Sato et al., Technical Report of ISEI Ser. A. No. 65 March 1996

Phase relations of phlogopite with and without enstatite up to 8 GPa : implication
to potassic magmatism and mantle metasomatism

Kiminori Sato, Tomoo Katsura and Eiji Ito
Institute for Study of the Earth's Interior, Okayama University,
Misasa, Tottori 682-02, Japan
Submitted to Earth and Planetary Science Letters, 1996
Abstract
Stability relations of phlogopite and phlogopite+enstatite systems without additional water have been
experimentally determined at pressures of 4 - 8 GPa and at temperatures of 1200 - 1500˚C by using auniaxial split-sphere apparatus. The phlogopite gradually dissociates into pyrope and Al2O3-deficientphlogopite at pressures above 5 GPa. The Al2O3 content in the phlogopite decreases from 14.4 wt% at
6 GPa to 12.9 wt% at 8 GPa and concurrently the mode of pyrope increases from <5 modal% at 5.GPato 20 - 30 % at 8 GPa, in the phlogopite system. Thus increase in pressure enhances two cation
substitutions, Al+Al=Mg+Si and Mg+2Al= []+2Si, in the phlogopite structure. The solidi of thephlogopite and phlogopite+enstatite systems reach maximum temperatures of 1350˚C and 1300˚C,respectively, at about 5 GPa. Pyrope is a residual phase under hypersolidus conditions in both thesystems above 5 GPa.
The stabilities of phlogopite determined in the present study indicate that phlogopite can be stablein garnet harzburgite in the subcratonic lithosphere down to 210 km depth. The observed
decomposition of the phlogopite into an assemblage of garnet and fluid or hydrous silicate melt
suggests that phlogopite can be secondarily formed by a reaction of garnet and upwelling metasomatic
agents near the base of continental lithosphere.
1. Introduction
It has been recognized that the stability of phlogopite and reaction of phlogopite with K2O bearing
fluid or hydrous silicate melt play important roles in the mantle processes as follows; (1) fluid that isreleased by break-down of phlogopite lowers the solidus temperature of peridotite and causes magma
generation [1], (2) fluid or hydrous silicate melt originated from deeper part of the mantle
metasomatizes peridotite and often forms secondarily phlogopite [2,3], and (3) ultrapotassic magma
would be produced by partial melting of phlogopite-bearing peridotites formed by the process (2) andthus could be a source of ultrapotassic magma, such as lamproites [4,5]. With regard to the process (2),phlogopite is often found in mantle xenoliths in various tectonic settings [3,6,7,8]. The phlogopiteinvolving reactions described above are directory related to the pressure-temperature stability limit ofphlogopite. Therefore, the determination of stability of phlogopite under the wide range of mantle
conditions is of essentially importance to understand the distribution of phlogopite, melting processes
and metasomatic phenomena in the upper mantle.
Many workers experimentally investigated the stability of phlogopite under upper mantle conditions[9, 10, 11, 12, 13, 14, 15, 16]. For example, Sudo and Tastumi [1] has reported that phlogopite reacts
with clinopyroxene and enstatite to produce K-amphibole (K2CaMg5Si8O22(OH)2), garnet andforsterite at pressures of 6 - 7 GPa in subsolidus condition, suggesting that the high pressure stabilityof phlogopite in lherzolite is limited by a univariant
reaction:
2phlogopite +diopside +2enstatite =K-amphibole +2garnet +2forsterite +fluid(1)
However, phase relations of phlogopite and phlogopite+enstatite systems above 4 GPa have been
still remained unclear. Without detailed information of melting relation of phlogopite, the following
processes and whereabouts in the mantle these processes take place cannot be discussed properly.
1Sato et al., Technical Report of ISEI Ser. A. No. 65 March 1996

First, phlogopite is often observed in garnet harzburgite xenoliths in kimberlites and is considered to
be formed secondarily by the reaction of metasomatic agents such as hydrous silicate melt or fluid andperidotites [2,3,7]. Secondly, composition of liquid generated by low degree of partial melting ofphlogopite-bearing rocks are indispensable to understand the formation of ultrapotassic magmas.
Third, concerning with the stability of phlogopite, the phase relations of phlogopite +enstatite are
required to discuss how phlogopite decomposes or forms in Ca-rich pyroxene-free peridotites orphlogopite-pyroxenites which may be presented in wedge mantle along subducting slab [17,18].
In the present study, the melting and subsolidus phase relations are experimentally investigated on
two systems, phlogopite and phlogopite + enstatite, without additional H2O at pressures of 4 - 8 GPa
and at temperatures of 1200 - 1500˚C. Based on these results, stability and secondary formation ofphlogopite in the subcratonic mantle are discussed together with lamproite petrogenesis.
2. Experimental and analytical procedure
2.1. Starting materials
Starting materials of phlogopite was natural specimen from Posu mine in Korean peninsula and its
chemical composition is shown in Table 1. Enstatite was crystallized at 1500˚C for 48 hours from
MgSiO3 glass. Examinations of two phases by electron microprobe analysis and powder X-ray
diffractmetry confirmed that both were single phases. These crystalline phases were ground and thendried at 110˚C for more than 24 hours. A single phase of phlogopite and an even weight mixture ofphlogopite and enstatite were used as starting materials for the high-pressure and high-temperatureexperiments. No water was added to the starting materials.
2.2. High-pressure experiments
High-pressure and high-temperature experiments were performed by utilizing a uniaxial splitsphere apparatus [19]. Tungsten carbide cubes of 11 mm truncation were used with a semi-sinteredmagnesia octahedron of 18 mm edge length as a pressure medium (11-18 system). Sample pressure
was calibrated against the oil pressure of the hydraulic press. Pressure calibration curve for the 11 -18
system was constructed by detecting phase the olivine - spinel transition in Fe2SiO4 [20] and theolivine - modified spinel transition and the modified spinel - spinel transitions in Co2SiO4 [21] at
1300˚.
Cell assembly is schematically shown in Fig. 1. Heating material was graphite with a straightcylindrical shape. A cylindrical sleeve of ZrO2 was placed outside the graphite heater as a thermal
insulator. The temperature was monitored using a W97%Re3%-W75%Re25% thermocouple with adiameter of 0.2 mm, without any correction for the pressure effect on e.m.f. Sample was charged in asealed Pt capsule of 1.2 or 1.5mm outside diameter, 0.8 or 1.0 mm inside diameter and of 1 - 2 mm
length. Temperature distribution in the sample container was measured by simultaneous measurementsof temperatures at the hot and the cold ends of the sample container. The temperature difference
between both ends was about 50˚C at 1300˚C.
Pressure was applied first and then the sample was heated to a desired temperature. Experimental
temperature and pressure were controlled within ±10˚C and ±0.05 GPa of the nominal values,
respectively. After kept for a desired duration, the sample was quenched by shutting off the electric
power under the working pressure. Temperature went down below 200˚C within a second. The
recovered sample was mounted in epoxy resin and made a polished section parallel to the longitudinal
direction of the cylindrical heater.
Run durations were 60-120 and 30-90 minutes at temperatures below and above 1300˚C,respectively (Table 2). The attainment of equilibrium was tested by conducting two run durations of 1
and 10 hours on the phlogopite+enstatite system at 6 GPa and 1300oC. The mineral assemblage
and chemical composition of 1 hour duration were found to be identical to those of 10 hours duration(Table 3). Thus it was concluded that equilibrium was attained in 1 hour running duration attemperatures above 1300˚C.
2.3. Analyses
Phases present were identified by means of electron microprobe analysis and micro-focused X-ray
2

Sato et al., Technical Report of ISEI Ser. A. No. 65 March 1996

diffractmetry. Chemical compositions of the phases were analyzed by using JEOL JXA-8800 electronprobe microanalyzer, operated at 15 Kev accelerating voltage. The beam current and diameter were
2X10-8A and 1 µm, respectively.
A criterion for occurrence of melting was the existence of quench crystal of phlogopite which
exhibits needle texture (Fig. 2a,b). When the quench phlogopite coexisted with solid phlogopite in theexperimental charge, the former has lower Al2O3 content than the latter. Therefore, the Al2O3 content
of phlogopite was also a good indicator to judge the occurrence of partial melting.
3. Results
3.1. Stability relations of phlogopite and phlogopite+enstatite systems
Table 2 summarizes the experimental results on the systems of phlogopite and phlogopite +enstatite. Figs. 3a and b show phase relations of the phlogopite and the phlogopite+enstatite systems,
respectively.
Phlogopite: Up to 4.5 GPa, phlogopite is stable under subsolidus conditions, and incongruently melts
into liquid+forsterite. Above 4.5 GPa, pyrope is present under both the subsolidus and hypersolidus
conditions. Under subsolidus conditions of 5 - 8 GPa, the starting phlogopite dissociates into an
assemblage of Al2O3-deficient phlogopite, pyrope and fluid. The amount of pyrope increases withincreasing pressure; less than 5 modal % at 5 GPa to 20-30 modal % at 8 GPa. Water drops wereactually observed in the run products under the subsolidus conditions, indicating the presence of fluidin the conditions. An assemblage of phlogopite +pyrope melts to forsterite+pyrope+liquid at pressures
between 5 and 8 GPa. However, forsterite disappears at temperatures 50 - 100˚C higher than thesolidus. The assemblage starts to melt at 1350˚C and 5 GPa, and at 1300˚C and 8 GPa, respectively.
Thus solidus curve has a negative dT/dP slope above 5 GPa. It is also noted that MgAl2O4 spinel
coexists with pyrope and forsterite in the limited hypersolidus condition around 5 GPa and 1400˚C(Fig. 3a).
Phlogopite + enstatite: Main features of the phase relations in this system are similar to those of thephlogopite system. For example, pyrope occurs both in the subsolidus and hypersolidus conditions
above 5 GPa, and the solidus curve has a negative dT/dP slope above 6 GPa. The solidus curve inthis system is about 50˚C lower than that of the phlogopite system at pressures of 4 - 6 GPa, but bothare very close at 8 GPa. Coexisting phases with melt just above the solidus are as followings; enstatite,
forsterite and phlogopite at 4 GPa, enstatite, forsterite, pyrope and phlogopite at pressure of 5 - 7 GPa,and enstatite, pyrope, and phlogopite above 7 GPa. Coexistence of solid and quench phlogopites wasobserved in the four runs conducted at 1350˚C and pressures of 4 GPa, 5 GPa and 6 GPa, and in that at
8 GPa and 1300˚C (Fig. 3b), indicating that partial melting of phlogopite occurs in this system justabove the solidus conditions. The partial melting of phlogopite in the system of phlogopite+enstatite
was confirmed at pressures of 1 - 2.5 GPa, and the temperature range in which melt and phlogopite
coexist was reported to be 30-40˚C [11]. The melting interval of phlogopite was found to be certainly
less than 100˚C as shown in Fig. 3. However, more specification was impossible, because thetemperature variation in the sample was about 50˚C in this study. Thus the melting interval is assumed
to be about 40˚C by extrapolation of those at the lower pressures [11] (Fig. 3b). An occurrence of verysmall amount of forsterite at 5 GPa and 1300˚C, which is marked in the field of enstatite+phlogopite+pyrope +fluid, is due to the thermal gradient through the sample and the running condition shouldhave been very close to the solidus.
3.2. Variation in the composition of phlogopite under subsolidus conditions
Chemical compositions of phlogopite are shown in Table 3. Figs. 4a and b show negative
correlation between Al+Al and Mg+Si and that between Mg+2Al and V (vacancy) +2Si in thephlogopites, respectively. The change in phlogopite composition due to the cation substitutions areshown in Fig. 5. Number of vacancy was calculated by subtracting the cation sum of Si, Al, Mg, Fe, Ti
and Mn in the phlogopite (represented as #1 in Table 3) from the total number of tetrahedral andoctahedral sites in the phlogopite structure. With increasing pressure, the cation numbers of Si and Mgof phlogopite in both systems increase, while that of Al decreases. Therefore, two substitutions, Al+Al-> Mg+Si and Mg+2Al -> []+2Si control the change in composition of phlogopite with pressure. In thephlogopite system, these compositional changes in phlogopite are corresponding to the exsolution of3

Sato et al., Technical Report of ISEI Ser. A. No. 65 March 1996

pyrope, because Mg:Al:Si ratio is 3:2:3 in pyrope and 3:1:3 in phlogopite. Namely, the exsolution ofpyrope reduces the relative abundance of Al to Mg and Si in phlogopite. In the phlogopite+enstatite
system, the degree of deficiency of Al2O3 component in phlogopite is higher than those of thephlogopite system at the same pressure (Fig. 5). This would be responsible to the larger amount ofpyrope crystallization in the phlogopite +enstatite system. In the former system, crystallization ofpyrope is prompted by the presence of enstatite.
The solidus of the enstatite+phlogopite system between 5 and 6 GPa is univariant curve on which 6
phases exist, provided that 5 components (Al2O3, SiO2, MgO, K2O and H2O) comprise the system.
This means that K2O and H2O behave as independent components in the system. This is consistent
with the observation that number of K in phlogopite formula increases at 1300˚C with elevatingpressure (Table 2).
3.3. Chemical composition of melt produced near solidus in the phlogopite+ enstatite system
The compositions of melts produced by partial melting near solidus in the phlogopite+enstatite
system are necessary to understand the generation of ultrapotassic magmas. We can estimate
compositions of the melts produced near solidus in phlogopite +enstatite system by analyzing thequench phlogopite which was the former liquid. All of Mg, Al and Si should be accommodated in thequench crystal of phlogopite, because no other quench crystals than phlogopite was observed in therun products. However, some excess potassium and water may not be accommodated in quench
crystals of phlogopite
Average chemical compositions at each pressure of quench crystals of phlogopite are shown in
Table 4, and are plotted on a MgO-SiO2-Al2O3 diagram as shown in Fig. 6. The compositions of melts
produced at 4 - 8 GPa [12] are more enriched in MgO, or more basic than those at 1 GPa, suggestingthat the compositions of melt is enriched in MgO component up to 6 GPa. In the pressure range of 6- 8
GPa, the melt composition becomes less aluminious according to pyrope fractionation.
4. Discussion
4.1. Stability of phlogopite In the mantle
The phase relations of the phlogopite+enstatite system are applicable to assess the stability ofphlogopite in the mantle, because the mineral assemblage on solidus of the phlogopite +enstatite
system contains three major mantle minerals, forsterite, enstatite and garnet at pressures of 4 - 7 GPa.
As addition of other component such as FeO reduce the stability of phlogopite, however, the stabilityof phlogopite determined in the present study should be regarded as 'maximum phlogopite stability'.
As phlogopite occurrence was reported in the mantle xenoliths from various tectonic settings, i.e.,
subcontinental lithosphere [3], the mantle beneath the back-arc region [4], and the oceanic mantle
beneath oceanic island [7], phlogopite is ubiquitously present in the mantle down to at least 200 km
depth. The phlogopite in the mantle is mostly caused by a secondary formation process in whichhydrous metasomatic agents scavenging potassium react with peridotites (e.g.,[3,7]). In order tounderstand the process of formation of secondary phlogopite, the phlogopite stability in the mantles
beneath various tectonic settings has to be clarified.
The phlogopite was shown to be stable in lherzolite up to about 6 GPa or 180 km depth by previous
studies [1, 11]. Based on the present experimental results, we here discuss the stability of phlogopite inphlogopite-pyroxenites in the wedge mantle being dragged by a down going slab, and garnet
harzburgite in the subcratonic mantle. Phlogopite is expected to be more stable in these rocks than inlherzolite, because the geotherms for these regions are lower than that for oceanic mantle and theserocks may not have so large amount of Ca-rich pyroxene as all of the coexisting phlogopite isconverted into K-amphibole according to the reaction (1).
Concerning the phlogopite pyroxenites, Sekine and Wyllie [17,18] advocated that the discrete
masses of phlogopite-orthopyroxenite or garnet-phlogopite-websterite are formed in the mantle justoverlying subducting slabs by hybridization of cool hydrous magma from down going slabs into
hotter overlying harzburgite or lherzolite, respectively. These phlogopite-bearing rocks are candidates
to transport water and potassium into the deep mantle along with the subducting slab. In phlogopiteorthopyroxenite under the thermal conditions of the normal subducting slab [22], phlogopite would
begin to break down gradually and to produce garnet and fluid at about 5 GPa or 150 km depth.
However, phlogopite could survive to at least 240 km depth, so that the present experimental results
4Sato et al., Technical Report of ISEI Ser. A. No. 65 March 1996

indicate that H2O and K2O could be transported to 240 km depth by phlogopite. In the garnetphlogopite-websterite, phlogopite reacts with Ca-rich pyroxene according to the reaction (1) at 6GPaor 180 km depth. However, if the amount of phlogopite is so large that phlogopite cannot be consumed
by the reaction (1), the remained phlogopite could survive to the depth as in the phlogopite
orthopyroxenite. Thus, both the rocks can be transporters of H2O and K2O and potential source rocksof potassic magma in the deep mantle as well. Actually the phlogopite-orthopyroxenite in paleo
subduction zones is regarded as a source rock of phlogopite-lamproite which erupted in continental
margins[23].
Phlogopite is often found in diamond-bearing harzburgite which is considered to constitute near thebase of subcratonic lithosphere of 150 - 200 km depths [7,24,25]. This is consistent with the present
experimental results as following. The stability of phlogopite in harzburgite constituting the
subcratonic mantle is defined by the intersection of the solidus of the phlogopite +enstatite system andsubcratonic geotherms (Fig. 7). For a typical geotherm for surface heat flow of 40 mWm-2[26], thestability limit of phlogopite is 7 GPa or 210 km (point A in Fig. 7). However, it should be notedthat the stability of phlogopite is limited to a shallower depth for a higher temperature distribution;e.g., 180 km depth for that of 44 mWm-2 surface heat flow (point B in Fig. 7).
4.2. Exsolution of garnet and production of fluid
Present study demonstrates that pyrope is present in both the subsolidus and hypersolidus conditionsat pressures above 5 GPa. The reaction to form pyrope is not univariant: the amount of pyrope
exsolved in the phlogopite and phlogopite + enstatite systems under subsolidus conditions increases
with increases in both pressure and temperature. Dehydration proceeds with the exsolution of pyrope
as the following schematic reaction;
Starting phlogopite=Al2O3-deficient phlogopite + pyrope + K2O +fluid (H2O)(2)
Occurrence of the dehydration was confirmed by observation of water drops in the sample charges.
Part of expelled potassium from phlogopite should be dissolved in the fluid phase (H2O) because H2O
fluid leaches K2O component together with SiO2 and Al2O3 [27,28], and the remainder ofpotassium would be in the grain boundary. The concentration of potassium along grain boundary wasconfirmed in a run of phlogopite+enatatite system (6GPa, 1300˚C and 10 hours). Increases in pressure
and temperature proceed the reaction (2) to the right hand side; e.g., increase in the amounts of pyrope
and released H2O fluid phase.
4.3. Secondary formation of phlogopite in the continental lithosphere
Replacement of garnet to phlogopite has been petrographically observed in the metasomatizedmantle xenoliths, which suggests phlogopite in peridotites from continental lithosphere are formed
secondarily (e.g., [3,7]). The metasomatizing agents rise adiabatically from underlying asthenosphere(Fig. 8b) and therefore should be enriched in potassium during upwelling through the undepleted
asthenosphere. These agents are inferred to be hydrous silicate melt in high-temperature conditions ora fluid in low temperature conditions. The present study reveals that phlogopite melts incongruently
into garnet and hydrous silicate melt and partially break down to garnet and fluid containing K2O
under subsolidus conditions. Thus the metasomatic agents may react with garnet in garnet peridotites
and then the secondary phlogopite is formed when they encounter the solidus of thephlogopite+enstatite system. As shown in Figs. 7a and b, the solidus and the typical geotherms for thecontinental mantle represented by the surface heat flows of 40 and 44 mWm-2 cross at about 6 GPa or
around 190 km depth, which corresponds to the base of subcratonic lithosphere. However, it should be
noted that the negative slope (dT/dP<0) of the solidus curve indicates deeper formation of secondary
phlogopite for the lower geotherms (Figs. 8a and b).
Stability of phlogopite is sensitive to change of the thermal state of continental lithosphere. For
example, if a thermal gradient increases from the surface heat flow of 40 mWm-2 to 44 mWm-2, thenphlogopite in the region of 170 - 200 km depths are melted out, as depicted by an area marked with acrossed pattern in Fig. 8c. As schematically illustrated in Fig. 8d, when temperature of a certain depth
rise from TA to TB, the following processes proceed: (1) at temperatures from TA to TB, phlogopite
decomposes gradually into garnet+ Al-depleted phlogopite+fluid to release fluid,5Sato et al., Technical Report of ISEI Ser. A. No. 65 March 1996

without melting and (2) at temperature TB, on the other hand, phlogopite begin to melt out.
These released fluid and silicate melt could serve as new metasomatic agents in the mantle:
Released fluid reduce the solidus temperature of a metasomatized peridotites to cause alkaline andhydrous magmas (e.g., kimberlite or lamproite magmatism); Released hydrous silicate melts makesecondary enrichment to form phlogopite and other minerals for the sources of alkaline magmas in theshallower part of the mantle.
4.4. Implication to lamproite petrogenesis
Lamproites are ultrapotassic rocks and some of them (olivine lamproite and leucite lamproite) havediamonds [29]. It is inferred from their potassium rich composition that phlogopite should have beenin their source material [8].
Recently, K. Sato [30] has revealed that (1) the olivine lamproite with 6 wt% H2O is saturated witholivine, orthopyroxene and garnet at 5.3 GPa and 1370˚C, and that (2) the magma with 12 wt% H2O issaturated with orthopyroxene and garnet at 5.7 GPa and 1340˚C. As these conditions are higher than
the solidus of the phlogopite+enstatite system determined in the present study, phlogopite in thesources should have been consumed during the partial melting process before magma segregation.
Thus, either phlogopite-bearing garnet harzburgite or phlogopite-bearing garnet orthopyroxenite canbe the source of olivine lamproite. The problem which rock is the source of the olivine lamproite canbe solved by a comparison of melt composition produced near solidi of two rocks with that of theolivine lamproite.
As seen in Fig. 6, the composition of melts produced just above the boundary curve of phlogopiteout in phlogopite-enstatite system at pressures from 5 to 8 GPa, under which conditions the systemrepresents phlogopite-garnet-orthopyroxenite, are different from that of olivine lamproite (point A).
On the other hand, the composition of melt produced near solidus from phlogopite harzburgite atpressures from 5 to 8 GPa has not been reported. However, Inoue [31] determined that the Mg/Si ratioof the initial melt produced by a partial melting of hydrous harzburgite up to 15 GPa. His results
approximately indicate the melt composition produced by partial melting of phlogopite harzburgite,
because phlogopite in the olivine lamproite source is not residual phase. The Mg/Si ratio in partial
melt of hydrous harzburgite under 5-6 GPa is in agreement with that of the olivine lamproites [29].
Therefore, the olivine lamproite magma is produced by partial melting of phlogopite-bearing garnet
harzburgite, but not by that of phlogopite-garnet-orthopyroxenite.
5. Conclusions
Melting and subsolidus phase relations in the systems of a natural phlogopite and thephlogopite+enstatite were investigated using the uniaxial split-sphere apparatus in the pressure rangeof 4 - 8 GPa and in the temperature range of 1200 - 1500˚C. Pyrope garnet forms in subsolidus andhypersolidus conditions above 5 GPa in both the systems. Fluid phase gradually forms undersubsolidus conditions, and the amount of fluid increase with elevating pressure and temperature inboth systems. Solidus temperatures of both the systems have maxima at about 5 GPa and reachnegative dP/dT slopes above the pressure. Phlogopite can be stable down to 180-210 km depth insubcalcic garnet harzburgite which constitute the subcratonic lithosphere mantle. Phlogopites could be
produced by the interaction between upwelling metasomatic agents and garnet in the host garnet
harzburgite near the base of continental lithosphere. If once secondary phlogopite has been formed in
the lower part of the subcontinental lithosphere, a metasomatic agent of fluid or hydrous silicate melt
could be produced by decomposition of phlogopite on athermal event which raises temperature. These
metasomatic agents would induce volcanism or metasomatized peridotites in the shallower parts of thesubcontinental lithosphere.

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