Retention of sodium during chondrule melting

0016-7037/91/$3.00

Geochunlca cf Cosmochrmrca AC/U Vol. 55, pp. 935-942 Copynghi 0 1991 Pergamon Press pk. Printed in U.S.A.

+ .Xl

Retention of sodium during chondrule melting ROGER H. HEWINS* Geological Sciences, Rutgers University,

New Brunswick,

NJ 08903, USA

(Received August 13, 1990; accepted in revisedjbrm December

I I, 1990)

Abstract-Type I chondrules in unequilibrated ordinary and carbonaceous chondrites tend to be enriched in refractory elements and depleted in volatiles relative to bulk CI. Type II chondrules show chondritic concentrations of major and minor lithophile elements with Na, in particular, at or slightly above the appropriate bulk-rock values. Element ratio diagrams for chondrule bulk compositions show that Type II chondrules plot on a mixing line between forsterite and a Na phase, with Na/Al 1: 1, whereas Type I compositions can be explained by mixing forsterite with melilite or CA1 or other refractory component with little Na. Calcium in Type II chondrules must have been added as an Al-free phase such as diopside. If their bulk compositions are manipulated, subtracting Ca as diopside and Fe + Mg as olivine, the residue is 90% albite and 10% silica (which would be in pyroxene). Albite was incorporated into the precursors of Type II chondrules, which clearly have not been depleted in Na although their initial temperatures overlap with those of Type I. There is no (negative) correlation between Type I liquidus temperatures (1450-1900°C) and Na/Al ratios and hence no indication of Na loss from the melt. If Type I precursors contained albite, the most aluminous chondrules would have suffered the most extreme Na loss, but these have the lowest liquidus temperatures. Their precursors were Na depleted, whereas those of Type II were Na enriched, as a function of the abundance of albite. The simplest way to obtain the bulk compositions of chondrules is to assemble condensates into precursors at different temperatures, and concentration of solids in the nebula or exceptionally rapid heating is required to preserve Na in chondrules after melting. Substantial exchange of Na occurred between chondrules and chondrite matrix during parent-body metamorphism to petrologic type 3.6.

the chondrules did not suffer extensive volatilization. Alkali abundances were explained as reflecting feldspathic minerals in the precursors, and GROSSMAN (1988) reported stronger evidence for this in ordinary chondrites. IKEDA ( 1983) identified a sodic feldspar component for chondrule precursors and suggested that Na-poor chondrules were assembled from higher temperature condensates than Na-rich, although some might have experienced alkali loss. RUBIN and WASSON (1987, 1988) and RUBIN et al. (1990) identified a Na component in chondrule precursors in Omans (C03) and Allende (CV3). Similarly, MISAWA and NAKAMURA (1988a,b) suggested alkali-bearing precursor phases for chondrules in Felix (C03) and Allende, and no evidence of alkali loss during melting. Somewhat different geochemical patterns are observed for different kinds of chondrules. Chondrules are generally classified by either texture and mineralogy (e.g., JONES, 1990) or by bulk composition (e.g., MCSWEEN, 1977a,b,c). The textures (granular, porphyritic, barred, radial) combined with major minerals (olivine, pyroxene) give names such as GO, PO, BO, PP, and RP for chondrules. Bulk composition names are Type I (highly magnesian, with Type IA silica-poor and olivine-rich, and Type IB silica-rich and pyroxene-rich), Type II (more ferroan), and Type III (radial pyroxene chondrules overlapping Type IB in composition). Many have noted that the lowest Na concentrations are found in refractory or Type I chondrules (e.g., KURAT et al., 1983; JONES and SCOTT, 1989; JONES, 1990; Lu et al., 1990). JONES and SCOTT ( 1989) and JONES ( 1990) explain higher Na in Type II than Type IA chondrules in Semarkona as due to either a volatile-rich

INTRODUCTION

CHONDRULES AREOF GREAT interest because of evidence that they formed in the solar nebula before the accretion of meteorite parent bodies. They attest to events energetic enough to melt solid precursors (GOODING et al., 1980; GROSSMAN, 1988), although the solar accretion disk appears to have been generally cool (WOOD and MORFILL, 1988). In the classical low-pressure, hydrogen-rich nebula the chondrule melt droplets should have lost most of the volatile elements such as Na by volatilization. Sodium loss is confirmed in many experimental studies, even at 1 atm, and higher oxygen fugacity than in the nebula (TSUCHIYAMA et al., 1981; RADOMSKY and HEWINS, 1990; and others). However, the chondrules show varied but quite high Na concentrations, and there is no clear consensus whether chondrules actually lost Na while molten or not. As GROSSMAN et al. (1988) point out, in the case of low Na abundances, it is important to know whether it was precursors or the droplets which were depleted. GROSSMAN and WASSON (1983a) established that chondrules were generated from heterogeneous precursors, probably nebular condensates, and behaved essentially as closed systems during melting. GROSSMAN et al., (1988) reported small depletions in moderately volatile lithophile elements in chondrules relative to bulk chondrite but concluded that

* Temporary address: Laboratoire de Mintralogie, Mustum National d’Histoire Naturelle, 61 rue de Buffon, 75005 Paris, France. 935

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R. H. Hewins

precursor in the Type II chondrules or volatile loss from the Type I chondrules. KURAT et al. (1983) have ascribed strong volatile depletions in the most refractory chondrules in Chainpur to vapor fractionation during chondrule formation. Most recently, LU et al. (1990) concluded that Semarkona Type I chondrules showed volatile loss during chondrule formation but that less refractory chondrules did not. In this paper, I analyze differences in Na concentrations in different groups of porphyritic olivine chondrules and show that they are due to the abundance of precursor albite, rather than the loss of Na from the melt. SODIUM

CONCENTRATIONS

IN CHONDRULES

Bulk compositions of chondrules of given textural types have been determined by electron microprobe defocusedbeam techniques for carbonaceous (MCSWEEN, 1977a,b,c) and ordinary (JONES and SCOTT, 1989; JONES, 1990) chondrites. Before addressing the question of causes of Na depletion (or enrichment), we must first establish how Na concentrations vary relative to chondrite abundances in various kinds of chondrules. Figure 1 shows CI-normalized lithophile major and minor element concentrations of magnesian (Type I) and ferroan (Type 11) porphyritic chondrules in carbonaceous and ordinary chondrites, arranged with refractory elements on the left and volatiles on the right. Inclined patterns are consistent with vapor-solid or vapor-liquid fractionation and flat patterns are not. The samples are (a) Type I PO/GO/POP/PP chondrules in Efremovka (MCSWEEN, 1977a,b), a CV3 chondrite, (b) Type II PO/B0 chondrules in CV3 chondrites (MCSWEEN, 1977a,b), (c) Type IA PO/GO chondrules in the LL3.0 chondrite Semarkona (JONES and SCOTT, 1989) and (d) Type II PO chondrules in Semarkona (JONES, 1990). All four plots (Fig. la-d) resemble bow-ties, because the high abundances of the intermediate elements Mg and Si leave little room for variation. The Mg/Si is always greater than chondritic, because these are olivine-rich chondrules, produced after concentration of forsterite (relative to fayalite, enstatite, silica. etc.) into chondrule precursors, perhaps by some evaporationcondensation fractionation process. Complementary materials with lower Mg/Si exist, e.g., in RP chondrules (see GROSSMAN and WASSON, 1983a). The Type I chondrules (Fig. la and c) are enriched in the refractories Ca, Al, and Ti up to 6XCI except for one or two each in Efremovka and Semarkona. They are generally depleted in volatiles relative to CI, although two Semarkona chondrules are slightly enriched in Na. In contrast, Type II chondrules in CV chondrites (Fig. lb) and Semarkona (Fig. Id) show generally flat patterns. The concentrations show a small variation about the CI value, except that Mg/Si is always greater than chondritic and Na in Semarkona Type 11 PO chondrules is always greater than chondritic. SODIUM

COMPONENTS

IN CHONDRULES

Figure 1 confirms previous indications that Type I chondrules are generally depleted in Na, whereas Type II are close to CI or even enriched. Is this due to fractionation in the precursors (as suggested by GROSSMAN and WASSON, 1983a) or volatile loss from the chondrule melts (as suggested by LU

et al., 1990)? Consider Figure 2, which shows the variation of Al,O,/MgO with Na*O/MgO for similar chondrule suites. The Type II PO chondrules in Semarkona (open circles) form a diagonal linear array (Fig. 2a) whereas the Type I PO chondrules plot close to the AlzO,/MgO axis. except for the two anomalously high Na points of Fig. Ic. Similarly, for carbonaceous chondrites (including Efremovka), there are two fields for Types I and II chondrules (Fig. 2b). Because only a few CV Type II chondrules (Fig. 1b) were analyzed by MCSWEEN (1977a,b), his CO Type II chondrules have been added to this plot. Although there are two chondrule fields, as for Semarkona, in Fig. 2b they overlap slightly. I discuss below whether this is due to metasomatism accompanying parent-body metamorphism or differences in precursors. Two Type III radial pyroxene chondrules plot with the Type 1 in Fig. 2b, consistent with the demonstration of MCSWEEN et al. (1983) that these form one composition population. Other chondrules from ordinary chondrites, although not classified as Types I and II, also plot as Na-rich and Na-poor groups (Fig. 2~). The solid diagonal line in Fig. 2 is for a Na/Al molar ratio of 1, passing through nepheline and albite with a finite concentration of MgO. This line is very close to a best-fit line, particularly for the Semarkona Type II chondrules, but also for a subset of the Manych and ALH 770 15 chondrules, and, to a lesser extent (see below), the CO Type II chondrules, and it is hard to believe that this is a coincidence. It means that all the Al in the Semarkona Type II chondrules is tied up in a stoichiometric Na-Al phase. and it makes it very unlikely that any Na was lost from the melt. It seems very improbable, for example, that Na was incorporated as some Al-free phase or simple oxide, and Al as a phase like melilite, and that Na loss from the melt was arrested precisely when the liquid Na/Al ratio was unity. Further, as pointed out by R. JONES (pers. comm.), if Na/Al were initially greater than unity, one would expect the chondrules with the lowest Na/ Mg to have the highest liquidus temperatures and therefore the greatest Na loss, producing a line trending not towards the origin as observed but towards the Al/Mg axis of Fig. 2. Forsterite (and enstatite and ferroan olivine) plots at the origin in Fig. 2. Since forsterite is the most abundant component in chondrules, relative to fayalite, etc., even for Type II chondrules, Fig. 2 shows that to a first approximation Type II chondrules may be regarded as mixtures of forsterite and the Na phase. Considering other chemical components, of course other minerals must also have been present in the precursors. Because the Na/Al ratio is 1: 1, Ca must have been present in the Type II precursors as some Al-free phase, the most probable of which, considering condensation sequences and the results of IKEDA ( 1983) is diopside. Following a suggestion from R. Jones, the Na phase has been identified. Assuming the main precursor minerals of Type 11 chondrules included olivine, diopside, and the Na phase, we subtract from average Type II chondrule composition (JONES, 1990) all the Ca as diopside and all the Fe t- Mg as olivine. Albite forms 90% of the residue, the rest of which is 10% silica which would have been part of precursor orthopyroxene. We can conclude that the Na precursor phase was albite, in accord with the results of IKEDA (I 983). Albite, like the other minerals used, is important in condensation schemes both

937

Retention of Na during chondrule melting

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FIG. 1, Bulk compositions (WY%)of chondrules normalized to CI values and arranged in order of increasing volatility: (a) Type I chondrules in Efremovka (MCSWEEN 1977a,b);(b) Type II in CV chondrites (MCSWEEN 1977a,b);(c) Type I in Semarkona (JONES and SCOTT, 1989); (d) Type II in Semarkona (JONES,1990). Type I are enriched in refractories and depleted in volatile% Type 11are chondritic.

classic (GROSSMAN, 1977) and recent (e.g., WOOD and HASHIMOTO,1988), and the low Al does not permit plagioclase solid solution. Nepheline with enstatite and silica (and no olivine) is also possible mathematically but is much less plausible given condensation sequences. Figure 2 shows, in addition to the Na/Al 1: 1 line, four lines radiating from the origin towards the average compositions of four types of CA1 (MACPHERSONet al., 1988) which may contain different amounts of secondary Na. Type I chondrules could be explained in Fig. 2 as mixtures of forsterite and Allende CAI, as analyzed by ~ACPHERSONet al. f 19881,with, of course, additional phases to explain other components. However, MISAWAand NAKAMURA(1988b) suggested from REE data that similar fractionations influenced the precursors of both chondrules and CAI, rather than chondrules containing specific CA1 material. The excess Al over that required for the Na-Al phase rather than being incorporated with Ca as real CAI more probably represents a precursor refractory phase such as spine1 or melilite, would plot close to bulk CA1

off to the right of these plots and would need to have been accompanied by minor amounts of a phase such as albite to supply Na. The Na phase is difficult to identify, given the small amount of Na in Type I PO chondrules (JONES and Sco-r-r, 1989), but, because of the large excess of silica, it is much more likely to be albite than, say, nepheline, which is in any case generally considered to be a very low temperature condensate. Mixing calculations (HEWINS, 199 1) used various refractory minerals plus olivine, pyroxene, etc., to match chondrule bulk compositions. Such calculations do not provide unique solutions for a given chondrule, but if melilite is used it may require either anorthite or diopside or spine1 to yield bulk compositions, whereas spine1 (+diopside + forsterite + enstatite + albite + fayalite) is consistently satisfactory. Taking spine1 as the Al phase also has the advantage that it is the only additional phase required over those which match bulk com~sitions for Semarkona Type II chondrules. Spine1 plus diopside plus forsterite are in accord with the refractory component in Semarkona formed “‘above the con-

938

R. H. Hewins cl. 3

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FIG. 2. Chondrule bulk compositions (wtW):(a) Type I (solid circles) and Type II (open circles) from Semarkona; (b) Type I (solid circles) and Type III (plus signs) from CV chondrites, and Type II (open circles) from CV and CO chondrites; (c) Porphyritic (solid circles), barred (open circles), and non-porphyritic (plus signs) chondrules from Manych LL3.4 (DODD, 1978a,b) and ALH 77015 L3.5 (NAGAHARA,198 1). Mixing lines from albite and CA1 compositions to forsterite (etc.) at the origin. Type II chondrule compositions show mixing of forsterite and albite and Type I forsterite-refractory-Na mixing.

densation temperature of enstatite” (GROSS-MAN and WASSON, 1983b), and the precursors of the JONES (1990) chondrules were assembled after spine1 was sequestered in other aggregates. The degree of separation between the suites of Na-rich and Na-poor chondrules in Fig. 2 is variable. The Na/Al 1: 1 line tends to represent an upper bound rather than a best fit to the Type II chondrules, as also observed by FUJIMAKI et al. (1981) and GROSSMAN and WASSON(1983a). The CO Type II chondrules deviate much more from the Na/Al 1: 1 line than the Semarkona Type II chondrules, possibly due to metamorphism, because CO chondrites record a metamor-

phism (e.g. MCSWEEN, 1977~; SCOTT and JONES, 1990) during which Na/Al ratios of chondrules changed (MCCOY et al., 199 l), and this is discussed below. Two chondrules in Semarkona (Fig. 2a) show intermediate Na/Al ratios (anomalously high for Type I PO), and this is unlikely to be due to metamorphism given a type 3.0 chondrite. According to J. GROSSMAN (pers. comm.), numerous published Semarkona chondrule analyses would plot between the two groups of Fig. 2a, possibly because those samples were not chosen at random but as archetypal representatives of the textural groups. This suggests that while archetypal Type II PO chondrule precursors acquired all their Al as albite and archetypal Type II PO chondrule precursors acquired very little Na, many Semarkona precursors acquired intermediate amounts of albite. The precursors of such chondrules with intermediate Na/Al ratios in Semarkona could record a transition from initial to abundant condensation of albite, either as a function of reaction kinetics (GROSSMAN and WASSON, 1983a) or effective temperature range (IKEDA, 1983; HEWINS, 1989), although diffusion of Na in metamorphosed chondrites could also transform pristine Type I and II Na/Al ratios to intermediate values. Figure 2c shows chondrules from the LL3.4 chondrite Manych (DODD, 1978a,b) and the L3.5 chondrite ALHA 770 15 (NAGAHARA, 198 1). These chondrules were not classified as Types I, II, and III, and, indeed, the high Na/Al and low Na/Al chondrules have overlapping ranges for Mg# (7090 for Na-poor and 50-90 for Na-rich chondrules). Similar results were obtained by IKEDA (1983). Given textural types (porphyritic, barred, and non-porphyritic) occur in both composition groups of Fig. 2c. Porphyritic chondrules (dots) are more abundant on the Semarkona Type II trend (Narich), BO chondrules on the I trend (Na-poor), and nonporphyritic on the II trend. Since radial pyroxene chondrules (Type III) are one of the main kinds of non-porphyritic chondrules, this is consistent with RP chondrules being assembled from different precursor phases in ordinary than in carbonaceous chondrites, in line with a previous suggestion based on MgO-FeO-Si02 values (RADOMSKY and HEWINS, 1990). Manych and ALH 770 15 show good separation (Fig. 2c) between Na-rich and Na-poor chondrules, especially considering Al-rich BO and non-porphyritic chondrules, despite the fact that these two chondrites are slightly metamorphosed. By analogy with the H3.6 chondrite Parnallee (MCCOY et al., 1990), the chondrules might have suffered slight modification of bulk compositions. Figure 2c suggests that Type II vs. Type I-III characteristics might be recognizable from Na/Al ratios, even when Fe/Mg ratios have been perturbed substantially, as in type 3.6 chondrites (MCCOY et al., 199 1). However, I show below that Na/Al ratios do change during such metamorphism. Therefore, we can conclude that these samples are not significantly metamorphosed and that some acquired Fe as a primary Fe olivine component accompanied by little Na. The chondrules in Fig. 2c may again have been selected using textural criteria, rather than at random. In particular, it is clear that granular-porphyritic Type I chondrules were not analyzed for Manych (DODD, 1978a,b). Type I granularporphyritic textures do occur in ALH 77015 in chondrules which are relatively Fe-rich, apparently not because of metamorphism but because of an unusual precursor situation,

939

Retention of Na during chondrule melting i.e., the presence of ferroan “relict” olivine in chondrules similar to Type I (see Figs. 5 and 6 in NAGAHARA, 1981). The Fe olivine component observed in the bulk compositions could be specifically the ferroan “relict” olivine observed inside the chondrules, which have crystallized magnesian olivine from the melt (NAGAHARA, 1981, 1983), and could have been introduced by seeding of molten droplets. Seeding has been shown to be a very plausible process (e.g., CONNOLLY and HEWINS, 1990). The addition of ferroan olivine seeds to Type 1 chondrule droplets produced from precursors formed before abundant albite condensation produces bulk compositions not consistent with continuous condensation. By contrast, two Semarkona Type I PO chondrules (Figs. lc and 2a) which require precursors rich in Na but not in Fe olivine could represent a more expected suite of condensates assembled after considerable albite condensation before the onset of fayalite. The two pathways (Fo-Fa-Ab and Fo-Ab-Fa) are consistent with random factors in assembly or seeding, but could also possibly be explained by different condensation conditions, e.g., causing fayalite precipitation at high temperatures (PALME and FEGLEY, 1990). TE~PE~TURE

AND COMPOSITION

It appears that unequilibrated Type II chondrules contain chondritic to superchondritic Na levels because albite was incorporated into the precursors (Fig. 2), and subsequently there was little or no Na loss. It is not yet entirely certain that Type I chondrules are Na-poor because their precursors contained only a little albite along with forsterite, spinel, etc., and have not suffered Na loss from the melt. Experiments show that the textures of porphyritic chondrules require initial temperatures slightly below their liquidus temperatures (HEWINS, 1988; HEWINS and RADOMSKY, 1990). Many authors believe that Type I chondrules have been heated to more extreme temperatures than Type II chondrules and therefore should have lost more Na than Type II, but, in fact, Type II chondrules and the least refractory of the Type I chondrules were heated to similar temperatures (WEWINS and RADOMSKY, 1990). In the case of Semarkona, Fig. 3 shows

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NaZO/A1203 FIG. 4. Calculated liquidus temperatures for Type I (solid circles), Type II (open circles), and Type III (plus signs) chondrules from CV chondrites plotted against NazO/A1203ratio. The absence of a negative correlation makes it unlikely that Na was lost from the chondrule melts.

that six of the Type I chondrules have distinctly higher liquidus temperatures, but five fall in the same range as the Type II. Furthermore, Type I initial temperatures could have been further below liquidus values than Type II (HEWINS and RADOMSKY, 1990). Figure 4 shows that liquidus temperatures for Type I chondrules in CV meteorites range from about 1450 to almost 1900°C. Although there is a small difference in average liquidus temperatures, which are upper bounds to chondrule initial temperatures, this is not enough to explain the wholesale difference in Na concentrations and Na/Al ratios between Type I and II chondrules. Consider the possibility that all chondrules initially contained abundant albite, and Na was lost in the case of the Type I chondrules. One would expect that those which had lost the most Na would have the highest liquidus temperatures, but there is no (negative) correlation between Na/Al ratios and liquidus temperature, ranging from 1450 to 1900°C in Fig. 4. If some liquids had been extensively evaporated, raising their calculated liquidus temperatures, one would expect higher Al in the chondrules with higher liquidus temperature, whereas, in fact, they have similar Na/Al ratios, lower Al/Mg, and lower absolute AI concentrations. Also, there is no reason to expect extensive Na loss from the Type I chondrules with liquidus temperatures of 1450-16OO’C when Type II chondrules (Na/Al 1: 1 molar) with equal and higher liquidus temperatures show a complete absence of such evaporation. If Na had been incorporated as albite, then chondrules which had lost the largest absolute amount of Na would plot the furthest from the l/l line of Fig. 2. Those with higher Al/Mg would have lost less Na because of their lower liquidus temperatures, leading to a curved distribution (R. JONES,pers. comm.). However, CV Type I chondrules, for example, have relatively constant Na/AI over a wide range of Al/Mg (dots in Fig. 2b), and their highest values for Na/Al are found amongst those with the lowest AI/Mg ratios, exactly the opposite of what we would expect with evaporation. Although some chondrules may have experienced some volatilization, there is no clear sign of systematic loss of Na from the chondrules, and low Na can be explained simply as low Na in the

940

R. H. Hewins

precursors. The correlation of Na with the refractory component in many chondrule suites, e.g., for Omans (RUBIN and WASSON, 1988) and Allende (RUBIN et al., 1990), is consistent with such a lack of evaporation in Type I chondrules. REDOX EFFECTS GROSSMAN (1988) points out that to prevent loss of Na, the oxygen fugacity of the ambient gas must be much higher than the canonical value (unless the volatile loss mechanism is imperfectly understood). Furthermore, Fe0 contents of chondrules and matrix grains are consistent with disequilibrium during heating in a canonical nebula or equilibrium in an extremely fractionated gas, which seems to require extreme dust or ice concentrations (GROSSMAN, 1988; WOOD et al., 1989). MATSUDA et al. (1990) note a correlation between K concentration and Fa in olivine for BO chondrules (but not for PO), which they explain in terms of enhanced alkali loss for more reduced samples. I have plotted Na, Na/Al, and Na/Ti against Fe/Mg, Fe/Cr, and Fe/Ti, and also Al/Mg (which might correlate with Na loss) against Fe/Mg, for primitive chondrules (CV and Semarkona). Although some chondrules may have experienced reduction and related volatile loss, no correlations were observed within Type I chondrule suites (which show a small range of Fe0 and Fe/Mg values) to support the concept of loss of Na as a function of reduction. Type II chondrules do have higher Na and Fe/ Mg than Type I, and correlations between alkalis and Fe0 or Fa do occur in some chondrule suites (MATSUDA et al., 1990). These patterns can be explained because both components (albite, ferroan olivine) will tend to be incorporated into precursors at low temperature. However, we also see evidence in Semarkona and ALH 77015 that either component may be incorporated almost to the exclusion of the other.

Type I chondrules and the rest of the chondrites, even for the LL3.6 chondrite. Figure 5a shows Na/Al ratios increasing with metamorphic grade. A few BO chondrules in ALH 770 15 (L3.5) seen in Fig. 2c likewise display slightly higher Na/Al levels than typical of Type I chondrules, although it cannot be ruled out that this is not a precursor effect. Type I chondrules for equilibrated type 4 and 5 chondrites plot close to but just below the forsterite-Na mixing line. Type II chondrules shown in Fig. 5b show a very slight complementary decrease in Na/Al ratios from the Semarkona Type II mixing line. Na/Al ratios in equilibrated chondrules are close to that of bulk Semarkona. The dispersion of Na/AI ratios for LL Type II chondrules (Fig. 5b) is less than for CO (Fig. 2b), although Fe/Mg equilibration is less complete for olivine in CO than in equilibrated LL chondrites. The CO Na/Al ratios are lower than for LL either because the bulk Na/Al ratio in CO is lower or because some chondrules had precursors with intermediate Na/Al ratios. We can conclude that many chondrules plotting between the forsterite-Na and forsteriterefractory mixing lines have suffered metasomatism during parent-body equilibration. BISCHOFF (1988) similarly found that Ca-Na exchange is significant for Al-rich chondrules by stage 3.8 and very extensive in equilibrated chondrites. CHONDRULE

FORMATION

The differences in bulk compositions for Type I and Type II chondrules are indicative of different precursors and there0. 15

METAMORPHISM 0. 05

This discussion shows that Type II chondrules are Na-rich in proportion to the amount of albite incorporated in the precursors and that the Type I chondrules are depleted because there was little of such a component in the precursors. Some chondrule data sets do not show two linear mixing arrays like those in Fig. 2 but do show points filling the field between the mixing lines. Such chondrules include the BO data of WEISBERG (1987) with chondrules from equilibrated chondrites and the CO chondrites, which are known to comprise a metamorphic sequence (MCSWEEN, 1977~; SCOTT and JONES, 1990; KECK and SEARS, 1987). I suggested above that filling the field below the mixing line may reflect metamorphism of chondrules, although there may also be chondrules with moderate amounts of albite in the precursors. MCCOY et al. ( 199 1) recently provided evidence of systematic composition changes to chondrules during metamorphism. I therefore evaluate their data for information on the behavior of Na. Figure 5a has the same format (A1,03/Mg0 vs. NazO/MgO) as Fig. 2 but shows a metamorphic sequence of Type I chondrules. These are from Semarkona LL 3.0 (JONES and SCOTT, 1989) and from Parnallee LL 3.6, Sevilla LL4, and Olivenza LL5 (MCCOY et al., 1991). The data of McCoy et al. (1991) document substantial exchange of Na, Fe, and Mg between

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FIG. 5. Metamorphism in LL chondrules (MCCOY et al., 1991). (a) Type I chondrules show increasing Na/Al with metamorphic grade, approaching bulk meteorite (and Type 11) values. (b) Type II chondrules show an extremely small decrease in Na/Al as bulk LL Na/ AI is only a little lower than that for Semarkona Type II chondrules. CO Type II chondrules (Fig. 2b) show a clearer decrease towards the lower bulk CO Na/AI.

941

Retention of Na during chondrule melting fore different conditions (temperature, pressure, and/or oxygen fugacity) or, in the case of random assembly, different combinations of conditions for the various precursor phases. Many Type I chondrules are almost free of fayalitic and Na components. Such Fe0 and Na as they contain might have been added by secondary modification of the precursors, for example, fayalite in Allende chondrules (PECK and WOOD, 1987) and Na-bearing components in CA1 (MACPHERSON et al., 1988), but they might also have originated as primary condensates before the abundant precipitation of such phases. The precursors could have been assembled from forsterite, enstatite, diopside, and spine1 (or other refractory phases) formed as condensates above or close to the condensation temperatures of albite and fayalite for the composition and pressure of the available gas. In contrast, fayalite and albite must have been abundantly condensed and assembled into the precursors of Type II chondrules. There appear to be some chondrules with abundant albite but little fayalite (in Semarkona) and some with abundant fayalite but little albite (in Manych and AI&I 77015). This suggests either different condensation sequences as a function of variations in physical parameters, or random transport of grain aggregates in and out of condensation zones, or late addition of foreign crystals to chondrule droplets. With different assumptions, fayalite may form after abundant albite precipitation (WOOD et al., 1988) or at high temperatures (PALME and FEGLEY, 1990). However, heterogeneous assembly of condensates is quite pIausible, and it now seems clear that some “relict” grains may be introduced into chondrule melt droplets, having profound effects on the textures produced (CONNOLLY and HEWINS, 1990). Such seeding is a simple direct way to modify chondrule bulk compositions by adding late condensation or even early chondrule formation products. The Type II chondrules should have lost much Na in a canonical solar nebula, as pointed out by WOOD et al. ( 1989). Even in relatively high oxygen fugacity experiments. synthetic chondrules lose significant Na (HEWINS, 1988). However, chondrules could escape volatile loss with extremely short heating times or by production in a thick clump with transient atmosphere (HEWINS, 1989). WOOD et al. (1989) and IOTAMURAand TSUCHIYAMA(1990) suggested a nebula with a high dust/gas ratio, a high gas pressure, and similar tar/ dust and ice/dust ratios due to collisions of icy planetesimals. Such scenarios are not unthinkable, but we must provide for both FeO- and Na-poor chondrules (Type I) as well as the Type II. This requires separate assembly of precursors, perhaps in environments with different gas characteristics rather than simply at different stages in a single cooling sequence. MATSUDA et al. ( 1990) suggested that chondrules may be self-buffering rather than controlled by ambient gas. We may perhaps combine these concepts by su~esting that chondrule precursor aggregates could be either purely silicate particles or microcomets containing silicates, ices, and tars. Rapid heating of the latter precursors would certainly produce a local transient non-canonical nebular environment. CONCLUSIONS Sodium was incorporated into Type II chondrule precursors as albite and was not significantly lost during melting. Type I chondrules contain very low Na and were not depleted

in Na during melting, as there is no correlation between Na/ Al ratios and liquidus temperatures over a range of 14501900°C. Retention of Na by chond~~es requires either exceptionaily rapid heating or formation in a solid-enriched nebular environment. Sodium contents of chondruies are significantly modified during metamorphism even by petrologic type 3.6, and chondrules in equilibrated (4-6) chondrites have essentially bulk-rock Na/AI ratios. thank C. Macrae, M. J. Carr, H. C. Connolly Jr., and P. M. Radomsky for assistance with software and plotting: A. E. Rubin, and H. Y. McSween for reviews; and R. H. Jones and

Acknowledgments-1

J. N. Grossman for very thorough and helpful reviews. Funding was provided by NASA grant NSG 9-35. Editorial handling: G. Faure

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