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941.
Multi-anvil and piston cylinder experiments were performed to determine the effect of both pressure and temperature on the diffusivities of several siderophile elements in a Fe90Ni10 core-analog alloy. Activation energies were calculated to be between 244 and 257 kJ/mol for Re, Pd, and Au at 10 GPa, and 264 kJ/mol for P at 1 GPa. It was found that pressure has a marked negative effect on the diffusivities of Au, Re, and Pd, and activation volumes for these elements were calculated to be between 3 and 6 cm3/mol at 10 GPa. The effect of both temperature and pressure on P diffusion is noticeably less, and the absolute diffusivity of phosphorus is consistently higher than that of the other elements. It is inferred that the reason for this difference is because P is diffusing via an interstitial mechanism as opposed to Re, Pd, and Au which occupy regular lattice sites in the crystal. The effect of pressure and temperature together with depth in the Earth suggests that these elements may continue to exhibit different behavior at more extreme conditions. The significance of these new results lies in the ability to place constraints on many time-dependent processes that pertain to the formation and evolution of planetary cores, as well as the formation and cooling histories of other metal-rich bodies in the solar system, such as meteorites. 相似文献
942.
Ana M. Dreher Roberto P. Xavier Bruce E. Taylor Sérgio L. Martini 《Mineralium Deposita》2008,43(2):161-184
The Igarapé Bahia Cu–Au deposit in the Carajás Province, Brazil, is hosted by steeply dipping metavolcano-sedimentary rocks
of the Igarapé Bahia Group. This group consists of a low greenschist grade unit of the Archean (∼2,750 Ma) Itacaiúnas Supergroup,
in which other important Cu–Au and iron ore deposits of the Carajás region are also hosted. The orebody at Igarapé Bahia is
a fragmental rock unit situated between chloritized basalt, with associated hyaloclastite, banded iron formation (BIF), and
chert in the footwall and mainly coarse- to fine-grained turbidites in the hanging wall. The fragmental rock unit is a nearly
concordant, 2 km long and 30–250 m thick orebody made up of heterolithic, usually matrix-supported rocks composed mainly of
coarse basalt, BIF, and chert clasts derived from the footwall unit. Mineralization is confined to the fine-grained matrix
and comprises disseminated to massive chalcopyrite accompanied by magnetite, gold, U- and light rare earth element (LREE)-minerals,
and minor other sulfides like bornite, molybdenite, cobaltite, digenite, and pyrite. Gangue minerals include siderite, chlorite,
amphibole, tourmaline, quartz, stilpnomelane, epidote, and apatite. A less important mineralization style at Igarapé Bahia
is represented by late quartz–chalcopyrite–calcite veins that crosscut all rocks in the deposit area. Fluid inclusions trapped
in a quartz cavity in the ore unit indicate that saline aqueous fluids (5 to 45 wt% NaCl + CaCl2 equiv), together with carbonic (CO2 ± CH4) and low-salinity aqueous carbonic (6 wt% NaCl equiv) fluids, were involved in the mineralization process. Carbonates from
the fragmental layer have δ13C values from −6.7 to −13.4 per mil that indicate their origin from organic and possibly also from magmatic carbon. The δ34S values for chalcopyrite range from −1.1 to 5.6 per mil with an outlier at −10.8 per mil, implying that most sulfur is magmatic
or leached from magmatic rocks, whereas a limited contribution of reduced and oxydized sulfur is also evident. Oxygen isotopic
ratios in magnetite, quartz, and siderite yield calculated temperatures of ∼400°C and δ18O-enriched compositions (5 to 16.5 per mil) for the ore-forming fluids that suggest a magmatic input and/or an interaction
with 18O-rich, probably sedimentary rocks. The late veins of the Igarapé Bahia deposit area were formed from saline aqueous fluids
(2 to 60 wt% NaCl + CaCl2 equiv) with δ18Ofluid compositions around 0 per mil that indicate contribution from meteoric fluids. With respect to geological features, Igarapé
Bahia bears similarity with syngenetic, volcanic-hosted massive sulfide (VHMS)-type deposits, as indicated by the volcano-sedimentary
geological context, stratabound character, and association with submarine volcanic flows, hyaloclastite, and exhalative beds
such as BIF and chert. On the other hand, the highly saline ore fluids and the mineral assemblage, dominated by magnetite
and chalcopyrite, with associated gold, U- and LREE-minerals and scarce pyrite, indicate that Igarapé Bahia belongs to the
Fe oxide Cu–Au (IOCG) group of deposits. The available geochronologic data used to attest syngenetic or epigenetic origins
for the mineralization are either imprecise or may not represent the main mineralization episode but a later, superimposed
event. The C, S, and O isotopic results obtained in this study do not clearly discriminate between fluid sources. However,
recent B isotope data obtained on tourmaline from the matrix of the fragmental rock ore unit (Xavier, Wiedenbeck, Dreher,
Rhede, Monteiro, Araújo, Chemical and boron isotopic composition of tourmaline from Archean and Paleoproterozoic Cu–Au deposits in the Carajás Mineral
Province, 1° Simpósio Brasileiro de Metalogenia, Gramado, Brazil, extended abstracts, CD-ROM, 2005) provide strong evidence of the
involvement of a marine evaporitic source in the hydrothermal system of Igarapé Bahia. Evaporite-derived fluids may explain
the high salinities and the low reduced sulfur mineral paragenesis observed in the deposit. Evaporite-derived fluids also
exclude a significant participation of magmatic or mantle-derived fluids, reinforcing the role of nonmagmatic brines in the
genesis of Igarapé Bahia. Considering this aspect and the geological features, the possibility that the deposit was generated
by a hydrothermal submarine system whose elevated salinity was acquired by leaching of ancient evaporite beds should be evaluated. 相似文献
943.
Chongbin Zhao Bruce E. Hobbs Alison Ord Peter Hornby Hans Mühlhaus Shenglin Peng 《Computers and Geotechnics》2008,35(3):357-371
In this paper, theoretical and numerical methods are used to investigate pore-fluid flow focused temperature distribution patterns around geological faults and cracks of any length-scales in hydrothermal systems. If the far field inflow is uniform and the long axis of an elliptical fault of any length-scale is parallel to the far field inflow direction, a complete set of analytical solutions has been presented for the pore-fluid velocity, stream function and excess pore-fluid pressure around the elliptical fault embedded in fluid-saturated porous media. Because the analytical solutions are explicitly expressed in the conventional Cartesian coordinate system, not only can they be used to gain a theoretical insight into the pore-fluid flow patterns around geological faults and large cracks, but also they can be used as valuable benchmark solutions for validating any numerical methods. After a finite element computational model is validated by comparing the numerical solutions with the present analytical solutions, it is used to investigate pore-fluid flow focused heat transfer around geological faults in hydrothermal systems. Some interesting conclusions in relation to the effects of geological faults on pore-fluid flow focused heat transfer have been made through both the theoretical and the numerical analyses. 相似文献
944.
Hydrogeological and hydrochemical characterization of the Voltaian Basin: the Afram Plains area,Ghana 总被引:1,自引:0,他引:1
In the Afram Plains area, groundwater is the main source of water supply for most uses. The area is underlain by aquifers
of the southern Voltaian sedimentary basin, which are predominantly sandstones, mudstones, conglomerates and shale. Ordinary
least squares regression analysis using 41 well-test data from aquifers in the Afram Plains portion of the Voltaian system
reveals that transmissivity, T, exists in a non-linear relationship with specific capacity, Sc. The analysis reveals that T = 0.769Sc1.075 with R
2 = 0.83 for aquifers in the area. The mudstone/conglomerate aquifer in the area appears to be the most variable in terms of
both specific capacity and transmissivity with transmissivity ranging from 0.18 to 197.7 m2/day and 0.5 and 148.5 m3/day/m, respectively. Horizontal fractures and joints resulting from secondary fracturing appear to be the main determinants
of both transmissivity and specific capacity in the area. PHREEQC modeling and mineral stability diagrams indicate that groundwater
quality in the Afram Plains area is controlled by the incongruent weathering of silicate minerals in the aquifers. These processes
concentrate calcium, sodium, magnesium, potassium, bicarbonate ion and quartz, leading to calcite, dolomite and aragonite
supersaturation at most locations. Hierarchical cluster analysis performed on the raw chemical data reveals two main water
types or facies: the calcium–sodium–chloride–bicarbonate facies, and the magnesium–potassium–sulfate–nitrate facies for the
southern and northern sections of the Afram Plains area, respectively. 相似文献
945.
Groundwater availability in the shallow aquifers of the southern voltaian system: a simulation and chemical analysis 总被引:1,自引:0,他引:1
A steady state groundwater flow simulation model was developed using available well data and general hydrogeological and geological
information, for the Afram Plains area, Ghana. The hydrochemistry of groundwater from wells in the area was then evaluated
to determine its suitability for irrigation and domestic uses. The assessment of the irrigation quality of groundwater from
this area was based on salinity (EC) and sodium adsorption ratios (SAR), residual sodium carbonate (RSC), and permeability
indices (PI). The simulation model reveals that groundwater in the Afram Plains area generally flows from the midsections
in the neighborhood of Tease and surrounding areas, where significant recharge takes place, to the outer regions and discharges
into the Volta Lake in the southern and eastern sections of the area. Flow magnitude and piezometric maps suggest that there
is probably of less potential for groundwater extraction for sustainable irrigation in the central regions of the area, when
compared to the other discharge areas. This study reveals that more than 70% of the samples analyzed fall within the C2–S1
category, referring to the medium level salinity and low sodium. Medium salinity waters may be used for irrigation on coarse
textured soils with good permeability. About 15% of the data fall within the C3–S2 category, referring to water of high salinity
and medium sodicity. High salinity, medium sodicity irrigation water cannot be used on fine-grained soils where drainage is
restricted. This is because restricted flow is likely to result in the accumulation of salts in the root zones of crops, leading
to salinity and soil clogging crisis. About 3% falls within the C3–S3 (high salinity, high sodicity) category. This category
requires special soil management including improved drainage, heavy leaching and the use of chemical amendments on the water.
Only one point plots within the extreme salinity–sodicity range. Concentrations of fluoride, arsenic and other natural elements
in the area generally fall well within the world health standards for domestic water. 相似文献
946.
Kenneth M. Brown Gerald J. George Gary W. Peterson Bruce A. Thompson James H. CowanJr. 《Estuaries and Coasts》2008,31(3):597-604
Oyster reefs (Crassostrea virginica) supply important ecosystem services to estuarine habitats in the northern Gulf of Mexico, but little is known of the role
of fish predators in controlling their structure or areal cover on soft sediments. At two sites and during fall and spring,
we employed gill nets and trot lines to remove black drum (Pogonias cromis) from experimental reefs, and assessed oyster survival in comparison to control reefs. Numbers and biomass of black drum
removed from reefs varied seasonally, among sites, and among removal methods. In the fall, black drum were rare at one site
and abundant at the other, but did not significantly lower oyster survival on control reefs at either site. In the spring,
black drum were common at both sites, and significantly lowered oyster survival on control reefs. Oysters and epizoic hooked
mussels comprised roughly a third of the fishes’ diet, and oyster mortality was closely related to the percentage of drum
feeding on oysters. There was little evidence of mortality from other predators of seed oysters like stone crabs or Southern
oyster drills, and a repeated measures analysis of variance indicated black drum biomass was significantly depressed on experimental
reefs during the experiments. Black drum thus appear to be potentially important predators on oyster reefs, but more work
needs to be done on what factors explain the temporal and spatial variation in their abundance and oyster consumption. 相似文献
947.
The primary aquifer on the Maputaland Coastal Plain in northern KwaZulu-Natal, South Africa, is the principal source of water for rivers, lakes and most of the wetlands in dry periods, and is recharged by these systems in wet periods. Modelling hydrologic conditions that control regional water-table depth can provide insight into the spatial patterns of wetland occurrence and of the persistence of wet conditions that control their character. This project used a groundwater model (MODFLOW) to simulate 10-year water-table fluctuations on the Maputaland Coastal Plain from January 2000 to December 2010, to contrast the conditions between wet and dry years. Remote sensing imagery was used to map “permanent” and “temporary” wetlands in dry and wet years to evaluate the effectiveness of identifying the suitable conditions for their formation using numerical modelling techniques. The results confirm that topography plays an important role on a sub-regional and local level to support wetland formation. The wetlands’ extent and distribution are directly associated with the spatial and temporal variations of the water table in relation to the topographical profile. Groundwater discharge zones in the lowland (1–50 masl) areas support more permanent wetlands with dominantly peat or high organic soil substrates, including swamp forest and most of the permanent open water areas. Most temporary wetlands associated with low-percentage clay occurrence are through-flow low-lying interdune systems characterised by regional fluctuation of the water table, while other temporary wetlands are perched or partially perched. The latter requires a more sophisticated saturated-unsaturated modelling approach. 相似文献
948.
Use of Hydrofluoric Acid Desilicification in the Determination of Highly Siderophile Element Abundances and Re‐Pt‐Os Isotope Systematics in Mafic‐Ultramafic Rocks
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James M.D. Day Christopher L. Waters Bruce F. Schaefer Richard J. Walker Simon Turner 《Geostandards and Geoanalytical Research》2016,40(1):49-65
Properly combining highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data, obtained by isotope dilution, with corresponding 187Os/188Os and 186Os/188Os measurements of rocks requires efficient digestion of finely‐ground powders and complete spike‐sample equilibration. Yet, because of the nature of commonly used methods for separating Os from a rock matrix, hydrofluoric acid (HF) is typically not used in such digestions. Consequently, some silicates are not completely dissolved, and HSE residing within these silicates may not be fully accessed. Consistent with this, some recent studies of basaltic reference materials (RMs) have concluded that an HF‐desilicification procedure is required to fully access the HSE (Ishikawa et al. (2014) Chemical Geology, 384, 27–46; Li et al. (2015) Geostandards and Geoanalytical Research, 39, 17–30). Highly siderophile element abundance and Os isotope studies of intraplate basalts typically target samples with a range of MgO contents (< 8 to > 18% m/m, or as mass fractions, < 8 to > 18 g per 100 g), in contrast to the lower MgO mass fractions (< 10 g per 100 g) of basalt and diabase RMs (i.e., BIR‐1, BHVO‐2, TDB‐1). To investigate the effect of HF‐desilicification on intraplate basalts, experiments were performed on finely ground Azores basalts (8.1–17 g per 100 g MgO) using a ‘standard acid digestion’ (2:1 mixture of concentrated HNO3 and HCl), and a standard acid digestion, followed by HF‐desilicification. No systematic trends in HSE abundances were observed between data obtained by standard acid digestion and HF‐desilicification. Desilicification procedures using HF do not improve liberation of the HSE from Azores basalts, or some RMs (e.g., WPR‐1). We conclude that HF‐desilicification procedures are useful for obtaining total HSE contents of some young lavas, but this type of procedure is not recommended for studies where Re‐Pt‐Os chronological information is desired. The collateral effect of a standard acid digestion to liberate Os, followed by HF‐desilicification to obtain Re and Pt abundances in samples, is that the measured Re/Os and Pt/Os may not correspond with measured 187Os/188Os or 186Os/188Os. 相似文献
949.