發(fā)布時間：2018-05-08 00:13

  本文選題：石灰?guī)r + 化學(xué)風(fēng)化��； 參考：《中國科學(xué)院大學(xué)(中國科學(xué)院廣州地球化學(xué)研究所)》2017年碩士論文

【摘要】：石灰?guī)r是廣布地表的一類重要巖石,其化學(xué)風(fēng)化過程制約著水圈的化學(xué)演化和生物地球化學(xué)循環(huán)。首先,地表出露的石灰?guī)r快速溶解后,向水體釋放大量的可溶性離子,一定程度上控制著其所在流域的水化學(xué)組成,進而制約著海水的化學(xué)演化。其次,石灰?guī)r化學(xué)風(fēng)化釋放的元素,如Mg、Ca、Sr、Ba廣泛參與生物地球化學(xué)循環(huán),因此,石灰?guī)r化學(xué)風(fēng)化還制約生物地球化學(xué)過程。此外,石灰?guī)r化學(xué)風(fēng)化是全球Sr循環(huán)的一個重要源區(qū),對地史時期的全球Sr循環(huán)具有重要影響。然而,目前鮮有研究報道碳酸鹽巖化學(xué)風(fēng)化過程中的元素遷移與再分配以及Sr同位素變化的控制機制。為了更好地認(rèn)識這一重要科學(xué)問題,本文以粵北地區(qū)的典型石灰?guī)r風(fēng)化剖面作為研究對象,基于高分辨率的礦物學(xué)、元素地球化學(xué)和Sr-Nd同位素的綜合研究,得出以下幾點認(rèn)識:1.原巖中方解石的溶解造成了堿土金屬元素Ca、Sr大量遷出剖面;而其它堿金屬-堿土金屬元素以及過渡金屬元素則發(fā)生了不同程度的富集,這可能是由黏土礦物的吸附作用所致。2.粵北石灰?guī)r化學(xué)風(fēng)化過程中,REE發(fā)生了顯著的富集與分異:在近中性的介質(zhì)中,碳酸根/碳酸氫根離子與REE形成穩(wěn)定絡(luò)合物,顯著提高了REE的遷移能力,促使風(fēng)化產(chǎn)物中的REE發(fā)生了顯著富集。然而,隨著原子序數(shù)的遞增,REE的離子半徑逐漸減小,造成HREE與碳酸根/碳酸氫根離子形成絡(luò)合物的穩(wěn)定性顯著大于LREE與碳酸根離子形成的絡(luò)合物,最終導(dǎo)致HREE的富集程度顯著大于LREE。3.該剖面上~(87)Sr/~(86)Sr比值呈現(xiàn)出較大的變化范圍(0.70979-0.72216),這與外來源區(qū)輸入沒有直接聯(lián)系。~(87)Sr/~(86)Sr比值與Sr含量的顯著負(fù)相關(guān)性說明,Sr同位素的演化主要受控于碳酸鹽礦物與硅酸鹽礦物在抗風(fēng)化強度以及Sr同位素組成方面的顯著差異:隨著風(fēng)化作用的進行,原巖中具有較低~(87)Sr/~(86)Sr比值的方解石中的Sr大量遷出剖面,而具有較高~(87)Sr/~(86)Sr比值的硅酸鹽礦物中的Sr殘留在風(fēng)化產(chǎn)物中,從而造成了~(87)Sr/~(86)Sr比值從剖面下部到上部呈現(xiàn)出逐漸增大的趨勢。
[Abstract]:Limestone is one of the most important rocks on the surface. Its chemical weathering process restricts the chemical evolution and biogeochemical cycle of hydrosphere. Firstly, after the limestone is dissolved rapidly, a large amount of soluble ions are released into the water body, which to some extent controls the hydrochemical composition of the basin, and then restricts the chemical evolution of the sea water. Secondly, the elements released by chemical weathering of limestone, such as MgCa-Sr-Ba, are widely involved in the biogeochemical cycle, so chemical weathering of limestone also restricts the biogeochemical process. In addition, the chemical weathering of limestone is an important source of the global Sr cycle, which has an important influence on the global Sr cycle during the geological history. However, there are few studies on the mechanism of element migration and redistribution during chemical weathering and the control mechanism of Sr isotopic variation in carbonate rocks. In order to better understand this important scientific problem, the typical limestone weathering profile in northern Guangdong is taken as the research object. Based on the comprehensive study of high resolution mineralogy, element geochemistry and Sr-Nd isotopes, the following points are obtained: 1. The dissolution of calcite in the original rock resulted in the mass migration of alkali earth metal element CaOSr out of the profile, while other alkali metal-alkali earth metal elements and transition metal elements were enriched to varying degrees, which may be due to the adsorption of clay minerals. In the chemical weathering process of limestone in northern Guangdong Province, there are significant enrichment and differentiation of REE: in near-neutral medium, carbonate / bicarbonate ions form a stable complex with REE, which significantly improves the migration ability of REE. The REE in the weathering products was enriched significantly. However, with the increase of atomic number, the ionic radius of HREE decreases gradually, which results in the stability of the formation complex of HREE with carbonate / bicarbonate ion is significantly higher than that of the complex formed by LREE and carbonate ion. As a result, the enrichment of HREE was significantly higher than that of LREE.3. In this section, there is a large range of variation in the ratio of Sr ~ (86) ~ + ~ 0.70979-0.72216 ~ 0.70979-0.72216 ~ 0.70979-0.72216 ~ 0.70979-0.72216 ~ 0.70979-0.72216 ~ (th), which has no direct relation with the input from the source region. The significant negative correlation between Sr ~ (86) ~ (Sr) ratio and Sr content indicates that the evolution of Sr isotopes is mainly controlled by carbonate minerals and silicate deposits. Significant differences in weathering strength and Sr isotopic composition: as weathering progresses, A large number of Sr migration profiles are found in calcite with a lower Sr ~ (87) ~ (- /) ~ + ~ (86) ~ (-1) Sr ratio in the original rock, while Sr residue in silicate minerals with a high Sr ~ (87) ~ (- /) ~ + ~ (86) Sr ratio is found in weathering products, which results in a gradual increase in the Sr ~ (86) ~ (Sr) ratio from the lower to the upper part of the profile.
【學(xué)位授予單位】：中國科學(xué)院大學(xué)(中國科學(xué)院廣州地球化學(xué)研究所)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：P512.1;P59

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