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Journal of Petrology 45(7) © Oxford University Press 2004; all rights reserved
Partial Melting of Spinel Lherzolite in the System CaO–MgO–Al2O3–SiO2 ± K2O at 1·1 GPa
RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, CANBERRA, A.C.T. 0200, AUSTRALIA
RECEIVED MAY 7, 2003; ACCEPTED JANUARY 22, 2004
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The compositions of multiply saturated partial melts are valuable for the thermodynamic information that they contain, but are difficult to determine experimentally because they exist only over a narrow temperature range at a given pressure. Here we try a new approach for determining the composition of the partial melt in equilibrium with olivine, orthopyroxene, clinopyroxene and spinel (Ol + Opx + Cpx + Sp + Melt) in the system CaO–MgO–Al2O3–SiO2 (CMAS) at 1·1 GPa: various amounts of K2O are added to the system, and the resulting melt compositions and temperature are extrapolated to zero K2O. The ‘sandwich’ experimental method was used to minimize problems caused by quench modification, and Opx and Cpx were previously synthesized at conditions near those of the melting experiments to ensure they had appropriate compositions. Results were then checked by reversal crystallization experiments. The results are in good agreement with previous work, and establish the anhydrous solidus in CMAS to be at 1320 ± 10°C at 1·1 GPa. The effect of K2O is to depress the solidus by 5·8°C/wt %, while the melt composition becomes increasingly enriched in SiO2, being quartz-normative above 4 wt % K2O. Compared with Na2O, K2O has a stronger effect in depressing the solidus and modifying melt compositions. The isobaric invariant point in the system CMAS–K2O at which Ol + Opx + Cpx + Sp + Melt is joined by sanidine (San) is at 1240 ± 10°C. During the course of the study several other isobaric invariant points were identified and their crystal and melt compositions determined in unreversed experiments: Opx + Cpx + Sp + An + Melt in the system CMAS at 1315 ± 10°C; in CMAS–K2O, Opx + Cpx + Sp + An + San + Melt at 1230 ± 10°C and Opx + Sp + An + San + Sapph + Melt at 1230 ± 10°C, where An is anorthite and Sapph is sapphirine. Coexisting San plus An in three experiments help define the An–San solvus at 1230–1250°C.
KEY WORDS: feldspar solvus; igneous sapphirine; mantle solidus; partial melting; systems CMAS and CMAS–K2O
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The simplest chemical system that contains all the major phases of peridotitic upper mantle (olivine, orthopyroxene, clinopyroxene, and an aluminous phase, one of plagioclase, spinel or garnet, depending on pressure) is CaO–MgO–Al2O3–SiO2 (CMAS). This simple system has been used to investigate the basic phase relations of partial melting in the upper mantle (O'Hara, 1968
; Kushiro, 1972; Presnall et al., 1979; Longhi, 1987; Gudfinnsson & Presnall, 1996; Herzberg & Zhang, 1998; Milholland & Presnall, 1998; Presnall, 1999; Liu & Presnall, 2000). However, with a typical four-phase lherzolite assemblage, the initial melting in CMAS is isobarically invariant; that is, the chemical potentials of all four components (CaO, MgO, Al2O3 and SiO2) in the melt are completely defined, as in
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Because all chemical potentials are known, the composition of the melt in equilibrium with all four solid phases is particularly valuable in constructing thermodynamic models of silicate melts. Unfortunately, the isobarically invariant melt composition is difficult to determine experimentally, because the five-phase assemblage exists only over an infinitely narrow temperature interval at a given pressure.
The traditional way around this problem is to bracket the isobaric invariant point by varying the experimental starting composition, to produce four-phase isobarically univariant assemblages (i.e. three solid phases plus melt as in Fo + Opx + Sp + Melt). Not only does this require a large number of experiments, but it is tedious to demonstrate equilibrium (Presnall et al., 1978; Liu & Presnall, 1990), as the solid phases (especially the pyroxenes) are multicomponent solid solutions with the potential to be compositionally unequilibrated. This method also requires some extrapolation to the isobaric invariant point, as this point is only bracketed.
Alternatively, it is possible to rely on experimental imperfections, such as temperature gradients or chemical impurities (e.g. some Na2O in the starting material or H2 diffusing through the Pt capsule to produce H2O in the experimental charge) to increase the effective variance of the system and obtain melt in equilibrium or quasi-equilibrium with four solid phases, which can then be analysed directly. This approach has been adopted by Presnall and colleagues (Presnall et al., 1979; Gudfinnsson & Presnall, 1996; Milholland & Presnall, 1998; Liu & Presnall, 2000). Although this seems empirically to work well for the system CMAS, this may not always be the case, and we have encountered insuperable difficulties in applying this approach to determining the effect of Cr2O3 on melting in the system CMAS–Cr2O3. Relying on semi-controlled imperfections to obtain a result is also intellectually unsatisfying, and the question arises of how much the imperfections affect results.
In this paper we report a logical improvement to this approach, which is to introduce another component into the system deliberately. The system is then studied as a function of the controlled concentration of this component, such that the composition of the CMAS isobaric invariant melt can be obtained by extrapolating to zero concentration of the added component. We chose K2O as the additional component (the K2O-method hereafter), as it is almost completely incompatible in all the solid phases (Ol, Opx, Cpx, Sp) at our chosen experimental pressure of 1·1 GPa. Having the extra component entering only the melt phase makes the extrapolation to the pure CMAS system simple. Also, K2O is an important constituent in several mantle-derived magma types (kimberlites, shoshonites, and so on), so that our data for the system CMAS–K2O generated as a by-product of our main purpose should be of some direct petrological relevance.
We show that the K2O-method produces a result in good agreement with the previous work of Presnall and colleagues (Presnall, 1976; Presnall et al., 1979; Walter & Presnall, 1994), and our improved accuracy allows for some refinement of this work.
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EXPERIMENTAL AND ANALYTICAL TECHNIQUES |
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Four types of experiments were performed:
- conventional direct partial melting experiments (DEs) in the system CMAS;
- sandwich experiments using the K2O-method (KEs), in which the solid assemblage Fo + Opx + Cpx + Sp was placed at the top and bottom of the capsule, with a K2O-containing melt composition as the ‘filling’;
- reversal experiments (REs-1) to study the crystallization of the CMAS melt composition deduced from the KEs;
- reversal experiments (REs-2) with Fo added; all but one of these (C-1789) used the sandwich configuration.
Starting materials
Table 1 summarizes the starting materials used in this study. Compositions were made from high-purity oxides (SiO2, Al2O3 and MgO) and carbonates (CaCO3 and K2CO3). An important strategy was to use pre-synthesized Opx and Cpx with compositions similar to those expected in the run products.
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Mix9 was prepared by combining a crystalline mixture of pure forsterite (Fo) and a crystalline mixture of Sp + Opx + Cpx in a proportion of 1:5 by weight. The Fo was synthesized at 1400°C and atmospheric pressure for 69 h; the mixture with Sp:Opx:Cpx in weight proportions of 1:2:2 was made in a 5/8 inch piston-cylinder press using a 3·5 mm diameter Pt capsule and a talc–Pyrex assembly at 1280°C and 1·1 GPa for 48 h. The phase proportion in the final Mix9 was close to 1:1:2:2 (Fo:Sp:Opx:Cpx). Crystalline mixture SEM02-1 containing Fo:Sp:Opx: Cpx = 1:1:1:1 (by weight) was made by crystallizing the decarbonated oxide mix at 1300°C and 1·1 GPa for 48 h in a Pt capsule. Mixtures SEM02-3, SEM02-4, SEM02-8 and SEM02-6 were melted at 1400°C and 1 atm for 20 min and then quenched to glass.
All mixtures were checked for compositions with electron microprobe (Ware, 1991). Mix9 was later stored in an oven at 110°C and other starting materials were stored in another oven at 150°C.
Piston-cylinder assemblies
All experiments were made in a conventional half-inch piston-cylinder apparatus (Boyd & England, 1960). The salt–Pyrex pressure assembly used in the preliminary DEs was described by Klemme & O'Neill (1997), except that, because of the generally lower pressure and temperature regime of this study, fired pyrophyllite replaced the MgO spacer underneath the capsule and mullite was used instead of high-purity alumina for the thermocouple tube. Further modification for the assembly used in the KEs and the REs were made because of possible H2 diffusion into the Pt capsule causing contamination of the runs by H2O. In these experiments the Pt capsule was held in an Fe2O3 sleeve, which was in turn surrounded by an alumina sleeve. At each end of the alumina sleeve, a ruby disc (0·5 mm thick) separated the Fe2O3 sleeve from other parts of the assembly. This structure prevents the Fe2O3 sleeve from being reduced by the graphite heater, or contaminating the thermocouple. Alumina spacers and then MgO spacers were positioned next to both ruby discs, to enhance the mechanical stability of the assembly. The thermocouple was protected by a combination of high-purity alumina tubing in the hot part of the assembly (
4 mm long), followed by mullite tubing above this. The Fe2O3 sleeve was made by cold pressing in a steel die and then sintered at atmospheric pressure and 850°C for 3 h, using acetone as a binder.
These salt–Pyrex assemblies have low friction and no pressure correction is required, considering the high temperatures and long run times used in this study (Green et al., 1966; Bose & Ganguly, 1995; Klemme & O'Neill, 1997).
Experimental procedures
For each experiment, 8–10 mg starting materials were loaded into a Pt capsule. The capsules used in the DEs were stored at 110°C for 6–8 h before welding. The Pt capsules for the KEs and REs were stored at 150°C for 6–8 h, then held in a steel block that had been pre-heated to 750°C while they were welded (Robinson et al., 1998).
All experiments were performed using the ‘piston-out’ method, i.e. the pressure was first raised to several MPa, then the temperature was raised to c. 450°C to soften the Pyrex sleeve. The pressure was then increased to c. 0·05 GPa higher than the desired pressure, the temperature was raised to the nominal temperature of the run, and finally the pressure was lowered to the required pressure (Johannes et al., 1971). Pressures were continuously monitored and adjusted, if necessary, which allowed each run to be controlled within ±0·02 GPa of the nominal pressure. Temperature was measured and controlled with Pt94Rh6–Pt70Rh30 thermocouple (type B), previously calibrated against the melting point of gold at atmospheric pressure; possible pressure effects on the e.m.f. of the thermocouple were neglected. The tip of the thermocouple, the upper ruby disc and the whole Pt capsule containing the experimental charge were all carefully placed in the 5-mm-long hot spot of the experimental assembly. Although temperature during experiments was controlled to ±1°C, the true temperature uncertainties are estimated to be c. ±5–10°C, mainly because of slight differences in sample position and power density through the graphite heater. Such an uncertainty is consistent with the variations in melt composition and other experimental parameters observed in these experiments. A discussion of the superior merits of type B thermocouples over other Pt/Rh thermocouples or W/Re thermocouples at temperatures near 1300°C has been given by Klemme & O'Neill (2000b).
Analytical methods
At the end of a run, the sample was sectioned longitudinally, mounted in epoxy and polished using a series of diamond pastes. Run products were carbon-coated and analysed on a Cameca electron microprobe at the Research School of Earth Sciences (RSES), ANU and/or on a JEOL 6400 scanning electron microprobe in energy dispersive mode (EDS) at the Electron Microprobe Unit (EMU) at ANU. Coexisting phases in all run products were identified by back-scattered electron imaging. Beam current was 1 nA, accelerating voltage was 15 keV and ZAF correction was applied in all analyses (Ware, 1991). A beam spot size of 1 µm was used for all crystalline phases, and both 1 µm and 10 µm beam spot sizes were used for glass analyses. Calibration was based on optimization to a large number of standards; to check this calibration and also as a monitor of any drift between analysing sessions, we repeatedly analysed three glass standards, GOR132G, T1G, and KL2G, which have comparable compositions to the phases present in this study (Jochum et al., 2000). The 185 analyses over 24 analytical sessions are summarized in Table 2. There is excellent agreement between our electron probe analyses and the recommended values.
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Besides the major oxides CaO, MgO, Al2O3, SiO2, and K2O expected in the run products, we also routinely analysed for FeO, Cr2O3, and Na2O in all phases. Either diffusion of Fe through the Pt capsule from the Fe2O3 sleeve or a leak in the capsule could result in Fe contamination. Na2O appears to be a good indicator of the purity of the starting materials. It concentrates in the melt, and 0·1–0·3 wt % was found in the glasses in this study, depending on the proportion of melt to solid in the experiment. Such a concentration is close to the limit of detection by EDS, and has been ignored as it has a minimal effect on the phase relations (Walter & Presnall, 1994
; Liu et al., in preparation). Lithium and boron have potentially been cryptic contaminants of previous piston-cylinder studies, as only recently have these elements become amenable to microbeam analysis. Li salts are sometimes used in noble-metal fabrication processes, and B could come from the Pyrex glass (it apparently diffuses readily through Pt). To check for any such contamination, Li and B were measured in the glasses in runs C-1422, C-1448 and C-1565 (see Table 3 for experimental conditions) by laser ablation–inductively coupled plasma–mass spectrometry at RSES, ANU. The Li contents were 4·1 ± 0·2 ppm, 4·1 ± 0·1 ppm and 2·8 ± 0·4 ppm, respectively. The B contents were slightly higher at 53 ± 2, 54 ± 2 and 52·3 ± 7 ppm. We assume these levels have a negligible effect on phase relations.
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Fe2O3 sleeve and H2O contamination
Following Robinson et al. (1998)
, Fe2O3 sleeves were used as H2-getters surrounding the Pt capsule in all experiments apart from the first few reported in this study (the DEs). The idea is that an oxidized substance such as Fe2O3 should react with H2, which may be produced in the piston-cylinder assembly by contaminant moisture reacting with the graphite heater, for example. The getter should minimize the ingress of H2 into the Pt capsule by diffusion, where it can react with silicates (e.g. alloying some Si into the Pt in FeO-free systems), thus forming H2O. According to Robinson et al. (1998), the use of Fe2O3 sleeves coupled with careful drying of capsules before and during welding can reduce the H2O content in experimental melts from 1 wt % to 0·1 wt %. The Fe2O3 sleeves after two experiments (C-1417 and C-1566; see Table 3 for experimental conditions) were checked by powder X-ray diffraction at the Geology Department, ANU and were found to be >99 wt % Fe2O3. This does not mean that the sleeves were unnecessary, however, as analysis of the Pt capsules (using the Cameca electron microprobe in wavelength-dispersive mode at RSES, ANU) showed an Fe diffusion gradient. An example is given in Fig. 1 (1340°C, 1·1 GPa, 72 h). Whereas Fe metal reached the middle point of the capsule wall, the experimental charge remained untouched. Longer run times or higher temperatures would result in Fe reaching the inner wall of the capsule, hence the need to analyse Fe routinely in run products.
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C-1621 contains only melt (see Table 3 for experimental conditions) and is ideal for using transmission infrared spectroscopy to determine the water content. Details of this procedure as implemented in this study will be reported by Liu et al. (in preparation). The average water content from seven analyses is 120 ppm, which is substantially lower than the 0·1% reported by Robinson et al. (1998)
and the 1100 ppm by Falloon et al. (1999); Baker et al. (1996) and Falloon et al. (2001) estimated an even larger amount, 0·5–1%, in their nominally anhydrous experiments. If the same total of H2O as in run C-1621 were concentrated into the melt phase in those runs with smaller proportions of melt, the H2O concentrations might be higher than in C-1621, but it seems unlikely that levels of H2O in any run with the Fe2O3 sleeves would exceed 0·1%.
Attainment of equilibrium
There is a fundamental dilemma to be addressed in designing experiments aimed at obtaining the compositions of partial melts at modest degrees of melt fraction. To optimize equilibration between melt and crystals, it is obviously advantageous to have the melt distributed between crystals, to maximize mutual contacts. This is the usual texture formed in simple experiments (the DEs of this study) with less than about 20–30% melt, in the absence of a temperature gradient. However, maximizing the contact between melt and crystals also maximizes the probability of quench modification by precipitation on the rims of crystals. In addition, melt so distributed is difficult to analyse accurately, as melt pockets are small and irregularly shaped. This latter problem becomes worse as the melt fraction decreases. The experimental solution to these problems has been to devise ways of separating the melt from the crystals, as in sandwich experiments (as in this study and Stolper, 1980; Takahashi & Kushiro, 1983; Fujii & Scarfe, 1985; Falloon & Green, 1987, 1988; Robinson et al., 1998) or by extraction into diamond aggregates or similar (Baker et al., 1992; Johnson & Kushiro, 1992; Kushiro & Hirose, 1992; Hirose & Kawamoto, 1995; Hirose, 1997). Another possible tactic is to exploit the temperature gradient present in many high-pressure experiments, particularly in multi-anvil experiments, to separate melt towards the hot end of the capsule (Takahashi, 1986). However, the inevitable consequence of any separation is to impair the chances of attaining equilibrium between melt and crystals.
The problems that this can cause are well illustrated in this study by two experiments, C-1580 and C-1576 (Table 3; see Fig. 2a), in which the initial composition chosen for the ‘filling’ part of the sandwich turned out not to be in equilibrium with the Fo + Opx + Cpx + Sp assemblage at the ends of the sandwich (the ‘bread’), despite the long run times used in these experiments. Instead, the run products consisted of three zones: (1) the initial Fo + Opx + Cpx + Sp assemblage, which in these two runs is unequilibrated with respect to the pyroxene compositions (particularly Opx; see Table 4); (2) the ‘filling’ part, comprising melt plus An + Sapph + San (C-1580) or, in the lower-temperature experiment (C-1576), melt plus An + Sapph + San + Sp + Opx, the latter of which has a very different composition from the Opx in the Fo + Opx + Cpx + Sp assemblage; (3) a narrow reaction zone of Opx, no more than 20 µm wide, between the two other zones (see below). These textures are shown in Fig. 2a; for comparison, the texture developed in a successfully equilibrated sandwich experiment is given in Fig. 2b. The conspicuous inability of the melts in the ‘filling’ to re-equilibrate with the solid Fo + Opx + Cpx + Sp assemblage in these two runs may be due to their high SiO2, and hence high viscosity.
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The large difference in the composition of Opx in the different zones of C-1576 is instructive. Because the composition of a solid solution phase depends on its chemical environment, it is unlikely to be the same in the two different parts of a sandwich experiment unless these parts are in equilibrium. We therefore paid particular attention in this study to analysing pyroxene compositions in the ‘filling’ part of the sandwich experiment, to check that they were indeed the same within analytical error as in the ‘bread’ layers. Some crystallization in the ‘filling’ part is inevitable unless the initial glass composition is exactly that expected at equilibrium; in fact, as regards attainment of equilibrium, an absence of crystals in the ‘filling’ would be an ambiguous result, in the same way that ‘no reaction’ in the determination of a subsolidus univariant reaction indicates either full equilibrium at the P–T condition or merely sluggish reaction kinetics.
Clearly, a key aspect of any partial melting experiment in which the melt is separated from the crystals is demonstration of equilibrium between melt and crystals. We shall point to some other runs where we believe such equilibrium was not obtained, based on the following criteria (as discussed above): (1) careful examination of the interface between the melt-rich and the crystal-rich parts; (2) comparison of compositions of solid-solution phases in the two parts. On the basis of these criteria, equilibrium was obtained in the majority of the sandwich experiments reported here. Application of these criteria may not be possible in diamond-aggregate or similar types of melt extraction experiments, in which case the results from these experiments must be viewed with a degree of caution.
Several lines of evidence indicate that local equilibrium was approached closely in the experiments of this study. In addition to being the same in different parts of the sandwich, the compositions of Opx and Cpx are generally fairly homogeneous: the most heterogeneous oxide component, Al2O3, in the most heterogeneous phase, Cpx, generally has a variation of <0·5 wt % one standard deviation, which is comparable with that observed in other simple-system experiments. The amounts of Al2O3 in both Opx and Cpx show small but smooth variations with temperature, and are in good agreement with the results of previous studies (see below). Similarly, the average difference between temperatures calculated from the Ca-geothermometer of Nickel et al. (1985) and the nominal experimental temperatures is only 23°C, consistent with equilibrium Ca exchange between Opx and Cpx.
The achievement of local equilibrium in this work is expected because the duration of our experiments is generally longer than that used in previous comparable studies. The results of previous near-liquidus experiments in the system CMAS (Presnall et al., 1978; Sen & Presnall, 1984; Liu & Presnall, 1990) suggested that a period of several hours is all that is required to establish reversals of phase boundaries. Walter & Presnall's (1994) experiments in the system CMAS–Na2O found that 48 h was long enough for the attainment of equilibrium at solidus temperatures. Most of our run durations were longer than this.
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EXPERIMENTAL RESULTS |
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Table 3 summarizes the starting materials, the run conditions, the techniques used, and the results in terms of phases present, for each experiment. The compositions of these phases are given in Table 4. Calculated temperatures from the two-pyroxene geothermometer of Nickel et al. (1985)
for those experiments containing both Opx and Cpx are shown in Table 3. There is a good correlation between calculated and experimental temperatures, although with a systematic offset towards the calculated temperatures being higher at low experimental temperatures, with a mean difference of 23°C (see Fig. 5).
The initial DEs bracket the solidus in the system CMAS between 1300°C (C-1555) and 1310°C at 1·1 GPa. The run at 1300°C contains a small amount of melt (too small to analyse). The melt may have been produced as a result of H2 diffusion into the capsule during the run, producing a trace of water by reducing SiO2 to Si, which alloys with Pt (Chen & Presnall, 1975). At slightly higher temperature (1310°C, C-1556), one phase (Cpx) disappears in agreement with the phase rule. We did not observe the four solid phases coexisting with melt over a range of temperature (20°), as did Presnall (1976) and Presnall et al. (1979). This is consistent with only small amounts of chemical impurities (such as H2O) in our run products, and minimal temperature gradients.
Fourteen experiments were carried out using the K2O-addition method (KEs). The full Sp-lherzolite assemblage coexisting with melt is observed in seven experiments between 1260 and 1310°C. The temperatures of these KEs are plotted against the K2O contents of the melts in Fig. 3. Within experimental uncertainty the relationship is linear, at least to 8 wt % K2O and 1270°C. Extrapolating K2O to zero gives a solidus temperature of 1319°C for the system CMAS. The change of melt composition with K2O is shown in Fig. 4. On a simple oxide weight percent (wt %) basis, SiO2 increases linearly with K2O, MgO and CaO both decrease linearly, and Al2O3 remains approximately constant, although in detail it appears to go up then down, with a maximum at 6 wt % K2O. The melt composition at the solidus in the CMAS system is found by extrapolation to zero K2O, as for the solidus temperature: we obtain (in wt %) SiO2 49·09, Al2O3 20·14, MgO 15·45 and CaO 15·33. The compositions of both Opx and Cpx at the CMAS solidus can be deduced similarly (see Table 5).
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We then tested these results by making up a glass with this composition and locating its liquidus and solidus (REs-1). The experiment at 1340°C was above the liquidus, whereas two runs at 1320°C (C-1639 and C-1565) crystallized Sp + Opx + Cpx ± An, with the residual melt being close to that in the starting material (Table 4). The compositions of the pyroxenes in C-1639 are in good agreement with the forward experiments, whereas those in C-1565, with a shorter run time and An present, have slightly higher Al2O3 (Table 4). These experiments confirm directly the peritectic nature of the melting reaction, as Fo was not crystallized. They also imply that the solidus of the phase assemblage Sp + Opx + Cpx + An is only slightly lower than that of the phase assemblage Fo + Sp + Opx + Cpx, in agreement with Kushiro (1972)
. Thus the run at 1310°C (C-1566) is almost completely crystallized. Although in one experiment (C-1565) melt coexists with four crystalline phases, the temperature interval over which this occurs is clearly very narrow (an infinitely narrow temperature interval is expected from the phase rule).
The REs-1 demonstrate empirically that simple crystallization experiments cannot in practice reverse the partial melting equilibrium when this is peritectic, as here (i.e. the melt is in reaction relationship with Fo). Accordingly, we tried reversal crystallization experiments with added Fo. These experiments (called REs-2) directly constrain the solidus of the Sp-lherzolite phase assemblage in the system CMAS at 1·1 GPa to be 1315 ± 10°C. The experiment at this temperature, C-1781, produced four crystalline phases plus melt; however, the melt composition is slightly less magnesian than the composition deduced from the KEs (Table 5), and is intermediate between this composition and that in equilibrium with Opx + Cpx + Sp + An. The Al2O3 content of the pyroxenes is also slightly higher than in other comparable runs. This raises the question of whether equilibrium between Fo and the ‘filling’ part of the sandwich was truly established. Clearly, equilibrium was not established in a similar experiment at the slightly lower temperature of 1310°C, C-1768; here the ‘filling’ crystallized completely to Sp + Opx + Cpx + An, with An separated from Fo by a narrow reaction zone of Opx + Cpx + Sp. The further implications of this experiment for the interpretation of results from melt-extraction experiments (such as the diamond-aggregate technique) will be discussed below.
Table 5 compares the result of this study with previous work in CMAS at similar pressures in the literature. There is a difference of 30°C between the solidus as determined here and the solidus defined by run 116-3 of Presnall (1976), which contains Fo + Opx + Cpx + Sp + Melt at 1350°C and 1·1 GPa. However, another run with the same starting composition reported by Presnall et al. (1979), their 122-8 at 1327°C and 1·1 GPa, contained Fo + Opx + Melt and is therefore above the solidus, which would be in agreement with our result. There is also good agreement as regards the solidus melt composition and the solidus pyroxene compositions. It may be noted that Presnall (1976) reported 8·4 ± 0·2 wt % Al2O3 for his Opx based on five analyses, but noted two outliers at 9·6 and 5·5 wt % Al2O3, indicating that his Opx was somewhat heterogeneous in composition. The higher amount (9·49 wt %) later reported by Walter & Presnall (1994) following re-analysis of this experiment may be due to the inclusion of high-Al2O3 outliers. The solidus temperature and melt composition reported by Kushiro (1972) at 1·0 GPa agree slightly less well, the main difference in melt composition being in CaO (Table 5). Kushiro (1972) also reported data for the phase assemblage Sp + Opx + Cpx + An + Melt; in agreement with this study, this isobaric invariant point occurs at essentially the same temperature at a melt composition only 1 wt % lower in MgO than Fo + Sp + Opx + Cpx + Melt.
We attempted to locate the isobaric invariant point in the system CMAS–K2O by exploring lower temperatures. An isobaric invariant point should exist at which sanidine joins Fo + Opx + Cpx + Sp in equilibrium with melt. Our first attempts consisted of two runs, C-1580 (1250°C) and C-1576 (1230°C), with the same initial melt composition SEM02-08 as used at 1260 and 1270°C. However, this melt composition crystallized An + Sapph + San at 1260°C, joined at 1250°C by Sp and Opx, the latter with very high Al2O3 (14 wt %). This assemblage is clearly out of equilibrium with the Fo + Opx + Cpx + Sp layers, with a narrow reaction zone of about 10–20 µm separating the two assemblages (see Fig. 2a).
With this experience, we tried again with a new initial melt composition (SEM02-10). This time equilibrium was approached throughout the charge, but Fo was reacted out, producing Sp + Opx + Cpx + An + Melt (with a trace of residual Fo) in C-1701 at 1240°C, and the isobarically invariant assemblage Sp + Opx + Cpx + An + San + Melt in C-1708 at 1230°C. A third attempt with another initial melt composition (SEM02-13) produced Sp + Opx + Cpx + San + Melt with a trace of residual Fo in C-1779 at 1250°C.
These and the higher-temperature results in the system CMAS–K2O are summarized in Fig. 6, a projection from Fo + Di onto the An–Or–Qz plane. Although we failed to locate the isobaric invariant point Fo + Sp + Opx + Cpx + San + Melt at 1·1 GPa directly, this point, which is eutectic-like on this projection, must occur at a melt composition between runs C-1585 at 1260°C and C-1779 at 1250°C, at a slightly lower temperature (i.e. 1240 ± 10°C).
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DISCUSSION |
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Compositions of Opx and Cpx
The compositions of both Opx and Cpx in the experimental run products vary in CaO and Al2O3 contents.
CaO in pyroxenes
The partitioning of CaO between Opx and Cpx is the basis of two-pyroxene geothermometry, the main means of estimating temperatures of equilibration in lherzolitic assemblages. To compare our data with previous work in the system CMAS, we have used the formulation of Nickel et al. (1985) to calculate temperatures from our experimental run products (Table 3 and Fig. 5a). The calculated temperatures are in good agreement with nominal experimental temperatures, although in detail there is a weak systematic tendency for the former to be higher at low experimental temperatures. The average difference between the calculated temperature and the nominal experimental temperature for the experiments reported here is 23° with a standard deviation of 17°. We also tested two other pyroxene geothermometers, equations (9) and (10) of Brey & Köhler (1990). Equation (9), which uses the exchange of Ca between Opx and Cpx, yields higher calculated temperatures, with an average difference of 65°C between the calculated temperatures and the experimental temperatures (Fig. 5b). Conversely, equation (10), which uses only the amount of Ca in Opx coexisting with Cpx, gives lower calculated temperatures, with an average difference of –39° (Fig. 5c). These equations were intended for use with chemically complex natural systems, but were formulated as simply as possible for ease of use. Figure 5b and c indicates that this simplicity may result in a loss of accuracy; consequently, more rigorous albeit tediously complicated two-pyroxene geothermometers could potentially give better results.
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Shown in Fig. 7 are differences between observed and calculated temperatures using the geothermometer of Nickel et al. (1985)
for experiments in the system CMAS at pressures from 1 atm to 3·4 GPa, from Longhi (1987), Walter & Presnall (1994), Gudfinnsson & Presnall (1996), Klemme & O'Neill (2000a), and this study (CMAS ± K2O). Similar diagrams using the two geothermometers of Brey & Köhler (1990), their equations (9) and (10), are shown in Fig. 8, with the addition of the CMAS–Na2O data from Walter & Presnall (1994). These plots raise a number of issues regarding potential problems with the experimental database in the CMAS system, in addition to the problem with the geothermometers. One consideration is the choice of thermocouple. The Nickel et al. (1985) and Brey & Köhler (1990) geothermometers are based on experiments by those workers using type B Pt/Rh thermocouples, as in this study, whereas the data from Presnall and co-workers were obtained using W/Re thermocouples. Thus the discrepancy with increasing pressure seen in Figs 7 and 8 may be due to an error in the pressure dependence of the Brey–Köhler geothermometers, or to differences in the effect of pressure on the e.m.f. of Pt/Rh thermocouples compared with W/Re thermocouples.
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A further complicating issue is the possibility of oxidation of W/Re thermocouples at lower pressures. To guard against this, Walter & Presnall (1994)
used a nitrogen flow at low pressures (
1·1 GPa) for their experiments in the system CMAS–Na2O, but this precaution does not appear to have been used in the earlier experiments of Presnall and co-workers in the system CMAS [re-analysed by Walter & Presnall (1994)]. Oxidation of W/Re thermocouples is thought not to be a problem at higher pressures, as collapse of the alumina tubing around the thermocouple wires should prevent entry of air. The calculated temperatures from the lower-pressure experiments with W/Re thermocouples with the N2 flow agree fairly well with the experiments using Pt/Rh thermocouples. However, earlier low-pressure experiments using W/Re thermocouples but without a N2 flow (system CMAS) have lower calculated temperatures, consistent with thermocouple drift caused by oxidation.
Al2O3 of pyroxenes in Sp-lherzolite
The alumina contents of Opx and Cpx in the Sp-lherzolite assemblage are controlled by the reactions
 | (5) |
 | (6) |
Previous work has shown that these equilibria are insensitive to pressure but strongly dependent on temperature, Al2O3 increasing with rise in temperature (Obata, 1976; Fujii, 1977; Danckwerth & Newton, 1978; Herzberg, 1978; Lane & Ganguly, 1980; Gasparik, 1984; Sen, 1985). Recently Klemme & O'Neill (2000a) re-fitted existing experimental data for reaction (5) together with their new data. The alumina contents of Opx coexisting with Fo + Sp in the systems CMAS and CMAS–K2O reported here agree well with this previous work (Fig. 9b), except for the data of Gudfinnsson & Presnall (1996), which plot at slightly lower values.
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and 
) vs inverse temperature for the Sp-lherzolite phase assemblage in the systems CMAS, CMAS–K2O and CMAS–Na2O. Data sources: WP94, Walter & Presnall (1994)The Al2O3 contents of Cpx show a very similar trend (Fig. 9a), but with a little more scatter. This extra scatter may be due to a slight pressure effect on this equilibrium, as seen in the experiments of Gasparik (1984)
. The partitioning of Al between Opx and Cpx does not vary with temperature (i.e. the temperature dependence of the Al isopleths is similar in Opx and Cpx), as demonstrated in Fig. 10. In experiments in the system CMAS–Na2O, the Cpx might be expected to have additional Al from a jadeite component (NaAlSi2O6). To test this, we also plotted (Fig. 10b) the data for the CMAS–Na2O system (Walter & Presnall, 1994) with the molar amount of Al associated with Na subtracted. Remarkably, this results in a greatly increased scatter in the plot, indicating that our expectation of increased Al associated with Na must be incorrect.
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Al2O3 in pyroxenes, particularly in Cpx, is always the most heterogeneous component in our experiments and anomalous Al2O3 contents are a sign of disequilibrium. An example is the subsolidus reversal experiment C-1789, in which the Al2O3 of the Cpx remains similar to that in the starting material, although the Opx in this run has changed its Al2O3 content to a lower value.
Al2O3 in pyroxenes of other phase assemblages
Reactions (5) and (6) show that, in the absence of Fo, the MgAl2SiO6 (MgTs) component in Opx and the CaAl2SiO6 (CaTs) component in Cpx increase. Therefore, the pyroxenes in the phase assemblage Sp + Opx + Cpx should have higher Al2O3 contents than those in the phase assemblage Fo + Opx + Cpx + Sp, as observed in our experiments (Fig. 11).
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A single governing reaction for the Al2O3 content in both pyroxenes coexisting with An + Sp can be written:
 | (7) |
The absence of either Sp or An would shift this reaction to the left-hand side so that MgTs in Opx and CaTs in Cpx decrease. Again, this is observed in our experiments (Fig. 11).
As the Al2O3 contents of pyroxenes depend on the identities of coexisting phases, the Al2O3 contents are useful indicators of equilibrium between the different parts of sandwich experiments. Although this kind of disequilibrium is clearly seen in a few experiments, in most runs the same Al2O3 contents in pyroxenes were indeed observed in the ‘filling’ and the ends of the sandwich.
The effect of K2O on melting relations of Sp-lherzolite in the system CMAS–K2O
Effect of K2O on melt compositions
We find that 1 wt % K2O in the melt depresses the solidus temperature of the Sp-lherzolite phase assemblage at 1·1 GPa by about 5·8°C (Fig. 2); concurrently, this amount of K2O decreases MgO by 1·30% and CaO by 1·03%, but increases SiO2 by 1·05% and Al2O3 by 0·28% (Fig. 3).
To compare the effect of K2O on melt composition with that of Na2O, we plot our experimental data in mole percent in Fig. 12, together with the data of Walter & Presnall (1994) in the system CMAS–Na2O, at 2·0 GPa. Walter & Presnall (1994) provided a global regression of their data to 3·5 GPa that shows that the difference in pressure between 1·1 and 2·0 GPa has a negligible effect in this context. For K, an increase of 1% molar increases Si by 0·95% and Al by 0·33%, and decreases Mg by 1·46% and Ca by 0·82%. For all cations, the effect of Na is similar in direction to that of K, but much weaker: a 1% molar increase in Na only increases Si by 0·1% and Al by 0·1%, and decreases Mg by 0·74% and Ca by 0·47%.
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Hence, the effect of K2O on modifying the composition of melts multiply saturated with Fo + Opx + Cpx + Sp in the system CMAS–K2O is very different from that of Na2O in the system CMAS–Na2O. This is illustrated in Fig. 13, in which the compositions of multiply saturated melts are plotted in the basalt tetrahedron projected from Ol (Fig. 13a) and Di (Fig. 13b). The data for the system CMAS–Na2O were taken from Walter & Presnall (1994)
at 2·0 GPa, the higher pressure being used because Na2O stabilizes plagioclase at the expense of spinel at only 2 wt % Na2O at 1·1 GPa. The difference in the behaviours of K2O and Na2O can be summarized under three points, as follows. (1) The addition of K2O drives the melt composition towards the Qz-normative field (defined by the join AbAnOr–Hy in Fig. 13b), the melt becoming Qz-normative at 2 wt % K2O. The addition of Na2O drives the melt composition towards the Ne-normative field (defined by the join AbAnOr–Ol in Fig. 13b), the melt becoming Ne-normative at 2 wt % Na2O. (2) K2O decreases normative Di, whereas Na2O has no effect on this normative component. (3) As a consequence, adding K2O eventually produces a corundum-normative melt at 8 wt % K2O. Na2O-rich melts never become corundum-normative.
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Effect of K2O on the partitioning of MgO between olivine and melt
The amount of MgO in a basaltic magma increases strongly with rise in temperature. Several workers have attempted to quantify this effect, using the partition coefficient for MgO between Melt and Ol (Leeman, 1978
; Ford et al., 1983; Gudfinnsson & Presnall, 2001). However, such single-element partition coefficients are generally expected to depend on other factors including the details of the melt composition (e.g. O'Neill & Eggins, 2002). Figure 14 shows that adding K2O to CMAS produces a very different trend from that established by the systems CMAS, CMAS–Na2O and CMAS–FeO (compare Gudfinnsson & Presnall, 2001, fig. 2). Clearly, empirical geothermometers, with a less than rigorous thermodynamic basis, have limited reliability, and should never be applied to compositions outside those used in their formulation.
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The MgO-partitioning ‘magmathermometer’ of Ford et al. (1983)
has been used recently by Danyushevsky et al. (1996), Falloon et al. (1999, 2001) and Falloon & Danyushevsky (2000). Falloon & Danyushevsky (2000) found that this formulation returns progressively larger errors in the calculated liquidus temperatures of basaltic melts as alkalis (Na2O + K2O) exceed 3 wt %. Our data can be used to check this phenomenon further. The result is shown in Fig. 15. Apparently the Ford et al. (1983) geothermometer cannot reproduce the experimental temperatures accurately either in the system CMAS or in the system CMAS–K2O (Fig. 15a). We also checked the effect of pressure on the geothermometer, with results shown in Fig. 15b. Evidently pressure affects the calculated temperature as well.
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Melt properties and phase relations
The CIPW norms of the experimentally observed melt compositions are summarized in Table 6. Melts with K2O contents below
4·1 wt % K2O are ol-normative. The melt becomes qz-normative at higher K2O. Melts with >8 wt % K2O are co-normative.
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Melt compositions observed in this study are plotted in Fig. 16. In the system CMAS there are two isobaric invariant points of interest, A (Fo + Sp + Opx + Cpx + Melt) and A* (Sp + Opx + Cpx + An + Melt). Points B, C and D are isobaric invariant points in the system CMAS–K2O, for the phase assemblages Fo + Sp + Opx + Cpx + San + Melt, Sp + Opx + Cpx + An + San + Melt and Sp + Opx + An + San + Sapph + Melt, respectively. All these invariant points are peritectic. From point A to point B the isobarically univariant phase assemblage is Fo + Sp + Opx + Cpx + Melt, whereas from point B to point C it is Sp + Opx + Cpx + San + Melt. At point C, the melt composition trend is joined by another melt composition trend emanating from point A*, with the isobarically univariant phase assemblage of Sp + Opx + Cpx + An + Melt. The isobarically univariant phase assemblage from point C to point D is Sp + Opx + San + An + Melt. Thermal maxima must be located between B and C, and between C and D. The temperature change is judged by our direct temperature observation or by the method of Presnall (1986)
. One interesting property of the system suggested by our experiments is that the univariant solidus curves have a relatively steep temperature drop (1320–1240°C) from A to B or from A* to C, but the temperature interval between A and A* and the corresponding univariant solidus curves A–B and A*–C is very small.
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The compositions of Melt, Cpx and Opx in the experiments with the phase assemblage Fo + Sp + Opx + Cpx + Melt in the system CMAS–K2O were fitted by regression to a second-degree polynomial of the form
 |
is the concentration in weight percent of oxide i in phase 
and T is in °C.
To calculate the partial melting reaction along the univariant curve of Fo + Sp + Opx + Cpx + Melt, the method of Walter et al. (1995) is used. The melting reaction is independent of the bulk composition as long as all phases are present. The calculated reaction coefficients are plotted against temperature in Fig. 17. The partial melting reactions for some special temperatures are listed in Table 8.
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Also listed in Table 8 are the partial melting reactions at the invariant points of the system CMAS (points A and A* in Fig. 16) and the invariant points of the system CMAS–K2O (points B, C and D in Fig. 16) which are directly calculated from the experimentally observed phase compositions using the method of Korzhinskii (1959)
. Partial melting is always peritectic along the univariant curve Fo + Sp + Opx + Cpx + Melt at 1·1 GPa in the system CMAS–K2O (Fig. 17). At high temperatures (from 1320°C down to 1265°C), Fo is in reaction relationship with melt. From 1276°C Sp becomes in reaction relationship.
Experiment C-1768: an analogue of a diamond-aggregate melt extraction experiment
Run C-1768 is a crystallization experiment on the melt composition determined for the isobaric invariant point Fo + Sp + Opx + Cpx + Melt from the KEs, with additional Fo added in the sandwich geometry. Importantly, this particular run is at 1310°C, just below the solidus (1320°C). The experimental charge was prepared with layers of Fo sandwiching a layer of crushed glass of the anticipated solidus melt composition (synthesized at 1 atm). Initially, of course, both layers have considerable porosity. This porosity would not be expected to survive at 1·1 GPa and 1310°C even in a subsolidus run (for example, a pure Fo charge would be expected to recrystallize to near 100% density in a few minutes at these conditions), and indeed it does not. However, the interesting observation is that the Fo layers are thoroughly impregnated with material that includes the components CaO and Al2O3 plus additional SiO2, leading to the crystallization of Cpx, Opx and Sp among the original Fo. These components must come from the glass layer, through the action of a metastable melt present during the initial stages of the run. The metastable melt is inferred because the distances involved (2 mm) are too large for solid state diffusion timescales. Whereas the initial olivine-only layers ended up with the assemblage Fo + Sp + Opx + Cpx, the glass layer crystallized to An + Sp + Opx + Cpx and is hence not in equilibrium with the Fo layers. No quenched melt was observed in the final run products (either Fo or glass starting layers), indicating that the run was, as expected, thoroughly subsolidus. We believe that the scenario whereby material migrates into the olivine layers may be analogous to the processes occurring in diamond-aggregate melt-extraction experiments.
The diamond-aggregate technique was used to study the near-solidus partial melting of mantle peridotite (Johnson & Kushiro, 1992; Hirose & Kushiro, 1993; Baker & Stolper, 1994; Baker et al., 1995; Kushiro, 1996). The results of one of these studies (Baker et al., 1995) was questioned by Falloon et al. (1996), who drew attention to possible pitfalls in the technique, particularly when applied to experiments with low degrees of partial melting (where it is potentially most useful). Falloon et al. (1997, 1999) then checked some of the melt compositions observed in these studies and concluded that the diamond-aggregate technique compounds rather than solves the experimental problems. However, Kushiro (2001) has recently pointed out that ‘the analyses given by Baker et al. (1995) were too low in alkalies because of wrong correction procedures (Hirschmann et al., 1998)’. Kushiro therefore concluded that the results of Baker et al. were ‘not inconsistent’ with the tests of Falloon et al. (1997); but as Falloon et al. were evidently studying a different composition, consistency is difficult to judge. Although Kushiro (2001) inferred from this that ‘The diamond aggregate method can be a reliable technique for obtaining and analyzing low-degree partial melt’, we suggest that more testing is needed.
Full evaluation of the diamond-aggregate technique is beyond the scope of this study, but the observations obtained on C-1768 can be used to throw further doubt on the reliability of the diamond-aggregate technique at low melt fractions. The existence of the CaO, Al2O3 and extra SiO2 amidst the Fo supports the argument of Falloon et al. (1996) that the initial intra-pore pressure can lead to metastable melt production. As another possible example of this, Hirose & Kushiro (1993) found melt in every single experiment of their diamond-aggregate study, despite several runs being at temperatures up to 50°C below the solidi of their two compositions as previously determined (Takahashi & Kushiro, 1983; Takahashi, 1986).
A defocused beam of approximately 40 µm x 50 µm was used to analyse areas in C-1768 from one end of the capsule to the other. The results are summarized in Fig. 18. Figure 18a shows that the amount of material infiltrating the Fo layers (as monitored by CaO and Al2O3) decreases away from the interface with the glass layer, but some still reaches the edge of the capsule. What is strange is the decrease in CaO/Al2O3 (Fig. 18b) from that in the initial glass (CaO/Al2O3 = 0·75), which suggests decoupling of CaO and Al2O3. The transient melt in the Fo layers appears to have a much lower ratio than equilibrium melt coexisting with either Fo + Sp + Opx + Cpx or Sp + Opx + Cpx + An. A low CaO/Al2O3 ratio is also observed in many of the diamond-aggregate studies at low degrees of partial melting, although a true comparison is obscured by the high Na2O contents of these melts. It would be valuable to test the diamond-aggregate method directly in the system CMAS at 1·1 GPa at subsolidus and near-solidus conditions. It is an accepted principle in the experimental petrology of subsolidus phase equilibria that the attainment of equilibrium should be tested by performing reversal experiments; this principle would seem to equally important in the case of melt-extraction experiments, which could be tested very effectively using the kind of reversals presented in this study.
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) and also another incomplete profile (•), extending to the glass layer.The anorthite–sanidine solvus
Anorthite, albite (Ab) and sanidine are the three main components of natural feldspars. At high temperatures there is complete solid solution across the joins An–Ab (plagioclase) and Ab–San (alkali feldspar), but the solvus between An and San extends to their solidus. Despite the usefulness of the An–San solvus for constraining thermodynamic models of ternary feldspar solid solutions (Nekvasil, 1994
), there are only two experimental studies on the binary An–San join at high temperatures (Ai & Green, 1989; Nekvasil & Carroll, 1993), which disagree on the compositions of the coexisting feldspars as well as on the temperature of the solidus. Ai & Green (1989) located the eutectic in the An–San binary at 1·0 GPa at An30San70 and 1215 ± 15°C, and found the maximum mutual solubility of 18% San in An (by mole) and 7% An in San (Fig. 19). Nekvasil & Carroll (1993), however, found the eutectic at 1·13 GPa at An19San81 and 1280 ± 10°C, and the maximum mutual solubility is 11% San in An and 9% An in San (Fig. 19).
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Determining the phase relations around the An–San solvus and solidus is experimentally more difficult than it may seem at first sight. The kinetics of the mixing/unmixing process at the solvus are too sluggish for direct experimental investigation at subsolidus conditions. The assemblage Anss + Sanss + Melt is isobarically invariant in the system CaAl2Si2O8–KAlSi3O8, and thus observing this assemblage directly is not theoretically possible in the pure system (this is of course exactly the same problem we confront in this study). Indeed, the reported observations of both the Ai & Green (1989)
and Nekvasil & Carroll (1993) on the solvus/solidus relations are inferences obtained by the bracketing technique. In the same way that adding K2O to the system CMAS enables us to constrain the phase relations around the Fo + Opx + Cpx + Sp + Melt invariant point, so the presence of MgO plus other minor CAS components in our experiments increases the variance of the system and enables us to observe Anss + Sanss + Melt directly. We are thus able to report some useful points on the solvus, and also to put some constraints on the solidus of the CaAl2Si2O8–KAlSi3O8 system at 1·1 GPa.
We have three runs that produced coexisting Anss and Sanss (C-1580 at 1250°C and C-1708 and C-1576 at 1230°C), which are shown projected onto the CaAl2Si2O8–KAlSi3O8 join in Fig. 19. The maximum mutual solubility of An and San observed here is higher than that of Ai & Green (1989) and much higher than that of Nekvasil & Carroll (1993). These new data suggest that the feldspar model calibrated by Fuhrman & Lindsley (1988) has the best performance at higher temperatures. As shown by Nekvasil (1994), all other models calculate much lower mutual solubilities of An and San.
Because MgO concentrates in the melt relative to Anss and Sanss, the eutectic in the MgO-free system CaAl2Si2O8–KAlSi3O8 at 1·1 GPa must be higher than 1250°C (run C-1580). Allowing for a representative slope dT/dP for silicate melting of about 100°C/GPa, our results are not consistent with the solidus temperature of 1215 ± 15°C at 1·0 GPa found by Ai & Green (1989), but are consistent with the 1280 ± 10°C at 1·13 GPa suggested by Nekvasil & Carroll (1993). The molar Ca/(Ca + K) ratios in our three melts in equilibrium with An + San are 0·31, 0·27 and 0·25. On the grounds that some of the Ca in the melt is presumably associated with components other than An, but all the K can be assigned to a San component, we surmise that the composition of the solidus (eutectic) melt in the binary CaAl2Si2O8–KAlSi3O8 would lie at lower molar Ca/(Ca + K), again consistent with the phase diagram of Nekvasil & Carroll (1993) (eutectic at An19San81), rather than that of Ai & Green (1989) (An30San70).
Sapphirine
In two of the sandwich experiments (C-1580 and C-1576), the middle melt-rich regions produced an assemblage containing sapphirine (Sapph); the phase assemblage consisted of An + San + Sapph + Melt at 1250°C (C-1580), which was joined by Opx and Sp at 1230°C to form the isobarically invariant assemblage An + San + Sapph + Sp + Opx + Melt. Neither assemblage is in equilibrium with the end regions, which consisted of the usual Fo + Opx + Cpx + Sp with no apparent melt, and with a texture showing minimal recrystallization, also indicative of no melt. However, the compositions of the pyroxenes in these regions are consistent with local equilibrium. Sapph is incompatible with Fo at 1·1 GPa and 1250°C, reacting to form Opx + Sp (e.g. Liu & Presnall, 2000). A semi-schematic phase diagram showing the subsolidus phase relations in the system MgO–Al2O3–SiO2 at 1250°C and 1·1 GPa is shown in Fig. 20.
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In C-1576, the Opx in the two regions was very different in composition; that in the subsolidus peridotitic assemblage had the usual 8·5 wt % Al2O3 (consistent with other experiments), whereas that in the sapphirine-bearing middle region had 14 wt %. The regions are separated by a thin band of orthopyroxene
10 µm in width (Fig. 2a). Remarkably, the composition of the orthopyroxene changes across this narrow band, from the 14 wt % of the An + San + Sapph + Sp + Opx + Melt assemblage on that side, to the 8·5 wt % on the other (see Fig. 2a).
The solubility of Al2O3 in Opx in equilibrium with Sapph + Qz (quartz) has recently been studied by Hollis & Harley (2002) at P–T conditions close to those of this study. The controlling reaction can be written
 | (8) |
Thus lowering the activity of SiO2 should move equilibrium (8) towards the right-hand side, lowering Al2O3 in Opx (see also Fig. 20). However, the Al2O3 content of the Opx in equilibrium with Sapph + Sp in our experiment C-1576 is significantly higher than found by Hollis & Harley (2002) at similar temperatures and pressures (e.g. their experiments R50a,b at 1250°C and 1·2 GPa have only 9·5–10·5 wt % Al2O3 in Opx). Our experiment, unlike those of Hollis & Harley (2002), is unreversed, so that it is possible that we are observing metastable Opx, rapidly crystallized from melt at the start of the experiment. However, the Opx and Sapph in this experiment are both homogeneous and euhedral (Fig. 2a).
The compositional variation of sapphirine in the MgO–Al2O3–SiO2 system can be considered by starting with the basic formula Mg4Al4Si2O20 (called 2:2:1 sapphirine). Most natural sapphirines are more aluminous than this, as a result of substitution of 2Al for Mg + Si (i.e. solid solution towards the 7:9:3 composition, Mg7Al9Si3O20; e.g. Liu & Presnall, 2000; Hollis & Harley, 2002); here, by contrast, the reverse substitution of Mg + Si for 2Al occurs, to produce sapphirines with less alumina than in 2:2:1, as found by Liu & Presnall (2000). Thus if the formula is represented as Mg4–xAl4+2xSi2–xO20, x is usually in the range 0–0·5 (Hollis & Harley, 2002), but here x is approximately –0·15 in C-1580 and –0·3 in C-1576. Figure 20 confirms that Sapphss in equilibrium with Opx should contain the minimum Al2O3 at a given P and T.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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In this study, we made forward partial melting experiments to determine the phase relations at the solidus of the spinel-lherzolite assemblage in the system CMAS both with and without K2O. We also made reversal crystallization experiments. The K2O method, thus, can be fully assessed both by internal consistency and by reference to the previous literature data (Presnall, 1976
; Presnall et al., 1979; Walter & Presnall, 1994).
Compositions of all phases, including both pyroxene solid solutions and the melt compositions in the system CMAS derived from the KEs by extrapolating to zero K2O, agree reasonably well with previous studies. The melt composition of Presnall et al. (1979) is almost the same as our extrapolated result from the KEs.
Agreement on the solidus temperature for a Sp-lherzolite phase assemblage in the system CMAS at 1·1 GPa is consistent between the KEs and the REs and it is around 1320°C, with a probable uncertainty of less than ±10°C. The slightly higher temperature of 1350°C determined by Presnall (1976), later used in Presnall et al. (1979), Walter & Presnall (1994) and Gudfinnsson & Presnall (1996, 2000, 2001), may be due to the W/Re thermocouples used in that study, either from oxidation or from calibration errors. The oxidation problem of the W/Re thermocouples forced Walter & Presnall (1994) to use a nitrogen flow down the thermocouple insulator for their experiments at lower pressures. Recently, Falloon et al. (2001) reported drift of W/Re thermocouples at 1·0 GPa to higher apparent temperature in the temperature range 1300–1400°C caused by oxidation. Commercially available W/Re thermocouples vary widely in their deviation from theoretical e.m.f.–temperature relations, requiring careful calibration of each batch (see Klemme & O'Neill, 2000b).
The obvious agreement on the pyroxene composition, the melt composition and the solidus among the extrapolation result from the KEs, the result of the REs and the literature data argues that the K2O method is generally successful. We will report on the application of this method to the determination of partial melting at the solidus in the system CMAS–Cr2O3 in another paper (Liu & O'Neill, in preparation).
Comparison with previous work on the system CMAS–Na2O shows that the effect of K2O on melting equilibria is much stronger than that of Na2O, both as regards temperature and melt composition. Increasing K2O causes the multiply-saturated solidus melt composition to trend towards silica enrichment, becoming quartz-normative above 4 wt % K2O. By contrast, the effect of Na2O at 1·1 GPa is for multiply-saturated melts to trend towards becoming more silica undersaturated; high Na2O melts are ne-normative (where ne is nepheline). It should be noted, however, that the activity of SiO2 in both systems remains nearly the same, being buffered by Fo + Opx [reaction (3)].
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ACKNOWLEDGEMENTS |
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We acknowledge the technical assistance of Bill Hibberson, Dean Scott, Nick Ware, Mike Shelley, Harry Kokkonen, Frank Brink, and Drs Roger Heady, Cheng Huang, Sue Kesson and Uli Troitzsch. Ruby discs were supplied by Dr John Mavrogenes. We benefited from constructive discussions with Professor David Green and Drs Jorg Hemann and Sue Kesson. This study is part of the Ph.D. thesis of L.X. funded by A. E. Ringwood Memorial Scholarship and Australian International Postgraduate Research Scholarship. L.X. also thanks Dr Shen-su Sun and Professor Qingchen Wang for their encouragement. We thank Leonid Danyushevsky and Richard Arculus for their reviews, and the latter for his editorial handling.
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FOOTNOTES |
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Corresponding author. Telephone: (61) 2 6125 5159. Fax: (61) 2 6125 5989. E-mail: hugh.oneill{at}anu.edu.au
* Present address: Geodynamics Research Center, Ehime University, Matsuyama 790-8577, Japan. 
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REFERENCES |
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TOPABSTRACTINTRODUCTIONEXPERIMENTAL AND ANALYTICAL...EXPERIMENTAL RESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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