55

MELTING IN THE MANTLE: PHASERELATIONSHIPS IN NATURAL AND SYNTHETIC

PERIDOTITE-H20 AND PERIDOTITE-H20-C02SYSTEMS AT HIGH PRESSURES

By A. L. BOETTCHER, BJORN O. MYSEN and P. J. MODRESKI*

Department of GeoscIences, The PennsylvanIa State University, University Park, Pennsylvania 16802, U.S.A.

ABSTRACT

To Investigate the proposal that kimberlites and associated rocks originate by anatexIs of mantle pendotltein a H2O-and COrrich environment, phase relationships offour peridotite (lherzolite) nodules and one websteritenodule were investigated to 12OD"C and 30 kbar in the presence of H20 and H2O-C02 vapors with controlledoxygen fugacIty. Solidus temperatures, which are a function of vanous parameters including bulk composition,total pressure. and aH20, are considerably (12D-2OD°e) lower than previous determinations. Melting of garnetlherzolIte, even in the presence of a vapor of 50/50 mol.~~ H20/C02 (XH;o = 0.5). begins at depths of less than150 km beneath the continental shields.

This Investigation was augmented by a study of the stabilIty of phlogopite in the system K20-MgO-CaO­AI20rSIOrH20 to 35 kbar at PeH,O <1; PTand - PT' The reactIOn phlogopite + enstatite" + dlOpslde"~ fors­terite + pyrope + liquid occurs at about 1350°C at 35 kbar. These temperatures are considerably higher thanthose of the vapor-saturated solIdil of the peridotite nodules, reflecting the absence of Fe and Na and the verylow values of aH20' In expenments with the nodules, phlogopite appeared only when the compositions werespiked with phlogopite. Amphiboles occurred in all experiments with the nodules at pressures below 20-25 kbar.These amphiboles become enriched in pargasite component relative to tschermakite component with increasIngpressure and decreasing temperature. Temperatures indIcated by the compositions of orthopyroxenes coexistingwith clInopyroxenes are approximately IOD°C lower than those obtained from previously publIshed pyroxenegeothermometers.

LIquids under condItions of XH;o ~ 0.6 are nch in Si02 and mimic andeSItes. For more CO2-rich com­posItIOns. liquIds are usually nepheline normatIve and rich in CaO and A120 3• resembling melilitite lavas, whichcommonly accompany kimberlites.

INTRODUCTION

As the scale of this Conference indicates, kimberlites are of considerable interest to

geologists---a fascination not commensurate with the meager abundance of these unusual

rocks. Petrologists are enthusiastic about kimberlites because they contain nodules that

may be xenoliths derived from the upper mantle, and they may also represent the chemically

extreme products of processes including anatexis of the mantle or differentiation of mantle­

derived magmas.

Several authors have reviewed the proposed mechanisms by which kimberlites originate

(e.g. MACGREGOR, 1970; DAWSON, 1972). These include (1) zone refining (HARRIS, 1957;

* Present address. AFWL/LRT. Kirtland A.F.B., New MexIco 87117, U.S.A.

855

856 A. L. BOETTCHER, B. O. MYSEN AND P. J. MODRESKI

HARRIS and MIDDLEMOST, 1970), (2) extreme fractional crystallization of basic magmas(e.g. MACGREGOR, 1970; O'HARA and YODER, 1967), and (3) anatexis of peridotite mantle(e.g. DAWSON, 1972).

The experimental results described herein are applicable to all of these mechanisms.However, we feel that the available evidence more nearly supports the third proposal.Zone melting would be an efficacious method for concentrating the "incompatible" elementsthat are characteristic of kimberlites, but DAWSON (1972) has pointed out that such aprocess, as we conceive of it, would result in large volumes of magma. Regarding thesecond process, the production of cognate nodules and late-state residual liquid (matrix)from fractional crystallization of a single parent magma is not easily reconcilable withradiometric determinations that reveal ages for the nodules that are considerably older thanthose of the matrices (e.g. DAWSON, 1972). Nevertheless, MACGREGOR (1970) postulates amodel in which differentiation occurs over periods of billions of years, thereby reconcilingthe old age of some nodules.

All of these models involve some stage of melting of, presumably, peridotite mantle.In addition, it is apparent that kimberlites evolve in a gas-rich mileau, as evidenced by thediatreme nature of parts of most kimberlites (e.g. DAWSON, 1972; HEARN, 1968). This gasappears to be COz- and HzO-rich as evidenced by the occurrence of hydrous and carbonateminerals (e.g. MCGETCHIN and BESANCON, 1973; DAWSON and POWELL, 1969a; OXBURGH,1964; MCGETCHIN et al., 1970; GURNEY and BERG, 1969), by evidence for carbonatiticmagmas (e.g. ZHABIN and CHEREPVSKAYA, 1965; DAWSON and HAWTHORNE, 1973), by thecomposition of fluid inclusion in olivine-bearing nodules (ROEDDER, 1965), and by thecomposition of volcanic gases (e.g. NORDLIE, 1971; SIGVALDASON and ELISSON, 1968). Waterin the mantle occurs predominantly in magmas, bound in hydrous minerals, or dissolved ina multicomponent (COrrich?) fluid phase. In all three instances, the activity of water(aH20) would be much less than unity (e.g. BURNHAM, 1967).

Consequently, we have undertaken an extensive study of melting and other phaserelations of peridotites in the presence of HzO-CO z vapor. This study was augmented byour investigation of synthetic phlogopite-bearing peridotites in the presence ofan HzO-richvapor and in the absence of vapor (HzO deficient). Previous studies on various peridotitecompositions have either been under anhydrous conditions (e.g. GREEN and RINGWOOD,1967; ITO and KENNEDY, 1967; KUSHIRO et al., 1968), or under conditions with a range ofHzO contents (e.g. KUSHIRO et aI., 1968; GREEN, 1973).

SCOPE AND PROCEDURE

Phase relationships to about 30 kbar and 1200°C have been determined in the presenceof HzO and HzO-CO z vapors with controlled a0

2and aH20 for four peridotite nodules

(including one from the Wesselton Mine) and a garnet websterite nodule. The compositionsand c.I.P.W. norms are listed in Table 1. These compositions more than span the ranges ofmost oxides reported in the analyses of fifteen garnet peridotite nodules by CARSWELL andDAWSON (1970). All experiments were in a piston-cylinder apparatus using either a 2.54 cmfurnace as described by BOETTCHER (in JOHANNES et al., 1971) or a 1.91 cm furnace describedby MYSEN (1973). The control of water activity (aH20) and oxygen activity (ao

2) and the

production ofHzO-COz vapors were achieved using the method of BOETTCHER et al. (1973).Phase relationships in the presence of nearly pure HzO vapor were investigated to tem-

TABLE I. COMPOSITIONS OF NODULES USED IN EXPERIMENTS

A B C D Ega-p(l) 618-138b.1 66PAL-3 66SAL-l 68SAL-7

Si02 45.7 43.7 45.1 44.8 45.6Ti02 0.05 0.20 0.13 0.52 0.80AI20 3 1.6 4.0 3.92 8.21 13.1Fe20 3 0.77 0.89 1.00 2.07 3.76FeO 5.21 8.09 7.29 7.91 5.85MgO 42.8 37.4 38.8 26.5 16.1CaO 0.70 3.50 2.66 8.12 1l.8Na20 0.09 0.38 0.27 0.89 1.27K20 0.04 0.01 0.02 0.03 0.02H 2O+ 1.28 0.14 0.07 0.11 0.35H2O- 0.28 0.10 0.12 0.15 0.32P 20 S 0.01 < 0.01 0.01 0.04 0.03MnO 0.09 0.12 0.14 0.19 0.16CO2 - - 0.01 0.01 0.09C - - 0.00 0.00 0.00F - - 0.01 0.01 0.01S - - 0.02 0.04

I0.05

Cr203 0.41 0.40 0.31 0.20 0.12NiO 0.26 0.24 0.25 0.20 0.06

Subtotal 99.29 99.17 I 100.14 100.05 100.03Less ° - - 0.01 0.02 0.03

Total 99.39 99.2 100.1 100.0 100.0Mg/(Mg + Fe2+) 0.94 0.89 0.90 0.86 0.83

CIPW(wt.%)[I A B C D E

IQuartz i 0.000 0.000 0.000 0.000 0.000Orthoclase

I

0.242 0.059 0.118 0.177 0.118Albite 0.787 3.330 2.285 7.616 10.831Anorthite 3.507 9.326 9.480 18.411 31.906Nepheline I 0.000 0.000 0.000 0.000 0.000Diopside

I(0.000) (6.535) (2.958) (17.327) (21.309)

wo 0.000 3.454 1.566 9.129 11.248en I 0.000 2.678 1.231 6.910 8.640fs

I0.000 0.403 0.161 1.288 1.421

Hypersthene (33.672) (13.267) (22.122) (7.001) (8.213)en 31.043 11.532 19.569 5.901 7.053fs 2.629 1.735 2.553 1.100 1.160

Olivme (60.360) (65.802) (61.294) (45.345) (20.508)fo 55.208 56.445 53.590 37.618 17.362fa

I

5.153 9.357 7.705 7.727 3.146Corundum 0.167 0.000 0.000 0.000 0.000Magnetite 0.000 1.319 1.464 3.016 5.510Ilmenite 0.097 0.380 0.247 0.988 1.538Apatite 0.023 0.023 0.023 0.093 0.070

Total i 100.001 100.012 99.992 99.973 100.002I

(A) ga-p(l}-peridotite (garnet lherzolite), Wesselton Mine, South Africa. Provided by D. A. Pretorius.Analyst: N. H. Suhr, The Pennsylvania State University.

(B) 618-138b.1-peridotite (spinel lherzolite), Hawaii (White, 1966). Analyst: N. H. Suhr, The PennsylvaniaState UnIversIty.

(C) 66PAL-3-pendotite (spinel lherzolite), Hawaii. Provided by E. D. Jackson. (See JACKSON and WRIGHT,1970, table 4.)

(D) 66SAL-I-peridotite (garnet lherzolite), Hawaii. Provided by E. D. Jackson. (See JACKSON and WRIGHT,1970, table 4.)

(E) 68SAL-7-garnet websterite, Hawaii. Provided by E. D. Jackson. (See JACKSON and WRIGHT, 1970,table 4.)

857

858 A. L. BOETTCHER, B. O. MYSEN AND P. J. MODRESKI

peratures near the vapor-saturated liquidii (~1200°C). However, because of the differentialsolubility of HzO and COz in silicate melts, the experiments with HzO~COz vapors wereconfined to temperatures near the beginning of melting to ensure a nearly constant andknown HzO/COz ratio in the vapor.

Additional data to 1350°C and 35 kbar were obtained in experiments on synthetic,phlogopite-bearing peridotites in the system KzO-Mgo-CaO~Alz03-SiOz-HzO atconditions of PeH20 ~ PT and PeH20 ~ PT in a piston-cylinder apparatus with a 1.27 cmfurnace (BOETTCHER and WYLLIE, 1968) and in an internally-heated gas-pressure apparatus(YODER, 1950). Chemical homogeneity of the phases and reversals of phase-boundarycurves were used as indicators of equilibrium. Run durations ranged from 48 hours nearthe beginning of melting of the nodules (< 900°C) to 3 hours at temperatures in excess of1200°C. Similar run durations were used in the synthetic system. About 1000 analyses ofphases in the run products were determined from many of the 529 experiments on thenodules using the computerized electron microprobe at the Geophysical Laboratory.Phases were also identified using the petrographic microscope and X-ray diffractiontechniques. Solidii were determined by the recognition of glass identified as quenchedliquid and easily distinguishable from quenched vapor.

RESULTS

PHASE RELATIONSHIPS OF NODULES + HzO VAPOR

The phase relationships of the four peridotite nodules in the presence of nearly pureHzO vapor (XH~O ~ 1) appear in Fig. 1. With the exception that rock A contains no spinel,the overall pattern is similar for all ofthe rocks, but the position ofthe boundaries is sensitiveto bulk chemical composition. The ratio Mg/(Mg + FeZ +) for the rock is the most signifi­cant factor in determining the temperature of the solidus. Less obvious is the observationthat lowering the CaO/Alz0 3 ratio at constant Mg/(Mg + Fe2+) lowers the solidustemperature at pressures in excess of 10 kbar. These solidii are considerably lower (120­200°C) than those reported for similar conditions by KUSHIRO et al. (1968) and by GREEN(1972). KUSHIRO (personal communication, 1973) informs us that failure to reach equilibriumin his short-run durations may explain the variance.

As in previous studies on basic rock compositions (LAMBERT and WYLLIE, 1972; ALLENet al., 1972), there is a reaction relationship between amphibole and garnet at high pressures.The trend is for the amphiboles to become richer in pargasite (Na) component with in­creasing pressure and richer in tschermakite (Ca) component at higher temperatures (seeFig. 2). ALLEN and BOETTCHER (1973) have shown that amphiboles are increasingly stableto higher pressures as the alkalinity of the rock increases, but the relationships are toocomplex to allow us to predict the exact stability of amphiboles relative to garnet- orliquid-bearing assemblages. Nevertheless, garnet appears at lower pressures in rocks withlower Mg/(Mg + Fe2+) because of the lower content of pyrope component (MYSEN andBOETTCHER, 1972).

MELTING IN THE MANTLE 859

A) go- p(1) + H2O B) 618-138bJ+H2 O

, ,,

0'~ 30

Blo 30

0.0 .0

e brc..>: ..>:

~ 20 e b c i 20

'"'" a V>

~ V> a''" 10 '"et et 10

< ' I" ' ..,f

a a '-700 900 f100 700 900 1100 1300

Temperoture,OC Temperoture,OC

0) 66SAL-ItH 2O C) 66PAL-3+H2O

I II ,

~ 30

~/0 30

~ ..0.0 .0

..>: ..>:

,.' ", +f,~ 20 a" i 20

'" '"V> e ,bV>

V> V>

'" 10 a '" 10et et, , I

'""> "a a700 900 1100 700 900 1100 1300

Temperature,OC Temperature,OC

FIG. 1. Pressure-temperature projectIOns for the four peridotite nodules (see Table I).a = 01 opx cpx amph yap liquid,a 1 = 01 opx cpx amph sp yap lIquid,all = 01 opx cpx amph sp ga yap liquid,b = 01 opx cpx sp yap liquid,c = 01 opx sp yap liquid,d = 01 opx cpx ga yap liquid,d 1 = 01 opx cpx ga sp yap liquid,e = subsolidus,f = above vapor-saturated lIqUidus.

o~090

+o

No

Z+oo~ 085

"oo

U

Amphiboles

Pendot Ite (0) + H20

Py =15 kbar

01 + Opx + Cpx +Amph+Sp:l: Ga + L+V

Temperature,OC

FIG 2. Isobaric change in the composItIOn of amphibole as a functIOn of temperature for pendotlte 0 (see Table I).

860 A. L. BOETTCHER, B. O. MYSEN AND P. J. MODRESKI

PHASE RELATIONSHIPS OF NODULES AND H 20-C02 VAPOR

Although CO2 is a major chemical component in many kimberlites, carbonatites, andrelated rocks, the main use of CO2 in these experiments is as diluent, lowering the XH;o(aH20)' Although there is evidence that CO2 is soluble in silicate melts at high pressures(HILL and BOETTCHER, 1970; EGGLER, 1973), the net effect of lowering aH20 by the additionof CO2 is to raise the temperatures of the vapor-saturated solidii (Fig. 3). Amphiboles arestable in the presence of liquid for values of XH~O as low as 0.25, but the upper temperaturelimit was not determined for reasons explained above. Nevertheless, work on basaltic andandesitic compositions (HILL and BOETTCHER, 1970; EGGLER and BURNHAM, 1973; ALLENand BOETTCHER, in preparation) suggests that maximum stability will occur for XH;o '" 0.5.

150

40 125

~

c..c 0~

~ 30 100 ~

Q) ~-~ Cl..:::J Q)en Clen 75Q)~

Q..

50

25

Temperature.oC

FIG. 3. Pressure-temperature projectIOn for nodule B + H 20-C02 vapor showmg vapor-saturated solIdil andgarnet- and amphibole-stability boundaries for a range of H20-C02 (XH~) values. The oceanic geotherm is from

RINGWOOD et al. (1964); the shield geotherm is from CLARK and RINGWOOD (1964).

STABILITY OF PHLOGOPITE

Mica occurred in none of the experiments described above. However, the meltingbehavior of phlogopite in the presence of combinations of aluminous enstatite, aluminousdiopside, olivine, spinel, pyrope, corundum, and ± H 20 vapor was studied to 35 kbar and1350°C (see MODRESKI and BOETTCHER, 1972; 1973). Some of the reactions governing thestability of phlogopite are shown in Fig. 4. Additional experiments with the incorporation

35

... 25

~~

~ 20...:J(/)(/)Ql 15..0.

10

MELTING IN THE MANTLE 861

Bulk Composition Products5 PhPy O"ORY" 0 Subsolidus

\1 PhSp [J"WET" ~ Hypersolidusb. PhSpEn EH Includes PyOPhEn 'OA') + Trace y

0'--'------'-----'--------'=----'--':.....-------'1000 1200 1400

Temperature, °C

FIG. 4. Pressure-temperature projection of selected vapor-absent ("dry") and vapor-present ("wet") runs on thecompositions phlogopItesopyropeso and phlogopitesoenstatite (25 wt./o Alz0 3)so, phlogopite6ospinel4 o,phlogopite4 ospinel 3senstatItezs, and phlogopItesoenstatite (10 wt.~~ Alz0 3)so (wt.~~). Also shown are the reac­tions forstente + pyrope~ enstatite" + spmel (MACGREGOR, 1964); pyrope~ enstatite" + sapphirine + silli­manIte (BOYD and ENGLAND, 1959); phlogopite~ forsterite + liquid, and phlogopite + vapor~ forsterite +liqUId (YODER and KUSHIRO, 1969); phlogopite + enstatite~ forsterite + liquid, and phlogopite + enstatite +vapor ~ forsterite + liquid (MODRESKI and BOETTCHER, 1972); phlogopite + pyrope ~ spinel + enstatite +liqUId, and phlogopIte + pyrope ~ forsterite + spmel + liquid (MODRESKJ and BOETTCHER, 1973); and the

contours for Alz0 3 content of enstatite coexisting wIth pyrope (BOYD and ENGLAND, 1964).

of diopside component (MODRESKI and BOETTCHER, 1973) suggest that the vapor-absentreaction:

phlogopitess + enstatitess + diopsidess ~ forsterite + pyrope + liquid

will occur at about 13500 at 35 kbar. This temperature is considerably higher than that ofthe solidii reported here for the peridotite nodules, probably reflecting the absence of Feand Na and the very low aH20 in the synthetic system. These results do, however, reveal thatphlogopitess is a very refractory hydrous mineral, stable to high temperatures and pressures.

A number of experiments were made on nodule C + H 20 spiked with 3, 6, and 10wt.%synthetic phlogopite (MODRESKI and BOETTCHER, 1972). Euhedral, seemingly primary,crystals of phlogopite occurred in all runs (up to at least 1150°C and 15 kbar). Wherephlogopite coexists with amphibole, the proportion of amphibole was reduced relative tothat in unspiked compositions. Microprobe analysis of such an amphibole coexisting withliquid and phlogopite at l000°C and 15 kbar showed it to contain 1.0% K 20. These resultssuggest that the presence of mica will not prohibit the crystallization of amphibole, even forrocks with low NazO contents such as in rock C (0.27%). Magmas crystallizing within thestability field of amphibole « 30 kbar) should contain amphibole, even if they are rich inphlogopite component. Alternatively, no mica was observed in any experiments with theunspiked nodules, even above the stability limits of amphibole.

862 A. L. BOETTCHER, B. O. MYSEN AND P. J. MODRESKI

COMPOSITIONS OF LIQUIDS

Quenched liquids (glasses) were analyzed with the electron microprobe using the methodof FINGER and HADIDIACOS (1971) and using a low beam current ("'0.015 j1A) and de­focussed electron beam (> 10 j1m). Most of the results discussed here were obtained onperidotite B + Hzo-COz between 7.5 and 25 kbar. As with all crystalline phases, Mgj(Mg + Fe) in the glasses increases with temperature, but this ratio is also a direct functionofthat in the rock. As is shown in Fig. 5 and Table 2, liquids in runs as much as 150aC above

Ne

CPI 01

Oi

Hy

lie

01

Qz

Normative (wt%) compositionsof quenched liquids

Peridot ite (8) + H20 + CO2

FIG 5. Normative compositions of quenched hquids for nodule B + H 2 0 CO2 projected onto CHAVES' (1971)olivIne-nephehne--diopside-hypersthene-quartz dIagram. The symbol lOB?IZ 5 indicates 1080' Cat 7.5 kbar withXH;o - 1.0. All Fe was calculated as Fe+ 2

; the effect of calculatIng all Fe as Fe+ 3 is shown for one POInt. Asillustrated in the inset, liquids formed In the presence of H 20-nch vapors are quartz normative; those in the

presence of COrrich (XH;o < 0.6) vapor contaIn normative nepheline.

the vapor-saturated solidii of the peridotites are rich in SiOz (quartz normative) to thehighest pressures investigated under conditions of XH~O > 0.6. At lower values of XH~O'

the liquids are undersaturated with respect to SiOz and rich in CaO and Alz0 3 . Similarchanges from SiOrrich to SiOz-impoverished liquids with decreasing aH20 were obtainedon the phlogopite-rich synthetic compositions (MODRESKI and BOETTCHER, 1972, 1973).This phenomenon is in part related to the change from congruent melting of enstatite(En~ L) at high pressures under anhydrous (low aH20) conditions (BoYD et ai., 1964) to"incongruent" melting at high aH20 (KUSHIRO et ai., 1968), producing a SiOz-rich liquid andforsterite (En + VH20~ Fo + L). Also, it results in part from a reduction in the proportionof SiOz-poor amphibole as aH20 is lowered by decreasing XH~O (Table 2).

MELTING IN THE MANTLE

TABLE 2. ANHYDROUS COMPOSITIONS OF QUENCHED LIQUIDS IN EXPERIMENTS COMPAREDTO ANALYSES OF LAVAS

863

-~,~-- -~- -~-

(3) (4) (5) i(6)lm-! (1) : (2)---:-----+- --- ~- ,

45.6 40.7 45.8 42.81 38.01SI02 , 62.2 , 57.9 'Ti02 : 0.8 I 0.7 1.0 0.6 0.7 3.91 1.53Al20 3 I 20.1 I 17.9 18.9 17.4 12.0 8.43 10.06Fe20 3 0.21 3.58FeO 1.7- 5.4- 6.0- 3.5- 9.9- 8.39 6.01MgO 2.1 4.5 5.9 14.5 18.3 20.66 17.67CaO 10.4 9.8 18.5 18.5 10.2 9.36 16.20Na20 2.1 3.5 3.8 4.6 2.8 3.39 2.86K20 0.2t O.lt 0.3t O.3t O.1t 1.91 1.79

0.74 1.10

M"~-t0.2 0.2 0.2 0.1 0.2

~0.15 0.28

Total 99.8: 100.0 100.0 100.2 100.0 I 99.96 99.09~ ----~ I

(I) Rock B + H20 (XH;o - 1.0) 940'C and 15 kbar.(2) Rock B + H20 - CO2(XH;o - 0.6) ll00°C and 10 kbar.(3) Rock B + H20 - CO2(XHV

O - 0.5) llOO°C and 10 kbar.(4) Rock B + H20 - CO2(XH~O - 0.25) 1070"C and 10 kbar.(5) Rock B + H20 - CO2(XH,o - 0.5) llOO°C and 20 kbar.(6) Olivine-augite nephelinite, Oldoinyo Loolmurwak (DAWSON and POWELL, 1969b) (contains 0.63%

H20+ and 0.82~~ CO2), (Calculated without H20 and CaC03 .)

(7) Average rushayite (DENAEYER, 1966 from DAWSON and POWELL, 1969b) (contains 0.28% H20-, 0.09%H 20+, and 0.39~~ CO2)

- Total Fe as FeO.t Not corrected for volatilIzation III microprobe.

The K20jNa20 value in kimberlites is > 1.0, in contrast to our liquids, but this ratiowould be increased if the proportion of K20 in the starting materials was larger. Forexample, in the experiments with phlogopite-spiked nodules, liquids formed by partialmelting contained up to 0.6% K20 when the K20 of the starting material was '" 1.0%.The relatively low Mgj(Mg + Fe) values of our liquids in experiments with rock B + H20­CO2 (as low as 0.44) result from the high a02(high relative to that proposed for kimberlitesby MITCHELL, 1973) in these particular runs (MYSEN and BOETTCHER, 1973).

The extent to which changing aC02 affects the compositions of liquids other than throughthe effect of changing aH20 could not be determined from these experiments. Nevertheless,HILL and BOETTCHER (1970) and EGGLER (1973) have demonstrated experimentally thatCO2 is appreciably soluble in silicate liquids at high pressures.

COMPOSITIONS OF CRYSTALLINE PHASES

Limited space allows us to discuss nothing but the most salient aspects of the hundredsof analyses obtained on the run products. Chemical trends in the amphiboles were discussedearlier (see also Fig. 2). Mgj(Mg + Fe) of all phases shows a positive correlation withtemperature and with Mgj(Mg + Fe) of the starting material. The Caj(Ca + Mg + Fe)value of orthopyroxene (Fig. 6) also increases systematically with temperature. DAVIS andBoYD (1966), on the basis of experiments on the enstatite--diopside composition join,

864 A. L. BOETTCHER, B. O. MYSEN AND P. J. MODRESKI

1300

120015 kbar

U1100 /"'-/_75100'0 VI.. . / \

Q)~

. /::1- 10000 /,. /~

Q)Q..

E 900 I

~/

I

800

700

5 10 15 20 25 30

[Ca/(Ca+ Mg +Feij X1000

FIG. 6. Compositions of orthopyroxenes in runs at 7 5 and 15 kbar.

suggested that the compositions of orthopyroxenes coexisting with clinopyroxene werequite insensitive to pressure and could be a useful geothermometer. Our results, some ofwhich appear in Fig. 6, are lower than those of DAVIS and BoYD and clearly demonstratethat the position of the orthopyroxene limb of the miscibility gap is sensitive to pressure.In addition, some of the difference undoubtedly results from the fact that DAVIS and BOYD

were dealing with an Fe-free system.

CONCLUSIONS

The beginning of melting of peridotite nodules in the presence of aqueous vapors occursat temperatures considerably lower than those previously reported. Even with XH;o aslow as 0.25, melting begins below 1050)C to depths of 150 km in the continental mantle(Fig. 3). Thus, as proposed on chemical grounds by GRIFFIN and MURTHY (1969), kimberliticmagmas may originate by anatexis of hydrous (phlogopite-bearing) mantle peridotite.

Compositions of the liquids are a function of pressure, temperature, bulk composition,ao" and aH,o' For H 20-rich vapors, anatexis to temperatures as much as 150CC above thesolidii of the peridotites produces SiOrrich melts. Together with the relative positions ofthe solidii of the peridotites and the liquidus of andesite (Fig. 7), it has been proposed thatanatexis can result in primary andesitic magmas (BOETTCHER, 1973). On the other hand, forXH;o < 0.6, the liquids are Si02-poor and enriched in A1 2 0 3 • CaO, and alkalis. TheseCO2-charged liquids may mimic kimberlite-{;arbonatite magmas and exhibit manychemical similarities to olivine melilitites and olivine nephelinites (see Table 2), which are

MELTING IN THE MANTLE

40

35

30

~ 25­.a-'<:

'5

'0

°60~0--::70::-0-~80::-0-~90'--0--,c!:OO'--O--"-'::::,=±:lO'="O~-:,:-l20'--0 --,.,L30'--,-l

Temperature. ·C

E:.:::

75~

0.Q)

Cl

25

865

FIG. 7. Pressure-temperature projectIOn of the vapor-saturated (XH~O - 1) solidii for nodules A-E and thevapor-saturated hqUldll for nodules E and C. The other vapor-saturated (XH~O - 1) liquidIi are from ALLEN et al.(1972). The continental geotherm is from CLARK and RINGWOOD (1964); the oceanic geotherm is by RINGWOOD

et al. (1964).

intimately spatially associated with many kimberlites (e.g. UKHANOV, 1965). In general,however, our liquids are too low in TiO z, KzO and NazO and too high in CaO and Alz0 3 .

The alkali and TiOz contents possibly could be increased by melting a phlogopite-bearingperidotite. In addition, experiments at higher pressures produce liquids with lower CaOand Alz0 3 contents (compare columns 3 and 5, Table 2) because these oxides would bemore strongly fractionated into garnets and more aluminous pyroxenes. Amphiboles occurin equilibrium with liquid in all runs below 2(}-25 kbar, even those spiked with as much as10

00 phlogopite. The absence of amphiboles and the common occurrence of phlogopite

must indicate a minimum depth offormation ofkimberlites of about 75 km. The intersectionof geotherms and solidii, as illustrated in Fig. 3, places additional constraints on formationof such melts. For example, under conditions of X~,o of ~ 0.25 to 0.50, melting of peridotitewould begin at depths of 10(}-175 km. This lies athwart the diamond ~ graphite equilibriumcurve (BERMAN and SIMON, 1955), and it is in concert with the minimum depths usuallyproposed for the genesis of kimberlites.

ACKNOWLEDGEMENTS

We are indebted to E. D. Jackson, D. A. Pretorius, and R. W. White for providing thevaluable nodules. The National Science Foundation supported this research through grantGA 12737.

866 A. L. BOETTCHER, B. 0. MYSEN AND P. J. MODRESKI

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