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1.
Basaltic glass and diabase were reacted with seawater at 70°C at 1 bar and 150°C at 500 bars to determine fluid composition and alteration mineralogy. All experiments were performed at a water/ rock mass ratio of 10.The changes in seawater chemistry depended on temperature and crystallinity of the basalt. The experiment at 70°C produced a slight but continuous loss of Mg, Na and K and enrichment of Ca and SiO2 in the seawater while pH decreased slowly. At 150°C, in contrast, Mg and SO4 were quickly and quantitatively removed while Ca, SiO2, Na, K, Fe, Mn and Ba were added to the seawater. pH rose to values between 5.5 and 6.5 after an initial drop to lower values. Basalt glass reacted more extensively at 150°C than diabase.Smectite was the major alteration product (iron-rich saponite) at 150°C for both the glass and diabase experiments. Smectite from the diabase experiment was well crystallized while that from the glass experiment was poorly crystallized. The smectites are similar to smectites found in altered oceanic ophiolitic basalts.  相似文献   

2.
Fresh mid-ocean ridge basalt glass and diabase have been reacted with seawater at 150–300°C, 500 bar, and water/rock mass ratios of 50, 62, and 125, using experimental apparatus which allowed on-line sampling of solution to monitor reaction progress. These experiments characterize reaction under what we have called “seawater-dominated” conditions of hydrothermal alteration.In an experiment at 300°C, basalt glass undergoing alteration removed nearly all Mg2+ from an amount of seawater 50 times its own mass. In the process, the glass was converted entirely to mixed-layer smectite-chlorite, anhydrite, and minor hematite. Removal of Mg from seawater occurred as a Mg(OH)2 component incorporated into the secondary clay. This produced a precipitous drop in solution pH early in the experiment, accompanied by a dramatic increase in the concentrations of Fe, Mn, and Zn in solution. As Mg removal neared completion and the glass was hydrolyzed, pH rose again and heavy metal concentrations dropped.At water/rock ratios of 62 and 125 and 150–300°C, the mineral assemblage produced was similar to that at a water/rock ratio of 50. Solution chemistry, however, contrasted with the earlier experiment in that Mg concentrations in solution were greater and pH lower. This caused significant leaching of heavy metals. At 300°C nearly all of the Na, Ca, Cu, Zn, and CO2 and most of the K, Ba, Sr, and Mn were leached from the silicates. H2S, Al, Si, and possibly Co were also significantly mobilized, whereas V, Cr, and Ni were not. Little or no seawater sulfate was reduced.Although submarine hot spring solutions sampled to date along mid-ocean ridges clearly come from rock-dominated hydrothermal systems, evidence from ocean floor metabasalts and from heat flow studies indicates that seawater-dominated conditions of alteration prevail at least locally both in axial hightemperature systems and in ridge flank systems at lower temperatures.  相似文献   

3.
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4.
Fresh tholeiitic basalt glass has been reacted with seawater at 150°C, (water/rock mass ratio of 10), and fresh diabase has been reacted with a Na-K-Ca-Cl fluid at 375°C (water/rock mass ratios of 1, 2, and 5) to understand better the role of temperature, basalt composition, and water/rock mass ratio on the direction and magnitude of B and Li exchange during basalt alteration. At 150°C, slight but nevertheless significant amounts of B and Li were removed from seawater and incorporated into a dominantly smectite alteration phase. At 375°C, however, B and Li were leached from basalt. B behaved as a “soluble” element and attained concentrations in solution limited only by the B concentration in basalt and the water/rock mass ratio. Li, however, was less mobile. For example, at water/rock mass ratios of 1, 2, and 5, the percent of Li leached from basalt was 58, 70, and 92% respectively. This suggests some mineralogic control on Li mobility during hydrothermal alteration of basalt, especially at low-water/rock mass ratios. In general, these results, as well as those for B, are consistent with the temperature-dependent chemistry of altered seafloor basalt and the chemistry of ridge crest hydrothermal fluids.Based on the distribution and chemistry of products of seafloor weathering, low (≤ 150°C) and high-temperature hydrothermal alteration of basalt, and the chemistry of ridge crest hydrothermal fluids, it was estimated that alteration of the oceanic crust is a Li source for seawater. This is not true for B, however, since the hot spring flux estimated for B is balanced by low-temperature basalt alteration. These data, coupled with B and Li flux estimates for other processes (e.g., continental weathering, clay mineral adsorption, authigenic silicate formation and formation of siliceous skeletal material) yield new insight into the B and Li geochemical cycles. Calculations performed here indicate relatively good agreement between the magnitude of B and Li sources and sinks. The geochemical cycle of B, however, may be affected by serpentinization of mantle derived peridotite in oceanic fracture zones. Serpentinites are conspicuously enriched in B and if the B source for these rocks is seawater, then an additional B sink exists which must be integrated into the B geochemical cycle. However, until more data are available in terms of areal extent of serpentinization, serpentite chemistry and isotopic composition, the importance of B in these rocks with respect to the B geochemical cycle remains speculative at best.  相似文献   

5.
The products of the 1974 eruption of Fuego, a subduction zone volcano in Guatemala, have been investigated through study of silicate melt inclusions in olivine. The melt inclusions sampled liquids in regions where olivine, plagioclase, magnetite, and augite were precipitating. Comparisons of the erupted ash, groundmass, and melt inclusion compositions suggest that the inclusions represent samples of liquids present in a thermal boundary layer of the magma body. The concentrations of H2O and CO2 in glass inclusions were determined by a vacuum fusion manometric technique using individual olivine crystals (Fo77 to Fo71) with glass inclusion compositions that ranged from high-alumina basalt to basaltic andesite. Water, Cl, and K2O concentrations increased by a factor of two as the olivine crystals became more iron-rich (Fo77 to Fo71) and as the glass inclusions increased in SiO2 from 51 to 54 wt.% SiO2. The concentration of H2O in the melt increased from 1.6 wt.% in the least differentiated liquid to about 3.5% in a more differentiated liquid. Carbon dioxide is about an order of magnitude less abundant than H2O in these inclusions. The gas saturation pressures for pure H2O in equilibrium with the melt inclusions, which were calculated from the glass inclusion compositions using the solubility model of Burnham (1979), are given approximately by P(H2O)(Pa)=(SiO2−48.5 wt.%) × 1.45 × 107. The concentrations of water in the melt and the gas saturation pressures increased from about 1.5% to 3.5% and from 300 to 850 bars, respectively, during pre-eruption crystallization.  相似文献   

6.
Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally Ca2SiO2[OH,F]4) and hydrogarnet assemblages require a water-rich fluid containing F−, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350–250°C, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400°C. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits.  相似文献   

7.
The solubility of quartz has been determined in borax buffer solutions having total boron concentrations of 0.10, 0.20, 0.40 and 0.60 mol kg?1 and over the temperature range 130–350°C at the saturated vapour pressure of the system. The first ionization constant of silicic acid was calculated from the solubility data and varied from ?logK1 = 8.88 (± 0.15) at 130°C to ?logK1 = 10.06 (± 0.20) at 350°C. The solubility of quartz in these solutions was due to the presence of the three species, H4SiO4, H3SiO4? and NaH3SiO4°. The equilibrium constant for the reaction, Na+ + H3SiO4? = NaH3SiO4° extended from log Kas = 1.18?1.40 (± 0.20) over the temperature interval 135–301°C. The formation of NaH3SiO4° ion pairs was concluded to contribute significantly to the solubility of quartz in alkaline hydrothermal solutions when pH > 8 and sodium concentration exceeds 0.10 mol kg?1.  相似文献   

8.
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9.
Major element, Sr isotope and trace element data for 16 elements are reported for various weathered zones in three submarine basalt pillows. During the initial stages of alteration, it appears that SiO2, Al2O3, CaO, S, and Ga are lost from the basalt, whereas Fe2O3, total Fe, MnO, K2O, H2O, Cl, B, Rb, and Cs increase. Sr87/Sr86 ratios also increase during weathering. MgO, Na2O, P2O5, Ba, Ni and Cu show significant (10–50%) but less consistent changes. TiO2, Zr, Sr and V show only minor changes (<7%). Zn, Cr, Co, Y and Nb show no changes outside experimental error.  相似文献   

10.
This study focuses on the production of convergent margin calc-alkaline andesites by crystallization–differentiation of basaltic magmas in the lower to middle crust. Previous experimental studies show that dry, reduced, subalkaline basalts differentiate to tholeiitic (high Fe/Mg) daughter liquids, but the influences of H2O and oxidation on differentiation are less well established. Accordingly, we performed crystallization experiments at controlled oxidized fO2 (Re–ReO2 ≈ ΔNi–NiO + 2) on a relatively magnesian basalt (8.7 wt% MgO) typical of mafic magmas erupted in the Cascades near Mount Rainier, Washington. The basalt was synthesized with 2 wt% H2O and run at 900, 700, and 400 MPa and 1,200 to 950 °C. A broadly clinopyroxenitic crystallization interval dominates near the liquidus at 900 and 700 MPa, consisting of augite + olivine + orthopyroxene + Cr-spinel (in decreasing abundance). With decreasing temperature, plagioclase crystallizes, Fe–Ti-oxide replaces spinel, olivine dissolves, and finally amphibole appears, producing gabbroic and then amphibole gabbroic crystallization stages. Enhanced plagioclase stability at lower pressure narrows the clinopyroxenitic interval and brings the gabbroic interval toward the liquidus. Liquids at 900 MPa track along Miyashiro’s (Am J Sci 274(4):321–355, 1974) tholeiitic versus calc-alkaline boundary, whereas those at 700 and 400 MPa become calc-alkaline at silica contents ≥56 wt%. This difference is chiefly due to higher temperature appearance of magnetite (versus spinel) at lower pressures. Although the evolved liquids are similar in many respects to common calc-alkaline andesites, the 900 and 700 MPa liquids differ in having low CaO concentrations due to early and abundant crystallization of augite, with the result that those liquids become peraluminous (ASI: molar Al/(Na + K + 2Ca) > 1) at ≥61 wt% SiO2, similar to liquids reported in other studies of the high-pressure crystallization of hydrous basalts (Müntener and Ulmer in Geophys Res Lett 33(21):L21308, 2006). The lower-pressure liquids (400 MPa) have this same trait, but to a lesser extent due to more abundant near-liquidus plagioclase crystallization. A compilation of >6,500 analyses of igneous rocks from the Cascades and the Sierra Nevada batholith, representative of convergent margin (arc) magmas, shows that ASI increases continuously and linearly with SiO2 from basalts to rhyolites or granites and that arc magmas are not commonly peraluminous until SiO2 exceeds 69 wt%. These relations are consistent with plagioclase accompanying mafic silicates over nearly all the range of crystallization (or remelting). The scarcity of natural peraluminous andesites shows that progressive crystallization–differentiation of primitive basalts in the deep crust, producing early clinopyroxenitic cumulates and evolved liquids, does not dominate the creation of intermediate arc magmas or of the continental crust. Instead, mid- to upper-crustal differentiation and/or open-system processes are critical to the production of intermediate arc magmas. Primary among the open-system processes may be extraction of highly evolved (granitic, rhyolitic) liquids at advanced degrees of basalt solidification (or incipient partial melting of predecessor gabbroic intrusions) and mixing of such liquids into replenishing basalts. Furthermore, if the andesitic-composition continents derived from basaltic sources, the arc ASI–SiO2 relation shows that the mafic component returned to the mantle was gabbroic in composition, not pyroxenitic.  相似文献   

11.
The concentrations of Na, Al, and Si in an aqueous fluid in equilibrium with natural albite, paragonite, and quartz have been measured between 350°C and 500°C and 1 to 2.5 kbar. Si is the dominant solute in solution and is near values reported for quartz solubility in pure H2O. At 1 kbar the concentrations of Na and Al remain fairly constant from 350°C to 425°C but then decrease at 450°C. At 2 kbar, Na increases slightly with increasing temperature while Al remains nearly constant. Concentrations of Si, Na, and Al all increase with increasing pressure at constant temperature.The molality of Al is close to that of Na and is nearly a log unit greater than calculated molalities assuming Al(OH)03 is the dominant Al species. This indicates a Na-Al complex is the dominant Al species in solution as shown by Anderson and Burnham (1983) at higher temperature and pressure. The complex can be written as NaAl(OH)04 ± nSiO2 where n is the number of Si atoms in the complex. The value of n is not well constrained but appears to be less than or equal to 3.The results indicate Al can be readily transported in pure H2O solutions at temperatures and pressures as low as 350°C and 1 kbar.  相似文献   

12.
The solubility of rhodonite, represented by the reaction MnSiO3 (rhodonite) + 2HCl0 = MnCl20 + SiO2 (quartz) + H2O, was investigated experimentally in the temperature range 400°–700°C at 1 and 2 kbar by rapid-quench hydrothermal techniques and the Ag-AgCl buffer methods. Variations in the molalities of associated hydrogen chloride (mHCl0) as a function of the molalities of total Mn indicate that Mn in the fluid in equilibrium with the assemblage rhodonite + quartz is predominantly associated as MnCl20. The Mn:Cl in the fluid ?2, indicating that Mn+2 is the dominant oxidation state.The solubility data were used to calculate the equilibrium constant of the above reaction as a function of temperature, pressure, and the difference in Gibbs free energy of formation between MnCl20 and HCl0. The equilibrium constants of solubility for Mn minerals for which thermochemical data are available were also calculated. Calculated mineral solubilities were used in conjunction with the data of Frantz et al. (1981) to calculate the composition of supercritical fluids in equilibrium with Mn-bearing phases and assemblages. At 400°C and 1000 bars, supercritical fluids in equilibrium with olivines of compositions similar to those present in MORB tend to be enriched in Mn, despite the low mole fraction of tephroite in the olivine. Supercritical fluids in equilibrium with the assemblage quartz-hematite-rhodonite at 500° and 400°C and 1000 bars show high concentrations of Mn relative to Fe. Manganese concentrations in the fluids increase with decrease in the mole fraction of H, whereas Fe concentrations decrease. The data indicate that H fugacity plays a significant role in the separation of Mn from Fe in chloride-bearing hydrothermal fluids at supercritical temperatures.  相似文献   

13.
Fresh mid-ocean ridge basalt of varying crystallinity has been powdered and reacted with seawater and an artificial Na-K-Ca-Cl solution at 200–500°C and 500–1000 bar in sealed gold capsules. Water/rock mass ratios of 1–3 were used and durations ranged from 2 to 20 months.These time periods were sufficient for most elements to approach a steady-state concentration in solution which was determined by equilibrium with alteration minerals (Mg, SiO2, SO4), by rate of formation of these minerals (Na, Ca), or by depletion from the rock (K, B, Ba). The resulting solutions closely resemble the brines from the basalt-seawater geothermal system at Reykjanes, Iceland. Mg was almost completely removed from seawater into the alteration products smectite, tremolite-actinolite, or talc. Sulfate also was removed to low concentrations, both by precipitation of anyhydrite and by reduction to sulfide. Net transfer of Na from seawater into solids occurred in most experiments by formation of sodic feldspar and possibly analcime. Sr was removed from seawater in some experiments but showed no change or a small gain in others. SiO2, Ca, K, Ba, B and CO2 were leached from basalt and enriched in solution. SiO2 concentrations were controlled by saturation with quartz at 300°C and above. The principal Ca-bearing phases which formed were anhydrite, the hydrated Ca-silicate truscottite, tremolite-actinolite, and possibly wairakite. No K-rich phases formed. For some minerals the crystallinity of the starting basalt affected the amount which formed.Removal of Mg from seawater into solid alteration products occurred rapidly and was balanced largely by leaching of Ca from basalt. Net transfer of Na from seawater into solids occurred more slowly and was balanced mainly by leaching of additional Ca from basalt. Thus, reaction between seawater and basalt at low water/rock ratios can be considered to consist of two exchanges: Mg for Ca, and Na for Ca.  相似文献   

14.
The speciation of CO2 in dacite, phonolite, basaltic andesite, and alkali silicate melt was studied by synchrotron infrared spectroscopy in diamond anvil cells to 1,000 °C and more than 200 kbar. Upon compression to 110 kbar at room temperature, a conversion of molecular CO2 into a metastable carbonate species was observed for dacite and phonolite glass. Upon heating under high pressure, molecular CO2 re-appeared. Infrared extinction coefficients of both carbonate and molecular CO2 decrease with temperature. This effect can be quantitatively modeled as the result of a reduced occupancy of the vibrational ground state. In alkali silicate (NBO/t = 0.98) and basaltic andesite (NBO/t = 0.42) melt, only carbonate was detected up to the highest temperatures studied. For dacite (NBO/t = 0.09) and phonolite melts (NBO/t = 0.14), the equilibrium CO2 + O2? = CO3 2? in the melt shifts toward CO2 with increasing temperature, with ln K = ?4.57 (±1.68) + 5.05 (±1.44) 103 T ?1 for dacite melt (ΔH = ?42 kJ mol?1) and ln K = ?6.13 (±2.41) + 7.82 (±2.41) 103 T ?1 for phonolite melt (ΔH = ?65 kJ mol?1), where K is the molar ratio of carbonate over molecular CO2 and T is temperature in Kelvin. Together with published data from annealing experiments, these results suggest that ΔS and ΔH are linear functions of NBO/t. Based on this relationship, a general model for CO2 speciation in silicate melts is developed, with ln K = a + b/T, where T is temperature in Kelvin and a = ?2.69 ? 21.38 (NBO/t), b = 1,480 + 38,810 (NBO/t). The model shows that at temperatures around 1,500 °C, even depolymerized melts such as basalt contain appreciable amounts of molecular CO2, and therefore, the diffusion coefficient of CO2 is only slightly dependent on composition at such high temperatures. However, at temperatures close to 1,000 °C, the model predicts a much stronger dependence of CO2 solubility and speciation on melt composition, in accordance with available solubility data.  相似文献   

15.
Experimentally reversed portlandite and brucite solubilities were determined between 300° and 600°C and 1 to 3 kbar. In the portlandite runs the molality of Ca decreases with increasing pressure at constant temperature. For instance, at 2 kbar log molalities at 300°, 400°, 500° and 600°C give values of −2.34, −2.71, −3.18 and −4.18, respectively. At 500°C, pressures of 1, 2 and 3 kbar yield values of −4.40, −3.18 and −2.65. Distribution of species in solution can be calculated with the aid of data from Helgeson and co-workers assuming Ca++ is the dominant Ca species. These calculated Ca concentrations are within ± 0.2 log units of experimental values in most cases. The solubility reaction is, in all likelihood: 2H+ + Ca(OH)2 ↔a3 Ca++ + 2H2O.Although the computed pH's are close to 2 units greater than neutral, the solutions apparently contained no significant Ca(OH)+ or Ca(OH)2sq.Concentrations of Mg in the brucite runs show a sigmoidal behavior at 2 kbar as a function of temperature with log molalities of Mg of −4.00, −4.28, −4.14 and −4.60 at 350°, 450°, 550° and 600°C, respectively. Values at 1 kbar are lower and decrease monotonically from 350° to 550°C. Based on available thermodynamic data for Mg++ it appears that Mg(OH)+ is the dominant Mg species in solution. The solubility reaction is proposed to be: H+ + Mg(OH)2↔a3 Mg(OH)+ + H2O.With the aid of data of Helgeson and co-workers values of the equilibrium constant for H2O + Mg++ ↔a3 Mg(OH)+ + H+ necessary to account for the measured solution compositions can be calculated. These calculations indicate Mg(OH)+ becomes dominant at temperatures above 450°C at 2 kbar and above 360°C at 1 kbar at neutral pH.  相似文献   

16.
Aluminous, high-temperature clay minerals form from alteration of tholeiitic basaltic glass and calcic plagioclase during hydrothermal venting on the crest of the East Pacific Rise at 21°N. The clay alteration assemblages are layered crusts (up to 1 mm thick) completely replacing glass and calcic plagioclase adjacent to surfaces exposed to hydrothermal fluids. The interiors of the affected basalt samples have unaltered appearances and oxygen isotopic compositions just slightly heavier than that of MORB. The surficial alteration crusts are mixtures of beidellitic smectite (aluminous, dioctahedral), randomly interstratified mixed-layer Al-rich chlorite/smectite, minor chlorite, an x-ray amorphous aluminosilicate material, and possible minor serpentine (amesite). A δ18O value of +4.1 ± 0.2%. (SMOW) is determined for the beidellitic smectite. Assuming that this smectite equilibrated with hydrothermal fluid having an oxygen isotope value between that of seawater (0%.) and 350°C hydrothermal fluid from EPR, 21°N vents (+1.6%.), an equilibration temperature between 290°C and 360°C is calculated for the beidellitic smectite. This is substantially higher than any previously reported temperature for an oceanic smectite. The mixed-layer Al-rich chlorite/smectite has a δ18O value of +3.5%., which corresponds to equilibration at 295°–360°C. The aluminous composition of the alteration assemblage is uncommon for clay minerals produced by submarine hydrothermal basalt alteration. We suggest that this assemblage is largely the product of high-temperature interaction between basalt glass + plagioclase and Mg-poor, acidic hydrothermal fluids, with possibly some contribution of Mg from bottom seawater, and that the aluminous clays either incorporate Al3+ remobilized from basalt by lowpH hydrothermal fluids, or are residual phases remaining after intense alteration of basaltic glass + plagioclase.  相似文献   

17.
Fluid inclusions have recorded the history of degassing in basalt. Some fluid inclusions in olivine and pyroxene phenocrysts of basalt were analyzed by micro-thermometry and Raman spectroscopy in this paper. The experimental results showed that many inclusions are present almost in a pure CO2 system. The densities of some CO2 inclusions were computed in terms of Raman spectroscopic characteristics of CO2 Fermi resonance at room temperature. Their densities change over a wide range, but mainly between 0.044 g/cm3 and 0.289 g/cm3. Their micro-thermometric measurements showed that the CO2 inclusions examined reached homogenization between 1145.5℃ and 1265℃ . The mean value of homogenization temperatures of CO2 inclusions in basalts is near 1210℃. The trap pressures (depths) of inclusions were computed with the equation of state and computer program. Distribution of the trap depths makes it know that the degassing of magma can happen over a wide pressure (depth) range, but mainly at the depth of 0.48 km to 3.85 km. This implicates that basalt magma experienced intensive degassing and the CO2 gas reservoir from the basalt magma also may be formed in this range of depths. The results of this study showed that the depth of basalt magma degassing can be forecasted from CO2 fluid inclusions, and it is meaningful for understanding the process of magma degassing and constraining the inorganogenic CO2 gas reservoir.  相似文献   

18.
Generation of CO2-rich melts during basalt magma ascent and degassing   总被引：1，自引：0，他引：1  
To test mechanisms of basaltic magma degassing, continuous decompressions of volatile-bearing (2.7–3.8 wt% H2O, 600–1,300 ppm CO2) Stromboli melts were performed from 250–200 to 50–25 MPa at 1,180–1,140 °C. Ascent rates were varied from 0.25 to ~1.5 m/s. Glasses after decompression show a wide range of textures, from totally bubble-free to bubble-rich, the latter with bubble number densities from 104 to 106 cm?3, similar to Stromboli pumices. Vesicularities range from 0 to ~20 vol%. Final melt H2O concentrations are homogeneous and always close to solubilities. In contrast, the rate of vesiculation controls the final melt CO2 concentration. High vesicularity charges have glass CO2 concentrations that follow theoretical equilibrium degassing paths, whereas glasses from low vesicularity charges show marked deviations from equilibrium, with CO2 concentrations up to one order of magnitude higher than solubilities. FTIR profiles and maps reveal glass CO2 concentration gradients near the gas–melt interface. Our results stress the importance of bubble nucleation and growth, and of volatile diffusivities, for basaltic melt degassing. Two characteristic distances, the gas interface distance (distance either between bubbles or to gas–melt interfaces) and the volatile diffusion distance, control the degassing process. Melts containing numerous and large bubbles have gas interface distances shorter than volatile diffusion distances, and degassing proceeds by equilibrium partitioning of CO2 and H2O between melt and gas bubbles. For melts where either bubble nucleation is inhibited or bubble growth is limited, gas interface distances are longer than volatile diffusion distances. Degassing proceeds by diffusive volatile transfer at the gas–melt interface and is kinetically limited by the diffusivities of volatiles in the melt. Our experiments show that CO2-oversaturated melts can be generated as a result of magma decompression. They provide a new explanation for the occurrence of CO2-rich natural basaltic glasses and open new perspectives for understanding explosive basaltic volcanism.  相似文献   

19.
Speciation of aqueous calcium chloride and the solubility of wollastonite represented by the reaction wollastonite + 2HCl° → CaCl2° + quartz + H2O were experimentally investigated at 1 and 2 kbar in the range 425–600°C using rapid-quench hydrothermal techniques and a modified Ag + AgCl buffer technique (Frantz and Popp, 1979). Variation in the measured concentration in HCl° as a function of total dissolved calcium was used to identify associated aqueous CaCl2° as the predominant calcium species in the fluid at temperatures above 500°C at 2 kbar. The data were used to calculate the equilibrium constant for the above reaction as a function of temperature and pressure, from which the difference in Gibbs free energy of formation between CaCl2° and HCl° at 1 and 2 kbar, 450°–600°C was calculated. Solubility constants for minerals in the system MgO-CaO-SiO2-H2O-HCl-CO2 were calculated using the data from this study and from Frantz and Popp (1979). Calculated mineral solubilities were used to calculate the solution compositions and solid alteration products resulting from interactions of a Ca-Mg silicate mineral (diopside) with hydrothermal solutions containing a range of different total chloride concentrations. High total chloride (2.0 m) in the solution results in Si-Mg enrichment in the solids and Ca enrichment in the fluid, whereas low total chloride (0.008 m) results in Mg enrichment in the solids and Ca-Si enrichment in the fluid.  相似文献   

20.
The initial 87Sr/86Sr ratios of twelve basalt flows of Jurassic age on Storm Peak in the Queen Alexandra Range are anomalously high and range from 0.7094–0.7133. The average value is 0.7112±0.0013 (1). The concentrations of rubidium and strontium have arithmetic means of 60.6±19.4 ppm and 128.8±11.9 ppm, respectively. The corresponding average Rb/Sr ratio is 0.47 which is also anomalously high for rocks of basaltic composition. In addition, these rocks have high concentrations of SiO2 (56.50%) and K2O (1.29%) and are depleted in Al2O3 (12.92%), MgO (3.44%) and CaO (7.91%) compared to average continental tholeiites. They are nevertheless classified as basalts on the basis of the composition of microphenocrysts.The initial 87Sr/86Sr ratios and all of the chemical parameters of the flows exhibit systematic stratigraphic variations. These are interpreted as indicating the occurrence of four eruptive cycles. In a typical cycle the initial 87Sr/86Sr ratios of successive flows and their concentrations of SiO2, FeO (total iron), Na2O, K2O, P2O5, Rb and Sr decrease in ascending stratigraphic sequence while the concentrations of TiO2, Al2O3, MgO, CaO and MnO increase upward. The initial 87Sr/86Sr ratios of the flows show a strong positive correlation with the strontium concentration. Similar correlations are observed between the initial 87Sr/86Sr ratios and all of the major oxide components. These relationships are incompatible with the hypothesis that these flows are the products of crystal fractionation of a-34 magma at depth under closed-system conditions. It is suggested that the flows resulted from the hybridization of a normal tholeiite basalt magma by assimilation of varying amounts of granitic rocks in the Precambrian basement which underlies the entire Transantarctic Mountain chain.Mixtures of two components having different 87Sr/86Sr ratios and differing strontium concentrations are related to each other by hyperbolic mixing equation. Such an equation was fitted by least squares regression of data points to a straight line in coordinates of initial 87Sr/86Sr and the reciprocals of the concentrations of strontium. This equation and plots of strontium versus other oxides were then used to estimate the chemical composition of the parent basalt magma and of the granitic contaminant by substituting reasonable estimates of their 87Sr/86Sr ratios. The chemical composition of the parent basalt (87Sr/86Sr=0.706) is generally compatible with that of average continental tholeiite, but is distinctive by having a low concentration of strontium (117 ppm). The chemical composition of the contaminant (87Sr/86Sr=0.720) is enriched in strontium (173 ppm), SiO2, FeO (total iron) and the alkalies but is depleted in Al2O3, MgO and CaO. The data for strontium indicate that the lava flows on Storm Peak contain between 20 and 40% of this granitic contaminant. The contamination of basalt magma is not a local event but is characteristic of the Jurassic basalt flows and diabase sills throughout the Transantarctic Mountains and in Tasmania.Laboratory for Isotope Geology and Geochemistry, Contribution No. 33.  相似文献   

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