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The Formation Conditions of Chondrules and Chondrites

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Science  20 Jun 2008:
Vol. 320, Issue 5883, pp. 1617-1619
DOI: 10.1126/science.1156561

Abstract

Chondrules, which are roughly millimeter-sized silicate-rich spherules, dominate the most primitive meteorites, the chondrites. They formed as molten droplets and, judging from their abundances in chondrites, are the products of one of the most energetic processes that operated in the early inner solar system. The conditions and mechanism of chondrule formation remain poorly understood. Here we show that the abundance of the volatile element sodium remained relatively constant during chondrule formation. Prevention of the evaporation of sodium requires that chondrules formed in regions with much higher solid densities than predicted by known nebular concentration mechanisms. These regions would probably have been self-gravitating. Our model explains many other chemical characteristics of chondrules and also implies that chondrule and planetesimal formation were linked.

Chondrules make up ∼20 to 80 volume % of most chondrites and formed at peak temperatures of ∼1700 to 2100 K (1). Chondrules in the different chondrite groups have distinct physical and chemical properties (2), as well as distinct age ranges (3), indicating that they formed in relatively local environments via a process that operated at least periodically between ∼1 and 4 million years after the formation of the solar system.

It was long thought that individual chondrules behaved as chemically closed systems during their formation, inheriting their compositions from their precursors (4, 5). However, for likely cooling rates of 10 to 1000 K/hour (1) and at the low pressures (total pressure ≈ 10–6 to 10–3 bars) of the solar protoplanetary disk (nebula), experiments (68), natural analogs (9, 10), and theoretical calculations (11, 12) all show that there should be extensive evaporation of major and minor elements, in the order S > Na, K > Fe > Si > Mg.

Elemental fractionations in chondrules are generally a function of volatility (4, 5). If evaporation in the nebula produced the alkali metal and Fe fractionations, the more volatile elements (such as S) should be entirely absent, which they are not. In addition, the fractionated elements should exhibit large and systematic isotopic fractionations, which they do not (13).

Here we demonstrate that chondrules did indeed behave as essentially closed systems during melting, at least for elements with volatilities less than or equal to that of Na. We also propose a means of resolving the apparent conflict between this result and experimental and theoretical expectations that chondrules should have suffered considerable evaporation during formation. Our conclusions have implications for mechanisms of dust concentration in the solar nebula, for chondrule formation, and for planetesimal formation.

Chondrules are dominated by olivine [(Mg,Fe)2SiO4], pyroxene [(Mg,Fe,Ca) SiO3], Fe-Ni metal, and quenched silicate melt (glass). Many of the more volatile elements (such as Na) can diffuse rapidly, particularly in melts and glasses. Therefore, it is possible that volatiles were completely lost when chondrules melted, and reentered the chondrules during cooling or even after solidification. However, Na clinopyroxene/glass ratios show that the Na contents of the final chondrule melts (now glass) had approximately their observed, relatively high abundances at temperatures of ∼1600 to 1200 K (1416).

Calculations suggest that chondrule melts could have been stabilized in the nebula by substantially enriching solids (chondrule precursors or other dust) relative to gas (11, 12). This also substantially increases the condensation temperatures of even highly volatile elements such as S (11, 12). Even in solid-enriched systems, there is an initial phase of evaporation when a chondrule melts, but subsequent chondrule/gas re-equilibration would erase any isotopic fractionations (12). If the enrichment of solids is high, little evaporation may be needed to reach chondrule/gas equilibrium, and the behavior of volatile elements during cooling would resemble closed-system behavior. However, even at a high total pressure of 10–3 bars, with a solids enrichment of 1000 relative to the solar composition, all the Na would evaporate at near-liquidus temperatures, and substantial recondensation only begins well below 1600 K (11). Locally enriching chondrule-sized or smaller solids by 1000 times on substantial spatial scales would have been difficult to achieve in the nebula with known mechanisms (17), except perhaps if levels of turbulence were very low (18).

It is evident from the above that the behavior of Na during cooling can constrain the degree to which solids were enriched during chondrule formation. Olivine is the highest-temperature liquidus phase in chondrules and is predicted to crystallize throughout much of their supersolidus cooling (11). Olivine phenocrysts in chondrules are generally zoned in major and minor elements, demonstrating that olivine cores did not maintain equilibrium with the liquid during cooling. Na does partition into olivine, albeit at low levels. Therefore, if Na could be measured in them, olivine phenocrysts should record any changes in the Na content of their host chondrules as they crystallized.

We analyzed bulk, mineral, and glass compositions in 26 Semarkona (classified as a LL3.0) chondrules (Table 1) (18) with a range of types (IA-IAB and IIA-IIAB-IIB, where I = FeO-poor, II = FeO-rich, A = olivine-dominated, B = pyroxene-dominated, and AB = intermediate). In general, chondrule olivines exhibit pronounced normal zoning in major (Mg and Fe) and minor (Na, Cr, Mn, and Ca) elements that is consistent with expectations for crystallization from their bulk chondrule melts (5, 16, 19). Na can diffuse rapidly in some minerals and glasses at high temperature, but the fact that its zoning roughly parallels that of Cr, Mn, and Ca demonstrates its primary nature. In addition, the shapes of Na zoning profiles around Na- and Fe-poor relict olivine grains (unmelted precursors) (20, 21) in type II chondrules are similar to or broader than those of Fe (22), suggesting that Na diffusion rates in these olivines were comparable to those of Fe. Except where grains are clearly relict, analyses of phenocrysts and microphenocrysts from the same chondrule fall on similar trends, but these trends commonly differ from chondrule to chondrule.

Table 1.

The petrologic types and estimated liquidus temperatures of the Semarkona chondrules studied, as well as the Mg number and Na olivine-melt KDs of the first (core) and last (rim) olivines to crystallize.

View this table:

The simple observation of measurable Na in the cores of chondrule olivines (Fig. 1 and Table 1) (18) is contrary to predictions (Fig. 1), even in what has previously been considered a highly dust-enriched system (1000 times that of the solar composition) (11). To determine how much Na was present in the chondrule melts during olivine crystallization, it is necessary to know the Na olivine-melt distribution coefficient (KD), measurements of which have yet to be published. However, clinopyroxene-glass partitioning demonstrates that the Na contents of the glass in the central regions of these chondrules were established before solidification (1416). In olivine-normative chondrules, olivine was generally still on the liquidus at the onset of clinopyroxene crystallization [calculated with MELTS (23)]. Consequently, we can estimate the olivine-liquid KD for Na at the end of crystallization for each chondrule as the ratio of the Na2O contents of the last olivine to form (the most Na- and Fe-rich phenocryst rims and microphenocrysts) to that in the glass (rim KDs, Table 1). The rim KDs are fairly similar for all chondrules, implying no strong compositional dependence.

Fig. 1.

Comparison of the Na2O radial zonation in an olivine phenocryst (squares) from chondrule C3 with predictions (18) for closed-system crystallization (dashed line) and a chondrule forming in a region of the nebula with a total pressure of 10–3 bars that is enriched in solids relative to a solar gas by a factor of 1000 (solid line). wt %, weight %. The predictions assume an olivine-melt KD of 0.0075 (Table 1). The diamond symbols represent microphenocrysts in the glass that probably most closely reflect the compositions of the last olivine to be in equilibrium with the glass.

To determine whether bulk chondrule Na abundances were very different at liquidus temperatures, when olivine began to grow (∼400 to 500 K above the clinopyroxene and final olivine crystallization temperatures), we calculated the apparent KDs for the cores of olivine grains. We did this for each chondrule, using their most Na- and Fe-poor core olivine compositions and their bulk Na content (core KDs, Table 1). Figure 2 compares the apparent core and rim KDs. Most chondrules fall close to the 1:1 line, as would be expected for closed-system behavior. The notable exceptions, which all fall well to the right of the 1:1 line and out of the figure, are the three type IAs and chondrule C5, which have very high core KDs. We suspect that this is because they contain unrecognized relatively Na-rich relict cores. The good correspondence between core and rim KDs for most chondrules, which cover several hundred degrees of cooling, suggests that (i) they do not have strong temperature or compositional dependences, and (ii) the Na contents of most chondrules did not vary dramatically during cooling. This inference is supported by the subparallel Na, Cr, Mn, and Ca zoning profiles in most olivine phenocrysts. Some chondrules do have core KDs that, for the quoted errors, are significantly lower than the rim KDs, which could be indicative of some Na gain by the melt during cooling, although the differences are still much less than predicted by the most solids-enriched model of (11).

Fig. 2.

The initial (core) and final (rim) olivine Na KDs for most of the Semarkona chondrules studied (Table 1). Four chondrules with large core KDs fall outside the figure.

There are several potential sources of uncertainty in estimating the KDs that could not be quantified (18). There is also a range of KDs even for those chondrules that lie close to the 1:1 line. Some of this variation could be due to the gain or loss of Na after crystallization (18). Despite these concerns, because most chondrules fall close to the 1:1 line, we infer that to first order they were essentially closed systems for Na during crystallization.

There is some uncertainty about the nebular conditions during chondrule formation. To estimate the range of solids densities that would have been necessary for roughly closed-system behavior of Na at liquidus temperatures, we calculated the equilibrium vapor pressures over the bulk chondrule silicate melts alone as well as over the silicate melts in equilibrium with pure Fe metal (18). The solids densities were estimated by calculating, for a given temperature and equilibrium pressure of Na (PNa), the density of solids required for 90% of the Na to remain condensed (Table 1). If the PH2 was >10–7 bars during chondrule formation, it will increase the lower limits on the densities (18).

The ranges of solids densities in Table 1 are much higher than previously considered for chondrule formation. In principle, the required solids densities could be lowered if the oxygen fugacity (Math) were enhanced. However, the Math levels in the calculations of the lower limits were well above the iron-wüstite buffer and, therefore, almost certainly already too high because chondrules contain metal. The apparent absence of chondrules exhibiting clear evidence for the open-system behavior of Na or isotopic evidence for evaporation indicates that the chondrules in chondrites formed almost exclusively in such high-density regions. Either there was a remarkably efficient process for concentrating chondrule precursors in regions with high solids densities, or only chondrules that formed in high-density regions accreted into chondrites.

On the basis of the lack of isotopic fractionations in chondrules, it has been estimated that chondrule-forming regions were at least 150 to 6000 km in radius (24). At the minimum liquidus densities we calculated (typically >10 g/m3), regions >4000 km in radius will be self-gravitating (18). Regions of this size and density should cool at rates that are consistent with those inferred for chondrules (18) and contain enough mass to produce a body of the size typically assumed for the chondrite parent asteroids (radius ∼50 km, density ∼3 g/cc). These observations raise the possibility that planetesimal formation and chondrule formation in high-density regions were linked. In any case, such high densities would promote the aggregation of larger objects that could then coalesce into asteroid-sized objects (25).

A close link between chondrule and planetesimal formation would explain how distinct chondrule populations were preserved in a turbulent nebula in which large-scale mixing would have occurred relatively rapidly. At near liquidus temperatures and inferred densities, there would be gas-mediated exchange between chondrules in close proximity to one another. The diversity of chondrule compositions would thus seem to be problematic, but equilibration time scales are poorly understood and would depend on vapor pressures, diffusion rates in the melts, and the magnitude of the initial compositional differences between chondrules. Even at equilibrium, differences in bulk chondrule compositions are possible to generate simply by varying the proportions of the equilibrium phases. The finite time scales of gaseous diffusion mean that if chondrule-forming regions were relatively large, microenvironments could exist within them (24). However, chondrite formation cannot have immediately followed chondrule formation, because at least some chondrules experienced multiple heating events (2), and the low-temperature matrix that cements chondrites must also be present before final assembly (18).

Shock (26) and current-sheet (27) heating models both predict thermal histories that are consistent with those inferred for chondrules. The solids densities we estimate are much higher than assumed in previous models (26, 27) but are more consistent with shock heating than with current sheets. Over the range of previously explored solids densities, chondrule cooling rates after shock heating are predicted to increase with increasing solids density. It remains to be seen whether the calculated cooling rates at the densities we estimate are consistent with those inferred for real chondrules. If not, other heating mechanisms, such as lightning or planetesimal collisions, will have to be explored.

Supporting Online Material

www.sciencemag.org/cgi/content/full/320/5883/1617/DC1

Methods

Fig. S1

Tables S1 to S5

References

References and Notes

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