Why meter-wide dikes at oceanic spreading centers?

Numerical models show that maximum dike width at oceanic spreading centers should scale with axial lithospheric thickness if the pre-diking horizontal stress is close to the Andersonian normal faulting stress and the stress is fully released in one dike intrusion. Dikes at slow-spreading ridges could be over 5 m wide and maximum dike width should decrease with increasing plate spreading rate. However, data from ophiolites and tectonic windows into recently active spreading ridges show that mean dike width ranges from 0.5 m to 1.5 m, and does not clearly correlate with plate spreading rate. Dike width is reduced if either the pre-diking horizontal stress difference is lower than the faulting stress or the stress is not fully released by a dike. Partial stress release during a dike intrusion is the more plausible explanation, and is also consistent with the fact that dikes intrude in episodes at Iceland and Afar. Partial stress release can result from limited magma supply when a crustal magma chamber acts as a closed source during dike intrusions. Limited magma supply sets the upper limit on the width of dikes, and multiple dike intrusions in an episode may be required to fully release the axial lithospheric tectonic stress. The observation of dikes that are wider than a few meters (such as the recent event in Afar) indicates that large tectonic stress and large magma supply sometimes exist.     

Sulphur isotope geochemistry of carbonatites

Sulphur isotopic data for sulphides and barite from several carbonatites (Mountain Pass, Oka, Magnet Cove, Bearpaw Mountains, Phalabora) show that individual carbonatites have different mean sulphide or barite isotopic compositions which deviate from the meteoritic mean $\text{δ}{}^{34}\text{S(0‰)}$.Classification of carbonatites in terms of T,?O₂ and pH during formation of the sulphur-bearing assemblages indicates that with decreasing T and increasing relative ?O₂ the mean δ³⁴S sulphide becomes increasing negative relative to the mean magma δ³⁴S. Only barite-free high temperature carbonatites (Phalabora) in which the mean δ^34S sulphide approaches the mean magmaδ³⁴S as a consequence of the paucity of oxidized anionic sulphur species in the magma can be used to directly estimate the mean isotopic composition of the source material.Barites from the Mountain Pass carbonatite show an increase in δ³⁴S with sequence of intrusion of the carbonatite units; dolomitic carbonatite (mean δ³⁴S, + 5.4‰), calcitic carbonatite (+ 4.8%.), silicified carbonatite (+ 6.9‰), tabular carbonatite dikes (+ 8.7‰), mineralized shear zones (+ 9.5‰). Within each of these units a spread of 6.8%. is evident. Isotopic trends in this low temperature (300°C) carbonatite are evaluated by treating the system as a hydrothermal fluid. The observed isotopic variations can be explained by removal of large amounts of sulphur from a fluid whose mean δ³⁴S is 0 to + 1‰     

Mixed messages in iron oxide–copper–gold systems of the Cloncurry district,Australia: insights from PIXE analysis of halogens and copper in fluid inclusions

Proterozoic rocks of the Cloncurry district in NW Queensland, Australia, are host to giant (tens to hundreds of square kilometers) hydrothermal systems that include (1) barren regional sodic–calcic alteration, (2) granite-hosted hydrothermal complexes with magmatic–hydrothermal transition features, and (3) iron oxide–copper–gold (IOCG) deposits. Fluid inclusion microthermometry and proton-induced X-ray emission (PIXE) show that IOCG deposits and the granite-hosted hydrothermal complexes contain abundant high temperature, ultrasaline, complex multisolid (type 1) inclusions that are less common in the regional sodic–calcic alteration. The latter is characterized by lower salinity three-phase halite-bearing (type 2) and two-phase (type 3) aqueous inclusions. Copper contents of the type 1 inclusions (>300 ppm) is higher than in type 2 and 3 inclusions (<300 ppm), and the highest copper concentrations (>1,000 ppm) are found both in the granite-hosted systems and in inclusions with Br/Cl ratios that are consistent with a magmatic source. The Br/Cl ratios of the inclusions with lower Cu contents are consistent with an evaporite-related origin. Wide ranges in salinity and homogenization temperatures for fluid inclusions in IOCG deposits and evidence for multiple fluid sources, as suggested by halogen ratios, indicate fluid mixing as an important process in IOCG genesis. The data support both leaching of Cu by voluminous nonmagmatic fluids from crustal rocks, as well as the direct exsolution of Cu-rich fluids from magmas. However, larger IOCG deposits may form from magmatic-derived fluids based on their higher Cu content.     

Field evidence of hydraulic connections between bedrock aquifers and overlying granular aquifers: examples from the Grenville Province of the Canadian Shield

Field evidence of hydraulic connections between a bedrock aquifer and an overlying granular aquifer in the Canadian Shield (Grenville Province) is presented. This issue is rarely considered and investigated despite its important hydraulic and chemical consequences and its widespread occurrence worldwide. The methodology employed is based on complementary field tests conducted at specific experimental sites instrumented both in the rock and in the overlying deposits. One of the bedrock sites revealed a natural hydraulic connection with the overlying granular aquifer caused by the weathered surface of the uppermost bedrock. Another site revealed an artificial hydraulic connection between the bedrock and the granular aquifer created by an improperly sealed casing. A regional study showed that hydraulic connections yield an erroneous interpretation of the true hydraulic properties of the tested aquifer. The detection of hydraulic connections is therefore essential to properly define well-capture areas and contamination conditions. It is recommended to practitioners that pumping tests be performed as well as hydrochemical comparisons of each existing aquifer unit. Falling-head permeability tests are also helpful in verifying the quality of the seal at the bedrock–casing contact. More effective procedural controls and better well-construction practices are necessary to reduce the risks of cross-contamination induced by defective seals.     

Liquefied vs stratified sediment mobilization processes: Insight from the South of the Barbados accretionary prism

This paper discusses the nature and origin of subsurface sediment mobilization processes in deep marine clay-rich environments. In the studied area of the southern Barbados accretionary prism, new geophysical acquisitions have emphasized the spectacular widespread development of mud volcanoes that are well-developed along ramp anticlines and along sigmoidal rises with trends that are oblique to the axes of the main folds of the accretionary wedge. On some active mud volcanoes, heat-flow measurements show high positive anomalies related to high fluxes of mud transfer. The mobilized solid fraction expelled by the mud volcanoes does not originate from a unique source bed but from various formations pierced by the mud conduits and is driven by the water phase. The area studied also exhibits trends of structures corresponding to sub-circular massive local uplifts of deformed but well-preserved stratified sediments (turbidites and hemipelagics). No piercing shale diapirs have been encountered in this area. Some of these local uplift structures are complicated by the development of collapses, calderas, and superimposed mud volcanoes. Mud volcanism corresponds fundamentally to fluid displacement (water and gas), whereas massive sedimentary uplift corresponds to large vertical displacements of stratified solid levels but for which the deep cause could be partly the intrusion of mud plugs. Both are dynamic phenomena controlled by the development of overpressure at depth, contributing to sedimentary mobilizations.     

Viral-like particles (VLPS) in the alga,Aureococcus anophagefferens (pelagophyceae), during 1999–2000 brown tide blooms in Little Egg Harbor, New Jersey

We assessed the presence and quantified the intracellular virus-like particles (VLPs) in natural populations ofAureococcus anophagefferens during the 1999–2000 brown tide blooms that occurred in New Jersey coastal waters. From displayed a wide range of ultrastructural changes from apparently healthy cells to those showing late stages of production of VLPs. VLP-infected cells usually had an electron dense plasma membrane and lacked the typical exocellular polysaccharide layer (EPS). The VLPs were similar in size (c. 140 nm) and morphology to those initially reported in natural populations ofA. anophagefferens and to brown tide viruses (BtVs) which were previously isolated and inoculated into laboratory cultures ofA. anophagefferens. VLP-infectedA. anophagefferens were found consistently throughout the brown tide blooms in both years in Little Egg Harbor. Percentages of VLP-infected cells were 8.1% at the beginning of a bloom, which decreased to less than 2% at the height of the blooms during both years, and increased at the end of the 2000 bloom to 2.5%. While these percentages appear low, the estimated VLP infection rate ofA. anophagefferens cells, which ranged from 0.83%–50% of the standing population, is comparable to other studies.     

A cryo-SEM study of aggregate and floc structure changes during clay settling and raking processes

The destabilization of kaolinite suspension by anionic flocculant addition occurs in three zones; free settling, hindered settling and compression which usually includes a final bed raking process in mineral processing practice. This paper reports changes in the kaolinite aggregate and floc structures in the different settling and raking zones by cryo-vitrification/cryo-SEM techniques with image analysis combining micro- and macro-flocs. Cryo-SEM images indicate that, even during free settling, fine clay particles are bridged predominantly in edge–edge (E–E) with some face–face (F–F) configurations forming single, small flocs and some chain structures. When these small flocs and chains settle into the hindered settling zone, the collision between flocs and chains results in “honeycomb” network structures formed with lateral chain-like extension. The settled bed consists of these honeycomb structures with both inter-aggregate and intra-aggregate trapped water and has relatively low bed density (e.g. < 12 wt.% for a 2 wt.% slurry). The effect of the raking process in dramatically improving thickener underflow solids has been extensively studied but the structural changes in flocs and aggregates in this process are less well defined. Raking the compression zone for 1 h at 3 rpm can release some of the trapped water in the “honeycomb” structure and the bed density for 2 wt.% slurry improves dramatically to more than 36 wt.%. Cryo-SEM illustrates the extensive restructuring of flocs from predominantly E–E to predominantly F–F in many areas. The STructural IMage ANalysis (STIMAN) software is used to combine a series of images at magnifications from 1000× to 8000×, including both macro- and micro-flocs. This structural analysis comparing the un-raked and raked bed samples gives increases in total particle area of 30% and in relative particle area of 6%. The relatively low energy rake action of the shear stress results in the disruption of the E–E chains and the honeycomb structure, partly releasing the trapped water and inducing some E–E to F–F aggregate restructuring are clearly illustrated in these results.     

Variations in grain‐size and sorting on two kaikoura beaches

Two exposed, high‐energy beaches on the Kaikoura coast of New Zealand are composed of sand and gravel derived from a greywacke terrain. Both beaches can be classified as mixed beaches although the sediment varies from dominantly gravel at the ends of the beach to dominantly sand at the centre, through transition zones in which sand and gravel are mixed. Sixty‐four surface samples were analysed for grain size; two sediment parameters, mean grain size (M_z) and sorting (σ_I), were calculated.

A striking feature of the cumulative frequency curves is that both unimodai and bimodal distributions include median sizes over the whole range of sampled material, even though bimodal samples display two strong modes in the sand and gravel grades. The general deficiency lof sediment dn the very coarse sand and granule classes (0 to — 2 _F ) noted by numerous authors in many parts of the world is apparent in the poorly‐sorted bimodal samples. However, the best‐sorted samples also occur in these two classes.

Mean grain size of samples ranges from medium sand (1.820) to medium pebbles (—4.7 _F ), and sorting ranges from very well sorted (0.250) to very poorly sorted (2.69 _F ). Mean erain size on the northern beach is significantly greater than on the southern beach, but values of sorting are comparable. The greater mean size on one beach compared with the other is thought to be a function of the grade of material supplied by local rivers; the similarity in sorting presumably reflects the similarity of the processes acting on the two beaches.

Mixed sand‐shingle beaches are relatively rare on a world scale but common in New Zealand. Sediment distributions along the Kaikoura beaches do not reveal a regular decrease in size away from the rivers which supply material to shore at present. Instead, the beaches are differentiated into a number of sediment zones composed of either sand, or mixed sand‐gravel, or gravel. On each beach a gravel zone is located furthest from the river outlets. Sorting generally improves toward the Kaikoura Peninsula. Explanations for these trends are not given. Variations in size and sorting across the two beaches do not show a well developed zonation because of the high level of wave energy which continually mixes the material across the beach.     

Product formation from the gas-phase reactions of OH radicals and O3 with a series of monoterpenes

The formation yields of nine carbonyl products are reported from the gas-phase OH radical-initiated reactions (in the presence of NO _x ) and the O₃ reactions with seven monoterpenes. The products were identified using GC/MS and GC-FTIR and quantified by GC-FID analyses of samples collected on Tenax solid adsorbent cartridges. The identities of products from camphene, limonene and -pinene were confirmed by comparison with authentic standards. Sufficient quantities of products from the 3-carene, limonene, -pinene, sabinene and terpinolene reactions were isolated to allow structural confirmation by proton NMR spectroscopy. The measured total carbonyl formation yields ranged from non-detectable for the OH radical reaction with camphene and the O₃ reactions with 3-carene and limonene to 0.5 for the OH radical reaction with limonene and the O₃ reaction with sabinene.