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71.
ERIC OLAUSSON BJÖRN SVENONIUS 《Boreas: An International Journal of Quaternary Research》1975,4(1):55-62
Contributions have been made to the theory of the relation between terrestrial magnetism and glaciations. The results are applied to the ice sheets of high-latitude regions. A review of Pleistocene palcomagnetic dating is given. 相似文献
72.
73.
The aluminous pyroxene, fassaite, occurs in two small tabularbodies within mafic plutonites of the Boulder Batholith nearits north-east margin twelve miles east of Helena, Montana.First described by Knopf & Lee (1957), the bodies are contact-metasomatizedlimestone septa, now magnesian-tactites, consisting chieflyof fassaite, spinel, garnet, vesuvianite, and clintonite. Lesscommon minerals include pargasite, diopside, wollastonite, sphene,perovskite, anorthite, forsterite, calcite and chlorite. Sometwenty-five microprobe analyses of the fassaite show it is variablein composition and largely consists of the components CaMgSi2O6(5383 per cent), CaAl2SiO6 (725 per cent), CaFeAlSiO6(828 per cent), and CaTiAl2O6 (07 per cent). Thestoichiometry generally requires that most of the iron is ferric,consistent with Mössbauer data taken on a typical sample.If fassaite analyses from these and other contact metamorphicrocks are plotted on a triangular diagram with Ca(Mg,Fe)Si2O6,CaAl2SiO6 and CaFeAlSiO6 as end-members, the distribution ofpoints offers no positive evidence for a solvus gap betweenfassaite and diopside as proposed by Ginzburg (1969). The mostaluminous fassaites occur with spinel-clintonite ± grossularand have 25 per cent of the Si replaced by Al, making them truepolymorphs of a garnet (i.e. Gr42And23Pp35). No unusual cationordering is detected in these fassaites by single-crystal X-rayphotographs or Mössbauer measurements. Smede's (1966) estimate of 34 km of stratigraphic coverfor the Boulder Batholith indicates pressures of approximately1 kb, in agreement with the occurrence of andalusite + K-feldsparin a hornfels at the Kokaruda Ranch complex. The partial assemblagesof grossular, epidote, perovskite, anorthite-wollastonite, anorthite-calcite,and fassaite-calcite require XCO2 = 0·12 ± 0·08and T = 570 ± 10 °C at these pressures. These pressuresand temperatures place this occurrence in the upper portionsof the hornblende-hornfels facies after Turner (1968), althoughthe low pressures and water-rich fluids permit assemblages (wollastonite,calcite-forsterite-diopside) that Turner lists as characteristicof the pyroxene-hornfels facies. 相似文献
74.
Historical geography was once a popular element of university curricula in New Zealand. It was also a conspicuous focus of research. Today however there is only one identifiable course in historical geography in New Zealand's university calendars – at Massey – and few writers have maintained an active research interest rooted in the sub‐discipline. This Comment suggests some reasons why now is a good time for New Zealand's geographers to reassess this state of affairs, and outlines five themes that might be pursued in the construction of more explicit historical geographies at the start of the third millennium. 相似文献
75.
Pyroxene-phyric basalts constitute a distinct petrologic groupin samples from lunar maria at both Apollo 12 and Apollo 15sites. Textures of pyroxene-phyric basalts from both sites aresimilar, but bulk compositions and mineralogy are somewhat different.Pyroxene-phyric basalts are characterized by large pyroxenephenocrysts with cores of magnesian pigeonite and mantles ofaugite grading to ferroaugite, usually set in a distinctly finergroundmass of iron-rich pyroxene, plagioclase, ilmenite, andother minor minerals. Olivine is scarce or absent modally andnormatively. Controversy has arisen over whether the porphyritictexture is a result of (1) a two-stage cooling history, e.g.phenocrysts formed at depth, and groundmass formed on extrusion;or (2) single stage, rapid cooling under supercooled conditions(cooling history here refers to cooling conditions imposed byexternal factors, and is not to be equated with crystallizationhistory). A study of six rocks belonging to this groupfrom Apollo 15 rake samples is reported here. A considerablerange of textures is present in these rocks, and they may beranked in order of decreasing late-stage cooling rate (15125,15666, 15682, 15118, 15684, 15116) on the basis of groundmasscrystal size. The same ranking is obtained from delta-beta (measuredin X-ray precession photos) of pigeonite and augite exsolvedfrom once homogeneous crystals, or of epitaxially overgrownaugite and pigeonite. The size of the phenocrysts in these rocks tends to be positivelycorrelated with the coarseness of the groundmass. Furthermorethe same correlation is evidently present in almost all otherApollo 12 and Apollo 15 pyroxene-phyric rocks. This constitutesa strong argument in favor of a single-stage cooling historyfor all pyroxene-phyric rocks, because the correlation wouldbe fortuitous for a two-stage cooling history. On the basisof this and many other arguments advanced previously and inthis paper, it is concluded that all the pyroxene-phyric rocksoriginated in a single-stage cooling process. A crystallization model for pyroxene-phyric rocks accounts forthe bimodal distribution of crystal size by two episodes ofsupercooling and nucleation during the continuous cooling process,the first of pyroxene, which nucleates homogeneously, the secondof plagioclase, which nucleates heterogeneously on the pyroxenephenocrysts. The more rapidly cooled rocks attained greaterdegrees of supercooling in both stages, hence greater nucleationrates and smaller crystals. 相似文献
76.
Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas) 总被引:1,自引:0,他引:1
CHRISTINA GLUNK CHRISTOPHE DUPRAZ OLIVIER BRAISSANT KIMBERLEY L. GALLAGHER ERIC P. VERRECCHIA PIETER T. VISSCHER 《Sedimentology》2011,58(3):720-736
Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments. 相似文献
77.
The Occurrence of Forsterite and Highly Oxidizing Conditions in Basaltic Lavas from Stromboli Volcano, Italy 总被引:1,自引:1,他引:0
CORTES JOAQUIN A.; WILSON MARJORIE; CONDLIFFE ERIC; FRANCALANCI LORELLA 《Journal of Petrology》2006,47(7):1345-1373
We report the occurrence of unusual, high-magnesium (Fo96) olivinephenocrysts in a basaltic lava and an ejected lithic block fromthe Upper Vancori period (13 ka) and the recent activity (20022003)of Stromboli volcano, Italy. The samples that contain this distinctivemineral chemistry are a shoshonitic basalt and a basaltic andesitewith anomalous bulk-rock chemical characteristics in which theiron is highly oxidized (68 wt % Fe2O3 and <1 wt %FeO). In other respects these samples are similar to the majorityof Stromboli basalts, characterized by the coexistence of olivine,clinopyroxene, plagioclase and FeTi oxides as phenocrysts,and clinopyroxene, plagioclase and FeTi oxides in thegroundmass. In the high-magnesium olivine samples, FeTioxides (pseudobrookite) typically occur as symplectitic intergrowthswith the olivine phenocrysts, indicating simultaneous growthof the two phases. We propose, as a paragenetic model, thatthe Fo96 olivine phenocrysts crystallized from a highly oxidizedbasaltic magma in which most of the iron was in the ferric state;hence, only magnesium was available to form olivine. The highlyoxidized state of the magma reflects sudden degassing of volatilephases associated with instantaneous, irreversible, transientdegassing of the magma chamber; this is postulated to occurduring periods of sudden decompression induced by fracturingof the volcanic edifice associated with paroxysmic activityand edifice collapse. KEY WORDS: Stromboli; Mg-rich olivine; oxygen fugacity; redox state of magmas; irreversible processes 相似文献
78.
79.
The imaginary wavefront of a common midpoint gather of seismic data is analysed in terms of elementary wavefronts. The wavefronts emitted by the sources are split into a sequence of elementary circular wavefronts. Each such wavefront is independently transmitted through the subsurface and received by a group of receivers associated with the sources. The theory is first demonstrated in the case of a one-layer medium and is then extended to the case of a multilayer medium. We show that pairs of imaginary stationary points F and F' occur in the first medium through which all the rays, corresponding to the group of sources and receivers associated with the elementary wavefront pass. The location of the points is determined by 2n equations with 2n unknowns; (2n - 1) of these equations correspond to n layers of the medium, and an additional equation is given by the geometry of the CMP gather. Applying Fermat's principle between the points F and F', we obtain an algorithm for CMP ray tracing. The wavefront sampling problem is considered but not theoretically solved. Results obtained in the general case (a multilayer medium with lateral velocity variation) are presented and compared with those obtained by other modelling techniques. 相似文献
80.