贵州碳酸盐岩风化壳中次生晶体石英硅、氧同位素特征及其成因探讨

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	贵州碳酸盐岩风化壳中次生晶体石英硅、氧同位素特征及其成因探讨
其他题名	Origin of secondary crystal quartz in weathering crusts of carbonate rocks in Guizhou province based on δ30Si and δ18O analyses
	姜立君
	2008-06-03
学位授予单位	中国科学院地球化学研究所
学位授予地点	地球化学研究所
学位名称	博士
关键词	硅同位素氧同位素风化壳碳酸盐岩贵州
摘要	在贵州碳酸盐岩风化壳中发现的晶体石英使风化壳及其所发育的地貌面的定年研究成为可能，对这种石英进行裂变径迹尝试性测年工作，发现它们具有很强的研究潜力，是目前为止在碳酸盐岩风化壳剖面中发现的唯一可作为测年对象的矿物。前期工作中进行的风化壳剖面物源判别及石英表面结构观察表明晶体石英是原位形成的，晶体形态指示它们可能是次生的，其裂变径迹测年结果远小于母岩年龄，从另一方面也证明了它们可能是次生的。然而，上述证据只能对贵州碳酸盐岩风化壳中晶体石英的成因进行初步的判定，即次生形成于剖面中，未能揭示其确切的成因。由于，晶体石英成因的确定是利用它们开展测年工作的前提和基础，所以当前急需对这类石英进行成因判别，以明确裂变径迹年龄值的年代学意义。本文在前人成果的基础上，首次对贵州碳酸盐岩风化壳中的晶体石英进行了较系统的研究，通过晶体形态观察、石英粒度分析、包裹体观察来判断其基本成因，利用硅、氧同位素的组成及与其它地质体的对比揭示晶体石英形成的硅质来源、流体性质、沉淀结晶环境等信息，主要获得了以下几方面的认识： 1、风化壳中石英颗粒的粒度特征新蒲和大兴剖面中提取出的石英多分布在粉砂(2 ~ 63 μm)和砂粒(>63 μm)粒级范围内，其中以粉砂粒级为主。石英的粒度分布曲线常呈多峰分布，与黄土-古土壤中石英的单峰分布曲线明显不同，说明剖面中的石英不是风成成因的，继而证明两风化壳剖面不是风尘堆积的产物。两剖面中石英颗粒的分选性较差，不同样品之间，石英的粒度分布曲线存在差异，可能是次生石英、母岩中的碎屑石英和燧石团块中的石英以不同比例混合的结果。鉴于以上可能性，与酸不溶物相比，土层的粒度分布曲线表现出较好的一致性，即在碳酸盐岩风化壳物源研究中，石英的粒度分布并不是理想的示踪手段。 2、风化壳中石英颗粒的包裹体观察及均一温度测量研究剖面中晶体石英的包裹体呈带状分布，多数为单相包体，小于5 μm，无法进行均一温度的测量。在某些次生加大石英或石英胶结物中包裹体很少或未见可供测温的两相包裹体，可能就是由于形成温度低(小于70 ˚C或地表温度)，生长缓慢造成的。研究剖面中晶体石英的包裹体特征与某些次生加大石英或石英胶结物中的包裹体类似，暗示晶体石英可能为低温条件下缓慢形成的，与其形态特征一致。 3、风化壳中晶体石英的硅同位素特征及其硅质来源探讨研究剖面中晶体石英的δ30Si值在0.8‰ ~ 1.7‰之间，与火山作用和热液(水)作用产物具有明显不同的硅同位素组成，表明与火山作用或热液(水)作用无直接关系，而与风化作用有关，可能由风化过程中产生的富30Si流体形成。有研究表明风化过程中溶解的二氧化硅通常来自硅酸盐矿物的化学风化而非石英的溶解，研究剖面中晶体石英的硅质来源以硅酸盐矿物风化释放的硅质流体为主。利用Ge/Si值评估植物作用能否为研究剖面中晶体石英的形成提供物质来源，结果表明植物作用对研究剖面的硅质贡献作用相对较小，在表土层中植物作用的影响可能较大。以Ti作为参比元素来评估风化作用能否为晶体石英提供硅质来源，结果表明剖面中存在稳定且可观的硅的迁出，说明剖面中的晶体石英具有稳定且充足的硅质来源。 4、风化壳中晶体石英的氧同位素特征及其形成条件分析研究剖面中晶体石英的δ18O值与高温成因石英及高、低温混合来源石英的δ18O值明显不同，在低温石英δ18O值范围内，暗示其形成于低温条件，与晶体石英的形态及包裹体的特征吻合。由于化学风化过程中W/R值很高，研究剖面中母岩以及粘土矿物的同位素组成对晶体石英δ18O值的影响可以忽略，即晶体石英的δ18O值可以粗略地反映大气降水的δ18O值，并可能进一步指示古气候信息。 5、风化壳中不同石英的硅、氧同位素特征及其成因指示石英的硅、氧同位素分析表明，尽管大兴剖面中圆状石英和棱角状石英具有与晶体石英相似的硅、氧同位素组成特征，但它们的硅-氧同位素相关关系不同，可以明显地将晶体石英与圆状石英和棱角状石英区分开，说明它们具有不同的成因。 6、晶体石英的形成与贵州碳酸盐岩风化成土的关系风化壳中晶体石英的形成与贵州碳酸盐岩风化成土的两阶段过程密切相关，酸不溶物中长石等硅酸盐矿物的风化能够为晶体生长提供硅质来源，碳酸盐岩风化形成的孔洞，则为晶体石英提供有利的生长环境和充分的生长空间。
其他摘要	The crystal quartz found in the weathering crusts of carbonate rocks in Guizhou Province makes it possible to date the weathering crusts or planation surface. Experiment carried out on the application of crystal quartz in fission track dating suggests that the crystal quartz has strong potentialities in the dating of weathering profiles. It is also the only mineral found in the studied weathering profiles that can be used in the chronology of weathering crusts. The provenance of weathering crusts and the surface structure characteristics of quartz suggest that the crystal quartz must be in situ origin. The results of fission track dating of crystal quartz are far younger than the ages of parent rocks, indicating the secondary origin of the crystal quartz, consistent with its crystal form. It is well-kown that the origin of the crystal quartz is the prerequisite and foundation for the application of crystal quartz in fission track dating, however, the specific origin of the crystal quartz is not clear. So it is imperative to determinate the specific origin of the crystal quartz and the value of fission track dating of crystal quartz. Based on the previous research, the crystal quartz in the weathering crusts of carbonate rocks is first systemically studied. The genetic investigation of crystal quartz is preliminary made from crystal shapes, grain size distribution and fluid inclusion investigation. By studying silicon and oxygen isotopic compositions, it can be confirmed about the silica source, fluid characteristic and deposit position. Some new ideas and conclusions have been obtained. 1. grain-size distribution of quartz in the weathering crusts The quartz isolated from Xinpu and Daxing profiles has high content of sand(>63 μm) and silt(2 ~ 63 μm) and the latter is in the majority. Curves of the grain-size distribution of quartz show multi-peak modal and different quartz has different grain-size distribution, distinctly different from those of loess or paleosol. Which indicate that the quartz in the weathering crusts do not have eolian origin, that is to say, the weathering crusts are not from eolian dust. The low degree of sorting suggests that the quartz in the studied weathering crusts may be mixed from secondary quartz, detrital quartz in parent rocks and quartz in chert noddles. Considering the complicated origins of the quartz the grain-size distribution of quartz grains is not a perfect method of tracing the origin of weathering cursts of carbonate rocks. 2. The fluid inclusion observation and the homogenization temperatures The fluid inclusions in crystal quartz are generally in zonal distribution, and mainly are single-phase (liquid) inclusions. The fluid inclusions in crystal quartz are usually smaller than 5 μm and can not be used to measure homogenization temperatures. The characteristics of the fluid inclusions in crystal quartz are similar to those in some quartz overgrowths or quartz cement, which suggest a slow growth at near-surface temperatures or temperatures no greater than 70 ˚C. The scarcity and the lack of two-phase fluid inclusions in the crystal quartz imply a slow growth at low temperature consistent with crystal shapes. 3. Silicon isotope compositions and silica source of the crystal quartz The δ30Si values of crystal quartz are between 0.8‰ and 1.7‰, obviously different from those of samples formed in volcanism and hydrothormalism. These differences suggest that the source of silica is irrelevant to volcanism or hydrothermalism and may be related to weathering. The crystal quartz may precipitate from the fluid rich in 30Si released during the weathering process. Considering the dissolve Si is generally from the dissolution of silicate minerals during the weathering process, the dissolution of the primary quartz and cherty noddles in the parent rocks must be tiny. The source of silica may be mostly from the weathering of silicate minerals. Ge/Si ratios used to estimate biogenic silicon in the profiles suggest that except the top soil biogenic silicon may have little contribution to the silica source of the crystal quartz in the studied profiles. Using Ti as the conservative element, the mobility of silicon suggests that a mass of silicon can be leached out during weathering process which can act as abundant and steady silica source of the crystal quartz. 4. Paleoenvironment and the oxygen isotope compositions of the crystal quartz The δ18O values of crystal quartz are distinctly different from those of quartz with high temperature origins and quartz mixed with high and low temperature origins. They are accordant with the δ18O values of quartz with low temperature origins suggesting that the crystal quartz form in low temperature consistent with the fluid inclusions observation and the crystal shapes of the quartz. In view of high W/R ratios, the isotopic compositions of the parent rocks and clays may have little influence on the δ18O values of crystal quartz. That is to say the δ18O values of crystal quartz approximately reflect the oxygen isotope compositions of precipitation, which can be used to indicate the paleoclimate. 5. Silicon and oxygen isotope compositions of different types of quartz and their genetic indication Although silicon and oxygen isotope compositions of other quartz are similar to those of the crystal quartz in the Daxing profile, the relationships between the silicon and oxygen isotopes are different. In this way the crystal quartz can be distinguished from the rounded quartz and pointed quartz, implying their different origins. 6. The relationship between the formation of crystal quartz and the weathering of carbonate rocks in Guizhou province The formation of crystal quartz correlates closely with the two-stage development model for red residua in the karst terrain in Guizhou province. In the second weathering stage the silica source of crystal quartz is from the the weathering of silicate minerals such as feldspar in “insoluble” constituents. Then the crystal quartz slowly precipitated in the caves in the bedrocks formed in the first weathering stage.
页数	104
语种	中文
文献类型	学位论文
条目标识符	http://ir.gyig.ac.cn/handle/352002/3250
专题	研究生_研究生_学位论文
推荐引用方式 GB/T 7714	姜立君. 贵州碳酸盐岩风化壳中次生晶体石英硅、氧同位素特征及其成因探讨[D]. 地球化学研究所. 中国科学院地球化学研究所,2008.

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