The NAIAD experiment for WIMP searches at Boulby mine and recent results

The NAIAD experiment (NaI Advanced Detector) for weakly interacting massive particle (WIMP) dark matter searches at Boulby mine (UK) is described. The detector consists of an array of encapsulated and unencapsulated NaI(Tl) crystals with high light yield. Six crystals are collecting data at present. Data accumulated by four of them (10.6 kg × year exposure) have been used to set upper limits on the WIMP–nucleon spin-independent and WIMP–proton spin-dependent cross-sections. Pulse shape analysis has been applied to discriminate between nuclear recoils, as may be caused by WIMP interactions, and electron recoils due to gamma background. Various calibrations of crystals are presented.     

Neutron background in large-scale xenon detectors for dark matter searches

Simulations of the neutron background for future large-scale particle dark matter detectors are presented. Neutrons were generated in rock and detector elements via spontaneous fission and (α,n) reactions, and by cosmic-ray muons. The simulation techniques and results are discussed in the context of the expected sensitivity of a generic liquid xenon dark matter detector. Methods of neutron background suppression are investigated. A sensitivity of 10⁻⁹–10⁻¹⁰ pb to WIMP-nucleon interactions can be achieved by a tonne-scale detector.     

The ClBrI composition of 3.23 Ga modified seawater: implications for the geological evolution of ocean halide chemistry

Fluid inclusion leachates obtained from vug and vein quartz samples from an Archean (3.23 Ga) Fe-oxide hydrothermal deposit in the west-central part of the Barberton greenstone belt, South Africa, were analyzed by ion chromatography for chloride, bromide, and iodide. The deposit, known as the ironstone pods, formed by seafloor hydrothermal activity and fluid discharge. Quartz is dominated by type I liquid-vapor, aqueous inclusions with a bimodal salinity distribution (0–0.25 M_Cl⁻ and 0.9–1.8 M_Cl⁻). Bulk analytical salinities range from 0.45 to 0.99 M_Cl⁻ represent averages of type I inclusions. Bulk fluid inclusion bromide and iodide concentrations are 1.44–3.32 mM and 0.01–0.12 mM, respectively. For comparison, modern seawater has halogen contents of 590 mM chloride, 0.9 mM bromide, and 0.5 μM total iodine. In the fluids from the ironstone pods, bromide and iodide are enriched relative to chloride, when compared with modern seawater.Approximate Br⁻Cl⁻ and I⁻Cl⁻ ratios of 3.2 Ga Barberton seawater are 2.5 × 10⁻³ and 40 × 10⁻⁶, respectively. Dispersion to higher values was caused principally by reaction with organic sediments whose trends are similar to those seen for modern vent fluids at unsedimented and sedimented ridges, relative to modern seawater. These halide ratios are greater than those of modern seawater, suggesting a change in the halide ratios of seawater over geological time. The analytical data are consistent with a model in which marine organic sedimentation has fractionated bromine and iodine out of seawater relative to chloride, thereby causing the halide ratios of seawater to decrease from high early and mid-Archean values towards their present day values.     

Genetic diversity and population structure of the franciscana dolphin,Pontoporia blainvillei,in Southern Buenos Aires,Argentina

The franciscana dolphin, Pontoporia blainvillei, is endemic to the coastal waters of the southwestern Atlantic Ocean and the most endangered dolphin in the area. Four Franciscana Management Areas (FMAs) are currently recognised; however, results of genetic studies suggest the requirement for additional FMAs and highlight the need for more detailed studies in the southern extreme of the species distribution. With this aim, we studied the genetic diversity and population structure of the species analysing an mtDNA control region fragment (434?bp) in 44 individuals collected in four sampling sites located in Southern Buenos Aires. Haplotype diversity (H?=?0.75?±?0.05) was mostly higher than the observed in endangered or near threatened odontocetes. Population structure analyses suggest that three different genetic populations should be recognised within FMA IV: Northern, Eastern and Southern Buenos Aires. Altogether, these results should be taken into account in future conservation plans for the species.     

A theoretical study of hydrothermal convection and the origin of the ophiolitic sulphide ore deposits of Cyprus

Hydrothermal circulation of seawater has been suggested as a mass transport mechanism for the formation of sulphide ore deposits in the ophiolitic rocks of Cyprus. Since ophiolitic sequences are generally regarded as fragments of oceanic crust and upper mantle, hydrothermal circulation of a form inferred from geological observations on Cyprus may be analogous to that thought to occur in oceanic crust at spreading ridges. The hypothesis that ore deposits were formed in ascending plumes of hot, buoyant fluid is examined by considering thermal convection in a permeable medium. To match the inferred pattern of circulation, finite amplitude convection in a cylindrical geometry is studied using finite difference approximations. These results combined with available geological and geochemical data are applied to understand better the physical controls on mineralisation.A simple model for the formation of the hydrothermal ore deposits of Cyprus is discussed. The model is semi-quantitatively reasonable in terms of vertical fluid flow rate, thermal structure, permeability and basal heat flow, and predicts volumes of maximum mineralisation similar to those observed. Three factors are identified which were important in confining mineralisation to a small volume immediately beneath the sea water/rock boundary: (1) hot fluid was confined to a narrow core zone of a rising plume, (2) the upward fluid flux was greatest in this same core zone, and (3) significant temperature decrease occurred within a thin surface boundary layer.     

Ophiolitic breccias associated with allochthonous oceanic crustal rocks in the East Ligurian Apennines,Italy — a comparison with observations from rifted oceanic ridges

In the East Ligurian segment of the North Apennines, eugeosynclinal sequences which contain ophiolitic rocks have been tectonically emplaced onto approximately coeval miogeosynclinal sediments. These allochthonous sequences represent the floor of a Mesozoic ocean which closed during the early Tertiary. The ophiolitic rocks consist of serpentinite, gabbro, pillowed and massive basalts, and breccias derived from these lithologies. They are overlain with depositional contacts by Upper Jurassic-Cretaceous pelagic cherts, limestones, and a shale/limestone sequence.The ophiolitic breccias attain thicknesses up to 100 m and strike lengths up to a few kilometres, and consist largely of unorganized accumulations of sand- to block-sized clasts. Compositions at specific horizons may range from oligomict breccias containing gabbro, basalt, or serpentinite fragments, to polymict breccias consisting of any mixture of these lithologies. Most of the breccias probably represent slow talus accumulations at the base of major submarine fault scarps which have exposed gabbro and serpentinite to submarine erosion. Direct exposure of gabbro and serpentinite on the ocean floor is also indicated by the occurrence of stratigraphically intact contacts between these lithologies and overlying pelagic sediments (generally cherts). The distribution and thickness of the breccias and volcanics, and the distribution of the gabbro and serpentinite, can vary greatly within distances of a few kilometres, thus producing complex heterogeneous sequences consisting of laterally impersistent lithological units.Recent observations and deep drilling of the Mid-Atlantic Ridge and other rifted ridges have revealed occurrences of significant thicknesses of basaltic, serpentinitic, and gabbroic breccias upon and within the volcanic layer of the oceanic crust, as well as the direct submarine exposure of plutonic rocks. It is therefore likely that the East Ligurian sequences represent parts of rifted ridge-generated crust. If so, then the complexity of the East Ligurian sequences suggests that the upper part of rifted ridge-generated crust may in places possess large variations in its stratigraphy over small (<10 km²) areas.Smooth, non-rifted (fast-spreading) ridges, which have very reduced topography and lack major fault scarps, should form ophiolitic complexes deficient in breccias containing fragments of plutonic igneous rocks. Most large ophiolitic complexes do not contain plutonic rock-bearing breccias, and were therefore probably formed at smooth ridges. The apparently preferential preservation of this type of ophiolitic complex, as opposed to the rifted ridge-type crust in East Liguria, may be related to the less pervasive and less intense fracturing of smooth ridges. This resulted in greater “cohesion” and lateral continuity of smooth ridge-generated crust during later tectonic emplacement into allochthonous positions in orogenic belts.     

Hydration, 18O enrichment and oxidation during ocean floor hydrothermal metamorphism of ophiolitic metabasic rocks from E. Liguria,Italy

During ocean-floor hydrothermal metamorphism of a 225 m thick allochthonous Jurassic sequence of ophiolitic pillow lavas and underlying material in E. Liguria, Italy, the rocks were hydrated, enriched in ¹⁸O and oxidised. H₂O⁺ contents increased from ~0.3 to 3.8 wt.%, δ¹⁸O values increased from ~ +6‰ to values as high as +13.2‰, and ($\text{Fe}{}_2\text{O}{}_3\text{FeO}$1) increased from 0.18 to ratios as high as 1.0. Both δ¹⁸O values and oxidation ratios decrease in the original direction of increasing depth. Pillow margins are consistently more enriched in ¹⁸O and are more oxidised than cores.These observations are qualitatively interpreted in terms of a non-isothermal, heterogeneous reaction model of interaction of basalt with oxygen-bearing sea water during flow through the packed bed of pillows. Fluid flow approximated undirectional downward motion, and occurred in the recharge part of a cycle of single pass convection. Mass transfer through the pile was by flow (infiltration metasomatism), whereas intra-pillow mass transfer was diffusional. Oxygen isotope exchange and oxidation did not occur under conditions of perfect incremental equilibrium. An integrated bulk volumetric water/rock ratio of ~2 × 10³:1 is estimated from the oxidation profile.     

Water quality monitoring: a combined approach to investigate gradients of change in the Great Barrier Reef, Australia

For the managers of a region as large as the Great Barrier Reef, it is a challenge to develop a cost effective monitoring program, with appropriate temporal and spatial resolution to detect changes in water quality. The current study compares water quality data (phytoplankton abundance and water clarity) from remote sensing with field sampling (continuous underway profiles of water quality and fixed site sampling) at different spatial scales in the Great Barrier Reef north of Mackay (20 degrees S). Five transects (20-30 km long) were conducted from clean oceanic water to the turbid waters adjacent to the mainland. The different data sources demonstrated high correlations when compared on a similar spatial scale (18 fixed sites). However, each data source also contributed unique information that could not be obtained by the other techniques. A combination of remote sensing, underway sampling and fixed stations will deliver the best spatial and temporal monitoring of water quality in the Great Barrier Reef.