Solubility of niobium in the system CaCO3–Ca(OH)2–NaNbO3 at 0.1 GPa pressure

Experimental data are presented for the solubility of NaNbO₃ in the ternary system CaCO₃-Ca(OH)₂-NaNbO₃ (or calcite-portlandite-lueshite) over the temperature range 500-950 °C at 0.1 GPa pressure. Near-liquidus phase relationships are given for the pseudo-binary join ([CaCO₃]₄₅[Ca(OH)₂]₅₅)_100-x-(NaNbO₃)_x; (0 <x <70 wt%). These data show the presence of a large field of calcite plus liquid and the absence of NaNbO₃ as a primary liquidus phase until the melts contain greater than 60 wt% NaNbO₃. The primary crystallization fields of calcite and NaNbO₃ are separated by a steep thermal valley located at about 55 wt% NaNbO₃. Quenched liquids contain calcite, portlandite, sodium carbonates and perovskite-structured calcium niobates. The maximum solubility of Nb₂O₅ in this system is estimated to be on the order of 48 wt% at 650 °C at 0.1 GPa pressure. The experiments show that perovskite-structured compounds rather than pyrochlore crystallize from these fluorine-free, water-rich melts.     

Re–Os isotopic constraints on magma mixing in the Peridotite Zone of the Stillwater Complex, Montana, USA

Chromite separates within the Peridotite Zone of the 2,700 Ma-old Stillwater Complex are characterized by low ¹⁸⁷Re/¹⁸⁸Os (0.009 to 1.74) and relatively high Os concentrations (8.74 to 78.2 ppb). Their calculated initial Os isotopic compositions likely reflect the compositions of the magmas from which they crystallized. The chromites show variable initial Os isotopic compositions (%_Os of +2.0 to +16.4) over the vertical extent of the Peridotite Zone, implicating at least two sources of Os. Both the range of %_Os and values of %_Os decrease upsection. These variations in %_Os were caused by mixing of variable proportions of two magmas having different Os isotopic compositions. One of the magmatic components was a more primitive magma with a nearly chondritic Os isotopic composition. The other magma had a radiogenic Os isotopic composition as a result of assimilation of crust, perhaps of sedimentary rocks beneath the Stillwater Complex. The gradual decrease in the initial %_Os values of the chromite layers with increasing stratigraphic height implies a decreasing relative contribution from the contaminated magma throughout the growth of the Peridotite Zone. Small variations in %_Os between different chromite occurrences within the H multicyclic unit reflects the petrologic requirement that chromite layers crystallize from slightly different proportions of the magmas, compared to chromite from olivine- and orthopyroxene-rich layers.     

Atmospheric chemistry of the monoterpene reaction products nopinone,camphenilone, and 4-acetyl-1-methylcyclohexene

Rate constants for the gas-phase reactions of OH radicals with nopinone (6,6-dimethylbicyclo[3.1.1]heptan-2-one) and camphenilone (3,3-dimethylbicyclo[2.2.1]heptan-2-one) and for the reactions of 4-acetyl-1-methylcyclohexene with OH and NO₃ radicals and O₃ have been measured at 296±2 K. The rate constants (cm³ molecule^–1 s^–1 units) obtained were, for reaction with the OH radical: nopinone, (1.43±0.37)×10^–11; camphenilone, (5.15±1.44)×10^–12; and 4-acetyl-1-methylcyclohexene, (1.29±0.33)×10^–10; for reaction with the NO₃ radical: 4-acetyl-1-methylcyclohexene, (1.05±0.38)×10^–11; and for reaction with O₃: 4-acetyl-1-methylcyclohexene, (1.50±0.53)×10^–16. These data are used to calculate the tropospheric lifetimes of these monoterpene atmospheric reaction products.     

Fan and playa areas in southern California and adjacent parts of Nevada

In bolsons in the desert regions of southern California and adjacent parts of Nevada, the area underlain by alluvial fan gravels and playa sediments is generally ?1·2 times the area being eroded to produce those sediments. In certain larger basins in the vicinity of Death Valley, however, the depositional area is only about half the size of the erosional area. This reflects the more active tectonic environment in these bolsons. Of the areas underlain by recent sediments in these bolsons, playas make up 2–6 per cent. Smaller playas are found in the Mojave region, and seem to be associated with sedimentary terranes. Conversely, igneous terranes support larger playas. Larger deviations of playa area from these averages are attributable to incorrect identification of bolson boundaries. Fine sediment is either able to pass through the bolson to the next down stream, or is being collected from areas upstream that were not considered to be part of the system. For example, the playa in Death Valley is unusually large. This is in part because the Death Valley playa has been deformed tectonically so parts of it are now eroding, and in part because the outlet of Lake Tecopa was downcut in the geologically recent past, so sediment once trapped there now reaches Death Valley. The size of the playa in Death Valley is still adjusting to these changes.     

THREE-DIMENSIONAL MESOSCALE MODELING OF METEOROLOGICAL FIELDS OVER ZHUJIANG(PEARL)RIVER DELTA

The CSU-RAMS-2A was used to simulate the meteorological fields over the Zhujiang River Delta in South China.Initialized from a horizontally homogeneous atmosphere,real topography and inhomogeneous surface boundary condi-tions,the model was run with thermal and terrain forcing.The modeling results of winter and summer cases are com-pared with those observed.The similarity of the predicted distributions of winds,temperatures and humidities to the ob-served patterns permits us to conclude that the mesoscale distribution of meteorological elements for the two study datesis the result of the thermal and dynamical forcing by the underlying surface and topography.     

The significance of global snow and ice cover for global change studies

The spatial extent and volume of global snow and ice cover at present and during glacial conditions are reviewed for each of the principal components (snow cover, land ice, sea ice and permafrost). The state of global monitoring of snow and ice conditions is shown to be adequate for some variables, but unsatisfactory for others. Recent trends are reported and expected changes projected for increasing concentrations of greenhouse gases are examined.     

Climatological and hydrographic influences on nearshore food webs off the southeastern United States: bacterioplankton dynamics

Bacterioplankton productivity, numbers, and cell specific activity were studied in nearshore waters of the southeastern U.S. continental shelf during seasons of maximum freshwater discharge. In April 1984, coastal waters were stratified from normal spring discharge and typical northeastward wind stress. In April 1985, shelf waters were vertically homogeneous due to below normal runoff and southwestward wind stress. In 1984, nearshore bacterial productivity ranged from 7.0 to 14.7 × 10⁶ cells l⁻¹ h⁻¹ and midshelf rates were 40–50% less. In 1985, nearshore productivity ranged from 0.9 to 2.4 × 10⁶ cells 1⁻¹ h⁻¹, and productivity was extremely patchy over the entire shelf. The cell-specific activity (thymidine incorporation per cell) suggests that although productivity was high in 1984, only a fraction of the bacterioplankton was actively growing or incorporating thymidine (0.9–2.9 × 10⁻²¹ mol cell⁻¹ h⁻¹). In 1985, a higher percentage of cells appeared to be active and incorporating thymidine (5–13 × 10⁻²¹mol cell⁻¹h⁻¹) even though productivity was low. Hydrographic conditions along the southeastern coastline may have had a significant impact on the overall community structure and carbon flow through the microbial food web. When coastal waters were stratified in 1984, bacterial biomass was a significant percentage (35–320%) of the phytoplankton biomass. During vertically homogeneous conditions of 1985, bacterial production and biomass were a small percentage (2–13%) of the phytoplankton production and biomass across the shelf. The interannual variation in the microbial food web was attributed to the interannual variability of the southeastern U.S. hydrology due to changes in freshwater discharge and wind direction and intensity. The ecological implications of these results extend to the potential impact of seasonal microbial food webs on nearshore allochothonous and autochothonous organics before removal from the southeastern U.S. coastline.     

Sapphirine and spinel phase relationships in the system FeO-MgO-Al2O3-SiO2-TiO2-O2 in the presence of quartz and hypersthene

Sapphirine and spinel can accommodate significant ferric iron and therefore the mineral equilibria involving these phases must be sensitive to a(O₂). In this paper we examine the theoretical phase relationships involving sapphirine and spinel in addition to sillimanite, garnet, cordierite, rutile, hematite-ilmenite solid solution (henceforth ilmenite), and magnetite-ulvospinel solid solution (henceforth magnetite), in the presence of quartz and hypersthene in the system FeO-MgO-Al₂O₃-SiO₂-TiO₂-O₂ (FMASTO), with particular reference to the topological inversion in P-T postulated by Hensen (Hensen 1986). Documented natural associations suggest that the appropriate topology for assemblages involving magnetite and ilmenite is Hensen's higher a(O₂) one, while, in contrast, the topology for assemblages involving ilmenite and rutile is the lower a(O₂) one. The exact configuration of the inversion between these two topologies remains uncertain because of uncertainties in the ferric/ferrous iron partitioning between sapphirine and spinel-cordierite at high temperatures. By comparison with experimental data and natural occurences, the sillimanite-sapphirine-cordierite-garnet-hypersthene-quartz assemblage is in equilibrium at about 1000°–1020° C and 7–8 kbars, while sapphirine-cordierite-spinel-garnet-hypersthene-quartz occurs at temperatures in excess of those attainable during crustal metamorphism, for ilmenite-rutile buffered assemblages. This implies that sapphirine-rutil-ehypersthene-quartz assemblages, as found in the Napier Complex, Antarctica, can only occur at > 1000° C. Also, spinel-rutile-hypersthene-quartz assemblages should not be found in rocks because temperatures in excess of 1100° C are expected to be involved in their formation. The temperatures of formation of spinel-sillimanite-sapphirine-garnethypersthene-quartz, sapphirine-spinel-cordierite-sillimanite-hypersthene-quartz, and sillimanite-spinel-cordieritegarnet-hypersthene-quartz in assemblages buffered by magnetite and ilmenite are less well constrained, but are likely to be in the range 900°–1000° C. These conclusions apply to rocks with compositions close to FMASTO; the perturbing effects of substantial concentrations of additional components, in particular Ca, mainly in garnet, and Zn and Cr, mainly in spinel, may invalidate these conclusions.     

The gas-phase reaction of ClONO2 with hydrogen chloride

The gas-phase reaction of ClONO₂ with HCl was investigated using two large-volume environmental chambers with analysis by in situ long pathlength Fourier transform infrared absorption spectroscopy. In these chambers the reaction was observed to proceed, at least in part, by heterogenous routes, and an upper limit to the rate constant for the homogeneous gas-phase reaction of geneous routes, and an upper limit to the rate constant for the homogeneous gas-phase reaction of $$k\left( {{\text{ClONO}}_{\text{2}} + {\text{HCl}}} \right) < 1.5 \times 10^{ - 19} {\text{ cm}}^{\text{3}} {\text{ molecule}}^{{\text{ - 1}}} {\text{ s}}^{{\text{ - 1}}}$$ Was derived at 298±2K. Assuming that this room-temperature upper limit to the rate constant is applicable to stratospheric temperatures, this homogeneous gas-phase reaction can be estimated to be of negligible importance as a ClONO₂ loss process in the stratosphere.