Journal of Petrology

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Journal of Petrology Volume 41 Number 4 Pages 583-603 2000
© Oxford University Press 2000

Transport and Storage of Potassium in the Earth’s Upper Mantle and Transition Zone: an Experimental Study to 23 GPa in Simplified and Natural Bulk Compositions

JÜRGEN KONZETT^,* and YINGWEI FEI

GEOPHYSICAL LABORATORY AND CENTER FOR HIGH-PRESSURE RESEARCH, CARNEGIE INSTITUTION OF WASHINGTON, 5251 BROAD BRANCH ROAD NW, WASHINGTON DC, 20015-1305, USA

Received March 22, 1999; Revised typescript accepted September 21, 1999

	ABSTRACT

TOP ABSTRACT INTRODUCTION COMPOSITION OF STARTING... EXPERIMENTAL AND ANALYTICAL... PREVIOUS EXPERIMENTAL WORK RESULTS DISCUSSION APPENDIX REFERENCES

 
We have investigated the stability and composition of potassium amphibole and its high-pressure breakdown product phase X in synthetic peralkaline and subalkaline KNCMASH (K₂O–Na₂O–CaO–MgO–Al₂O₃–SiO₂–H₂O) and natural KLB-1 peridotite bulk compositions between 10 and 23 GPa at 800–1800°C. In the KNCMASH system, potassium amphibole reaches its upper pressure stability limit at 13–15 GPa at 1400°C. In the natural KLB-1 bulk composition, potassium amphibole breaks down between 12 and 13 GPa at 1200°C. Phase X is a hydrous potassium–magnesium silicate with variable stoichiometry, a general formula K_2–xMg₂Si₂O₇H_x with x = 0–1, and a maximum possible H₂O content of 3·5 wt %. Electron microprobe analytical totals suggest H₂O contents of 1–2 wt % and a decrease in H₂O contents with increasing pressure. In both KNCMASH and KLB-1 systems, phase X coexists with Mg₂SiO₄ + garnet + high-Ca clinopyroxene + low-Ca clinopyroxene ± fluid. Phase X breaks down between 20 and 23 GPa at 1500–1700°C to form K-hollandite + -Mg₂SiO₄ + majorite + Ca-perovskite + fluid. The upper temperature stability limit of phase X was located in the subalkaline KNCMASH system between 1400 and 1600°C at 14 GPa and at >1700°C at 20 GPa, the latter being at least 200°C above an average current mantle adiabat. Thus, phase X could store and transport both water and potassium not only in subduction zone settings, but also in convecting mantle down to the transition zone–lower-mantle boundary. Phase X would also be an eminently suitable host for Rb, Cs, Ba or Pb.

KEY WORDS: experimental study; high pressure; phase X; potassium

	INTRODUCTION

TOP ABSTRACT INTRODUCTION COMPOSITION OF STARTING... EXPERIMENTAL AND ANALYTICAL... PREVIOUS EXPERIMENTAL WORK RESULTS DISCUSSION APPENDIX REFERENCES

 
The hydrous potassic phases (HPP) phlogopite and K-amphibole are major storage sites for potassium in the Earth’s upper mantle. Both host the incompatible trace elements with large ionic radii (Rb, Ba or Pb), which can occupy large and highly coordinated lattice positions in phlogopite and K-amphibole (Basu, 1978; Foley et al., 1995; Ionov et al., 1997). Phlogopite is an important component in sources of kimberlites, lamproites, lamprophyres and K-rich basaltic rocks (e.g. Wilkinson & LeMaitre, 1986; Esperança & Holloway, 1987; Rogers, 1992; Mitchell, 1995, and references therein; Sato, 1997). K-richterite has been suggested as a possible source of K, Ti and high field strength elements (HFSE) for kimberlites and ultrapotassic rocks (Foley, 1992; Taylor et al., 1994). Phlogopite is stable in Al-rich metasedimentary and peridotitic bulk compositions. In Al-rich bulk compositions phlogopite breaks down at relatively low P (and T) to form phengite ± K-feldspar and possibly K,Mg-rich fluid (Massonne & Schreyer, 1989; Massonne, 1992). In peridotitic bulk compositions, phlogopite is stable to at least 6 GPa and 1100°C with orthopyroxene + clinopyroxene + olivine (Konzett & Ulmer, 1999; see also Wendlandt & Eggler, 1980; Mengel & Green, 1989, for experiments at lower P) and to at least 12 GPa and 1350°C with phlogopite + clinopyroxene (Luth, 1997). Phlogopite is stable above the solidus of natural lherzolite at P 3 GPa (Wendlandt & Eggler, 1980). Experiments with phlogopite + enstatite (Sato et al., 1997) suggest above-solidus stability of phlogopite through continuous melting reactions between 4 and 8 GPa that form olivine + pyrope. Although it is difficult to distinguish hydrous melts from fluids at high P, K-richterite is likely to be unstable above the solidus in either subalkaline or peralkaline bulk compositions (Konzett et al., 1997; Konzett & Ulmer, 1999). However, K-richterite probably controls the P–T location of the solidus by incongruent melting (Gilbert & Briggs, 1974; Foley, 1991).

K-amphibole with a composition close to KNaCaMg₅Si₈O₂₂(OH)₂ can form as a high-P breakdown product of phlogopite in peridotitic bulk compositions at P > 6 GPa (Trønnes et al., 1988; Luth, 1997; Konzett & Ulmer, 1999). In Na-free systems the K-amphibole KKCaMg₅Si₈O₂₂(OH)₂ is present (Sudo & Tatsumi, 1990; Luth, 1997; Inoue et al., 1998; Yang et al., 1999). In peralkaline bulk compositions [mica–amphibole–rutile–ilmenite–diopside (MARID), lamproites] phase relations from natural rocks and from high-P experiments suggest a continuous stability of K-amphibole + phlogopite from 0·1 MPa to at least 8·5 GPa (Mitchell & Bergman, 1991, and references therein; Konzett et al., 1997).

Experimental studies of the KCMSH and KCMASH systems (Luth, 1997; Inoue et al., 1998) show that K-amphibole breaks down at high pressures to a hydrous K-rich silicate that is capable of transporting alkalis and water to even greater depths than does amphibole. The structure, stoichiometry and compositional variability of this phase—termed phase X (Luth, 1995)—are still unknown and its stability field is poorly constrained. The aims of our study are to (1) better constrain the stability field of phase X, especially its high-pressure stability limit; and (2) determine the chemical variability of phase X and coexisting phases. This will permit us to assess the potential of phase X as a storage site for water and alkalis in the mantle transition zone and to trace mechanisms of potassium and water recycling into the mantle.

	COMPOSITION OF STARTING MATERIALS

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Our subalkaline and peralkaline starting materials are mixes of high purity (99·95% purity) synthetic oxides or silicates and carbonates, and cover the full range of bulk compositions that can stabilize K-amphibole and its breakdown products (Table 1). They represent peridotitic and MARID-type (Dawson & Smith 1977) or lamproitic bulk composition, respectively. The phase relations of these bulk compositions at P < 10 GPa have been described by Konzett et al. (1997) and Konzett & Ulmer (1999). A simplified K₂O–Na₂O–CaO–MgO–Al₂O₃–SiO₂–H₂O (KNCMASH) system was examined to avoid the potential influence of fO₂ on phase relations in an Fe-bearing system at very high pressures as a result of stabilization of phases by Fe³⁺ partitioning. Despite this simplification, the system is still complex enough to permit all important exchange reactions (MgSiAl_-2, NaSiCa_-1Al_-1, CaMg_-1, KNa_-1) among the silicate phases. Additional experiments were conducted with KLB-1 peridotite (Takahashi, 1986) doped with 10 wt % synthetic K-richterite, to locate the amphibole phase X transition in a natural peridotitic bulk composition.

View this table: [in this window] [in a new window]   Table 1: Compositions of synthetic and natural starting materials

 

	EXPERIMENTAL AND ANALYTICAL TECHNIQUES

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Experiments (Table 2) were performed with Walker-type and split-sphere MA-8 multianvil devices at the Geophysical Laboratory (GL) and at the Bayerisches Geoinstitut (BG), respectively, using prefabricated pyrophyllite gaskets and MgO octahedra. Assembly sizes and furnace materials are as follows: GL: 10/5 (10 mm edge length of octahedra/5 mm truncated edge length of WC cubes) and 8/3 assemblies to 15 and 23 GPa using Re heaters; BG: 14/7 assemblies using stepped LaCrO₃ heaters. Pre-dried starting materials were placed in 1·55 mm or 1·00 mm (GL, for 8/3 assemblies) outer diameter Pt₁₀₀ capsules and welded shut immediately. For the KLB-1 starting material, an additional inner graphite capsule (approximate dimensions after runs: wall thickness 300 µm, bottom and lid 200 µm) was added. To minimize T gradients and phase separation as a result of thermal diffusion, the length of experimental charges ranged between 200 and 600 µm. T gradients were 20°C/100 µm for 8/3 assemblies and <10°C/100 µm for 10/5 assemblies (Bertka & Fei, 1997, and unpublished data, 1999). Temperatures were measured with W3%Re–W25%Re thermocouples without correcting for the pressure effect on e.m.f. Both pressure and temperature were computer controlled during the runs. Detailed descriptions of the GL and BG experimental and calibration procedures have been given by Bertka & Fei (1997) and Rubie et al. (1993), respectively.

View this table: [in this window] [in a new window]   Table 2: Summary of experimental run conditions and run products

 

Sample capsules from completed experiments were embedded in epoxy resin and ground to expose the center of the charges. Most phase compositions were analyzed with an electron microprobe at analytical conditions of 15 kV and 20 nA. Phase X, which was found to be extremely susceptible to beam damage and loss of alkalis, was analyzed with 5 nA beam current and a rastered electron beam as large as the size of phase X grains permitted (typically 10–20 µm). Counting times of 20 s on peaks and 10 s on backgrounds of the X-ray lines were ratioed to a combination of synthetic oxide (Si, Mg, Al), synthetic mineral (Na) and natural mineral (Ca, K) standards. Data were corrected on-line using the PRZ correction procedure. After standardization, no peak search procedures were performed on phase X grains, to minimize residence time of the electron beam. Microprobe analyses of phlogopite and amphibole–pyribole were recalculated assuming stoichiometric OH. No recalculation was attempted for phase X because H₂O was not determined quantitatively.

To search for structural OH in phase X, Raman spectra were recorded at the GL with a Dilor XY confocal micro Raman spectrometer equipped with a cryogenic Wright Model CCD. The excitation source was the 514 nm line of a Coherent Innova Model 90-5 Ar⁺ laser operating at 150 mW power using an integration time of 600 s.

	PREVIOUS EXPERIMENTAL WORK

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Phase X was described as a breakdown product of K-amphibole at P > 14 GPa between 1100 and 1400°C in the KCMSH system by Inoue et al. (1995a, 1998). Luth (1995, 1997) observed phase X in the system phlogopite–diopside at P 11 GPa. Based on secondary ionization mass spectrometry (SIMS) measurements of OH combined with microprobe analyses, Inoue et al. (1995a) proposed a formula of K₄Mg₈Si₈O₂₅(OH)₂ for phase X. Experimental results of Luth (1997) and Inoue et al. (1998) show a wide range in K₂O contents (10·2–19·4 wt % K₂O) and oxide totals (91·5–98·2 wt %) at relatively constant Si:Mg or Si:(Mg + Al) ratios of 1:1. Phase X may be the ‘amphibole-like mineral’ reported by Trønnes (1990) as a breakdown product of phlogopite between 11 and 12 GPa. The formula given by Trønnes (1990)—K_3·3Mg_6·5Al_0·5Si₇O₂₂(OH)₂—is similar to the composition of phase X reported by Luth (1997) and Inoue et al. (1998).

	RESULTS

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Petrography and chemical homogeneity of the phases
All starting materials readily recrystallize to mineral grains 10–50 µm in size at T 1100°C and to grains 10 µm in size in lower T runs (Fig. 1). HPPs are typically euhedral to subhedral but phase X also forms irregular grains that contain numerous olivine, clinopyroxene, or amphibole inclusions. Many phase X grains show irregularly spaced cleavage (Fig. 1b). A mixed-chain hydrous pyribole (sensu Veblen, 1981) was present in three runs, along with K-richterite. The former occurs as lath-shaped to needle-like crystals up to 100 µm x 20 µm in size. In high-P runs, K-hollandite appears as small (20 µm x 5 µm) needle-like crystals, and Ca-perovskite forms irregular patches up to 20 µm in size dispersed in the matrix or is present as inclusions in garnet. The phase distribution within individual capsules is inhomogeneous, and melt or quenched fluid is most abundant in the hotter part of the capsule. In experiments on the peralkaline bulk compositions, evidence for quench crystallization is present over a T interval of 200°C. The modal amounts of quench vary from <3% at 10 GPa and 1100°C to 20% at 10 GPa and 1350°C. Near the hot ends of the capsules, along the solid–liquid and quench interface, garnet and clinopyroxene often display larger grain sizes compared with cooler parts of the capsule. Inhomogeneities in grain size and phase distribution can be ascribed to grain maturation and chemical diffusion in a temperature gradient (Lesher & Walker, 1988) aided by the water content of the systems. With the exceptions of large garnets and phase X, neither systematic zoning of individual mineral grains nor differences in phase compositions are observed between tops and bottoms of the capsules. In low-T runs, diffuse Al-rich cores in garnets can be ascribed to incomplete equilibration. Phase X may show strong zoning with respect to K/(K + Na). This zoning is fairly regular, with K-rich cores and Na-rich rims, or is patchy and irregular. These compositional inhomogeneities are independent of run duration and temperature.

View larger version (96K): [in this window] [in a new window]   Fig. 1. Back-scattered electron photomicrographs of phase X-bearing assemblages taken from the centers of experimental charges (tops of the images point to the hotter end of the capsules). (a) Run JKW7 at 14 GPa and 1300°C showing euhedral crystals of phase X in part with heterogeneity in K/(K + Na): light areas are K rich, whereas dark areas are more Na rich. (b) Run JKW9 at 12 GPa and 1300°C with coexisting phase X and K-richterite. Phase X shows irregular cleavage and numerous inclusions of Kr and hiCapx. Holes in the sample surface are due to mechanical abrasion during polishing. (c) Run JKW47 at 20 GPa and 1500°C showing poikiloblastic phase X coexisting with majoritic garnet and -Mg2SiO4; abbreviations are given in Table A1.

 

Phase relations
Peralkaline KNCMASH
In the peralkaline KNCMASH system, HPPs are stable to at least 20 GPa and 1300°C, along with garnet + Mg₂SiO₄ + high-Ca clinopyroxene ± low-Ca clinopyroxene ± K-hollandite ± Ca-perovskite (Fig. 2). With increasing pressure, the first HPP to disappear is phlogopite, which is stable at 10 GPa between 1100°C and 1350°C along with K-richterite, but absent from run JKW17 at 11 GPa and 1300°C. In runs at 11 GPa and 1300°C, and 13 GPa and 1100°C K-richterite is the only stable HPP. At higher pressures, amphibole is joined by phase X as a result of continuous amphibole breakdown. At 15 GPa and 1100°C, and 14 GPa and 1300°C, the upper pressure stability limit of K-richterite is reached and amphibole is replaced by phase X as the HPP. Within the spacing of experimental data points, both phase X-in and K-richterite-out reactions have negative slopes with 5 MPa/K < dP/dT < 15 MPa/K for phase X-in and 10 MPa/K for K-richterite-out. At 20 GPa and 1300°C, K-hollandite appears as the first anhydrous potassic phase as a result of continuous phase X breakdown. Between 18 and 20 GPa, high-Ca clinopyroxene breaks down, and its diopside and jadeite components form Ca-perovskite and sodium-garnet solid solution, respectively.

View larger version (22K): [in this window] [in a new window]   Fig. 2. P–T diagram of experimental results in the peralkaline KNCMASH system (Tables 1 and 2). Phases present in the experimental charges are represented by black sectors within the run symbol; phases not detected are denoted by white sectors (inset upper left); abbreviations are given in Table A1; ACMA average current mantle adiabat (see text); - to ß-Mg2SiO4 and ß- to -Mg2SiO4 transitions according to Morishima et al. (1994) and Katsura & Ito (1989), respectively; experimental data at P < 10 GPa from Konzett et al. (1997) for comparison.

 

The high-temperature stability limit of all HPPs is reached between 1300 and 1400°C. At conditions of 8 GPa and 1400°C, and 13 GPa and 1400°C the stable assemblage is garnet + high-Ca clinopyroxene + Mg₂SiO₄ + quench (the term ‘quench’ is used to denote a mixture of unidentified and mostly K-rich phases that crystallized from a solute-rich fluid or a hydrous melt upon quenching). The spacing of experimental data points (Fig. 2) precludes discussion of the slope of the K-phase-out reaction(s), but in accordance with results at P 8·5 GPa (Konzett et al., 1997), we chose a positive slope. Because of the difficulty in distinguishing quenched melts from solute-rich fluids at high pressures based on textural evidence (see Konzett et al., 1997) we did not attempt to locate the position of the solidus. The mixed-chain hydrous pyribole was found in runs at 10 GPa and 15 GPa (Table 3), either with K-richterite + phlogopite or K-richterite alone (see Finger et al., 1998; Konzett & Fei, 1998).

View this table: [in this window] [in a new window]   Table 3: Average analyses of K-richterite and mixed-chain hydrous pyribole

 

Subalkaline KNCMASH and KLB-1
In the subalkaline system, K-richterite is stable to 13 GPa along with olivine + garnet + high-Ca clinopyroxene ± low-Ca clinopyroxene in an assemblage resembling a metasomatized lherzolite (Fig. 3). The apparent lack of low-Ca clinopyroxene in run Ma95sB is probably due to small grain sizes and the sparse occurrence of this phase in subalkaline runs. At P 14 GPa, K-richterite is replaced by phase X. The slope of the K-richterite-out reaction was assumed to be slightly negative, in accordance with the results of K-amphibole breakdown in the KCMSH system obtained by Inoue et al. (1998). Phase X is stable to at least 20 GPa. At this pressure, phase X may coexist with K-hollandite as a result of H₂O partitioning constraints (see below), and with Ca-perovskite produced by the breakdown of high-Ca clinopyroxene. At 23 GPa, phase X is absent and K-hollandite carries the K in the system. The upper T stability limit of phase X is between 1400 and 1600°C at 14 GPa, in an assemblage without a solid K-rich phase: high-Ca clinopyroxene + garnet + low-Ca clinopyroxene + quench. At 20 GPa, the K-phase-out reaction must occur at T > 1700°C, which is at least 200°C above an average current mantle adiabat (ACMA) as defined by a ß transition in Mg₂SiO₄ at 17·9 GPa and 1475°C (Katsura & Ito, 1989) and a perovskite + wüstite transition at 23·2 GPa and 1530°C (Ito & Takahashi, 1989).

View larger version (25K): [in this window] [in a new window]   Fig. 3. P–T diagram of experimental results in the subalkaline KNCMASH (pie symbols) and the KLB-1 (square symbols) systems (Tables 1 and 2). Meaning of symbols, ACMA and ß transitions in Mg2SiO4 as in Fig. 2; K-amph out I98 and L97 indicate K-amphibole-out reactions in the KCMSH and KCMASH systems according to Inoue et al. (1998) and Luth (1997), respectively; filled arrows bracket position of H2O-saturated solidus for KLB-1 peridotite according to Kawamoto et al. (1996) (K96).

 

In the K-richterite-doped peridotite KLB-1, HPPs coexist with garnet + low-Ca clinopyroxene + high-Ca clinopyroxene + Mg₂SiO₄. K-richterite is stable at 12 GPa and 1200°C after a run duration of 72 h. X-ray mapping also showed that a K-rich phase is dispersed within graphite, along the interface between experimental charge and inner graphite capsule (see Konzett & Ulmer, 1999). At 13 GPa and 1200°C, after an identical run time, no HPP could be identified and X-ray mapping showed that all K was concentrated at the charge–graphite interface in diffuse grain boundary films. Because of their extremely small grain size, the K-carrying phase(s) could not be identified. Small amounts of alkaline and possibly CO₂-rich fluid that probably formed by amphibole breakdown and/or graphite oxidation evidently destabilized phase X with increasing run durations. In a run that lasted only 48 h, phase X was stabilized at 14 GPa and 1200°C (see Table 9, below). Thus, the amphibole breakdown for starting material KLB-1 was placed between 12 and 13 GPa at 1200°C, which is 1 GPa below the amphibole phase X transition in the subalkaline KNCMASH system (Fig. 3).

View this table: [in this window] [in a new window]   Table 9: Representative analyses of coexisting phases from natural KLB-1 + 10% Kr

 

Mineral chemistry
K-amphibole and hydrous pyribole
In both peralkaline and subalkaline systems, the K-rich amphibole is K-richterite (Leake, 1978; revised Leake et al., 1997), which increases in K per formula unit (p.f.u.) and K/(K + Na) with P (Table 3, Fig. 4), as shown by Konzett et al. (1997). Above 8 GPa, amphibole has K in the M(4) site, with a maximum K(M4) of 0·51 ± 0·02 at 13 GPa and 1300°C in the subalkaline system. In the peralkaline system, K-richterite is the only HPP at 13 GPa and 1100°C and, hence, the K contents of amphibole depend only upon the bulk K contents. The lack of coexisting HPP can explain the sudden increase in K p.f.u. of amphibole and the deviation from the almost linear K p.f.u.–P trend (Fig. 4). In the subalkaline system, K-richterite is the only K phase at P 9 GPa, and therefore the increase in K p.f.u. and K/(K + Na) must be compensated by a decrease in the modal amount of amphibole within that P range (assuming constant or increasing K in the fluid phase). The amphibole contains small amounts of Al both as Al(IV) and Al(VI), and Al_tot decreases with increasing P to 0·09 and 0·12 p.f.u. in the subalkaline and peralkaline systems, respectively (Fig. 4). The Tschermak component decreases with increasing P, but it is sensitive to changes in the coexisting Mg-phase assemblage. This may explain the reversals in the observed Al(VI)–P trend (e.g. phl-out at >10 GPa and > 8 GPa in the peralkaline and subalkaline systems). The decrease of the Tschermak (tk) component can be attributed to a model reaction

in which garnet and high-Ca clinopyroxene grow at the expense of amphibole. Reaction (1) is equivalent to a reaction

which buffers the Tschermak content of calcic amphibole in tonalitic systems below the albite + quartz = jadeite reaction (Schmidt, 1993).

View larger version (25K): [in this window] [in a new window]   Fig. 4. Selected mineral chemical parameters of K-richterite as a function of P at constant T (1100°C) for peralkaline and subalkaline KNCMASH systems; each point represents an average of 5–13 individual amphibole analyses (Table 3).

 

In the KLB-1 starting composition, amphibole at its upper P stability limit is still close to K-richterite end-member composition (see Table 9, below) with 1·07–1·13 K p.f.u. and 0·11–0·13 Al p.f.u. The X_Mg for amphibole is 0·95 with X_Mg^Kr > X_Mg^loCapx > X_Mg^ol > X_Mg^hiCapx > X_Mg^ga.

A mixed-chain hydrous pyribole (Thompson, 1981; Veblen, 1981) was found with K-richterite ± phase X at 10 and 15 GPa. According to Finger et al. (1998), the pyribole belongs to the M_nPM_(n-1)P series with n = 1 and contains alternating single and double tetrahedral chains. The pyribole is a combination of 2 omphacite + 1 K-richterite, which ideally would yield

Pyribole compositions at 10 GPa are slightly enriched in Na and Al(VI) and depleted in Mg and Ca (Fig. 5), which can be explained by combined Tschermak and plagioclase exchange

operating on the pyroxene-layers within the pyribole structure. Pyribole formed at 15 GPa is poorer in Al and Na and richer in Ca and Mg relative to the ideal 2 omphacite + K-richterite combination, indicating the operation of reaction (3) in the opposite direction, which shifts the composition towards the hypothetical end-member KNaCa₃Mg₇Si₁₂O₃₄(OH)₂ (Fig. 5). These compositional changes can be rationalized in terms of the end-member reactions

each involving K-richterite and a clinopyroxene end-member. The decrease in Al and increase in K/(K + Na) (Table 3) with increasing P is consistent with the behaviour of K-richterite, phase X (see below) and phlogopite (Konzett et al., 1997).

View larger version (22K): [in this window] [in a new window]   Fig. 5. Compositional variation of mixed-chain hydrous pyribole in terms of Na–Ca and Mg–Al; , hypothetical pyribole end-member compositions (see text for explanation).

 

Phlogopite
Phlogopite is near the end-member composition, with a small excess of 0·02–0·04 Si p.f.u. (Table 4), indicating limited solid-solution with K(Mg_2·50·5)Si₄O₁₀(OH)₂ (Seifert & Schreyer, 1971).

View this table: [in this window] [in a new window]   Table 4: Average analyses of phlogopite

 

Phase X
Phase X is a disilicate (Libau, 1982), containing corner-sharing SiO₄ tetrahedra and has a stoichiometry A_2–xB₂Si₂O₇H_x with A = K or Na, B = Mg, Al or Ca, and x = 0–1, ranging from anhydrous A₂B₂Si₂O₇ to AB₂Si₂O₇H with a maximum possible H₂O content of 3·51 wt % and a vacancy on the A-position (H. Yang & J. Konzett, unpublished data, 1999; Konzett & Yang, 1998). Variable H₂O contents have been observed in phase E (Kudoh et al., 1993), phase D (Frost & Fei, 1998), hydrous wadsleyite (Kudoh et al., 1996) or hydrous modified spinel (Inoue et al., 1995b). The presence of OH in phase X in both subalkaline and peralkaline systems was confirmed by laser Raman spectroscopy. Raman spectra show strong peaks in the OH-stretching region at 3600 cm^-1 (Fig. 6). This is consistent with low average analytical oxide totals in the range of 96–98 wt % in most runs (Table 5). In the silicate stretching region, phase X spectra are characterized by major peaks at 635–640 cm^-1 and 895–903 cm^-1 (Fig. 6). The composition of phase X may be inhomogeneous in K/(K + Na), which may range between 0·15 and 0·88 in an individual run (Table 5). This compositional inhomogeneity is restricted to the K/(K + Na) ratio and occurs even if the coexisting phases are well equilibrated. Other chemical characteristics such as Al or Ca contents and Mg/Si ratios show no correlation with K/(K + Na). A decrease in the degree of inhomogeneity can be observed with increasing pressure and K₂O contents. Because the inhomogeneity of the phase X composition is independent of run duration and temperature, it cannot be easily explained by a failure of the runs to attain equilibrium. It might instead be a quench effect promoted by high Na contents of phase X. This effect may result from the difference of the ionic radii of K and Na,which leads to a lattice collapse during pressure release. Excluding Na-rich rims, an increase in K₂O and a decrease in Na₂O with increasing P (and T) is observed (Fig. 7). In the subalkaline bulk composition, the highest K₂O contents of phase X (20–25 wt %) are correlated with the highest oxide totals of >99 wt % (Fig. 8). Although oxide totals can be affected by the analytical technique (i.e. variable electron beam raster size adjusted to grain size of phase X) this correlation indicates decreasing H₂O contents of phase X with increasing P. The effect may be explained by an increasing ability of coexisting phases—especially Mg₂SiO₄—to incorporate H₂O (Kohlstedt et al., 1996), which changes H₂O partitioning and leads to a continuous dehydration of phase X. The increase in K of phase X with pressure is consistent with other studies (Luth, 1997; Inoue et al., 1998) and parallels results for phlogopite and K-richterite (Konzett & Ulmer, 1999).

View larger version (17K): [in this window] [in a new window]   Fig. 6. Unoriented laser Raman spectrum of phase X from run JKW7 (14 GPa and 1300°C, peralkaline KNCMASH) in the silicate (200–1200 cm-1) and the OH-stretching region (3500–3750 cm-1).

 

View this table: [in this window] [in a new window]   Table 5: Average and representative analyses of phase X

 

View larger version (21K): [in this window] [in a new window]   Fig. 7. Selected mineral chemical parameters of phase X as a function of P and T. •, Runs at 1300°C; , runs at 1100°C; , runs at 1200°C; x, runs at 900°C; , runs at 1700°C; , runs at 1600°C; open bars, composition range of phase X (Luth,1997); open stars, compositions of phase X obtained by Inoue et al. (1998); in (e) and (g), and • are analyses of K-rich and Na-rich grains/grain areas of phase X (see Table 5), respectively; in (a) and (c), only K-rich phase X analyses are plotted.

 

View larger version (24K): [in this window] [in a new window]   Fig. 8. Plot of wt % (K2O + Na2O) vs oxide totals obtained from electron microprobe analyses for phase X from the subalkaline KNCMASH system. Only K-rich phase X analyses are plotted.

 

Phase X contains from 0·4 to 2·6 wt % Al₂O₃ and from 0·05 to 0·5 wt % CaO, respectively, values consistent with observations by Luth (1997) and Inoue et al. (1998). Al decreases with increasing P at constant T, but varies little with T (Fig. 7). The increase of Al in phase X in the peralkaline bulk composition at 20 GPa reflects the stabilization of K-hollandite. The molar Si/(Mg + Al) ratios of individual phase X analyses are consistently closer to 1·0 than are the Si/Mg ratios. This suggests a replacement of six-coordinated Mg by Al through a coupled substitution to maintain charge balance. Although there are several possibilities, such as Al₂Mg_-3 or AlAlMg_-1Si_-1, the structure analysis gives no indication of ^[IV]Al and the most likely substitution is AlMg_-1K_-1. This exchange introduces a further vacancy on the K position for charge balance and would lead to a theoretical phase X end-member ₂(MgAl)Si₂O₇H or K(MgAl)Si₂O₇ for the anhydrous end-member. The Ca content of phase X does not exceed 0·8 wt % (Fig. 7) and can be explained by a simple CaMg_-1 exchange.

Garnet
All synthesized garnets are Mg-rich pyrope–grossular solid solutions with X_Ca 0·45 (Table 6). Garnet analyses consistently show >3 Si p.f.u., indicating the presence of a majorite component as a result of Mg₃Al₂Si₃O₁₂ + MgSiAl_-2 = Mg₃[MgSi]Si₃O₁₂. The amount of majorite component increases with P and T, consistent with results of previous studies (e.g. Ringwood, 1967; Kanzaki, 1987; Luth, 1997). A small amount of excess Si is also introduced through a Na-garnet component as a result of a continuous reaction

(Ringwood & Major, 1971; Irifune et al., 1994). Na increases with increasing P (Fig. 9) and above the upper P stability limit of high-Ca clinopyroxene at >18 GPa (Oguri et al., 1997), garnet represents the major subsolidus host for Na as a result of a reaction of jadeite component (see Gasparik, 1989)

The reason for the anomalously high Na in garnets from JKW47 is not clear; it might be due to incomplete equilibration. Potassium contents of garnet are <800 ppm at pressures up to 23 GPa.

View this table: [in this window] [in a new window]   Table 6: Average analyses of garnet

 

View larger version (16K): [in this window] [in a new window]   Fig. 9. Plot of Na p.f.u. vs Si p.f.u. for garnets from the subalkaline and peralkaline KNCMASH systems.

 

High-Ca clinopyroxene
High-Ca clinopyroxene is essentially a binary diopside–jadeite solid solution with <0·25 Na p.f.u. and minor amounts of Mg(M2) in most cases (Table 7). Na varies little between 10 and 15 GPa but decreases strongly near the upper P stability limit of clinopyroxene to 0·06 Na p.f.u., equivalent to 5 mol % jadeite component. This reflects a continuous reaction of jadeite component to form Na-garnet (see above). Averaged K contents range between 0·2 and 0·8 wt % K₂O. These lack systematic trends with either P or T, instead reflecting the strong dependence of K in high-Ca clinopyroxene upon the coexisting assemblage (Luth, 1997).

View this table: [in this window] [in a new window]   Table 7: Average analyses of high-Ca clinopyroxene

 

K-hollandite and Ca-perovskite
K-hollandite is close to stoichiometric KAlSi₃O₈ with K p.f.u. of 0·97–0·99 and small amounts of Mg and Ca (Table 8). Ca-perovskite is close to CaSiO₃ with negligible MgSiO₃ component. All analyzed Ca-perovskites contain 0·3–5·5 wt % Al₂O₃, 0–0·4 wt % Na₂O and 0·4–1·0 wt % K₂O (Table 8). In the absence of Fe³⁺, possible mechanisms for incorporation of Al into the Ca-perovskite structure are ^[XII]Al^[VI]Al^[XII]Ca_-1^[VI]Si_-1 (Andrault et al., 1998) or ^[XII]Al^[XII]Na^[XII]Ca_-2 (Kesson et al., 1995). The small number of analyses and the relatively large scatter of the data do not allow us to assess whether or not K can enter the perovskite structure.

View this table: [in this window] [in a new window]   Table 8: Average and representative analyses of Ca-perovskite and K-hollandite

 

	DISCUSSION

TOP ABSTRACT INTRODUCTION COMPOSITION OF STARTING... EXPERIMENTAL AND ANALYTICAL... PREVIOUS EXPERIMENTAL WORK RESULTS DISCUSSION APPENDIX REFERENCES

 
K-amphibole breakdown
Experiments in natural KLB-1 and in the KNCMASH system indicate an upper P stability limit of K-amphibole at 12–14 GPa, corresponding to 340–400 km depth. In the KLB-1 system, amphibole is close to pure K-richterite with only minor K on M(4) even at its upper P stability limit (Table 9). Assuming KNaCaMg₅Si₈O₂₂(OH)₂ and KHMg₂Si₂O₇ as amphibole and phase X compositions, continuous reactions that produce phase X from K-amphibole breakdown can be written as

with model end-member reactions

These reactions involve a continuous change in garnet composition as a result of CaMg exchange and limited MgSiO₃ solubility to form majorite component in garnet. The small amounts of Na in phase X can be ascribed to reactions forming NaHMg₂Si₂O₇ component instead of jadeite component in clinopyroxene. Reactions involving NaHMg₂Si₂O₇ component can be written as

Reaction (9) has olivine as an additional reactant, which will shift the reaction to lower P and T compared with reactions (7) and (8). Initial amphibole breakdown to an NaHMg₂Si₂O₇ component would be consistent with the fact that phase X is richest in Na at the lowest P at which phase X is stable. Reactions (7) and (8) produce free fluid because the K/H ratio of phase X is higher than that of amphibole. The amount of water produced per mol of amphibole consumed increases for phase X compositions with decreasing H content. Analytical totals of microprobe analyses suggest that phase X contains less than the theoretical maximum amount of H. This is supported by SIMS analyses of phase X by Inoue et al. (1998), which yielded 1·7 ± 0·1 wt % H₂O, equivalent to K_1·5Mg₂Si₂O₇H_0·5. Moreover, because of the possibility of electron beam damage and alkali diffusion, water contents inferred from microprobe analytical totals are maximum values. Analyses of phase X grains formed at 20 GPa yield the highest K₂O values and display analytical totals indistinguishable from 100 wt %. This suggests that H₂O decreases in phase X with increasing P, which could be explained by changing H₂O partitioning with coexisting nominally anhydrous phases, especially Mg₂SiO₄ high-pressure polymorphs and melts or fluids. P–T dependent variations in the H₂O contents of phase D were inferred on the basis of variable analytical oxide totals by Frost & Fei (1998) and may also be present in phase E (Kudoh et al., 1993), hydrous wadsleyite (Kudoh et al., 1996), and hydrous spinel (Inoue et al., 1995b). The importance of H₂O partitioning for the phase relations is illustrated by comparing data for runs JKW47 and JKW64 at 20 GPa and 1500 and 1600°C. In JKW47, phase X coexists with K-hollandite + 20 vol. % Mg₂SiO₄. In JKW64, phase X is the only K phase, and it coexists with <5 vol. % Mg₂SiO₄. Because of the much larger amount of Mg₂SiO₄ present in JKW47 compared with JKW64 (and assuming constant D_H2O^{Mg₂SiO₄–phase X}), too little H₂O is available to form phase X, and an additional anhydrous K phase—K-hollandite—forms.

In the KLB-1 bulk composition K-amphibole breaks down between 12 and 13 GPa at 1200°C, equivalent to depths of 360–390 km. Within this depth interval, the to + ß transition occurs for olivine (Fo₉₀) (Katsura & Ito 1989). Because of the high solubility of H₂O in Mg₂SiO₄ (0·1–0·15 wt % in ol at 10–13 GPa; 2·1–2·4 wt % in ß-Mg₂SiO₄ at 14–15 GPa; Young et al., 1993; Kohlstedt et al., 1996), the generation of a free H₂O fluid from K-amphibole breakdown is unlikely, as was noted by Inoue et al. (1998).

Stability of hydrous potassic phases along the ACMA
Experiments of this study combined with those by Konzett et al. (1997) indicate that in peralkaline bulk compositions hydrous potassic phases break down at temperatures slightly below an ACMA defined by 17·9 GPa and 1475°C for the ß transition in Mg₂SiO₄ and 23·2 GPa and 1530°C for the perovskite + magnesiowüstite transition (see Agee, 1998). Because the breakdown of HPPs in the peralkaline KNCMASH system occurs 50–100°C below the ACMA, only large amounts of F can stabilize HPPs along the ACMA (Foley, 1991). Otherwise, a hydrous melt will carry most of the K under P–T conditions of an ACMA, and the K content of the solid residue will reflect partitioning between high-Ca clinopyroxene and melt. In run Ma104M at 13 GPa and 1400°C the quench has 6·6 ± 0·5 wt % K₂O (n = 3) compared with 0·45 ± 0·02 wt % K₂O in high-Ca clinopyroxene with a resulting D_K^{hiCapx–liquid} of 0·07 (because of the possibility that H₂O-soluble K-rich material was lost from the quench as a result of H₂O saturation, the D value has to be considered a maximum value). This value is consistent with D_K^{hiCapx–liquid} of 0·02–0·15 obtained by Luth (1997) and Tsuruta & Takahashi (1998) from experiments in a simplified phlogopite + diopside and a dry natural alkali basalt system. In our experiments the K content of high-Ca clinopyroxene is <0·7 wt % K₂O, even at P = 18 GPa. This is consistent with the hypothesis of Luth (1997) that in a peridotitic bulk composition high-Ca clinopyroxene cannot accommodate significant K in the presence of an HPP (either solid or melt). The formation of K-rich (up to 1·7 wt % K₂O; Harlow & Veblen, 1991) high-Ca clinopyroxene found as diamond inclusions probably requires the presence of C- and K-rich (carbonatitic) melts as suggested by Harlow (1997). In metabasaltic compositions, however, omphacitic high-Ca clinopyroxene coexisting with phengite can accommodate up to 1·1 wt % K₂O (Schmidt, 1996).

In the subalkaline bulk composition, the stability of HPPs extends to temperatures significantly higher than those of an ACMA, above the solidus of H₂O-bearing peridotite (Fig. 3; Kawamoto et al., 1996; Kawamoto & Holloway, 1997). Luth (1997) reported the assemblage phase X + olivine + garnet + clinopyroxene + liquid at 11 GPa and 1600°C, which suggests a potential supersolidus stability of phase X. In our experiments, phase X coexists with small amounts of quench over a temperature interval of at least 200°C. The textures do not demonstrate whether the quench formed by crystallization from a hydrous melt or a solute-rich fluid, and changes in X_Mg of the phases as a result of the presence of melt (Kawamoto et al., 1996) could not be used in the KNCMASH system. Nevertheless, our data indicate that phase X is stable under the P–T conditions of convecting mantle, and that it coexists with hydrous peridotite melt over a wide pressure range. Thus, a supersolidus stability of phase X should control the large ion lithophile element (LILE) budget of coexisting partial melts. Although no trace element partition coefficients exist for phase X, it should, like other HPPs, store and retain large ions such as Cs, Rb, Ba or Pb in the large potassium lattice position (Kramers et al., 1983; Rosenbaum, 1993; Irifune et al., 1994; Ionov et al., 1997).

Potassium recycling in subduction zones
Experimental studies suggest that within subducted oceanic crust [mid-ocean ridge basalt (MORB), andesites, graywackes] the major subsolidus potassium carrier is phengite, which is stable to 10 GPa and breaks down to form K-hollandite + K-rich fluid at P > 10 GPa (Domanik & Holloway, 1996; Schmidt, 1996). K-feldspar would survive initial stages of subduction only under fluid-absent conditions but could react to form K-cymrite (hydrous hexasanidine KAlSi₃O₈.nH₂O) under fluid-present conditions and remain stable in the hydrated form to 8 GPa (George E. Harlow, personal communication, 1999). Thus, potassium and water transport are likely to be coupled to 300 km depths in Al-rich metasedimentary bulk compositions. Dehydration and/or melting reactions within the slab involving HPPs such as continuous phengite dehydration, K-MORB melting or Ca-amphibole and phlogopite breakdown at <3 GPa (Domanik & Holloway, 1996; Schmidt, 1996) can provide K and H₂O and stabilize HPPs within the peridotitic mantle wedge. With increasing P, the succession is phlogopite (± Ca-amphibole) K-richterite phase X, with final dehydration of phase X to K-hollandite at P 20 GPa or 600 km. Decoupling of K and H₂O in the mantle wedge therefore will occur at the base of the transition zone, or 300 km deeper than in the subducting slab. Any K transferred to the mantle wedge below 150 km will be lost for recycling by arc magmatism, and dragged down into the lower mantle in K-hollandite.

For most subduction zone geometries (see Davies & Stevenson, 1992; Schmidt & Poli, 1998) the partially molten zone that feeds arc volcanism lies above a region of the mantle wedge in which phlogopite is stable. Xenolithic evidence (e.g. Swanson et al., 1987; Canil & Scarfe, 1988; McGibbon et al., 1988; Ionov & Hofmann, 1995; Ertan & Leeman, 1996) confirms the presence of phlogopite in subarc mantle. Assuming that isotherms parallel the subducting slab, it is difficult to transport phlogopite-bearing peridotite into the melting zone of the wedge (>1200°C at 3 GPa for phlogopite melting; see Wendlandt & Eggler, 1980). Lateral transport of phlogopite through a mechanism such as that proposed by Davies & Stevenson (1992) for amphibole is unsuitable because of the much higher P stability of phlogopite compared with amphibole. On the basis of experimentally derived partition coefficients, LaTourrette et al. (1995) showed that phlogopite in the residue of an arc magma would be inconsistent with typical trace element patterns of arc lavas (e.g. Saunders et al., 1991) because residual phlogopite would strongly retain LILE. Thus, the enrichment in LILE of arc lavas might be due to either complete extraction of phlogopite with the degree of LILE enrichment controlled by the modal amount of phlogopite in the source or by addition of LILE-enriched melts or fluids rising from the slab through channels without pervasive phlogopite formation.

	APPENDIX

TOP ABSTRACT INTRODUCTION COMPOSITION OF STARTING... EXPERIMENTAL AND ANALYTICAL... PREVIOUS EXPERIMENTAL WORK RESULTS DISCUSSION APPENDIX REFERENCES

 

View this table: [in this window] [in a new window]   Table A1: List of mineral end-members, abbreviations and #formulae used in this study

 

	ACKNOWLEDGEMENTS

 
Part of the multianvil experiments were performed at the Bayerisches Geoinstitut under the EC ‘Human Capital and Mobility—Access to Large Scale Facilities’ programme (Contract ERBCHGECT940053 to D. C. Rubie). Sincere thanks go to Dave Rubie for providing access to the high-pressure equipment, and especially to Max Schmidt for sacrificing his time and nerves in an effort to protect the equipment from destruction while it was being used by J.K. We would also like to thank Bjørn Mysen for his help with Raman spectroscopy, and Hexiong Yang for helpful comments on an early version of the manuscript. Eiichi Takahashi kindly provided a sample of KLB-1. Reviews by George Harlow, Bob Luth, and Anne Peslier helped to improve the manuscript and are gratefully acknowledged. This work was supported by the Swiss National Science Foundation, the NSF Center for High Pressure Research, and the Carnegie Institution of Washington.

	FOOTNOTES

 
^*Corresponding author. Present address: Institut für Mineralogie und Petrographie, Universität Innsbruck, Innrain 52, A-6020 Innsbruck, Austria. Telephone: +43-512-507-5506. Fax: +43-512-507-2926. e-mail: juergen.konzett{at}uibk.ac.at

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