Equilibrium isotopic fractionation of copper during oxidation/reduction, aqueous complexation and ore-forming processes: Predictions from hybrid density functional theory
Equilibrium isotopic fractionation of copper during oxidation/reduction, aqueous complexation and ore-forming processes: Predictions from hybrid density functional theory
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DOI：
10.1016/j.gca.2013.04.030
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发表时间：
2013-10
影响因子：
5
通讯作者：
D. Sherman
中科院分区：
文献类型：
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作者：
D. Sherman
铜以两种同位素存在：65Cu（~（63）Cu）（~（63）Cu）（~（69）Cu）（~（63）Cu）（~（69）Cu）（~（65）Cu）（~（69）Cu）（~（69）Cu）（~（63）Cu）（~（69）Cu）（~（63）Cu）（~（69）Cu）（~（63）Cu）（~（69）Cu）（~（69）Cu）（~（63）Cu）（~（69）Cu）（~（69）Cu）（~（63）Cu）（~（63）Cu）（~（69）Cu）（~（69）Cu）（~（63）Cu）（~（63）Cu）（~（63）（~（69）Cu）（~（63）Cu）（~（63）Cu）（~（63）Cu）（~（63）Cu）（~次生矿物、地表沃茨和含氧地下沃茨中铜的同位素组成比原生硫化物重1 - 12 ‰。氧化态和络合的变化应产生大量的铜物种之间的同位素分馏，但目前还不清楚在何种程度上所观察到的铜同位素的变化反映平衡分馏。本文采用周期密度泛函理论和分子杂化密度泛函理论计算了黄铜矿（CuFeS2）、赤铜矿（Cu2O）、碲铜矿（CuO）和水溶液中Cu+、Cu +2络合物的约化分配函数比，预测了Cu+氧化成Cu +2和溶解态Cu络合的平衡同位素分馏。在水环境中的各种铜（II）配合物中，有一个显着的（1.3 ‰）范围内的约化分配函数比。黄铜矿（CuFeS2）氧化和同成分溶解成溶解态Cu +2（以Cu（H2O）5 + 2计）产生65 - 63 δ（Cu +2-CuFeS2）= 3.1 ‰;但是，在此情况下，黄铜矿的氧化/溶解过程是不一致的，因此，所观察到的同位素分馏作用较小，而赤铜矿（Cu2O）的二次沉淀则使溶解态65 Cu在65 - 63 δ之间进一步富集（Cu +2-Cu2O）为1.2 ‰。然而，铜的沉淀将有利于重同位素+1.0 ‰，使溶解的Cu同位素变轻。这些是平衡分馏的上限估计值。因此，在表生环境中所观察到的溶解态Cu +2（或Cu +2矿物）与原生Cu+硫化物之间的巨大分馏（9 ‰）必然反映了瑞利分馏（开放体系）或动力学分馏。最后，先前提出的（Asael等人，2009）使用黄铜矿中的δ 65Cu来估计在层状沉积物容矿铜矿中运输Cu的流体的氧化态。
Copper exists as two isotopes:65Cu (∼30.85%) and63Cu (∼69.15%). The isotopic composition of copper in secondary minerals, surface waters and oxic groundwaters is 1–12‰ heavier than that of copper in primary sulfides. Changes in oxidation state and complexation should yield substantial isotopic fractionation between copper species but it is unclear to what extent the observed Cu isotopic variations reflect equilibrium fractionation. Here, I calculate the reduced partition function ratios for chalcopyrite (CuFeS2), cuprite (Cu2O), tenorite (CuO) and aqueous Cu+, Cu+2complexes using periodic and molecular hybrid density functional theory to predict the equilibrium isotopic fractionation of Cu resulting from oxidation of Cu+to Cu+2and by complexation of dissolved Cu. Among the various copper(II) complexes in aqueous environments, there is a significant (1.3‰) range in the reduced partition function ratios. Oxidation and congruent dissolution of chalcopyrite (CuFeS2) to dissolved Cu+2(as Cu(H2O)5+2) yields65–63δ(Cu+2–CuFeS2) = 3.1‰ at 25 °C; however, chalcopyrite oxidation/dissolution is incongruent so that the observed isotopic fractionation will be less. Secondary precipitation of cuprite (Cu2O) would yield further enrichment of dissolved65Cu since65–63δ(Cu+2–Cu2O) is 1.2‰ at 25 °C. However, precipitation of tenorite (CuO) will favor the heavy isotope by +1.0‰ making dissolved Cu isotopically lighter. These are upper-limit estimates for equilibrium fractionation. Therefore, the extremely large (9‰) fractionations between dissolved Cu+2(or Cu+2minerals) and primary Cu+sulfides observed in supergene environments must reflect Rayleigh (open-system) or kinetic fractionation. Finally the previously proposed (Asael et al., 2009) use of δ65Cu in chalcopyrite to estimate the oxidation state of fluids that transported Cu in stratiform sediment-hosted copper deposits is refined.