Mechanisms of fine extinction band development in vein quartz: new insights from correlative light and electron microscopy

Fine extinction bands (FEBs) (also known as deformation lamellae) visible with polarized light microscopy in quartz consist of a range of nanostructures, inferring different formation processes. Previous transmission electron microscopy studies have shown that most FEB nanostructures in naturally deformed quartz are elongated subgrains formed by recovery of dislocation slip bands. Here we show that three types of FEB nanostructure occur in naturally deformed vein quartz from the low-grade metamorphic High-Ardenne slate belt (Belgium). Prismatic oriented FEBs are defined by bands of dislocation walls. Dauphiné twin boundaries present along the FEB boundaries probably formed after FEB formation. In an example of two sub-rhombohedral oriented FEBs, developed as two sets in one grain, the finer FEB set consists of elongated subgrains, similar to FEBs described in previous transmission electron microscopy studies. The second wider FEB set consists of bands with different dislocation density and fluid-inclusion content. The wider FEB set is interpreted as bands with different plastic strain associated with the primary growth banding of the vein quartz grain. The nanometre-scale fluid inclusions are interpreted to have formed from structurally bounded hydroxyl groups that moreover facilitated formation of the elongate subgrains. Larger fluid inclusions aligned along FEBs are explained by fluid-inclusion redistribution along dislocation cores. The prismatic FEB nanostructure and the relation between FEBs and growth bands have not been recognized before, although related structures have been reported in experimentally deformed quartz.     

Siliceous Cementation of Chlorite-Coated Grains in the Permian Sandstone Gas Reservoirs, Ordos Basin

<正>Objective It has long been controversial that whether authigenic chlorite coatings in sandstone reservoirs can prevent precipitation of siliceous cements.It is commonly believed that chlorite coatings(also called chlorite films,chlorite linings,or chlorite rims)may prevent quartz overgrowth,and thus help the preservation of original pores in sandstone reservoirs.Recently,however,this assumption has been challenged by reservoir geologists.This dispute cannot be solved by mere analysis of thin sections,nor by chemical equations and diagenesis analysis.The main objective of the present contribution is to shed light on this     

Trace Element Determination of Single Fluid Inclusions in Quartz by Laser Ablation ICP-MS

Single fluid inclusions in quartz from a Pb-Zn-Ag carbonate replacement deposit were selected for trace element determination by laser ablation ICP-MS. Spikes in element intensities were noted between first breached fluids versus subsequent analyses, suggesting that accurate element concentrations may not be determined in smaller fluid inclusions when only one analysis is obtained before the fluid is exhausted. Elemental concentrations in the fluid inclusions were determined by external standardisation using solutions sealed in microcapillary tubes. Standards and single natural inclusion analyses give repeatabilities (%RSD) of ˜ 20% for Rb and Sr. Rubidium and strontium concentrations range from 0.56-5.07 μg ml^-1 and 1.12-27.4 μg ml^-1, respectively, whereas Zn and Ag are below detection limits (< 10 ng ml^-1). The results suggest that nearly all Zn and Ag are removed by the time hydrothermal fluids precipitate gangue minerals.     

Carbonatite melt–CO2 fluid inclusions in mantle xenoliths from Tenerife, Canary Islands: a story of trapping, immiscibility and fluid–rock interaction in the upper mantle

Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO₂ (Type A); carbonate-rich CO₂–SO₂ mixtures (Type B); and polyphase inclusions dominated by silicate glass±fluid±sp±silicate±sulfide±carbonate (Type C). Type A inclusions commonly exhibit a “coating” (a few microns thick) consisting of an aggregate of a platy, hydrous Mg–Fe–Si phase, most likely talc, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the “coating”, towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid–wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO₂–H₂O–NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO₂–SO₂ fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 °C, the mixed CO₂–H₂O–NaCl fluid inside the Type A inclusions were separated into CO₂-rich fluid and H₂O–NaCl brine. At T<650 °C, the residual silicate melt reacted with the host olivine, forming a reaction rim or “coating” along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO₂ fluid. The homogenization temperatures of +2 to +25 °C obtained from the Type A CO₂ inclusions reflect the densities of the residual CO₂ after its reactions with the olivine host, and are unrelated to the initial fluid density or the external pressure at the time of trapping. The latter are restricted by the estimated crystallization temperatures of 1000–1200 °C, and the spinel lherzolite phase assemblage of the xenolith, which is 0.7–1.7 GPa.     

A method for predicting swelling pressure of compacted bentonites

An approach for predicting swelling pressure of bentonites based on thermodynamic relationships between swelling pressure and suction is presented in this paper. The proposed method requires sorption isotherm data of the bentonites. A series of swelling pressure tests were performed on compacted specimens of bentonite-sand mixtures with different bentonite contents, water contents, and dry densities. The sorption isotherm of the pure bentonite was measured using a chilled-mirror hygrometer. It is found that the method works well for the bentonite-sand mixtures tested. Several published data on bentonites that have been proposed to be used as buffer and sealing material for nuclear waste repository were collected and used to verify the method. The proposed method is found to be also applicable for other bentonites of different types and therefore, can be used to predict swelling pressure of bentonites.     

Timescales of geological processes: Preface

正1. Introduction One of the major challenges in Geoscience is to understand how the formation and evolution of the Earth System are governed by timescales e that is, how the various geological processes that continue to contribute to its present-day structure and composition operated in the deep past. The traditional view of such processes refers to events that occur at immense spatial scales and over hundreds of millions of years, constrained in most cases by the ages of rocks determined using isotopic dating methods or the fossil record. However, the modern view of geological processes has increasingly acknowledged that their durations can be significantly shorter than previously thought possible, or indeed detectable