The geology and petrology of Bouvetøya,south Atlantic Ocean

The Quaternary island Bouvetøya is situated near the triple junction on the South Atlantic Ridge. It is the top of a volcano of about 2000 km⁸, evolved beyond the stage of caldera collapse. Basic, intermediate and peralkaline silicic rocks of a transitional rock series are recorded. Major and trace element chemistry, model calculations and petrographic observations do not favour a closed system fractional crystallization from one parent. The rocks may have evolved from slightly different parents by fractional crystallization under varying P,T and P_H2O conditions, or from one parent modified by crustal contamination and/or volatile transfer.     

The major element composition of the upper mantle estimated from the composition of lherzolites

The compilation of analyses of continental and oceanic spinel Iherzolites show that these two types of Iherzolites have very similar compositions. Their composition range differ from that of African garnet Iherzolites, and the data suggest that the mantle beneath Africa has an anomalous composition. If the composition of the upper mantle may be estimated from that of Iherzolites, the compositions of spinel Iherzolite should form the basis for this estimate. It is suggested that the compositions of spinel Iherzolite represent both undepleted and depleted compositions, and a representative composition for the primitive mantle is proposed on this basis.     

Geology of the Norwegian Shelf between 62°N and 69°N

This paper reviews the exploration history, scientific theories and regional geology of the Norwegian shelf between 62° N and 69° N. The Tertiary section of this shelf can be subdivided into three units. The lower two units show depositional maxima near land while the uppermost section is thickest near the shelf edge. The Pre-Tertiary section is structured into several sub-basin and structural high trends.     

Structure and properties of fluorine-bearing aluminosilicate melts: the system Na2O-Al2O3-SiO2-F at 1 atm

The chemical interaction between fluorine and highly polymerized sodium aluminosilicate melts [Al/(Al+Si)= 0.125–0.250 on the join NaAlO₂-SiO₂] has been studied with Raman spectroscopy. Fluorine is dissolved to form F^– ions that are electrically neutralized with Na⁺ or Al³⁺. There is no evidence for association of fluorine with either Si⁴⁺ or Al³⁺ in four-fold coordination and no evidence of fluorine in six-fold coordination with Si⁴⁺ in these melt compositions. Upon solution of fluorine nonbridging oxygens are formed and are a part of structural units with nonbridging oxygen per tetrahedral cations (NBO/T) about 2 and 1. The proportions of these two depolymerized units in the melts increase systematically with increasing F/(F+O) at constant Al/(Al+Si) and with decreasing Al/(Al+Si) at constant F/(F+O). Depolymerization (increasing NBO/T) of silicate melts results from a fraction of aluminum and alkalies (in the present study; Na⁺) reacting to form fluoride complexes. In this process an equivalent amount of Na⁺ (orginally required for Al-³⁺charge-balance) or Al³⁺ (originally required Na⁺ to exist in tetrahedral coordination) become network-modifiers.The structural data have been used to develop a method for calculating the viscosity of fluorine-bearing sodium aluminosilicate melts at 1 atm. Where experimental viscosity data are available, the calculated and measured values are within 5% of each other.A method is also suggested by which the liquidus phase equilibria of fluorine-bearing aluminosilicate melts may be predicted. In accord with published experimental data it is suggested, for example, that — on the basis of the determined solubility mechanism of fluorine in aluminosilicate melts — with increasing fluorine content of feldspar-quartz systems, the liquidus boundaries between aluminosilicate minerals (e.g., feldspars) and quartz shift away from silica.     

Phase relations governing the derivation of alkaline basaltic magmas from primary magmas at high pressures

A comparison between the variation trend of alkaline basaltic magmas within the CaO-MgO-Al₂O₃-SiO₂ system and experimentally estimated phase relations for this system at high pressures, suggests an olivine reaction relationship, which may explain the transition from primary magmas in equilibrium with olivine to alkaline basaltic magmas in which olivine does not form at high pressures. This reaction relationship is considered to be due to a transition from positive to negative crystallization with respect to olivine along the four phase curve where olivine, diopside, pyrope garnet and liquid are initially in equilibrium. The bimineralic, eclogitic character of alkaline basaltic compositions at high pressures is interpreted as being due to the presence of a thermal minimum on the three phase surface, where dioside and pyrope garnet are in equilibrium with liquid.     

Water content of a granite magma deduced from the sequence of crystallization determined experimentally with water-undersaturated conditions

The sequence of crystallization in a biotite-granite from the Bohus batholith of Norway and Sweden, deduced from its texture, was magnetite, plagioclase, microcline, quartz, and finally biotite. Several sequences of crystallization were determined experimentally at 2 kb in the presence of varying only for H₂O contents below 1.2% by weight. The rock was fused to a homogeneous glass, and each experiment included samples of finely crushed rock and glass. The samples were reacted in Ag-Pd capsules with measured H₂O content in coldseal pressure vessels with NNO buffer. With excess H₂O (more than 6.5%) the crystallization interval extends from 865° C to 705° C. In the H₂O-deficient region, the solidus temperature remains unchanged as long as a trace of vapor is present, but the liquidus temperature increases as H₂O content decreases; with 0.8 % H₂O the liquidus temperature is 1125° C, the crystallization interval is 420° C, and a separate aqueous vapor phase is evolved only a few degrees above the solidus at 705° C. The biotite phase boundary increases slightly from 845° C with excess H₂O to 875° C with 1% H₂O, and it intersects the steep phase boundaries for quartz and feldspars; the sequence of crystallization changes at each intersection point. Similar diagrams at various pressures for related rock compositions involving muscovite, biotite and amphibole will provide grids useful in defining limits for the water content of granitic and dioritic magmas. Applications are considered for the Bohus batholith, other granitic rocks, and rhyolites. The Bohus magma could have been formed by crustal anatexis as a mobile assemblage of H₂O-undersaturated liquid and residual crystals with initial total H₂O content less than 1.2%, or it could have been derived by fractionation of a more basic parent with low H₂O content from mantle or subduction zone, but it could not have been derived from a primary andesite generated from mantle peridotite. We consider it unlikely that the H₂O content of large granitic magma bodies exceeds about 1.5% H₂O; these magmas are H₂O-undersaturated through most of their histories. Uprise and progressive crystallization of magma bodies produces H₂O-saturation around margins and in the upper regions of magma chambers. H₂O-saturated rhyolitic and dacitic magmas with phenocrysts can be tapped from the upper parts of the magma chambers.     

Apparent palaeomagnetic low-inclination excursion in a pre-consolidated continental shelf sediment

The presence of an almost exact 180° change in declination in a piston-core from the continental margin off the west coast of Norway is associated with two distributions of positive inclination of the remanent directions which could have been interpreted to represent an excursion of the geomagnetic field. The over-consolidated top section, defined by anomalously low-inclination directions, reveals a magnetic fabric typical of slurries or deposits affected by water currents, as opposed to an anomalous fabric in the lower section associated with magnetic directions coinciding with the present geomagnetic field. The sampling area reveals seismic features indicative of slumping, erosion and redeposition, which are also reflected by micropalaeontological evidence. It is concluded that the pre-consolidated top-section has been transported by slumping, the anomalously low-dipping directions resulting from processes acting during the initial consolidation.     

Interpretation of the magnetic field from a sphere in the wavenumber domain

A Fourier transformation of the magnetic field from a magnetized sphere allows a particularly simple interpretation of the parameters of the sphere. The depth to the centre and total magnetic moments of the sphere are related to the intercept and slope of the power spectrum. The horizontal centre and direction of magnetization are related to slope and intercept of the phase spectra in two perpendicular, horizontal directions. Examples with artificial data contaminated by various noise components are presented.