Experimental investigation of H2O and Cl− solubilities in F-enriched silicate liquids; implications for volatile saturation of topaz rhyolite magmas

To interpret the degassing of F-bearing felsic magmas, the solubilities of H₂O, NaCl, and KCl in topaz rhyolite liquids have been investigated experimentally at 2000, 500, and ≈1 bar and 700° to 975 °C. Chloride solubility in these liquids increases with decreasing H₂O activity, increasing pressure, increasing F content of the liquid from 0.2 to 1.2 wt% F, and increasing the molar ratio of ((Al + Na + Ca + Mg)/Si). Small quantities of Cl⁻ exert a strong influence on the exsolution of magmatic volatile phases (MVPs) from F-bearing topaz rhyolite melts at shallow crustal pressures. Water- and chloride-bearing volatile phases, such as vapor, brine, or fluid, exsolve from F-enriched silicate liquids containing as little as 1 wt% H₂O and 0.2 to 0.6 wt% Cl at 2000 bar compared with 5 to 6 wt% H₂O required for volatile phase exsolution in chloride-free liquids. The maximum solubility of Cl⁻ in H₂O-poor silicate liquids at 500 and 2000 bar is not related to the maximum solubility of H₂O in chloride-poor liquids by simple linear and negative relationships; there are strong positive deviations from ideality in the activities of each volatile in both the silicate liquid and the MVP(s). Plots of H₂O versus Cl⁻ in rhyolite liquids, for experiments conducted at 500 bar and 910°–930 °C, show a distinct 90° break-in-slope pattern that is indicative of coexisting vapor and brine under closed-system conditions. The presence of two MVPs buffers the H₂O and Cl⁻ concentrations of the silicate liquids. Comparison of these experimentally-determined volatile solubilities with the pre-eruptive H₂O and Cl⁻ concentrations of five North American topaz and tin rhyolite melts, determined from melt inclusion compositions, provides evidence for the exsolution of MVPs from felsic magmas. One of these, the Cerro el Lobo magma, appears to have exsolved alkali chloride-bearing vapor plus brine or a single supercritical fluid phase prior to entrapment of the melt inclusions and prior to eruption. Received: 6 November 1995 / Accepted: 29 January 1998     

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.     

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.     

Origin of carbonatite magma during the evolution of ultrapotassic basite magma

Clinopyroxene phenocrysts in fergusite from a diatreme in the Dunkel’dyk potassic alkaline complex in the southeastern Pamirs, Tajikistan, and from carbonate veinlets cutting across this rock contain syngenetic carbonate, silicate, and complex melt inclusions. The homogenization of the silicate and carbonate material of the inclusions with the complete dissolution of daughter crystalline phases and fluid in each of them occur simultaneously at 1150?1180°C. The pressures estimated using fluid inclusions and mineral geobarometers were 0.5–0.7 GPa. The behavior of the inclusions during their heating and their geochemistry are in good agreement with the origin of carbonate melts via liquid immiscibility. Carbonatite magma was segregated at the preservation of volatile components (H₂O, CO₂, F, Cl, and S) in the melt, and this resulted in the crystallization of H₂O-rich minerals and carbonates and testifies that the magma was not intensely degassed during its ascent to the surface. The silicate melts are rich in alkalis (up to 4 wt % Na₂O and 12 wt % K₂O), H₂O, F, Cl, and REE (up to 1000 ppm), LREE, Ba, Th, U, Li, B, and Be. The diagrams of the concentrations of incompatible elements of these rocks typically show deep Nb, Ta, and Ti minima, a fact making them similar to the unusual type of ultrapotassic magmas: lamproites of the Mediterranean type. These magmas are thought to be generated in relation to subduction processes, first of all, the fluid transport of various components from a down-going continental crustal slab into overlying levels of the mantle wedge, from which ultrapotassic magmas are presumably derived.     

Experimental study of eclogite melting with participation of the H<Subscript>2</Subscript>O-CO<Subscript>2</Subscript>-KCl fluid at 5 GPa

In order to model the processes of formation of the highly alkaline (potassic) melts during the partial melting of the eclogite nodules in kimberlites, experiments on the melting of the model and natural eclogites in presence of the H₂O-CO₂ and H₂O-CO₂-KCl fluids at 5 GPa and 1200 and 1300°C are performed. A comparative analysis of the phase relations in the systems with H₂O-CO₂ and H₂O-CO₂-KCl demonstrate that KCl in the fluid equilibrated with eclogites intensifies their melting. It is related to both high Cl concentration in the forming silicate melt (2.0–5.5 wt %) and its enrichment in K₂O owing to the K-Na exchange reactions with the immiscible chloride melt. Because of these reactions, the K₂O/Cl ratio in the melts increases with the KCl content in the system and reaches 2.5–3.5 in the silicate melts coexisting with the immiscible chloride liquid. However, the ratio KCl/(H₂O + CO₂ + KCl) in the fluid does not influence on the ratio K₂O/Cl in the melts. Thus, the solubility KCl in the melts, apparently, does not depend on presence of the H₂O-CO₂ fluid, at least, within the concentration range used in the experiments (up to 20 wt %). The experiments show that the deliberated chloride liquid is necessary to form the potassium-rich chlorine-bearing silicate melts during the eclogite melting. It corresponds to the KCl content in the system above 5 wt %.     

A simple thermodynamic model for estimating the solubility of H2O in magmas

A symmetrical, strictly regular solution model is used to estimate H₂O solubilities in silicate melts. The standard state chemical potential of dissolved H₂O and the adjustable parameter in the activity coefficient are determined by least squares analyses of data on H₂O solubility in silicate melts. The adjustable parameter in the expression for the activity coefficient (In) is a function only of the anhydrous melt composition and eleven values are provided for melts ranging in composition from picrite to rhyolite. At the 95% confidence level, the model should estimate H₂O contents to within 4.8% of the amount present if the amount present is less than 10 wt.%. This compares to the reproducibility of 2.25% of the amount present for experimental determinations. To apply the model to rocks and magmas estimates ofT, P, and the fugacity of H₂O are required.Variation of the H₂O content of the melt changes the activity of other components. Knowledge of this variation removes the requirement that the fugacity of H₂O be estimated. Application of the properties of exact differentials to the Gibbs function for the hydrous melt provides an expression relating the chemical potential of a feldspar component to the H₂O content of the melt. This expression contains a second adjustable parameter which depends on the anhydrous melt composition. Using this second expression, the H₂O content can be estimated ifT, P, and feldspar composition are known. Data are too meagre to evaluate the quantitative success of the second method.     

Magma storage prior to the 1912 eruption at Novarupta, Alaska

New analytical and experimental data constrain the storage and equilibration conditions of the magmas erupted in 1912 from Novarupta in the 20th century's largest volcanic event. Phase relations at H₂O+CO₂ fluid saturation were determined for an andesite (58.7 wt% SiO₂) and a dacite (67.7 wt%) from the compositional extremes of intermediate magmas erupted. The phase assemblages, matrix melt composition and modes of natural andesite were reproduced experimentally under H₂O-saturated conditions (i.e., P_H2O=P_TOT) in a negatively sloping region in T-P space from 930 °C/100 MPa to 960 °C/75 MPa with fO₂~NNO+1. The H₂O-saturated equilibration conditions of the dacite are constrained to a T-P region from 850 °C/50 MPa to 880 °C/25 MPa. If H₂O-saturated, these magmas equilibrated at (and above) the level where co-erupted rhyolite equilibrated (~100 MPa), suggesting that the andesite-dacite magma reservoir was displaced laterally rather than vertically from the rhyolite magma body. Natural mineral and melt compositions of intermediate magmas were also reproduced experimentally under saturation conditions with a mixed (H₂O + CO₂) fluid for the same range in P_H2O. Thus, a storage model in which vertically stratified mafic to silicic intermediate magmas underlay H₂O-saturated rhyolite is consistent with experimental findings only if the intermediates have X_H2O^fl=0.7 and 0.9 for the extreme compositions, respectively. Disequilibrium features in natural pumice and scoria include pristine minerals existing outside their stability fields, and compositional zoning of titanomagnetite in contact with ilmenite. Variable rates of chemical equilibration which would eliminate these features constrain the apparent thermal excursion and re-distribution of minerals to the time scale of days.     

The behavior of chlorine and sulfur during differentiation of the Mt. Somma-Vesuvius magmatic system

Summary Reheated silicate melt inclusions in volcanic rock samples from Mt. Somma-Vesuvius, Italy, have been analyzed for 29 constituents including H₂O, S, Cl, F, B, and P₂O₅. This composite volcano consists of the older Mt. Somma caldera, formed between 14 and 3.55 ka before present, and the younger Vesuvius cone. The melt inclusion compositions provide important constraints on pre-eruptive magma geochemistry, identify relationships that relate to eruption behavior and magma evolution, and provide extensive evidence for magmatic fluid exsolution well before eruption. The melt inclusion data have been categorized by groups that reflect magma compositions, age, and style of eruptions. The data show distinct differences in composition for eruptive products older than 14.0 ka (pre-caldera rocks) versus eruptive products younger than 3.55 ka. Moreover, pre-caldera eruptions were associated with magmas relatively enriched in SiO₂, whereas eruptions younger than 3.55 ka (i.e., the syn- and post-caldera magmas which generated the Somma caldera and the Vesuvius cone) were derived from magmas comparatively enriched in S, Cl, CaO, MgO, P₂O₅, F, and many lithophile trace elements. Melt inclusion data indicate that eruptive behavior at Vesuvius correlates with pre-eruptive volatile enrichments. Most magmas associated with explosive plinian and subplinian events younger than 3.55 ka contained more H₂O, contained significantly more S, and exhibited higher (S/Cl) ratios than syn- and post-caldera magmas which erupted during relatively passive interplinian volcanic phenomena. Received January 10, 2000 Revised version accepted July 17, 2000     

The occurrence and distribution of Mo and molybdenite in unaltered peralkaline rhyolites from Pantelleria,Italy

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

Formation of extremely F-rich hydrous melt fractions and hydrothermal fluids during differentiation of highly evolved tin-granite magmas: a melt/fluid-inclusion study

Quartz crystals from topaz–zinnwaldite–albite granites from Zinnwald (Erzgebirge, Germany) contain, in addition to primary and secondary fluid inclusions (FIs), abundant crystalline silicate-melt inclusions (MIs) with diameters up to 200 m. These MIs represent various stages of evolution of a highly evolved melt system that finally gave rise to granite-related Sn–W mineralization. The combination of special experimental techniques with confocal laser Raman-microprobe spectroscopy and EMPA permits precise measurement of elevated contents of H₂O, F, and B in re-homogenized MIs. The contents of H₂O and F were observed to increase from 3 to 30 and 1.9 to 6.4 wt%, respectively, during magma differentiation. However, there is a second MI group, very rich in H₂O, with values up to 55 wt% H₂O and an F concentration of approximately 3 wt%. Ongoing enrichment of volatiles H₂O, F, B, and Cl and of Cs and Rb can be explained in terms of magma differentiation triggered by fractional crystallization and thus, is suggested to reflect elemental abundances in natural magmas, and not boundary-layer melts. Partitioning between melt and cogenetic fluids has further modified the magmatic concentrations of some elements, particularly Sn. The coexistence of two types of MIs with primary FIs indicates fluid saturation early in the history of magma crystallization, connected with a continuous sequestration of Sn, F, and B. The results of this study provide additional evidence for the extraordinary importance of the interplay of H₂O, F, and B in the enrichment of Sn during magma differentiation by decreasing the viscosity of and increasing the diffusivity in the melts as well as by the formation of various stable fluoride complexes in the melt and coexisting fluid.

---

Copper partitioning between granitic silicate melt and coexisting aqueous fluid at 850 °C and 100 MPa

Experiments on the partitioning of Cu between different granitic silicate melts and the respective coexisting aqueous fluids have been performed under conditions of 850 °C, 100 MPa and oxygen fugacity（f O₂） buffered at approaching Ni–Ni O（NNO）. Partition coefficients of Cu（DCu= cfluid/cmelt） were varied with different alumina/alkali mole ratios [Al₂O₃/（Na₂O·K₂O）, abbreviated as Al/Alk], Na/K mole ratios, and Si O₂ mole contents. The DCu increased from 1.28 ± 0.01 to 22.18 ± 0.22 with the increase of Al/Alk mole ratios（ranging from 0.64 to 1.20）and Na/K mole ratios（ranging from 0.58 to 2.56）. The experimental results also showed that DCuwas positively correlated with the HCl concentration of the starting fluid.The DCuwas independent of the Si O₂ mole content in the range of Si O₂ content considered. No DCuvalue was less than 1 in our experiments at 850 °C and 100 MPa, indicating that Cu preferred to enter the fluid phase rather than the coexisting melt phase under most conditions in the melt-fluid system, and thus a significant amount of Cu could be transported in the fluid phase in the magmatichydrothermal environment. The results indicated that Cu favored partitioning into the aqueous fluid rather than themelt phase if there was a high Na/K ratio, Na-rich, peraluminous granitic melt coexisting with the high Cl-fluid.     

Trace element geochemistry of the 1991 Mt. Pinatubo silicic melts, Philippines: Implications for ore-forming potential of adakitic magmatism

The dacite pumice erupted from Mt. Pinatubo on June 15, 1991 (whole-rock, rhyolitic groundmass glasses and homogenized melt inclusions) has been analyzed using inductively coupled plasma-mass spectrometry (ICP-MS), nanosecond and femtosecond laser ablation ICP-MS and secondary ion mass spectrometry (SIMS) to evaluate its ore-forming potential. Data suggest that adakite magmas are metal-rich and concentrate ore metals during magmatic differentiation. Sulfides segregate in limited amounts under the hydrous, oxidizing conditions typical of adakitic magmas resulting in incompatible behavior for Au (6-22 ppb), Cu (26-77 ppm), and Pb, Mo, As, and Sb in melts of dacitic to rhyolitic compositions. Metal transfer from this adakite magma to the coexisting aqueous phase was favored by the peraluminous composition of the rhyolitic melt and high aqueous chloride concentrations. Mass balance calculations suggest that the pre-eruptive aqueous phase could have extracted a minimum of 100 t Au and 5 × 10⁵ t Cu from the Mt. Pinatubo magma. Our data suggest that intrusives having adakitic signatures are genetically associated with Au-Cu and Cu-Mo mineralization, auriferous porphyry copper deposits, and epithermal gold veins. High H₂O, Cl, Sr/Y, Pb/Ce, Mo/Ce, As/Ce and Sb/Ce in Mt. Pinatubo melts reflect the contribution of deep fluids derived from subducted sediments and altered MORBs in the dacite genesis. The slab-derived fluids carrying mobile elements are likely responsible for the enrichment of adakite magmas in gold, associated metals and H₂O, and may explain the exceptional ore-forming potential of adakite magmatism.     

Segregation of magmatic fluids and their potential in the mobilization of platinum-group elements in the South Kawishiwi Intrusion,Duluth Complex,Minnesota — Evidence from petrography,apatite geochemistry and coexisting fluid and melt inclusions

Pegmatitic and other felsic rock pockets and dike-like intrusions are abundant in the South Kawishiwi Intrusion of the Duluth Complex, including the basal, Cu–Ni–PGE mineralized units. These occurrences are found as pockets, pods or as veins and contain abundant accessory apatite and quartz. Quartz hosts primary fluid inclusions as well as silicate melt inclusions. Combined microthermometry and Raman spectroscopy helped to determine the bulk composition of primary fluid inclusions that are CO₂-rich (95 mol%) and contain small amounts of H₂O (4.5 mol%), CH₄ (0.4 mol%) and trace N₂, respectively. This combined technique also made it possible to measure total homogenization temperatures of the inclusions (Th_tot = ~ 225 ± 10 °C), otherwise not detectable during microthermometry. Silicate melt inclusions have been quenched to produce homogeneous glasses corresponding to the original melt. Composition of the entrapped melt is granitoid, peraluminous and is very poor in mafic components. We interpret the melt as a product of partial melting of the footwall rocks due to the contact effect of the South Kawishiwi Intrusion. The presence of CO₂ in the vapor bubbles of the quenched melt inclusions and petrographic evidence suggest that the fluid and melt inclusion assemblages are coeval. The composition of the fluid and melt phase implies that the fluid originates from the mafic magma of the South Kawishiwi Intrusion and the fluid and melt phases coexisted as a heterogeneous melt–fluid system until entrapment of the inclusions.Coexistence of primary fluid and melt inclusions makes it possible to calculate a minimum entrapment pressure (~ 1.7 kbar) and thus estimate formation depth (~ 5.8 km) for the inclusions. Chlorine is suggested to behave compatibly in the silicate melt phase in the fluid–melt system represented by the inclusions, indicated by the high (up to 0.3%) Cl-concentrations of the silicate melt and CO₂-rich nature of the fluid.Apatite halogen-contents provide further details on the behavior of Cl. Apatite in pegmatitic pockets often has elevated Cl-concentrations compared to troctolitic rocks, suggesting enrichment of Cl with progressive crystallization. Systematic trends of Cl-loss at some differentiated melt pockets suggest that in some places Cl exsolved into a fluid phase and migrated away from its source. The segregation of Cl from the melt is probably inhibited by the presence of CO₂-rich fluids until the last stages of crystallization, increasing the potential for the development of late-stage saline brines.Platinum-group minerals are often present in microcracks in silicate minerals, in late-stage differentiated sulfide veinlets and in association with chlorapatite, indicating the potential role of Cl-bearing fluids in the final distribution of PGEs.     

Phase-equilibrium geobarometers for silicic rocks based on rhyolite-MELTS. Part 1: Principles,procedures, and evaluation of the method

Constraining the pressure of crystallization of magmas is an important but elusive task. In this work, we present a method to derive crystallization pressures for rocks that preserve glass compositions (either glass inclusions or matrix glass) representative of equilibration between melt, quartz, and 1 or 2 feldspars. The method relies on the well-known shift of the quartz–feldspar saturation surface toward higher normative quartz melt compositions with decreasing pressure. The critical realization for development of the method is the fact that melt, quartz and feldspars need to be in equilibrium at the liquidus for the melt composition. The method thus consists of calculating the saturation surfaces for quartz and feldspars using rhyolite-MELTS over a range of pressures, and searching for the pressure at which the expected assemblage (quartz+1 feldspar or quartz+2 feldspars) is found at the liquidus. We evaluate errors resulting from uncertainties in glass composition using a series of Monte Carlo simulations for a quartz-hosted glass inclusion composition from the Bishop Tuff, which reveal errors on the order of 20–45 MPa for the quartz+2 feldspars constraint and on the order of 25–100 MPa for the quartz+1 feldspar constraint; we suggest actual errors are closer to the lower bounds of these ranges. We investigate the effect of fluid saturation in two ways: (1) By applying our procedure over a range of water contents for three glass compositions; we show that the effect of fluid saturation is more important at higher pressures (~300 MPa) than at lower pressures (~100 MPa), but reasonable pressure estimates can be derived irrespective of fluid saturation for geologically relevant H₂O concentrations >3 wt% and (2) by performing the same type of pressure determinations with a preliminary version of rhyolite-MELTS that includes a H₂O–CO₂ mixed fluid phase; we use a range of H₂O and CO₂ concentrations for two compositions characteristic of early-erupted and late-erupted Bishop Tuff glass inclusions and demonstrate that calculated pressures are largely independent of CO₂ concentration (for CO₂ <1,000 ppm), at least for relatively high H₂O contents, as expected in most natural magmas, such that CO₂ concentration can be effectively neglected for application of our method. Finally, we demonstrate that pressures calculated using the rhyolite-MELTS geobarometer compare well with those resulting from H₂O–CO₂ glass inclusion barometry and Al-in-hornblende barometry for an array of natural systems for which data have been compiled from the literature; the agreement is best for quartz-hosted glass inclusions, while matrix glass yields systematically lower rhyolite-MELTS pressures, suggestive of melt evolution during eruptive decompression.     

The enhanced element enrichment in the supercritical states of granite–pegmatite systems

In this paper, we show that supercritical fluids have a greater significance in the generation of pegmatites, and for ore-forming processes related to granites than is usually assumed. We show that the supercritical melt or fluid is a silicate phase in which volatiles; principally H₂O are completely miscible in all proportions at magmatic temperatures and pressures. This phase evolves from felsic melts and changes into hydrothermal fluids, and its unique properties are particularly important in sequestering and concentrating low abundance elements, such as metals. In our past research, we have focused on processes observed at upper crustal levels, however extensive work by us and other researchers have demonstrated that supercritical melt/fluids should be abundant in melting zones at deep-crustal levels too. We propose that these fluids may provide a connecting link between lower and upper crustal magmas, and a highly efficient transport mechanism for usually melt incompatible elements. In this paper, we explore the unique features of this fluid which allow the partitioning of various elements and compounds, potentially up to extreme levels, and may explain various features both of mineralization and the magmas that produced them.

     

The competing models for the origin and internal evolution of granitic pegmatites in the light of melt and fluid inclusion research

In this paper we discuss the main petrogenetic models for granitic pegmatites and how these models have evolved over time. We suggest that the present state of knowledge requires that some aspects of these models to be modified, or absorbed into newer ones. Pegmatite formation and internal evolution have long supposed the need for highly water- and flux-enriched magmas to explain the differences between pegmatites and other intrusives of similar major element composition. Compositions and textural characteristics of fluid and melt inclusions in pegmatite minerals provide strong evidence for such magmas. Furthermore, we show that melt inclusion research has increased the number of potential flux components, which may include H₂O, OH^?, CO₂, HCO ₃ ^? , CO ₃ ^2? , SO ₄ ^2? , PO ₄ ^3? , H₃BO₃, F , and Cl, as well as the elements Li, Na, K, Rb, Cs, and Be, herein described as melt structure modifiers. In this paper we emphasize that the combined effect which these components have on the properties of pegmatite melts is difficult to deduce from experimental studies using only a limited number of these components. The combination and the amount of the different magmatic species, together with differences in the source region, and variations in pressure and temperature cause the great diversity of the pegmatites observed. Some volatile species, such as CO ₃ ^2? and alkalis, have the capacity to increase the solubility of H₂O in silicate melt to an extraordinary degree, to the extent that melt-melt-fluid immiscibility becomes inevitable. It is our view that the formation of pegmatites is connected with the complex interplay of many factors.     

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.     

C�CO�CH�CS fluids and granitic magma: how S partitions and modifies CO2 concentrations of fluid-saturated felsic melt at 200?MPa

Hydrothermal volatile-solubility and partitioning experiments were conducted with fluid-saturated haplogranitic melt, H₂O, CO₂, and S in an internally heated pressure vessel at 900°C and 200?MPa; three additional experiments were conducted with iron-bearing melt. The run-product glasses were analyzed by electron microprobe, FTIR, and SIMS; and they contain ??0.12 wt% S, ??0.097 wt% CO₂, and ??6.4 wt% H₂O. Apparent values of log f _O2 for the experiments at run conditions were computed from the [(S⁶⁺)/(S⁶⁺+S^2?)] ratio of the glasses, and they range from NNO ?0.4 to NNO?+?1.4. The C?CO?CH?CS fluid compositions at run conditions were computed by mass balance, and they contained 22?C99?mol% H₂O, 0?C78?mol% CO₂, 0?C12?mol% S, and <3 wt% alkalis. Eight S-free experiments were conducted to determine the H₂O and CO₂ concentrations of melt and fluid compositions and to compare them with prior experimental results for C?CO?CH fluid-saturated rhyolite melt, and the agreement is excellent. Sulfur partitions very strongly in favor of fluid in all experiments, and the presence of S modifies the fluid compositions, and hence, the CO₂ solubilities in coexisting felsic melt. The square of the mole fraction of H₂O in melt increases in a linear fashion, from 0.05 to 0.25, with the H₂O concentration of the fluid. The mole fraction of CO₂ in melt increases linearly, from 0.0003 to 0.0045, with the CO₂ concentration of C?CO?CH?CS fluids. Interestingly, the CO₂ concentration in melts, involving relatively reduced runs (log f _O2????NNO?+?0.3) that contain 2.5?C7?mol% S in the fluid, decreases significantly with increasing S in the system. This response to the changing fluid composition causes the H₂O and CO₂ solubility curve for C?CO?CH?CS fluid-saturated haplogranitic melts at 200?MPa to shift to values near that modeled for C?CO?CH fluid-saturated, S-free rhyolite melt at 150?MPa. The concentration of S in haplogranitic melt increases in a linear fashion with increasing S in C?CO?CH?CS fluids, but these data show significant dispersion that likely reflects the strong influence of f _O2 on S speciation in melt and fluid. Importantly, the partitioning of S between fluid and melt does not vary with the (H₂O/H₂O?+?CO₂) ratio of the fluid. The fluid-melt partition coefficients for H₂O, CO₂, and S and the atomic (C/S) ratios of the run-product fluids are virtually identical to thermodynamic constraints on volatile partitioning and the H, S, and C contents of pre-eruptive magmatic fluids and volcanic gases for subduction-related magmatic systems thus confirming our experiments are relevant to natural eruptive systems.     

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.     

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).