排序方式： 共有37条查询结果，搜索用时 15 毫秒
1.
Calcite dendrite crystals are important but poorly understood components of calcite travertine that forms around many hot springs. The Lýsuhóll hot-spring deposits, located in western Iceland, are formed primarily of siliceous sinters that were precipitated around numerous springs that are now inactive. Calcite travertine formed around the vent and on the discharge apron of one of the springs at the northern edge of the area. The travertine is formed largely of two types (I and II) of complex calcite dendrite crystals, up to 1 cm high, that grew through the gradual addition of trilete sub-crystals. The morphology of the dendrite crystals was controlled by flow direction and the competition for growth space with neighbouring crystals. Densely crowded dendrites with limited branching characterize the rimstone dams whereas widely spaced dendrites with open branching are found in the pools. Many dendrite bushes in the pools nucleated around plant stems. Growth of the dendrite crystals was seasonal and incremental. Calcite precipitation was driven by rapid CO2 degassing of CO2-rich spring waters during the spring and summer. During winter, when snow covered the ground and temperatures were low, opal-A precipitated on the exposed surfaces of the dendrites. Segmentation of dendrite branches by discontinuities coated with opal-A and overgrowth development around sub-crystals resulted from this seasonal growth cycle. The calcite dendrite crystals in the Lýsuhóll travertine differ in morphology from those at other hot springs, such as those at Lake Bogoria, Kenya, and Waikite in New Zealand. Comparison with the calcite dendrite crystals found at those sites shows that dendrite morphology is site-specific and probably controlled by carbonate saturation levels that, in turn, are controlled by the rate of CO2 degassing and location in the spring outflow system. 相似文献
2.
The Perseverance Ultramafic Complex, Western Australia: The Product of a Komatiite Lava River 总被引：3，自引：0，他引：3
The Perseverance ultramafic complex is a body of olivine-richkomatiitic rocks spatially associated with the Agnew nickeldeposit, in the Agnew-Wiluna greenstone belt of the ArchaeanYilgarn Block in Western Australia. The complex consists ofa central lenticular body, up to 700 m thick, of olivine adcumulates,flanked by laterally extensive sheet-like bodies of olivineorthocumulates and spinifextextured komatiite flows. Rocks progressivelyfurther away from the central lens have chemical compositionsreflecting higher original proportions of komatiite liquid tocumulus olivine. Parent liquids had MgO contents between 25and 32% MgO, approximately chondritic Al/Ti ratios and HREEpatterns, and moderate depletion in LREE. Olivines within the adcumulate lens show a progressive increasein forsterite content from Fo93 at the bottom to Fo94?5, atthe top. Calculated original olivine compositions in the flankingrocks are similar to those at the base of the central lens.Original olivine nickel contents show a symmetrical variationfrom maximum values of 3500 ppm at the top of the central lens,through minimum values of 1000 ppm at the base and margins ofthe central lens to intermediate values in the distal rocks.The complex as a whole shows evidence for nickel depletion relativeto other komatiite suites. These observations are explained in terms of prolonged eruptionand flow of komatiitic lava down a major flow channel or lavariver. Adcumulates crystallized on the floor and sides of thecentral channel, which was formed at an early stage by thermalerosion of floor rocks. Episodic overflow of the central channelproduced distal ‘flood plain’ rocks consisting ofolivine orthocumulates and layered flows. Lavas became moremagnesian and nickel-rich with time, giving rise to the observedspatial variation in primary olivine composition. Nickel depletionof the earliest lavas is attributed to pre-eruption segregationof large volumes of immiscible Fe-Ni-sulfide, which were concentratedto form the underlying Agnew nickel deposit. 相似文献
3.
4.
5.
Deinstitutionalisation of psychiatric patients since the 1980s has introduced a new population and a new land use to New Zealand's urban areas. This paper reviews the geography of deinstitutionalisation, and investigates whether the North American phenomenon of a service-dependent ghetto can be identified in Dunedin. 相似文献
6.
This paper reports on log transportation as a health-related issue in the Hokianga district of Northland at a time when forestry activity in the area is expanding and radical reforms to New Zealand's roading network have been proposed. Drawing on technical information and key informant interviews, the potential impacts of logging trucks on Hokianga roads are considered, the costs associated with forestry traffic are estimated, and the routes most likely to be affected are identified. Possible responses to the log transport problem are discussed, and it is concluded that rural communities such as Hokianga are vulnerable to both logging truck activity and changes in government policy. 相似文献
7.
8.
Abstract. .Series of experimental trawls repeated over the same area during both 12 and 24 h cycles coupled with an analysis of the stomach contents of the bib demonstrated quantitative and qualitative variations in the predator's diet. The population studied feeds continuously over 24 h, the coefficient of "fullness" is relatively constant, and freshly eaten prey can be observed both during the day and at night. The proportion of brown shrimp in the food increases after high tide, especially when this occurs at night, coinciding with the shrimp's period of maximum activity. However, the overall impact of the bib on the shrimp population depends more on the abundance of the former than on variations in their individual food intake. 相似文献
9.
THE WRITING'S ON THE WALL 总被引：1，自引：0，他引：1
Graffiti can be a marker of territory. Youthful grafffiti writers in Auckland have adopted ‘tagging’, a form of graffiti writing linked to the Americanisation of popular culture in New Zealand. Regularities in the presence of ‘tags’ are documented through field observation in suburban and central city locations and related to the daily activities of the taggers. Self-proclamation, not the marking out of turf appears to underlie the territorial dimensions of graffiti in Auckland's urban space. 相似文献
10.
Diagenetically enhanced bedding in argillaceous platform limestones: stratified cementation and selective compaction 总被引：2，自引：0，他引：2
ROBIN G. C. BATHURST 《Sedimentology》1987,34(5):749-778
A field and petrographic study has been made of 34 massive beds in argillaceous limestones of open marine platform facies in the U.K. Lithologies include grainstones, packstones, wackestones and lime mudstones. The rocks are of Silurian, Carboniferous and Jurassic ages. Additional information was obtained from other limestones in the U.K., the U.S.A., Canada and continental Europe. The beds are parts of sequences composed of couplets of strata, fissile limestones alternating with hard limestones. In the fissile limestones the effects of mechanical compaction and pressure-dissolution have been concentrated, whereas in the hard limestones they are minimal or absent. Bedding planes visible in outcrop are diagenetic in origin and lie in the middle parts of the fissile limestones where compaction has been most severe. The features produced by pressure-dissolution are dissolution seams and fitted fabric: there are no stylolites. The original carbonate sediments were bioturbated and any structures produced by flowing water were destroyed. The vertical distribution of the bedding planes bears no relation to primary depositional bedding planes which are rare or absent. It is inferred that the strata which were to become the hard limestones were selectively cemented before mechanical compaction had been completed. Thenceforth, mechanical compaction and then pressure-dissolution were concentrated in the less cemented strata: these became the fissile limestones. Pressure-dissolution acted late in the diagenetic history and provided only an insignificant part, if any, of the carbonate for cementation. It is concluded that the orientation of beds (couplets) is parallel to successive sea floors and that the sediments that eventually became single beds accumulated synchronously. Similar couplets in platform limestones of the Mississippian and Pennsylvanian of the U.S.A. extend over thousands of square kilometres. The signal that controlled the initial selective cementation must have been widespread and synchronous and also syndepositional in its timing but otherwise cannot be further defined on the basis of the data so far collected. The presumed order of events was (1) accumulation of carbonate sediment, terrigenous clay and organic matter, (2) hydrodynamic reworking and bioturbation. the latter finally overprinting the former, (3) selective cementation of the more carbonate-rich strata yielding couplets, each consisting of a relatively well-cemented stratum and a poorly cemented stratum, (4) mechanical compaction concentrated in the less cemented strata, (5) pressure-dissolution concentrated in the same strata. 相似文献