John R. Holloway

Experimental determination of the fluid-absent melting relations in the pelitic system

In order to provide additional constraints on models for partial melting of common metasediments, we have studied experimentally the melting of a natural metapelite under fluid-absent conditions. The starting composition contains quartz, plagioclase, biotite, muscovite, garnet, staurolite, and kyanite. Experiments were done in a halfinch piston-cylinder apparatus at 7, 10, and 12 kbar and at temperatures ranging from 750° to 1250° C. The following reactions account for the mineralogical changes observed at 10 kbar between 750° and 1250° C: Bi+Als+Pl+Q=L+Gt+(Kf), Ky=Sill, Gt+Als=Sp+Q, Gt=L+Sp+Q, and Sp+Q=L+Als.The compositions of the phases (at T>875° C) were determined using an energy-dispersive system on a scanning electron microscope. The relative proportions of melt and crystals were calculated by mass balance and by processing images from the SEM. These constraints, together with other available experimental data, are used to propose a series of P-T, T-XH2O, and liquidus diagrams which represent a model for the fluid-present and fluid-absent melting of metapelites in the range 2–20 kbar and 600°–1250° C.We demonstrate that, even under fluid-absent conditions, a large proportion (≈40%) of S-type granitic liquid is produced within a narrow temperature range (850°–875° C), as a result of the reaction Bi+Als+Pl+Q=L+Gt(+/-Kf). Such liquids, or at least some proportion of them, are likely to segregate from the source, leaving behind a residue composed of quartz, garnet, sillimanite, plagioclase, representing a characteristic assemblage of aluminous granulites.The production of a large amount of melt at around 850° C also has the important effect of buffering the temperature of metamorphism. In a restitic, recycled, lower crust undergoing further metamorphism, temperature may reach values close to 1000° C due to the absence of this buffering effect. Partial melting is the main process leading to intracontinental differentiation. We discuss the crustal cross-section exposed in the North Pyrenean Zone in the context of our experiments and modelling.

Trace element partitioning between amphibole, phlogopite, and basanite melt

Abstract We have measured amphibole-melt and phlogopite-melt partition coefficients ( D ) for 22 trace elements in experimentally crystallized natural basanites with the ion microprobe. The synthesized phases display an exceptional degree of homogeneity for both major and trace elements, as demonstrated by the ratio of the standard deviation to the mean counting statistics uncertainty of the measurements. In pargasitic hornblende, actinides are highly incompatible ( D = 0.001), LILE and HFSE are mildly incompatible ( D = 0.04 – 0.2and0.1 – 0.2, respectively), and REE partition coefficients vary from 0.05 to 0.6, with a maximum near Ho. Except for the LILE ( D = 0.1 – 3.7), phlogopite partition coefficients are generally lower, especially the REE ( D ≈ 0.01). The partitioning results are consistent with a model in which the variation in partition coefficient with ionic radius results from the crystal lattice strain induced by the size misfit of the substituting trace element. This result predicts a decrease in Youngs Modulus ( E ) with increasing size of the cation sites in the crystal lattice, and E derived for the largest site in both amphibole and phlogopite agree well with experimentally determined bulk mineral values. The ability to model partitioning with an elastic strain model provides an important link between trace element partitioning and the macroscopic properties of minerals. Relative to an anhydrous peridotite, partial melting of an amphibole or phlogopite bearing peridotite will result in no Th-U fractionation, slight LILE depletions, and, aside from Ti, no significant HFSE depletions. Thus, barring the addition of any slab components besides H 2 O, partial melting of hydrated peridotite is not a plausible explanation for any of the geochemical features commonly associated with subduction zone magmas.

Fugacity and Activity of Molecular Species in Supercritical Fluids

Naturally occuring gases (fluids) in metamorphic and igneous systems are known to be mixtures of several volatile species, usually including H2O, CO2, CO, H2, CH4, H2S or SO2 in major or minor amounts. The thermodynamic interpretation of experimental results, the thermodynamic calculation of reactions, and the accurate determination of fluid compositions recorded by natural assemblages require knowledge of the thermodynamic properties of the geologically important fluid-phase species. Because very few accurate experimental determinations of these properties exist, it seems necessary to rely on an equation of state which can be used to calculate thermodynamic properties of fluid mixtures in geologically important pressure-temperature regions.

Experimental determination of the solubility of carbon dioxide in molten basalt at low pressure

We report the first measurements of CO_2 solubility in molten basalt at pressures comparable to those at which submarine basalts erupt. A basalt from the Juan de Fuca ridge was equilibrated with CO_2-rich vapor at 1200°C, 100–1500 bar for up to four hours. After quenching, the glass was analyzed for dissolved carbonate ions by infrared spectroscopy. No forms of dissolved CO_2 other than carbonate were detected. CO_2 solubility is roughly a linear function of pressure at these low pressures. The experimentally determined solubility differs from previous estimates based on CO_2 concentrations of submarine glasses, on CO_2 solubilities in basaltic liquids at significantly higher pressures, and on CO_2 concentrations of glasses equilibrated with H_2O-CO_2 vapor. Our results are compatible with those obtained previously at higher pressures on a molten Kilauea tholeiite only if there is a significant positive dependence of carbonate solubility on temperature. CO_2 contents of mid-ocean ridge glasses measured by infrared spectroscopy are generally higher than would be expected based on solubilities at the hydrostatic pressures for the water depths from which the glasses were recovered, but the lowest dissolved CO_2 contents agree with the experimentally determined solubilities. We propose that submarine glasses with low CO_2 contents were quenched from magmas that were able to degas because they rose slowly from depth. The common occurrence of glasses with dissolved CO_2 contents in excess of the experimentally determined solubility suggests they were quenched from magmas that ascended too rapidly to degas fully. In conjunction with our solubility data, the highest CO_2 contents allow minimum estimates of depths to magma chambers. Depths of ⩾ 2.3 km beneath the ridge are indicated for the East Pacific Rise at 21°N, in agreement with geophysical constraints.

The pressure and temperature dependence of carbon dioxide solubility in tholeiitic basalt melts

The solubility of carbon dioxide in tholeiitic melt (1921 Kilauea basalt ) was determined under experimental conditions of 1 kbar, 1200°C; 10 and 15 kbar and 1300–1600°C. We examined the solubility at pressure and temperature conditions intermediate to those reported in previous studies, and, in particular, we addressed the effect of temperature on carbon dioxide solubility. Two different carbon sources were used in the experiments, silver oxalate and a mixture of carbonate minerals, to examine the effects of dissolved silver on carbon dioxide solubility. Three analytical methods were employed to measure accurately and precisely the dissolved carbon in the run products: ( 1 ) Fourier transform micro-infrared spectroscopy, ( 2 ) secondary ion mass spectrometry, and ( 3 ) bulk carbon analysis with a Perkin Elmer Elemental Analyzer. The first two methods are micro-beam techniques which allowed for assessment of sample homogeneity. Consistent with previous solubility studies, infrared analyses showed that carbon is dissolved in basaltic melt in the form of carbonate. However, our experimental results differ from the previous solubility study in that we demonstrate carbon dioxide solubility is temperature independent. At 1 kbar and 1200°C, carbon dioxide solubility is 543 ppm; at 10 kbar and 1300, 1400, and 1500°C, carbon dioxide solubility is approximately 0.77 ± .07 wt%; and at 15 kbar and 1400, 1450, 1500, 1550, and 1600°C, the solubility is approximately 1.21 ± .13 wt%. Dissolved silver does not appear to affect the solubility. These results invalidate previous models for carbon dioxide solubility. We have developed a new model which describes the pressure and temperature dependence of carbon dioxide solubility for tholeiitic basalts. Regression of the solubility data for the reaction CO2vapor + O2−melt = CO32−melt gives a heat of solution (ΔH0 at 1 kbar and 1473 K) of 5.20 ± 4.30 kJ/mol and the change in partial molar volume ΔV0[CO32−melt− O2−melt of 23.14 ± 1.03 cm3/mol. Application of this model suggests that fluid-saturated partial melting of the MORB source region cannot be supported.

Fluids in the evolution of granitic magmas: Consequences of finite CO2 solubility

The small but finite solubility of CO 2 in granitic magmas under crustal conditions, together with the common occurrence of CO 2 in likely magma source materials, suggests that granitic magmas will often be accompanied by a CO 2 -H 2 O fluid phase during their ascent in the crust. Polybaric and isobaric calculations have been made for model systems with varying total volatile content, initial CO 2 /H 2 O ratios, crystallization rates, and closed-system or open-system conditions. The calculations demonstrate that the presence of CO 2 in an evolving magma system can result in greatly differing values of H 2 O activity (and hence H 2 O content, phase equilibria, and physical properties of the magma). Specifically, if the mass ratio CO 2 /H 2 O is ≥0.4 and the initial mass ratio of total volatiles to silicate magma is ≥0.05, then, if little or no loss of the fluid phase occurs during magma evolution, the activity of H 2 O will remain nearly constant. This is in strong contrast to all other possible cases in which the activity of H 2 O increases rapidly with decreasing pressure and (or) anhydrous phase crystallization, invariably reaching a value of unity. It is also demonstrated that if CO 2 is present in a fluid phase in the magma source region, then there will be a fluid present throughout the evolutionary history of the magma. The presence of fluid bubbles in the magma should considerably alter many properties of the magma system such as heat transfer, mass transfer, and viscosity.

The stability and composition of phengitic muscovite and associated phases from 5.5 to 11 GPa: Implications for deeply subducted sediments

Abstract The stability and composition of phengitic muscovite was investigated from 5.5–11 GPa, 700–1150°C in synthesis experiments performed in a multianvil apparatus. Starting materials consisted of natural minerals with a bulk composition similar to that of a K-deficient, intermediate dioctahedral-trioctahedral mica. Phengitic muscovite was found to be stable from 5.5–11 GPa at 900°C. Phengite melting occurs between 1075–1150°C at 7–8 GPa and 1000–1050°C at 10 GPa. At 10–11 GPa, 800°C octahedral cation deficient (OCD) muscovite and K-hollandite are observed rather than phengite. The average phengite content of muscovite is positively correlated with pressure ranging from 3.65 Si pfu at 5.5 GPa to 3.81 Si pfu at 11 GPa. The maximum phengite content of stable muscovite appears to be 3.80–3.85 Si pfu. Most phengite examined exhibits at least minor solid solution towards phlogopite, averaging 2.04 ± 0.06 (2σ) octahedral cations pfu. The hydrous phases phengite, lawsonite, topaz-OH, and Mg-pumpellyite occur between 6–8 GPa, 700–900°C. With increasing pressure lawsonite and Mg-pumpellyite dehydrate to form garnet between 8–9 GPa. With increasing temperature Mg-pumpellyite, lawsonite, and topaz-OH devolatilize to form garnet and kyanite between 900–1000°C at 7–8 GPa. Phengitic muscovite and topaz-OH would be stable in hydrous sediments in cool mature subduction zones to depths exceeding 360 km while lawsonite and Mg-pumpellyite would be stable to 240–300 km allowing the transport of H2O contained in these phases deep into the upper mantle. In warmer subduction zones the dehydration of Mg-pumpellyite, lawsonite, and topaz-OH; as well as the melting of phengite would result in fluid release at depths of 180–240 km. The presence of phengite at far greater depths than the zone of melt generation beneath arcs (100–150 km) requires a mechanism such as the partitioning of K, Be, B, Ba, and Rb into migrating fluids rather than the simple dehydration of phengite beneath arcs in order to provide for the transfer of these slab signature elements from phengite into arc magmas.

Experimental evidence for a hydrous transition zone in the early Earth's mantle

Abstract Partition coefficients of H2O between β and γ phases of olivine stoichiometry and coexisting ultra mafic melt have been estimated to be > 0.1 ± 0.04 (1σ) and 0.04, respectively; based on experiments at 15–16.5 GPa, 1300–1500°C in a hydrous KLB-1 peridotite system. The high H2O contents of β (1.5–3 wt%) and γ phases (0.7 wt%) would form a reservoir for H2O after cooling and crystallization of a hydrous magma ocean. Subsequent upwelling of this hydrous mantle would release H2O at the β phase-olivine boundary near 400 km depth, inducing partial melting of the peridotite to produce hydrous ultramafic magma. Most subducting hydrous minerals dehydrate at pressures shallower than 6.5 GPa if the down-dragged hydrous peridotite follows a P-T path hotter than 900°C at 8 GPa and cannot re-hydrate the transition zone. Therefore, the above proposed partial melting would gradually deplete the H2O reservoir, which is consistent with the decrease in the activity of ultramafic magmatism and the apparent degree of melting of komatiites from the Archean to the Mesozoic.

Water Sources for Subduction Zone Volcanism: New Experimental Constraints

Despite its acknowledged importance, the role of water in the genesis of subduction zone volcanism is poorly understood. Amphibole dehydration in subducting oceanic crust at a single pressure is assumed to generate the water required for melting, but experimental constraints on the reaction are limited, and little attention has been paid to reactions involving other hydrous minerals. Experiments on an oceanic basalt at pressure-temperature conditions relevant to subducting slabs demonstrate that amphibole dehydration is spread over a depth interval of at least 20 kilometers. Reactions involving other hydrous minerals, including mica, epidote, chloritoid, and lawsonite, also release water over a wide depth interval, and in some subduction zones these phases may transport water to deep levels in the mantle.

Graphite-CH4-H2O-CO2 equilibria at low-grade metamorphic conditions

Equilibrium calculations and published phase equilibria are used to show that at temperatures below 400 °C and pressures above 300 bar, the fluids coexisting with graphite in the C-O-H system consist either of CO2-H2O mixtures or CH4-H2O mixtures. Bulk fluid compositions consisting of CH4-H2O will unmix to methane-rich fluid and H2O-rich liquid at temperatures below about 325 °C. Compositions on the CO2-H2O join will unmix only at temperatures below about 275 °C. Oxygen fugacity in the CH4-H2O-graphite three-phase region is fixed (at constant P and T ) and approximately equal to that of the quartz-magnetite-fayalite assemblage. In the CO2-H2O-graphite three-phase region, oxygen fugacity is about three orders of magnitude greater. From the end stages of diagenesis to temperatures of 300 °C (and possibly 400 °C in salt-rich systems), many metamorphic rocks may contain CH4-H2O fluids rather than CO2-H2O mixtures. Thus, metamorphic reactions involving carbonate minerals would involve CH4, graphite, and H2O rather than CO2. The immiscibility between CH4 and H2O could result in the common occurrence of methane (natural gas) in low-grade metamorphic terrains.