高温实验模拟Mg/Ca浓度对文石质生物碎屑白云石化过程的影响

Mg/Ca concentration effecting on dolomitization process of aragonitic bioclastics by high-temperature experimental simulation

高温(150℃以上)合成白云石实验被认为是了解自然条件下白云石成因的有效途径。选择自然条件下对白云石成因有重要影响的Mg/Ca值(摩尔浓度比)指标,分别采用鹿角珊瑚和大琵琶螺两种文石质生物碎屑为先驱反应物,在220℃高温环境下对不同Mg/Ca值(0.5、0.7、1.0、1.2、1.5)和不同反应时间(0.5、1.0、1.5、2、3、6、8、14 h)条件下文石质生物碎屑的白云石化过程开展研究。结果表明,在高温条件下白云石化过程可分为诱导阶段、快速反应阶段和平稳阶段,这一反应过程未明显受到先驱文石质反应物的影响,反映了流体Mg/Ca浓度对白云石化产物具有明显的控制作用。随着反应溶液Mg/Ca值的提高,白云石化的诱导期缩短、速率加快,产物白云石中CaCO_(3)的摩尔分数降低且有序度提高,表明高Mg^(2+)浓度能够促进Mg^(2+)快速交代文石中的Ca^(2+)并完成白云石的有序化过程。研究结果为解释自然条件下Mg/Ca浓度对白云石化过程的影响提供了基础理论支撑。

High-temperature(above 150℃) dolomite synthesis experiment is considered to be an effective way to understand the genesis of natural dolomites. Under the consideration of the Mg/Ca value(molar concentration ratio) index that has an important impact on the genesis of natural dolomites, our experiment used two kinds of aragonitic bioclasts(Acropora, Ficus) as the precursor reactants in the 220℃ conditions to study the dolomitization process under different Mg/Ca concentration ratios(0.5, 0.7, 1.0, 1.2, 1.5) and reaction times(0.5, 1.0, 1.5, 2, 3, 6, 8, 14 h) respectively. The results show that the dolomitization process under high temperature can be divided into induction stage, rapid reaction stage and stable stage. This reaction process was not obviously affected by the precursor aragonite reactants, implying that the Mg/Ca concentration should have an obvious influence on the dolomitized products. With the increase of Mg/Ca ratio in the reaction solution, the induction period of dolomitization was shortened and the rate of dolomitization was accelerated, and the mole fraction of CaCO_(3) in the product dolomile was reduced and the order degree was increased, which indicates that high Mg^(2+)concentration can rapidly promote the replacement of Ca^(2+)by Mg^(2+)in aragonite lattice and complete the ordering process of the dolomites. This results provide basic theoretical support for explaining the influence of Mg/Ca mole concentration on natural dolomitization process.