Major and trace elements and interelement ratios at DSDP Leg 81 Holes

Fifty-two samples of basalt from the four holes drilled on the Leg 81 transect across the Rockall margin were analyzed by X-ray fluorescence for Rb, Sr, Y, Zr, and Nb. On the basis of these results 13 samples were chosen for major and supplementary trace-element analysis. The results show no progressive change in the character of the volcanism, from Hole 555 in the continental domain through Holes 552 and 553A in the dipping reflector sequence to Hole 554A on the outer high. Two distinct magma types are present, apparently reflecting heterogeneity of the underlying mantle, but both types are present in both Holes 553A and 555, while Hole 552 and Hole 554 are each composed of a single type. Both magma types have a clear ocean-floor basalt signature when examined by discrimination diagrams, as does the basalt from Deep Sea Drilling Project Site 112, which formed at the same time as the Leg 81 basalts slightly farther south along the spreading center. In contrast, the basalts of East Greenland, formed at the same time, are more enriched in incompatible elements and have a within-plate geochemical signature, as is found in some basalts of Iceland today. Clearly the present distinction in geochemistry between the basalts of Iceland and those erupting well south on the Reykjanes Ridge was already established when continental splitting took place.

Age models, ostracods and trace elements of five gravity cores from the NW Black Sea

New Mg/Ca, Sr/Ca, and published stable oxygen isotope and 87Sr/86Sr data obtained on ostracods from gravity cores located on the northwestern Black Sea slope were used to infer changes in the Black Sea hydrology and water chemistry for the period between 30 to 8 ka B.P. (calibrated radiocarbon years). The period prior to 16.5 ka B.P. was characterized by stable conditions in all records until a distinct drop in d18O values combined with a sharp increase in 87Sr/86Sr occurred between 16.5 and 14.8 ka B.P. This event is attributed to an increased runoff from the northern drainage area of the Black Sea between Heinrich Event 1 and the onset of the Bølling warm period. While the Mg/Ca and Sr/Ca records remained rather unaffected by this inflow; they show an abrupt rise with the onset of the Bølling/Allerød warm period. This rise was caused by calcite precipitation in the surface water, which led to a sudden increase of the Sr/Ca and Mg/Ca ratios of the Black Sea water. The stable oxygen isotopes also start to increase around 15 ka B.P., although in a more gradual manner, due to isotopically enriched meteoric precipitation. While Sr/Ca remains constant during the following interval of the Younger Dryas cold period, a decrease in the Mg/Ca ratio implies that the intermediate water masses of the Black Sea temporarily cooled by 1-2°C during the Younger Dryas. The 87Sr/86Sr values drop after the cessation of the water inflow at 15 ka B.P. to a lower level until the Younger Dryas, where they reach values similar to those observed during the Last Glacial Maximum. This might point to a potential outflow to the Mediterranean Sea via the Sea of Marmara during this period. The inflow of Mediterranean water started around 9.3 ka B.P., which is clearly detectable in the abruptly increasing Mg/Ca, Sr/Ca, and 87Sr/86Sr values. The accompanying increase in the d18O record is less pronounced and would fit to an inflow lasting ~100 a.

Rare earth elements of ROV sample GeoB12338-2 from the Makran accretionary complex

Authigenic carbonates forming at an active methane-seep on the Makran accretionary prism mainly consist of aragonite in the form of microcrystalline, cryptocrystalline, and botryoidal phases. The d13Ccarbonate values are very negative (-49.0 to -44.0 per mill V-PDB), agreeing with microbial methane as dominant carbon source. The d18Ocarbonate values are exclusively positive (+ 3.0 to + 4.5 per mill V-PDB) and indicate precipitation in equilibrium with seawater at bottom water temperatures. The content of rare earth elements and yttrium (REE + Y) determined by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and solution ICP-MS varies for each aragonite variety, with early microcrystalline aragonite yielding the highest, cryptocrystalline aragonite intermediate, and later botryoidal aragonite the lowest REE + Y concentrations. Shale-normalised REE + Y patterns of different types of authigenic carbonate reflect distinct pore fluid compositions during precipitation: Microcrystalline aragonite shows high contents of middle rare earth elements (MREE), reflecting REE patterns ascribed to anoxic pore water. Cryptocrystalline aragonite exhibits a seawater-like REE + Y pattern at elevated total REE + Y concentrations, indicating higher concentrations of REEs in pore waters, which were influenced by seawater. Botryoidal aragonite is characterised by seawater-like REE + Y patterns at initial growth stages followed by an increase of light rare earth elements (LREE) with advancing crystal growth, reflecting changing pore fluid composition during precipitation of this cement. Conventional sample preparation involving micro-drilling of carbonate phases and subsequent solution ICP-MS does not allow to recognise such subtle changes in the REE + Y composition of individual carbonate phases. To be able to reconstruct the evolution of pore water composition during early diagenesis, an analytical approach is required that allows to track the changing elemental composition in a paragenetic sequence as well as in individual phases. High-resolution analysis of seep carbonates from the Makran accretionary prism by LA-ICP-MS reveals that pore fluid composition not only evolved in the course of the formation of different phases, but also changed during the precipitation of individual phases.

Analysis of elements at DSDP Holes 70-506 and 70-509B

Sediments in the area of the Galapagos hydrothermal mounds are divided into two major categories. The first group, pelagic sediments, are nannofossil oozes with varying amounts of siliceous microfossils. The second group are hydrothermal sediments consisting of manganese-oxide crust fragments and green nontronitic clay granules. Hydrothermal sediments occur only in the upper half to two-thirds of the cores and are interbedded and mixed with pelagic sediments. Petrologic evidence indicates that hydrothermal nontronite forms as both a primary precipitate and as a replacement mineral of pre-existing pelagic sediment and hydrothermal manganese-oxide crust fragments. In addition, physical evidence supports chemical equations indicating that the pelagic sediments are being dissolved by hydrothermal solutions. The formation of hydrothermal nontronite is not merely confined to the surface of mounds, but also occurs at depth within their immediate area; hydrothermal nontronite is very likely forming today. Geologically speaking, the mounds and their hydrothermal sediments form almost instantaneously. The Galapagos mounds area is a unique one in the ocean basins, where pelagic sediments can be diagenetically transformed, dissolved, and replaced, possibly within a matter of years.