Petrology and geochemistry of plagiogranite in the Canyon Mountain ophiolite,Oregon

Plagiogranites in the Canyon Mountain ophiolite, Oregon, include a wide range of rock types ranging from diorite to trondhjemite. The plagiogranites are mostly concentrated as an intrusive sill swarm at the top of a section of gabbroic cumulates. The plagiogranites are typically low in K₂O and high in Na₂O, and are enriched 10–20 times chondrites in REE, and overlap with abundances in basic rocks from Canyon Mountain. All samples of plagiogranite are relatively depleted in LREE, with more silicic samples characterized by a slightly lesser degree of LREE depletion. Total REE content is not consistently correlated with contents of major and other trace elements. Fractional crystallization of basaltic magma may give rise to plagiogranites; however this model applied to Canyon Mountain plagiogranites is discounted because of the significant volume of plagiogranites relative to basic rocks, and the complete overlap of REE abundances of the basic rocks and the plagiogranites. The latter is also a major reason for rejecting the hypothesis of silicate liquid immiscibility in the generation of the plagiogranites. Field observations coupled with major-element and trace element chemistry lend support to a model by which the plagiogranites were produced by partial melting of basic rocks under hydrous conditions. REE data for the plagiogranites were used in calculations to delimit source REE contents. Relevant parameters in the calculations were estimated from experimentally determined phase relations of basalt under hydrous conditions. The resulting calculated source patterns are similar to those of basic rocks in ophiolites and oceanic settings, and suggest boundary conditions for the model. Partial melting as suggested for the Canyon Mountain plagiogranites probably occurred at relatively shallow depths (i.e., total pressures less than 5 kb).     

Optimization of the freeze pipe arrangement and the necessary refrigeration plant capacity by a femcomputer program

The number and the arrangement of freeze pipes and the energy needed to freeze a certain amount of soil are important factors for the economic success of a freeze project. A thermal design in which these factors are considered is based on the solution of a nonlinear unsteady heat conduction equation including phase transition. The equation is solved by means of a finite-element-method (FEM), considering boundary conditions related to artificial ground freezing.

In this paper the basic mathematical techniques to deal with the transient heat conduction problem, with temperature-dependent soil properties, and the release of latent heat are described. The significance of the convective heat transfer coefficient and the temperature distribution in the coolant running through the freeze pipes are shown and their dependencies to other factors as refrigeration plant capacity or type of flow in the pipes are considered. Finally an example is presented.     

Lateral coherence of longitudinal wind components in strong winds

A combination of lateral coherence measurements of wind speed at five locations suggests that the decay constant is a monotonically increasing function of the ratio of separation to height, under neutral conditions.     

Growth of Mytilus edulis in net bags transferred to different localities in a eutrophicated Danish fjord

The growth rate has been measured in Mytilus edulis transferred in net bags to seven localities in the brackish Danish fjord, Limfjorden, in which certain areas are heavily eutrophicated. The increase in shell length, shell weight and flesh body weight was measured after growth periods of 14–18 days. The increase in, e.g., flesh dry weight ranged from twice the start weight to a four-fold increase of the start weight. The net growth efficiencies were estimated to be between 54 and 73%. Algal concentration was in no case the limiting growth factor, but low growth rates were observed in areas with seasonal oxygen depletion and release of toxic H₂S from the sediments. It is suggested that measurements of actual growth in M. edulis can be a useful technique in the study of biological effects in marine recipients.     

Hydrochemistry of the Lake Magadi basin,Kenya

New and more complete compositional data are presented for a large number of water samples from the Lake Magadi area, Kenya. These water samples range from dilute inflow (<0.1 g/kg dissolved solids) to very concentrated brines (>300 g/kg dissolved solids). Five distinct hydrologic stages can be recognized in the evolution of the water compositions: dilute streamflow, dilute ground water, saline ground water (or hot spring reservoir), saturated brines, and residual brines. Based on the assumption that chloride is conserved in the waters during evaporative concentration, these stages are related to each other by the concentration factors of about 1:28:870:7600:16,800.Dilute streamflow is represented by perennial streams entering the Rift Valley from the west. All but one (Ewaso Ngiro) of these streams disappear in the alluvium and do not reach the valley floor. Dilute ground water was collected from shallow pits and wells dug into lake sediments and alluvial channels. Saline ground water is roughly equivalent to the hot springs reservoir postulated by Eugster (1970) and is represented by the hottest of the major springs. Saturated brines represent surficial lake brines just at the point of saturation with respect to trona (Na₂CO₃.NaHCO₃.2H₂O), while residual brines are essentially interstitial to the evaporite deposit and have been subjected to a complex history of precipitation and re-solution.The new data confirm the basic hydrologic model presented by Eugster (1970) which has now been refined, particularly with respect to the early stages of evaporative concentration. Budget calculations show that only bromide is conserved as completely as chloride. Sodium follows chloride closely until trona precipitation, whereas silica and sulfate are largely lost during the very first concentration' step (dilute streamflow-dilute ground water). A large fraction of potassium and all calcium plus magnesium are removed during the first two concentration steps (dilute streamflow-dilute ground water-saline ground water). Carbonate and bicarbonate are the dominant anions, and mechanisms by which they are extracted from the solution include precipitation of alkali and alkaline-earth carbonates, and degassing, as well as precipitation and re-solution of efflorescent crusts. Much sulfate is apparently lost from solution by sorption as well as subsurface reduction.Seasonal runoff, principally from the valley floor north of Lake Magadi, is considered to be the principal recharge to the Magadi ground water system. Evaporative concentration is the overall process responsible for the chemical evolution of the brines. This includes not only simple evaporation, but also mineral precipitation as films and cements in the unsaturated zone, re-solution, and reprecipitation of efflorescent crusts, with consequent recycling of salts. In fact, a large fraction of the solutes are acquired through dissolution of efflorescent crusts.Data were obtained for borehole brines from as deep as 297 m. They show the existence of two distinct brine bodies below the present lake, one shallow, coexistent with bedded salts, and highly concentrated (260 g/kg average dissolved solids), and the other deeper in lacustrine sediments or fractured lavas, and only half as concentrated.