Fracture geometry of pseudotachylyte generation zones: a study of shear fractures formed during seismic events

The orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the Ikertôq shear belt, West Greenland are described, and the fracturing sequence is inferred. The generation zones occur in strongly foliated gneisses, and consist of concordant, usually paired, principal displacement shear fractures (paired shears), on which most melt was generated, and a system of minor shear fractures. Minor shear fracturing was most intense between principal displacement shear fractures, and most melt intruded from these paired shears inwards with respect to the zone. Intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures, is believed to be contemporaneous with minor shear fracturing in the generation zones.Generation zones affect intact rocks, and individual principal displacement shear fractures have a maximum displacement of 4.6 m. Paired principal displacement shear fractures are up to 3 m apart, and have a maximum observed length of 1 km. In all the zones described, the fracturing appears to be the result of a single episode of slip. Both dextral and sinistral generation zones occur within the study area, and have different geometries. In each case the displacement is strike-slip.     

The interstellar dust as a precursor of Ca,Al-rich inclusions in carbonaceous chondrites

(1) The observed anomalies in meteoritic oxygen isotope compositions are not due to an incomplete mixing of several dust or gas-plus-dust components in the solar nebula. If they were, other elements would display similar anomalies. (The FUN inclusions in Allende appear to be exceptions to this premise.) (2) The anomalies must therefore stem from differing degrees of incomplete exchange of oxygen isotopes between the primordial gas and dust components of the nebula. The dust is more likely to have been the¹⁶O-enriched component. (3) Since the isotopic difference between dust and gas probably could not have been preserved if the dust was ever completely vaporized in the nebula, the Ca,Al-rich inclusions (CAI's) in carbonaceous chondrites are unlikely to be condensates, but instead are distillation residues. (4) If so, the observed depletion of super-refractory elements in the Group II CAI's cannot have been accomplished by fractional condensation in the solar nebula. (5) Then this depletion, and a number of other properties of the components of primitive meteoritic material, must be relics of pre-solar system fractionations among different populations of interstellar dust grains.     

Regionalization of global electromagnetic induction data: a theoretical model

Systematic differences are noted between those global response functions (Q = I₁/E₁; $\text{W}\text{= Z/H}$) derived from single observatories and those derived from global averages. Using a simple first-order model to simulate global scale lateral heterogeneities, we argue that reasonable differences in the depth to the conductosphere (d (average) = 400 km in one hemisphere, 600 km in the other) result in significant differences in the response functions at single observatories. These differences appear to be easily resolvable within the expected error-bars of actual observations. At this point it is believed that regional differences in global response parameters can indeed be used to infer large-scale differences in the depth to the conductosphere, providing that systematic biases introduced in data processing and interpretation are minimized.     

Lithification analysis — Experimental observations

Thin sections prepared from laboratory compacted dry sediment mixtures are compared with selected thin sections from natural sedimentary rocks. Compression of a quantity of calcium carbonate bioclasts at a pressure of 1,000 kg/cm² gives rise to plastic deformation of grain contacts, penetration contacts, and the reduction of porosity and permeability. Compressing a mixture of carbonate fragments and quartz grains at a pressure of 1,000 kg/cm² causes the quartz grains to penetrate the softer carbonate bioclasts. Compression of a dry, well-rounded quartz sand at a confining pressure of 3,000 kg/cm² leads to the crushing of the majority of the grains. An artificial sediment consisting of a mixture of quartz spheres and gem fragments demonstrates upon compaction by a pressure of 3,000 kg/cm² the capacity of the harder gem minerals to penetrate quartz grains by way of plastic deformation of the grain contacts. Differences in hardness between dolomite and calcite/aragonite fragments are responsible for the creation of lithification textures of the dolomite crystals penetrating the softer carbonate grains.

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Zusammenfassung Dünnschliffe von Sedimentmischungen, die einer Trockenkompaktion unterzogen wurden, werden mit ausgewählten Dünnschliffen natürlicher Sedimentgesteine verglichen. Kompression von Kalziumkarbonatbioklasten mit einem Druck von 1000 kg/cm² bewirkt die plastische Deformierung der Kornkontakte, Entstehung von Eindruckkontakten und beträchtliche Reduktion der Porosität und Permeabilität. Bei Kompression einer Mischung von Karbonatfragmenten und Quarzkörnern mit einem Druck von 1000 kg/cm² dringen die Quarzkörner in die weicheren Karbonatbioklaste ein. Kompaktion eines trockenen, gut gerundeten Quarzsandes mit einem Druck von 3000 kg/cm² bewirkt das Zerbrechen des größten Teiles dieser Quarzkörner. Ein künstliches Sediment aus einer Mischung von Quarzkörnern und Edelsteinfragmenten zeigt, daß bei Kompaktion mit einem Druck von 3000 kg/cm² die härteren Edelsteinmineralien in die Quarzkörner eindringen. Unterschiede in der Mineralhärte zwischen Dolomit und Kalzit- und Aragonit-Fragmenten sind verantwortlich für das Auftreten von Lithifikationstexturen von Dolomitkristallen in den weicheren Karbonatkörnern.

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Résumé Des lames minces préparées à partir de mélanges artificiels de sédiments compactés à l'état sec, forment la base d'une comparaison avec des lames minces tirées des roches sédimentaires naturelles sélectionnées. La compression exercée par des biofragments composés d'aragonite, d'une valeur de 1000 kg/cm², imprime une déformation plastique des contacts intergranulaires, la formation de contacts par impressionement, et une réduction de la porosité et de la perméabilité. La compression d'un mélange de bio-fragments carbonatés avec une quantité de grains de quartz, sous une pression de 1000 kg/cm² donne aux grains de quartz la capacité de pénétrer les grains carbonatés moins dûrs. La compression d'un sable composé des grains bien-arrondis, complètement sec, à 3000 kg/cm² a comme résultat l'éclatement de la plupart des grains. Un mélange sédimentaire artificiel de grains de quartz et de fragments de pierres précieuses montre, après la compression sous 3000 kg/cm², la capacité des pierres précieuses (minéraux avec une dureté spécifique plus grande que celle du quartz) de pénétrer les grains de quartz grâce à la déformation plastique des contacts intergranulaires. Les différences dans la durété spécifique de la dolomite et de la calcite/aragonite sont-responsables de la création des textures de lithification, où des cristaux de dolomite pénétrent les grains carbonatés moins durs.

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Momentum and energy balance equations for compacting sediments

Coupled energy and momentum balance equations are derived for a one-dimensional sequence of compacting sediments. These transient, nonlinear partial differential equations represent the one-dimensional equations of state for an accumulating sedimentary basin. A numerical solution is presented which provides a first-order approximation for porosity, temperature, and fluid pressures in the northern Gulf of Mexico. It seems that compaction disequilibrium is the primary mechanism for development of excess fluid pressures. Furthermore, the coupling of the equations demonstrates that temperature and pressure cannot be treated independently as may have been done in diagenetic studies. Some areas for further investigations are indicated.     

Dynamical formulation of Broecker''s model for marine cycles of biologically incorporated elements

Broecker presented a steady-state, two-box model which emphasizes the role of kinetic factors in determining the chemical composition of sea water. Unlike thermodynamic models, Broecker's model suggests that sizable temporal variation in the composition of sea water and rates of sedimentation may have taken place in the past 100 million years. To describe the evolution of the ocean's chemical system and interpret variations observed in the sedimentary record, we have formulated a dynamical model. Mass-balance consideration leads to a set of coupled nonlinear differential equations. The equations are linearized and solved for step function changes in the rate of river input and in the rate of vertical mixing. This simple model of the ocean's chemical system is shown to be stable against oscillations. Using data for the modern ocean, the response times for P, C, Si, Ca, and Ba are calculated to be of the order of 10⁴ to 10⁶ yr for changes in river and other inputs and of the order 10¹ to 10² yr for a change in the rate of vertical mixing. Analog-simulation techniques, discussed for the situation of the two-box model, provide a powerful tool for treating nonlinearities and systems with more than two components. In the final section, consequences of the dynamical formulation are compared with parameters appearing in Broecker's steady-state formulation.     

The transformation behaviour of bornite

Phase transformations in bornite have been observed dynamically by in situ experiments in a transmission electron microscope. A series of superstructure phases is formed with decreasing temperature and a new polymorph with a 3a supercell is described. Ordering behaviour with the formation of antiphase domains is proposed as an alternative to the generally accepted twinning mechanism for the transformation to the 2a type.Work carried out while on leave at Department of Mineralogy and Petrology, Cambridge     

A crustal suture and lineament in North Africa

A possible suture between an old craton in West Africa and the Arabian—Nubian shield is marked by a persistent zone of Phanerozoic weakness. This zone extends between the Nile and Niger deltas and passes through the Qattara embayment, along the Bahariya fault, through the Kufra basin, between the Tibesti and Jabal al Uwaynat massifs, through the Bahr el Ghazal and Lake Chad areas, and along the Benoue trough.     

Chemical fractionations in meteorites—IX. C3 chondrites

Four C3V chondrites (Grosnaja, Kaba, Mokoia, Vigarano) and three C3O chondrites (Felix, Kainsaz, and Lancé) were analyzed by radiochemical neutron activation for 17 trace elements. Both classes show a typical chondritic step pattern, reflecting loss of volatiles during chondrule formation. Elements condensing above 1300 K (U, Re, Ir, Ni) are present in essentially Cl chondrite proportions, while moderately volatile elements condensing between 1300 K and 800 K (Ge, Rb, Ag) are depleted by a factor of 0.44. However, elements condensing below 700 K (S, Cs, Bi, Tl, Br, Se, Te, In, Cd) are depleted to a still greater degree, and more so in the Ornans subclass (factor of 0.24, except Cd 0.007) than in the Vigarano subclass (factor of 0.29). This additional depletion may be due to a slight (less than 3-fold) dust-gas fractionation, by settling of dust to the median plane of the solar nebula. Among other chondrite classes, ordinary chondrites show a similar depletion, but C2 chondrites do not. Possibly the undepleted meteorites formed in one of the convection zones of the nebula predicted by Cameron and Pine, whereas the depleted meteorites formed in a quiescent region.The condensation of chalocophile elements as a function of H₂S partial pressure is discussed, in an attempt to explain the drastic difference in Cd abundance between the two subclasses. It appears that the $\text{H}{}_2\text{S}\text{H}{}_2$ ratio is the key variable. C3O's seem to have condensed in a region where enough metallic Fe was present to buffer the H₂S pressure, while C3V's condensed in a more oxidized region, where H₂S was in excess. Accretion temperatures, for an assumed nebular pressure of 10^?5 atm, were between 415 and 430 K for C3O's and less than 440 K for C3V's.Two slightly volatile elements, Sb and Au, show variable depletion, presumably reflecting variable loss during chondrule formation. Indeed, their depletion correlates with the abundance of iron-poor olivine, a measure of the peak temperature and time during chondrule formation.     

Chemical classification of iron meteorites—VIII. Groups IC. IIE,IIIF and 97 other irons

Concentrations of Ni, Ga, Ge and Ir in 106 iron meteorites are reported. Three new groups are defined: IC, IIE and IIIF containing 10, 12 and 5 members, respectively, raising the number of independent groups to 12. Group IC is a cohenite-rich group distantly related to IA. Group IIE consists of those irons previously designated Weekeroo Station type and five others having similar compositions though diverse structures. The IIE irons are compositionally similar to the mesosiderites and pallasites, and the three groups probably formed at similar heliocentric distances. The mixing of the globular IIE silicates with the metal probably occurred during shock events. Group IIIF is a well-defined group of low-Ni and low-Ge irons. The compositions of these groups are summarized as follows:

     

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Group	Ni (%)	Ga (ppm)	Ge (ppm)	Ir (ppm)
IC	6.1–6.8	42–54	85–250	0.07–10
IIE	7.5–9.7	21–28	62–75	0.5–8
IIIF	6.8–7.8	6.3–7.2	0.7–1.1	1.3–7.9