Isotopic fractionation of Mg2+(aq), Ca2+(aq), and Fe2+(aq) with carbonate minerals

Rustad, James R.; Casey, William H.; Yin, Qing‐Zhu; Bylaska, Eric J.; Felmy, Andrew R.; Bogatko, Stuart; Jackson, Virgil E.; Dixon, David A.

doi:10.1016/j.gca.2010.08.018

Cited by 191 publications

(234 citation statements)

References 75 publications

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“…4a). Values for calcite from theoretical calculations performed by Rustad et al (2010) on molecular clusters are higher than our calculations by $4‰ at 300 K. This is to be expected because the computational method is different, involving molecular clusters instead of crystal cell and the B3-LYP hybrid functional instead of PBE. For comparison, similar calculations of [Ca(H 2 …”

Section: Calciumcontrasting

confidence: 53%

“…A variety of isotopes and minerals have been investigated using first-principles methods based on density functional theory (DFT), such as stable isotopes O and C in carbonates (Schauble et al, 2006), and H, O and Si in silicates . Theoretical studies of calcium isotopic fractionation have also been conducted on sulfates (Griffith et al, 2008) and carbonates (Rustad et al, 2010;Wang et al, 2017). In addition, theoretical approaches make it possible to determine the isotopic fractionation properties of specific sites in the crystal structure (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Site-specific equilibrium isotopic fractionation of oxygen, carbon and calcium in apatite

Aufort

Ségalen

Gervais

et al. 2017

Geochimica et Cosmochimica Acta

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The stable isotope composition of biogenic apatite is an important geochemical marker that can record environmental parameters and is widely used to infer past climates, biomineralization processes, dietary preferences and habitat of vertebrates. In this study, theoretical equilibrium isotopic fractionation of oxygen, carbon and calcium in hydroxyapatite and carbonate-bearing hydroxyapatite is investigated using first-principles methods based on density-functional theory and compared to the theoretical isotopic fractionation properties of calcite, CO 2 and H 2 O. Considering the variability of apatite crystal-chemistry, special attention is given to specific contributions of crystal sites to isotopic fractionation. Significant internal fractionation is calculated for oxygen and carbon isotopes in CO 3 between the different structural sites occupied by carbonate groups in apatite (typically 7‰ for both 18 O/ 16 O and 13 C/ 12 C fractionation at 37°C). Compared with calcite-water oxygen isotope fractionation, occurrence of A-type substitution in apatite structure, in addition to the main B-type substitution, could explain the larger temperature dependence of oxygen isotope fractionation measured at low temperature between carbonate in apatite and water. Theoretical internal fractionation of oxygen isotopes between carbonate and phosphate in B-type carbonated apatite ($8‰ at 37°C) is consistent with experimental values obtained from modern and well-preserved fossil bio-apatites. Concerning calcium, theoretical results suggest a small fractionation between apatite and calcite (À0.17‰ at 37°C). Internal fractionation reaching 0.8‰ at 37°C occurs between the two Ca sites in hydroxyapatite. Furthermore, the Ca isotopic fractionation properties of apatite are affected by the occurrence of carbonate groups, which could contribute to the variability observed on natural samples. Owing to the complexity of apatite crystal-chemistry and in light of the theoretical results, measurements of site-specific isotopic fractionation properties could improve our understanding and the interpretation of isotopic records in apatites.

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Section: Calciumcontrasting

confidence: 53%

Section: Introductionmentioning

confidence: 99%

Site-specific equilibrium isotopic fractionation of oxygen, carbon and calcium in apatite

Aufort

Ségalen

Gervais

et al. 2017

Geochimica et Cosmochimica Acta

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“…Inasmuch as β factors obtained here for Mg and Ca are in good agreement with those obtained by more sophisticated ab initio methods (Rustad et al 2010), our electrostatic model may be considered as an alternative tool which allows to…”

Section: Discussionsupporting

confidence: 78%

“…It is seen that our values for lithium isotopes are close to the first set of results calculated by Yamaji et al (2001) or fall between their two sets. The same refers to β factors for magnesium and calcium isotopes, which are compared with results of Rustad et al (2010). The calculated 1000·ln(β) values as a function of temperature are listed in Table 5 and plotted in Fig.…”

Section: Resultsmentioning

confidence: 99%

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Reduced partition function ratios for isotopes of hydrated alkali and alkaline earth ions calculated by a simple electrostatic model

Czarnacki¹,

Hałas²

2011

Central European Geology

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] + (n = 3-18, M = Li, Mg, Ca, K) which enables to calculate ion vibration frequency of the ground state. In this model the considered ion with a reduced mass vibrates in quasispherical well formed by the ion-dipole attractive potential and repulsive valence potential, these simplifications allowed to solve one dimensional Schrödinger equation, whilst the calculated ground state was considered as one of triply degenerated state of the three dimensional motion of the ion vs. hydration shell. The reduced partition function ratios were calculated from the vibration frequencies using Urey's (1947) harmonic approximation formula.The results obtained in this way are in good agreement with those obtained by much more laborious ab initio molecular orbital methods, like SCF Hartree-Fock, DFT, MP2, etc. Moreover, we were able to extend calculations to hydrated Li and K ions surrounded with two shells of water molecules. These results are the first estimations of the upper limit of isotope fractionation in water solutions, which are 99.3‰ for Li and only 2.5‰ for K isotopes.

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Constraining dolomitization by Mg isotopes: A case study from partially dolomitized limestones of the middle Cambrian Xuzhuang Formation, North China

Yang

Shen

Lang

et al. 2016

Geochem Geophys Geosyst

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The “dolomite problem” refers to the rare dolomite formation in modern oceans that is in sharp contrast to the widespread ancient dolostone in rock record, as well as failure of laboratory inorganic dolomite precipitation at near Earth‐surface temperature. Novel Mg isotope systematics provides a promising tool in resolving the “dolomite problem”. Here, we develop a protocol to place constraints on the dolomitization process by using Mg isotopes. In this study, we measured Mg isotopic compositions ( δ26Mg) of two batches of partially dolomitized limestone samples from the middle Cambrian Xuzhuang Formation in North China. δ26Mg varies between −0.55‰ and −3.18‰, and shows a negative linear correlation with 1true[Mgtrue], suggesting that δ26Mg can be described by a binary mixing between the calcite and dolomite components. Mg isotopic composition of the dolomite component ( δ26Mgdol) for the lower sample set that is collected from a 4 m stratigraphic interval containing three high‐frequency ribbon rock‐packstone cycles is −1.6‰, while δ26Mgdol for the upper sample set (from a thick sequence of ribbon rock) is significantly higher (−0.3‰). However, neither mineralogical and elemental compositions, carbon and oxygen isotopes, nor crystal morphologies of dolomite provides diagnostic criteria to differentiate these two batches of samples. δ26Mgdol of the Xuzhuang limestone is simulated by the Advective Flow (AF) and the Diffusion‐Advection‐Reaction (DAR) models. The AF model assumes that Mg is transported by advective fluid flows, while the DAR model simulates a contemporaneous seawater dolomitization process, in which Mg is delivered by diffusion. The AF modeling result indicates that δ26Mg of the dolomitization fluid is +0.4‰ and +1.7‰ for the lower and upper sample sets, respectively. These values are significantly higher than modern and Cenozoic seawater Mg isotopic composition, suggesting that the dolomitization fluid is not contemporaneous seawater. The AF model also predicts spatially heterogeneous δ26Mgdol with progressive enrichment in 26Mg along the fluid flow pathway. In the DAR model, both dolomite content and δ26Mgdol of the lower sample set can be simulated by using seawater Mg isotopic composition of −0.75‰, thus contemporaneous seawater dolomitization may explain δ26Mgdol of the Xuzhuang limestone. Furthermore, the DAR model demonstrates spatially homogeneous δ26Mgdol. To differentiate the AF and DAR models, samples from multiple sections are required. Nevertheless, this study implies that Mg isotope might be a useful tool in the study of dolomitization.

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Isotopic fractionation of Mg2+(aq), Ca2+(aq), and Fe2+(aq) with carbonate minerals

Cited by 191 publications

References 75 publications

Site-specific equilibrium isotopic fractionation of oxygen, carbon and calcium in apatite

Site-specific equilibrium isotopic fractionation of oxygen, carbon and calcium in apatite

Reduced partition function ratios for isotopes of hydrated alkali and alkaline earth ions calculated by a simple electrostatic model

Constraining dolomitization by Mg isotopes: A case study from partially dolomitized limestones of the middle Cambrian Xuzhuang Formation, North China

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