现代长江沉积物地球化学组成的不均一性与物源示踪

长江沉积物从源到汇过程研究的关键是揭示长江沉积物的组成特征,准确识别其时空组成变化规律,这也是东部边缘海海陆相互作用研究和长江水系构造演化研究的关键.本文研究一些常用的元素地球化学参数在现代长江水系的河漫滩和悬浮沉积物中组成的时空变化特点.在稀土元素(REE)组成上,下游近河口段干流悬浮沉积物在季节性时间尺度上比较稳定,可以代表现代长江入海沉积物的平均组成,而长江主要支流和干流间REE组成变化较大,化学相态分析可以更好地揭示特定物源区的REE组成特征以及由矿物分异引起的REE参数变化.相比REE而言,悬浮物中Al/Ti与Zr/Rb比值更能敏感地指示不同物源区的源岩组成信息,且减弱水动力分选引起的粒级和矿物分异影响.长江入河口的干流悬浮物作为整个流域风化剥蚀细颗粒物质的平均混合,具有更好的源区平均组成的示踪特性;但在不同季节也存在不同的主导性源区,主要受控于流域内季风降雨区的迁移.在河流沉积物源汇过程重建中,要特别关注流域物质风化和输运的自然过程可能引起的这些地球化学参数变化.     

长江水系沉积物碎屑矿物组成及其示踪意义

长江碎屑矿物组成研究表明,轻矿物以石英、长石和岩屑为主,不同支流轻矿物组成特征不同,成熟度指数平均是2.0,一般干流高于支流,成熟度随沉积物搬运距离增加而增大。QFL及QtFL三角图解显示长江沉积物主要来自再旋回造山带物源区,流域风化剥蚀速度较快,不同支流物质汇入干流,使得干流轻矿物组成复杂多变而难以和支流区别。重矿物含量从长江上游至下游呈递减趋势,其主要组合是磁铁矿-普通角闪石-普通辉石-石榴子石-绿帘石-褐铁矿-钛铁矿。红柱石和磷灰石是金沙江沉积物的特征矿物组合;蓝晶石是岷江流域的特征矿物;涪江的特征矿物是榍石;汉江的特征矿物组合是磷灰石、紫苏辉石和硅镁石;锆石是湘江的特征矿物。不同流域的特征矿物指示其源岩性质。上游的雅砻江、大渡河以及岷江等支流沉积物对中、下游干流沉积物的贡献较弱。涪陵以上长江流域风化作用强烈,母岩主要是沉积岩类(碎屑岩、泥岩);其下流域沉积物中近源弱风化物质明显增加,其源岩类型体现为岩浆岩和变质岩类;而金沙江攀枝花地区及湘江、沅江沉积物则更多来自流域内广泛分布的大片变质岩类。     

长江流域沉积物黏土矿物组合特征及物源指示意义

通过X射线衍射方法系统分析了长江流域(干流和主要支流)表层沉积物(及部分表土)的黏土矿物,结果显示:长江流域黏土矿物主要由伊利石、绿泥石、高岭石和蒙脱石组成,伊利石含量最高,蒙脱石含量最低;长江干流上、中、下游黏土矿物含量基本一致,支流表现出明显区别,与流域不同的源岩类型和风化强度相对应.长江干流伊利石结晶度和化学指数...     

长江口沉积物化学相态分析及主泓变迁的指示

元素地球化学是沉积物源判别和环境研究的重要手段,但河口海岸地区沉积动力环境复杂多变,人类活动影响强烈,全岩沉积地球化学的示踪研究存在局限性和多解性。选择长江下游干流悬浮物、东海陆架表层沉积物以及长江口具有一百多年沉积记录的ZK6孔,通过化学相态分析（1 N HCl处理）,探究酸溶态微量元素组成特征及其对河口环境变迁的指示。相较于钻孔全岩样品,酸溶态Sr/Ba比能更可靠地反映河口古盐度和海陆相沉积环境的变化。ZK6孔沉积物酸溶态稀土元素（REE）主要赋存于Mn氧化物中,Mn、ΣREE含量、Ce/Ce*以及Sr/Ba比在1899—2007年间呈三段式变化,主要反映长江河口流路分汊和主泓位置改变引起的河口沉积环境变化,进而影响河口环境中活跃元素和次生组分在沉积地层中的保存记录。该研究对今后深化认识复杂河口环境下微量元素地球化学行为以及微量元素示踪海洋环境变化具有借鉴意义。     

长江上游水系沉积物锶-钕同位素组成及物源示踪

泥沙资料表明,现代长江干流沉积物主要源自上游地区。因此,长江上游干支流沉积物主控关系及其源汇过程在长江水系沉积物物源示踪研究中极为重要。为探讨上述过程,详细测定了上游水系沉积物Sr-Nd同位素组成。结果显示,金沙江及闽江沉积物具有较高的εNd(0)值,主要受控于流域内大面积分布的峨眉山玄武岩的高εNd(0)背景值;嘉陵江水系沉积物具有相对较低的εNd(0)值,反映了其流域内源岩对沉积物Nd同位素组成的控制;与Nd同位素组成相比,水系沉积物87Sr/86Sr值具有更大的变化范围,表明除源岩因素外,沉积物Sr同位素组成受更为复杂的因素制约。支流与干流沉积物Sr-Nd同位素组成对比表明,长江上游干流沉积物主要来源于金沙江流域内的源岩,金沙江流域内的表壳岩系主导了上游干流沉积物的Sr-Nd同位素组成。     

长江川江段现代沉积物的重矿物组合特征

川江是长江上游物源的主要汇集河段,其重矿物组合特征对长江的形成与演化研究非常重要。通过对金沙江、川江主河段及各支流沉积物进行系统的样品采集和重矿物组合特征分析发现,川江段干流重矿物组合稳定,以赤褐铁矿-磁铁矿-钛铁矿-绿帘石-角闪石-辉石-绿泥石为主。川江段干流与支流在重矿物组合方面存在显著不同,各主要支流的重矿物组合也有明显差异。金沙江、川江段及川江段的主要支流的沉积物重矿物特征与其源区的表壳岩系显示出极好的一致性。通过比较发现,川江段干流重矿物组合主要受金沙江制约,尽管有众多的支流汇入,但并未改变干流的重矿物组合特征,说明宜宾以上的表壳岩系对长江上游的重矿物组合特征具有支配意义。     

长江流域悬浮物磁性特征及其物源指示意义

通过分析长江干流及主要支流表层悬浮物的环境磁学参数特征(类型、含量及粒度)及其空间变化,探讨环境磁学对河流物源示踪的意义.结果表明,长江悬浮物磁性矿物特征以磁铁矿为主,含有少量赤铁矿及针铁矿.上游不完整反铁磁性矿物含量高于中下游,反映四川盆地广泛分布的紫红色砂页岩的影响.受攀枝花钒钛磁铁矿影响,干流磁性矿物含量上游高于中下游,表现为在攀枝花处迅速增加,向下游逐渐降低并在经过三峡后显著降低,说明三峡大坝对上游磁性矿物有明显的拦截作用.支流中雅砻江、汉江及湘江较高的磁性矿物含量分别受控于流域出露的源岩及工农业等人为影响.长江悬浮物xfd％大于5％,表明样品中含有超顺磁颗粒.受流域地势及气候影响,磁性颗粒的粒径从上游至下游逐渐变细.长江悬浮物磁性特征能够在一定程度上反映物源特征,但长江地质条件复杂,气候类型空间变异大,且流域人为影响较大(三峡大坝等),使得利用磁性矿物特征示踪物源受到限制.     

长江口晚新生代沉积物的物源研究: REE和Nd同位素制约*

长江三角洲地区第四纪以来堆积了200多米厚的碎屑沉积物,主要由河湖相和滨浅海相组成,构成了多个沉积旋回。选择长江口地区一个320m深的PD钻孔,运用多通道等离子体质谱MC ICP-MS方法,开展沉积物中的REE和Nd同位素组成分析,研究了上新世以来三角洲地区沉积物物源的变化。沉积物中REE和Nd同位素组成具有明显的变化规律,Ce呈弱的负异常,介于0.83~0.99之间,而Eu呈现中等亏损,在0.53~0.73之间变化。岩芯中上新统沉积物中Ce异常变化大,而Eu亏损相对第四系沉积物更显著。¹⁴³ Nd/¹⁴⁴ Nd比值在钻孔中变化较小,介于0.511975~0.5122367之间,平均值为0.512062。相关分析揭示粒度和化学风化对Nd同位素组成影响小。REE和Nd同位素判别图解揭示河口地区上新统沉积物主要来自长江流域中、下游的近源物源区,而第四系沉积物的物源虽然存在一定的变化,但是总体上与上新统沉积物来源明显不同,主要来自更广泛的流域物源区,尤其是长江上游的风化物质被大量输运到河口三角洲地区。在第四纪构造和气候因素控制下,古长江水系具有不同的演化阶段,流域源岩经历的风化作用强度也不同,因此河流沉积物的源汇过程也相应地发生变化。     

长江中下游干流河底沉积物环境磁性特征

通过多参数磁性测量,分析探讨了长江中下游干流3种不同粒级(<2mm,<0.28mm和<0.125mm)河底沉积物中磁性矿物的类型、含量、颗粒变化及空间分布特征。3种粒级中,磁性矿物主要富集在<0.125mm的细质沉积物中。对细质沉积物的分析表明,长江中下游干流河底沉积物的磁性矿物含量较长江口高近10倍,类型以亚铁磁性矿物磁铁矿为主,颗粒以假单畴-多畴为主,超顺磁性颗粒含量较低。从中游到下游,磁性矿物含量呈下降趋势,颗粒呈变细趋势。干流磁性矿物含量远高于支流,颗粒远粗于支流,支流泥沙的汇入不断影响干流沉积物的磁性特征。     

长江河流沉积物磁铁矿化学组成及其物源示踪

运用电子探针分析了长江干流和主要支流河漫滩沉积物中磁铁矿的元素组成.磁铁矿中的FeO平均含量稍高于其标准组成,而Fe2O3平均含量则明显低于标准组成;Ti、Al、Cr、V、Mn、Mg、Co和Zn等元素在磁铁矿中含量变化大,不同支流的磁铁矿的元素组成不同,同一取样点不同样品磁铁矿的元素组成变化也较大.金沙江、湘江、汉江及长江干流磁铁矿与钛磁铁矿、钛尖晶石、钒钛磁铁矿和铬铁矿等出溶交生,TiO2、Cr2O3和V2O3等元素含量高且变化大.金沙江磁铁矿富Mg、Al和Cr;大渡河、雅砻江和岷江磁铁矿中微量元素含量大多低于0.5%;涪江、汉江磁铁矿富Ti和V,而湘江磁铁矿富Ti和Al;总体上,长江干流上游磁铁矿富Ti,而下游磁铁矿中Ti、Al、Cr、V、Mg和Mn含量低于0.15%.干流磁铁矿的元素组成变化反映主要支流源岩组成及对干流影响程度的差异.     

Geochemistry of rare earth elements in the mainstream of the Yangtze River,China

Water, suspended matter, and sediment samples were taken from 8 locations along the Yangtze River in 1992. The concentration and speciation (exchangeable, bound to carbonates, bound to Fe–Mn oxides, bound to organic matter, and residual forms) of rare earth elements (La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu) were determined by instrumental neutron activation analysis (INAA).The contents of the soluble fraction of REEs in the river are low, and REEs mainly reside in particulate form. In the particles, the chondrite-normalized distribution patterns show significant LREE enrichment and Eu-depletion. While normalized to shales, both sediments and suspended matter samples show relative LREE enrichment and HREE depletion. REEs are relatively enriched in fine-grained fractions of the sediments.The speciation characteristics of REEs in the sediments and suspended matter are very similar. The amount of the five forms follows the order: residual>>bound to organic matter∼bound to Fe–Mn oxides>bound to carbonates>>exchangeable. About 65 to 85% of REEs in the particles exist in the residual form, and the exchangeable form is very low. High proportions of residual REEs reveal that REEs in sediments and suspended matter are controlled by their abundances in the earth's crust. Carbonate, Fe–Mn oxide and organic fractions of REEs in sediments account for 2.4–6.9%, 5.2–11.1%, and 7.3–14.0% of the total contents respectively. They are similar to those in the suspended matter. This shows that carbonates, Fe–Mn oxides and organic matter play important roles during the particle-water interaction processes. By normalization to shales, the 3 forms of REEs follow convex shapes according to atomic number with middle REE (Sm, Eu, and Tb) enrichment, while light REE and heavy REE are depleted.     

Composition of rare earth elements in settling particles collected in the highly productive North Pacific Ocean and Bering Sea: Implications for siliceous-matter dissolution kinetics and formation of two REE-enriched phases

Settling particles were sampled monthly for 1 year using an automated time-series sediment trap positioned at similar depths at two sites of high diatomaceous productivity in the North Pacific Ocean and Bering Sea. The particles were analyzed for rare earth elements (REEs) by inductively coupled plasma mass spectrometry (ICP-MS) with and without chemical treatment of the bulk samples to isolate siliceous fractions. The REE composition of the bulk samples is explained largely by the contribution of two distinct components: (i) carbonate with a higher REE concentration, a negative Ce anomaly and lighter REE (LREE) enrichment; (ii) opal with a lower REE concentration, a weaker negative Ce anomaly and heavier REE (HREE) enrichment.The siliceous fractions of settling particles are characterized by high Si/Al ratios (30-190), reflecting high diatom productivity at the studied sites. The La/Al ratio of the siliceous fraction is close to that of the upper crust, but the Lu/Al and Lu/La ratios are significantly higher than those of the upper crust or airborne particles, indicating the presence of excess HREEs in the siliceous fraction. Diatoms are believed to be important carriers of HREEs.The Ce anomaly, Eu anomaly, slope of the REE pattern, and ΣREE of the siliceous fraction vary exponentially with decreasing total mass flux. They can be well-reproduced according to the differential dissolution kinetics of elements in the order of Ce < lighter REEs (LREEs) < Eu = heavier REEs (HREEs) < Si from settling particles, where the dissolution rate is critically reduced through particle aggregation. This order is consistent with the vertical distribution of dissolved REEs and Si in oceans. The differential dissolution kinetics leads to HREE enrichment of the original diatoms and REE enrichment of dissolved diatoms. The Lu/Si ratio of the siliceous fraction of settling particles recovered from some of the highest diatom fluxes is identical to that of the two elements dissolved in deep seawater, providing further evidence for the dissolution of siliceous matter in deep water.     

Geochemistry of the Rare Earth Elements in Natural Terrestrial Waters：A Review of What Is Currently Knows

The range of observed chemical compositions of natural terrestrial waters varies greatly especially when compared to the essentially constant global composition of the oceans.The concentrations of the REEs in natural terrestrial waters also exhibit more variation than what was reported in seawater,In terrestrial waters ,pH values span the range from acid up to alkaline,In addition,terrestrial waters can range from very dilute waters through to highly concentrated brines.The REE concentrations and their behavior in natural terrestrial waters reflect these compositional ranges,Chemical weathering of rocks represents the source of the REEs to natural terrestrial waters and ,consequently,the REE signature of rocks can impart their REE signature to associated waters,In addition,Because of the typical low solubilities of the REEs both surface and solution complexation can be important in fractionating REEs in aqueous solution.Both of these processes are important in all natural terrestrial waters,however,their relative importance varies as a function of the overall solution composition,In alkaline waters,for example,Solution complexation of the REEs with carbonate ions appears to control their aqueous distributions whereas in acid waters,the REE signature of the labile fraction of the REEs is readily leached from the rocks.In circumneutral pH waters,both processes appear to be important and their relative significance has not yet been determined.     

Assimilation and partial melting of continental crust: evidence from the mineralogy and geochemistry of autoliths and xenoliths

This paper presents a model for the partial melting of quartz diorite and greywacke in the upper crust based on the mineralogy and geochemistry of enclaves within the Loch Doon granitic intrusion of southern Scotland. The melting of quartz diorite was modelled using autoliths, which represent fragments of cogenetic igneous rocks that became incorporated in the fractionating magma. Compared to their quartz diorite parents, the autoliths are enriched to varying degrees in some elements (notably Rb, Nb, Ta, Sm, Y, Yb) and depleted in others (Sr, and Ba); Eu and P are also depleted in the more assimitated autoliths. The compositions of melts that could be derived from assimilation of the autoliths have also been calculated: their REE patterns reveal a light REE enrichment, low concentrations of heavy REEs (1–3 x chondrite) and a positive Eu anomaly. The calculated degrees of melting vary from 35% in the least assimilated to 84% in the most assimilated autolith (assuming a bulk distribution coefficient of 10 for the most compatible element). Results from modelling of xenolith compositions (derived from metasediments) are also reported, but because of uncertainties in the composition of the parental sediment, these data are subject to larger errors. They do, however, indicate that resultant partial melts are distinctly different from those derived by partial melting of autoliths. In particular, the REE pattern of a greywacke-derived melt shows a slight enrichment in light REEs, greater concentrations of heavy REEs (10 x chondrite) and a small negative Eu anomaly. The calculated degrees of melting of the xenoliths fall in the range of 66–88% (assuming a bulk distribution coefficient of 10 for the most compatible element). The results have direct implications for assimilation and melting of the upper crust. By taking into account how the nature of residual phases is likely to change with depth, it can be demonstrated that some Archaean tonalite gneisses could represent liquids derived by partial melting of igneous material.     

Multiple luminescent spectroscopic methods applied to the two related minerals,leucophanite and meliphanite

The photo-, cathodo-, radio- and ionoluminescence of two related beryllium minerals, leucophanite and meliphanite, have been determined. Meliphanite is an efficient luminescent mineral and has similar luminescence to leucophanite. Both leucophanite and meliphanite efficiently transfer energy to all activators in the sample either from the conduction or through a charge transfer band. We infer that coupling of the luminescence centre and lattice is important for interpreting rare earth element (REE) luminescence. Furthermore, changes in the relative intensity between REE emissions during sample degradation by CL demonstrate that each REE interacts differently with the host. Conversely, when a specific energy level of one REE is targeted in photoluminescence, no energy transfer to other REEs is observed despite many REEs having similarly spaced energy levels. This demonstrates that absorption and luminescence within an individual REE can be considered as intra-ion energy cascades, with little or no energy transfer to other REEs. Anomalous behaviour in the temperature dependence of IL in one leucophanite sample is interpreted as phase transitions in ice hosted in fluid inclusions. A comparison of luminescence of powder and cleavage fragment reveals that some emissions in the UV–blue region are surface related and do not represent bulk luminescence.     

广西某地花岗岩风化壳中稀土元素特征与iREE矿床成矿机制

为了解风化壳中离子交换相稀土元素的特征,对广西某地花岗岩风化壳剖面样品进行了X射线衍射及主量、稀土元素地球化学特征的研究.剖面自上而下可划分为腐殖土层(A₁)、亚粘土层(A₂)、网纹状风化层(B₁)和全风化层(B₂);自A₁至B₂,粘土矿物的含量和化学风化蚀变指数快速降低;与母岩相比A₁、A₂、B₁中全相Ce、Nd和HREE相对富集,B₂中全相稀土与母岩特征相似,所有样品的离子交换相HREE亏损,Y相对富集;离子交换相轻、重稀土一起富集在B₂中.据此推测,花岗岩中褐帘石、榍石等易风化的稀土矿物为离子交换相稀土提供了主要的物源,锆石、磷钇矿等难风化的稀土矿物的残留及表生稀土矿物的形成使全相HREE相对富集;离子交换相轻、重稀土元素的分馏程度随风化程度的增加而变化.      

Fractionation characteristics of rare earth elements (REEs) linked with secondary Fe,Mn, and Al minerals in soils

Soil secondary minerals are important scavengers of rare earth elements (REEs) in soils and thus affect geochemical behavior and occurrence of REEs. The fractionation of REEs is a common geochemical phenomenon in soils but has received little attention, especially fractionation induced by secondary minerals. In this study, REEs (La to Lu and Y) associated with soil-abundant secondary minerals Fe-, Al-, and Mn-oxides in 196 soil samples were investigated to explore the fractionation and anomalies of REEs related to the minerals. The results show right-inclined chondrite-normalized REE patterns for La–Lu in soils subjected to total soil digestion and partial soil extraction. Light REEs (LREEs) enrichment features were negatively correlated with a Eu anomaly and positively correlated with a Ce anomaly. The fractionation between LREEs and heavy REEs (HREEs) was attributed to the high adsorption affinity of LREEs to secondary minerals and the preferred activation/leaching of HREEs. The substantial fractions of REEs in soils extracted by oxalate and Dithionite-Citrate-Bicarbonate buffer solutions were labile (10 %–30 %), which were similar to the mass fraction of Fe (10 %–20 %). Furthermore, Eu was found to be more mobile than the other REEs in the soils, whereas Ce was less mobile. These results add to our understanding of the distribution and geochemical behavior of REEs in soils, and also help to deduce the conditions of soil formation from REE fractionation.     

Post-depositional redistribution processes and their effects on middle rare earth element precipitation and the cerium anomaly in sediments in the South Korea Plateau,East Sea

We sampled two box-core sediments from the slope of the eastern South Korea Plateau (SKP) in the East Sea (Sea of Japan) at water depths of 1400 and 1700 m. Two chemical fractions of extractable (hydroxylamine/acetic acid) and residual rare earth elements (REEs) together with Al, Ca, Fe, Mg, Mn, P, S, As, Mo, and U were analyzed to assess the post-depositional redistribution of REEs. Extractable Fe and Mn are noticeably abundant in the oxic topmost sediment layer (<3 cm). However, some trace elements (e.g., S, As, Mo, U) are more abundant at depth, where redox conditions are different. Analysis of upper continental crust (UCC)-normalized (La/Gd)_UCC, (La/Yb)_UCC, and (Ce/Ce^*)_UCC revealed that the extractable REE is characterized by middle REE (MREE) enrichment and a positive cerium (Ce) anomaly, different from the case of the residual fraction which shows slight enrichment in light REEs (LREEs) with no Ce anomaly. The extractable MREEs seem to have been incorporated into high-Mg calcite during reductive dissolution of Fe oxyhydroxides. In the top sediment layer, the positive Ce anomaly is attributed to Ce oxide, which can be mobilized in deeper oxygen-poor environments and redistributed in the sediment column. In addition, differential concentrations of Ce and other LREEs in pore water appear to result in variable (Ce/Ce^*)_UCC ratios in the extractable fraction at depth.     

REE geochemistry of fine-grained sediments from major rivers around the Yellow Sea

To investigate the distribution pattern and controlling factors of rare earth elements (REEs) in riverine sediments, river mouth sediments were collected at five geographically different rivers around the Yellow Sea. Two- (1 M HCl leached and residual fractions) and five-step sequential extraction schemes (the SEDEX method) were applied to size-separated sediments < 20 µm. For the total REE composition, patterns normalized relative to the upper continental crust (UCC) showed light REE (LREE) and middle (MREE) enrichments in the Korean river sediments and MREE enrichment in the Chinese river sediments. LREE and MREE enrichments in 1 M HCl leached fractions played a major role in the distribution patterns of the total compositions in all the river sediments. About half of LREE enrichment in Korean river sediments was explained by the residual fraction. Comparison of the REE composition with major elements (Al, Fe, Ca, P) in each fraction of the SEDEX scheme revealed that MREE enrichment could be explained by reactive iron minerals, including goethite and hematite, although REE/Fe ratios showed different trends among the rivers due to different major REE-host iron minerals. After extracting reactive irons, authigenic phosphate, and carbonate, the sequential 1 M HCl fraction indicated that LREE enrichment in Korean river sediments may have originated from clay minerals, such as chlorite. These observations suggest that LREE enrichment may be a good tracer, while MREE enrichment should be used cautiously considering diagenetic modification, when using the REE composition to identify the sources of terrestrial materials.     

New data on distribution of REEs in sulfide minerals and Sm-Nd dating of ore genesis of layered basic intrusions

This paper presents data on the distribution of REEs in sulfide minerals from ore-bearing gabbronorites in the Penikat layered intrusion and the results of their isotopic-geochronological Sm-Nd study. A new procedure for determination of REEs in the samples without preliminary separating and concentrating was tested on standard samples to be further used for analysis of sulfide minerals. Analysis of the spectra of the REE distribution in sulfides represents a distribution trend that is similar to the already studied bulk rock and allows deducing that the character of the REE distribution in sulfide minerals from gabbronorites in the Penikat layered intrusion was inherited from the parent magma melt; while the formation of sulfides took place at the stage of rock crystallization. The performed complex studies allow considering that sulfides can be successfully used together with the rock-forming minerals in Sm-Nd dating of ore-bearing mafite-ultramafite intrusions.