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591.
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Robert P. Wintsch Anne F. O'Connell Barbara L. Ransom Mark J. Wiechmann 《Contributions to Mineralogy and Petrology》1981,77(3):207-213
Disseminated carbon has been extracted from 19 samples of arkosic rocks from the chlorite, biotite and garnet zones in the
Narragansett basin, and analysed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). XRD of these samples yields
only the (002) peak which may be as much as four times wider than the silicon internal standard peak and is always skewed
to low angles 2θ. All (002) peaks were digitized and the four moments calculated. The results suggest that width at half-height
(W1/2) provides a poorer estimate of peak shape than coefficients of skewness and kurtosis. The positive correlation of these coefficients
with grain size determined by SEM in the range of 0.1–100 μm suggest a direct relationship between peak shape and grain size.
No variable measured in this study correlates with metamorphic grade. However, W1/2, skewness and kurtosis of the (002) diffraction peak of the carbonaceous material in the biotite and garnet zones correlates
inversely with modal carbon. This suggests that the volatilization reaction: C+2H2=CH4 may be an important step in the mechanism of graphitization. Variable fugacities of H2 and CH4 and/or variable permeabilities may be responsible for the non-uniform development of graphite crystallinity in the greenschist
facies rocks in Rhode Island. 相似文献
595.
Janez K. Lapajne Barbara Sket Motnikar Blaz Zabukovec Polona Zupancic 《Journal of Seismology》1997,1(1):73-85
A progress report on the mapping effort for construction of a peak ground acceleration (PGA) map of Slovenia for 475-year return period for rock and firm soil is presented. The methodology is similar to that recently applied in Central and Eastern United States. It is based on historical seismicity spatially smoothed to different length scales. The procedure is described by Frankel (1995). He uses the characteristic earthquake recurrence relationship and in his firt version four different seismicity models. We also use four seismicity models. But instead of characteristic earthquake recurrence, we use the doubly truncated exponential magnitude-frequency relationship; no evidence of characteristic earthquakes in Slovenia has been found yet. Three of our models are similar to Frankel's first three models. Model 1 uses spatially smoothed activity rate based on magnitude 3.7 and above events since 1880. Model 2 deals with spatially smoothed activity rate based on magnitude 5.0 abd above events since 1690. Model 3 smoothes the observed activity over the entire region; it represents a uniform seismicity zone. Frankel skipped this model in his latest version (Frankel et al. 1996). In model 4, we introduce a new approach of calculating seismic activity rate taking into account released seismic energy. The ground motion attenuation model of Pugliese and Sabetta (1989) is used for all models. PGA maps for models 1, 2, 3 and 4 have been calculated, and a weighted mean map derived from them. A map of model 1 has been compared with the corresponding source zone map; the two maps do not differ significantly. A worst-case map derived from all four models has also been produced. 相似文献
596.
597.
Artificial substrate collectors were used to sample settled brachyuran megalopae in Terrebonne Bay, Louisiana, northern Gulf
of Mexico. Three taxa,Callinectes sapidus, Uca spp., andRhithropanopeus harrisii, were the most common settlers. Settlement occurred almost year-round, although settlement densities were highest in late
summer and early fall. In 1990, all three taxa had several simultaneous settlement peaks; in 1991, one simultaneous settlement
peak occurred. FewerC. sapidus andUca spp. settled in 1991 than in 1990, but settlement abundance ofR. harrissii was similar between years. Local weather events, such as heavy precipitation, may have influenced the lower abundances ofC. sapidus andUca spp. in 1991. The megalopae ofC. sapidus andUca spp., which re-invade the estuary from higher salinity offshore waters where larval development occurs, may have been more
affected by the lower 1991 estuarine salinity values than the larvae ofR. harrissii, which are retained within the estuary throughout larval development. Settlement ofC. sapidus andUca spp. was correlated with maximum tidal height, tidal amplitude, salinity, temperature, and lunar phase. In 1990, settlement
ofC. sapidus was correlated with lunar declination cycles; settlement peaks occurred during equatorial minimum amplitude tides. The settlement
ofR. harrisii was associated with tidal amplitude and lunar phase. Associations between environmental variables and settlement of megalopae
were not consistent between the two years sampled. The opposing or reinforcing effects of various environmental variables
on settlement, e.g., tidal height and rainfall, are superimposed on long-term hydrologic cycles so that dramatically altered
cycles of settlement occur among years. 相似文献
598.
599.
Barbara Louise Endemao Walker 《The Professional geographer》2001,53(2):160-177
As processors and marketers of fish, women fishtraders in the Fanti town of Cape Coast, Ghana have become powerful financers and owners of canoes, nets, and other fishing equipment. Since the 1960s, when motors were first introduced to Ghana's artisanal canoe fleet, two interrelated processes have occurred in Cape Coast. First, Ghana's fisheries have become increasingly exploited and—in the case of some species—overfished. Second, the social relations of production in the artisanal sector have shifted from being socially embedded to being more market‐based and impersonal. I argue that two recent Women in Development (WID) projects in particular have contributed to the breakdown of fishtraders’ traditional economic networks and livelihood strategies: (a) loan schemes that target women's associations, and (b) the 1985 Intestate Succession Law, which reconfigured inheritance rights. These WID projects, based on western notions of gender and the household, have created disharmony and mistrust among Cape Coast's fishtraders rather than promoting their “development.” The breakdown of fishtraders’ labor and marketing organizations has resulted in increasingly desperate strategies to get fish, increased degradation of Ghana's marine environment, and uncertainty for the future of the coastal economy. 相似文献
600.
B. Lafrance Barbara E. John James S. Scoates 《Contributions to Mineralogy and Petrology》1996,122(4):431-440
Plagioclase recrystallization microstructures and petrofabrics in the unmetamorphosed, 1.43 Ga Poe Mountain anorthosite,
Wyoming, are indicative of very high-temperature deformation and recrystallization during the emplacement of the anorthosite
body. The Poe Mountain anorthosite consists of a core of recrystallized, massive anorthosite transitional with a series of
layered anorthositic cumulates at the margin of the intrusion. Irregular grain boundaries and dissected grain microstructures
in the massive core and transitional anorthosites suggest that the anorthositic rocks recrystallized by “fast” grain boundary
migration and possibly subgrain rotation recrystallization, at very high temperatures (≈1050°C) during emplacement of the
intrusion in the mid-crust (3 kbar). The deformation and recrystallization of the Poe Mountain anorthosite was continuous
from subliquidus to subsolidus temperature conditions during the emplacement of the intrusion. Anorthosites with the lowest
modal percentages of ferromagnesian minerals and Fe-Ti oxides are always the most recrystallized. This suggests that melt
interstitial to the plagioclase-crystal framework was removed during deformation and recrystallization of the intrusion. Bulging
of plagioclase grain boundaries around Fe-Ti oxides together with deformed oikocrystic ferromagnesian minerals and plagioclase
chadacrysts indicate that the deformation and recrystallization of the intrusion continued after the crystallization of the
interstitial melt minerals.
Received: 28 February 1995/Accepted: 20 July 1995 相似文献