Crystal morphologies and pyroxene compositions in boninites and tholeiitic basalts at DSDP Holes 60-458 and 60-459B

Not all boninites are glassy lavas. Those of Hole 458 in the Mariana fore-arc region are submarine pillow lavas and more massive flows in which glass occurs only in quenched margins. Pillow and flow interiors have abundant Plagioclase spherulites, microlites, or even larger crystals but can be recognized as boninites by (1) occurrence of bronzite, (2) presence of augite-bronzite microphenocryst intergrowths, and (3) reversal of the usual basaltic groundmass crystallization sequence of plagioclase-augite to augite-plagioclase. The latter is accentuated by sharply contrasting augite and Plagioclase crystal morphologies near pillow margins, a consequence of rapid cooling rates. This crystallization sequence appears to be a consequence of boninites having higher SiO2 and Mg/Mg + Fe than basalts but lower CaO/Al2O3. Microprobe data are used to illustrate the effects of rapid cooling on the compositions of pyroxene and microphenocrysts in a glassy boninite sample and to estimate temperatures of crystallization of coexisting bronzite and augite. A range from 1320°C to 1200°C is calculated with an average of 1250°C. This is higher by 120°-230° than the known range for western Pacific arc tholeiites and by over 300° than for calc-alkalic andesites. Boninites of Hole 458 lack olivine and clinoenstatite but are otherwise chemically and petrographically similar to boninites that have these minerals. In order to distinguish the two types, the Hole 458 lavas are here termed boninites and the others are termed olivine boninites. Arc tholeiite pillow lavas from Holes 458 and 459B are briefly described and their textures compared to fractionated, moderately iron-enriched, abyssal tholeiites. Massive tholeiite flows contain striking quartz-alkali feldspar micrographic intergrowths with coarsely spherulitic textures resulting from in situ magmatic differentiation. Such intergrowths are rare in massive abyssal tholeiites cored by DSDP and probably occur here because arc tholeiites have higher normative quartz at comparable degrees of iron enrichment - a result of higher oxygen fugacities and earlier separation of titanomagnetite - than abyssal tholeiites.

Pyrolyzable carbon, temperature maximum, production index and lithology for DSDP Leg 96 holes

Sediments from Deep Sea Drilling Project Sites 615, 617, 618, 619, and 620-623 were subjected to pyrolysis. The sediments are immature with respect to petroleum generation as determined by production index values of less than 0.1 and Tmax values of 460-480°C. The amount of pyrolyzable organic matter was moderately low as compared to typical petroleum source rocks. The immature organic matter present does not appear to contain a significant proportion of woody material as shown by the low gas-generating potential. Typical overbank sediments from Sites 617 and 620 generally show higher P2 values (500-800 µg hydrocarbon per g dry weight sediment) than typical channel-fill sediments from Sites 621 and 622 (P2 = 450-560 µg/g). Tmax for both types of sediment remained very constant (462-468 °C) with a slight elevation (+ 15°C) occurring in samples containing lignite. The highest P2 values occurred in sections described as turbidites. Very low P2 values (about 50 µg/g) occurred in sands. P2 values for shallower sections of basin Sites 618 and 619 tended to be higher (900-1000 µg/g) and decreased in deeper, more terrigenous sections of Site 619. Preliminary experiments indicate that microbiological degradation of sediment organic matter causes a decrease in P2. Pyrolyzable organic matter from lower fan Site 623 appears to increase with depth in two different sediment sequences (40-85 and 95-125 m sub-bottom). Organic matter type, as shown by pyrolysis capillary gas chromatography (GC) patterns, was generally the same throughout the well, with much more scatter occurring in the deepest sections (130-155 m sub-bottom). One major and two minor organic matter types could be recognized in both fan and basin sites drilled on Leg 96.

(Table 1) Coarse-fraction (0.1 to 0.05 mm) mineralogy at DSDP Leg 55 Holes

This chapter was previously intended to trace volcanic episodes through the Neogene and Pleistocene geological history recorded in the sedimentary sections drilled on the Emperor seamounts. Drilling disturbance, poor core recovery, and incomplete stratigraphic sections recovered from the seamounts have frustrated that plan, however. Moreover, the Leg 55 sedimentologists found in their smear-slide studies that transported island-arc tephra is scarce in the sediments, if present at all. So we have restricted our objective to description of the volcaniclastic admixture in sediments, as determined by mineralogical and geochemical data. We studied geochemistry of bulk samples (see Murdmaa et al., 1980), coarse-fraction mineralogy, and additional smear slides. The results obtained, however, do not tell much more about the volcaniclastic matter than did shipboard core descriptions.