Earth and Planetary Science Letters, 64 (1983) 305-315 305 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

[61

Phase relations of amphibole, amphibole-carbonate, and phlogopite-carbonate peridotite: petrologic constraints on the

asthenosphere

M. Olafsson * and David H. Eggler Department of Geosciences, Geochemistry-Mineralogy Section, Pennsylvania State University, University Park, PA 16802 ( U.S.AA

Received November 23, 1982 Revised version received March 30, 1983

The subsolidus mineralogy and the isobarically-invariant solidus of peridotite in the presence of small amounts of H20 and CO 2 have been determined from 13.5 to 26 kbar pressure. The composition of fluid buffered by amphibole peridotite, below 17.5 kbar, is CO2-rich; amphibole melts completely, although incongruently, near the solidus, contributing materially to the nephelinitic character of the liquids. Fluid-absent amphibole peridotite also melts to nephelinitic liquid. Between 17.5 kbar and 22 kbar, amphibole-carbonate peridotite, without fluid, melts to alkalic, very silica-undersaturated liquid. Above 22 kbar, fluid buffered by carbonate peridotite, with accessory phlogopite, is H20-rich. Quenched liquids could not be analyzed, but they are inferred to be carbonate-rich by analogy with synthetic systems. Buffering o f f o 2 by graphite-carbonate-silicates maintains these phase relations if graphite is present.

The subsolidus change of peridotite carbonate mineralogy from low-pressure dolomite to high-pressure magnesite has been bracketed at 975°C, 28 kbar.

Suboceanic thermal models are consistent with the existence of a relatively deep, partially-molten asthenosphere even beneath old (180 m.y.) lithosphere, but melts should be exclusively carbonate-rich. Beneath younger lithosphere, a shallower zone should change, with increasing depth, from nephelinitic to carbonate-rich. Amounts of liquid are proportional to amounts of volatiles present, because amphibole, phlogopite, and carbonates melt quantitatively at or close to the solidus; in particular, 0.1% water in amphibole peridotite represents about 5% nephelinitic melt.

1. Introduction

The composition of oceanic asthenosphere is reflected in compositions of basalts that form by relatively large-percentage melting of astheno- spheric peridotite at mid-ocean ridges. Heavy iso- tope signatures of MORB reflect a homogeneous asthenospheric reservoir (e.g. [1]). That reservoir also contains small amounts of volatiles, which are unimportant to generation of MORB, but which may cause partial melting of a layer underlying

* Present address: Nordic Volcanological Institute, University of Iceland, Jardfraedahus v/Hringbraut , 101 Reykjavik, Ice- land.

0012-821X/83/$03.00 © 1983 Elsevier Science Publishers B.V.

much of the ocean basins. There is some debate on the nature of those volatiles--whether they are CO 2 or CO2-CO [2], as analyzed from glass inclu- sions in phenocrysts [3], or whether they are both CO 2 and H20, as analyzed in fresh, quenched pillow basalts [3,4]. We proceed on the assumption that both CO 2 and H20 are present. Our experi- ments are in any case applicable to asthenosphere that incontrovertibly does contain both H20 and CO 2, namely asthenosphere beneath island arcs, back-arc basins, and ocean islands [5].

The volatile species H20 and CO 2 are domi- nant, among C-O-H species, at mantle oxidation states ranging from the relatively oxidized quartz- fayalite-magnetite (QFM) buffer to below the

306

magnetite-wustite (MW) buffer [6-9]. That fo~ re- gion also includes the stability fields of carbonates, part of the stability fields of graphite and di- amond, the range usually associated with ter- restrial basalts [10], and the range calculated from nodules in kimberlites [11]. The coexistence of graphite, peridotite minerals, and carbonates de- fines fo2 for the assemblages enstatite-magnesite- olivine-graphite (EMOG) and graphite-enstatite- diopside-olivine-dolomite (GEDOD) [8,9] (equa- tions refer to equilibria involving pure end-mem- ber phases; P in bar, T in K):

logfo 2 ( E M O G ) = 7.83 - 2378_......~1T + 0.054( P ~ 1 )

logfo 2 (GEDOD) = 7.68 - ~ + 0.068

We consider EMOG-GEDOD f o ' s to be more reasonable than lower oxidation states, approach- ing the iron-wustite (IW) buffer, because below EM O G- GEDOD H20 and CH 4 are dominant volatile species and carbonates are not stable [6,7,9]. At no oxidation states is CO quantitatively important except in the complete absence of H20 or CH 4 [6,9].

Although theory of melting of peridotite in the presence of small amounts of H20 and of CO 2 has been developed [12-15], few experiments are avail- able on melting under fluid-buffered conditions or under fluid-absent conditions. Here such experi- ments are reported, with emphasis on solidi, al- though numerous analyses of crystalline and liquid phase compositions have been made [16].

2. The vapor-buffered mantle solidus

2.1. Experiment

Starting material was a spinel lherzolite from Hawaii [17], the same sample as Rock B studied by Mysen and Boettcher [18]. To overcome practi- cal difficulties in weighing small amounts of H20 into small capsules, an amphibole-bearing starting material was prepared by reacting peridotite and 10% H20 at 800°C and 15 kbar pressure for 20 hours. This procedure produced a fine-grained

amphibole peridotite, in which the water is bound in amphibole. After this material was ground un- der acetone, microscopic examination revealed that the majority of the grains were less than 15 ~m in diameter.

Calculation of modes of subsolidus amphibole peridotite from analysis of each phase (olivine, clinopyroxene, orthopyroxene, spinel, amphibole) indicated a modal abundance of 15.0_+0.5% amphibole. Because amphibole contains 2% H20 , the peridotite should contain 0.3 + 0.01 wt.% H20. Gravimetric analysis of 120 mg of the amphibole peridotite indeed showed 0.3% H20.

About 8-mg portions of the amphibole peri- dotite starting material, with appropriate amounts of dried Ag2C204 as a source for CO 2, were con- tained in welded capsules. For 3 subsolidus runs at high CO 2 contents at 13.5 kbar, additional H20 (0.5-1.4 additional wt.%) was added. For runs bracketing the solidus at 25.3 kbar, more volatiles were added to increase the potential amount of liquid (0.8% H20, 2.9% CO 2 at 1060°; 2.4% H20, 5.6% CO: at 1030°C).

All experiments were accomplished in solid- media apparatus with initial 2 kbar overpressuriza- tion. Talc-Pyrex-graphite-crushable alumina as- semblies were used, 2.54-cm-diameter (13-mm fur- nace ID) at pressures to approximately 22 kbar, and 1.27-cm-diameter (3.2-mm furnace ID) at higher pressures. Pressures have been corrected by - 11% for 2.54-cm assemblies and by - 16% for i.27-cm assemblies, as calibrated against the reac-

.tion enstatite + magnesite = forsterite + CO 2 [19]. Nominal temperatures, measured with Pt- Pt90Rhl0 thermocouples, are reported.

Several runs were made to determine the fo2 of the 2.54-cm assembly. Magnetite, wustite, and NiO were loaded in Pt capsules with 10% H 2 0 and run for 5 hours at 1180°C and 13.5 kbar pressure. The fH2 was found to be controlled by assemblies to yield fo 2 in the H20-bearing capsules that is above NNO buffer and below HM buffer. With CO 2 added to sample capsules, fo: must be lower than in the buffer capsules, because fH20 is lower; it is believed that in fact fo 2 is close to the QFM buffer.

Capsule materials were 1.8-mm-diameter Ags0Pds0 tubing at temperatures below 1250°C and Pt or iron-presaturated Pt at higher tempera-

tures. Presaturation was accomplished by heating wustite-containing capsules at 1 atm (1150°C, fo2 = 10 -12) for 8 hours; wustite was removed by ultrasonic cleaning. Capsules were run horizon- tally in the 1.27 cm assemblies and vertically, in batches of 5 or 6 per assembly, in the 2.54-cm assemblies.

Run duration varied from 180 to 900 minutes, depending upon temperature.

2.2. Phase characterization

Charges were examined in oil-moints with a petrographic microscope and in polished sections prepared for the electron microprobe. The polished mounts enabled observations of the textural rela- tionships between the quenched liquid and miner- als in back-scattered electron (BSE) images.

Images and quantitative analyses were obtained on an ETEC Autoprobe. Quenched liquids were analyzed with a defocussed beam (5-15 #m in diameter), provided that areas of liquid were suffi- ciently large. Liquid rarely quenched to glass in experiments close to the solidus, but rather to microcrystalline patches. These patches in BSE images are discrete, rather than grain-boundary films [16], and represent at least 15% of the charge under most conditions. Glass was detected in high-temperature runs. Compositions of glass or quenched liquid were judged to be reasonable if two criteria were fulfilled: (1) the peridotite bulk composition could be reasonably mass-balanced to compositions of all individual phases by least- squares techniques; (2) olivine-liquid Fe-Mg parti- tioning was reasonably close to that predicted [20], considering uncertainties in analysis and in the (unknown, but presumably low) ferric iron content of the liquid.

2.3. Equifibrium and iron-loss

Phase boundaries reported here were not re- versed, but several features indicate that equi- librium was attained. Several runs were made at temperatures closely above and below the solidus at 13.5 kbar pressure, at various molar CO2/(CO 2 + H 2 0 ) (hereafter called CO2) volatile contents (Fig. 1), and with different run durations. Results

307

. . . . . . ~ - - - ~ . . . . r . . . . i

1500

F--

1100

olw + L 1500

. . . . . . . . . . o0~- out

oliv Iopx+ L •

- - , , - . . . . .

" : - • oliv+opx+spin+L I •

-II . . . . . . . l - . . . . iUpx'0ut oltv+opx+cpx+spin+ L lOliv,,opx,,spin.

opx,,,, \ = =m • omp~,,L--,~ ~:!x _=/= " " • • • • spin / I ohv+opx* cpx+ spin+ omph ,v .V I / • •

9001 I I , _ --/ .- I ~o 08 06 04 02 oo

CO 2 / (COz+ H20), tool

Fig. 1. Phase re la t ions of a n amph ibo l e - sp ine l lherzol i te com-

pos i t i on as a f u n c t i o n o f C O : / ( C O 2 + H : O ) volat i le c o m p o s i -

t ion a t 13.5 k b a r pressure . N e a r l y all runs (see text) c o n t a i n e d

0.3 wt.% H 2 0 . Some supe r so l idus runs (wi th l iquid, L) a t

C O 2 / ( C O 2 + H 2 0 ) > 0.1 m a y have c o n t a i n e d f luid (v).

are consistent: amphibole was stable at the solidus but never above it, and liquid was present only above the nominal solidus. Microprobe analyses were also frequently obtained from cores and rims of olivines as a test of homogeneity within charges. It was found that grains within charges were ho- mogeneous, with the exception that large grains had slight reverse zoning (e.g., Mg no. = 0.909 at a core but 0.913 20 ftm away at a point about 2 #m from a rim).

To lessen iron loss, the majority of runs were carried out in AgsoPds0 capsules (below 1250°C). The slight reverse zoning in olivine, mentioned above, could indicate loss of iron from samples. On the other hand, least-square calculations, of the type mentioned above, indicated residual errors for iron no larger than errors for other elements, indicating, it can be argued, that significant iron was not lost from samples to AgsoPds0 capsules.

308

At temperatures in excess of 1250°C mass-balance calculations suggest that charges did not lose iron to iron-presaturated Pt capsules and probably even gained a little.

2.4. Results at 13.5 kbar

At 13.5 kbar, where phase relations were studied most extensively, amphibole is the only volatile- bearing phase stable at the solidus. A carbonation reaction occurs only at subsolidus temperatures, and for the starting material, containing only 0.01% K20, K is incorporated in amphibole, so phlogo- pite is not present.

Phase relations are plotted as a function of temperature and CO 2 volatile composition (not fluid composition) in Fig. 1. The relations conform reasonably closely to those predicted from theory and previous indirect data [ 14,15,21]. The diagram shows that fluid at the solidus, which is buffered by the amphibole peridotite phase assemblage, has CO 2 = 0.8. The first-produced liquid contains CO 2 and H 2 0 with CO2 = 0.1. For bulk compositions

more H20-rich than CO2=0.1 , amphibole melts over a supersolidus interval up to the melting point of fluid-absent amphibole peridotite, 1150°C. (Our experiments are consistent with Green's [22] determination of that melting point.) Supersolidus runs with CO 2 > 0.1 may have contained fluid

(certainly, near-solidus runs with CO 2 > 0.1 would be predicted to contain fluid [21]), but quenched fluid was not detected, presumably because the CO2-rich fluid dissolved negligible amounts of silicates.

The nature of melting near the fluid-buffered solidus is well illustrated from modes of individual runs (Fig. 2). The modes were calculated by least- squares mass balance of microprobe analyses of all phases [16]. It is apparent, in Fig. 2, that the 15% amphibole in subsolidus peridotite melts incon- gruently but completely near the solidus, to about 15% liquid, by the probable reaction:

opx + amph + spinel = ol + cpx + melt

That reaction is consistent with the modes (Fig. 2) and with previous interpretations of melting of assemblages that include amphibole [23,24]. The analyzed quenched melts are nepheline-normative

at temperatures to at least 1250°C, as melts dominated by melting of amphibole are predicted to be [23,24]. (The liquid produced from fluid-ab- sent amphibole peridotite at 1150°C is also nepheline-normative.) Once about 15% liquid forms near the solidus, the amount of liquid changes very little until a temperature in excess of 1300°C is achieved, at which point the modal curves are inflected (Fig. 2) and melts become tholeiitic. That point also approximately corre- sponds to the volatile-absent solidus.

2.5. Results at higher pressures

The solidus for peridotite was extended from 13.5 kbar to higher pressures (Fig. 3a) in runs on the amphibole peridotite material with 0.7 wt.% CO 2 added (bulk CO 2 = 0.5). The solidus, amphi-

bole-buffered at 13.5 kbar, where fluid CO 2 = 0.8 (Fig. 1), drops about 60°C near 17.5 kbar. At that point (1060°C), log fo2 of the carbonated peri- dotite would be - 9.45 if the assemblage were also in equilibrium with graphite ( G E D O D buffer). Fluid composition for that fo 2 can be calculated to be CO 2 = 0.9 (Fig. 3b), using modified Redlich- Kwong (MRK) equations of state [6]. Above 17.5 kbar, the subsolidus assemblage is a spinel-garnet

t solidus (this sludy) onhydrous solldus

= _.~__ m_ . . . . . ~ - - ~ ' ~ . m _

40

• . - op~_x _ _ _

zo I . omph ------

l 900 1100 1]00 1500

Temperoture, °C

Fig. 2. Modal proportions of phases in experiments on an amphibole-spinel lherzolite composition at 13.5 kbar containing approximately 0.3 wt.% H20 and 0.7 wt.% CO 2 (molar CO2/(CO 2 + H20) = 0.5). Compositions containing less vola- tiles are expected to show the same near-solidus relations between modal amphibole and modal liquid but at reduced modal contents.

309

(a) I 1 ~1 1 1

/ I I Ol * opx " cpx I

I • gO.l~ / • c o r b * v I

I I / c o r b - # / OUt

°t' 0 0 I 0 0 0

0 I

, ~ . ~ -b , g, o o l9 'o o ~.'"g l

0mph-cQrb/ o °c= o/do= . . . . . o ~o 11 " / f I / ~ ' ~ " OpX" CpX" ~ .' II$'" I ~ ( - :ou¢

(b)

t

(c) ' • ~ / ~ ' I

/ . . . ' "

• ol*opx+cpx. / go+l~+CO~/~ ... ."

ol +q~+epx+ ~ .." ar~.eorb+~ L_ .'"

, e ~ ." . d , ~ p x ' , c p x ~

/ / ol~ox*cpx* / ~*omph /

' | i . l , 0 1 ~ 1 ~ I l I - 0 9 0 0 I lOO

~o uoo ~)oo ~ Xi Temperature, *C Temperature, "C

4 0

3 0

2 0 " ~

m-

Q .

IO

Fig. 3. (a) Phase relations of a peridotite composition in the presence of small amounts of H20 and CO 2. Phase relations for volatile contents of approximately 0.3 wt.~v HzO and 0.7 wt.~0 CO z (molar CO2/(CO 2 + H20) = 0.5); a few runs contained higher volatile contents (see text). Two subsolidus regions contain fluid (v) buffered in composition by an amphibolitization or decarbonation reaction. Amphibole (amph) and phJogopite (ph) melt out very near the solidus. Percent figures refer to percent melting for the bulk composition cited. Heavy line is the solidus; the solidus is applicable provided that the molar CO2/(CO 2 + H20) approximately = 0.1-0.8 and that compositions contain less than about 0.3 wt.% H20 and less than about 7 wt.% CO 2. (b) Compositions of fluid along the solidus of (a), in equilibrium with graphite, in mol fraction (see text). (c) The solidus of fluid-absent peridotite containing CO 2 or H20, or both, estimated from Fig. 3a and other sources. The dotted lines are approximate suboceanic geotherms for lithospheric ages of 180 m.y. (upper) [451 and 30 m.y. [44] (lower).

peridotite with amphibole and carbonate. No free fluid is present for any bulk compositions, which include the composition studied, that contain less than 0.3% H20 and less than at least 5% CO 2. For such compositions, all H20 can be bound in amphibole and all CO 2 in carbonate (dolomite).

The amphibole-carbonate peridotite is stable at the solidus up to about 22 kbar, where the solidus temperature is deflected another 60°C (Fig. 3a). That deflection coincides with the high-pressure stability limit of amphibole (as well as, approxi- mately, with the high-pressure subsolidus stability of spinel), resulting in a subsolidus assemblage, at pressures greater than 22 kbar, of olivine, two pyroxenes, garnet, carbonate, and trace phlogo- pite. Phlogopite was identified optically in only one subsolidus run, but because no other minerals contained detectable K20, by microprobe analy- sis, all runs in the phiogopite field are interpreted

to actually contain phlogopite. Because the bulk composition contains only 0.01% K20, represent- ing 0.09% potential phlogopite, and because phlogopite contains 4.3% H20, bulk compositions containing less than 0.004% H20 would contain no fluid phase. Bulk compositions containing more than 0.004% H20, like those used in the experi- ments, would contain fluid that is buffered to a very low CO 2 by the carbonate peridotite buffer [12-14]. A low CO 2 is also consistent with MRK calculations for fluids at fo2 of GEDOD or, above 28.5 kbar (Fig. 4), EMOG assemblages. The carbonate-buffered solidus is necessarily in close proximity to the hydrous solidus. The solidus de- termined at 25.3 kbar (Fig. 3a) is a carbonate- buffered solidus, in close proximity to the hydrous solidus of Kushiro et al. [25]. Because of the agreement, the solidus is confidently extrapolated to 40 kbar (Fig. 3a).

310

0 , , ,

to to ~0

IX.

i i r i

/

moo '," di • "'/ t'~ ~)0 " ' ......... 17I• .... / pe

en * dol ridotite Iidus

20

I0 , , i . I 8 0 0 I O 0 0 1200

Temperature, °C Fig. 4. Pressure-temperature projection of the reaction that produces a subsolidus phase change from dolomite peridotite to magnesite peridotite. The solid line was extrapolated, from the runs shown, by thermodynamic methods. The dotted line shows the calculated position of the reaction for pure phases rather than for solid solutions. The peridotite solidus is after Fig. 3a.

2.6. The fluid-absent peridotite solidus

The solidus of peridotite containing volatiles but no fluid is shown in Fig. 3c. Its position between 18 and 22 kbar coincides with that in Fig. 3a and, below 19 kbar, with that in Fig. l (for CO 2 = 0.0 only) and with that of Green [22], but above 22 kbar, its position can only be estimated from experiments on a dolomite-phlogopite peri- dotite [21]. The solidus at 30 kbar is also con- strained to be lower in temperature than that of magnesite peridotite [26].

It should be noted that subsolidus peridotite below 18 kbar contains only H 2 0 ( < 0.3%), be- tween 18 kbar and 22 kbar H20 and CO 2 (H20 < 0.3%), and above 22 kbar H 2 0 and CO 2 (amount of H20 dependent on amount of K20, but for "normal" peridotite compositions with low K, very low).

3. Subsolidus oeridotite carbonate mineralogy

3.1. Experiment

The reaction diopside + magnesite-- enstatite + dolomite was investigated in solid-media ap-

paratus, using piston-out technique and 1.27-cm- diameter sample assemblies of talc, boron nitride, graphite, and inner rods of crushable alumina. Nomina l temperatures , measured with Pt- Pt90Rhl0 thermocouples, are reported, as well as nominal pressures; for this assembly, no pressure correction is required [27]. Because results of anhydrous runs indicated a very slow reaction rate, hydrous reversal runs were made at a temper- ature (975°C)just below the hydrous solidus. For each experiment, a finely-ground mechanical mix- ture of natural dolomite and magnesite and syn- thetic diopside and clinoenstatite was encapsu- lated, with H20, in 2-ram Pt tubing. Assemblies buffered fo2 in capsules between N N O and HM [15], well within the carbonate stability field. No graphite was observed.

In runs at 33.2 and 31.0 kbar, ensta t i te+ dolomite reacted completely to magnesite + diopside (Table 1). In runs at 28.8 and 27.2 kbar, which constitute a reversal bracket on the reaction, reaction was not complete, but the direction was clearly indicated by peak heights on X-ray dif- fractograms.

3.2. Calculation

The reaction was extrapolated from 1248 K (28200 bar) to higher and lower temperatures from the relation:

AG(P,T) = 0 = AG°(28200,1248) + AC~dT 248

r ac,~ - ( T - 1248) AS~'24 ~ - T f I 2 4 8 T dT

+ ( P - 28200)AV o

+ R T l n ( X¢"Xa°l X~' Xma* )( ~'¢.'tdo.............~ I "/d i'Ym a g ]

Heat capacities and molar volumes are from Helz [23], except molar volume of orthoenstatite [29]. Compositions and activity coefficients of coexist- ing enstatite and diopside solid solutions are from Lindsley et al. [30], and activities of coexisting dolomite and magnesite solid solutions were calculated, assuming an asymmetric Margules model, from a 27-kbar solvus [31].

TABLE 1

Experiments on the reaction diopside + magnesite = enstatite + dolomite

Pressure Temperature Time %H20 Products (kbar) (°C) (hr) (wt.%)

33.2 975 24 16,5 mag, di 31.0 975 24 9,2 mag, di 28.8 975 24 9.0 mag, di, en, dol

(mag + di grew) 27.2 975 24 9.6 mag, di, en, dol

(dol + en grew)

The extrapolated curve is shown in Fig. 4 (see also Eggler and Baker [9]), as is the calculated position of a curve representing reaction of pure phases.

4. Discussion

4.1. Subsolidus carbonate mineralogy

The reversal at 975°C (Fig. 4) is in excellent agreement with solid-media runs reported by Brey et al. [32], but the P-T slope of the reaction is much shallower. Part of that discrepancy could be attributed to uncertainties in heat capacities, in extrapolations of heat capacities to temperatures above 1000 K, in molar volumes, and in activities of solid solutions. The discrepancy is probably due to uncertainty in the molar entropy of one or more phases, however, most likely of dolomite, due to order-disorder effects [31].

Both determinations of the reaction enstatite + dolomite = magnesite + diopside, which divides the field of dolomite per]dot]re from that of mag- nesite per]dot]re, are considerably lower in pres- sure than an earlier determination of the reaction [33].

4.2. The fluid-buffered solidus

Compositions of fluids buffered by amphibole or by carbonate along the fluid-buffered solidus, as discussed in 2.5, are shown in Fig. 3B. The solidus (Fig. 3a) is applicable to all volatile con- tents for which buffering capacity is not exceeded.

311

For amphibole peridotite, that capacity is about 0.3% H20. For dolomite per]dot]re, the capacity for CO 2 buffering is proportional to modal di- opside in the peridotite but is typically 770 CO 2. For magnesite peridotite, the capacity for CO 2 buffering is enormous because it is proportional to modal olivine in the rock--typically 2070 CO 2. As discussed in 2.5, phlogopite has a buffering capac- ity of 0.0470 H 2 0 per 170 phlogopite (or per 1000 ppm K). Even fertile peridotites, with a few hundred ppm K, would have little H20 buffering capacity. The unlikelihood of a phlogopite buffer is also evident from the absence in peridotites of kalsilite, a key mineral in the high-pressure buffer- ing assemblage [34].

The fluid-buffered solidus of this study (Fig. 3a) can be compared to that of Wyllie [12l and of Brey et al. [32]. Wyllie's solidus, which was con- structed from theoretical considerations and litera- ture data, is unconstrained by direct experiment. It differs from that shown here in that the high-pres- sure (> 30 kbar) H 2 0 - C O 2 solidus lies at lower temperatures than the H20 solidus and in that a thermal maximum occurs on Wyllie's solidus, in a P-T region without fluid-buffering. That region lacks buffering because neither amphibole nor carbonate is shown as stable at the solidus. Our diagram (Fig. 3a) has no thermal maximum, be- cause both amphibole and dolomite are stable on the solidus. The thermal maximum does not ap- pear on the solidus of Brey et al. [32]; data have not appeared, however, to assess how constrained that solidus may be.

4.3. Near-solidus melts

Because the fluid-buffered solidus is, at con- stant pressure, invariant (a zone of invariant vapor composition, or ZIVC, in the terminology of Eg- gler [15]), melting should proceed like any isobari- cally-invariant sol idus--a phase should melt com- pletely and a proportionately large amount of liquid should be produced. We agree with Brey et al. [32] that amphibole, dolomite, and phlogopite should melt very near the solidus, although where two of those phases are present, one must melt before the other. It does not necessarily follow that the composition of the liquid produced is

312

dominated by the composition of a first-melting phase, but amphibole components certainly dominate the near-solidus melt of amphibole peri- dotite (see above), melts derived from phlogopite peridotite are potash-rich [34], and melts derived from carbonated peridotite are carbonate-rich [12,15].

The nephelinitic nature of near-solidus melts of fluid-buffered (CO2-rich fluid) amphibole peri- dotite and of fluid-absent amphibole-peridotite found in this study is broadly consistent with interpretation [22] of melting of vapor-absent amphibole peridotite and with experiments on nat- ural [18] and synthetic [15] CO2-rich peridotite compositions. A melt of amphibole-carbonate peridotite was also found to be nephelinitic, pre- sumably because subsolidus modal amphibole ex- ceeded modal carbonate. No compositions could be obtained on melts produced in the phlogopite- carbonate peridotite field. Near-solidus melts are presumed to be carbonate-rich at 25-30 kbar, by analogy with phase relations in synthetic systems [12,15,34] and may be potassic; higher-tempera- ture melts may or may not be carbonate-rich or potassic, depending on relative amounts of CO 2 and K in the source peridotite and the (unknown) melting relations. Some experiments suggest that the higher-temperature melts can be melilititic, depending on amounts and proportions of dis- solved H20 and CO 2 [35,36]. At pressures above 30 kbar, near-solidus melts probably are olivine- rich and kimberlitic, by analogy with the system KAISiO4-MgO-SiO2-H20-CO 2 [34]. Phase rela- tions of a kimberlite composition [37] are also consistent with its derivation from phlogopite- carbonate peridotite at 50-60 kbar.

5. Applications

5.1. Mantle redox conditions: graphite and carbonates

Because phase relations in Fig. 1 and 3a were determined at fo2 above QFM in the absence of graphite, it may be asked if the phase relations apply to a mantle that, from evidence in sub- marine basalts and in peridotite nodules [2], does contain graphite.

Graphite in fact is non-diagnostic of redox con- ditions, because it is stable from the fo2 of its coexistence with CO2-CO--the CCO buffer, which for most P-T conditions is between QFM and MW [38]-- to fo2'S of IW and below. Coexistence of graphite, carbonates, and peridotite minerals--the buffers EMOG and G ED O D (Section 1)--defines fo2, however, at values somewhat lower than CCO [7,9]. Because such values are only marginally lower than those studied and because C dissolves in melts as CO32-, not as reduced carbon, phase relations (Figs. 1 and 3) should indeed apply to the upper mantle whether or not graphite is present.

It may be argued (e.g. [7]) that estimates of highly reduced conditions in the upper mantle exclude carbonates from consideration as mantle minerals. We feel that the existence of carbonate- rich magmas (kimberlite, carbonatite) necessitates existence of carbonates and hence necessitatesfo 's, at least in those source reservoirs, near the buffers EMOG and GEDOD. It is gratifying that recent estimates of fo 'S of nodules and megacrysts in kimberlites [11,38] are near EMOG-GEDOD.

At fo 'S lower than MW, fluid species in equi- librium with graphite or diamond are H20 and C H 4 [6,7,9]. Because CH 4 dissolves readily in melts [9], but not as CO~-, production of carbonate-rich melts under quite reduced conditions is unlikely.

5.2. Partial melt in the asthenosphere

The seismic low-velocity, high-attenuation zone (LVZ) and the high electrical conductivity layer (HCL) have traditionally been interpreted to be due to partial melting. That traditional view is not universally accepted today. The seismic properties have also been attributed to solid-state processes - -gra in boundary relaxation or dislocation motion (reviewed in Arculus et al [39])--or trapped water [40], although the latter is unlikely because water should be contained either in amphibole or melt (Fig. 3a, c). The seismic zone is even believed to be considerably suppressed and to be shallower than previously calculated when anisotropy in horizon- tal and vertical velocities is allowed in the upper 220 km [41]. The HCL has been attributed to an interconnected film of graphite [431.

In one sense, it hardly matters if melt can be

detected geophysically, only if it is present. Solidi in Fig. 3a, c can be compared to plate models in which lithosphere, as defined by depth to a partic- ular isotherm, is typically defined with a 1200°C isotherm (e.g. [43]). The comparison suggests that oceanic asthenosphere is indeed partially molten below that boundary layer, which deepens with lithospheric age (e.g. [44]). Unfortunately, such models are not independent of petrologic input, nor i re models based on conduction [44]. The independently-derived equilibrium oceanic geo- therm of Sclater et al. [45] (age > 180 m.y., mantle heat flow = 31 roW/m2), however, is also con- sistent with presence of melt even beneath old lithosphere, because it crosses the solidi (Fig. 3a, c) at 115-130 km, at temperatures in excess of 1100°C. Such depths are consistent with estimated plate thickness [44,45].

Melts in the asthenosphere should be zoned compositionally both vertically and laterally. Par- tially-melted asthenosphere beneath relatively young lithosphere (e.g. along the 30-m.y. geo- therm, Fig. 3c) that is stabilized against vertical migration [46,47] should range in composition from nephelinite at 50-60 km depth to carbonate-rich at 70-150 km depth (4.3). At greater distances from spreading axes, more alkalic melts (melilite nephelinite?) should be underlain by carbonate- rich melts, and at even greater distances (180-m.y. geotherm, Fig. 3c), carbonate-rich melts alone should exist.

Because phlogopite, amphibole, and carbonates melt near the solidus (Fig. 3a, c), amounts of melt are necessarily proportional to the volatile content of the asthenosphere. The relationship is not sim- ple, however, due to incongruent melting reac- tions. For amphibole peridotite, 0.1% water repre- sents about 5% amphibole and about 5% potential melt (Fig. 2). (Any water present in subsolidus fluid, rather than in amphibole, represents a sec- ond-order effect, because the fluid is CO2-rich ). For carbonate peridotite, 0.1% CO 2 represents about 0.2% carbonate and perhaps 0.2-0.4% potential carbonate-rich melt.

5.3. Fluid migration in the asthenosphere

Current petrochemical models for generation of alkaline magmas include metasomatic addition of

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lithophilic and other elements to the source region shortly before the melting event. The metasomatic agent may be a fluid or a melt, although probably a fluid (H20-CO2) is usually envisioned.

Phase relations in Fig. 3 offer two constraints to the existence and composition of fluids: ( l ) ex- istence of fluids in relatively hot asthenosphere (e.g., deeper than 50 km along the 30-m.y. geo- therm, Fig. 3c) is impossible, because melt is pres- ent; (2) fluids cannot exist in cooler, subsolidus asthenosphere or in lithosphere unless abundances exceed limits set by Fig. 3c, and compositions of fluids must be buffered as in Fig. 3a and b. For example, fluid in the phlogopite-carbonate peri- dotite zone can exist only if water is present in excess of that required in potential phlogopite; if fluid does exist, it must be water-rich (Fig. 3b) because of the large buffering capacity of carbonate peridotite. The only reasonable exception to buffering control would be flow through vein or dike channels with completely carbonatized walls.

Acknowledgements

This research was supported by the National Science Foundation, Earth Sciences Section, grants EAR-7715704 and 8022262.

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