高硫酸盐矿山废水的处理方法研究进展

高硫煤矿闭坑后成为埋藏在地表以下深部的产酸污染场,对相邻含水层和周边环境造成了潜在危害,且因采动效应影响和矿井空间分布复杂导致相邻含水层的污染过程与污染程度评价困难。以某闭坑矿区为研究对象,选取SO₄ ²⁻作为特征污染物,考虑多煤层采动裂隙对含水层结构的破坏影响,运用数值模拟技术对闭坑矿区酸性矿井水中SO₄ ²⁻污染迁移特征进行研究,并分析流体扩散系数对含水层污染物运移的影响。结果表明：SO₄ ²⁻在二叠系童子岩组砂岩含水层中水平迁移面积随时间增大而增大,增大速率逐渐减小,垂向渗透系数扩大10倍,空间变异性增强;闭坑5、10、15 a后,最大水平迁移距离分别为215、414、612 m,最大垂向迁移距离分别达到50、65、70 m;而扩散系数越大,产酸时间越长,SO₄ ²⁻迁移距离和污染范围将随之增大,相较于水平方向,垂向上SO₄ ²⁻浓度变化对扩散系数的敏感度更高。基于模拟结果,结合闭坑矿区实际条件,针对性提出了“源头减量−过程阻断−末端治理”的综合防控与治理方案。研究结果将为该闭坑煤矿酸性矿井水污染治理与防控提供理论基础,也为其他同类矿井提供科学借鉴。

     

CO2注气压力对瓦斯扩散系数影响规律实验研究

煤层CH₄解吸效率低、扩散慢的特点严重制约着煤层瓦斯抽采的效率,为解决低透气性煤层瓦斯抽采困难的问题,选取晋城赵庄煤矿煤样,研究不同注气压力对驱替CH₄过程的影响以及驱替过程中CH₄扩散系数的变化规律,利用自主研发的CO₂驱替CH₄试验平台,在0.6、0.8、1.0 MPa等不同注气压力条件下分别进行CO₂驱替CH₄实验。结果表明：驱替压力越大,达到最大CH₄排放量的时间越短,CO₂突破时间越快,置换效率越大,驱替效果越好;CH₄气体驱替过程分为3个阶段,先急剧增加再缓慢增加最后保持平稳;在同一注气压力下,瓦斯扩散系数随时间呈先增大后减小的变化规律,注气压力为0.6、0.8、1.0 MPa时,瓦斯扩散系数的最大值分别为2.27×10^-5、3.36×10^-5、4.62×10^-5 cm²/s。从实验结果可知,不同注气压力下,CO₂对CH₄主要起到驱替作用、置换吸附-解吸作用及稀释驱替作用;每个阶段的CH₄气体运移情况不同,根据实验阶段合理调整注气流量、压力等参数,使注驱技术搭配更高效。研究结果对CO₂深埋与瓦斯（煤层气）高效抽采具有理论指导意义。     

On the origin of the western Mediterranean Sea basins

This is a report of a symposium organized by the Netherlands Commission for the Upper Mantle Project. The data relative to the generation of the western Mediterranean Sea basins, presented during this symposium, are summarized in the Appendix.

Several modes of origin have been discussed:

1. (1) the basins are remnants of a former larger ocean;

2. (2) they are formed in the wake of drifting continental blocks;

3. (3) by an erosion and denudation of a continental crust;

4. (4) by an upheaval and later subsidence of an ocean floor; or

5. (5) by sub crustal erosion of a continental crust.

It is concluded that, although many data are in agreement with the drift model, this process cannot have been the sole agent in the generation of the basins.     

An experimental study of calcite dissolution rates at acidic conditions and 25 °C in the presence of NaPO3 and MgCl2

Dissolution rates of single calcite crystals were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed at 25 °C in aqueous HCl solutions over the bulk solution pH range −1 to 3 and in the presence of trace concentrations of aqueous NaPO₃ and MgCl₂. These salts were chosen for this study because aqueous magnesium and phosphate are known to strongly inhibit calcite dissolution at neutral to basic pH. Reactive solutions were undersaturated with respect to possible secondary phases. Neither an inhibition or enhancement of calcite dissolution rates was observed in the presence of aqueous MgCl₂ at pH 1 and 3. The presence of trace quantities of NaPO₃, which dissociates in solution to Na⁺ and H₂PO₄⁻, decreased the overall calcite dissolution rate at pH≤2. This contrasting behavior could be attributed to the different adsorption behavior of these dissolved species. As calcite surfaces are positively charged in acidic solutions, aqueous Mg²⁺ may not adsorb, whereas aqueous phosphate, present as either the anion H₂PO₄⁻ or the neutral species H₃PO₄⁰, readily adsorbs on calcite surfaces leading to significant dissolution inhibition.     

重金属污染土壤修复的Pb2+钝化产物稳定性

自然状态下土壤中重金属元素是否稳定存在是重金属污染固化修复技术中的核心问题。本文以常见重金属离子Pb²⁺为例,分析土壤中重金属离子与常见离子基团结合的稳定性。将第一性原理应用于Pb的存在形态和稳定性的分析,计算了PbCO₃、PbSO₄、PbCl₂、Pb₃（PO₄）₂、PbAl₂O₄和Pb₃Fe₂（PO₄）₄的自由能、能带和态密度。首先推断Pb²⁺和CO₃^2-、SO₄^2-、Cl^-、PO₄^3-等土壤中常见阴离子的结合稳定性,再判断加入金属阳离子Al³⁺和Fe³⁺对体系稳定性的影响。结果表明：铅化合物自由能从高到低趋势为PbCl₂、PbAl₂O₄、PbCO₃、PbSO₄、Pb₃（PO₄）₂、Pb₃Fe₂（PO₄）₄,其相应的结构稳定性顺序从大到小为Pb₃Fe₂（PO₄）₄、Pb₃（PO₄）₂、PbSO₄、PbCO₃、PbAl₂O₄、PbCl₂。通过对能带和态密度的分析,首先确定了SO₄^2-和PO₄^3-的引入能够增强含Pb体系的稳定性,进一步加入金属阳离子Fe³⁺会使体系更稳定。推测SO₄^2-、PO₄^3-为治理Pb²⁺污染合适的官能团,应选择容易释放SO₄^2-、PO₄^3-的物质作为合适的钝化剂。在实际应用中可选择磷酸二氢钠、无水硫酸钠、脱硫石膏等作为钝化剂。     

Studying the migration behaviour of selenate in Boom Clay by electromigration

For the disposal of high-level waste (HLW) in a deep geological formation as Boom Clay, safety assessment studies have shown that long lived ⁷⁹Se is one of the more critical fission products. Therefore, the knowledge of its migration properties (diffusion, retention) through the geological barrier (Boom Clay) is of paramount importance. The migration behaviour of selenium strongly depends on its speciation. Under reducing conditions, selenide would be the dominant species and selenium migration would mainly be controlled by the low solubility of Se(−II)-bearing minerals. However Se species are often found in redox disequilibrium and more oxidized species might also coexist. Therefore, the study of selenate migration requires attention, as it might be the most mobile selenium species in the host rock. Electromigration experiments performed with a ⁷⁵Se-labeled selenate in Boom Clay indicate a high mobility for this species. The apparent diffusion coefficient (D_app) of selenate in Boom Clay is estimated from electromigration experiments performed under different electric fields. Using two independent approaches, the value of D_app for selenate is shown to fall in the range from 1.7×10⁻¹¹ to 6.2×10⁻¹¹ m² s⁻¹. Moreover, no reduction of selenate in Boom Clay was observed.     

福建长乐地区海水入侵(上溯)对水化学变化的影响

海水入侵是沿海地区主要环境地质问题,分析沿海地下水和河水水化学特征,对圈定海水入侵范围及判断影响程度具有重要意义。通过在福建长乐市海水入侵含水层和闽江口海水上溯区采取23件水样进行室内常规离子测试,获得了海水入侵(上溯)区水中主要离子含量。利用海水稀释线分析主要离子与Cl离子之间的相对变化,对比离子实际浓度与理想浓度的差异,分析海水入侵对水化学变化的影响。研究结果显示,随着海水与淡水混合,水化学类型从海水到淡水依次为Na-Cl型水-Na-Cl·HCO₃型水-Na·Ca-HCO₃·Cl型水-Ca-HCO₃型水;引起水化学类型和TDS变化的主要因素为Na⁺和Cl^-的增加;Ca²⁺、Mg²⁺高于海水稀释线和理想浓度线,Na⁺、K⁺在海水稀释线和理想浓度线上下跳跃：Br^-/Cl^-与海水稀释线吻合较好,可以作为判断海水入侵范围的依据。     

Rotation and strain of linear and planar structures in three-dimensional progressive deformation

Rotation and progressive strain have been studied for a sheet embedded in a matrix which undergoes rotational three-dimensional strain under constant volume conditions. The mathematics gives explicit information on the following features:

1. (1) The length and position (relative to a defined coordinate system) of the principal axes of the strain ellipsoid at any stage of the progressive deformation.

2. (2) The position and length of the principal axes in any plane intersecting the strain ellipsoid, also at any stage of the deformation.

3. (3) The position and length of passive markers which initially coincided with the principal axes in an intersecting plane. This is of consequence for the distinction between passively rotating structures and actively forming structures.

4. (4) The shear strain parallel to an intersecting plane or sheet, as indicated by the angular difference between the normal to an intersecting plane at any time and the marker at the same time which initially, however, was parallel to the normal. This layer-parallel shear causes boudins to rotate and the axial plane of buckles to tilt.

The relationships have been expressed quantitatively in the bulk of the paper and illustrated in diagrams. The analysis presented is basic for the study of the deformational behavior of competent sheets of rocks embedded in less competent ones.     

长江口沉积物-水界面砷的迁移转化机制与微生物调控分析

为探析长江口沉积物-水界面砷的迁移转化机制,本文分析了2019年夏季长江口4个站位上覆水和间隙水中总As浓度及形态的剖面变化特征,耦合氧化还原敏感元素(Fe、Mn和S)的剖面变化剖析了沉积物-水界面砷循环的Fe-Mn-S控制机制,同时结合砷相关功能基因探讨了沉积物-水界面砷迁移转化的微生物调控过程,估算了沉积物-水界面总As的扩散通量。结果表明,除A7-4站位外,长江口其他3个站位间隙水总As以As³⁺为主要存在形态,且总As浓度均在上覆水中为最低值(0.748~1.57 μg·L^-1),而在间隙水中随着深度增加而逐渐增加并在6~9 cm深度达到峰值(7.14~26.9 μg·L^-1)。间隙水总As及As³⁺浓度的剖面变化趋势与溶解态Fe²⁺、Mn²⁺相似,其均在中间层出现高值,说明沉积物Fe/Mn还原带砷的释放可能是随固相Fe(Ⅲ)或Mn(Ⅳ)的还原而转移到间隙水中的。氧化层和Fe/Mn还原带过渡区间隙水砷浓度与砷异化还原菌功能基因arrA和arsC丰度存在对应关系(除A1-3站外),说明砷异化还原菌将溶解As⁵⁺或固相As⁵⁺还原为溶解As³⁺可能是该过渡层砷迁移转化的另一重要过程。硫酸盐还原带的间隙水总As和As³⁺浓度降低,但由于间隙水的低S^2-浓度不利于砷硫化物生成,因此深层间隙水砷可能与铁硫矿物结合而被移除。底层环境氧化还原条件是影响沉积物-水界面砷迁移转化的重要因素,随底层水DO浓度的降低,砷迁移转化更倾向于微生物还原控制。长江口沉积物-水界面总As的扩散通量为1.18×10^-7~2.07×10^-7 μmol·cm^-2·s^-1,均表现为沉积物间隙水中总As向上覆水释放,即沉积物是研究区域水体总As的来源之一。     

The Oslo Rift: P-T relations and lithospheric structure

Extrusion temperatures for basaltic lavas in the Permo-Carboniferous Oslo Rift, estimated from whole rock major element compositions, are estimated to be 1270 to 1340°C. This means that magmatism during the Oslo rifting event was not associated with a large temperature anomaly in the underlying upper mantle. Partial melting is believed to be caused by a combination of crustal extension, a weak temperature anomaly in the underlying asthenosphere, and/or high fluid-contents in the mantle source region (“wet-spot”). Petrological and gcochemical data imply that large masses of cumulate rocks were deposited in the deep crust during the Oslo rifting event. The densities and seismic velocities (V_p) of these cumulate rocks are estimated to be 2.8–3.5 g/cm³ and 7.5–8.0 km/s. A rough estimate suggests that cumulus minerals alone account for a net transfer of at least 2 × 10¹⁷ kg of magmatic material from the mantle into the deep crust. In addition comes material representing

1. (a) cumulate minerals corresponding to eroded magmatic surface and subsurface rocks

2. (b) intercumulus material, and

3. (c) magmas crystallized to completion in the deep crust.

Estimates based exclusively on geophysical data tend to underestimate the true transfer of mass into the lower crust as gabbroic cumulate rocks, and melts crystallizing to completion in the lower crust have densities and seismic velocities similar to those of lower crustal wallrocks.     

Diffusion in silicate melts: II. Multicomponent diffusion in CaOAl2O3SiO2 at 1500°C and 1 GPa

Chemical diffusion profiles in molten CaO---Al₂O₃---SiO₂ have been measured over a large range of compositions at 1500°C and l GPa. The diffusion profiles have been inverted for effective binary diffusion coefficients (EBDCs) and for the chemical diffusion matrix. The EBDCs are shown to depend strongly on both composition and direction of diffusion in composition space. The dependence of EBDCs on direction in composition space, which for the system studied here can be as large as a factor of seven, severely limits the applicability of EBDCs to interdiffusion in any direction other than the one used to derive the EBDCs.

The chemical diffusion matrix for molten CaO---Al₂O₃---SiO₂ was determined using diffusion profiles from two or three mutually orthogonal diffusion couples in the ternary composition space. All features of the diffusion profiles shown in this work can be reproduced by representing the chemical fluxes in the three-component system as a linear combination of concentration gradients via a 2 × 2 diffusion matrix. Chemical diffusion in molten CaO---Al₂O₃---SiO₂ shows clear evidence of strong diffusive coupling among the components. This can be seen in the uphill diffusion profiles of components that were initially uniform, in the fact that the apparent rate of diffusion of some components is a strong function of direction in composition space, and most quantitatively in the magnitude of off-diagonal elements of the diffusion matrix relative to the magnitude of the diagonal elements. SiO₂ for example, is found to be strongly coupled with CaO in relatively silicic melts, whereas Al₂O₃ is strongly coupled with CaO in less silicic melts. Furthermore, the coupling of CaO with either Al₂O₃ or Si02 reverses sign between more and less polymerized compositions. Interdiffusion profiles in natural melts have numerous features that suggest similar coupling between Al₂O₃ and CaO and between SiO₂ and CaO.     

Metals in a lateritic peat deposit — A case study from Sri Lanka

Peat is known to show metal enrichment. The geochemical enrichment factor (GEF) of U on peat humic acid is, for example, 10,000. Apart from U, peat accumulates a large number of metals including Cu, V, Ni, Cr, Zn, Pb, Zr, Mo, etc.

A large laterite bound peat deposit in Sri Lanka was studied for its distribution of metals. Most metals, in particular Be, Zr, Li and the majority of the transition series metals, exhibit a strong positive correlation with K, Al, Fe and Mn. A noteworthy feature observed was the poor correlation of the metals with organic carbon, as contrasted with a very good correlation with elemental components of clay. The laterites found around the peat deposit appear to be the possible source materials for the metals in the peat deposit. The geochemical distribution of metals in the peat is governed by:

1. (1) the nature of the organic matter;

2. (2) clay component;

3. (3) pH and Eh changes;

4. (4) marine-continental sediment mixing;

5. (5) nature of source materials for the metals.

Most of the metals appeared to be bound to the clay matrix, coated perhaps with hydrous Fe- and Mn-oxides. The existence of brine-rich marine-based sediments in the peat deposit had an important bearing on the geochemical distribution of the metals, as evidenced by the fact that in saline conditions most metals form chloride complexes, particularly under the prevailing acidic conditions. The metals thus mobilized interact with humic acids. The peat deposit under investigation provides an ideal setting for the study of the distribution of metals in a laterite-clay-peat mixture.     

地下水系统中镍污染和天然胶体共迁移特征

为了对地下水系统中天然胶体与Ni²⁺的共迁移特征进行研究,通过静态吸附实验和石英砂模拟含水层介质柱实验研究了土壤胶体对Ni²⁺在地下水中运移的影响,以及pH、离子强度（IS）、有机质等对土壤胶体吸附Ni²⁺的影响。结果表明：随着pH值升高,土壤胶体对Ni²⁺的吸附量增加;离子强度的增加会显著地降低土壤胶体吸附Ni²⁺的能力;腐殖酸（HA）的存在会增强胶体对Ni²⁺的吸附能力;在有胶体的情况下,Ni²⁺穿透砂柱的时间会缩短,吸附能力增强,吸附量增加,但当离子强度增加时,虽然Ni²⁺穿透砂柱的时间也被缩短,但是吸附量却降低。     

广西红水河中下游澄江地区地下水地球化学特征

以西南岩溶地下水污染调查评价中广西红水河中下游澄江地区地下水数据为基础,对该地区26件样品的水化学类型、化学成分含量特征、成分间相关特征以及化学成分的空间分布特征及影响因素进行了研究。结果表明：该地区地下水水化学类型为Ca-HCO₃型或Ca·Mg-HCO₃型;地下水中测试指标背景含量均未超过Ⅲ类水标准;大部分测试指标在该地区地下水中分布较均匀,只有Zn、Se和Hg在局部富集,分布不均匀;该地区地下水中元素含量与pH值没有明显的相关关系,而总硬度、HCO^-3、Ca²⁺、Mg2⁺、K⁺、Na⁺、SO^2-₄、NO^-3、I^-等元素间则显示出一定的相关关系,地下水中起决定性作用的是HCO^-3、Ca²⁺、Mg²⁺;该地区地下水中元素含量空间上具有片状分布特点,主要受地质背景、地理条件和人类活动的综合作用所制约。     

Monitoring of movement of potential earthquake areas with precise distance measuring and leveling systems

Whether monitoring crustal movements in localized volcanic areas along known fault lines, or over large crustal-movement areas, the geodesist has been restricted by the measurement accuracy of the instruments used, the accumulation of errors, the lack of reliable air refraction information and the problem of finding proper measurement procedures and mathematical solutions to assure that the inherent errors of the measurement-mathematical procedures do not exceed any conceivable ground movement.

Recent technological advances have placed new instruments and systems at the disposal of the geodesist, so that is now feasible to measure and analyze these micro and macro crustal movements within the accuracies required.

The paper describes three such systems:

1. (1) The Wild Electronic Theodolite T-2000 with a highly precise distance-measurement instrument, the DI-4S, together with a data collector, the GRE-3, which are connected to a computer and a plotter to measure and analyze both micro and macro crustal movements.

2. (2) The Wild NAK-2 level with an antimagnetic compensator which increases the accuracy in the height/velocity monitoring of vertical crustal movements by virtual elimination of the influence of natural or man-made magnetic fields on the automatic level.

3. (3) The use of analytical photogrammetry employing both terrestrial and aerial photography to monitor crustal movements.

By taking advantage of these new instruments and systems, the scientists capability to provide crustal movement data for use in the analysis and prediction of micro or macro crustal movement is greatly enhanced.     

Refinements in BaSO4 to CO2 Preparation and δ18 O Calibration of the Sulfate Reference Materials NBS-127, IAEA SO-5 and IAEA SO-6

Refinements have been made to achieve over 99% yield in the conversion of CO to CO₂ in order to improve the reproducibility and accuracy of δ¹⁸ O measurements in sulfates. BaSO₄ (10-15 mg) was mixed with an identical amount of spectrographic-grade graphite and loaded into a Pt boat. The mixture was gradually heated to 1100 °C to reduce sulfate to CO and CO₂; the former gas was simultaneously converted to CO₂ by a glow discharge between Pt electrodes immersed in a magnetic field (produced by a pair of external neodymium magnets). A small memory effect was noticed during the analysis (less than 0.3‰ per 10‰ difference in δ¹⁸ O between two subsequently analysed samples). The memory effect, however, was suppressed by repetitive preparation of the same specimen. CO₂ produced in this way from sulfate reference samples was analysed on a dual inlet and triple collector mass spectrometer along with CO₂ equilibrated with VSMOW, GISP and SLAP water reference samples. To avoid large departures of measured isotope ratios from ¹⁸O/¹⁶O of the working calibrator we used CO₂ gas prepared from ocean water sulfate for this purpose. The calibrated δ¹⁸ O values (in ‰) obtained in this way for NBS-127, IAEA SO-5 and IAEA SO-6 reference materials were 8.73 ± 0.05, 12.20 ± 0.07 and -10.43 ± 0.12, respectively.     

Solubility of silica polymorphs in electrolyte solutions, I. Activity coefficient of aqueous silica from 25° to 250°C, Pitzer''s parameterisation

Within the framework of Pitzer's specific interaction model, interaction parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solubility measurements. The values, at 25°C, of the Pitzer interaction parameter (λ_SiO2(aq)−i) determined in this study are the following: 0.092 (i = Na⁺), 0.032 (K⁺), 0.165 (Li⁺), 0.292 (Ca²⁺, Mg²⁺), −0.139 (SO₄²⁻), and −0.009 (NO₃⁻). A set of polynomial equations has been derived which can be used to calculate λ_SiO2(aq)−i for these ions at any temperature up to 250°C. A linear relationship between the aqueous silica-ion interaction parameters (λ_SiO2(aq)−i) and the surface electrostatic field (Z_i/r_e,i) of ions was obtained. This empirical equation can be used to estimate, in first approximation, λ_SiO2(aq)−i if no measurements are available. From this parameterisation, the calculated activity coefficient of aqueous silica is 2.52 at 25°C and 1.45 at 250°C in 5 m NaCl solution. At lower concentrations, e.g. 2 m NaCl, the activity coefficient of silica is 1.45 at 25°C and 1.2 at 250°C. Hence, in practice, it is necessary to take into account the activity coefficient of aqueous silica (λ_SiO2(aq)≠1) in hydrothermal solutions and basinal brines where the ionic strength exceeds 1. A comparison of measured [Marshall, W.L., Chen, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solubility behaviour in aqueous mixed electrolyte solutions to 300°C. Geochim. Cosmochim. Acta 46, 289–291.] and computed amorphous silica solubility, using this parameterisation, shows a good agreement. Because the effect of individual ions on silicate and silica polymorph solubilities are additive, the present study has permitted to derive Pitzer interaction parameters that allow a precise computation of γ_SiO2(aq) in the Na---K---Ca---Mg---Cl---SO₄---HCO₃---SiO₂---H₂O system, over a large range of salt concentrations and up to temperatures of 250°C.     

沙尘暴对天山托木尔峰青冰滩72号冰川环境的影响

基于沙尘暴发生前后在天山托木尔峰青冰滩72号冰川末端采集的7个大气气溶胶样品和在水文断面采集的9个融水径流样品,探讨了沙尘暴事件对天山冰川区大气环境和水环境的影响。结果表明,Ca²⁺和SO₄^2-是青冰滩72号冰川大气和融水径流的主要离子,当沙尘暴发生时,这两种介质中的离子浓度显著升高,而且升高存在同时性。由于影响因素不同,浓度随时间变化的趋势不同,大气环境对沙尘事件的响应能力强,水环境则对温度变化的响应能力更强。通过相关性分析和主成分分析认为,青冰滩72号冰川大气中Ca²⁺、Mg²⁺、Na⁺、K⁺、Cl^-和SO₄^2-主要以自然源为主;NO₃^-和NH₄⁺则以人为源为主。轨迹分析发现,由源于东欧、西伯利亚经中亚南下的冷气团引发的大风降温天气以及区域沙尘暴对冰川区环境有显著影响。     

Early diagenetic reactions in interbedded pelagic and turbiditic sediments in the Nares Abyssal Plain (western North Atlantic): Consequences for the composition of sediment and interstitial water

Measured pore-water concentrations of iron in interbedded pelagic and turbiditic sediments from the Nares Abyssal Plain are in excellent agreement with sediment colour and measured redox potential. The organic carbon content of these sediments appears to define the redox conditions and consequently the porewater and solid-phase concentration of constituents that are involved in early diagenetic reactions. In the turbiditic sediments the concentration of NO₃⁻ generally goes to zero within a sediment depth of 1 m, whereas at 8 m in a pelagic core from the same area the concentration of NO₃⁻ is still higher than it is in the bottom water. The pore-water concentration of Mn²⁺ in the turbiditic sediments increases sharply down to a depth of approximately 3 m and from thereon remains nearly constant due to saturation with respect to Mn, Ca-CO₃. The pore water of the turbiditic sediments is also saturated with respect to calcite. The few “diagenetic spikes” in the pore-water concentration of NO₃⁻ and Mn²⁺ and the concentration/depth profile of dissolved iron, H₄SiO₄ and phosphate all clearly demonstrate the inhomogeneous nature of interbedded pelagic and turbiditic sediments. The simultaneous occurrence of peaks of dissolved iron/silica and of sediment intervals with a relatively high organic carbon content is attributed to enhanced early diagenetic reactions associated with the decomposition of organic matter in these specific intervals. Linked with these reactions is the irregular pore-water concentration of phosphate, which is shown to originate partly from the oxidation of organic matter, but mainly from the desorption of phosphate from iron oxide. Potential concentrations of phosphate are calculated from the stoichiometric early diagenetic reactions and compared with measured concentrations. Due to the unique combination of low porosity and relatively high sedimentation rates, the sediments from the Nares Abyssal Plain are an ideal basis for the study of such interbedded sequences of pelagic and turbiditic deposits.     

Diffusion-controlled corona reaction and overstepping of equilibrium in a garnet granulite, Yenisey Ridge, Siberia

Diffusion modelling is applied to layered garnet–pyroxene–quartz coronas, formed by a pressure-induced reaction between plagioclase and primary pyroxene in a metabasic granulite. The reconstructed reaction involves some change in composition of reactant minerals. The distribution of minerals between layers is satisfactorily explained by diffusion-controlled reaction with local equilibrium, in which the diffusion coefficient for Al was smaller than those for Fe, Mg and Ca by a factor of approximately four. Diffusion of Mg towards plagioclase implies a chemical-potential gradient for MgO component in a direction opposite to the changing Mg content of garnet; this is explained by the influence of Al₂O₃ on the chemical potential of the pyrope end-member. Grain-boundary diffusion is suggested to have operated, possibly with composition gradients different from those in the bulk minerals. Chemical-potential differences across the corona are estimated from the variation in garnet composition, enabling affinity (the free energy change driving the reaction) to be estimated as 6.9±1.8  kJ per 24-oxygen mole of garnet produced. This implies that the pressure for equilibrium among the minerals was overstepped by 1.4±0.4  kbar. The probable P–T conditions of reaction were in the range 650–790  °C, 8–10  kbar. Assuming a timescale of reaction between 10⁶ and 10⁸ years, estimated diffusion coefficients for Fe, Mg and Ca are in the range 9×10⁻²³ to 5×10⁻²⁰ m² s⁻¹. These are consistent with experimental values in the literature for solid-state diffusion, including grain-boundary diffusion.