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.

Geochemistry of selected minerals at DSDP Leg 65 Holes

The majority of the basalts drilled on Leg 65 in the Gulf of California are aphyric to sparsely phyric massive flows ranging in average thickness between 5 meters in the upper part of the sections in Holes 483 and 483B, where they are interlayered with sediment, and 14 meters in Hole 485A, where interlayered sediments constitute more than half of the section. Massive flows interlayered with pillows are generally less than 4 meters thick. The pillow lavas recovered are more phyric (up to 15 modal%) and contain two to three generations of plagioclase and olivine ± clinopyroxene. Plagioclase generally exceeds 60% of any given phenocryst assemblage. Resorbed olivine, clinopyroxene, and plagioclase megacrysts may reflect a high-pressure stage, the phenocrysts crystallizing in the main magma chamber and the skeletal microphenocrysts in dikes. Precise measurements of length/width ratios of different phenocryst types and compositions show low aspect ratios and large crystal volumes for early crystals and high ratios and low volumes for late crystals grown under strong undercooling conditions. The minerals examined show wide ranges in composition: in particular, plagioclase ranges from An92 to An36; clinopyroxene ranges from Ca41Mg51Fe8 in the cores of phenocrysts to Ca40**36 Mg45**49Fe15**20 in the groundmass; and olivine ranges from Fo86 to Fo81. The wide range in mineral compositions, together with evidence of disequilibrium based on textures and comparisons of glass and mineral compositions, indicate complex crystallization histories involving both polybaric crystal fractionation and magma mixing.

Geochemistry of basalts at DSDP Leg 73 Holes

During Deep Sea Drilling Project Leg 73 (South Atlantic), basaltic pillow lava, flows, and sills were encountered in Holes 519A, 520, 522B, and 524. Paleomagnetic data indicate that the basalts from Holes 519A (magnetic Anomaly 51) and 522B (Anomaly 16) have ages of about 12 m.y. and about 38 m.y., respectively. The major- and trace- (including rare-earth-) element characteristics of the Hole 519A basalts (a total of 27 m) demonstrate that these basalts are typical normal-type mid-ocean-ridge basalts (N-type MORB). In composition the basalts overlap olivine tholeiites from other normal Mid-Atlantic Ridge segments. Both the spectra of incompatible, or less-hygromagmatophile elements (such as Ti, V, Y, and Zr) and REE abundances indicate that these basalts are the result of a low-pressure fractionation of olivine, spinel, and Plagioclase prior to eruption. In Hole 520 only 1.7 m of basalt were recovered from a total drilling depth of 10.5 m. These pillow basalts crystallized from fairly evolved (N-type MORB) tholeiitic melts. In total, 19 m of basaltic pillow lavas and flows were penetrated in Hole 522B. Thirteen cooling units were distinguished on the basis of glassy margins and fine quench textures. In contrast to Holes 519A and 520, the basalts of the Hole 522B ridge section can be divided into two major groups of tholeiites: (1) Cooling Units 1 through 12 and (2) Cooling Unit 13. The basalts in this ridge section are also N-type MORBs but are generally more differentiated than those of Holes 519A and 520. The lowermost basalts (Cooling Unit 13) have the most primitive composition and make up a compositional group distinct from the more evolved basalts in the twelve units above it. Hole 524 was drilled on the south flank of the Walvis Ridge and thus provided samples from a more complex part of the South Atlantic seafloor. Three different basaltic rock suites, interlayered with volcanic detrital sediments, were encountered. The rock suites are, from top to bottom, an alkali basaltic pillow lava; a 16-m-thick alkaline diabase sill with an age of about 65 m.y. (according to K-Ar dating and planktonic foraminifers); and a second sill that is approximately 9 m thick, about 74 m.y. in age, and tholeiitic in composition, thus contrasting strongly with the overlying alkaline rocks. The alkali basalts of Hole 524 show chemical characteristics that are very similar to the basaltic lavas of the Tristan da Cunha group volcanoes, which are located approximately 400 km east of the Mid-Atlantic Ridge crest. Thus, the Walvis Ridge may plausibly be interpreted as a line of hot-spot alkaline volcanoes.

Chemical profiles in sediment and basalt at DSDP Hole 74-525A

Forty sediment and four basement basalt samples from DSDP Hole 525A, Leg 74, as well as nine basalt samples from southern and offshore Brazil, were subjected to instrumental neutron activation analysis. Thirty-two major, minor, and trace elements were determined. The downcore element concentration profiles and regression analyses show that the rare earth elements (REE) are present in significant amounts in both the carbonate and noncarbonate phases in sediments; Sr is concentrated in the carbonate phase, and most of the other elements determined exist mainly in the noncarbonate phase. The calculated partition coefficients of the REE between the carbonate phase and the free ion concentrations in seawater are high and increase with decreasing REE ionic radii from 3.9 x 10**6 for La to 15 x 10**6 for Lu. Calculations show that the lanthanide concentrations in South Atlantic seawater have not been changed significantly over the past 70 Ma. The Ce anomaly observed in the carbonate phase is a redox indicator of ancient seawater. Study of the Ce anomaly reveals that seawater was anoxic over the Walvis Ridge during the late Campanian. As the gap between South America and West Africa widened and the Walvis Ridge subsided from late Campanian to late Paleocene times, the water circulation of the South Atlantic improved and achieved oxidation conditions about 54 Ma that are similar to present seawater redox conditions in the world oceans. The chemical compositions of the basement rocks correspond to alkalic basalts, not mid-ocean ridge basalts (MORBs). The results add more evidence to support the hypothesis that the Walvis Ridge was formed by a series of volcanos moving over a "hot spot" near the Mid-Atlantic Ridge. From the chemical composition and REE pattern, one 112 Ma old basalt on the Brazilian continental shelf has been identified as an early stage MORB. To date, this is the oldest oceanic tholeiite recovered from the South Atlantic. This direct evidence indicates that the continental split between South America and Africa commenced > 112 Ma.