New experimental data for the system CaO-MgO-SiO2-H2O and a synthesis of inferred phase relations

New experimental data for the system CaO-MgO-SiO2-H2O and a synthesis of inferred phase relations

Geochimica et Cosmochimica Acta. 1975. Vol. 39, pp. 1413 to 1421. Pergamon Press. Prmted in Great Br~tam New ex~r~e~ta1 data for the system [email protected]

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Geochimica et Cosmochimica Acta. 1975. Vol. 39, pp. 1413 to 1421. Pergamon Press. Prmted in Great Br~tam

New ex~r~e~ta1

data for the system [email protected] synt~~is of inferred phase relations

and a

RICHARD D. WARNER Institute of Meteoritics, The University of New Mexico, Albuquerque, New Mexico 87131. U.S.A. (Received

4 September

1974; accepted

in wised


14 March


Abstract-Subsolidus and vapor-saturated liquidus phase relations for a portion of the system CaOMg0-Si02-HzO, as inferred from experimental data for the composition regions CaMgSi,O,Mg,SiO,-SiO,-H,O and CaMgSi,O,-Mg,SiO,-Ca,MgSi,O, (merwinite)-H,O, are presented in pressure-temperature projection. Sixteen invariant points and 39 univariant reactions are defined on the basis of the 1 atm and 10 kbar (vapor-saturated) liquidus diagrams. Lack of experimental control over many of the reactions makes the depicted relations schematic in part. An invariant point involving orthoens~tite, protoenstatite, pigeonite, and diopside (all solid solutions) occurs at low pressure (probably between 1 and 2 kbar). At pressures below this invariant point, orthoenstatite breaks down at high temperature to the assemblage diopside + protoenstatite; with increasing temperature, the latter assemblage reacts to form pigeonite. At pressures above the invariant point, pigeonite forms according to the reaction diopside + orthoenstatite = pigeonite, and the assemblage diopside + protoenstatite is not stable. At 1 atm, both pigeonite and protoenstatite occur as primary liquidus phases, but at pressures above 6-7 kbar orthoenstatite is the only Ca-poor pyroxene polymorph which appears on the vapor-saturated liquidus surface. At pressures above approximately 108 kbar, only diopside, forsterite, and merwinite occur as primary liquidus phases in the system CaMgSizO,-Mg,SiOhCa3MgSi,0s-H,O, in the presence of an aqueous vapor phase. At pressures between 1atm and 10.2 kbar, both akermanite and monticellite also occur as primary liquidus phases. Comparison of the 1atm and 10 kbar vapor-saturated liquidus diagrams suggests that melilite basalt bears a low pressure, or shallow depth, relationship to m~ti~llite-daring ultrabasites.

IN THIS paper

of the system CaO-MgO-Si02-H,O have been reported by KU~HIRO (1969), for the 20 kbar isobar, and WARNER (1973), for the 10 kbar isobar, while

an attempt is made to construct internally consistent phase diagrams for portions of the 1 atm melting relations have been discussed by, system CaD-MgO-Si02-Hz0 considered to be of among others, RICKERand OSBORN(1954) and KUSHIRO significant petrologic import. Of particular concern (1972). Supplementing the above data are experare the phase relationships involving silica (quartz, imental results at 2 and 5 kbar (primarily) reported tridymite, cristobalite), Ca-poor pyroxene (pigeonite, in this paper. orthoenstatite, protoenstatite), diopside, forsterite, Many of the univariant reactions considered in this monticellite, merwinite, akermanite, a water-saturated paper lack experimental control or verification. Howsilicate liquid, and an aqueous vapor phase. These ever, univariant phase wmpositions are, in general, minerals exist on a number of reaction curves which quite well known, and provide the current basis for are of importance in discussing the genesis of certain postulating the existence and relative P-T location types of mafic and ultramafic rocks. of certain. otherwise indeterminate, univariant reacIn deriving the phase relationships presented here, tions. Thus, analytic expressions for the solvi bounding data from a number of sources were used. Informathe monticellite-forsterite and diopside-orthoenstatite tion concerning polymorphism of Ca-poor pyroxene two-phase regions (WARNER and LUTH, 1973; has been reported by ATLAS (1952), BOYD and 1974) provide excellent control on the compositions SCHARER (1964), B~YD et al. (1964), KUSHIRO and of these solid solution phases as functions of temperaYODER (1970), SMITH (1969), SMYTH (1974), YANG ture and pressure. By interpolation between the 1 atm (1973), and YANG and FOWER (1972). Stability relaliquidus diagram(R~cz~ and OSBORN,1954; KUSHIRO, tions of akermanite and related assemblages have 1972) and the 10 kbar vapor-saturated liquidus been presented by YODER (1968, 1973) and DE WYS diagram (WARNER, 1973), a fair measure of control (1972). Vapor-~turated melting relations for portions on compositions of uni~riant silicate liquids is GXA.39srn 1




obtained. Unfortunately, vapor-phase compositions in this pressure interval are not available. To a first approximation, KUSHIRO et al. (1968) and WARNER and LUTH (1974) suggest that the vapor phase is nearly pure H,O which has a Si02: (CaO + MgO) ratio consistently higher than the corresponding anhydrous bulk composition.

Table I. (Continued)

EXPERIMENTAL TECHNIQUES The experiments reported in this paper were performed in an internally-heated pressure vessel, modified after YODER (1950), and utilizing argon gas as the pressure medium. Details as to method of hydrothermal experimentation herein employed have been previously described by WARNER(1973). Starting materials for the experiments were mechanical mixtures of crystalline end-member phases (preparation described in WARNER,1973). Following each experiment, fragments of the run product were immersed in oil and examined with a petrographic microscope. Subsolidus assemblages were additionally finely ground and characterized using a focusing Nonius Guinier-de Wolff diffraction camera set for CuK, radiation. To confirm the polytype of enstatite in certain hypersolidus runs, a Norelco high angle diffractometer with a curved crystal monochromator and CuK, radiation was utilized.


Results of selected synthesis experiments for the system CaO-MgO-SiO,-H,O are presented in Table 1. The data are organized according to their bearing on specific univariant reactions, as tabulated under Table 2. Particular runs having a bearing on more than one reaction may thus be repeated in appropriate subsections of Table I. Table I. Experimental results

Component designations: q = SiO,; fo = Mg,SiO,; en = MgSiO, ; di = CaMgSi,O,; me = Ca,MgSi,O,. Phase designations: Q = P-quartz; Fo = forsterite; Cen = low clinoenstatite; Pr = protoenstatite; En = orthoenstatite; Pi = pigeonite; Di = diopside; MO = monticellite; Ak = akermanite; Me = merwinite; L = silicate liquid; V = aqueous vapor. t ss = Subscript denotes solid solution phases; * = largely or wholly inverted to low clinoenstatite. Anhydrous experiments performed in piston-cylinder apparatus.

Data for the system CaO-Mg&SiO,-HZ0


Table 2. invariant and univariant equilibria

TB: 2 20

PP. E", 9, L, v





21 EM!+Y-L


TO: Pr.b,3,i,Y Q-T?









rmary TF' Pr.En,Fo,L,v 3




16 17








24 25 26

25 -32 3s

Fig. 1. Schematic pressur~temperature projection of vapor-saturated equilibria in the system CaMgSi,O,Mg,SiO,-SiO,-H20. Lettering of invariant points and numbering of univariant reactions are in accordance with Table 2. Data of TUTTLE and ENGLAND (1955) and KUSHIRO and Y~DER (1970) used for reactions (1) and (5), respectively. Data of KUSHIRO et al. (1968) and CARMAN (1969), both supplemented by data from this study, used for reactions (2) and (21), respectively. Reactions (3). (4), (ll), (17) and (18) based on data from this study; all other reactions schematic. Invariant points T, and T, correspond to invariant points B and E, respectively, of CARMAN (1969). all of the invariant points indicated in Table 2 are of the degenerate type. That is, coincidence in composition of two phases (such as En = Pr), or a colinearity in composition space of three phases, or a coplanarity of four phases occurs. Consequently, in the vicinitv of the invariant point, two or more reaction curves coincide. Phases enclosed in parentheses are those which, although involved in the invariant equilibrium under consideration. do not participate in that particular univariant reaction. The invariant and univariant equilibria summarized in Table 2 are shown in P-T projection in Fig. 1, for the composition region CaMgSi,O,-Mg,SiOpSiO,-H20, and Fig. 3, for the composition region CaMgSi,O,Mg,SiO,-Ca,MgSi,O,-H20. Univariant reactions are indicated as numbered lines which correspond to the numbered reactions listed in Table 2; invariant points are designated by letters which also correspond to Table 2. The geometrical arrangement of univariant reactions about each invariant point is in accord with the rules of SCHREINEMAKERS (1915-i 925) THE COMPOSITION REGION CaMgSi,O,-Mg,SiO,-SiO,-HZ0


The P-quartz = tridymite and orthoenstatite enstatite inversion curves

Table 2 summarizes inferred invariant and univariant phase quilibrium relationships for the composition joins (I) ~MgSi~O~-Mg~SiO~-SiO~-H~O and (2) CaMgSimon-Mg~SiO~-~~MgSi*Os-H~O (see WARNER, Fig. 1, 1973). With the exception of the binary equilibrium, B,

No data were obtained in this study on the pquartz = tridymite inversion curve [reaction (l)]; the curve shown in Fig. 1 is taken from TUTTLE and ENGLAND (1955). The orth~nstatite = prot~nstatite inversion curve for MgSiO, was investigated by BOYD

Tr = tridymite; all other phase designations as in Table

= proto-



et al. (1964), who found, on the basis of 5-min runs, a dT/dP slope of 84” + lOC/kbar for this reaction. However, based on 24 hr runs, KUSHIROet al. (1968) found instead that the inversion curve trends toward higher pressures (dT/dP - SO”C/kbar). Results obtained in the present study [reaction (2) Table l] show that the inversion curve passes near 1100°C at 2 kbar and 1250°C at 5 kbar; this is in good agreement with the curve presented by KUSHIRO et al. (1968). In the experiments reported here, multiply twinned clinoenstatite, sometimes accompanied by minor amounts of protoenstatite (identified by X-ray), was the phase present at room temperature. It is presumed to have formed by inversion of protoenstatite during quenching.

Reactions involving Ca-poor pyroxenes Except for data outlining the lower stability limit of pigeonite at moderate to high pressures (KUSHIRO and YODER,1970), experimental information defining P-T curves generated by polymorphism of Ca-poor pyroxene is lacking. At 1 atm the assemblages diopside + orthoenstatite, diopside + protoenstatite, and diopside + pigeonite occur with increasing temperature (ATLAS,1952; KUSHIRO,1972). These assemblages are related by the reactions Ens = Prss + Di,, and Prsa + Di,, = Pi,,. Atlas suggested a temperature of about 1100°C for the reaction Enrs = Prss + Di,, at 1 atm; Bon, and SCHAIRER(1964) noting a change in slope along the diopside solvus limb near llOOC, concurred with this determination. Pertinent data in this study [reaction (3), Table l] indicate that the P-T curve for this reaction passes between 1240°C 1 kbar and 1235”C, 2 kbar. The limits of solid solution at 1240°C 1 kbar are approximately 22mol% Mg,SizOe in Di,, and 4mol% CaMgSizO, in Pr,,. [Some of the results obtained at 1240°C 1 kbar present a problem in interpretation since the assemblages En, + Prss and Di,, + Prss are not compatible. One plausible explanation stems from the well-documented experimental observation that protoenstatite is non-quenchable (e.g. SMITH, 1969; SMYTH,1974). SMYTH(1974) reported that rapid quenching of protoenstatite yields twinned low clinoenstatite and orthoenstatite. In the system CaMgSi,O,-Mg,Si,O,, protoenstatite solid solution passes through an Ens, + Prss two-phase region on cooling (Fig. 2a and b), so that upon quenching to room temperature, a twophase orthoenstatite + low clinoenstatite assemblage would be produced. Compositions in the stability field Di,, + Pras may thus quench to a three-phase assemblage, namely diopside + orthoenstatite + low clinoenstatite. Why the latter assemblage should be

- cot.qo,




Fig. 2. Temperature-composition sections for the Ca-poor region of the join CaMgSi,O,-Mg,Si,O,. Isobars (a) and (b) illustrate inferred subsolidus pyroxene phase relations at pressures, respectively, less than and greater than invariant point B (Fig. 1). Abbreviations: Di,,, diopside solid solution; En,,, orthoenstatite solid solution; Pr,,, protoenstatite solid solution; Pi,,, pigeonite solid solution.

obtained for the composition di,,en,,, but not for d&en,,, is not presently clear.] The reaction Prss + Di,, = Pi,, is not precisely located by experiment. ATLAS(1952) on the basis of 1 atm experiments, indicated a temperature of about 1250°C for this reaction, while KUSHIRO(Fig. 2, 1972) by extrapolation of the Prss + Pi,, two-pyroxene field from higher temperatures, suggested a lower stability limit for Pi,, between 1200 and 1300°C at this pressure. YANG (1973) demonstrated a temperature of 1276°C (1 atm) for the disappearance of the stability field of pigeonite in the system anorthitediopsideenstatite-silica, but cautioned that the presence of minor Al,O, in pyroxene solid solutions “may alter considerably the temperature of this transformation” (p. 492) from that obtained in the Al,O,-free system. One-kilobar experiments reported here place the reaction Prss + Di,, = Pi,, [reaction (4) Table l] between 1240 and 1280°C. (At 128o”C, 1 kbar, as at 1240°C 1 kbar, the quench products obtained with MgSiO,rich compositions are confusing, and suggest the existence of serious quench problems in this part of the system.) At 1280°C 1 kbar, diopside coexisting with pigeonite contains about 30 mol% Mg,SizOb in solid solution, and pigeonite coexisting with diopside dissolves about 20 mol% CaMgSi,O,. Reaction (4) has a shallower dT/dP slope than reaction (3): the two curves should thus intersect at a low pressure (probably between 1 and 2 kbar), generating an invariant point (B in Table 2 and Fig. 1) marking the disappearance of the two-phase coexistence Prss + Di,,. From this invariant point, two other reactions arise: Enss + Di,, = Pi,, and Enss = Prss + Pi,,. The first reaction [reaction (5), Table 2 and Fig.

Data for the system Ca0-MgO-Si02-HZ0 I)] denotes the lower stability limit of pigeonite at pressures above invariant point B. The second reaction is required on the basis of the compositional relationships among the polymorphs of Ca-poor pyroxene, but is of little petrologic interest. Reaction (5) was investigated in the pressure range 2-20 kbar by KUSHIROand YOIXR (1970), using a gas-media apparatus at and below 10 kbar, and a piston-cylinder apparatus above 10.5 kbar. Discrepant results were obtained between the gas-media and solid-media apparatus experiments. Furthermore, many of the results which they obtained with the gas-media apparatus were different depending on the nature of the starting material. For example, at 1250 and 1300°C at 2 kbar, 1325” at 5 kbar, and 1450” at 10 kbar, a “mixture of clinoenstatite and diopside solid solutions was converted to a mixture of orthoenstatite and diopside solid solutions” (p. 228). However, under the same experimental conditions, “glass was crystallized to pigeonite clinopyroxene, and pigeonitic clinopyroxene was unchanged” (pp. 227-228). In a study of the diopside-orthoenstatite two-phase region in the system CaMgSi206-Mg,Si,Ob (WARNER and LUTH, 1974), mixtures of end-member diopside and clinoenstatite converted to an assemblage diopside solid solution + orthoenstatite solid solution at all temperatures in the range 9m13OO”C at 2, 5, and 10 kbar, corroborating Kushiro and Yoder’s results with clinoenstatite + diopside starting mixtures. The conversion of clinoenstatite + diopside into orthoenstatite + diopside in both studies suggests the probable stability of the latter assemblage (as opposed to pigeonite). The persistence of pigeonite in Kushiro and Yoder’s other experiments could reflect problems of metastability, perhaps related to metastable crystallization from the glass medium, or, in the pistoncylinder experiments, a wider stability range under non-hydrostatic pressure conditions. The curve for reaction (5) shown in Fig. 2 has been constructed in accordance with Kushiro and Yoder’s gas-media apparatus experiments with clinoenstatite-diopside and orthoenstatitediopside starting mixtures. A tentative summary of subsolidus pyroxene phase relations for the join ~MgSi~O~-Mg~Si~O~ is presented in Fig. 2. The isobar of Fig. 2a is at a pressure less than that of invariant point B (Fig. I), and is similar in overall aspect to the 1 atm isobar (KWSHIRO, Fig. 2, 1972; YANG and FOSTER,Fig. 3, 1972). The isobar of Fig. 2b is at pressure greater than B; isobars at much higher pressures would differ only in that no Prss, Pr,, + Ens%,and Pr,, + Pi,, fields would be encountered (e.g. KUSHIROand YODER,Fig. 19, 1970). Compositionally, pigeonite is the most Ca-rich of the polymorphic forms of C&poor pyroxene, while pro-


tenstatite is the most C&poor. Orthoens~tite is thus less Ca-rich than coexisting pigenoite (Fig. 2b), but is more Ca-rich than coexisting -protoenstatite (Fig. 2a, b). Melting reactions Since it is apparent from the preceding section that reactions (3)-+(6), and thus invariant point B, are only approximately located, invariant points generated by intersections of melting reactions (3)--t(6) must also be considered approximations as to their location on a P-T diagram, although the nature of the invariant equilibria may be well defined. On the vapor-saturated liquidus surface for CaMgSi,06 Mg,Si04-SiO,-H,O, the incipient melting reaction involves two pyroxenes and silica. Invariant points QA, QB( and Qc (Fig. 1 and Table 2) arise when this two-pyroxene-silica melting curve [reactions (7), (9), (lo), and (11)] intersects subsolidus reactions (4) (Pr, + Di,, = Pi,,), (3) (En,, = Pr,, + Di3 and 1 (Q = Tr), respectively. Similarly, invariant points To and T, in the ternary system Mg0-SiO,-HZ0 are located at intersections of the enstatitesilica melting curve [reactions (19), (20), and (21)] with the Q = Tr and En = Pr inversion curves, respectively. pernary invariant points TD and T, are equivalent to invariant points B and E, respectively, of CARMAN(p. 67, 1969).] At 1 atm, the melting reactions analogous to (7) and (8) are Di,, + Tr = Pi,, + L (1373°C) and Pi,, + Tr = Prss + L)(T> 1395°C-K~~~~o, Fig. 1, 1972). With increasing pressure, these melting curves trend rapidly toward lower tem~ratures, and the univariant liquid compositions migrate to very SiOz-rich compositions. Both features are a direct consequence of the pronounced effect of pressure on lowering melting temperatures in the system SiO,-H,O: 2 kbar “depresses the freezing point of SiOZ from 1720 to 1120°C” (KENNEDYet af., p. 510, 1962). Also with decreasing temperature, the pyroxene solvus widens, which means that the incipient melting reaction should become eutectic (i.e. Di,, + Ca-poor pyroxene + silica + V = L) at a low pressure probably beiow 2 kbar). Above Qc this reaction is Di,, + Enss + Q + V = L, and has been located at approximately 1100°C at 2 kbar, between 1050 and 1100°C at 5 kbar (Table l), and 1030 + 15°C at 10 kbar (WARNER, 1973). A similar P-T curve is generated by the analogous reaction in the C&free system [reaction (Zl), Table 11, although this reaction must occur at least a few degrees above the quaternary reaction. Reaction (12), which begins at invariant point QB, is inferred from theoretical considerations, the principal control being the relative compositions of the phases participating in that invariant equilibrium. Invariant



point Qn is generated when this reaction intersects the Q = Tr inversion curve; reaction (13), arising at Qn, terminates at ternary invariant point TB. Quaternary invariant points QE and QF arise from intersections of the olivine peritectic curve [reactions (14). (16) and (17)] with reactions (4) (Pr,, + Di,, = Pi,,) and (3) (En,, = Prss + Di,,), respectively. The 1atm reactions corresponding to (14) and (15) are Di,, + Pi,, = Fo,, + L (1385°C) and Pi,, = Prss + Foss + L (1420” < T < 1430”C-KUSHIRO, Fig. 1, 1972). Reaction (17) which originates at QF, lies between 1250 and 1300°C at 2 kbar, and between 1200 and 1245’C at 5 kbar (Table 1). This reaction has been located at 1230 + 20°C at 10 kbar by WARNER (1973) and at 1220 + 15°C at 20 kbar by KUSHIRO(1969). Reaction (18) which also originates at QF, is deduced from chemographic relations at that invariant point. This reaction is constrained to lie below 1300°C at 2 kbar (Table l), but at a temperature above that of reaction (17). Summary

The invariant and univariant equilibria portrayed in Fig. 1 may be summarized as follows. At 1 atm, both pigeonite and protoenstatite (as well as diopside, forsterite, tridymite, and cristobalite) occur as primary liquidus phases. Invariant point QB marks the appearance of a primary phase field for orthoenstatite on the vapor-saturated liquidus, while invariant point QE marks the disappearance of a primary phase field for pigeonite. If, as is shown in Fig. 1, QB occurs at lower pressure than QE, then for P,, < P < PQE, protoenstatite, orthoenstatite, and pigeonite will all be primary liquidus phases under vapor-saturated conditions. The primary phase field for protoenstatite pinches out at ternary invariant point Tr. At pressures above that of T,, and at least to 20 kbar (KUSHIRO,1969), orthoenstatite is the only Ca-poor pyroxene occurring as a primary liquidus phase in the vapor-saturated system. In a similar manner, invariant point Qc marks the appearance of B-quartz as a primary liquidus phase, whereas tridymite is eliminated from the vapor-saturation surface at a pressure where the silica + vapor = liquid reaction in the system Si02-H,O intersects reaction (I), Q = Tr (1.5 kbar according to KENNEDYet al.. 1962). THE COMPOSITION REGION CaMgSi,O,-M&GO,-Ca,MgSi,O,-H,O Subsolidus reactions

Inferred phase equilibria for the composition region diopsideforsterite-merwinitewater are shown in Fig. 3. T, is an invariant point in the anhydrous ter-



Fig. 3. Schematic pressure-temperature projection of vapor-saturated equilibria in the system CaMgSi,O,Mg,SiO,-Ca,MgSi,O,-H,0. Lettering of invariant points and numbering of univariant reactions is in accordance with Table 2. Data of YODER(1968), supplemented by data from this study, used for reactions (24), (25), (26) and (38); all other reactions schematic. Invariant points

T, and T, correspond to invariant points C and B, respectively, of YODER(1968).

nary system CaO-MgO-Si02 involving the solid phases diopside, akermanite, monticellite, and forsterite; it is equivalent to invariant point C (1095”C, 10.6 kbar) of YODER(1968; 1973). The P-T curves for univariant reactions (24)-(26) were outlined by YODEX (Figs. 72 and 75, 1968). Experimental data obtained for reaction (24), Di,, + MO,, = Ak,, + Fo,, (Table l), are in good agreement with Yoder’s results. A critical experiment at 1050°C 10 kbar suggests that the 10 degree heating-cooling cycle of the run brackets reaction (25) Di,, + Me = Ak,, [almost precisely at the position reported by YODER(1968)]. There is some doubt as to the exact nature of this reaction, for akermanite probably crystallizes slightly off the endmember composition, CazMgSizO, (SCHAIRERet al., 1967; YODER,1968). This would complicate, but not significantly alter, the relations shown at invariant points T,, Ti and Q,. Disparate results have been obtained with respect to reaction (26), which represents the maximum upper thermal stability of MO, at a given pressure. Whereas YODER (1968) located this reaction near 1200°C at 10 kbar, WARNER and LUTH (1973) found no evidence for this reaction in hydrous experiments up to 1300°C at 10 kbar. However, anhydrous experiments with a piston-cylinder


Data for the system Ca*[email protected],-H20 apparatus (Table 1) indicate that Fo,, + Me, and not MO,, is stable at 12%135O”C, 10 kbar. It is possible that non-hy~os~~c pressure, as is obtained in the piston~ylinder experiments, stabilizes the assemblage Fo, + Me at temperatures below the (hydros~tic) equilibrium curve, in analogous manner to the apparent stabilization of pigeonite with respect to the assemblage Di,, + Enss (KUSHIROand YODER, 1970). As shown by YODER(1968), reaction (26) possesses an extreme negative dT/dP slope (approximately - IbO”C/kbar), so that moderate discrepancies in pressure could produce large differences in the determined reaction tempature; this, too, may contribute to the conflicting results.

Ternary invariant point TI corresponds to invariant point B (1065X, 10.2 kbar) of YDDER (1968). The other invariant points indicated in Fig. 3 are postulated on the basis of the vapor-saturated liquidus diagram at 10 kbar (WARNER, Fig. 5, 1973), and arise from intersections of a number of univariant melting curves with subsolidus reactions (24)-(26). Invariant point T, signifies the disappearance of a primary phase field for akermanite on the vapor-saturated liquidus; similarly, QJ marks the disappearance of monticellite as a primary liquidus phase. Thus, at pressures above that of Q, (_ 10%kbar), only diop side, forsterite, and merwinite occur as primary liquidus phases in this portion of the system CaOMgO-Si02-H20, while at pressures below that of Ti (10.2 kbar) akermanite and monticellite also occur as primary liquidus phases. Singular reactions

In order that the vapor-saturated liquidus diagram at 10 kbar (WARNER,Fig. 5, 1973), and the P-T relations shown in Fig. 3, both be consistent with the 1 atm liquidus diagram of RICKERand C&BORN(Fig. 1, 1954), the postulation of certain singular quilibria are required. These equilibria are generated along a univariant reaction whenever compositions of univariant liquids migrate into coplanarity (in composition space) with three other phases (here two crystalline phases and an aqueous vapor phase). A list of the postulated singular equilibria is given in Table 3; since there is minimal control as to their P-T location, they have not been indicated in Fig. 3. The singular point which occurs among the univariant equilibrium D&, Ak,,, Fo,,, L, and V is, petrologic&y, of most interest here. This singular point is generated when the univariant silicate liquid becomes coplanar ~~m~sitionally) with Ak,, Fo,, and V. At pressures less than the singular equilibrium, the uni-

Table 3. Singular equilibria ““lvafiant TeXtloll BtPZP, Ak,fW,s+k!&-L







variant reaction is eutectic, Di,, + Ak,, + Fo,, + V = L [at 1 atm the reaction (1357°C) is Di, + Ak,, + Fo,, = L (RICK and OSBORN,Fig. 1, 1954)]. At pressures greater than the singular equilibrium, the reaction is of the peritectic type, Ak,, + Fo,, + V = Di,, + L. The composition join Ak,,-Fo,,-V thus constitutes a thermal divide at lower pressures (below the singular point), but not at higher pressures; the petrologic implication of this will be discussed subsequently. DISCUSSION The two-liquid region

GRIEG (1927) outlined a region of liquid immiscibility in the silica-rich portion of the system CaCr Mg&SiOz at 1 atm. KUSHIRO (1969) and WARNER (I 973) found no evidence for such a two-liquid region on the vapor-saturated liquidus surface at 20 and 10 kbar, respectively. This feature is encountered only at very high temperature (approximately 1700°C at 1 atm), however, and its absence from the vaporsaturated liquidus surface is not surprising considering the much lower temperatures involved. To the writer’s knowledge, there is no experimental information concerning the effect of pressure alone on the stability of the two-liquid region. In the absence of information to the contrary, it may be presumed that a twoliquid region persists in the a~ydrous system, at least to moderate, and possibly to high, pressures. A field of liquid immiscibility should thus be present in the vapor-absent region of the hydrous system. It is probable that at extremely low pressures (P < 1 kbar) the two-liquid region will intersect the vapor-saturated liquidus surface. EfSect of FeSiO, on inferred pyroxene

phase relations

In terms of natural pyroxenes, specifically those that have crystallized from basaltic mamas, the phase relations of importance are those that prevail as FeSiO, is added to this system. Al~ough knowledge of the phase relations within the pyroxene quad-



rilateral is incomplete, sufficient information is available from documented crystallization trends of natural pyroxenes and from recent experimental investigations (e.g. LINDSLEYand MUNOZ, 1969; SMITH, 1972; TURN~CK et al., 1973; Ross et al., 1973) to deduce the following: (i) The miscibility gap between augite and Ca-poor pyroxene shrinks with increasing Fe/(Fe + Mg) ratio. For example, Ross et al. (1973) reported that, at the 1 atm solidus, the width of the gap narrows from about 20”/, Wo at Fe/(Fe + Mg) = 0.3 (1175°C) to about 10% Wo at Fe/(Fe + Mg) = 0.85 (960°C). At high pressure (1516 kbar), SMITH (1972) found that the solvus is crested between 91>95o”C for compositions along the synthetic join En, ,Fs,,-wollastonite

involving akermanite (RICKER and OSBORN,Fig. 1, 1954). The eutectic crystallization of diopside, forsterite, and akermanite provides a synthetic analogue of melilite basalt. In contrast, on the 10 kbar vaporsaturated liquidus surface, the boundary curve along which forsterite and diopside are coprecipitated leads to a peritectic point, where forsterite reacts with liquid to form diopside and monticellite in the presence of an aqueous vapor phase (WARNER, Fig. 5, 1973). The shift in the liquidus phase boundaries between 1 atm and 10 kbar implies that melilite basalt bears a low-pressure, or shallow depth, relationship to monticellite-bearing igneous rocks, such as monticellite alnoite. Of pertinence to this relation will be the singular point, discussed in a preceding section, where the eutectic reaction Di,, + Ak,, + Fo,, + V = L changes to a peritectic reaction, Ak,, + Fo,, + V = Di,, + L. At piessures below that of this singular point, a temperature maximum occurs along the boundary curve L(Ak,,, Fo,,, V) near the akermaniteforsterite join; this thermal maximum acts as a barrier preventing derivation of monticellite-saturated liquids from ‘melilite basalt’ magmas. At pressures above that of the singular equilibrium, this maximum no longer occurs, and it is possible to pass to liquids which may precipitate monticellite. However, it should be emphasized that this conclusion is restricted to vaporsaturated conditions, which do not necessarily apply

[Fe/(Fe + Mg) = 0.85]. (ii) The temperature of the lower stability limit of pigeonite is appreciably decreased with increasing Fe/ (Fe + Mg) ratio. A drop of about 300°C occurs between the iron-free system and a natural composition (BROWN, 1968) having an Fe/(Fe + Mg) ratio = 0.56 (KUSHIRO and YODER, 1970). The fact that the cooling curve for basaltic magma intersects the hypersthene-pigeonite inversion interval at an Fe/ (Fe + Mg) ratio generally about 0.3 is consistent with this observation. (iii) The diopside-protoenstatite two-phase region pinches out rapidly with increasing Fe/(Fe + Mg). in nature. This is necessitated by the fact that during crystallization-differentiation of basaltic magma the coexistence Acknowledgements-The author would like to thank W, augite + hypersthene passes directly into the coexisC. LUTH, for his supervision and encouragement throughtence augite + pigeonite, without intersecting an augite out this study, P. R. GORWN, for his technical assistance. + protohypersthene stability region. Moreover, the and J. H. CARMANand J. R. WEIDNER,for providing much: assemblage augite + pigeonite has been recorded for needed guidance in the early stages of this investigation. Fe/(Fe + Mg) ratios as low as 0.15 (POLDERVAART Special thanks are due to D. H. LINDSLEY,D. C. PRESNALL, and J. C. STEINER,whose comments materially improved and HESS,p. 478, 1951). Of pertinence to this discusthe present manuscript. This work was supported by NSF sion is the Papuan inverted protoenstatite occurrence Grant GA 4164 to W. C. Luth and by a National Academy reported by Dallwitz et al. (1966). The presence of of Sciences/National Research Council Resident Research both orthoenstatite and protoenstatite (now inverted Associateship to the author. to clinoenstatite) phenocrysts with consistent compositional differences is interpreted by them to indiREFERENCES cate intersection of the protoenstatite-orthoenstatite ATLASL. (1952) The polymorphism of MgSiO, and solidinversion curve (actually a narrow two-phase region state equilibria in the system MgSiO,-CaMgSi,O,. J. within the quadrilateral) with the liquidus curve of Geol. 60, 125147. BOYD F. R., ENGLANDJ. L. and DAVIS B. T. C. (1964) the magma at a point where the pyroxene Fe/(Fe + Effects of pressure on the melting and polymorphism Mg) ratio equalled 0.13 (DALLWITZ et al., p. 399, of enstatite, MgSiO,. J. Geophys. Res. 69, 2101-2109. 1966). For augite-saturated compositions, protohyBOYD F. R. and SCHAIRERJ. F. (1964) The system MgSiO,persthene should thus be eliminated at an Fe/(Fe + CaMgSi,O,. J. Petrol. 5, 2755309. Mg) ratio 50.13. BROWNG. M. (1968) Experimental studies on inversion relations in natural pigeonitic pyroxenes. Carnegie Inst.

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