scholarly journals Bromine Isotope Variations in Magmatic and Hydrothermal Sodalite and Tugtupite and the Estimation of Br Isotope Fractionation between Melt and Sodalite

Minerals ◽  
2021 ◽  
Vol 11 (4) ◽  
pp. 370
Author(s):  
Hans G. M. Eggenkamp ◽  
Michael A. W. Marks ◽  
Pascale Louvat ◽  
Gregor Markl
Keyword(s):  
Late Stage ◽  
Isotope Fractionation ◽  
Isotope Composition ◽  
Fractionation Factor ◽  
Data Set ◽  
Geothermal Fluids ◽  
A Value ◽  
Isotope Fractionation Factor ◽  
Very High ◽  
Fractionation Factors

We determined the bromine isotope compositions of magmatic and hydrothermal sodalite (Na8Al6Si6O24Cl2) and tugtupite (Na8Al2Be2Si8O24Cl2) from the Ilímaussaq intrusion in South Greenland, in order to constrain the Br isotope composition of the melt and hydrothermal fluids from which these minerals were formed. Early formed magmatic sodalite has high Br contents (138 ± 10 µg/g, n = 5) and low δ81Br values (+0.23 ± 0.07‰). Late stage hydrothermal sodalite has lower Br contents (53±10 µg/g, n = 5) and higher δ81Br values (+0.36 ± 0.08‰). Tugtupite that forms at even later stages shows the lowest Br contents (26 ± 2 µg/g, n = 2) and the highest δ81Br values (+0.71 ± 0.17‰). One hydrothermal sodalite has a Br concentration of 48 ± 9 µg/g and an exceptionally high δ81Br of 0.82 ± 0.12‰, very similar to the δ81Br of tugtupites. We suggest that this may be a very late stage sodalite that possibly formed under Be deficient conditions. The data set suggests that sodalite crystallises with a negative Br isotope fractionation factor, which means that the sodalite has a more negative δ81Br than the melt, of −0.3 to −0.4‰ from the melt. This leads to a value of +0.5 to +0.6‰ relative to SMOB for the melt from which sodalite crystallises. This value is similar to a recently published δ81Br value of +0.7‰ for very deep geothermal fluids with very high R/Ra He isotope ratios, presumably derived from the mantle. During crystallisation of later stage hydrothermal sodalite and the Be mineral tugtupite, δ81Br of the residual fluids (both melt and hydrothermal fluid) increases as light 79Br crystallises in the sodalite and tugtupite. This results in increasing δ81Br values of later stage minerals that crystallise with comparable fractionation factors from a fluid with increasingly higher δ81Br values.

scholarly journals Measurement on Diffusion Coefficients and Isotope Fractionation Factors by a Through-Diffusion Experiment

Minerals ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 208
Author(s):  
Takuma Hasegawa ◽  
Kotaro Nakata ◽  
Rhys Gwynne
Keyword(s):  
Groundwater Flow ◽  
Diffusion Coefficients ◽  
Isotope Fractionation ◽  
Free Water ◽  
Effective Porosity ◽  
Diffusion Experiment ◽  
Fractionation Factor ◽  
Through Diffusion ◽  
Geological Formations ◽  
Fractionation Factors

For radioactive waste disposal, it is important that local groundwater flow is slow as groundwater flow is the main transport medium for radioactive nuclides in geological formations. When the groundwater flow is very slow, diffusion is the dominant transport mechanism (diffusion-dominant domain). Key pieces of evidence indicating a diffusion-dominant domain are the separation of components and the fractionation of isotopes by diffusion. To prove this, it is necessary to investigate the different diffusion coefficients for each component and the related stable isotope fractionation factors. Thus, in this study, through-diffusion and effective-porosity experiments were conducted on selected artificial materials and natural rocks. We also undertook measurements relating to the isotope fractionation factors of Cl and Br isotopes for natural samples. For natural rock samples, the diffusion coefficients of water isotopes (HDO and H218O) were three to four times higher than those of monovalent anions (Cl−, Br- and NO3−), and the isotope fractionation factor of 37Cl (1.0017–1.0021) was slightly higher than that of free water. It was experimentally confirmed that the isotope fractionation factor of 81Br was approximately 1.0007–1.0010, which is equivalent to that of free water. The enrichment factor of 81Br was almost half that of 37Cl. The effective porosity ratios of HDO and Cl were slightly different, but the difference was not significant compared to the ratio of their diffusion coefficients. As a result, component separation was dominated by diffusion. For artificial samples, the diffusion coefficients and effective porosities of HDO and Cl were almost the same; it was thus difficult to assess the component separation by diffusion. However, isotope fractionation of Cl and Br was confirmed using a through-diffusion experiment. The results show that HDO and Cl separation and isotope fractionation of Cl and Br can be expected in diffusion-dominant domains in geological formations.

scholarly journals Technical note: Lithium isotopes in dolostone as a palaeo-environmental proxy – an experimental approach

2019 ◽  
Vol 15 (2) ◽  
pp. 635-646 ◽  
Author(s):  
Holly L. Taylor ◽  
Isaac J. Kell Duivestein ◽  
Juraj Farkas ◽  
Martin Dietzel ◽  
Anthony Dosseto
Keyword(s):  
Isotope Fractionation ◽  
Isotope Composition ◽  
Relative Proportion ◽  
Significant Proportion ◽  
Technical Note ◽  
Fractionation Factor ◽  
Calcium Carbonates ◽  
Marine Carbonates ◽  
Li Isotope ◽  
Environmental Proxy

Abstract. Lithium (Li) isotopes in marine carbonates have considerable potential as a proxy to constrain past changes in silicate weathering fluxes and improve our understanding of Earth's climate. To date the majority of Li isotope studies on marine carbonates have focussed on calcium carbonates. The determination of the Li isotope fractionation between dolomite and a dolomitizing fluid would allow us to extend investigations to deep times (i.e. Precambrian) when dolostones were the most abundant marine carbonate archives. Dolostones often contain a significant proportion of detrital silicate material, which dominates the Li budget; thus, pretreatment needs to be designed so that only the isotope composition of the carbonate-associated Li is measured. This study aims to serve two main goals: (1) to determine the Li isotope fractionation between Ca–Mg carbonates and solution, and (2) to develop a method for leaching the carbonate-associated Li out of dolostone while not affecting the Li contained within the detrital portion of the rock. We synthesized Ca–Mg carbonates at high temperatures (150 to 220 ∘C) and measured the Li isotope composition (δ7Li) of the precipitated solids and their respective reactive solutions. The relationship of the Li isotope fractionation factor with temperature was obtained: 103ln⁡αprec-sol=-(2.56±0.27)106(1)/T2+(5.8±1.3) Competitive nucleation and growth between dolomite and magnesite were observed during the experiments; however, there was no notable effect of their relative proportion on the apparent Li isotope fractionation. We found that Li isotope fractionation between the precipitated solid and solution is higher for Ca–Mg carbonates than for Ca carbonates. If the temperature of a precipitating solution is known or can be estimated independently, the above equation could be used in conjunction with the Li isotope composition of dolostones to derive the composition of the solution and hence make inferences about the past Li cycle. In addition, we also conducted leaching experiments on a Neoproterozoic dolostone and a Holocene coral. Results show that leaching with 0.05 M hydrochloric acid (HCl) or 0.5 % acetic acid (HAc) at room temperature for 60 min releases Li from the carbonate fraction without a significant contribution of Li from the siliciclastic detrital component. These experimental and analytical developments provide a basis for the use of Li isotopes in dolostones as a palaeo-environmental proxy, which will contribute to further advance our understanding of the evolution of Earth's surface environments.

scholarly journals Hydrogen isotope fractionation in the photolysis of formaldehyde

2008 ◽  
Vol 8 (5) ◽  
pp. 1353-1366 ◽  
Author(s):  
T. S. Rhee ◽  
C. A. M. Brenninkmeijer ◽  
T. Röckmann
Keyword(s):  
Isotope Ratio ◽  
Isotope Fractionation ◽  
Isotopic Fractionation ◽  
Photochemical Oxidation ◽  
Atmospheric Conditions ◽  
Isotopic Enrichment ◽  
Fractionation Factor ◽  
Removal Processes ◽  
Molecular Channel ◽  
Fractionation Factors

Abstract. Experiments investigating the isotopic fractionation in the formation of H2 by the photolysis of CH2O under tropospheric conditions are reported and discussed. The deuterium (D) depletion in the H2 produced is 500(±20)‰ with respect to the parent CH2O. We also observed that complete photolysis of CH2O under atmospheric conditions produces H2 that has virtually the same isotope ratio as that of the parent CH2O. These findings imply that there must be a very strong concomitant isotopic enrichment in the radical channel (CH2O+hν → CHO+H) as compared to the molecular channel (CH2O+hν → H2+CO) of the photolysis of CH2O in order to balance the relatively small isotopic fractionation in the competing reaction of CH2O with OH. Using a 1-box photochemistry model we calculated the isotopic fractionation factor for the radical channel to be 0.22(±0.08), which is equivalent to a 780(±80)‰ enrichment in D of the remaining CH2O. When CH2O is in photochemical steady state, the isotope ratio of the H2 produced is determined not only by the isotopic fractionation occurring during the photolytical production of H2 (αm) but also by overall fractionation for the removal processes of CH2O (αf), and is represented by the ratio of αm/αf. Applying the isotopic fractionation factors relevant to CH2O photolysis obtained in the present study to the troposphere, the ratio of αm/αf varies from ~0.8 to ~1.2 depending on the fraction of CH2O that reacts with OH and that produces H2. This range of αm/αf can render the H2 produced from the photochemical oxidation of CH4 to be enriched in D (with respect to the original CH4) by the factor of 1.2–1.3 as anticipated in the literature.

Sign in / Sign up

Export Citation Format

Share Document