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81.
Identifying Accessory Mineral Saturation during Differentiation in Granitoid Magmas: an Integrated Approach 总被引:19,自引:4,他引:15
HOSKIN PAUL W. O.; KINNY PETER D.; WYBORN DOONE; CHAPPELL BRUCE W. 《Journal of Petrology》2000,41(9):1365-1396
Numerical reconstructions of processes that may have operatedduring igneous petrogenesis often model the behaviour of importanttrace elements. The geochemistry of these trace elements maybe controlled by accessory mineral saturation and fractionation.Determination of the saturation point of accessory mineralsin granitoid rocks is ambiguous because assumptions about crystalmorphology and melt compositions do not always hold. An integratedapproach to identifying accessory mineral saturation involvingpetrography, whole-rock geochemical trends, saturation calculationsand mineral chemistry changes is demonstrated here for a compositionallyzoned pluton. Within and between whole-rock samples of the BoggyPlain zoned pluton, eastern Australia, the rare earth element(REE)-enriched accessory minerals zircon, apatite and titaniteexhibit compositional variations that are related to saturationin the bulk magma, localized saturation in intercumulus meltpools and fractionation of other mineral phases. Apatite isidentified as having been an early crystallizing phase overnearly the whole duration of magma cooling, with zircon (andallanite) only saturating in more felsic zones. Titanite andmonazite did not saturate in the bulk magma at any stage ofdifferentiation. Although some trace elements (P, Ca, Sc, Nb,Hf, Ta) in zircon exhibit compositional variation progressingfrom mafic to more felsic whole-rock samples, normalized REEpatterns and abundances (except Ce) do not vary with progressivedifferentiation. This is interpreted to be a result of limitationsto both simple xenotime and complex xenotime-typecoupled substitutions. Our data indicate that zircon REE characteristicsare not as useful as those of other REE-rich accessory mineralsas a petrogenetic indicator. KEY WORDS: saturation; zircon; apatite; titanite; magma differentiation; trace elements; REE patterns 相似文献
82.
Community-based economic self-reliance has become critical in many African countries because of the severity of prevailing soci-economic and environmental constraints. Concerns over food security have encouraged a re-evaluation of the importance of indigenous technical knowledge. This study examines a rural area in Zimbabwe where members of the local community have established a successful beekeeping association to improve their livelihoods. The history of the initiative, its broader impact on the community and the role played by external facilitation feature in the study. 相似文献
83.
The increase in the number of mountain bikers world‐wide has raised a number of issues associated with impacts and appropriate management strategies. Studies in the USA, UK and New Zealand have indicated a growing concern with environmental impacts, demonstrated attitudes of other recreationalists to mountain bikers and identified a number of management issues. This article presents the findings from field research conducted during the southern summer of 1997/8. The research was carried out in association with the Palmerston North City Council (PNCC) and the Manawatu Mountain Bike Club. The aims of the research were to investigate the demographics of mountain bikers and related management issues. Key findings of the field research were that: access to mountain biking opportunities was a problem; information provision was poor; there was conflict between user groups; there was a lack of agreement on whether there should be dedicated or multi‐use tracks. The article discusses how the research findings have been used to assist in the creation of an outdoor recreation and tourism strategy for Palmerston North. A model for managing recreation /tourism activities is also suggested. 相似文献
84.
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86.
PETER MARIENFELD 《Boreas: An International Journal of Quaternary Research》1992,21(2):169-186
The fjord system of Scoresby Sund on the east coast of Greenland has been the subject of two research cruises by RV Polarstern in 1988 and 1990. Most of the year, the fjord is covered by sea-ice. Sediment input takes place mostly via drifting icebergs during the short summer period. Depending on the distance to glaciers, surface sediments carry varying proportions of coarse ice-rafted debris (IRD). The degree of sediment reworking by scouring icebergs is controlled by the depth of the fjord, with the most intense reworking in areas shallower than about 450 m depth. Both IRD contribution and intensity of sediment scouring clearly control the distribution pattern of benthic organisms. 相似文献
87.
PETER EMIL KALAND KNUT KRZYWINSKI BJØRG STABELL 《Boreas: An International Journal of Quaternary Research》1984,13(2):243-258
In three different areas in western Norway, large errors are obtained in the radiocarbon dates from lacustrine sediments close to marine/lacustrine sediment boundaries. Differences occur between radiocar-bon and pollenanalytic dates and between dates at isolation/ingression contacts for lakes of the same altitude above the present sea level. Younger radiocarbon dates are also obtained below older ones in undisturbed sediments. When divergent dates occur, the radiocarbon dates always seem to be the youngest. Large differences are also found between NaOH soluble and insoluble fractions of the same sediment samples. Insoluble fractions generally yield younger dates than the soluble. Differences are not found, however, for dates younger than about 8,000 B.P. The dating errors are connected to periods with more oligotrophic conditions with isoetides. Their roots penetrate older sediments. Due to contamination of the organic part of the sediment from partly decomposed roots, some radiocarbon dates will be too young. The isoetide vegetation and the dating errors disappear when the lakes become dystrophic. 相似文献
88.
PETER JOHANSSON 《Boreas: An International Journal of Quaternary Research》1988,17(4):541-552
Traces of ice-dammed lakes are found along the southern part of the Saariselkä mountain range. Various shore marks, outlet channels and fine-grained sediments are indications of open water. The shallow basins were more or less filled by dead ice. At the beginning of deglaciation the meltwaters discharged over the mountain ridge to the north. As the ice margin receded toward the southwest new outlets were opened and the meltwaters discharged to the east and southeast. The ice-dammed lakes existed and the deglaciation took place during the period 9,500-9,300 B.P. The annual rate of retreat of the ice margin averaged 130–170 m per year. 相似文献
89.
Abstract Reconstructions of the relative positions of the Indian, African, and Antarctic plates and their uncertainties are given for the times of selected magnetic anomalies that could be identified on adjacent pairs of these plates. Among the most certain reconstructions are those for the Antarctic and African plates, which can be determined directly from recently published magnetic anomalies from both sides of the Southwest Indian Ridge. As Patriat and his colleagues reported, there was an important change in direction and a decrease in rate of separation between Africa and Antarctica between the times of anomalies 33 and 20. India moved rapidly away from both Africa and Antarctica in the Late Cretaceous and early Tertiary periods, but slowed markedly near the time of anomaly 20 (≅ 45 Myr). The positions of the Indian plate with respect to the others are poorly constrained between the times of anomaly 5 (≅ 10 Myr) and anomaly 23 (≅ 54 Myr), but using the reconstructions of the African and Antarctic plates, the uncertainties can be reduced. Despite the relatively large uncertainties, the positions of anomalies 5, 6, and 13 on the Antarctic and Indian plates apparently cannot be described by the same parameters that describe the history of separation of Australia and Antarctica. Therefore, Stein and Okal's contention that Australia and India lie on separate plates appears to be valid not only for the present, but for the last 35 Myr. 相似文献
90.
Expanding spread profile at the northern Jan Mayen Ridge 总被引:1,自引:0,他引:1
An expanding spread seismic profile at the central northern Jan Mayen Ridge, ESP-5, has yielded a crustal seismic velocity distribution which is similar to observations from the thinned continental crust at the Norwegian continental margin. The profile reveals a post-early Eocene sedimentary sequence, about 1. 5 km thick, overlying 1 km of volcanic extrusives and interbedded sediments. Below, there are about 3 km of pre-opening sediments above the seismic basement. The results indicate that the main ridge block is underlain by a thinned crust, possibly only 13.5 km thick. The results are compatible with a continental nature for the main ridge complex. 相似文献