bluuebunny/crossref_metadata_2025_split huggingface dataset:
Creep constitutive laws for rocks with evolving structure
Authors: Evans Brian 
Published: April 2008

Abstract
Zones of localized strain are common geological features at all spatial scales, are persistent in time and have important implications for rock strength. Technical advances now allow mechanical tests to be carried out to high strain, and, thus, there is both need and the opportunity to formulate constitutive laws for rocks as their structure (i.e. fabric, texture and mineralogy) changes. This paper reviews some attempts to describe quantitatively the mechanical behaviour of calcite rocks deformed to large strains. To include the effect of evolving structure on strength, three or more interlinked laws are needed: an evolution equation describing the rate of change for each structural variable; a kinetic equation relating mechanical and thermodynamic loading and the structural variables to the rate of strain; and a kinematic equation involving a time integral of inelastic strain rate. Structure variables may be explicitly identified or implicitly determined without identification. Appropriate explicit state variables might include aspects of the dislocation microstructure or the grain size, as are common in studies of plasticity in metals. But because natural tectonic situations are more complex, a much broader class of state variables will be needed. Among these additional variables might be crystal lattice preferred orientation, progress variables for metamorphic reactions, solid-solution chemistry, porosity and pore fluid fugacity.
Geochemical variations and element transfer during shear-zone development and related episyenites at middle crust depths: insights from the Mont Blanc granite (French — Italian Alps)
Authors: Rossi M. , Rolland Y. , Vidal O. , Cox S. F. 
Published: April 2008

Abstract

This paper highlights the relationships between the formation of shear zones, associated quartz-rich veins and their quartz-depleted alteration haloes (‘episyenites’) that have formed in the Mont Blanc Massif during the Alpine orogeny. The shear zones are steeply dipping and formed late (18–13 Ma) during collisional orogeny, at mid-crustal depths (5 ± 1 kbar, 400 ± 50 °C) during uplift of the Mont Blanc Massif. Between the shear zones, nearly undeformed granite contains widely dispersed, subhorizontal veins with a quartz-dominant quartz + albite + chlorite + adularia assemblage. They do not intersect the shear zones and are surrounded by quartz-depleted alteration haloes up to several metres wide. The compositions of the shear zones and the vein-alteration haloes (episyenites) show substantial departures from the bulk composition of the host rock. Shear zones are characterized by greenschist facies assemblages (epidote-, chlorite- or K-white-micabearing assemblages). Each shear zone type is featured by a specific chemical change: depletions in K
2
O, and enrichments in Fe
2
O
3
and CaO (epidote-); with depletions in CaO, Na
2
O, K
2
O and slight SiO
2
enrichments (white mica-chlorite-); with depletions in SiO
2
, CaO, Na
2
O, K
2
O and enrichments in MgO (phlogopite-chlorite shear zones). Episyenites are characterized by chemically induced porosity enhancement due to dissolution of magmatic quartz and biotite, with subsequent partial infilling of pore spaces by quartz, chlorite, albite and adularia. The vein arrays have accommodated minor vertical stretching in the Mont Blanc Massif, probably at the same time as the adjacent shear zones were accommodating more substantial vertical stretching in the massif. Coupled quartz dissolution in the wallrock alteration haloes and quartz precipitation in veins could be interpreted to reflect local mass transfer between wallrock and veins during essentially closed-system behaviour in the relatively underformed granite domains between shear zones. In contrast, shear zones probably develop in opened systems due to their kilometric length.
Terrane processes at the margins of Gondwana: introduction
Authors: Vaughan Alan P. M. , Leat Philip T. , Pankhurst Robert J. 
Published: April 2008

Abstract
The process of terrane accretion is vital to the understanding of the formation of continental crust. Accretionary orogens affect over half of the globe and have a distinctively different evolution to Wilson-type orogens. It is increasingly evident that accretionary orogenesis has played a significant role in the formation of the continents. The Pacific-margin of Gondwana preserves a major orogenic belt, termed here the ‘Australides’, which was an active site of terrane accretion from Neoproterozoic to Late Mesozoic times, and comparable in scale to the Rockies from Mexico to Alaska, or the Variscan-Appalachian orogeny. The New Zealand sector of this orogenic belt was one of the birthplaces of terrane theory and the Australide orogeny overall continues to be an important testing ground for terrane studies. This volume summarizes the history and principles of terrane theory and presents 16 new works that review and synthesize the current state of knowledge for the Gondwana margin, from Australia through New Zealand and Antarctica to South America, examining the evolution of the whole Gondwana margin through time.
Kinematic history of western Marie Byrd Land, West Antarctica: direct evidence from Cretaceous mafic dykes
Authors: Siddoway Christine S. , Sass Louis C. , Esser Richard P. 
Published: April 2008

Abstract

Intracontinental deformation occurred in West Antarctica during the final stages of plate convergence along the Cretaceous Gondwana margin. In western Marie Byrd Land, 115 Ma to 95 Ma A-type granitoids and mafic dykes record a change in plate kinematics. The magmatism typically is viewed as a record of extension leading to orthogonal break-up between New Zealand and Marie Byrd Land by
c.
67 Ma. This paper presents new kinmatic and
40
Ar/
39
Ar age data for a mafic dyke array in the Ford Ranges, a region >1000 km
2
dominated by plutonic and metamorphic bedrock. The mean dyke trend of N16W corresponds to a maximum finite strain axis orientated N74E, highly oblique to the N58E-trending margin and to on-land crustal structures defined from airborne geophysics.
40
Ar/
39
Ar emplacement ages for most dykes fall between 114 Ma and 97 Ma, coeval with emplacement of a gneiss dome at 101-96 Ma and with development of mylonitic shear zones at 100–95 Ma in coastal western Marie Byrd Land. The oblique orientation of maximum finite strain with respect to large faults, geophysical lineaments and the rifted margin of western Marie Byrd Land is consistent with transcurrent tectonics along this segment of the Gondwana margin at
c.
100 Ma.
Terrane and basement discrimination in northern Britain using sulphur isotopes and mineralogy of ore deposits
Authors: Lowry D. , Boyce A. J. , Fallick A. E. , Stephens W. E. , Grassineau N. V. 
Published: April 2008

Abstract
This study of four well characterized and adjacent terranes in Northern Britain outlines the sulphur isotope variations, assesses the overall importance of crustal and mantle sulphur, and presents a model that can be applied to terrane distinction throughout the North Atlantic Caledonides. The characteristics of metal components within the mineralization provide additional information that can be related to the nature of underlying basement and events from the onset of sedimentation to the cessation of mineralization within stratigraphically linked packages of rock.

The δ
34
S data show that the dominant crustal units in each terrane, whether upper crustal sediments or cratonic basement, provide the main alternative sulphur source to the mantle and act also as the main contaminant of subcrustal melts. The δ
34
S values of granitoid-related mineralization are either within the subcrustal melt-range of −3‰ to +3‰ or deviate toward the values of major crustal units in the terrane, i.e. toward
34
S depletion in the Southern Uplands and toward
34
S enrichment in the Lakesman and Grampian terranes. More complex mineralization in the Northern Highland terrane is linked to the presence of thick North Atlantic craton beneath upper crustal metasediments. Across the region the vein systems beyond the influence of magmatic components represent homogenized sulphur, metals and fluids from local upper crustal units. The sulphur isotope data and style of mineralization for the British terranes are compared with terranes of similar age along strike in Eastern Canada revealing notable correlations.
A reassessment of the tectonic zonation of the Uralides: implications for metallogeny
Authors: Herrington R. J. , Puchkov V. N. , Yakubchuk A. S. 
Published: April 2008

Abstract
A review of the structural zonation of the ‘oceanic’ Urals shows that only its westernmost Sakamara, Tagil and Magnitogorsk zones reveal the presence of thrust structures, whereas in the East Uralian megazone and Trans-Uralian zone, the classic zonation rather reflects late- or post-collisional granitic welding and strike-slip displacement of the orogen for 100–300 km. This sinistral strike-slip displacement is responsible for the lens-shapes structure of the individual zones in the Urals.
Metallogenically, these orogen-parallel faults and the eastern boundary of the East European craton control the distribution of the orogenic Au deposits. Restoration of the individual zones into their pre-strike-slip fault positions suggests that the Urals contains only two magmatic arcs, one in the west and one in the east. The western Tagil-Magnitogorsk immature arc hosts a variety of chromite, Alaska-type PGE and major VMS deposits. The eastern Valerianovka arc effectively stitches together the Kazakh-Tien Shan structures and is host to important copper-gold and giant iron(-copper) skarn deposits.
The geodynamic evolution of the Urals can be observed with the generation of the immature Tagil-Magnitogorsk magmatic arc in the Late Ordovician. Metallogenic zoning of the VMS deposits supports the petrological data that the arc developed due to eastward subduction inside the oceanic back-arc basin that existed in the rear of the Kazakhstan-Tien Shan arcs. In the late Palaeozoic, these arcs collided with each other and were together thrust onto the East European craton. Syncollisional granitoid intrusions welded the magmatic arcs, which were soon displaced into presently observed fragments along the post-collisional orogen-parallel strike-slip faults.
The terrestrial record of stable sulphur isotopes: a review of the implications for evolution of Earth’s sulphur cycle
Authors: Farquhar James , Wing Boswell A. 
Published: April 2008

AbstractThe observation of anomalous (non mass-dependent) sulphur isotope compositions in Archaean and early Proterozoic rocks but not in rocks younger than approximately 2 Ga has been interpreted to reflect fundamental change in the terrestrial sulphur cycle, in atmospheric chemistry, and in atmospheric oxygen content. Similar non mass-dependent sulphur isotope compositions in present-day samples (atmospheric aerosols and ice-core horizons containing remnants of stratosphere-piercing volcanic eruptions) are interpreted to carry information about modern atmospheric chemistry and transport. The interpretation of these observations hinges on our understanding of the processes that produce non mass-dependent sulphur isotope compositions, the processes that transport and transfer the isotopic signals throughout the sulphur cycle, and the processes that act to preserve or erase these isotopic signals once they are established. The growing dataset and hypotheses related to non mass-dependent sulphur are evaluated, emphasizing that which remains to be learned about the evolution of the record, the compositions of key reservoirs, and the transfer of the signal from the atmosphere to the surface and ultimately to the deep Earth.
Reactive iron enrichment in sediments deposited beneath euxinic bottom waters: constraints on supply by shelf recycling
Authors: Raiswell R. , Anderson T. F. 
Published: April 2008

Abstract

Modern and ancient euxinic sediments are often enriched in iron that is highly reactive towards dissolved sulphide, compared to continental margin and deep-sea sediments. It is proposed that iron enrichment results from the mobilization of dissolved iron from anoxic porewaters into overlying seawater, followed by transport into deep-basin environments, precipitation as iron sulphides, and deposition into sediments. A diagenetic model shows that diffusive iron fluxes are controlled mainly by porewater dissolved iron concentrations, the thickness of the surface oxygenated layer of sediment and to a lesser extent by pH and temperature. Under typical diagenetic conditions (pH < 8, porewater Fe
2+
= 10
−6
g cm
−3
) iron can diffuse from the porewaters in continental margin sediments to the oxygenated overlying seawater at fluxes of 100–1000 μg cm
−2
a
−1
. The addition of reactive iron to deep-basin sediments is determined by the magnitude of this diffusive flux, the export efficiency (ɛ) of recycled iron from the shelf, the ratio of source area (
S
) to basin sink area (
B
) and the trapping of reactive iron in the deep basin. Values of ɛ are poorly constrained but modern enclosed or semi-enclosed sedimentary basins show a wide variation in
S/B
ratios (0.25–13) where the shelf source area is defined as sediments at less than 200 m water depth. Diffusive fluxes in the range 100–1000 μg cm
−2
a
−1
are able to produce the observed reactive iron enrichments in the Black Sea, the Cariaco Basin and the Gotland Deep for values of ɛ ×
S/B
from 0.1–5. Transported reactive iron can be trapped physically and/or chemically in deep basins. Physical trapping is controlled by basin geometry and chemical capture by the presence of euxinic bottom water. The
S/B
ratios in modern basins may not be representative of those in ancient euxinic/semi-euxinic sediments but preliminary data suggest that ɛ ×
S/B
in ancient euxinic sediments has a similar range as in modern euxinic sediments.
The formation of economic porphyry copper (-gold) deposits: constraints from microanalysis of fluid and melt inclusions
Authors: Heinrich C. A. , Halter W. , Landtwing M. R. , Pettke T. 
Published: April 2008

Abstract

This paper summarizes our current understanding of the formation of porphyrystyle Cu ± Au ± Mo deposits, in the light of data obtained by direct analysis of the ore metals in individual fluid and melt inclusions using laser-ablation ICP mass spectrometry. An integrated study of the evolution of the calcalkaline Farallón Negro Volcanic Complex hosting the Bajo de la Alumbrera porphyry Cu-Au deposit (Argentina), and supplementary fluid-chemical data from Bingham (Utah) and other examples, permit a quantitative re-assessment of the fundamental processes controlling the key economic parameters of porhyry-style ore deposits. Deposit size (total metal content) is optimized by exsolution of a relatively dense (>0.3 g cm
−3
) single-phase fluid or a two-phase brine + vapour mixture from a moderately large hydrous pluton, possibly with an intermediate step involving the scavenging of the ore-forming elements in a magmatic sulphide melt. Emplacement mechanism, magma-chamber dynamics and possibly an additional source of sulphur are probably more decisive for the formation of a large deposit than sheer pluton volume and elevated Cu contents in the melts. Primary bulk ore grade is determined by temperature-controlled precipitation of ore minerals, which is optimized where a large magmatic fluid flux is cooled through 420–320 °C over a restricted vertical flow distance. Bulk metal ratios of the deposits, exemplified by the economically important Au/Cu ratio in the ore, are primarily controlled by the magmatic source defining the composition of the fluids before they reach the deposit site, although selective precipitation may contribute to metal zoning within orebodies.
Quantitative determination of hydraulic properties of fractured rock using seismic techniques
Authors: Liu Enru , Chapman Mark , Hudson John A. , Tod Simon R. , Maultzsch Sonja , Li Xiang-Yangi 
Published: April 2008

Abstract
There have been significant advances over the last ten years in the use of the seismic anisotropy concept to characterize subsurface fracture systems. Measurements of seismic anisotropy are now used to deduce quantitative information about the fracture orientation and the spatial distribution of fracture intensity. Analysis of the data is based upon various equivalent medium theories that describe the elastic response of a rock containing cracks or fractures in the long wavelength limit. Conventional models assume scale/frequency independence and hence cannot distinguish between micro-cracks and macrofractures. The latter, however, control the fluid flow in many oil/gas reservoirs, as the fracture size and spacing (hence fracture storability) are essential parameters for reservoir engineers. Recently, a new equivalent medium theory for modelling of wave propagation in media with multi-scale fractures has been presented. The model predicts velocity dispersion and attenuation due to a squirt-flow mechanism at two different scales: the grain scale (micro-cracks and equant matrix porosity) and formation-scale fractures. Application of this model to field data shows that fracture density and fracture size can be inverted successfully from the frequency dependence of the time delay between split shear waves. The derived fracture length matches independent observations from borehole data. This paper presents the results of the latest development in the seismic characterization of natural fractures, with an emphasis on the quantitative determination of fracture sizes.
Precise numerical modelling of physical transport in strongly heterogeneous porous media
Authors: Xie Zhongqiang , Mackay Rae , Cliffe K. Andrew 
Published: April 2008

Abstract
A library comprised of precise numerical simulation results for two-dimensional flow and advective transport through statistically-equivalent, structured, porous media is being created to investigate upscaling procedures for contaminant dispersal and physical transport at large space and long time-scales. Several technical challenges were overcome to achieve high precision in the velocity fields and particle paths. A Cholesky Decomposition method for efficient media generation has been extended to generate ‘artefact’-free, areally-extensive random fields of hydraulic conductivity variation. An efficient mixed finite-element method, capable of handling periodic boundary conditions was used to compute the flow distributions through the generated media. The method permits exact solution of particle trajectories. Comparison of particle migration patterns with available analytical solutions confirms persistent non-Fickian behaviour of particle migration at large space and times-scales, as well as confirming the accuracy of the simulations. Boundary effects on particle trajectories are found to be significant and cannot be removed totally from the steady-state flow fields. Periodicity, both parallel and perpendicular to the flow direction, results in particle trajectories that are almost periodic and so are inappropriate for transport studies of realistic media. A compromise solution has been adopted whereby only the domain boundaries parallel to the flow are periodic.
Fabric development and the smectite to illite transition in Upper Cretaceous mudstones from the North Sea: an image Analysis Approach
Authors: Worden R. H. , Charpentier D. , Fisher Q. J. , Aplin A. C. 
Published: April 2008

Abstract
In this study, Upper Cretaceous Shetland Group mudstone cuttings from a range of depths in the Northern North Sea, have been studied using X-ray diffraction, mercury porosimetry and electron microscopy. Millimetre to micrometre mudstone textures have been quantified using image analysis of backscattered electron microscope images. Relatively shallow samples (1615 m) have isotropic mudstone fabric (no alignment of clay minerals), are dominated by smectite and have porosity values of approximately 35%. In contrast, more deeply buried samples (3300 m) have developed an anisotropic fabric (distinct alignment of clay minerals), are dominated by illite and have porosity values of approximately 22%. The change in mineralogy is due to smectite replacement by illite, which occurs simultaneously with porosity-loss and fabric development during progressive burial. Image analysis of differentially buried mudstones has proved to be a rapid, flexible and quantitative method for characterizing mudstone textures. The coincidence of mineralogical evolution with textural development and compaction implies that the transformation of smectite to illite occurs by dissolution and precipitation and that chemically facilitated compaction may contribute to porosity loss.
The μ2M project on quantifying the effects of biofilm growth on hydraulic properties of natural porous media and on sorption equilibria: an overview
Authors: Brydie J. R. , Wogelius R. A. , Merrifield C. M. , Boult S. , Gilbert P. , Allison D. , Vaughan D. J. 
Published: April 2008

Abstract
The physical and chemical effects of bacterial biofilm formation upon hydraulic conductivity, mineral-solution interactions and the formation of biogenic mineral precipitates have been studied over a wide range of scales, from microscopic to macroscopic. Several novel pieces of equipment have been designed, constructed and commissioned in order to measure the physical effects of biofilms upon fluid flow through fractures and porous media, the overall effects of biofilm formation upon mineral surface reactivity, and the imaging and identification of mineral precipitates formed due to the presence of biofilm and bacterial cell surface polymers on a quartz surface. This paper presents an overview of key experimental methods and selected results; further experimental information is being published elsewhere.

Biofilm formation with quartz sand in artificial groundwater resulted in a two orders of magnitude reduction in hydraulic conductivity under bench-scale constant head conditions. However, under quasi-environmental conditions within macroscopic centrifuge experiments, a reduction of 21% was measured, revealing differences in measurements and, hence, the value of the macroscopic experimental work in scaling from micro to macro.
In-situ
microscopic evaluation of biofilms within simulated quartz rock fractures and in porous media reveal only a small percentage of the biomass to be in direct contact with the mineral surface, allowing mineral chemistry to be predominantly controlled by mineral surface reactivity, rather than by a diffusion-limited mineral-biofilm-solution interface. This is true even when a mineral surface is apparently completely covered by biofilm. The alteration of mineral surface drastically increases the kinetics of surface-coordinated trace metal precipitate formation by providing nucleation sites upon extracellular biopolymers and cell wall polymers. Over geological time-scales, these processes, particularly the formation of thermodynamically stable pore-blocking mineral precipitates, are envisaged to markedly change the flow paths, flow rates and interaction of migrating geofluids (water, petroleum, ore-forming solutions) with minerals and rocks.