Model System Controls on Conditions for Formation of Magnesiocarbonatite and Calciocarbonatite Magmas from the Mantle

Transcription

1 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 PAGES Model System Controls on Conditions for Formation of Magnesiocarbonatite and Calciocarbonatite Magmas from the Mantle PETER J. WYLLIE AND WOH-JER LEE DIVISION OF GEOLOGICAL AND PLANETARY SCIENCES, CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CA 91125, USA RECEIVED SEPTEMBER 30, 1997; REVISED TYPESCRIPT ACCEPTED MAY 21, 1998 Experimental data indicate that carbonate-rich magmas may be that magnesiocarbonatite magmas can precipitate sövites (calgenerated at depths greater than ~70 km by partial melting of ciocarbonatite rocks). carbonated peridotite. The near-solidus magmas lie on the liquidus field boundary between silicates and carbonates. Liquid compositions are dominated by the system CaCO 3 MgCO 3, and precise compositions (e.g. Ca/Mg) are defined by the peridotite mineralogy KEY WORDS: carbonatite; carbonated peridotite; dolomite; liquid im- (e.g. harzburgite, lherzolite, wehrlite); alkali contents reflect directly miscibility the peridotite composition. These liquids are dolomitic, with Ca/ (Ca + Mg) between 0 7 and 0 5 from 2 GPa to at least 7 GPa. At conditions of mantle melting, there is a large separation between the silicate carbonate liquid immiscibility volume, the INTRODUCTION silicate carbonate liquidus field boundary, and probable liquid paths. The formation of carbonate-rich liquids immiscible with Magnesiocarbonatites, calciocarbonatites, and natro- silicate magmas in the mantle is therefore unlikely, which denies the carbonatites have all been proposed as primary magmas from the mantle. Primary magnesiocarbonatite (dogeneration of immiscible CaCO 3 ocelli and primary natrocarbonatite lomitic) magmas are supported by experiments (Wyllie magmas. Rising carbonate-rich magmas retaining equilibrium with & Huang, 1976a; Eggler, 1978; Wallace & Green, 1988; mantle lherzolite will react, crystallize and release CO 2 vapor at Thibault et al., 1992; Sweeney, 1994; Dalton & Presnall, depths of ~70 km, increasing clinopyroxene/orthopyroxene in the 1997), and by field occurrences (Harmer & Gittins, 1997). rock. Primary magnesiocarbonatite magmas (dolomitic) can be Experiments of Dalton & Wood (1993) were interpreted erupted explosively from this depth. Given sufficient magma, lherzolite as supporting strong field-based arguments for primary can be converted to wehrlite by this decarbonation reaction. At calciocarbonatite magmas (e.g. Gittins, 1989; Bailey, shallower depths, wehrlite (but no other peridotite) can coexist with 1993; Stoppa & Cundari, 1995). The formation of natrocarbonatite magma relatively enriched in Ca/Mg. If metasomatism carbonatite magmas within the mantle by liquid imof lherzolite to wehrlite can occur through a depth of tens of miscibility or by fractionation of sodic dolomitic magma kilometers, our new data at 1 GPa confirm an earlier proposal that was explored from experiments of Koster van Groos primary calciocarbonatite magmas can be generated at some depth (1975), Wallace & Green (1988), and Sweeney et al. between 70 km and 40 km, but indicate considerably higher silicate (1995). Le Bas (1989) considered natrocarbonatites to be components. The shallowest magmas contain a maximum of 73 parental to others. The occurrence of ocelli or globules wt % CaCO 3 (equivalent to 89% CaCO 3 in the carbonate of calcite in mantle xenoliths has been considered as components of the liquid), with 18% silicate components at 1 evidence for the formation of immiscible CaCO 3 -rich GPa. Phase relations in the system CaO MgO CO 2 H 2 O show melts in the mantle (e.g. Pyle & Haggerty, 1994; Kogarko Corresponding author. Telephone: (626) Fax: (626) wjl@gps.caltech.edu Oxford University Press 1998

2 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 NOV & DEC 1998 et al., 1995; Seifert & Thomas, 1995). In a series of papers (Wyllie et al., 1990; Lee & Wyllie, 1994, 1996, 1997a, 1997b, 1998a; Lee et al., 1994, 1998a), we have presented experimental data related to the compositions of carbonate-rich liquids formed in peridotite CO 2, and on silicate carbonate liquid immiscibility. Here we summarize experimental data from the model system CaO MgO SiO 2 CO 2 (Lee et al., 1998a), and compare liquids from this system with those from carbonated peridotites. These liquids are then compared in a complex pseudoquaternary projection with the compositions of immiscible silicate carbonate liquids. These phase diagrams provide a framework for illustrating possible and improbable petrological processes, and particular attention is paid to the conditions of formation of primary carbonatite magmas. subsolidus reaction A to form dolomite, or melted by reactions B and B in the presence of CO 2 vapor; the curves meet at point Q. At higher pressures, vapor- absent dolomite lherzolite melts along curve D forming a dolomite-rich liquid; if there were sufficient CO 2 present for forsterite to react out, then dolomite websterite V melts along solidus E. [See Wyllie & Huang (1976a), Eggler (1978), and Lee & Wyllie (1997a) for detailed accounts of these and other reactions.] The partly schematic vapor-saturated liquidus surface at pressure Q in Fig. 3a is constructed using data from Huang & Wyllie (1976), Eggler (1978), and Lee et al. (1998a). [See Wyllie & Huang (1976b, fig. 1) for the mixtures that closely bracket the silicate carbonate field boundary at 2 7 GPa as shown, rather than as drawn by Eggler (1976, 1978, fig. 13).] Readers should note the limited solubility of silicates in the liquid (from magnesite to dolomite), and the temperature profile along the sil- icate carbonate boundary similar to that of the carbonate liquidus (Fig. 1). Also noteworthy is the curvature of the field boundary away from calcite as it extends to the field for quartz. All model carbonated peridotites must begin to melt at points along this field boundary. The invariant liquid Q (Fig. 3a) coexisting with Cc (dolomitic) and all three lherzolite minerals at pressure Q (Fig. 2) should be noted. At higher pressures (Fig. 3b), Q is replaced by two vapor-saturated points, C and E, corresponding to the liquids of univariant curves for melting of dolomite harzburgite V and dolomite websterite V (Fig. 2). The vapor-absent melting reaction for dolomite lherzolite (D, Figs 2 and 4) is situated behind the contoured liquidus surface. At lower pressures, the liquidus field for orthopyroxene shrinks, and point Q is CARBONATE-RICH LIQUIDS IN CaO MgO SiO 2 CO 2 Phase relationships in the system CaO MgO SiO 2 CO 2 (Wyllie & Huang, 1976a; Eggler, 1978; Dalton & Presnall, 1997, with Al 2 O 3 added) have provided the basis for understanding processes involved in partial melting of peridotite CO 2 systems, and the quaternary field bound- aries and liquid compositions have proved to be the dominant controls on major-element liquid paths in the whole-rock systems, as illustrated and discussed below. Liquid compositions from carbonated peridotite as- semblages with CO 2 at the solidus are given by isobaric eutectics and peritectics on the silicate carbonate field boundary on the vapor-saturated liquidus surface. Wyllie & Huang (1976a) determined the position of this boundary at 2 7 GPa, located the invariant liquid points, and sketched their changing positions as a function of pressure. This information was integrated into an internally consistent array of univariant decarbonation and melting curves for the carbonate peridotite system. The phase relationships of the carbonate-rich liquid compositions are dominated by the liquidus and solidus for CaCO 3 MgCO 3 (CC MC), which are shown in Fig. 1. Readers should note the continuous solid solution between calcite and dolomite, and the minimum liquidus temperature, with pressure decreasing from 2 7 to 1 GPa equivalent to depths of about 90 km to 40 km, approaching the continental Moho (Irving & Wyllie, 1975; Byrnes & Wyllie, 1981). The pronounced liquidus minimum changes composition from 63 to 71 wt % CC with decreasing pressure. Figure 2 for system CaO MgO SiO 2 CO 2 includes measured and estimated univariant reactions rep- resenting reactions of peridotites with CO 2 (V) or the stable carbonate, dolomite/calcite solid solution (Do/Cc). Below invariant point Q, lherzolite CO 2 is carbonated by Fig. 1. Comparison of phase relationships for the join CaCO 3 MgCO 3 at 2 7 GPa (Irving & Wyllie, 1975) and 1 GPa (Byrnes & Wyllie, 1981). CC, calcite; MC, magnesite; Cc, Do, and Mc, solid solutions of calcite, dolomite, and magnesite; Pe, periclase; L, liquid; V, vapor. 1886

3 WYLLIE AND LEE PRIMARY CARBONATE-RICH MAGMAS FROM THE MANTLE among melilitites and kimberlites (Dalton & Presnall, 1997; Moore & Wood, 1997). CARBONATE-RICH LIQUIDS IN PERIDOTITE CO 2 Figure 5 shows the primary mineral fields on the vaporsaturated liquidus surface of Fig. 3, projected as in Fig. 4c. The silicate carbonate field boundary is defined by the bold line. Experimentally measured compositions of liquids from model systems and natural rocks at various pressures are also projected, with the additional components FeO, Na 2 O, K 2 O and Al 2 O 3 taken into account as shown. The filled diamonds are the temperature minima in the carbonate system at 2 7 GPa and 1 GPa (Fig. 1), showing composition increasing by ~8 wt % CaCO 3 through a pressure decrease of 1 7 GPa; we expect a similar composition change to be reflected on the temperature profile of the silicate carbonate field boundary in the multicomponent system. Fig. 2. Univariant reaction boundaries for carbonated peridotite, Experimental results from most natural peridotites represented by the system CaO MgO SiO 2 CO 2 (see Wyllie & Huang, 1976a; Eggler, 1978; Lee & Wyllie, 1997a). Q, invariant point involving (group 1, which includes some more extreme varieties forsterite (Fo), orthopyroxene (Opx), clinopyroxene (Cpx), dolomite/ with amphibole and phlogopite) cluster within a few per magnesian calcite (Do/Cc), liquid (L), and vapor (V). Curve A represents cent of the estimated silicate carbonate invariant points decarbonation reaction of dolomite lherzolite, Do + Opx = E, C, and F (Figs 3 and 5) (Wallace & Green, 1988; Cpx + Fo + V; B B, C, E, and F represent respectively vapor-saturated solidi of CO 2 lherzolite, dolomite harzburgite, dolomite Thibault et al., 1992; Sweeney, 1994). The six analyses websterite, and (dolomite magnesian calcite) wehrlite; D, vapor-absent (open squares) reported by Dalton & Wood (1993) contain solidus for dolomite lherzolite; G, conversion reaction between much lower SiO 2, higher CaO/MgO, and lower alkalis dolomite and magnesite (lherzolite/websterite), Mc + Cpx = Do + Opx (Fo can be present as a non-reacting phase). (Lee & Wyllie, 1997a, fig. 12) than all other analyses. The four richest in MgO (groups 2 and 3) are liquids from carbonate harzburgite and wehrlite between 3 and 2 2 GPa. The two richest in CaO (group 4) are replaced by the two points F and B. F is shown in Fig. 3c, liquids from experiments at 1 5 GPa produced by reand B has migrated up the Cpx Fo field boundary away peated reaction of orthopyroxene with dolomitic liquid, from carbonates to higher temperatures (see B in Fig. 2). which was equivalent to that derived from lherzolite (with B is the liquid produced from lherzolite V, and F is H 2 O added to lower the solidus temperature). The liquids produced from dolomite (Do/Cc) wehrlite, which is the coexist with olivine, clinopyroxene and magnesian calcite. only carbonated peridotite that melts at pressures below The equivalent liquids from carbonate wehrlite (with V) Q (Figs 2 and 3c; reaction 6y, Wyllie & Huang, 1976a, are represented in the model system at 2 7 GPa by point fig. 10). F (Fig. 3c, slightly richer in CaO than the invariant liquid The significant curve for the progressive melting of Q (F is coincident with Q at 2 8 GPa; Figs 3a and 5). vapor-absent dolomite lherzolite under mantle con- The trend of liquid compositions (from carbonate ditions is D D, projected in Fig. 4. Dolomite melts out wehrlite) almost directly toward CaCO 3 with decreasing at point D (in a narrow temperature interval for a pressure from 3 to 1 5 GPa, represented by the Dalton & multicomponent rock) forming dolomitic carbonatite Wood (1993) sequence of analyses, diverges significantly liquid (amount controlled by per cent CO 2 present). With from our measured position of the silicate carbonate increasing temperature the liquid, in equilibrium with field boundary at 2 7 GPa. We have located a portion lherzolite, becomes progressively poorer in CO 2 (Fig. 4a of the field boundary at 1 GPa using a combination of and b) as its composition changes from D to D, the experimental brackets and quenched liquid analyses, with CO 2 -free haplobasalt eutectic in the system Fo Di En results shown in Fig. 5 (Lee & Wyllie, 1998b). The (Fig. 4c). Experimental determination of the curvature silicate carbonate field boundary does shift toward CaCO 3, and the isobaric invariant liquid F from mag- nesian calcite wehrlite is considerably enriched in CaO/ MgO compared with its value at 2 7 GPa. Liquid F at on this field boundary at various pressures will show how the liquid varies in CO 2, Ca/Mg, and Si/(Ca + Mg) as a function of temperature, variables that are important 1887

4 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 NOV & DEC 1998 Fig. 3. Isobaric vapor-saturated liquidus surface for the system CaO MgO SiO 2 CO 2 [based on Huang & Wyllie (1976) and Lee et al. (1998a)]. (a) At pressure Q (Fig. 2), liquidus field boundaries and liquidus isotherms (contour interval 50 C; temperature of liquid Q is 1230 C). All liquids on this surface coexist with minerals and a vapor phase (CO 2 ). Fields for primary minerals are separated by field boundaries, which meet in isobaric invariant peritectics and eutectics. The liquid compositions at each of these points are those formed when the three primary minerals in the liquidus areas meeting at each point begin to melt in the presence of CO 2 vapor. The key field boundary for carbonated peridotites is the carbonate silicate boundary extending from a point near MgCO 3, subparallel to and near the carbonate join, MgCO 3 CaCO 3, down to a temperature minimum near dolomite reflecting that on the binary carbonate join (Fig. 1), and then up in temperature as it swings away from the carbonate join and over a Cc Cpx temperature maximum toward the quartz liquidus field. Behind the contoured liquidus surface the vaporabsent liquidus volume is subdivided by surfaces, lines and points, but we consider these only in Fig. 4. En, enstatite; Wo, wollastonite; La, larnite; Qz, quartz; 2L, oxide oxide two-liquid field (see Figs 1 and 2 for abbreviations of other phases). Italic characters indicate liquidus minerals. (b) and (c) Silicate carbonate liquidus field relationships near dolomite at pressures above Q (b), and below Q (c). The dashed line between the Cc and Do fields is the temperature minimum associated with that in CaCO 3 MgCO 3, Fig. 1; see contours of (a). Points C, E, and F indicate compositions of carbonate-rich liquids coexisting with carbonated harzburgite, websterite, and wehrlite (Fig. 2). With decreasing pressure, E and C in (b) approach each other, meeting at liquid Q (a) at pressure Q (Fig. 2). At lower pressures, the Opx field shrinks as liquid B (Fig. 2) migrates up the Cpx Fo, leaving liquid F on the silicate carbonate field boundary (c). 1 GPa contains (wt %) 73% CaCO 3, 9% MgCO 3, and pressures up to at least 7 GPa (Dalton & Presnall, 1997). 18% CaMgSi 2 O 6 (representing silicate components), The steep liquidus surfaces for Fo, Opx, and Cpx rising compared with Dalton & Wood s 1 5 GPa liquids with from the silicate carbonate field boundary indicate that 81 86% CaCO 3, 14 11% (Mg,Fe,Mn)CO 3, and ~3% the liquids remain carbonate rich through a couple of (SiO 2 + Al 2 O 3 ). The carbonate component of both li- hundred degrees above the solidus, as confirmed by the quids contains ~89% CaCO 3, but we emphasize that analyses of Moore & Wood (1997) for the vapor-absent the model-system liquid F contains much higher SiO 2, boundary D D (Fig. 4). Trace element distributions and lower absolute content of CaCO 3. We suspect that between rock and liquid will vary depending on the the analyses of Dalton & Wood (1993) have suffered compositions of coexisting amphibole and other minerals, from the quenching problems that plague investigation but the dominance of the dolomitic carbonate component of systems with carbonate-rich liquids, and we are very for all peridotite types above pressure Q is independent sympathetic to such problems (e.g. Lee & Wyllie, 1994, of coexisting minerals. figs 3 and 4). Wallace & Green (1988) analyzed a sodic dolomitic carbonatite liquid coexisting with amphibole peridotite, and others have related this to a stability field for SILICATE CARBONATE LIQUID sodic dolomitic liquids within the phase boundaries for IMMISCIBILITY IN MANTLE amphibole. Model systems indicate that sodic dolomitic liquid is not restricted to this area. Such liquids are the SYSTEMS normal products of partial melting of peridotite with CO 2 Lee & Wyllie (1996, 1997a, 1997b) have mapped the (with or without H 2 O) through a very wide range of phase fields associated with silicate carbonate liquid im- 1888

5 WYLLIE AND LEE PRIMARY CARBONATE-RICH MAGMAS FROM THE MANTLE Fig. 4. Schematic phase relationships (in mol %) for the vapor-absent melting of dolomite lherzolite at pressures >Q, illustrated in three sections or projections. The first liquid has composition D on the vapor-absent silicate carbonate surface (Fig. 2). With progressive melting the liquid coexisting with (Fo + Opx + Cpx) follows the field boundary D D to the eutectic in the system Fo En Di. (a) CO 2 -saturated liquidus surface and volumes of primary minerals intersected by the triangle CaMgO 2 SiO 2 CO 2 that bisects the tetrahedron, with mineral compositions and progressive melting curve D D projected onto the triangle (based on Fig. 3). Di, diopside. (b) Minerals, melting curve D D, and vapor-absent silicate carbonate liquidus surface (the narrow area between CC and MC) projected from SiO 2 to CaO MgO CO 2. Point D and three short lines from it are located on the silicate carbonate surface (corresponding to the bold line through point D in (a), below and behind the vaporsaturated liquidus surface. The lines separate the surface into the fields of carbonates coexisting with forsterite (right), clinopyroxene (left), and orthopyroxene (bottom triangle, DEC). (c) Curve D D and the vapor-saturated silicate carbonate liquidus boundary (Fig. 3) projected from CO 2 onto CaO MgO SiO 2. phase relationships in natural rock experiments. Figure 6 shows the major features at mantle conditions in the pseudoquaternary tetrahedron CaO ( MgO + FeO ) (Na 2 O + K 2 O) (SiO 2 + Al 2 O 3 + TiO 2 ), projected from CO 2. For detailed construction and description, the reader is referred to Lee & Wyllie (1998a; FeO total iron content expressed as oxide). The silicate carbonate field boundary from Fig. 5 is reproduced in the corresponding projection on the front face. The dark-shaded surface is the extension of this field boundary through the tetrahedron, contoured with curves for constant wt % ( MgO+FeO ); this separates the liquidus volumes for silicates (to the left) from those for carbonates (to the right). The base of the tetrahedron is the Hamilton projection, and the silicate carbonate liquid miscibility gap drawn is intersected by the silicate carbonate liquidus field boundary (Kjarsgaard & Hamilton, 1988, 1989a; Lee & Fig. 5. Comparison of (i) experimental, near-solidus carbonate-rich Wyllie, 1996, 1997b). Results of Baker & Wyllie (1990) liquids from carbonated peridotites ( GPa), (ii) the projected and Lee & Wyllie (1997a) showed decrease in the size of vapor-saturated liquidus field boundaries at pressure Q (based on Fig. 3a), and (iii) a portion of the 1 GPa silicate carbonate field the miscibility gap with increasing Mg/Ca, which is boundary with liquid F for the solidus of magnesian calcite wehrlite consistent with the results of Freestone & Hamilton (compare Fig. 3c). Filled diamonds represent liquidus minima for (1980). Lee & Wyllie (1998a) therefore sketched the light- CaCO 3 MgCO 3 at 2 7 and 1 GPa (Fig. 1). Liquids in equilibrium with various lherzolites, group 1: Β, Wallace & Green (1988); Χ, Thibault shaded surface as the locus of coexisting liquids, with the et al. (1992); Ε, Sweeney (1994). Φ, liquids from Dalton & Wood dashed line as the critical curve, and contours for constant (1993). Group 2: liquids coexisting with harzburgite (3 2 8 GPa). Group wt % ( MgO+FeO ). This surface encloses a third vol- 3: liquids coexisting with wehrlite ( GPa). Group 4: liquids ume in the tetrahedron. The positions of these two coexisting with wehrlite (with H 2 O added, 1 5 GPa). surfaces and shapes of the three volumes vary with pressure and bulk composition (e.g. Al/Si, Mg/Fe, Na/ miscibility in model systems through a range of pressures, K, % CO 2 ), and the variables may have competing effects to provide a framework for understanding the complex on the shape of the immiscibility volume (i.e. miscibility 1889

6 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 NOV & DEC 1998 Kimberlites Melilitites Alkali basalts Silicate liquidus NEPH Na 2 O + K 2 O MgO + FeO* SiO 2 + Al 2 O 3 + TiO 2 CaO Framework at 2.5 GPa Projected from CO Wt% Carbonate liquidus Dalton & Wood (1993): group 2 from harzburgite, and groups 3 and 4 from wehrlite with pressure decreasing from 2 5 to 1 5 GPa. This figure illustrates the separation of Dalton & Wood s liquids from the main lherzolite group, 1, and the divergence of their liquid trend for carbonated wehrlite with decreasing pressure from the high-pressure silicate carbonate boundary. The position of the 1 GPa silicate carbonate field boundary shown in Fig. 5 diminishes the divergence slightly. Figure 6 was constructed for conditions and low CO 2 contents that we consider appropriate for most mantle processes. Baker & Wyllie (1990) concluded that under normal mantle conditions, magma paths might range from dolomitic carbonatite (stippled volume 1) to primitive nephelinites (NEPH), or alkali basalts (shaded oval). Magmas in the mantle are accordingly unlikely to reach the immiscibility volume and exsolve a carbonatite magma, except perhaps for conditions where magmas of extreme compositions were developed. Fig. 6. Generalized phase diagram illustrated in composition tetrahedron CaO (MgO + FeO ) (Na 2 O + K 2 O) (SiO 2 + Al 2 O 3 + TiO 2 ), projected from CO 2, showing dark- and light-shaded liquidus Forbidden zones for magmas surfaces for the silicate carbonate field boundary and liquid miscibility The immiscibility volume between silicate- and cargap at 2 5 GPa. There is also a small area above the dark surface for silicate oxide field boundary. Contours indicate wt % (MgO + FeO ) bonate-rich magmas and the carbonate liquidus volume of the two surfaces. [See Lee & Wyllie (1998a) for details of construction are both forbidden zones for magmas derived from silicate and interpretation.] The small stippled volumes (groups 1 4) delineate parents. Silicate CO 2 magmas may reach either of the the experimentally measured liquid compositions from carbonated two surfaces in Fig. 6, but then they are forced to remain peridotites, as in Fig. 5. Group 1 intersects the dark surface at the bold curve (compare Fig. 5). Selected rock compositions are also shown, on the surface as they either exsolve a carbonate-rich situated within the silicate volume. These rocks contain low alkalis and magma, or precipitate a carbonate. The large liquidus over ~25% (MgO + FeO ) (recalculated CO 2 free), as shown by their volume for primary calcite even at 1 GPa should be projections down onto the base of the tetrahedron; they are well removed from both the silicate carbonate liquidus boundary and the noted (compare Figs 6, 5, and 3a). We conclude that no immiscibility volume. Shaded oval, alkali basalts (e.g. Frey et al., carbonatite magma with composition inside the volume 1978); Μ (NEPH), magnesian nephelinite (Clague & Frey, 1982); Φ, can be generated in the mantle. kimberlites (Mitchell, 1989); Β, melilitites (Brey, 1978). gap). For attempts to define specific processes for specific PETROLOGICAL APPLICATIONS magmas under particular conditions, it is necessary to Some of the petrological applications from the phase determine the precise location of these phase elements. relationships outlined here relate to the near-isobaric Brooker & Holloway (1997) have reported that the size solidus ledge associated with invariant point Q (Fig. 2). of the magnesian miscibility gap increases remarkably The depth and temperature of point Q decreases with with increased CO 2 content; however, free CO 2 cannot additional components. For CaO MgO Al 2 O 3 SiO 2 exist with mantle peridotite deeper than level Q (Fig. 2), CO 2 it is at 2 6 GPa and 1230 C (Dalton & Presnall, because it reacts to form dolomite. Kjarsgaard & Ham- 1997), and for natural lherzolite it is near GPa ilton (1989b) reported immiscibility at 0 6 GPa in mag- and C (Wyllie & Rutter, 1986; Wyllie, 1987; nesian compositions (melilititic) with a tie-line extending Falloon & Green, 1989). Q in Fig. 2 corresponds to a directly across the silicate volume in Fig. 6. The miscibility depth of ~90 km, but for natural rocks the corresponding gap may be increased in systems relatively rich in potas- depth is ~70 km. Therefore in the following applications sium (P. Ulmer, personal communication, 1997). of the model system results, we will refer to 70 km for The small stippled volumes in Fig. 6 delineate the the solidus ledge. experimentally measured liquid compositions from car- The applications also involve relating the carbonatebonated peridotites, as in Fig. 5, but showing in addition rich liquids to carbonatite magmas and rocks. The comtheir alkali contents (which vary with bulk rock com- positions of carbonate-rich liquids have been expressed position and mineralogy). Group 1 contains liquids from in many different ways, and their positions in diagrams four different lherzolites. The other groups are from such as Fig. 3 or 5 shift a few per cent, depending on 1890

7 WYLLIE AND LEE PRIMARY CARBONATE-RICH MAGMAS FROM THE MANTLE whether compositions are expressed in terms of weight ponent, as established in the model system (Figs 3 5), or mole %. Also, their CaCO 3 content has been expressed and confirmed by experiments with natural lherzolites as either percentage of total bulk composition, or as (Figs 5 and 6). This has the potential for intrusion percentage of the carbonate component of the liquid, or eruption as an explosive primary magma, through numbers which can be significantly different as the silicate evolution of CO 2 vapor at the solidus ledge near 70 km content increases. As defined by Woolley & Kempe (Wyllie & Huang, 1976a). (1989), for rocks with >50% modal carbonates, calcio- Dalton & Wood s (1993) analytical results (groups 3 carbonatites have CaO/(CaO + MgO + FeO + Fe 2 O 3 and 4 in Figs 5 and 6) have been widely adopted + MnO) >0 8 (by weight); if this ratio is <0 8 the rock to support the proposal that metasomatic reaction of is magnesiocarbonatite (except for Fe-rich rocks), i.e. dolomitic carbonatite magma from lherzolite (D in Fig. 4) dolomitic. For the model system, the boundary cor- would convert lherzolite to wehrlite near the solidus ledge, responds to 77 wt % CaCO 3 in the carbonate component that this metasomatic process simultaneously enriched the of the liquid. carbonatite in Ca/Mg, and that continuation of this The silicate carbonate liquidus boundary limits the process as magnesiocarbonatite rose through wehrlitemaximum CaCO 3 content of carbonatite magmas, and sheathed channels generated primary calciocarbonatite we do not believe that these limits can be modified much magma at lower pressures. In terms of the model system, by the additional components of igneous processes. The this involves the migration of the carbonate wehrlite results in Figs 3 and 5 suggest that the maximum CaCO 3 peritectic from Q to F (Fig. 3), and then on to F at 1 content of carbonatite magmas at mantle pressures is 73 GPa (Fig. 5). We doubt that the reacted liquid comwt % (see F, Fig. 5). Lee & Wyllie (1996, 1997b, 1998a) position changes along the low-sio 2, low-alkali path concluded from experimental data that immiscible car- toward CaCO 3 to the extent claimed by Dalton & Wood bonatite liquids at mantle pressures would have a max- (Figs 5 and 6), and maintain that the changing position imum CaCO 3 content of ~80 wt %. We do not see how of F to F in the model system is the dominant control immiscible liquids with compositions near pure CaCO 3 on the liquid composition in the metasomatic reaction. could be generated under any conditions, which suggests However, we have confirmed that the magma changes that rounded calcite ocelli and globules in mantle xeno- from magnesiocarbonatite (dolomitic) to calcioliths are crystalline (Lee & Wyllie, 1996). carbonatite at some depth between 70 km and 40 km, A selection of kimberlites and melilitites are plotted in and Dalton & Wood s (1993) process for generating Fig. 6. They are situated well within the silicate volume, primary calciocarbonatite magmas is possible. It requires far removed from both surfaces, indicating that im- very extensive metasomatism through tens of kilometers. miscible liquids were not involved in their generation. If The calciocarbonatite magma so generated contains a the magmas contained CO 2, as usually assumed, then maximum of 73% CaCO 3 (89% of carbonate cominitial melting must have been at points such as E, C, F ponents) and up to 18% silicate components. or D in Figs 3 and 4, but situated on the dark-shaded Because of the prevalence of crystal accumulation silicate carbonate field boundary according to their alkali (Wyllie & Tuttle, 1960), carbonatite magmas are unlikely contents. The magmas have therefore experienced con- to precipitate rocks of the same composition. Calciosiderable change in composition, and possibly a wide carbonatite magmas will precipitate sövites (calciotemperature interval of progressive melting, to reach carbonatite rocks). Fanelli et al. (1986) defined a large these positions in the tetrahedron so far removed from liquidus field for calcite in the system CaO the silicate carbonate boundary. The wide variation in MgO CO 2 H 2 O at 0 2 GPa. With H 2 O present to lower CaO/MgO among these rocks may be a reflection of the liquidus temperature below the crest of the dolomite variations in the curvature of vapor-absent field bound- solvus (compare Fig. 1), liquids with Ca/Mg = 1 prearies such as D D in Fig. 4, as a function of pressure. cipitate calcite (only slightly magnesian) through a Figure 5 indicates that a variety of potential primary considerable temperature interval (Lee et al., 1986, carbonatite magmas might be formed under mantle 1998b). Harmer & Gittins (1997) reported that conditions. The only way we can see to generate na- addition of Na 2 CO 3 to dolomite had a similar effect. trocarbonatite liquids from peridotite rocks at the mantle Magnesiocarbonatite magmas can precipitate sövites. conditions of Fig. 6 is for the silicate magma to exsolve such liquids by reaching the miscibility surface near compositions with relatively high Na/Ca, and low Mg/ Ca (see Lee & Wyllie, 1997a). The geometry of the phase volumes makes this appear extremely unlikely. ACKNOWLEDGEMENTS Partial melting of carbonated mantle peridotite at We thank reviewers T. J. Falloon, J. Meen, Y. Thibault, depths greater than ~70 km generates a magnesiocarbonatite and B. J. Wood for their stimulating comments. This melt, dominated by calcic dolomite com- research was supported by the Earth Science section 1891

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