English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Site 450 is located in the Parece Vela Basin 76 km east of the IPOD Trough, a portion of the presumed former spreading center of the basin. Drilled in 4707 meters of water, the hole yielded 265.7 meters of volcaniclastic sediments and sedimentary rocks beneath the surficial 67.3-meter layer of pelagic clay. As at all other Leg 59 sites, pumice fragments and volcaniclastic deposits were sampled in the hope that shore-based analyses of these materials would provide (1) clues concerning the tectonic and volcanic histories of the site and (2) data bearing upon the complex processes of arc-sundering and back-arc spreading. The volcanic-glass sampling program proved disappointing at the other sites. Holes 448 and 451, drilled in the Palau-Kyushu and West Mariana ridges, respectively, yielded great abundances of volcanic glass; unfortunately, volcaniclastic depositional rates had been so great that the sediments had all accumulated over indeterminant short spans of time. Depositional rates and time horizons could not be determined precisely for either of these ridge sites because neither sedimentary column contains fossils from more than a single clearly defined biostratigraphic zone. At Site 449, on the other hand, volcaniclastic debris occurs in a sedimentary column that represents 4 or 5 m.y. of deposits; however, these glasses are only a minor component of the clays and oozes and were not sampled in sufficient volume to be studied according to the methods we used for the glasses of Site 450. It may well be fruitful to analyze the Site 449 glasses in the future. Site 450 is situated far enough from volcaniclastic arc provenance so that the depositional rates were slow. Good biostratigraphic control indicates that volcanic debris accumulated during at least 3 m.y. of the earliest middle Miocene (Fig. 1). Volcanic glasses were sampled at 21 horizons in Hole 450. The uppermost 3 samples are from ashy horizons in the pelagic clay unit overlying the main volcaniclastic unit from which the other 18 samples were taken. At the University of Illinois in Chicago we examined these materials optically, paying particular attention to refractive indices (RI) as a good first approximation of silica content; the samples throw some interesting light on the tectonic, volcanic, and sedimentologic history of the Parece Vela Basin and West Mariana Ridge and indicate promising avenues for future work. LABORATORY ANALYSIS Subsamples of 1to 2-cm volumes were washed free of their fin< fractions by immersion in 90 ml of distilled water, ultrasonic disag gregation, and decantation. This treatment was repeated until the supernatant water was clear of fines. The samples from the ashy horizons in the pelagic clay unit contained little coarse material, and additional 1to 2-cm volumes had to be processed to provide sufficient glass for analysis. Small quantities of the washed sample were mounted on glass slides in an oil of known refractive index. Under transmitted light, a standard set of microscopic observations was made: the respective percentages of lithic, crystal, and glass grains; the mean grain sizes of the glass and of the combined lithic and crystal components; and the respective percentages of the colored and colorless glass fragments. These values are plotted in Figure 1 and presented in Table 1. In the determinations of refractive indices, the only glass fragments analyzed were those barren of crystalline inclusions and those that were completely isotropic. Colored fragments invariably displayed a higher refractive index than did colorless shards. To determine the range of index for each color population, between 15 and 20 fragments of each color were examined. If fewer than 90% of the fragments in a color population had indices less than that of the index oil, another mount was made in an oil with an index 0.004 higher than that of the first oil. This process was repeated until the upper limit of all color populations had been determined. The lower limits of the refractive indices of each population were determined in similar fashion. In order to avoid observer bias and premature interpretation, we conducted analyses in random sequence on the entire suite of samples.

u.s. government printing office ebooks

Tectonic, Volcanic, and Sedimentologic Significance of Volcanic Glasses from Site 450 in the Eastern Parece Vela Basin, Deep Sea Drilling Project Leg 59