ18OSIMS results.Finally, combined laser-heating (U-Th)/He geochronology, SHRIMP-SI δ18O measurements (δ18OSIMS-gth), and electron microprobe analysis of goethites from the Quadrilatero Ferrifero and Carajas sites produced two extensive Cenozoic paleoclimatic records for the continental interior of Brazil, spanning from ~ 70 Ma to 900 ka at Carajas and from ~ 40 Ma to 600 ka at the QF. The results identify major climatic shifts in continental Brazil, compatible with global climatic changesobserved in the δ18O record of oceanic sediments. Periods of optimum climatic conditions (Early to Mid-Eocene, Late Oligocene, and Mid-Miocene) correspond to more frequent precipitation of weathering minerals, stoichiometrically pure goethites, and δ18OSIMS-gth shifts towards lighter isotopic values. These shifts are consistent with warm monsoonal conditions in the continental interior. Abrupt shifts in the d18OSIMS-gth record towards more positive values occur during global glaciation (Oi-1 and Mi-1 glaciations), periods that also correspond to less frequent precipitation of Al-rich goethites suggestive of dryer continental environments. The combined goethite (U-Th)/He– δ18OSIMS-gth record provides a robust new way of investigating continental paleoclimates on Earth.

Iron isotope systematics during igneous differentiation in lavas from Kīlauea and Mauna Loa, Hawai'i

<p>A wide range of novel, non-traditional, stable isotope systems have been developed over the last decade, largely as a result of the advent of multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS), and continue to provide valuable new insights in the earth, environmental and planetary sciences. The platinum (Pt) stable isotope system represents a potentially powerful but, as yet, unexplored addition to this suite of stable isotope tracers. Pt has six naturally occurring isotopes, and can exist in a range of oxidation states. The geochemical behaviour of Pt coupled with the relatively large mass difference (ca. 2%) between the abundant heavy and light isotopes and its variable oxidation states leads to potential applications in tracing a range of natural processes. In particular, the strong elemental partitioning of Pt between metals and silicates makes the Pt stable isotope system uniquely suited to tracing processes of Earth’s accretion and differentiation. This study aims to develop new techniques for measurement of Pt stable isotopes in geological samples, and to apply these to terrestrial and meteorite samples to attempt to resolve outstanding questions relating to Earth’s early evolution. A technique was developed for measurement of Pt stable isotope ratios using multiple collector inductively coupled plasma mass spectrometry (MCICPMS), employing a ¹⁹⁶Pt–¹⁹⁸Pt double-spike to correct for instrumental mass fractionation. Results are reported in terms of δ¹⁹⁸Pt, which represents the per mil difference in the ¹⁹⁸Pt/¹⁹⁴Pt ratio from the IRMM-010 Pt isotope standard. A range of analytical tests were conducted and show that this approach has a reproducibility of ca. ±0.04 %∘ on δ¹⁹⁸Pt (i.e., ±0.01%∘ amu⁻¹) for Pt solution standards, and is insensitive to minor amounts of matrix that may be retained after chemical purification of Pt. Measurements of Pt solution standards conducted using two different MC-ICPMS instruments showed resolvable variations, which suggest that natural fractionations exist that exceed the reproducibility of the technique. Techniques were also developed for dissolution of natural samples and chemical separation of Pt. Geological standards were digested using a nickel sulphide fire assay technique, which pre-concentrates the highly siderophile elements in a NiS bead that is readily dissolved in acid. This was followed by chemical separation of Pt from digested samples using anion exchange chemical techniques. Elution curves were constructed for a range of synthetic rock matrices. These tests show that Pt separation is achieved with >90% Pt yield and ca. 95% purity. Analytical tests show that this level of Pt separation is sufficient for accurate determination of Pt stable isotope ratios by double-spike MC-ICPMS. These techniques were then applied to 11 international geological standard reference materials representing mantle peridotites, igneous samples, and Pt ore materials. The reproducibility in natural samples was determined by processing multiple replicate digestions of a standard reference material, and was shown to be ca. ±0.08%∘ (2 sd). Pt stable isotope data for the full set of reference materials have a range of δ¹⁹⁸Pt values with offsets of up to 0.40%∘ from the IRMM-010 standard, which are readily resolved with this technique. Mantle samples yielded the lightest (most negative) isotopic compositions of the terrestrial standards, with igneous and Pt ore samples defining a continuous trend towards zero, which is consistent with the IRMM-010 standard being derived from a Pt ore. These results demonstrate the potential of the Pt isotope system as a tracer in geochemical systems. The techniques developed above were then applied to investigate an outstanding problem relating to Earth’s accretion and differentiation. Highly siderophile elements (HSE) are strongly partitioned into the cores of terrestrial planets during core formation, and the abundances of HSE in Earth’s mantle compared with primitive meteorites have provided key constraints on models of Earth’s early evolution. Two leading models to explain the HSE abundances in the silicate Earth involve either a late-veneer of chondritic material that was added after core formation or core formation in a deep magma ocean. The platinum (Pt) stable isotope system represents a novel tool for investigating these processes. Using the techniques developed above, Pt stable isotope ratios were measured in a range of meteorite samples, including enstatite, ordinary and carbonaceous chondrites, primitive achondrites, achondrites and iron meteorites, as well as additional terrestrial mantle xenolith samples. Our data set reveals that the Pt stable isotopic composition of Earth’s mantle overlaps with all of the chondrite groups. Primitive achondrite and ureilite samples revealed the heaviest compositions of all meteorite groups. These data suggest that metal–silicate differentiation produces an isotopic fractionation for Pt, with heavy isotopes being preferentially retained in the silicate phase. Thus, Earth’s mantle is expected to have been significantly enriched in the heavy isotopes of Pt during core formation, even if metal–silicate differentiation took place in a magma ocean. The absence of a large fractionation between chondrites, representing the composition of the undifferentiated Earth, and the mantle suggests that the signature of core formation in the mantle has been subsequently overprinted. Considering the overlap between the Pt stable isotopic compositions of the mantle and chondrites, the most likely means for overprinting the composition of the mantle is by addition of a chondritic late-veneer. Mixing calculations show that addition of 0.5% of Earth’s mass by a late-veneer of chondritic material would be sufficient to overprint highly fractionated Pt stable isotope signatures resulting from core-formation.</p>

A study on the separation of lithium isotopes was carried out with an ion exchange resin having 1,7,13-trioxa-4,10,16-triazacyclooctadecane (N3O3) as an anchor group. The lighter isotope,6Li concentrated in the resin phase, while the heavier isotope,7Li is enriched in the fluid phase. Upon column chromatography [0.6 cm (I. D.)×20 cm (height)] using 1.0M ammonium chloride solution as an eluent, single separation factor, α, 1.068 (6Li/7Li)resin/(6Li/7Li)solution was obtained by theGlueckauf method from the elution curve and isotope ratios.

The geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological and temperature heterogeneities originating in both the upper and lower mantle. Recently, non-traditional stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity (e.g., major and trace elements, radiogenic isotopes), because mineral- and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. Here, we investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle temperature and/or lithological heterogeneity. We use a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites), responding to changes in mantle mineralogy, oxygen fugacity, temperature and pressure. We find that, given current analytical precision, the stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of >10% MORB-like pyroxenite in the mantle source, given current analytical precision. Magnesium and Ca stable isotopes show most sensitivity to a garnet-bearing source lithology, and Fe and Cr stable isotopes are potentially sensitive to the presence of MORB-like pyroxenite in the mantle source, although the behaviour of Cr isotopes is comparatively under-constrained and requires further work to be applied with confidence to mantle melts. When comparing the magnitude and direction of predicted equilibrium isotopic fractionation of peridotite and pyroxenite melts to natural MORB and OIB data, we find that aspects of the natural data (including the mean Mg-Ca-Fe-V isotopic composition of MORB, the range of Mg-Ca isotopic compositions seen in MORB data, the mean Mg-Ca-Cr isotopic composition of OIB, and the range of Mg-V-Cr isotopic compositions in OIB data) can be matched by equilibrium isotope fractionation during partial melting of peridotite and pyroxenite sources – with pyroxenite required even for some MORB data. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. We conclude that the five stable isotope systems considered here have potential to be powerful complementary tracers to other geochemical tracers of the source lithology of erupted basalts. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these novel stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts.

The implementation of the carbon peaking and carbon neutrality strategy has led to a steady increase in the supply of lithium resources. Brine is one of the important sources of lithium, and the extraction of Li from carbonate-type brine is particularly straightforward. Research into the source of materials and hydrological processes of brine is crucial for the sustainable development of lithium in carbonate-type brine. As a fluid-mobile and metallogenic element, lithium has a significant mass difference between its stable isotopes (7Li and 6Li), leading to isotopic fractionation. In this study, we analyzed the hydrochemistry and Li isotope compositions of samples collected from a Li-rich salt lake (Bangor Co) in the Qinghai-Tibetan Plateau. The samples included lake brines, recharge rivers, cold springs, and salt minerals (hydromagnesites). The Li content in the various types of water varied significantly, ranging from 0.06 mg/L to 198.10 mg/L, showing a variation of 4 orders of magnitude. Water samples exhibit a wide range of δ7Li values, varying from 4.89‰ to 16.02‰. Notably, the lowest and highest values are observed in cold springs. Additionally, the concentrations and δ7Li values in hydromagnesite differ across various relative ages. The hydrochemistry indicated that the recharge water is influenced by rock weathering, but the lake brine is influenced by evaporation concentration. The analysis of trace elements and Li isotopic data reveals that rock weathering, geothermal systems, salt minerals, and freshwater, primarily from early geothermal activities and the redissolution of carbonate minerals, contribute to the Li in salt lake brine. Boron isotopes and lithium isotopes of lake brines are found to vary differently. The δ7Li in brine is increased significantly by adsorption of hydromagnesite. And 11B gradually accumulates in hydromagnesite. This study has demonstrated that hydromagnesite plays a crucial role in influencing the characteristics of Li in brine.

Non-volatile vs volatile behaviours of halogens during the AD 79 plinian eruption of Mt. Vesuvius, Italy

The Nd-isotope analytical technique is a powerful tool for studying many geological processes: particularly the evolution of the upper mantle as seen through the isotope and trace element geochemistry of mantle-derived volcanic rocks1–3, and magmatic processes along destructive plate margins4,5. However, previous studies of continental crust6–8 have been largely restricted to Archaean examples and concerned primarily with dating their time of derivation from the upper mantle. In the present study we investigated the Nd- and Sr-isotope characteristics of rocks involved in a relatively young (800–450 Myr) orogenic event. Such events represent critical stages in the evolution of most of the Earth's continental crust—new material is added from the mantle, and pre-existing crust is remobilized by erosion and sedimentation, deformation and magmatic activity. Nd- and Sr-isotopes were used to outline the range in age and geochemical characteristics of rock sequences and provinces within both the upper mantle and the pre-existing crust which were sampled during the orogeny, and hence provided the major components of what is now a stable segment of continental crust.

The behavior of nickel (Ni) isotopes during magmatic differentiation is not adequately explored. Here, we find that tholeiitic rocks in the Kīlauea Iki (KI) lava lake, Hawai'i, show progressively lighter Ni isotopic compositions with increasing magmatic differentiation, whereas calc‐alkaline rocks from the thick Kamchatka arc (30–45 km), located at the convergent boundary of the Eurasian and Pacific plates show increasing Ni isotope values as MgO and Ni decrease. Forty‐three global intermediate‐felsic continental igneous rocks analyzed in this study display large Ni isotopic variations, with the Eoarchean samples having light Ni isotopic compositions that fall in the trend defined by the KI lavas, and the post‐Eoarchean samples showing systematically heavier Ni isotopic compositions overlapping those of Kamchatka arc rocks. The isotopic dichotomy results from the crystallization of isotopically heavy magnetite during low‐pressure differentiation of KI lavas, whereas the participation of sulfide separation that removes isotopically light Ni during high‐pressure differentiation of magmas traversing thick continental crust. Combined with Rhyolite‐MELTS and sulfur concentration at sulfide saturation simulations, we demonstrate that the Ni isotope fractionation during magmatic differentiation is mainly controlled by the tempo of magnetite crystallization and sulfide formation, which is a function of pressure, oxygen fugacity, and water activity. High‐pressure calc‐alkaline differentiation usually suppresses magnetite crystallization while stabilizing sulfide, leading to heavy Ni isotopic compositions for the evolved magmas, significantly deviating from the low‐pressure fractionation trend seen in the KI lavas. Ni isotopes can be used in the future as a tracer of magmatic differentiation and processes of continent formation and differentiation.