Equilibrium Ca isotope fractionation and the rates of isotope exchange in the calcite-fluid and aragonite-fluid systems at 25 °C
Equilibrium Ca isotope fractionation and the rates of isotope exchange in the calcite-fluid and aragonite-fluid systems at 25 °C
复制标题
25 °C 下方解石-流体和霰石-流体系统中的平衡 Ca 同位素分馏和同位素交换速率
DOI：
10.1016/j.epsl.2022.117985
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发表时间：
2023
影响因子：
5.3
通讯作者：
V. Mavromatis
中科院分区：
文献类型：
--
作者：
A. Harrison;A. Heuser;V. Liebetrau;A. Eisenhauer;J. Schott;V. Mavromatis
方解石和文石的 Ca 同位素组成可以帮助了解它们的形成条件。然而，对天然样品的 Ca 同位素组成的准确解释需要精确了解固相和液相之间的平衡 Ca 同位素分馏。在本研究中，使用 42 Ca、 43 Ca 和 44 Ca 的三同位素方法来估计 CaCO 3 矿物方解石和文石与 Ca 2+ 水合离子之间的 Ca 平衡同位素分馏。反应流体富含 43 Ca，并与天然 Ca 同位素分布的合成方解石和文石在 25°C 下平衡长达 3745 小时。使用 MC-ICP-MS 测量固体和流体的同位素组成，方解石-Ca 2+(aq) 和文石-Ca 2+(aq) 的估计平衡分馏为 Δ 44/42 Ca 固体-流体分别=−0.02±0.13‰和−0.80±0.10‰。对反应固体的结构观察表明文石实验中的同位素平衡是通过广泛的奥斯特瓦尔德熟化发生的，在实验结束时产生大的结晶针状体。相比之下，方解石在实验过程中没有表现出可观察到的尺寸增加。方解石的同位素交换率与之前研究中报道的相似，并且比远非平衡方解石溶解率低约 4 个数量级。文石的钙同位素交换率比方解石更快，这是由通过溶解/沉淀反应发生的更大程度的奥斯特瓦尔德熟化驱动的。这项研究的结果表明，处于化学平衡但同位素不平衡的方解石和文石晶体的 Ca 同位素组成可能会在没有明显成岩作用的证据的情况下发生显着改变，特别是在方解石的情况下。然而，Ca 同位素组成改变的程度很大程度上取决于环境条件，例如流体：固体比率和促进流体输送的固体渗透性。
The Ca isotope composition of calcite and aragonite can provide insights into their formation conditions. The accurate interpretation of the Ca isotope composition of natural samples, however, requires precise knowledge of the equilibrium Ca isotope fractionation between the solid and fluid phases. In this study, the three-isotope method with 42 Ca, 43 Ca and 44 Ca has been used to estimate the equilibrium isotope fractionation of Ca between the CaCO 3 minerals calcite and aragonite and the Ca 2+ aquo ion. Reactive fluids were enriched with 43 Ca and equilibrated with synthetic calcite and aragonite of natural Ca isotope distribution up to 3745 h at 25° C. The isotopic composition of solids and fluids was measured using MC-ICP-MS and the estimated equilibrium fractionation for calcite-Ca 2+(aq) and aragonite-Ca 2+(aq) was Δ 44/42 Ca solid-fluid=− 0.02±0.13‰ and− 0.80±0.10‰, respectively. Textural observations of the reacted solids suggest that isotope equilibration in aragonite experiments occurs via extensive Ostwald ripening, yielding large crystalline needles at the end of the experiments. In contrast, calcite did not exhibit an observable increase in size during the course of the experiments. Isotope exchange rates in the case of calcite are similar to those reported in previous studies and∼ 4 orders of magnitude lower than the far-from-equilibrium calcite dissolution rate. Calcium isotope exchange rates for aragonite are more rapid than calcite driven by a greater extent of Ostwald ripening occurring via dissolution/precipitation reactions. The results of this study suggest that the Ca isotope compositions of calcite and aragonite crystals in chemical equilibrium, but isotope disequilibrium, with natural fluids could be significantly altered without overt evidence of diagenesis, especially in the case of calcite. The extent to which Ca isotope compositions are altered, however, would depend strongly on the environmental conditions, such as fluid: solid ratio and permeability of the solid facilitating fluid transport.
影响因子：
5
作者：
Higgins, J. A.;Blattler, C. L.;Swart, P. K.
通讯作者：
Swart, P. K.