首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 265 毫秒
1.
We determined total CO2 solubilities in andesite melts with a range of compositions. Melts were equilibrated with excess C-O(-H) fluid at 1 GPa and 1300°C then quenched to glasses. Samples were analyzed using an electron microprobe for major elements, ion microprobe for C-O-H volatiles, and Fourier transform infrared spectroscopy for molecular H2O, OH−, molecular CO2, and CO32−. CO2 solubility was determined in hydrous andesite glasses and we found that H2O content has a strong influence on C-O speciation and total CO2 solubility. In anhydrous andesite melts with ∼60 wt.% SiO2, total CO2 solubility is ∼0.3 wt.% at 1300°C and 1 GPa and total CO2 solubility increases by about 0.06 wt.% per wt.% of total H2O. As total H2O increases from ∼0 to ∼3.4 wt.%, molecular CO2 decreases (from 0.07 ± 0.01 wt.% to ∼0.01 wt.%) and CO32− increases (from 0.24 ± 0.04 wt.% to 0.57 ± 0.09 wt.%). Molecular CO2 increases as the calculated mole fraction of CO2 in the fluid increases, showing Henrian behavior. In contrast, CO32− decreases as the calculated mole fraction of CO2 in the fluid increases, indicating that CO32− solubility is strongly dependent on the availability of reactive oxygens in the melt. These findings have implications for CO2 degassing. If substantial H2O is present, total CO2 solubility is higher and CO2 will degas at relatively shallow levels compared to a drier melt. Total CO2 solubility was also examined in andesitic glasses with additional Ca, K, or Mg and low H2O contents (<1 wt.%). We found that total CO2 solubility is negatively correlated with (Si + Al) cation mole fraction and positively correlated with cations with large Gibbs free energy of decarbonation or high charge-to-radius ratios (e.g., Ca). Combining our andesite data with data from the literature, we find that molecular CO2 is more abundant in highly polymerized melts with high ionic porosities (>∼48.3%), and low nonbridging oxygen/tetrahedral oxygen (<∼0.3). Carbonate dominates most silicate melts and is most abundant in depolymerized melts with low ionic porosities, high nonbridging oxygen/tetrahedral oxygen (>∼0.3), and abundant cations with large Gibbs free energy of decarbonation or high charge-to-radius ratio. In natural silicate melt, the oxygens in the carbonate are likely associated with tetrahedral and network-modifying cations (including Ca, H, or H-bonds) or a combinations of those cations.  相似文献   

2.
The solubility of CO2 in dacitic melts equilibrated with H2O-CO2 fluids was experimentally investigated at 1250°C and 100 to 500 MPa. CO2 is dissolved in dacitic glasses as molecular CO2 and carbonate. The quantification of total CO2 in the glasses by mid-infrared (MIR) spectroscopy is difficult because the weak carbonate bands at 1430 and 1530 cm−1 can not be reliably separated from background features in the spectra. Furthermore, the ratio of CO2,mol/carbonate in the quenched glasses strongly decreases with increasing water content. Due to the difficulties in quantifying CO2 species concentrations from the MIR spectra we have measured total CO2 contents of dacitic glasses by secondary ion mass spectrometry (SIMS).At all pressures, the dependence of CO2 solubility in dacitic melts on xfluidCO2,total shows a strong positive deviation from linearity with almost constant CO2 solubility at xCO2fluid > 0.8 (maximum CO2 solubility of 795 ± 41, 1376 ± 73 and 2949 ± 166 ppm at 100, 200 and 500 MPa, respectively), indicating that dissolved water strongly enhances the solubility of CO2. A similar nonlinear variation of CO2 solubility with xCO2fluid has been observed for rhyolitic melts in which carbon dioxide is incorporated exclusively as molecular CO2 (Tamic et al., 2001). We infer that water species in the melt do not only stabilize carbonate groups as has been suggested earlier but also CO2 molecules.A thermodynamic model describing the dependence of the CO2 solubility in hydrous rhyolitic and dacitic melts on T, P, fCO2 and the mol fraction of water in the melt (xwater) has been developed. An exponential variation of the equilibrium constant K1 with xwater is proposed to account for the nonlinear dependence of xCO2,totalmelt on xCO2fluid. The model reproduces the CO2 solubility data for dacitic melts within ±14% relative and the data for rhyolitic melts within 10% relative in the pressure range 100-500 MPa (except for six outliers at low xCO2fluid). Data obtained for rhyolitic melts at 75 MPa and 850°C show a stronger deviation from the model, suggesting a change in the solubility behavior of CO2 at low pressures (a Henrian behavior of the CO2 solubility is observed at low pressure and low H2O concentrations in the melt). We recommend to use our model only in the pressure range 100-500 MPa and in the xCO2fluid range 0.1-0.95. The thermodynamic modeling indicates that the partial molar volume of total CO2 is much lower in rhyolitic melts (31.7 cm3/mol) than in dacitic melts (46.6 cm3/mol). The dissolution enthalpy for CO2 in hydrous rhyolitic melts was found to be negligible. This result suggests that temperature is of minor importance for CO2 solubility in silicic melts.  相似文献   

3.
Experimental study of the effect of SiO2 on Ni solubility in silicate melts   总被引：1，自引：0，他引：1  
A. A. Borisov 《Petrology》2006,14(6):530-539
The solubility of Ni in silicate melts with variable SiO2 content was studied at a total pressure of 1 atm within a wide range of temperature and oxygen fugacity. The maximum solubility of Ni (minimum activity coefficient of NiO) was observed in melts with ～55–57 wt % SiO2, regardless of temperature and oxygen fugacity. Melts beyond this range showed significantly lower Ni solubility and, correspondingly, higher NiO activity coefficients. The analysis of our results and literature data led us to the conclusion that the NBO/T (number of nonbridging oxygen atoms per tetrahedrally coordinated atom) is inadequate to describe the effect of melt composition on Ni solubility.  相似文献   

4.
The solubility behavior of H2O in melts in the system Na2O-SiO2-H2O was determined by locating the univariant phase boundary, melt = melt + vapor in the 0.8-2 GPa and 1000°-1300°C pressure and temperature range, respectively. The NBO/Si-range of the melts (0.25-1) was chosen to cover that of most natural magmatic liquids. The H2O solubility in melts in the system Na2O-SiO2-H2O (XH2O) ranges between 18 and 45 mol% (O = 1) with (∂XH2O/∂P)T∼14-18 mol% H2O/GPa. The (∂XH2O/∂P)T is negatively correlated with NBO/Si (= Na/Si) of the melt. The (∂XH2O/∂T)P is in the −0.03 to +0.05 mol% H2O/°C range, and is negatively correlated with NBO/Si. The [∂XH2O/∂(NBO/Si)]P,T is in the −3 to −8 mol% H2O/(NBO/Si) range. Melts with NBO/Si similar to basaltic liquids (∼0.6-∼1.0) show (∂XH2O/∂T)P<0, whereas more polymerized melts exhibit (∂XH2O/∂T)P>0. Complete miscibility between hydrous melt and aqueous fluid occurs in the 0.8-2 GPa pressure range for melts with NBO/Si ≤0.5 at T >1100°C. Miscibility occurs at lower pressure the more polymerized the melt.  相似文献   

5.
Solubility mechanisms of water in depolymerized silicate melts quenched from high temperature (1000°-1300°C) at high pressure (0.8-2.0 GPa) have been examined in peralkaline melts in the system Na2O-SiO2-H2O with Raman and NMR spectroscopy. The Na/Si ratio of the melts ranged from 0.25 to 1. Water contents were varied from ∼3 mol% and ∼40 mol% (based on O = 1). Solution of water results in melt depolymerization where the rate of depolymerization with water content, ∂(NBO/Si)/∂XH2O, decreases with increasing total water content. At low water contents, the influence of H2O on the melt structure resembles that of adding alkali oxide. In water-rich melts, alkali oxides are more efficient melt depolymerizers than water. In highly polymerized melts, Si-OH bonds are formed by water reacting with bridging oxygen in Q4-species to form Q3 and Q2 species. In less polymerized melts, Si-OH bonds are formed when bridging oxygen in Q3-species react with water to form Q2-species. In addition, the presence of Na-OH complexes is inferred. Their importance appears to increase with Na/Si. This apparent increase in importance of Na-OH complexes with increasing Na/Si (which causes increasing degree of depolymerization of the anhydrous silicate melt) suggests that water is a less efficient depolymerizer of silicate melts, the more depolymerized the melt. This conclusion is consistent with recently published 1H and 29Si MAS NMR and 1H-29Si cross polarization NMR data.  相似文献   

6.
The sodium solubility in silicate melts in the CaO-MgO-SiO2 (CMS) system at 1400 °C has been measured by using a closed thermochemical reactor designed to control alkali metal activity. In this reactor, Na(g) evaporation from a Na2O-xSiO2 melt imposes an alkali metal vapor pressure in equilibrium with the molten silicate samples. Because of equilibrium conditions in the reactor, the activity of sodium-metal oxide in the molten samples is the same as that of the source, i.e., aNa2O(sample) = aNa2O(source). This design also allows to determine the sodium oxide activity coefficient in the samples. Thirty-three different CMS compositions were studied. The results show that the amount of sodium entering from the gas phase (i.e., Na2O solubility) is strongly sensitive to silica content of the melt and, to a lesser extent, the relative amounts of CaO and MgO. Despite the large range of tested melt compositions (0 < CaO and MgO < 40; 40 < SiO2 < 100; in wt%), we found that Na2O solubility is conveniently modeled as a linear function of the optical basicity (Λ) calculated on a Na-free basis melt composition. In our experiments, γNa2O(sample) ranges from 7 × 10−7 to 5 × 10−6, indicating a strongly non-ideal behavior of Na2O solubility in the studied CMS melts (γNa2O(sample) ? 1). In addition to showing the effect of sodium on phase relationships in the CMS system, this Na2O solubility study brings valuable new constraints on how melt structure controls the solubility of Na in the CMS silicate melts. Our results suggest that Na2O addition causes depolymerization of the melt by preferential breaking of Si-O-Si bonds of the most polymerized tetrahedral sites, mainly Q4.  相似文献   

7.
The solubility of H2O–CO2–Cl-containing fluids of various concentrations (0, 3, 10, and 23 wt % of HCl and from 0 to ~8–15 wt % of CO2) in dacite, phonolite, and rhyolite melts at 1000°C and 200 MPa was studied in experiments. It was shown that the Cl concentration in the melt increased substantially from rhyolite to phonolite and dacite (up to 0.25, 0.85, and 1.2 wt %, respectively). The introduction of CO2 into the system resulted in an increase in the Cl content in the melt composition by 20–25%. One may suppose that Cl reactivity in a fluid increases in the presence of CO2 to cause growth of the Cl content in the melt. The introduction of CO2 into the system considerably affects the content of H2O in aluminosilicate melts as well. Thus, the addition of CO2 decreases the H2O content in the melt by ~0.5–1.0 wt %. The decrease in the H2O content in an aluminosilicate melt is probably caused by fluid dilution with CO2 resulting in a decrease in the H2O mole fraction and fugacity in the fluid.  相似文献   

8.
Small hexagonal and triangular platelets of molybdenite (MoS2), 5 to 25 m in diameter, were identified in phenocrysts and matrix glass of unaltered felsic volcanic rocks from Pantelleria, Italy. The MoS2 occurs commonly in pantellerites (peralkaline rhyolites), rarely in pantelleritic trachytes, and never in trachytes. The occurrence of euhedral MoS2 platelets in all phenocryst phases, in matrix glass, and even in some melt inclusions indicates that MoS2 precipitated directly from the peralkaline melt. Despite MoS2 saturation, the melt (glass) contains greater than 95% of the Mo in Pantellerian rocks: X-ray fluorescence analyses of 20 whole rocks and separated glasses show that whole rocks consistently contain less Mo than corresponding matrix glasses, the differences being in proportion to phenocryst abundances. The Mo contents increase with differentiation from trachytes (2–12 ppm) to pantellerites (15–25 ppm) and correlate positively with incompatible elements such as Th, Y, and Nb. The Mo concentrations, as determined by secondary ion mass spectrometry, are essentially the same in matrix glasses and melt inclusions, showing that Mo did not partition strongly into a volatile fluid phase during outgassing. The high Mo contents of the pantellerites (relative to metaluminous magmas with 1–5 ppm) may be due to several factors: (1) the enhanced stability of highly charged cations (such as Mo6+, U4+, and Zr4+) in peralkaline melts; (2) the rarity of Fe-Ti oxides and litanite into which Mo might normally partition; (3) reduced volatility of Mo in low fO2, H2O-poor (1–2 wt%) peralkaline magmas. Geochemical modeling indicates that the precipitation of MoS2 can be explained simply by the drop in temperature during magmatic differentiation. The occurrence of MoS2 in pantellerites may result from their high Mo concentrations and low redox state (Ni/NiO=-2.5) relative to metaluminous magmas, causing them to reach MoS2 saturation at magmatic temperatures. The apparent absence of MoS2 microphenocrysts in more oxidized, metaluminous rhyolites may indicate that Mo is dissolved primarily as a hexavalent ion in those magmas.  相似文献   

9.
The effect of alkalis on the solubility of H2O and CO2 in alkali-rich silicate melts was investigated at 500 MPa and 1,250 °C in the systems with H2O/(H2O + CO2) ratio varying from 0 to 1. Using a synthetic analog of phonotephritic magma from Alban Hills (AH1) as a base composition, the Na/(Na + K) ratio was varied from 0.28 (AH1) to 0.60 (AH2) and 0.85 (AH3) at roughly constant total alkali content. The obtained results were compared with the data for shoshonitic and latitic melts having similar total alkali content but different structural characteristics, e.g., NBO/T parameter (the ratio of non-bridging oxygens over tetrahedrally coordinated cations), as those of the AH compositions. Little variation was observed in H2O solubility (melt equilibrated with pure H2O fluid) for the whole compositional range in this study with values ranging between 9.7 and 10.2 wt. As previously shown, the maximum CO2 content in melts equilibrated with CO2-rich fluids increases strongly with the NBO/T from 0.29 wt % for latite (NBO/T = 0.17) to 0.45 wt % for shoshonite (NBO/T = 0.38) to 0.90 wt % for AH2 (NBO/T = 0.55). The highest CO2 contents determined for AH3 and AH1 are 1.18 ± 0.05 wt % and 0.86 ± 0.12 wt %, respectively, indicating that Na is promoting carbonate incorporation stronger than potassium. At near constant NBO/T, CO2 solubility increases from 0.86 ± 0.12 wt % in AH1 [Na/(Na + K)] = 0.28, to 1.18 ± 0.05 wt % in AH3 [Na/(Na + K)] = 0.85, suggesting that Na favors CO2 solubility on an equimolar basis. An empirical equation is proposed to predict the maximum CO2 solubility at 500 MPa and 1,100–1,300 °C in various silicate melts as a function of the NBO/T, (Na + K)/∑cations and Na/(Na + K) parameters: \({\text{wt}}\% \;{\text{CO}}_{2} = - 0.246 + 0.014\exp \left( {6.995 \cdot \frac{\text{NBO}}{T}} \right) + 3.150 \cdot \frac{{{\text{Na}} + {\text{K}}}}{{\varSigma {\text{cations}}}} + 0.222 \cdot \frac{\text{Na}}{{{\text{Na}} + {\text{K}}}}.\) This model is valid for melt compositions with NBO/T between 0.0 and 0.6, (Na + K)/∑cation between 0.08 and 0.36 and Na/(Na + K) ratio from 0.25 to 0.95 at oxygen fugacities around the quartz–fayalite–magnetite buffer and above.  相似文献   

10.
We report results of experiments constraining oxygen isotope fractionations between CO2 vapor and Na-rich melilitic melt at 1 bar and 1250 and 1400°C. The fractionation factor constrained by bracketed experiments, 1000.lnαCO2-Na melilitic melt, is 2.65±0.25 ‰ (±2σ; n=92) at 1250°C and 2.16±0.16 ‰ (2σ; n=16) at 1400°C. These values are independent of Na content over the range investigated (7.5 to 13.0 wt. % Na2O). We combine these data with the known reduced partition function ratio of CO2 to obtain an equation describing the reduced partition function ratio of Na-rich melilite melt as a function of temperature. We also fit previously measured CO2-melt or -glass fractionations to obtain temperature-dependent reduced partition function ratios for all experimentally studied melts and glasses (including silica, rhyolite, albite, anorthite, Na-rich melilite, and basalt). The systematics of these data suggest that reduced partition function ratios of silicate melts can be approximated either by using the Garlick index (a measure of the polymerization of the melt) or by describing melts as mixtures of normative minerals or equivalent melt compositions. These systematics suggest oxygen isotope fractionation between basalt and olivine at 1300°C of approximately 0.4 to 0.5‰, consistent with most (but not all) basalt glass-olivine fractionations measured in terrestrial and lunar basalts.  相似文献   

11.
Trevor H. Green  John Adam 《Lithos》2002,61(3-4):271-282
The solubility of Ti- and P-rich accessory minerals has been examined as a function of pressure and K2O/Na2O ratio in two series of highly evolved silicate systems. These systems correspond to (a) alkaline, varying from alkaline to peralkaline with increasing K2O/Na2O ratio; and (b) strongly metaluminous (essentially trondhjemitic at the lowest K2O/Na2O ratio) and remaining metaluminous with increasing K2O/Na2O ratio (to 3). The experiments were conducted at a fixed temperature of 1000 °C, with water contents varying from 5 wt.% at low pressure (0.5 GPa), increasing through 5–10 wt.% at 1.5–2.5 GPa to 10 wt.% at 3.5 GPa. Pressure was extended outside the normal crustal range, so that the results may also be applied to derivation of hydrous silicic melts from subducted oceanic crust.

For the alkaline composition series, the TiO2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure but is unchanged with increasing K content (at fixed pressure). The P2O5 content of the alkaline melts at apatite saturation increases with increased pressure at 3.5 GPa only, but decreases with increasing K content (and peralkalinity). For the metaluminous composition series (termed as “trondhjemite-based series” (T series)), the TiO2 content of the melt at Ti-rich mineral saturation decreases with increasing pressure and with increasing K content (at fixed pressure). The P2O5 content of the T series melts at apatite saturation is unchanged with increasing pressure, but decreases with increasing K content. The contrasting results for P and Ti saturation levels, as a function of pressure in both compositions, point to contrasting behaviour of Ti and P in the structure of evolved silicate melts. Ti content at Ti-rich mineral saturation is lower in the alkaline compared with the T series at 0.5 GPa, but is similar at higher pressures, whereas P content at apatite saturation is lower in the T series at all pressures studied. The results have application to A-type granite suites that are alkaline to peralkaline, and to I-type metaluminous suites that frequently exhibit differing K2O/Na2O ratios from one suite to another.  相似文献   


12.
Thermodynamic properties of PbO-SiO2 melts, obtained from published data and calculated from freezing point depressions, reflect the gradual polymerization of silicate anions in the melt as the SiO2PbO ratio is increased. The free energy of mixing curve at 1000°C has a minimum at 40 mole % SiO2 and is convex-upward between 72 and 98 mole % SiO2. The latter is an indication of metastable liquid immiscibility. The free energy minimum is correlated with the maximum in the distribution of nonbridging oxygens in the melt. In SiO2-poor melts, the activities of PbO and SiO2 (pure liquid standard states) show sharp negative deviations from ideality. The PbO activity reflects the paucity of free oxygen species in the melt whereas the SiO2 activity reflects the depolymerized state of the silicate anions. In more SiO2-rich melts, the activity of SiO2 shows a positive deviation from ideality which is qualitatively correlated to a polymerization parameter. The heat of mixing term has a minimum of ?2000 cal at 35 mole % SiO2 and a maximum of +200 cal at 90 mole % SiO2. The minimum is associated with the exothermic heat effect obtained during the reaction (O0) + (O2?) = 2(O?), whereas the maximum corresponds to the endothermic heat effect obtained when coordination polyhedra of oxygens form around the Pb cation. The entropy of mixing curve has the same form but is systematically smaller than a theoretical curve calculated on the assumption of random mixing of oxygen species. The discrepancy is due to the entropy loss obtained by the clustering of oxygen species to form complex silicate species.  相似文献   

13.
Carbon dioxide solubilities in H2O-free hydrous silicate melts of natural andesite (CA), tholeiite (K 1921), and olivine nephelinite (OM1) compositions have been determined employing carbon-14 beta-track mapping techniques. The CO2 solubility increases with increasing pressure, temperature, and degree of silica-undersaturation of the silicate melt. At 1650° C, CO2 solubility in CA increases from 1.48±0.05 wt % at 15 kbar to 1.95±0.03 wt % at 30 kbar. The respective solubilities in OM1 are 3.41±0.08 wt % and 7.11±0.10 wt %. The CO2 solubility in K1921 is intermediate between those of CA and OM1 compositions. At lower temperatures, the CO2 contents of these silicate melts are lower, and the pressure dependence of the solubility is less pronounced. The presence of H2O also affects the CO2 solubility (20–30% more CO2 dissolves in hydrous than in H2O-free silicate melts); the solubility curves pass through an isothermal, isobaric maximum at an intermediate CO2/(CO2+H2O) composition of the volatile phase. Under conditions within the upper mantle where carbonate minerals are not stable and CO2 and H2O are present a vapor phase must exist. Because the solubility of CO2 in silicate melts is lower than that of H2O, volatiles must fractionate between the melt and vapor during partial melting of peridotite. Initial low-temperature melts will be more H2O-rich than later high-temperature melts, provided vapor is present during the melting. Published phase equilibrium data indicate that the compositional sequence of melts from peridotite +H2O+CO2 parent will be andesite-tholeiite-nephelinite with increasing temperature at a pressure of about 20 kbar. Examples of this sequence may be found in the Lesser Antilles and in the Indonesian Island Arcs.  相似文献   

14.
To investigate the influence of temperature and composition on the diffusivities of dissolved carbon dioxide and argon in silicate melts, diffusion experiments were performed at magmatic pressure and temperature conditions in (a) albite melts with excess Na2O (0-8.6 wt%) and a constant Si/Al ratio of 3, and (b) albite70quartz30 to jadeite melts with decreasing SiO2 content and a constant Na/Al ratio of 1. We obtained diffusion coefficients at 500 MPa and 1323-1673 K. In the fully polymerized system Ab70Qz30 - Jd, the change in composition only has a weak effect on bulk CO2 diffusivity, but Ar diffusivity increases clearly with decreasing SiO2 content. In the system Ab + Na2O, bulk CO2 and Ar diffusivity increase significantly with gradual depolymerisation. The relatively small change in composition on molar basis in the depolymerized system leads to a significantly larger change in diffusivities compared to the fully polymerized Ab70Qz30-Jd join. Within error, activation energies for bulk CO2 and Ar diffusion in both systems are identical with decreasing silica content (Ab + Na2O: 159 ± 25 kJ mol−1 for bulk CO2 and 130 ± 8 kJ mol−1 for Ar; Ab70Qz30-Jd: 163 ± 16 kJ mol−1 for bulk CO2 and 148 ± 15 kJ mol−1 for Ar) even though this results in depolymerisation in one system and not the other.Although there is a variation in CO2 speciation with changing composition as observed in quenched glasses, it has previously established that this is not a true representation of the species present in the melt, with the ratio of molecular CO2 to carbonate decreasing during quenching. Thus, diffusion coefficients for the individual CO2 species cannot be directly derived by measuring molecular CO2 and CO32- concentration-distance profiles in the glasses. To obtain diffusivities of individual CO2 species, we have made two assumptions that (1) inert Ar can be used as a proxy for molecular CO2 diffusion characteristics as shown by our previous work and (2) the diffusivity of CO32− can be calculated assuming it is identical to network forming components (Si4+ and Al3+). This is derived from viscosity data (Eyring eqn.) and suggests that CO32− diffusion would be several orders of magnitude slower than molecular CO2 diffusion.The systematics of measured bulk CO2 diffusivity rates and comparison with the Ar proxy all suggest that the faster molecular CO2 species is much more dominant in melts than measurements on resulting quenched glasses would suggest. This study has confirmed an observation of surprisingly consistent bulk CO2 diffusivity across a range of natural compositions were Ar diffusivity significantly increases. This is consistent with an actual increase in molecular CO2 mobility (similar to Ar) that is combined with an increase in the proportion of the slower carbonate in the melt.These results demonstrate that the CO2 diffusion and speciation model provides an insight into the transport processes in the melt and is promising and an alternative tool to in situ speciation measurements at magmatic conditions, which at the moment are technically extremely difficult. We present the first high pressure high temperature in situ MIR spectra of a CO2 bearing albitic glass/melt suggesting that molecular CO2 is a stable species at high temperature, which is qualitatively consistent with the modelled CO2 speciation data.  相似文献   

15.
Hydrothermal experiments were carried out at 2 kbar water pressure, 700 °–800 ° C, with the objective of determining the level of dissolved Zr required for precipitation of zircon from melts in the system SiO2-Al2O3-Na2O-K2O. The saturation level depends strongly upon molar (Na2O + K2O)/Al2O3 of the melts, with remarkably little sensitivity to temperature, SiO2 concentration, or melt Na2O/ K2O. For peraluminous melts and melts lying in the quartz-orthoclase-albite composition plane, less than 100 ppm Zr is required for zircon saturation. In peralkaline melts, however, zircon solubility shows pronounced, apparently linear, dependence upon (Na2O + K2O)/Al2O3, with the amount of dissolvable Zr ranging up to 3.9 wt.% at (Na2O + K2O)/Al2O3 = 2.0. Small amounts (1 wt.% each) of dissolved CaO and Fe2O3 cause a 25% relative reduction of zircon solubility in peralkaline melts.The main conclusion regarding zirconium/zircon behavior in nature is that any felsic, non-peralkaline magma is likely to contain zircon crystals, because the saturation level is so low for these compositions. Zircon fractionation, and its consequences to REE, Th, and Ta abundances must, therefore, be considered in modelling the evolution of these magmas. Partial melting in any region of the Earth's crust that contains more than 100 ppm Zr will produce granitic magmas whose Zr contents are buffered at constant low (< 100 ppm) values; unmelted zircon in the residual rock of such a melting event will impart to the residue a characteristic U- or V-shaped REE abundance pattern. In peralkaline, felsic magmas such as those that form pantellerites and comendites, extreme Zr (and REE, Ta) enrichment is possible because the feldspar fractionation that produces these magmas from non-peralkaline predecessors does not drive the melt toward saturation in zircon.Zircon solubility in felsic melts appears to be controlled by the formation of alkali-zirconosilicate complexes of simple (2:1) alkali oxide: ZrO2 stoichiometry.  相似文献   

16.
Solubility and solution mechanisms in silicate melts of oxidized and reduced C-bearing species in the C-O-H system have been determined experimentally at 1.5 GPa and 1400 °C with mass spectrometric, NMR, and Raman spectroscopic methods. The hydrogen fugacity, fH2, was controlled in the range between that of the iron-wüstite-H2O (IW) and the magnetite-hematite-H2O (MH) buffers. The melt polymerization varied between those typical of tholeiitic and andesitic melts.The solubility of oxidized (on the order of 1-2 wt% as C) and reduced carbon (on the order of 0.15-0.35 wt% as C) is positively correlated with the NBO/Si (nonbridging oxygen per silicon) of the melt. At given NBO/Si-value, the solubility of oxidized carbon is 2-4 times greater than under reducing conditions. Oxidized carbon dioxide is dissolved as complexes, whereas the dominant reduced species in melts are CH3-groups forming bonds with Si4+ together with molecular CH4. Formation of complexes results in silicate melt polymerization (decreasing NBO/Si), whereas solution of reduced carbon results in depolymerization of melts (increasing NBO/Si).Redox melting in the Earth’s interior has been explained with the aid of the different solution mechanisms of oxidized and reduced carbon in silicate melts. Further, effects of oxidized and reduced carbon on melt viscosity and on element partitioning between melts and minerals have been evaluated from relationships between melt polymerization and dissolved carbon combined with existing experimental data that link melt properties and melt polymerization. With total carbon contents in the melts on the order of several mol%, mineral/melt element partition coefficients and melt viscosity can change by several tens to several hundred percent with variable redox conditions in the range of the Earth’s deep crust and upper mantle.  相似文献   

17.
P. Bhalla  F. Holtz  R.L. Linnen  H. Behrens 《Lithos》2005,80(1-4):387-400
The aim of this experimental study was to determine the solubility of cassiterite in natural topaz- and cassiterite-bearing granite melts at temperatures close to the solidus. Profiles of Sn concentrations at glass–crystal (SnO2) interface were determined following the method of (Harrison, T.M., Watson, E.B., 1983. Kinetics of zircon dissolution and zirconium diffusion in granitic melts of variable water content. Contributions to Mineralogy and Petrology 84, 66–72). The cassiterite concentration calculated at the SnO2–glass interface is the SnO2 solubility. Experiments were performed at 700–850 °C and 2 kbar using a natural F-bearing peraluminous granitic melt with 2.8 wt.% normative corundum. Slightly H2O-undersaturated to H2O-saturated melt compositions were chosen in order to minimize the loss of Sn to the noble element capsule walls. At the nickel–nickel oxide assemblage (Ni–NiO) oxygen fugacity buffer, the solubility of cassiterite in melts containing 1.12 wt.% F increases from 0.32 to 1.20 wt.% SnO2 with an increasing temperature from 700 to 850 °C. At the Ni–NiO buffer and a given corundum content, SnO2 solubility increases by 10% to 20% relative to an increase of F from 0 to 1.12 wt.%. SnO2 solubility increases by 20% relative to increasing Cl content from 0 to 0.37 wt.% in synthetic granitic melts at 850 °C. We show that Cl is at least as important as F in controlling SnO2 solubility in evolved peraluminous melts at oxygen fugacities close to the Ni–NiO buffer. In addition to the strong effects of temperature and fO2 on SnO2 solubility, an additional controlling parameter is the amount of excess Al (corundum content). At Ni–NiO and 850 °C, SnO2 solubility increases from 0.47 to 1.10 wt.% SnO2 as the normative corundum content increases from 0.1 to 2.8 wt.%. At oxidizing conditions (Ni–NiO +2 to +3), Sn is mainly incorporated as Sn4+ and the effect of excess Al seems to be significantly weaker than at reducing conditions.  相似文献   

18.
A. A. Borisov 《Petrology》2008,16(6):552-564
Model silicate melts with variable Al2O3 and SiO2 contents were experimentally saturated with alkalis at a total pressure of 1 atm and temperatures of 1300–1470°C, using the crucible supported loop technique. It was shown that Al2O3 content has little influence on the degree of silicate melt saturation with K and Na. In contrast, SiO2 content strongly affects the solubility of alkalis in silicate melts. Model calculations were performed to evaluate the behavior of alkalis during the contamination/mixing of basic and silicic magmas.  相似文献   

19.
A large body of recent work has linked the origin of Si-Al-rich alkaline glass inclusions to metasomatic processes in the upper mantle. This study examines one possible origin for these glass inclusions, i.e., the dissolution of orthopyroxene in Si-poor alkaline (basanitic) melt. Equilibrium dissolution experiments between 0.4 and 2 GPa show that secondary glass compositions are only slightly Si enriched and are alkali poor relative to natural glass inclusions. However, disequilibrium experiments designed to examine dissolution of orthopyroxene by a basanitic melt under anhydrous, hydrous and CO2-bearing conditions show complex reaction zones consisting of olivine, ± clinopyroxene and Si-rich alkaline glass similar in composition to that seen in mantle xenoliths. Dissolution rates are rapid and dependent on volatile content. Experiments using an anhydrous solvent show time dependent dissolution rates that are related to variable diffusion rates caused by the saturation of clinopyroxene in experiments longer than 10 minutes. The reaction zone glass shows a close compositional correspondence with natural Si-rich alkaline glass in mantle-derived xenoliths. The most Si-and alkali-rich melts are restricted to pressures of 1 GPa and below under anhydrous and CO2-bearing conditions. At 2 GPa glass in hydrous experiments is still Si-␣and alkali-rich whereas glass in the anhydrous and CO2-bearing experiments is only slightly enriched in SiO2 and alkalis compared with the original solvent. In the low pressure region, anhydrous and hydrous solvent melts yield glass of similar composition whereas the glass from CO2-bearing experiments is less SiO2 rich. The mechanism of dissolution of orthopyroxene is complex involving rapid incongruent breakdown of the orthopyroxene, combined with olivine saturation in the reaction zone forming up to 60% olivine. Inward diffusion of CaO causes clinopyroxene saturation and uphill diffusion of Na and K give the glasses their strongly alkaline characteristics. Addition of Na and K also causes minor SiO2 enrichment of the reaction glass by increasing the phase volume of olivine. Olivine and clinopyroxene are transiently stable phases within the reaction zone. Clinopyroxene is precipitated from the reaction zone melt near the orthopyroxene crystal and redissolved in the outer part of the reaction zone. Olivine defines the thickness of the reaction zone and is progressively dissolved in the solvent as the orthopyroxene continues to dissolve. Although there are compelling reasons for supporting the hypothesis that Si-rich alkaline melts are produced in the mantle by orthopyroxene – melt reaction in the mantle, there are several complications particularly regarding quenching in of disequilibrium reaction zone compositions and the mobility of highly polymerized melts in the upper mantle. It is considered likely that formation of veins and pools of Si-rich alkaline glass by orthopyroxene – melt reaction is a common process during the ascent of xenoliths. However, reaction in situ within the mantle will lead to equilibration and therefore secondary melts will be only moderately siliceous and alkali poor. Received: 24 August 1998 / Accepted: 2 December 1998  相似文献   

20.
The effects of F, B2O3 and P2O5 on the H2O solubility in a haplogranite liquid (36 wt. % SiO2, 39 wt. % NaAlSi3O8, 25 wt. % KAlSi3O8) have been determined at 0.5, 1, 2, and 3 kb and 800, 850, and 900°C. The H2O solubility increases with increasing F and B content of the melt. The H2O solubility increase in more important at high pressure (2 and 3 kb) than at low pressure (0.5 kb). At 2 kb and 800°C, the H2O solubility increases from 5.94 to 8.22 wt. % H2O with increasing F content in the melt from 0 to 4.55 wt. %, corresponding to a linear H2O solubility increase of 0.53 mol H2O/mol F. With addition of 4.35 wt. % B2O3, the H2O solubility increases up to 6.86 wt. % H2O at 2 kb and 800°C, corresponding to a linear increase of 1.05 mol H2O/mol B2O3. The results allow to define the individual effects of fluorine and boron on H2O solubility in haplogranitic melts with compositions close to that of H2O-saturated thermal minima (at 0.5–3 kb). Although P has a dramatic effect on the phase relations in the haplogranite system, its effect on the H2O solubility was found to be negligible in natural melt compositions. The concominant increase in H2O solubility and F can not be interpreted on the basis of the available spectroscopic data (existence of hydrated aluminofluoride complexes or not). In contrast, hydrated borates or more probably boroxol complexes have been demonstrated in B-bearing hydrous melts.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号