the origin of chondrules and chondrites: debris …escott/sanders n scott 2012...the origin of...

The origin of chondrules and chondrites: Debris from low-velocity impacts between

molten planetesimals?

Ian S. SANDERS1,* and Edward R. D. SCOTT2

1Department of Geology, Trinity College, Dublin 2, Ireland2Hawai’i Institute of Geophysics and Planetology, University of Hawai’i at Manoa, Honolulu, Hawai’i 96822, USA

*Corresponding author. E-mail: [email protected]

(Received 11 June 2012; revision accepted 17 September 2012)

Abstract–We investigate the hypothesis that many chondrules are frozen droplets of sprayfrom impact plumes launched when thin-shelled, largely molten planetesimals collided at lowspeed during accretion. This scenario, here dubbed ‘‘splashing,’’ stems from evidence thatsuch planetesimals, intensely heated by 26Al, were abundant in the protoplanetary disk whenchondrules were being formed approximately 2 Myr after calcium-aluminum-rich inclusions(CAIs), and that chondrites, far from sampling the earliest planetesimals, are made frommaterial that accreted later, when 26Al could no longer induce melting. We show how‘‘splashing’’ is reconcilable with many features of chondrules, including their ages, chemistry,peak temperatures, abundances, sizes, cooling rates, indented shapes, ‘‘relict’’ grains, igneousrims, and metal blebs, and is also reconcilable with features that challenge the conventionalview that chondrules are flash-melted dust-clumps, particularly the high concentrations of Naand FeO in chondrules, but also including chondrule diversity, large phenocrysts,macrochondrules, scarcity of dust-clumps, and heating. We speculate that type I (FeO-poor)chondrules come from planetesimals that accreted early in the reduced, partially condensed,hot inner nebula, and that type II (FeO-rich) chondrules come from planetesimals thataccreted in a later, or more distal, cool nebular setting where incorporation of water-ice withhigh D17O aided oxidation during heating. We propose that multiple collisions and repeatedre-accretion of chondrules and other debris within restricted annular zones gave eachchondrite group its distinctive properties, and led to so-called ‘‘complementarity’’ and metaldepletion in chondrites. We suggest that differentiated meteorites are numerically rarecompared with chondrites because their initially plentiful molten parent bodies were mostlydestroyed during chondrule formation.

INTRODUCTION

Chondrules are igneous-textured grains that makeup 50% or more by volume of most chondrites. Many ofthem are frozen droplets of magma, typically 0.1–2 mmacross, rounded to lobate in shape, and composed largelyof the Mg-rich silicate minerals olivine, (Mg,Fe)2SiO4,and pyroxene, (Mg,Fe)SiO3 (Zanda 2004; Scott and Krot2007). They tend to be either FeO-poor (type Ichondrules) or FeO-rich (type II chondrules). Theirigneous textures suggest cooling from near-liquidustemperatures and solidifying over a matter of hours.Evidently, they were present in the solar nebula, orprotoplanetary disk (the disk of gas and dust that

surrounded the infant Sun, from which the planets laterdeveloped) prior to accreting to the surfaces of growingchondrite parent asteroids. They co-accreted with otherdisk materials including small grains and droplets ofFe-Ni metal and sulfide, refractory objects calledcalcium-aluminum-rich inclusions (CAIs), mineralfragments, and dust including micron-scale grains ofstardust surviving from the presolar molecular cloud. Aschondrites account for five out of six meteorites falling toEarth, the formation of chondrules was apparently aprocess that affected a substantial fraction of the solidmaterial in the solar nebula.

There is no consensus on how chondrules weremade: two fundamentally different approaches have

� The Meteoritical Society, 2012. 2170

Meteoritics & Planetary Science 47, Nr 12, 2170–2192 (2012)

doi: 10.1111/maps.12002

come to dominate current discussion. The first contendsthat clumps of dust in the disk were transformed directlyto chondrules by rapid melting, probably as a result ofshock-induced heating in the nebula. This idea wassuggested half a century ago (Wood 1963, p. 165) in aseminal paper whose title we adopt here, and the meltingof dust-clumps, whether by shock or by some othermeans, has since become the prevailing theory forchondrule formation. It has been advocated, tacitly if notovertly, by many authors, including Taylor et al. (1983),Wood (1988), Grossman (1988), Wasson (1993), Rubin(2000), Shu et al. (2001), Boss and Durisen (2005),Lauretta and McSween (2006), Scott (2007), Alexanderet al. (2008), and Ruzicka et al. (2012a).

The second approach imagines that chondrules wereproduced when large volumes of molten rock becamesplashed and dispersed into the nebula as showers ofspray. Specifically, it has come to embrace a hypothesis,dating from about 30 yr ago, that chondrules originatedin great plumes of droplets launched by collisionsbetween planetesimals that had been intensely heatedand melted by the decay of 26Al (Zook 1980, 1981;Wanke et al. 1981, 1984). This second hypothesis laydormant for many years, almost completelyovershadowed by the first, and its re-awakening has beenslow. Sanders (1996) attempted to revive interest, and theidea has also been promoted by LaTourrette andWasserburg (1998), Chen et al. (1998), Lugmair andShukolyukov (2001), and Hevey and Sanders (2006).Sanders and Taylor (2005) reviewed the hypothesis indetail. While the production of chondrules from moltenplanetesimals has never had a majority following, fewtoday would dismiss it out of hand, and recently,Asphaug et al. (2011) enhanced its credibility with acomputer simulation of how chondrules might formfrom magma released as a result of collisions.

A great deal more is known today about the earlysolar system than was known 30 yr ago. The past5–10 yr in particular have witnessed majorimprovements in mass spectrometry and meteoritechronology, significant developments in the modeling ofplanetesimal melting and of conditions in the solarnebula, and a substantial increase in the number andvariety of meteorites available for study. The samerecent period has also seen much exchange of ideasbetween meteorite researchers, astronomers, andplanetary scientists. Observations of disks aroundyoung stars, discoveries of extra-solar planetarysystems, and computer simulations of orbital dynamicshave revolutionized our thinking on the formation ofthe asteroid belt. Together, these advances are bringinginto focus a new picture of the young solar system thatis significantly different from the one that has beenconventionally portrayed.

In this article, we examine that new picture and howit might be extended to embrace the formation ofchondrules and the assembly of chondritic asteroids. Webegin by showing how precise meteorite chronology nowindicates that planetesimals first melted long beforechondrules were made, consistent with 26Al having beenan internal planetesimal heat source. We argue thatmolten planetesimals dominated the population ofbodies in the inner solar system for the first 2 Myr, andwe propose that the inevitable collisions and mergersbetween them led to ‘‘splashing’’ with ejecta plumesbearing swarms of chondrule droplets and other debris,which later accreted to chondritic parent bodies. Weevaluate the established petrographic, compositional,and experimental evidence for chondrule formation, andfind it to be consistent with this collision scenario.Importantly, we find some of the evidence hard toreconcile with the conventional view that chondrulesbegan as dust-clumps. In the last section of the article,we discuss how the ‘‘splashing’’ hypothesis may relate tothe broader picture of planetesimal evolution in theyoung disk, speculating on the nature of the moltenprecursor planetesimals, and on the origin of somehitherto poorly understood features of chondrites.

A NEW PARADIGM FOR CHONDRITES

ANCHORED IN 26AL HEATING

The Conventional View of Chondrites

Chondrites have conventionally been interpreted asaggregates of primitive materials that were assembled atthe very start of the solar system from the same reservoirof nebular dust as went to make the Sun (e.g., Wood1988). This view stems from the remarkable similaritybetween the chemical composition of chondrites and thatof the Sun’s photosphere for all elements other than a fewthat normally occur in gases (e.g., H, He, C, N, Ar). Theview is reinforced by the presence in chondrites of CAIs,which are the oldest dated objects with a solar systemisotopic signature (Amelin et al. 2002, 2010). Indeed, thetime of CAI formation has now been widely adopted asdefining the start of the solar system (t = 0). In addition,chondrites contain pristine grains of stardust that are evenolder than the solar system (e.g., Hoppe 2008). Thus,conventional thinking holds that chondrules, along withCAIs, were created directly from clumps of nebular dustat the outset, before the first planetesimals (presumed tobe the chondrite parent bodies) had accreted. It furtherholds that after their accretion, some chondriticplanetesimals became overheated, melted, anddifferentiated into molten metal cores and basaltic crusts,the sources, respectively, of iron and basaltic meteorites(e.g., Lauretta andMcSween 2006).

Origin of chondrules and chondrites 2171

However, this intuitive and long-establishedinterpretation of meteorites has recently been challengedby chronological evidence, which suggests thatchondrules were made after, and not before, the parentbodies of differentiated meteorites had melted.

The Chronology of Molten Cores and Chondrules

Most iron meteorites now appear to come fromplanetesimals that had accreted and melted extremelyearly, perhaps within 1 Myr of CAI formation(t < 1 Myr) because their e182W values are very low andwithin error of the initial e182W of CAIs (Burkhardtet al. 2008). In the early solar system, e182W was risingdue to the radioactive decay of 182Hf to 182W (half-life8.9 Myr). Tungsten is a siderophile (iron-loving) element,whereas hafnium is lithophile (silicate-loving). The e182Wvalues of iron meteorites became fixed, therefore, whenmolten metal segregated into planetesimal cores. At thatstage, the radioactive hafnium, being lithophile, wasremoved from close proximity to metal and transferredto the silicate mantle of each planetesimal wheresubsequent radiogenic 182W accumulated. A recentestimate of the time of core formation based on e182W inmagmatic iron meteorites is 0.3 ± 1.2 Myr after CAIs(Kruijer et al. 2011). Even more recently, Burkhardtet al. (2012) revised the value of initial e182W in CAIs,and they inferred that while the cores of some parentbodies (notably the IVB group) formed at tapproximately 0.3 Myr, the cores of others continued toform up to about t = 2 Myr. The timing of coreseparation is critical to constraining early disk evolution,and is the culmination of painstaking efforts tounderstand and refine the Hf-W chronometer by manyworkers (Horan et al. 1998; Kleine et al. 2005;Markowski et al. 2006; Schersten et al. 2006; Qin et al.2008; Burkhardt et al. 2008). In particular, Markowskiet al. (2006) recognized the need to correct e182W valuesfor the effects of long-term exposure to cosmic rays;uncorrected values had previously given erroneously oldages.

By contrast, 26Al-26Mg dating suggests thatchondrules are mostly 1.5–2.5 Myr younger than CAIs.Following the pioneering work of Hutcheon andHutchison (1989), about 100 chondrule dates based on26Al-26Mg internal isochrons have now been published.A recent review of them by Kita and Ushikubo (2012)shows that more than 90% of those from unequilibrated(type 3.0) chondrites (64 determinations) fall within the1.5–2.5 Myr age range, with just three dating fromt < 1.5 Myr. The same age difference betweenchondrules and CAIs of about 2 Myr has been measuredindependently by 207Pb-206Pb dating (e.g., Amelin et al.2002, 2010; Connelly et al. 2008), and by 182Hf-182W

dating (Kleine et al. 2008). As the chondrite parentasteroids were assembled after the youngest chondruleswithin them had formed, i.e., probably more than about2.5 Myr after CAIs, then far from being the first bodiesto have accreted, as is conventionally assumed, they wereperhaps among the last to have done so.

The late accretion of chondritic asteroids was alreadysuspected on petrographic grounds long before the recentchronological evidence became known. Fragmentsdeemed to be of planetary igneous rock were identified inchondrites by Kurat and Kracher (1980), Hutchison et al.(1988), and Kennedy et al. (1992). The last of theseauthors reported a 2 mm chip of high Mn ⁄Fe basalt in theParnallee chondrite, which they interpreted as beingderived from a high Mn ⁄Fe planetesimal that had alreadymelted and broken up before Parnallee’s parent body hadaccreted. In addition, Ruzicka et al. (1995) reported anunusual silica pyroxenite clast in Bovedy (L3), which theyregarded as having a planetary igneous origin. Morerecently, Sokol et al. (2007) reviewed the occurrence of awide variety of differentiated igneous rock fragments inchondrites, and Ruzicka et al. (2012b) reported furthersilica-rich clasts of supposed igneous origin. On a relatednote, Libourel and Krot (2007) discovered small pieces oftexturally equilibrated olivine rock inside chondrules,which they interpreted as tiny fragments of earlierplanetesimals that had been metamorphosed and thenbroken up by impacts before being incorporated intochondrules. Although Whattam et al. (2008) questionedthat interpretation, the petrographic evidence, like thechronological evidence, clearly points to high-temperatureplanetesimal processing prior to chondrite accretion.

Heating by 26Al: The Key to the New Chronology

The cause of early melting and core formation, some1–2 Myr before chondrules were made, is not hard tofind. Ever since evidence for live 26Al was discovered inCAIs, it has been realized that the decay energy from thisshort-lived isotope (half-life 0.72 Myr) would have beenmore than sufficient to melt the fully insulated interiorsof planetary bodies that accreted early enough, whileradioactive heating was intense (Lee et al. 1977). Thecorollary is that the chondrite parent bodies, as they didnot melt, accreted later, after the 26Al had largelydecayed and lost its capacity to cause melting. Thisexplanation is reinforced in the following paragraphs bya simple quantitative analysis of the likely effects of 26Alheating on the timing of initial planetesimal meltdown,the timing of chondrule formation, and the timing ofchondrite accretion.

Our estimate of the energy available in 26Al to heatthe first crop of planetesimals at t = 0 is about 6.6 kJper gram of dry dust. This estimate requires knowledge

2172 I. S. Sanders and E. R. D. Scott

of the concentration of Al in the dust, of the initial ratioof 26Al ⁄ 27Al, and of the heat released by each decayingatom of 26Al. The concentration of Al is not knownprecisely, but would presumably have been more than0.85 wt.% (the level in CI chondrites, which areextensively hydrated; Lodders and Palme 2009). Weconservatively, although somewhat arbitrarily, choose avalue of 1.2 wt.%, which corresponds roughly to theconcentration of Al in dehydrated CI chondrite, and isclose to the concentration of Al in most anhydrouschondrite groups (Lodders and Fegley 1998). We assumethat the initial value of 26Al ⁄ 27Al in the disk wasuniformly 5 · 10)5, the so-called canonical value in CAIs(Jacobsen et al. 2008; MacPherson et al. 2010). Auniform distribution of canonical 26Al in the disk isindicated by the identical 26Mg ⁄ 24Mg in the Earth, theMoon, Mars, and bulk chondrites (Thrane et al. 2006);by the correlation between the initial 26Mg ⁄ 24Mg and the26Al ⁄ 26Mg ages of a suite of chondrules studied byVilleneuve et al. (2009); and by time intervals betweenspecific events measured using the 26Al ⁄ 26Mgchronometer being corroborated by other chronometers(e.g., Connelly et al. 2008). We are aware that Larsenet al. (2011) reported significant variation in 26Mg ⁄ 24Mgin objects with solar 27Al ⁄ 24Mg, and proposed that theinitial 26Al ⁄ 27Al in parts of the inner solar system whereplanetesimals accreted and where chondrules formedmay have been substantially lower than the canonicallevel where CAIs were made. However, Wasserburget al. (2011) found a wide variation in the initial26Mg ⁄ 24Mg of different CAIs with identical canonical26Al ⁄ 27Al, which leaves an open verdict for the casemade by Larsen et al. (2011). Finally, we take the decayenergy per atom of 26Al as 3.1 MeV (Castillo-Rogezet al. 2009). A plausible 10% uncertainty in both theinitial 26Al ⁄ 27Al, and in the wt. % Al, leaves ourestimated initial radioactive energy at 6.6 ± 1 kJ g)1.

In addition to 26Al, the short-lived isotope 60Fe mayhave contributed to radioactive heating. However, theinitial concentration of 60Fe and whether it wasuniformly distributed in the disk remain unknown (Teluset al. 2012). The ratio at t = 0 of 60Fe ⁄ 56Fe (1.5 · 10)6)assumed by Sanders and Taylor (2005) now seems fartoo high. Telus et al. (2011) suggest that it was between 3and 5 · 10)7, making the contribution of 60Fe to heating<0.5 kJ g)1. Moreover, as 60Fe’s half-life of 2.6 Myr(Rugel et al. 2009) is more than three times longer thanthat of 26Al (0.72 Myr), its contribution to overallheating during the critical first 2 Myr would have beentrivial, and we therefore ignore it in this paper.

Figure 1a shows the temporal decline in energystored as 26Al in each gram of dry primitive dust,starting from the initial 6.6 kJ g)1, through almost sevenhalf-lives during the first 5 Myr. To put this decline in

perspective, 6.6 kJ g)1 is about four times larger than the1.6 kJ g)1 needed to fully melt the insulated interior of aplanetesimal at a temperature of 1850 K. The estimate of1.6 kJ g)1 assumes starting from cold (250 K), withspecific heat capacity, Cp = 837 J kg)1 K)1, and latentheat of fusion = 2.56 · 105 J kg)1 (Hevey and Sanders2006). Thus, planetesimals that accreted during the firsttwo half-lives of 26Al, or roughly during the first1.5 Myr, would have had the potential to becomecompletely molten in their fully insulated interiors.

Figure 1b shows the time it would have taken to reachthe onset of melting (the solidus temperature,approximately 1425 K) and also the completion of melting(the liquidus temperature, approximately 1850 K) of thefully insulated interior (i.e., with zero heat loss) as afunction of the time of cold planetesimal accretion,

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Fig. 1. a) Exponential decline with time of the potentialthermal energy stored as 26Al in a gram of ‘‘dry’’ primitivedust. b) Time at which solidus (1425 K) and liquidus (1850 K)temperatures are reached in the fully insulated interior (deeperthan approximately 5 km at t = 1 Myr to deeper thanapproximately 20 km at t approximately 5 Myr—see Figs. 2and 4) of a planetesimal as a function of the time of its cold(250 K) instantaneous accretion and assuming no meltmigration during heating. Arrows A, B, C, and D illustrate thetiming of initial and total melting following cold accretion att = 0, 0.75, 1.4, and approximately 2 Myr, respectively (seetext for explanation). Accretion-time intervals labeled 1, 2, 3,and 4 relate to the fields shown in Fig. 4. The lower edge of thegray zone is the 1850 K liquidus calculated using latent heatand specific heat capacity values from Ghosh and McSween(1999), which are greater than those adopted here.


assuming no migration of the 26Al heat source. Withaccretion at t = 0 (arrow ‘‘A’’ in Fig. 1b), heating wouldhave been rapid and the liquidus would have been reachedby about t = 0.3 Myr. This is in good agreement with thetiming of earliest planetesimal melting and core formation,shown by 182W-deficit dating of ironmeteorites (Burkhardtet al. 2008, 2012; Kruijer et al. 2011) and, although errorsin the dating are large, such early melting clearly endorsesthe assumption that 26Alwas the heat source.

With accretion at t = 0.75 Myr (arrow ‘‘B’’), theinitial heating rate would have been half that for arrow‘‘A,’’ but rapid enough for total internal melting to havebeen achieved by t = 1.5 Myr.

As a third example, with accretion at tapproximately 1.5 Myr (arrow ‘‘C’’), the insulatedinterior of a planetesimal would have carried just enough26Al to reach the liquidus, but melting would not havebeen completed until after t approximately 5 Myr. Thisexample may explain the paucity of chondrules that datefrom before t approximately 1.5 Myr (Kita andUshikubo 2012). Assuming that chondrules (regardlessof their formation mechanism) were produced in largenumbers before t approximately 1.5 Myr, the scarcity ofthose old chondrules in meteorites must reflect their poorsurvival rate. We imagine that such chondrules accretedto planetesimals before t approximately 1.5 Myr andbecame buried in their insulated interiors where theywould later have been melted down and destroyed. If thisexplanation is correct, then it implies that these pre-1.5 Myr chondrules, once made, did not linger in space,but accreted to planetesimals promptly and were thencedestined for a magmatic grave.

Finally, with accretion after about t approximately2 Myr (arrow ‘‘D’’), the level of 26Al would have beentoo low to have heated the planetesimal’s interior to thesolidus, so no melting at all would have taken place. Thetiming is consistent with the evidence that chondrites(which of course did not melt) accreted after about tapproximately 2.5 Myr (by when chondrule productionwas in decline), and again corroborates the view that26Al was the main heat source within planetesimals.

In summary, the timing of core formation beforet = 1 Myr, the scarcity of chondrules made before t = 1.5Myr, and the accretion of chondrites after t = 2.5 Myr,combine to uphold our conviction that planetesimalheating by 26Al was a key factor in the evolution ofplanetesimals in the infant solar system.

The Structure of Molten Planetesimals

To visualize the changing internal structure of amolten planetesimal, we use the results of Hevey andSanders (2006) who presented simulations of the heating,melting, and cooling of initially cold, porous

planetesimals that accreted instantaneously. Their modelis based on a radiogenic heat budget of 6.4 kJ g)1 of dustat t = 0, which is very close to the value of 6.6 kJ g)1 weestimate here. We note that they used an incorrect 26Aldecay energy (4 MeV). That value wrongly includesapproximately 1 MeV of energy that is not deposited asheat, but is lost in neutrinos. However, the error wasfortuitously compensated by a lower concentration of Al(0.9 wt.%) compared with the 1.2 wt.% we use here.

As an example of their results, Fig. 2a illustrates thechanging temperature profile within a planetesimal thataccreted cold (250 K) at t = 0 and had a radius of 50 km(after early sintering and shrinkage). After a little over0.3 Myr of heating, with the 26Al heat source evenlydistributed, the interior deeper than approximately 5 kmwould have become uniformly hot and 50% molten(approximately 1725 K). At this stage, the interior isassumed to have lost rigidity and become cohesionlessmagmatic slurry undergoing turbulent thermalconvection. With continued intense heating beyond0.3 Myr, the magma is assumed to have remained at sub-liquidus temperatures, but to have increased in volume asthe overlying rigid carapace was melted upward from itsbase and its thickness reduced from approximately 5 kmat t = 0.3 Myr to just 0.5 km by t = 0.5 Myr (Fig. 2b).By that time, the rate of conductive heat loss through theresidual crust would have reached a maximum, equalingthe rate of internal heat production, and the crust’sthickness would have been at a minimum. Thereafter, withheat production lower than heat loss, no further meltingwould have occurred, and the crust would have thickened,slowly at first but ever more rapidly, over the next 2 or3 Myr and beyond. A cartoon of the state of theplanetesimal at t = 2 Myr is shown in Fig. 3.

The model predicts that during maximum heat loss,only about 2 m of porous, unconsolidated, andextremely insulating dust separated solid, sintered rockat 700 K from the surface at 250 K. However, thisprediction assumes that all accretion was completedinstantaneously at t = 0. In reality, at least someaccretion would have continued after the initialaggregation of material. Beyond about t approximately2 Myr, with the 26Al heat source fading, any such lateaccretion would not have melted, but accumulated as acoating of loose, or weakly consolidated, dusty debris,which could have attained a considerable thickness.

What if the radius had been much smaller than the50 km chosen in Fig. 2? Hevey (2001) showed that a bodywith a 20 km radius would have melted substantially byt = 0.3 Myr, and its crust would have thinned down to aminimum of 1.5 km by t = 0.5 Myr, but its high surface-to-volume ratio would then have led to rapid cooling,and the body would have been largely solid by t = 3 Myr.A body 10 km in radius would scarcely have melted at all.


In a comparable thermal model, Moskovitz and Gaidos(2011) predicted similar melting behavior.

What if the radius had been larger? Going up in size,if the radius had been doubled, and was 100 km instead50 km, the ratio of internal heat production to surfacearea would also have doubled, giving twice the heat flow

and halving the thickness of insulating crust to a mere250 m during the period of peak heat loss betweent = 0.5 and t = 1.5 Myr. In this case, the crust mayperhaps easily have foundered, exposing incandescentmagma at the surface. With a still larger radius, theinsulating carapace would have become even thinner andeven more susceptible to foundering.

Figure 4 shows the effects of a planetesimal’s radiuson its predicted melting behavior combined with theeffects of the timing of its accretion discussed above(Fig. 1b). Four fields, numbered (1) to (4), correspond tothe four accretion time intervals shown in Fig. 1b.Planetesimals starting in field (1) would have becomeextensively molten before t = 1.5 Myr, with very thincrusts as depicted in Figs. 2 and 3. Such planetesimals, wesuggest below, would have been potential sources ofchondrules by impact splashing. Planetesimals starting infield (2) would also have undergone extensive internalmelting, but beneath a thicker insulating crust than forfield (1), and with a longer heating period, generallybecoming molten from t approximately 1.5 Myr up to tapproximately 5 Myr depending on the time of accretion.Planetesimals starting in field (3) would have melted onlypartially, and they probably would have remained rigid. Itis possible that the melt fraction (basalt magma) wouldhave migrated upward, and that these planetesimalsincluded the parent asteroids for primitive achondrites likethe lodranites and ureilites. Planetesimals from field (4)would never have melted, although they may have become

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Fig. 2. a) Temperature profiles at selected times inside a planetesimal with a 50 km radius and zero porosity that accreted at t = 0and a temperature of 250 K and was heated by 26Al decay. Broken lines are profiles during heating (until 0.5 Myr) and continuouslines are profiles during cooling. Convection began soon after t = 0.3 Myr, and by t = 0.5 Myr the molten, convecting interiorhad expanded to within about 0.5 km of the surface (after Hevey and Sanders 2006). b) Depth of solid rock and crust (gray) forthe same body as a function of time.

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Fig. 3. Cartoon showing the internal structure of the 50 kmradius planetesimal exemplified in Fig. 2 at t = 2 Myr. Thecore is assumed to be fully formed, but it is possible that smalldroplets of metal may have been held in suspension byturbulent convection (symbolized by curved arrows) in themagma ocean. The base of the crust is arbitrarily taken as thelevel at which the interior is 50% molten; the lower crust, withless than 50% melting, is deemed to be rigid. The 2 m of dustshown on the surface is predicted by instantaneous accretion; inreality, continuous accretion probably led to a considerablethickness of cool, loose, dusty debris, particularly after aboutt = 2 Myr when 26Al heating was very weak.


heated and metamorphosed. They would have becomechondrite parent bodies. Figure 4 can be regarded as arefinement of the related, but oversimplified and rathermisleading, two-field diagram presented by Hevey andSanders (2006, fig. 6) in which planetesimals were showneither to have melted or not melted. It also bearssimilarities to fig. 5 of Moskovitz and Gaidos (2011),although the latter has later accretion times for givenoutcomes because it assumes 4 MeV, and not 3.1 MeV, asthe decay energy per atom of 26Al.

The thermal model of Hevey and Sanders (2006)assumes that the internal 26Al heat source remains evenlydistributed at all stages during heating and cooling.However, some authors question this assumption, arguingthat basalt migrates rapidly upward as soon as it isgenerated (e.g., Moskovitz and Gaidos 2011). As nearly allaluminum enters the basaltic melt fraction, the removal ofsuch melt would also remove the heat source, forestallfurther internal heating, and invalidate the pattern ofmelting shown in Fig. 2. Wilson and Goodrich (2012) evensuggested that basalt migration away from the zone ofpartial melting was so rapid that ‘‘high degrees of mantlemelting never occurred in any asteroids.’’

While we acknowledge that basalt was removed inthe case of the ureilite parent body, which we believe tohave accreted late, in field 3 of Fig. 4, we cannot acceptthat basalt migrated from its source in all cases ofplanetesimal melting. Global magma oceans and very

high melt fractions almost certainly did develop withinsome young planetesimals. They were inferred by Tayloret al. (1993) who argued that the IVB iron meteoritescrystallized from liquid iron that contained <1 wt.%sulfur (Goldstein et al. 2009) at >1770 K, a temperatureat which primitive silicate material would have been wellover 50% molten. Taylor et al. (1993) also noted that, ifpallasite olivine represents unmelted residue, then thesilicate melt fraction must have been between 70% and90%. Keil et al. (1989) argued that the unusual textureof the Shallowater aubrite indicates a molten enstatitemagma ocean at 1850 K. So while the Hevey andSanders model is necessarily simplified, the evidence formagma oceans with high melt fractions suggests that themodel is not wildly wrong. The issue of precisely howglobal magma oceans were created is a matter for futureinvestigation; for now, we merely speculate that themechanism may possibly have been linked to gradualaccretion with the newly added material at an early stage(e.g., t < 1.5 Myr) continually ‘‘dissolving’’ in anyhighly radioactive rising basalt magma (see Kleine et al.2012), or it was perhaps linked to the onset of convectionbefore significant melting had occurred, facilitated by apossible substantial reduction in bulk viscosity (Schollingand Breuer 2009). Regardless of the details of themelting mechanism, we suspect that substantially molteninteriors were the norm rather than the exception inplanetesimals that accreted within field (1) of Fig. 4.

The Prevalence of Early Molten Planetesimals

We suggest that early accretion leading to extensivemelting and core formation before t approximately1.5 Myr was widespread in the protoplanetary disk.Between 100 and 150 separate parent bodies appear to berepresented by the meteorites in the world’s collections,one body for each meteorite group plus many morerepresented by ungrouped meteorites (Meibom and Clark1999; Burbine et al. 2002). Only about one in five of theseinferred bodies is chondritic. The rest of them, some fourof every five, melted and became differentiated bodies. Ifthe low e182W measured in magmatic iron meteorites isrepresentative of the timing of melting in general, then it isconceivable, indeed likely, that most of the material in theinner solar system existed as molten, or partially molten,planetesimals by the close of the first 1.5 Myr.

THE PRODUCTION OF CHONDRULES

The Case for Making Chondrules from Molten

Planetesimals

The above discussion points to a young solar nebulapopulated by largely molten spheres of primitive magma

Pla

nete

sim

al r

adiu

s (

km)

20

40

60

80

100

00 0.5 1 1.5 2 2.5

Time of accretion (Myr after CAIs)

interior will meltcompletely

after t = 1.5 Myrunder thicker

crust

interior will meltcompletely

before t = 1.5 Myrunder thin crust

interior will partially melt

aftert = 2.5 Myr

interior willremain solid

1 2 3 4

Fig. 4. Plot showing the eventual outcome of heating by 26Aldecay in planetesimals as a function of radius and time ofinstantaneous cold accretion. The boundaries that delineate thefour different fields are interpolated from Hevey and Sanders(2006, fig. 6) and Fig. 1b. Field (1) delimits planetesimals thatwill become substantially molten beneath a thin (e.g., <1 km)insulating crust before t approximately 1.5 Myr. Suchplanetesimals, we argue, will be prime candidates for burstinginto chondrule spray if disrupted by impact betweent approximately 1.5 and t approximately 2.5 Myr.


undergoing turbulent convection at near-liquidustemperatures, each enclosed by a thin outer shell of rigid,thermally conducting crust, and coated by a layer ofunconsolidated dust (Fig. 3). Molten metal possiblyformed suspended globules that would eventuallysegregate as cores. As, over time, these planetesimalsmust have been increasing in size and decreasing innumber as a result of mutual collisions and mergers (thenecessary early steps along the stochastic road to planetformation), we deduce that many of the collisions wouldhave launched huge plumes of molten droplets(chondrule spray), mixed with loose dust from theplanetesimal surfaces, into the disk. Incidentally, whilesuch collision plumes have commonly been described inthe literature as a ‘‘planetary’’ setting for chondruleformation, this is misleading. A plume might moreappropriately be regarded as a special case of a nebularsetting, albeit a rather local and ephemeral one.

This scenario for chondrule formation, which is aclear alternative to the conventional shock-melting ofdust-clumps, is the main subject of this article and wewill now explore it in detail, amplifying and developingthe case for it made recently by Asphaug et al. (2011).These authors noted that most collisions would havebeen oblique glancing blows in which much of the‘‘overlapping’’ portion of the smaller body (theprojectile) would have been engulfed by the larger body(the target), while the ‘‘overshooting’’ portion wouldhave undergone catastrophic decompression andexpanded slowly down-range as a huge fan-shapedplume of droplets and debris.

Asphaug et al. (2011) emphasized that encountervelocities between merging planetesimals at the time ofchondrule formation would generally have been close to,or less than, the combined escape velocity of the mergingpair, namely tens of meters to a few hundred meters persecond (the escape velocity in meters per second isnumerically about the same as the planetesimal’s radiusin kilometers). At these very low velocities, no shockmelting would have occurred; almost the entire enthalpyfor melting would already have been generated by thedecay of 26Al and stored as magma in the planetesimals.The kinetic energy of impact would have causedmechanical disruption, but would have been negligiblecompared with 26Al decay as a cause of melting.

The splashing model is, thus, quite different from, andmust not be confused with, the production of impact meltspherules by energetic, high-velocity collisions. Such meltdroplets include crystal-bearing lunar spherules, evidentlymade during excavation of the huge impact basins on theMoon (Symes et al. 1998; Ruzicka et al. 2000), andterrestrial impact spherules of which those from theEltanin site include beautiful coalesced droplets similar tocompound chondrules (Kyte et al. 2010). They could also

include the chondrules in the CB chondrites, believed to becondensates from a giant impact plume about 4 Myr afterCAIs (Rubin et al. 2003; Krot et al. 2005).

We consider in the following paragraphs how thepredictions of the ‘‘splashing’’ scenario might bereconciled with what is known or can be inferred aboutthe formation of chondrules. We initially reflect on thefollowing properties of chondrules: chondrule ages;chemical compositions, ‘‘peak’’ temperatures andabundances; chondrule sizes; mutually indented shapes;cooling rates; relict grains; igneous rims; and metalinclusions.

Chondrule Ages Are Mostly 1.5–2.5 Myr After CAIsA strength of the splashing model is that it can

explain why chondrules are mostly between 1.5 and2.5 Myr younger than CAIs (Connelly et al. 2008; Kleineet al. 2008; Kita and Ushikubo 2012). 26Al-inducedplanetesimal meltdown means that splash-generatedchondrules would have been made from as soon as thefirst planetesimals had melted, i.e., from aboutt = 0.3 Myr onward. However, as explained above, wesuggest that most chondrules made before t = 1.5 Myrwere destroyed because they accreted promptly andbecame buried inside planetesimals that were still highlyradioactive and destined to melt.

Those few chondrules that survive from beforet = 1.5 Myr, such as some very old ones in Allende(Connelly et al. 2011), may fortuitously have becomeembedded in bodies that were too small to retain heat, orembedded in the cool outer layers of larger bodies(LaTourrette and Wasserburg 1998), or perhaps inbodies where low temperatures were buffered by a largeamount of ice. As these kinds of low-temperature settingwould perhaps also have been necessary for thepreservation of CAIs, which date from t = 0, it may beno coincidence that the oldest chondrules appear tosurvive in meteorites (i.e., CV chondrites) with a largeabundance of CAIs.

Why would chondrule production have declinedrapidly after about t = 2.5 Myr? Chondrule productionby splashing would have continued for as long as moltenplanetesimals with thin crusts were colliding andmerging. By t approximately 2.5 Myr, heat loss byconduction would have far exceeded internal heatgeneration by 26Al decay, and crusts would probablyhave been growing thicker, cooler, and mechanicallystronger as their underlying magma oceans crystallized(Fig. 2b). In addition, following many collisions and theproduction of a correspondingly large volume ofchondrule-rich debris, thick accumulations of this debrismay have built up on the surfaces of remaining intactplanetesimals, rendering those planetesimals lesssusceptible to bursting open on impact. In this light, the


duration of chondrule formation between t = 1.5 Myrand t = 2.5 Myr would appear to coincide with a periodof transition from a disk populated mainly by moltenbodies of various sizes, to a disk with fewer, largerbodies (planetary embryos, perhaps) alongside a suite ofnewly accreted ‘‘second generation’’ chondritic bodies.

We note that chondrule production did not endabruptly at t = 2.5 Myr; many CR chondrules have agesof around 3 Myr after CAIs (Kita and Ushikubo 2012),based on their very low initial 26Al/27Al ratios, and theirPb-Pb ages.

Chondrules Have ‘‘Primitive’’ Chemistry, They CooledFrom Near-Liquidus Temperatures, and They AreAbundant

These three classic features of chondrules are clearlycompatible with the splashing model. The essentiallyunfractionated ‘‘primitive’’ chemistry of most chondrules,including their flat rare-earth element profiles (Joneset al. 2005), is consistent with the high degree of meltingwe envisage in molten planetesimal interiors prior to theirdisruption (Fig. 3). The high subliquidus temperaturesfrom which most chondrules cooled (inferred fromtheir igneous textures) is consistent with their derivationfrom hot convecting magma oceans where thetemperature was buffered just below the liquidus bysteady radioactive heating off-set by efficient convectiveand conductive heat loss. The high volume fraction ofchondrules, amounting to 80% or more by volume insome ordinary chondrites, is seen as a simple consequenceof the dominance of the molten planetesimals thatsupplied those chondrules.

Chondrule Sizes Are Mostly in the Range 0.1–2 mmAcross

The sizes of most chondrules are consistent withdroplet sizes expected from the spraying or spattering oflarger volumes of magma. They fall in the rangeobserved widely in naturally formed droplets of magmasuch as those, known as Pele’s tears, produced in basaltlava fountains at Kilauea volcano on Hawai’i. Otherexamples include the famous orange glass spherules atthe Apollo 17 site on the Moon (also presumed to haveformed in a lava fountain), and droplets produced byenergetic impacts and shock melting, such as the crystal-bearing lunar spherules, and terrestrial impact-meltspherules mentioned above. Also, chondrule-sizeddroplets were produced experimentally by the spatteringof melt in a solar furnace (King 1983). Finally, in theirmodel, Asphaug et al. (2011) showed that chondrule-sized droplets result from equating the total surfaceenergy of droplets in the expanding plume with theenergy associated with the catastrophic disruption anddecompression of the interior of the molten planetesimal.

The Inferred Time for Cooling to the Solidus WasTypically Several Hours

The successful experimental replication of chondruletextures under controlled rates of cooling shows thatnear-liquidus droplets probably cooled and solidifiedover a matter of hours, and not seconds, nor days(Lofgren 1989; Hewins and Radomsky 1990; Radomskyand Hewins 1990). This timescale was also inferred froma study of zoning in metal grains in CR chondrites(Humayun 2012), and it is in broad agreement with thekind of time needed for an impact plume to expand andcool (Asphaug et al. 2011).

Chondrules in Some Meteorites Have MutuallyIndented Shapes

Hutchison and Bevan (1983) noted that somechondrules in Tieschitz (H3) are apparently moldedagainst neighboring chondrules, indicating that they werestill hot and plastic, if not liquid, when they came intocontact. Holmen and Wood (1986) reported similartextures in other chondrites. This remarkable feature wasalso described by others (Sanders and Hill 1994;Hutchison 1996; Zanda 2004), but it has received onlylimited attention, perhaps because of arguments that thedeformed shapes could have resulted from thecompaction of chondrules into voids by shock (Sneydet al. 1988; Scott et al. 1992).

Recently, Metzler (2011, 2012) described spectacularexamples of unshocked, mutually molded chondrules inlarge clusters, which he called ‘‘cluster chondrites.’’ Theyoccur as clasts up to 10 cm across in ordinary chondritebreccias. He noted ‘‘this rock type consists of a mixtureof deformed and undeformed chondrules and ischaracterized by low abundances of inter-chondrulematrix, low abundances of distinct chondrule fragments,and restricted variations of chondrule sizes.’’ Hewinset al. (2012) have since reported similar clustering ofchondrules in Semarkona.

Cluster chondrites are consistent with the splashingmodel. They suggest that molten chondrules in denseswarms, as would be expected in an impact plume,aggregated either into ‘‘sticky clusters’’ at least 10 cmacross, like giant compound chondrules made frommany thousands of individuals, or they accreted rapidlyto the surface of the target body, forming a blanket ofmolded chondrule rock in the manner envisaged byAsphaug et al. (2011). In the latter case, if the targetbody were to have become the projectile in a latercollision, then fragments of this welded chondruleblanket would have been launched and may eventuallyhave come to reside in the kind of chondritic brecciadescribed by Metzler. The accretion of hot chondrulesdirectly onto a planetesimal surface is suggested by theparallel orientation of flattened molded chondrules seen,


for example, in the Bovedy (L3) chondrite (Sanders andHill 1994).

The good size sorting observed by Metzler issignificant. Size sorting has been observed widely inchondrites, but its origin remains unclear. In the case ofcluster chondrites, it appears that the sorting happenedlocally, and very rapidly, during the brief interval betweenthe formation and accretion of a batch of chondrules.

Many Chondrules Contain So-Called Relict Grains(Xenocrysts)

Xenocrysts (foreign crystals) within a chondrule arerecognized because they are chemically out of equilibriumwith other crystals in the same chondrule and may alsohave anomalous oxygen isotopic compositions (Joneset al. 2005; Ushikubo et al. 2012). They were first reportedby Nagahara (1981) and Rambaldi (1981). Nagaharanoted that these peculiar crystals imply that the chondrulehost did not condense from a cooling vapor, so must havebeen melted, and thus xenocrysts became dubbed ‘‘relictgrains’’ and were widely assumed to provide evidence forprecursor dust-clumps.

Connolly and Hewins (1995) showed experimentallythat chondrule liquids tend to wet and swallow up dustgrains that impinge on them. We suggest, therefore, thatxenocrysts were trapped and engulfed by melt droplets inflight, having been launched into the impact plume fromthe loose surficial regolith. In this context, perhapssmaller dust particles in the plume may similarly havebeen engulfed, and so become the seed crystals deemednecessary for the development of porphyritic textures inchondrules, as such seed crystals may not have beenubiquitous in the convecting magma prior to collision.

Some Chondrules Appear to Have Been Melted MoreThan Once

Textural evidence in chondrules for ‘‘multiple re-heating’’ events, such as igneous rims, has been widelyreported (e.g., Rubin and Krot 1996). In the context ofcollision plumes, much of the re-launched loose debriswill have been derived from older chondrules. Thus,chondrules which bear evidence of having been througha few distinct heating and melting phases, may simplyhave been caught up in collision plumes more than once.Their coarse-grained rims may either be the envelopingmantles of compound chondrules, or simply be the resultof heating of former dusty rims while suspended withinthe incandescent plume. Also, it is possible that, withturbulent motion in the expanding plume, an individualchondrule may have moved from a hot to a cooler regionand back again. Incidentally, we note that crystal-bearing lunar spherules, which originated in impactplumes, in some cases also show chondrule-like featuresthat suggest re-heating (Ruzicka et al. 2000).

Some Chondrules Contain Blebs of MetalSmall blebs of metal inside chondrules (e.g., Wasson

and Rubin 2010) may have origins that are consistentwith the splashing model. Tiny globules of metal mayhave been kept in suspension in the magma ocean byturbulent convection (Fig. 3). Some of these globules ofmetal may have rained down late into the magma oceanas the overlying crust, including any late-accretedmaterial, was melted from below and thinned down(Fig. 2). Another possible explanation for metal insidechondrules is that miniscule droplets of metal spray maysimply have become engulfed, rather like xenocrysts,within silicate droplets in the expanding impact plume.

Problems with Making Chondrules from Clumps of Dust

We now consider a number of additional features ofchondrules that appear to pose difficulties for theconventional view that chondrules began as clumps ofdust. We discuss sodium in chondrule olivine, oxidizediron in chondrules, chondrule diversity, abnormally largecrystals in chondrules, giant chondrules, the scarcity ofdust-clumps, and the cause of melting. In all cases, weshow that the features can be explained in the context ofcollision and splashing.

Sodium Has Surprisingly High Concentrations inOlivine Crystals Within Chondrules

Alexander et al. (2008) reported high concentrationsof Na in chondrule olivine which, together with an absenceof isotopically mass fractionated isotopes of alkalis, pointto high gas pressures and very closely spaced chondrules(high chondrule densities) in enormous clouds measuringhundreds to several thousand kilometers across. Theyinferred that these large chondrule clouds were generatedby shock-melting of equally large clouds of precursordust-clumps that would quickly have collapsed under theirown gravity and become new planetesimals.

We question the feasibility of their scenario.Conventional dust-heating models do not explain hownebular processes could have concentrated the dust-clumps to the required extent, nor how nebular shockheating, or any other external heat source, could haveheated and melted such a huge mass of dust (equivalentto an entire planetesimal) in what was effectively a singleevent. In addition, the energy would need to have beendelivered at just the right moment—after the cloud hadbecome gravitationally unstable, but before its collapsewas completed. Finally, it is difficult to see how coldprimitive matrix dust could have been mixed in with thehot chondrules prior to accretion.

By contrast, a molten planetesimal collision wouldhave created an enormous (planetesimal-scale) transientdense cloud of droplets. The droplets would have been


immersed in their own Na-saturated vapor and thereforecapable of retaining high Na concentrations duringolivine crystallization, at least during the early stages ofplume growth. Dispersal of droplets after solidificationin the cooling, expanding cloud would have allowedmixing of chondrules from different impacts andaddition of primordial dust containing presolar grains.The dust could have come directly from the nebula or itcould have been derived as recycled dust from the poorlyconsolidated regolith of the disrupted planetesimals.

In a parallel study to that of Alexander et al. (2008),Hewins et al. (2012) reported similarly high levels of Nain chondrule olivine, and also reported Na in meltinclusions in the olivine, in the mesostasis, and in bulkchondrules, all in Semarkona (LL 3.0). They inferredpartial evaporative loss of Na from the moltenchondrules, followed by its later re-condensation and itsincorporation in chondrule mesostases. UnlikeAlexander et al. (2008), they were not committed to theidea of conventional shock melting, and suggested as analternative that chondrules may have formed in ‘‘debrisclouds after protoplanetary collisions,’’ but they did notelaborate on how the melt droplets formed.

Chondrules in enstatite chondrites are not only Na-rich but contain evidence for sulfidation of silicates andmetal-sulfide nodules (e.g., Rubin 1983). Lehner et al.(2011) and Petaev et al. (2012) propose that ordinaryferromagnesian chondrules reacted with an S-rich and H-poor gas above 1000�C.Such conditions,we suggest,wouldhave been more easily created within an impact-generatedplume rather thanwithin the conventional solar nebula.

Chondrules Contain Iron as FeOType II chondrules contain significant levels of FeO.

Even type I chondrules are rarely completely free of it.FeO poses a serious problem for the production ofchondrules from dust-clumps in the nebula because thestabilization of FeO in chondrule melts requires ambientgas that is several orders of magnitude more oxidizingthan the standard hydrogen-rich nebula. In an ongoingeffort to resolve this conundrum, Fedkin and Grossman(2006, 2010), Fedkin et al. (2012), and Grossman et al.(2012) reaffirmed the view of Alexander et al. (2008) thatchondrules were created in close proximity to each other(to retard evaporation of volatile elements like Na), butalso concluded that chondrules were enveloped byoxidizing, H2O-bearing, gas to stabilize FeO. Theytentatively suggested that such a setting might have existedin the aftermath of collisions involving icy planetesimals.This setting for chondrule formation is difficult toreconcile with the shock melting of dust-clumps becausethe timing of the heating event would need to havecoincided precisely with the brief, ephemeral existence ofthe H2O-enriched collision plume.

On the other hand, the production of type IIchondrules by collision and splashing of moltenplanetesimals poses no problems, provided of course thatthe planetesimals were made of FeO-bearing magma. Wereturn to this issue in the final section of the article.

Chondrules Are Chemically DiverseWhile chondrules have primitive chemistry in a broad

sense, they are not identical, and they vary particularly intheir contents of olivine and pyroxene. Their chemicaldiversity presents problems for the dust-clump hypothesis.A millimeter-sized chondrule precursor clump mighttypically have contained between 106 and 109 dust grainsbetween 1 and 10 microns across. In a well-mixed disk,therefore, all dust-clumps might be expected to havesampled much the same statistically representativeselection of available grains and so shared the sameprimitive chemical composition. Thus, olivine-rich andpyroxene-rich chondrules are very unlikely to have formedfrom clumps of micrometer-sized dust grains becausenebular processes cannot conceivably have sorted the dustinto millimeter-sized monomineralic aggregates.

We suggest, in the context of the splashing model,that magmatic processes in molten planetesimals prior tocollision could have generated liquids ranging fromolivine-rich to pyroxene-rich. With high degrees ofmelting, olivine would have been the sole liquidus phase.Olivine has an atomic Si ⁄Mg ratio of 0.5, which is lessthan solar (0.9), so its removal by crystal settling from aprimitive unfractionated magma ocean would havedriven the residual melt toward pyroxenitic compositionsfor which the Si ⁄Mg ratio is 1.

Mostefaoui et al. (2002) observed that olivine-richchondrules (lower Si ⁄Mg) were made, in general, earlierthan pyroxene-rich chondrules (higher Si ⁄Mg). Tachibanaet al. (2003) and Kita et al. (2005) confirmed thiscorrelation between Si ⁄Mg and age with more precisemeasurements and an extended data set. Tachibana et al.(2003) attempted to explain the correlation in terms ofconventional flash-melting of dust balls in an open systemover a number of cycles. With each cycle, differentialevaporation of Si relative to less-volatile Mg meant thatsuccessive generations of dust-grain precursors becameprogressively enriched in condensates with higher Si ⁄Mg.

While we agree that evaporation and condensationprocesses in the nebula may have been important, wecannot see how, in physical terms, the chondrules (withlow Si ⁄Mg) were removed from the system followingeach cycle. We prefer to explain the correlation in termsof olivine crystal fractionation. We suggest that overtime the Si ⁄Mg ratio of the magma oceans in manyplanetesimals increased ‘‘in step’’ at roughly the samerate, and so did the Si ⁄Mg of the chondrules made fromthose magma oceans by collision and splashing.


In support of this explanation, we draw attention toevidence in differentiated meteorites that magmaticfractionation driven by crystal settling probably did occurduring the first few million years. Baker et al. (2012)reported a deficit in d26Mg in main group pallasites thatcorresponds to separation of crystals from liquid at around1 Myr after CAIs. A similar result was obtained from theEagle Station pallasite by Villeneuve et al. (2011). On theHED, parent body crystal fractionation, albeit ofpyroxene rather than olivine, led to igneous differentiationover 1 or 2 Myr, reflected in a steady increase in d26Mggoing from the most primitive diogenites to cumulateeucrites (Schiller et al. 2011).

Some Chondrules Contain Abnormally Large andOscillatory-Zoned Crystals

Some single olivine crystals in chondrules areunusually large and may occupy >90 vol.% of thechondrule. They seem unlikely to have crystallized fromchondrule-sized melt droplets and more probablyoriginated in much larger volumes of melt. Largerounded olivine grains that occupy almost all thechondrule (e.g., fig. 1e in Jones et al. 2000) are especiallydifficult to form by conventional mechanisms involvingdust-clumps. However, rounded crystals may form inconvecting magma under conditions that periodicallyrequire dissolution as well as growth. In some terrestrialsettings, phenocrysts develop rounded and embayedshapes, apparently for this reason.

Olivine grains are also found as large (<1 mm)isolated single crystals in chondrite matrices and theirorigin has not been satisfactorily explained. Steele (1989)and Weinbruch et al. (2000) argued that they are nebularcondensates, but Jones et al. (2000) found traces of low-Capyroxene and mesostasis attached to large rounded forsteritecrystals, and inferred that they formed in chondrules. Wesuggest that the large isolated crystals in the matrix, like thosein chondrules, were in suspension in asteroidal magmaoceans at the time of collision and disruption.

Oscillatory chemical zoning is present in some largecrystals of olivine and pyroxene in both chondrules andchondrite matrices (Steele 1995; Jones and Carey 2006;McCanta et al. 2009; Blinova et al. 2011). Oscillatoryzoning in some elements such as phosphorus (McCantaet al. 2009) may result from disequilibrium crystallization,but oscillatory zoning in Fe andMg (Blinova et al. 2011) ishard to explain by crystal growth in a droplet, and mayperhaps be attributed to variations in the environment of agrowing crystal that was suspended in convecting magma(Shore and Fowler 1996).

Macrochondrules ExistSome chondrules, called macrochondrules or mega-

chondrules, are more than one centimeter across, yet

have compositions and textures like those of normalmillimeter-sized chondrules (Binns 1967; Prinz et al.1988; Hill 1993; Bridges and Hutchison 1997; Ruzickaet al. 1998). Macrochondrules would be difficult to formby a nebular flash-melting process because of the need totransfer heat rapidly to the center of what wouldpresumably have been a large (e.g., golf-ball sized),porous, and thermally insulating dust-clump withoutglazing or vaporizing the outside first. Also, as the rise intemperature would generally be proportional to the ratioof the surface area to the mass of a dust-clump, then aflash-melting event that delivers just the right amount ofenergy to make millimeter-sized chondrules would barelyaffect the temperature of a centimeter-sized clump. Weprefer to interpret macrochondrules simply as large blobsof magma that failed to be shaken and dispersed intonormal-sized chondrule droplets in the collision plume.

Dust-Clumps Are Rarely Observed in ChondritesIf chondrules had formed from precursor dust-

clumps, we might expect to find small lumps of suchmaterial in the interchondrule matrix. Although there isevidence for them in some meteorites (Rubin 2011, pp.553 and 554), millimeter-sized aggregates that mightrepresent plausible precursor dust-clumps are reallyrather rare (e.g., Scott and Krot 2007). Moreover, matrixis typically more FeO-rich than chondrules, so it is aninappropriate starting material.

Some authors have reported dust-rich chondruleswhich they interpret as incipiently melted products ofnebular flash-heating (Nettles et al. 2001; Ruzicka et al.2012a). Certainly, these objects display clear evidence oflow levels of intergrain melt, but we suggest that they arenot necessarily products of dust-clump melting. We canenvisage three possible ways that they might have beengenerated in the collision and splashing scenario. First,they may be fragments broken from the incipientlymelted lower rigid crust (Fig. 3) of the disruptedprojectile. Second, they may be some sort of weldedaccretionary lapilli that grew in the impact plume ofdroplets and dust; this is how we would interpretchondrule Beg-6 reported by Ruzicka et al. (2012a).Third, they may be melt droplets that became choked inflight with dust grains, which is perhaps just a specialcase of the second example. In both the second and thirdexamples, the principal source of dust would have beenthe loose accumulation of regolith covering the collidingplanetesimals.

A Plausible Heat Source for Melting Dust-ClumpsRemains Elusive

The identification of a heat source capable ofmelting nebular dust-clumps on the required scale hasproved to be somewhat intractable. The currently


favored nebular shock-melting model (Desch andConnolly 2002; Boss and Durisen 2005; Morris andDesch 2010) claims to provide an appropriate thermalhistory for the inferred chondrule cooling rates, andMorris et al. (2012) similarly suggest that bow shocksaround planetary embryos may provide a plausiblethermal regime for making chondrules. Nevertheless, asdiscussed here, shock melting is difficult to reconcile withsuch observations as chondrule diversity, macrochondrules,and the retention of Na and FeO in chondrules. Also,shock melting should lead to significant isotopicfractionation effects, but these are not observed. Forexample, Fedkin et al. (2012) calculated that at enormouslyinflated PH2O (·550, to maintain iron as FeO rather thanmetal) and with huge dust enrichment (·600, to retardevaporation), all iron would evaporate during shock-waveheating, that olivine would grow in the unevaporated Mg-rich melt droplets before re-condensation, and largeisotopic fractionation effects between olivine and glasswould be preserved. As such isotopic fractionation is notseen, they concluded that chondrules were unlikely tohave formed in nebular shocks.

Turning briefly to other postulated sources of rapidnebular heating, the once fashionable X-wind model(Shu et al. 1997, 2001) in which proximity to theprotosun is linked to melting, seems beset withinsurmountable problems and has now been largelydiscredited (Desch et al. 2010). Electrical dischargeheating (Love et al. 1995), electromagnetic radiativeheating (Eisenhour et al. 1994), gamma-ray bursts(McBreen and Hanlon 1999), and heating in protostellarjets (Liffman and Brown 1996), along with other heatsources mentioned in a review by Rubin (2000), all haveserious shortcomings and are not considered viable.

The splashing model, in total contrast, has a clearlyunderstood and quantifiable heat source in 26Al, andobviates the need to find a mechanism for the rapidheating of dust in the nebula.

CHONDRULES AND THE EVOLUTION OF

PLANETESIMALS

We now explore in the context of the splashingmodel, the nature of the planetesimals that preceded theperiod of chondrule formation and the properties of thechondrite parent bodies that postdated chondruleformation.

The Nature of the Postulated ‘‘Precursor’’ Planetesimals

to Chondrules

If the molten planetesimal splashing hypothesis iscorrect, then type I (FeO-poor) and type II (FeO-rich)chondrules suggest, respectively, that the precursor

planetesimals themselves were compositionally bimodal,being either volatile-depleted and reduced with most oftheir iron in metal, or volatile-bearing with much of theiriron as FeO.

What Was the Planetesimal Source for Type I(FeO-Poor) Chondrules?

We tentatively suggest that type I chondrules camefrom molten parent bodies which were the same as, orsimilar to, the parent bodies of certain iron meteorites.Many iron meteorites are extremely depleted inmoderately volatile siderophile elements, and their parentplanetesimals had generally melted within the firstmillion years (Burkhardt et al. 2008). As Bland andCiesla (2010) noted, they may have acquired theirvolatile-depleted chemical signature by accreting earlyfrom partially condensed matter in the still-hot innerregion of the infant disk, perhaps close to 1 AU (Bottkeet al. 2006) where the ambient gas would have beenhydrogen-rich and reducing.

In this light, we wonder whether the giant hit-and-run collision postulated by Yang et al. (2007) to accountfor the IVA iron meteorite parent body may havecreated an enormous cloud of type I chondrules. Thatcollision, it has been argued, left in its path a string ofsecondary planetesimals, one of which (the IVA body)was purportedly a sphere of molten metal perhaps300 km in diameter covered by a veneer of silicate rock.One particular IVA meteorite, Muonionalusta, has aPb-Pb age of 4565.3 ± 0.1 Myr (Blichert-Toft et al.2010), suggesting that the metal was solid by that timeand that the postulated giant collision happenedsomewhat earlier, consistent with the age of chondrules.We note that the published age for Muonionalusta maybe about 1 Myr too old, as it is based on an assumed238U ⁄ 235U ratio of 137.88 that is probably too high(Connelly et al. 2011). Nevertheless, the uncertainty inits age does not affect our contention that the postulatedgiant IVA collision could have been synchronous withchondrule formation.

The idea that volatile depletion is a signature of veryearly planetesimal accretion from a hot partiallycondensed nebula is supported not only by certain ironmeteorites but also by the basaltic achondrites. TheHED meteorites and the angrites are both highlydepleted in volatile elements, and recent revision of theirSr isotope systematics (Hans et al. 2011; Kleine et al.2012; Moynier et al. 2012) suggests that their parentbodies accreted, like the iron meteorite parent bodies,close to the time of formation of CAIs. An earlierproposal by Halliday and Porcelli (2001) that volatiledepletion in the angrites and HEDs might have resultedfrom giant impacts 2 or 3 Myr after CAIs now seems lesslikely.


On the same theme, Scott and Sanders (2009)appealed to a reservoir of volatile-depleted bodies datingfrom the time of CAI formation as the refractory Mn-poor end-member of a mixing line with Mn-rich CI-likedust to explain the whole-rock Mn-Cr isochron forcarbonaceous chondrites.

What Was the Planetesimal Source of Type II (FeO-rich) Chondrules?

We suggest that, in contrast to the reduced andvolatile-depleted planetesimals that gave rise to type Ichondrules, type II chondrules came from planetesimalsthat were initially made from aggregates of dust andwater-ice. On becoming heated by 26Al, the ice wouldhave melted to water, which would then have reacted,while still at a low temperature, with anhydrous silicategrains and metal to produce minerals such as serpentineand magnetite. On further heating, this oxidized suite ofminerals would have remained oxidized as it becamedehydrated and finally melted.

The accretion of water-ice would have required acold nebular setting. This setting may have developedlater than the time of accretion of the volatile-poor typeI planetesimals, after the initially hot nebula had cooledsufficiently for water-ice to condense, or perhaps itexisted farther from the Sun, where it had always beensuitably cold. In the latter case, subsequent orbitalmigration presumably brought oxidized and reducedplanetesimals close together, because type I and type IIchondrules are found side by side in many meteorites.

We note that there are limits to the amount of water-ice that could have been incorporated into a planetesimalif the planetesimal were later to have melted. The latentheat absorbed by vaporizing ice is a massive 2.6 kJ g)1,and the specific heat of steam is 2 J g)1 K)1. Thus, withonly 6.6 kJ g)1 available in the dust, a planetesimalaccreting at t = 0 from equal masses of ice and dustcould not have melted in time to make chondrules bysplashing, and perhaps it never would have melted.However, somewhat lower mass fractions of ice couldhave led to different levels of oxidation of iron as well aspermitting melting in time to make chondrules.

Planetesimals would have grown over a period oftime, both from gradual accumulation of nebular dust,and from mergers with other planetesimals. Thus, theywould have been composite bodies that incorporatedboth reduced and oxidized iron prior to their eventualmeltdown. This might explain why the distinctionbetween type I and type II chondrules based on theirFe ⁄ (FeO + MgO) values, particularly in ordinarychondrites, is not always sharp.

Independent evidence suggesting that water-ice was aprincipal cause of oxidized iron in meteorites comes froma consideration of D17O in water-ice. Water-ice in the

disk is thought to have been isotopically very heavy.Values of d17O and d18O close to +180&, i.e., on a slope 1line passing through Earth, were measured in tiny grainsof a nanoscale-mixed magnetite-sulfide phase scattered inthe matrix of the pristine carbonaceous chondrite Acfer094 (Sakamoto et al. 2007). These grains show just howenriched in the heavy isotopes of oxygen water-ice mayconceivably have been. If water-ice supplied the oxygenthat stabilized iron as FeO in meteorites, as we propose,then the value of D17O in a meteorite might be expectedto correlate with its degree of oxidation.

Such a correlation is observed. A progressive increasein oxidation state, expressed as molar FeO ⁄ (FeO+ MgO)in equilibrated pyroxene or olivine, going from the Echondrites, through the H, L, and LL chondrites to the Rchondrites, correlates with increasing D17O (Fig. 5). Theseequilibrated compositions are the average of a range ofFeO ⁄ (FeO + MgO) and D17O values that would haveexisted in the chondrules and matrix of the originalunequilibrated assemblage in each case.

The correlation line of Fig. 5 does not extend tobulk carbonaceous chondrites, which have negativevalues of D17O. Nevertheless, chondrules withinindividual carbonaceous chondrites have recently beenfound to show a good correlation between D17O andFeO ⁄ (FeO + MgO). The correlation has been observedin chondrules in CO chondrites (Tenner et al. 2011), CRchondrites (Schrader et al. 2011; Tenner et al. 2012), andin Acfer 094 (Ushikubo et al. 2012). Again, thiscorrelation appears to corroborate the hypothesis thatoxidation was induced by H2O derived from water-icewith high D17O.

The correlation between oxidation state and D17O isnot confined to chondrites and chondrules. In ureilites,FeO ⁄ (FeO + MgO) in olivine correlates with D17O

00 10 20 30 40

1

2

3

ELEH

H

R

FeO/(FeO + MgO) in silicate (mol %)

LLL

17O

(

)%

Fig. 5. Plot of mean molar FeO ⁄ (FeO + MgO) in equilibratedsilicates of five chondrite groups against mean D17O of thosegroups.


(Clayton and Mayeda 1996; fig. 6 of Goodrich andDelaney 2000). Explaining its cause has proven difficult;Goodrich and Wilson (2011) inferred that the correlationwas somehow inherited from the state of the parent bodyprior to melt extraction. In line with that view, wespeculate here that the ureilite parent body accreted in aheterogeneous way, with local internal variation in itscontent of water-ice. We envisage that the higher theice content in a given subvolume of the parent body,the higher would have been the final values ofFeO ⁄ (FeO + MgO) and of D17O in that subvolume, andwe suspect that these values would have persisted inthe solid residue following melt extraction. The sameprocess appears also to have operated in theacapulcoite ⁄ lodranite parent body, where a similarcorrelation is observed (McCoy et al. 1997; Greenwoodet al. 2012).

The Accretion of Chondritic Asteroids

Finally, in the context of the collision and splashingmodel, we speculate on the origin of a number of well-known features of chondrites that, hitherto, have largelydefied a satisfactory explanation, and we discuss theirwider implications. These features include: (1) thedistinctive characteristics of each chondrite group, (2) theso-called ‘‘complementarity’’ between chondrules andmatrix, (3) the depletion of metal in most chondrites, and(4) the huge mismatch between the large number ofmeteorites that are chondritic, and the small number ofparent bodies that are chondritic.

Why Is Each Chondrite Group Distinctive?Each of the 15 chondrite groups is thought to come

from its own separate parent body, characterized by aunique suite of chondrules and other components, andhaving distinctive petrographic, chemical, and isotopicfeatures (e.g., Rubin 2000, 2010; Jones et al. 2005; Scottand Krot 2007). To account for each group’s properties,Jones (2012) inferred that ‘‘multiple reservoirs ofchondrite components were present in the protoplanetarydisk, and that these were separated spatially, temporally,or both, such that limited mixing occurred between theseparate reservoirs’’ before and during accretion of therespective chondrite parent bodies. Aware that turbulentmixing and radial drift in the disk are likely to havequickly destroyed a reservoir’s identity and led to thewidespread homogenization of disk materials, Jones(2012) states that ‘‘the problem of maintaining such aseparation over an extended time period is currently oneof the biggest conundrums associated with our overallpicture of the early history of the solar system.’’ Wood(1988) also acknowledged this problem.

As a possible solution to the puzzle, we imagine thatthe materials in each reservoir were stored for most ofthe time, not in the nebula as dispersed dust, chondrules,and other tiny objects that could easily have been mixedradially, but as loose or poorly consolidated outer layerson planetesimals whose orbital radii remained more orless constant. Thus, we envisage each reservoir as acircum-solar annulus whose planetesimal population wasthe main carrier of its unique chemical and isotopicpeculiarities. We imagine, over time, a succession ofplanetesimal collisions and mergers within an individualannulus, such that each impact plume injected into itslocal zone a fresh batch of chondrules, along with olderrecycled debris from the regoliths and crusts, and thatthe whole mixture was promptly accreted to new, orexisting, planetesimals before there was time for it tohave been dispersed throughout the disk. In this way, thedistinctive chemical, petrographic, and isotopic traits ofeach ‘‘reservoir’’ would have been kept largely intact andeventually preserved in each individual chondrite parentbody (=chondrite group). Also, as each annulus wouldhave remained an essentially closed chemical system, thescrambling of materials within it could not have erasedits initial near-solar chemistry despite more than 2.5 Myrof processing.

Undoubtedly, there would have been some exchangeof dust and chondrules between neighboring annularreservoirs, and the degree of intermixing may well haveincreased with the passage of time as planetesimalsbecame fewer and larger, and underwent orbitalmigration. It is perhaps for this reason that particularkinds of chondrule are not always confined to singlegroups, but may be present in a number of groups (e.g.,H, L, and LL), albeit in different proportions in each.

How many of our imagined chondrule-formingcollision events are recorded in an individual chondrite?Statistical peaks in the distribution of chondrule ages(Villeneuve et al. 2009) and the clustering of oxygenisotope compositions in Mg-rich olivine in chondrules(Libourel and Chaussidon 2011) hint that the numbermay have been in single figures.

Why Do Chondrules and Matrix Display Compositional‘‘Complementarity’’?

In some carbonaceous chondrites, particularly inthose of the CR group, the chondrules and matrix arechemically quite distinct from each other, yet the twocomplement each other, such that when mixed together,they have almost perfect solar, i.e., CI-like, elementratios (Hezel and Palme 2010). This remarkablerelationship, dubbed ‘‘complementarity,’’ suggests thatchondrules and matrix were formed from a single volumeof starting material with solar-composition. Hezel and


Palme (2010) envisage a local part of the nebula wherethe chemically distinct refractory chondrules andvolatile-rich dust were created from the same batch ofprimitive dust, and then promptly recombined and addedto the chondrite parent body. With this kind of local,self-contained process in mind, Palme et al. (2011)claimed that ‘‘complementarity’’ rules out the productionof chondrules from molten planetesimals.

However, we suggest that ‘‘complementarity’’ neednot have been the outcome of local processing, and thatit can be reconciled with ‘‘splashing.’’ We imagine thateach evolving annulus of planetesimals and dustremained an essentially closed chemical system, asoutlined above. With each collision and shower of hotchondrule droplets, evaporation and recondensationwould have led to a net transfer of the more volatileelements from the chondrules to the surrounding finedust. If the chondrules and dust remained within theirhost annulus then, sooner or later, they would have beenreunited. In this way, even with many successive phasesof collision and re-accretion, a complementaryrelationship between chondrules and matrix would havebeen preserved.

Why Are Some Chondrite Groups Strongly Depleted inSiderophile Elements?

Chondrite groups other than the H, EH, andCH ⁄CB groups are depleted to varying degrees in Fe, Ni,and other siderophile elements relative to the CIchondrites (e.g., fig. 2 in Krot et al. 2003). The depletionis particularly marked in the L and LL groups; in the LLgroup, Mg-normalized molar siderophile elementconcentrations are at roughly half their CI levels (Krotet al. 2003).

We speculate here that this well-known metaldepletion is a simple consequence of planetesimalmergers. In a typical oblique-impact merger, asportrayed by Asphaug et al. (2011), the molten metalliccore of the projectile body would have becomesubstantially embedded inside the larger target body,while the ejecta plume would have been dominated bymaterial from the projectile’s silicate mantle (Fig. 6). If,with successive collisions, the metal repeatedly showedthis preference for joining the larger body of the mergingpair, then the ejecta would have become progressivelydepleted in metal and enriched in silicate, and so wouldthe chondrite parent bodies constructed from that ejecta.The corollary is that the target bodies, and ultimately theplanets, would have a proportionately higher fraction ofmetal than the solar average. The process we envisage isthe same, albeit on a much smaller scale, as thatenvisaged for the formation of the Moon by giantimpact, which led to a large deficit of iron within theMoon and an increase of iron in the Earth’s core.

Mars, like the Moon, may be depleted in metal as itsdensity is low relative to that of the Earth, even afterallowing for differences in the internal pressures of the twoplanets. This suggests that Mars possibly grew fromplanetesimals that were predominantly depleted insiderophile elements, like the L and LL chondrites. Wespeculate that such depletion of metal may be linked to therecent explanation for the small size of Mars proposed byWalsh et al. (2011). These authors postulated that Jupiterand Saturn migrated toward the Sun until their orbitsreached a 3:2 mean-motion resonance and then migratedout to near their present locations. This so-called ‘‘GrandTack’’ would have cleared the disk of most planetesimalsas far in as 1.5 AU, leaving in its wake a mere smatteringof scattered planetesimals from which Mars could grow.We speculate that the residual, thinned out suite ofplanetesimals may have been biased toward second-generation chondritic bodies with siderophile deficits.

Why Are Five out of Six Meteorite Falls Chondrites,When Only One in Five Parent Bodies Is Chondritic?

Chondritic meteorites are common, accounting forbetween 80% and 85% of observed falls, yet as we notedearlier, they appear to have been sourced from only about20% of the 100–150 inferred meteorite parent bodies. Thisdisparity has led to considerable debate on thecomposition of asteroids (Burbine et al. 2002). We suspectthat the high abundance of chondrites among meteorites isa true reflection of their actual abundance in the asteroidbelt, and we postulate that the large number ofdifferentiated parent bodies that have been sampled is noindication of the abundance of these bodies in the asteroidbelt today, but is a legacy of the situation that existed inthe infant solar system before chondrule formation.

The ‘‘splashing’’ hypothesis contends that as theearly molten bodies merged together and grew in size,eventually to produce planets, they released swarms ofchondrules and dust, which aggregated to makechondritic asteroids. The latter, second-generation

debris plume of droplets and dustdepleted in metal

projectile’smetal core

Fig. 6. Cartoon showing a cross-section through a collidingpair of planetesimals following an oblique impact duringaccretion, based on results of Asphaug et al. (2011). The ironcore of the impactor is shown largely embedded in the targetbody, leaving the ejecta plume depleted in iron metal and othersiderophile elements.


bodies, it appears, came to dominate the tiny mass ofmaterial that escaped being subsumed into planets, andthey now reside, somewhat battered and brecciated, inthe asteroid belt. Of the original molten bodies, perhapsVesta alone survives intact, while the many others arerepresented only by fragments of iron, stony iron, andrare achondritic material not from Vesta, as eitherisolated pieces or assembled into asteroidal rubble piles.We imagine that by t approximately 2.5 Myr, when 26Alhad lost its potency and chondrule formation was inrapid decline, most of the original molten planetesimalshad already merged into larger bodies and disappeared.As John Wood (2000) so aptly put it, the beginning was‘‘swift and violent.’’

SUMMARY AND FURTHER WORK

We find it remarkable that the chronologicalevidence for the timing of early planetesimal meltdown(by t approximately 0.3 Myr) and the timing ofchondrite accretion (after t approximately 2.5 Myr)coincide so well with the timing of these processescalculated from the inferred 26Al heat source in nebulardust. This coincidence, along with the evidence in ourmeteorite collections for a very large number of earlymolten bodies, has led us to envisage the inner solarsystem during its first 2 Myr as being populated with agreat abundance of substantially molten planetesimals.We might name those first approximately 2 Myr, fromwhich so little tangible evidence survives, the solarsystem’s ‘‘meltdown era.’’ As planetary embryos wereprobably already forming during this period, the moltenplanetesimals must have been continuously colliding andmerging, becoming fewer in number, and growing largerin size. Within this conceptual framework for the youngdisk, chondrule production from the ‘‘splashing’’ ofmolten planetesimals seems an inevitable consequence,with new generations of chondritic planetesimals beingspawned from the debris ejected and dispersed duringmergers.

We believe that the ‘‘splashing’’ hypothesis can bereconciled with much of what we understand aboutchondrules, including their ages, chemical compositions,peak temperatures, abundances, sizes, cooling rates,indented shapes, ‘‘relict’’ grains, igneous rims, blebs ofmetal, retention of Na, presence of FeO, diversity, andlarge phenocrysts, as well as other issues that constrainchondrule origins such as the formation ofmacrochondrules, the scarcity of dust-clumps, and theneed for a feasible heat source. However, we contendthat several of these chondrule properties, most notablythe concentration of Na in olivine, challenge the long-standing conventional interpretation of chondrules asshock-melted dust-clumps. We speculate that the

bimodal division of chondrules into types I and IIreflects a bimodal chemical division of moltenplanetesimals, such that reduced refractory planetesimalssupplied type I chondrules and may also have been thesource of many iron meteorites, while oxidized volatile-bearing planetesimals supplied type II chondrules andprobably incorporated varying amounts of water-ice,reflected in the correlation between D17O values andFeO ⁄ (FeO + MgO) in some groups of meteorite.Finally, we propose that at the close of the ‘‘meltdownera,’’ chondritic planetesimals began to appear. Weimagine that each chondritic parent planetesimal is amixture of chondrules and debris launched and re-accreted more than once in a discrete chemicallyrestricted annulus in the disk, to account for its uniquetraits, its broad solar composition, its level of metaldepletion and, in some cases, its ‘‘complementarity’’between chondrules and matrix.

So where do we go next in the quest to test anddevelop the scenario presented here?

Conventional ideas about chondrule formation arepredicated on the intuitive, but now largely discredited,belief that chondrites are samples of the very firstplanetesimals to have formed. We submit that thisconceptual framework has contributed a great deal tothe understanding of disk processes, and that we wouldbe presumptuous to claim that all chondrules were madeby ‘‘splashing.’’ After all, some chondrule-like objects inmeteorites (CAIs in particular) are widely believed tooriginate in a high-temperature nebular setting that wasnot an impact plume. Nevertheless, we believe thatmeteorite chronology and the likelihood of 26Al heatinghave brought about a new paradigm, which holds morepromise than the conventional view for understandingthe evolution of the nascent disk in general, and theorigin of chondrules in particular.

To move forward, we need thermal and petrologicalmodels of planetesimal evolution that can accommodatecollision and accretion history as well as differentmechanisms of heat loss from magma oceans, not justduring the heating stage but also during cooling. Weneed more sophisticated models of impact plumes, to tryto understand how particles within them would haveevolved thermally and spatially under a range of collisionconditions including different impact angles, encountervelocities, planetesimal sizes, planetesimal internalstructures, and regolith thicknesses. We need furtherprecise chronology for chondrules and differentiatedmeteorites to strengthen the evidence for meltdownbefore chondrule formation. In particular, the possibilitythat the 26Al-26Mg chronometer may be flawed (Larsenet al. 2011), and that the 26Al heat source may have beenmuch weaker than assumed, needs thoroughinvestigation. Also, we need to check our model’s


prediction that type II chondrules are younger than theoldest type I chondrules.

On a broader front, we need to better understand thedichotomy of chondrites and why virtually alldifferentiated meteorites are derived from materials thatisotopically resemble ordinary and enstatite chondrites,with so few genetically related to carbonaceous chondrites(Warren 2011). We also need to investigate whether ourmodel can shed light on the scarcity of meteorite brecciascarrying both chondritic and differentiated components,whether it can explain why the cooling rates of magmaticiron meteorites are surprisingly high, or why basalticmeteorites did not crystallize at the same time as iron coressegregated (e.g., Kleine et al. 2012), and whether it can tellus anything about the virtual absence of meteorites madefrom olivine rock (i.e., from planetesimal mantles) thatmight complement irons, stony-irons, and basalticachondrites. Finally, all the above avenues of potentialfuture enquiry should intersect the paths being followed inthe pursuit of numerical models of planetesimal accretionand orbital evolution (e.g., Bottke et al. 2006; Johansenet al. 2007; Walsh et al. 2011). We are optimistic that allapproaches will eventually converge on an agreed, self-consistent picture of the first critical steps of planetarydevelopment in the solar system.

Acknowledgements—We are grateful to Stuart Agrell whoinspired us both as students, to Bob Hutchison who wassteadfast in his conviction that differentiated planetarybodies existed before chondrites, and to John Wooddespite his conviction that they did not. We are indebtedto Klaus Keil, whom we honored at the Workshop on theFormation of the First Solids in the Solar System, for hisfriendship and distinguished leadership in meteoritics andcosmochemistry. We acknowledge financial support fromTrinity College Dublin (IS) and the NASACosmochemistry program (ES). We thank Alan Rubinand Alex Ruzicka for their constructive reviews.

Editorial Handling—Dr. Alexander Krot

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