【作者】 王小敏；

【导师】 李伟强；

【作者基本信息】 南京大学 ， 矿物学、岩石学、矿床学， 2018， 硕士

【摘要】 白云岩化在全球镁循环过程中发挥了重要作用,镁是白云岩的重要组成元素,且沉积型白云岩的镁同位素比值存在显著的自然变化。因此白云岩镁同位素组成有可能被用于指示全球镁循环和海水化学成分。但是,由于白云岩被普遍认为无法从海水直接沉淀而成,而属于成岩过程的产物。白云岩镁同位素是否可以指示同沉积海水的化学和同位素组成尚存不确定性。另一方面,白云岩常产出于碳酸盐台地环境,而局限湖盆在碳酸盐台地普遍发育,当前我们对这种环境中白云岩Mg同位素的地球化学行为仍知之甚少。因此,有必要对白云岩形成过程中早期成岩作用对其Mg同位素影响,以及在白云岩化对局限碳酸盐台地海水封闭的Mg同位素响应进行深入探讨。为了研究早期成岩过程对白云岩Mg同位素的影响,我们选取了位于以色列耶路撒冷附近的Shoresh剖面白垩纪阿尔布期Soreq组和Givat Ye’arim组的白云岩地层做系统研究。该套白云岩记录了典型的早期成岩作用,具体证据包括:(1)地层具有较大的C同位素波动现象,且白云岩δ13C值和Mn含量呈显著的负相关性;(2)白云石的d(104)值和离子有序度具有显著的变化,且白云岩的Sr含量和有序度之间呈负相关关系。白云岩的Mn含量和C同位素的负相关关系暗示白云岩初始形成于Mn(Ⅳ)还原带,其形成过程中伴随有机质的Mn还原反应以及有机碳和Mn进入碳酸盐晶格的过程。而白云石d(104)值、离子有序度、和Sr含量的变化和相关关系指示白云岩成岩过程中存在程度不一的重结晶作用。剖面白云岩的δ26Mg值变化区间为-2.28‰至-1.78‰。而白云岩的Mg同位素和Mn还原作用(δ13C值和Mn含量)及白云石重结晶作用(Sr含量和白云石有序度)的指标之间没有相关性。说明白云岩Mg同位素不受早期成岩作用影响,可以很好继承其沉积初始阶段(成岩作用的最开始阶段)的Mg同位素特征。对以色列中部Shoresh剖面的白云岩研究表明白云岩Mg同位素可以反映白云岩形成时同期海水的Mg同位素组成,在此基础之上,我们针对以色列南部地区的白垩纪阿尔布期Hevyon组地层做了详细的沉积学和Mg同位素研究。Hevyon组形成于连陆碳酸盐岩台地,属于典型的局限性盆地。对Hevyon组碳酸盐岩地层的详细野外和室内观察显示,其白云岩含量自下而上逐渐增加,并且与微生物沉积物出现及后生动物化石消失相联系,反映其沉积过程中的多次水体深度波动现象和海水补充事件。沉积相分析、地球化学指标(Ni/Co、V/Cr和Ce/Ce*)指示的厌氧还原环境以及沉积结构和C-Sr同位素组成的综合证据表明,白云岩化作用具有准同生和微生物参与的特征。该地层白云岩的δ26Mg值由地层下部的-1.99 ± 0.12‰向中部升高至-1.52 ± 0.02‰,随后镁同位素在地层上部降低至-2.06± 0.18‰。白云岩Mg同位素的波动记录了碳酸盐岩台地和广海之间连通性的周期性变化,是判断盆地局限性变化的指标。利用瑞利分馏模型,可将白云岩Mg同位素和Mg/Ca比值结合,推算出白垩纪阿尔布期的全球海水Mg/Ca比值介于1.5至1.0之间。更多还原

【Abstract】 Dolomite plays an important role in global Mg cycling and Mg isotope composition of dolomite has been proposed as a potential tracer for global Mg cycling and seawater chemistry changes.However,dolomite formation is complicated and sedimentary dolomite was generally regarded as a diagenetic product,yet many details about the effects of early diagenesis on Mg isotope composition of dolomite remain unclear.The mid-Cretaceous Albian Soreq and Givat Ye’arim Formations in the west Jerusalem contain massive dolostones that are exceptionally well preserved.Dolomite samples from this section show large variations in 813C and Mn contents that are negatively correlated,indicating degradation of organic matters and mineralization of organic carbon in Mn(Ⅳ)reduction zone of soft sediment during dolomite formation.A clear link between Mn(Ⅳ)reduction and dolomite precipitation was presented based on geochemical signatures in rock records.Dolomite samples also have large variations in d(104)value and the degree of cation ordering.And the latter one was negatively correlated with Sr contents,which indicates that dolomite recrystallization occurred during diagenesis.δ26Mg of the over fifty dolomite samples from this section vary from-2.28‰ to-1.78‰.However Mg isotope compositions of the dolomite do not show correlations with either indicators of organic matter degradation(δ13C and Mn content),or indicator of dolomite recrystallization(Sr content),suggesting that Mg isotopes have a conservative behavior since initial precipitation of dolomite(or proto-dolomite)during the very early stage of diagenesis.We suggest that Mg isotopes in dolomite formed in the stage of Mn(Ⅳ)reduction should be buffered by contemporary seawater due to the shallowness of Mn(Ⅳ)reduction zone in platform sediments,and this is fundamentally different from dolomitization associated with bacteria sulfate reduction or methanogenesis where Mg supply is limited by diffusion.Furthermore,overall Mg isotopes in dolomite are robust against resetting by recrystallization during burial.Mg isotope compositions of platform dolomite that show variable and negatively correlated δ13C and Mn signatures therefore could be considered as potential archives for reconstructing seawater Mg isotope compositions.Significant Mg isotope fractionation occurs between dolomite and aqueous solution.More important,the applicability of Mg isotopes in dolomites in studies of ancient seawater requires a detailed understanding on the behavior of Mg isotopes during dolomitization in a variety of geological settings.The mid-Cretaceous Albian Hevyon Formation in southern Israel was deposited within a typical hinterland attached carbonate platform setting,and provides an ideal case for investigating the Mg isotope behavior during dolomitization in environments of frequent basin restrictions.The abundance of dolomite increases upward in the Hevyon Formation,and correlates with the appearance of microbial deposits and disappearance of metazoans,reflecting water level fluctuations in multiple exposure and recharging events.In conjunction with sedimentary facies analysis,multiple geochemical proxies(Ni/Co,V/Cr,Ce/Ce*and δ13C)indicate development of anoxic conditions,together with textural indicators and 87Sr/86Sr ratios,suggest that dolomitization was microbial and penecontemporaneous in a shallow low circulation water body.δ26Mg of dolomite increases from-1.99 ± 0.12‰ in the lower part of the sequence to a value of-1.52 ± 0.02‰ in the middle interval,then decreases back to-2.06 ± 0.180‰ in the upper unit,which tracks the fluctuating marine connection and provides a powerful tracer for the level of restriction.Paring Mg isotopes with Mg/Ca ratios of the dolomite,it is possible to reconstruct the seawater Mg/Ca ratio during the Albian.更多还原