The formation of saturn's satellites and rings,as influenced by saturn's contraction history

We have used Pollack et al.'s 1976 calculations of the quasi-equilibrium contraction of Saturn to study the influence of the planet's early high luminosity on the formation of its satellites and rings. Assuming that the condensation of ices ceased at the same time within Jupiter's and Saturn's primordial nebulae, and using limits for the time of cessation derived for Jupiter's system by Pollack and Reynolds (1974) and Cameron and Pollack (1975), we arrive at the following tentative conclusions. Titan is the innermost satellite at whose position a methane-containing ice could condense, a result consistent with the presence of methane in this satellite's atmosphere. Water ice may have been able to condense at the position of all the satellites, a result consistent with the occurrence of low-density satellites close to Saturn. The systematic decrease in the mass of Saturn's regular satellites with decreasing distance from Saturn may have been caused partially by the larger time intervals for the closer satellites between the start of contraction and the first condensation of ices at their positions and between the start of contraction and the time at which Saturn's radius became less than a satellite's orbital radius. Ammonia ices, principally NH₄SH, were able to condense at the positions of all but the innermost satellites.Water ice may bave been able to condense in the region of the rings close to the end of the condensation period. We speculate that the rings are unique to Saturn because on the one hand, temperatures within Jupiter's Roche limit never became cool enough for ice particles to form before the end of the condensation period and on the other hand, ice particles formed only very early within Uranus' and Neptune's Roche limits, and were eliminated by gas drag effects that caused them to spiral into the planet before the gas of these planets' nebula was eliminated. Gas drag would also have eliminated any rocky particles initially present inside the Roche limit.We also derive an independent estimate of several million years for the time between the start of the quasi-equilibrium contraction of Saturn and the cessation of condensation. This estimate is based on the density and mass characteristics of Saturn's satellites. Using this value rather than the one found for Jupiter's satellites, we find that the above conclusions about the rings and the condensation of methane-and ammonia-containing ices remain valid.     

Cautions and Suggestions for Geochemical Sampling in Fractured Rock

Collecting water samples for geochemical analyses in open bedrock boreholes or in discrete intervals of boreholes intersected by multiple fractures is likely to yield ambiguous results for ground water chemistry because of the variability in the transmissivity, storativity, and hydraulic head of fractures intersecting the borehole. Interpreting chemical analyses of water samples collected in bedrock boreholes requires an understanding of the hydraulic conditions in the borehole under the ambient flow regime in the aquifer as well as during sampling. Pumping in open boreholes, regardless of the pumping rate and the location of the pump intake, first draws water from the borehole and then from fractures intersecting the borehole. The time at which the volumetric rate of water entering the borehole from fractures is approximately equal to the pumping rate can be identified by monitoring the logarithm of drawdown in the borehole as a function of the logarithm of time. Mixing of water entering the borehole from fractures with water in the borehole must be considered in estimating the time at which the pump discharge is representative of aquifer water. In boreholes intersected by multiple fractures, after the contribution from the borehole volume has diminished, the contribution of fractures to the pump discharge will be weighted according to their transmissivity, regardless of the location of the pump intake. This results in a flux-averaged concentration in the pump discharge that is biased by the chemical signature of those fractures with the highest transmissivity. Under conditions where the hydraulic head of fractures varies over the length of the borehole, open boreholes will be subject to ambient flow in the water column in the borehole. In some instances, the magnitude of the ambient flow may be similar to the designated pumping rate for collecting water samples for geochemical analyses. Under such conditions, the contributions to the pump discharge from individual fractures will be a function not only of the transmissivity of the fractures, but also of the distribution of hydraulic head in fractures intersecting the borehole. To reduce or eliminate the deleterious effects of conducting geochemical sampling in open boreholes, a straddle-packer apparatus that isolates a single fracture or a series of closely spaced fractures is recommended. It is also recommended that open boreholes be permanently outfitted with borehole packers or borehole liners in instances where maintaining the hydraulic and chemical stratification in the aquifer is of importance. In a field example, a comparison of results from sampling in an open borehole and in discrete intervals of the same borehole showed dramatic differences in the concentrations of chemical constituents in the water samples, even though chemical field parameters stabilized prior to both open borehole and discrete interval sampling.     

The influence of thermochemical convection on the fixity of mantle plumes

A general feature of both isochemical and thermochemical studies of mantle convection is that horizontal plume velocities tend to be smaller than typical convective velocities, however, it is not clear which system leads to a greater fixity of mantle plumes. We perform two- and three-dimensional numerical calculations and compare both thermochemical and isochemical cases with similar convective vigor to determine whether presence of a dense component in the mantle can lead to smaller ratios of horizontal plume velocity to surface velocity. We investigate different viscosity and density contrasts between chemical components in the thermochemical calculations, and we perform isochemical calculations with both free-slip and no-slip bottom boundary conditions. We then compare both visually and quantitatively the results of the thermochemical and isochemical calculations to determine which leads to greater plume fixity. We find that horizontal plume velocities for thermochemical calculations are similar to those from isochemical calculations with no-slip bottom boundary conditions. In addition, we find that plumes tend to be more fixed for isochemical cases with free-slip bottom boundary conditions for two-dimensional calculations, however, in three dimensions, we find that plume fixity is similar to that observed in thermochemical calculations.     

Preliminary investigation of the surficial sediments in the Cap-Breton Canyon (southwest France) and the surrounding continental shelf

A total of 120 grab samples of the surficial sediments in the Cap-Breton submarine canyon and surrounding continental shelf were collected and analyzed by grain-size sieving. A Q-mode Factor Analysis was made on the grain-size data in order to define the most meaningful facies types. Four distinct lithological facies were found to exist: silt and clay, very fine sand, fine sand, and coarse sand. Comparison with previous work and a ¹⁴C date on the silt and clay facies showed that the facies are not contemporaneous. The sands and coarse sands on the shelf were emplaced during the pre-Würm and Würm regressions, and later probably reworked during the Holocene (Flandrian) transgression. The silty clays found in the canyon and on the shelf to the south are younger and represent sediments brought in as suspended load by the Adour and other nearby rivers during the Holocene (Flandrian) transgression.     

Aqueous uptake of uranium onto pyrite surfaces; reactivity of fresh versus weathered material

In order to better understand the interaction between aqueous uranium and pyrite (FeS₂) the uptake of uranium onto the surfaces of both weathered and freshly generated pyrite surfaces was examined using batch sorption experiments. Analysis was performed using X-ray photoelectron spectroscopy (XPS). The results clearly indicate that freshly polished pyrite surfaces are efficient scavengers of uranium from solution, while weathered surfaces exhibit only limited uptake. Results also indicate partial reduction of uranium at the pyrite surfaces, with a heterogeneous distribution of U(IV) and U(VI) species.     

Timing and setting of skarn and iron oxide formation at the Smältarmossen calcic iron skarn deposit, Bergslagen, Sweden

Abundant iron oxide deposits including banded iron formations, apatite iron oxide ores, and enigmatic marble/skarn-hosted magnetite deposits occur in the Palaeoproterozoic Bergslagen region, southern Sweden. During the last 100 years, the latter deposit class has been interpreted as contact metasomatic skarn deposits, metamorphosed iron formations, or metamorphosed carbonate replacement deposits. Their origin is still incompletely understood. At the Smältarmossen mine, magnetite was mined from a ca. 50-m-thick calcic skarn zone at the contact between rhyolite and stratigraphically overlying limestone. A syn-volcanic dacite porphyry which intruded the footwall has numerous apophyses that extend into the mineralized zone. Whole-rock lithogeochemical and mineral chemical analyses combined with textural analysis suggests that the skarns formed by veining and replacement of the dacite porphyry and rhyolite. These rocks were added substantial Ca and Fe, minor Mg, Mn, and LREE, as well as trace Co, Sn, U, As, and Sr. In contrast, massive magnetite formed by pervasive replacement of limestone. Tectonic fabrics in magnetite and skarn are consistent with ore formation before or early during Svecokarelian ductile deformation. Whereas a syngenetic–exhalative model has previously been suggested, our results are more compatible with magnetite formation at ca. 1.89 Ga in a contact metasomatic skarn setting associated with the dacite porphyry.     

Restrictions on Deep Flow Across the Shelf-break and the Role of Submarine Canyons in Facilitating such Flow

The shelf-break acts as a separator between the coastal ocean and the open ocean. Circulation (particularly deep near-bottom flow) is restricted from crossing the bathymetry. Eddies become elongated in the region of the shelf-break restricting exchange. An estimate of the horizontal eddy diffusivity over the shelf-break of less than 10m²s^-1 is found from a numerical model. Various mechanisms are responsible for the weak cross-isobath flow that does occur. One is the increase of the Rossby number over small-scale topography such as submarine canyons. Along-shore flow (in the direction opposite to Kelvin wave propagation) generates upwelling through submarine canyons. A review of upwelling through submarine canyons is given. The deep cross-shelf flow generated by the canyons is shown to be as significant as the wind-driven upwelling in some regions. Examples for the reduction of flow across the shelf-break and for upwelling through canyons are taken from the West Coast of Vancouver Island.     

Zircon M127 – A Homogeneous Reference Material for SIMS U–Pb Geochronology Combined with Hafnium,Oxygen and,Potentially, Lithium Isotope Analysis

In this article, we document a detailed analytical characterisation of zircon M127, a homogeneous 12.7 carat gemstone from Ratnapura, Sri Lanka. Zircon M127 has TIMS‐determined mean U–Pb radiogenic isotopic ratios of 0.084743 ± 0.000027 for ²⁰⁶Pb/²³⁸U and 0.67676 ± 0.00023 for ²⁰⁷Pb/²³⁵U (weighted means, 2s uncertainties). Its ²⁰⁶Pb/²³⁸U age of 524.36 ± 0.16 Ma (95% confidence uncertainty) is concordant within the uncertainties of decay constants. The δ¹⁸O value (determined by laser fluorination) is 8.26 ± 0.06‰ VSMOW (2s), and the mean ¹⁷⁶Hf/¹⁷⁷Hf ratio (determined by solution ICP‐MS) is 0.282396 ± 0.000004 (2s). The SIMS‐determined δ⁷Li value is ?0.6 ± 0.9‰ (2s), with a mean mass fraction of 1.0 ± 0.1 μg g^?1 Li (2s). Zircon M127 contains ~ 923 μg g^?1 U. The moderate degree of radiation damage corresponds well with the time‐integrated self‐irradiation dose of 1.82 × 10¹⁸ alpha events per gram. This observation, and the (U–Th)/He age of 426 ± 7 Ma (2s), which is typical of unheated Sri Lankan zircon, enable us to exclude any thermal treatment. Zircon M127 is proposed as a reference material for the determination of zircon U–Pb ages by means of SIMS in combination with hafnium and stable isotope (oxygen and potentially also lithium) determination.