Whole-rock and mineral geochemistry of ODP Hole 176-735B gabbros

We report mineral chemistry, whole-rock major element compositions, and trace element analyses on Hole 735B samples drilled and selected during Leg 176. We discuss these data, together with Leg 176 shipboard data and Leg 118 sample data from the literature, in terms of primary igneous petrogenesis. Despite mineral compositional variation in a given sample, major constituent minerals in Hole 735B gabbroic rocks display good chemical equilibrium as shown by significant correlations among Mg# (= Mg/[Mg + Fe2+]) of olivine, clinopyroxene, and orthopyroxene and An (=Ca/[Ca + Na]) of plagioclase. This indicates that the mineral assemblages olivine + plagioclase in troctolite, plagioclase + clinopyroxene in gabbro, plagioclases + clinopyroxene + olivine in olivine gabbro, and plagioclase + clinopyroxene + olivine + orthopyroxene in gabbronorite, and so on, have all coprecipitated from their respective parental melts. Fe-Ti oxides (ilmenite and titanomagnetite), which are ubiquitous in most of these rocks, are not in chemical equilibrium with olivine, clinopyroxene, and plagioclase, but precipitated later at lower temperatures. Disseminated oxides in some samples may have precipitated from trapped Fe-Ti-rich melts. Oxides that concentrate along shear bands/zones may mark zones of melt coalescence/transport expelled from the cumulate sequence as a result of compaction or filter pressing. Bulk Hole 735B is of cumulate composition. The most primitive olivine, with Fo = 0.842, in Hole 735B suggests that the most primitive melt parental to Hole 735B lithologies must have Mg# 0.637, which is significantly less than Mg# = 0.714 of bulk Hole 735B. This suggests that a significant mass fraction of more evolved products is needed to balance the high Mg# of the bulk hole. Calculations show that 25%-45% of average Eastern Atlantis II Fracture Zone basalt is needed to combine with 55%-75% of bulk Hole 735B rocks to give a melt of Mg# 0.637, parental to the most primitive Hole 735B cumulate. On the other hand, the parental melt with Mg# 0.637 is far too evolved to be in equilibrium with residual mantle olivine of Fo > 0.89. Therefore, a significant mass fraction of more primitive cumulate (e.g., high Mg# dunite and troctolite) is yet to be sampled. This hidden cumulate could well be deep in the lower crust or simply in the mantle section. We favor the latter because of the thickened cold thermal boundary layer atop the mantle beneath slow-spreading ridges, where cooling and crystallization of ascending mantle melts is inevitable. These observations and data interpretation require reconsideration of the popular concept of primary mantle melts and relationships among the extent of mantle melting, melt production, and the composition and thickness of igneous crust.

Planktonic and benthic foraminiferal abundances in ODP Holes 133-819A and 133-821A

To reveal changes in the oceanic environment on the continental slope adjacent to the Great Barrier Reef, east of Cairns (NE Australia), planktonic and benthic foraminiferal abundances were counted and planktonic percentages (P/B ratios) were determined in sediments from two sites. Counts of planktonic and benthic specimens per gram of sediment over the last glacial/interglacial cycle at the shallowest Site 821, located in a water depth of 212 m just below the core of Subtropical Lower Water, show high abundances in the last glacial compared with the Holocene interglacial. We interpret the apparent increase in abundances during the last glacial as mainly a consequence of fluctuations in the intensity of flow of Subtropical Lower Water along the outer shelf edge and upper slope. During the lowstand in sea level, the increased flow winnowed the sediments, concentrating the foraminiferal skeletons. The P/B ratios are low throughout, with the highest values occurring during the Holocene interglacial and glacial stage 2. This suggests that some upwelling might have occurred during glacial stage 2. The relatively deeper water Site 819 is located in 565.2 m of water in a zone of mixing between Subtropical Lower Water and Antarctic Intermediate Water. The studied record at this site represents middle to upper Quaternary sediments, but it was interrupted by a hiatus just above stage 15 (Alexander et al., this volume); stages 7 through 13 are missing. Below the hiatus (isotopic stages 15 through 21), the foraminiferal abundances are low, while above the hiatus, the highest abundances occur in isotopic stage 6. In addition, a major change in the P/B ratio occurs across the unconformity. Below the hiatus, the ratios are low and resemble the values of the top of Site 821; but above it, ratios rapidly fluctuate, with a tendency for high values during glacial periods. We interpret the changes across the hiatus as having been caused by a shift in the position of the mixing zone between subsurface Subtropical Lower Water and Antarctic Intermediate Water. The mixing zone of these watermasses was farther down the slope in isotopic stages 15 through 21. This is indicated by the low P/B ratios, similar to the values found in the top of Site 821, which presently is bathed in subtropical waters. Above the hiatus, the influence of Antarctic Intermediate Water increased, as inferred from the high P/B ratios.