Geological setting of hydrothermal activity at 12°50′N on the East Pacific Rise: A submersible study

A detailed submersible investigation of a 20-km segment of the East Pacific Rise near 12°50′N between the Orozco and Clipperton fracture zones has resulted in the localization of 24 active hydrothermal vent fields and over 80 sites of sulfide accumulations. The active vents range from low-temperature vents characterized by exotic benthic communities to high-temperature “black smokers” and the deposition of polymetallic sulfides. The study is based upon a combination of fine scale topography obtained using the SEABEAM sonar system on N/O “Jean Charcot”, camera lowerings along the axis using the RAIE vehicle, and 32 dives by the submersible “Cyana” operating from N/O “Le Suroit”. The observations made between the Orozco and Clipperton fracture zones show topographic highs situated along the strike of the accreting plate segment separated by a small ridge offset at 11°49′N. This offset divides this portion of the ridge into two separate spreading segments each of which has a primary topographic high along strike. Secondary highs are associated with each segment of the ridge separated by either small offsets (or relay zones) or in some cases, zones where spreading centers overlap. Dives made on the tops of both primary highs (12°50′N and 11°30′N) confirm the presence inferred from previous surface work of high-temperature vent fields while one reconnaissance dive (14°20′N) near the Orozco fracture zone/ridge axis intersection reveals the absence of any hydrothermal activity in the present or recent past. The vast majority of vent fields investigated were found at the topographic high near 12°50′N, are associated with the most recent period of volcanism, and are confined to lava ponds situated within the axial graben.     

Instrumental temperature records at two sites in Central Canada: 1768 to 1910

As a result of affiliation between the Hudson's Bay Company and the Royal Society a relatively large number of instrumental temperature records are available from York Factory and Churchill Factory on the southwest of Hudson Bay beginning in 1768. The nature of these records, details of the instruments and information about the observers are presented. The major difficulty with the records is that the number of observations and the time of observation varied considerably. Adjustment factors were calculated for all of the combinations using a modern record maintained at the Churchill airport. By combining the Hudson's Bay Company record with data recorded by members of the Royal Canadian Mounted Police after 1852, and up to 1910, a long and relatively continuous record of daily and monthly average temperatures has been created for Central Canada.     

Intrinsic oxygen fugacities of diogenites and mesosiderite clasts

Oxygen fugacities of diogenite and mesosiderite clast material were measured with the double ZrO₂ cell technique between 800° and 1150°C. The samples were taken from large clasts in the diogenites Johnstown (En₇₃) and Tatahouine (En₇₅), and the mesosiderites Estherville (En₈₁), West Point (Fo₈₈) and Emery (En₆₈). Fugacity values for all except Emery plot near the wüstite-iron buffer curve and are interpreted as indicating similar source regions and environments of crystallization for the two suites. Emery orthopyroxene records a lower fugacity, close to the fayalite-quartz-iron buffer curve, probably as a result of equilibration with the mesosiderite matrix assemblage. The similarity of redox conditions experienced by mesosiderite orthopyroxenite and diogenites is not sufficient to require a single parent body and, if the common achondrites were derived from Vesta, mesosiderites probably came from a different body.     

Hydrogen speciation in synthetic quartz

The dominant hydrogen impurity in synthetic quartz is molecular H₂O. H-OH groups also occur, but there is no direct evidence for the hydrolysis of Si-O-Si bonds to yield Si-OH HO-Si groups. Molecular H₂O concentrations in the synthetic quartz crystals studied range from less than 10 to 3,300 ppm (H/Si), and decrease smoothly by up to an order of magnitude with distance away from the seed. OH^? concentrations range from 96 to 715 ppm, and rise smoothly with distance away from the seed by up to a factor of three. The observed OH^? is probably all associated with cationic impurities, as in natural quartz. Molecular H₂O is the dominant initial hydrogen impurity in weak quartz. The hydrolytic weakening of quartz may be caused by the transformation H₂O + Si-O-Si → 2SiOH, but this may be a transitory change with the SiOH groups recombining to form H₂O, and the average SiOH concentration remaining very low. Synthetic quartz is strengthened when the H₂O is accumulated into fluid inclusions and cannot react with the quartz framework.     

Calculation of velocity skewness in real and artificial plant canopies

Wind velocities within a plant canopy are much more strongly skewed than those of the air flow above. We have examined the governing Eulerian equations for the velocity products u _i, u _j u_k using data from a wind tunnel study with an artificial canopy consisting of an array of 5 cm lengths of monofilament fishing line, and from measurements in corn (Zea mays L).Simple parameterizations for pressure-velocity correlations, and for the quadruple velocity products allowed reasonably accurate calculations of the third moments using measured profiles of the mean velocity, variance and covariance fields. Comparisons of individual terms in the rate equations for ovu _i, u _j u _krevealed that diffusion (from above) and mean shear were most important in creating large skewness in the canopy. A drag term also contributed but was of lesser importance. These terms were balanced by return-to-isotropy and a turbulence interaction term. A quasi-Gaussian approximation considerably underestimated the magnitude of the fourth moments within the canopy.     

Isotopic study of the Manaslu granite (Himalaya,Nepal): inferences on the age and source of Himalayan leucogranites

A detailed isotopic study of the Manaslu leucogranite was carried out. A U-Pb age of 25 Ma and a whole rock Rb-Sr age isochron of 18 Ma were obtained, suggesting that the magmatic activity lasted at least 7 Ma. Initial Sr isotopic ratios are very high (0.740 to 0.760) and initial Nd isotopic ratios are low ( _Nd ⁱⁿ : –13 to –16), and they show the existence of large isotopic variations even at the metre scale. These are not the result of perturbations by fluids but rather they reflect the initial isotopic heterogeneity of the source material which has not been obliterated by magmatic processes (e.g. fusion, mixing by convection). These results also support the crustal origin of this leucogranite. The Tibetan slab paragneisses, whose Sr and Nd isotopic ratios are very similar to those of the granite at an age of 20 Ma, are the most probable parental material. Nd model ages for both the leucogranite and the gneisses are in the range 1.5–2 Ga. A model of formation of the Manaslu granite by coalescence of different batches of magma is in agreement with the present data.     

Numerical study of wind energy characteristics over heterogeneous terrain — Central Israel case study

A numerical mesoscale meteorological model has been applied over the heterogeneous terrain of central Israel in order to study wind energy characteristics of three typical synoptic situations. The supportive nature of this method for observationally oriented wind energy studies has been emphasized. Mesoscale forcing effects on the availability of wind energy and on the exponent, p, in the vertical wind power law are evaluated.     

Traveling screens as sampling gear for vertical distribution studies

A sampling technique has been developed for increasing the information gathered during routine monitoring of impingement at water intake structures. Samples from impingement catches on traveling screens were taken from the sluiceway of the Brunswick Steam Electric Plant near Southport, North Carolina during the screen wash process so as to divide the catch into vertical catch components. Each component represented 1.2 m of the water column. Results showed differences in day and night vertical distributions of impinged organisms according to the spatial preferences of each species. Impingement during daytime was lower than during night. Impingement of surface-oriented species during daytime occurred at mid-depth, midwater species near the bottom, and bottom species were seldom impinged. During the night surface-oriented species were impinged at the surface, midwater species at mid-depth, and bottom species near the bottom. Residue (animals and debris) which remained within the screen wash system from collection of the previous sample, and those organisms which became impinged on the screens during retrieval of the sample, were used to calculate the rate of “continuous impingement” on the traveling screens. These rates were compared to the impingement catch in order to determine biases in the vertical catch components. Comparison of these rates indicated that impingement estimates determined by routine monitoring methods may under-estimate true impingement rates for certain species. We have concluded that this sampling technique for monitoring impingement at intake structures will increase knowledge of the local biologic system while minimizing the cost of obtaining the information. The technique will also aid in pinpointing specific impingement problems which may be corrected through modifications to the design of the intake structure.     

Les Quartzites à Silicates Calciques et Scheelite: Préconcentrations Familières ou Pièges pour un Tungstène Etranger lié à L'Hydrothermalisme Périgranitique? Exemple du Nord-Est Transmontain (Portugal)

Three groups of Sn-W veins occur in the contact aureole of composite granitic plutons South of Bragança (Trás-os-Montes, Portugal). Thin layers of calcic quartzites with or without scheelite are interbedded in the Ordovician and Silurian metamorphic country-rocks. Could they represent synsedimentary preconcentrations and a likely source for the W-deposits? The geologic features and the trace-elements distribution (W, Sn, Be, B, Li, F, Rb) in the metasedimentary and granitic rocks support the following conclusions: 1) the scheelite-bearing calcic quartzites are tactites, that occur only in the contact aureole of the granites; 2) the dynamo-metamorphic country-rocks are weakly enriched in W and impoverished in Sn compared to the "clarke" values; 3) the trace-elements concentration is higher in the contact aureole (X 2) than outwards; 4) the quartzites are scheelite-bearing only in the vicinity of the wolframite veins. Thus, scheelite appears related to mineralizing hydrothermal processes. Through their chemical composition and their mechanical properties, the calcic quartzites appear as traps for a tungsten mainly foreign to the metamorphic rocks close to the granites. Thus, the source of W lies deeper in lower crust levels or in the upper-mantle.