Carbon Isotope Fractionation in Magnesian Calcite: Effects of Precipitation Rate and Mg Content

本文首次实验研究了镁含量和沉淀速率对镁方解石中13C的影响。镁质方解石（Mg-方解石）是一种MgCO3含量约大于10摩尔的方解石，是浅海沉积物的重要组成部分，也是白云化过程中镁和CO3的主要来源。令人惊讶的是，镁方解石和HCO3之间的13C分馏从未被确定。已经开发了在恒温、化学和同位素组成的溶液中以恒定速率析出方解石和文石的实验程序。85次实验得到方解石-HCO3和文石-HCO3 13C富集因子平均分别为1.0和2.5%。方解石- hco3富集因子（cl-HCO3-）随温度（100 ~ 40℃）或降水速率（103.0 ~ 105.2 umol m2hr1）变化不显著。文石- hco3富集因子（ar-HCO3）与温度呈弱相关，ar-HCO3降低0.020%oC-1。只有在100℃和25℃的实验中，ar-HCO3与降水速率呈弱负相关。该项目包括大约30个实验，在25℃下，Mg/Ca比为5到20的溶液中，在2到5 μ mol m2 hr1的沉淀速率范围内，产生含有7到20摩尔% MgCO3的Mg方解石。确定这些镁-方解石- hco313c分馏关系将使研究人员能够更好地量化海洋沉积物和水域的原始同位素组成，这是研究全球变化和碳酸盐成岩作用的重要信息。

This is the first experimental study of the effect of magnesium content and precipitation rate on the 13C of magnesian calcite. Magnesian calcite (Mg-calcite), calcite with roughtly greater than 10 mole percent MgCO3, is an important and often dominant component of shallow marine sediments, and serves as a Mg and CO3 source for subsequent dolomitization. Surprisingly, the 13C fractionation between Mg-calcite and HCO3 has never been determined. Experimental procedures have been developed for the precipitation of calcite and aragonite at constant rates in solutions of constant temperature and chemical and isotopic composition. Eighty-five previous experiments yield calcite- HCO3 and aragonite-HCO3 13C enrichment factors averaging 1.0 and 2.5% respectively. Calcite-HCO3 enrichment factors ( cl-HCO3-) do not vary significantly with temperature (10o to 40oC) or precipitation rate (103.0 to 105.2 umol m2hr1). Aragonite-HCO3 enrichment factors ( ar-HCO3) show a weak correlation with temperature, suggesting that ar-HCO3 decreases 0.020%oC-1. Only in experiments at 10o and 25oC does ar-HCO3 exhibits a weak inverse correlation with precipitation rate. The project includes roughly 30 experiments at 25oC over a range of precipitation rates between 2 and 5 umol m2 hr1 for solutions having Mg/Ca ratios of 5 to 20, producing Mg-calcites with 7 to 20 mole % MgCO3. Determination of these Mg-calcite-HCO3 13C fractionation relations will permit researchers to better quantify the original isotopic composition of marine sediments and waters, vital information for studies of global change and carbonate diagenesis.