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951.
Oxygen isotope geothermometry of coexisting minerals from five mafic granulites does not reflect the peak temperature of metamorphism as determined by other methods. Exchange of 18O between phases during slow cooling has (a) lowered the δ 18O of clinopyroxene relative to orthopyroxene in the presence of hornblende, (b) raised the δ 18O of quartz in a manner which is partly dependent on quartz abundance, and (c) lowered the δ 18O of plagioclase in some rocks. 相似文献
952.
953.
Hydrostratigraphic modeling of a complex,glacial-drift aquifer system for importation into MODFLOW 总被引:4,自引:0,他引:4
Deposition from at least three episodes of glaciation left a complex glacial-drift aquifer system in central Illinois. The deepest and largest of these aquifers, the Sankoty-Mahomet Aquifer, occupies the lower part of a buried bedrock valley and supplies water to communities throughout central Illinois. Thin, discontinuous aquifers are present within glacial drift overlying the Sankoty-Mahomet Aquifer. This study was commissioned by local governments to identify possible areas where a regional water supply could be obtained from the aquifer with minimal adverse impacts on existing users. Geologic information from more than 2,200 existing water well logs was supplemented with new data from 28 test borings, water level measurements in 430 wells, and 35 km of surface geophysical profiles. A three-dimensional (3-D) hydrostratigraphic model was developed using a contouring software package, a geographic information system (GIS), and the 3-D geologic modeling package, EarthVision. The hydrostratigraphy of the glacial-drift sequence was depicted as seven uneven and discontinuous layers, which could be viewed from an infinite number of horizontal and vertical slices and as solid models of any layer. Several iterations were required before the 3-D model presented a reasonable depiction of the aquifer system. Layers from the resultant hydrostratigraphic model were imported into MODFLOW, where they were modified into continuous layers. This approach of developing a 3-D hydrostratigraphic model can be applied to other areas where complex aquifer systems are to be modeled and is also useful in helping lay audiences visualize aquifer systems. 相似文献
954.
The 1886 eruption of Tarawera, New Zealand, was unusual for a Plinian eruption because it involved entirely basaltic magma, originated in a 17-km-long fissure, and produced extremely overthickened proximal deposits with a complex geometry. This study focuses on an 8-km-long segment cutting across Mount Tarawera where over 50 point-source vents were active during the 5.5-h eruption. A detailed characterization of the proximal deposits is developed and used to interpret the range of styles and intensities of the vents, including changes with time. We identify the four vents that contributed most heavily to the Plinian fall and evaluate the extent to which current volcanic plume models are compatible with the depositional patterns at Tarawera. Three proximal units are mapped that have phreatomagmatic, magmatic, and phreatomagmatic characteristics, respectively. Within the magmatic proximal unit, beds of like character are grouped into packages and delineated on scaled cross sections. Package dispersal is quantified by measuring the linear thickness half-distance (t1/2) in the planes of the fissure walls. Most packages have localized dispersals (low t1/2), indicating that Strombolian-style activity dominated most vents. The more widely dispersed packages (high t1/2) reflect contributions from additional transport regimes that were more vigorous but still contributed considerable material to the proximal region. We conclude that the geometry of the observed proximal deposits requires three modes of fall transport: (1) fallout from the upper portions of the Plinian plumes produced principally by vents in four craters; (2) sedimentation from the margins of the lower portions of the Plinian plumes including the jets and possibly the lower convective regions; and (3) ejection by weak Strombolian-style explosions from vents that did not contribute significant volumes of particles to the high plume. We suggest that the curvature of the velocity profile across the jet region of each plume (1–4 km height) was important, and that the lower velocity at the margins allowed proximal deposition of a large volume of material with a wide grain-size range. 相似文献
955.
956.
957.
Mapping and analysing virtual outcrops 总被引:1,自引:0,他引:1
Immo Trinks Phillip Clegg Ken McCaffrey Richard Jones Richard Hobbs Bob Holdsworth Nick Holliman Jonny Imber Steve Waggott Robert Wilson 《Visual Geosciences》2005,10(1):13-19
Laser scanning is a very efficient way to generate realistic, high-resolution digital models of 3-D geological outcrops. This paper discusses the methodologies involved in the creation and analysis of virtual outcrops, based on laser scanner data. The visualisation of the laser scanner data as a photorealistic 3-D object is described. Geological features picked out on the virtual outcrop (e.g. fractures, faults or bedding planes) can be extrapolated outward, into space, and inward, into the subsurface, using tension surfaces.Electronic Supplementary Material Supplementary material is available in the online version of this article at Reviewed by: J.D. Clemens, D. Yuen 相似文献
958.
Y. H. Zhou J. L. Chen X. H. Liao C. R. Wilson 《Geophysical Journal International》2005,162(2):390-398
959.
960.
Alan J. Wilson David R. Cooke Benjamin J. Harper Cari L. Deyell 《Mineralium Deposita》2007,42(5):465-487
The alkalic porphyry gold–copper deposits of the Cadia district occur in the eastern Lachlan Fold Belt of New South Wales,
Australia. The district comprises four porphyry deposits (Ridgeway, Cadia Quarry, Cadia Hill, and Cadia East) and two iron–copper–gold
skarn deposits (Big Cadia and Little Cadia). Almost 1,000 tonnes of contained gold and more than four million tonnes of copper
have been discovered in these systems, making Cadia the world’s largest known alkalic porphyry district, in terms of contained
gold. Porphyry gold–copper ore at Cadia is associated with quartz monzonite intrusive complexes, and is hosted by central
stockwork and sheeted quartz–sulfide–(carbonate) vein systems. The Cadia porphyry deposits are characterized by cores of potassic
and/or calc–potassic alteration assemblages, and peripheral halos of propylitic alteration, with late-stage phyllic alteration
mostly restricted to fault zones. Hematite dusting is an important component of the propylitic alteration assemblage, and
has produced a distinctive reddening of feldspar minerals in the volcanic wall rocks around the mineralized centers. Sulfide
mineralization is strongly zoned at Ridgeway and Cadia East, with bornite-rich cores surrounded by chalcopyrite-rich halos
and peripheral zones of pyrite mineralization. The Cadia Hill and Cadia Quarry deposits have chalcopyrite-rich cores and pyrite-rich
halos, and Cadia Hill contains a high-level bornite-rich zone. Distinctive sulfur isotopic zonation patterns have been identified
at Ridgeway, Cadia Hill, and Cadia East. The deposit cores are characterized by low δ34Ssulfide values (−10 to −4‰), consistent with sulfide precipitation from an oxidized (sulfate-predominant) magmatic fluid at 450 to
400°C. Pyrite grains that occur in the propylitic alteration halos typically have δ34Ssulfide values near 0‰. There is a gradual increase in δ34Ssulfide values outwards from the deposit cores through the propylitic halos. Water–rock interaction during propylitic alteration
caused magmatic sulfate reduction and concomitant oxidation of ferrous iron-bearing minerals, resulting in enrichment of 34S in pyrite and also producing the distinctive reddened, hematite-rich alteration halos to the Cadia deposits. These results
show that sulfur isotope analyses have potential applications in the exploration of alkalic porphyry-style deposits, with
zones of depleted δ34Ssulfide values most prospective for high-grade mineralization. 相似文献