Water and magmas; a mixing model

A model for the mixing of H₂O and silicate melts has been derived from the experimentally determined effects of H₂O on the viscosity (fluidity), volumes, electrical conductivities, and especially the thermodynamic properties of hydrous aluminosilicate melts. It involves primarily the reaction of H₂O with those O^?2 ions of the melt that are shared (bridging) between adjacent (Al, Si)O₄ tetrahedra to produce OH^? ions. However, in those melts that contain trivalent ions in tetrahedral coordination, such as the Al³⁺ ion in feldspathic melts, the model further involves exchange of a proton from H₂O with a non-tetrahedrally coordinated cation that must be present to balance the net charge on the AlO₄ group. This cation exchange reaction, which goes essentially to completion, results in dissociation of the H₂O and is limited only by the availability of H₂O and the number of exchangeable cations per mole of aluminosilicate.In the system NaAlSi₃O₈-H₂O, upon which this thermodynamic model is based, there is 1 mole of exchangeable cations (Na⁺) per mole (GFW) of NaAlSi₃O₈, consequently ion exchange occurs for H₂O contents up to a 1:1 mole ratio (X^m_w = mole fraction H₂O = 0.5). For mole fractions of H₂O greater than 0.5, no further exchange can occur and the reaction with additional bridging oxygens of the melt produces 2 moles of associated OH^? ions per mole of H₂O dissolved. These reactions lead to a linear dependence of the thermodynamic activity of H₂O (a^m_w) on the square of its mole fraction (X^m_w) for values of X^m_w, up to 0.5 and an exponential dependence on X^m_w at higher H₂O contents. Thus, for values of $\text{X}{}^{\mathrm{m}}{}_{\mathrm{w}}\text{? 0.5, a}{}^{\mathrm{m}}{}_{\mathrm{w}}\text{= k(X}{}^{\mathrm{m}}{}_{\mathrm{w}}\text{)}{}^2$, where k is a Henry's law constant for the dissociated solute.Extension of the thermodynamic model for NaAlSi₃O₈-H₂O to predict H₂O solubilities and other behavior of compositionally more complex aluminosilicate melts (magmas) requires placing these melts on an equimolal basis with NaAlSi₃O₈. This is readily accomplished using chemical analyses of quenched glasses by normalizing to the stoichiometric requirements of NaAlSi₃O₈, first in terms of equal numbers of exchangeable cations for mole fractions of H₂O up to 0.5 and secondly in terms of 8 moles of oxygen for higher H₂O contents. Chemical analyses of three igneous-rock glasses, ranging in composition from tholeiitic basalt to lithium-rich pegmatite, were thus recast and the experimental H₂O solubilities were computed on this equimolal basis. The resulting equimolal solubilities are all the same, within experimental error, as the solubility of H₂O in NaAlSi₃O₈ melt calculated from the thermodynamic relations.The equivalence of equimolal solubilities implies that the Henry's law constant (k), which is a function of temperature and pressure, is independent of aluminosilicate composition over a wide range. Moreover, as a consequence of the Gibbs-Duhem relation and the properties of exact differentials, it is clear that the silicate components of the melt, properly defined, mix ideally. Thus, a relatively simple mixing model for H₂O in silicate melts has led to a quantitative thermodynamic model for magmas that has far-reaching consequences in igneous petrogenesis.     

Melting of albite and dehydration of brucite in H2O–NaCl fluids to 9 kbars and 700–900°C: implications for partial melting and water activities during high pressure metamorphism

 The melting reaction: albite_(solid)+ H₂O_(fluid) =albite-H₂O_(melt) has been determined in the presence of H₂O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H₂O–CO₂ fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X _(H2O) ) melting relations in the two systems are almost coincident. By contrast, H₂O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 ^°C higher in H₂O–NaCl brines than in H₂O–CO₂ fluids for X _(H2O) =0.8 and ∼100 ^°C higher for X _(H2O) =0.5. The melting temperature of albite in H₂O–NaCl fluids of X _(H2O)=0.8 is ∼100 ^°C higher than in pure water. The P–T curves for albite melting at constant H₂O–NaCl show a temperature minimum at about 5 kbar. Water activities in H₂O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H₂O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a _(H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996     

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.     

Calculation of partial melting equilibria in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O (NCKFMASH)

A thermodynamic model for haplogranitic melts in the system Na₂O–CaO–K₂O–Al₂O₃–SiO₂–H₂O (NCKASH) is extended by the addition of FeO and MgO, with the data for the additional end‐members of the liquid incorporated in the Holland & Powell (1998) internally consistent thermodynamic dataset. The resulting dataset, with the software thermocalc , is then used to calculate melting relationships for metapelitic rock compositions. The main forms for this are P–T and T–X pseudosections calculated for particular rock compositions and composition ranges. The relationships in these full‐system pseudosections are controlled by the low‐variance equilibria in subsystems of NCKFMASH. In particular, the solidus relationships are controlled by the solidus relationships in NKASH, and the ferromagnesian mineral relationships are controlled by those in KFMASH. However, calculations in NCKFMASH allow the relationships between the common metapelitic minerals and silicate melt to be determined. In particular, the production of silicate melt and melt loss from such rocks allow observations to be made about the processes involved in producing granulite facies rocks, particularly relating to open‐system behaviour of rocks under high‐grade conditions.     

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.     

Melting of plagioclase-quartz assemblages at 2 kbar water pressure

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

Water solubility in haplogranitic melts coexisting with H2O-H2 fluids

The water solubility in haplogranitic melts (normative composition Ab₃₉Or₃₂Qz₂₉) coexisting with H₂O-H₂ fluids at 800 and 950 °C and 1, 2 and 3 kbar vapour pressure has been determined using IR spectroscopy. The experiments were performed in internally heated pressure vessels and the hydrogen fugacity (f _H2) was controlled using the double capsule technique and oxygen buffer assemblages (WM and IW). Due to the limited lifetimes of these oxygen buffers the water solubility was determined from diffusion profiles (concentration-distance profiles) measured with IR spectroscopy in the quenched glasses. The reliability of the experimental strategy was demonstrated by comparing the results of short- and long-duration experiments performed with pure H₂O fluids. The water solubility in Ab₃₉Or₃₂Qz₂₉ melts equilibrated with H₂O-H₂ fluids decreases progressively with decreasing f _H2O, as f _H2 (or X _H2) increases in the fluid phase. The effect of H₂ on the evolution of the water solubility is similar to that of CO₂ or another volatile with a low solubility in the melt and can be calculated in a first approximation with the Burnham water solubility model. Recalculation of high temperature water speciation for AOQ melts coexisting with H₂O-H₂ fluids at 800 °C, 2 kbar suggests that the concentrations of molecular H₂O are proportional to f _H2O (calculated using available mixing models), indicating Henrian behaviour for the solubility of molecular H₂O in haplogranitic melts. Received: 29 June 1998 / Accepted: 10 March 1999     

Melting, carbonic fluids and water recycling in the deep crust: an example from the Limpopo Belt, South Africa

The proposed retrograde orthoamphibole isograd in the Southern Marginal Zone of the Limpopo Belt separates hydrated, amphibolite grade metapelites from their granulite grade precursors and provides an intriguing geological dilemma. Widespread rehydration of metapelitic granulites under conditions of 660–600 °C and ≥0.6 GPa, and CO₂-dominated fluid-inclusion populations appear to suggest thorough flushing of the high-grade crust with an externally derived carbonic fluid. However, past studies of the carbon and oxygen isotope geochemistry of the hydrated rocks have not demonstrated the involvement of any voluminous out of equilibrium’ fluid in the evolution of the rocks. This contribution proposes a model wherein the hydrating fluids are derived from crystallizing anatectic leucosomes, generated by in situ fluid-absent biotite melting along the prograde path. Model equilibrium fluid compositions suggest that reaction between this melt-derived H₂O and biogenic graphite produced CO₂-rich fluid compositions and potentially high fluid:rock ratios at the wet granite solidus. Declining temperature resulted in fluid compositions shifting to higher XH₂O, with the precipitation of graphite essentially at the sites of initial fluid generation, thereby preserving original (pre-metamorphic) isotopic heterogeneities. The hydration pattern of the Southern Marginal Zone appears to be a function of melt migration. In the hydrated zone, leucosomes generally approximate minimum melt compositions and in this zone H₂O was effectively recycled between the prograde and retrograde assemblages. In contrast, leucosomes in the granulite grade portion of the terrane have lost a K₂O- and H₂O-rich melt fraction, and although some hydration has occurred in this zone, orthopyroxene is generally preserved in metapelites. In a general context, in situ crystallization of graphitic partially melted source rocks has the potential to produce high fluid-rock ratios at temperatures close to the wet granite solidus. This single process holds the potential for widespread retrogression of formerly high-grade assemblages, at a variety of aH₂O values, without external fluid input.     

Electrical conductivity of hydrous basaltic melts: implications for partial melting in the upper mantle

The Earth’s uppermost asthenosphere is generally associated with low seismic wave velocity and high electrical conductivity. The electrical conductivity anomalies observed from magnetotelluric studies have been attributed to the hydration of mantle minerals, traces of carbonatite melt, or silicate melts. We report the electrical conductivity of both H₂O-bearing (0–6 wt% H₂O) and CO₂-bearing (0.5 wt% CO₂) basaltic melts at 2 GPa and 1,473–1,923 K measured using impedance spectroscopy in a piston-cylinder apparatus. CO₂ hardly affects conductivity at such a concentration level. The effect of water on the conductivity of basaltic melt is markedly larger than inferred from previous measurements on silicate melts of different composition. The conductivity of basaltic melts with more than 6 wt% of water approaches the values for carbonatites. Our data are reproduced within a factor of 1.1 by the equation log σ = 2.172 − (860.82 − 204.46 w ^0.5)/(T − 1146.8), where σ is the electrical conductivity in S/m, T is the temperature in K, and w is the H₂O content in wt%. We show that in a mantle with 125 ppm water and for a bulk water partition coefficient of 0.006 between minerals and melt, 2 vol% of melt will account for the observed electrical conductivity in the seismic low-velocity zone. However, for plausible higher water contents, stronger water partitioning into the melt or melt segregation in tube-like structures, even less than 1 vol% of hydrous melt, may be sufficient to produce the observed conductivity. We also show that ~1 vol% of hydrous melts are likely to be stable in the low-velocity zone, if the uncertainties in mantle water contents, in water partition coefficients, and in the effect of water on the melting point of peridotite are properly considered.     

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

The solubility behavior of H₂O in melts in the system Na₂O-SiO₂-H₂O was determined by locating the univariant phase boundary, melt = melt + vapor in the 0.8-2 GPa and 1000°-1300°C pressure and temperature range, respectively. The NBO/Si-range of the melts (0.25-1) was chosen to cover that of most natural magmatic liquids. The H₂O solubility in melts in the system Na₂O-SiO₂-H₂O (X_H2O) ranges between 18 and 45 mol% (O = 1) with (∂X_H2O/∂P)_T∼14-18 mol% H₂O/GPa. The (∂X_H2O/∂P)_T is negatively correlated with NBO/Si (= Na/Si) of the melt. The (∂X_H2O/∂T)_P is in the −0.03 to +0.05 mol% H₂O/°C range, and is negatively correlated with NBO/Si. The [∂X_H2O/∂(NBO/Si)]_P,T is in the −3 to −8 mol% H₂O/(NBO/Si) range. Melts with NBO/Si similar to basaltic liquids (∼0.6-∼1.0) show (∂X_H2O/∂T)_P<0, whereas more polymerized melts exhibit (∂X_H2O/∂T)_P>0. Complete miscibility between hydrous melt and aqueous fluid occurs in the 0.8-2 GPa pressure range for melts with NBO/Si ≤0.5 at T >1100°C. Miscibility occurs at lower pressure the more polymerized the melt.     

The Distribution of H2O between Cordierite and Granitic Melt: H2O Incorporation in Cordierite and its Application to High-grade Metamorphism and Crustal Anatexis

Experiments defining the distribution of H₂O [D_w = wt % H₂O(melt)/wt% H₂O(crd)]) between granitic melt and coexisting cordieriteover a range of melt H₂O contents from saturated (i.e. coexistingcordierite + melt + vapour) to highly undersaturated (cordierite+ melt) have been conducted at 3–7 kbar and 800–1000°C.H₂O contents in cordierites and granitic melts were determinedusing secondary ion mass spectrometry (SIMS). For H₂O vapour-saturatedconditions D_w ranges from 4·3 to 7 and increases withrising temperature. When the system is volatile undersaturatedD_w decreases to minimum values of 2·6–5·0at moderate to low cordierite H₂O contents (0·6–1·1wt %). At very low aH₂O, cordierite contains less than 0·2–0·3wt % H₂O and D_w increases sharply. The D_w results are consistentwith melt H₂O solubility models in which aH₂O is proportionalto X_w² (where X_w is the mole fraction of H₂O in eight-oxygenunit melt) at X_w     

Lamprophyres of the Tomtor Massif: A result of mixing between potassic and sodic alkaline mafic magmas

The paper presents data on inclusions in minerals of the least modified potassic lamprophyres in a series of strongly carbonatized potassic alkaline ultramafic porphyritic rocks. The rocks consist of diopside, kaersutite, analcime, apatite, and rare phlogopite and titanite phenocrysts and a groundmass, which is made up, along with these minerals, of potassic feldspar and calcite. The diopside and kaersutite phenocrysts display unsystematic multiple zoning. Chemically and mineralogically, the rock is ultramafic foidite and most likely corresponds to monchiquite. Primary and secondary melt inclusions were found in diopside, kaersutite, apatite, and titanite phenocrysts and are classified into three types: sodic silicate inclusions with analcime, potassic silicate inclusions with potassic feldspar, and carbonate inclusions, which are dominated by calcite. Heating and homogenization of the inclusions show that the potassic lamprophyres crystallized from a heterogeneous magma, with consisted of mixing mafic sodic and potassic alkaline magmas enriched in a carbonatite component. The composition of the magmas was close to nepheline and leucite melanephelinite. The minerals crystallized at 1150–1090°C from the sodic melts and at 1200–1250°C from the potassic ones. The sodic mafic melts were richer in Fe than the potassic ones, were the richest in Al, Mn, SO₃, Cl, and H₂O and poorer in Ti and P. The potassic mafic melts were not lamproitic, as follows from the presence of albite in the crystallized primary potassic melt inclusions. The diopside, the first mineral to crystallize in the rock, started to crystallize in the magmatic chamber from sodic mafic melt and ended to crystallize from mixed sodic–potassic melts. The potassic mafic melts were multiply replenished in the chamber in relation to tectonic motions. The ascent of the melts to the surface and rapidly varying P–T parameters of the magma were favorable for multiple separations of carbonatite melts from the alkaline mafic ones and their mixing and mingling.     

Solubility of water-hydrogen fluid in haplogranite,albite, and sodium disilicate melts

Solubility curves of water-hydrogen fluid were studied using a high-pressure gas apparatus at a pressure of 200 MPa under variable fluid composition in haplogranite (Ab ₃₉ Or ₃₂ Qtz ₂₉, 950°C), Na-disilicate (Na₂Si₂O₅, 950°C), and albite melts (1200°C). The mole fraction of hydrogen in experiments was controlled directly by Ar-H₂ mixtures using a specially designed cell with a Shaw membrane. $ X_{H_2 }^{Ar - H_2 } $ X_{H_2 }^{Ar - H_2 } ranged from 0 to 1. In some experiments with haplogranite and Na-disilicate melts under oxidizing conditions, in order to increase the accuracy of experimental parameters, the fugacities of oxygen and hydrogen were controlled using the double-capsule technique and the solid-phase buffer mixtures Ni-NiO (NNO) and Co-CoO (CCO). The addition of H₂ to the H₂O-saturated systems ($ X_{H_2 }^{H_2 O - H_2 } $ X_{H_2 }^{H_2 O - H_2 } ≥ 0.012) results in the appearance of a distinct maximum on the solubility curves at $ X_{H_2 }^{H_2 O - H_2 } $ X_{H_2 }^{H_2 O - H_2 } = 0.05–0.07 (H₂ mole fractions were calculated for real H₂O-H₂ mixtures of real gases), and the maximum content of H₂O-H₂ fluid increases relative to the H₂O-saturated melts by 1.51 wt % for haplogranite melt at $ X_{H_2 } $ X_{H_2 } = 0.063, 2.68 wt % for albite melt at $ X_{H_2 } $ X_{H_2 } = 0.066, and 3.54 wt % for Na-disilicate melt at $ X_{H_2 } $ X_{H_2 } = 0.067. A further increase in H₂ content in the gas mixture decreases the solubility of H₂O-H₂ fluid in the melts, and under pure H₂ pressure, the contents of fluid components are 0.08 wt % in haplogranite melt and 0.06 wt % in albite melt. The ¹H NMR study of aluminosilicate and Na-silicate glasses obtained under the pressure of H₂O and H₂O-H₂ fluids suggests different mechanisms of the dissolution of H₂O and H₂O-H₂ fluids in magmatic melts. In addition to the spectra of dissolved water fluid, the spectra of quenched glasses synthesized under H₂O-H₂ fluid pressure exhibited a narrow line of molecular hydrogen with a width at half height of 1.8–2.0 kHz at $ X_{H_2 } $ X_{H_2 } ≥ 0.653 for albite and $ X_{H_2 } $ X_{H_2 } ≥ 0.063 for Na-disilicate and two lines at $ X_{H_2 } $ X_{H_2 } ≥ 0.063 for the haplogranite composition.     

Transport of low-aH2O dehydration products to melt sites via reaction-zone networks, Milford Sound, New Zealand

The gabbroic/dioritic Pembroke Hornblende Granulite (PHG) of Milford Sound displays a geometrically simple mesoscopic network of sub‐planar garnet reaction zones (GRZ) in which the meta‐igneous hornblende granulite has been depleted of Na, Si, and H₂O, and c. 25 vol.% almandine‐rich garnet has formed. Some studies postulate the initial presence of melt along the centres of all GRZ, explaining the frequent absence of feldspathic veins by selective melt loss. A more parsimonious model is necessitated by structural evidence and, together with chemical data, suggests a relationship between mid‐range metasomatic transport and anatexis. The Pembroke outcrops show a process of incipient melting of gabbro/diorite in an environment of relatively low aH₂O in lithologies that have limited free quartz. A non‐equilibrium steady state is proposed, in which a sodic dehydration fluid moves some distance via the GRZ network towards areas of partial melting. Only in these areas are Na and Si reconstituted as albite, with more garnet as byproduct, having avoided the need for melt percolation. The combined structural and chemical evidence directs a focus on mass transport in low‐aH₂O gabbroic environments. In subsequent events of shearing and complete transposition, both sets of garnet – the atypical GRZ residue and partial melt melanosomes – were inherited by the Milford Gneiss ‘facies’ of the PHG.     

Paragonite stability at 700°C in the presence of H<Subscript>2</Subscript>O–NaCl fluids: constraints on H<Subscript>2</Subscript>O activity and implications for high pressure metamorphism

We carried out reversed piston-cylinder experiments on the equilibrium paragonite = jadeite + kyanite + H₂O at 700°C, 1.5–2.5 GPa, in the presence of H₂O-NaCl fluids. Synthetic paragonite and jadeite and natural kyanite were used as starting materials. The experiments were performed on four different nominal starting compositions: X(H₂O)=1.0, 0.90, 0.75 and 0.62. Reaction direction and extent were determined from the weight change in H₂O in the capsule, as well as by optical and scanning electron microscopy (SEM). At X(H₂O)=1.0, the equilibrium lies between 2.25 and 2.30 GPa, in good agreement with the 2.30–2.45 GPa reversal of Holland (Contrib Miner Petrol 68:293–301, 1979). Lowering X(H₂O) decreases the pressure of paragonite breakdown to 2.10–2.20 GPa at X(H₂O)=0.90 and 1.85–1.90 GPa at X(H₂O)=0.75. The experiments at X(H₂O) = 0.62 yielded the assemblage albite + corundum at 1.60 GPa, and jadeite + kyanite at 1.70 GPa. This constrains the position of the isothermal paragonite–jadeite–kyanite–albite–corundum–H₂O invariant point in the system Na₂O–Al₂O₃–SiO₂–H₂O to be at 1.6–1.7 GPa and X(H₂O)~0.65±0.05. The data indicate that H₂O activity, a(H₂O), is 0.75–0.86, 0.55–0.58, and <0.42 at X(H₂O)=0.90, 0.75, and 0.62, respectively. These values approach X(H₂O)², and agree well with the a(H₂O) model of Aranovich and Newton (Contrib Miner Petrol 125:200–212, 1996). Our results demonstrate that the presence or absence of paragonite can be used to place limits on a(H₂O) in high-pressure metamorphic environments. For example, nearly pure jadeite and kyanite from a metapelite from the Sesia Lanzo Zone formed during the Eo-Alpine metamorphic event at 1.7–2.0 GPa, 550–650°C. The absence of paragonite requires a fluid with low a(H₂O) of 0.3–0.6, which could be due to the presence of saline brines.     

Phase relations to 3 kbar in the systems edenite + H2O and edenite + excess quartz + H2O

Amphiboles approached edenite (NaCa₂Mg₅Si₇AlO₂₂(OH)₂), richterite (Na₂CaMg₅Si₈O₂₂(OH)₂), tremolite (□Ca₂Mg₅Si₈O₂₂(OH)₂) solid solutions were studied by conventional hydrothermal techniques employing the bulk compositions edenite, and edenite + additional quartz, all with excess H₂O. For the stoichiometric edenite bulk composition + excess H₂O, the equilibrium phase assemblage is diopside + Na-phlogopite + forsterite + fluid at, and just above the amphibole high-temperature limit at 850 ± 5°C, 500 bar, and 880 ± 5°C, 1000 bar. The breakdown temperature of sodic phlogopite is 855 ± 3°C at 500 bar, and 890 ± 5°C at 700 bar, producing nepheline + plagioclase (or melt), additional forsterite and fluid. Diopside and Na-phlogopite solid solution coexist over a broad P_fluid-T region, even within the amphibole field, where they are associated with an edenite-richterite (-tremolite) solid solution of approximate composition Ed₃₅Rc₅₀Tr₁₅.In the system edenite + 4 quartz + excess H₂O, nearly pure tremolite and albite coexist stably between 670° and 830°C at 1000 bar and give way to the possibly metastable assemblage diopside + talc + albite below 670°C. In the presence of albite, tremolite reacts to produce diopside + quartz + enstatite + fluid above 830°C at 1000 bar. For the investigated silica-rich bulk composition, amphibole P_fluid-T stability is divided by the albite melting curve into a tremolite + albite field, and a tremolite + aqueous melt field. Substantial equilibrium solid solution of tremolite towards edenite or richterite was not observed for silica-excess bulk compositions. Metastable edenite-rich amphiboles initially synthesized change to tremolite with increasing run length in the presence of free SiO₂.Edenitic amphibole is stable only over a very limited temperature range in silica-undersaturated environments, thus accounting for its rarity in nature. Na-phlogopite solid solutions are also disfavored by high a_SiO2; even for nepheline-normative lithologies, a hypothesized rapid low-temperature conversion to vermiculite or smectite could partly explain the scarcity of sodic phlogopite in rocks.     

Water solubility in a calcium aluminosilicate melt

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

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.     

Interaction of model peridotite with H2O-KCl fluid: Experiment at 1.9 GPa and its implications for upper mantle metasomatism

Mineralogical and geochemical data suggest that chloride components play an important role in the transformation and partial melting of upper mantle peridotites. The effect of KCl on the transformation of hydrous peridotite rich in Al₂O₃, CaO, and Na₂O was examined in experiments aimed at studying interaction between model NCMAS peridotite with H₂O-KCl fluid under a pressure of 1.9 GPa, temperatures of 900–1200°C, and various initial H₂O/KCl ratios. The experimental results indicate that KCl depresses the solidus temperature of the hydrous peridotite: this temperature is <900°C at 1.9 GPa, which is more than 100°C lower than the solidus temperature (1000–1025°C) of hydrous peridotite in equilibrium with KCl-free fluid. The reason for the decrease in the melting temperature is that the interaction of KCl with silicates prevails over the effect of chloride on the water activity in the fluid. Experimental data highlight the key role of Al₂O₃ as a component controlling the whole interaction process between peridotite and H₂O-KCl fluid. Garnet, spinel, and pargasite-edenite amphibole in association with aluminous orthopyroxene are unstable in the presence of H₂O-KCl fluid at a chloride concentration in the fluid as low as approximately 2 wt % and are replaced by Cl-bearing phlogopite (0.4–1.1 wt % Cl). Interaction with H₂O-KCl fluid does not, however, affect clinopyroxene and forsterite, which are the Al poorest phases of the system. Chlorine stabilizes phlogopite at relatively high temperatures in equilibrium with melt at temperatures much higher than the solidus (>1200°C). The compositional evolution of melt generated during the melting of model peridotite in the presence of H₂O-KCl fluid is controlled, on the one hand, by the solubility of the H₂O-KCl fluid in the melt and, on the other hand, by phlogopite stability above the solidus. At temperatures below 1050°C, at which phlogopite does not actively participate in melting reactions, fluid dissolution results in SiO₂-undersaturated (35–40 wt %) and MgO-enriched (up to 45 wt %) melts containing up to 4–5 wt % K₂O and 2–3 wt % Cl. At higher temperatures, active phlogopite dissolution and, perhaps, also the separation of immiscible aqueous chloride liquid give rise to melts containing >10 wt % K₂O and 0.3–0.5 wt % Cl. Our experimental results corroborate literature data on the transformation of upper mantle peridotites into phlogopite-bearing associations and the formation of ultrapotassic and highly magnesian melts.     

The beginnings of hydrous mantle wedge melting

This study presents new phase equilibrium data on primitive mantle peridotite (0.33 wt% Na₂O, 0.03 wt% K₂O) in the presence of excess H₂O (14.5 wt% H₂O) from 740 to 1,200°C at 3.2–6 GPa. Based on textural and chemical evidence, we find that the H₂O-saturated peridotite solidus remains isothermal between 800 and 820°C at 3–6 GPa. We identify both quenched solute from the H₂O-rich fluid phase and quenched silicate melt in supersolidus experiments. Chlorite is stable on and above the H₂O-saturated solidus from 2 to 3.6 GPa, and chlorite peridotite melting experiments (containing ~6 wt% chlorite) show that melting occurs at the chlorite-out boundary over this pressure range, which is within 20°C of the H₂O-saturated melting curve. Chlorite can therefore provide sufficient H₂O upon breakdown to trigger dehydration melting in the mantle wedge or perpetuate ongoing H₂O-saturated melting. Constraints from recent geodynamic models of hot subduction zones like Cascadia suggest that significantly more H₂O is fluxed from the subducting slab near 100 km depth than can be bound in a layer of chloritized peridotite ~ 1 km thick at the base of the mantle wedge. Therefore, the dehydration of serpentinized mantle in the subducted lithosphere supplies free H₂O to trigger melting at the H₂O-saturated solidus in the lowermost mantle wedge. Alternatively, in cool subduction zones like the Northern Marianas, a layer of chloritized peridotite up to 1.5 km thick could contain all the H₂O fluxed from the slab every million years near 100 km depth, which suggests that the dominant form of melting below arcs in cool subduction zones is chlorite dehydration melting. Slab P–T paths from recent geodynamic models also allow for melts of subducted sediment, oceanic crust, and/or sediment diapirs to interact with hydrous mantle melts within the mantle wedge at intermediate to hot subduction zones.