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71.
Linglin Chu Andreas Enggist Robert W. Luth 《Contributions to Mineralogy and Petrology》2011,162(3):565-571
To examine the effect of KCl-bearing fluids on the melting behavior of the Earth’s mantle, we conducted experiments in the
Mg2SiO4–MgSiO3–H2O and Mg2SiO4–MgSiO3–KCl–H2O systems at 5 GPa. In the Mg2SiO4–MgSiO3–H2O system, the temperature of the fluid-saturated solidus is bracketed between 1,200–1,250°C, and both forsterite and enstatite
coexist with the liquid under supersolidus conditions. In the Mg2SiO4–MgSiO3–KCl–H2O systems with molar Cl/(Cl + H2O) ratios of 0.2, 0.4, and 0.6, the temperatures of the fluid-saturated solidus are bracketed between 1,400–1,450°C, 1,550–1,600°C,
and 1,600–1,650°C, respectively, and only forsterite coexists with liquid under supersolidus conditions. This increase in
the temperature of the solidus demonstrates the significant effect of KCl on reducing the activity of H2O in the fluid in the Mg2SiO4–MgSiO3–H2O system. The change in the melting residues indicates that the incongruent melting of enstatite (enstatite = forsterite + silica-rich
melt) could extend to pressures above 5 GPa in KCl-bearing systems, in contrast to the behavior in the KCl-free system. 相似文献
72.
This study demonstrates that a hydrous, halide bearing silicate melt is a viable medium for diamond growth. Experiments were
conducted in the MgO–SiO2–H2O–C ± KCl ± NaCl system, which was used as a model for harzburgitic mantle. In no case did we observe crystals that could
be interpreted as spontaneously nucleated, but growth of diamond on seed crystals at 1,400–1,600°C and 7 GPa in experiments
of 4 h duration was observed. The addition of KCl to the system produced crystallization of diamond at temperatures as low
as 1,400°C. At higher temperatures, larger growth features were produced than those that seen in the KCl-free system at the
same conditions. The NaCl-bearing system is different; in these experiments, the diamond seed crystals show evidence of possible
dissolution and layer growth, albeit more subdued growth than in the KCl system. Therefore, NaCl may be an inhibitor of diamond
growth in a hydrous silicate melt. Based on these results, hydrous silicate melts could play a role in formation of diamond
in either deep subduction zones, or above slabs imbricated against a lithospheric ‘root’ in the sub-continental lithospheric
mantle. The water and halide necessary for their formation could be transported into the mantle in hydrous phases such as
serpentine in subducting lithospheric slabs. Dehydration of serpentine at >200 km depth would release hydrous, halide-bearing
fluids into the overlying mantle wedge or lithospheric root, triggering melting at conditions similar to those of the formation
of natural diamond. 相似文献
73.
Extracellular polymeric substances (EPS) are continuously produced by bacteria during their growth and metabolism. In soils, EPS are bound to cell surfaces, associated with biofilms, or released into solution where they can react with other solutes and soil particle surfaces. If such reaction results in a decrease in EPS bioaccessibility, it may contribute to stabilization of microbial-derived organic carbon (OC) in soil. Here we examined: (i) the chemical fractionation of EPS produced by a common Gram positive soil bacterial strain (Bacillus subtilis) during reaction with dissolved and colloidal Al species and (ii) the resulting stabilization against desorption and microbial decay by the respective coprecipitation (with dissolved Al) and adsorption (with Al(OH)3(am)) processes. Coprecipitates and adsorption complexes obtained following EPS-Al reaction as a function of pH and ionic strength were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The stability of adsorbed and coprecipitated EPS against biodegradation was assessed by mineralization experiments for 1100 h. Up to 60% of the initial 100 mg/L EPS-C was adsorbed at the highest initial molar Al:C ratio (1.86), but this still resulted only in a moderate OC mass fraction in the solid phase (17 mg/g Al(OH)3(am)). In contrast, while coprecipitation by Al was less efficient in removing EPS from solution (maximum values of 33% at molar Al:C ratios of 0.1-0.2), the OC mass fraction in the solid product was substantially larger than that in adsorption complexes. Organic P compounds were preferentially bound during both adsorption and coprecipitation. Data are consistent with strong ligand exchange of EPS phosphoryl groups during adsorption to Al(OH)3(am), whereas for coprecipitation weaker sorption mechanisms are also involved. X-ray photoelectron analyses indicate an intimate mixing of EPS with Al in the coprecipitates, which is not observed in the case of EPS adsorption complexes. The incubation experiments showed that both processes result in overall stabilization of EPS against microbial decay. Stabilization of adsorbed or coprecipitated EPS increased with increasing molar Al:C ratio and biodegradation was correlated with EPS desorption, implying that detachment of EPS from surface sites is a prerequisite for microbial utilization. Results indicate that the mechanisms transferring EPS into Al-organic associations may significantly affect the composition and stability of biomolecular C, N and P in soils. The observed efficient stabilization of EPS might explain the strong microbial character of organic matter in subsoils. 相似文献
74.
Cornel E. J. de Ronde Gary J. Massoth David A. Butterfield Bruce W. Christenson Junichiro Ishibashi Robert G. Ditchburn Mark D. Hannington Robert L. Brathwaite John E. Lupton Vadim S. Kamenetsky Ian J. Graham Georg F. Zellmer Robert P. Dziak Robert W. Embley Vesselin M. Dekov Frank Munnik Janine Lahr Leigh J. Evans Ken Takai 《Mineralium Deposita》2011,46(5-6):541-584
Brothers volcano, of the Kermadec intraoceanic arc, is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world. It has two distinct vent fields, known as the NW Caldera and Cone sites, whose geology, permeability, vent fluid compositions, mineralogy, and ore-forming conditions are in stark contrast to each other. The NW Caldera site strikes for ??600?m in a SW?CNE direction with chimneys occurring over a ??145-m depth interval, between ??1,690 and 1,545?m. At least 100 dead and active sulfide chimney spires occur in this field and are typically 2?C3?m in height, with some reaching 6?C7?m. Their ages (at time of sampling) fall broadly into three groups: <4, 23, and 35?years old. The chimneys typically occur near the base of individual fault-controlled benches on the caldera wall, striking in lines orthogonal to the slopes. Rarer are massive sulfide crusts 2?C3?m thick. Two main types of chimney predominate: Cu-rich (up to 28.5?wt.% Cu) and, more commonly, Zn-rich (up to 43.8?wt.% Zn). Geochemical results show that Mo, Bi, Co, Se, Sn, and Au (up to 91?ppm) are correlated with the Cu mineralization, whereas Cd, Hg, Sb, Ag, and As are associated with the dominant Zn-rich mineralization. The Cone site comprises the Upper Cone site atop the summit of the recent (main) dacite cone and the Lower Cone site that straddles the summit of an older, smaller, more degraded dacite cone on the NE flank of the main cone. Huge volumes of diffuse venting are seen at the Lower Cone site, in contrast to venting at both the Upper Cone and NW Caldera sites. Individual vents are marked by low-relief (??0.5?m) mounds comprising predominately native sulfur with bacterial mats. Vent fluids of the NW Caldera field are focused, hot (??300°C), acidic (pH????2.8), metal-rich, and gas-poor. Calculated end-member fluids from NW Caldera vents indicate that phase separation has occurred, with Cl values ranging from 93% to 137% of seawater values. By contrast, vent fluids at the Cone site are diffuse, noticeably cooler (??122°C), more acidic (pH?1.9), metal-poor, and gas-rich. Higher-than-seawater values of SO4 and Mg in the Cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water/rock interactions. Iron oxide crusts 3?years in age cover the main cone summit and appear to have formed from Fe-rich brines. Evidence for magmatic contributions to the hydrothermal system at Brothers includes: high concentrations of dissolved CO2 (e.g., 206?mM/kg at the Cone site); high CO2/3He; negative ??D and ??18OH2O for vent fluids; negative ??34S for sulfides (to ?4.6??), sulfur (to ?10.2??), and ??15N2 (to ?3.5??); vent fluid pH values to 1.9; and mineral assemblages common to high-sulfidation systems. Changing physicochemical conditions at the Brothers hydrothermal system, and especially the Cone site, occur over periods of months to hundreds of years, as shown by interlayered Cu?+?Au- and Zn-rich zones in chimneys, variable fluid and isotopic compositions, similar shifts in 3He/4He values for both Cone and NW Caldera sites, and overprinting of ??magmatic?? mineral assemblages by water/rock-dominated assemblages. Metals, especially Cu and possibly Au, may be entering the hydrothermal system via the dissolution of metal-rich glasses. They are then transported rapidly up into the system via magmatic volatiles utilizing vertical (??2.5?km long), narrow (??300-m diameter) ??pipes,?? consistent with evidence of vent fluids forming at relatively shallow depths. The NW Caldera and Cone sites are considered to represent stages along a continuum between water/rock- and magmatic/hydrothermal-dominated end-members. 相似文献
75.
Harris E. Mason Robert S. Maxwell Susan A. Carroll 《Geochimica et cosmochimica acta》2011,75(20):6080-6093
We present the results of a series of experiments designed to probe the interactions between Al and the amorphous silica surface as a function of thermodynamic driving forces. The results from 27Al single pulse magic angle spinning (SP/MAS) and 27Al{1H} rotational echo double resonance (REDOR) allow us to identify the reaction products and constrain their structure. In all cases, despite low Al and Si concentrations we observe the formation of metastable aluminosilicates. Results from low temperature experiments indicate that despite thermodynamic driving forces for the formation of gibbsite we observe the precipitation of separate octahedrally coordinated Al (Al[6]) and tetrahedrally coordinated Al (Al[4]) silicate phases. At higher temperatures the Al[4] silicate phase dominates the speciation. Structural models derived from the NMR data are also proposed, and the results are discussed as they relate to previous work on Al/Si cycling. 相似文献
76.
Woodrow B. Thompson Carol B. Griggs Norton G. Miller Robert E. Nelson Thomas K. Weddle Taylor M. Kilian 《Quaternary Research》2011,75(3):552-565
Excavations in the late-glacial Presumpscot Formation at Portland, Maine, uncovered tree remains and other terrestrial organics associated with marine invertebrate shells in a landslide deposit. Buds of Populus balsamifera (balsam poplar) occurred with twigs of Picea glauca (white spruce) in the Presumpscot clay. Tree rings in Picea logs indicate that the trees all died during winter dormancy in the same year. Ring widths show patterns of variation indicating responses to environmental changes. Fossil mosses and insects represent a variety of species and wet to dry microsites. The late-glacial environment at the site was similar to that of today's Maine coast. Radiocarbon ages of 14 tree samples are 11,907 ± 31 to 11,650 ± 50 14C yr BP. Wiggle matching of dated tree-ring segments to radiocarbon calibration data sets dates the landslide occurrence at ca. 13,520 + 95/−20 cal yr BP. Ages of shells juxtaposed with the logs are 12,850 ± 65 14C yr BP (Mytilus edulis) and 12,800 ± 55 14C yr BP (Balanus sp.), indicating a marine reservoir age of about 1000 yr. Using this value to correct previously published radiocarbon ages reduces the discrepancy between the Maine deglaciation chronology and the varve-based chronology elsewhere in New England. 相似文献
77.
Robert Šajn Josip Halamić Zoran Peh Lidija Galović Jasminka Alijagić 《Journal of Geochemical Exploration》2011
The objectives of this study are as follows: (a) an assessment of the geochemical background signature of the Drava Valley before the industrial revolution; (b) an evaluation of anthropogenic geochemical influences on the alluvial plains and river terraces in the valley; and (c) a determination of the spatial distribution of trace elements in the alluvial soils of the Drava River downstream of the Austrian–Slovenian border to the confluence of Mura and Drava Rivers. 相似文献
78.
Przemyslaw Dera John D. Lazarz Vitali B. Prakapenka Madison Barkley Robert T. Downs 《Physics and Chemistry of Minerals》2011,38(7):517-529
Single-crystal X-ray diffraction experiments with SiO2 α-cristobalite reveal that the well-known reversible displacive phase transition to cristobalite-II, which occurs at approximately
1.8 GPa, can be suppressed by rapid pressure increase, leading to an overpressurized metastable state, persisting to pressure
as high as 10 GPa. In another, slow pressure increase experiment, the monoclinic high-pressure phase-II was observed to form
at ~1.8 GPa, in agreement with earlier in situ studies, and its crystal structure has been unambiguously determined. Single-crystal
data have been used to refine the structure models of both phases over the range of pressure up to the threshold of formation
of cristobalite X-I at ~12 GPa, providing important constraints on the feasibility of the two competing silica densification
models proposed in the literature, based on quantum mechanical calculations. Preliminary diffraction data obtained for cristobalite
X-I reveal a monoclinic unit cell that contradicts the currently assumed model. 相似文献
79.
Paleosol (fossilized soil) geochemistry can provide a record of paleoclimatic conditions due to the relationships between pedogenic processes and climate. In this study, paleosols from the late Paleocene Goler Formation of Southern California were used to determine the paleoclimatic conditions active during pedogenesis. The enrichment and retention of soluble elements (Ca, Mg and Na) and the mobilization of Fe and Mn within the paleosol profiles suggest a climate with strongly seasonal precipitation. Similarly, variations in the Mn content and δ13C ratio of pedogenic carbonate nodules also reflect seasonal precipitation. Regional paleotopography was likely an important control on the paleoclimate of the Goler Formation due to rain shadow effects. Although the size and location of these ancient mountains are poorly constrained, the identification of climatic effects specifically associated with these variables provides new constraints. 相似文献
80.
This paper studies the long-run economic effects of severe weather on regional economies. A catastrophic event, such as a
hurricane, will have an effect on both the directly impacted region and adjacent regions. With dramatically increasing damage
from catastrophic weather events over the past few decades, comprehensive assessment of the long-run economic impact of natural
disasters across the broader region becomes more important than ever for planning for post-disaster recovery. We estimate
the long-run effect of Hurricane Katrina on the unemployment rate of Houston, TX by employing time-series and fixed-effect
models. Using Dallas as a control, we find that Katrina is associated with a higher long-run unemployment rate in Houston
than would otherwise have been expected. This implies that the hurricane-generated adverse relative effects on Houston. Our
findings suggest that areas that are geographically proximate to the directly impacted region can sustain lasting negative
economic consequences. 相似文献