An occurrence of secondary sulfides from Leg 37 basalts     

N. D. MacRae  M. E. Fleet  A. J. Andrews 《Contributions to Mineralogy and Petrology》1977,63(3):309-317

Two types of secondary sulfides occur on the walls of a vesicle in slightly altered basalt recovered from site 332A of DSDP Leg 37. The first consists of euhedral crystals of pure pyrite, with occasional small partial overgrowths of fibrous chalcopyrite, embedded within smectite (altered glass) which lines the vesicle. The second type occurs as small composite sulfide spherules lightly attached to the surface of the smectite. The spherules are composed of pyrite crystallites arranged in approximate pyritohedral symmetry, with marcasite crystallites decorating the pyritohedron edges. Precession camera photographs and optical studies suggest the marcasite and pyrite grew simultaneously from a pyrite nucleus, with marcasite adopting the pyrite growth habit. These spherules differ sharply from the magmatic sulfide spherules previously described.  相似文献   

Holocene environmental changes in the alpine zone,northern San Juan Mountains,Colorado: Evidence from bog stratigraphy and palynology   总被引：1，自引：0，他引：1  

J.T. Andrews  P.E. Carrara  F.B. King  R. Stuckenrath 《Quaternary Research》1975,5(2):173-197

Cores from five high alpine basins in the northern San Juan Mountains show several fluctuations in lithology. Typically, peats are interbedded with coarser clastic sediments or else woody peats alternate with fibrous peat. Twenty ¹⁴C dates provide radiometric control. Sediment rates averaged about 2.5 cm/100 yr but varied at the different sites between 1.19 and 50 cm/100 yr. Rates were lower during the middle of the Holocene. Basal radiocarbon dates indicate that these high (ca. 3600 m a.s.l.) northeasterly facing cirques were icefree by 9000 BP. There is some evidence in the cores for a short climatic reversal sometime between 8000 and 7000 BP. A major change occurred in the high basins very close to 5000 BP and thereafter there are several intervals of increased clastic sedimentation which may be related to Neoglacial climatic fluctuations. Analysis of a 2.15 m core near Hurricane Basin indicates significant fluctuation of pollen and macrofossils occurred during the 9000 ± year record. The Picea/Pinus ratios are used to delimit changes in the apparent elevation of the site: the ratios indicate that a short drop of “treeline” occurred about 8000 BP and then remained near present level until about ≥1800 BP when the apparent elevation of the site rose. Macrofossils indicate that spruce was present in the Hurricane Basin (and others) at specific periods and confirms the general results of the Picea/Pinus ratios. The San Juan Mountains do not possess a glacial Neoglacial record but the stratigraphy of these high cirque basins can be used to define glacial stades (cf. Jardine, 1972). The interpreted climatic response record on vegetation and sediment flux has both similarities and differences from other records in the western mountains of North America.  相似文献   

CORRESPONDENCE     

H. J. Andrews.  Patrik  Mogensen P. O. Fagerholm 《The Photogrammetric Record》1955,1(5):76-77

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Divergent evolution in fluviokarst landscapes of central Kentucky     

Jonathan D. Phillips  Linda L. Martin  Viva G. Nordberg  William A. Andrews 《地球表面变化过程与地形》2004,29(7):799-819

Central Kentucky is characterized by a mixture of karst and ?uvial features, typically manifested as mosaic of karst‐rich/channel‐poor (KRCP) and channel‐rich/karst‐poor (CRKP) environments. At the regional scale the location and distribution of KRCP and CRKP areas are not always systematically related to structural, lithological, topographic, or other controls. This study examines the relationship of KRCP and CRKP zones along the Kentucky River gorge area, where rapid incision in the last 1·5 million years has lowered local base levels and modi?ed slopes on the edge of the inner bluegrass plateau. At the scale of detailed ?eld mapping on foot within a 4 km² area, the development of karst and ?uvial features is controlled by highly localized structural and topographic constraints, and can be related to slope changes associated with retreat of the Kentucky River gorge escarpment. A conceptual model of karst/?uvial transitions is presented, which suggests that minor, localized variations are suf?cient to trigger a karst–?uvial or ?uvial–karst switch when critical slope thresholds are crossed. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

The 1994 Sefidabeh (eastern Iran) earthquakes revisited: new evidence from satellite radar interferometry and carbonate dating about the growth of an active fold above a blind thrust fault   总被引：2，自引：0，他引：2  

B. Parsons  T. Wright  P. Rowe  J. Andrews  J. Jackson  R. Walker  M. Khatib  M. Talebian  E. Bergman  E. R. Engdahl 《Geophysical Journal International》2006,164(1):202-217

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An intercomparison of ESR and uranium series ages for quaternary speleothem calcites     

P.L. Smart  B.W. Smith  H. Chandra  J.N. Andrews  M.C.R. Symons 《Quaternary Science Reviews》1988,7(3-4)

Equivalent dose (ED) estimates for speleothem calcite derived by ESR measurements of the g = 2.0005 signal have a typical precision of ±7% (minimum ±2%), and depend on the signal strength, radiation sensitivity and the presence of interfering signals. Underestimation of ED will occur if the crushing signal at g = 2.0001 is not removed by acid etching, while overestimation may be caused if the short-lived radiation induced interfering signal at g = 2.0020 is not removed by annealing. Quoted errors for ESR ages are rarely better than ±10% and are typically ±15% because of uncertainties in ED, calibration of dosimeters and gamma source, and in the a-value. Serial analyses along the growth axis of single speleothems, and between speleothems interbedded in a sequence demonstrate excellent stratigraphic concordance. Intercomparison of ESR and uranium series ages for samples derived from 11 geographically diverse sites show good agreement with no indication of systematic error. Although validation is more difficult for samples beyond the limit of dating, the results appear encouraging, but recrystallisation and changes in the external radiation dose may cause problems.  相似文献   

Rn solution by the circulating fluids in a “hot dry rock” geothermal reservoir     

J.N. Andrews  N. Hussain  A.S. Batchelor  K. Kwakwa   《Applied Geochemistry》1986,1(6)

The mechanism of²²²Rn release into fracture fluids by direct alpha-recoil, lattice and grain boundary/micro-crack diffusion is discussed. Experimental measurements of²²²Rn release into surrounding air and water phases have been made for crystalline rock specimens with well defined surface areas. The²²²Rn flux from an infinite plane surface and hence the effective diffusion length of²²²Rn in the rock matrix has been estimated.The²²²Rn flux from plane crystalline rock surfaces has been used in conjunction with a simple hydrological model of the reservoir to calculate the²²²Rn content of the return fluids of a geothermal doublet circulation system. For given production rate and piezometric difference between the injection and production wells, the²²²Rn content of the return fluid is dependent upon the distribution of flow path lengths and fracture apertures in the reservoir. Matching of the calculated and experimental²²²Rn contents of the return fluids has been used to select appropriate parameters for the reservoir model and hence to estimate the extent of the heat-transfer surface. The model estimates the fracture width of the flow paths, total swept surface area and fracture volume within the reservoir.  相似文献   

The automated photometric determination of total fluorine in mineral exploration     

R. Fuge  M.J. Andrews 《Journal of Geochemical Exploration》1985,23(3):293-297

An analytical method previously applied to the determination of fluorine in rocks and minerals would seem to be ideal for exploration samples such as soils, stream sediments and vegetation. The determination which utilises the Zr-xylenol orange complex is performed on acidified leachates following sodium carbonate fusion. This digestion technique is rapid, simple and amenable to batch sample processing. The method, which is applicable to samples containing from about 10 ppm to over 3% fluorine, is essentially interference free.  相似文献   

Close interval sampling of interstitial silicate and porosity in marine sediments     

Daniel Andrews  Barry T. Hargrave 《Geochimica et cosmochimica acta》1984,48(4):711-722

A method was developed to measure porosity and dissolved interstitial silicate at millimeter intervals or less in a sediment core. In cores from Emerald Basin (Scotian Shelf), interstitial concentrations near the sediment surface did not drop rapidly to bottom-water concentrations as measured in bottle casts (28 μM) but remained as high as 166 μM in the upper 0.5 mm of sediment High rates of benthic silicate release were measured which could not be accounted for by interstitial concentration gradients or by ventilation of macro-invertebrate burrows. The silicate discontinuity observed between the sediments and water column suggests that a diffusive sublayer exists in a zone of viscous flow above the sediment surface. This is possible only if a surface reaction is primarily responsible for silicate release. By assuming a linear concentration gradient across this diffusive sublayer, the silicate release rates were used to estimate the thickness of the sublayer to be about 2 mm.  相似文献   

Reply to A. S. Dyke's discussion     

J.T. Andrews  W.W. Shilts  G.H. Miller 《Quaternary Research》1984,22(2):253-258

In our opinion the amino acid data are consistent with the till/nonglacial stratigraphy. We reject Dyke's proposal that the plotting of data in 0.04 increments is appropriate as an unwarranted interpretation that errors are cumulative. We also see no grounds for accepting his alternative interpretation that the groups of amino acid ratios reflect various transport (read temperature) histories of a single population of Bell Sea shells. It is our opinion that the relative sequence of marine incursions in Hudson Bay is reliable and we repeat that the evidence favors one or more deglacial events. We stress that the ages of the units between the Tyrrell Sea and Bell sea end membrers are interpolated and that the chronology of events is currently based on the assumption that the Bell Sea represents marine incursion at the onset of marine isotope stage 5.Dyke has raised a number of points which have concerned us since we started our joint research on the aminostratigraphy of the Hudson Bay Lowlands. The answer to many questions will come, not from the amino acid results per se, but from detailed litho-, and biostratigraphic logging of the thousands of kilometers exposed along the large rivers that drain into James Bay and southwestern Hudson Bay. This work is presently going on. Let us say in conclusion that analysis of a further 63 shells and shell fragments resulted in a virtually identical frequency distribution to that discussed in our paper. We are currently evaluating the stratigraphic integrity of these results. Field expeditions by the Geological Survey of Canada in 1982 and 1983 into the Hudson Bay Lowlands were specifically designed to log new sections and make additional shell collections. We hope to report on these new data in due course.  相似文献