Vesicles,water, and sulfur in Reykjanes Ridge basalts

Dredge hauls of fresh submarine basalt collected from the axis of the Reykjanes Ridge (Mid-Atlantic Ridge) south of Iceland were taken aboard R/ V TRIDENT in 1967 and 1971. The samples show systematic changes as the water depth of collection (and eruption) decreases: radially elongate vesicles and concentric zones of vesicles appear at about 700 m depth and are conspicuous to shallow water; the smoothed volume percent of vesicles increases from 5% at 1000 m, 10% at 700 m, to 16% at 500 m, and the scatter in degree of vesicularity increases in shallower water; specific gravity decreases from 2.7±0.1 at 1000 m to 2.3±0.3 at 100 m. Bulk sulfur content for the outer 2 cm averages 843 ppm up to a depth of 200 m, then drops off rapidly in shallower water owing to degassing. Sulfur content below 200 m is independent of depth (or geographic position), and the melt is apparently saturated with sulfur, but the excess cannot escape the lava unless another vehicle carries it out. Only shallower than 200 m, where intense vesiculation of other gases occurs can excess sulfur be lost from the lava erupting on the sea floor. H₂O^+110° averages about 0.35 percent and H₂O^+150° about 0.25 percent, and both apparently decrease in water shallower than 200 m as a result of degassing. H₂O⁺ (below 200 m) decreases with distance from Iceland or increasing depth, presumably as a result of either adsorption of water on the surface of shallower, more vesicular rocks; or more likely due to the presence of the Iceland hot mantle plume supplying undifferentiated primordial material, relative to lavas of the Reykjanes Ridge supplied from the low velocity layer already depleted in volatiles and large lithophile elements. The H₂O^+110°/S ratio of lava erupting below 200 m water depth ranges from 3 to 5 which is comparable to reliable gas analyses from oceanic basaltic volcanoes. Publication authorized by the Director, U.S. Geological Survey.     

Datation 87Rb-87Sr du massif granitique du Mendic et des porphyroïdes a l'est de la Montagne Noire —un exemple de relation entre pluton et volcans

In the East of Montagne Noire (South Massif Central), the granitic Mendic massif and the porphyroïds have been dated as 510±20 m.y. with a corresponding initial ratio of 0.706±0.001. Within the limits of the analytical errors, we cannot distinguish between two separate isochrons. The Mendic-porphyroïds represents a volcano-plutonic association. The age of 510 m.y. is somewhat younger than the one determined on the orthogneisses of the Central zone (530 m.y.). It can be connected to a post-orogenic magmatism of the cadomian orogenesis. At last the Sr of the minerals of the Mendic massif has been partially remobilised during the hercynian orogeny (285 m. y.).     

Composition of metal in type III carbonaceous chondrites and its relevance to the source-assignment of lunar metal

The metal in seven Type III carbonaceous chondrites has been measured for concentrations of Ni, Co and Cr. Cobalt in kamacite is 3.2 to 5.5 times greater than in taenite on composite grains containing both phases. No correlation was found between the metal compositions and sub-type classification. Ni and Co contents of kamacite from several of the Type III's studied fall outside of the range for these elements in bulk meteoritic metal and are relevant to the assignment of a meteoritic vs a non-meteoritic origin for lunar metal particles in the fines and breccias.     

塔里木板块西南缘晚侏罗世古地磁研究

塔里木板块西南缘托云、奥依塔格晚侏罗世古地磁研究,获得了托云地区特征磁化方向:Ds=1.5°,Is=50.8°;古地磁极:λ=81.0°,ψ=247.0°,古纬度31.5°N。奥依塔格地区特征磁化方向:Ds=118.4°,Is=55.9°;古地磁极:λ=4.9°,ψ=120.6°,古纬度=36.4°N。从古地磁结果认为:①托云和奥依塔格相对托云地区产生顺时针旋转;②这两个地区古纬度为31°～36°左右,与塔里木平均古纬度30°N是基本一致的,但与现在纬度差达8°～10°左右,造成纬度差主要原因是构造运动造成的;③塔里木晚侏罗世古地磁亦与华北、华南基本一致,说明晚侏罗世塔里木、华北、华南三板块已联合成一个整体,拼合成统一的中国大陆,其位于热带—亚热带古气候环境。     

Comparison of the anisotropic behaviour of undeformed sandstones under dry and saturated conditions

This article presents a systematic analysis of the anisotropic behaviours of the Bentheim and Rothbach sandstones using ultrasonic P-wave velocity, electrical conductivity and magnetic susceptibility measurements. For each sandstone, the data were obtained from three core samples drilled perpendicularly to each other and tested in dry- and water-saturated conditions. For acoustic and magnetic investigations, the same statistical analysis was applied in order to present the data on comparable stereoplots. Surprisingly, the Bentheim sandstone which appeared homogeneous at macroscopic scale showed a stronger elastic and electrical anisotropy than the Rothbach sandstone in which cross-laminations were clearly identified, as confirmed by a sedimentary magnetic fabric. A discussion on the velocity contrasts between dry and saturated samples led us to consider two different origins for the observed anisotropies. First, by comparing electrical and acoustic properties in the Bentheim sandstone, we conclude that the nature of the anisotropic behaviour is linked to the anisotropy of pore shape: the inclusion model developed by Kachanov (Kachanov, M., 1993. Elastic solids with many cracks and related problems. Advances in Applied Mechanics, vol. 30. Academic Press, Boston, MA, pp. 259–445) accounts for our observations if one considers that the pore space is made of parallel flat pores with moderate pore aspect ratio. Second, acoustic, electrical and magnetic properties indicate that the observed anisotropy in the Rothbach sandstone can be attributed to the matrix, and more specifically to cementation: we modified the Dvorkin and Nur (Geophysics 61 (5) (1996) 1363) model of cemented granular media by introducing a spatially variable contact length, and the model suggests that a very small variability of cemented contact length is enough to account for the observed P-wave velocity anisotropy. We emphasise the fact that combining several kinds of measurements is of great help in capturing the nature of the anisotropic behaviour of porous rocks.     

A chlorine-36 study of regional groundwater flow and vertical transport in southern Nevada

Chlorine-36 data for groundwater from the Death Valley regional flow system is interpreted in the context of existing conceptual models for regional groundwater flow in southern Nevada. Chlorine-36 end member compositions are defined for both recharge and chemically evolved groundwater components. The geochemical evolution of ³⁶Cl is strongly controlled by water-rock interaction with Paleozoic carbonate rocks that comprise the regional aquifer system, resulting in chemically evolved groundwater that is characteristically low in ³⁶Cl/Cl and high in Cl. Groundwater from alluvial and volcanic aquifers that overlie the regional carbonate aquifer are generally characterized by high ³⁶Cl/Cl and low Cl signatures, and are chemically distinct from water in the regional carbonate aquifer. This difference provides a means of examining vertical transport and groundwater mixing processes. In combination with other geochemical and hydrogeologic data, the end members defined here provide constraints on aquifer residence times and mixing ratios.     

Calculation of Well Index for Nonconventional Wells on Arbitrary Grids

Wells are seldom modeled explicitly in large scale finite difference reservoir simulations. Instead, the well is coupled to the reservoir through the use of a well index, which relates wellbore flow rate and pressure to grid block quantities. The use of an accurate well index is essential for the detailed modeling of nonconventional wells; i.e., wells with an arbitrary trajectory or multiple branches. The determination of a well index for such problems is complicated, particularly when the simulation grid is irregular or unstructured. In this work, a general framework for the calculation of accurate well indices for general nonconventional wells on arbitrary grids is presented and applied. The method entails the use of an accurate semianalytical well model based on Green's functions as a reference single phase flow solution. This result is coupled with a finite difference calculation to provide an accurate well index for each grid block containing a well segment. The method is demonstrated on a number of homogeneous example cases involving deviated, horizontal and multilateral wells oriented skew to the grid. Both Cartesian and globally unstructured multiblock grids are considered. In all these cases, the method is shown to provide results that are considerably more accurate compared to results using standard procedures. The method is also applied to heterogeneous problems involving horizontal wells, where it is shown to be capable of approximating the effects of subgrid heterogeneity in coarse finite difference models.     

Imagerie sismique de la zone de collision hercynienne dans le Sud-Est du Massif armoricain (projet Armor 2/programme Géofrance 3D)

The structure of the Hercynian collision zone in the southeast of the Armorican Massif is illustrated by a 70-km long deep seismic profile acquired in September 2000. The profile images a previously unknown south-dipping thrust that brought the Champtoceaux Domain on top of the Central Armorican Domain during Carboniferous times. Dextral strike-slip motions along the South Armorican Shear Zone, which is downward cut by the thrust zone, are partly coeval with northward thrusting. A major discontinuity, hidden by the thrust front, is also imaged in the lower crust between the Champtoceaux area and the Central Armorican Domain. These new data lead to a structural and kinematic re-interpretation of this part of the Hercynian collision zone. To cite this article: A. Bitri et al., C. R. Geoscience 335 (2003).