Determining mineral weathering rates based on solid and solute weathering gradients and velocities: application to biotite weathering in saprolites

Chemical weathering gradients are defined by the changes in the measured elemental concentrations in solids and pore waters with depth in soils and regoliths. An increase in the mineral weathering rate increases the change in these concentrations with depth while increases in the weathering velocity decrease the change. The solid-state weathering velocity is the rate at which the weathering front propagates through the regolith and the solute weathering velocity is equivalent to the rate of pore water infiltration. These relationships provide a unifying approach to calculating both solid and solute weathering rates from the respective ratios of the weathering velocities and gradients. Contemporary weathering rates based on solute residence times can be directly compared to long-term past weathering based on changes in regolith composition. Both rates incorporate identical parameters describing mineral abundance, stoichiometry, and surface area.

Weathering gradients were used to calculate biotite weathering rates in saprolitic regoliths in the Piedmont of Northern Georgia, USA and in Luquillo Mountains of Puerto Rico. Solid-state weathering gradients for Mg and K at Panola produced reaction rates of 3 to 6×10⁻¹⁷ mol m⁻² s⁻¹ for biotite. Faster weathering rates of 1.8 to 3.6×10⁻¹⁶ mol m⁻² s⁻¹ are calculated based on Mg and K pore water gradients in the Rio Icacos regolith. The relative rates are in agreement with a warmer and wetter tropical climate in Puerto Rico. Both natural rates are three to six orders of magnitude slower than reported experimental rates of biotite weathering.     

冷冻固定化优势菌群处理含甲醛苯酚废水

包气带土壤中氨氮污染迁移规律已成为近年来国内外学者研究的重点,目前在氨氮污染迁移规律研究中采用柱实验模拟以及软件模拟的方法较多,土壤生物技术则广泛应用于污染物降解领域,而在污染物迁移规律研究中应用较少。本文采用变性梯度凝胶电泳(DGGE)技术、16S rRNA序列分析技术以及典范对应分析相结合,对华北平原3个典型氨氮污染区土壤表层至包气带剖面微环境的细菌垂直分布特征及群落结构进行研究。结合污染区土壤理化性质分析,认为包气带土壤剖面的细菌群落中存在着与氮循环、硫酸盐代谢等过程偶联的优势细菌类群,说明土壤微环境中细菌群落分布明显受氨态氮、硝态氮、亚硝态氮和硫酸盐的分布影响,进一步表明污染土壤优势菌群的群落结构信息是描述包气带土壤环境氨氮污染物迁移规律的重要参数。     

Mechanism of feldspar weathering—II. Observations of feldspars from soils

Examination of the surface morphology (via scanning electron microscopy) and surface composition (via X-ray photoelectron spectroscopy) of sodic plagioclase and potash feldspar grains taken from four different soils, provides little or no evidence for the existence of a tightly adhering protective surface layer of altered composition on the feldspar surface. Grains, from which all adhering clay has been removed by ultrasonic cleaning, exhibit the same chemical composition in the outermost few tens of angstroms as the underlying feldspar. Aluminum-rich ‘clay’ coatings which continue to adhere to the grains after ultrasonic treatment are patchy, highly hydrous, and unlikely to act as major diffusion-limiting, and thus protective, barriers. Attack by dissolution of the feldspar surface is non-uniform and follows a definite etching sequence characterized by the development and growth of distinctive etch pits. This dissolution sequence can be reproduced by treating fresh feldspars in the laboratory with strong HF-H₂SO₄^? solutions and, thus, the sequence is unaffected by the composition of the attacking solution. All of our results suggest that the dissolution of feldspar during weathering is controlled by selective chemical reaction at the feldspar-solution interface and not by uniform diffusion through a protective surface layer.     

Climatic and tectonic controls on chemical weathering in the New Zealand Southern Alps

Climatic and tectonic controls on the relative abundance of solutes in streams draining the New Zealand Southern Alps were investigated by analyzing the elemental and Sr isotope geochemistry of stream waters, bedload sediment, and hydrothermal calcite veins. The average relative molar abundance of major cations and Si in all stream waters follows the order Ca²⁺ (50%) > Si (22%) > Na⁺ (17%) > Mg²⁺ (6%) > K⁺ (5%). For major anions, the relative molar abundance is HCO₃⁻ (89%) > SO₄²⁻ (7%) > Cl⁻ (4%). Weathering reactions involving plagioclase and volumetrically small amounts of hydrothermal calcite define the ionic chemistry of stream waters, but nearly all streams have a carbonate-dominated Ca²⁺ and HCO₃⁻ mass-balance. Stream water Ca/Sr and ⁸⁷Sr/⁸⁶Sr ratios vary from 0.173 to 0.439 μmol/nmol and from 0.7078 to 0.7114, respectively. Consistent with the ionic budget, these ratios lie solely within the range of values measured for bedload carbonate (Ca/Sr = 0.178 to 0.886 μmol/nmol; ⁸⁷Sr/⁸⁶Sr = 0.7081 to 0.7118) and hydrothermal calcite veins (Ca/Sr = 0.491 to 3.33 μmol/nmol; ⁸⁷Sr/⁸⁶Sr = 0.7076 to 0.7097).Streams draining regions in the Southern Alps with high rates of physical erosion induced by rapid tectonic uplift and an extremely wet climate contain ∼10% more Ca²⁺ and ∼30% more Sr²⁺ from carbonate weathering compared to streams draining regions in drier, more stable landscapes. Similarly, streams draining glaciated watersheds contain ∼25% more Sr²⁺ from carbonate weathering compared to streams draining non-glaciated watersheds. The highest abundance of carbonate-derived solutes in the most physically active regions of the Southern Alps is attributed to the tectonic exhumation and mechanical denudation of metamorphic bedrock, which contains trace amounts of calcite estimated to weather ∼350 times faster than plagioclase in this environment. In contrast, regions in the Southern Alps experiencing lower rates of uplift and erosion have a greater abundance of silicate- versus carbonate-derived cations. These findings highlight a strong coupling between physical controls on landscape development and sources of solutes to stream waters. Using the Southern Alps as a model for assessing the role of active tectonics in geochemical cycles, this study suggests that rapid mountain uplift results in an enhanced influence of carbonate weathering on the dissolved ion composition delivered to seawater.     

岩溶关键带C-N耦合循环与碳酸盐岩风化——以重庆雪玉洞观测站为例

本文尝试利用重庆丰都雪玉洞岩溶关键带观测站2015年7月~2016年6月大气N湿沉降、地下水化学以及δ¹⁵N-NO₃^-、δ¹⁸O-NO₃^-和δ¹³C同位素等数据,探讨岩溶关键带C-N耦合循环过程与环境效应。结果表明：1）观测站大气N湿沉降通量为20.9×10³kg N/a,其中NH₄⁺-N和NO₃^--N分别占湿沉降量的52%和48%;2）地下水δ¹⁵N-NO₃^-和δ¹⁸O-NO₃^-分别介于2.5 ‰~5.6 ‰和3.3 ‰~15.6 ‰,大气沉降N和土壤N是地下水NO₃^-的主要来源;3）变化于-13.7 ‰~-10.4 ‰与0.59~0.62的地下水δ¹³C_DIC与（Ca²⁺+Mg²⁺）/HCO₃^-摩尔比证实岩溶关键带C-N耦合循环的存在,并控制碳酸盐岩的风化过程;4）C-N耦合循环输出的NO₃^--N和DIC-C分别为11.4×10³kg N/a和287.1×10³kg C/a,其中大气沉降与硝化过程形成的HNO₃风化碳酸盐岩形成的DIC占DIC输出总量的3.5%。因此,岩溶关键带的C-N耦合循环不但导致地下水NO₃^-的污染,而且扰动了碳酸盐岩的风化过程。

     

Chemical weathering in the Krishna Basin and Western Ghats of the Deccan Traps, India: Rates of basalt weathering and their controls

Rates of chemical and silicate weathering of the Deccan Trap basalts, India, have been determined through major ion measurements in the headwaters of the Krishna and the Bhima rivers, their tributaries, and the west flowing streams of the Western Ghats, all of which flow almost entirely through the Deccan basalts.Samples (n = 63) for this study were collected from 23 rivers during two consecutive monsoon seasons of 2001 and 2002. The Total dissolved solid (TDS) in the samples range from 27 to 640 mg l⁻¹. The rivers draining the Western Ghats that flow through patches of cation deficient lateritic soils have lower TDS (average: 74 mg l⁻¹), whereas the Bhima (except at origin) and its tributaries that seem to receive Na, Cl, and SO₄ from saline soils and anthropogenic inputs have values in excess of 170 mg l⁻¹. Many of the rivers sampled are supersaturated with respect to calcite. The chemical weathering rates (CWR) of “selected” basins, which exclude rivers supersaturated in calcite and which have high Cl and SO₄, are in range of ∼3 to ∼60 t km⁻² y⁻¹. This yields an area-weighted average CWR of ∼16 t km⁻² y⁻¹ for the Deccan Traps. This is a factor of ∼2 lower than that reported for the Narmada-Tapti-Wainganga (NTW) systems draining the more northern regions of the Deccan. The difference can be because of (i) natural variations in CWR among the different basins of the Deccan, (ii) “selection” of river basin for CWR calculation in this study, and (iii) possible contribution of major ions from sources, in addition to basalts, to rivers of the northern Deccan Traps.Silicate weathering rates (SWR) in the selected basins calculated using dissolved Mg as an index varies between ∼3 to ∼60 t km⁻² y⁻¹, nearly identical to their CWR. The Ca/Mg and Na/Mg in these rivers, after correcting for rain input, are quite similar to those in average basalts of the region, suggesting near congruent release of Ca, Mg, and Na from basalts to rivers. Comparison of calculated and measured silicate-Ca in these rivers indicates that at most ∼30% of Ca can be of nonsilicate origin, a likely source being carbonates in basalts and sediments.The chemical and silicate weathering rates of the west flowing rivers of the Deccan are ∼4 times higher than the east flowing rivers. This difference is due to the correspondingly higher rainfall and runoff in the western region and thus reemphasises the dominant role of runoff in regulating weathering rates. The silicon weathering rate (SWR) in the Krishna Basin is ∼15 t km⁻² y⁻¹, within a factor of ∼2 to those in the Yamuna, Bhagirathi, and Alaknanda basins of the Himalaya, suggesting that under favourable conditions (intense physical weathering, high runoff) granites and the other silicates in the Himalaya weather at rates similar to those of Deccan basalts. The CO₂ consumption rate for the Deccan is deduced to be ∼3.6 × 10⁵ moles km⁻² y⁻¹ based on the SWR. The rate, though, is two to three times lower than reported for the NTW rivers system; it still reinforces the earlier findings that, in general, basalts weather more rapidly than other silicates and that they significantly influence the atmospheric CO₂ budget on long-term scales.     

Exploring weathering and regolith transport controls on Critical Zone development with models and natural experiments

The architecture of the Critical Zone, including mobile regolith thickness and depth to the weathering front, is first order controlled by advance of a weathering front at depth and transport of sediment at the surface. Differences in conditions imposed by slope aspect in the Gordon Gulch catchment of the Boulder Creek Critical Zone Observatory present a natural experiment to explore these interactions. The weathering front is deeper and saprolite more decayed on north-facing than on south-facing slopes. Simple numerical models of weathering front advance, mobile regolith production, and regolith transport are used to test how weathering and erosion rates interact in the evolution of weathered profiles. As the processes which attempt are being made to mimic are directly tied to climate variables such as mean annual temperature, the role of Quaternary climate variation in governing the evolution of Critical Zone architecture can be explored with greater confidence.     

Exploring links between vadose zone hydrology and chemical weathering in the Boulder Creek critical zone observatory

Understanding the relationship between subsurface flow paths on hillslopes and chemical weathering of bedrock is fundamental to understanding the timing and mechanisms that weather bedrock to saprolite. The link between chemical weathering of bedrock and contact time with reactive water along flow paths motivates this study. Water drives the chemical alteration of rock into saprolite, yet connected porosity generally declines with depth into the weathered profile. Saprolite formation, therefore, reflects coupled weathering and permeability development over time. This study uses numerical modeling and soil-moisture monitoring to explore the hydrology of the unsaturated zone and the influence of fracture density, hillslope gradient, and permeability contrasts within the saprolite development horizon on saprolite development.     

基于结构体的峨眉山玄武岩风化程度评价（Ⅳ）：风化指数FF

提要：岩石化学风化程度评价指标应该同时满足以下三个基本条件,即与风化程度之间的关系简单明确、对风化程度变化具有足够的敏感性和易于确定和不易受到人为因素影响。作为铁镁质岩石的主要代表,（峨眉山）玄武岩风化程度评价除考虑可引起组分淋失与富集的水解反应外,还应充分考虑二价铁的氧化反应。新鲜峨眉山玄武岩中并存的二价铁和三价铁的含量总体稳定,910个样品的FeO、Fe₂O₃平均含量分别为8.45%和5.15%,以均匀、随机的方式赋存于辉石、橄榄石、杏仁体中的绿泥石、磁铁矿及火山玻璃中。风化玄武岩、玄武岩斜坡地下水及新鲜玄武岩浸泡液的地球化学研究结果表明,FeO及Fe₂O₃含量对峨眉山玄武岩风化程度的敏感性明显高于其他组分,同时铁又是玄武岩风化过程中活动性最差的元素之一。三价铁和二价铁的摩尔数比值（FF）适合于峨眉山玄武岩整个风化过程的风化程度判别,比既有风化指数具有更高的分辨率,尤其是对风化初期玄武岩。     

Chemical weathering and related controls on surface water chemistry in the Absaroka Mountains,Wyoming

Chemical relationships among surface waters, soils and rocks were investigated in the drainage basin of the North Fork of the Shoshone River in northwestern Wyoming. The area is underlain entirely by andesitic volcanic rocks. Smectite is the only clay mineral forming in soils over much of the area, although minor kaolinite occurs in a few areas of higher-than-average rainfall.Mass-balance calculations relating stream water chemistry to rock alteration indicate that controls on the chemistry of surface waters take place not in the soil zone but in the altered rock zone. The dominant weathering process which controls the water chemistry is slight alteration of large volumes of rock, rather than development of chemical equilibria involving secondary phases in the soil zone. The altered rock is enriched in feldspars and depleted in ferromagnesian minerals compared to fresh rock. The high rate of physical erosion of the area is enough to remove the residue, reexpose the bedrock, and continue the weathering process.     

Basalt weathering and trace elements migration in the boreal Arctic zone

Chemical weathering of basalts in the Putorana Plateau, Central Siberia, has been studied by combining chemical and mineralogical analysis of solids (rocks, soils, river sediments and suspended matter) and fluid solution chemistry in order to quantify CO₂ consumption and to assess the major factors controlling basic rock weathering under permafrost-dominated taiga climate. The chemical status of  40 major and trace elements (TE) in pristine boreal rivers and interstitial solutions of permafrost soils has been investigated using in-field ultrafiltration procedure. This revealed strong relationships between concentration of TE and that of major inorganic components of colloids, i.e., Fe or Al. Decomposing plant litter and permafrost thawing are considered important sources of most major and trace elements in Arctic rivers during summertime.     

Francolite geochemistry—compositional controls during formation,diagenesis, metamorphism and weathering

Francolite in unaltered Tertiary phosphorites is highly substituted with about 1.2% Na, 0.25% Sr, 0.36% Mg and 2.7% SO₄ within the structure in substitution for Ca and PO₄. These substitutions reflect the composition of the solution from which francolite forms and are not influenced by the mechanism, rate or redox conditions of formation. The remarkably constant composition for a wide range of samples reflects the relative constant composition of sea water, during Tertiary times. Francolite in sulphate-reducing sediments forms in an uppermost zone where bioturbation maintains the concentrations of Na, Sr, Mg and SO₄ at sea water concentrations. The structura-CO₂ content of unaltered francolites vary from 1.4% to 6.3%, in response to variations in pH. The maximum level of carbonate substitution is limited by the disruption of the francolite structure at levels above 6.3% CO₂. For the samples considered here F/P₂O₅ ratios vary with X-ray CO₂ contents showing that PO₄^3? is replaced by (CO₃^2? + F?) and not by CO₃^2? alone. In Phosphoria Formation francolite the Na/P₂O₅ ratios and X-ray CO₂ contents show areal trends related to burial depth and thermal metamorphism. These two processes together with weathering and fresh water diageneses explain all observed variation in francolite composition.     

成都粘土地球化学特征及其对物源和风化强度的指示

提要：成都平原晚更新世成都粘土地球化学分析表明,不同剖面样品的化学组成具有较好的一致性,以SiO2（平均75.24%）、Al2O3（平均14.12%）、Fe2O3（平均5.81%）为主,地球化学特征与UCC基本相似。与北方黄土相比,成都粘土具有较高的TiO2/Al2O3、Zr/Al、Zr/Ti和Y/Al以及较低的Eu/Eu?鄢、LaN/SmN和LaN/YbN比值,这些特征与成都平原第四系深层土壤样品组成一致,表明成都粘土来源于当地,与北方黄土物源不同。成都粘土CIA平均值为78.59,高于北方黄土平均值（69.34）,经历了中期的K迁移阶段,在风化过程中Ca、Na、Mg和K是主要的活动性元素。与北方黄土相比,成都粘土Na、Mg、K和Ca亏损较大,表明经历了更强的化学风化。     

包气带土层对氮素污染地下水的防护能力试验研究

包气带土层能够有效滞留和吸附污染物,是阻隔污染物进入地下水的重要屏障。通过对北京市典型地区包气带土体进行土柱淋溶试验,研究包气带土层对氮素污染地下水的防护能力,实验结果显示,包气带土层对氮素的截污性能具有明显的分段性。粘质粉土对氮素污染地下水具有较强的截留和防护能力,粉砂截留和防护能力相对较差。经计算粘质粉土和粉砂对再生水的净化能力分别为83.3%、29.1%。包气带土层颗粒粒径愈大,污染物浓度高,水动力条件好,包气带土层的防护能力愈差。     

CO2 and sulfuric acid controls of weathering and river water composition

Reactions of CO₂ with carbonate and silicate minerals in continental sediments and upper part of the crystalline crust produce HCO₃⁻ in river and ground waters. H₂SO₄ formed by the oxidation of pyrite and reacting with carbonates may produce CO₂ or HCO₃⁻. The ratio, ψ, of atmospheric or soil CO₂ consumed in weathering to HCO₃⁻ produced depends on the mix of CO₂ and H₂SO₄, and the proportions of the carbonates and silicates in the source rock. An average sediment has a CO₂ uptake potential of ψ = 0.61. The potential increases by inclusion of the crystalline crust in the weathering source rock. A mineral dissolution model for an average river gives ψ = 0.68 to 0.72 that is within the range of ψ = 0.63 to 0.75, reported by other investigators using other methods. These results translate into the CO₂ weathering flux of 20 to 24 × 10¹²mol/yr.     

温湿气候区结晶岩与碎屑岩腐岩特征及其发育过程

发育于斜坡非饱和带的结晶岩与碎屑岩腐岩多夹于残坡积土层和风化岩带之间,保留有裂隙及层理等构造,但质地松软。腐岩结构体与母岩色差显著,保留原岩结构和构造特征,矿物成分以次生粘土矿物、次生石英及褐铁矿最为常见。腐岩次生微孔隙发育,具有低密度、高孔隙度、高渗透率、高容水度及高持水度特征。腐岩发育是通过低渗透岩石结构体风化前锋的渐进性扩展实现的,非饱和带高湿度空气形成的凝结水和降水入渗形成的非饱和渗流是该过程的核心动力。气态水分子能够扩散到低渗透岩石结构体表面内侧一定宽度范围内的孔隙系统之中凝结为液态水并溶蚀造岩矿物。雨季入渗降水在结构体表面形成的薄膜水流通过分子扩散机制接受其内侧结构体粒间溶液中的溶出组分并将其携入饱水带,同时恢复粒间溶液的侵蚀动力。当上述过程循环到一定程度时,结构面内侧便形成一定厚度的腐岩壳。腐岩壳在雨季直接吸收、贮存流经其表面的液态渗入水,在核心石外侧形成富水结构层并在旱季向包气带或其内侧的风化前锋提供水汽。随着时间推移,腐岩壳越来越厚,核心石最终消失;当某一范围内的岩石结构体全部演变为腐岩结构体时,该范围内的岩体也就完成了向腐岩体的转变。     

Climatic and lithologic controls on the temporal and spatial variability of CO2 consumption via chemical weathering: An example from the Australian Victorian Alps

A detailed geochemical study on river waters of the Australian Victorian Alps was carried out to determine: (i) the relative significance of silicate, carbonate, evaporite and sulfide weathering in controlling the major ion composition and; (ii) the factors regulating seasonal and spatial variations of CO₂ consumption via silicate weathering in the catchments. Major ion chemistry implies that solutes are largely derived from evaporation of precipitation and chemical weathering of carbonate and silicate lithologies. The input of solutes from rock weathering was determined by calculating the contribution of halite dissolution and atmospheric inputs using local rain and snow samples. Despite the lack of carbonate outcrops in the study area and waters being undersaturated with respect to calcite, the dissolution of vein calcite accounts for up to 67% of the total dissolved cations, generating up to 90% of dissolved Ca and 97% of Mg. Dissolved sulfate has δ³⁴S values of 16 to 20‰_CDT, indicating that it is derived predominantly from atmospheric deposition and minor gypsum weathering and not from bacterial reduction of FeS₂. This militates against sulphuric acid weathering in Victorian rivers. Ratios of Si vs. the atmospheric corrected Na and K concentrations range from ~ 1.1 to ~ 4.3, suggesting incongruent weathering from plagioclase to smectite, kaolinite and gibbsite.Estimated long-term average CO₂ fluxes from silicate weathering range from ~ 0.012 × 10⁶ to 0.039 × 10⁶ mol/km²/yr with the highest values in rivers draining the basement outcrops rather than sedimentary rocks. This is about one order of magnitude below the global average which is due to low relief, and the arid climate in that region. Time series measurements show that exposure to lithology, high physical erosion and long water–rock contact times dominate CO₂ consumption fluxes via silicate weathering, while variations in water temperature are not overriding parameters controlling chemical weathering. Because the atmospheric corrected concentrations of Na, K and Mg act non-conservative in Victorian rivers the parameterizations of weathering processes, and net CO₂ consumption rates in particular, based on major ion abundances, should be treated with skepticism.     

Diffusive transport of water in porous feldspars from granitic saprolites: In situ experiments using FTIR spectroscopy

A novel experimental cell was developed for in situ measurements of transport phenomena in porous media using Fourier-Transform Infrared (FTIR) Spectroscopy. The technique was employed at ambient pressure in the temperatures range of 11–44 °C to study the H₂O → D₂O exchange between water-saturated weathered feldspars (bulk porosity of 5–19 vol% for feldspar) from granitic saprolites and a surrounding aqueous liquid. Such measurements are an important step for understanding internal weathering reactions of feldspars in soils and aquifers. Effective diffusion coefficients D_eff for water in water-saturated porous feldspars were determined assuming one-dimensional diffusion in a quasi-homogeneous medium. The values of D_eff vary from 7.2 × 10⁻¹⁰ to 1.9 × 10⁻¹¹ m²/s and are 1–2 orders of magnitude lower than the diffusion coefficients (D) of protons and molecular H₂O in liquid water. The activation energy for the H₂O → D₂O exchange process in porous feldspars ranges from 7.8 to 18.8 kJ/mol.The results imply that the effective diffusivity of water is mainly controlled by physical properties of the feldspars like porosity, pore connectivity, pore geometry and distribution. Perthitic feldspars with homogeneous pore distribution in the albitic lamellas have diffusional tortuosity factors X = D/D_eff between 3 and 10 while alkali feldspars with inhomogeneously distributed and disconnected pores have much higher X values up to 129. Diffusion anisotropy has been verified for a vein perthite with diffusion perpendicular to the lamellas being faster by 0.3–0.5 log units than within the lamellas. It has to be emphasized that the study is based only on few selected feldspars, including perthitic feldspar, and additional work on samples with different weathering stages is needed to test the importance of the different parameters controlling diffusive transport in the pore system.     

硅酸盐化学风化强度评估的沉积物指标与方法研究进展*

风化作用是源-汇沉积体系中的重要环节,气候、构造、地形、植被和岩性在不同程度上控制着地表硅酸盐化学风化,量化硅酸盐化学风化强度有助于开展全球性的实例研究对比。本研究归纳总结了基于碎屑沉积物的化学风化强度指标,包括岩石学和矿物学指标、元素地球化学指标和非传统稳定同位素指标,并指出了指标在应用中存在的问题。这些指标中:砂质沉积物骨架颗粒组成、粉砂级碎屑的矿物组成和矿物表面结构特征从矿物组成和结构上直观地反映了化学风化强度,运用不多但值得关注;黏土矿物组合和主微量元素指标如CIA、Rb/Sr、α^AlE等在实例研究中运用广泛,同时也显现出易受物源和水动力分选影响等弊端;新近开发的利用锂、硼、钾、镁、硅等同位素示踪化学风化强度的方法具有广阔的应用前景,同位素的分馏机理有待完善。源-汇体系中的其他过程如物源供给、水动力分选、成岩作用、再旋回作用、成土作用、物理淋滤和生物利用等会影响化学风化指标的使用效果,通过多指标的综合运用和相互验证,可以有效提升化学风化强度评估的准确性。     

Mineralogy and geochemistry of clay fractions from different saprolites,Egypt: implications for the source of sedimentary kaolin deposits

The clay fractions of saprolites from granites, basalt, and schists in Egypt were subjected to mineralogical and geochemical investigations to examine the effect of source rock on the composition of the saprolites and the possibilities of these saprolites as a source of the nearby sedimentary kaolin deposits. The clay fractions of the studied saprolites show mineralogical and geochemical variations. Saprolites from the granites consist of kaolinite, while saprolites from the basalts are composed entirely of smectite. Schists-derived saprolites are composed of kaolinite in some cases and of a mixture of kaolinite, illite, and chlorite in the other. Saprolite from the basalt is characterized by relatively higher contents of TiO₂ and Ni compared to the saprolites from granites. Saprolites from granites have higher contents of Ba, Li, Pb, Sr, Th, Y, and Zr compared to those of the saprolites from the basalts and schists. Saprolites from different schists show variations in the distributions of many constituents, such as TiO₂, Cr, Ni, Ba, Y, and Zr. Although chondrite-normalized rare earth elements (REE) patterns are characterized by relative enrichments in the light rare earth elements (LREE) compared to the heavy rare earth elements (HREE) in all saprolites, granitic saprolites show negative Eu anomalies, while saprolite from basalt has no Eu anomaly. REE patterns of the saprolites from schists exhibit slight positive Ce anomalies and slight to moderate negative Eu anomalies. Weathering of saprolites from the basalt and metasediments is classified as the bisiallitization type, while weathering of saprolite from the granite is allitization type. Saprolites from schists vary from the bisiallitization (Aswan and Abu Natash) and allitization (Khaboba) types. Saprolites from the Khaboba schist can be considered the possible source of the Carboniferous kaolin deposits in the Hasber and Khaboba areas of Sinai, based on the similarity in the mineralogy and geochemistry of major, trace, and REE between the saprolites and the deposits. On the other hand, Carboniferous sedimentary kaolin deposits in the Abu Natash area, as well as the Cretaceous kaolin deposits in all areas of Sinai, might have been derived from the nearby schist saprolites, based on the similarity in the mineralogy and geochemistry between the saprolites and the kaolin deposits. Granites from the Arabian-Nubian Shield (ANS) and East Sahara Craton (ESC) are the possible sources of the pisolitic and plastic kaolin deposits in the Kalabsha area (Aswan), as indicated by the similarity in the mineralogy and geochemistry of the granitic saprolites and the kaolin deposits.