Rare-earth element geochemistry of Madras granulites

Archaean granulites from the type charnockite locality of Pallavaram, Madras City, Tamil Nadu, southern India consist of a bimodal suite of basic and silicic orthogneisses, associated with metasedimentary gneisses (khondalites). Charnockite is the dominant rock type. Basic granulites display a tholeiitic trend of strong iron enrichment accompanied by an increase in the concentration of the rare earth elements (REE), and the development of appreciable negative europium anomalies. These trends are considered to reflect low pressure fractional crystallisation of pyroxene-plagioclase assemblages under conditions of lowf _{O 2}. Ultramafic granulites may represent the cumulus material of such a process. The silicic granulites (charnockites) are generally K₂O rich and in marked contrast to the low K₂O (tonalitic) silicic gneisses which dominate most granulite facies terrains. Their REE abundances, however, embrace the complete range of patterns observed in both K-rich and K-poor Archaean gneisses. The presence of a large number of pre-granulite facies potassic pegmatites in the area suggests metasomatism of an originally less potassic suite of rocks. Trace element considerations lead to a model whereby metasomatism and partial fusion of silicic gneisses in the terrain preceded the granulite facies metamorphic event. This sequence of events is best related to fluctuations in the composition of metamorphic fluids in the lower crust.     

Crustal stress in Iceland

Hydrofracturing stress measurements have been carried out to about 0.4 km in two boreholes in Quaternary volcanic rocks in Reykjavik, Iceland, on the flank of the Reykjanes-Langjökull continuation of the Mid-Atlantic Ridge. The measurements indicate a dominant orientation of _{H max} approximately perpendicular to the axial rift zone, in contrast to earthquake focal mechanism solutions from within the axial rift zone of the Reykjanes Peninsula. In one hole (H32) a depth-dependent change in stress orientation is indicated, with ₁ horizontal above a depth of about 0.25 km, and vertical below it; however the orientation of _{H max} remains unchanged. The data thus suggest reconciliation of an apparent conflict between the dominantly compressive indications of shallow overcoring stress measurements and dominant extension as required by focal mechanism solutions. The measured stresses are supported by the more reliable of overcoring measurements from southeast Iceland, and by recent focal mechanism solutions for the intraplate Borgarfjördur area. A fundamental change in crustal stresses appears therefore to occur as a function of distance from the axis of the axial rift zone. The data seem reasonably explicable in terms of a combination of thermoelastic mechanisms associated with accretion and cooling of spreading lithosphere plates. Stresses directly associated with the driving mechanisms of plate tectonics apparently do not dominate the observed stress pattern.     

Geochemistry and petrogenesis of Archaean metavolcanic amphibolites from Fiskenaesset,S. W. Greenland

The meta-volcanic amphibolites closely associated with the Fiskenaesset anorthosite complex can be subdivided on the basis of trace element patterns or mineral chemistry; by far the most abundant type has light rare-earth element (LREE) depleted REE patterns and displays a wide range in trace element abundances. Chemically comparable amphibolites can be recognised throughout the ca. 2800 M.yr. West Greenland terrain.The geochemistry of the basaltic amphibolites is dominantly controlled by fractional crystallisation processes, although variable degrees of partial melting may also be important. The required crystal extract (plagioclase dominated) in the proposed fractionation scheme is very similar to the primary mineralogy of cumulates of the Fiskenaesset complex and trace element models support a genetic relationship between the anorthosite complex and enclosing host amphibolites.The application of trace element discrimination to assign tectonic environment in the Archaean is arguable. However, details of the trace element chemistry (especially chondritic La/Ta ratios) are taken to suggest, out of a range of likely tectonic environments, an ocean floor, rather than island arc, affinity for the Fiskenaesset amphibolites. The large ion lithophile (LIL) elements display erratic distribution patterns, but are generally enriched relative to the REE. This appears not to be related to high-grade metamorphism but may be a relict feature of seafloor alteration. The association of the cumulate sequence with meta-volcanic amphibolites and metasediments probably represents an ocean floor assemblage emplaced into the lower crust during crustal accretion.     

Subcritical crack propagation in rocks: theory,experimental results and applications

The micromechanisms of tensile fracture are reviewed, with particular emphasis on the influence of chemical effects on fracture controlled by pre-existing cracks (stress corrosion). A fracture mechanism map for quartz is presented which was constructed using a combination of theoretical insights and experimental data. The manner in which stress corrosion will modify the predictions of fracture mechanism maps is discussed by reviewing the numerous theories of stress corrosion. Experimental data are presented on stress corrosion in tensile deformation of quartz, quartz rocks, calcite rocks, basaltic rocks, granitic rocks and other geological materials. Although the experimental evidence for stress corrosion is overwhelming, very few data were obtained under conditions that simulate those in the bulk of the earth's crust and so the extent of its geophysical significance is yet to be fully established. Examples are given, however, of how invoking stress corrosion as a rate-controlling deformation mechanism sheds new light on extremely diverse geophysical phenomena, such as: predicting the strength and sliding friction properties of rocks, modelling earthquake rupture, the stability of hot, dry-rock geothermal reservoirs, stimulation of oil and gas reservoirs, the crack-seal mechanism of rock deformation and low stress dilatancy, fracture mechanics of lunar rocks, magmatic intrusions and the relaxation of internal stresses in rock.     

Analysis of fluoride, chloride, nitrate and sulphate in natural waters using ion chromatography

The analysis of anion components of thirty natural water samples using an ion-chromatographic method were compared to analyses for F, Cl, NO₃ and SO₄ obtained by presently used techniques. Of these thirty samples, five were replicates. Precision estimates were also calculated from synthetic solution replicates. In addition, a comparison of results obtained through separator columns of two different lengths (250 mm and 500 mm) was completed. An inter-anion interference test indicated that, at pH 6, analysis of low concentrations of any one of the four anions studied was not affected by large concentrations of the other three anions. Reported detection limits for the ion-chromatographic technique are one to two orders of magnitude below routine methods. Replicate sample analysis indicated that a relative standard deviation below 1 percent was possible for F, Cl, NO₃ and SO₄.     

Vibrational structure of the S 2 − luminescence in scapolite

An analysis has been made of the strong yellow luminescence of S ₂ ^? in the silicate mineral scapolite. The emission spectrum is dependent upon excitation wavelength, indicating that S ₂ ^? occupies several different sites. The vibrational constants for the ground state average 609 cm^?1 for ω″₀, while ω″₀χ″₀=2.2cm^?1. For the excited state ω′₀=399cm^?1 and ω′₀χ′₀=1.0cm^?1. The intensities of the vibrational bands are described by a simple harmonic oscillator calculation.     

Tectonophysical implications of rock stress determinations

Knowledge of spatial variations (concerning both magnitude and orientation) and temporal variations of the stress tensor at various points in the earth's crust is crucial to our understanding of the fundamental nature of tectogenesis, The means by which this data can be acquired forms the principal subject of a newly developed field of endeavor concerned with thein- situ measurement of stress. This field of research is expanding, with rapid advances in development of techniques, numbers of personnel, and acquisition of field data. Although primarily motivated by engineering and mining operations (because of the immediate applicability of this data to subsurface ground control), the geologic implications — thus far largely ignored — appear enormous. Determinations of stress have been made in every continent. The purpose of this paper is to emphasize the geological (and geophysical) relevance of current stress measurement work, which imposes conditions with which adequate theories of tectogenesis must comply.

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Zusammenfassung Die Kenntnis räumlicher Veränderungen (sowohl hinsichtlich der Größe wie der Richtung) und zeitlicher Veränderungen des stress tensor an verschiedenen Punkten der Erdkruste ist entscheidend für unser Verständnis der Grundzüge der Tektogenese. Die Mittel, mit deren Hilfe diese Daten gewonnen werden können, bilden den Hauptgegenstand eines neu entwickelten Tätigkeitsbereiches, der sich mitin situ-Messungen des stress befaßt. Mit dem schnellen Fortschritt der technischen Entwicklung, steigendem Personalstand und zunehmendem Anfall von Geländeergebnissen dehnt sich dieses Forschungsgebiet immer weiter aus. Obwohl zunächst Ingenieur- und Bergbautätigkeit (wegen der unmittelbaren Anwendbarkeit dieser Daten auf die Untertagekontrolle) Ausgangspunkt waren, erscheinen uns die — bisher weitgehend außer acht gelassenen — geologischen Folgerungen sehr erheblich. Stress-Bestimmungen werden auf jedem Kontinent angestellt. Der Zweck dieser Arbeit ist es, die geologische (und geophysikalische) Relevanz der gegenwärtigen Stress-Messungen hervorzuheben, mit deren Ergebnissen entsprechende Theorien der Tektogenese in Einklang stehen müssen.

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Résumé La connaissance des variations de l'espace (concernant aussi bien la magnitude que l'orientation) et des variations temporelles du «stress tensor» à des points différents de la croûte de la terre est décisive à notre entendement de la nature fondamentale de la tectogenèse. Les moyens par lesquels ces dates peuvent être obtenues, sont le sujet principal d'une sphère d'activité récemment développée, qui s'occupe des mesuragesin situ du stress. Ce domaine de recherches s'élargi avec le développement rapide des techniques, nombre du personnel, et acquisition des dates de champs. Bienque primairement motivé par des opérations ingénieures et minières (à cause de l'applicabilité immédiate de ces dates au contrôle du sous-sol), les conclusions géologiques — largement ignorées jusqu'à présent — apparaîssent énormes. Des déterminations de stress ont été exécutées dans tous les continents. Le but de ce travail est donc de souligner l'importance géologique (et géophysicale) des travaux de mesurage de stress en cours, imposant des conditions avec lesquelles des théories adéquates de tectogenèse doivent être mises d'accord.

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