Dissolution and depletion of ferromagnesian minerals from Holocene tephra layers in an acid bog,New Zealand,and implications for tephra correlation

This study examines the depletion of ferromagnesian silicate minerals from a sequence of thin, distal, mainly rhyolitic tephra layers of Holocene age preserved in an acid peat bog (Kopouatai), North Island, New Zealand. The rate of such depletion has been fast, as indicated by the complete loss of biotite from one tephra layer (Kaharoa Tephra), in which it is normally dominant, in only ca. 770 yr. Chemical dissolution is advocated as the likely cause for the depletion, with amphiboles and other mineral grains commonly showing etch pits, microcaves, and other characteristic surface solution features. Theoretical thermodynamic and kinetic models show a marked increase in the rate of dissolution of all ferromagnesian minerals under conditions of low pH (< 4), but that where silica concentrations in solution are high the relative proportions of minerals remaining are unaffected. However, where concentrations of dissolved silica are low, as in most bog environments, the relative proportions of ferromagnesian minerals are affected as well as absolute amounts being decreased. Amphiboles are depleted relative to pyroxenes, consistent with kinetic studies. The results show that the identification and correlation of tephras on the basis of relative abundances of ferromagnesian minerals alone may be unreliable, and emphasise the need to use multiple criteria in such studies.     

An Attempt to Predict the Failure Time of Abandoned Mine Pillars

Summary  Left in place pillars of abandoned mines are subject to weathering (e.g action of water, bacteria) that degrades their mechanical strength and eventually leads to collapse. A simple weathering model is proposed, that is governed by two parameters: the rate of progression of weathered front and the rate of degradation of the compressive strength with time. Both plane strain and axisymmetric analyses are performed and closed form solutions of the variation with time of the bearing capacity of the pillar are given. Experimental data of the tests conducted on gypsum and anhydrite specimens attacked by water are presented. It is shown that in order to fit the experimental data a third parameter must be introduced. New closed form solutions are given and the data are used for estimating the time to failure of abandoned gypsum mines in Northern Italy. Authors’ address: Dr. Riccardo Castellanza, Research Assistant, Department of Structural Engineering, Milan University of Technology (Politecnico), Piazza Leonardo da Vinci 32, 20133 Milan, Italy     

The effect of rock moisture on Schmidt hammer rebound: tests on rock samples from Marion Island and South Africa

In an assessment of the influence of internal rock moisture content on Schmidt hammer readings, rebound (R) values are found to decrease with increasing moisture content. For samples of basalt, sandstone and dolerite the maximum decrease in R‐values is found between oven dry values and saturated rock rebound values, the magnitude of which varies from 2 to 10 points on the R‐scale. A quartzite block has the greatest decline of 6 points at 60 per cent saturation. For certain rock types under differing site‐to‐site field moisture conditions the moisture effect can be a significant factor in the interpretation of the relative state of weathering from rebound values. Copyright © 2002 John Wiley & Sons, Ltd.     

大兴安岭阿龙山地区流纹岩风化的地球化学特征

通过大兴安岭地区两个流纹岩风化剖面的研究发现，大兴安岭地区的岩石风化过程具有特殊性。风化过程分为两个阶段，即以化学风化为主导的基质风化阶段和以机械风化为主导的斑晶风化阶段，第二阶段显示出与正常风化剖面相反的风化趋势。低温和长冬使第二风化阶段延长，并呈现逆风化趋势。风化过程中Fe和Al始终处于流失状态。上述特征的产生是由原岩的结构及成分、气候因素和生态系统等多种因素作用的结果。     

Monitoring of rapid salt weathering in the central Namib Desert using limestone blocks

Pre-weighed blocks of a Jurassic Limestone were exposed on the ground surface in the coastal Namib Desert for a period of 2 years. The environment is both salty and foggy. Some of the blocks suffered extensive disintegration, and laboratory analyses (including geochemistry, XRD and SEM) indicate that the weathered samples have a high halite (sodium chloride) content. Cycles of wetting and drying associated with the frequent fog events of the area cause cycles of halite crystallization. Rocks exposed at the surface absorb salts from the surrounding desert surface and then disintegrate, contributing to planation of the landscape.     

Erosion rates of alpine bedrock summit surfaces deduced from in situ Be and Al

We have measured the concentration of in situ produced cosmogenic ¹⁰Be and ²⁶Al from bare bedrock surfaces on summit flats in four western U.S. mountain ranges. The maximum mean bare-bedrock erosion rate from these alpine environments is 7.6 ± 3.9 m My⁻¹. Individual measurements vary between 2 and 19 m My⁻¹. These erosion rates are similar to previous cosmogenic radionuclide (CRN) erosion rates measured in other environments, except for those from extremely arid regions. This indicates that bare bedrock is not weathered into transportable material more rapidly in alpine environments than in other environments, even though frost weathering should be intense in these areas. Our CRN-deduced point measurements of bedrock erosion are slower than typical basin-averaged denudation rates ( 50 m My⁻¹). If our measured CRN erosion rates are accurate indicators of the rate at which summit flats are lowered by erosion, then relief in the mountain ranges examined here is probably increasing.

We develop a model of outcrop erosion to investigate the magnitude of errors associated with applying the steady-state erosion model to episodically eroding outcrops. Our simulations show that interpreting measurements with the steady-state erosion model can yield erosion rates which are either greater or less than the actual long-term mean erosion rate. While errors resulting from episodic erosion are potentially greater than both measurement and production rate errors for single samples, the mean value of many steady-state erosion rate measurements provides a much better estimate of the long-term erosion rate.     

类石材料--灰泥的冻融过程

Serious failure on the slope of rock ground can be caused by a cyclic action of freezing and thawing in the cold regions. The frost susceptibility and the effect of freezing and thawing onthe rock material, however, have not been well investigated. In order to find out the freezing effect on the rock materials, mortar specimens are frozen as a pseudo-rock material under the constant rate of freezing by means of controlling the temperature of both ends of specimen. The freezing process is given one-dimensionally to the cylindrical samples in the laboratory to simulate the in-situ freezing phenomena in the natural ground. Formation of ice lens, frost heave and water intake during freezing process are observed on the mortar specimen under constant freezing rate, which probably causes cracks or large deformation in the real rock ground. The values of the velocity of elastic wave propagation are compared before and after freezing process to estimate the degree of weathering due to freezing and thawing.     

The role of moisture cycling in the weathering of a quartz chlorite schist in a tropical environment: findings of a laboratory simulation

Long‐term weathering of a quartz chlorite schist via wetting and drying was studied under a simulated tropical climate. Cubic rock samples (15 mm × 15 mm × 15 mm) were cut from larger rocks and subjected to time‐compressed climatic conditions simulating the tropical wet season climate at the Ranger Uranium Mine in the Northern Territory, Australia. Fragmentation, moisture content and moisture uptake rate were monitored over 5000 cycles of wetting and drying. To determine the impact of climatic variables, five climatic regimes were simulated, varying water application, temperature and drying. One of the climatic regimes reproduced observed temperature and moisture variability at the Ranger Uranium Mine, but over a compressed time scale. It is shown that wetting and drying is capable of weathering quartz chlorite schist with changes expected over a real time period of decades. While wetting and drying alone does produce changes to rock morphology, the incorporation of temperature variation further enhances weathering rates. Although little fragmentation occurred in experiments, significant changes to internal pore structure were observed, which could potentially enhance other weathering mechanisms. Moisture variability is shown to lead to higher weathering rates than are observed when samples are subjected only to leaching. Finally, experiments were conducted on two rock samples from the same source having only subtle differences in mineralogy. The samples exhibited quite different weathering rates leading to the conclusion that our knowledge of the role of rock type and composition in weathering is insufficient for the accurate determination of weathering rates. Copyright © 2005 John Wiley & Sons, Ltd.     

我国热带亚热带部分地区花岗岩和片麻岩中黑云母风化研究

综合应用Ｘ射线衍射、扫描电镜、透射电镜及偏光显微镜等技术手段研究了我国热带、亚热带部分地区花岗岩和片麻岩中黑云母的风化现象。结果表明，化学风化产物主要有黑云母／蛭石不规则间层矿物、水黑云母、蛭石、０．７ｎｍ埃洛石及铁的氢氧化物（氧化物）等。影响黑云母风化及次生产物形成的主要因素包括黑云母的晶体结构和化学成分、气候、风化壳母岩的矿物成分等。     

Textural and Geochemical Characteristics of Proglacial Sediments: A Case Study in the Foreland of the Nelson Ice Cap, Antarctica

This paper presents a detailed study on the textural and geochemical characteristics of the proglacial sediments near the edge of modern Nelson Ice Cap, Antarctica. The grain size distributions of the proglacial sediments are characteristic of glacigenic deposits, but very different from those of aeolian and lacustrine sediments. Moreover, the grain size distributions of the proglacial sediments are fractal with a dimension of about 2.9, and the fractal dimensions can be used as another summary statistical parameter for quantifying the relative amounts of coarse and fine materials. Correlations between the absolute element abundances of the proglacial sediments are very weak due to mineral partitioning and other effects of glacial processes, but correlations between the element/Rb ratios are statistically significant. This finding indicates that element/Rb ratios can be used to reduce or eliminate the effects of glacial processes, evaluate geochemical data and determine the sediment provenance in the for