Integration of micro-gravity and geodetic data to constrain shallow system mass changes at Krafla Volcano, N Iceland     

Elske de Zeeuw-van Dalfsen  Hazel Rymer  Glyn Williams-Jones  Erik Sturkell  Freysteinn Sigmundsson 《Bulletin of Volcanology》2006,68(5):420-431

New and previously published micro-gravity data are combined with InSAR data, precise levelling and GPS measurements to produce a model for the processes operating at Krafla volcano, 20 years after its most recent eruption. The data have been divided into two periods: from 1990 to 1995 and from 1996 to 2003 and show that the rate of deflation at Krafla is decaying exponentially. The net micro-gravity change at the centre of the caldera is shown, using the measured free air gradient, to be −85 μGal for the first and −100 μGal for the second period. After consideration of the effects of water extraction by the geothermal power station within the caldera, the net gravity decreases are −73±17 μGal for the first and −65±17 μGal for the second period. These decreases are interpreted in terms of magma drainage. Following a Mogi point source model, we calculate the mass decrease to be ∼2×10¹⁰ kg/year reflecting a drainage rate of ∼0.23 m³/s, similar to the ∼0.13 m³/s drainage rate previously found at Askja volcano, N. Iceland. Based on the evidence for deeper magma reservoirs and the similarity between the two volcanic systems, we suggest a pressure-link between Askja and Krafla at deeper levels (at the lower crust or the crust-mantle boundary). After the Krafla fires, co-rifting pressure decrease of a deep source at Krafla stimulated the subsequent inflow of magma, eventually affecting conditions along the plate boundary in N. Iceland, as far away as Askja. We anticipate that the pressure of the deeper reservoir at Krafla will reach a critical value and eventually magma will rise from there to the shallow magma chamber, possibly initiating a new rifting episode. We have demonstrated that by examining micro-gravity and geodetic data, our knowledge of active volcanic systems can be significantly improved.Editorial responsibility: A. Harris  相似文献   

Brainstorming, Modified Logframes and the Conversion of Research Hypotheses into Field Questions: Reflections from Team-based Fieldwork in Eastern India     

Stuart Corbridge  Glyn Williams  Manoj Srivastava  René Véron 《Singapore journal of tropical geography》2003,24(2):242-257

This paper considers some practical problems associated with organising large‐scale comparative field research in eastern India. The focus of the paper is on the use of brainstorming and “modified logframes” as two means by which hypotheses about the working of the local state from the point of view of the rural poor could be turned into concrete field questions. The paper is less concerned with ethical and positional issues relating to team‐based research in “the tropics” (on this, see Williams et al., 2003a) than with the equally important if apparently more prosaic issues relating to the flawed but necessary search for objectivity and rigour in comparative field studies.  相似文献   

A spectrophotometric study of neodymium(III) complexation in chloride solutions     

Art A MigdisovA.E Williams-Jones 《Geochimica et cosmochimica acta》2002,66(24):4311-4323

The formation constants of neodymium complexes in chloride solutions have been determined spectrophotometrically at temperatures of 25 to 250°C and a pressure of 50 bars. The simple ion, Nd³⁺, is dominant at 25°C, whereas NdCl²⁺ and NdCl₂⁺ are the dominant species at elevated temperatures. Equilibrium constants were calculated for the following reactions:Nd³⁺ + Cl⁻ = NdCl²⁺ β₁,Nd³⁺ + 2 · Cl⁻ = NdCl⁺₂ β₂.The values of β₁ were found to be identical within experimental error to the values reported by Gammons et al. (1996) but substantially different from those proposed by Stepanchikova and Kolonin (1999). The values of β₂ obtained in this study agree relatively well with those of Gammons et al. (1996); differences are greatest at intermediate temperature and reach a maximum of one half an order of magnitude at 200°C.Theoretical estimates of β₁ and β₂ by Haas et al. (1995) using the revised Helgeson-Kirkham-Flowers (HKF) equation of state predict lower stability of NdCl²⁺ and NdCl₂⁺ at temperatures above 150°C than determined in this study. A new fit to the HKF equation of state is therefore proposed, which yields values for β₁ and β₂ similar to those obtained experimentally.Using the formation constants reported in this study, we predict that typical seafloor hydrothermal vent fluids will contain a maximum concentration of Nd of ∼2 ppb. This value is several orders of magnitude lower than would be required to explain the levels of Nd mobility commonly reported for seafloor hydrothermal systems and suggests that other ligands may be more important than Cl in transporting rare earth elements in the Earth’s crust.  相似文献   

Response to the comment by J. Schijf and R.H. Byrne on “An experimental study of the solubility and speciation of neodymium (III) fluoride in F-bearing aqueous solutions”     

A. Migdisov Art  A.E. Williams-Jones 《Geochimica et cosmochimica acta》2008,72(22):5578-5579

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A fluid inclusion study of vug minerals in dawsonite-altered phonolite sills,Montreal, Quebec: implications for HFSE mobility     

Eileen Vard  Anthony E. Williams-Jones 《Contributions to Mineralogy and Petrology》1993,113(3):410-423

Ordovician limestones in the Francon quarr on the island of Montreal, Quebec, are host to three sills of Cretaceous age composed of phonolite that has been extensively altered to dawsonite. An interesting feature of the sills is the presence of abundant vugs containing a wide variety of minerals, including several in which one or more high field strength elements (Zr, Hf, Nb, Ti) is a major component. The most important of these latter minerals is weloganite, a rare strontium zirconium hydrous carbonate, first identified in the Francon, quarry. Four types of inclusions have been recognized in vug minerals: aqueous, aqueous-carbonic, carbonic and solidbearing. Aqueous inclusions homogenize at temperatures mainly between 70° and 170°C and between 230° and 390°C. The homogenization temperatures of primary inclusions cluster around 350°C. Aqueous inclusions and the aqueous phase in aqueous-carbonic inclusions have salinities ranging between 10 and 24 eq.wt.% NaCl equivalent. Primary aqueous-carbonic inclusions have low XCO₂ (<0.03), whereas secondary aqueous-carbonic inclusions can have high XCO₂ (>0.7); carbonic inclusions are all secondary. Nahcolite, dawsonite and weloganite occur as daughter minerals or trapped solids. Nahcolite and possibly natron or mirabilite appear to form in frozen inclusions. Analyses of fluid inclusion decrepitates detected high concentrations of Na, Cl, Al, S, and C. The extraordinarily high concentration of Al in the fluid (possibly exceeding 1 wt.%) suggests a pH of approximately 10. Pressure and temperature conditions, estimated from stratigraphic reconstruction and the isochores of primary aqueous fluid inclusions, were 450 bar and 360 to 400°C, respectively. The relatively high temperatures and compositions of primary fluid inclusions suggest that vug filling was the result of mineral precipitation from an orthomagmatic fluid. A model is proposed in which a partially crystallized phonolite melt started exsolving a homogeneous low XCO₂ fluid immediately prior to or after intrusion. Sodium, aluminium, chlorine, fluorine, sulphur and HFSE elements such as Zr, Hf, Nb and Ti were partitioned into the hydrous phase, in the case of Zr, possibly to a concentration of 300 ppm. The near horizontal orientation of the sills and the chilled margins, produced by quenching of the magma, created a tight seal that inhibited escape of the fluids. As a result, the phonolite stewed in its own juices long after crystallization, giving rise to widespread replacement of primary igneous minerals by dawsonite, and precipitation of this and other minerals in vugs. Once the sills had colled to temperatures between 200 and 300°C, the aqueous fluid exsolved a high CO₂ fluid which was trapped as the secondary three-phase type II and type III inclusions. Decreasing temperature is considered to have been the principal control of mineralization, although in the case of the lower temperature minerals, decreased bicarbonate or carbonate ion activity, and a lower dielectric constant, as a result of CO₂ exsolution, may have played a role in the deposition of HFSE-bearing minerals.  相似文献   

Thermal metamorphism and H2O-CO2-NaCl immiscibility at Patapedia,Quebec: evidence from fluid inclusions     

Anthony E. Williams-Jones  Dexter R. Ferreira 《Contributions to Mineralogy and Petrology》1989,102(2):247-254

The methamorphic history of the Patapedia thermal zone, Gaspé, Quebec, is re-evaluated in the light of results obtained from a study of fluid inclusions contained in quartz phenocrysts of felsic dyke rocks. The thermal zone is characterised by calc-silicate bodies that have outwardly telescoping prograde metamorphic isograds and display extensive retrograde metamorphism with associated copper mineralization. Three distinct fluid inclusion types are recognized: a low to moderate salinity, high density aqueous fluid (Type I); a low density CO₂ fluid (Type II); and a high salinity, high density aqueous fluid (Type III). Fluid inclusion Types I and II predominate whereas Type III inclusions form <10% of the fluid inclusion population. All three fluid types are interpreted to have been present during prograde metamorphism. Temperatures and pressures of metamorphism estimated from fluid inclusion microthermometry and isochore calculations are 450°–500° C and 700–1000 bars, respectively. A model is proposed in which the metamorphism at Patapedia was caused by heat transferred from a low to moderate salinity fluid of partly orthomagmatic origin (Type I inclusions). During the early stages, and particularly in the deeper parts of the system, CO₂ produced by metamorphism was completely miscible in the aqueous hydrothermal fluid and locally resulted in high XCO₂ fluids. On cooling and/or migrating to higher levels these latter fluids exsolved high salinity aqueous fluids represented by the Type III inclusions. Most of the metamorphism, however, took place at temperature-pressure conditions consistent with the immiscibility of CO₂ and the hydrothermal fluid and was consequently accompanied by the release of large volumes of CO₂ vapour which is represented by Type II inclusions. The final stage of the history of the Patapedia aureole was marked by retrograde metamorphism and copper mineralization of a calcite-free calc-silicate hornfels in the presence of a low XCO₂ fluid.  相似文献   

Alteration, HFSE mineralisation and hydrocarbon formation in peralkaline igneous systems: Insights from the Strange Lake Pluton, Canada   总被引：1，自引：0，他引：1  

Stefano Salvi  Anthony E. Williams-Jones 《Lithos》2006,91(1-4):19-34

Two characteristics of peralkaline igneous rocks that are poorly understood are the extreme enrichment in HFSE, notably Zr, Nb, Y and REE, and the occurrence of fluid inclusions dominated by methane and higher hydrocarbons. Although much of the HFSE enrichment can be explained by magmatic processes, the common intense alteration of the parts of the peralkaline intrusions most enriched in these elements suggests that hydrothermal processes also play an important role in HFSE enrichment. Likewise, although the origin of the higher order hydrocarbons that occur as inclusions in these rocks is still debated, there is strong evidence that at least in some cases their formation involved hydrothermal processes. The issues of HFSE enrichment and hydrocarbon formation in peralkaline intrusions are examined using data from the Strange Lake pluton, a small, middle-Proterozoic intrusion of peralkaline granite in northeast Canada. This pluton contains some of the highest concentrations of Zr, REE and Y ever reported in an igneous body, and is characterised by abundant hydrocarbon-dominated fluid inclusions in rocks that have been hydrothermally altered, including those that form a potential HFSE ore zone. We show that HFSE at Strange Lake were partly concentrated to near exploitable levels as a result of their transport in a high salinity magmatic aqueous liquid, and that this fluid coexisted immiscibly with a carbonic phase which reacted with hydrogen and iron oxides generated during the associated hydrothermal alteration to produce hydrocarbons via a Fischer–Tropsch synthesis. As a result, hydrocarbons and HFSE mineralization are intimately associated. We then go on to show that hydrothermal alteration, HFSE mineralisation and hydrocarbons are also spatially associated in other peralkaline complexes, and present a model to explain this association, which we believe may be applicable to any peralkaline intrusion where HFSE enrichment was accompanied by calcium metasomatism, hematisation and hydrothermal fluorite. We also suggest that, even where these criteria are not satisfied, hydrothermally enriched HFSE and hydrocarbons will be intimately associated simply because they are products of the same initial magmatic fluid. Finally, we speculate that the association of HFSE and hydrocarbons may in some cases actually be genetic, if, as seems possible, unmixing or effervescence of a reduced carbonic fluid from the original magmatic fluid caused changes in temperature, pH, fO₂ or the activity of volatile ligands sufficient to induce the deposition of HFSE minerals.  相似文献   

The solubility of gold in H₂O-H₂S vapour at elevated temperature and pressure   总被引：1，自引：0，他引：1  

Denis Yu. Zezin  Artashes A. Migdisov  Anthony E. Williams-Jones 《Geochimica et cosmochimica acta》2011,75(18):5140-5495

This experimental study sheds light on the complexation of gold in reduced sulphur-bearing vapour, specifically, in H₂O-H₂S gas mixtures. The solubility of gold was determined in experiments at temperatures of 300, 350 and 365 °C and reached 2.2, 6.6 and 6.3 μg/kg, respectively. The density of the vapour varied from 0.02 to 0.22 g/cm³, the mole fraction of H₂S varied from 0.03 to 0.96, and the pressure in the cell reached 263 bar. Statistically significant correlations of the amount of gold dissolved in the fluid with the fugacity of H₂O and H₂S permit the experimental data to be fitted to a solvation/hydration model. According to this model, the solubility of gold in H₂O-H₂S gas mixtures is controlled by the formation of sulphide or bisulphide species solvated by H₂S or H₂O molecules. Formation of gold sulphide species is favoured statistically over gold bisulphide species and thus the gold is interpreted to dissolve according to reactions of the form: