醋酸提取沉积物非碳酸盐来源Mg的实验探究——以西宁盆地中中新世-上新世沉积地层为例

摘要

古环境研究中沉积物碳酸盐成分的提取常使用不同浓度的稀醋酸进行.已有的研究表明沉积物的非碳酸盐组分也会对稀醋酸提取组分产生一定影响.查明非碳酸盐组分对稀醋酸溶解Mg的贡献十分重要,可为碳酸盐化学组成定量研究提供必要的约束,也是进一步开展沉积物碳酸盐Mg同位素研究的基础.然而目前对稀醋酸提取非碳酸盐来源Mg的研究尚十分缺乏.本研究选取了5个来自西宁盆地莫家庄剖面中中新世-上新世沉积地层碳酸盐含量各异的沉积物样品开展醋酸溶解非碳酸盐来源Mg的实验研究.实验结果表明,全岩样品的稀醋酸提取组分含有非碳酸盐来源的Ca和Mg,特别是在低碳酸盐含量(＜1％)样品中,非碳酸盐组分的影响十分显著.进一步的多相态连续提取实验表明,交换态和硅酸盐是醋酸提取组分非碳酸盐Mg的主要来源.较高Mg/Ca比值的硅酸盐和交换态端元与低Mg/Ca比值的方解石端元混合,可以解释各样品醋酸提取组分的Mg/Ca比值与Ca、Mg含量成分特征.醋酸提取组分相对较高的Mg/Al比值及其变化表明硅酸盐Mg可能主要来自非晶格位置Mg的溶解(如粘土层间).醋酸提取组分中的Mg/Al比值显著高于全岩沉积物本身的Mg/Al比值,所以不宜采用固定的全岩沉积物元素比值或者标准物质成分(如上部大陆地壳、标准页岩)来校正在醋酸提取组分中硅酸盐溶解对Mg的贡献.%One important precondition for the carbonate composition-based quantitative analyses for paleo-reconstructions is to choose adequate selective leaching reagents for carbonates extractions.The widely adopted diluted acetic acid leaching method is being challenged by contributions from non-carbonate fractions based on multiple clues including strontium isotopes.Such investigations for non-carbonate magnesium in acetic acid leachate,however,are quite limited,hindering the future application of bulk sample carbonate Mg isotope into the studies of weathering and climate change.Herein,we use a series of experimental analyses to investigate the sources of non-carbonate Mg in diluted acetic acid leachate.The materials are from MJZ section(36°41′07″N,102°04′15″E) in the northeast part of Xining Basin,NW China.This section with 336 m in thickness includes mid-Miocene to Pliocene fluvial sediments,spanning from 12.8 Ma to 4.8 Ma according to detailed paleomagnetism analyses.First,we choose five representative mudstone/siltstone samples,and analyze their inorganic carbon content by carbon analyzer(SHIMADZU TOC-L).The 5 samples are named as ES-1,ES-2,ES-3,ES-4 and ES-5 in sequence(from 116.4 m,158.8 m,186.4 m,89.7 m and 216.7 m of the MJZ section,respectively)according to their carbon content from high to low.And then the carbon contents are compared with the CO2 amount calculated by the Ca and Mg concentrations (ICP-OES,Leeman Labs Prodigy-H) in acetic acid leachate.The results show good correlation among samples with high carbonate contents but poor among those with low carbonate contents,suggesting that there are significant amounts of non-carbonate Mg and Ca leached by acetic acid in the low carbonate samples.Therefore,we design a 4-step sequential extraction to check the sources of these non-carbonate Mg.We first separate each of the 5 samples into 3 parallel samples,one is left as blank and other two are treated with KCl and NH4Ac solutions,respectively,in order to extract the exchangeable fractions and examine the difference between the two exchangeable agents.Next,we use 1 mol/L acetic acid to extract the carbonate fractions.After that,continuous 1 mol/L acetic acid leaching for 2 times are applied in order to check the capability of acetic acid to dissolving non-carbonate fractions when carbonates are exhausted.The final step is 2 mol/L HCl leaching under 80 ℃.According to the result of exchangeable fraction leaching,the difference in agents leads to little difference in the composition of leachates except that some carbonate may dissolve in NH4Ac.By comparing the first acetic acid leachates with and without exchangeable fraction extractions,the contributions of exchangeable fractions to the Mg in acetic acid leachate are estimated to be more than 30％ for all the samples and more than 50％ for low carbonate content samples.Another source of non-carbonate Mg might be from silicate,since other fractions such as those bounded to organic matters or Fe-Mn oxides are not likely to provide Mg cations to acetic acid leachate.The silicate fractions provide as an endmember high in Mg/Ca ratios,supported by the generally inverse correlation between Mg/Ca ratios and carbonate content in the first acetic acid leaching step,and the increasing Mg/Ca ratios during continuous acetic acid leaching.The acetic acid leachate has high and relatively stable Mg/Al ratios,suggesting that Mg-rich minerals such as chlorite,which is presented in the XRD diffractogram of bulk sediments(unpublished data),are dissolved in acetic acid.The quite limited Al concentrations as well as high Mg/Al ratios which is higher than the Mg/Al ratios of the heated HCl leachates and UCC mean that acetic acid doesn't destroy the lattice of silicate minerals and the Al-O bonds.So this fraction of Mg is likely to be mainly derived from clays interlayers,such as the brucite layers of chlorites.The low Al and high Mg/Al ratios in acetic acid leachate indicate the incongruent dissolution of silicate minerals by acetic acid,suggesting that using the overall compositions of standard substances,bulk sediments or diluted acid insoluble residues to calibrate the silicate-derived Mg is improper.The mixing between silicate and exchangeable endmember and calcite endmember on the plot of(CaCO3 +MgCO3)contents versus Mg/Ca molar ratios explains the variation of Ma/Ca ratios and Ca and Mg concentrations in acetic acid leachate especially among low carbonate content samples.