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首页> 外文期刊>Contributions to Mineralogy and Petrology >Oxygen isotope exchange and disequilibrium between calcite and tremolite in the absence and presence of an experimental C–O–H fluid
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Oxygen isotope exchange and disequilibrium between calcite and tremolite in the absence and presence of an experimental C–O–H fluid

机译：在不存在和存在实验性C–O–H流体的情况下，方解石与透闪石之间的氧同位素交换和不平衡

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Oxygen isotope exchange between minerals during metamorphism can occur in either the presence or the absence of aqueous fluids. Oxygen isotope partitioning among minerals and fluid is governed by both chemical and isotopic equilibria during these processes, which progress by intragranular and intergranular diffusion as well as by surface reactions. We have carried out isotope exchange experiments in two- and three-phase systems, respectively, between calcite and tremolite at high temperatures and pressures. The two-phase system experiments were conducted without fluid either at 1 GPa and 680 °C for 7 days or at 500 MPa and 560 °C for 20 days. Extrapolated equilibrium fractionations between calcite and tremolite are significantly lower than existing empirical estimates and experimental determinations in the presence of small amounts of fluid, but closely match calculated fractionations by means of the increment method for framework oxygen in tremolite. The small fractionations measured in the direct calcite–tremolite exchange experiments are interpreted by different rates of oxygen isotope exchange between hydroxyl oxygen, framework oxygen and calcite during the solid–solid reactions where significant recrystallization occurs. The three-phase system experiments were accomplished in the presence of a large amount of fluid (CO2+H2O) at 500 MPa and 560 °C under conditions of phase equilibrium for 5, 10, 20, 40, 80, 120, 160, and 200 days. The results show that oxygen isotope exchange between minerals and fluid proceeds in two stages: first, through a mechanism of dissolution-recrystallization and very rapidly; second, through a mechanism of diffusion and very slowly. Synthetic calcite shows a greater rate of isotopic exchange with fluid than natural calcite in the first stage. The rate of oxygen diffusion in calcite is approximately equal to or slightly greater than that in tremolite in the second stage. A calculation using available diffusion coefficients for calcite suggests that grain boundary diffusion, rather than volume diffusion, has been the dominant mechanism of oxygen transport between the fluid and the mineral grains in the later stage.
机译：在变质过程中，矿物之间的氧同位素交换可以在存在或不存在含水流体的情况下发生。在这些过程中，矿物和流体之间的氧同位素分配受化学和同位素平衡的支配，其通过颗粒内和颗粒间扩散以及表面反应来进行。我们分别在高温和高压下在方解石和透闪石之间的两相和三相系统中进行了同位素交换实验。两相系统实验是在无流体的情况下在1 GPa和680°C下进行7天，或在500 MPa和560°C下进行20天。在存在少量流体的情况下，方解石和透闪石之间的外推平衡分馏比现有的经验估计和实验确定要低得多，但通过透闪石中骨架氧的增量法，计算出的分馏非常接近。直接方解石-透闪石交换实验中测得的细小馏分可以通过固-固反应中发生重结晶的羟基氧，骨架氧和方解石之间不同的氧同位素交换速率来解释。三相系统实验是在500 MPa和560°C的大量流体（CO2 + H2O）存在下，在5、10、20， 40、80、120、160和200天。结果表明，矿物与流体之间的氧同位素交换过程分为两个阶段：第一，通过溶解-重结晶机制，并且非常迅速。其次，是通过扩散机制，而且非常缓慢。在第一阶段，合成方解石与流体的同位素交换速率比天然方解石高。在第二阶段，方解石中氧的扩散速率大约等于或略大于透闪石中的氧扩散速率。使用方解石的可用扩散系数进行的计算表明，在随后的阶段中，晶界扩散而不是体积扩散一直是流体与矿物晶粒之间氧气传输的主要机理。

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    《Contributions to Mineralogy and Petrology》 |2004年第6期|683-695|共13页
  • 作者

    Y.-F. Zheng; M. Satir; P. Metz;

  • 作者单位

    School of Earth and Space Sciences University of Science and Technology of ChinaInstitute for Mineralogy Petrology and Geochemistry University of Tuebingen;

    Institute for Mineralogy Petrology and Geochemistry University of Tuebingen;

    Institute for Mineralogy Petrology and Geochemistry University of Tuebingen;

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