Heavier Isotopes Of Magnesium Research Articles

Heavy magnesium isotopes in the Gangdese Magmatic Belt: Implications for magmatism in the Mesozoic subduction system of southern Tibet

The Gangdese magmatic belt of southern Tibet preserves important records of the Mesozoic Neo-Tethyan orogeny. Here we investigate a newly identified Late Cretaceous hornblende gabbro pluton using zircon U–Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf-O-Mg isotopes with a view to evaluate the magma source characteristics and fluid evolution. The geochronological data and Sr-Nd-Hf isotopes suggest that the Late Cretaceous gabbro crystallized in the Cenomanian stage of the Late Cretaceous (ca. 98 Ma), from magma generated by partial melting of the depleted mantle. We report the first set of Mg isotopic data for the Late Cretaceous and Late Triassic arc-type gabbroid rocks in the Gangdese magmatic belt, where both rocks show homogeneous heavy Mg isotopic compositions with δ26Mg values ranging from 0.01 to 0.12‰ and − 0.09 to 0.09‰, respectively. Their δ26Mg values are higher than the average mantle values (−0.25 ± 0.07‰), suggesting that 26Mg-rich fluids were involved in their mantle sources. Based on Sr-Nd-Hf-O isotopic data and published information on Mg isotopic reservoir, we argue that talc-rich serpentinite fluids played an important role in producing the heavy Mg isotopes in these arc-related intrusions. Integrating data from previous studies and this study, we propose that the northward subduction of the Neo-Tethys oceanic lithosphere and formation of arc-type magmas provide a viable model to account for the heavy Mg isotopic compositions through fluid-rock interaction within the subduction channel. This further indicates that the northward subduction of the Neo-Tethys oceanic lithosphere commenced in the Late Triassic or earlier rather than in the Early Cretaceous (ca. 145 Ma).

Intensity of methane seepage reflected by relative enrichment of heavy magnesium isotopes in authigenic carbonates: A case study from the South China Sea

Changing conditions including steep redox gradients result in different Ca-Mg-carbonates forming at methane seeps as a consequence of sulfate-driven anaerobic oxidation of methane (SD-AOM). Since Mg2+ is one of the main cations in Ca-Mg-carbonates, Mg isotope composition may reflect the mode of mineral authigenesis at seeps. Seep carbonates were collected from the Shenhu area and the Southwest (SW) Taiwan basin in the South China Sea. The carbonate phases in Shenhu samples are dolomite accompanied by accessory Mg-calcite, while SW Taiwan samples consist of Mg-calcite and dolomite. Correspondingly, Shenhu carbonates show systematically higher Mg/Ca ratios. Low δ13C values of seep carbonates confirm their derivation from the oxidation of methane, δ18O values reflect formation in equilibrium with coeval seawater. The δ26Mg value of a reference sample of biodetrital carbonate (−4.28‰) is lower than those of seep carbonates (−3.25 to −2.95‰). Since only little variability of δ26Mg values of pore waters is expected based on previous work, differences in the δ26Mg values of seep carbonates were apparently caused by changing degrees of isotopic fractionation during precipitation. Trends between δ26Mg values and Mg/Ca ratios and between δ26Mg and δ13C values suggest that Mg isotope fractionation was controlled by a kinetic mechanism affecting the incorporation of Mg2+ ions into the carbonate lattice in the course of SD-AOM. By consuming sulfate and by producing sulfide, SD-AOM reduces the energy differences for the dehydration of ions of light and heavy Mg isotopes, lowering isotope fractionation. Additionally, the two trends have been found to be steeper for Shenhu samples, suggesting more pronounced SD-AOM in the Shenhu area. Our study indicates that Mg isotope composition of methane-derived carbonates is affected by the process that drives carbonate precipitation – SD-AOM. Future work is required to confirm the utility of Mg isotopes as a new proxy for this major biogeochemical process.

The effect of lanthanides on photosynthesis, growth, and chlorophyll profile of the green alga Desmodesmus quadricauda.

Lanthanides (La, Gd, Nd, Ce) accumulated in the green alga Desmodesmus quadricauda but their intracellular localizations were distinctly different: lanthanum and gadolinium were localized in cytoplasm, while neodymium and cerium were in the chloroplast. The effect of lanthanum and neodymium, as representatives of these two groups, on growth, chlorophyll content and photosynthetic rate at different light intensities was studied. At the lowest light intensity used (50µmol photons m-2s-1), in the presence of lanthanides (Nd), growth was enhanced by as much as 36% over lanthanide free control, and the photosynthetic rate increased by up to 300%. At high light intensities (238, 460, and 750µmol photons m-2 s-1), photosynthetic rate increased markedly, but there was no significant difference between rates in the presence or absence of lanthanides. However, growth, measured as a percentage of dry weight, if compared with lanthanide free control, increased at all light intensities (31, 39, and 20%, respectively). The total amount of chlorophyll after lanthanide treatment increased by up to 21% relative to the control culture, mainly due to an increase in the level of chlorophyll b. Addition of lanthanides caused a change in the chlorophyll a/b ratio from 4.583 in control cultivation, to 1.05. Possible mechanisms of lanthanide-induced photosynthetic change, alterations in photosynthetic structures, and increases in growth are discussed and compared with findings in higher plants. The hypothesis that the lanthanide effect could be due to formation of lanthanide-pheophytins was not confirmed as lanthanide pheophytins were not found in D. quadricauda. Furthermore, we have shown that the preferential incorporation of heavy isotopes of magnesium, namely 25Mg and 26Mg, into chlorophyll during photosynthesis that occurred in controls was diminished in the presence of lanthanides.

Super and massive AGB stars – III. Nucleosynthesis in metal-poor and very metal-poor stars – Z = 0.001 and 0.0001

We present a new grid of stellar models and nucleosynthetic yields for super-AGB stars with metallicities Z=0.001 and 0.0001, applicable for use within galactic chemical evolution models. Contrary to more metal rich stars where hot bottom burning is the main driver of the surface composition, in these lower metallicity models the effect of third dredge-up and corrosive second dredge-up also have a strong impact on the yields. These metal-poor and very metal-poor super-AGB stars create large amounts of He4, C13 and N14, as well as the heavy magnesium isotopes Mg25 and Mg26. There is a transition in yield trends at metallicity Z approximately 0.001, below which we find positive yields of C12, O16, N15, Al27 and Si28, which is not the case for higher metallicities. We explore the large uncertainties derived from wind prescriptions in super-AGB stars, finding approximately 2 orders of magnitude difference in yields of Ne22, Na23, Mg24,25,26, Al27 and our s-process proxy isotope g. We find inclusion of variable composition low temperature molecular opacities is only critical for super-AGB stars of metallicities below Z approximately 0.001. We analyze our results, and those in the literature, to address the question: Are super-AGB stars the polluters responsible for extreme population in the globular cluster NGC 2808? Our results, as well as those from previous studies, seem unable to satisfactorily match the extreme population in this globular cluster.

Silicon isotopic fractionation of CAI-like vacuum evaporation residues

Calcium-, aluminum-rich inclusions (CAIs) are often enriched in the heavy isotopes of magnesium and silicon relative to bulk solar system materials. It is likely that these isotopic enrichments resulted from evaporative mass loss of magnesium and silicon from early solar system condensates while they were molten during one or more high-temperature reheating events. Quantitative interpretation of these enrichments requires laboratory determinations of the evaporation kinetics and associated isotopic fractionation effects for these elements. The experimental data for the kinetics of evaporation of magnesium and silicon and the evaporative isotopic fractionation of magnesium is reasonably complete for Type B CAI liquids (Richter F. M., Davis A. M., Ebel D. S., and Hashimoto A. (2002) Elemental and isotopic fractionation of Type B CAIs: experiments, theoretical considerations, and constraints on their thermal evolution. Geochim. Cosmochim. Acta 66, 521–540; Richter F. M., Janney P. E., Mendybaev R. A., Davis A. M., and Wadhwa M. (2007a) Elemental and isotopic fractionation of Type B CAI-like liquids by evaporation. Geochim. Cosmochim. Acta 71, 5544–5564.). However, the isotopic fractionation factor for silicon evaporating from such liquids has not been as extensively studied. Here we report new ion microprobe silicon isotopic measurements of residual glass from partial evaporation of Type B CAI liquids into vacuum. The silicon isotopic fractionation is reported as a kinetic fractionation factor, α Si, corresponding to the ratio of the silicon isotopic composition of the evaporation flux to that of the residual silicate liquid. For CAI-like melts, we find that α Si = 0.98985 ± 0.00044 (2 σ) for 29Si/ 28Si with no resolvable variation with temperature over the temperature range of the experiments, 1600–1900 °C. This value is different from what has been reported for evaporation of liquid Mg 2SiO 4 (Davis A. M., Hashimoto A., Clayton R. N., and Mayeda T. K. (1990) Isotope mass fractionation during evaporation of Mg 2SiO 4. Nature 347, 655–658.) and of a melt with CI chondritic proportions of the major elements (Wang J., Davis A. M., Clayton R. N., Mayeda T. K., and Hashimoto A. (2001) Chemical and isotopic fractionation during the evaporation of the FeO–MgO–SiO 2–CaO–Al 2O 3–TiO 2–REE melt system. Geochim. Cosmochim. Acta 65, 479–494.). There appears to be some compositional control on α Si, whereas no compositional effects have been reported for α Mg. We use the values of α Si and α Mg, to calculate the chemical compositions of the unevaporated precursors of a number of isotopically fractionated CAIs from CV chondrites whose chemical compositions and magnesium and silicon isotopic compositions have been previously measured.

Magnesium-Isotope Fractionation During Plant Growth

Magnesium is an essential nutrient, which activates more enzymes than any other mineral element and, thus, plays an important role in biogeochemical cycles. With three stable isotopes naturally abundant (24Mg, 78.992%; 25Mg, 10.003%; 26Mg, 11.005%), magnesium stable isotope fractionation may provide insights into these cycles. Here, we detail for the first time the magnesium stable-isotope distribution in a higher plant, wheat (Triticum aestivum L.), during its growth cycle. Wheat plants were grown in a limiting nutrient supply hydroponically, some being left to mature through senescence and others detopped at maturity for collection of exudates. Measurements of the magnesium isotopic composition of chlorophylls, seeds, shoots, roots, leaves, exudates, and the limiting nutrient solution over time show that the plant appears to establish an isotopic equilibrium with the nutrient available to it and that the plant (in particular, the seeds and exudates) becomes enriched in the heavy isotopes of magnesium in a mass-dependent relationship as the plant reaches maturity. The preference of the plants for heavy magnesium isotopes suggests that a difference might exist in the bioavailable magnesium of agricultural and natural soils due to the periodic removal of heavy magnesium isotopes by harvest.

Reaction rate uncertainties: NeNa and MgAl in AGB stars

We study the effect of uncertainties in the proton-capture reaction rates of the NeNa and MgAl chains on nucleosynthesis due to the operation of hot bottom burning (HBB) in intermediate-mass asymptotic giant branch (AGB) stars. HBB nucleosynthesis is associated with the production of sodium, radioactive Al26 and the heavy magnesium isotopes, and it is possibly responsible for the O, Na, Mg and Al abundance anomalies observed in globular cluster stars. We model HBB with an analytic code based on full stellar evolution models so we can quickly cover a large parameter space. The reaction rates are varied first individually, then all together. This creates a knock-on effect, where an increase of one reaction rate affects production of an isotope further down the reaction chain. We find the yields of Ne22, Na23 and Al26 to be the most susceptible to current nuclear reaction rate uncertainties.

An experimental study of magnesium-isotope fractionation in chlorophyll-a photosynthesis

Measurements are presented of the magnesium isotopic composition of chlorophyll-a, extracted from cyanobacteria, relative to the isotopic composition of the culture medium in which the cyanobacteria were grown. Yields of 50–93% chlorophyll-a were achieved from the pigment extracts of Synechococcus elongatus, a unicellular cyanobacteria. This material was then digested using concentrated nitric acid to extract magnesium. Separation was accomplished using columns of cation-exchange resin, which achieved a 103 ± 10% yield of magnesium from chlorophyll-a. This procedure ensured accurate measurement of the magnesium-isotopic ratios without isobaric interferences using a multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). We find a slight depletion in the heavier isotopes of magnesium in chlorophyll-a relative to culture medium, early growth phase: Δ 26Mg = −0.71(±0.35)‰ and Δ 25Mg = −0.37(±0.18)‰; late growth phase: Δ 26Mg = −0.53(±0.20)‰ and Δ 25Mg = −0.26(±0.11)‰, due to an apparent mass-dependent fractionation. We suggest that the small fractionation results from chelation during intracellular processes. A likely candidate for this chelation step involves the magnesium-chelatase enzyme, which mediates the insertion of magnesium to the tetrapyrrole ring during chlorophyll-a biosynthesis. Proof of this hypothesis can be tested with biological controls whereby steps in the enzymatic pathways of chlorophyll synthesis are selectively suppressed.

The Uncertainties in the22Ne+α‐Capture Reaction Rates and the Production of the Heavy Magnesium Isotopes in Asymptotic Giant Branch Stars of Intermediate Mass

We present new rates for the 22Ne(alpha, n)25Mg and 22Ne(alpha,gamma)26Mg reactions, with uncertainties that have been considerably reduced compared to previous estimates, and we study how these new rates affect the production of the heavy magnesium isotopes in models of intermediate mass Asymptotic Giant Branch (AGB) stars of different initial compositions. All the models have deep third dredge-up, hot bottom burning and mass loss. Calculations have been performed using the two most commonly used estimates of the 22Ne + alpha rates as well as the new recommended rates, and with combinations of their upper and lower limits. The main result of the present study is that with the new rates, uncertainties on the production of isotopes from Mg to P coming from the 22Ne + alpha-capture rates have been considerably reduced. We have therefore removed one of the important sources of uncertainty to effect models of AGB stars. We have studied the effects of varying the mass-loss rate on nucleosynthesis and discuss other uncertainties related to the physics employed in the computation of stellar structure, such as the modeling of convection, the inclusion of a partial mixing zone and the definition of convective borders. These uncertainties are found to be much larger than those coming from 22Ne + alpha-capture rates, when using our new estimates. Much effort is needed to improve the situation for AGB models.

Nucleosynthesis on the Asymptotic Giant Branch: A comparison between codes

We present calculations of the thermally pulsing asymptotic giant branch (TP-AGB) phase of stellar evolution made with the fully implicit STARS code. These are the first detailed evolution calculations of the AGB phase to be made in this manner. Prelimi- nary results suggest deeper third dredge-up at lower core masses than found in previous calculations without convective overshooting. We present a comparison between results computed with STARS and with the MSSSP code. We examine the physical differences between the models produced and assess their impact on the production of the heavy magnesium isotopes. Possible effects on the isotopic ratios of s-process elements are also discussed.

Adsorption and isotope effects by ion exchange with 1-aza-12-crown-4 bonded Merrifield peptide resin

Magnesium isotope effects were investigated by chemical ion exchange with synthesized 1-aza-12-crown-4 bonded Merrifield peptide resin using elution chromatography. The capacity of azacrown ion exchanger was 0.89 meq/g dry resin. The heavier isotopes of magnesium were enriched in the resin phase, while the lighter isotopes were enriched in the solution phase. The hydration effect is less than the complexation and isotope mass effects. The single stage separation factor was determined according to the method of Glueckauf from the elution curve and isotopic assays. The separation factors of 24Mg 2+– 25Mg 2+, 24Mg 2+– 26Mg 2+, and 25Mg 2+– 26Mg 2+ were 1.012, 1.023, and 1.011, respectively.

Production of Aluminium and the Heavy Magnesium Isotopes in Asymptotic Giant Branch Stars

AbstractWe investigate the production of aluminium and magnesium in asymptotic giant branch models covering a wide range in mass and composition. We evolve models from the pre-main sequence, through all intermediate stages, to near the end of the thermally-pulsing asymptotic giant branch phase. We then perform detailed nucleosynthesis calculations from which we determine the production of the magnesium and aluminium isotopes as a function of the stellar mass and composition. We present the stellar yields of sodium and the magnesium and aluminium isotopes. We discuss the abundance predictions from the stellar models in reference to abundance anomalies observed in globular cluster stars.

Adsorption and Isotope Effects by Ion Exchange with Aqueous-2-Aminomethyl-18-Crown-6 Bonded Merrifield Resin

Magnesium isotopes effects were investigated by chemical ion exchange using synthesized 2-aminomethyl-18-crown-6 (AM18C6) bonded Merrifield peptide resin. It was found that separation factors larger those reported previously were obtained, and the hydration and isotope mass effects are more significant than that of the complexation. The capacity of the crown ion exchanger was 2.3 meq/g dry resin. The adsorption capacity of the resin for magnesium ion was 26.8 mg/g dry resin at pH 7. The heavier isotopes of magnesium were enriched in the solution phase, while the lighter isotopes were enriched in the resin phase. The separation factors of 24Mg– 25Mg, 24Mg– 26Mg, and 25Mg– 26Mg were 1.0085, 1.0162, and 1.0081, respectively.

Magnesium isotope effect in the aqueous-monoazacrown bonded merrifield peptide resin

Separation of magnesium isotopes was investigated by chemical ion exchange with synthesized 1-aza-15-crown-5 bonded Merrifield peptide resin using elution chromatography. The capacity of novel monoazacrown ion exchanger was 2.3 meq g−1 dry resin. The heavier isotopes of magnesium are concentrated in the resin phase, while the lighter isotopes are enriched in the solution phase. The glass ion-exchange column used in our experiment was 35-cm long with an inner diameter of 0.2 cm, and 2.0 M NH4Cl solution was used as an eluent. The single-stage separation factor was determined according to the method of Glueckauf from the elution curve and isotopic assays. The separation factors of 24Mg2+–25Mg2+, 24Mg2+–26Mg2+, and 25Mg2+–26Mg2+ were 1.014, 1.026, and 1.012, respectively.

Magnesium isotope separation by ion exchange using hydrous manganese(IV) oxide

The magnesium isotope effects were investigated by chemical ion exchange with a hydrous manganese(IV) oxide. The capacity of manganese(IV) oxide was 0.5 meq g −1. The distribution coefficient of magnesium ions on the MnO 2 was determined by a batch method. The heavier isotopes of magnesium were enriched in the solution phase, while the lighter isotopes were enriched in the hydrous MnO 2 phase. The separation factor was determined according to the method of Glueckauf from the elution curve and isotopic assays. The separation factors of 24Mg 2+– 25Mg 2+, 24Mg 2+– 26Mg 2+, and 25Mg 2+– 26Mg 2+ isotope pair fractionations were 1.011, 1.021, and 1.011, respectively.

Syntheses and separating properties of triazacrown and AM18C6 bonded Merrifield peptide resins

Separation of lithium and magnesium isotopes by cation exchange elution chromatography was carried out with a synthesized 1,13,16-trioxa-4,7,10-triazacyclooctadecane (N 3O 3)-4,7,10-trimerrifield peptide resin and with a 2 ′-aminomethyl-18-crown-6 (AM18C6) bonded Merrifield peptide resin. The resins have a capacity of 0.1 and 2.3 meq/g dry resin. A single stage separation factor of lithium isotopes, 1.018 was obtained by the Glueckauf theory from the elution curve and isotopic assays. The heavier isotope, 7Li was concentrated in the resin phase, while the lighter isotope, 6Li concentrated in the solution phase. On the other hand, the heavier isotopes of magnesium were concentrated in the solution phase, while the lighter isotopes were concentrated in the resin phase. The separation factors of 24Mg– 25Mg, 24Mg– 26Mg, and 25Mg– 26Mg isotope pair fractionations were 1.012, 1.022, and 1.012, respectively.

Synthesis of AM15C5 Bonded Merrifield Peptide Resin and its Separating Properties

Abstract A 2-aminomethyl-15-crown-5 (AM15C5) bonded Merrifield peptide resin was synthesized. The capacity of this crown ion exchanger was 2.25 meq/g of dry resin. The heavier isotopes of magnesium were enriched in the solution phase, while the lighter isotopes were enriched in the resin phase. The hydration and isotope mass effects dominated over those of the complexation and properties of the exchanger. The separation factors of 24Mg2+/25Mg2+, 24Mg2+/26Mg2+, and 25Mg2+/26Mg2+ isotope pair fractionations were 1.0012, 1.0023, and 1.0011, respectively.

Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions: experiments, theoretical considerations, and constraints on their thermal evolution

Experiments exposing Type B calcium-, aluminum-rich inclusion (CAI)-like melts at high temperatures to high vacuum or reducing hydrogen-rich gas mixtures were used to determine the rates and consequences of elemental and isotopic fractionation by evaporation. Silicon and magnesium were found to evaporate much faster than calcium and aluminum, and the resulting residual liquid trajectories in composition space are reproduced via a thermodynamic model for the saturation vapor pressure of the evaporating species. Isotopic fractionations associated with evaporation were measured for magnesium. The resulting relationship between fraction of magnesium lost and enrichment of the residue in the heavy isotopes of magnesium follows a Rayleigh fractionation curve with a fractionation factor that is close to, but not exactly, the theoretically expected value. The rate of evaporation is found to be a strong function of temperature, oxygen fugacity, and melt composition, which can be understood and modeled in terms of the dependence of the saturation vapor pressures on these variables. The relationship between evaporation rate, which we measure, and calculated saturation vapor involves empirical evaporation coefficients that we find to be significantly less than one (∼0.1). Analytical and numerical models are used to characterize how diffusion in both the melt and in the surrounding gas affects evaporation rates and the degree of chemical and isotopic fractionation. The experimental data and theoretical considerations are combined to give a parameterization of the rates and consequences of evaporation of Type B CAI-like liquids, which is then used to translate the measured isotopic fractionation of Type B CAIs into constraints on their thermal history. Cooling rates of the order of 10°C per hour are indicated.

The enrichment of magnesium isotopes by AM15C5 bonded ion exchanger

Summary Magnesium isotope enrichment was investigated by chemical ion exchange with a synthesized 2′-aminomethyl-15-crown-5(AM15C5) bonded Merrifield peptide resin using elution chromatography. The capacity of the novel crown ion exchanger was found to be 2.3 meq/g dry resin. The heavier isotopes of magnesium were enriched in the solution phase, while the lighter isotopes were enriched in the resin phase. The separation factor was determined according to the method of Glueckauf from the elution curve and isotopic assays. The separation factors of 24Mg2+–25Mg2+, 24Mg2+–26Mg2+, and 25Mg2+–26Mg2+ isotope pair fractionations were 1.013, 1.024, and 1.012, respectively.

Separation of Magnesium Isotopes by Chemical Exchange with Manganese (IV) Oxide

Separation of magnesium isotopes was investigated by chemical ion exchange with manganese (IV) oxide using an elution chromatographic technique. The capacity of manganese (IV) oxide ion exchanger was 0.5 meq/g. The heavier isotopes of magnesium were concentrated in the manganese oxide phase, while the lighter isotopes were enriched in the solution phase. The glass ion exchange column used in our experiment was 35 cm long with inner diameter of 0.2 cm, and 1.0 M CH3COONH4 solution was used as an eluent. The single stage separation factor was determined according to the method of Glueckauf from the elution curve and isotopic assays. The separation factors of 24Mg2+-25Mg2+, 24Mg2+-26Mg2+, and 25Mg2+-26Mg2+ were 1.008, 1.014, and 1.006, respectively.