Volatile element zonation in Campanian Ignimbrite magmas (Phlegrean Fields, Italy): evidence from the study of glass inclusions and matrix glasses

The distribution of H₂O, F, Cl and S in the Campanian Ignimbrite (CI) magma chamber was investigated through study of primary glass inclusions and matrix glasses from pumices of the Plinian fall deposit. The eruption, fed by trachytic to phono-trachytic magmas, mainly produced a trachytic non-welded to partially welded tuff, underlain by a minor cogenetic fallout deposit. The entire chemical variability of the eruptive products is well represented in the pumices of the Plinian fall deposit, which we divide into a basal Lower Fall Unit (LFU) and an overlying Upper Fall Unit (UFU). Primary glass inclusions were only found in clinopyroxenes associated with the LFU pumice and contain a mean of 1.60ǂ.32 wt% H₂O (analysed by FTIR), 0.11ǂ.08 wt% F, 0.37ǂ.03 wt% Cl and 0.08ǂ.04 wt% SO₃ (EMP analysis); CO₂ concentrations were below the FTIR detection limit (10-20 ppm). The coexisting matrix glasses contain similar amounts of halogens and sulfur but less water (~0.60 wt%). Partially degassed matrix glasses from UFU pumices contain a mean of 0.30ǂ.02 H₂O, 0.28ǂ.10 F, 0.04ǂ.02 SO₃ and 0.80ǂ.04 wt% Cl. To reconstruct the total amount of volatiles dissolved in the most evolved trachytes we have used experimental solubility data and mass balance calculations concerning the amount of crystal fractionation required to produce the most evolved trachyte from the least evolved trachyte; these yield an estimated pre-eruptive magma volatile content (H₂O + Cl + F) of ~5.5 wt% for the most evolved magmas. On the basis of new determinations of Cl solubility limits in hydrous trachytic melts coexisting with an aqueous fluid phase + hydrosaline melt (brine), we suggest that the upper part of the magma chamber which fed the CI eruption was fluid(s) saturated and at a minimum depth of ~2 km. Variations in eruptive style (Plinian fallout, pyroclastic flows) do not appear to be related to significant variations in pre-eruptive volatile contents.     

Solubility of Au in Cl- and S-bearing hydrous silicate melts

The solubility of Au in Cl- and S-bearing hydrous rhyodacitic and andesitic melts has been experimentally investigated at 1050 °C, 200 MPa and log fO₂ close to the Ni/NiO solid oxygen buffer (NNO). The concentrations of Au in the experimental glasses have been determined using Laser Ablation ICP-MS (LA) with special efforts to avoid incorporation of Au micronuggets in the analysis. It is concluded that metal micronuggets are an experimental artefact and produced by Au partitioning into the fluids during heating with consequent precipitation on fluid dissolution in the melting glass powder. Hence, the micronuggets do not represent quench phases and must be excluded from the analysis. The micro-analytical data obtained by LA show that Au concentrations vary from ∼0.2 to ∼2.5 ppm by weight, generally consistent with the literature data for other melt compositions. The measured Au concentrations increase with increasing amounts of Cl and S dissolved in the silicate melt and show a correlation with the apparent activities of Cl and S in the system. The apparent activities of Cl and S are defined by the simplified linear relationship between volatile concentrations in the melt and activity of volatiles. The maximum activity (a^∗ = 1) is assumed to be reached at the saturation of the systems in respect of Cl-rich brine or FeS liquid for Cl and S, respectively. The dependence of Au solubility on the concentrations/activities of Cl and S at the fixed redox conditions shows that Au may form not only oxide- but also Cl- and S-bearing complexes in silicate melts. Furthermore, it indicates that exsolution of S and Cl from the melt by degassing/segregation/crystallization processes may lead to mobilization and extraction of Au into the fluid, liquid and/or mineral phase(s).     

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.     

The effect of Cl on Pt solubility in haplobasaltic melt: Implications for micronugget formation and evidence for fluid transport of PGEs

Platinum solubility was determined in a haplobasaltic, diopside-anorthite melt at 1523 K and 0.2 GPa as a function of oxygen fugacity and chlorine content. Synthetic glass powder of an An₄₂Di₅₈ composition was sealed in a platinum or platinum-iridium alloy capsule and equilibrated with a solid CaCl₂ and MgCl₂ chlorine source, water and the noble-metal capsule. All experiments were run in an internally-heated pressure-vessel equipped with a rapid-quench device with oxygen fugacity controlled by the water content and intrinsic hydrogen fugacity of the autoclave (MnO-Mn₃O₄). Resultant glasses were analyzed by isotope dilution ICP-MS and LA-ICP-MS to determine the solubility and distribution of Pt and assess potential Cl-complexation of Pt in the melt.Experiments with run durations longer than 96 h show Pt solubilities consistent with solubilities determined for the equivalent Cl-free diopside-anorthite system, under the same P-T conditions. These results indicate that chlorine has no discernable effect on Pt solubility and there is no evidence of Pt-Cl complexing in the silicate melt from 0.6 to 2.75 wt% Cl (saturation).However, products from short run duration experiments (<96 h) contain Pt concentrations which are orders of magnitude higher than those of the Pt-free starting glass and of the experimental products of the longer run duration experiments. These anomalously high levels are most pronounced in the shortest experiments and Pt concentration decreases with increasing run duration. It is suggested that this excess platinum is dissolved within the Cl-bearing fluid during the heating stages of the experiment and is left behind as the fluid dissolves into the melt leaving small amounts of Pt as “micronuggets”, increasing the bulk Pt concentration. With increasing run duration the platinum appears to migrate out of the melt, back to the capsule walls, decreasing the amount of Pt contained within the glass. This behavior offers compelling evidence that Cl-bearing fluids have the capacity to transport significant amounts of Pt under magmatic conditions.Mass balance calculations on the excess amount of Pt in the glass (above inherent solubility) in short duration experiments lead to an estimation of Pt concentration in the Cl-bearing fluid ranging from tens to a few hundred ppm, versus ppb levels in the melt. The correspondingly high estimated apparent partition coefficients of 10³-10⁴ suggest that Cl-bearing fluids can be highly efficient at enriching and transporting platinum in mafic magmatic-hydrothermal ore-forming systems. These values strongly contrast with recent experimental results in felsic systems, highlighting the potential importance of melt composition on partitioning, the need for composition specific partitioning experiments, as well as a detailed understanding of Pt distribution in experimental products.     

The behaviour of boron in a peraluminous granite-pegmatite system and associated hydrothermal solutions: a melt and fluid-inclusion study

Detailed analyses of melt and fluid inclusions combined with an electron-microprobe survey of boron-bearing minerals reveal the evolution of boron in a highly evolved peraluminous granite-pegmatite complex and the associated high- and medium-temperature ore-forming hydrothermal fluids (Ehrenfriedersdorf, Erzgebirge, Germany). Melt inclusions in granite represent embryonic pegmatite-forming melts containing about 10 wt% H₂O and 1.8 wt% B₂O₃. These melts are also enriched in F, P, and other incompatible elements such as Be, Sn, Rb, and Cs. Ongoing differentiation and volatile enrichment drove the system into a solvus, where two pegmatite-forming melts coexisted. The critical point is at about 712 °C, 100 MPa, 20 wt% H₂O and 4.1 wt% B₂O₃. Cooling and concomitant fractional crystallisation from 700 to 500 °C induced development of two conjugate melts, an H₂O-poor (A-melt) and an H₂O-rich melt (B-melt) along the opening solvus. Boron is a major element in both melts and is preferentially partitioned into the H₂O-rich melt. Temperature-dependent distribution coefficients D_boron^{B - melt/A - melt} D_{{\rm{boron}}}^{{\rm{B - melt/A - melt}}} are 1.3 at 650 °C, 1.5 at 600 °C, and 1.8 at 500 °C. In both melts, boron concentrations decreased during cooling because of exsolution of a boron-rich hypersaline brine throughout the pegmatitic stage. Boromuscovite containing up to 8.5 wt% was another sink for boron at this stage. The end of the melt-dominated pegmatitic stage was attained at a solidus temperature of around 490 °C. Fluid inclusions of the hydrothermal stage reveal trapping temperatures of 480 to 370 °C, along with varying densities and highly variable B₂O₃ contents ranging from 0.20 to 2.94 wt%. A boiling system evolved, indicating a complex interplay between closed- and open-system behaviour. Pressure switched from lithostatic to hydrostatic and back, generating hydrothermal convection cells where meteoric waters were introduced and mixed with magmatic fluids. Boron-rich solutions originated from magmatic fluids, whereas boron-depleted fluids were mainly of meteoric origin. This highlights the potential of boron for discriminating fluids of different origin. Tin is continuously enriched during the evolution because tin and boron are cross-linked by formation of boron-, fluorine- and tin-fluorine-bearing complexes and is finally deposited within quartz-cassiterite veins during the transition from closed- to open-system behaviour. Boron does not only trace the complex evolution of the Ehrenfriedersdorf complex but exerts, together with H₂O, F and P, an important control on the physical and chemical properties of pegmatite-forming melts, and particularly on the formation of a two-melt solvus at low pressure. We discuss this with respect to experimental results on H₂O solubility and the critical behaviour of the haplogranite-water system which contained variable concentrations of volatiles.     

Fluid and magmatic processes in the formation of the Ary-Bulak ongonite massif (eastern Transbaikalia)

The paper discusses the formation conditions of the Ary-Bulak ongonite massif (eastern Transbaikalia). Studies of melt and fluid inclusions have shown that, along with crystalline phases and a silicate melt, ongonitic magma contained aqueous–saline fluids of different types, fluoride melts compositionally similar to fluorite, sellaite, cryolite, chiolite, and more complex aluminum fluorides as well as silicate melts with abnormal Cs and As contents. An ongonite melt crystallized with the participation of P–Q fluids as vapor solutions, presumably NaF-containing and slightly admixed with chlorides. We studied the properties and composition of brine inclusions from Ca- and F-rich rocks on the margin of the massif. Depending on the thermophysical properties of the host rocks and ongonite melt, the duration of its crystallization has been estimated for a magma chamber of the size and shape of the Ary-Bulak massif. Magma chamber cooling has been modeled, and the density, viscosity, and Rayleigh number of the ongonite melt have been estimated from the composition of silicate glasses in melt inclusions. These data strongly suggest intense convection in the residual magma chamber lasting for centuries. We have calculated possible fluid overpressure during the crystallization and degassing of the ongonite melt in a closed magma chamber.Calcium- and fluorine-rich aphyric and porphyritic rocks on the southwestern margin of the massif might have formed by the following mechanism. Local decompression in the magma chamber quenched an oxygen-containing calcium fluoride melt accumulated at the crystallization front, and then these rocks altered during the interaction with fluids. When penetrating the marginal zone, a P–Q magmatic fluid which coexisted with the melt in the residual chamber cooled and changed its composition and properties. This caused the fluid to boil and segregate into immiscible phases: a vapor solution and a brine extremely rich in Cl, F, K, Cs, Mn, Fe, and Al. The fluoride and silicate liquids were immiscible; the silicate melts had abnormal Cs and As contents; changes in the composition and properties of the magmatic fluids caused them to boil and produce brines. All this is evidence for complex fluid–magma interaction and heterogeneous ongonitic magma during the crystallization of the Ary-Bulak rocks. These processes were favored by the low viscosity and high mobility of the F- and water-rich ongonite melt, intense melt convection in the residual chamber, and rising fluid pressure during its degassing.     

Partitioning of F between H2O and CO2 fluids and topaz rhyolite melt

Fluid/melt distribution coefficients for F have been determined in experiments conducted with peraluminous topaz rhyolite melts and fluids consisting of H₂O and H₂O+CO₂ at pressures of 0.5 to 5 kbar, temperatures of 775°–1000°C, and concentrations of F in the melt ranging from 0.5 to 6.9 wt%. The major element, F, and Cl concentrations of the starting material and run product glasses were determined by electron microprobe, and the concentration of F in the fluid was calculated by mass balance. The H₂O concentrations of some run product glasses were determined by ion microprobe (SIMS). The solubility of melt in the fluid phase increases with increasing F in the system; the solubility of H₂O in the melt is independent of the F concentration of the system with up to 6.3 wt% F in the melt. No evidence of immiscible silica- and fluoriderich liquids was detected in the hydrous but water-undersaturated starting material glasses (8.5 wt% F in melt) or in the water-saturated run product glasses. F concentrates in topaz rhyolite melts relative to coexisting fluids at most conditions studied; however, D_F (wt% F in fluid/wt% F in melt) increases strongly with increasing F in the system. Maximum values of D_F in this study are significantly larger than those previously reported in the literature. Linear extrapolation of the data suggests that D_F is greater than one for water-saturated, peraluminous granitic melts containing 8 wt% F at 800° C and 2 kbar. D_F increases as temperature and as (H₂O/H₂O+CO₂) of the fluid increase. For topaz rhyolite melts containing 1 wt% F and with H₂O-rich fluids, D_F is independent of changes in pressure from 2 to 5 kbar at 800° C; for melts containing 1 wt% F and in equilibrium with CO₂-bearing fluids the concentrations of F in fluid increases with increasing pressure. F-and lithophile element-enriched granites may evolve to compositions containing extreme concentrations of F during the final stages of crystallization. If F in the melt exceeds 8 wt%, D_F is greater than one and the associated magmatic-hydrothermal fluid contains >4 molal F. Such F-enriched fluids may be important in the mass transport of ore constituents, i.e., F, Mo, W, Sn, Li, Be, Rb, Cs, U, Th, Nb, Ta, and B, from the magma.     

Concentration of ore elements in magmatic melts and natural fluids as deduced from data on inclusions in minerals

Based on intergration of the published data on composition of inclusions in minerals and quenched glasses, the mean concentrations of 24 ore elements have been calculated for magmatic silicate melts formed in main geodynamic settings of the Earth and in natural fluids. The mean glass compositions normalized to the primitive mantle correlate with the partition coefficient between olivine and the basic melt. It is established that the degree of enrichment in ore elements depending on geodynamic setting is controlled by various contribution of fluids to the element transfer and accumulation. The ratios of element contents in each geodynamic setting to the mean concentrations of elements over all settings in the Earth have been calculated.     

Partitioning behavior of chlorine and fluorine in the system apatite-silicate melt-fluid

The partitioning behavior of Cl among apatite, mafic silicate melt, and aqueous fluid and of F between apatite and melt have been determined in experiments conducted at 1066 to 1150 °C and 199-205 MPa. The value of D_Cl^apatite/melt (wt. fraction of Cl in apatite/Cl in melt) ≈0.8 for silicate melt containing less than ∼3.8 wt.% Cl. At higher melt Cl contents, small increases in melt Cl concentration are accompanied by large increases in apatite Cl concentration, forcing D_Cl^apatite/melt to increase as well. Melt containing less than 3.8% Cl coexists with water-rich vapor; that containing more Cl coexists with saline fluid, the salinity of which increases rapidly with small increases in melt Cl content, analogous to the dependency of apatite composition on melt Cl content. This behavior is due to the fact that the solubility of Cl in silicate melt depends strongly on the composition of the melt, particularly its Mg, Ca, Fe, and Si contents. Once the melt becomes “saturated” in Cl, additional Cl must be accommodated by coexisting fluid, apatite, or other phases rather than the melt itself. Because Cl solubility depends on composition, the Cl concentration at which D_Cl^apatite/melt and D_Cl^fluid/melt begin to increase also depends on composition. The experiments reveal that D_F^apatite/melt ≈3.4. In contrast to Cl, the concentration of F in silicate melt is only weakly dependent on composition (mainly on melt Ca contents), so D_F^apatite/melt is constant for a wide range of composition.The experimental data demonstrate that the fluids present in the waning stages of the solidification of the Stillwater and Bushveld complexes were highly saline. The Cl-rich apatite in these bodies crystallized from interstitial melt with high Cl/(F + OH) ratio. The latter was generated by the combined processes of fractional crystallization and dehydration by its reaction with the relatively large mass of initially anhydrous pyroxene through which it percolated.     

Metasomatism in mantle xenoliths from Gees,West Eifel,Germany: evidence for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment

 We have investigated new samples from the Gees mantle xenolith suite (West Eifel), for which metasomatism by carbonatite melt has been suggested. The major metasomatic change is transformation of harzburgites into phlogopite-rich wehrlites. Silicate glasses are associated with all stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, variably alkaline glass present exclusively within the xenoliths. Si-Al-rich glasses (up to 72 wt% SiO₂ when associated with orthopyroxene (Opx) are usually interpreted in mantle xenoliths as products of decompressional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to phlogopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those related to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Both phases crystallize as microlites from the glass. The chemical composition of the Si-Al-enriched glass shows that it cannot be derived from decompressional melting of the Gees xenoliths, but must have been present prior to their entrainment in the host magma. Simple mass-balance calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + microlite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55–60 wt% SiO₂), characterized by high Al₂O₃ (ca. 18 wt%). The obtained composition is very similar to high-alumina, calc-alkaline melts that should form by AFC-type reactions between basalt and harzburgite wall rock according to the model of Kelemen (1990). Thus, we suggest that the Si-Al-enriched glasses of Gees, and possibly of other suites as well, are remnants of upper mantle hybrid melts, and that the Gees suite was metasomatized by silicate and not carbonatite melts. High-Mg, high-Ca composition of metasomatic olivine and clinopyroxene in mantle xenoliths have been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple function of silica activity. High-Ca compositions are attained at low a SiO₂ and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespective of whether it is a carbonatite or a silicate melt. Such low a SiO₂ is recorded by the microlites in the Gees Si-Al-rich glasses. Our results imply that xenolith suites cannot confidently be related to carbonatite metasomatism if the significance of silicate glasses, when present, is not investigated. Received: 2 March 1995 / Accepted: 12 June 1995     

The effect of CO<Subscript>2</Subscript> on the solubility of aqueous chloride fluid in dacite,phonolite, and rhyolite melts

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.     

金在花岗质熔体中溶解度的初步实验研究

以合成花岗岩、水、不同浓度的盐酸溶液、氢氟酸溶液作为反应初始物,在850℃,100 MPa,接近于NNO的条件下开展了金在不同花岗质熔体中溶解度的实验研究,实验固液相产物中的金含量使用石墨炉原子吸收法测定。实验结果显示,金在花岗质熔体中的溶解度变化范围为1.87～156.62μg/g,流体相中金的溶解度为0.31～6.92μg/g;金在熔体相中的溶解度较其在共存液相中的高。花岗质熔体相中金的溶解度明显受熔体化学组成的影响,过碱性富钠花岗质熔体中金的溶解度明显高些;金在花岗质熔体中的溶解度随着熔体中Na2O/K2O摩尔比增大而增大;在氟氯共存岩浆体系中,氟含量变化对金在熔体相中的溶解度影响不明显,而液相中氯含量增大有利于提高金在流体相中的含量。     

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.     

Partitioning behavior of chlorine and fluorine in the system apatite-melt-fluid. II: Felsic silicate systems at 200 MPa

Hydrothermal experiments were conducted to determine the partitioning of Cl between rhyolitic to rhyodacitic melts, apatite, and aqueous fluid(s) and the partitioning of F between apatite and these melts at ca. 200 MPa and 900-924 °C. The number of fluid phases in our experiments is unknown; they may have involved a single fluid or vapor plus saline liquid. The partitioning behavior of Cl between apatite and melt is non-Nernstian and is a complex function of melt composition and the Cl concentration of the system. Values of D_Cl^apat/melt (wt. fraction of: Cl in apatite/Cl in melt) vary from 1 to 4.5 and are largest when the Cl concentrations of the melt are at or near the Cl-saturation value of the melt. The Cl-saturation concentrations of silicate melts are lowest in evolved, silica-rich melts, so with elevated Cl concentrations in a system and with all else equal, the maximum values of D_Cl^apat/melt occur with the most felsic melt. In contrast, values of D_F^apat/melt range from 11 to 40 for these felsic melts, and many of these are an order of magnitude greater than those applying to basaltic melts at 200 MPa and 1066-1150 °C. The Cl concentration of apatite is a simple and linear function of the concentration of Cl in fluid. Values of D_Cl^fluid/apat for these experiments range from 9 to 43, and some values are an order of magnitude greater than those determined in 200-MPa experiments involving basaltic melts at 1066-1150 °C.In order to determine the concentrations and interpret the behavior of volatile components in magmas, the experimental data have been applied to the halogen concentrations of apatite grains from chemically evolved rocks of Augustine volcano, Alaska; Krakatau volcano, Indonesia; Mt. Pinatubo, Philippines; Mt. St. Helens, Washington; Mt. Mazama, Oregon; Lascar volcano, Chile; Santorini volcano, Greece, and the Bishop Tuff, California. The F concentrations of these magmas estimated from apatite-melt equilibria range from 0.06 to 0.12 wt% and are generally equivalent to the concentrations of F determined in the melt inclusions. In contrast, the Cl concentrations of the magmas estimated from apatite-melt equilibria (e.g., ca. 0.3-0.9 wt%) greatly exceed those determined in the melt inclusions from all of these volcanic systems except for the Bishop Tuff where the agreement is good. This discrepancy in estimated Cl concentrations of melt could result from several processes, including the hypothesis that the composition of apatite represents a comparatively Cl-enriched stage of magma evolution that precedes melt inclusion entrapment prior to the sequestration of Cl by coexisting magmatic aqueous and/or saline fluid(s).     

Sulphur solubility in andesitic to basaltic melts: implications for Hekla volcano

The solubility of sulphur in sulphide-saturated, H₂O-bearing basaltic–andesitic and basaltic melts from Hekla volcano (Iceland) has been determined experimentally at 1,050°C, 300 and 200 MPa, and redox conditions with oxygen fugacity (logfO₂) between QFM−1.2 and QFM+1.1 (QFM is a quartz–fayalite–magnetite oxygen buffer) in the systems containing various amounts of S and H₂O. The S content of the H₂O-rich glasses saturated with pyrrhotite decreases from 2,500 ppm in basalt to 1,500 ppm in basaltic andesite at the investigated conditions. Furthermore, the reduction of water content in the melt at pyrrhotite saturation and fixed T, P and redox conditions leads to a decrease in S concentration from 2,500 to 1,400 ppm for basaltic experiments (for H₂O decrease from 7.8 to 1.4 wt%) and from 1,500 to 900 ppm (for H₂O decrease from 6.7 to 1.7 wt%) for basaltic andesitic experiments. Our experimental data, combined with silicate melt inclusion investigations and the available models on sulphide saturation in mafic magmas, indicate that the parental basaltic melts of Hekla were not saturated with respect to sulphide. During magmatic differentiation, the S content in the residual melts increased and might have reached sulphide saturation with 2,500 ppm dissolved S. With further magma crystallization, the S concentration in the melt was controlled by the sulphide saturation of the magma, decreasing from ~2,500 to 900 ppm S.     

An experimental study of bromine behaviour in water-saturated silicic melts

To assess the effect of the melt composition on bromine concentrations in magmas, we have investigated bromide solubility for water-saturated, iron-free silicic melts with variable Na+K/Al and Si/Al molar ratios (albite, haplogranite, rhyolite, and pantellerite). The experiments were performed in rapid quench cold-seal autoclaves over a range of pressure (1, 1.5, and 2 kbar) and temperature (900, 1000, and 1080 °C) with run durations from 5 to 7 days. A series of natural volcanic glasses and melt inclusions hosted in magmatic minerals were analysed together with the synthetic glasses by PIXE (proton-induced X-ray emission). The Br concentrations range from 5360 to 7850 ppm for albite, from 2800 to 3900 ppm for haplogranite, from 4300 to 5900 ppm for rhyolite, and from 9745 to 11,250 ppm for pantellerite. Br concentrations are negatively correlated with pressure in H₂O-saturated silicic melts and vary with (Na+K)/Al molar ratio with a minimum value at the ratio close to unity. Br behaves similarly to chlorine for all of these melt compositions. The bromide solubility is similar in albitic and rhyolitic melts, which implies that D^f/m is nearly the same for both compositions and is applicable for natural rhyolites as suggested in our previous study (Bureau et al., 2000). This means that the volcanic Br contribution to the atmosphere may be significant. In natural obsidian samples and MI hosted in quartz, olivine, and leucite, the Br concentration varies from < 3 to 28 ppm, with the highest concentrations in pantelleritic melts. We attribute the low Br concentrations of natural melts to a low initial abundance of this halogen in the Earth mantle. However, because Br behaves as an incompatible element before water exsolution, our results imply that magmas could contain much more dissolved Br before eruption and water degassing than the few ppm usually measured in volcanic rocks. Br behaviour during magma crystallisation is controlled by its partitioning into the H₂O-rich fluid phase when this occurs. In addition, its potential high solubility in silicate melts makes it a very sensitive chemical tracer of magma contamination by seawater and Br-rich material. This infers that the investigation of Br behaviour in subduction-zone samples may help for a better understanding of volatiles cycling between the Earth reservoirs.     

硅酸盐-盐（硫酸盐）流体不混溶：蒙古南部Mushugai-Khuduk含火成碳酸岩杂岩体矿物中的熔体包裹体研究

Crystalline and melt inclusions were studied in garnet,diopside,potassium feldspar,and sphene from the garnet syenite porphyry of the carbonatite-bearing complex Mushugai-Khuduk,southern Mongolia.Phlogopite,clinopyroxene,albite,potassium feldspar,spheric,wollastonite,magnetite,Ca and Sr sulfates,fluorite,and apatite were identified among the crystalline inclusions. The melt inclusions were homogenized at 1010～1080℃and analyzed on an electron microprobe.Silicate,salt,and combined silicate- salt melt inclusions were found.Silicate melts show considerable variations in SiO_2 concentration(56 to 66wt% ),high Na_2O K_2O (up to 17wt% ),and elevated Zr,F,and C1 contents.In terms of bulk rock chemistry,the silicate melts are alkali syenites.During thermometric experiments,salt melt inclusions quenched into homogeneous glasses of predominantly sulfate compositions containing no more than 1.3wt% SiO_2.These melts are enriched in alkalis,Ba,Sr,P,F,and C1.The investigation of the silicate and salt melt inclusions in minerals of the garnet syenite porphyries indicate that these rocks were formed under influence of the processes of crystallization differentiation and magma separation into immiscible silicate and salt(sulfate)liquids.     

High pressure fluids in the system MgO–SiO2–H2O under upper mantle conditions

Fluids and melts have been trapped and analysed in high pressure experiments in the model mantle system MgO-SiO₂-H₂O at 6 to 10.5 GPa and 900 to 1,200 °C. The fluid/melt traps consisted of a diamond layer that was added to the experimental charge and was separate from the silicate phases. The recovered diamond traps were analysed by laser ablation - ICP - MS. Starting materials were synthetic mixtures of brucite, talc and silica with variable Mg/Si containing 11-31 wt% H₂O. Experiments on a serpentine starting composition [Mg₃Si₂O₅(OH)₄] result in MgO/SiO₂ weight ratios in the subsolidus fluids close to 1 at 6 GPa and close to 2 at 9 GPa. Melt compositions at 6 and 9 GPa have MgO/SiO₂ ratios close to that of forsterite. At a single pressure the amount of dissolved silicate in the fluid increases steadily with increasing temperature up to 1,150 °C, where a sudden increase of both SiO₂ and MgO is observed. This discrete step marks the solidus, which is more clearly developed at 6 than at 9 GPa. Thus, hydrous melts within the model mantle subsystem Mg₂SiO₄-Mg₂Si₂O₆-H₂O are chemically distinct from aqueous fluids up to at least 9 GPa, corresponding to 300 km depth. Extrapolation of the current data set implies that total convergence between fluid and melt along the solidus probably occurs at 12-13 GPa (~400 km), i.e. close to the Earth's mantle transition zone. Beneath cratons, interactions of hydrous fluids with upper mantle lithologies cause relative silica depletion (olivine enrichment) at depths greater than 200 km and silica (orthopyroxene) enrichment at shallower depths.     

Carbon dioxide in silicate melts: A molecular dynamics simulation study

We have performed a series of molecular dynamics simulations aimed at the evaluation of the solubility of CO₂ in silicate melts of natural composition (from felsic to ultramafic). In making in contact within the simulation cell a supercritical CO₂ phase with a silicate melt of a given composition, we have been able to evaluate (i) the solubility of CO₂ in the P-T range 1473-2273 K and 20-150 kbar, (ii) the density change experienced by the CO₂-bearing melt, (iii) the respective concentrations of CO₂ and species in the melt, (iv) the lifetime and the diffusivity of these species and (v) the structure of the melt around the carbonate groups. The main results are the following:(1) The solubility of CO₂ increases markedly with the pressure in the three investigated melts (a rhyolite, a mid-ocean ridge basalt and a kimberlite) from about ∼2 wt% CO₂ at 20 kbar to ∼25 wt% at 100 kbar and 2273 K. The solubility is found to be weakly dependent on the melt composition (as far as the present compositions are concerned) and it is only at very high pressure (above ∼100 kbar) that a clear hierarchy between solubilities occurs (rhyolite < MORB < kimberlite). Furthermore at a given pressure the calculated solubility is negatively correlated with the temperature.(2) In CO₂-saturated melts, the proportion of carbonate ions is positively correlated with the pressure at isothermal condition and is negatively correlated with the temperature at isobaric condition (and vice versa for molecular CO₂). Furthermore, at fixed (P, T) conditions the proportion of carbonate ions is higher in CO₂-undersaturated melts than in the CO₂-saturated melt. Although the proportion of molecular CO₂ decreases when the degree of depolymerization of the melt increases, it is still significant in CO₂-saturated basic and ultrabasic compositions at high temperatures. This finding is at variance with experimental data on CO₂-bearing glasses which show no evidence of molecular CO₂ as soon as the degree of depolymerization of the melt is high (e.g. basalt). These conflicting results can be reconciled with each other by noticing that a simple low temperature extrapolation of the simulation data predicts that the proportion of molecular CO₂ in basaltic melts might be negligible in the glass at room temperature.(3) The carbonate ions are found to be transient species in the liquid phase, with a lifetime increasing exponentially with the inverse of the temperature. Contrarily to a usual assumption, the diffusivity of carbonate ions into the liquid silicate is not vanishingly small with respect to that of CO₂ molecules: in MORB they differ from each other by a factor of ∼6 at 1473 K and only a factor of ∼2 at 2273 K. Although the bulk diffusivity of CO₂ is governed primarily by the diffusivity of CO₂ molecules, the carbonate ions contribute significantly to the diffusivity of CO₂ in depolymerized melts.(4) Concerning the structure of the CO₂-bearing silicate melt, the carbonate ions are found to be preferentially associated with NBO’s of the melt, with an affinity for NBOs which exceeds that for BOs by almost one order of magnitude. This result explains why the concentration in carbonate ions is positively correlated with the degree of depolymerization of the melt and diminishes drastically in fully polymerized melts where the number of NBO’s is close to zero. Furthermore, the network modifier cations are not randomly distributed in the close vicinity of carbonate groups but exhibit a preferential ordering which depends at once on the nature of the cation and on the melt composition. However at the high temperatures investigated here, there is no evidence of long lived complexes between carbonate groups and metal cations.     

俄罗斯Kurile-Kamchatka地区安山质熔体的成分、挥发分组成和微量元素

Melt inclusions in minerals from some volcanoes of the Kurile-Kamchatka region were examined.The studied basaltic andesites and andesites were sampled from volcanoes of the Central Kamchatka depression(Shiveluch and Bezymyannyi),Eastern Kamchatka volcanic belt(Avachinskii and Karymskii),and Iturup Island,Southern Kuriles(Kudryavyi).Basalts of the 1996 eruption of the Karymskii volcanic center and dacites of Dikii Greben'volcano,Southern Kamchatka were also studied.More than 260 melt inclusions from 31 rock samples were homogenized,and quenched glasses were analyzed using electron and ion microprobes.The compositions of melt inclusions in andesitic phenoerysts vary in silica contents from 56 to 80wt%.Al_2 O_3 ,FeO,MgO,CaO decrease and Na_2O and K_2O increase with increasing SiO_2.Many inclusions(about 80% )are dacitic or rhyolitic.However,the compositions of silicic glasses(>65wt% SiO_2)in andesites significantly differ in TiO2,FeO,MgO,CaO,and K_2O contents from those in dacites and rhyolites.High-potassium melts(K_2O 3.8～6.8wt% )with various SiO_2 from 51.4 to 77.2wt% were found in minerals of all volcanoes studied.This indicates a contribution of a component selectively enriched in potassium to magmas of the whole region.A great compositional diversity of melt inclusions in plagioelase phenocrysts from the Bezymyannyi andesites suggests a complex history of plagioclase crystallization and magma evolution in the andesite formation.Melts from different volcanoes strongly vary in volatile contents.The highest H_2O contents are found in the melts from Shiveluch(3.0～7.2wt%,4.7wt% on average)and Avachinskii (4.7～4.8wt%);while those are lower in melts of Kudryavyi(0.1～2.6wt% ),Dikii Greben'(0.4～1.8wt%),and Bezymyannyi (<1wt%).Chlorine contents are also variable.The lowest values are found in the Bezymyannyi melts(0.09wt% on average),the highest Cl contents are typical of melt inclusions in minerals from the Karymskii andesites(0.26wt% on average).The melts from Avachinskii,Dikii Greben',Kudryavyi,and Shiveluch show intermediate Cl contents(0.13～0.20wt% ).The pressure of 350～1600 bar determined by CO_2 fluid inclusions in plagioclase from the Shiveluch andesites suggests a magma chamber at a depth of 1.5～6 km. Concentrations of 17 elements were determined in glasses of melt inclusions in plagioclases from five volcanoes(Avachinskii, Bezymyannyi,Dikii Greben',Kudryavyi,and Shiveluch).The studied melts show similar trace-element patterns with Nb and Ti minima and B,K,Be,and Li maxima.The melts are close to typical island arc magmas by Sr/Y,La/Yb,K/Ti,and Ca/St ratios, and have some specific regional geochemical features.REE patterns sensitive to degree of magma differentiation indicate that Kudryavyi magmas are most primitive,while Shiveluch magmas are most evolved.