(Table 1) Estimated e-Nd(0) input calculated using the relative fraction of ODP Hole 105-647A amphibole grains originated from different provinces of the North America

Many marine radiogenic isotope records show both spatial and temporal variations, reflecting both the degree of mixing of distinct sources in the oceans and changes in the distribution of chemical weathering on the continents. However, changes in weathering and transport processes may themselves affect the composition of radiogenic isotopes released into seawater. The provenance of physically weathered material in the Labrador Sea, constrained through the use of Ar-Ar ages of individual detrital minerals, has been used to estimate the relative contributions of chemically weathered terranes releasing radiogenic isotopes into the Labrador Sea. A simple box-model approach for balancing observed Nd-isotope variations has been used to constrain the relative importance of localised input in the Labrador Sea, and the subsequent mixing of Labrador Sea Water into North Atlantic Deep-Water. The long-term pattern of erosion and deep-water formation around the North Atlantic seems to have been a relatively stable feature since 1.5 Ma, although there has been a dramatic shift in the nature of physical and chemical weathering affecting the release of Hf and Pb isotopes. The modelled Nd isotopes imply a relative decrease in water mass advection into the Labrador Sea between 2.4 and 1.5 Ma, accompanied by a decrease in the rate of overturning, possibly caused by an increased freshwater input into the Labrador Sea.

Grain size percentiles and Mk values of coastal sediments from Schleswig Holstein, western Baltic Sea

In einer Fülle sedimentpetrographischer Arbeiten wird versucht, aus der Korngrößenverteilung und der Mineralzusammensetzung von Sanden Schlüsse auf ihre Herkunft, ihre Transportrichtung oder ihr Ablagerungsmilieu abzuleiten, die für die Lösung geologischer und ebenso auch wasserbaulicher Probleme nötig sind. Diese Literatur steckt noch voller Widersprüche und Fehlschlüsse. In der vorliegenden Arbeit wird daher versucht, den Mechanismus des Sandtransports vom Grundsätzlichen her besser verständlich zu machen. Das geschieht anhand zweier ausgewählter und eingehend untersuchter Beispiele aus dem Küstenbereich der westlichen Ostsee unter Einbeziehung der Erfahrungen an vielen Vergleichsproben aus verschiedensten Sedimentationsräumen. Unentbehrlich für das Verständnis der transportbedingten Veränderungen an den Sanden ist das sog. 'Äquivalenzprinzip' (Abschnitt 2). Es stellt fest, daß es in einem von einer Strömung transportierten Sediment immer Körner zwar verschiedener Korngröße, aber auch entsprechend verschiedener Dichte und/oder Kornform gibt, die miteinander transportiert und abgelagert werden, weil unter den herrschenden hydraulischen Bedingungen diese Eigenschaften einander voll kompensieren. In Abschnitt 3 wird kurz die von Rittenhouse angegebene Methode geschildert, mit der man an natürlichen Sedimenten unter der sehr allgemein gehaltenen 'Äquivalenzbedingung' gleicher Transportierbarkeit bestimmen kann, welches Korngrößenverhältnis ein bestimmtes Verhältnis der Dichten kompensieren kann. Die von Rittenhouse am Beispiel von Flußsanden gefundene Funktion zwischen der Dichte der Körner und ihrem Äquivalenzverhältnis gegen Quarzkörner wird hier als erste Näherung auch für die Transportverhältnisse von Strandsanden zugrunde gelegt. In Abschnitt 6 wird gezeigt, daß das auch gerechtfertigt ist. In Abschnitt 4 wird eine allgemein brauchbare Methode abgeleitet, mit der man nicht nur unter stark vereinfachenden Annahmen, sondern auch an Sanden mit realen, stets komplexen Korn-größenverteilungen die Folgen des Äquivalenzprinzips für die Verteilung von Mineralen verschiedener Dichte berechnen kann. Für jede Serie von Sanden, deren Korngrößenverteilungen entlang des Transportweges eine bestimmte, von den Transportbedingungen abhängige Entwicklung durchmachen, ergibt sich damit eine Kurvenschar, die beschreibt, wie sich die Mengen von Mineralien mit verschiedenen Dichten in den einzelnen Korngrößenklassen dabei ändern müßten, vorausgesetzt, daß sie im gesamten Korngrößenbereich gleich verfügbar wären. Diese Kurvenschar ist die 'Charakteristik' des betreffenden Transportfalles. Durch den Vergleich zwischen den nach der Charakteristik in den einzelnen Klassen zu erwartenden Mineralmengen mit den in dem betrachteten Transportfall tatsächlich gefundenen läßt sich deren relative, d. h. auf die Menge des Quarzes bezogene 'Verfügbarkeit' berechnen. Sie wird durch die sog. 'hydraulischen Verhältnisse' (Rittenhouse) ausgedrückt, die im Gegensatz zu den 'Klassenverhältnissen' von der Korngrößensonderung beim Transport unabhängig und nur von der Zusammensetzung des Ausgangsmaterials bestimmt sind, solange beim Transport allein das Äquivalenzprinzip wirksam ist. In den untersuchten Fällen von Sandtransport an zwei Strandabschnitten der westlichen Ostsee (Abschnitt 5) zeigte dieser Vergleich (Abschnitt 6), daß die beobachtete Verteilung von Schwermineralen nicht allein durch Transportsonderung unter Gültigkeit des Äquivalenzprinzips erklärt werden kann, sondern daß dabei offenbar auch mechanische Zerkleinerung der Körner während des Transports mitgewirkt haben muß. Nur ein solcher, von der Transportsonderung unabhängiger Effekt kann als Transportrichtungs-Kriterium benutzt werden, wenn die Entwicklung der Korngrößenverteilungen allein keine Entscheidung erlaubt. Wie die Beispiele zeigen, läßt sich Klarheit über die bisher noch sehr umstrittene Frage nach dem Ausmaß der transportbedingten mechanischen Zerkleinerung von Sandkörnern nur gewannen, wenn in Zukunft versucht wird, bei der Bearbeitung natürlicher Beispiele den Einfluß der stets vorhandenen Transportsonderung auf Veränderungen des Mineralbestandes unter Anwendung des Äquivalenzprinzips rechnerisch auszuschalten. Über dieses wesentlichste Ergebnis hinaus erlauben die dargestellten Zusammenhänge auch eine kritische Stellungnahme zu den oben erwähnten allgemeinen Problemen und führen zu methodischen und sachlichen Verbesserungsvorschlägen für weitere Untersuchungen an klastischen Sedimenten.

Low-temperature magnetic remanence and hysteresis measurements on sediment core GeoB4901-8 from the Niger deep-sea fan

Low-temperature (LT) magnetic remanence and hysteresis measurements, in the range 300-5 K, were combined with energy dispersive spectroscopy (EDS) in order to characterize the magnetic inventory of strongly diagenetically altered sediments originating from the Niger deep-sea fan. We demonstrate the possibility of distinguishing between different compositions of members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series on a set of five representative samples, two from the upper suboxic and three from the lower sulfidic anoxic zone of gravity core GeoB 4901. Highly sensitive LT magnetic measurements were performed on magnetic extracts resulting in large differences in the magnetic behavior between samples from the different layers. This emphasizes that both Fe-Ti oxide phases occur in different proportions in the two geochemical environments. Most prominent are variations in the coercivity sensitive parameter coercive field (BC). At room-temperature (RT) hysteresis loops for all extracts are narrow and yield low coercivities (6-13 mT). With decreasing temperature the loops become more pronounced and wider. At 5 K an approximately 5-fold increase in BC for the suboxic samples contrasts a 20-25-fold increase for the samples from the anoxic zone. We demonstrate that this distinct increase in BC at LT corresponds to the increasing proportion of the Ti-rich hemoilmenite phase, while Fe-rich (titano-)magnetite dominates the magnetic signal at RT. This trend is also seen in the room-temperature saturation isothermal remanent magnetization (RT-SIRM) cycles: suboxic samples show remanence curves dominated by Fe-rich mineral phases while anoxic samples display curves clearly dominated by Ti-rich particles. We show that the EDS intensity ratios of the characteristic Fe Kalpha and Ti Kalpha lines of the Fe-Ti oxides may be used to differentiate between members of the magnetite-ulvöspinel and ilmenite-hematite solid solution series. Furthermore it is possible to calculate an approximate composition for each grain if the intensity ratios of natural particles are linked to well-known standards. Thus, element spectra with high Fe/Ti intensity ratios were found to be rather typical of titanomagnetite while low Fe/Ti ratios are indicative of hemoilmenite. The EDS analyses confirm the LT magnetic results, Fe-rich magnetic phases dominate in the upper suboxic environment whereas Ti-rich magnetic phases comprise the majority of particles in the lower anoxic domain: The mineral assemblage of the upper suboxic environments is composed of magnetite (~19%), titanomagnetite (~62%), hemoilmenite (~17%) and ~2% other particles. In the lower anoxic sediments, reductive diagenetic alteration has resulted in more extensive depletion of the (titano-)magnetite phase, resulting in a relative enrichment of the hemoilmenite phase (~66%). In these strongly anoxic sediments stoichiometric magnetite is barely preserved and only ~5% titanomagnetite was detected. The remaining ~28% comprises Ti-rich particles such as pseudobrookite or rutile.

Mineralogy, lithology and stable isotope composition of diagenetic siliceous rocks from ODP Leg 129 sites

Ocean Drilling Program Leg 129 recovered chert, porcellanite, and radiolarite from Middle Jurassic to lower Miocene strata from the western Pacific that formed by different processes and within distinct host rocks. These cherts and porcellanites formed by (1) replacement of chalk or limestone, (2) silicification and in-situ silica phase-transformation of bedded clay-bearing biosiliceous deposits, (3) high-temperature silicification adjacent to volcanic flows or sills, and (4) silica phase-transformation of mixed biosiliceous-volcaniclastic sediments. Petrologic and O-isotopic studies highlight the key importance of permeability and time in controlling the formation of dense cherts and porcellanites. The formation of dense, vitreous cherts apparently requires the local addition and concentration of silica. The influence of permeability is shown by two examples, in which: (1) fragments of originally identical radiolarite that were differentially isolated from pore-water circulation by cement-filled fractures were silicified to different degrees, and (2) by the development of secondary porosity during the opal-CT to quartz inversion within conditions of negligible permeability. The importance of time is shown by the presence of quartz chert below, but not above, a Paleogene hiatus at Site 802, indicating that between 30 and 52 m.y. was required for the formation of quartz chert within calcareous-siliceous sediments. The oxygen-isotopic composition for all Leg 129 carbonate- and Fe/Mn-oxide-free whole-rock samples of chert and porcellanite range widely from d18O = 27.8 per mil to 39.8 per mil vs. V-SMOW. Opal-CT samples are consistently richer in 18O (34.1 per mil to 39.3 per mil) than quartz subsamples (27.8 per mil to 35.7 per mil). Using the O-isotopic fractionation expression for quartz-water of Knauth and Epstein (1976) and assuming d18Opore water = -1.0 per mil, model temperatures of formation are 7°-26°C for carbonate-replacement quartz cherts, 22°-25°C for bedded quartz cherts, and 32°-34°C for thermal quartz cherts. Large variations in O-isotopic composition exist at the same burial depth between co-existing silica phases in the same sample and within the same phase in adjacent lithologies. For example, quartz has a wide range of isotopic compositions within a single breccia sample; d18O = 33.4 per mil and 28.0 per mil for early and late stages of fracture-filling cementation, and 31.6 per mil and 30.2 per mil for microcrystalline quartz precipitation within enclosed chert and radiolarite fragments. Similarly, opal-CT d101 spacing varies across lithologic or diagenetic boundaries within single samples. Co-occurring opal-CT and chalcedonic quartz in shallowly buried chert and porcellanite from Sites 800 and 801 have an 8.7 per mil difference in d18O, suggesting that pore waters in the Pigafetta Basin underwent a Tertiary shift to strongly 18O-depleted values due to alteration of underlying Aptian to Albian-Cenomanian volcaniclastic deposits after opal-CT precipitation, but prior to precipitation of microfossil-filling chalcedony.

Chemistry and mineralogy of ODP Leg 124 sediment samples

From X-ray mineralogical studies and chemical analyses of the whole rocks and the fine fractions (<2 µm) of ten to fifteen samples at each site of ODP Leg 124, two major sources were identified in the sedimentary components of the Celebes and Sulu basins: (1) a terrestrial and continental contribution; (2) a volcanic influx that gives way to well-defined volcanic units or to a dilute contamination, consisting of coarse-grained minerals (Plagioclase, pyroxene, olivine, spinel) or a smectitic-rich fraction produced by the alteration of volcanic glasses and ashes. The continental signature increases the amount of quartz in the rocks and the phyllitic association is complex: micas, kaolinite, disordered interstratified clay-minerals. The chemical compositions of the bulk rocks and the fractions <2 µm are more potassic and aluminum-rich. The volcanic imprint depends on the grain-size and chemical properties of the components. Ca/Na contents highly variable compared to the K content of the bulk composition are due to the presence of coarse-grained volcanic Plagioclase. The fractions <2 µm are more magnesian than in the continental regime. The diagenesis is revealed by the crystallization of zeolites, the fixation of magnesium into the smectites that depletes the pore fluids in this element. Smectitization of the disordered interstratified clay minerals enriches the alkalinity of the pore fluids. Some deep formations of the Sulu Basin are affected by a thermal event, but no thermal event was recognized in the Celebes Basin.

Geochemistry of vein rocks of ODP Hole 118-735B

Rock samples from Hole 735B, Southwest Indian Ridge, were examined to determine the principal vein-related types of alteration that occurred, the nature of fluids that were present, and the temperatures and pressures of these fluids. Samples studied included veined metagabbro, veined mylonitic metagabbro, felsic trondhjemite, and late-stage leucocratic diopside-bearing veins. The methods used were standard petrographic analysis, mineral chemical analysis by electron microprobe, fluid inclusion petrography and analysis by heating/freezing techniques and laser Raman microspectroscopy, and oxygen isotopic analyses of mineral separates. Alteration in lithologic Units I and II (above the level of Core 118-735B-3OR; approximately 140 meters below the seafloor) is dominated by hydration by seawater-derived fluids at high temperature, up to about 700°C, and low water/rock ratio, during and immediately after pervasive ductile deformation. Below Core 118-735B-30R, pervasive deformation is less common, and brittle veining and brecciation are the major alteration styles. Leucocratic centimeter-scale veins, often containing diopside and plagioclase, were produced by interaction of hot (about 500°C) seawater-derived fluid and gabbro. The water/rock ratio was locally high at the veins and breccia zones, but the integrated water/rock ratio for the lower part of the hole is probably low. Accessory hydrous magmatic or deuteric phases formed from magmatic volatiles in some gabbro and in trondhjemite. Most subsequent alteration was affected by fluids that were seawater-derived, based on isotopic and chemical analyses of minerals and analyses of fluid inclusions. Many early-generation fluid inclusions, associated with high-temperature veining, contain appreciable methane as well as saline water. The source of methane is unclear, but it may have formed as seawater was reduced during low water/rock interaction with ultramafic upper mantle or ultramafic and mafic layer 3. Temperatures of alteration were calculated on the basis of coexisting mineral chemistry and isotopic values. Hydrothermal metamorphism commenced at about 720°C and continued to about 550°C. Leucocratic veining took place at about 500°C. Alteration within brecciated horizons was also at about 500° to less than 400°C, and the trondhjemite was altered at about 550° to below 490°C. Pressures calculated from a diopside-bearing vein, based on a combination of fluid inclusion and isotopic analysis, were 90 to 100 MPa. This pressure places the sample, from Core 118-735B-70R in Unit V, at about 2 km below the seafloor.