Magnesium Isotope Fractionation Research Articles

Magnesium isotopic fractionation during post-serpentinization alteration: Implications for arc and oceanic Mg cycles

In recent years, numerous geodynamic studies have concentrated on serpentinites and the serpentinization processes that occur within subduction zones. However, as an important host phase for Mg, whether Mg isotopic fractionation occurs during peridotite serpentinization is still unclear. In our study, we conducted high-precision Mg isotope analyses on serpentinized peridotites and forearc serpentinites from the Mariana subduction zone. Except for one sample affected by strong late weathering with clay formation, which had an anomalously high δ26Mg value (0.01 ± 0.04‰), the moderately serpentinized peridotites (−0.24 ± 0.04‰, 2SD, n = 3) and deep (>40 mbsf) serpentinites (−0.24 ± 0.02‰, 2SD, n = 3) have Mg isotopic compositions consistent with mantle peridotites (−0.25 ± 0.04‰). Shallow serpentinites display slightly higher δ26Mg values (−0.19 ± 0.04‰, 2SD, n = 15) than mantle peridotites, and these values tend to increase toward the seafloor. These high δ26Mg values of forearc serpentinites cannot be attributed to serpentinizing fluid metasomatism and mantle wedge serpentinization processes, while post-serpentinization alteration involving brucite dissolution played a dominant role. Rayleigh fractionation simulations suggest that the heavy Mg isotopes are preferentially allocated to serpentinites during the post-serpentinization alteration process, where the calculated ∆26Mgbrucite-fluid varies from 0.1‰ to 0.8‰. This study deduces that the process of serpentinite dehydration in the subarc mantle cannot significantly change the Mg isotopic composition of arc lavas because serpentinite itself has a mantle-like Mg isotopic composition. Moreover, the fluids released by the brucite dissolution process have a low δ26Mg value, making an important contribution to the Mg budget of modern ocean seawater.

Magnesium isotope fractionation during natural travertine deposition from Baishuitai, SW China

Travertine samples deposited in Earth's surface environments can be used as an effective archive for paleo-climatic reconstruction. As a common element in carbonates, magnesium (Mg) and its isotopic composition in travertine could provide useful information for evaluating paleo-environment changes. In this study, we investigate the Mg isotope systematics in both endogenic travertines (mainly calcite) and spring/stream waters at Baishuitai, Yunnan, SW China. Our results show a systematic increase in δ26Mg value from −1.37 to −1.26‰ for water samples downstream, but varied δ26Mg values between −4.12 and −3.95‰ (average −4.02‰) for solid carbonates, thus a corresponding fractionation Δ26Mgcalcite-water between −2.76 to −2.59‰ (mean value of −2.69‰). Therefore, the solid carbonates preferentially incorporate light Mg isotopes during travertine formation. More interestingly, the Mg distribution coefficient (KMg/Ca) between travertine and water exhibits two variation trends with the calcite deposition rate (Rp) along the canal, which can be explained by the change of calcite formation mechanism from direct nucleation to precipitation via amorphous calcium carbonate (ACC) intermediate. In the upper-stream, the direct nucleation of calcite results in the rapid incorporation of Mg ions into crystal lattice, while a relatively slow precipitation of calcite downstream would incorporate Mg via ACC formation pathway in a quasi-equilibrium pattern. This is consistent with the grain size distribution and crystal morphology observed under SEM. Our results show the important control of water Mg/Ca ratios on the calcite precipitation during travertine formation, and imply the potential and complexity of using Mg isotopes of travertine deposits to reconstruct paleo-environments.

No biological effect on magnesium isotope fractionation during stromatolite growth

The growth and morphology of stromatolites have been previously linked to diverse microbial activities. However, the role of microbial metabolisms on carbonate formation during stromatolite growth remains controversial. Magnesium isotopes have been proposed to serve as a tracer of microbial carbonate formation, implying a potential biological isotope fractionation. To further elucidate whether Mg isotope fractionation is modified during microbial carbonate formation, this study reports Mg isotope compositions of Holocene stromatolites and pore waters from Lagoa Salgada, a coastal ephemeral lake in Brazil. The stromatolitic carbonates are composed of high-Mg calcites characterized by extremely positive inorganic δ13C values, up to +20‰, and variable δ26Mg values, ranging from −2.98‰ to −0.68‰. Multiple pieces of evidence consistently demonstrate changes in microbial metabolism resulting from ecosystem fluid chemistry evolution. However, the direction of Mg isotope fractionation associated with carbonate precipitation remained invariable despite the changes in microbial activity. Mineralogical features indicate that the stromatolitic carbonates formed via 2-D nucleation. An isotopic mass balance calculation based on the observed variations in δ26Mg values and Mg concentrations of associated pore waters argues for an Mg isotopic equilibrium fractionation factor of −2.60‰ associating with the carbonate formation at Lagoa Salgada, well-matching experimental values for abiotic calcite precipitation. Thus, the observed δ26Mg variability of stromatolitic carbonates is primarily controlled by the changes of physicochemical conditions in the ambient fluid. We infer that during the formation of stromatolites, a consortium of different microbial communities produces extracellular polymeric substances, which serve as a substrate for carbonate nucleation from ambient fluid without significantly affecting the fractionation of Mg-isotopes. Our findings shed light on the effects of microbial processes on carbonate formation during stromatolite growth and improve the current understanding of the Mg isotope record in microbial carbonates.

Magnesium isotope fractionation in differentiation of mafic-alkaline-carbonatitic magma and Fe-P-REE-rich melt at Bayan Obo, China

The Bayan Obo area in the North China Craton (NCC) contains the largest rare earth element (REE) resources in the world. The key to this gigantic and efficient REE accumulation has been the focal point of century-long research efforts. Intense differentiation of mantle-derived alkaline magmas is envisaged as a highly viable process for REE enrichment based on the first discovery of mafic-alkaline-carbonatitic suite in the Bayan Obo region. Geochronological and Sr-Nd-Hf-O isotopic analyses revealed that these rocks derived from a common mantle source and evolved with magmatic differentiation at 1.31 Ga to 1.32 Ga. Elemental geochemistry and Mg isotopes in this magma series (alkaline diabase sill, syenite dyke and carbonatite) unambiguously support the differentiation model. The earliest ferrocarbonatite, with nearly undifferentiated REE concentrations, has mantle-like δ26Mg (−0.27 ± 0.03‰), and therefore can be considered as the original product of the mantle derived alkaline magma at Bayan Obo. The subsequent magnesiocarbonatites have a LREE enrichment feature, which is consistent with the alkaline diabase sills. The magnesiocarbonatites possess light Mg isotopic composition (from − 0.42‰ to − 0.50‰), whereas the alkaline diabase sills have heavy Mg isotopic composition (from − 0.03‰ to − 0.05‰). This rock assemblage should present two end members, which differentiated likely by immiscibility in the chamber from a common alkaline magma. The subsequent syenite dyke has no obvious Mg isotopic variation (from − 0.44‰ to − 0.39‰) during differentiation from the above alkaline magma. Notably, as the most important REE hosts, fine-grained ferrocarbonatites show large Mg isotopic variations (from − 0.85‰ to + 0.13‰) and present positive correlations with total MgO and CaO contents, but negative with total Fe2O3, P2O5 and REE contents. In fine-grained ferrocarbonatite, closely associated apatite, monazite, hematite and magnetite minerals occur as a special intergrowth texture. This unique paragenetic assemblage is likely the mineralization record of a Fe-P-REE-rich melt, which shows particular affinity of hydrothermal fluid and light Mg isotope in carbonatitic magma. By contrast, magnesian siderite phenocrysts as the early cumulates in carbonatitic magma are enriched in heavy Mg isotope (from − 0.07‰ to + 0.21‰). Immiscibility of Fe-P-REE-rich melt and fractional crystallization of carbonate minerals are responsible for large Mg isotope fractionation and giant REE enrichment in the Bayan Obo deposit.

Magnesium isotope fractionation between calcite and aqueous solutions under elevated temperatures of 98–170 °C

The incomplete understanding of Mg isotope fractionation behavior between calcite and aqueous solutions has limited the interpretation and application of Mg isotope data in natural carbonates. In this study, we performed a series of aragonite-to-calcite conversion experiments under hydrothermal conditions to investigate the effects of temperature and organic ligands on elemental and isotopic partitioning of Mg between calcite and aqueous solutions. The experiments produced magnesian calcite via a dissolution-and-precipitation process. The Mg distribution coefficients between solid and aqueous solution (DMg, 0.08 to 0.21) showed negative correlations with Sr distribution coefficients (DSr, 0.04 to 0.48), as well as the proportion of calcite in the bulk solid, which indicates mixing of calcite (Mg-rich and Sr-poor) and aragonite (Sr-rich and Mg-poor) in the solid phases. The apparent Mg isotope fractionation factors between solid and aqueous solutions (Δ26Mgsolid-soln) show a positive correlation with reaction temperatures, where Δ26Mgsolid-soln increases from ca. −1.84 ‰ at 98 °C to −1.35 ‰ at 170 °C. In addition, the presence of oxalate contributed to slightly lower Δ26Mgsolid-soln values, which is interpreted to be related to the isotopic effect of Mg-oxalate speciation. Based on the experimental results, we propose a temperature-dependent function for Mg isotope fractionation between calcite and free Mg-aquo ions (i.e., Mg(H2O)62+): Δ26Mgcal-Mg(aq) = (−0.17 ± 0.01) × 106/T2- (0.52 ± 0.08). This equation is applicable to a wide range of temperatures, especially at hydrothermal conditions, thus enabling interpretations of Mg isotope data from calcite of hydrothermal and burial origin.

Determination of the equilibrium magnesium isotope fractionation factors between brucite and aqueous inorganic and organic species

Magnesium (Mg) is a major element in seawater, rock-forming minerals, and biological systems. Stable Mg isotopes fractionate during silicate weathering and carbonate mineralization, and hence are a promising tool to trace these processes. Magnesium can be present in natural aqueous solutions as a number of distinct inorganic and organic complexes including MgHCO3+, MgCO30, MgSO40, Mg(OH)+, Mg(citrate)− and Mg(EDTA)2−, in addition to Mg(H2O)62+ commonly referred to as Mg2+. The formation of these species can significantly alter the fractionation of Mg isotopes between minerals and natural fluids. To quantify these effects, isotope exchange experiments were performed at bulk chemical equilibrium between brucite and aqueous solutions containing different organic (citrate, EDTA) and inorganic (SO4−) ligands at 25 °C. The ‘three isotope’ method was used to determine the equilibrium Mg isotope fractionation factors Δeq26Mg between brucite and several major aqueous magnesium species. The experimentally measured equilibrium Mg isotope fractionation factor between brucite and aqueous Mg2+ was found to be Δeq26Mgbrucite-Mg2+ = -0.35 ± 0.39‰. First-principle calculations to retvieve the brucite β-factor were performed consistently with the calculations of Pinilla et al. (2015) for Mg2+(aq) β-factor. The combination of both studies yield values of Δeq26Mgbrucite-Mg2+ between +0.3 and +0.8 ± 1.0‰, which is the lowest theoretical estimate of this constant obtained to date. An average value Δeq26Mgbrucite-MgSO40 = 0.48 ± 0.16‰ was retrieved from the experiments for the isotope fractionation between brucite and aqueous MgSO40. Mg isotope equilibrium fractionation factors between brucite and aqueous Mg(citrate)− and between brucite and aqueous Mg(EDTA)2− retrieved from the experiments performed in the presence of these organic ligands are Δeq26Mgbrucite-Mg(citrate)− = 0.35 ± 0.21, and Δeq26Mgbrucite-Mg(EDTA)2− = 2.41 ± 0.20‰.The experimental values determined in this study for Δeq26Mgbrucite-Mg2+ agree with the experimental values reported by Li et al. (2014). There is also an excellent agreement between the experimental values of this study and Li et al. (2014) with the density functional theory (DFT) estimates from Schott et al. (2016) for Δeq26MgMg2+-Mg(EDTA)2−. In contrast, the Mg isotope fractionation factor measured in this study between aqueous Mg2+ and both aqueous Mg sulphate or citrate species is significantly smaller than predictions from the ab initio calculations reported by Schott et al. (2016). The results of the present study confirm that the mineral-fluid equilibrium fractionation of Mg isotopes is strongly dependent on the identity of the inorganic or organic ligands present in the aqueous fluid and the nature of the complexes, (e.g. inner-sphere versus outer-sphere complexes), formed by magnesium with these ligands. Therefore, Mg speciation in natural fluids and the structure of aqueous Mg complexes have to be known for an accurate interpretation of Mg isotopic signatures in natural environments.

Isotopic Composition and Fractionation of Stable Magnesium Isotopes in Relation to Fruits Growing in Different Regions of Russia

Background: Present study on determining the region of origin of raw materials, the authenticity of organic food products is relevant. The determination of the natural content of stable isotopes are not able to answer these questions. The solution can be obtained by studying the ratios and fractionation of stable isotopes, which provide information on the transformations of chemical elements in biological, ecological and geochemical studies. Methods: During 2020-2021 obtained data on the content of magnesium isotopes in apple fruits of 14 varieties from three growing regions of the Russian Federation. Previously, over 10 years, a database was formed of more than 500 samples on the level of potassium, calcium, magnesium, phosphorus and nitrogen cations, depending on the variety and phase of fruit development. Result: Research presents the results of determining the natural content of stable magnesium isotopes in apples, the isotopic composition and their fixation, depending on different regions of the growth of raw materials. The isotopic composition of apples (26Mg/24Mg) varies from 0.1669047 to 0.1782649; the deviation of the magnesium isotopic composition from the conventional standard varies within 25.52-43.52; their chemical fractionation has value: in Central Zone of Russia-185.2-212.3; South of Russia-248.0-311.5; North Caucasus-182.7-214.8.

Magnesium isotope fractionation during carbonate precipitation associated with bacteria and extracellular polymeric substances

Magnesium (Mg) is abundant on Earth's surface aquatic environments and plays an important role during the precipitation of biogenic CaCO3 minerals. Moreover, Mg isotopes are increasingly used to study a number of bio-induced mineralization processes of calcite. In this study, Mg isotope signatures during calcite and aragonite precipitation in the presence of Curvibacter sp. HJ-1 and of their extracellular polymeric substances (EPS) were investigated and the evolution of mineralogical and liquid composition were tracked over time. The results showed that light Mg isotopes were preferentially incorporated into the precipitated solids in all the experiments with and without bacteria. Significant Mg isotope fractionation was associated with the transformation of amorphous calcium carbonate (ACC) to crystal carbonate (Δ26Mgcrystals-ACC = −0.9‰ and −0.7‰ in biotic and EPS experiments, respectively), and obvious Mg isotope fractionation was noted between solids and liquid (Δ26Mgsolid-liquid achieved −2.2‰ and −2.0‰ in biotic and EPS experiments, respectively). Besides, δ26Mg values were significantly correlated with pH, Mg content in liquid and solid, bacterial density and presence of EPS in biotic experiments. These findings indicated that strain HJ-1 had an effect on Mg isotope fractionation during calcite and aragonite precipitation. The difference in Mg isotope fractionation may be a new tool for understanding the biologically mediated effects on Mg-bearing carbonate precipitation, and serves as a useful alternative for aqueous Mg isotope, which is vital for reconstructing past environmental changes.

Magnesium Isotope Fractionation During Silicate Weathering: Constrains From Riverine Mg Isotopic Composition in the Southeastern Coastal Region of China

AbstractMg isotopic composition of river water is dominantly controlled by the dissolution of both silicate and carbonate sources and a series of biogeochemical processes. However, the relative importance of source and isotopic fractionation control at basin/global scale is poorly constrained. This study presents the Mg isotopic composition of river water and suspended load in river draining silicate rocks in the southeastern coastal region of China. The fractionation effect of silicate weathering on Mg isotopes is documented in both dissolved and solid phases. Mg isotopic composition of rivers draining silicate rocks exhibit ∼0.3‰ δ26Mg difference, the release of Mg from Mg‐rich minerals and formation of clays are the dominant processes controlling Mg isotopic composition of river water. The variation of Mg isotopic compositions of suspended load is closely related to the species of secondary clays (illite and chlorite); the fractionation direction during illite and chlorite formation contrasts, isotopically heavy Mg preferentially incorporates into illite while light Mg incorporates into chlorite. Furthermore, the negative correlation between 1/Mg and Mg isotopic compositions of river water indicates the Mg re‐distribution and isotope fractionation between weathering solutions and secondary clays during silicate weathering. Such isotope fractionation‐induced Mg isotope variations could be employed to estimate the contribution of Mg from silicate weathering at basin/continental scale.

Magnesium isotope fractionation during alkaline brine evaporation and implications for Precambrian seawater chemistry

The chemical evolution of the ocean is one major component of the puzzle of how climate and life have co-evolved over the Earth's history. A “soda ocean” with high alkalinity, high pH, and low calcium concentration in Precambrian has been proposed to explain the emergence and evolution of early life. However, this hypothesis has not been widely accepted due to the lack of reliable tracers for the chemical composition of Precambrian seawater. Evaporite is formed during seawater/brine evaporation and thus has been widely used to reconstruct the ancient seawater/brine chemical composition. Here, evaporation experiments were conducted using Qinghai Lake (QHL) water, a modern soda lake, to provide an analogy of the Precambrian “soda ocean” evaporation and investigate the mineralogical and Mg isotopic signatures of alkaline brine-derived evaporites. Our results show that the evaporation path of QHL water overall covers the stages of hydrous Mg carbonates (hydromagnesite), halite, and bloedite [Na2Mg(SO4)2·4H2O] precipitation. The precipitation of hydrous Mg carbonates and bloedite is distinct from the modern seawater evaporation and is accompanied by the removal of up to 85% Mg from brine. The brine gradually becomes enriched in heavy Mg isotopes during evaporation due to the preferential incorporation of light Mg isotopes into precipitates. The fractionation of Mg isotopes is dominantly controlled by the bond structure during hydrous Mg carbonates and bloedite precipitation, and the latter is also slightly influenced by the kinetics in the highly concentrated brine. The Mg isotope fractionation during bloedite dissolution is limited due to the rapid congruent dissolution. The significant Mg isotopic fractionation observed within the hydrous Mg carbonates precipitation during the earliest QHL water evaporation indicates the potential Rayleigh distillation of Mg by alkalinity in the “soda ocean”, which is absent in the Phanerozoic oceans. Therefore, the large Mg isotope fractionation during alkaline brine evaporation can be applied to test the existence of Precambrian “soda oceans”.

Magnesium isotopic fractionation during basalt differentiation as recorded by evolved magmas

Magnesium isotopic fractionation during basalt differentiation is commonly thought to be negligible at current levels of analytical precision. However, Mg isotopic variation of ∼0.4‰ (δ26Mg) can be observed in some highly evolved volcanic rocks with MgO contents of <5 wt.%, suggesting that detectable Mg isotopic fractionation may occur during late-stage basalt differentiation. Here we examine this possibility with a Mg isotopic study of a suite of well-characterized cogenetic alkaline volcanic rocks from St. Helena Island (South Atlantic), which vary from primitive nepheline-normative basalt to highly evolved trachyandesite with MgO contents decreasing from 15.72 wt.% to 0.81 wt.%. Our results show that the basalt samples which only experienced segregation of olivine and clinopyroxene have a narrow δ26Mg range (−0.23‰ to −0.32‰), while the evolved rocks saturated with Fe–Ti oxides (MgO < 5 wt.%) display larger Mg isotopic variation with δ26Mg vary to higher (−0.14‰) or lower (−0.36‰) values relative to mantle value (δ26Mg=−0.25±0.04‰). For most of the Fe–Ti oxide saturated samples, their δ26Mg values are positively correlated with TiO2 contents and Ti/Ti* ratios and negatively correlated with total-alkali contents. This indicates that detectable Mg isotopic fractionation occurred in MgO-poor samples, probably through fractional crystallization of Mg-bearing Fe–Ti oxides. Further Mg isotopic analysis of Fe–Ti oxide phenocrysts (titanomagnetite) in the evolved samples reveals that titanomagnetite has remarkably higher δ26Mg value (+0.15‰ to +0.52‰) than the corresponding bulk sample and silicate minerals. Fractional crystallization of such isotopically heavy titanomagnetite is thus expected to drive the residual magma progressively enriched in light Mg isotopes. In order to reproduce the observed Mg isotopic variation in St. Helena evolved samples, quantitative modeling requires a titanomagnetite-melt Mg isotopic fractionation factor (Δ26MgTi-Mgt−melt) of ∼0.6 ‰, which is among our measured δ26Mg difference value (0.41–0.73‰) between titanomagnetite and bulk samples. When combining our results with published data for alkaline lavas from the Antipodes volcano (New Zealand), contrasting patterns of Mg isotope fractionation emerge during late-stage basalt differentiation. Quantitative modeling shows that the positive and negative δ26Mg–MgO correlations shown by evolved lavas could be related to segregation of isotopically heavy titanomagnetite and isotopically light ilmenite, respectively, depending on redox state of the magma system. Therefore, this study highlights that basalt differentiation involving separation of Fe–Ti oxides may induce resolvable Mg isotopic fractionation, and the δ26Mg values of compositionally evolved rocks should be used with caution in studies of petrogenesis.

Magnesium and carbon isotope fractionation during hydrated Mg-carbonate mineral phase transformations

The fractionation of carbon and magnesium isotopes is a potentially useful tracer of natural weathering in ultramafic catchments and engineered CO2 storage. To evaluate the use of carbon and magnesium isotopes as tracers of ultramafic weathering and CO2 storage, we assessed the carbon and magnesium isotope fractionation between hydrous Mg-carbonate minerals and fluid during a mineral phase transformation from nesquehonite [MgCO3·3H2O] to dypingite [Mg5(CO3)4(OH)2· ∼ 5–8H2O], two common products of ultramafic rock weathering. Batch reactor experiments containing nesquehonite were conducted at 5 °C, 25 °C, and 35 °C and the evolution of mineralogical composition, fluid composition, and isotopic composition were tracked over time. At 5 °C, the solid remained nesquehonite throughout the experiments, and isotopic equilibrium did not appear to be achieved between the solid and the fluid phase for either carbon or magnesium. At 25 °C, and 35 °C a transformation from nesquehonite to dypingite occurred by dissolution and re-precipitation, which resulted in extensive exchange of Mg and C between solid and fluid. The phase transformation caused the initial C and Mg isotopic composition of the solid phase to be overwritten. The extensive isotopic exchange during the phase transformation suggests C and Mg isotopes likely obtained approximate isotopic equilibrium between dypingite and fluid. For dypingite, the Δ13Cdyp-DIC was 4.74 ± 0.12‰ (VPDB) and 4.47 ± 0.17‰ (VPDB) at 25 and 35 °C, respectively. The Δmeas26Mgdyp-fluid between solid and the bulk fluid was −0.76‰, and −0.98 ± 0.08‰ for the 25 and 35 °C experiments, respectively. There was no clear impact of temperature on Mg or C isotope fractionation. The calculated Δcalc26Mgdyp-Mg2+ between dypingite and the Mg2+ aquo species rather than bulk aqueous Mg values were positive. This indicates that if dypingite is formed by the incorporation of the free Mg2+ ion in the solid, the solid is preferentially enriched in the isotope of higher mass (26Mg). This is opposite to anhydrous Mg-bearing carbonate minerals, which tend to be depleted in 26Mg relative to the forming fluid. These data will help improve interpretation of carbon and magnesium isotope compositions measured in natural and engineered ultramafic weathering environments, and may help to trace the fate of anthropogenic CO2 during engineered CO2 storage efforts.

Phyllosilicate controls on magnesium isotopic fractionation during weathering of granites: Implications for continental weathering and riverine system

Continental weathering is a fundamental process in releasing magnesium (Mg) from crystalline rocks to the hydrosphere and biosphere. Mg isotopes can be substantially mobilized, re-distributed, and fractionated during weathering, and therefore can be used as a powerful tool to trace the biogeochemical cycle of Mg. Causes of significant Mg isotopic fractionation and behaviors during silicate weathering are still not well understood, hindering further application of the Mg isotopes to probe different geological processes. In this study, we demonstrate that dissolution and formation of phyllosilicates are the main control of Mg isotopic fractionation during sub-tropical weathering of granite. Furthermore, different formation and dissolution mechanisms for the same mineral phase could also cause variations in magnitude and directionality of fractionation. In incipient weathering, supergene phyllosilicates form mainly through topotactic transformation. Vermiculitization of parental chlorite tends to release 24Mg and causes significant 26Mg enrichment in the saprock. In an advanced stage of weathering, Mg isotopic compositions of supergene phyllosilicates are more influenced by the interaction with the soil solutions. Minerals formed mainly through a dissolution-precipitation mechanism with Mg in neoformed phyllosilicates dominantly sourced from the contemporary soil solutions. 26Mg would be firstly incorporated into neoformed phyllosilicates, such as vermiculite, interstratified biotite/vermiculite and chlorite/vermiculite. Therefore, soil solutions became more enriched in 24Mg with depth in the pedolith, from which relatively 24Mg-rich phyllosilicates would form. However, in the saprolite, precipitation of illite may have preferentially scavenged 24Mg, enriching the soil solutions with 26Mg. Varying relative abundances of different phyllosilicate minerals along the profile could cause large variations in the Mg isotopic compositions of regolith. Our study shows that Mg isotopic composition of the slightly weathered materials could be significantly heavy. Hence, entrainment of 26Mg-rich but slightly weathered materials could be an alternative to explain the high δ26Mg as recorded in some sedimentary rocks, especially of aeolian source. Whereas low δ26Mg widely archived in groundwater and river water could be alternatively explained by interaction with the saprock and 26Mg scavenging during phyllosilicate transformation, instead of severe depletion of 26Mg in soil solutions due to intense weathering and vast formation of secondary minerals, as previously suggested. Comprehensive characterization of the weathering processes and the resultant products is essential to interpret the observed Mg isotopic fractionation and trace the biogeochemical cycle of Mg.

Compositional and pressure controls on calcium and magnesium isotope fractionation in magmatic systems

Stable isotope fractionation in magmatic systems depends on equilibrium isotope fractionation between different phases. However, current equilibrium stable isotope theory mostly assumes ideal crystal structures and simple chemical compositions, but it is unclear how the pressure and complex compositional variations in natural microscopic mineral structures affect inter-mineral stable isotope fractionation and thus control stable isotope fractionation in macroscopic magmatic systems. Here, we calculate the CaO and MgO bond lengths controlled by pressure and compositional variations of coexisting garnet (Grt) and clinopyroxene (Cpx) in the Roberts Victor eclogites from the Kaapvaal Craton and use these data as a proof of concept to interpret their inter-mineral Ca and Mg isotopic compositions (Δ44/40CaGrt-Cpx and Δ26MgGrt-Cpx). Our results show that the CaO difference between Grt and Cpx (ΔCaOGrt-Cpx) shows a significant increase with CaO content from 3.4 to 13.6 wt.% in Grt and with pressure from 2.9 to 6.9 GPa. ΔCaOGrt-Cpx has an excellent negative correlation with inter-mineral Ca isotope fractionation corrected for temperature effect (Δ44/40CaGrt-Cpx × T2/106), indicating that inter-mineral Ca isotope fractionation is controlled by pressure and compositional variations of the Grt through effects on the bond lengths. Inter-mineral Mg isotope fractionation corrected for temperature effect (Δ26MgGrt-Cpx × T2/106) in these eclogites shows a negative correlation with pressure but no obvious correlations with the mineral compositions, suggesting the dominant role of pressure effect in addition to temperature. The MgO bond length of Grt increases by about 0.02 Å with increasing CaO content in Grt of these eclogites, implying a mild compositional effect on inter-mineral Mg isotope fractionation. The results suggest that pressure and compositional variations in minerals control the equilibrium stable isotope fractionation between minerals. Utilizing the Ca isotope fractionation factor controlled by crystal chemistry of garnet, our modelling indicates that partial melting of eclogite in the mantle could not significantly fractionate Ca isotopes and, therefore, that low δ44/40Ca values in previously reported basalts cannot be attributed to the involvement of eclogite in their sources. This suggests that crystal chemistry exerts major controls on isotope fractionation in magmatic systems in addition to temperature.

Magnesium isotope fractionation reflects plant response to magnesium deficiency in magnesium uptake and allocation: a greenhouse study with wheat

AimsMagnesium (Mg) deficiency is detrimental to plant growth. However, how plants respond to Mg deficiency via regulation of Mg uptake and allocation is yet not fully understood. In this study, we tested whether Mg isotope compositions (δ26Mg) associated with Mg mass balance of the plants could be used as an indicator to trace Mg uptake and subsequent translocation processes under sufficient and low-Mg supply conditions. We aimed at using stable isotope fractionation as a novel proxy for nutrient uptake and cycling in plants.MethodsWe grew wheat plants (Triticum aestivum) in a greenhouse under control (1 mM Mg) and low-Mg supply (0.05 mM Mg) conditions, respectively. The Mg concentrations and isotope compositions in roots, stems, leaves and spikes/grains at different growth stages were analyzed.ResultsWheat plants were systematically enriched in heavy Mg isotopes relative to the nutrient solution regardless of Mg supply conditions. With crop growth, the δ26Mg of the whole plants, as well as each plant organ, gradually shifted towards higher values in the control. However, the δ26Mg value of the whole plants in the low-Mg supply did not vary significantly. In addition, the wheat stems and spikes showed continuous enrichment of lighter Mg isotopes in the low-Mg supply than those in the control.ConclusionsAs reflected from Mg isotope compositions, the Mg supply in the growth media could affect the Mg uptake and subsequent translocation processes in plants. Changes in δ26Mg indicated that wheat plants likely regulated their Mg uptake strategy by switching between active and passive pathways during their life cycle. When Mg supply was low, a more negative δ26Mg value of the spikes suggested a potentially enhanced remobilization of Mg from leaves to spikes. Our results showed that Mg stable isotopes can provide new insights into plants’ response to nutrient shortage.

Experimental constraints on magnesium isotope fractionation during abiogenic calcite precipitation at room temperature

Magnesium (Mg) isotopes in carbonate minerals are a useful proxy for paleoclimate studies, but interpretations are often limited by an inadequate understanding of the various factors controlling Mg isotopic fractionation during carbonate formation. Previous work has studied a number of parameters including aqueous chemistry, mineralogy, temperature, and precipitation rate. However, little is known about the impact of solid/solution ratio, calcite growth mechanism, and crystal morphology on isotope fractionation. In this work, two groups of seeded chemo-stat calcite precipitation experiments were conducted at 25 °C to explore the potential impact of crystal growth and morphology on the fractionation of Mg isotopes. Group-1 experiments (G1) contained nine individual runs that were performed under identical physicochemical conditions, except for solid/solution ratio and the length of an experiment. The isotope fractionation between precipitated calcite and aqueous solution is limited, with Δ26Mgcal-sol ranging from −2.58 to −2.40‰ and an average of −2.49 ± 0.12‰ (2SD, n = 9). The Group-2 experiments (G2) contained 3 paired runs with solution Mg/Ca molar ratios of 0.5, 2.0, and 5.0, and yielded Δ26Mgcal-sol values that ranged from −2.69 to −2.36‰ with an average of −2.62 ± 0.25‰ (2SD, n = 6).The average Δ26Mgcal-sol value for both sets of experiments is −2.54 ± 0.22‰ (2SD, n = 15), and it is independent of precipitation rate, solution Mg/Ca molar ratio, solid/solution ratio, amount of overgrowth, and mol% Mg content in overgrowth. The form and texture of the calcite overgrowths in our experiments range from {104} rhombohedra with smooth crystal faces containing few macrosteps to {104} rhombohedra containing extensive evidence for 2-D nucleation on crystal faces, to more steeply sided rhombohedra {0kl}. While significant changes in crystal morphology are related to solid/solution ratio and solution composition in the G1 and G2 experiments, respectively, there was no difference in Mg isotope systematics, suggesting that crystal morphology does not affect the Mg isotopic composition of calcite within the range of features investigated and 2-D nucleation may be less affected by calcite growth kinetics than a spiral growth mechanism. Integrating our results with previous published values, an equilibrium isotopic fractionation factor of −2.47 ± 0.09‰ (weighted average ± weighted 2SD, n = 70) between calcite and aqueous solutions is derived at room temperature.

Magnesium-Isotope Fractionation in Chlorophyll-a Extracted from Two Plants with Different Pathways of Carbon Fixation (C3, C4).

Relatively few studies have been focused so far on magnesium–isotope fractionation during plant growth, element uptake from soil, root-to-leaves transport and during chlorophylls biosynthesis. In this work, maize and garden cress were hydroponically grown in identical conditions in order to examine if the carbon fixation pathway (C4, C3, respectively) might have impact on Mg-isotope fractionation in chlorophyll-a. The pigment was purified from plants extracts by preparative reversed phase chromatography, and its identity was confirmed by high-resolution mass spectrometry. The green parts of plants and chlorophyll-a fractions were acid-digested and submitted to ion chromatography coupled through desolvation system to multiple collector inductively coupled plasma-mass spectrometry. Clear preference for heavy Mg-isotopes was found in maize green parts (∆26Mgplant-nutrient 0.65, 0.74 for two biological replicates, respectively) and in chlorophyll-a (∆26Mgchlorophyll-plant 1.51, 2.19). In garden cress, heavy isotopes were depleted in green parts (∆26Mgplant-nutrient (−0.87)–(−0.92)) and the preference for heavy isotopes in chlorophyll-a was less marked relative to maize (∆26Mgchlorophyll-plant 0.55–0.52). The observed effect might be ascribed to overall higher production of energy in form of adenosine triphosphate (ATP), required for carbon fixation in C4 compared to C3, which could reduce kinetic barrier and make equilibrium fractionation prevailing during magnesium incorporation to protoporphyrin ring.

Magnesium isotope fractionation during hydrothermal seawater-basalt interaction

Fluid-rock interactions in hydrothermal systems at or near mid-oceanic ridges (MOR) play a major role in determining the composition of the oceanic crust and seawater. To quantify the processes that govern cation exchange in these environments we have experimentally studied the isotopic evolution of δ26/24Mg in the fluid phase during seawater-basalt interaction at 250 and 290 °C. Mass balance constraints indicate that isotopically heavy Mg was preferentially incorporated into non-exchangeable (octahedral) sites in secondary clay minerals such as saponite (Mg-rich smectite), leaving residual fluids enriched in light Mg isotopes. The magnitude of fractionation observed during smectite precipitation in our experiments (εSmectite-Liquid26/24) ranged from 0.35‰ to 0.42‰. This observation, which contrasts with the preferential uptake of light Mg isotopes into biogenic and inorganic marine carbonates, highlights the potential utility of Mg isotopes as tracers of the precipitation dynamics of authigenic Mg-silicate and Mg-carbonate phases. Furthermore, although Mg isotopic fractionation is often masked by the almost complete removal of Mg in high temperature marine hydrothermal systems, our experiments demonstrate that it does become significant at lower temperatures where Mg removal by clay formation is incomplete. Under such conditions, this fractionation will create isotopically light fluids due to smectite precipitation, thus potentially represents an important component of the marine Mg isotope inventory.

지표환경에서의 마그네슘 동위원소 분별

최근 분석법 및 질량분석기의 발달로 인해 비전통 안정동위원소의 지구과학적 활용이 두드러지지만, 이에 대한 국내에서의 활용은 미진한 실정이다. 마그네슘 동위원소는 전 지구적 탄소순환, 해수 화학, 양분순환(nutrient cycling)을 이해하는 지시자로 활용될 수 있으며, 이를 위해서는 Mg 순환과정 중 발생하는 동위원소분별에 대한 이해가 선행되어야 한다. 따라서 본 논평에서는 지표환경에서 마그네슘 동위원소 분별을 일으키는 요인(주요 광물 용해, 이차광물 생성, 흡착, 식물 섭취 및 탄산염 광물 침전)에 대하여 소개하고자 한다. 먼저 마그네슘 동위원소의 개념, 질량분석기를 활용한 분석법을 살펴보고 마지막으로 최근 연구결과를 통한 지표환경에서 마그네슘 동위원소 분별을 일으키는 요인 및 기작에 대하여 살펴보고자 한다.

Experimental constraints on Mg isotope fractionation during clay formation: Implications for the global biogeochemical cycle of Mg

The direction and magnitude of magnesium (Mg) isotope fractionation attendant to the formation of clay minerals is fundamental to the use of Mg isotopes to decipher the biogeochemical cycling of Mg in the critical zone and for the oceanic Mg budget. This study provides experimental data on the Mg fractionation factor for two smectite-group minerals (stevensite and saponite) at temperatures relevant for Earth surface processes. The resultant solids were characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) to confirm the mineralogy and crystallinity of the product. A series of experiments were performed to asses the impact of temperature and pH on isotope fractionation. Bulk solid samples were treated with ammonium chloride to remove exchangeable Mg in order to distinguish the Mg isotopic fractionation between these sites and octahedral sites.All bulk and residual solids were enriched in 24Mg compared to the initial solution and δ26Mg values of the exchangeable pool were lower than, or within error of, the initial solution. Final solutions were either within error of, or enriched in, 26Mg compared to the initial solution, depending on the fraction of Mg removed from solution (fMg). For experiments with small or negligible fMg, increasing the pH resulted in a higher reaction rate and reduced fractionation from the initial solution. This could point to a kinetic effect, but the composition of the residual solid (Mg/(Li+Mg) ratio) was also dependent on pH. The change in the composition was reflected in the wavenumber of the Mg3-OH stretch in FT-IR data, which is a proxy for bond strength, and suggests an equilibrium control. An equilibrium control is further supported by the observation of reduced fractionation compared to the initial solution with increasing temperature. Rayleigh and batch fractionation models were fitted to the data giving fractionation factors of 0.9991 and 0.9990 respectively.We compare our results with existing field and experimental data and suggest that the apparent contradictions surrounding the direction of Mg isotope fractionation into phyllosilicate minerals could be due to the similarity of Mg bond lengths between clay octahedral sites and dissolved Mg. Thus small changes in mineral structure or initial solution conditions may result in a change in bond length sufficient to alter the direction of fractionation, implying that the magnitude and direction of Mg isotope fractionation into clay minerals could be dependent on local field conditions. Alternatively, if the precipitation of secondary clay minerals in the field preferentially incorporates light Mg, as observed in this experimental study, this implies the contribution of carbonate weathering to dissolved Mg fluxes has been underestimated, with major implications for the global biogeochemical cycle of Mg.