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LISA M. CAMPBELL NOELLA J. GRAY ZOE A. MELETIS JAMES G. ABBOTT JENNIFER J. SILVER 《Geographical review》2006,96(1):97-121
ABSTRACT. This article contributes to a recent and growing body of literature exploring the nature of fieldwork in human geography. Specifically, we critically examine the role of gatekeepers in providing access to “the field,” based on existing conceptualizations of gatekeepers in the literature and on our own experiences with gatekeepers. We argue that the concept of gatekeepers has been oversimplified, in that relationships between researchers and gatekeepers are often assumed to be unidirectional—with gatekeepers controlling or providing access by researchers—and predominantly static in form and time. Although we accept the necessity and advantages of working through gatekeepers, our experiences suggest that relationships with them are highly complex and evolve over time, with sometimes unexpected implications for research. In gathering and analyzing data, researchers become gatekeepers themselves, what we are calling “keymasters.” Reconceptualizing the gatekeeper‐researcher relationship will contribute to ongoing efforts to more fully understand field‐workers as undertaking a practice inherently political, personal, and linked to the production of knowledge. 相似文献
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Water in Albitic Glasses 总被引：2，自引：1，他引：2
Infrared spectroscopy has been calibrated to provide a preciseand accurate method for determining the concentrations of molecularwater and hydroxyl groups in hydrous albitic glasses. At totalwater contents less than 4 wt.%, most of the water is dissolvedas hydroxyl groups; at higher total water contents, molecularwater becomes the dominant species. For total water contentsabove 4 wt.%, the amount of water dissolved as hydroxyl groupsis nearly constant at about 2 wt.% and additional water is incorporatedas molecular water. These trends in the concentrations of theH-bearing species are similar to those observed in other silicateglasses using infrared and NMR spectroscopies. The ratio ofmolecular water to hydroxyl groups at a given total water contentis independent of the pressure and only weakly dependent onthe temperature of equilibration. Molecular water and hydroxyl group concentrations in glassesprovide constraints on the dissolution mechanisms of water insilicate liquids. Several mixing models involving homogeneousequilibria of the form H2O+O2– = 2OH– among meltspecies in albitic melts have been developed. These models canaccount for the measured species concentrations if the effectsof non-ideal behavior or mixing of polymerized units are included,or by allowing for several different anhydrous species. We used two thermodynamic models of hydrous albitic melts tocalculate phase equilibria. The first assumes that Henry's lawholds for molecular water in albitic liquids; i.e. that theactivity of molecular water in the melt is proportional to itsmole fraction as determined by infrared spectroscopy. The seconddescribes the speciation and thermodynamics of hydrous albiticmelts using the formalism of a strictly regular solution. Thesemodels can account reasonably well for the position of the vapor-saturatedsolidus of high albite and the pressure and temperature dependenceof the solubility of water in albitic melts. To the extent thatthese models are successful, our approach provides a directlink between measured species concentrations in hydrous albiticglasses and the macroscopic thermodyn amic properties of theNaAlSi3O8-H2O system. 相似文献
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REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs 总被引：10，自引：1，他引：10
Rare earth element (REE) patterns of plutonic rocks across theCretaceous Peninsular Ranges batholith vary systematically westto east, transverse to its long axis and structural trends andgenerally parallel to asymmetries in petrologic, geochronologicand isotopic properties. The batholith can be ivided into threedistinct parallel longitudinal regions, each defined by distinctREE pattern types. An abrupt transition occurs between rockswith slightly fractionated REE patterns in the western (coastal)region and rocks with middle to heavy REE fractionated and depletedpatterns in the central region. Further to the east a secondtransition to strongly light REE enriched rocks occurs. Theslopes of the REE patterns within each of these regions arelargely independent of rock type. The first REE transition isclosely coupled to regional discontinuities in other parameters:elimination of negative Eu anomalies, an increase in Sr content,and a marked restriction in petrologic diversity. This transitionoccurs over a range of initial 87Sr/86Sr ratios and 18O values,but approximately correlates to a major shift in the emplacementstyle of the batholith from a stationary arc to a rapidly eastward-migrating(cratonward) arc. The sense of the regionally consistent REE trends cannot beexplained by crystallization, assimilation, combined crystallization-assimilation,or mixing processes. The consequences of assimilation and high-leveldifferentiation are not observed generally, despite the sensitivityof the REE to these processes. Geochemical and petrologicalfeatures argue that the partial melting of mafic source rocksin which plagioclase-rich (gabbroic) residual assemblages abruptlygave way laterally and downward to garnet-bearing (eclogitic)residual assemblages produced all the changes associated withthe first REE transition. The change in residual assemblagesfrom gabbroic to eclogitic was superimposed on source regionsalready zoned in light REE abundances, 87Sr/86Sr and 18O. Temperatureand pressure constraints on the source regions place them ina subcrustal location. The calcic nature of the batholith andthe dominance of tonalite and low-K2O granodiorite in all itsregions argue that the source materials are broadly basalticin composition. Experimental studies are consistent with thegeneration of the abundant tonalitic magmas by the partial meltingof basalt under both low and high pressure conditions. Arc basaltssuch as those commonly erupted in modern island arcs and continentalmargins are inferred to have provided much of the source materialand the heat. Additional high-18O components are needed in themore easterly source regions. These materials must be distributedso as to contribute equally to the range of magmas that occurin a given local region, and must preserve the calcic natureof batholithic sources. Altered basalts of ancient oceanic crustand possibly their associated metasediments, previously incorporatedinto the lithosphere beneath the continental margin during earliercycles of subduction, most readily satisfy these constraints. The REE geochemistry of the central and eastern regions of thebatholith differs from that of oceanic island arcs in the presenceof strongly heavy REE depleted and fractionated magmas. A modelis proposed in which arc basalts accumulate beneath a crustallayer. Melting of accumulated material at low pressure producesmagmas of the western region. Where thickening of the basalticunderplate is sufficient to form eclogitic assemblages, eclogite-derivedmagmas of the central and eastern region are produced. The abrupttransition to eclogite-derived magmas that suggests a processdriven by a density instability is responsible for their origin. The Peninsular Ranges batholith appears to be representativeof a major differentiation process in which mantle-derived basaltis remelted, contributing its more sialic fractions to the continentalcrust and leaving its mafic to ultramafic residues in the mantle.This process preserves the sialic character of the continentalcrust and may play a significant role in its growth and evolution.The batholith and the processes that produced it may be a moreappropriate basis than immature oceanic island arcs on whichto construct models of continental growth and evolution. 相似文献
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