Theoretical calculation of equilibrium Mg isotope fractionations between minerals and aqueous solutions
Theoretical calculation of equilibrium Mg isotope fractionations between minerals and aqueous solutions
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矿物与水溶液平衡镁同位素分馏的理论计算
DOI：
10.1016/j.chemgeo.2018.04.005
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发表时间：
2018-06-05
期刊：
影响因子：
3.9
通讯作者：
Liu, Yun
中科院分区：
文献类型：
--
作者：
Gao, Caihong;Cao, Xiaobin;Liu, Yun
矿物与溶液的平衡Mg同位素分馏计算与实验研究存在很大的差异。为了澄清这个令人困惑的问题，一个新设计的基于团簇模型的量子化学方法，即，体积可变簇模型（VVCM）用于提供含镁碳酸盐（方解石、文石、白云石和菱镁矿）、无定形碳酸钙（ACC）、水镁石和含水物质（即，Mg-（aq）（2+）、MgHCO3（aq）+和MgOH（aq）+）。我们发现，本地配置采样的水性物种是必不可少的，以提供矿物和溶液之间的精确分馏。基于声子的周期边界方法也用于几种矿物，它获得了与VVCM结果非常相似的分馏。尽管采用了完全不同的方法，但我们的结果与Pinilla等人（2015）的结果非常接近。这两种方法都考虑了局域组态无序的影响。然而，这两个是显着不同的碳酸盐与溶液的情况下，一些实验结果。实验流体中各种含Mg物质的存在、水合Mg 2+直接掺入固体中、Mg 2+浓度效应以及中间前体（例如，ACC）是导致不匹配的几种可能原因。相对于共存的Mg ~（2+）水溶液，我们发现ACCs将富集重Mg同位素，即，类似于25摄氏度时的千分之1.45，与先前的实验估计一致。还预测了水镁石和溶液之间的平衡Mg同位素分馏因子。此外，我们应用VVCM预测了高温相之间的Mg同位素分馏，即，镁橄榄石、透辉石、顽火辉石、透闪石和尖晶石。预测的β因子大小顺序为尖晶石>透闪石>透辉石>顽火辉石>镁橄榄石。该研究为进一步认识积累的镁同位素数据奠定了基础。
There are large discrepancies existing in equilibrium Mg isotope fractionation calculations and experimental investigations for cases related to mineral vs. solution. To clarify this confusing issue, a newly designed clustermodel- based quantum chemistry method, i.e., volume variable cluster model (VVCM), is used to provide equilibrium Mg isotope fractionation factors between Mg-bearing carbonates (calcite, aragonite, dolomite and magnesite), amorphous calcium carbonates (ACCs), brucite and aqueous species (i.e., Mg-(aq)(2+), MgHCO3(aq)+ and MgOH(aq)+). We find that local configuration sampling of aqueous species is essential to provide precise fractionations between mineral and solution. The phonon-based periodic boundary method is also used for several minerals and it obtains very similar fractionations with VVCM results.Our results are very close to those of Pinilla et al. (2015) although via completely different approaches. Both of them have included the effect of local configuration disorder. However, both of them are significantly different from some of experimental results for cases of carbonates vs. solutions. The existence of various Mg-bearing species in fluids of experiments, the direct incorporation of hydrated Mg2+ into the solids, the Mg2+ concentration effect, and the existence of intermediate precursors (e.g., ACCs) are several possible causes for the mismatches. Relative to coexisting aqueous Mg2+, we find that ACCs will enrich heavy Mg isotopes, i.e., similar to 1.45 parts per thousand at 25 degrees C, agreeing with previous experimental estimation. Equilibrium Mg isotope fractionation factors between brucite and solutions are also predicted. Besides, we applied VVCM to predict the Mg isotope fractionations between high-temperature phases, i.e., forsterite, diopside, enstatite, tremolite and spinel. The predicted beta factors are in the order of spinel > tremolite > diopside > enstatite > forsterite. This study provides a base for understanding the accumulating Mg isotope data.