【作者】 黄康俊；

【导师】 鲍征宇； 藤方振；

【作者基本信息】 中国地质大学 ， 地球化学， 2013， 博士

【摘要】 作为主要的造岩元素之一,镁有3个稳定同位素(24Mg、25Mg、26Mg),由于它们之间的相对质量差较大导致镁同位素在很多地质过程中会发生明显的质量相关分馏。因此,镁同位素有作为地球化学“示踪剂”的先决条件。目前镁同位素同位素研究已成为国际上地球化学领域新兴的研究热点。其中,地球各主要储库的Mg同位素组成特征和地质过程中Mg同位素的行为机制是两个关键的基础性问题。然而,由于缺乏高精度的Mg同位素数据和开展系统地研究,低温水-岩相互作用过程中的镁同位素行为,以及该个过程对陆壳和洋壳的Mg同位素组成的影响程度都尚不清楚。因此,本论文针对主要的低温地球化学过程,如大陆风化过程、黄土沉积形成过程以及洋壳蚀变过程中的镁同位素行为进行了系统的研究,从而为更好认识Mg地球化学循环过程以及应用Mg同位素地来示踪地质过程提供理论依据。为了实现以上研究目标,我们对典型的并有良好研究基础的经历过低温地球化学过程的样品,中国海南岛玄武岩风化剖面、国际大洋科学钻探计划(ODP)801站的蚀变洋壳剖面、全球主要黄土沉积区的黄土沉积物,进行了系统的矿物学、主微量元素、传统同位素和Mg同位素组成分析研究。通过这些研究得到以下几点主要认识：(1)通过测试分析来自海南新生代玄武岩风化剖面的一套富集粘土矿物的风化残余土的镁同位素组成来探讨大陆风化过程中的镁同位素行为。与未风化的玄武岩母岩相比(δ26Mg=-0.36‰),上覆的风化残余土强烈的亏损Mg(即τTh,Mg从-99.1%到-92.9%),并且具有高度不一致的镁同位素组成,δ26Mg在-0.49‰到+0.40%0之间。在风化剖面3m以下的部分,随着深度逐渐变浅,风化残余土的镁浓度和镁同位素组成均呈现增加的趋势。然而,在风化剖面3m以上的部分,越靠近表层的风化残余土,它们的镁浓度和镁同位素组成均越低。该风化剖面中镁浓度和镁同位素组成的变化趋势可以通过与次生矿物有关的吸附和解吸附过程来解释。也就是,在风化过程中,原生矿物溶解释放到流体中的Mg离子,通过吸附作用被玄武岩风化后形成的高岭土矿物固定,在这个过程中重的Mg同位素优先被高岭土等次生矿物摄取。这可以解释大部分风化残余土具有比母岩更重的镁同位素组成,并且来自风化剖面下半部分的残余土的镁同位素组成与高岭土矿物含量具有显著的正相关关系。此外,由于雨水等外来流体的加入,风化剖面靠近地表的部分经历了更强的风化作用,同时外来流体淋溶的作用使得流体中具有更低水化能的离子(如,K+、Rb+、CS+、Sr2+等)通过阳离子交换作用把之前吸附在高岭土矿物表面的重的Mg同位素被释放出来。来自风化剖面上半部分的残余土的镁同位素组成与那些具有低水化能的元素(如,Sr和Cs)之间的负相关关系,支持解吸附作用导致镁同位素分馏。该研究结果第一次强调了与粘士矿物有关的吸附与解吸附过程对极端风化过程中镁同位素的行为起到显著的影响。这个过程导致镁同位素分馏为解释河流水具有显著不均一的镁同位素组成提供了新的观点。汇编比较来自不同气候带风化产物的镁同位素组成数据揭示在大陆风化过程中的镁同位素在原生矿物溶解为主的初始风化阶段分馏程度十分有限；而在以次生矿物形成为主的高级风化阶段能导致明显的镁同位素分馏。(2)黄土沉积作为研究第四纪环境变化和估计上地壳平均化学组成重要的陆地地质档案。为了探讨黄土沉积能否作为有效的上地壳平均组成“代理”来正确的评估具有十分不均一组成的上地壳的Mg同位素平均组成,以及弄清控制黄土Mg同位素组成的因素。我们测试分析了来自中国、阿根廷和欧洲等地19个黄土样品的镁同位素组成,这些黄土样品之前已经用来来评估上地壳的平均化学组成。结果显示这些来自全球的黄土样品具有十分不均一的镁同位素组成,它们的δ26Mg值为-1.64‰到+0.25‰,平均值为-0.89‰,比来自地壳和地幔的硅酸岩的镁同位素组成都要轻。此外,这些黄土样品的MgO含量和δ26Mg值都与CaO/Al2O3比值呈正相关关系,可以解释为碳酸盐岩和二次硅酸岩矿物之间的二原混合。在黄土沉积形成过程中,(1)源区物质的不均一性,(2)风尘搬运过程发生的矿物分选作用,(3)化学风化作用,均可能影响黄土物质中富镁的矿物相(硅酸盐和碳酸盐矿物)的相对组成,进而影响黄士的镁同位素组成。其中,风尘搬运过程中发生的矿物分选作用和沉积过程中经历的化学风化作用都趋向使黄土中粘士矿物的相对比例增加,从而使黄士的镁同位素组成变重。这些结果指示由于黄土沉积形成过程中不同组份的混合、分选以及同位素分馏作用已经改变了黄土原始物质的镁同位素组成,所以黄土的镁同位素组成已经不能代表上地壳的平均镁同位素组成。但是,来自同一地区黄土的镁同位素组成与它们的古气候变化代理指标,如化学蚀变指数(CIA)和粒径变化指标SiO2/TiO2摩尔比值之间的具有明显的相关关系,这指示黄土的镁同位素组成具有作为示踪古气候变化代理的潜力。(3)蚀变洋壳记录着洋壳在蚀变过程中的相关信息,为揭示许多元素的全球物质循环过程提供关键性信息。蚀变洋壳的镁同位素组成是认识地幔、地壳以及水圈之间的镁循环的一个关键纽带,但是蚀变洋壳的镁元素和同位素通量到目前为止还不清楚。本次研究的主要目的是通过分析位于西太平洋的国际大洋科学钻探计划(ODP)801站位基底玄武岩剖面的镁同位素组成来研究低温条件下海水与洋壳玄武岩之间相互作用过程中的镁同位素行为,并评估俯冲进入地幔之前洋壳的镁同位素平均组成。研究结果显示,与未蚀变的洋壳相比,这些经历过低温海水蚀变作用的洋壳明显亏损镁,并且亏损程度(AMgO)随深度增加而呈现逐渐减弱的趋势。这些蚀变玄武岩具有十分不均一的镁同位素组成(δ26Mg从-2.82%到+0.19‰),其中蚀变程度更高的火山碎屑(VCL)混合样的镁同位素组成(δ26Mg值在-1.01到0.14‰之间)比蚀变程度较低的玄武岩熔岩混合样(FLO)的(δ26Mg值在-0.54到-0.02‰之间)变化范围更大,其平均组成更重。代表整个ODP801站玄武岩基底的平均组成的SUPER混合样(δ26Mg=-0.02‰)具有明显比新鲜玄武岩更重的镁同位素组成。这些特征指示洋壳在低温条件下与海水之间的相互作用会导致镁同位素在蚀变洋壳中重新分配。通过新鲜玄武岩与蚀变形成的次生含镁矿物之间的混合模拟计算可以很好的解释这些蚀变玄武岩的δ26Mg组成与MgO含量之间的相关关系,这表明蚀变玄武岩的镁同位素组成是受蚀变形成的次生矿物相控制的。这些次生矿物相在蚀变洋壳中的分布主要是海水与洋壳相互反应过程中水-岩比值、蚀变温度以及氧化程度等因素联合作用产生的结果。例如,随着水-岩比值的升高,蚀变程度增强,蚀变洋壳中碳酸盐岩脉的含量逐渐升高；蚀变环境越偏向还原条件,蚀变形成的次生矿物中皂石和黄铁矿的含量越高,导致蚀变玄武岩的铁同位素组成越轻,而镁同位素组成越重；此外,随着蚀变温度逐渐逐渐降低,蚀变形成的产物中碳酸盐矿物的含量越高,进入其中的Mg含量也越高,导致蚀变洋壳的镁同位素组成变轻。根据ODP801站的蚀变洋壳数据估算的低温海水蚀变过程向海洋输入的Mg通量大约为2×1010mol/yr,这可能是自新生代以来的海水中的Mg浓度和Mg/Ca比值持续升高的原因之一。另外,具有十分不均一的镁同位素组成的蚀变洋壳和大洋沉积物通过俯冲作用循环进入地幔,可能会导致地幔物质的镁同位素组成出现不同程度的不均一性。更多还原

【Abstract】 Magnesium is a fluid-mobile, major element in both the mantle and the crust, and has three isotopes (24Mg,25Mg,26Mg) with relative mass difference of～8%between24Mg and26Mg, which can potentially lead to large mass-dependent Mg isotope fractionation. This makes Mg isotopes have several advantages to serve as promising geochemical tracers. As new geochemical tracers, Mg isotopes have attracted more and more attentions from international geologists. At present, the studies on Mg isotope geochemistry are focusing on estimating the Mg isotopic compositions of the major geochemical resvervoirs and exploring the behavior of Mg isotopes during the different geological processes. However, due to the lack of high-precision Mg isotope data and the systematic studies, the behavior of Mg isotopes during the low temperature water-rock interaction processes, and the degree of influence of these processes on Mg isotopic compositions of continental crust and oceanic crust are still unclear. In order to provide a theoretical basis for better understanding of Mg geochemical cycle and the application of Mg isotopes as geochemical tracers for the geological processes, this thesis mainly focuses on the behavior of Mg isotopes during the major low temperature gaochmical processes, such as continental weathering, oceanic crust alteration and loess deposit formation. Consequently, detailed case studies are presented from basalt weathering profile in Hainan Island, alterated oceanic crust in the ODP Site801, and loess samples from the major loess deposits in the world, considering mineralogical abundance, major and trace element concentration, and isotopic composition, and the controls on the behaviors of Mg isotopes. The main knowledges based on these studies are following:(1) Magnesium isotopic compositions of a set of clay-rich saprolites developed on the Neogene tholeiitic basalt from Hainan island in southern China have been measured in order to document the behavior of Mg isotopes during continental weathering. Compared with unaltered basalts (δ26Mg=-0.36%o), the overlying saprolites are strongly depleted in Mg (i.e., τTh,Mg=-99.1to-92.9), and display highly variable δ26Mg, ranging from-0.49%o to+0.40‰. Magnesium concentration and δ26Mg value of the saprolites display a general increasing trend upwards in the lower part of the profile, but a decreasing trend towards the surface in the upper part. The variations of Mg concentration and isotopic composition in this weathering profile can be explained through adsorption and desorption processes,(a) adsorption of Mg to kaolin minerals (kaolinite and halloysite), with preferential uptake of heavy Mg isotopes onto kaolin minerals; and (b) desorption of Mg through cation exchange of Mg with the relatively lower hydration energy cations in the upper profile. Evidence for adsorption is supported by the positive correlation between δ26Mg and the modal abundance of kaolin minerals in saprolite of the lower profile, while negative correlations between δ26Mg and concentrations of lower hydration energy cations (e.g., Sr and Cs) in the upper profile support the desorption process. Our results highlight that adsorption and desorption of Mg on clay minerals play an important role in the behavior of Mg isotopes during extreme weathering, which may help to explain the large variation in Mg isotopic composition of river waters. Compilation of Mg isotopic data of weathered products reveals that behavior of Mg isotopes during continental weathering can be considered as a two-stage process,(a) limited Mg isotope fractionation during primary mineral dissolution in the incipient stage of weathering and (b) large Mg isotope fractionation during secondary mineral formation in the advanced stage of weathering.(2) Loess deposits serve as important continental archives for studying Quaternary climatic variations and for estimating the average chemical composition of the upper continental crust. Here, we report high-precision Mg isotopic data for nineteen loess samples from China, Argentina and Europe, which were previously used to estimate the composition of the upper continental crust. The results show that global loess have heterogeneous Mg isotopic compositions, with δ26Mg ranging from-1.64%o to+0.25‰and a weighted average of-0.89%o, which is lighter than both crust and mantle silicates. MgO content and826Mg of loess positively correlate with CaO/Al2O3ratio, which can be explained by a two-component mixing between carbonates and secondary silicate minerals. The large variation in Mg isotopic composition of loess results from the variations in the relative proportions of carbonates to clays in loess. Three factors:(a) source heterogeneity,(b) eolian sorting during transport and (c) chemical weathering after deposition, play important roles in controlling the distribution of carbonates and clays in loess. Both processes of eolian sorting and chemical weathering during formation of loess deposit tend to enrich clays in loess and in turn drive the Mg isotopic composition of loess toward heavier values. Significant correlations between δ26Mg values of loess from the same loss deposit and climatic indices such as CIA and SiO2/TiO2molar ratio indicate that Mg isotopic composition of loess may provide insights into paleoclimatic changes. However, our results suggest that Mg isotopic composition of loess may not represent the average Mg isotopic composition of the upper continental crust due to mixing and/or sorting of isotopically distinct components and isotope fractionation during loess deposit formation.(3) The altered oceanic crust (AOC) records the alteration processes and reveals much information about the global geochemical cycles of many elements such as Mg, which has high abundance in the solid Earth and ocean. The AOC therefore plays a central role in the global Mg cycle; however, the Mg isotope geochemistry of the AOC remains poorly constrained. The Mg isotopic composition of the altered oceanic crust from the ODP Site801in the western Pacific have been measured in order to investigate the behaviors of Mg isotopes during low-temperature interaction of seawater with oceanic basalt, and to estimate the flux of Mg isotopes of the oceanic crust that is recycled at subduction zones for the first time. Our results show that the AOC are depleted in Mg compared to fresh mid-ocean ridge basalt (MORB), and the change in MgO content (AMgO) decreases with depth. In addition, these AOC samples display highly variable δ26Mg, ranging from-2.82%o to+0.19%o. The SUPER composite, which represents the bulk upper oceanic basement sections of the ODP Site801, has a heavier826Mg value (=-0.02%o) relative to the bulk silicate earth or fresh MORB (δ26Mg=-0.25±0.08%o) previously reported by Teng et al (2010a). Composite samples prepared from volcanoclastics (VCL,δ26Mg=-1.01to0.14‰) and contained more Mg-rich secondary minerals have larger variations in826Mg compared to composite samples prepared from basaltic flows (FLO,δ26Mg=-0.54to-0.02%o), and on average is heavier than the later one. All these findings suggest that Mg isotopes have significantly fractionated during during low-temperature interaction of seawater with oceanic crust. Moreover, δ26Mg values of the AOCs positively correlated with their MgO contents. This correlationship can be explained by a three-component mixing among the fresh MORB, secondary carbonates and secondary silicate minerals, reflecting that Mg isotopic composition of the AOC is controlled by the relative proportion of primary and secondary Mg-rich mineral. The distribution of these secondary minerals in the AOC mainly results from combination of the water-rock ratio, temperature of alteration, and seawater oxidation during alteration of seawater with oceanic crust. For examples, the degree of alteration increases with the wate-rock ratio, and in turn produces higher abundance of the secondary carbonates in the AOC and lighter826Mg values; additionally, the AOC contains more secondary silicate minerals such as saponite and pyrite, and in turn has heavier826Mg value when the reaction environment trends to deoxidation; furthermore, the temperature of alteration decreases will induces higher abundance of carbonate and lighter Mg isotopic composition of the AOC. Based on the date from this study, the flux of Mg input from low-temperature alteration of oceanic crust is estimated about2×1010mol/yr, suggesting that this process might be the main factor driving the Mg concentration and Mg/Ca raio of seawater increasing over the past100million years. More importantly, the process of the AOC with highly heterogenous Mg isotopic composition along with the marine sediment recycled into mantle through subduction may lead to Mg isotopic composition of the mantle with heterogeneity.更多还原