Journal of Petrology | Volume 41 | Number 3 | Pages 363-386 | 2000
© Oxford University Press 2000
Effect of Carbon Dioxide on Dehydration Melting Reactions and Melt Compositions in the Lower Crust and the Origin of Alkaline Rocks
DEPARTMENT OF GEOLOGY AND GEOLOGICAL SCIENCES, COLORADO SCHOOL OF MINES, GOLDEN, CO 80401, USA
Received October 1, 1998; Revised typescript accepted September 3, 1999
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL METHODSEXPERIMENTAL RESULTSANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Dehydration melting experiments of alkali basalt associated with the Kenya Rift were performed at 0·7 and 1·0 GPa, 850–1100°C, 3–5 wt % H2O, and fO2 near nickel–nickel oxide. Carbon dioxide [XCO2 = molar CO2/(H2O + CO2) = 0·2–0·9] was added to experiments at 1025 and 1050°C. Dehydration melting in the system alkali basalt–H2O produces quartz- and corundum-normative trachyandesite (6–7·5 wt % total alkalis) at 1000 and 1025°C by the incongruent melting of amphibole (pargasite–magnesiohastingsite). Dehydration melting in the system alkali basalt–H2O–CO2 produces nepheline-normative tephriphonolite, trachyandesite, and trachyte (10·5–12 wt % total alkalis). In the latter case, the solidus is raised relative to the hydrous system, less melt is produced, and the incongruent melting reaction involves kaersutite. The role of carbon dioxide in alkaline magma genesis is well documented for mantle systems. This study shows that carbon dioxide is also important to the petrogenesis of alkaline magmas at the lower pressures of crustal systems. Select suites of continental alkaline rocks, including those containing phonolite, may be derived by low-pressure dehydration melting of an alkali basalt–carbon dioxide crustal system.
KEY WORDS: alkali basalt; alkaline rocks; carbon dioxide; dehydration melting; phonolite
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL METHODSEXPERIMENTAL RESULTSANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Dehydration melting, in which a melting reaction that involves a hydrous phase defines the solidus, is an important magma-generating process in continental crust (e.g. Thompson, 1982
). Much recent effort examines the origin of granite by simulating dehydration melting in a variety of protoliths, including graywackes (Vielzeuf & Montel, 1994; Patiño Douce & Beard, 1996; Skjerlie & Johnston, 1996), pelitic rocks (Patiño Douce & Johnston, 1991; Carrington & Harley, 1995; Patiño Douce, 1996), felsic igneous rocks (Rutter & Wyllie, 1988; Skjerlie & Johnston, 1992, 1993), mixed, interlayered lithologies (Skjerlie et al., 1993; Skjerlie & Patiño Douce, 1995), and mafic gneisses and amphibolites (Ellis & Thompson, 1986; Patiño Douce & Beard, 1995). Other studies explore the origin of tonalite and trondhjemite by dehydration melting of felsic gneisses (Johnston & Wyllie, 1988) and basalts and amphibolites (Hacker, 1990; Beard & Lofgren, 1991; Rapp et al., 1991; Rushmer, 1991; Winther & Newton, 1991; Wolf & Wyllie, 1991, 1994; Rapp & Watson, 1995; Winther, 1996; Springer & Seck, 1997).
Water in hydrous phases is a critical constituent in these dehydration melting studies. After H2O, carbon dioxide is probably the second most important volatile species at volcanic centers (e.g. Blank & Brooker, 1994; Jambon, 1994). Within certain systems, such as continental rifts, CO2 even exceeds H2O in importance (Bailey, 1980; Bailey & Macdonald, 1987). In petrologic systems, therefore, CO2 can be an important component in the melting of crustal lithologies. Few studies have explored the role of CO2 in the partial melting of these systems. In early, vapor-present basalt melting experiments, Holloway & Burnham (1972) and Allen & Boettcher (1978) used CO2 as a diluent to lower fH2O. Holloway & Burnham (1972) melted tholeiite at 0·8 GPa to produce dacites and andesites whereas Allen & Boettcher (1978) investigated amphibole stability in andesite and basalt as a function of fH2O and fO2. Neither study addressed the effect of CO2 on the phase relations. Springer & Seck (1997) investigated dehydration melting of granulite facies metagabbros and metabasalts at crustal conditions, including one alkali basalt composition. Tonalite is produced at <20% melting and dacite at >25% melting. To duplicate the composition of metamorphic fluids, they added CO2 to their experiments. According to Springer & Seck (1997), use of two different volatile mixtures [molar H2O/(H2O + CO2) = 0·5 and 0·75] did not significantly affect the results. However, most of their experiments were performed with one volatile mixture [molar H2O/(H2O + CO2) = 0·5], and no systematic evaluation of the effect of changes in CO2 content on dehydration melting was conducted.
Our understanding of how CO2 influences dehydration melting in continental crust is surprisingly limited. In particular, we do not know how the composition of partial melts and residual lithologies changes in response to variations in CO2 content. The purpose of this study, therefore, is to examine systematically the effect of CO2 on dehydration melting. Naturally occurring alkali basalt is used as starting material in experiments that simulate near-solidus dehydration melting of lower continental crust. Alkali basalt is a desirable material for studying dehydration melting because the lower continental crust is broadly mafic in composition (e.g. Okrusch et al., 1979; Rivalenti et al., 1981; Rudnick & Taylor, 1987; Wendlandt et al., 1991; Downes, 1993) and, in many places, alkaline (e.g. Griffin et al., 1979; Loock et al., 1990; Sinigoi et al., 1994; Hay et al., 1995a). Several previous dehydration melting studies of continental crust focused specifically on naturally occurring, metamorphosed alkali basalts (Rapp et al., 1991; Rushmer, 1991; Wolf & Wyllie, 1991, 1994; Rapp & Watson, 1995; Springer & Seck, 1997). Use of alkali basalt establishes a common ground with these studies as a preliminary step toward understanding the effects of CO2 on dehydration melting. Because the beginning of melting is important, we focus on experiments near the solidus and do not explore phase relationships across the entire interval between the solidus and liquidus. The results of these experiments bear upon our understanding of dehydration melting and melting reactions in continental crust and the origin of alkaline rocks by dehydration melting.
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EXPERIMENTAL METHODS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL METHODSEXPERIMENTAL RESULTSANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Starting material
A specimen of Miocene alkali basalt, sample KM314 from the East Kenya Plateau (Bellieni et al., 1986
), was selected for study (Table 1). Hay & Wendlandt (1995) and Hay et al. (1995b) suggested that alkali basalt compositionally similar to KM314 is a suitable parent to Plateau-type flood phonolite in the Kenya segment of the East African rift. Alkali basalt KM314 contains a moderate amount of Na2O and K2O, and the mg-number of 0·58 suggests the basalt is not a primary mantle-derived magma.
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Sample KM314 is a holocrystalline, porphyritic–aphanitic basalt. Phenocrysts of subhedral, embayed olivine (Fo81–85) and euhedral, glomeroporphyritic clinopyroxene (Ca45Mg43Fe12) occur in an intergranular, pilotaxitic matrix of plagioclase (An41), olivine (Fo46–68), clinopyroxene (Ca46Mg37–42Fe11–17), and ulvöspinel (Bellieni et al., 1986). Less than 1 vol. % xenocrystic material, composed of alkali feldspar and very fine-grained aggregates of pyroxene, is present. Olivine phenocrysts (12 vol. %) are partially altered to iddingsite plus spinel. Clinopyroxene phenocrysts (1 vol. %) are unaltered. Thin, calcite-filled veins make up <1% of the rock.
Alkali basalt KM314 is part of a cycle of early fissural volcanism in the East Kenya Plateau. Most magmas in this cycle are interpreted as primary or slightly evolved (Bellieni et al., 1986). The basalt occurs 
200 km east of the axis of the Kenya rift (Bellieni et al., 1986) and is coeval with the 50 000 km3 of flood phonolites (14–11 Ma) that filled the early rift structure in central and southern Kenya (Baker, 1987).
The rock was broken into smaller fragments and ground in a ball mill, including fragments with altered phenocrysts and calcite veins. The ground material was subsequently hand ground in an agate mortar under acetone for 3·5 h. For temperature reversal experiments or experiments with CO2-rich compositions, the material was hand ground an additional 2·5 h. The long grinding time was necessary to reduce the grain size of olivine phenocrysts in the starting material. Olivine phenocrysts persisted in preliminary experiments performed in the system alkali basalt–H2O–CO2 with starting material that was ground for a shorter time. No evidence of SiO2 contamination of the starting material was observed in any of the experiments. Rock powder for experiments was stored in a drying oven at 100°C to prevent adsorption of moisture.
Double-distilled, deionized H2O was added to capsules using a microsyringe. Carbon dioxide was added to experiments through the thermal breakdown of encapsulated silver oxalate. The volatile content of these experiments is reported as XCO2 [molar CO2/(H2O + CO2)]. This quantity includes the volatile content of the starting material (Table 1).
Silver oxalate was prepared by mixing dilute solutions of reagent grade silver nitrate and oxalic acid dihydrate. The precipitate was subsequently dried, characterized by X-ray diffraction, and stored in a desiccator under a vacuum. The storage vial was covered by foil to minimize exposure to light and, consequently, oxidation of the silver. Stoichiometry of silver oxalate was periodically checked by weighing a known amount of the material before and after thermal breakdown in an oven.
The oxidation state of most experiments with added volatiles was not controlled with solid-state buffers because the furnace assembly possesses an intrinsic oxidation state near that of the nickel–nickel oxide buffer, as documented by the experiments of Hay & Wendlandt (1995). The oxidation state of the lower-crustal phonolite source region is believed to be moderately oxidized as suggested by the mineral compositions, the high ratio of ferric to ferrous iron in phonolites, and the results of phonolite melting experiments (Hay & Wendlandt, 1995). In addition, the upper mantle and lower crust are believed to be characterized by fO2 values near those of the quartz–fayalite–magnetite buffer (Luth, 1989, and references cited therein).
Experimental procedure and apparatus
Starting materials were sealed in Au75Pd25 capsules of 3 mm diameter for experiments at temperatures at and above 1025°C. For experiments at lower temperatures, gold capsules of 2 mm diameter, or gold capsules of 2 mm inside those of 3 mm diameter, were used (Tables 2 and 3). Gold capsules generally failed at temperatures of 1025°C and higher at the extended run times required for near-solidus experiments. Gold double capsules of 2 mm inside 3 mm diameter were used in a few experiments with solid-state buffers.
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Experiments were performed in an end-loaded solid-media piston-cylinder apparatus (Boyd & England, 1960). The anhydrous furnace assembly (1·25 cm diameter) consisted of a cylindrical graphite resistance heater enclosed in an outer sleeve of calcium fluoride as the pressure-transmitting medium. These components were stored in a drying oven at 100°C to prevent adsorption of moisture. Capsules were centered in the graphite heater at the furnace hot spot. Capsules were isolated from the graphite furnace by crushable magnesia tubing and solid filler rods inserted above and below the capsule, respectively. Lead foil and MoS2 grease were used to lubricate the tungsten carbide core.
Temperature was measured with a Pt–Pt90Rh10 thermocouple and maintained within 2°C of the setpoint. No correction was applied for the effect of pressure on the e.m.f. output of the thermocouple. A ceramic disk of 0·5 mm thickness was used to separate gold capsules from the thermocouple. Estimated uncertainties are ±15°C and ±0·05 GPa (Hay & Wendlandt, 1995). Experiments were brought to temperature and pressure using the ‘hot piston-in’ technique. Each experiment was quenched by shutting off the power, resulting in a temperature drop of several hundred degrees in a few seconds. The capsule was removed from the furnace assembly, cleaned with hydrochloric acid, and inspected for integrity. The potential presence of vapor in the capsule was determined by inspecting the capsule for ‘puffing’. In addition, the capsule was weighed, punctured, heated to 100°C for at least 1 h, and reweighed to determine whether vapor was present.
Near-solidus phase relationships in the system alkali basalt–H2O were evaluated for a range of water-undersaturated and water-saturated conditions at 0·7 and 1·0 GPa and 800–1100°C (Table 2). A few experiments were also conducted at 0·5 GPa. Experiments in the system alkali basalt–H2O–CO2 were conducted at 0·7 GPa, 1025°C and 1·0 GPa, 1025 and 1050°C (Table 3). These temperatures were selected because they were the lowest temperatures in the system alkali basalt–H2O at which glass is produced in quantities sufficient for electron probe microbeam analysis (EPMA) and because they are the approximate temperatures predicted for phonolite genesis (Hay & Wendlandt, 1995).
Analytical methods
Experimental charges were analyzed using optical microscopy, scanning electron microscopy, and EPMA techniques. Determination of the solidus is based on visual identification of glass in grain mounts and, therefore, the solidus is only approximately located because of the difficulty of recognizing small amounts of glass in the experimental products.
The EPMA data were collected with a JEOL 8900 Superprobe at the US Geological Survey, Lakewood, CO. Mineral and glass data were acquired with a 15 kV accelerating voltage and a 20 nA beam current. A 1 µm beam diameter was used on minerals. The widest beam diameter allowed by proximity of surrounding phases was used on glass. Beam diameters for glass analysis ranged from 5 to 20 µm.
Diffusion of sodium during EPMA analysis of hydrous silicate glasses is a well-documented problem (e.g. Morgan & London, 1996). The effect of sodium diffusion within and from the sample was minimized by obtaining sodium data first. In addition, sodium K
line intensity vs time curves were obtained for each sample. These curves were used to extrapolate the Na2O content in glasses to zero-time values. Corrections obtained in this manner range from 11 to 31% of the measured Na2O. Similar Na2O corrections were obtained in several analyses by decreasing the beam current to 1–2 nA.
Standards for the EPMA analyses were well-characterized natural silicates, nonsilicates, and synthetic materials. Data were reduced using CITZAF matrix correction routines (Armstrong, 1989). Standard mineral formula calculation schemes were used for clinopyroxene, olivine, plagioclase, and apatite (Deer et al., 1966). The Fe2O3 content of titaniferous magnetite was calculated according to Carmichael (1967). Amphibole mineral formulae were calculated, and mineral names assigned, on the anhydrous basis of 23 oxygens by the software AMPH (Rameshwar Rao & Subba Rao, 1996) according to International Mineralogical Association (IMA) guidelines (Leake, 1997). Two formulae based on stoichiometric limits were determined for each amphibole analysis. One formula, calculated assuming 15 cations exclusive of sodium and potassium, contains the minimum estimate of ferric iron consistent with stoichiometry. For amphiboles synthesized in this study, no ferric iron is calculated following the first formula and amphiboles classify as magnesiohastingsite (system alkali basalt–H2O) and kaersutite (system alkali basalt–H2O–CO2). The second formula, calculated assuming 13 cations exclusive of calcium, sodium, and potassium, contains the maximum estimate of ferric iron. The resulting amphibole formula contains 0–0·874 atoms of ferric iron per formula unit, a range consistent with naturally occurring calcic and kaersutitic amphiboles (Stout, 1972; Spear, 1980; Popp et al., 1995b). These amphiboles classify as pargasite (system alkali basalt–H2O) and kaersutite (system alkali basalt–H2O–CO2). Representative amphibole formulae reported in this study are mean values of formulae calculated with maximum and minimum ferric iron contents (Spear & Kimball, 1984; Leake, 1997). Averaged amphibole formulae also classify as magnesiohastingsite, pargasite, and kaersutite.
Evaluation of equilibrium
Several lines of evidence support a close approach to equilibrium in experiments, including time studies, temperature reversals, and textural and compositional evidence. Three near-solidus experiments with no added volatiles were conducted at 975°C and 1·0 GPa for durations of 9, 48, and 193 h. Each of the three experiments produced the same mineral assemblage, although the last experiment produced a more homogeneous distribution of euhedral crystals. Subsequent experiments with added volatiles were planned and executed based on these results. Successful temperature reversals were performed in experiments with added volatiles for the amphibole-out and phlogopite-out reaction curves. No temperature reversals were conducted for higher-temperature reactions because of iron loss problems.
Textural evidence for equilibrium includes the presence of euhedral, unzoned crystals and, except for crystal settling in experiments with large amounts of melt, the homogeneous distribution of crystals throughout the charge. Compositional criteria include equilibrium iron–magnesium partitioning between olivine and melt that is consistent with phase equilibria studies (Roeder & Emslie, 1970) and replicate EPMA analysis of phases distributed throughout the experimental charge, suggesting compositional homogeneity.
Temperature reversal experiments in the system alkali basalt–H2O–CO2 were attempted to confirm important trends in melt compositions. These experiments were not successful. Because of the long duration of these experiments, iron was lost to the Au75Pd25 container walls in quantities large enough to change the phase assemblage by destabilizing titaniferous magnetite and olivine. The durations of experiments in the systems alkali basalt–H2O and alkali basalt–H2O–CO2, however, were devised based on the results of time studies and temperature reversal experiments without added volatiles. The success of temperature reversals for experiments without added volatiles, therefore, suggests that experiments with added volatiles also represent a close approach to equilibrium.
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EXPERIMENTAL RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL METHODSEXPERIMENTAL RESULTSANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Near-solidus phase assemblages for the system alkali basalt–H2O are summarized in Table 2. Amphibole and phlogopite are both present throughout the range of temperature and water contents explored. The following phase relationships are observed at 0·7 GPa. Plagioclase disappears from the assemblage between 975 and 1000°C at an H2O content of
5 wt %. Olivine appears above the solidus between 1000 and 1025°C at an H2O content of 3 wt %. Both reactions coincide with the incongruent melting of amphibole as indicated by the decreased quantities of amphibole and increased clinopyroxene and melt quantities. At 1100°C and 5 wt % H2O, trace amounts of both amphibole and phlogopite are still present. Near-solidus phase assemblages for the system alkali basalt–H2O–CO2 at 0·7 GPa, 1025°C and 1·0 GPa, 1025 and 1050°C are summarized in Table 3. Amphibole + clinopyroxene + olivine + phlogopite + plagioclase + apatite + titaniferous magnetite + liquid is the stable assemblage for most of these experiments. The addition of CO2 to the system alkali basalt–H2O at these pressures and temperatures adds plagioclase to the assemblage amphibole + clinopyroxene + olivine + phlogopite + apatite + titaniferous magnetite + liquid and raises the solidus temperature.
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ANALYTICAL RESULTS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL METHODSEXPERIMENTAL RESULTSANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Amphibole
Amphibole is present throughout the range of temperature, pressure, and fluid composition investigated. The EPMA analyses, chemical formulae, and IMA nomenclature (Leake, 1997
) are presented in Table 4. Amphiboles synthesized in the system alkali basalt–H2O are magnesiohastingsite or pargasite. Amphiboles synthesized in the system alkali basalt–H2O–CO2 are kaersutite with one exception. Amphibole in experiment 106, synthesized with XCO2 = 0·46, is a pargasite. Titanium in the chemical formula distinguishes between pargasite (Ti > 0·50) and kaersutite (Ti 0·50). Amphibole in experiment 106, containing 0·494 cations titanium in the chemical formula, is nearly a kaersutite and closer in composition to amphiboles synthesized with CO2 than to amphiboles synthesized without CO2.
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Amphibole compositions are displayed on a plot of tetrahedral aluminum vs A-site occupancy in Fig. 1. Amphiboles synthesized in other alkali basalt melting studies and compositions of naturally occurring hornblende and kaersutite are plotted for comparison. Tetrahedral aluminum contents among all of the synthetic amphiboles compare favorably. The A-site occupancy of amphiboles in this study is greater than, but overlaps with, the A-site occupancy of most of the other synthetic amphiboles. Amphiboles synthesized with added CO2 in this study display among the highest A-site occupancies (Fig. 1), comparable with those of naturally occurring kaersutite (Fig. 1). Amphiboles synthesized from a hydrous basanite (Fujinawa & Green, 1997) display even greater A-site occupancy (Fig. 1).
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: amphiboles synthesized in system basalt–H2O; •: amphiboles synthesized in system basalt–H2O–CO2). Shown for comparison are synthetic amphibole compositions crystallized from amphibolite of alkali basalt composition (
: Allen et al., 1975
: Rapp & Watson, 1995
: Fujinawa & Green, 1997
, Leake, 1968Amphibole apparently exhibits systematic changes in composition as a function of temperature at 0·7 GPa and 5 wt % H2O (Fig. 2). The Al2O3 content increases from 975 to 1025°C. The SiO2 and MgO contents increase and the TiO2, FeO, and Na2O contents decrease from 975 to 1000°C, but these trends appear to reverse from 1000 to 1025°C. These apparent compositional reversals coincide with the appearance of olivine and increased amounts of clinopyroxene from the incongruent melting of amphibole.
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, 3 wt % H2O; •, 5 wt % H2O; open cross: 10 wt % H2O). Amphibole compositions synthesized with 5 wt % H2O are connected by a line to illustrate the systematic changes. Uncertainties expressed as standard deviations for compositional data at 5 wt % H2O in Table Figure 3 illustrates the changes in amphibole formulae as a function of XCO2 in the system basalt–H2O–CO2. Several trends are apparent in Fig. 3. The Si, [6]Al, Fe3+, and mg-number appear to decrease whereas the [4]Al, Ti, Ca, and A-site occupancy apparently increase as XCO2 increases at 1·0 GPa and 1050°C. The mg-number decreases because Fe2+ increases and Mg decreases (Table 4). Despite some scatter in the data and the limited size of the data set, the trends exhibited at 0·7 GPa, 1025°C and 1·0 GPa, 1025°C are consistent with those exhibited at 1·0 GPa and 1050°C. Sodium in amphibole reflects the influence of temperature and XCO2. Sodium increases as XCO2 increases at 1025°C and 1·0 GPa, but this trend reverses at 1050°C (Table 4). These systematic changes are different from the changes imposed by temperature (Fig. 2) and are not, therefore, explained by a raised solidus temperature with addition of CO2. However, compositional changes as a result of increasing XCO2 at constant temperature cannot be isolated from those changes occurring as a result of changing melt fraction and composition.
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, 1·0 GPa, 1025°C; 
, 1·0 GPa, 1050°C). Amphibole compositions synthesized at 1·0 GPa, 1050°C are connected by a line to illustrate the systematic changes.
The changes in amphibole chemistry imposed by CO2 suggest that several coupled substitution mechanisms are occurring simultaneously, including the stuffing substitution (Si 
NaA-site + [4]Al) and oxy-amphibole substitutions. The following oxy-amphibole substitution mechanisms accommodate Fe3+ and Ti4+ in naturally occurring kaersutitic amphiboles (Popp & Bryndzia, 1992):
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; Popp et al., 1995a, 1995b). Substitution of components such as ferri-tschermakite or magnesiorichterite to accommodate Fe3+ is not significant in kaersutite (Popp et al., 1995b). The [6]Al3+ decrease and Ti4+ increase in amphibole synthesized in this study (Fig. 3) are consistent with oxy-amphibole substitution reaction (2). The decrease in Fe3+ and FeO (total) in synthetic amphibole, however, are contrary to oxy-amphibole substitution reaction (1).
Clinopyroxene
Clinopyroxene occurs throughout the range of temperature, pressure, and fluid composition investigated. The EPMA analyses, chemical formulae, and IMA nomenclature (Morimoto et al., 1988) are presented in Table 5. With one exception, all of the clinopyroxene is aluminian diopside. Clinopyroxene synthesized in one experiment with a high H2O content (10·4 wt %) contains less aluminum and is a diopside.
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Clinopyroxene displays systematic changes in composition as a function of temperature at 0·7 GPa and 5 wt % H2O (Fig. 4). The TiO2, Al2O3, and FeO contents increase and the SiO2, MgO, and mg-number decrease from 975 to 1100°C (Table 5). The CaO and Na2O contents remain constant in this temperature interval. The compositional trends of clinopyroxene exhibit an apparent inflection between 1000 and 1025°C (Fig. 4), coinciding with the appearance of olivine and increased amounts of clinopyroxene from the incongruent melting of amphibole.
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Clinopyroxene also displays systematic changes in composition from 3 wt % to 10 wt % H2O at 0·7 GPa and 1000–1025°C (Fig. 4). These changes are the opposite of those observed for increasing temperature at 5 wt % H2O. The systematic changes in pyroxene composition as a function of water content (Fig. 4) coincide with increased modal quantities of amphibole.
Systematic changes in clinopyroxene chemistry with increased CO2 are the opposite of the changes observed with increasing temperature. The SiO2, MgO and mg-number increase, and TiO2, Al2O3, and FeO decrease as XCO2 increases (Fig. 5). The CaO and Na2O contents remain relatively constant as XCO2 increases (Table 5).
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Olivine
In the system basalt–H2O at 0·7 GPa, olivine composition ranges from Fo66 at 1025°C to Fo78 at 1100°C. The composition of olivine in the system basalt–H2O–CO2 also ranges between Fo66 and Fo78, but no systematic relationship between olivine chemistry and CO2 content is evident. A table containing the EPMA analyses is available as an electronic supplement.
Phlogopite
Mica is present in small amounts throughout the range of temperature, pressure, and fluid composition investigated. Although readily identified in oil mounts by its bird’s-eye extinction and habit, mica was impossible to locate and analyze with EPMA analysis. Low relief, low birefringence, and lack of color suggest the mica is phlogopite. In some experiments, mica exhibits a reddish brown color characteristic of a high titanium content (Deer et al., 1966).
Plagioclase
Plagioclase in the system basalt–H2O is andesine (An48) with 2 mol % orthoclase. Plagioclase in the system basalt–H2O–CO2 is labradorite (An53–55) containing similar amounts of orthoclase. A table containing the EPMA analyses is available as an electronic supplement.
Titaniferous magnetite
The EPMA analyses and chemical formulae for titaniferous magnetite containing ulvöspinel (Deer et al., 1966) are presented in Table 6. Titaniferous magnetite displays systematic compositional changes as a function of temperature at 0·7 GPa and 5 wt % H2O (Fig. 6a). The TiO2, Al2O3, and MgO contents increase from 1000 to 1100°C whereas FeO (total iron) decreases. The MgO content exhibits an apparent maximum between 1000 and 1025°C (Fig. 6a) that coincides with the appearance of olivine from the incongruent melting of amphibole and/or phlogopite. Systematic changes in titaniferous magnetite composition with increasing XCO2 (Fig. 6b) are different from the temperature trends. The TiO2 and MgO contents increase whereas FeO (total iron) decreases as XCO2 increases.
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Glass
The EPMA analyses and CIPW norms for glass are presented in Table 7. The CIPW norms and mg-numbers in these tables are calculated with iron as FeO. Representative glass compositions are plotted in the total alkali–silica diagram of Le Bas et al. (1986)
for experiments at 0·7 and 1·0 GPa (Fig. 7a and b, respectively). The composition of the starting material is also plotted in these diagrams for comparison.
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Dehydration melting in the system basalt–H2O produces uniform melt compositions. Glasses synthesized at 0·7 GPa, 1025°C with a range of H2O contents (from 3 to 5 wt %, Fig. 7a) plot as the same trachyandesite composition in the total alkali–silica diagram. Glasses synthesized at lower temperature (1000°C and 5 wt % H2O, plots on the boundary between trachyandesite and andesite in Fig. 7a), higher pressure (1·0 GPa and 5 wt % H2O, Fig. 7b), and greater H2O content (1000°C and 10 wt % H2O, plots as andesite in Fig. 7a) are of similar composition. These glasses contain normative quartz and hypersthene and similar amounts of normative albite and orthoclase (Table 7). Differences among the CIPW norms of these glasses are because P2O5 was not analyzed in two of the glasses (experiments 99 and 100 in Table 7). Glass synthesized at higher temperature (1100°C, 0·7 GPa in Fig. 7a and 1050°C, 1·0 GPa in Fig. 7b) and 5 wt % H2O plots as basaltic trachyandesite. Increased melting at these temperatures produces glass that is closer in composition to the starting material than to the trachyandesitic glasses synthesized at lower temperature.
Dehydration melting in the system basalt–H2O–CO2 produces uniform melt compositions over a range of XCO2. Glasses synthesized at 0·7 GPa, 1025°C with a range of H2O (3–5 wt %), CO2 (2–12 wt %), and XCO2 (0·22–0·49) contents are nepheline-normative and plot near the junction of the phonolite, tephriphonolite, trachyte, and trachyandesite fields (Fig. 7a). Glass synthesized at 1·0 GPa, 1025°C, XCO2 = 0·34 is trachytic (Fig. 7b).
Glasses in these experiments exhibit several important petrologic trends. Basalt melting at 0·7 and 1·0 GPa, 1025°C, and a range of water-undersaturated, CO2-absent conditions generates trachyandesites (Fig. 7). Addition of CO2 (0·2 < XCO2 < 0·5) shifts melt compositions toward phonolite. The trend at 1·0 GPa, 1050°C is similar to the 1025°C trend but shifted to lower SiO2 contents in the diagram (Fig. 7b). Melt compositions range from basaltic trachyandesite (XCO2 = 0·01) through trachyandesite (XCO2 = 0·53) to tephriphonolite (XCO2 < 0·86).
Glasses synthesized in this study show other systematic changes in composition in response to temperature and XCO2 variations. The TiO2, FeO, MgO, and CaO contents increase between 1000°C and 1100°C whereas the SiO2 and K2O contents decrease (Fig. 8). The Al2O3 and Na2O contents display small maxima at 1025°C. The compositional changes with increasing XCO2 are different from the temperature trends. The SiO2, Al2O3, Na2O, and K2O contents increase, and the FeO, MgO, CaO and perhaps the TiO2 contents decrease as XCO2 increases (Fig. 9).
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Modal proportions of minerals and melt
Phase proportions in selected experiments as a function of temperature (at 0·7 GPa and 5 wt % H2O) and XCO2 (at 0·7 GPa, 1025°C, and 3 wt % H2O) are presented in Fig. 10. These modes are calculated from EPMA data using least-squares regression analysis (Albarède & Provost, 1977). Error in the modal data is estimated from the error of the least-squares regression (
, the sum of the squares of the residuals) and originates from phase heterogeneity and uncertainty in EPMA data. Most of the modal data presented in this study exhibits < 1·0, a value that corresponds to an error of 6 wt %. Larger errors exist for some phases and are identified in the following discussion.
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Melting proceeds with rising temperature as amphibole decreases in quantity, clinopyroxene, olivine and liquid increase, and plagioclase disappears (Fig. 10a). The amount of titaniferous magnetite in the melting interval remains constant. The apparent decrease in clinopyroxene quantity between 975 and 1000°C is believed to be an artifact of a larger error in the near-solidus modal data than in the higher temperature modes. Small amounts of melt are present at 975°C, but these do not occur in sufficient quantities for EPMA analysis and do not appear in the modal diagram. Consequently, in the least-squares regression analysis at 975°C, some components that reside in glass are used instead to make additional clinopyroxene.
Melting diminishes with increasing XCO2, a likely indication that the solidus temperature is increasing (Fig. 10b). Changes in the modes of the residual minerals with increased XCO2 are different from the changes imposed by temperature. Plagioclase reappears and, along with clinopyroxene and olivine, increases in quantity from XCO2 = 0 to XCO2 = 0·22 as amphibole decreases. Plagioclase, clinopyroxene, olivine, and titaniferous magnetite proportions remain relatively constant, within the 6 wt % uncertainty of the least-squares regression analysis, from XCO2 = 0·22 to XCO2 = 0.46. The apparent increase in the quantity of amphibole in this interval coincides with increased error (10 wt %) of the least-squares regression for amphibole modal data at XCO2 = 0·46.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL METHODSEXPERIMENTAL RESULTSANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Melting reactions
Dehydration melting of amphibole does not proceed uniformly. Phase relationships in the system alkali basalt–H2O (Fig. 10a) suggest that amphibole melts incongruently below 1000°C at water-undersaturated conditions by the reaction
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30 wt %). Plagioclase disappears from the residual mineral assemblage between 975 and 1000°C. Amphibole changes from pargasite to magnesiohastingsite and clinopyroxene exhibits systematic compositional changes (TiO2, Al2O3 and FeO increase, and SiO2 and MgO decrease, while FeO decrease and MgO increase in amphibole). Reaction (3) is consistent with dehydration melting reactions reported by Wolf & Wyllie (1994) and with incongruent melting reactions expected for amphibole.
Amphibole continues to melt incongruently above 1000°C, but the phase relationships depicted in Fig. 10a suggest a different reaction:
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Decreased melting in the system alkali basalt–H2O–CO2 suggests that CO2 raises the solidus temperature relative to the system alkali basalt–H2O. A different melting reaction, however, accompanies this change in melting. Phase relationships in the system alkali basalt–H2O–CO2 (Fig. 10b) suggest that the following reaction proceeds as XCO2 increases from 0 to 0·22:
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Comparison with dehydration melting studies
Several recent experimental studies (Rapp et al., 1991; Rushmer, 1991; Wolf & Wyllie, 1991, 1994; Rapp & Watson, 1995; Springer & Seck, 1997) examined dehydration melting of naturally occurring, metamorphosed alkali basalts (amphibolites). Although those metabasalts display a range of major oxide and normative mineral contents, results of those studies and the melting phase relationships of this study bear some similarities.
Rushmer (1991) reported a solidus at 0·8 GPa of 925°C. Rapp et al. (1991) reported a solidus of 975°C between 0·5 and 1·5 GPa, whereas Springer & Seck (1997) reported a solidus of 700–750°C in the same pressure interval. This broad range in solidus temperature reflects the range of bulk chemistry of the starting materials used in these dehydration melting studies. The solidus determined for KM314 lies between 800°C and 850°C at 0·7 GPa and 5 wt % water (Table 2), a temperature that is intermediate to and consistent with the solidus temperature determined in those other studies.
The thermal stability of amphibole varies among the dehydration melting studies, from 975°C at 1·0 GPa (Wolf & Wyllie, 1994) to 1085°C at 1·0 GPa (Rapp & Watson, 1995). The stability of amphibole in KM314 is consistent with these dehydration melting studies. Only Rapp et al. (1991) reported mica as a hydrous phase that is present in addition to amphibole. They described biotite as present in the melting residue at 1000°C and 0·8 GPa. Wolf & Wyllie (1994) reported two amphiboles present in their 1·0 GPa experiments, an aluminous hornblende and a calcic hornblende. Both amphibole and phlogopite are stable in the melting residue of KM314.
Effect of CO2 on dehydration melting
Glasses synthesized in the system alkali basalt–H2O plot as tonalite in a tight cluster near the junction of the tonalite, trondhjemite, and granodiorite fields in the normative feldspar diagram of Barker (1979) (Fig. 11a). The one outlier is a glass synthesized under water-saturated conditions. Glasses produced in the other dehydration melting studies of alkali basalt (Rushmer, 1991; Wolf & Wyllie, 1994; Rapp & Watson, 1995; Springer & Seck, 1997) also plot as tonalite and trondhjemite in Fig. 11a. Most of these latter glasses contain 10–20 wt % normative orthoclase and exhibit a broad range of albite and anorthite content (10–65 wt % normative anorthite). The Wolf & Wyllie (1994) glasses are distinctly different and contain a significantly greater proportion of normative anorthite. Wolf & Wyllie (1994) attributed this difference to the high anorthite content of plagioclase (An90) in their starting material.
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Hydrous glasses synthesized in this study are near the center of the glass compositions plotted in Fig. 11a. Glasses synthesized in the system alkali basalt–H2O–CO2 over a range of XCO2, however, plot as granite in Fig. 11a. These glasses contain twice the amount of normative orthoclase (30–40 wt %) as any of the hydrous glasses synthesized without CO2 [glasses of Rushmer (1991)
, Wolf & Wyllie (1994), and Rapp & Watson (1995)]. The experiments of Holloway & Burnham (1972) and Springer & Seck (1997) generated tonalites rather than granites despite the presence of CO2 (XCO2 0·4 and 0·5, respectively). This contrast is due to the difference in total alkali content among the bulk compositions of the starting materials that were used [4·5 wt % in this study vs 2·5 wt % used by Holloway & Burnham (1972) and 2·2 wt % used by Springer & Seck (1997)]. In addition, Holloway & Burnham (1972) performed vapor-present experiments whereas most experiments in this study were performed under vapor-absent dehydration melting conditions.
With rising temperature at constant CO2 content, hydrous glasses become richer in normative anorthite (dashed arrow labeled SS in Fig. 11b). Helz (1976) identified a similar trend for water-saturated, CO2-absent melting of nepheline- and quartz-normative basalts (dashed arrows H and K, respectively, in Fig. 11b). Glasses synthesized with increased amounts of CO2 at constant temperature define a trend opposite to these temperature trends. Normative anorthite content decreases and normative orthoclase content increases with increasing XCO2 at constant temperature (solid arrow in Fig. 11b). This trend is not an artifact of decreased melting that accompanies the raising of the solidus by adding CO2. Simply raising the solidus temperature increases SiO2 saturation in the melt, whereas the glasses in the system alkali basalt–H2O–CO2 become undersaturated in SiO2.
The same glass compositions plotted in Fig. 11a are also plotted in the total alkali–silica diagram of Fig. 12. Starting materials for experiments producing these glasses are also plotted. In this diagram, most of the published partial melt compositions for basalt–H2O systems define distinct lineages that span the classic calc-alkaline trend. The data of Rapp & Watson (1995) differ in that they define a trend that begins with basaltic trachyandesite and extends to trachyandesite and rhyolite. In our study, glasses in the system alkali basalt–H2O span the same lineage as described by Rapp & Watson (1995) but encompass a broader range of igneous rock compositions (basalt through trachybasalt and basaltic trachyandesite to trachyandesite). The different trends observed in the alkali basalt–H2O system reflect differences in total alkali content among the bulk compositions of the starting materials that were used.
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Glass compositions shift dramatically in the system alkali basalt–H2O–CO2 to the alkaline compositions of tephriphonolite, trachyandesite, and trachyte, near the phonolite field (Fig. 12). Bailey & Macdonald (1987) suggested that the interaction of CO2-bearing fluids with different source rocks produced distinct alkaline lavas in the Kenya rift. Our model provides a specific mechanism with which to evaluate the petrogenesis of alkaline rocks, particularly those that occur within the Kenya rift.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL METHODSEXPERIMENTAL RESULTSANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The role of carbon dioxide in alkaline magma genesis is well documented for mantle systems. This study shows that carbon dioxide is also important to the petrogenesis of alkaline magmas at the lower pressures of crustal systems. Dehydration melting of a hydrous crustal system yields SiO2-oversaturated, peraluminous melts. With carbon dioxide present, the melts become SiO2 undersaturated and alkaline rich. As with many mantle systems, carbon dioxide raises the solidus temperature of the crustal system.
Amphibole melting reactions change with XCO2 and temperature. Incongruent melting of amphibole is still an important process in the CO2-present system, but the residual amphibole is kaersutite rather than pargasite or magnesiohastingsite. Kaersutite is, therefore, not restricted to mantle systems. Dehydration melting in the CO2-present system may also explain titanium partitioning between kaersutitic megacrysts and hydrous alkali basalt melts.
Melt compositions approaching phonolite are possible at low-pressure melting conditions with carbon dioxide (Fig. 12). Dehydration melting of a protolith containing more alkalis than our experimental system may produce melt compositions equaling phonolite. The calc-alkaline trend indicates a water-dominated system that is independent, to some degree, of initial basalt composition. With carbon dioxide, magma lineages become alkaline. We predict that select suites of continental alkaline rocks are derived from low-pressure dehydration melting or equilibrium crystallization of an alkali basalt–carbon dioxide crustal system. Potential suites to investigate include alkaline rocks in nappe sequences containing a source of carbon dioxide and alkaline rocks in continental rifts, particularly the alkaline lavas of the Kenya rift.
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ACKNOWLEDGEMENTS |
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This contribution is dedicated in loving memory to my mother, Irene Kaszuba. We thank Dr L. Morbidelli for supplying the sample of alkali basalt used as starting material in the experiments. We also thank Dr Mike Baker for providing the least-squares regression analysis software used to calculate phase proportions. Journal reviews by T. H. Green and B. Mysen are gratefully acknowledged. This work was supported by National Science Foundation Grant EAR-9316197 and by the Department of Geology and Geological Engineering at the Colorado School of Mines.
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FOOTNOTES |
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*Corresponding author. Present address: Environmental Science and Technology Division, Los Alamos National Laboratory, E-ET, Mail Stop J514, Los Alamos, NM 87545, USA. Telephone: +1-(505)-665-7832. Fax: +1-(505)-665-4955. e-mail: jkaszuba{at}lanl.gov
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REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONEXPERIMENTAL METHODSEXPERIMENTAL RESULTSANALYTICAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
|---|
Albarède, F. & Provost, A. (1977). Petrological and geochemical mass-balance equations: an algorithm for least-square fitting and general error analysis. Computers and Geosciences 3, 309–326.
Allen, J. C. & Boettcher, A. L. (1978). Amphiboles in andesite and basalt: II. Stability as a function of P–T–fH2O–fO2. American Mineralogist 63, 1074–1087.[Abstract]
Allen, J. C., Boettcher, A. L. & Marland, G. (1975). Amphiboles in andesite and basalt: I. Stability as a function of P–T–fO2. American Mineralogist 60, 1069–1085.
Armstrong, J. T. (1989). CITZAF: Combined ZAF and phi–rho (Z) electron beam correction programs. Pasadena, CA: California Institute of Technology.
Bailey, D. K. (1980). Volcanism, earth degassing and replenished lithosphere mantle. Philosophical Transactions of the Royal Society of London, Series A 297, 309–322.
Bailey, D. K. & Macdonald, R. (1987). Dry peralkaline felsic liquids and carbon dioxide flux through the Kenya rift zone. In: Mysen, B. O. (ed.) Magmatic Processes: Physicochemical Principles. Geochemical Society Special Publication 1, 91–105.
Baker, B. H. (1987). Outline of the petrology of the Kenya rift alkaline province. In: Fitton, J. G. & Upton, B. G. J. (eds) Alkaline Igneous Rocks. Geological Society, London, Special Publication 30, 293–311.
Barker, F. (1979). Trondhjemite: definition, environment and hypothesis of origin. In: Barker, F. (ed.) Trondhjemites, Dacites and Related Rocks. Amsterdam: Elsevier, pp. 1–12.
Beard, J. S. & Lofgren, G. E. (1991). Dehydration melting and water-saturated melting of basaltic and andesitic greenstones and amphibolites at 1, 3, and 6·9 kbar. Journal of Petrology 32, 365–401.
Bellieni, G., Brotzu, P., Morbidelli, L., Piccirillo, E. M. & Traversa, G. (1986). Petrology and mineralogy of Miocene fissural volcanism of the East Kenya Plateau. Neues Jahrbuch für Mineralogie, Abhandlungen 154, 153–178.[Web of Science]
Blank, J. G. & Brooker, R. A. (1994). Experimental studies of carbon dioxide in silicate melts: solubility, speciation, and stable carbon isotope behavior. In: Carroll, M. R. & Holloway, J. R. (eds) Volatiles in Magmas. Mineralogical Society of America, Reviews in Mineralogy 30, 157–186.
Boyd, F. R. & England, J. L. (1960). Apparatus for phase-equilibrium measurements at pressures up to 50 kilobars and temperatures up to 1750°C. Journal of Geophysical Research 65, 741–748.
Carmichael, I. S. E. (1967). The iron–titanium oxides of salic volcanic rocks and their associated ferromagnesian silicates. Contributions to Mineralogy and Petrology 14, 36–64.
Carrington, D. P. & Harley, S. L. (1995). Partial melting and phase relations in high-grade metapelites: an experimental petrogenetic grid in the KFMASH system. Contributions to Mineralogy and Petrology 120, 270–291.[Web of Science]
Deer, W. A., Howie, R. A. & Zussman, J. (1966). An Introduction to the Rock-forming Minerals. London: Longman, pp. 424–433.
Downes, H. (1993). The nature of the lower continental crust of Europe: petrological and geochemical evidence from xenoliths. Physics of the Earth and Planetary Interiors 79, 195–218.[Web of Science]
Dyar, M. D., Mackwell, S. J., McGuire, A. V., Cross, L. R. & Robertson, J. D. (1993). Crystal chemistry of Fe3+ and H+ in mantle kaersutite: implications for mantle metasomatism. American Mineralogist 78, 968–979.[Abstract]
Ellis, D. J. & Thompson, A. B (1986). Subsolidus and partial melting reactions in the quartz-excess CaO + MgO + Al2O3 + SiO2 + H2O system under water-excess and water-deficient conditions to 10 kb: some implications for the origin of peraluminous melts from mafic rocks. Journal of Petrology 27, 91–121.
Fujinawa, A. & Green, T. H. (1997). Partitioning behavior of Hf and Zr between amphibole, clinopyroxene, garnet and silicate melts at high pressure. European Journal of Mineralogy 9, 379–391.
Griffin, W. L., Carswell, D. A. & Nixon, P. H. (1979). Lower-crustal granulites and eclogites from Lesotho, Southern Africa. In: Boyd, F. R. & Meyer, H. O. A. (eds) The Mantle Sample: Inclusions in Kimberlites and other Volcanics. Proceedings of the Second International Kimberlite Conference, 2. Washington, DC: American Geophysical Union, pp. 59–86.
Hacker, B. R. (1990). Amphibolite-facies-to-granulite-facies reactions in experimentally deformed, unpowdered amphibolite. American Mineralogist 75, 1349–1361.[Abstract]
Hay, D. E. & Wendlandt, R. F. (1995). The origin of Kenya Rift Plateau-type flood phonolites: results of high-pressure/high-temperature experiments in the systems phonolite–H2O and phonolite–H2O–CO2. Journal of Geophysical Research 100, 401–410.
Hay, D. E., Wendlandt, R. F. & Keller, G. R. (1995a). Origin of Kenya Rift Plateau-type flood phonolites: integrated petrologic and geophysical constraints on the evolution of the crust and upper mantle beneath the Kenya Rift. Journal of Geophysical Research 100, 10549–10557.
Hay, D. E., Wendlandt, R. F. & Wendlandt, E. D. (1995b). The origin of Kenya Rift Plateau-type flood phonolites: evidence from geochemical studies for fusion of lower crust modified by alkali basaltic magmatism. Journal of Geophysical Research 100, 411–422.
Helz, R. T. (1976). Phase relations of basalts in their melting ranges at PH2O = 5 kb: Part II. Melt compositions. Journal of Petrology 17, 139–193.
Holloway, J. R. & Burnham, C. W. (1972). Melting relations of basalt with equilibrium water pressure less than total pressure. Journal of Petrology 13, 1–29.
Jambon, A. (1994). Earth degassing and large-scale geochemical cycling of volatile elements. In: Carroll, M. R. & Holloway, J. R. (eds) Volatiles in Magmas. Mineralogical Society of America, Reviews in Mineralogy 30, 479–517.[Abstract]
Johnston, A. D. & Wyllie, P. J. (1988). Constraints on the origin of Archean trondhjemites based on phase relationships of Nuk gneiss with H2O at 15 kbar. Contributions to Mineralogy and Petrology 100, 35–46.[Web of Science]
Le Bas, M. J., Le Maitre, R. W., Streckeisen, A. & Zanettin, B. (1986). A chemical classification of volcanic rocks based on the total alkali–silica diagram. Journal of Petrology 27, 745–750.
Leake, B. E. (1968). A catalog of analyzed calciferous and subcalciferous amphiboles together with their nomenclature and associated minerals. Geological Society of America, Special Paper 98, 210 pp.
Leake, B. E. (Chairman) (1997). Nomenclature of amphiboles: Report of the Subcommittee on Amphiboles of the International Mineralogical Association Commission on New Minerals and Mineral Names. Mineralogical Magazine 61, 295–321.[Abstract]
Loock, G., Stosch, H.-G. & Seck, H. A. (1990). Granulite facies lower crustal xenoliths from the Eifel, West Germany: petrological and geochemical aspects. Contributions to Mineralogy and Petrology 105, 25–41.[Web of Science]
Luth, R. W. (1989). Natural versus experimental control of oxidation states: effects on the composition and speciation of C–H fluids. American Mineralogist 74, 50–57.[Abstract]
Morgan, G. B., VI & London, D. (1996). Optimizing the electron microprobe analysis of hydrous alkali aluminosilicate glasses. American Mineralogist 81, 1176–1185.[Abstract]
Morimoto, N., Fabries, J., Ferguson, A. K., Ginzburg, I. V., Ross, M., Seifert, F. A., Zussman, J., Aoki, K. & Gottardi, G. (1988). Nomenclature of pyroxenes. American Mineralogist 73, 1123–1133.[Abstract]
Nono, A., Déruelle, B., Demaiffe, D. & Kambou, R. (1994). Tchabal Nganha volcano in Adamawa (Cameroon): petrology of a continental-alkaline lava series. Journal of Volcanology and Geothermal Research 60, 147–178.[Web of Science]
Okrusch, M., Schröder, B. & Schnütgen, A. (1979). Granulite-facies metabasite ejecta in the Laacher See area, Eifel, West Germany. Lithos 12, 251–270.[Web of Science]
Patiño Douce, A. E. (1996). Effects of pressure and H2O content on the compositions of primary crustal melts. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 11–21.[Web of Science]
Patiño Douce, A. E. & Beard, J. S. (1995). Dehydration melting of biotite gneiss and quartz amphibolite from 3 to 15 kbar. Journal of Petrology 36, 707–738.
Patiño Douce, A. E. & Beard, J. S. (1996). Effects of P, f(O2) and Mg/Fe ratio on dehydration melting of model metagreywackes. Journal of Petrology 37, 999–1024.
Patiño Douce, A. E. & Johnston, A. D. (1991). Phase equilibria and melt productivity in the pelitic system: implications for the origin of peraluminous granitoids and aluminous granulites. Contributions to Mineralogy and Petrology 107, 202–218.[Web of Science]
Popp, R. K. & Bryndzia, L. T. (1992). Statistical analysis of Fe3+, Ti, and OH in kaersutite from alkalic igneous rocks and mafic mantle xenoliths. American Mineralogist 77, 1250–1257.[Abstract]
Popp, R. K., Virgo, D. & Phillips, M. W. (1995a). H deficiency in kaersutitic amphiboles: experimental verification. American Mineralogist 80, 1347–1350.[Abstract]
Popp, R. K., Virgo, D., Yoder, H. S., Jr, Hoering, T. C. & Phillips, M. W. (1995b). An experimental study of phase equilibria and Fe oxy-component in kaersutitic amphibole: implications for the fH2 and aH2O in the upper mantle. American Mineralogist 80, 534–548.[Abstract]
Rameshwar Rao, D. & Subba Rao, T. V. (1996). AMPH: a program for calculating formulae and for assigning names to the amphibole group of minerals. Computers and Geosciences 22, 931–933.
Rapp, R. P. & Watson, E. B. (1995). Dehydration melting of metabasalt at 8–32 kbar: implications for continental growth and crust–mantle recycling. Journal of Petrology 36, 891–931.
Rapp, R. P., Watson, E. B. & Miller, C. F. (1991). Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites. Precambrian Research 51, 1–25.[Web of Science]
Rivalenti, G., Garuti, G., Rossi, A., Siena, F. & Sinigoi, S. (1981). Existence of different peridotite types and of a layered igneous complex in the Ivrea Zone of the Western Alps. Journal of Petrology 22, 127–153.
Roeder, P. L. & Emslie, R. F. (1970). Olivine–liquid equilibrium. Contributions to Mineralogy and Petrology 29, 275–289.[Web of Science]
Rudnick, R. L. & Taylor, S. R. (1987). The composition and petrogenesis of the lower crust: a xenolith study. Journal of Geophysical Research 92, 13981–14005.
Rushmer, T. (1991). Partial melting of two amphibolites: contrasting experimental results under fluid-absent conditions. Contributions to Mineralogy and Petrology 107, 41–59.[Web of Science]
Rutter, M. J. & Wyllie, P. J. (1988). Melting of vapour-absent tonalite at 10 kbar to simulate dehydration-melting in the deep crust. Nature 331, 159–160.
Sen, C. & Dunn, T. (1994). Dehydration melting of a basaltic composition amphibolite at 1·5 and 2·0 GPa: implications for origin of adakites. Contributions to Mineralogy and Petrology 117, 394–409.[Web of Science]
Sinigoi, S., Quick, J. E., Clemens-Knott, D., Mayer, A., Demarchi, G., Mazzucchelli, M., Negrini, L. & Rivalenti, G. (1994). Chemical evolution of a large mafic intrusion in the lower crust, Ivrea–Verbano Zone, northern Italy. Journal of Geophysical Research 99, 21575–21590.
Skjerlie, K. P. & Johnston, A. D. (1992). Vapor-absent melting at 10 kbar of a biotite- and amphibole-bearing tonalitic gneiss: implications for the generation of A-type granites. Geology 20, 263–266.
Skjerlie, K. P. & Johnston, A. D. (1993). Fluid-absent melting behaviour of an F-rich tonalitic gneiss at mid-crustal pressures: implications for the generation of anorogenic granites. Journal of Petrology 34, 785–815.
Skjerlie, K. P. & Johnston, A. D. (1996). Vapour-absent melting from 10 to 20 kbar of crustal rocks that contain multiple hydrous phases: implications for anatexis in the deep to very deep continental crust and active continental margins. Journal of Petrology 37, 661–691.
Skjerlie, K. P. & Patiño Douce, A. E. (1995). Anatexis of interlayered amphibolite and pelite at 10 kbar: effect of diffusion of major components on phase relations and melt fraction. Contributions to Mineralogy and Petrology 122, 62–78.[Web of Science]
Skjerlie, K. P., Patiño Douce, A. E. & Johnston, A. D. (1993). Fluid absent melting of a layered crustal protolith: implications for the generation of anatectic granites. Contributions to Mineralogy and Petrology 114, 365–378.[Web of Science]
Spear, F. S. (1980). NaSi exchange equilibrium between plagioclase and amphibole: an empirical model. Contributions to Mineralogy and Petrology 72, 33–41.[Web of Science]
Spear, F. S. & Kimball, C. (1984). RECAMP—a FORTRAN IV program for estimating Fe3+ contents in amphiboles. Computers and Geoscience 10, 317–325.
Springer, W. & Seck, H. A. (1997). Partial fusion of basic granulites at 5 to 15 kbar: implications for the origin of TTG magmas. Contributions to Mineralogy and Petrology 127, 30–45.[Web of Science]
Stout, J. H. (1972). Phase petrology and mineral chemistry of coexisting amphiboles from Telemark, Norway. Journal of Petrology 13, 99–145.
Thompson, A. B. (1982). Dehydration melting of pelitic rocks and the generation of H2O-undersaturated granitic liquids. American Journal of Science 282, 1567–1595.
Vielzeuf, D. & Montel, J. M. (1994). Partial melting of metagraywackes: Part I. Fluid-absent experiments and phase relationships. Contributions to Mineralogy and Petrology 117, 375–393.[Web of Science]
Wendlandt, R. F., Baldridge, W. S. & Neumann, E.-R. (1991). Modification of lower crust by continental rift magmatism. Geophysical Research Letters 18, 1759–1762.[Web of Science]
Winther, K. T. (1996). An experimentally based model for the origin of tonalitic and trondhjemitic melts. Chemical Geology 127, 43–59.[Web of Science]
Winther, K. T. & Newton, R. C. (1991). Experimental melting of hydrous low-K tholeiite: evidence on the origin of Archaean cratons. Bulletin of the Geological Society of Denmark 39, 213–228.[Web of Science]
Wolf, M. B. & Wyllie, P. J. (1991). Dehydration-melting of solid amphibolite at 10 kbar: textural development, liquid interconnectivity and applications to the segregation of magmas. Mineralogy and Petrology 44, 151–179.[Web of Science]
Wolf, M. B & Wyllie, P. J. (1994). Dehydration-melting of solid amphibolite at 10 kbar: the effects of temperature and time. Contributions to Mineralogy and Petrology 115, 369–383.[Web of Science]
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