Element analyses and isotope composition of basalts from the West Valley segment, Juan de Fuca Ridge, northeast Pacific

The 50 km-long West Valley segment of the northern Juan de Fuca Ridge is a young, extension-dominated spreading centre, with volcanic activity concentrated in its southern half. A suite of basalts dredged from the West Valley floor, the adjacent Heck Seamount chain, and a small near-axis cone here named Southwest Seamount, includes a spectrum of geochemical compositions ranging from highly depleted normal (N-) MORB to enriched (E-) MORB. Heck Seamount lavas have chondrite-normalized La/Sm en -0.3, 87Sr/86Sr = 0.70235 - 0.70242, and 206Pb/204Pb = 18.22 - 18.44, requiring a source which is highly depleted in trace elements both at the time of melt generation and over geologic time. The E-MORB from Southwest Seamount have La/Sm en -1.8, 87Sr/86Sr = 0.70245 - 0.70260, and 206Pb/204Pb = 18.73 - 19.15, indicating a more enriched source. Basalts from the West Valley floor have chemical compositions intermediate between these two end-members. As a group, West Valley basalts from a two-component mixing array in element-element and element-isotope plots which is best explained by magma mixing. Evidence for crustal-level magma mixing in some basalts includes mineral-melt chemical and isotopic disequilibrium, but mixing of melts at depth (within the mantle) may also occur. The mantle beneath the northern Juan de Fuca Ridge is modelled as a plum-pudding, with "plums" of enriched, amphibole-bearing peridotite floating in a depleted matrix (DM). Low degrees of melting preferentially melt the "plums", initially removing only the amphibole component and producing alkaline to transitional E-MORB. Higher degrees of melting tap both the "plums" and the depleted matrix to yield N-MORB. The subtly different isotopic compositions of the E-MORBs compared to the N-MORBs require that any enriched component in the upper mantle was derived from a depleted source. If the enriched component crystallized from fluids with a DM source, the "plums" could evolve to their more evolved isotopic composition after a period of 1.5-2.0 Ga. Alternatively, the enriched component could have formed recently from fluids with a lessdepleted source than DM, such as subducted oceanic crust. A third possibility is that enriched material might be dispersed as "plums" throughout the upper mantle, transported from depth by mantle plumes.

Mineral composition and experimental results from XRD analyses of sediments from ODP Leg 190 sites

X-ray diffraction analyses of the clay-sized fraction of sediments from the Nankai Trough and Shikoku Basin (Sites 1173, 1174, and 1177 of the Ocean Drilling Program) reveal spatial and temporal trends in clay minerals and diagenesis. More detrital smectite was transported into the Shikoku Basin during the early-middle Miocene than what we observe today, and smectite input decreased progressively through the late Miocene and Pliocene. Volcanic ash has been altered to dioctahedral smectite in the upper Shikoku Basin facies at Site 1173; the ash alteration front shifts upsection to the outer trench-wedge facies at Site 1174. At greater depths (lower Shikoku Basin facies), smectite alters to illite/smectite mixed-layer clay, but reaction progress is incomplete. Using ambient geothermal conditions, a kinetic model overpredicts the amount of illite in illite/smectite clays by 15%-20% at Site 1174. Numerical simulations come closer to observations if the concentration of potassium in pore water is reduced or the time of burial is shortened. Model results match X-ray diffraction results fairly well at Site 1173. The geothermal gradient at Site 1177 is substantially lower than at Sites 1173 and 1174; consequently, volcanic ash alters to smectite in lower Shikoku Basin deposits but smectite-illite diagenesis has not started. The absolute abundance of smectite in mudstones from Site 1177 is sufficient (30-60 wt%) to influence the strata's shear strength and hydrogeology as they subduct along the Ashizuri Transect.

Representative and whole rock analyses of rocks from Filchnerfjella, Antarctica

The geological history of Filchnerfjella and surrounding areas (2°E to 8°E) in central Dronning Maud Land, East Antarctica, is constructed from metamorphic and igneous petrology, and structural investigations. The geology of Filchner-fjella consists mainly of metamorphic rocks accompanied by intrusive rocks. Two stages of metamorphism can be recognized in this area. The earlier stage metamorphism is defined as a porphyroblast stage (garnet, hornblende, and sillimanite stable), and the later one is recognized as a symplectic stage (orthopyroxene and cordieritestable). Taking metamorphic textures and geothermobarometries into account, the rocks experienced an early high-P/medium-T followed by a low-P and high-T stage. Partial melting took place during the low-P/high-T stage, because probable melt of leucocratic gneiss contains cordierite. The field relationships and petrography of the syenite at Filchnerfjella are similar to those of post-tectonic plutons from central Dronning Maud Land, and most of the post-tectonic intrusive rocks have within-plate geochemical features. The structural history in Filchnerfjella and surrounding areas can be divided into the Pan-African stage and the Meso to Cenozoic stage that relates to the break-up of Gondwana.

Age models, organic analyses and major-element analyses of sediment cores on a transect of the tropical African rainbelt

The distribution of rainfall in tropical Africa is controlled by the African rainbelt**1, which oscillates on a seasonal basis. The rainbelt has varied on centennial to millennial timescales along with changes in Northern Hemisphere high-latitude climate**2, 3, 4, 5, the Atlantic meridional overturning circulation**6 and low-latitude insolation**7 over the past glacial-interglacial cycle. However, the overall dynamics of the African rainbelt remain poorly constrained and are not always consistent with a latitudinal migration**2, 4, 5, 6, as has been proposed for other regions**8, 9. Here we use terrestrially derived organic and sedimentary markers from marine sediment cores to reconstruct the distribution of vegetation, and hence rainfall, in tropical Africa during extreme climate states over the past 23,000 years. Our data indicate that rather than migrating latitudinally, the rainbelt contracted and expanded symmetrically in both hemispheres in response to changes in climate. During the Last Glacial Maximum and Heinrich Stadial 1, the rainbelt contracted relative to the late Holocene, which we attribute to a latitudinal compression of atmospheric circulation associated with lower global mean temperatures**10. Conversely, during the mid-Holocene climatic optimum, the rainbelt expanded across tropical Africa. In light of our findings, it is not clear whether the tropical rainbelt has migrated latitudinally on a global scale, as has been suggested**8,9.

Mineralogical, sedimentological and paleontological analyses of ODP Hole 127-797B upper Quarternary sediments

Nearly continuous cores of Quaternary fine-grained sediments with distinct dark-light colored cycles were recovered from Sites 794, 795, and 797 in the basinal parts of the Japan Sea during Leg 127. A comparison of gray value (darkness) profiles supplemented by visual inspection of core photographs between sites indicated that most of the dark and light layers were correlatable between sites, and that two of the dark layers lie close to adjacent marker ash layers. These observations indicate that deposition of dark and light layers resulted from basin-wide synchronous events. In order to understand the origin of these dark-light cycles, petrographical, mineralogical, compositional, and paleontological studies were carried out on closely spaced samples from the upper Quaternary sediments recovered from Site 797. Age model was constructed based on comparison between variation in diatom abundance and the standard oxygen isotope curve of Imbrie et al. (1984), the latter was interpolated between the five age controlled levels established at Site 797. The two curves show similar patterns which enabled us to "tune" the sediment ages to the oxygen isotope stages. We have to use variation in diatom abundance as a substitute for oxygen isotope curve since oxygen isotopic data are not available at the studied sites. Bottom water oxygenation conditions were estimated based on two criteria: (1) the degree of lamina preservation and (2) the ratio of Corg to Stot. The surface water productivity was deduced from the Corg and biogenic silica content. Results suggest that the bottom water oxygenation level and the surface water productivity varied significantly in response to the glacial-interglacial cycles. Glacio-eustatic sea-level changes and subsequent changes in water circulation in the Japan Sea appear to have been responsible for these variations and consequent changes in sediment composition throughout the Quaternary.

Bulk rock magnetic measurements and thermomagnetic analyses of sediment core GeoB4901-8 from the southeastern flank of the Niger deep-sea fan

Diagenesis has extensively affected the magnetic mineral inventory of organic-rich late Quaternary sediments in the Niger deep-sea fan. Changes in concentration, grain size, and coercivity document modifications of the primary magnetic mineral assemblages at two horizons. The first front, the modern iron redox boundary, is characterized by a drastic decline in magnetic mineral content, coarsening of the grain size spectrum, and reduction in coercivity. Beneath a second front, the transition from the suboxic to the sulfidic anoxic domain, a further but less pronounced decrease in concentration and bulk grain size occurs. Finer grains and higher coercive magnetic constituents substantially increase in the anoxic environment. Low- and high-temperature experiments were performed on bulk sediments and on extracts which have also been examined by X-ray diffraction. Thermomagnetic analyses proved ferrimagnetic titanomagnetites of terrigenous provenance as the principal primary magnetic mineral components. Their broad range of titanium contents reflects the volcanogenic traits of the Niger River drainage areas. Diagenetic alteration is not only a grain size selective process but also critically depends on titanomagnetite composition. Low-titanium compounds are less resistant to diagenetic dissolution. Intermediate titanium content titanomagnetite thus persists as the predominant magnetic mineral fraction in the sulfidic anoxic sediments. At the Fe redox boundary, precipitation of authigenic, possibly bacterial, magnetite is documented. The presence of hydrogen sulfide in the pore water suggests a formation of secondary magnetic iron sulfides in the anoxic domain. Grain size-specific data argue for a gradual development of a superparamagnetic and single-domain iron sulfide phase in this milieu, most likely greigite.

Mineralogical and stable isotope analyses of carbonates from the eastern Black Sea

Authigenic carbonates associated with cold seeps provide valuable archives of changes in the long-term seepage activity. To investigate the role of shallow-buried hydrates on the seepage strength and fluid composition we analysed methane-derived carbonate precipitates from a high-flux hydrocarbon seepage area ("Batumi seep area") located on the south-eastern Black Sea slope in ca. 850 m. In a novel approach, we combined computerized X-ray tomography (CT) with mineralogical and isotope geochemical methods to get additional insights into the three-dimensional internal structure of the carbonate build-ups. X-ray diffractometry revealed the presence of two different authigenic carbonate phases, i.e. pure aragonitic rims associated with vital microbial mats and high-Mg calcite cementing the hemipelagic sediment. As indicated by the CT images, the initial sediment has been strongly deformed, first plastic then brittle, leading to brecciation of the progressively cemented sediment. The aragonitic rims on the other hand, represent a presumably recent carbonate growth phase since they cover the already deformed sediment. The stable oxygen isotope signature indicates that the high-Mg calcite cement incorporated pore water mixed with substantial hydrate water amounts. This points at a dominant role of high gas/fluid flux from decomposing gas hydrates leading to the deformation and cementation of the overlying sediment. In contrast, the aragonitic rims do not show an influence of 18O-enriched hydrate water. The differences in d18O between the presumably recent aragonite precipitates and the older high-Mg cements suggest that periods of hydrate dissociation and vigorous fluid discharge alternated with times of hydrate stability and moderate fluid flow. These results indicate that shallow-buried gas hydrates are prone to episodic decomposition with associated vigorous fluid flow. This might have a profound impact on the seafloor morphology resulting e.g. in the formation of carbonate pavements and pockmark-like structures but might also affect the local carbon cycle.

Multi-proxy analyses of sediment core GeoB13801-2

Over the Uruguayan shelf and uppermost slope the coalescence of northward flowing Subantarctic Shelf Water and southward flowing Subtropical Shelf Water forms a distinct thermohaline front termed the Subtropical Shelf Front (STSF). Running in a SW direction diagonally across the shelf from the coastal waters at 32°S towards the shelf break at ca. 36°S, the STSF represents the shelf-ward extension of the Brazil-Malvinas Confluence zone. This study reconstructs latitudinal STSF shifts during the Holocene based on benthic foraminifera d18O and d13C, total organic carbon, carbonate contents, Ti/Ca, and grain-size distribution from a high-accumulation sedimentary record located at an uppermost continental-slope terrace. Our data provide direct evidence for: (1) a southern STSF position (to the South of the core site) at the beginning of the early Holocene (>9.4 cal ka BP) linked to a more southerly position of the Southern Westerly Winds in combination with restricted shelf circulation intensity due to lower sea level; (2) a gradual STSF northward migration (bypassing the core site towards the North) primarily forced by the northward migration of the Southern Westerly Winds from 9.4 cal ka BP onwards; (3) a relatively stable position of the front in the interval between 7.2 and 4.0 cal ka BP; (4) millennial-scale latitudinal oscillations close to 36°S of the STSF after 4.0 cal ka BP probably linked to the intensification in El Niño Southern Oscillation; and (5) a southward migration of the STSF during the last 200 years possibly linked to anthropogenic influences on the atmosphere.

Mineralogical and geochemical analyses of serpentines from ODP Hole 125-778A

Thirty-five samples from Hole 778A were prepared for X-ray diffraction (XRD) mineralogical analyses and for chemical analyses of major and trace elements. Most of the selected samples were silt- and sand-sized sedimentary serpentinites or microbreccias except for a soft clast of mafic rock, a hard clast of massive serpentinized peridotite, and a pebble of consolidated, undeformed serpentine microbreccia that contained planktonic foraminifers. Both mineralogical and geochemical analyses allow discrimination of three groups among the analyzed samples. These groups correspond to three stratigraphic intervals present along the drilled section. Group A contains the upper samples (lithologic Unit I). These consist of poorly consolidated serpentine muds carrying hard-rock clasts (serpentinized peridotites, metabasalts). They are characterized by the following mineralogical assemblage: serpentine, Fe-oxides and hydroxides, aragonite, and halite. They exhibit variable SiO2, MgO contents, but are characterized by a SiO2/MgO ratio near 1. CaO content is high in relation to development of aragonite. Al2O3 content is low. Relatively high K2O, Na2O, and Sr contents are present, presumably in relation to interactions with seawater. Group B (30-77 mbsf) contains samples exhibiting very homogeneous chemical and mineralogical compositions. They consist of serpentinite microbreccias exhibiting frequent shear structures. Hard-rock clasts are also present (serpentinized peridotites, metabasalts, one possible chert fragment). The mineralogy of the Group B samples is characterized by the presence of serpentine and authigenic minerals: hydroxycarbonates and hydrogrossular. Calcite and chlorite are also present, but all the samples lack aragonite. Their chemical compositions are remarkably similar to compositions of their parent rocks. Group C contains silt- and sand-sized serpentine and serpentine microbreccias, which are locally rich in red clasts, probably strongly altered (oxidized?) mafic fragments. Intervals having clasts of more diverse origin than those higher in the section were recovered. Clast lithology includes serpentinized peridotites, metabasalts, metavolcaniclastite, meta-olivine gabbro, and amphibolite sandstone. Mineralogy and geochemistry reflect these compositions. Serpentine content of the samples is less than in previous groups. Correlatively, sepiolite, palygorskite, and chlorite-smectite are mineral phases present in the analyzed samples. Accessory igneous minerals (amphiboles, pyroxenes, hematite) also were found. The chemical compositions of most of Group C samples differ from that of massive serpentinized peridotites. The main differences are (1) higher SiO2, CaO, TiO2 and Al2O3 contents, (2) a SiO2/MgO ratio greater than 1, and (3) a negative correlation between Al2O3, and MgO, Cr, and Ni. These characteristics suggest new constraints relative to the flow structure of the flank of Conical Seamount.

Whole-rock geochemical analyses of serpentinized harzburgites from ODP Site 209-1270

Small-scale shear zones are present in drillcore samples of abyssal peridotites from the Mid-Atlantic ridge at 15°20'N (Ocean Drilling Program Leg 209). The shear zones act as pathways for both evolved melts and hydrothermal fluids. We examined serpentinites directly adjacent to such zones to evaluate chemical changes resulting from melt-rock and fluid-rock interaction and their influence on the mineralogy. Compared to fresh harzburgite and melt-unaffected serpentinites, serpentinites adjacent to melt-bearing veins show a marked enrichment in rare earth elements (REE), strontium and high field strength elements (HFSE) zirconium and niobium. From comparison with published chemical data of variably serpentinized and melt-unaffected harzburgites, one possible interpretation is that interaction with the adjacent melt veins caused the enrichment in HFSE, whereas the REE contents might also be enriched due to hydrothermal processes. Enrichment in alumina during serpentinization is corroborated by reaction path models for interaction of seawater with harzburgite-plagiogranite mixtures. These models explain both increased amounts of alumina in the serpentinizing fluid for increasing amounts of plagiogranitic material mixed with harzburgite, and the absence of brucite from the secondary mineralogy due to elevated silica activity. By destabilizing brucite, nearby melt veins might fundamentally influence the low-temperature alteration behaviour of serpentinites. Although observations and model results are in general agreement, due to absence of any unaltered protolith a quantification of element transport during serpentinization is not straightforward.