The Transition from Carbonate to Silicate Melts in the CaO MgO SiO 2 CO 2 System

Transcription

1 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 PAGES The Transition from Carbonate to Silicate Melts in the CaO MgO SiO 2 CO 2 System K. R. MOORE AND B. J. WOOD DEPARTMENT OF EARTH SCIENCES, WILLS MEMORIAL BUILDING, UNIVERSITY OF BRISTOL, QUEEN S ROAD, BRISTOL BS8 1RJ, UK RECEIVED SEPTEMBER 30, 1997; REVISED TYPESCRIPT ACCEPTED JUNE 23, 1998 The compositions of melts in equilibrium with a lherzolite mineral INTRODUCTION assemblage were determined in the analogue system CaO The common association of carbonatites with silica- MgO SiO 2 CO 2 at 3 GPa. Carbonate liquids coexist with olivine undersaturated alkaline igneous rocks has led to many and two pyroxenes between the solidus for carbonated lherzolite at speculative petrogenetic schemes for their generation C and 1450 C. The Ca/(Ca + Mg) ratio of these melts These include limestone assimilation (Shand, 1945) to is 0 64 and the main effects of rising temperature are increasing produce silica-undersaturated rocks, crystal fractionation SiO 2 content (from <4 3 to 7 5 wt %) and decreasing CO 2 to produce carbonatite from a primary CO2 -rich silicate content. Between 1475 C and ~1525 C the SiO 2 content of the melt (Watkinson & Wyllie, 1971) and liquid immiscibility liquid increases dramatically from 10 to 30% and, thereafter, the to produce both rock types from an intermediate parent CO 2 content decreases rapidly as the CO 2 -absent invariant point ( Freestone & Hamilton, 1980). It has become apparent (at >1700 C) is approached. The progression from carbonate to from isotopic studies (e.g. Powell et al., 1966; Bell & silicate liquids is, therefore, abrupt and the field of transitional Blenkinsop, 1987a, 1987b) that the primitive magma is compositions (10 30% SiO 2 ) is restricted to very narrow temof mantle origin. An understanding of the genesis of perature intervals at pressures greater than the solidus ledge. All associated carbonate and silicate rocks therefore depends liquids appear to be miscible. In the context of upwelling magma, on determining the compositions of melts that are proour results provide possible insight into the origins of complexes that duced from peridotitic compositions under upper-mantle are considered to contain primary carbonatites. The solidus ledge conditions. This study is aimed at the elucidation of between 2 5 and 3 GPa acts as a filter for both carbonatites and melting behaviour in mantle analogue systems. transitional melt compositions. Carbonatites, which have a wide Qualitative interpretation of the behaviour of carstability field at 3 GPa, may rise through the mantle if they are bonated peridotite at solidus and supersolidus conditions isolated from lherzolite by wallrock reaction and production of can be derived from the model system CaO wehrlite. Transitional carbonate silicate melts must also, however, MgO SiO 2 CO 2 (CMS CO 2 ) (Fig. 1). Decarbonation react with the mantle at low pressures. This fact, combined with the small range of physical conditions over which they are generated reactions (Wyllie & Huang, 1975a, 1975b, 1976; Eggler, and their higher (than carbonatite) viscosity, means that they rarely 1978) define the stability fields of carbonate in lherzolitic reach crustal levels. Low-CO mantle. The following reaction separates regions in the 2 silicate melts, in contrast, are not required to react extensively en route to the surface and are abundant. mantle where CO 2 exists as a free fluid phase and where We suggest that the binary nature of some carbonatite complexes it resides in mineral phases (dolomite): may be controlled by the compositions of primary mantle melts 2Mg 2 SiO 4 +CaMgSi 2 O 6 +2CO 2 =4MgSiO 3 +CaMg(CO 3 ) 2. (1) produced at pressures greater than the solidus ledge. fo di fluid en dolomite KEY WORDS: carbonatites; primary liquids; CO 2 saturation; metasomatism At higher pressures, dolomite reacts with orthopyroxene to produce magnesite as the carbonate phase stable in lherzolite: Corresponding author. Telephone: , ext. 4780, Fax: K.R.Moore@bris.ac.uk Oxford University Press 1998

2 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 NOV & DEC 1998 CaMg(CO 3 ) 2 +2MgSiO 3 =2MgCO 3 +CaMgSi 2 O 6. (2) dolomite en magnesite di At CO 2 saturation the solidus remains close to that of the vapour-absent solidus, with relatively little CO 2 dissolved in the melt to pressures in excess of 2 GPa. Then, between 2 5 and 2 8 GPa the solidus bends sharply to lower temperatures as increasing amounts of CO 2 become dissolved in the melt (Wyllie & Huang, 1976; Eggler, 1978; White & Wyllie, 1992). The melt at the solidus in this region approximates a calcic dolomite carbonatite. The position of the back-bend is, in the simple system, constrained by the intersection of the carbonation reaction (1) with the solidus at ~2 8 GPa. Liquid compositions may be represented by projection from CO 2 onto the CaO MgO SiO 2 side of the compositional tetrahedron, as shown in Fig. 2. At 2 GPa, the first melt of carbonated lherzolite (at peritectic T) is approximately basaltic, containing less than ~5 wt % CO 2 (Wyllie & Huang, 1976). Carbonate-rich liquids, which exist near the solidus as small fraction melts, Fig. 1. Diagram showing phase relations for carbonated lherzolite in cannot be in equilibrium with lherzolite at this pressure, the system CaO MgO SiO 2 CO 2 [after Eggler (1978) and White & Wyllie (1992)]. A prominent ledge in the solidus occurs where the and instead coexist with olivine and clinopyroxene lower decarbonation reaction intersects the decarbonation reaction at (Fig. 2a). The association of carbonate liquid with the (I). phases olivine and clinopyroxene has been used to link the frequent occurrence of wehrlite veins in peridotite to low-pressure metasomatism by ascending carbonate melts react with this phase, enriching the melt in the CaCO 3 (Dalton & Wood, 1993; Rudnick et al., 1993). Above 2 5 component and evolving CO 2 : GPa, the peritectic T moves rapidly in the approximate 5MgSiO 3 +2CaMg(CO 3 ) 2 = direction of the vapour-saturated eutectic E (Wyllie & en dolomitic melt (3) Huang, 1976; Eggler, 1978), merging to generate two 3Mg 2 SiO 4 +CaMgSi 2 O 6 +CaCO 3 +3CO 2. new peritectics A and B at ~3 GPa (Fig. 2b). In the context fo di Ca-rich fluid of carbonatites and associated rocks, the important point melt is that carbonate melts produced at 3 GPa can be in equilibrium with harzburgite and lherzolite mineral Wallrock reaction to produce wehrlite increases the Ca/ assemblages in the upper mantle. Thus carbonatite is a (Ca + Mg) ratio of the residual carbonate melt, a process viable primary melt of peridotite at a range of depths that could, according to Dalton & Wood (1993) extend that may extend to >100 km. to Ca/(Ca + Mg) ratios as high as 0 87, approximating Given that carbonate melt and carbonated silicate those observed in many carbonatites. melts can be produced at pressures near 3 GPa, the At pressures greater than those of the solidus ledge relationships between them and the compositional paths of Fig. 1 and at temperatures considerably higher than taken en route to the surface remain to be established. the solidus at 3 GPa, harzburgite and lherzolite mineral Melts in equilibrium with orthopyroxene-bearing as- assemblages are in equilibrium with CO 2 -rich silicate semblages are dolomitic and have a maximum Ca/ melts. Although the relationships between these melts (Ca + Mg) at 3 GPa of ~0 7 (Dalton & Wood, 1993). and eruptive rocks are uncertain, it has been demonstrated that some carbonated silicate rocks are plausible Carbonatite rocks have Ca/(Ca + Mg) averages of 0 5 and 0 86 for dolomitic carbonatites and calcio-car- primary mantle melts. For example, Brey & Green (1977) bonatites, respectively [based on the data of Woolley & and Brey (1978) have found that olivine nephelinite and Kempe (1989)], with the latter being more common. olivine melilitite may be generated from lherzolitic mantle Current experimental data indicate that calcio-carbonatites cannot equilibrate with harzburgite or lherzolite links between the different rock types of the carbonate at pressures between 2 7 and 3 5 GPa. The petrogenetic and must evolve from a primary mantle melt by fractional alkali silicate association remain unclear, however. In crystallization or wallrock reaction. At low pressure the this study we have attempted to elucidate them by latter is due to the instability of orthopyroxene in carbonate melt (Fig. 2a) so that ascending carbonate melts to silicate melts at 3 GPa in the CaO MgO SiO 2 CO investigating the nature of the transition from carbonate

3 MOORE AND WOOD TRANSITION FROM CARBONATE TO SILICATE MELTS Table 1: Bulk starting compositions (wt %) and run conditions; all runs were at 3GPa Bulk MgO SiO 2 CaO CO 2 (a) (b) (c) Run Bulk t (h) T ( C) Products 50 a opx, L 58 a opx, L 59 a L 61 a ol, opx, cpx, L 67 a ol, opx, L 68 a ol, opx, L 69 a ol, opx, L 70 a opx, L 77 b ol, opx, cpx, L 79 b ol, opx, L 80 b ol, opx, L 81 b opx, L 105 c 0: ol, opx, cpx, L Fig. 2. Phase relations in the system CaO MgO SiO 2 CO 2 (mol %) (a) at 2 GPa [after Wyllie & Huang (1976)] and (b) at 3 GPa [after Eggler (1978)]. The stability field of orthopyroxene expands dramatically with increasing pressure, with the 2 GPa peritectic T moving towards the carbonate silicate field boundary and reaching it at pressure I (Fig. 1). At pressures greater than I the peritectic T is replaced by peritectics A and B near the eutectic E. system, the simplest model for the range of primary melts generated from carbonated lherzolite. the base of the capsule so that it did not dissociate with loss of CO 2 during welding. Proportions of silicate and carbonate were chosen to be comparable with previous studies and to produce large enough melt pools (normally >50 μm diameter) to be analysed by electron microprobe. Packed capsules were stored open-ended overnight in a drying oven to ensure that no atmospheric water absorbed by the powders during packing was retained. The weight of capsules was noted before and after arc-welding to confirm that there had been insignificant decarbonation of the starting mixture during the welding process. Experiments were performed in a piston-cylinder apparatus using barium carbonate and crushable alumina pressure media. The capsule was surrounded by a tightly packed mixture of haematite and Pyrex to inhibit ingress of hydrogen into the capsule. Temperature was monitored and controlled directly above the capsule by the use of a calibrated W Re thermocouple. Using the pistonin technique, with a pressure correction of 10%, experiments were run at 3 GPa and temperatures between 1300 and 1500 C for 3 h. Experiments above 1500 C were run for 2 h and those at 1600 C for 40 min (Table 1). Quenching was performed by turning off the power supply to the graphite furnace. The resulting charge was mounted in epoxy resin and prepared for analysis. EXPERIMENTAL PROCEDURE Three starting compositions were used (Table 1): (a) MgSiO CaMg(CO 3 ) (wt %); (b) MgSiO Mg 2 SiO CaMgSi 2 O CaMg(CO 3 ) ; (c) MgSiO 3 17 Mg 2 SiO 4 31 CaMgSi 2 O 6 42 CaMg(CO 3 ) These were prepared by mixing and grinding analytical grade oxides and carbonates under ethanol to produce silicate and carbonate powders. These were packed into 2 mm o.d. platinum capsules in layers with carbonate at Charges were ground and impregnated with resin at 1945

4 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 NOV & DEC 1998 frequent intervals to minimize plucking of carbonate and Table 2: Microprobe analyses (wt %) of all run quenched silicate phases, which can be severe in such experiments. They were then polished by hand, using products diamond pastes and oiled-based lubricants on polishing T/bulk n MgO SiO 2 CaO Total Ca pads uncontaminated with water, to produce a flat surface 1300a for electron microprobe analysis. opx Analysis was carried out using the JEOL 8600 electron L microprobe in the Geology Department at the University (0 11) (0 31) (0 17) (0 25) 1400a of Bristol. Compositions of silicate phases were obtained ol using spot analyses and a beam current of 15 na. Melts, opx with their high carbonate content, devolatilize easily L (0 09) (0 20) (0 14) (0 16) under the electron beam and were analysed using a beam 1425a current of 5 na and a diffuse spot (beam rastered over ol opx an area up to 20 μm 2 ). Compositions of liquids that L quenched to dendritic crystals were obtained by averaging (0 11) (0 17) (0 12) (0 15) a large sample number of melt analyses (Table 2). Anaol a lyses were carried out at 15 kv using olivine (SJIO), quartz opx and wollastonite as standards, with results processed using cpx ZAF procedures. To check for errors introduced by not L (0 14) (0 17) (0 14) (0 17) analysing for CO 2 we performed a number of analyses 1475a of carbonate standards and of our quenched melts in ol which neighbouring points were processed by (a) ignoring opx L CO 2 completely and (b) adding oxygen to the ZAF (0 25) (0 32) (0 28) (0 29) correction until the analysis totalled 100%. Method (a) 1500a is the one that is normally adopted, but method (b) should opx L be more accurate if one is certain that the surface is (0 12) (0 23) (0 15) (0 19) absolutely flat and only one phase is being analysed. The 1550a results show that method (a) gives a linear increase in opx L the error in Ca/(Ca + Mg) with CO 2 content from 0 0 (0 08) (0 16) (0 10) (0 14) at 0 0% CO 2 to at 50% CO 2. Thus, for essentially 1600a L pure dolomitic melts Ca/(Ca + Mg) appears to be (0 03) (0 04) (0 02) (0 06) overestimated by ~ Data in Table 2 have not been 1300b corrected for this effect. ol opx cpx L (0 05) (0 06) (0 06) (0 05) 1400b RESULTS Enstatite dolomite join ol opx L An experiment run at 1250 C using starting composition (0 08) (0 20) (0 12) (0 17) (a), MgSiO (top layer) CaMg(CO 3 ) (base 1450b layer) wt %, produced liquid in such small amounts that ol opx it could not be reliably analysed and is therefore not L included in the tables. From this we inferred that the (0 07) (0 10) (0 07) (0 09) 0 50 solidus for composition (a) lies very close to 1250 C, in 1500b opx agreement with previous work (Eggler, 1978, 1987; L Wyllie, 1987). Near-solidus melting in this composition (0 07) (0 10) (0 08) (0 09) 0 47 produced enstatite and a dolomitic melt of homogeneous 1600c ol appearance and a Ca/(Ca + Mg) ratio of 0 51 (Table 2, opx Fig. 3). The texture of the quenched melt remains homo- cpx geneous up to 1400 C, at which temperature hopper L (0 07) (0 12) (0 06) (0 11) crystals of olivine appear and orthopyroxenes have a corroded appearance. Although all melt compositions contain some silica, this is the first textural evidence observed that silicate phases are participating in the is 1 SE of liquid composition. melting reaction. At 1450 C equant crystals of forsteritic Ca = Ca/(Ca + Mg) mol. T/bulk, temperature and starting composition of runs (see Table 1 for detail); n, number of analyses.value in parentheses 1946

5 MOORE AND WOOD TRANSITION FROM CARBONATE TO SILICATE MELTS Fig. 3. Melt compositions expressed as the molar ratio of divalent cations [Ca/(Ca + Mg)] as a function of temperature. Points labelled Fig. 4. Major element liquid compositions in equilibrium with lher- (a), (b) and (c) correspond to starting compositions (Table 1). Error bars zolite as a function of temperature. SiO 2 content of the liquid increases refer to 3 SE. The larger symbols indicate liquids that coexist with a gradually, from 4 3 wt % at 1300 C to 7 5 wt % at 1450 C, until at lherzolite mineral assemblage ± 25 C it begins to rise dramatically. At 1600 C a carbonated silicate melt with SiO 2 = 37 wt % and Ca/(Ca + Mg) = 0 36 is produced. This implies that the whole spectrum of melt compositions between carbonate and silicate can be produced in equilibrium with olivine and diopsidic clinopyroxene join orthopyroxene olivine and two pyroxenes at pressures above the solidus ledge in the in the silicate phase assemblage and the dolomitic melt mantle. Maximum standard errors are 0 06 wt % at 1300 C, 0 17 wt has a Ca/(Ca + Mg) ratio of 0 55 (Fig. 3). Clinopyroxene % at 1450 C and 0 12 wt % at 1600 C. Open symbols indicate the reacts out at 1475 C as Ca/(Ca + Mg) of the liquid volatile-absent liquid composition, after Kushiro (1968), taken from Eggler (1978). reaches Between 1475 and 1500 C extensive melting of silicate phases dramatically changes the nature of the liquid from being dominantly carbonate to a silicate It is clear from our results on compositions (a) and (b) composition, which quenches to a heterogeneous mixture that, despite the evolution of gas from the capsules on of crystals rather than to a glass. In all cases gas was puncturing, the liquids generated in this study were not produced on rupture of the capsule, indicating that either all CO 2 saturated. This is required by the fact that, excess CO 2 was present and/or CO 2 was evolved during isobarically, we have generated phase assemblages conquenching. taining four condensed phases (ol opx cpx L) over a Melt composition (Fig. 3) varies little over the first temperature interval of C (Fig. 4). We have C above the solidus, with SiO 2 content in- therefore determined part of the four-phase cotectic at creasing from 5 to 8 wt %. Then, in the temperature 30 kbar, that part in equilibrium with carbonate melts interval between 1475 C and 1500 C the SiO 2 content and hence close to CO 2 saturation. To characterize this more than triples as the liquid changes from carbonate to cotectic more fully we performed experiments on a CO 2 - silicate, with concomitant lowering of the Ca/(Ca + Mg) poor composition (c), MgSiO 3 17 Mg 2 SiO 4 31 ratio from 0 57 to At temperatures higher than CaMgSi 2 O 6 42 CaMg(CO 3 ) 2 10 (wt %). At 1600 C this 1500 C olivine reacts out and orthopyroxene is left as produces a carbonated silicate melt containing 37% SiO 2 the liquidus phase. (Table 2, Fig. 4), with Ca/(Ca + Mg) of 0 36 (Fig. 3), which coexists with olivine, orthopyroxene and clinopyroxene. Haplolherzolite dolomite Composition (b) was chosen as a simple analogue of carbonated lherzolite to generate near-solidus liquids in equilibrium with a lherzolitic residuum. The melt at DISCUSSION 1300 C does indeed coexist with olivine, orthopyroxene Figures 4 and 5 imply that all liquid compositions between and abundant clinopyroxene, and has Ca/(Ca + Mg) of carbonate and CO 2 -free silicate can be produced iso (Table 2, Fig. 3). Clinopyroxene disappears at barically in equilibrium with lherzolite (in the simple 1400 C, and the melt, coexisting with olivine and orthoperatures. For starting compositions with fixed carbonate system CMS CO 2 ) by partial melting at varying tempyroxene, begins to precipitate large silicate quench crystals during quenching from 1450 C. At 1500 C the content, melt compositions lie on the four-phase cotectic carbonated silicate melt coexists with orthopyroxene only. projected in Fig. 5 and the CO 2 content of the liquid 1947

6 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 NOV & DEC 1998 additional components such as Na 2 O, H 2 O, P 2 O 5 and F ( Jago & Gittins, 1991) preliminary experiments on Naand P-bearing compositions in our laboratory suggest that the field of transitional melts remains narrow even in bulk compositions more closely approximating the natural system. These results demonstrate, therefore, that the association of carbonatites with nephelinites melilitites could be a primary feature of mantle melting and that transitional compositions should in this case, as observed, be rare. While recognizing that carbonate silicate liquid immiscibility is frequently observed in nature (e.g. Dawson, 1966; Le Bas, 1977; Kjarsgaard & Peterson, 1991; Macdonald et al., 1993; Dawson et al., 1994), the field Fig. 5. Diagram (mol %) showing experimental results projected from CO 2 onto the CaO MgO SiO 2 face of the tetrahedron. Melt observations do not demonstrate that it is the major compositions found in equilibrium with lherzolite (Ε) at 1300, 1450 petrogenetic process. The silicate suite of Shombole, and 1600 C were used to construct the ol cpx opx L cotectic (dashed for example, includes nephelinites that contain irregular line). Melt compositions in equilibrium with harzburgite are also shown (Χ). The end-point of the cotectic, corresponding to the volatile-free calcite-rich bodies interpreted as immiscible liquid system, was taken from Eggler (1978), after Kushiro (1968). (Peterson, 1989). This is clearly, however, a late-stage immiscibility, which is not necessarily related to the decreases with rising temperature. The end-point of the generation of the main carbonate and silicate end-mem- cotectic, corresponding to the volatile-free system, was bers. taken from Eggler (1978), after Kushiro (1968). The rapid Analyses of carbonatites and associated rocks from a change in composition at 1475 ± 25 C is the result of number of carbonatite complexes are plotted, in promelting generated in a system with few phases and there jection, in Fig. 6 together with the experimentally pro- is no evidence that carbonate and CO duced liquid compositions in equilibrium with lherzolite 2 -free silicate liquids are immiscible liquids. at 3 GPa. As noted by Bailey (1989), primary carbonate Comparison of the trace of the CO melts formed experimentally from carbonated lherzolite 2 -undersaturated cotectic at 3 GPa with the CO at high pressure closely approximate natural dolomitic 2 -saturated peritectic (ol + opx + cpx + L) at 2 GPa in Fig. 2a (Wyllie & carbonatites. As previously shown by Brey & Green Huang, 1976) shows that the latter has similar Ca/ (1975), observed olivine melilitite compositions also fall (Ca + Mg) to the low P within the range of liquids that can coexist with the CO2 end of the 3 GPa cotectic. This demonstrates that the progression from carbonate model lherzolite mineral assemblage at ~3 GPa. The to silicate melt in equilibrium with lherzolite can occur nephelinite phonolite association could, in this simplified either by raising temperature isobarically or by holding projection, be interpreted in terms of fractional crysthe peridotite just above its solidus and lowering pressure tallization (dominantly olivine) from a CO 2 -bearing sil- at CO 2 saturation. In the latter case, the initial liquid icate parent, whereas calciocarbonatites would be follows the trace of the peritectic from CO 2 -rich (at high produced from primary dolomitic carbonatites partly by pressure) to CO 2 -poor melts at low pressure. Olafsson & wallrock reaction (Dalton & Wood, 1993) and partly by Eggler (1983) noted a similar progression over the pres- fractional crystallization (Harmer & Gittins, 1997). sure interval kbar in the presence of both H 2 O Bailey (1989, 1993) has provided extensive evidence, and CO 2. based on xenocrysts, that many effusive calcio- and As can be seen from Figs 2 and 5, the progression from magnesio-carbonatites are of mantle origin unrelated, as carbonate to silicate melts both iso- and polybarically suggested above, to liquid immiscibility. As such car- is abrupt and the field of transitional compositions in bonate melts are the first liquids to form above the mantle equilibrium with lherzolite is small. At 3 GPa, for exthat they are the earliest components of high-level com- solidus at high pressure their low viscosities should ensure ample, melts are carbonatitic over the temperature range C and SiO 2 rich from about 1525 to 1800 C. plexes. Carbonatites with accompanying silicate vol- The field of transitional melt compositions, that is, those canism are, however, frequently observed to be late liquids having SiO 2 content between 10 and 30 wt %, rather than early in the intrusive sequence. Advocates of a in equilibrium with lherzolite is restricted to a narrow primary origin for carbonatites in complexes cite thermal temperature interval of ~50 C. Similarly, near-solidus death as the delaying factor in their emplacement (Bailey, melts shift from carbonate to silicate over the very small pressure interval GPa as shown in Fig. 1. Although the transitional region must be broadened by 1987). That is, near-solidus liquids formed at pressures greater than the solidus ledge (Fig. 7) ascend through the mantle only until they encounter the ledge. An ascending 1948

7 MOORE AND WOOD TRANSITION FROM CARBONATE TO SILICATE MELTS Fig. 6. Diagram (mol %) comparing the lherzolite melting path at 3 GPa (dashed line from Fig. 5) with naturally occurring rock compositions projected by adding alkalis and Ca; Fe, Mn, Ni, Cr and Mg; Al and Si. Data used in calculations: carbonatite from Woolley & Kempe (1989); kimberlite from Bergman (1987); olivine melilitites from Lohmann (1964), Spencer (1969), Jackson & Wright (1970) and Brey (1978); nephelinites from Simonetti & Bell (1995); African melilitites, nephelinites and phonolites from Le Bas (1977) and Baker (1987). carbonate melt that maintains equilibrium with surrounding lherzolite will crystallize and solidify by the reaction of orthopyroxene with the liquid to produce olivine, clinopyroxene and carbon dioxide vapour, effectively metasomatizing the mantle (Wyllie, 1980; Schneider & Eggler, 1986; Wallace & Green, 1988; Haggerty, 1989; Meen et al., 1989; Dalton & Wood, 1993) at the depth of reaction. Successive melts meeting the ledge and freezing extend the altered zone until a reaction-lined conduit is formed, which allows ascending carbonatite melts to rise to the point at which evolution of CO 2 accelerates the melt sufficiently for it to take off to the surface (Bailey, 1985). Thus, the delaying factor is the time interval between the production of the first small fraction melt that freezes at the ledge and the first small fraction melt that ascends to crustal levels. Wyllie (1980) envisioned released vapour as enhancing the prospects for crack propagation through overlying lithosphere in tension to produce an initial channel to the surface. In this scenario, the delaying factor incorporates the time taken for vapour pressure to increase to levels that will exert a sufficient force on the overlying mantle. At higher degrees of melting, silicate melts ascend and by-pass the ledge where previously formed carbonatite melts may have ponded, metasomatizing the surrounding mantle as they solidify. The silicates that by-pass the ledge ascend to form the large pile of volcanics that make up the vast majority of complexes. As melts transitional between carbonate and silicate are stable only in a very small P T region, their ascent, as shown in Fig. 7, is also likely to be influenced by the solidus ledge. Transitional melts [Ca/(Ca + Mg) of , SiO %] should intersect the solidus Fig. 7. P T diagram showing ascent of experimentally produced liquids through a simplified mantle. Numbers refer to the Ca/(Ca + Mg) ratio (mol) the melts (having 4 31, 7 45 and 36 84% SiO 2, respectively) in equilibrium with the lherzolite mineral assemblage. Only the most silica-rich liquids [Ca/(Ca + Mg) < 0 4] may ascend without extensive alteration to the surface. All others must intersect the solidus ledge, which extends across a temperature interval of >200 C. Metasomatism of the mantle by these liquids produces wehrlite. Decarbonation reaction (1). ledge and participate in wallrock reaction, thus being trapped by the overlying mantle. The filtering effect is likely to be more severe in this case than for carbonatites because of the smaller P T field over which the trans- itional melts are generated and their higher viscosities. This effect, when combined with their narrow P T field of stability, could account for the lack of recorded oc- currences of rocks transitional in composition between carbonatites and nephelinites. CONCLUSIONS We have produced liquid compositions in the system CaO MgO SiO 2 CO 2 that show that the complete com- positional range of melts from magnesiocarbonatite [Ca/ (Ca + Mg) = 0 64] to volatile-free silicate melt [Ca/ (Ca + Mg) = 0 27] is likely to be in equilibrium with the lherzolite mineral assemblage at 3 GPa. In the presence of carbonate the solidus is at ~1250 C and the CO 2 -free invariant point (Fo En Di L) is between 1700 C and 1800 C. The carbonate and silicate endmembers are connected by a four-phase cotectic field 1949

8 JOURNAL OF PETROLOGY VOLUME 39 NUMBER 11 & 12 NOV & DEC 1998 boundary along which CO 2 content (and therefore P CO2 ) Bailey, D. K. (1987). Mantle metasomatism perspective and prospect. decreases with increasing temperature. In: Fitton, J. G. & Upton, B. G. J. (eds) Alkaline Igneous Rocks. Geological Society, London, Special Publication 30, We determined the isobaric melting paths for two bulk Bailey, D. K. (1989). Carbonate melts from the mantle in the volcanoes compositions in the system CaO MgO SiO 2 CO 2. The of south-east Zambia. Nature 338, data show that, for a peridotite with low initial carbonate Bailey, D. K. (1993). Petrogenetic implications of the timing of alkaline, content, a small amount of carbonatitic melt [Ca/(Ca carbonatite and kimberlite igneous activity in Africa. South African + Mg) = 0 64]) coexists with olivine + orthopyroxene Journal of Geology 96, clinopyroxene from the solidus at 1250 C to ~1475 C. Baker, B. H. (1987). Outline of the petrology of the Kenya rift alkaline Melt composition in this temperature interval changes province. In: Fitton, J. G. & Upton, B. G. J. (eds) Alkaline Igneous Rocks. Geological Society, London, Special Publication 30, steadily, with SiO 2 content increasing from 4 3% at Bell, K. & Blenkinsop, J. (1987a). Archean depleted mantle: evidence 1300 C to 7 5 wt % at 1450 C. Between 1475 C and from Nd and Sr initial isotopic ratios of carbonatites. Geochimica et ~1525 C the melt SiO 2 content increases rapidly to 28%, Cosmochimica Acta 51, thereafter rising to 37% (with 15% CO 2 ) at 1600 C. Bell, K. & Blenkinsop, J. (1987b). Nd and Sr isotopic compositions of Carbonated silicate melts have decreasing CO 2 contents East African carbonatites: implications for mantle heterogeneities. from 1600 C to the ternary (CaO MgO SiO Geology 15, ) invariant Bergman, S. C. (1987). Lamproites and other potassium-rich igneous point between 1700 C and 1800 C. There is no evidence rocks: a review of their occurrence, mineralogy and geochemistry. that the rapid change in melt composition at 1475 ± In: Fitton, J. G. & Upton, B. G. J. (eds) Alkaline Igneous Rocks. Geological 25 C is influenced by liquid immiscibility. Society, London, Special Publication 30, The data relate to one possible explanation for the Brey, G. (1978). Origin of olivine melilitites chemical and experimental binary nature of carbonatite complexes. Melts dominated constraints. Journal of Volcanology and Geothermal Research 3, by carbonate and silicate compositions are both produced Brey, G. & Green, D. H. (1975). The role of CO 2 in the genesis of over wide temperature intervals. Transitional comolivine melilitite. Contributions to Mineralogy and Petrology 49, positions are produced over a much narrower tem- Brey, G. & Green, D. H. (1977). Systematic study of liquidus phase perature interval (50 C in the CMS CO 2 system) and relations in olivine melilitite + H 2 O + CO 2 at high pressures and must, like carbonatites themselves, react with the mantle petrogenesis of an olivine melilitite magma. Contributions to Mineralogy at pressures lower than those of the solidus ledge. Thus, and Petrology 61, carbonate melts, formed at high pressure, can only reach Dalton, J. A. & Wood, B. J. (1993). The compositions of primary the surface after extensive metasomatism of the overlying carbonate melts and their evolution through wallrock reaction in the mantle. Earth and Planetary Science Letters 119, mantle and production of a wehrlitic pathway. Trans- Dawson, J. B. (1966). Oldoinyo Lengai an active volcano with sodium itional compositions need also to react a pathway to carbonatite lava flows. In: Tuttle, O. F. & Gittins, J. 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