Modelling quartz cementation of quartzose sandstones

Cementation of quartzose sandstones is modelled assuming that the main source of silica is quartz dissolved at stylolites. The cementation process is shown to operate in one of two different regimes depending on the Damköhler number for diffusion. The regime, where diffusion of silica from the stylolites is a faster process than precipitation, is characterized by a nearly constant supersaturation between the stylolites. This regime, which spans the depth interval of quartz cementation for close stylolites, allows for approximate analytical expressions for the porosity evolution as a function of time and temperature. An expression is derived for the temperature where half the initial porosity is lost during constant burial along a constant thermal gradient. This expression is used to study the sensitivity of all parameters which enter the cementation process. The cementation process is shown to be particularly sensitive to the activation energy for quartz dissolution. The expression for the porosity decrease under constant burial is generalized to any piecewise linear burial and temperature history. The influence of the burial histories on the cementation process is then studied.     

Analysis of digital elevation data for palaeosurfaces in south-western Sweden

A 3873-km² large rectangular area of the Precambrian basement in south-western Sweden has been investigated regarding gross morphology of palaeosurfaces. GIS and digital elevation data were used to construct maps of relative and absolute relief and E–W trending height profiles. Three different palaeosurfaces were recognised and delimited; the very even sub-Cambrian peneplain in the east at an intermediate altitudinal position, an uplifted and broken part of the sub-Cambrian peneplain in the centre at the highest present elevation, and in the western coastal areas a joint-aligned valley landscape with high relative relief, probably an exhumed Mesozoic etch-surface. Phanerozoic faulting has dissected the different palaeosurfaces into tectonic blocks, which are shown from trend surface analysis to be tilted in different directions and to different degrees, characteristic and decisive for the morphological development.     

Modelling thermal transients from magmatic underplating—an example from the Vøring margin (NE Atlantic)

The thermal impact of several kilometre-thick magmatic underplating in the lower continental crust is studied with analytical and numerical methods. Simple analytical solutions are derived for the thermal transient in the case of an infinite depth below the underplate and also for the case of a finite depth (down to the asthenosphere). It is shown that these solutions lead to simple approximations for when the transient surface heat flow is at its maximum, what the maximum is, and for how long the transient lasts. Even though these solutions assume that the underplate is emplaced instantaneously, they are useful in the interpretation of underplating over finite time spans. A numerical scheme is suggested for the modelling of underplating that handles both short time intervals as well as long intervals. The scheme treats magmatic underplating in a mass and energy conservative manner, and it is compared against the analytical solutions. Finally, the analytical and numerical results for thermal transients are applied to a transect from the Vøring margin (NE Atlantic), with respect to various degrees of early Cenozoic magmatic intrusion. It appears that more than half of the lower crustal body (LCB) in the Vøring margin must be magmatic underplating for the vitrinite reflectance to be substantially higher than for the non-magmatic case, where the LCB is assumed to comprise Caledonian crust.     

TWO-PHASE OIL MIGRATION IN COMPACTING SEDIMENTARY BASINS MODELLED BY THE FINITE ELEMENT METHOD

The upstream-weighted finite element method with lumped mass matrix is applied to the modelling of oil migration in compacting sedimentary basins. An implicit formulation is made in Lagrangian co-ordinates of a pressure, saturation and a temperature equation, which is based on immiscible two-phase flow of oil and water. The formulation accounts for the compaction of the sediments, the generation of oil from solid organic material (kerogen), the eventual pore space generated by kerogen breakdown, and the density variations of the fluids which may set up thermal convection. The model is validated by comparison with results from a one-dimensional (1D) fractional flow-based migration model. A 2D case example showing oil expulsion from source rocks, and the filling of a trap is presented. The mass balance of the model is easily checked because all oil in the basin originates from breakdown of kerogen. Compared with other alternatives, the simple upstream-weighted finite element method is suggested as a possible first choice for a numerical method for the modelling of oil migration in compacting sedimentary basins. It easily deals with the complex geometry of a basin, it yields reasonably good results, is simple to implement, and the same implementation applies to all spatial dimensions. © 1997 by John Wiley & Sons, Ltd.     

Fungal colonies in open fractures of subseafloor basalt

The deep subseafloor crust is one of the few great frontiers of unknown biology on Earth and, still today, the notion of the deep biosphere is commonly based on the fossil record. Interpretation of palaeobiological information is thus central in the exploration of this hidden biosphere and, for each new discovery, criteria used to establish biogenicity are challenged and need careful consideration. In this paper networks of fossilized filamentous structures are for the first time described in open fractures of subseafloor basalts collected at the Emperor Seamounts, Pacific Ocean. These structures have been investigated with optical microscopy, environmental scanning electron microscope, energy dispersive spectrometer, X-ray powder diffraction as well as synchrotron-radiation X-ray tomographic microscopy, and interpreted as fossilized fungal mycelia. Morphological features such as hyphae, yeast-like growth and sclerotia were observed. The fossilized fungi are mineralized by montmorillonite, a process that probably began while the fungi were alive. It seems plausible that the fungi produced mucilaginous polysaccharides and/or extracellular polymeric substances that attracted minerals or clay particles, resulting in complete fossilization by montmorillonite. The findings are in agreement with previous observations of fossilized fungi in subseafloor basalts and establish fungi as regular inhabitants of such settings. They further show that fossilized microorganisms are not restricted to pore spaces filled by secondary mineralizations but can be found in open pore spaces as well. This challenges standard protocols for establishing biogenicity and calls for extra care in data interpretation.     

Chemistry, reaction mechanisms and micro-structures during retrograde metamorphism of gedrite-biotite-plagioclase bearing rocks from Bergslagen, south-central Sweden

The Proterozoic rhyolitic volcanics constituting the foot-wall rocks in the Stollberg ore-field, Bergslagen, south-central Sweden, locally contain gedrite altered to chlorite and serpentine, biotite altered to chlorite and plagioclase altered to epidote.

The intergrowths between the host gedrite and the chlorite/serpentine inclusions are oriented with the a^* of gedrite parallel to the c^* of serpentine and chlorite. The biotite has been altered to chlorite by brucitization of both the K-interlayer and talc-like layer. In both cases the net change in volume during chloritization is small.

The assumption that Al is conserved during alteration of gedrite and biotite agrees very well with the micro-structures and orientation relations observed by transmission electron microscopy. Normalizing the chlorite to 1.00 mole, the overall chemical change that took place during the retrograde metamorphism of the Stollberg rocks can be written as: 0.84Ged+0.14Bio+0.65Mg+4.76H₂O+0.42H=1.00Chl+0.57Alb+0.72Fe+0.01Na+0.12K+0.01Ti+0.05Mn+0.95H₄SiO₄ The reaction results in ca 9% increase in volume for the solid phases. Thus, a slightly acidic Mg-rich fluid started the reaction and, upon leaving the system, the metasomatic fluid was enriched in Na, Fe, K, and Si.     

Experimental estimation of velocities and anisotropy of a series of Swedish crystalline rocks and ores

To provide a guide for future deep (<1.5 km) seismic mineral exploration and to better understand the nature of reflections imaged by surface reflection seismic data in two mining camps and a carbonatite complex of Sweden, more than 50 rock and ore samples were collected and measured for their seismic velocities. The samples are geographically from the northern and central parts of Sweden, ranging from metallic ore deposits, meta‐volcanic and meta‐intrusive rocks to deformed and metamorphosed rocks. First, ultrasonic measurements of P‐ and S‐wave velocities at both atmospheric and elevated pressures, using 0.5 MHz P‐ and S‐wave transducers were conducted. The ultrasonic measurements suggest that most of the measured velocities show positive correlation with the density of the samples with an exception of a massive sulphide ore sample that shows significant low P‐ and S‐wave velocities. The low P‐ and S‐wave velocities are attributed to the mineral texture of the sample and partly lower pyrite content in comparison with a similar type sample obtained from Norway, which shows significantly higher P‐ and S‐wave velocities. Later, an iron ore sample from the central part of Sweden was measured using a low‐frequency (0.1–50 Hz) apparatus to provide comparison with the ultrasonic velocity measurements. The low‐frequency measurements indicate that the iron ore sample has minimal dispersion and attenuation. The iron ore sample shows the highest acoustic impedance among our samples suggesting that these deposits are favourable targets for seismic methods. This is further demonstrated by a real seismic section acquired over an iron ore mine in the central part of Sweden. Finally, a laser‐interferometer device was used to analyse elastic anisotropy of five rock samples taken from a major deformation zone in order to provide insights into the nature of reflections observed from the deformation zone. Up to 10% velocity‐anisotropy is estimated and demonstrated to be present for the samples taken from the deformation zone using the laser‐interferometery measurements. However, the origin of the reflections from the major deformation zone is attributed to a combination of anisotropy and amphibolite lenses within the deformation zone.     

Alkaline fluid circulation in ultramafic rocks and formation of nucleotide constituents: a hypothesis

Seawater is constantly circulating through oceanic basement as a low-temperature hydrothermal fluid (<150°C). In cases when ultramafic rocks are exposed to the fluids, for instance during the initial phase of subduction, ferromagnesian minerals are altered in contact with the water, leading to high pH and formation of secondary magnesium hydroxide, among other – brucite, that may scavenge borate and phosphate from seawater. The high pH may promote abiotic formation of pentoses, particularly ribose. Pentoses are stabilized by borate, since cyclic pentoses form a less reactive complex with borate. Analyses have shown that borate occupies the 2' and 3' positions of ribose, thus leaving the 5' position available for reactions like phosphorylation. The purine coding elements (adenine, in particular) of RNA may be formed in the same general hydrothermal environments of the seafloor.