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

首次实验研究了镁含量和沉淀率对镁方解石~(13)C的影响。镁方解石(镁方解石)，方解石，粗略大于10摩尔百分的碳酸镁，是浅海沉积物中重要且往往占主导地位的组分，是后续白云岩作用的镁和二氧化碳来源。令人惊讶的是，镁方解石和HCO3之间的13C分馏从未被确定。建立了方解石和文石在恒温、化学和同位素组成的溶液中恒速沉淀的实验方法。以往的85次实验中，方解石-HCO3和文石-HCO3的13C平均富集率分别为1.0%和2.5%。方解石-HCO3-的浓缩系数(CL-HCO3-)不随温度(100℃至40℃)或沉淀率(103.0至105.2微克分子数m2hr1)而显著变化。文石-HCO3的富集度(Ar-HCO3)与温度的相关性较弱，表明Ar-HCO3降低了0.020%OC-1。只有在10o和25oC的实验中，Ar-HCO3才表现出与降水速率的弱负相关。该项目包括大约30个实验，在25oC下，对于镁/钙比为5至20的溶液，在2至5umol m2 HR1的沉淀速率范围内，生成含7至20摩尔%碳酸镁的镁方解石。这些镁-方解石-HCO3 13C分馏关系的确定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.