Selected major elements and trace elements of ferrar dolerite clasts from sediment core CRP-1 (Table 1,2)

One of the objectives of the Cape Roberts Project is to study the tectonic history of the western Ross Sea region. Timing of the uplift of the Transantarctic Mountains, which are adjacent to the drillsite, will be a component of the tectonic studies (International Steering Committee, 1994; Cale Roberts Science Team, 1998a). The study of the clast samples from the core will be an important means of providing insight into the timing of uplift of the Transantarctic Mountains. Tholeiitic igneous rocks of the Jurassic (180 Ma) Ferrar large igneous province (FLIP) are widespreaded along the Transantarctic Mountains and have the potential to provide distinct indicators of erosion during uplift of the mountains. In the Transantarctic Mountains adjacent to the Cape Roberts drill site the FLIP is represented by lavas and pyroclastic of the Kirkpatrick basalts and by thick Ferrar dolerite sills which intrude the Beacon Supergroup sediments and, occasionally, the granitic basement rocks. In the Prince Albert Mountains, the youngest Kirkpatrick basalt lava is over 150 m thick, and has a very distinct high TiO2 chemical composition which is unique in the FLIP. If such rocks can be identified in the core they may provide precise timing of the initiation of uplift and denudation of the Transantarctic Mountains. Here we report on an examination of 20 Ferrar dolerite clasts. This brief report is intended as a pilot study to the examination of FLIP clasts from older drillcore.

Chemistry of very young basalts from ODP Hole 106-648B

Basalts in Hole 648B, located in the rift valley of the Mid-Atlantic Ridge at 23°N in crust estimated to be less than 100,000 years old, are mainly fresh, but small amounts of secondary phases are found on fracture surfaces and in alteration halos within the rocks. The halos are defined by dark bands 1-4 mm thick that have developed parallel to fracture surfaces or pillow margins and which in some cases have migrated some centimeters into the rock. The dark bands are the principal locus of secondary phases. The secondary phases are olive-green and yellow protoceladonites, of composition and structure intermediate between celadonite and iron-rich saponite, red (Mn-poor) to opaque (Mn-rich) iron oxyhydroxides, mixtures of protoceladonite and iron oxyhydroxide, and rare manganese oxides. These phases occur mainly as linings or fillings of open spaces in the basalt within the dark bands. Sulfides and intersertal glass are the only primary phases that can be seen to have been altered. Where dark bands have migrated into the rock, the rock behind the advancing band is almost devoid of secondary phases, implying redissolution. The potassium and magnesium in the secondary phases could have been supplied from ambient seawater. The aluminum in the protoceladonites must have been derived from local reaction of intergranular glass. The source of iron and silica could have been intergranular glass or low temperature mineralizing solutions of the type responsible for the formation of deposits of manganese oxides and iron oxyhydroxides and silicates on the seafloor.

Analysis of minerals from Arctic Ocean sediments

The Arctic Ocean System is a key player regarding the climatic changes of Earth. Its highly sensitive ice Cover, the exchange of surface and deep water masses with the global ocean and the coupling with the atmosphere interact directly with global climatic changes. The output of cold, polar water and sea ice influences the production of deep water in the North Atlantic and controls the global ocean circulation ("the conveyor belt"). The Arctic Ocean is surrounded by the large Northern Hemisphere ice sheets which not only affect the sedimentation in the Arctic Ocean but also are supposed to induce the Course of glacials and interglacials. Terrigenous sediment delivered from the ice sheets by icebergs and meltwater as well as through sea ice are major components of Arctic Ocean sediments. Hence, the terrigenous content of Arctic Ocean sediments is an outstanding archive to investigate changes in the paleoenvironment. Glazigenic sediments of the Canadian Arctic Archipelago and surface samples of the Arctic Ocean and the Siberian shelf regions were investigated by means of x-ray diffraction of the bulk fraction. The source regions of distinct mineral compositions were to be deciphered. Regarding the complex circumpolar geology stable christalline shield rocks, active and ancient fold belts including magmatic and metamorphic rocks, sedimentary rocks and wide periglacial lowlands with permafrost provide a complete range of possible mineral combinations. Non- glaciated shelf regions mix the local input from a possible point source of a particular mineral combination with the whole shelf material and function as a sampler of the entire region draining to the shelf. To take this into account, a literature research was performed. Descriptions of outcropping lithologies and Arctic Ocean sediments were scanned for their mineral association. The analyses of glazigenic and shelf sediments yielded a close relationship between their mineral composition and the adjacent source region. The most striking difference between the circumpolar source regions is the extensive outcrop of carbonate rocks in the vicinity of the Canadian Arctic Archipelago and in N Greenland while siliciclastic sediments dominate the Siberian shelves. In the Siberian shelf region the eastern Kara Sea and the western Laptev Sea form a destinct region defined by high smectite, (clino-) pyroxene and plagioclase input. The source of this signal are the extensive outcrops of the Siberian trap basalt in the Putorana Plateau which is drained by the tributaries of the Yenissei and Khatanga. The eastern Laptev Sea and the East Siberian Sea can also be treated as one source region containing a feldspar, quartz, illite, mica, and chlorite asscciation combined with the trace minerals hornblende and epidote. Franz Josef Land provides a mineral composition rich in quartz and kaolinite. The diverse rock suite of the Svalbard archipelago distributes specific mineral compositions of highly metamorphic christalline rocks, dolomite-rich carbonate rocks and sedimentary rocks with a higher diagenetic potential manifested in stable newly built diagenetic minerals and high organic maturity. To reconstruct the last 30,000 years as an example of the transition between glacial and interglacial conditions a profile of sediment cores, recovered during the RV Polarstern" expedition ARK-VIIIl3 (ARCTIC '91), and additional sediment cores around Svalbard were investigated. Besides the mineralogy of different grain size fractions several additional sedimentological and organo-geochemical Parameterswere used. A detailed stratigraphic framework was achieved. By exploiting this data set changes in the mineral composition of the Eurasian Basin sediments can be related to climatic changes. Certain mineral compositions can even be associated with particular transport processes, e.g. the smectitel pyroxene association with sea ice transport from the eastern Kara Sea and the western Laptev Sea. Hence, it is possible to decipher the complex interplay between the influx of warm Atlantic waters into the Southwest of the Eurasian Basin, the waxing and waning of the Svalbard1Barents- Sea- and Kara-Sea-Ice-Sheets, the flooding of the Siberian shelf regions and the surface and deep water circulation. Until now the Arctic Ocean was assumed to be a rather stable System during the last 30,000 years which only switched from a completely ice covered situation during the glacial to seasonally Open waters during the interglacial. But this work using mineral assemblages of sediment cores in the vicinity of Svalbard revealed fast changes in the inflow of warm Atlantic water with the Westspitsbergen Current (< 1000 years), short periods of advances and retreats of the marine based Eurasian ice sheets (1000-3000 years), and short melting phases (400 years?). Deglaciation of the marine-based Eurasian and the land-based north American and Greenland ice sheets are not simultaneous. This thesis postulates that the Kara Sea Ice Sheet released an early meltwater signal prior to 15,000 14C years leading the Barents Sea Ice Sheet while the western land-based ice sheets are following later than 13,500 14C years. The northern Eurasian Basin records the shift between iceberg and sea-ice material derived from the Canadian Arctic Archipelago and N-Greenland and material transported by sea-ice and surface currents from the Siberian shelf region. The phasing of the deglaciation becomes very obvious using the dolomite and quartd phyllosilicate record. It is also supposed that the flooding of the Laptev Sea during the Holocene is manifested in a stepwise increase of sediment input at the Lomonosov Ridge between the Eurasian and Amerasian Basin. Depending on the strength of meltwater pulses from the adjacent ice sheets the Transpolar Drift can probably be relocated. These movements are traceable by the distribution of indicator minerals. Based on the outcome of this work the feasibility of bulk mineral determination can be qualified as excellent tool for paleoenvironmental reconstructions in the Arctic Ocean. The easy preparation and objective determination of bulk mineralogy provided by the QUAX software bears the potential to use this analyses as basic measuring method preceding more time consuming and highly specialised mineralogical investigations (e.g. clay mineralogy, heavy mineral determination).

Concentrations and isotopic compositions of low molecular weight hydrocarbons in hydrothermal fluids from ODP Field, Middle Valley, northern Juan de Fuca Ridge

The presence of sedimentary organic matter blanketing midocean ridge crests has a potentially strong impact on metal transport in hydrothermal vent fluids. To constrain the role of organic matter in metal mobility during hydrothermal sediment alteration, we reacted organic-rich diatomaceous ooze from Guaymas Basin, Gulf of California, and organic-poor hemipelagic mud from Middle Valley, northern Juan de Fuca Ridge, with seawater and a Na-Ca-K-Cl fluid of seawater chlorinity, at 275° to 400°C, 350 to 500 bars, and initial fluid: sediment mass ratios ranging from 1.6 to 9.8. Reaction of these fluids with both sediment types released CO2 and high concentrations of ore-forming metals (Fe, Mn, Zn, Pb) to solution. Relatively low concentrations of Cu were observed in solution and likely reflect the reducing conditions that resulted from the presence of sedimentary organic matter. Both the concentrations of CO2 and dissolved metals were lower in fluids reacted with Middle Valley sediment compared with aqueous concentrations in fluids reacted with Guaymas Basin sediment. During alteration of both sediment types, metal concentrations varied strongly as a function of temperature, increasing by up to an order of magnitude over the 75°C range of each experiment. Major element fluid chemistry and observed alteration assemblages suggest that during hydrothermal alteration of organic-lean sediment from Middle Valley a feldspar-quartz-illite mineral assemblage buffered in situ pH. In contrast, data from the experimental alteration of organic-rich Guaymas Basin sediment suggest that a calcite-plagioclase-quartz assemblage regulated in situ pH. Fluid speciation calculations suggest that in situ pH during Guaymas Basin sediment alteration was lower than during alteration of Middle Valley sediment and accounts for the substantially greater metal mobility at a given temperature and pressure during the former experiment. Comparison of our results with the results of basalt alteration experiments indicate that except for Cu, hydrothermal sediment alteration results in equal or greater concentrations of ore-forming metals at a given temperature and pressure. Accordingly, the presence of ore-forming metals in fluids currently venting from sediment-covered hydrothermal systems at concentrations substantially lower than in fluids from bare-rock systems may reflect chemical reequilibration during subsurface cooling within the sediment pile.

Geochemistry of silicate melt inclusions from ODP Holes 137-504B and 140-504B

Geochemical data from plagioclase-hosted silicate melt inclusions from Leg 140, Hole 504B diabase dikes are reported. Hand-picked plagioclase grains were heated to 1260°-1280°C to remelt the glass inclusions and to infer trapping temperatures. The samples were then polished to expose the inclusions, which were analyzed by electron and ion microprobes. Inclusion compositions are mainly in equilibrium with the host plagioclase and are more depleted in incompatible elements than the host rock. Simple crystal-liquid equilibrium calculations show that the melt inclusions could have been in equilibrium with depleted abyssal peridotite diopsides, whereas whole-rock basalt compositions generally could not have been. The melt inclusions are significantly more depleted than normal (N-type) mid-ocean-ridge basalt (MORB) and are consistent with being produced by 8%-16% incremental or open-system melting with 2% residual porosity in the peridotite source. These magmas were formed during pressure-release melting of the mantle over a range of depths between 30 and 15 km.

Chemical composition of basalts from ODP Leg 115

Leg 115 of the Ocean Drilling Program recovered basalts from four locations along the hotspot track that leads from the Deccan flood basalts in India to Reunion Island in the western Indian Ocean (Sites 706, 707, 713, and 715). The drilled basalts range in age from 35 Ma (Site 706) to 64 Ma (Site 707), and including the Deccan basalts (66 to 68 Ma), Mauritius Island (0.2 to 8 Ma), and Reunion Island (0 to 2 Ma), seven sites are provided for sampling the volcanic record of the 5000-km-long hotspot track. Chemical and age comparisons indicate that Site 707 lavas correlate with basalt units near the top of the Deccan flood basalt sequence. The lavas of Site 715 (55 to 60 Ma) are most similar to the islands of Mauritius and Reunion. Site 713 basalts (48 Ma) are similar to the earliest lavas of the Deccan province, and Site 706 basalts are intermediate in chemistry between those of central Indian spreading-ridge basalts and Reunion. Differences in lava compositions along the hotspot track can be related to variable mixing of plume and asthenospheric mantle, depending on the changing position of spreading-ridge segments and the hotspot during the opening of the Indian Ocean. Alternatively, time-dependent changes in the composition of hotspot melts may be due to a decrease in partial melting of a heterogeneous plume or to intrinsic changes in the composition of material supplied by the plume.