Geochemistry on sediment profile PS69/335

During RV Polarstern cruise ANT-XXIII/4 in 2006, a gravity core (PS69/335-2) and a giant box core (PS69/335-1) were retrieved from Maxwell Bay off King George Island (KGI). Comprehensive geochemical (bulk parameters, quantitative XRF, Inductively Coupled Plasma Mass Spectrometry) and radiometric dating analyses (14C, 210Pb) were performed on both cores. A comparison with geochemical data from local bedrock demonstrates a mostly detrital origin for the sediments, but also points to an overprint from changing bioproductivity in the overlying water column in addition to early diagenetic processes. Furthermore, ten tephra layers that were most probably derived from volcanic activity on Deception Island were identified. Variations in the vertical distribution of selected elements in Maxwell Bay sediments further indicate a shift in source rock provenance as a result of changing glacier extents during the past c. 1750 years that may be linked to the Little Ice Age and the Medieval Warm Period. Whereas no evidence for a significant increase in chemical weathering rates was found, 210Pb data revealed that mass accumulation rates in Maxwell Bay have almost tripled since the 1940s (0.66 g cm-2 yr-1 in AD 2006), which is probably linked to rapid glacier retreat in this region due to recent warming.

Geochemistry and fluxes at DSDP Hole 83-504B

This chapter documents the chemical changes produced by hydrothermal alteration of basalts drilled on Leg 83, in Hole 504B. It interprets these chemical changes in terms of mineralogical changes and alteration processes and discusses implications for geochemical cycling. Alteration of Leg 83 basalts is characterized by nonequilibrium and is heterogeneous on a scale of centimeters to tens or hundreds of meters. The basalts exhibit trends toward losses of SiO2, CaO, TiO2; decreases in density; gains of MnO, Na2O, CO2, H2O+ , S; slight gains of MgO; increased oxidation of Fe; and variable changes in A12O3. Some mobility of rare earth elements (REE) also occurred, especially the light REE and Eu. The basalts have lost Ca in excess of Mg + Na gains. Variations in chemical trends are due to differing water/rock ratios, substrate control of secondary mineralogy, and superimposition of greenschist and zeolite facies mineralogies. Zeolitization resulted in uptake of Ca and H2O and losses of Si, Al, and Na. These effects are different from the Na uptake observed in other altered basalts from the seafloor attributed to the zeolite facies and are probably due to higher temperatures of alteration of Leg 83 basalts. Basalts from the transition zone are enriched in Mn, S, and CO2 relative to the pillow and dike sections and contain a metal-sulfide-rich stockwork zone, suggesting that they once were located within or near a hydrothermal upflow zone. Samples from the bottom of the dike section are extensively fractured and recrystallized indicating that alteration was significantly affected by local variations in permeability.

Chemistry of secondary minerals from the deep sheeted dike complex of ODP Holes 137-504B and 140-504B

Dolerites sampled from the lower sheeted dikes from Hole 504B during Ocean Drilling Program Legs 137 and 140, between 1562.4 and 2000.4 mbsf, were examined to document the mineralogy, petrography, and mineral parageneses associated with secondary alteration, to constrain the thermal history and composition of hydrothermal fluids. The main methods used were mineral chemical analyses by electron microprobe, X-ray diffraction, and cathodoluminescence microscopy. Temperatures of alteration were estimated on the basis of single and/or coexisting mineral chemistry. Permeability is important in controlling the type and extent of alteration in the studied dike section. At the meter-scale, intervals of weakly altered dolerites containing fresh olivine are interpreted as having experienced restricted exposure to hydrothermal fluids. At the centimeter- or millimeter-scale, alteration patches and extensively altered halos adjacent to veins reflect the permeability related to intergranular primary porosity and cracks. Most of the sheeted dike alteration in this case resulted from non-focused, pervasive fluid-rock interaction. This study confirms and extends the previous model for hydrothermal alteration at Hole 504B: hydrothermal alteration at the ridge axis followed by seawater recharge and off-axis alteration. The major new discoveries, all related to higher temperatures of alteration, are: (1) the presence of hydrothermal plagioclase (An80-95), (2) the presence of deuteric and/or hydrothermal diopside, and (3) the general increasing proportion of amphiboles, and particularly magnesio-hornblende with depth. We propose that the dolerites at Hole 504B were altered in five stages. Stage 1 occurred at high temperatures (less than 500° to 700°C) and involved late-magmatic formation of Na- and Ti-rich diopside, the hydrothermal formation of Na, Ti-poor diopside and the hydrothermal formation of an assemblage of An-rich plagioclase + hornblende. Stage 2 occurred at lower temperatures (250°-320°C) and is characterized by the appearance of actinolite, chlorite, chlorite-smectite, and/or talc (in low permeability zones) and albite. During Stage 3, quartz and epidote precipitated from evolved hydrothermal fluids at temperatures between 310° and 320°C. Anhydrite appeared during Stage 4 and likely precipitated directly from heated seawater. Stage 5 occurred off-axis at low temperatures (250°C) with laumontite and prehnite from evolved fluids.

Abundance and composition of Fe-Mn nodules and host sediments in the Pacific Ocean

Data on internal structure, distribution, and chemical composition of iron-manganese nodules from the central part of the South Pacific are reported. Nodules with relatively high contents of Fe, Ti, Co, and Pb were found. Formation of these nodules in pelagic regions of the ocean with low sedimentation rates is tentatively ascribed by the authors to leaching of Fe, Mn, and some minor elements during submarine lava outflow and to geochemical mobility of these elements. The role of diagenetic re-distribution of ore elements during formation of nodules is also discussed.

(Table 1, page 220), Spot chemical analyses of a section of nodule 5133.7

Microscopic and electron probe examination of some manganese nodules show that they consist of segregations of manganese-iron oxides in an interstitial material almost free of manganese but rich in iron and silicates. The segregations are widely spaced in the volcanic cores of the nodules but become more abundant towards their outer crusts where they form the centres of linked polygons of interstitial materials. Most of the minor elements are concentrated in the segregations compared to the interstitial materials. It is suggested that the structures observed result partly from solution and reprecipitation of elements in the original volcanic cores of the nodules and partly from the replacement and coating of these cores by manganese-iron oxides precipitated from sea water.