First-principle partitioning and disequilibrium of chromium in garnet – clinopyroxene assemblage

2020 ◽  
Author(s):  
Benoit Dubacq ◽  
Sarah Figowy ◽  
Yves Noël ◽  
Philippe D'Arco
Keyword(s):  
Ab Initio ◽  
Mineral Composition ◽  
Partition Coefficients ◽  
Thermodynamic Description ◽  
Element Distribution ◽  
Trace Element Distribution ◽  
Mineral Assemblages ◽  
Predicted Values ◽  
Trivalent Element

<p><span>Using partition coefficients is extremely useful to model melting processes and fluid-rock interactions. However, partition coefficients values remain scarce in regard of their sensitivity to mineral composition and to the variability of mineral composition. In addition, the inferred equilibrium between phases is not necessarily reached, even in high-grade metamorphic conditions associated to melting. Disequilibrium may dramatically hamper the effective mobility of species and lead to element distribution far from the predicted values.</span></p><p><span>This contribution aims at estimating partition coefficients for chromium (Cr) between garnet and clinopyroxene, and testing them in natural rocks of various metamorphic grades. As a poorly mobile trivalent element, Cr is chosen as a proxy to rare earth elements.</span></p><p><span>Theoretical partition coefficients for Cr between garnet and clinopyroxene are calculated </span><span><em>ab initio</em></span><span> from structures where Cr</span><sup><span>3+</span></sup><span> is modelled as a defect in Al</span><sup><span>3+</span></sup><span> sites using CRYSTAL17 (Dovesi et al., 2014) and the thermodynamic description of Dubacq and Plunder (2018). Results are compared to electron microprobe measurements in mineral assemblages containing tens to thousands of ppm of Cr, where element mapping brings much information. </span></p><p><span>Results of </span><span><em>ab initio</em></span><span> computations highlight the role of crystal-chemistry over the strain field around point defects, controlling the dynamics of the Cr</span><sup><span>3+</span></sup><span> = Al</span><sup><span>3+ </span></sup><span>exchange between clinopyroxene and garnet. As expected, the partitioning of Cr between garnet and clinopyroxene depends strongly on the grossular and pyrope content: Cr incorporates grossular preferentially to jadeite, but jadeite incorporates Cr preferentially to pyrope.</span></p><p><span>Comparison between predicted and measured partition coefficients allowed to estimate the deviation from equilibrium. Disequilibrium is evidenced even for samples metamorphosed around 850°C, as shown by the distribution of Cr-rich and Cr-depleted domains. Disequilibrium is attributed to slow diffusivity of Cr in fluid and at grain boundaries during crystal growth, leading to interface-coupled dissolution-precipitation.</span></p><p><span></span></p><p align="justify"><span>Dovesi, R., Orlando, R., Erba, A., Zicovich‐Wilson, C. M., Civalleri, B., Casassa, S., ... & </span><span>Noël, Y. (2014). CRYSTAL14: A program for the ab initio investigation of crystalline solids. </span><span><em>International Journal of Quantum Chemistry</em></span><span>, </span><span><em>114</em></span><span>(19), 1287-1317.</span></p><p><span>Dubacq, B., & Plunder, A. (2018). Controls on trace element distribution in oxides and silicates. </span><span><em>Journal of Petrology</em></span><span>, </span><span><em>59</em></span><span>(2), 233-256.</span></p>

scholarly journals Simultaneous cryo X-ray ptychographic and fluorescence microscopy of green algae

2015 ◽  
Vol 112 (8) ◽  
pp. 2314-2319 ◽  
Author(s):  
Junjing Deng ◽  
David J. Vine ◽  
Si Chen ◽  
Youssef S. G. Nashed ◽  
Qiaoling Jin ◽  
...  
Keyword(s):  
Trace Elements ◽  
Fluorescence Microscopy ◽  
Green Algae ◽  
Element Distribution ◽  
Trace Element Distribution ◽  
X Rays ◽  
Binding Affinities ◽  
Reconstruction Algorithms ◽  
X Ray

Trace metals play important roles in normal and in disease-causing biological functions. X-ray fluorescence microscopy reveals trace elements with no dependence on binding affinities (unlike with visible light fluorophores) and with improved sensitivity relative to electron probes. However, X-ray fluorescence is not very sensitive for showing the light elements that comprise the majority of cellular material. Here we show that X-ray ptychography can be combined with fluorescence to image both cellular structure and trace element distribution in frozen-hydrated cells at cryogenic temperatures, with high structural and chemical fidelity. Ptychographic reconstruction algorithms deliver phase and absorption contrast images at a resolution beyond that of the illuminating lens or beam size. Using 5.2-keV X-rays, we have obtained sub–30-nm resolution structural images and ∼90-nm–resolution fluorescence images of several elements in frozen-hydrated green algae. This combined approach offers a way to study the role of trace elements in their structural context.

scholarly journals Partition coefficients of trace elements between carbonates and melt and suprasolidus phase relation of Ca-Mg-carbonates at 6 GPa

2020 ◽  
Vol 105 (6) ◽  
pp. 922-931 ◽  
Author(s):  
Melanie J. Sieber ◽  
Franziska Wilke ◽  
Monika Koch-Müller
Keyword(s):  
Trace Elements ◽  
Rare Earth ◽  
Rare Earth Elements ◽  
Partial Melting ◽  
Partition Coefficients ◽  
Divalent Cations ◽  
Major And Trace Elements ◽  
Element Distribution ◽  
Phase Relations ◽  
Trace Element Distribution

Abstract The presence of Ca-Mg-carbonates affects the melting and phase relations of peridotites and eclogites in the mantle, and (partial) melting of carbonates liberates carbon from the mantle to shallower depths. The onset and composition of incipient melting of carbonated peridotites and carbonated eclogites are influenced by the pure CaCO3-MgCO3-system making the understanding of the phase relations of Ca-Mg-carbonates fundamental in assessing carbon fluxes in the mantle. By performing high-pressure and high-temperature experiments, this study clarifies the suprasolidus phase relations of the nominally anhydrous CaCO3-MgCO3-system at 6 GPa showing that Ca-Mg-carbonates will (partially) melt for temperatures above ~1300 °C. A comparison with data from thermodynamic modeling confirms the experimental results. Furthermore, partition coefficients for Li, Na, K, Sr, Ba, Nb, Y, and rare earth elements between calcite and dolomitic melt, Ca-magnesite and dolomitic melt, and magnesite and dolomitic melt are established. Experiments were performed at 6 GPa and between 1350 to 1600 °C utilizing a rotating multi-anvil press. Rotation of the multi-anvil press is indispensable to establish equilibrium between solids and carbonate liquid. Major and trace elements were quantified with EPMA and LA-ICP-MS, respectively. The melting temperature and phase relations of Ca-Mg-carbonates depend on the Mg/Ca-ratio. For instance, Ca-rich carbonates with a molar Mg/(Mg+Ca)-ratio (XMg) of 0.2 will transform into a dolomitic melt (XMg = 0.33–0.31) and calcite crystals (XMg = 0.19–0.14) at 1350–1440 °C. Partial melting of Mg-rich carbonates (XMg = 0.85) will produce a dolomitic melt (XMg = 0.5–0.8) and Ca-bearing magnesite (XMg = 0.89–0.96) at 1400–1600 °C. Trace element distribution into calcite and magnesite seems to follow lattice constraints for divalent cations. For instance, the compatibility of calcite (XMg = 0.14–0.19) for Sr and Ba decreases as the cation radii increases. Ca-Mg-carbonates are incompatible for rare earth elements (REEs), whereby the distribution between carbonates and dolomitic melt depends on the Mg/Ca ratio and temperature. For instance, at 1600 °C, partition coefficients between magnesite (XMg = 0.96) and dolomitic melt (XMg = 0.8) vary by two orders of magnitudes from 0.001 to 0.1 for light-REEs to heavy-REEs. In contrast, partition coefficients of REEs (and Sr, Ba, Nb, and Y) between magnesite (XMg = 0.89) and dolomitic melt (XMg = 0.5) are more uniform scattering marginal between ~0.1–0.2 at 1400 °C.

Phase relations of Ca-Mg-carbonates and trace element partition coefficients between carbonates and dolomitic melt at 6 and 9 GPa

2020 ◽  
Author(s):  
Melanie Sieber ◽  
Franziska D.H. Wilke ◽  
Hans-Josef Reichmann ◽  
Monika Koch-Müller
Keyword(s):  
Trace Element ◽  
Partial Melting ◽  
Partition Coefficients ◽  
Divalent Cations ◽  
Thermal Structure ◽  
Melting Behavior ◽  
Element Distribution ◽  
Phase Relations ◽  
Trace Element Distribution ◽  
Melt Composition

<p>Calcite (CaCO<sub>3</sub>) and magnesite (MgCO<sub>3</sub>) are among the most common carbonates on Earth. The presence of Ca‑Mg‑carbonates in the mantle affects the melting and phase relations of peridotites and eclogites and (partial) melting of carbonates liberates carbon from the mantle to shallower depths. The onset of melting and the incipient melt composition of carbonated peridotites and carbonated eclogites are influenced by the pure CaCO<sub>3</sub>‑MgCO<sub>3</sub>‑system. Thus, a deeper insight into the phase relations and melting behavior of the CaCO<sub>3</sub>‑MgCO<sub>3</sub>‑system is crucial to better understand the carbon cycle in the Earth’s mantle.</p><p>We performed quenched multi-anvil experiments at 6 and 9 GPa to (a) examine the phase relations of the nominally anhydrous CaCO<sub>3</sub>‑MgCO<sub>3</sub>‑system and to (b) establish partition coefficients for Li, Na, K, Sr, Ba, Nb, Y and rare earth elements (REEs) between carbonates and dolomitic melt. We used a rotating multi-anvil press to overcome quenching problems as observed in previous studies. Rotation of the multi‑anvil press is, additionally, indispensable to establish equilibrium between solid carbonates and dolomitic melt.</p><p>The melting temperature and phase relations of Ca‑Mg‑carbonates depend on the Mg/Ca‑ratio. For instance, at 6 GPa Ca‑rich carbonates with a molar Mg/(Mg+Ca)‑ratio (<em>X</em><sub>Mg</sub>) of 0.2 will transform into a dolomitic melt (<em>X</em><sub>Mg</sub>=0.33‑0.31) and calcite crystals (<em>X</em><sub>Mg</sub>=0.19‑0.14) at 1350‑1440 ℃. Partial melting of Mg‑rich carbonates (<em>X</em><sub>Mg</sub>=0.85) will produce a dolomitic melt (<em>X</em><sub>Mg</sub>=0.5‑0.8) and Ca‑bearing magnesite (<em>X</em><sub>Mg</sub>=0.89‑0.96) at 1400‑1600 ℃. Trace element distribution into Ca-Mg-carbonates depends on <em>X</em><sub>Mg</sub>, temperature and seems to follow lattice constraints for divalent cations. Partition coefficients of REEs between magnesite (Ca<sub>0.11</sub>Mg<sub>0.89</sub>CO<sub>3</sub>) and dolomitic melt (Ca<sub>0.5</sub>Mg<sub>0.5</sub>CO<sub>3</sub>) at 6 GPa and 1400 ℃ are uniform scattering marginal between 0.1‑0.2. The partition coefficient of Lu (D=0.1) is unmodified to lower Ca-content in magnesite (Ca<sub>0.04</sub>Mg<sub>0.96</sub>CO<sub>3</sub>) and higher temperature (1600 ℃), but the partition coefficients between such Ca-poor magnesite and dolomitic melt (Ca<sub>0.2</sub>Mg<sub>0.8</sub>CO<sub>3</sub>) decrease continuously from heavy-REEs to light‑REEs from 0.1 to 0.001, respectively.</p><p>Our findings have important implications for the cycle of carbon and trace elements in the mantle because Ca‑Mg‑carbonates will (partially) melt at 6 GPa and temperatures above ~1300 ℃ producing a dolomitic melt. Consequently, CO<sub>2</sub> will be liberated by partial melting of an upwelling carbonated mantle at a depth of ~200 km considering the thermal structure of the upper mantle. The results also affirm that carbonates are stable in the subducting slab even for hot subduction zone geothermal gradients unless carbonate-bearing lithologies in the slab are infiltrated by aqueous fluids.</p>

Methylmercury and Trace Element Distribution in the Organs of Stenella coeruleoalba Dolphins Stranded on the French Mediterranean Coast

2014 ◽  
Vol 8 (1) ◽  
pp. 35-48 ◽  
Author(s):  
Emmanuel Wafo ◽  
Véronique Risoul ◽  
Thérèse Schembri ◽  
Véronique Lagadec ◽  
Frank Dhermain ◽  
...  
Keyword(s):  
Trace Elements ◽  
Trace Element ◽  
Marine Pollution ◽  
Element Distribution ◽  
Mediterranean Coast ◽  
Trace Element Distribution ◽  
Slight Reduction ◽  
Stenella Coeruleoalba ◽  
Contamination Levels

The main objective of this study was to evaluate the contamination by mercury (Hg), methylmercury (Me-Hg), cadmium (Cd), selenium (Se), zinc (Zn), copper (Cu), iron (Fe) and manganese (Mn) in dolphins stranded on the French Mediterranean coast. The distributions of these contaminants in the organs of dolphins have also been studied. Overall, contamination levels varied according to the following sequence: liver > kidney > lung > muscle, except for cadmium (kidney > liver > lung > muscle). Size and sex of animals were also considered. Young dolphins were less impacted with trace elements than adults, except for copper. Among the studied parameters, the most important appeared to be the size of mammals. In addition, in the case of mercury and selenium, the sex of mammals seemed to be also relevant. The correlations between the concentrations of trace elements suggest the existence of detoxification processes. Since 1990s, using dolphins for tracing marine pollution, a slight reduction in the burden of the considered trace elements could be noted.

TRACE-ELEMENT DISTRIBUTION IN GIANT SPODUMENE CRYSTALS IN GRANITIC PEGMATITES FROM THE BLACK HILLS, SOUTH DAKOTA AND NEWRY, MAINE

2019 ◽  
Author(s):  
Stephan R. Hlohowskyj ◽  
◽  
Mona-Liza C. Sirbescu ◽  
James J. Student ◽  
Niels Hulsbosch ◽  
...  
Keyword(s):  
South Dakota ◽  
Trace Element ◽  
Element Distribution ◽  
Black Hills ◽  
Trace Element Distribution ◽  
Granitic Pegmatites
Sign in / Sign up

Export Citation Format

Share Document