Basalt analyses from different DSDP Holes in the Mozambique Basin and Ridge and from the Southwest Indian Ridge

Basalts from DSDP Sites 248, 249, 250 and 251 in the southwestern Indian Ocean formed in a complex tectonic region affected by the separation of Africa and South America. The different ages and variable geochemical features of these DSDP basalts probably reflect this tectonic complexity. For example, Site 251 on the flanks of the Southwest Indian Ridge is represented by normal MORB which probably originated at the Southwest Indian Ridge. Site 250 in the Mozambique Basin includes an older incompatible- element enriched unit which may represent basalt associated with the Prince Edward Fracture Zone; the upper unit is normal MORB. Basalts at Site 248 also in the Mozambique Basin are geochemically very unlike MORB and have strong continental affinities; they are also comparable in age to some of the continental Karroo basalts. They appear to be related to a subcontinental mantle source or to contamination by continental basement associated with the tectonic elevation of the Mozambique Ridge. Basalts from Site 249 on the Mozambique Ridge are relatively weathered but appear to be normal MORB. Their age, location, and composition are consistent with their origin at an early Cretaceous rift which has been postulated to have separated the Falkland Plateau from the Mozambique Ridge.

Geochemistry in the mid Atlantic Ocean ridge basalts

The main objective of Leg 82 of the Glomar Challenger was to document mantle heterogeneity in the vicinity of, and away from, a so-called hot spot: the Azores Triple Junction. One of the geochemical tools that permits, at least in part, the recognition of mantle heterogeneities uses hygromagmaphile elements, those elements that have an affinity for the liquid. This tool is presented in terms of an extended Coryell-Masuda plot, which incorporates within the rare earth elements the hygromagmaphile transition elements Th, Ta, Zr, Hf, Ti, Y, and V. The extended Coryell-Masuda plot is used to summarize our knowledge of mantle heterogeneity along the ridge axis at zero-age. It is also used by choosing those hygromagmaphile elements that can be analyzed on board by X-ray fluorescence spectrometry to give preliminary information on the enriched or depleted character of recovered samples. Shore-based results, which include analyses of most of the hygromagmaphile elements measured either by X-ray spectrometry or neutron activation analysis, confirm the shipboard data. From the point of view of comparative geochemistry, the variety of basalts recovered during Leg 82 provides a good opportunity to test and verify the classification of the hygromagmaphile elements. Analyses from Leg 82 provide new data about the relationship between extended rare earth patterns (enriched or depleted) that can be estimated either by La/Sm ratio or Nb/Zr (or Ta/Hf) ratios: samples from Hole 556 are depleted (low Nb/Zr ratio) but have a high 206Pb/ 204Pb (19.5) ratio; in Hole 558 a moderately enriched basalt unit with a La/Sm (= Nb/Zr) ratio (chondrite normalized) of 2 has a high 206Pb/204Pb (20) ratio. One of the most interesting results of Leg 82 lies in the crossing patterns of extended Coryell-Masuda plots for basalts from the same hole. This result enhances the notion of local mantle heterogeneity versus regional mantle heterogeneity and is confirmed by isotope data; it also favors a model of short-lived, discrete magma chambers. The data tend to confirm the Hayes Fracture Zone as a southern limit for the influence of Azores-type mantle. Nevertheless, north of the Hayes Fracture Zone, the influence of a plumelike mantle source is not simple and probably requires an explanation more complex than a contribution from a single fixed hot spot.

Geochemical analysis of surface sediments from the ocean floor and from sea ice in the northern Atlantic and Arctic Oceans

Barium in marine terrigenous surface sediments of the European Nordic Seas is analysed to evaluate its potential as palaeoproductivity proxy. Biogenic Ba is calculated from Ba and Al data using a conventional approach. For the determination of appropriate detrital Ba/Al ratios a compilation of Ba and Al analyses in rocks and soils of the catchments surrounding the Nordic Seas is presented. The resulting average detrital Ba/Al ratio of 0.0070 is similar to global crustal average values. In the southern Nordic Seas the high input of basaltic material with a low Ba/Al ratio is evident from high values of magnetic susceptibility and low Al/Ti ratios. Most of the Ba in the marine surface sediments is of terrigenous and not of biogenic origin. Variability in the lithogenic composition has been considered by the application of regionally varying Ba/Al ratios. The biogenic Ba values are comparable with those observed in the central Arctic Ocean, they are lower than in other oceanic regions. Biogenic Ba values are correlated with other productivity proxies and with oceanographic data for a validation of the applicability in paleoceanography. In the Iceland Sea and partly in the marginal sea-ice zone of the Greenland Sea elevated values of biogenic Ba indicate seasonal phytoplankton blooms. In both areas paleoproductivities may be reconstructed based on Ba and Al data of sediment cores.

Geochemistry of ODP Hole 176-735B igneous and hydrothermal veins

During Legs 118 and 176, Ocean Drilling Program Hole 735B, located on Atlantis Bank on the Southwest Indian Ridge, was drilled to a total depth of 1508 meters below seafloor (mbsf) with nearly 87% recovery. The recovered core provides a unique section of oceanic Layer 3 produced at an ultraslow spreading ridge. Metamorphism and alteration are extensive in the section but decrease markedly downward. Both magmatic and hydrothermal veins are present in the core, and these were active conduits for melt and fluid in the crust. We have identified seven major types of veins in the core: felsic and plagioclase rich, plagioclase + amphibole, amphibole, diopside and diopside + plagioclase, smectite ± prehnite ± carbonate, zeolite ± prehnite ± carbonate, and carbonate. A few epidote and chlorite veins are also present but are volumetrically insignificant. Amphibole veins are most abundant in the upper 50 m of the core and disappear entirely below 520 mbsf. Felsic and plagioclase ± amphibole ± diopside veins dominate between ~50 and 800 mbsf, and low-temperature smectite, zeolite, and prehnite veins are present in the lower 500 m of the core. Carbonate veinlets are randomly present throughout the core but are most abundant in the lower portions. The amphibole veins are closely associated with zones of intense crystal plastic deformation formed at the brittle/ductile boundary at temperatures above 700°C. The felsic and plagioclase-rich veins were formed originally by late magmatic fluids at temperatures above 800°C, but nearly all of these have been overprinted by intense hydrothermal alteration at temperatures between 300° and 600°C. The zeolite, prehnite, and smectite veins formed at temperatures <100°C. The chemistry of the felsic veins closely reflects their dominant minerals, chiefly plagioclase and amphibole. The plagioclase is highly zoned with cores of calcic andesine and rims of sodic oligoclase or albite. In the felsic veins the amphibole ranges from magnesio-hornblende to actinolite or ferro-actinolite, whereas in the monomineralic amphibole veins it is largely edenite and magnesio-hornblende. Diopside has a very narrow range of composition but does exhibit some zoning in Fe and Mg. The felsic and plagioclase-rich veins were originally intruded during brittle fracture at the ridge crest. The monomineralic amphibole veins also formed near the ridge axis during detachment faulting at a time of low magmatic activity. The overprinting of the igneous veins and the formation of the hydrothermal veins occurred as the crustal section migrated across the floor of the rift valley over a period of ~500,000 yr. The late-stage, low-temperature veins were deposited as the section migrated out of the rift valley and into the transverse ridge along the margin of the fracture zone.

Magnetic susceptibility and ice-rafted debris in surface sediments of the Nordic Sea

Magnetic susceptibility and ice-rafted debris of surface sediments in the Nordic Seas were investigated to reconstruct source areas and recent transport pathways of magnetic minerals. From the distribution of magnetic susceptibility and ice-rafted debris and published data on petrographic tracers for iceberg drift, we reconstructed a counter-clockwise iceberg drift pattern during cooler phases in the Holocene, which is similar to conceptual and numerical models for Weichselian iceberg drift. The release of basaltic debris at Scoresby Sund played a significant role for the magnetic signature of stadial/interstadial events during isotope stage 3 recorded in sediment cores of the Nordic Seas.

Annotated record of the detailed examination of Mn deposits from DSDP Site 162, Leg 16 (Cores 16-162-1, 16-162-2, 16-162-8, 16-162-9 and 16-162-15)

DSDP 162 is located due north of DSDP 161 on the lower west flank of the East Pacific Rise about 3900 km west of the crest. It is in the Clarion-Clipperton block, about 80 km south of the Clarion Fracture Zone. The site lies at the extreme northern edge of the zone of thick sediments that parallels the equator in the Pacific and marks the region of high biological productivity.

Amino acid racemization for several DSDP samples (Table 1)

Kinetic parameters for the epimerization of isoleucine in multispecific foraminiferal asemblages were used to establish the effects of burial depth and the geothermal gradient on the extent of reaction. It was observed that with a little as thirty meters of burial in a normal thermal regime there were differences between the extent of epimerization measured and that which would have been predicted for thermal equilibrium with bottom water temperatures. As would be expected, these differences are greatest when the heat flow (the geothermal gradient) and/or the sedimentation rates are highest. These effects were observed in most of the DSDP samples studied, and have been used to estimate the average heat flux since the time of sample deposition. Occasional anomalous effects were observed which could not be related to past or present heat flux. These were determined to be due to such geologic occurrences as slumping and reworking or to recent sample contamination. Other problems emerged related to bottom water temperatures including changes over geologic time which are unknown and could not be deduced. Thus, the presence of epimerization anomalies in DSDP cores as noted above limits the effectiveness of amino acid geochronology in such cores, unless these anomalies can be recognized as ab initio.

(Table 3, page 280) Chemical composition of micronodules from a series of stations and sediment depths from Areas C, F, G and K in the Pacific Ocean from R/V Sonne Cruise SO06

Compositional data for coexisting manganese nodules, micronodules, sediments and pore waters from five areas in the equatorial and S.W. Pacific have been obtained. This represents the largest study of its type ever undertaken to establish the distribution of elements between the various phases within the sediment column. The composition of manganese nodules, micronodules and sediments (on a carbonate-free basis) shows marked differences between the equatorial high productivity zone and the low productivity region of the S.W. Pacific. In the case of the nodules, th is reflects an increased supply of transition elements (notably Ni, Cu and Zn) to the nodules as a result of the in situ dissolution of siliceous tests within the sediment column in the equatorial Pacific high productivity zone. Micronodules display similar, but somewhat different, compositions to those of the associated nodules in each area. Micronodule composition is therefore influenced by the same basic factors that control nodule composition, but is modified by dissolution of the micronodules in situ within the sediment column. Locally, as in the area immediately south of the Marquesas Fracture Zone, the micronodule population is contaminated by small, angular volcanic rock fragments; this leads to apparently anomalous micronodule compositions. Micronodules appear to be a transient feature in the sediment column, especially in the equatorial Pacific. Dissolution of micronodules in the sediment column therefore represents an important source of elements for the growth of manganese nodules in the equatorial Pacific. Sediment composition is markedly influenced by the carbonate content. On a carbonate-free basis, the sediments from the equatorial high productivity zone are quite distinct in composition from those in the S.W. Pacific. This reflects differences in the lithology of the sediments. In the Aitutaki Passage, the local influence of volcanoclastic material in sediment composition has been established. The major cations and anions in pore waters measured here show no major differences between equatorial and S.W. Pacific sediments. Silica is, however, higher in equatorial Pacific pore waters reflecting the dissolution of siliceous tests in these sediments.

Magnetic properties of the oceanic igneous rocks (Table 1)

Hysteresis measurements have been carried out on a suite of ocean-floor basalts with ages ranging from Quaternary to Cretaceous. Approximately linear, yet separate, relationships between coercivity (Bc) and the ratio of saturation remanence/saturation magnetization (Mrs/Ms) are observed for massive doleritic basalts with low-Ti magnetite and for pillow basalts with multi-domain titanomagnetites (with x= 0.6). Even when the MORB has undergone lowtemperature oxidation resulting in titanomaghemite, the parameters are still distinguishable, although offset from the trend for unoxidized multidomain titanomagnetite. The parameters for these iron oxides with different titanium content reveal contrasting trends that can be explained by the different saturation magnetizations of the mineral types. This plot provides a previously underutilized and non-destructive method to detect the presence of low-titanium magnetite in igneous rocks, notably MORB.

Annotated record of the detailed examination of Mn deposits from DSDP Site 32, Leg 5 (Cores 5-32-13, 5-32-14)

Site 32 was proposed by the Pacific Advisory Panel at a location over a strong positive magnetic anomaly (Number 13 on the Pittman-Heirtzler scale, 38 million years) where samples of the basement and the basal sediment would be of value in testing hypotheses for origin of the linear magnetic anomalies from this part of the Pacific. Comparison of this site, south of the Pioneer Fracture Zone, with later sites north of the Fracture Zone would be the basis for evaluating the discontinuity formed by the Pioneer.

(Table 1) K, Rb and Sr concentrations and Sr isotopic composition of sediments and igneous rocks from DSDP Hole 30-286

To evaluate the possible contribution of ocean floor sediments during the genesis of the volcanism of Vanuatu (New Hebrides) active margin, we have determined the balance of Sr isotopes and K, Rb and Sr contents for the stratigraphic column of site 286 (leg 30, DSDP). This site is located on the oceanic plate that will be subducted. Analyses have been performed on sedimentary and igneous rocks, before and after acid leaching. The Sr isotopic data do not support the occurrence of some continental component in arc magmas of this active margin which is really intraoceanic. It is demonstrated that the d'Entrecasteaux fracture zone results from the intense fracturing of typical oceanic crust. The analyses of the volcanogenic components of the sediments show a change in the source of volcanoclastic detritus from the Loyalty islands in the Eocene to the volcanic arc of Vanuatu (New Hebrides) during Pliocene and Quaternary times. The determined balance of Sr isotopes and of K, Rb, Sr contents, may be used for calculation of multicomponent melting mixing models for the origin of Vanuatu arc magmas, but we emphasize that in these models the Sr isotopes cannot be considered as an appropriated tracer of sediment contribution.

Oxygen isotope ratios amd Li concentrations in spilitized basaltic rocks of DSDP Hole 83-504B (Table 5)

Li-delta18O-SiO2 relationships have been examined for suites of spilitized basaltic rocks (DSDP 504B; Xigaze Ophiolite; Blanco Fracture Zone; Greater Caucasus; Rhenohercynian Fold Belt) and intra-plate evolved tholeiites (Northern Hessian Depression and Vogelsberg, W Germany; Mount Falla, Transantarctic Mountains). Relative to unaltered MORB and intra-plate primary olivine tholeiites, both the spilitic rocks and the evolved tholeiites are characterized by Li and 18O enrichment. For the spilitic rocks, Li and 18O enrichment is accompanied by a loss of SiO2 as a result of seawater hydrothermal alteration, whereas the evolved tholeiites have gained SiO2, Li and 18O from fractionation of mafic phases and assimilation of crustal rocks. On Li vs. SiO2 and delta18O vs. SiO2 diagrams, the two rock groups plot largely in distinct fields, suggesting the possibility of so distinguishing between such lithologies in the ancient rock record. Mafic granulite xenoliths from the Northern Hessian Depression have elevated Li and 18O abundances at low SiO2 contents. Even after correction for extraction of felsic components, their Li-delta18O-SiO2 signatures plot within the field of spilitic protoliths, suggesting that the lower crust in this region contains relics of spilitic rocks from a former oceanic crust.