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Naomi Marks Peter Schiffman Robert A. Zierenberg Hjalti Franzson Gudmundur Ó. Fridleifsson 《Journal of Volcanology and Geothermal Research》2010,189(1-2):172-190
The Reykjanes geothermal system is a seawater-recharged hydrothermal system that appears to be analogous to seafloor hydrothermal systems in terms of host rock type and low water/rock alteration. The similarities make the Reykjanes system a useful proxy for seafloor vents. At some time during the Pleistocene, the system was dominated by meteoric water recharge, and fluid composition at Reykjanes has evolved through time as a result of changing proportions of meteoric water influx as well as differing pressure and temperature conditions. The purpose of this study is to characterize secondary mineralization, degree of metasomatic alteration, and bulk composition of cuttings from well RN-17 from the Reykjanes geothermal system. The basaltic host rock includes hyaloclastite, breccia, tuff, extrusive basalt, diabase, as well as a marine sedimentary sequence. The progressive hydrothermal alteration sequence observed with increasing depth results from reaction of geothermal fluids with the basaltic host rock. An assemblage of greenschist facies alteration minerals, including actinolite, prehnite, epidote and garnet, occurs at depths as shallow as 350 m; these minerals are commonly found in Icelandic geothermal systems at temperatures above 250 °C (Bird and Spieler, 2004). This requires hydrostatic pressures that exceed the present-day depth to boiling point curve, and therefore must record alteration at higher fluid pressures, perhaps as a result of Pleistocene glaciation. Major, minor, and trace element profiles of the cuttings indicate transitional MORB to OIB composition with limited metasomatic shifts in easily mobilized elements. Changes in MgO, K2O and loss on ignition indicate that metasomatism is strongly correlated with protolith properties. The textures of alteration minerals reveal alteration style to be strongly dependent on protolith as well. Hyaloclastites are intensely altered with calc-silicate alteration assemblages comprising calcic hydrothermal plagioclase, grandite garnet, prehnite, epidote, hydrothermal clinopyroxene, and titanite. In contrast, crystalline basalts and intrusive rocks display a range in alteration intensity from essentially unaltered to pervasive and nearly complete albitization of igneous feldspar and uralitization of clinopyroxene. Hydrothermal anorthite (An92–An98) occurs in veins in the most altered basalt cuttings and is significantly more calcic than igneous feldspar (An48–An79). Amphibole compositions change from actinolite to hornblende at depth. Hydrothermal clinopyroxene, which occurs in veins, has greater variation in Fe content and is systematically more calcic than igneous pyroxene and also lacks uralitic textures. Solid solutions of prehnite, epidote, and garnet indicate evolving equilibria with respect to aluminum and ferric iron. 相似文献
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Ó. Pueyo Anchuela J. F. Ipas-Lloréns H. Gil Garbí A. M. Casas-Sainz A. Pocoví Juan M. L. Garijo J. G. Fernández-Martos J. Fernández-Cascán 《Environmental Earth Sciences》2014,72(11):4291-4302
Assumptions of simple geometries for alluvial deposits (related to main rivers) can preclude determination of complexities that are revealed as fundamental controls in underground flow. Although subhorizontal contacts between alluvial and substratum materials are expected, previous sedimentary or erosional processes can result in irregular geometries, only accessible through detailed analysis. The studied case presents the 3D reconstruction of a Quaternary terrace in the proximities of Sabiñánigo (Southern Pyrenees, Northeastern Spain), lying on homogeneous Eocene marls. The studied area is located in an industrialized area supporting strongly pollutant chemical industry (chlorinated pesticides) for the past 50 years. The objective of the study was to constrain the groundwater pathway along the area to subsequently analyze leakage of chemicals to the water table. In order to reveal preferred flow paths, a detailed characterization of the internal structure of the alluvial deposits (0.5–13 m thick) and their relations with the Tertiary substratum was carried out by means of ground penetrating radar (GPR, 50 and 100 MHz antennas). The obtained models permit identifying several topographic highs in the alluvial/substratum contact, determining preferred pathways in water flow, favoring particular conditions associated with low levels of the water table. The presented results support the interest of application of GPR surveys in order to characterize the groundwater pathways in expected homogeneous areas and their importance in order to establish the contaminant surveillance network. 相似文献
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Gudmundur Pálmason 《Geophysical Journal International》1973,33(4):451-481
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In a global examination of mid-ocean ridge basalt (MORB) glasscompositions, we find that Na8–Fe8–depth variationsdo not support modeling of MORBs as aggregates of melt compositionsgenerated over a large range of temperature and pressure. However,the Na8–Fe8 variations are consistent with the compositionalsystematics of solidus melts in the plagioclase–spinellherzolite transition in the CaO–MgO–Al2O3–SiO2–Na2O–FeO(CMASNF) system. For natural compositions, the P–T rangefor melt extraction is estimated to be 1·2–1·5GPa and 1250–1280°C. This P–T range is a closematch with the maximum P–T conditions for explosive pressure-releasevaporization of carbonate-bearing melts. It is proposed thatfracturing of the lithosphere induces explosive formation andescape of CO2 vapor. This provides the vehicle for extractionof MORBs at a relatively uniform T and P. The upper portionof the CO2-bearing and slightly melted seismic low-velocityzone flows toward the ridge, rises at the ridge axis to lower-lithospheredepths, melts much more extensively during this rise, and releasesMORB melts to the surface driven by explosively escaping CO2vapor. The residue and overlying crust produced by this meltingthen migrate away from the ridge axis as new oceanic lithosphere.The entire process of oceanic lithosphere creation involvesonly the upper 140 km. When lithospheric stresses shift fractureformation to other localities, escape of CO2 ceases, the vehiclefor transporting melt to the surface disappears, and ridgesdie. Inverse correlations of Na8 vs Fe8 for MORB glasses areexplained by mantle heterogeneity, and positive variations superimposedon the inverse variations are consistent with progressive extractionof melts from short, ascending melting columns. The uniformlylow temperatures of MORB extraction are not consistent withthe existence of hot plumes on or close to ocean ridges. Inthis modeling, the southern Atlantic mantle from Bouvet to about26°N is relatively homogeneous, whereas the Atlantic mantlenorth of about 26°N shows significant long-range heterogeneity.The mantle between the Charlie Gibbs and Jan Mayen fracturezones is strongly enriched in FeO/MgO, perhaps by a trappedfragment of basaltic crust. Iceland is explained as the productof this enrichment, not a hot plume. The East Pacific Rise,Galapagos Ridge, Gorda Ridge, and Juan de Fuca Ridge samplemantle that is heterogeneous over short distances. The mantlebeneath the Red Sea is enriched in FeO/MgO relative to the mantlebeneath the northern Indian Ocean. 相似文献
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Simulation of subsurface heterogeneity is important for modeling subsurface flow and transport processes. Previous studies have indicated that subsurface property variations can often be characterized by fractional Brownian motion (fBm) or (truncated) fractional Levy motion (fLm). Because Levy-stable distributions have many novel and often unfamiliar properties, studies on generating fLm distributions are rare in the literature. In this study, we generalize a relatively simple and computationally efficient successive random additions (SRA) algorithm, originally developed for generating Gaussian fractals, to simulate fLm distributions. We also propose an additional important step in response to continued observations that the traditional SRA algorithm often generates fractal distributions having poor scaling and correlation properties. Finally, the generalized and modified SRA algorithm is validated through numerical tests. 相似文献
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A geochemical evaluation of the Szc-Halimba-Kisld area, Hungary, covering an area of more than 200 km2 is presented using different statistical and geostatistical methods. The study area is a representative example of allochtonous karst bauxite accumulation. The three groups of deposits studied here have been explored and mined since 1950. Several thousand boreholes have been drilled, and bauxite cores were analyzed for the five main chemical components. A total of 80,000 pleces of analytical data were processed, followed by a geological examination of borehole logs and of mining excavations.The quantitative geochemical evaluation of the data set led to both geochemical and practical results: The geochemical behavior of the allochtonous, clastic karst bauxite deposits differs essentially from that of the autochtonous and parautochtonous ones, as well as that of the lateritic bauxite deposits. The deposits of the study area can be split into several subsequent geochemical-sedimentological units, each representing an event of bauxite transport and accumulation. Clear regional patterns can be revealed in the composition of these units. The geostatistically measured chemical variability of the geochemical units is rather different, the lowest units showing the smallest variability. The interrelations of the main chemical components are weaker and more irregular in the studied deposits than in the autochtonous lateritic bauxite deposits. Additional local genetic features, such as transport routes, can be delineated by the methods applied. Within each deposit, local changes of chemical composition and of its variability can be determined more precisely. These results can be used in bauxite prospecting and exploration, because areas of high or low bauxite quality can be predicted. 相似文献
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Dean C Presnall Gudmundur H GudfinnssonMichael J Walter 《Geochimica et cosmochimica acta》2002,66(12):2073-2090
We propose a model for the generation of average MORBs based on phase relations in the CaO-MgO-Al2O3-SiO2-CO2 system at pressures from 3 to 7 GPa and in the CaO-MgO-Al2O3-SiO2-Na2O-FeO (CMASNF) system at pressures from ∼0.9 to 1.5 GPa. The MELT seismic tomography (Forsyth et al., 2000) across the East Pacific Rise shows the largest amount of melt centered at ∼30-km depth and lesser amounts at greater depths. An average mantle adiabat with a model-system potential temperature (Tp) of 1310°C is used that is consistent with this result. In the mantle, additional minor components would lower solidus temperatures ∼50°C, which would lower Tp of the adiabat for average MORBs to ∼1260°C. The model involves generation of carbonatitic melts and melts that are transitional between carbonatite and kimberlite at very small melt fractions (<0.2%) in the low-velocity zone at pressures of ∼2.6 to 7 GPa in the CMAS-CO2 system, roughly the pressure range of the PREM low-velocity zone. These small-volume, low-viscosity melts are mixed with much larger volumes of basaltic melt generated at the plagioclase-spinel lherzolite transition in the pressure range of ∼0.9 to 1.5 GPa.In this model, solidus phase relations in the pressure range of the plagioclase-spinel lherzolite transition strongly, but not totally, control the major-element characteristics of MORBs. Although the plagioclase-spinel lherzolite transition suppresses isentropic decompression melting in the CMAS system, this effect does not occur in the topologically different and petrologically more realistic CMASNF system. On the basis of the absence of plagioclase from most abyssal peridotites, which are the presumed residues of MORB generation, we calculate melt productivity during polybaric fractional melting in the plagioclase-spinel lherzolite transition interval at exhaustion of plagioclase in the residue. In the CMASN system, these calculations indicate that the total melt productivity is ∼24%, which is adequate to produce the oceanic crust. The residual mineral proportions from this calculation closely match those of average abyssal peridotites.Melts generated in the plagioclase-spinel lherzolite transition are compositionally distinct from all MORB glasses, but do not have a significant fractional crystallization trend controlled by olivine alone. They reach the composition field of erupted MORBs mainly by crystallization of both plagioclase and olivine, with initial crystallization of either one of these phases rapidly joined by the other. This is consistent with phenocryst assemblages and experimental studies of the most primitive MORBs, which do not show an olivine-controlled fractionation trend. The model is most robust for the eastern Pacific, where an adiabat with a Tp of ∼1260°C is supported by the MELT seismic data and where the global inverse correlation of (FeO)8 with (Na2O)8 is weak. Average MORBs worldwide also are well modeled. A heterogeneous mantle consisting of peridotite of varying degrees of major-element depletion combined with phase-equilibrium controls in the plagioclase-spinel lherzolite transition interval would produce the form of the global correlations at a constant Tp, which suggests a modest range of Tp along ridges. Phase-composition data for the CMASNF system are presently not adequate for quantitative calculation of (FeO)8-(Na2O)8-(CaO/Al2O3)8 systematics in terms of this model. The near absence of basalts in the central portion of the Gakkel Ridge suggests a lower bound for Tp along ridges of ∼1240°C, a potential temperature just low enough to miss the solidus for basalt production at ∼0.9 GPa. An upper bound for Tp is poorly constrained, but the complete absence of picritic glasses in Iceland and the global ridge system suggests an upper bound of ∼1400°C. In contrast to some previous models for MORB generation that emphasize large potential temperature variations in a relatively homogeneous peridotitic mantle, our model emphasizes modest potential temperature variations in a peridotitic mantle that shows varying degrees of heterogeneity. Calculations indicate that melt productivity changes from 0 to 24% for a change in Tp from 1240 to 1260°C, effectively producing a rapid increase to full crustal thickness or decrease to none as ridges appear and disappear. 相似文献
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Dr. Gudmundur E. Sigvaldason 《Contributions to Mineralogy and Petrology》1969,20(4):357-370
The chemistry of basalts from the Icelandic Rift Zone is discussed on the basis of 34 analysis, both new and previously published. It is concluded that a distinct chemical grouping is present between alkalibasalts and tholeiitic basalts.The tholeiitic basalts can furthermore be grouped into batches with slightly different primitive chemistry but identical differentiation trends.The chemical variation within a group of basalts from the same volcanic center can be related to a time scale in such a way, that the youngest (historic) lavas all show advanched differentiation, but the lavas known to be oldest have the most primitive chemistry.The possibility is suggested, that the volcanic activity along the Icelandic Rift Zone represents an independent cycle of chemical differentiation, distinct from the Tertiary volcanic activity along the Wyville Thompson Ridge.Attention is drawn to a fairly well defined grouping of basalts from different parts of the Midatlantic Ridge using the plot combined iron against magnesia. 相似文献
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Gudmundur E. Sigvaldason 《Contributions to Mineralogy and Petrology》1958,6(2):100-107
Zusammenfassung Es wird ein kleines, wahrscheinlich extrusives Liparitvorkommen (Quellkuppe) in der Nähe von Reykjavik beschrieben, das auf tertiärem Plateaubasalt aufliegt.Im Felde wurden zwei Gesteinsvarietäten beobachtet, eine helle und eine dunkle, wobei sich die helle als ein Umwandlungsprodukt, wahrscheinlich unter dem Einfluß hydrothermaler Lösungen der vulkanischen Nachphase, erwies. Die Umwandlung besteht hauptsächlich darin, daß eine ursprünglich vorhandene glasige Grundmasse auskristallisiert ist. Dabei wurde ein großer Teil des Eisens, Calciums und Magnesiums weggeführt.In den äußeren Teilen des Vorkommens wurden zwei Varietäten von Sphärolithen beobachtet, nämlich Albitsphärolithe und Albit-Quarz-Sphärolithe. 相似文献