Experimental calibration of Mg isotope fractionation between dolomite and aqueous solution and its geological implications
Experimental calibration of Mg isotope fractionation between dolomite and aqueous solution and its geological implications
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白云石与水溶液之间镁同位素分馏的实验校准及其地质意义
DOI：
10.1016/j.gca.2015.02.024
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发表时间：
2015-05
影响因子：
5
通讯作者：
Johnson Clark M.
中科院分区：
文献类型：
--
作者：
Li Weiqiang;Beard Brian L.;Li Chengxiang;Xu Huifang;Johnson Clark M.
通过220℃、160℃和130℃的水热实验，标定了白云石和镁水溶液之间的镁同位素分馏系数。水热实验包括使用不同的原料合成白云石，以及使用有序较差的原始白云石进行交换实验。合成白云石的形貌与起始矿物学有关，说明白云石的合成途径不同。水热法合成的白云石最初是细粒的无序或无序的白云石，随着时间的推移，这些白云石重结晶为较粗的有序白云石。用87Sr/86Sr比值和25 mg示踪剂监测了同位素交换，这表明在大多数热液实验结束时，白云岩与水溶液之间的同位素交换接近完成。由合成和交换实验得到的白云石与水溶液之间的镁同位素分馏系数随时间收敛，与白云石的形态无关，表明白云石达到了同位素平衡。综合合成和交换实验结果，有序白云石的镁同位素分馏系数为：Δ2 6 mg Dolo-Aq=-0.1554(±0.0096)×10 6/T2，其中T为开尔文。相反，有序白云岩的Δ为26mgDolo-Aq分馏因子，比有序白云岩低0.25mgDolo-Aq分馏因子，这归因于有序白云岩中较长的Mg-O键。实验校准的Δ26毫克Dolo-AQ分馏系数介于Schauble(2011年)和Rustad等人(2010年)计算的系数之间。利用本研究的Δ26 mg-T函数外推到较低温度的Δ26 mg Dolo-AQ分馏系数与通过对Δ钻芯样品的镁同位素组成进行模拟得到的ODP26 mg Dolo-AQ分馏系数相匹配。研究表明，在白云岩沉淀过程中发生了明显的镁同位素分馏。这些结果表明，白云岩中的镁同位素是研究镁全球循环和白云石化作用的有用工具。
Abstract Hydrothermal experiments at 220, 160, and 130° C were performed to calibrate the Mg isotope fractionation factor between dolomite and aqueous Mg. Hydrothermal experiments included synthesis of dolomite using different starting materials, as well as exchange experiments that used poorly-ordered proto-dolomite. The morphology of synthesized dolomite was dependent on starting mineralogy, suggesting that dolomite was synthesized by different pathways. Hydrothermally synthesized dolomite was initially fine-grained disordered or poorly-ordered dolomite that, with time, recrystallized to coarser-grained ordered dolomite. Isotopic exchange was monitored using 87 Sr/86 Sr ratios and 25 Mg tracers, and these indicated near-complete isotope exchange between dolomite and aqueous solutions at the end of most hydrothermal experiments. The Mg isotope fractionation factor between dolomite and aqueous solution obtained from synthesis and exchange experiments converged with time and was independent of dolomite morphology, suggesting attainment of isotopic equilibrium. Combining results from synthesis and exchange experiments, the temperature dependent Mg isotope fractionation factor for ordered dolomite is: Δ 26 Mg dolo-aq=-0.1554 (±0.0096)× 10 6/T 2 where T is in Kelvin. In contrast, poorly-ordered dolomite has a Δ 26 Mg dolo-aq fractionation factor that is up to 0.25‰ lower than that of ordered dolomite, and this is attributed to longer Mg–O bonds in imperfectly ordered dolomite. The experimentally calibrated Δ 26 Mg dolo-aq fractionation factors lie between those calculated by Schauble (2011) and Rustad et al.(2010). The Δ 26 Mg dolo-aq fractionation factor extrapolated to lower temperatures using the Δ 26 Mg-T function of this study matches the Δ 26 Mg dolo-aq fractionation factor obtained by modeling of Mg isotope compositions of ODP drill core samples. This study shows that significant Mg isotope fractionation occurs during dolomite precipitation. These results collectively demonstrate that Mg isotopes in dolomite are a useful tool for studying Mg global cycling and dolomitization.
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