The speciation of water in silicate melts

Previous models of water solubility in silicate melts generally assume essentially complete reaction of water molecules to hydroxyl groups. In this paper a new model is proposed that is based on the hypothesis that the observed concentrations of molecular water and hydroxyl groups in hydrous silicate glasses reflect those of the melts from which they were quenched. The new model relates the proportions of molecular water and hydroxyl groups in melts via the following reaction describing the homogeneous equilibrium between melt species: H₂O_molecular (melt) + oxygen (melt) = 2OH (melt). An equilibrium constant has been formulated for this reaction and species are assumed to mix ideally. Given an equilibrium constant for this reaction of 0.1–0.3, the proposed model can account for variations in the concentrations of molecular water and hydroxyl groups in melts as functions of the total dissolved water content that are similar to those observed in glasses. The solubility of molecular water in melt is described by the following reaction: H₂O (vapor) = H₂O_molecular (melt).These reactions describing the homogeneous and heterogeneous equilibria of hydrous silicate melts can account for the following observations: the linearity between f_H2O and the square of the mole fraction of dissolved water at low total water contents and deviations from linearity at high total water contents; the difference between the partial molar volume of water in melts at low total water contents and at high total water contents; the similarity between water contents of vapor-saturated melts of significantly different compositions at high pressures versus the dependence on melt composition of water solubility in silicate melts at low pressures; and the variations of viscosity, electrical conductivity, the diffusivity of “water,” the diffusivity of cesium, and phase relationships with the total dissolved water contents of melts.This model is thus consistent with available observations on hydrous melt systems and available data on the species concentrations of hydrous glasses and is easily tested, since measurements of the concentrations of molecular water and hydroxyl groups in silicate glasses quenched from melts equilibrated over a range of conditions and total dissolved water contents are readily obtainable.     

Solubility and solution mechanism of H2O in alkali silicate melts and glasses at high pressure and temperature

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 (X_H2O) ranges between 18 and 45 mol% (O = 1) with (∂X_H2O/∂P)_T∼14-18 mol% H₂O/GPa. The (∂X_H2O/∂P)_T is negatively correlated with NBO/Si (= Na/Si) of the melt. The (∂X_H2O/∂T)_P is in the −0.03 to +0.05 mol% H₂O/°C range, and is negatively correlated with NBO/Si. The [∂X_H2O/∂(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 (∂X_H2O/∂T)_P<0, whereas more polymerized melts exhibit (∂X_H2O/∂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.     

Peralkaline silicic melts of island arcs, active continental margins, and intraplate continental settings: Evidence from the investigation of melt inclusions in minerals and quenched glasses of rocks

Based on the analysis of data on the composition of melt inclusions in minerals and quenched glasses of igneous rocks, we considered the problems of the formation of peralkaline silicic magmas (i.e., whose agpaitic index, the molar ratio AI = (Na₂O + K₂O)/Al₂O₃, is higher than one). The mean compositions of peralkaline silicic melts are reported for island arcs and active continental margins and compared with the compositions of melts from other settings, primarily, intraplate continental areas. Peralkaline silicic rocks are rather common in the latter. Such rocks are rare in island arcs and active continental margins, but agpaitic melts were observed in inclusions in phenocrysts of plagioclase, quartz, pyroxene, and other minerals. Plagioclase fractionation from an alkali-rich melt with AI < 1 is considered as a possible mechanism for the formation of peralkaline silicic melts (Bowen’s plagioclase effect). However, the analysis of available experimental data on plagioclase-melt equilibria showed that natural peralkaline melts are almost never in equilibrium with plagioclase. For the same reason, the melting of the majority of crustal rocks, which usually contain plagioclase, does not produce peralkaline melts. The existence of peralkaline silicic melt inclusions in plagioclase phenocrysts suggests that plagioclase can crystallize from peralkaline melts, and the plagioclase effect may play a certain role. Another mechanism for the formation of peralkaline silicic magmas is the melting of alkali-rich basic and intermediate rocks, including the spilitized varieties of subalkali basalts.     

Water in Albitic Glasses

Infrared spectroscopy has been calibrated to provide a preciseand accurate method for determining the concentrations of molecularwater and hydroxyl groups in hydrous albitic glasses. At totalwater contents less than 4 wt.%, most of the water is dissolvedas hydroxyl groups; at higher total water contents, molecularwater becomes the dominant species. For total water contentsabove 4 wt.%, the amount of water dissolved as hydroxyl groupsis nearly constant at about 2 wt.% and additional water is incorporatedas molecular water. These trends in the concentrations of theH-bearing species are similar to those observed in other silicateglasses using infrared and NMR spectroscopies. The ratio ofmolecular water to hydroxyl groups at a given total water contentis independent of the pressure and only weakly dependent onthe temperature of equilibration. Molecular water and hydroxyl group concentrations in glassesprovide constraints on the dissolution mechanisms of water insilicate liquids. Several mixing models involving homogeneousequilibria of the form H₂O+O^2– = 2OH^– among meltspecies in albitic melts have been developed. These models canaccount for the measured species concentrations if the effectsof non-ideal behavior or mixing of polymerized units are included,or by allowing for several different anhydrous species. We used two thermodynamic models of hydrous albitic melts tocalculate phase equilibria. The first assumes that Henry's lawholds for molecular water in albitic liquids; i.e. that theactivity of molecular water in the melt is proportional to itsmole fraction as determined by infrared spectroscopy. The seconddescribes the speciation and thermodynamics of hydrous albiticmelts using the formalism of a strictly regular solution. Thesemodels can account reasonably well for the position of the vapor-saturatedsolidus of high albite and the pressure and temperature dependenceof the solubility of water in albitic melts. To the extent thatthese models are successful, our approach provides a directlink between measured species concentrations in hydrous albiticglasses and the macroscopic thermodyn amic properties of theNaAlSi₃O₈-H₂O system.     

Melting of plagioclase-quartz assemblages at 2 kbar water pressure

The melting of plagioclase and quartz has been investigated at P _H2O =2kb. A single crystal of plagioclase was surrounded by quartz powder and water. A reaction rim consisting of glass and of An-rich plagioclase developed around unchanged starting plagioclase. Microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase. For example at 850° C the approximate X _An of melt is 0.3 and that of plagioclase is 0.8. The temperature interval between beginning of melting and complete melting of cotectic compositions is 100° C or more for quartz-plagioclase mixtures with plagioclases between An 40 and An 75. In comparison to the system Ab-An-H₂O the plagioclase melting loop is somewhat wider in the investigated system Qz-Ab-An-H₂O but the temperature interval is much smaller (100° C) than in the system Ab-An-H₂O (200° C). The solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments. The results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi-component rocks than in the pure plagioclase system. This finding is important for the development of albite rich rocks from more basic compositions.     

Effects of F,B2O3 and P2O5 on the solubility of water in haplogranite melts compared to natural silicate melts

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.     

Solubility of carbon dioxide in melts of andesite,tholeiite, and olivine nephelinite composition to 30 kbar pressure

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.     

An investigation of disequilibrium growth processes of plagioclase in the system anorthite-albite-water by methods of numerical simulation

Compositional zoning of plagioclase is useful as a recorder of dynamic geological conditions if the mechanisms of crystal growth are known. Although the present lack of quantitative information on specific kinetic processes limits their accuracy, numerical simulations of phenoycryst plagioclase growth are useful both for identifying the most influential kinetic processes (for example, diffusion) that should receive priority in experimental measurements and for designing informative growth experiments. The interaction of kinetic processes at a crystal face is so complex that the overall result cannot be assessed intuitively. A primary purpose of these papers is to explore this interaction in the plagioclase system as quantitatively as data permit.The growth of a single face of a plagioclase crystal in an infinite melt was simulated in computer models for: (1) anhydrous and hydrous plagioclase melts; (2) for different undercoolings; and (3) for both interface-controlled and melt-transport controlled growth. Major uncertainties include the velocity and nonequilibrium partitioning laws in the interface-controlled model, and transport coefficients for melt components. Comparison of computed models with published growth velocity data for anhydrous melts was used to estimate a transport coefficient (with the form for diffusion), and that coefficient was extrapolated to hydrous melts on the basis of the Stokes-Einstein relationship.The results of simulations suggest that undercoolings reasonable for plutonic systems could result in deviations of crystal composition from that in equilibrium with the melt of several mole % An; geothermometers based on the assumption of equilibrium partitioning will be in error significantly. Similarly, the volatile content and composition of melt trapped during growth would deviate significantly from bulk melt properties. The velocity of crystal growth and deviation of crystal composition from equilibrium show low sensitivity to water content because larger water contents result in greater accumulation of water at the interface and a consequent depression of effective undercooling.The large magnitude of the derived transport parameter suggests that local convection as well as diffusion may occur during growth in the anhydrous system. The addition of water to the system reduces viscosity and increases the density gradients near the crystal, making local convection even more probable. Our meagre knowledge of transport by diffusion and convection in the melt is probably the most important factor limiting the accuracy of growth simulations.     

The effects of potassium addition on the rate of quartz dissolution in the CMAS and CAS systems

Quartz dissolution in melts in the KCAS and KCMAS systems results in the formation of a silica- and potassium-enriched boundary layer next to the dissolving crystals. The presence of potassium in CAS melts has no discernible effect on dissolution rate compared with that in K-free melts with otherwise similar composition despite a small decrease in the diffusivity of silica in the potassium-bearing melts. The decrease in silica diffusivity is offset by an increase in the solubility of silica in the K-bearing melts. Addition of potassium to CMAS melts results in a large decrease in the dissolution rate of quartz. Even though the solubility of silica is enhanced, the addition of potassium leads to large changes in the structure of the melt in the boundary layer (as measured by NBO/T), which results in a large decrease in the diffusivity of silica and thus slower dissolution. There is significant diffusive coupling of Al₂O₃, CaO and MgO during dissolution, which leads to local uphill diffusion of these components. K₂O is decoupled from the other components, as shown by its much thicker diffusion zone. Potassium moves through the boundary layer as a result of two homogeneous reactions: uphill diffusion in which potassium diffuses into the silica-enriched melt adjacent to the dissolving quartz crystal and downhill diffusion in the region furthest from the crystal–melt interface where SiO₂ and K₂O diffuse away from the interface together.     

Viscosity of flux-rich pegmatitic melts

Viscosity experiments were conducted with two flux-rich pegmatitic melts PEG0 and PEG2. The Li₂O, F, B₂O₃ and P₂O₅ contents of these melts were 1.04, 4.06, 2.30 and 1.68 and 1.68, 5.46, 2.75 and 2.46 wt%, respectively. The water contents varied from dry to 9.04 wt% H₂O. The viscosity was determined in internally heated gas pressure vessels using the falling sphere method in the temperature range 873–1,373 K at 200 and 320 MPa pressure. At 1,073 K, the viscosity of water-rich (~9 wt% H₂O) melts is in the range of 3–60 Pa s, depending on the melt composition. Extrapolations to lower temperature assuming an Arrhenian behavior indicate that highly fluxed pegmatite melts may reach viscosities of ~30 Pa s at 773 K. However, this value is a minimum estimation considering the strongly non-Arrhenian behavior of hydrous silicate melts. The experimentally determined melt viscosities are lower than the prediction of current models taking compositional parameters into account. Thus, these models need to be improved to predict accurately the viscosity of flux-rich water bearing melts. The data also indicate that Li influences significantly the melt viscosity. Decreasing the molar Al/(Na + K + Li) ratio results in a strong viscosity decrease, and highly fluxed melts with low Al/(Na + K + Li) ratios (~0.8) have a rheological behavior which is very close to that of supercritical fluids.     

The influence of melt structure on trace element partitioning near the peridotite solidus

This experimental study examines the mineral/melt partitioning of Na, Ti, La, Sm, Ho, and Lu among high-Ca clinopyroxene, plagioclase, and silicate melts analogous to varying degrees of peridotite partial melting. Experiments performed at a pressure of 1.5 GPa and temperatures of 1,285 to 1,345 °C produced silicate melts saturated with high-Ca clinopyroxene, plagioclase and/or spinel, and, in one case, orthopyroxene and garnet. Partition coefficients measured in experiments in which clinopyroxene coexists with basaltic melt containing ~18 to 19 wt% Al₂O₃ and up to ~3 wt% Na₂O are consistent with those determined experimentally in a majority of the previous studies, with values of ~0.05 for the light rare earths and of ~0.70 for the heavy rare earths. The magnitudes of clinopyroxene/melt partition coefficients for the rare earth elements correlate with pyroxene composition in these experiments, and relative compatibilities are consistent with the effects of lattice strain energy. Clinopyroxene/melt partition coefficients measured in experiments in which the melt contains ~20 wt% Al₂O₃ and ~4 to 8 wt% Na₂O are unusually large (e.g., values for Lu of up to 1.33±0.05) and are not consistent with the dependence on pyroxene composition found in previous studies. The magnitudes of the partition coefficients measured in these experiments correlate with the degree of polymerization of the melt, rather than with crystal composition, indicating a significant melt structural influence on trace element partitioning. The ratio of non-bridging oxygens to tetrahedrally coordinated cations (NBO/T) in the melt provides a measure of this effect; melt structure has a significant influence on trace element compatibility only for values of NBO/T less than ~0.49. This result suggests that when ascending peridotite intersects the solidus at relatively low pressures (~1.5 GPa or less), the compatibility of trace elements in the residual solid varies significantly during the initial stages of partial melting in response to the changing liquid composition. It is unlikely that this effect is important at higher pressures due to the increased compatibility of SiO₂, Na₂O, and Al₂O₃ in the residual peridotite, and correspondingly larger values of NBO/T for small degree partial melts.Editorial responsibility: T.L. Grove     

Mechanism of water solubility in silicate melts of the albite-nepheline system

Water solubility and speciation have been studied in melts from the system albite-nepheline. Water solubility was determined at P_H2O = 200 MPa and T = 1200ºC using Karl Fischer titration (KFT) and weight loss methods. It increases from 5.8?±?0.1 wt. % in the albite melt to 7.25?±?0.25 % in the nepheline melt. The solubility dependence has a sigmoidal shape with a steep increase in the Ab40–Ab60 compositional range. The densities of the hydrous glasses have been measured using the thermal-gradient method. Infrared (IR) combination bands of molecular H₂O and hydroxyl groups in these glasses demonstrate a systematic shift to lower frequencies with increasing alkalinity. The data are consistent with the appearance of aluminate species (Al-O-Al fragments) appearing at 40–60 wt.% nepheline (Ab60–Ab40).     

Water solubility in a calcium aluminosilicate melt

We have measured the water solubility between 1 atmosphere and 5 kilobars for a calcium aluminosilicate melt of molar composition CaO 0.28, Al₂O₃ 0.06, SiO₂ 0.66 (An₉Wo₃₈Qz₅₃). The water contents were measured via thermogravimetric analysis of isobarically quenched glasses, and range from 0.121 wt% H₂O near 1 aim to 9.25 wt% H₂O at 5 kilobars. The molar water solubility lies between those of SiO₂ and albite melts below around three kilobars, and crosses the albite solubility curve above this pressure. The present results are compared with data in the literature on related calcium aluminosilicate melts. There seems to be little variation of water solubility with composition for calcium aluminosilicate melts, unlike analogous alkali aluminosilicate compositions. Examination of the data suggests that there may be a maximum in molar water solubility along the albite-anorthite join.     

Cumulate xenoliths from St. Vincent, Lesser Antilles Island Arc: a window into upper crustal differentiation of mantle-derived basalts

In order to shed light on upper crustal differentiation of mantle-derived basaltic magmas in a subduction zone setting, we have determined the mineral chemistry and oxygen and hydrogen isotope composition of individual cumulus minerals in plutonic blocks from St. Vincent, Lesser Antilles. Plutonic rock types display great variation in mineralogy, from olivine–gabbros to troctolites and hornblendites, with a corresponding variety of cumulate textures. Mineral compositions differ from those in erupted basaltic lavas from St. Vincent and in published high-pressure (4–10 kb) experimental run products of a St. Vincent high-Mg basalt in having higher An plagioclase coexisting with lower Fo olivine. The oxygen isotope compositions (δ¹⁸O) of cumulus olivine (4.89–5.18‰), plagioclase (5.84–6.28‰), clinopyroxene (5.17–5.47‰) and hornblende (5.48–5.61‰) and hydrogen isotope composition of hornblende (δD = −35.5 to −49.9‰) are all consistent with closed system magmatic differentiation of a mantle-derived basaltic melt. We employed a number of modelling exercises to constrain the origin of the chemical and isotopic compositions reported. δ¹⁸O_Olivine is up to 0.2‰ higher than modelled values for closed system fractional crystallisation of a primary melt. We attribute this to isotopic disequilibria between cumulus minerals crystallising at different temperatures, with equilibration retarded by slow oxygen diffusion in olivine during prolonged crustal storage. We used melt inclusion and plagioclase compositions to determine parental magmatic water contents (water saturated, 4.6 ± 0.5 wt% H₂O) and crystallisation pressures (173 ± 50 MPa). Applying these values to previously reported basaltic and basaltic andesite lava compositions, we can reproduce the cumulus plagioclase and olivine compositions and their associated trend. We conclude that differentiation of primitive hydrous basalts on St. Vincent involves crystallisation of olivine and Cr-rich spinel at depth within the crust, lowering MgO and Cr₂O₃ and raising Al₂O₃ and CaO of residual melt due to suppression of plagioclase. Low density, hydrous basaltic and basaltic andesite melts then ascend rapidly through the crust, stalling at shallow depth upon water saturation where crystallisation of the chemically distinct cumulus phases observed in this study can occur. Deposited crystals armour the shallow magma chamber where oxygen isotope equilibration between minerals is slowly approached, before remobilisation and entrainment by later injections of magma.     

Fractional crystallization of high-K arc magmas: biotite- versus amphibole-dominated fractionation series in the Dariv Igneous Complex,Western Mongolia

Many studies have documented hydrous fractionation of calc-alkaline basalts producing tonalitic, granodioritic, and granitic melts, but the origin of more alkaline arc sequences dominated by high-K monzonitic suites has not been thoroughly investigated. This study presents results from a combined field, petrologic, and whole-rock geochemical study of a paleo-arc alkaline fractionation sequence from the Dariv Range of the Mongolian Altaids. The Dariv Igneous Complex of Western Mongolia is composed of a complete, moderately hydrous, alkaline fractionation sequence ranging from phlogopite-bearing ultramafic and mafic cumulates to quartz–monzonites to late-stage felsic (63–75 wt% SiO₂) dikes. A volumetrically subordinate more hydrous, amphibole-dominated fractionation sequence is also present and comprises amphibole (±phlogopite) clinopyroxenites, gabbros, and diorites. We present 168 whole-rock analyses for the biotite- and amphibole-dominated series. First, we constrain the liquid line of descent (LLD) of a primitive, alkaline arc melt characterized by biotite as the dominant hydrous phase through a fractionation model that incorporates the stepwise subtraction of cumulates of a fixed composition. The modeled LLD reproduces the geochemical trends observed in the “liquid-like” intrusives of the biotite series (quartz–monzonites and felsic dikes) and follows the water-undersaturated albite–orthoclase cotectic (at 0.2–0.5 GPa). Second, as distinct biotite- and amphibole-dominated fractionation series are observed, we investigate the controls on high-temperature biotite versus amphibole crystallization from hydrous arc melts. Analysis of a compilation of hydrous experimental starting materials and high-Mg basalts saturated in biotite and/or amphibole suggests that the degree of K enrichment controls whether biotite will crystallize as an early high-T phase, whereas the degree of water saturation is the dominant control of amphibole crystallization. Therefore, if a melt has the appropriate major-element composition for early biotite and amphibole crystallization, as is true of the high-Mg basalts from the Dariv Igneous Complex, the relative proximity of these two phases to the liquidus depends on the H₂O concentration in the melt. Third, we compare the modeled high-K LLD and whole-rock geochemistry of the Dariv Igneous Complex to the more common calc-alkaline trend. Biotite and K-feldspar fractionation in the alkaline arc series results in the moderation of K₂O/Na₂O values and LILE concentrations with increasing SiO₂ as compared to the more common calc-alkaline series characterized by amphibole and plagioclase crystallization and strong increases in K₂O/Na₂O values. Lastly, we suggest that common calc-alkaline parental melts involve addition of a moderate pressure, sodic, fluid-dominated slab component while more alkaline primitive melts characterized by early biotite saturation involve the addition of a high-pressure potassic sediment melt.     

An experimental study of the effects of melt composition on plagioclase-melt equilibria at 5 and 10 kbar: implications for the origin of magmatic high-An plagioclase

An experimental investigation of plagioclase crystallization in broadly basaltic/andesitic melts of variable Ca# (Ca/(Ca+Na)*100) and Al# (Al/(Al+Si)*100) values and H₂O contents has been carried out at high pressures (5 and 10 kbar) in a solid media piston-cylinder apparatus. The H₂O contents of glasses coexisting with liquidus or near-liquidus plagioclases in each experiment were determined via an FTIR spectroscopic technique. This study has shown that melt Ca# and Al#, H₂O content and crystallization pressure all control the composition of liquidus plagioclase. Increasing melt Ca# and Al# increase An content of plagioclase, whereas the effect of increasing pressure is the opposite. However, the importance of the role played by each of these factors during crystallization of natural magmas varies. Melt Ca# has the strongest control on plagioclase An content, but melt Al# also exerts a significant control. H₂O content can notably increase the An content of plagioclase, up to 10 mol % for H₂O-undersaturated melts, and 20 mol % for H₂O-saturated melts. Exceptionally calcic plagioclases (up to An₁₀₀) in some primitive subduction-related boninitic and related rocks cannot be attributed to the presence of the demonstrated amounts of H₂O (up to 3 wt %). Rather, they must be due to the involvement of extremely refractory (CaO/Na₂O>18) magmas in the petrogenesis of these rocks. Despite the refractory nature of some primitive MORB glasses, none are in equilibrium with the most calcic plagioclase (An₉₄) found in MORB. These plagioclases were likely produced from more refractory melts with CaO/Na₂O = 12–15, or from melts with exceptionally high Al₂O₃(>18%). Magmas of appropriate compositions to crystallize these most calcic plagioclases are sometimes found as melt inclusions in near liquidus phenocrysts from these rocks, but are not known among wholerock or glass compositions. The fact that such melts are not erupted as discrete magma batches indicates that they are effectively mixed and homogenized with volumetrically dominant, less refractory magmas. The high H₂O contents (∼ 6 wt%) in some high-Al basaltic arc magmas may be responsible for the existence of plagioclases up to An₉₅ in arc lavas. However, an alternative possibility is that petrogenesis involving melts with abnormally high CaO/Na₂O values (> 8) may account for the presence of highly anorthitic plagioclases in these rocks. Received: 31 August 1993 / Accepted: 20 May 1994     

Crystallization of primitive basaltic magmas at crustal pressures and genesis of the calc-alkaline igneous suite: experimental evidence from St Vincent,Lesser Antilles arc

Near-liquidus crystallization experiments have been carried out on two basalts (12.5 and 7.8 wt% MgO) from Soufriere, St Vincent (Lesser Antilles arc) to document the early stages of differentiation in calc-alkaline magmas. The water-undersaturated experiments were performed mostly at 4 kbar, with 1.6 to 7.7 wt% H₂O in the melt, and under oxidizing conditions (ΔNNO = −0.8 to +2.4). A few 10 kbar experiments were also performed. Early differentiation of primitive, hydrous, high-magnesia basalts (HMB) is controlled by ol + cpx + sp fractionation. Residual melts of typical high-alumina basalt (HAB) composition are obtained after 30–40% crystallization. The role of H₂O in depressing plagioclase crystallization leads to a direct relation between the Al₂O₃ content of the residual melt and its H₂O concentration, calibrated as a geohygrometer. The most primitive phenocryst assemblage in the Soufriere suite (Fo_89.6 olivine, Mg-, Al- and Ti-rich clinopyroxene, Cr–Al spinel) crystallized from near-primary (Mg# = 73.5), hydrous (∼5 wt% H₂O) and very oxidized (ΔNNO = +1.5–2.0) HMB liquids at middle crustal pressures and temperatures from ∼1,160 to ∼1,060°C. Hornblende played no role in the early petrogenetic evolution. Derivative HAB melts may contain up to 7–8 wt% dissolved H₂O. Primitive basaltic liquids at Soufriere, St Vincent, have a wide range of H₂O concentrations (2–5 wt%).     

Fractionation of major elements between coexisting H2O-saturated silicate melt and silicate-saturated aqueous fluids in aluminosilicate systems at 1-2 GPa

From experimental data in the systems Na₂O-Al₂O₃-SiO₂-H₂O, K₂O-Al₂O₃-SiO₂-H₂O at 1100°C, and CaO-Al₂O₃-SiO₂-H₂O at 1200°C in the 1-2 GPa pressure range, the solution behavior of the individual oxides in coexisting H₂O-saturated silicate melts and silicate-saturated aqueous fluids appears to be incongruent. Recalculated on an anhydrous basis, in the CaO-Al₂O₃-SiO₂-H₂O system, CaO^fluid/CaO^melt < 1, whereas in the Na₂O-Al₂O₃-SiO₂-H₂O and K₂O-Al₂O₃-SiO₂-H₂O systems, K₂O^fluid/K₂O^melt and Na₂O^fluid/Na₂O^melt both are greater than 1. The aqueous fluids are depleted in alumina relative to silicate melt.In the Na₂O-Al₂O₃-SiO₂-H₂O, K₂O-Al₂O₃-SiO₂-H₂O, and CaO-Al₂O₃-SiO₂-H₂O systems, fluid/melt partition coefficients for the individual oxides range between ∼0.005 and 0.35 depending on oxide, bulk composition and pressure. The alkali partition coefficients are about an order of magnitude higher than that of CaO. Alumina and silica partition coefficient values in the CaO-Al₂O₃-SiO₂-H₂O system are 10-20% of the values for the same oxides in the Na₂O-Al₂O₃-SiO₂-H₂O and K₂O-Al₂O₃-SiO₂-H₂O systems.Positive correlations among individual partition coefficients and oxide concentrations in the aqueous fluids are consistent with complexing in the fluid that involves silicate polymers associated with alkalis and alkaline earths and aluminosilicate complexes where alkalis and alkaline earths may serve to charge-balance Al³⁺, which is, perhaps, in tetrahedral coordination. Alkali aluminosilicate complexes in aqueous fluid appear more stable than Ca-aluminosilicate complexes.     

Phase relationships in the system KAlSiO4-Mg2SiO4-SiO2-H2O as a model for hybridization between hydrous siliceous melts and peridotite

The system KAlSiO₄-Mg₂SiO₄-SiO₂-H₂O includes model representatives of (1) hydrous siliceous magma from subducted oceanic crust — the eutectic liquid in KAlSi₃O₈-SiO₂-H₂O, and (2) the overlying mantle peridotite — the assemblage forsterite+enstatite (Fo+En). In a series of partly schematic isobaric isothermal sections, the products of hybridization between the model materials at pressures between 20 and 30 kbar have been determined. The liquid dissolves peridotite components with little change in composition. Hybridization is not a simple mixing process, because of the incongruent melting of peridotitic assemblages with phlogopite (Ph). Hybridization causes solidification of the liquid, with products a sequence of three mineral assemblages: Ph, Ph+quartz (Qz), and Ph+En. The products represent an absolute geochemical separation and local concentration of all potassium from the liquid. Hybridization is accompanied by H₂O-saturation of melts, and evolution of aqueous fluid. Although there are significant differences between the melt composition and that of the magma rising from subducted oceanic slab, and between Fo+En and the mantle rock, extrapolation of the results suggests that the conclusions can probably be extended to mantle conditions with sodium in the melt, and jadeitic clinopyroxene included in the hybrid products.     

高温高压条件下实验变形石英闪长岩微观结构与熔体特征研究

本文以高温高压条件下石英闪长岩流变实验样品为研究对象,利用偏光显微镜进行微观结构观察,研究了样品在实验温度压力条件下的变形机制与斜长石结构对流变强度的影响;通过透射电镜能谱与电子探针,分析了熔体分布和成分特征,讨论了角闪石脱水熔融的影响因素与脱水熔融对岩石流变的影响。结果表明,随着温度升高,岩石从脆塑性过渡域逐渐向高温位错攀移和动态重结晶为主的塑性域转化。在高温条件下,角闪石出现了脱水与部分熔融,脱水熔融的熔体分布和成分体现出非均匀与非平衡熔融的特点,空间分布上,熔体主要出现在角闪石和黑云母矿物颗粒的边缘以及角闪石和长石颗粒之间的区域内;成分分布上,熔体的成分与参与熔融的矿物成分密切相关。角闪石边缘的熔体和黑云母边缘的熔体具有低硅铝、高铁镁特征,斜长石边缘的熔体具有高硅铝、低铁镁的特征,处于角闪石和斜长石颗粒中间的熔体,其成分间于斜长石与角闪石成分之间。实验中出现的非平衡非均匀部分熔融可以解释混合岩中的浅色体与暗色体的成因,富硅熔体可以形成富硅铝的花岗质岩石,而贫硅富铁镁的熔体可以形成基性岩。角闪石的脱水熔融程度依赖于样品的封闭条件,处于封闭环境的样品,角闪石不易脱水熔融,而处于开放环境时,角闪石脱水熔融显著。拆离断层带及其附近具备这样的开放环境,有利于角闪石发生脱水熔融。实验力学数据和微观结构显示,随机分布的斜长石对岩石强度影响并不明显,但斜长石的长轴方向与最大主应力方向呈大角度相交(近90°)会显著强化岩石的强度,这意味着岩石组构与主应力方向大角度相交或呈垂直方向时,不利于岩石变形和拆离断层的形成,反之,均匀岩石或岩石组构与最大主应力方向小角度相交,有利于岩石的变形,容易发育拆离断层。