Reconciling the elemental and Sr isotope composition of Himalayan weathering fluxes: insights from the carbonate geochemistry of stream waters

Determining the relative proportions of silicate vs. carbonate weathering in the Himalaya is important for understanding atmospheric CO₂ consumption rates and the temporal evolution of seawater Sr. However, recent studies have shown that major element mass-balance equations attribute less CO₂ consumption to silicate weathering than methods utilizing Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations. To investigate this problem, we compiled literature data providing elemental and ⁸⁷Sr/⁸⁶Sr analyses for stream waters and bedrock from tributary watersheds throughout the Himalaya Mountains. In addition, carbonate system parameters (P_CO2, mineral saturation states) were evaluated for a selected suite of stream waters. The apparent discrepancy between the dominant weathering source of dissolved major elements vs. Sr can be reconciled in terms of carbonate mineral equilibria. Himalayan streams are predominantly Ca²⁺-Mg²⁺-HCO₃⁻ waters derived from calcite and dolomite dissolution, and mass-balance calculations demonstrate that carbonate weathering contributes ∼87% and ∼76% of the dissolved Ca²⁺ and Sr²⁺, respectively. However, calculated Ca/Sr ratios for the carbonate weathering flux are much lower than values observed in carbonate bedrock, suggesting that these divalent cations do not behave conservatively during stream mixing over large temperature and P_CO2 gradients in the Himalaya.The state of calcite and dolomite saturation was evaluated across these gradients, and the data show that upon descending through the Himalaya, ∼50% of the streams evaluated become highly supersaturated with respect to calcite as waters warm and degas CO₂. Stream water Ca/Mg and Ca/Sr ratios decrease as the degree of supersaturation with respect to calcite increases, and Mg²⁺, Ca²⁺, and HCO₃⁻ mass balances support interpretations of preferential Ca²⁺ removal by calcite precipitation. On the basis of patterns of saturation state and P_CO2 changes, calcite precipitation was estimated to remove up to ∼70% of the Ca²⁺ originally derived from carbonate weathering. Accounting for the nonconservative behavior of Ca²⁺ during riverine transport brings the Ca/Sr and ⁸⁷Sr/⁸⁶Sr composition of the carbonate weathering flux into agreement with the composition of carbonate bedrock, thereby permitting consistency between elemental and Sr isotope approaches to partitioning stream water solute sources. These results resolve the dissolved Sr²⁺ budget and suggest that the conventional application of two-component Ca/Sr and ⁸⁷Sr/⁸⁶Sr mixing equations has overestimated silicate-derived Sr²⁺ and HCO₃⁻ fluxes from the Himalaya. In addition, these findings demonstrate that integrating stream water carbonate mineral equilibria, divalent cation compositional trends, and Sr isotope inventories provides a powerful approach for examining weathering fluxes.     

Chemical weathering in the plain and peninsular sub-basins of the Ganga: Impact on major ion chemistry and elemental fluxes

Concentrations of major ions, Sr and ⁸⁷Sr/⁸⁶Sr have been measured in the Gomti, the Son and the Yamuna, tributaries of the Ganga draining its peninsular and plain sub-basins to determine their contribution to the water chemistry of the Ganga and silicate and carbonate erosion of the Ganga basin. The results show high concentrations of Na and Sr in the Gomti, the Yamuna and the Ganga (at Varanasi) with much of the Na in excess of Cl. The use of this ‘excess Na’ (Na∗ = Na_riv − Cl_riv) a common index of silicate weathering yield values of ∼18 tons km⁻² yr⁻¹ for silicate erosion rate (SER) in the Gomti and the Yamuna basins. There are however, indications that part of this Na∗ can be from saline/alkaline soils abundant in their basins, raising questions about its use as a proxy to determine SER of the Ganga plain. Independent estimation of SER based on dissolved Si as a proxy give an average value of ∼5 tons km⁻² yr⁻¹ for the peninsular and the plain drainages, several times lower than that derived using Na∗. The major source of uncertainty in this estimate is the potential removal of Si from rivers by biological and chemical processes. The Si based SER and CER (carbonate erosion rate) are also much lower than that in the Himalayan sub-basin of the Ganga. The lower relief, runoff and physical erosion in the peninsular and the plain basins relative to the Himalayan sub-basin and calcite precipitation in them all could be contributing to their lower erosion rates.Budget calculations show that the Yamuna, the Son and Gomti together account for ∼75% Na, 41% Mg and ∼53% Sr and ⁸⁷Sr of their supply to the Ganga from its major tributaries, with the Yamuna dominating the contribution. The results highlight the important role of the plain and peninsular sub-basins in determining the solute and Sr isotope budgets of the Ganga. The study also shows that the anthropogenic contribution accounts for ?10% of the major ion fluxes of the Ganga at Rajmahal during high river stages (October). The impact of both saline/alkaline soils and anthropogenic sources on the major ion abundances of the Ganga is minimum during its peak flow and therefore the SER and CO₂ consumption rates of the river is best determined during this period.     

Plant and mycorrhizal driven silicate weathering: Quantifying carbon flux and mineral weathering processes at the laboratory mesocosm scale

The rise of vascular land plants in the Paleozoic is hypothesized to have driven lower atmospheric CO₂ levels through enhanced weathering of Ca and Mg bearing silicate minerals and rocks. However, this view overlooks the co-evolution of roots and mycorrhizal fungi, with many of the weathering processes ascribed to plants potentially being driven by the combined activities of roots and fungi. Here mesocosm scale controlled laboratory experiments quantifying the effects of plant and fungal evolution on silicate rock weathering under ambient and elevated CO₂ concentrations are described. A snapshot is presented of C allocation through roots and mycorrhizal fungi and biological activity associated with geochemical changes in weathered mineral substrates via transfer of elements from solid phases into solution.     

Minerals formed and mineral formation from volcanic ash by weathering

Weathering of volcanic ash and pumice is unique regarding minerals formed and mineral formation. These minerals include allophane, imogolite, opaline silica and some halloysites, and have distinctive structures and properties. Studies on the surface and subsurface weathering of various volcanic ashes in different settings are reviewed. This review illustrates that relatively rapid weathering of finely comminuted volcanic ash well reflects the effects of climate, vegetation and time, and produces various combinations of weathering products. The moisture regime, the nature and amount of organic matter incorporated, the deposition of volcanic ash as overburden, and the chemical and mineralogical composition of parent ash are important in controlling the mineral formation in weathered volcanic ash.     

Anthropogenic impacts on mineral weathering: A statistical perspective

Correspondence Analysis was adopted as tool for investigating the statistical structure of hydrochemical and weathering datasets of groundwater samples, with the main purpose of identifying impacts on mineral weathering caused by anthropogenic activities, namely fertilizing of farmlands. The hydrochemical dataset comprised measured concentrations of major inorganic compounds dissolved in groundwater, namely bicarbonate, silica (usually by-products of chemical weathering), chloride, sulphate and nitrate (typically atmospheric plus anthropogenic inputs). The weathering dataset consisted of calculated mass transfers of minerals being dissolved in loess sediments of a region located in SW Hungary (Szigetvár area), namely Na-plagioclase, calcite and dolomite, and of pollution-related concentrations of sodium, magnesium and calcium. A first run of Correspondence Analysis described groundwater composition in the study area as a system of triple influence, where spots of domestic effluents-dominated chemistries are surrounded by areas with agriculture-dominated chemistries, both imprinted over large regions of weathering dominated chemistries. A second run revealed that nitrification of N-fertilizers is promoting mineral weathering by the nitric acid reaction (anthropogenic pathway), in concurrence with the retreating of weathering by carbonic acid (natural pathway). It also indicated that dolomite and calcite are being players in a dedolomitization process driven by dissolution of gypsum fertilizers and nitrification of N-fertilizers.     

Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory

Located in the uplands of the Valley and Ridge physiographic province of Pennsylvania, the Susquehanna/Shale Hills Critical Zone Observatory (SSHO) is a tectonically quiescent, first-order catchment developed on shales of the Silurian Rose Hill Formation. We used soil cores augered at the highest point of the watershed and along a subsurface water flowline on a planar hillslope to investigate mineral transformations and physical/chemical weathering fluxes. About 25 m of bedrock was also drilled to estimate parent composition. Depletion of carbonate at tens of meters of depth in bedrock may delineate a deep carbonate-weathering front. Overlying this, extending from ∼6 m below the bedrock-soil interface up into the soil, is the feldspar dissolution front. In the soils, depletion profiles for K, Mg, Si, Fe, and Al relative to the bedrock define the illite and chlorite reaction fronts. When combined with a cosmogenic nuclide-derived erosion rate on watershed sediments, these depletion profiles are consistent with dissolution rates that are several orders of magnitudes slower for chlorite (1-5 × 10⁻¹⁷ mol m⁻² s⁻¹) and illite (2-9 × 10⁻¹⁷ mol m⁻² s⁻¹) than observed in the laboratory. Mineral reactions result in formation of vermiculite, hydroxy-interlayered vermiculite, and minor kaolinite. During weathering, exchangeable divalent cations are replaced by Al as soil pH decreases.The losses of Mg and K in the soils occur largely as solute fluxes; in contrast, losses of Al and Fe are mostly as downslope transport of fine particles. Physical erosion of bulk soils also occurs: results from a steady-state model demonstrate that physical erosion accounts for about half of the total denudation at the ridgetop and midslope positions. Chemical weathering losses of Mg, Na, and K are higher in the upslope positions likely because of the higher degree of chemical undersaturation in porewaters. Chemical weathering slows down in the valley floor and Al and Si even show net accumulation. The simplest model for the hillslope that is consistent with all observations is a steady-state, clay weathering-limited system where soil production rates decrease with increasing soil thickness.     

Inorganic nutrient inputs from mineral weathering in two Scottish upland ecosystems

Uptake of K, Ca, Mg and Na by vegetation in two upland ecosystems, one situated in andesitic parent material and the other on mica-schists, has been studied in relation to the mineralogy of the soils and the rates of weathering of base cations. Rates of weathering were calculated by two methods: (1) long-term rates were calculated from losses of the elements in soil horizons using Zr as an internal index; (2) current rates were calculated from input-output budgets using rain-water and stream chemistry over a 3 a period. Vegetation uptake can be related in a general way to mineralogy and weathering trends. Most of the K released from the soil appears to be taken up by the vegetation and the content of K in grass-dominated plant communities is related mainly to the availability of K from the weathering of mica. High levels of Ca in the streams indicate an overabundance of Ca which is being released mainly by weathering of plagioclase feldspar. Amounts of Mg in the vegetation are related to variations in the content of chlorite which is easily weathered at both sites.     

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.     

Trace elements and mineral chemistry of silicified wood from Thailand: colours and elemental distribution

Thai silicified woods were examined using electron probe microanalysis, yielding chemical data that characterised the samples into two groups: low and high silica contents (82—94 wt% and 94—98 wt%). The elements analysed in order of abundance include Si > Fe > Ca > Na > Al > Ti > K > Mg > Mn > Zr. Iron plays a major role in the colour range (red, orange, yellow, brown, grey and black) of the samples. Calcium is associated with Fe in the darker colours of the wood. Pseudo-crystallochemistry has been used for the substitution of trace elements for Si⁴⁺ in silica polymorphs. The atomic channels that run parallel to the c-axis of silica polymorphs or lattice defects, or even the charge balance for trivalent-ion substitution for Si⁴⁺, can accommodate monovalent ions (K⁺ and Na⁺). Vacant and atomic cavities, which are charged balanced by trivalent ions [Al³⁺ or Fe³⁺ substituting for Si⁴⁺], are commonly occupied by divalent ions (Ca²⁺, Mg²⁺ and Mn²⁺). Quadrivalent ions, Ti⁴⁺ and Zr⁴⁺ are non-structurally incorporated but form clusters of mineral inclusions in the samples. Several other trace-element contents are also in the form of mineral/fluid inclusions hosted in the woods.     

帕米尔弧形构造带东北缘晚新生代构造活动的沉积与重矿物变化响应

帕米尔弧形构造带东北缘晚新生代沉积特征的研究对于理解帕米尔构造带弧形扩展机制具有重要意义。本文通过对帕米尔弧形构造带东北缘的晚新生代沉积地层的沉积特征对比研究和重矿物采样与组份分析,建立了帕米尔弧形构造带东北缘地区的晚新生代沉积联合柱状对比图和重矿物组份变化曲线。研究结果表明,晚新生代沉积与重矿物变化主要记录了帕米尔弧形构造带东北缘的两期构造活动。第一期构造活动发生在安居安组沉积时期,帕米尔弧形构造带东北缘的和什拉甫剖面、英科1井表现为粗碎屑沉积和对应的不稳定重矿物爆发,而南部的甫沙2井中则并未观察到,这一事件可能反映了喀什-叶城走滑系统在和什拉甫剖面附近的中新世构造启动。第二期构造活动发生在阿图什组沉积期间,帕米尔弧形构造带东北缘全区域出现了一套粗碎屑沉积和对应的不稳定重矿物爆发,反映了帕米尔弧形构造带东北缘在上新世以来大规模向北楔入塔里木盆地内部的构造过程。

     

Electron transfer at the mineral/water interface: Selenium reduction by ferrous iron sorbed on clay

The mobility and availability of the toxic metalloid selenium in the environment are largely controlled by sorption and redox reactions, which may proceed at temporal scales similar to that of subsurface water movement under saturated or unsaturated conditions. Since such waters are often anaerobic and rich in Fe²⁺, we investigated the long-term (?1 month) kinetics of selenite sorption to montmorillonite in the presence of Fe²⁺ under anoxic conditions. A synthetic montmorillonite was used to eliminate the influence of structural Fe. In the absence of aqueous Fe²⁺, selenite was sorbed as outer-sphere sorption complex, covering only part of the positive edge sites, as verified by a structure-based MUSIC model and Se K-edge XAS (X-ray absorption spectroscopy). When selenite was added to montmorillonite previously equilibrated with Fe²⁺ solution however, slow reduction of Se and formation of a solid phase was observed with Se K-edge XANES (X-ray absorption near-edge spectroscopy) and EXAFS (extended X-ray absorption fine-structure) spectroscopy. Iterative transformation factor analysis of XANES and EXAFS spectra suggested that only one Se reaction product formed, which was identified as nano-particulate Se(0). Even after one month, only 75% of the initially sorbed Se(IV) was reduced to this solid species. Mössbauer spectrometry revealed that before and after addition and reduction of Se, 5% of total sorbed Fe occurred as Fe(III) species on edge sites of montmorillonite (≈2 mmol kg⁻¹). The only change observed after addition of Se was the formation of a new Fe(II) species (15%) attributed to the formation of an outer-sphere Fe(II)-Se sorption complex. The combined Mössbauer and XAS results hence clearly suggest that the Se and Fe redox reactions are not directly coupled. Based on the results of a companion paper, we hypothesize that the electrons produced in the absence of Se by oxidation of sorbed Fe(II) are stored, for example by formation of surface H₂ species, and are then available for the later Se(IV) reduction. The slow reaction rate indicates a diffusion controlled process. Homogeneous precipitation of an iron selenite was thermodynamically predicted and experimentally observed only in the absence of clay. Interestingly, half of Fe was oxidized in this precipitate (Mössbauer). Since DFT calculations predicted the oxidation of Fe at the water-FeSe solid interface only and not in the bulk phase, we derived an average particle size of this precipitate which does not exceed 2 nm. A comparison with the Mössbauer and XAS spectra of the clay samples demonstrates that such homogenous precipitation can be excluded as a mechanism for the observed slow Se reduction, emphasizing the role of abiotic, heterogeneous precipitation and reduction for the removal of Se from subsurface waters.     

The short term climatic sensitivity of carbonate and silicate weathering fluxes: Insight from seasonal variations in river chemistry

Large seasonal variations in the dissolved load of the headwater tributaries of the Marsyandi river (Nepal Himalaya) for major cations and ⁸⁷Sr/⁸⁶Sr ratios are interpreted to result from a greater dissolution of carbonate relative to silicate at high runoff. There is up to a 0.003 decrease in strontium isotope ratios and a factor of 3 reduction in the Si(OH)₄/Ca ratio during the monsoon. These variations, in small rivers sampling uniform lithologies, result from a different response of carbonate and silicate mineral dissolution to climatic forcing. Similar trends are observed in compiled literature data, from both Indian and Nepalese Himalayan rivers. Carbonate weathering is more sensitive to monsoonal runoff because of its faster dissolution kinetics. Silicate weathering increases relative to carbonate during the dry season, and may be more predominant in groundwater with longer water-rock interaction times. Despite this kinetic effect, silicate weathering fluxes are dominated by the monsoon flux, when between 50% and 70% of total annual silicate weathering flux occurs.     

用稀盐酸浸取方法探讨玄武岩风化产物中REE与Al、P、Fe和Mn的依存关系

用2 mol/L HCl对海南新生代玄武岩风化剖面上的风化产物进行了浸取实验,通过浸取出来的稀土元素(REE)比例及其REE与Al、P、Fe和Mn的关系探讨风化产物中REE的赋存状态.结果显示,大部分REE、Al和Mn(＞50%)、相当一部分P(20%～40%)和Fe(-20%)被稀盐酸浸出.这些盐酸提取出来的REE(不包括Ce)根据含量不同与其他元素有不同的对应关系,较高的REE含量(＞250μg/g)与Al显示出良好的正相关关系,而与Fe、P和Mn相关性不明显;较低的REE含量(＜150μg/g)与Fe和P显示很好的正相关关系,与Mn正相关性不明显,与Al存在负相关关系;Ce与Mn存在较好的正相关关系,与Al、P和Fe没有明显的相关性.结合风化产物全样的相应元素分布特征可以判断,这些风化产物中高含量的REE(不包含Ce)是以比较松散的形式存在,并容易被流体淋滤出去,而正常含量的REE主要赋存于Fe氧化物/氢氧化物和次生磷酸盐矿物中,Ce的赋存则与Mn氧化物/氢氧化物有密切关系.同时也做了原岩的淋洗实验,发现Al、P及REE等元素被大量析出,因此不支持用2 mol/L以上的酸对岩石样品进行淋洗前处理.     

Climatic control on clay mineral formation: Evidence from weathering profiles developed on either side of the Western Ghats

Many physico-chemical variables like rock-type, climate, topography and exposure age affect weathering environments. In the present study, an attempt is made to understand how the nature of clay minerals formed due to weathering differs in tropical regions receiving high and low rainfall. Clay mineralogy of weathering profiles in west coast of India, which receives about 3 m rainfall through two monsoons and those from the inland rain-shadow zones (<200 cm rainfall) are studied using X-ray diffraction technique. In the west coast, 1:1 clays (kaolinite) and Fe—Al oxides (gibbsite/goethite) are dominant clay minerals in the weathering profiles while 2:1 clay minerals are absent or found only in trace amounts. Weathering profiles in the rain shadow region have more complex clay mineralogy and are dominated by 2:1 clays and kaolinite. Fe—Al oxides are either less or absent in clay fraction. The kaolinite—smectite interstratified mineral in Banasandra profiles are formed due to transformation of smectites to kaolinite, which is indicative of a humid paleoclimate. In tropical regions receiving high rainfall the clay mineral assemblage remains the same irrespective of the parent rock type. Rainfall and availability of water apart from temperature, are the most important factors that determine kinetics of chemical weathering. Mineral alteration reactions proceed through different pathways in water rich and water poor environments.     

Pb isotopic ratios and elemental abundances for selective leachates from near-surface till: implications for mineral exploration

Pb isotope ratios obtained from fine-grained fractions ( < 63 and < 2 gmm from near-surface ( < 1 m depth) till surrounding ore deposits show isotopic overprinting from the underlying sulfide mineralization, and provide a new approach to mineral exploration for massive sulfide deposits (VMS) in glaciated terrains.In this study, Pb isotopic measurements, and selective leaching of 6 near-surface till samples down-ice from the Chisel Lake (Manitoba) and Manitouwadge (Ontario) VMS deposits were carried out in order to determine the location and nature of the Pb within till. Elemental abundances from selective leachates for all 6 samples display similar patterns and show that chalcophile elements (Cu, Ni, Pb and Zn), derived predominantly from the underlying VMS deposits, occur as (i) adsorbed/exchangeable metals; (ii) associated with oxyhydrous Fe and Mn; (iii) crystalline Fe oxides, and/or (iv) silicate. Despite the relative proximity of some of the till samples to the VMS deposits, only a very small component of the chalcophile elements is present as sulfide. This result is consistent with those from studies of weathered (oxidized) tills, which show that labile minerals such as sulfides have been completely destroyed and their chemical constituents reprecipitated or scavenged locally by clay-sized phyllosilicates and secondary oxides/hydroxides.Pb isotopic ratios for selective leachates from till samples with VMS-like (anomalous) signatures are similar to those from ore (galena) within the proximal VMS deposits. This indicates that the Pb is of a secondary nature and was probably scavenged and deposited after destruction of original sulfide minerals during till formation. The lack of a predominant sulfide-held Pb component within the selective leachates supports this interpretation. In contrast, Pb isotopic ratios for the same selective leachates from “background” samples are significantly higher and show that the Pb is not derived from proximal VMS deposits but from a more radiogenic source.Till samples were also leached using 2.5 M HCl (ldconventional” leaching). The Pb isotope ratios from the conventional leachates are similar to those obtained from the selective leachates, and show a large difference in Pb isotopic ratios between anomalous and background samples. We propose, therefore, that the conventional leaching rather than selective leaching or complete dissolution of a particular grain-size fraction be adopted for mineral exploration purposes using glacial sediments.The results from this study support the effective use of Pb isotope ratios from near-surface till as an exploration tool despite the weathered nature of the latter. We feel that this represents a more cost-effective technique over traditional geochemical prospecting methods, if used in conjunction with Pb abundance data.     

浙江龙游沐尘早白垩世石英二长岩体的成因: 镁铁质包体及寄主岩的元素与Sr-Nd同位素地球化学证据

沐尘岩体呈北北东向出露于浙江龙游县沐尘至遂昌县双溪口一带,为早白垩世晚期(112Ma)岩浆活动的产物。岩体主体岩性为石英二长岩,岩体中普遍发育形态多样的暗色镁铁质微粒包体。包体岩性主要为黑云母二长闪长岩,包体多呈椭球形或卵形等塑性形态,大小不一,从几厘米到几十厘米不等。主量元素组成上,寄主石英二长岩具有中酸性、准铝质、富碱、富钾等特征;镁铁质包体则偏基性、贫钾。微量和稀土元素组成上,寄主岩富集Rb、K、Th、U,贫Sr、P、Nb、Ta、Ti,且Zr、Hf含量相对较高,具中-强的铕负异常(Eu/Eu^*=0.12~0.60)。镁铁质包体具有相似的微量元素特征,但相对富集Sr、P,贫Zr、Hf,铕负异常中等或不明显(Eu/Eu^*=0.43~0.93)。寄主岩及镁铁质包体具有相似的初始Sr、Nd同位素组成,I_Sr分别为0.7062~0.7065和0.7058~0.7070,ε_Nd(t)值均偏高,分别为-3.19~-2.43和-2.60~0.58。在主量元素氧化物比值相关图解及微量元素与同位素协变图解上,镁铁质包体与寄主岩之间呈现出良好的协变关系,从地球化学角度为成岩过程中存在岩浆混合作用提供了可靠证据。温压计算表明沐尘岩体为温度偏高(797~851℃)的中深成岩体(6~7km)。综合岩石学、元素地球化学与Sr-Nd同位素组成特征,表明沐尘石英二长岩及镁铁质包体最可能是在引张构造背景下,由亏损的地幔组分及其诱发的地壳物质部分熔融形成的长英质岩浆经混合后,并经进一步的分异演化形成。     

Effects of temperature on silicate weathering: Solute fluxes and chemical weathering in a temperate rain forest watershed,Jamieson Creek,British Columbia

Chemical weathering of silicate minerals has long been known as a sink for atmospheric CO₂, and feedbacks between weathering and climate are believed to affect global climate. While warmer temperatures are believed to increase rates of weathering, weathering in cool climates can be accelerated by increased mineral exposure due to mechanical weathering by ice. In this study, chemical weathering of silicate minerals is investigated in a small temperate watershed. The Jamieson Creek watershed is covered by mature coniferous forest and receives high annual precipitation (4000 mm), mostly in the form of rainfall, and is underlain by quartz diorite bedrock and glacial till. Analysis of pore water concentration gradients indicates that weathering in hydraulically unsaturated ablation till is dominated by dissolution of plagioclase and hornblende. However, a watershed scale solute mass balance indicates high relative fluxes of K and Ca, indicating preferential leaching of these solutes possibly from the relatively unweathered lodgement till. Weathering rates for plagioclase and hornblende calculated from a watershed scale solute mass balance are similar in magnitude to rates determined using pore water concentration gradients.When compared to the Rio Icacos basin in Puerto Rico, a pristine tropical watershed with similar annual precipitation and bedrock, but with dissimilar regolith properties, fluxes of weathering products in stream discharge from the warmer site are 1.8 to 16.2-fold higher, respectively, and regolith profile-averaged plagioclase weathering rates are 3.8 to 9.0-fold higher. This suggests that the Arrhenius effect, which predicts a 3.5- to 9-fold increase in the dissolution rate of plagioclase as temperature is increased from 3.4° to 22 °C, may explain the greater weathering fluxes and rates at the Rio Icacos site. However, more modest differences in K and Ca fluxes between the two sites are attributed to accelerated leaching of those solutes from glacial till at Jamieson Creek. Our findings suggest that under conditions of high rainfall and favorable topography, weathering rates of silicate minerals in warm tropical systems will tend to be higher than in cool temperate systems, even if the temperate system is has been perturbed by an episode of glaciation that deposits regolith high in fresh mineral surface area.     

Chemical weathering, river geochemistry and atmospheric carbon fluxes from volcanic and ultramafic regions on Luzon Island, the Philippines

We investigated rates of chemical weathering of volcanic and ophiolitic rocks on Luzon Island, the Philippines. Luzon has a tropical climate and is volcanically and tectonically very active, all factors that should enhance chemical weathering. Seventy-five rivers and streams (10 draining ophiolites, 65 draining volcanic bedrock) and two volcanic hot springs were sampled and analyzed for major elements, alkalinity and ⁸⁷Sr/⁸⁶Sr. Cationic fluxes from the volcanic basins are dominated by Ca²⁺ and Mg²⁺ and dissolved silica concentrations are high (500-1900 μM). Silica concentrations in streams draining ophiolites are lower (400-900 μM), and the cationic charge is mostly Mg²⁺. The areally weighted average CO₂ export flux from our study area is 3.89 ± 0.21 × 10⁶ mol/km²/yr, or 5.99 ± 0.64 × 10⁶ mol/km²/yr from ophiolites and 3.58 ± 0.23 × 10⁶ mol/km²/yr from volcanic areas (uncertainty given as ±1 standard error, s.e.). This is ∼6-10 times higher than the current best estimate of areally averaged global CO₂ export by basalt chemical weathering and ∼2-3 times higher than the current best estimate of CO₂ export by basalt chemical weathering in the tropics. Extrapolating our findings to all tropical arcs, we estimate that around one tenth of all atmospheric carbon exported via silicate weathering to the oceans annually is processed in these environments, which amount to ∼1% of the global exorheic drainage area. Chemical weathering of volcanic terranes in the tropics appears to make a disproportionately large impact on the long-term carbon cycle.     

河北承德偏硅酸矿泉水成因模式:岩石风化与水岩作用证据

【研究目的】承德地处京津冀水源涵养功能区,矿泉水资源丰富,研究其赋存分布与形成机制对矿泉水可持续利用与水源涵养优化具有重要意义。【研究方法】采用岩石地球化学和水化学分析、化学风化指数、矿物表面微观形态分析,同位素示踪等方法系统梳理了研究区地下水偏硅酸空间分异的影响因素,从岩石风化与水化学耦合角度探讨了偏硅酸矿泉水的成藏机制。【研究结果】结果表明：研究区常温水体偏硅酸含量达30 mg/L以上样品占比达5.16%,地热水偏硅酸平均含量达61.76 mg/L。偏硅酸矿泉水成藏受岩石风化和地质构造控制,风化酸性介质影响,水化学形成作用制约。风化敏感程度愈高,易风化矿物含量愈高的含水介质赋存地下水偏硅酸含量愈高。研究区硅酸盐岩总体处于初等化学风化——长石类矿物和辉石等镁铁质矿物风化形成高岭石、蒙脱石和伊利石阶段。构造深部幔源CO₂、工矿活动和人类生产生活输入的外源硫酸和硝酸共同参与岩石风化脱硅过程,偏硅酸矿泉水、地热水温泉出露处多为构造复合部位或主干断裂与次级断裂的交汇部位。【结论】承德市偏硅酸矿泉水成因模式可概化为构造断裂深循环淋溶型、风化裂隙浅循环淋溶型和层间孔隙裂隙-补给富集埋藏型3类。植被覆盖较好的玄武岩、火山碎屑岩、陆源碎屑岩流域山前宽缓沟谷与导水断裂交汇带,侵入岩导水导热断裂带、侵入岩与围岩接触带,花岗岩、片麻岩和陆源碎屑岩与碳酸盐岩接触带为偏硅酸矿泉水开采潜力区。创新点：（1）从岩石风化与水岩作用水化学耦合角度探讨了偏硅酸矿泉水的成藏机制;（2）系统总结了承德市偏硅酸矿泉水空间分布的影响因素与成因模式。     

Carbon river fluxes and weathering CO2 consumption in the Congo and Amazon river basins

Data on carbon river fluxes recently obtained by the authors for the Congo basin within the framework of the PIRAT Program (INSU-CNRS/ORSTOM) are compared with results previously obtained for the Amazon basin. A special interest is devoted to the bicarbonate river fluxes and to their relationships with river discharges. The flux of atmospheric and soil CO₂ consumed by rock weathering is estimated to be 3.1 × 10⁵ and 0.5 × 10⁵ moles/a/km² respectively for the Amazon and the Congo basin. These CO₂ fluxes represent, respectively, 67.4% and 74.7% of the total bicarbonate river fluxes. A comparison to other large river basins shows that this contribution is directly related to the proportion of carbonate rock areas. A transfer function between the weathering CO₂ flux and the river discharge is calculated for each basin and allows the reconstitution of the variations of this flux using the river discharge fluctuations during the last century. These interannual CO₂ fluctuations present average increasing trends of 10% for Amazon basin and only 0.7% for the Congo basin during the last century.