Empirical equations to predict the sulfur content of a mafic magma at the time of sulfide saturation have been developed based
on several sets of published experimental data. The S content at sulfide saturation (SCSS) can be expressed as:
where T is in degrees Kelvin, X is mole fraction and P is in kbar. The squared multiple correlation coefficient (r2) for the equation is 0.88. Application of the equation to data from sulfide-saturated mid-ocean ridge basalts (MORB) samples
show that the SCSS is closely predicted for primitive MORBs, but that accuracy decreases for lower T (<1,130°C) and more evolved MORB samples. This suggests that because the calibrations are based on anhydrous experimental
runs done at temperatures of 1,200°C and above, it is not possible to extrapolate them to significantly lower temperatures
and hydrous conditions. Because the SCSS of a primitive MORB magma increases with decreasing P, sulfide saturation in MORB appears to be a function of the degree of en route assimilation of S from country rocks as well
as the degree of fractional crystallization in shallow staging chambers. Application of the equation to the high-T
impact melt sheet that produced the Sudbury Igneous Complex and associated Ni–Cu sulfide ores indicates that sulfide-saturation
was reached at ~1,500°C, well above the start of orthopyroxene crystallization at ~1,190°C. This would permit ample time for
the gravitational settling and collection of immiscible sulfide liquid that produced the high-grade ore bodies. The development
of a platinum group element (PGE)-enriched layer in the Sonju Lake Intrusion of the Duluth Complex is thought to be due to
the attainment of sulfide saturation in the magma after a period of fractional crystallization. Using the composition of the
parent magma of the Sonju Lake Intrusion the presented equation indicates that sulfide saturation would have been reached
at ~60% crystallization, when iron oxide was a liquidus mineral; the prediction is in agreement with field evidence which
indicates that PGE-enrichment occurs in the oxide-rich gabbro zone. Contamination and mixing processes that may be related
to the attainment of sulfide saturation in mafic magmas can also be evaluated. Mixing of a siliceous melt and a liquid of
olivine tholeiite composition, similar to that thought to be a reasonable parental composition for many Duluth Complex intrusions,
can induce sulfide saturation at mixing ratios in excess of ~0.1. If the contaminant contains low quantities of sulfur the
mixing ratio required to promote saturation is reduced. Mixing of mafic magmas at various stages of fractionation is evaluated
using magma compositions that are thought to be appropriate for the generation of the Merensky Reef in the Bushveld Complex.
Magma mixing is shown to be an effective process for the attainment of sulfide saturation, depending strongly on the sulfur
concentrations of the end-member magmas. 相似文献
Natural analogue investigations aim to understand key phenomena and processes in natural systems related to those expected to occur in radioactive waste repositories. One of the key applications of natural analogue studies has been the possibility to test the geochemical models to be used to describe the migration of radionuclides in a future radioactive waste repository system. To this end, several geochemical modelling testing exercises (commonly denoted as blind predictive modelling, BPM) have formed an integral part of these studies over the last decade.
We have reviewed, discussed and compared the results obtained from geochemical modelling BPM exercises carried out within six natural analogue studies: Poços de Caldas, Cigar Lake, Maqarin, El Berrocal, Oklo and Palmottu. To make this comparison meaningful, we present the main geochemical characteristics of each site in order to highlight the most relevant mineralogical and hydrochemical differences. The elements selected for discussion are: Sr, Ba, Sn, Pb, Se, Ni, Zn, REEs, Th and U. We have based our discussion on the results obtained from the calculated aqueous speciation as well as by comparing solubility calculations with the actually observed concentrations.
Results can be differentiated into two categories of elemental behaviour:
1. those elements like Th and U under reducing conditions that can be fairly well described by assuming solubility control exerted by pure solid phases as their oxyhydroxides;
2. elements such as Sr, Zn, REEs and U under oxidising conditions for which the association to major geochemical components of the system must be considered in order to explain their concentrations in groundwaters.
Additionally, we discuss the main improvements made to the thermodynamic databases and the geochemical calculation methodologies due to the BPM exercises. Furthermore, the most important characterisation geochemical data needed to complete predictive solubility and speciation calculations are identified. 相似文献
The gradient solar pond technique is an economic separating process employed in the inorganic chemical industrial production of salt lake chemical engineering processes. In this paper,a novel isothermal evaporation experimental method was employed to simulate the evaporation phase equilibrium for the reciprocal quaternary system( LiCl + NaCl + Li_2 SO_4+ Na_2 SO_4+ H_2 O) at 348. 15 K to serve as a useful guide for lithium salt production via the depth solar ponds. The isothermal evaporation equilibrium solubilities and physicochemical properties,including the densities and pH values,were experimentally investigated. The dry-salt phase diagram,water-phase diagram,and the diagram of the physicochemical properties versus composition were plotted with respect to the experimental data. The drysalt phase diagram consists of three invariant points,seven univariant solubility curves,and five crystallization regions,specifically halite( NaCl,Ha),thenardite( Na_2 SO_4,Th),double salt( Li_2 SO_4·Na_2 SO_4,Db_2),lithium sulfate monohydrate( Li_2 SO_4·H_2 O,Ls),and lithium chloride monohydrate( LiCl·H_2 O,Lc). Based on Pitzer and its extended HMW model,the Pitzer single salt parameters,mixing ion interaction parameters,and thermodynamic stable equilibrium constants for the quaternary system at 348. 15 K were obtained. The calculated phase diagram and experimental isothermal phase diagram at 348. 15 K exhibited a great difference. Based on these results,the isothermal evaporation phase diagram can truly reflect the salt sedimentary in salt lakes and can be applied as a useful guide for the solar pond process. 相似文献
Experimental studies on the solubility,density,refractive index,and pH value in the aqueous system containing potassium,magnesium,and borate at 308. 15 K were determined with the method of isothermal dissolution equilibrium. Based on the experimental results,the diagrams of solubility,density,refractive index,and pH value in this system were plotted. We found that there were one eutectic point and two crystallization regions corresponding to the large area of inderite( L + Mg_2 B_6 O_(11)·15 H_2 O) and the relatively small area of potassium borate tetrahydrate( L + K_2 B_4 O_7·4 H_2 O),respectively. Neither double salt nor solid solution was found in this system. The physicochemical properties( density,refractive index,and pH value) in solution at 308. 15 K changed regularly with increasing potassium borate concentration. The calculated values of density and refractive index using empirical equations of the aqueous system were in good agreement with the experimental values. 相似文献