Mineralogy and Petrology of a Tactite near Helena, Montana     

SHEDLOCK  ROBERT J.; ESSENE  ERIC J. 《Journal of Petrology》1979,20(1):71-97

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(53–83 per cent), CaAl₂SiO₆ (7–25 per cent), CaFeAlSiO₆(8–28 per cent), and CaTiAl₂O₆ (0–7 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 3–4 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 X_CO2 = 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   

Landscape Change and Environmental Histories     

ERIC PAWSON  TOM BROOKING 《New Zealand geographer》2000,56(2):52-56

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   

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 (2002–2003)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 (6–8 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 Fe–Ti oxides as phenocrysts,and clinopyroxene, plagioclase and Fe–Ti oxides in thegroundmass. In the high-magnesium olivine samples, Fe–Tioxides (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   

Atmospheric deposition to a high-elevation forest at Whiteface Mountain, New York, USA   总被引：1，自引：0，他引：1  

ERIC K. MILLER  JEANNE A. PANEK  REW J. FRIEDLAND  JOHN KADLECEK  VOLKER A. MOHNEN 《Tellus. Series B, Chemical and physical meteorology》1993,45(3):209-227

  相似文献   

ANALYSIS OF THE COMMON MIDPOINT GATHER BY DECOMPOSITION INTO ELEMENTARY WAVEFRONTS1     

PIERRE THORE  ERIC DE BAZELAIRE 《Geophysical Prospecting》1991,39(4):453-471

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   

The impact of observed precipitation upon the transport of aerosols from South Asia     

ERIC M. WILCOX  V. RAMANATHAN 《Tellus. Series B, Chemical and physical meteorology》2004,56(5):435-450

  相似文献   

'Thus it was in former days…'     

ERIC ROBINSON 《Geology Today》1988,4(4):138-139

  相似文献   

The National Westminster Tower: a geologist's choice     

ERIC ROBINSON 《Geology Today》1993,9(6):227-229

  相似文献   

EXPLORING MARKET‐BASED DEVELOPMENT: MARKET INTERMEDIARIES AND FARMERS IN CALAKMUL,MEXICO*     

ERIC KEYS 《Geographical review》2005,95(1):24-46

ABSTRACT. Market intermediaries play important roles in the development of tropical‐forest frontiers but are often overlooked in the assessment of land‐change dynamics. Consistent with research beyond land‐change studies, intermediaries are found to be a pivotal element in land‐use and land‐cover change in southeastern Mexico. They have stimulated commercial chili cultivation in this development frontier, providing transportation and other services to smallholders who could otherwise not enter the chili market. This role comes at the cost of a near monopoly on chili marketing. The various roles played by these intermediaries, or coyotes, the means by which they operate, and the consequences for smallholders and land use are detailed for the Calakmul Municipality, Campeche, Mexico.  相似文献   

Commentary on the note by Vincent Courtillot and Paul R. Renne,On the ages of flood basalt events: [C. R. Geoscience 335 (2003) 113–140]     

Eric Buffetaut 《Comptes Rendus Geoscience》2003,335(9):773-774

  相似文献