Melting of Peridotites through to Granites : A Simple Thermodynamic Model in the System KNCFMASHTOCr

A new set of thermodynamic models is presented for calculating phase relations in bulk compositions extending from peridotite to granite, from 0 001 to 70 kbar and from 650 C to peridotite liquidus temperatures, in the system K2O–Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3– Cr2O3 (KNCFMASHTOCr). The models may be used to calculate phase equilibria in partial melting of a large range of mantle and crustal compositions. They provide a good fit to experimental phase relation topologies and melt compositions across the compositional range of the model. Compared with the preliminary model of Jennings, E. S. & Holland, T. J. B. (2015) (A simple thermodynamic model for melting of peridotite in the system NCFMASOCr. Journal of Petrology 56, 869–892) for peridotite–basalt melting relations, the inclusion of K2O and TiO2 allows for better modelling of small melt fractions in peridotite melting, and in reproducing rutile-bearing eclogite melting at high pressures. An improved order–disorder model for spinel is now incorporated. Above 10 kbar pressure, wet partial melting relations may be significantly affected by the dissolution of silicates in aqueous fluid, so the set of models includes an aqueous low-density silicatebearing fluid in addition to a high-density H2O-bearing silicate melt. Oxygen fugacity may be readily calculated for the whole range of bulk compositions investigated, and the effect of water content on melt fO2 is assessed.


INTRODUCTION
The modelling of melting relations in rock bulk compositions is an important goal in petrology.Modelling makes it possible to predict mineral þ melt assemblages at pressures, temperatures and compositions where existing experimental data must be interpolated.Model predictions may even, with caution, be extrapolated to regimes where experimental constraints would be challenging or time-consuming to obtain.Partial melting plays a major role in both modulating and probing Earth processes, the former by advecting heat and matter, and the latter by influencing seismic velocities.Consequently, the capacity to make accurate predictions of partial melting relations has great significance for Earth system science.
In order to model partial melting, a thermodynamic description is needed for each of the phases involved.The silicate melt is perhaps the most difficult.This is fundamentally because there is no obvious way to write the entropy of the melt, unlike in a crystalline solid, for which the configurational entropy of cation mixing on lattice sites is well defined.However, the problem is exacerbated by the enormous extent of composition space that melts are able to occupy, relative to any solid solution.Thermodynamic modellers have previously handled this problem by modelling subsets of this potential composition space, relevant to melt in a limited range of settings.
Using this approach, partial melting can now be handled in a wide range of contexts using different thermodynamic models.Basaltic melting can be addressed with the MELTS models of Ghiorso & Sack (1995), extended by Ghiorso et al. (2002), or with those of Jennings & Holland (2015), on which the work in this paper is based.Models generating melt of wet granitic composition include those of Holland & Powell (2001) and White et al. (2008), as well as those of Gualda et al. (2012) who extended the MELTS package, as rhyolite-MELTS, in an attempt to deal more effectively with quartz-and K-feldspar-bearing liquids, albeit at the expense of problematic adjustments to the enthalpies of solid quartz and K-feldspar.The models of Green et al. (2016) represent the melting of metabasite rocks, generating a melt more calcic than a typical granitic melt.
Between them, the models listed above can be used to represent partial melting in most geological contexts.The present study describes a simple new thermodynamic model that extends the scope and application of the Holland & Powell (2011) dataset to melting in a wide variety of rocks from peridotite to granite.It modifies and extends the earlier model of Jennings & Holland (2015).It offers, therefore, an alternative calibration to the existing rhyolite-MELTS package, which currently cannot be used to model partial melts in equilibrium with biotite-or hornblende-dominated assemblages, or near-solidus phase equilibria, as explained on the MELTS web page (http://melts.ofm-research.org/).

THERMODYNAMIC MODELS
Full details of all models are given in the Appendix.Thermodynamic data for all of the end-members used here are presented in file tc-ds633.txton the website listed in the Appendix, as are activity-composition (a-X) relations for the phases involved.
A Monte Carlo approach was adopted for calibrating the models, as described in Green et al. (2016).In the Monte Carlo simulations, filters were applied, both as temperature brackets for phase equilibrium features such as the solidus and for melt compositions expressed in terms of the end-member proportions.Initial estimates for values of parameters (with an associated variation range) were taken, and runs of THERMOCALC were made with relatively wide bracket constraints on phase boundaries and melt compositions.These were progressively refined by narrowing the constraint brackets until all the observations could be satisfied within the estimated uncertainties.Rather than fitting with respect to a large database of individual experimental runs, the Monte Carlo procedure was constrained using a few key runs from each of a set of experimental studies, covering the required range of bulk composition.
This step involves review and interpretation of the available experimental data in order to draw out the salient features of phase relations.When a database of petrological experiments is used to characterize trends in thermodynamic properties, the data typically contain inconsistencies as well as large amounts of redundancy with respect to the variation in properties along some of the relevant axes of pressure, temperature and composition.Meanwhile, along other axes, the variations are highly correlated and are individually underconstrained.Further, such databases generally contain biased weightings, particularly in favour of experiments at 1 atm.Though the interpretation of the phase relations that we adopt is necessarily non-unique, we consider this manual pre-processing to be a valuable step in understanding the data and extracting thermodynamic meaning from them.The constraints interpreted on the basis of these key experiments were either temperature brackets for phase equilibrium boundaries or melt compositions.Constraints were chosen with a view to being able to calculate pseudosections at the bulk compositions of each experimental study that would closely resemble the experimentallydetermined phase relations.Monte Carlo runs were made by successively imposing the constraints above until 100 successes at each stage were achieved through varying the set of parameters.The resulting model, while satisfying the constraints, may still be non-unique, but is sufficient within the scope of the objectives in this system.Parameters varied in the fitting are the pairwise interaction energies (W) and modifications to the enthalpies of end-members from the Holland & Powell (2011) dataset (H mod ).Below, the interaction energy for mixing between end-members mm and nn is denoted as W mm-nn .

Solid phases in KNCFMASHTOCr
Thermodynamic data on mixing models for pyroxenes, olivine and garnet are based on those used in Jennings & Holland (2015).They are extended by inclusion of K and Ti in cpx, Ti in garnet, and Na 2 O in opx to allow for better expressing phase relations at small melt fractions.Spinel in the Jennings & Holland (2015) model used a simplified random mixing model based on Bryndzia & Wood (1990).A new model for spinel is presented here (see Appendix) which incorporates orderdisorder in a manner similar to that of Hill & Sack (1987) and Sack & Ghiorso (1991a, 1991b).It involves the endmembers spinel (MgAl 2 O 4 ), inverse spinel (AlMgAlO 4 ), hercynite (FeAl 2 O 4 ), inverse hercynite (AlFeAlO 4 ), inverse magnetite (Fe 3þ FeFe 3þ O 4 ), normal magnetite (FeFe 3þ Fe 3þ O 4 ), picrochromite (MgCr 2 O 4 ) and qandilite (MgTiMgO 4 ).The ternary plagioclase mixing model from Holland & Powell (2003) is used here, allowing incorporation of K 2 O in plagioclase and CaO in alkali feldspars.Olivines in the new model incorporate Ca via a monticellite end-member (CaMgSiO 4 ) and include Fe-Mg disordering on M2 and M1 sites.Ilmenite in the system FeO-Fe 2 O 3 -TiO 2 is incorporated using the model in White et al. (2000).
Mixing parameters for pyroxenes and garnet were refined in the Monte Carlo fit, at the same time as melt parameters, using experimental equilibria as described below.

A simple thermodynamic model for melts
The melt model is based on the basaltic model of Jennings & Holland (2015).However, the new model is somewhat more complex, having been modified to allow for the enormous compositional range spanned by granitic, tonalitic and basaltic melts.A Temkin-type model is adopted, in which the melt structure can be considered to consist of mixing sites, analogous to a solid solution, but with the number and occupancy of sites varying as a function of melt composition.The sites, and the mixing units contributed to each site by the melt end-members, are shown in Table 1.On the site designated F, random mixing of the nine species AlSi 2 , AlSi, Si p , Si o 2 , Si q 4 , Cr, Ti, Fe 3þ and CaAl 2 Si takes place, representing the networking behaviour of silicate melts.The three different types of Si units reflect the presence of chains (Si p ), isolated tetrahedra (Si o 2 ) and 3dimensional network (Si q 4 ) elements.Random mixing of Mg, Fe, Al and Ca are assumed on separate, octahedrallike, M-sites.The A-sites are occupied by the alkali elements Na and K, which form large cations.A hypothetical site V, of multiplicity 2, is composed of just vacancies h and H.This makes it possible to represent H 2 O in a way that maintains compatibility with earlier work on granitic melts (e.g.Nicholls, 1980;Holland & Powell, 2001).
The melt model uses 12 end-members (Table 1 ).The end-member ctL is a fictive species (the Gibbs energy of ctL is constructed from ctL ¼ woL þ slL-1/4qL) whose amount in the melt is found by solving the equilibrium relation for the internal melt reaction ctL ¼ woL þ slL-1/4qL.The model is treated as a slightly asymmetric solution (van Laar), with ideal entropic mixing on sites, and a generally non-ideal enthalpy of mixing between pairs of end-members.
Compared with the Jennings & Holland (2015) model, the changes are: 1) diopside liquid has been replaced by wollastonite liquid; 2) sillimanite liquid is introduced; 3) quartz, forsterite and fayalite liquid endmember formula sizes have doubled; 4) K-jadeite and H 2 O liquid are incorporated; and 5) an additional ctL end-member is introduced.The end-members and their model structural elements are as shown in Table 1.The formulae for qL, foL and faL were doubled so that relations in more polymerised granitic melts could be modelled well.The fictive species ctL was introduced to lower the Gibbs energy in some melt composition regions, and was essential in modelling plagioclase phase relations in granitic melts.
The model was developed in two stages.Firstly, the H 2 O-absent system was calibrated using the experimental constraints discussed in Jennings & Holland (2015), as well as many additional ones given in the Appendix.Secondly, H 2 O-bearing equilibria were added using the constraints from hydrous experiments listed in the Appendix.The fitting made use of the melting temperatures and melt compositions of KLB-1 peridotite (Takahashi, 1986;Takahashi et al., 1993;Davis et al., 2009Davis et al., , 2011)), G2 quartz-rutile eclogite (Pertermann & Hirschmann, 2003a, 2003b), Leg 45-395A-8-1-9 basalt (Fujii & Kushiro, 1977), RE46 Icelandic basalt (Yang et al., 1996), Mix1G pyroxenite (Hirschmann et al., 2003), GS104-2-1 basalt (Tormey et al., 1987) and ARP74 basalt (Fujii & Bougault, 1983) to improve the Gibbs energy contribution of each end-member in the H 2 O-absent system.To extend the model to more Sirich compositions, the melting temperatures of albitequartz and sanidine-quartz as well as the dry R1 granite composition used by Whitney (1975) are incorporated in the fitting together with the many other equilibrium constraints listed in the Appendix.
For bulk compositions containing iron, a bulk composition ferric: ferrous ratio needs setting via application of an appropriate Fe 3þ / P Fe ratio.For 1 atm experiments (e.g.RE46 basalt) Fe 3þ / P Fe ¼ 0Á09 was used, based on QFM-buffered experiments.For highpressure piston-cylinder experiments values of Fe 3þ / P Fe ¼ 0Á033, or about one third of that for QFM were used in the absence of good experimental evidence.A value of 0Á033 was also used in Jennings & Holland (2015) for mantle peridotite.The fitted melt parameters are not affected significantly by varying these ratios, as the hmL end-member is determined from the experimentally determined Fe 3þ / P Fe ratio in melts by Kress & Carmichael (1991), Jayasuriya et al. (2004) andO'Neill et al. (2006).
In the Monte Carlo fitting, the parameters that were varied were the H mod terms of all end-members and the regular solution interaction energies (Ws) among them.After the initial stages where pyroxene terms were tuned, only melt parameters were varied in the final fits.Some interaction energies were initially taken as known, using values from binary and ternary subsystems, and then allowed to vary to see if any adjustments were needed.Some Ws benefitted from a modest pressure-dependence.

Silicate-bearing aqueous fluid
Below 10 kbar, it is sufficient to treat aqueous fluid as pure H 2 O.However, as discussed above, at higher The non-ideal interaction energies must be calibrated.
Values for W qL-h2oL , W jdL-h2oL , W kjL-h2oL , W woL-h2oL , and W foL-h2oL were found by initially fitting to wet melting experiments for quartz, albite, sanidine, wollastonite and forsterite, respectively.These values were then used as starting values in the Monte Carlo fitting of melting equilibria (see Appendix) and adjusted.W slL-h2oL was determined from the fitting process.The remaining Ws were assigned arbitrary values similar to those of W foL-h2oL .The H mod terms were adjusted in the Monte Carlo fitting of the H 2 O-bearing system.Although this simple model is crude, it does at least allow for some solubility of silicates in the fluid, increasing with pressure, and appears to yield solubilities that are consistent with measurements to modest (10-12 kbar) pressures: Kennedy et al. (1962) for quartz-H 2 O; Goranson (1931Goranson ( , 1932)); Morey & Hesselgesser (1951); Burnham & Jahns (1962); and the data of Stewart in Clark (1966), Yoder et al. (1957), Lambert et al. (1969) for albite-H 2 O and san-H 2 O; and additionally Burnham (1960) and Anovitz & Blencoe (1997, 1999) for albite-sanidine-quartz-H 2 O. Examples for quartz-H 2 O and albite-H 2 O are shown in Fig. 1.This aqueous model should be used only as an approximate vehicle to lower the H 2 O activities in calculations and not to estimate mineral solubilities.Although this approximation is an improvement over assuming pure H 2 O, a more realistic model awaits development.
Although the water contents of melt are not perfectly modeled for pure albite and pure quartz wet melting, the water contents of haplogranitic melts (Qz 28 Ab 38 Or 34 ) as given by the experiments of Holtz et al. (1992Holtz et al. ( , 1995) ) are in good agreement with the aqueous silicate melt and solute-bearing fluid model used here (Fig. 2).

Mantle melting using KLB-1 as a bulk composition
The supra-solidus phase relations are an improvement over the earlier models of Jennings & Holland (2015), and show a much simpler sequence that matches experiments well.The new models include K 2 O and TiO 2 in the melts, and they now agree with the liquidus temperatures of Takahashi (1986) and Takahashi et al. (1993).The change in end-members used, together with the new spinel model, has eliminated the appearance of small amounts of spinel which persisted up to liquidus temperatures in the earlier work.Figure 3 shows the revised P-T pseudosection for KLB-1 composition.The widening of the garnet þ spinel peridotite field at low temperature is more marked, but this is highly sensitive to both ferric iron and chromium content.Orthopyroxene disappears along the solidus at pressures near 30 kbar rather than 40 kbar as in the earlier model, a change stemming from the improved fitting of garnet and pyroxene compositions as a function of pressure and temperature.As noted earlier in Jennings & Holland (2015), the exact pressure of opx disappearance remains highly sensitive to bulk composition variations.
The models predict the sensitivity of the solidus to Na 2 O and K 2 O (Fig. 4) rather well in comparison to the Experimental data from Kennedy et al. (1962) for quartz, and from Goranson (1931Goranson ( , 1932)), Morey & Hesselgesser (1951), 15 kbar experiments (cf. Hirschmann, 2000).The diagram shows the good agreement between the calculated and experimental solidus of KLB-1 and reproduces the trends in lowering of melting temperatures with addition of alkalis in experimental studies.The dashed lines were constructed by addition of K 2 O and Na 2 O to the bulk composition.

Melting of Icelandic basalt RE46
Yang et al. (1996) performed experiments on melting and crystallization of an Icelandic basalt at ambient pressure with oxygen fugacity controlled by the QFM buffer.They documented the crystallization sequence liquid-olivine-olivine þ plagioclase-olivine þ plagioclase þ clinopyroxene over the interval 1245 C down to 1198 C. A calculated pseudosection for RE46 (with Fe 3þ / P Fe ¼ 0Á09) is shown in Fig. 5, along with the experimental temperatures for incoming of olivine, plagioclase and pyroxene at 1 bar pressure.The model shows the change of liquidus phase from olivine to pyroxene with increasing pressure, a feature commonly reported in experimental studies of such basalts (e.g.Fujii & Kushiro, 1977;Fujii & Bougault, 1983).
Melting of G2 eclogite at 30 kbar Pertermann & Hirschmann (2003a)[COMP: pls tag Pertermann & Hirschmann (2003a) as a reference] have determined melting relations for an eclogite at 30 kbar.Melting begins at 1300 C and rutile and then quartz melt out by 1360 C and both garnet and clinopyroxene remain to the liquidus around 1500 C. Calculations with the new model reproduce this behaviour quite well (Fig. 6a).On an up-temperature path at 30 kbar, the model predicts that quartz melts out at 1330 C and garnet outlasts cpx, whereas in the experiments garnet disappears just before cpx, but the difference is probably within combined experimental and model calculation errors.The compositions of the liquid are shown in Fig. 6b and are in good agreement with experiments, except for FeO and CaO that fall slightly low and high respectively in the calculations.(Hirose & Kushiro, 1993;Kushiro, 1996).

Melting of high-Al basalt composition TA-1
Baker & Eggler (1983) performed experiments on an Aleutian high-Al basalt, bulk composition AT-1 in the range 0-30 kbar.The phase relations were somewhat complex and difficult to decipher in the experiments.At 1 atm the crystallization sequence was determined as plagioclase (1250 C), olivine (1150 C) and cpx (1110 C), with olivine only stable below around 10 kbar (Fig. 7a).
The solidus was determined at 2 kbar and 910 C. Just below 20 kbar pressure there is a change from plagioclase to garnet on the liquidus.Figure 7a delineates the incoming and outgoing of garnet, pyroxene, plagioclase and olivine, as proposed by Baker & Eggler (1983) on the basis of their experiments.
Predictive calculations with the new model (neither the experiments of Baker & Eggler (1983), nor any other high-Al basalts were used in the fitting) reproduce these features rather well and can help make sense of the complexities.In Fig. 7b we show the equivalent phase limits for comparison with the interpretation of Baker & Eggler (1983).Near 20 kbar, the plagioclase-out and garnet-out curves intersect and run sub-parallel to the clinopyroxene-out curve, rather than all three curves intersecting at a point as in Fig. 7a.A complex region involving pigeonite is located between 4 and 9 kbar.The experimental diagram of Baker & Eggler (1983) has a region marked as possible opx, which, according to our calculations, may reflect inverted pigeonite on cooling.The calculations match the experimental solidus at 2 kbar, the loss of olivine at around 10 kbar, and the appearance of garnet on the liquidus at around 18 kbar.The complete pseudosection is shown in Fig. 8.
Wet melting of ksp-q and ab-q to high P As an example of the new melt model applied to more silica (and H 2 O)-rich compositions, the melting of albite þ quartz and sanidine þ quartz in both wet and dry situations is shown in Fig. 9.As a consequence of allowing dissolved silicate in the aqueous fluid, the wet melting curves are modeled well up to 25 kbar.In contrast, by assuming pure H 2 O fluid, the curves deviate to lower temperatures progressively above 10 kbar.Application to higher pressures awaits a more sophisticated model for silicate solubility.
8 kbar as a function of water content.Figure 10a shows Whitney's data and his interpretation of phase equilibria at 2 kbar for water contents up to 12 wt % H 2 O. Figure 10b shows the calculations for Whitney's granite composition at 2 kbar for comparison.The melting temperatures are a few degrees low, but in acceptable agreement, and the water contents at saturation agree well.The experimental location of the pl þ ksp þ q þ liq field at low temperatures is only poorly constrained.
The new model appears to perform remarkably well in covering the range in compositions from dry peridotite melting at temperatures of up to 1900 C to wet granitic compositions below 600 C (see Figs 3,[8][9][10].

Wet melting of tonalite 101
Application of the model to wet melting of natural compositions is illustrated with respect to the melting of tonalite 101 from the Sierra Nevada batholith in experiments by Piwinskii (1968) and later at higher pressures by Stern et al. (1975).A calculated pseudosection for tonalite 101 with 15 wt % H 2 O (as in the experiments) is presented in Fig. 11a.We have only used the mineral phases observed in the experimental studies with addition of a small amount of ilmenite to satisfy the bulk composition.Solidus and liquidus are shown with bolder lines.With 15 wt % H 2 O, nominally watersaturated, the solidus is in general agreement with the schematic interpretation of Stern et al. (1975), Fig. 11b, which was based on Piwinskii's (1968) data at 1-3 kbar.
Because very small amounts of magnetite are involved at the calculated liquidus, an 'effective liquidus' would correspond to the first appearance of plagioclase.The presence of magnetite and/or ilmenite is highly sensitive to both the bulk Fe 3þ /Fe 2þ ratio and to the value for the melt W hmL-h2oL interaction energy.The amounts of magnetite and ilmenite are small (less than 1%) and their stability limits quite uncertain.Because the bulk composition contains 15 wt % H 2 O, the magnitude of W hmL-h2oL has a large effect on magnetite stability, and the value used was chosen to make magnetite stable at the liquidus so as to agree with the experiments of  Phases: g, garnet; cpx, clinopyroxene; pl, plagioclase; ru, rutile; q, quartz; ilm, ilmenite; pig, pigeonite; opx, orthopyroxene, liq, liquid.The main features were compared with the experiments of Baker & Eggler (1983) in Fig. 7. Interesting features are: the solvus top for cpx and pig at around 1175 C and the field of pigeonitic pyroxenes in the region 1100-1230 C and 4-13 kbar.
Piwinskii (1968) on several similar granites and granodiorites.Reducing W hmL-h2oL from 60 to 50 kJ/mol would have the effect of just taking magnetite stability from the liquidus.Because of the uncertainties attending the magnetite and ilmenite stabilities, Fig. 11c has been drawn to show the non-oxide phase relations.Figure 11a also shows that fluid is exhausted at higher pressures and temperatures (above the dashed curves in Fig. 11a, c).This, and the sensitivity of accessory and minor phases to small variations in bulk composition, especially in such H 2 O-rich conditions, suggests difficulties might be anticipated in experimental equilibration of such assemblages and in the interpretation of the run products.Several features are different when compared with the experimental phase diagram: biotite and hornblende are less stable and the presence of pyroxenes is limited to rather low pressures, although in the experiments of Piwinskii (1968) pyroxenes were not observed, although they have been inferred in the interpretation of Stern et al. (1975) in Fig. 11b.
The lower temperature breakdown of biotite and hornblende in the calculations may be related to inadequacies in the activity models for these phases, or to the possible presence of fluorine in the natural materials used in the experiments.It may also be possible that capsule leakage in nominally water-saturated experiments is partly responsible for differences between calculated and experimental phase relations.In calculations with only 0Á8 wt % H 2 O in the range 4-6 kbar (Fig. 11d) the assemblages are fluid-absent just above the solidus and biotite and quartz stabilities are considerably extended relative to fluid saturation, as is the liquidus temperature.In addition, two pyroxenes are present over a large range of the melting interval.Excess H 2 O is unlikely to be a common scenario for the generation of tonalitic rocks and the common occurrence of two pyroxenes and hornblende is typical in those generated under granulite facies conditions (Palin et al., 2016).The diagrams using an earlier melt model in Palin et al. (2016) are not recalculated here as the current model reproduces them quite closely, albeit with differences at high temperatures because the new melt model includes ferric iron and titanium and, therefore, allows calculations up to the liquidus.

Melting of average pelite in KFMASH
As a final example, we compare the melt model with the earlier calculations of White et al. (2014) for an average KFMASH pelite composition.The relationships among the phases garnet-cordierite-biotite-sillimanite-K-feldspar-orthopyroxene-quartz and melt are superimposed as dashed lines on Fig. 12, which is taken from Fig. 4 of White et al. (2014).Phase relations were calculated with the new melt and fluid solubility model, but keeping all other phase properties as in White et al. (2014), so that the effect of the new models may be compared.The position of the solidus below 6 kbar is virtually unaffected, and the principal differences to be seen are the enhanced upper stability of K-feldspar by around 20 C, and the small lowering of the solidus temperature at high pressures.

OXYGEN FUGACITY OF MELTS
As outlined in Jennings & Holland (2015) the oxygen fugacity of dry melts may be estimated quite well if the Fe 3þ to Fe(total) ratio is known.The model allows calculation of the Fe 3þ /RFe ratio of melts through the internal equilibrium among melt end-members 4 hmL þ qL ¼ 2 faL þ O 2 and it was found that, given the interaction energies in the melt which had been determined from the experimental phase equilibrium data, only one parameter needed refining.This parameter was the enthalpy increment (H mod ) to the hmL end-member, which was fitted to the measurements reported in Kress & Carmichael (1991), Jayasuriya et al. (2004) andO'Neill et al. (2006).The quality of the fit may be seen in Fig. 13.The experimental melts measured in Kress & Carmichael (1991) range in logfO 2 from -8.5 to -0.6 and in temperature from 1200 to 1630 C. The calculations require solving for the proportion of the internal endmember ctL and must be done iteratively.We offer a program (fO2melt) that accepts melt compositions (including an estimate of Fe 2 O 3 ) and pressure and temperature and which outputs logfO 2 as well as density, entropy and Gibbs energy of the melt.
Oxygen fugacity estimation may also be extended to hydrous melt compositions.Figure 13 shows that the effect of adding 10 wt % H 2 O to all the melt compositions causes a shift in logfO 2 by 0Á7 log units at oxidizing conditions and up to nearly 1Á5 log units at more reducing conditions.The accuracy of estimating logfO 2 in wet melts is tempered largely by our poor knowledge of the mixing parameter for W hmL-h2oL .In this study, a value of 60 kJ/mol was found to bring magnetite onto the liquidus for tonalite 101 (see above), but this is not well constrained.A smaller value would reduce the oxygen fugacity offset between dry melts and melts with 10 wt % H 2 O. Regardless of this uncertainty, influx of H 2 O into a magma would cause an increase in its intrinsic logfO 2 and removal of water the reverse.Other factors such as gain or loss of hydrogen might be significant, but just adding H 2 O into a magma would require a reduction in the Fe 3þ to Fe(total) ratio if the oxygen fugacity were to be buffered by, for example, magnetite þ quartz þ ferrous silicate at close to the QFM buffer.

SUMMARY
The revised melt model is remarkable in being able to simultaneously reproduce phase equilibrium features (pressures and temperatures of melting, assemblage field boundaries, etc.) in compositions as diverse as lherzolite, basalt, eclogite, tonalite, granite and pelite under wet and dry conditions.It is based on a simplified hypothetical pseudo-structure (framework elements and interstitial cations in the melt) coupled with a nearly symmetrical solution model among a chosen endmember set.It may be unreasonable to expect perfect agreement with experimental phase equilibrium data given the over-simplified assumptions used in the model.Nevertheless, the model appears capable of describing a very wide range of features in diverse bulk compositions and should provide a useful tool for investigating a variety of petrological problems.It is hoped that this model will be modified in future as more is understood about the thermodynamics of mixing in molten silicate solutions.These melt models have their limitations.The range in compositions currently encompasses the more common natural rock suites (peridotite through basalt and andesite to granite/rhyolite and pelitic compositions) but the models have not been tested on highly alkaline rocks.Additionally, the uncertainty in dealing with solubility of silicate in aqueous fluid limits use of the models to less than 20 kbar, with caution extendable to 30 kbar.Oxidation states of H 2 O-bearing melts may be assessed approximately, but refinement awaits further calibration.
The ideal activities are then given by mixing on the various sites as: Non-ideal interactions are expressed in terms of regular solution energies W i-j ¼ a þ bT þ cP in kJ for endmembers i and j, as in Table A1.Approximate uncertainties are given in parentheses.Only the non-zero interaction energies are listed.
The enthalpies of end-members are those in the Holland & Powell (2011) dataset, with increments expressed as a difference (H i mod ¼ H i À H i, ref ).Where an end-member is made from a linear combination of others H i mod is relative to that combination (Table A2).As before, the increment is expressed as in kJ, and the van Laar parameter a as a ratio relative to 1Á0.Approximate uncertainties from Monte Carlo runs are a i (0Á2), b i (0Á0002), c i (0Á002) a i (0Á05).
The hmL (Fe 2 O 3 ) component was calibrated as discussed in Jennings & Holland (2015).Enthalpy was fitted to measured Fe 3þ /RFe of basalts with low K 2 O and low P 2 O 5 from Kress & Carmichael (1991), where logfO 2 was calculated using the all-melt equilibrium 1 =2 qL þ 4 hmL ¼ faL þ O 2 , using the thermodynamic data and activity model presented here.

PLAGIOCLASE
Plagioclase feldspars are treated here using the ternary feldspar model from Holland & Powell (2003).

OLIVINE
Olivines are treated as simple two-site symmetric solid solutions (M1M2SiO 4 ) between forsterite (MgMgSiO 4 ), fayalite (FeFeSiO 4 ), ordered cfm (MgFeSiO 4 ) and monticellite (MgCaSiO 4 ), with non-ideal mixing terms as follows: The W terms are derived from W Fe-Mg ¼ 4Á5 kJ per site following Wiser & Wood (1991).The other parameters were adjusted to fit the olivine-monticellite solvus of Warner & Luth (1973).The Fe-Mg ordering is assumed to be unimportant, so the Gibbs energy of the cfm end-member is given as just G cfm ¼ (G fo þG fa )/2 with no increment.

CLINOPYROXENE
Clinopyroxene is modelled after Green et al. (2012b) and Jennings & Holland (2015) with additions for Ti (Cabuffonite) and K (K-jadeite).The end-members used are given in Table A3, with ideal activities being given by a mixing-on-sites model.The thermodynamic data were adjusted to give the small Ti and K contents seen in KLB-1 and in granulite facies metabasites.
The non-ideality is expressed as an asymmetric van Laar model, using the formalism of Holland & Powell (2003).Ws, H i mod and asymmetry parameters are given in Tables A4 and A5.Ws involving Fe, Fe 3þ , Na and Cr end-member non-idealities are determined using the approach in Powell et al. (2014).Symbols and values are explained under the melt section above.

ORTHOPYROXENE
Orthopyroxene is also modelled after Jennings & Holland (2015), with addition of Mg-buffonite and orthojadeite.The thermodynamic data were adjusted to give the small Ti and Na contents seen in KLB-1 and in granulite facies metabasites.The end-members used are given in Table A6.The non-ideality is expressed as an asymmetric van Laar model, which is given in Tables A7 and  A8, with symbols and values as for clinopyroxene.

GARNET
Garnets are modelled as in Jennings & Holland (2015) and Miller et al. (2016), with addition of a Ti end-member (tig) and by including asymmetric mixing.The model interaction energies for garnet are as follows (in kJ): Table A4: Interaction energies for mixing of clinopyroxene end-members (kJ) Table A5: H mod i increments to Gibbs energies, and asymmetry parameters a for mixing, of clinopyroxene end-members The asymmetry parameters (van Laar) are grossular (2Á5) and andradite (2Á5), with all others set at unity.
The interaction energies have been modified from those in Jennings & Holland (2015) and adjusted in the Monte Carlo fitting of garnet compositions in the assemblages used.

SPINEL
Spinel is modelled as a two-site solid solution in which order-disorder may occur between the octahedral M and tetrahedral T sites.The end-members in Mg-Fe-Al-Fe 3þ -Cr-Ti-O used are as follows There are four compositional parameters (x ¼ Fe/ (Fe þ Mg), y ¼ Fe 3þ /(Alþ Fe 3þ ), c ¼ X Cr, M , t ¼ 2X Ti, M ) and three order parameters (Q1 ¼ X Mg, T À X Mg, M , Q2 ¼ X Fe, T À X Fe, M , Q3 ¼ X Fe3þ, T À X Fe3þ, M ).The activity model is similar to that of Sack & Ghiorso (1991a, 1991b), but we reduce the entropy of mixing on the M site by a factor of 2 as reasoned in Holland & Powell (1996), following Carpenter & Salje (1994).
Ideal activities are calculated from a sp ¼ X Mg;T X Al;M a hc ¼ X Fe;T X Al;M a nmt ¼ X Fe;T X Fe3þ;M a pcr ¼ X Mg;T X Cr;M Monte Carlo methods were used to determine the spinel mixing parameters so that the following experimental constraints were met: 1. Ordering of simple end-members (spinel, hercynite, magnetite and magnesioferrite) using data from Harrison & Putnis (1996) and Andreozzi et al. (2000).2. In MAS, the equilibrium py þ fo ¼ opx þ sp was made to pass through the brackets of Danckwerth & Newton (1978) and Gasparik & Newton (1984).3. The solvus between magnetite and hercynite (Turnock & Eugster, 1962) was fitted.4. The complex 'quad' solvus between Al-poor spinels and Al-rich spinels in the system Mg-Al-Fe-Fe 3þ was fitted to experimental tie lines in Lehmann & Roux (1986).5.The Fe-Mg distribution between olivine and spinel for Al-Cr spinels of Engi (1983) were fitted.
The final set of interaction energies (in kJ/mol) used is The increments (H i mod values, in kJ/mol) to the dataset Gibbs energies for end-members are given as: isp : H mod In the above sp, isp, hc, and ihc are made ordered before adding the H i mod values, which are for disordering.For nmt and imt the H i mod values are added to the equilibrium Gibbs energies from the dataset.

EXPERIMENTAL CONSTRAINTS USED FOR THE MELT MODEL
1. Beginning of melting of plagioclase peridotite at 1 bar, with a temperature bracket of 1120-1180 C (Takahashi et al., 1993).
Fig.3.P-T pseudosection calculated for KLB-1 in the KNCFMASTOCr system.Phases: g, garnet; sp, spinel; pl, plagioclase; ol, olivine; cpx, clinopyroxene; opx, orthopyroxene; liq, liquid.In comparison with the earlier study ofJennings & Holland (2015) the spinel-garnet transition widens more dramatically at low temperatures and spurious spinel does not appear at high temperatures.Note that the model, as that ofJennings & Holland (2015), predicts a negative slope for the solidus just below the plagioclase to spinel transition pressure.

Fig. 7 .Fig. 8 .
Fig. 7. Experiments on high-Al basalt AT-1.(a) Experiments: open symbols: noble-metal-encased samples; filled circles: graphiteencased samples.Figure adapted fromBaker & Eggler (1983).(b) Calculated boundaries (from Fig.8) for AT-1 to compare with experiments.The dashed line connects the pigeonite-cpx solvus crest to the cpx-out boundary where garnet joins.Although their diagram, reproduced here, shows the liquidus at 1250 C, their data at 1 atm show it to be at 1270 C, almost identical to the calculations.The more reduced graphite-encased samples and more oxidized noble-metal-encased samples may differ slightly in their phase boundaries.

Fig. 12 .
Fig. 12. Calculated P-T pseudosection for KFMASH pelite from figure 4 of White et al. (2014), with fields from the new melt and fluid model superimposed as dashed lines for comparison.Note that the ksp-out boundary has moved up-temperature by around 20 C.

Table 1 :
Melt end-members used in this study melt end-member is treated as a single molecule that mixes with the other molecular endmembers on a single site.The fluid is considered to be a dilute mixture in which all melt components mix ideally amongst themselves (e.g.W jdL-kjL ¼ 0), but each endmember mixes non-ideally with H 2 Calculated solubility of H 2 O in haplogranite granite melt (Qz 28 Ab 38 Or 34 wt %) compared with the experimental data of Holtz et al. (1995).
Fig. 10.Comparison of (a) experiments of Whitney (1975) and (b) calculations for haplogranite at various water contents at 2 kbar.

Table A1 :
Interaction energies for melt end-members (kJ)

Table A2 :
H mod

Table A7 :
Interaction energies for mixing of orthopyroxene end-members (kJ)

Table A3 :
Clinopyroxene end-members in the model

Table A6 :
Orthopyroxene end-members in the model