Phase relations of a F-enriched peraluminous granite: an experimental study of the Kymi topaz granite stock,southern Finland

Phase relations of F-rich Kymi equigranular topaz granite have been investigated experimentally at 100–500 MPa as a function of water activity and F content. Fluorite and topaz can crystallize as liquidus phases in F-rich peraluminous systems, but the F content of the melt should exceed 2.5–3.0 wt% for the crystallization of topaz. In peraluminous F-bearing melts containing more than 1 wt% F, topaz and muscovite are expected to be the first F-bearing phases to crystallize at high pressure, whereas fluorite and topaz should crystallize first at low pressure. Following reaction models the saturation of fluorite and topaz: CaAl₂Si₂O₈ (plagioclase) + 2[AlF₃]_melt = CaF₂ (fluorite) + 2Al₂SiO₄F₂ (topaz). The obtained partition coefficient for F between biotite and glass D(F)^Bt/glass ranges from 1.89 to 0.80 (average value 1.29) and can be used as an empirical fluormeter to determine the F content of coexisting melts. Combined petrological and experimental studies of the Kymi equigranular topaz granite indicate that plagioclase was the liquidus phase at nearly water-saturated (fluid-saturated) conditions and that the F content of the melt was at least 2 wt%. The mean F content of natural biotite (3.92 wt%) suggests that the late-stage crystallization of biotite occurred in melts containing about 3 wt% F. The early crystallization of biotite and the presence of muscovite in our experiments at 200 MPa contrasts with the late-stage crystallization of biotite and the absence of muscovite in the natural assemblage, indicating that crystallization pressure may have been lower than 200 MPa for the granite. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

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.

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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.     

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     

H2O diffusion in dacitic and andesitic melts

The diffusion of water in dacitic and andesitic melts was investigated at temperatures of 1458 to 1858 K and pressures between 0.5 and 1.5 GPa using the diffusion couple technique. Pairs of nominally dry glasses and hydrous glasses containing between 1.5 and 6.3 wt.% dissolved H₂O were heated for 60 to 480 s in a piston cylinder apparatus. Concentration profiles of hydrous species (OH groups and H₂O molecules) and total water (C_H2Ot = sum of OH and H₂O) were measured along the cylindrical axis of the diffusion sample using IR microspectroscopy. Electron microprobe traverses show no significant change in relative proportions of anhydrous components along H₂O profiles, indicating that our data can be treated as effective binary interdiffusion between H₂O and the rest of the silicate melt. Bulk water diffusivity (D_H2Ot) was derived from profiles of total water using a modified Boltzmann-Matano method as well as using fittings assuming a functional relationship between D_H2Ot and C_H2Ot. In dacitic melts D_H2Ot is proportional to C_H2Ot up to 6 wt.%. In andesitic melts the dependence of D_H2Ot on C_H2Ot is less pronounced. A pressure effect on water diffusivity could not be resolved for either dacitic or andesitic melt in the range 0.5 to 1.5 GPa. Combining our results with previous studies on water diffusion in rhyolite and basalt show that for a given water content D_H2Ot increases monotonically with increasing melt depolymerization at temperatures >1500 K. Assuming an Arrhenian behavior in the whole compositional range, the following formulation was derived to estimate D_H2Ot (m²/s) at 1 wt.% H₂O_t in melts with rhyolitic to andesitic composition as a function of T (K), P (MPa) and S (wt.% SiO₂):

     

Partitioning of F between aqueous fluids and albite granite melt and its petrogenetic and metallogenetic significance

The fluid/melt partitioning experiments on fluorine were carried out in the system albite-H₂O-HF atP = 100 MPa, 770°C ≤T≤800°C: and wt = 2% −6% conditions. The concentrations of fluorine in quenched glasses (melt) were determined by electron microprobe and those of fluorine in the coexisting aqueous fluid were calculated by the method of mass balance. The result shows that the fluorine was concentrated in granitic melt relative to the coexisting fluid. The partition coefficient D_F(wt _F ^F1 /wt _F ^Mt ) ranges from 0.35 to 0.89. It increases with increasing fluorine content in the system. This means that there is not just one single value of partition coefficient for fluorine in the granitic melt-fluid system. The partitioning behavior of fluorine in this system depends critically on fluorine and proton (H⁺) concentrations. Our data suggest that F-rich granitic melts exist in nature and that fluorine may not be an important complexing agent of metal elements in F-bearing fluids. The project was financially supported by both the National Natural Science Foundation of China (No. 49603048) and the State Key Laboratory of Mineral Deposit Research, Nanjing University.     

Liquidus Equilibria in the System K2O-Na2O-Al2O3-SiO2-F2O-1-H2O to 100 MPa: II. Differentiation Paths of Fluorosilicic Magmas in Hydrous Systems

We investigated phase equilibria in the six-component systemNa₂O–K₂O–Al₂O₃–SiO₂–F₂O_–1–H₂Oat 100 MPa to characterize differentiation paths of naturalfluorine-bearing granitic and rhyolitic magmas. Topaz and cryoliteare stable saturating solid phases in calcium-poor systems.At 100 MPa the maximum solidus depression and fluorine solubilityin evolving silicic melts are controlled by the eutectics haplogranite–cryolite–H₂Oat 640°C and 4 wt % F, and haplogranite–topaz–H₂Oat 640°C and 2 wt % F. Topaz and cryolite form a binaryperalkaline eutectic at 660°C, 100 MPa and fluid saturation.The low-temperature nature of this invariant point causes displacementof multiphase eutectics with quartz and alkali feldspar towardsthe topaz–cryolite join and enables the silicate liquidusand cotectic surfaces to extend to very high fluorine concentrations(more than 30 wt % F) for weakly peraluminous and subaluminouscompositions. The differentiation of fluorine-bearing magmasfollows two distinct paths of fluorine behavior, depending onwhether additional minerals buffer the alkali/alumina ratioin the melt. In systems with micas or aluminosilicates thatbuffer the activity of alumina, magmatic crystallization willreach either topaz or cryolite saturation and the system solidifiesat low fluorine concentration. In leucogranitic suites precipitatingquartz and feldspar only, the liquid line of descent will reachtopaz or cryolite but fluorine will continue to increase untilthe quaternary eutectic with two fluorine-bearing solid phasesis reached at 540°C, 100 MPa and aqueous-fluid saturation.The maximum water solubility in the haplogranitic melts increaseswith the fluorine content and reaches 12· 5 ±0· 5 wt % H₂O at the quartz–cryolite–topazeutectic composition. A continuous transition between hydrousfluorosilicate melts and solute-rich aqueous fluids is not documentedby this study. Our experimental results are applicable to leucocraticfluorosilicic magmas. In multicomponent systems, however, thepresence of calcium may severely limit enrichment of fluorineby crystallization of fluorite. KEY WORDS: granite; rhyolite; topaz; cryolite; magmatic differentiation     

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.     

Cordierite as a sensor of fluid conditions in high-grade metamorphism and crustal anatexis

Cordierite H₂O and CO₂ volatile saturation surfaces derived from recent experimental studies are presented for P–T conditions relevant to high‐grade metamorphism and used to evaluate fluid conditions attending partial melting and granulite formation. The volatile saturation surfaces and saturation isopleths for both H₂O and CO₂ in cordierite are strongly pressure dependent. In contrast, the uptake of H₂O by cordierite in equilibrium with melts formed through biotite dehydration melting, controlled by the distribution of H₂O between granitic melt and cordierite, D_w[D_w = wt% H₂O (melt)/wt% H₂O(Crd)], is mainly temperature dependent. D_w = 2.5–6.0 for the H₂O contents (0.4–1.6 wt percentage) typical of cordierite formed through biotite dehydration melting at 3–7 kbar and 725–900 °C. This range in D_w causes a 15–30% relative decrease in the total wt% of melt produced from pelites. Cordierite in S‐type granites are H₂O‐rich (1.3–1.9 wt%) and close to or saturated in total volatiles, signifying equilibration with crystallizing melts that achieved saturation in H₂O. In contrast, the lower H₂O contents (0.6–1.2 wt percentage) preserved in cordierite from many granulite and contact migmatite terranes are consistent with fluid‐absent conditions during anatexis. In several cases, including the Cooma migmatites and Broken Hill granulites, the cordierite volatile compositions yield a_H2O values (0.15–0.4) and melt H₂O contents (2.2–4.4 wt%) compatible with model dehydration melting reactions. In contrast, H₂O leakage is indicated for cordierite from Prydz Bay, Antarctica that preserve H₂O contents (0.5–0.3 wt%) which are significantly less than those required (1.0–0.8 wt%) for equilibrium with melt at conditions of 6 kbar and 860 °C. The CO₂ contents of cordierite in migmatites range from negligible (< 0.1 wt%) to high (0.5–1.0 wt%), and bear no simple relationship to preserved cordierite H₂O contents and a_H2O. In most cases the cordierite volatile contents yield total calculated fluid activities (a_H2O + a_CO2) that are significantly less than those required for fluid saturation at the P–T conditions of their formation. Whether this reflects fluid absence, dilution of H₂O and CO₂ by other components, or leakage of H₂O from cordierite is an issue that must be evaluated on a case‐by‐case basis.     

Source and H2O content of high-MgO magmas in island arc settings: an experimental study of a primitive calc-alkaline basalt from St. Vincent, lesser antilles arc

Liquidus phase relationships have been determined for a high-MgO basalt (STV301: MgO=12.5 wt%, Ni=250 ppm, Cr=728 ppm) from Black Point, St Vincent (Lesser Antilles arc). Piston-cylinder experiments were conducted between 7.5 and 20 kbar under both hydrous and oxidizing conditions. AuPd capsules were used as containers. Compositions of supraliquidus glasses and mass-balance calculations show that Fe loss is < 10% in the majority of experiments. Two series of water concentrations in melt were investigated: (i) 1.5 wt% and (ii) 4.5 wt% H₂O, as determined by SIMS analyses on quenched glasses and with the by difference technique. The Fe³⁺/Fe²⁺ partitioning between Cr-Al spinel and melt and olivine-spinel equilibria show that oxidizing fO₂ were imposed (NNO + 1.5 for the 1.5 wt% H₂O series, NNO + 2.3 for the 4.5 wt% H₂O series). For both series of water concentrations, the liquid is multiply-saturated with a spinel lherzolite phase assemblage on its liquidus, at 1235°C, 11.5 kbar (1.5 wt% H₂O) and 1185°C, 16 kbar (4.5 wt% H₂O). Liquidus phases are homogeneous and comparable to typical mantle compositions. Mineral-melt partition coefficients are generally identical to values under anhydrous conditions. The modal proportion cpx/opx on the liquidus decreases from the 1.5 wt% to the 4.5 wt% H₂O series. The experimental data are consistent with STV301 being a product of partial melting of lherzolitic mantle. Conditions of multiple saturation progressively evolve toward lower temperatures and higher pressures with increasing melt H₂O concentration. Phase equilibria constraints, i.e., the necessity of preserving the mantle signature seen in high-MgO and picritic arc basalts, and glass inclusion data suggest that STV301 was extracted relatively dry (∼ 2 wt% H₂O) from its mantle source. However, not all primary arc basalts are extracted under similarly dry conditions because more hydrous melts will crystallize during ascent and will not be present unmodified at the surface. From degrees of melting calculated from experiments on KLB-1, extraction of a 12.5 wt% MgO melt with ∼ 2 wt% H₂O would require a H₂O concentration of 0.3 wt% in the sub-arc mantle. For mantle sources fluxed with a slab-derived hydrous component, extracted melts may contain up to ∼ 5.5 wt% H₂O.     

S-rich apatite-hosted glass inclusions in xenoliths from La Palma: constraints on the volatile partitioning in evolved alkaline magmas

The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO₃ (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce₂O₃. Sulfur is incorporated in apatite via several independent exchange reactions involving (P⁵⁺, Ca²⁺) vs. (S⁶⁺, Si⁴⁺, Na⁺, and Ce³⁺). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from <0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO₃ (sulfate-saturated systems). The chlorine partition coefficients (D_Cl^{apatite/glass}) range from 0.4 to 1.3 (average D_Cl^{apatite/glass} = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions D_F^{apatite/glass} decreases from 35 to 3. However, most of our data display a high partition coefficient (~30) close to D_F^{apatite/glass} determined experimentally in felsic rock. D_S^{apatite/glass} decreases from 9.1 to 2.9 with increasing SO₃ in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (>0.2 wt% SO₃). D_S^{apatite/glass} is only slightly dependent on the melt composition and can be expressed as: SO_{3 apatite} (wt%) = 0.157 * ln SO_{3 glass} (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and haüyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of haüyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.     

Solubility of tin in (Cl, F)-bearing aqueous fluids at 700 °C, 140 MPa: A LA-ICP-MS study on synthetic fluid inclusions

Synthetic fluid inclusions in quartz were grown from cassiterite-saturated fluids in cold-seal pressure vessels at and subsequently analyzed by laser ablation-ICP-MS. Most inclusions were synthesized using a new technique that allows entrapment of fluid that had no immediate contact to the capsule walls, such that potential disequilibrium effects due to alloying could be avoided. Measured Sn solubilities increase with increasing ligand concentration in the fluid, ranging from 100 to 800 ppm in NaCl-bearing fluids (5-35 wt% NaCl), from 70 to 2000 ppm in HF-bearing fluids (0.5-3.2 m HF), and from 0.8 to 11 wt% in HCl-bearing fluids (0.5-4.4 m HCl). Two runs performed with the in-situ cracking method after 1 week of pre-equilibration demonstrate that the speed of hydrogen diffusion through the capsule wall relative to that of fluid inclusion formation is a critical factor in f_O2-dependent solubility studies. Graphical evaluation of the solubility data suggests that Sn may have been dissolved as Sn(OH)Cl in the NaCl-bearing fluids, as Sn(OH)Cl and SnCl₂ in the HCl-bearing fluids, and as SnF₂ in the HF-bearing fluids. Experiments with NaF-bearing fluids produced an additional melt phase with an approximate composition of 53 wt% SiO₂, 25 wt% H₂O, 14 wt% NaF and 8 wt% SnO, which caused the composition of the coexisting fluid to be buffered at 0.5 wt% NaF and 150 ppm Sn. Fluorine-rich, peralkaline melts may therefore serve as important transport media for Sn in the final crystallization stages of tin granites. Based on the available cassiterite-solubility data in fluids and melts, in natural granite systems is estimated to be in the order of 0.1-4 (depending on their aluminosity), suggesting that Sn is not easily mobilized by magmatic-hydrothermal fluids. This interpretation is in accordance with the high degrees of Sn-enrichment commonly observed in highly fractionated melt inclusions. is primarily controlled by the HCl concentration in the fluid, which in turn is a function of the aluminum saturation index of the magma. Compared to HCl, the effect of fluorine on is subordinate.     

The system NaAlSi3O8−H2O−H2 (1200° C, 2 kbar): the solubility and interaction mechanism of fluid species with melt

The H₂O and H₂ solubilities in an albite melt at 1200° C and 2 kbar over the entire range of gas phase composition, from pure hydrogen to pure water were studied in gas-media pressure vessels. The water solubility initially increases with increasing hydrogen content until a maximum of 9.19 wt% H₂O atXH ₂ ^v =0.1 is reached, withXH ₂ ^v >0.1 the water solubility decreases. The hydrogen solubility curve has a maximum atXH ₂ ^v =0.42 where the concentration reaches 0.206 wt% H₂O. Over the entire compositional range¹H NMR (nuclear magnetic resonance) spectra show distinct absorption lines due to protons bound to OH groups and to isolated firmly bound water molecules. In NMR and Raman spectra there were no bands attributable to the H–H vibrations of molecular hydrogen. The X-ray photo-electronic spectra of hydrogen-bearing glasses show the Si2p (99 eV) band which corresponds to the zero-valency silicon. The formation of OH groups and molecular water during interaction between hydrogen-bearing fluids and melts under reducing conditions has a qualitative effect, the same as for water dissolution. Another point of interest is that hydrogen-bearing melts undergo more depolymerization than do hydrous melts.     

The probable occurrence of interstitial Al in hydrous,F-bearing and F-free aluminosilicate melts

Experiments carried out on the system SiO₂-NaAlSi₃O₈-KAlSi₃O₈(Qz-Ab-Or) at 1 kbar in the presence of H₂O and F show that the quartz-alkali feldspar field boundary is progressively displaced towards the feldspar join as F contents increase from 0 to 4 wt. %F. Increasing , in the absence of F, has already been shown to have a similar effect (Tuttle and Bowen 1958; Luth, Jahns, and Tuttle 1964). The increased size of the quartz field in the F-bearing system compared to the hydrous system is believed to be caused by progressive removal of Al from the tetrahedral network of the melt by complexing with F. The residual network in the melt is thus enriched in Si and this stabilizes precipitation of quartz rather than feldspar for certain bulk compositions. The common presence of quench cryolite (Na₃AlF₆) in certain experiments carried out with 4 wt.% F supports this interpretation and indicates that some Al in the melt may be present in six-fold coordination with F^–. The effect of H₂O in the absence of F may be similar, with Al being progressively removed from four-fold coordination as more H₂O is dissolved in the melt. Although a proportion of Al in hydrous melts may occur in six-fold coordination, dry melts predominantly contain Al in four-fold coordination. This major difference in Al complexing may be one of the main causes for differences in the high-pressure phase relations of wet and dry Albearing silicate systems.     

Interaction of granitic and basic magmas: experimental observations on contamination processes at 10 kbar with H2O

Experiments designed to simulate the interaction of juxtaposed rhyolitic and basic magmas were conducted at 10 kbar with H₂O, using reaction-couples consisting of Westerly granite (WG) against basalt (DW-1) and WG against a synthetic mafic glass (SMG, enriched in MgO and Na₂O relative to DW-1). Each couple was run with 5 and 10 wt% H₂O corresponding respectively to H₂O-undersaturated and H₂O-oversaturated conditions. Experiments were run for 42–44 h at 920° C, above the liquidus of WG and within the melting intervals of DW-1 and SMG. WG was run above the basic material in all but one experiment. The composition of the granitic melt was altered only through material exchange with the adjacent basic melts, whereas that of the basic melts also changed (relative to the bulk basic composition) due to partial crystallization. Some crystallization also occurred within the zone of interaction. For control, the basic compositions were also run alone under the same conditions as the reaction-couple experiments. The crystalline phase assemblages in the basic ends of the coupled experiments differed from those produced from the basic materials alone, demonstrating interaction with the granite melt. Moreover, compositional gradients within the basic ends of coupled experiments are indicated by changes in phase assemblage and compositions with distance from the interface with WG. Microprobe analyses of glass collected along the length of the capsules confirm published observations that alkali diffusion is very fast: K₂O and Na₂O homogenized throughout the capsules in less than the two-day run times. This, coupled with the fact that introduction of K₂O into SMG stabilized biotite, produced the result that after interaction the bulk basic material (melt+crystals) contained more K₂O than the coexisting felsic melt. Only very gentle gradients for CaO, FeO, and MgO are preserved in our experiments, in contrast with published anhydrous results, suggesting that the difference in activity coefficients for these components between basic and felsic melts is reduced by the introduction of H₂O. Gradients for SiO₂ and Al₂O₃ are of comparable length to those of the divalent cations, confirming earlier results that the diffusivities of the network-formers limit the rate of diffusion of Ca, Fe, and Mg.     

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 %.     

Melt-vapor solubilities and elemental partitioning in peraluminous granite-pegmatite systems: experimental results with Macusani glass at 200 MPa

Vapor-saturated experiments at 200 MPa with peraluminous, lithophile-element-rich rhyolite obsidian from Macusani, Peru, reveal high miscibility of H₂O and silicate melt components. The H₂O content of melt at saturation (11.5+-0.5 wt.%) is almost twice that predicted by existing melt speciation models. The corresponding solubility of melt components in vapor decreases from 15 wt.% dissolved solids (750°–775° C) to 9 wt.% at 600° C. With regard to major and most minor components, macusanite melt dissolves congruently in vapor. Among the elements studied (B, P, F, Li, Rb, Cs, Be, Sr, Ba, Nb, Zr, Hf, Y, Pb, Th, U, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Tm), only boron has a vapor/melt partition coefficient (D[B]) consistently 1 at superliquidus temperatures (>645° C). Phosphorus and fluorine behave similarly, with D[P] and D[F]<0.5. Little or no significant vapor/melt fractionation is evident among most periodic groups (alkalis, alkaline earths, Zr/Hf, or the REE). The temperature dependence of vapor/melt partition coefficients is generally greatest for cations with charge +3 (except Nb and U); most vapor/melt partition coefficients for trace elements increase with decreasing temperature to the liquidus. Crystallization proceeds by condensation of crystalline phases from vapor; most coexisting melts are aphyric. Changes in the major element content of melt are dominated by the mineral assemblage crystallized from vapor, which includes subequal proportions of white mica, quartz, albite, and orthoclase. The volumetric proportion of (mica + or-thoclase)/albite increases slightly with decreasing T, creating a sodic, alkaline vapor. Vapor deposition of topaz (T500° C), which consumes F from melt, returns K/Na ratios of melt to near unity with the vapor-deposition of albite. The abundances of most trace elements in residual melt change little with the crystallization of major phases, but in some cases are strongly controlled by the deposition of accessory phases including apatite (T550° C), which depletes the melt in P and REE. Below the liquidus, boron increasingly favors the vapor over melt with decreasing temperatures.     

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.     

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).     

Anatexis and fluid regime of the deep continental crust: New clues from melt and fluid inclusions in metapelitic migmatites from Ivrea Zone (NW Italy)

We investigate the inclusions hosted in peritectic garnet from metapelitic migmatites of the Kinzigite Formation (Ivrea Zone, NW Italy) to evaluate the starting composition of the anatectic melt and fluid regime during anatexis throughout the upper amphibolite facies, transition, and granulite facies zones. Inclusions have negative crystal shapes, sizes from 2 to 10 μm and are regularly distributed in the core of the garnet. Microstructural and micro‐Raman investigations indicate the presence of two types of inclusions: crystallized silicate melt inclusions (i.e., nanogranitoids, NI), and fluid inclusions (FI). Microstructural evidence suggests that FI and NI coexist in the same cluster and are primary (i.e., were trapped simultaneously during garnet growth). FI have similar compositions in the three zones and comprise variable proportions of CO₂, CH₄, and N₂, commonly with siderite, pyrophyllite, and kaolinite, suggesting a COHN composition of the trapped fluid. The mineral assemblage in the NI contains K‐feldspar, plagioclase, quartz, biotite, muscovite, chlorite, graphite and, rarely, calcite. Polymorphs such as kumdykolite, cristobalite, tridymite, and less commonly kokchetavite, were also found. Rehomogenized NI from the different zones show that all the melts are leucogranitic but have slightly different compositions. In samples from the upper amphibolite facies, melts are less mafic (FeO + MgO = 2.0–3.4 wt%), contain 860–1700 ppm CO₂ and reach the highest H₂O contents (6.5–10 wt%). In the transition zone melts have intermediate H₂O (4.8–8.5 wt%), CO₂ (457–1534 ppm) and maficity (FeO + MgO = 2.3–3.9 wt%). In contrast, melts at granulite facies reach highest CaO, FeO + MgO (3.2–4.7 wt%), and CO₂ (up to 2,400 ppm), with H₂O contents comparable (5.4–8.3 wt%) to the other two zones. Our results represent the first clear evidence for carbonic fluid‐present melting in the Ivrea Zone. Anatexis of metapelites occurred through muscovite and biotite breakdown melting in the presence of a COH fluid, in a situation of fluid–melt immiscibility. The fluid is assumed to have been internally derived, produced initially by devolatilization of hydrous silicates in the graphitic protolith, then as a result of oxidation of carbon by consumption of Fe³⁺‐bearing biotite during melting. Variations in the compositions of the melts are interpreted to result from higher T of melting. The H₂O contents of the melts throughout the three zones are higher than usually assumed for initial H₂O contents of anatectic melts. The CO₂ contents are highest at granulite facies, and show that carbon‐contents of crustal magmas are not negligible at high T. The activity of H₂O of the fluid dissolved in granitic melts decreases with increasing metamorphic grade. Carbonic fluid‐present melting of the deep continental crust represents, together with hydrate‐breakdown melting reactions, an important process in the origin of crustal anatectic granitoids.