Major and trace elements at DSDP Leg 74 Holes

Deep Sea Drilling Project Leg 74 drilled basement on the Walvis Ridge at Sites 525, 527, and 528. These sites are located on the crest and flanks of the segment of the Ridge about 68 to 70 m.y. old in the central province of the Ridge. Each site has a number of distinct subaqueous flows separated by sediment layers. Although variation in geochemistry among units and sites is related in part to alteration or crystal fractionation, some is caused by small-scale compositional variation in the mantle source of the basalts. Leg 74 basalts are similar to other basalts recovered from the Walvis Ridge and the Rio Grande Rise. They show distinct compositional differences to mid-ocean ridge basalts in general, to those recovered from the South Atlantic at this latitude, and to basalts presently erupting in Tristan da Cunha. The composition of the Walvis Ridge basalts does not suggest simple mixtures of present-day MORB and Tristan da Cunha melts. If the Walvis Ridge represents the trace of the Tristan da Cunha hot spot as the plates separated, then the composition of the mantle source has differed at different times in the past, which suggests mantle heterogeneity.

(Table 1) Basalt geochemsitry at DSDP Hole 72-516F

Basalts from Hole 516F, DSDP Leg 72 on the Rio Grande Rise are tholeiitic in character but differ from normal mid-ocean ridge basalts in the South Atlantic in higher concentrations of incompatible elements such as Ti, K, V, Sr, Ba, Zr, Nb, and light rare-earth elements and in lower concentrations of Mg, Cr, and Ni. They contrast with previously reported basalts from the Rio Grande Rise, which were highly alkalic in character. The Rio Grande Rise basalts from Hole 516F (age 84.5 Ma) are generally similar to basalts from the eastern end of the Walvis Ridge (80-100 Ma). It is suggested that they either originated, like the Walvis Ridge, from a mantle hot spot that is different from the present-day hot spot (Tristan da Cunha) and that has changed composition with time, or from a spreading center that was shallow and chemically influenced by the adjacent hot spot, similar to the present-day Mid-Atlantic Ridge near the Azores and Tristan da Cunha.

Silicate mineralogy of basalts at DSDP Leg 54 Holes

Basalts drilled from the East Pacific Rise, OCP Ridge, and Siqueiros fracture zone during Leg 54 are texturally diverse. Dolerites are equigranular at Sites 422 and 428 and porphyritic, with phenocrysts of plagioclase (An69.73) and Ca-rich clinopyroxene (Ca42Mg48Fe10) at Site 427. The East Pacific Rise lavas and some of those from the OCP Ridge are fine-grained and porphyritic. The majority of the large crystals are clustered skeletal glomerocrysts of plagioclase An64-77), together with olivine (Fo80-87), Ca-rich clinopyroxene, or both. Euhedral phenocrysts of plagioclase, together with olivine, Carich clinopyroxene, and Cr-Al spinel in some cases, occur in most of the fine-grained lavas. These phenocrysts are small (maximum dimension <1 mm in all but one sample), sparse (combined modal amount <1% in all samples), and distinctive from the megacrysts which characterize many ocean-floor lavas. In two East Pacific Rise lavas, zoned plagioclase (An83 cores) is the sole phenocryst phase. In other porphyritic lavas from all the main East Pacific Rise and OCP Ridge units drilled during Leg 54, the plagioclase phenocrysts contain cores of bytownite (An79-87) surrounded by more-sodic feldspar (An67-77). Core/rim relationships vary from continuous normal zoning, through discontinuous zoning, to extensive resorption of the calcic cores in some samples. The compositions of the plagioclase calcic cores are systematically related to those of the glomerophyric plagioclase and olivine in the lavas containing them. Furthermore, only one compositional population of calcic cores occurs in each rock. The possible causes of these relationships are far from clear. Magma mixing, although superficially applicable, is inconsistent with important aspects of the phenocryst mineralogy of these particular lavas. A more satisfactory model to explain both phenocryst zoning and rapid glomerocryst growth immediately before extrusion may be constructed by postulating influx of water into the upwelling magmas within Layer 3 of the oceanic crust beneath the East Pacific Rise, and subsequent loss of part of this water during effervescence within feeder dykes between Layer 3 and the ocean floor. It is shown that this model is fully consistent with published data on water and carbon dioxide contents and ratios in the pillow-margin glasses, vesicles, and phenocryst inclusions of ocean-floor basalts. The evidence for the precipitation of plagioclase- dominated crystalline assemblages from these magmas in the upper part of Layer 3 is concordant with recent geophysically based modeling of the structure of the East Pacific Rise. Calcium-rich clinopyroxenes in dolerites from the OCP Ridge and Siqueiros fracture zone show radial, oscillatory, and sector-zoning. In Sample 428A-5-2 (Piece 5a), the compositional trends resulting from this zoning closely resemble those of the pyroxenes in some lunar lavas. The controls on crystallization of interstitial pigeonite - epitaxial upon augite - in this rock are discussed. Both sector-zoning of the augite and nucleation of pigeonite within microvolumes of magma with a low Ca(Mg + Fe) ratio appear to be important factors.

Geotechnical properties of sediments at DSDP Site 75-532

During Leg 75 of the Deep Sea Drilling Project (DSDP) from the D/V Glomar Challenger, a 200-m deep hole was drilled at Hole 532A on the eastern side of Walvis Ridge at a water depth of 1331 m. Sediment cores were obtained by means of a hydraulic piston corer. All of the cores from this boring were designated for geotechnical studies and were distributed among eight institutions. The results of laboratory studies on these sediment cores were compiled and analyzed. Sediment properties, including physical characteristics, strength, consolidation, and permeability were studied to evaluate changes as a function of depth of burial. It was concluded that the sediment profile to the explored depth of 200 m at Walvis Ridge consists of approximately 50 m of foram-nannofossil marl (Subunit 1a) over 64 m of diatom-nannofossil marl (Subunit 1b) over nannofossil marl (Subunit 1c) to the depth explored. All three sediment units appear to be normally consolidated, although some anomalies seem to exist to a depth of 120 m. No distinct differences were found among the sediment properties of the three subunits (1a, 1b, and 1c) identified at this site.