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Displaying 31 - 33 from the 33 total records
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Schreibersite Growth and Its Influence on the Metallography of Coarse-Structured Iron Meteorites
Roy S. Clarke, Jr. and Joseph I. Goldstein
80 pages, 28 figures, 20 tables
1978 (Date of Issue: 14 April 1978)
Number 21, Smithsonian Contributions to the Earth Sciences
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The role that schreibersite growth played in the structural development process in coarse-structured iron meteorites has been examined. The availability of many large meteorite surfaces and an extensive collection of metallographic sections made it possible to undertake a comprehensive survey of schreibersite petrography. This study was the basis for the selection of samples for detailed electron microprobe analysis. Samples containing representative structures from eight chemical Groups I and IIAB meteorites were selected.

Electron microprobe traverses were made across structures representative of the observed range of schreibersite associations. Particular emphasis was placed on schreibersite-kamacite interface compositions. An analysis of these data has led to a comprehensive description of the structural development process.

Massive schreibersite, one of the four major types of schreibersite encountered, may be accounted for by equilibrium considerations. Subsolidus nucleation and growth with slow cooling from temperatures at least as high as 850? C, and probably much higher, explain the phase relationships that one sees in meteorite specimens. The retention of taenite in the octahedrites establishes that bulk equilibrium did not extend as low as 550? C. Schreibersite undoubtedly continued in equilibrium with its enclosing kamacite to lower temperatures.

A second type of schreibersite to form is homogeneously nucleated rhabdite. It nucleated in kamacite in the 600? C temperature range, either as a consequence of low initial P level or after local P supersaturation developed following massive schreibersite growth.

A third type of schreibersite is grain boundary and taenite border schreibersite. It formed at kamacite-taenite interfaces, absorbing residual taenite. Nucleation took place successively along grain boundaries over a range of temperatures starting as high as 500? C or perhaps slightly higher. Grain boundary diffusion probably became an increasingly important factor in the growth of these schreibersites with decreasing temperature.

The fourth type of schreibersite is microrhabdite. These schreibersites nucleated homogeneously in supersatuated kamacite at temperatures in the 400? C range or below.

P diffusion controlled the growth rate of schreibersite. The Ni flux to a growing interface had to produce a growth rate equal to that established by the P flux. This was accomplished by tie line shifts that permitted a broad range of Ni growth rates, and these shifts account for the observed range of Ni concentrations in schreibersite. Equilibrium conditions pertained at growth interfaces to temperatures far below those available experimentally. Kinetic factors, however, restricted mass transfer to increasingly small volumes of material with decreasing temperature.

Underwater Television Survey of the Atlantic Outer Continental Margin near Wilmington Canyon
Daniel J. Stanley and Peter Fenner
54 pages, 18 figures
1973 (Date of Issue: 2 August 1973)
Number 11, Smithsonian Contributions to the Earth Sciences
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The study summarizes the results of an underwater television survey of the sea floor at the shelfbreak and at the head of Wilmington Canyon off the Middle Atlantic States. Distributions are shown for bottom currents and sedimentary structures, suspended matter, bottom firmness, bottom lithology, shell percentage, and fauna. A number of man-made objects on the sea floor in this area is documented. The Appendix is a listing of observations made at 26 stations. These data, based on direct visual observation of the sea floor, serve to complement earlier marine geological investigations made in this region. Mapping shows that surficial outer shelf and canyon head sediments at the shelfbreak are undergoing modification by both bottom current processes and bioturbation and that at the present time the Wilmington Canyon is receiving sediment mainly from the adjacent margin. Gravel, oyster banks, and near-vertical cavernous cliffs occurring at depths between 100 and 200 m are relict features related to eustatic changes of sea level that affected the outer continental margin during the Pleistocene.

Volcanic Eruption at Metis Shoal, Tonga, 1967-1968: Description and Petrology
William G. Melson, Eugene Jarosewich and Charles A. Lundquist
18 pages, 13 figures, 3 tables
1970 (Date of Issue: 16 October 1970)
Number 4, Smithsonian Contributions to the Earth Sciences
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The 1967-1968 eruption of Metis Shoal, Tonga, was evidentially similar to the frequent shallow submarine eruptions of the inner island arc of Tonga. The eruption began about 10 December 1967, and an island eventually emerged; by 19 February 1968, the island had been eroded to beneath wave base. The eruptions were characterized by explosions of steam and ash which hurled bombs a few to several hundred feet into the air. The rocks ejected are pumiceous dacites which, for their silica content, have unusually low alkali contents and rare earth-element contents. The chemical characteristics of the dacite are hard to account for by partial melting of an ocean-ridge basalt parent. The peculiar properties of the dacite appear to characterize other Tongan lavas and support the idea that Tonga is part of a distinct petrographic province.

Displaying 31 - 33 from the 33 total records

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