黄钾铁矾中溴氯的配分行为及对火星沉积岩的启示

Partitioning behavior of Br and Cl during jarosite precipitation and its implications for sedimentary rock on Mars

美国国家航空航天局机遇号火星车在火星子午线平原的原位样品中探测到Br元素和Br/Cl比值超过3个数量级的波动,但Br的赋存状态和富集机制却不清楚.已有的研究发现黄钾铁矾形成时会选择性地富集Br从而造成Br和Cl的分异,然而对于该过程中Br^-和Br^-的地球化学行为还缺乏系统研究.我们通过25℃下氧化亚铁硫酸盐的方法,合成了系列具有Br^-和Cl^-浓度梯度的黄钾铁矾,获取了Br^-和Cl^-在黄钾铁矾^-溶液中的分配规律以及卤素置换对黄钾铁矾结构的影响.实验结果表明,在黄钾铁矾生成时Br^-和Cl^-具有截然不同的分配行为,即Br^-倾向于进入固相而Cl^-倾向于留存在溶液中.在相同起始浓度下,进入黄钾铁矾的Br^-比Cl^-高约2个数量级;当Br^-和Cl^-共存于起始溶液时,Br^-会明显干扰黄钾铁矾对Cl^-的纳入.Br^-和Cl^-置换黄钾铁矾中的羟基位置,不改变其基本结构.黄钾铁矾中的Br/Cl比值较初始溶液高约2个数量级.火星子午线平原富含铁镁硫酸盐矿物的沉积岩中,Br/Cl比值的变化不单受到卤盐蒸发沉淀的影响,还可能受到成岩过程中铁硫酸盐(例如黄钾铁矾)沉淀及其溶解的控制.纳入卤素的黄钾铁矾在溶液中的稳定性及其对卤素的释放,有待进一步研究.

National Aeronautics and Space Administration（NASA）＇s Opportunity rover landed at Meridiani Planum on Mars in 2004, and discovered the first sedimentary record on Mars called Burns formation. The detection of magnesium-and iron-sulfates, jarosite（KFe3（SO4）2（OH）6）, and hematite（Fe2O3） in the outcrop of Burns formation indicates that aqueous activities have been once present in this region. The Alpha Particle X-ray Spectrometer（APXS） onboard the rover detected enrichment of elemental Br in rock and soil samples which varied by three orders of magnitude, and primarily controlled the variations of Br/Cl ratios in these samples. Although aqueous related processes have been suggested to explain the enrichment of Br, the speciation of Br and the mechanisms for Br variations are poorly constrained. Jarosite has been reported to be able to preferentially incorporate Br^-over Cl^-during precipitation, and result in Br enrichment and Br/Cl fractionation. However, the partitioning behavior of Br^-and Cl^-during jarosite formation and how the incorporation of halogens would influence properties of jarosite are not well known. In this work, we synthesized a series of halogen bearing jarosite with Br^-and Cl^-concentration gradients by oxidation of ferrous sulfate at room temperature. After synthesis, we used X-ray diffraction（XRD）, Raman spectrometer, infrared spectrometer（IR）, scanning electron microscope（SEM） to analyze structure information and surface morphology of jarosite solids. Anion and cation concentrations in solution and solid were analyzed by ion chromatography（IC）, atomic absorption spectrophotometer（AAS） and X-ray fluorescence（XRF）. Our work shows that Br and Cl contents in the synthesized jarosite are positively correlating with its initial concentrations in solutions. Starting at the same concentrations in solution, incorporation of Brin jarosite is about two orders of magnitude higher than that of Cl^-. Coexisting Br^-can significantly interfere with Clpartitioning into jarosite. The distribution coefficients of Br^-and Cl^-are negatively correlated with their initial concentrations in solutions, and in the Br^-and Cl^-coexisting setting, the distribution coefficients of Cl^-decrease as the initial Br^-concentration increase in the solution. Therefore, during precipitation of jarosite, Br^-prefers to participate into jarosite while Cl^-prefers to remain in solution. Incorporation of halide anions into jarosite directly affected the range of hydroxyl and water in Raman spectra, indicating that Br^-and Cl^-substitute for OH position in jarosite without changing its fundamental structure. We calculated that in our experiment settings, Br/Cl ratios in jarosite are about two orders of magnitude higher than that of initial solutions. Therefore, if jarosite precipitates from a brine containing Br^-and Cl^-, it can enrich Br over Cl and bearing significant higher Br/Cl ratio signature comparing to the initial brine. For sedimentary outcrop composed of substantial amount of jarosite, Br/Cl ratios may not be controlled solely by evaporation and precipitation of halide evaporites, but might also be influenced by precipitation and dissolution of halogen bearing jarosite. The aqueous stability of halogen bearing jarosite and the possible release of halide anions during jarosite dissolution require further evaluation.