Abstract Geopressure zones immediately subjacent to platform carbonate rocks can be modelled to serve as proximal sources of hydrothermal fluids for epigenetic Pb-Zn deposits in sedimentary basins. In some geopressure zones of the Gulf of Mexico, geothermal gradients can be as high as 10°C 100 m −1 . This arises because the water-saturated geopressured shale masses act as thermal insulators. Geopressure zones may have sufficient fluid pressure to rupture overlying strata, providing a vertical conduit for hot mineralized brine to migrate directly into carbonate host rocks.
Varves are sand-clay sedimentary couplets deposited annually in glacial lakes. Previous work has shown that the Fourier spectra of the individual bed thicknesses are intractable, or at least require significant filtering prior to analysis. This is, in part, because the data are not strictly periodic but are quasiperiodic. Therefore, a natural approach to their analysis is the use of techniques of non-linear dynamics. Varve sequences are too short (typically n[approximately]100) to allow time series analyses using the correlation function. The authors show results obtained using a predictor technique for many sets of unfiltered varve-data ranging in age form Quaternary to Precambrian. These are interpreted to show deterministic control of sedimentation, beyond the obvious annual cycle. In contrast, rhythmic sediments from very different environments (e.g. turbidites) show no deterministic signal. A series composed of scaled Quaternary varve data combined with scaled Quaternary tree-ring data also shows a strong deterministic signal. These data are consistent with the postulated existence of a climate attractor.
Abstract Silicate minerals grown from glasses, and rapidly cooled melts, often have non-compact branching or ‘spherulitic’ morphology. The branching patterns are observed in volcanic rocks, glasses, meteorites, slags and sometimes in shallow level intrusive rocks. Experiments, observations, theory and simulations all support the concept that the crystal morphology is the result of growth under diffusion limited conditions. We show that in a silicate melt under appropriate conditions the equations for heat transfer and chemical-diffusion reduce to the Laplace equation. This means that the temperature or chemical gradient is a steady state field. Interaction between this field and a random variable (Brownian motion of growth species) is modelled and yields complex branching objects. The growing cluster affects the field such that an in-filled structure cannot be formed. The branching structures of the model crystal are remarkably similar to those formed in nature, and to those produced in laboratory experiments, implying that the model captures the essence of the branching-growth process.
The Archean tholeiitic Kinojévis suite is characterized by an iron-enrichment trend and abundant Fe–Ti oxides in its evolved basalts, andesites, and rhyolites. The rare-earth-element (REE) patterns of the suite remain flat from the basalts through to the rhyolites, with the development of small, negative Eu anomalies. Quantitative modelling of the trace elements from little-altered samples is consistent with the mineralogy, suggesting that the suite was produced through fractional crystallization of olivine, pyroxene, plagioclase, and Fe–Ti oxides. The evolved rhyolites are interpreted as having developed by greater than 90% fractional crystallization in a high-level magma chamber.The calc-alkaline Blake River Group conformably overlies the Kinojévis rocks and is characterized by enrichment in alkalis and silica. The REE patterns are light rare-earth-element (LREE) enriched, and the felsic rocks have prominent negative Eu anomalies. Geochemical modelling shows that the suite could have developed either through fractional crystallization dominated by plagioclase and clinopyroxene or by assimilation of tonalite, coupled with fractional crystallization.
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