Petrological and trace element geochemical features of the Okete Volcanics,western North Island,New Zealand

The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al₂O₃, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.     

ToF-SIMS and SEM study on the preferential oxidation of chalcopyrite

Chalcopyrite is known to be slow reacting mineral in hydrometallurgical systems and is considered one of the most inert sulphide minerals with respect to leaching. Such character of chalcopyrite seems to be linked to a formation of a passive layer on its surface. This work reports that freshly fractured chalcopyrite surfaces exhibit highly selective reactivity depending on the exposed fracture planes. ToF-SIMS was used to qualitatively characterize various fracture planes in freshly fractured chalcopyrite particles, prior to and after hydrometallurgical treatment. It was found that, prior to treatment, certain areas exhibited pronounced contamination from atmospheric hydrocarbons; whereas, others were highly unreactive and remarkably free from adventitious hydrocarbon contamination. The positive ion spectra recorded from these areas were found to be dominated by peaks from Fe- and Cu-elements and related compounds. The negative ion spectra for the reactive areas on the other hand showed a high content of oxidized (sulphur) species.The differences between the areas of low and high reactivity, as detected after leaching, were more subtle than prior to leaching; whereas, SEM analysis showed clear evidence for selective attack of ferric sulphate to specific planes. Furthermore, it was shown that, when chalcopyrite is in intimate contact with pyrite, it experiences an enhanced oxidation compared to when there is no electric contact with pyrite.Attempts were made to explain the preferential oxidation observed based on the different chemistry of the fracture surfaces.     

Variability in in situ shear strength of gassy muds

Torque measurements are made by divers with a vane apparatus to a sediment depth of 136 cm in the high-porosity, gassy sediments of Eckernförde Bay. Corrected shear strength values calculated from torque measurements are quite variable in the Eckernförde Bay sediments, varying from less than 0.5 kPa in the top 10 cm of sediment to 4–5 kPa at 136 cm sediment depth. Variability increases markedly below 60 cm sediment depth, probably because of the presence of methane gas bubbles within the sediment fabric.     

Crustal structure of the deepwater west Niger Delta passive margin from the interpretation of seismic reflection data

The interpretation of 2D and 3D seismic reflection data complemented with gravity data allows the crustal architecture of the deepwater west Niger Delta passive margin to be defined. The data show that the area is underlain by oceanic crust that is characterised by a thickness of 5–7 km and by internal reflectivity consisting of both dipping and sub-horizontal reflectors. Some of the dipping reflections can be traced up to the top of the basement where they offset it across a series of minor to major thrust faults. Other internal reflections are attributed to extensional shear zones and possibly due to intrusions in the lower crust. The Moho can be correlated as a discrete reflection over >70% of the study area. It is generally smooth, but localised relief of up to 1 km is observed. The southeastern part of the study area is dominated by a zone of SW–NE striking basement thrusts.     

Evaluation of sediment heterogeneity using microresistivity imaging and X-radiography

Microresistivity imaging and X-radiography of 3-cm-thick sediment core slabs collected by divers from Eckernförde Bay, Germany, indicate subtle delineations and features created by hydrodynamic and biological processes within the top 20 cm of the sediment. Variations in the images of both techniques are controlled by the physical properties of the sediment. The X-radiographs record attenuation by solids throughout the 3-cm thickness of the sediment slab, while microresistivity images depend on pore water in terms of amount, salinity, and distribution. The microresistivity method is shown to detect layered structures and the presence of shells within the highporosity sediment.     

Small-scale fluctuations in acoustic and physical properties in surficial carbonate sediments and their relationship to bioturbation

 Measurements of physical and acoustic properties within the top 40 cm of sediment indicate that there are three classes of parameter variability relevant to acoustic bottom scattering at the Marquesas Keys and Dry Tortugas sites. The three classes of variability, spatially and temporally different in terms of scale, are generated by biological and physical processes acting on sediments. Interplay of bioturbation, trawling and storm events create fine laminations of sand–silt–clay and high gravel-size mollusk shell content at the Marquesas Keys site; the predominance of biological processes creates strong surficial gradients and lateral variability in sediment properties at the Dry Tortugas site.     

High-frequency acoustic observations of benthic spatial and temporal variability

A bottom-mounted sonar operating at 40 kHz has been used to measure the variation of bottom acoustic scattering over extended time intervals at two shallow sites as part of the Coastal Benthic Boundary Layer Special Research Project. The acoustic data were analyzed using a correlation method that measures the spatial and temporal dependence of benthic change. The rate of decorrelation was two orders of magnitude more rapid at a sandy site near Panama City, Florida, USA, than at a silty site in Eckernförde Bay, Germany, and both sites were characterized by hot spots or localized regions of activity.