Meteoritics & Planetary Science 47, Nr 12, 2170–2192 (2012) doi: 10.1111/maps.12002

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, Ireland 2Hawai’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 spray from impact plumes launched when thin-shelled, largely molten planetesimals collided at low speed during accretion. This scenario, here dubbed ‘‘splashing,’’ stems from evidence that such planetesimals, intensely heated by 26Al, were abundant in the protoplanetary disk when chondrules were being formed approximately 2 Myr after calcium-aluminum-rich inclusions (CAIs), and that chondrites, far from sampling the earliest planetesimals, are made from material 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, igneous rims, and metal blebs, and is also reconcilable with features that challenge the conventional view that chondrules are flash-melted dust-clumps, particularly the high concentrations of Na and 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 that accreted in a later, or more distal, cool nebular setting where incorporation of water-ice with high D17O aided oxidation during heating. We propose that multiple collisions and repeated re-accretion of chondrules and other debris within restricted annular zones gave each chondrite group its distinctive properties, and led to so-called ‘‘complementarity’’ and metal depletion in chondrites. We suggest that differentiated meteorites are numerically rare compared with chondrites because their initially plentiful molten parent bodies were mostly destroyed during chondrule formation.

INTRODUCTION surrounded the infant Sun, from which the planets later developed) prior to accreting to the surfaces of growing Chondrules are igneous-textured grains that make chondrite parent asteroids. They co-accreted with other up 50% or more by volume of most chondrites. Many of disk materials including small grains and droplets of them are frozen droplets of magma, typically 0.1–2 mm Fe-Ni metal and sulfide, refractory objects called across, rounded to lobate in shape, and composed largely calcium-aluminum-rich inclusions (CAIs), mineral of the Mg-rich silicate minerals olivine, (Mg,Fe)2SiO4, fragments, and dust including micron-scale grains of and pyroxene, (Mg,Fe)SiO3 (Zanda 2004; Scott and Krot stardust surviving from the presolar molecular cloud. As 2007). They tend to be either FeO-poor (type I chondrites account for five out of six meteorites falling to chondrules) or FeO-rich (type II chondrules). Their Earth, the formation of chondrules was apparently a igneous textures suggest cooling from near-liquidus process that affected a substantial fraction of the solid temperatures and solidifying over a matter of hours. material in the solar nebula. Evidently, they were present in the solar nebula, or There is no consensus on how chondrules were protoplanetary disk (the disk of gas and dust that made: two fundamentally different approaches have

The Meteoritical Society, 2012. 2170 Origin of chondrules and chondrites 2171 come to dominate current discussion. The first contends In this article, we examine that new picture and how that clumps of dust in the disk were transformed directly it might be extended to embrace the formation of to chondrules by rapid melting, probably as a result of chondrules and the assembly of chondritic asteroids. We shock-induced heating in the nebula. This idea was begin by showing how precise meteorite chronology now suggested half a century ago (Wood 1963, p. 165) in a indicates that planetesimals first melted long before seminal paper whose title we adopt here, and the melting chondrules were made, consistent with 26Al having been of dust-clumps, whether by shock or by some other an internal planetesimal heat source. We argue that means, has since become the prevailing theory for molten planetesimals dominated the population of chondrule formation. It has been advocated, tacitly if not bodies in the inner solar system for the first 2 Myr, and overtly, by many authors, including Taylor et al. (1983), we propose that the inevitable collisions and mergers Wood (1988), Grossman (1988), Wasson (1993), Rubin between them led to ‘‘splashing’’ with ejecta plumes (2000), Shu et al. (2001), Boss and Durisen (2005), bearing swarms of chondrule droplets and other debris, Lauretta and McSween (2006), Scott (2007), Alexander which later accreted to chondritic parent bodies. We et al. (2008), and Ruzicka et al. (2012a). evaluate the established petrographic, compositional, The second approach imagines that chondrules were and experimental evidence for chondrule formation, and produced when large volumes of molten rock became find it to be consistent with this collision scenario. splashed and dispersed into the nebula as showers of Importantly, we find some of the evidence hard to spray. Specifically, it has come to embrace a hypothesis, reconcile with the conventional view that chondrules dating from about 30 yr ago, that chondrules originated began as dust-clumps. In the last section of the article, in great plumes of droplets launched by collisions we discuss how the ‘‘splashing’’ hypothesis may relate to between planetesimals that had been intensely heated the broader picture of planetesimal evolution in the and melted by the decay of 26Al (Zook 1980, 1981; young disk, speculating on the nature of the molten Wa¨ nke et al. 1981, 1984). This second hypothesis lay precursor planetesimals, and on the origin of some dormant for many years, almost completely hitherto poorly understood features of chondrites. overshadowed by the first, and its re-awakening has been slow. Sanders (1996) attempted to revive interest, and the A NEW PARADIGM FOR CHONDRITES idea has also been promoted by LaTourrette and ANCHORED IN 26AL HEATING Wasserburg (1998), Chen et al. (1998), Lugmair and Shukolyukov (2001), and Hevey and Sanders (2006). The Conventional View of Chondrites Sanders and Taylor (2005) reviewed the hypothesis in detail. While the production of chondrules from molten Chondrites have conventionally been interpreted as planetesimals has never had a majority following, few aggregates of primitive materials that were assembled at today would dismiss it out of hand, and recently, the very start of the solar system from the same reservoir Asphaug et al. (2011) enhanced its credibility with a of nebular dust as went to make the Sun (e.g., Wood computer simulation of how chondrules might form 1988). This view stems from the remarkable similarity from magma released as a result of collisions. between the chemical composition of chondrites and that A great deal more is known today about the early of the Sun’s photosphere for all elements other than a few solar system than was known 30 yr ago. The past that normally occur in gases (e.g., H, He, C, N, Ar). The 5–10 yr in particular have witnessed major view is reinforced by the presence in chondrites of CAIs, improvements in mass spectrometry and meteorite which are the oldest dated objects with a solar system chronology, significant developments in the modeling of isotopic signature (Amelin et al. 2002, 2010). Indeed, the planetesimal melting and of conditions in the solar time of CAI formation has now been widely adopted as nebula, and a substantial increase in the number and defining the start of the solar system (t=0). In addition, variety of meteorites available for study. The same chondrites contain pristine grains of stardust that are even recent period has also seen much exchange of ideas older than the solar system (e.g., Hoppe 2008). Thus, between meteorite researchers, astronomers, and conventional thinking holds that chondrules, along with planetary scientists. Observations of disks around CAIs, were created directly from clumps of nebular dust young stars, discoveries of extra-solar planetary at the outset, before the first planetesimals (presumed to systems, and computer simulations of orbital dynamics be the chondrite parent bodies) had accreted. It further have revolutionized our thinking on the formation of holds that after their accretion, some chondritic the asteroid belt. Together, these advances are bringing planetesimals became overheated, melted, and into focus a new picture of the young solar system that differentiated into molten metal cores and basaltic crusts, is significantly different from the one that has been the sources, respectively, of iron and basaltic meteorites conventionally portrayed. (e.g., Lauretta and McSween 2006). 2172 I. S. Sanders and E. R. D. Scott

However, this intuitive and long-established dating (Kleine et al. 2008). As the chondrite parent interpretation of meteorites has recently been challenged asteroids were assembled after the youngest chondrules by chronological evidence, which suggests that within them had formed, i.e., probably more than about chondrules were made after, and not before, the parent 2.5 Myr after CAIs, then far from being the first bodies bodies of differentiated meteorites had melted. to have accreted, as is conventionally assumed, they were perhaps among the last to have done so. The Chronology of Molten Cores and Chondrules The late accretion of chondritic asteroids was already suspected on petrographic grounds long before the recent Most iron meteorites now appear to come from chronological evidence became known. Fragments planetesimals that had accreted and melted extremely deemed to be of planetary igneous rock were identified in early, perhaps within 1 Myr of CAI formation chondrites by Kurat and Kracher (1980), Hutchison et al. (t < 1 Myr) because their e182W values are very low and (1988), and Kennedy et al. (1992). The last of these within error of the initial e182W of CAIs (Burkhardt authors reported a 2 mm chip of high Mn ⁄ Fe basalt in the et al. 2008). In the early solar system, e182W was rising Parnallee chondrite, which they interpreted as being due to the radioactive decay of 182Hf to 182W (half-life derived from a high Mn ⁄ Fe planetesimal that had already 8.9 Myr). Tungsten is a siderophile (iron-loving) element, melted and broken up before Parnallee’s parent body had whereas hafnium is lithophile (silicate-loving). The e182W accreted. In addition, Ruzicka et al. (1995) reported an values of iron meteorites became fixed, therefore, when unusual silica pyroxenite clast in Bovedy (L3), which they molten metal segregated into planetesimal cores. At that regarded as having a planetary igneous origin. More stage, the radioactive hafnium, being lithophile, was recently, Sokol et al. (2007) reviewed the occurrence of a removed from close proximity to metal and transferred wide variety of differentiated igneous rock fragments in to the silicate mantle of each planetesimal where chondrites, and Ruzicka et al. (2012b) reported further subsequent radiogenic 182W accumulated. A recent silica-rich clasts of supposed igneous origin. On a related estimate of the time of core formation based on e182Win note, Libourel and Krot (2007) discovered small pieces of magmatic iron meteorites is 0.3 ± 1.2 Myr after CAIs texturally equilibrated olivine rock inside chondrules, (Kruijer et al. 2011). Even more recently, Burkhardt which they interpreted as tiny fragments of earlier et al. (2012) revised the value of initial e182W in CAIs, planetesimals that had been metamorphosed and then and they inferred that while the cores of some parent broken up by impacts before being incorporated into bodies (notably the IVB group) formed at t chondrules. Although Whattam et al. (2008) questioned approximately 0.3 Myr, the cores of others continued to that interpretation, the petrographic evidence, like the form up to about t = 2 Myr. The timing of core chronological evidence, clearly points to high-temperature separation is critical to constraining early disk evolution, planetesimal processing prior to chondrite accretion. and is the culmination of painstaking efforts to understand and refine the Hf-W chronometer by many Heating by 26Al: The Key to the New Chronology workers (Horan et al. 1998; Kleine et al. 2005; Markowski et al. 2006; Scherste´n et al. 2006; Qin et al. The cause of early melting and core formation, some 2008; Burkhardt et al. 2008). In particular, Markowski 1–2 Myr before chondrules were made, is not hard to et al. (2006) recognized the need to correct e182W values find. Ever since evidence for live 26Al was discovered in for the effects of long-term exposure to cosmic rays; CAIs, it has been realized that the decay energy from this uncorrected values had previously given erroneously old short-lived isotope (half-life 0.72 Myr) would have been ages. more than sufficient to melt the fully insulated interiors By contrast, 26Al-26Mg dating suggests that of planetary bodies that accreted early enough, while chondrules are mostly 1.5–2.5 Myr younger than CAIs. radioactive heating was intense (Lee et al. 1977). The Following the pioneering work of Hutcheon and corollary is that the chondrite parent bodies, as they did Hutchison (1989), about 100 chondrule dates based on not melt, accreted later, after the 26Al had largely 26Al-26Mg internal isochrons have now been published. decayed and lost its capacity to cause melting. This A recent review of them by Kita and Ushikubo (2012) explanation is reinforced in the following paragraphs by shows that more than 90% of those from unequilibrated a simple quantitative analysis of the likely effects of 26Al (type 3.0) chondrites (64 determinations) fall within the heating on the timing of initial planetesimal meltdown, 1.5–2.5 Myr age range, with just three dating from the timing of chondrule formation, and the timing of t < 1.5 Myr. The same age difference between chondrite accretion. chondrules and CAIs of about 2 Myr has been measured Our estimate of the energy available in 26Al to heat independently by 207Pb-206Pb dating (e.g., Amelin et al. the first crop of planetesimals at t=0 is about 6.6 kJ 2002, 2010; Connelly et al. 2008), and by 182Hf-182W per gram of dry dust. This estimate requires knowledge Origin of chondrules and chondrites 2173

of the concentration of Al in the dust, of the initial ratio 8 of 26Al ⁄ 27Al, and of the heat released by each decaying atom of 26Al. The concentration of Al is not known 6 precisely, but would presumably have been more than 1.6 kJ/g is the energy needed to melt a 0.85 wt.% (the level in CI chondrites, which are planetesimal interior completely extensively hydrated; Lodders and Palme 2009). We 26 4 conservatively, although somewhat arbitrarily, choose a value of 1.2 wt.%, which corresponds roughly to the 2 concentration of Al in dehydrated CI chondrite, and is close to the concentration of Al in most anhydrous a 0 chondrite groups (Lodders and Fegley 1998). We assume 26 27 that the initial value of Al ⁄ Al in the disk was 4 solid )5 D uniformly 5 · 10 , the so-called canonical value in CAIs 2 (Jacobsen et al. 2008; MacPherson et al. 2010). A 3 partial 26 C melt uniform distribution of canonical Al in the disk is 1.5 indicated by the identical 26Mg ⁄ 24Mg in the Earth, the 2 1850 K 1 1425 K Moon, Mars, and bulk chondrites (Thrane et al. 2006); 250 K B by the correlation between the initial 26Mg ⁄ 24Mg and the 26Al ⁄ 26Mg ages of a suite of chondrules studied by 0.5 1 magma ocean Villeneuve et al. (2009); and by time intervals between

0 A b Time of accretion (Myr after CAIs)Time Al (kJ/g) Energy as 26 26 0 specific events measured using the Al ⁄ Mg 1234 5 chronometer being corroborated by other chronometers Time (Myr after CAIs) (e.g., Connelly et al. 2008). We are aware that Larsen Fig. 1. a) Exponential decline with time of the potential 26 24 et al. (2011) reported significant variation in Mg ⁄ Mg thermal energy stored as 26Al in a gram of ‘‘dry’’ primitive in objects with solar 27Al ⁄ 24Mg, and proposed that the dust. b) Time at which solidus (1425 K) and liquidus (1850 K) initial 26Al ⁄ 27Al in parts of the inner solar system where temperatures are reached in the fully insulated interior (deeper planetesimals accreted and where chondrules formed than approximately 5 km at t = 1 Myr to deeper than approximately 20 km at t approximately 5 Myr—see Figs. 2 may have been substantially lower than the canonical and 4) of a planetesimal as a function of the time of its cold level where CAIs were made. However, Wasserburg (250 K) instantaneous accretion and assuming no melt et al. (2011) found a wide variation in the initial migration during heating. Arrows A, B, C, and D illustrate the 26Mg ⁄ 24Mg of different CAIs with identical canonical timing of initial and total melting following cold accretion at 26Al ⁄ 27Al, which leaves an open verdict for the case t = 0, 0.75, 1.4, and approximately 2 Myr, respectively (see text for explanation). Accretion-time intervals labeled 1, 2, 3, made by Larsen et al. (2011). Finally, we take the decay 26 and 4 relate to the fields shown in Fig. 4. The lower edge of the energy per atom of Al as 3.1 MeV (Castillo-Rogez gray zone is the 1850 K liquidus calculated using latent heat et al. 2009). A plausible 10% uncertainty in both the and specific heat capacity values from Ghosh and McSween initial 26Al ⁄ 27Al, and in the wt. % Al, leaves our (1999), which are greater than those adopted here. estimated initial radioactive energy at 6.6 ± 1 kJ g)1. In addition to 26Al, the short-lived isotope 60Fe may have contributed to radioactive heating. However, the perspective, 6.6 kJ g)1 is about four times larger than the initial concentration of 60Fe and whether it was 1.6 kJ g)1 needed to fully melt the insulated interior of a uniformly distributed in the disk remain unknown (Telus planetesimal at a temperature of 1850 K. The estimate of ) et al. 2012). The ratio at t =0of60Fe ⁄ 56Fe (1.5 · 10 6) 1.6 kJ g)1 assumes starting from cold (250 K), with )1 )1 assumed by Sanders and Taylor (2005) now seems far specific heat capacity, Cp = 837 J kg K , and latent ) too high. Telus et al. (2011) suggest that it was between 3 heat of fusion = 2.56 · 105 Jkg 1 (Hevey and Sanders ) and 5 · 10 7, making the contribution of 60Fe to heating 2006). Thus, planetesimals that accreted during the first <0.5 kJ g)1. Moreover, as 60Fe’s half-life of 2.6 Myr two half-lives of 26Al, or roughly during the first (Rugel et al. 2009) is more than three times longer than 1.5 Myr, would have had the potential to become that of 26Al (0.72 Myr), its contribution to overall completely molten in their fully insulated interiors. heating during the critical first 2 Myr would have been Figure 1b shows the time it would have taken to reach trivial, and we therefore ignore it in this paper. the onset of melting (the solidus temperature, Figure 1a shows the temporal decline in energy approximately 1425 K) and also the completion of melting stored as 26Al in each gram of dry primitive dust, (the liquidus temperature, approximately 1850 K) of the starting from the initial 6.6 kJ g)1, through almost seven fully insulated interior (i.e., with zero heat loss) as a half-lives during the first 5 Myr. To put this decline in function of the time of cold planetesimal accretion, 2174 I. S. Sanders and E. R. D. Scott assuming no migration of the 26Al heat source. With planetesimals that accreted instantaneously. Their model accretion at t=0 (arrow ‘‘A’’ in Fig. 1b), heating would is based on a radiogenic heat budget of 6.4 kJ g)1 of dust have been rapid and the liquidus would have been reached at t=0, which is very close to the value of 6.6 kJ g)1 we by about t=0.3 Myr. This is in good agreement with the estimate here. We note that they used an incorrect 26Al timing of earliest planetesimal melting and core formation, decay energy (4 MeV). That value wrongly includes shown by 182W-deficit dating of iron meteorites (Burkhardt approximately 1 MeV of energy that is not deposited as et al. 2008, 2012; Kruijer et al. 2011) and, although errors heat, but is lost in neutrinos. However, the error was in the dating are large, such early melting clearly endorses fortuitously compensated by a lower concentration of Al the assumption that 26Al was the heat source. (0.9 wt.%) compared with the 1.2 wt.% we use here. With accretion at t = 0.75 Myr (arrow ‘‘B’’), the As an example of their results, Fig. 2a illustrates the initial heating rate would have been half that for arrow changing temperature profile within a planetesimal that ‘‘A,’’ but rapid enough for total internal melting to have accreted cold (250 K) at t=0 and had a radius of 50 km been achieved by t = 1.5 Myr. (after early sintering and shrinkage). After a little over As a third example, with accretion at t 0.3 Myr of heating, with the 26Al heat source evenly approximately 1.5 Myr (arrow ‘‘C’’), the insulated distributed, the interior deeper than approximately 5 km interior of a planetesimal would have carried just enough would have become uniformly hot and 50% molten 26Al to reach the liquidus, but melting would not have (approximately 1725 K). At this stage, the interior is been completed until after t approximately 5 Myr. This assumed to have lost rigidity and become cohesionless example may explain the paucity of chondrules that date magmatic slurry undergoing turbulent thermal from before t approximately 1.5 Myr (Kita and convection. With continued intense heating beyond Ushikubo 2012). Assuming that chondrules (regardless 0.3 Myr, the magma is assumed to have remained at sub- of their formation mechanism) were produced in large liquidus temperatures, but to have increased in volume as numbers before t approximately 1.5 Myr, the scarcity of the overlying rigid carapace was melted upward from its those old chondrules in meteorites must reflect their poor base and its thickness reduced from approximately 5 km survival rate. We imagine that such chondrules accreted at t = 0.3 Myr to just 0.5 km by t=0.5 Myr (Fig. 2b). to planetesimals before t approximately 1.5 Myr and By that time, the rate of conductive heat loss through the became buried in their insulated interiors where they residual crust would have reached a maximum, equaling would later have been melted down and destroyed. If this the rate of internal heat production, and the crust’s explanation is correct, then it implies that these pre- thickness would have been at a minimum. Thereafter, with 1.5 Myr chondrules, once made, did not linger in space, heat production lower than heat loss, no further melting but accreted to planetesimals promptly and were thence would have occurred, and the crust would have thickened, destined for a magmatic grave. slowly at first but ever more rapidly, over the next 2 or Finally, with accretion after about t approximately 3 Myr and beyond. A cartoon of the state of the 2 Myr (arrow ‘‘D’’), the level of 26Al would have been planetesimal at t = 2 Myr is shown in Fig. 3. too low to have heated the planetesimal’s interior to the The model predicts that during maximum heat loss, solidus, so no melting at all would have taken place. The only about 2 m of porous, unconsolidated, and timing is consistent with the evidence that chondrites extremely insulating dust separated solid, sintered rock (which of course did not melt) accreted after about t at 700 K from the surface at 250 K. However, this approximately 2.5 Myr (by when chondrule production prediction assumes that all accretion was completed was in decline), and again corroborates the view that instantaneously at t = 0. In reality, at least some 26Al was the main heat source within planetesimals. accretion would have continued after the initial In summary, the timing of core formation before aggregation of material. Beyond about t approximately t = 1 Myr, the scarcity of chondrules made before t =1.5 2 Myr, with the 26Al heat source fading, any such late Myr, and the accretion of chondrites after t =2.5Myr, accretion would not have melted, but accumulated as a combine to uphold our conviction that planetesimal coating of loose, or weakly consolidated, dusty debris, heating by 26Al was a key factor in the evolution of which could have attained a considerable thickness. planetesimals in the infant solar system. What if the radius had been much smaller than the 50 km chosen in Fig. 2? Hevey (2001) showed that a body The Structure of Molten Planetesimals with a 20 km radius would have melted substantially by t = 0.3 Myr, and its crust would have thinned down to a To visualize the changing internal structure of a minimum of 1.5 km by t = 0.5 Myr, but its high surface- molten planetesimal, we use the results of Hevey and to-volume ratio would then have led to rapid cooling, Sanders (2006) who presented simulations of the heating, and the body would have been largely solid by t = 3 Myr. melting, and cooling of initially cold, porous A body 10 km in radius would scarcely have melted at all. Origin of chondrules and chondrites 2175

ab 0 0 1 Myr 3 Myr crust

10 5 Myr 10 10 Myr 7 Myr

20 20

Depth (km) 30 30 magma ocean

40 40 0.1 Myr 0.3 Myr 0.2 Myr 0.4 Myr 50 0.5 Myr 50 500 1000 1500 2000 1234 Temperature (K) Time (Myr after CAIs) Fig. 2. a) Temperature profiles at selected times inside a planetesimal with a 50 km radius and zero porosity that accreted at t=0 and a temperature of 250 K and was heated by 26Al decay. Broken lines are profiles during heating (until 0.5 Myr) and continuous lines are profiles during cooling. Convection began soon after t=0.3 Myr, and by t=0.5 Myr the molten, convecting interior had expanded to within about 0.5 km of the surface (after Hevey and Sanders 2006). b) Depth of solid rock and crust (gray) for the same body as a function of time.

~ 2 m of dust and halving the thickness of insulating crust to a mere 250 m during the period of peak heat loss between sintered solid t = 0.5 and t = 1.5 Myr. In this case, the crust may upper crust perhaps easily have foundered, exposing incandescent partially molten but rigid lower magma at the surface. With a still larger radius, the crust insulating carapace would have become even thinner and even more susceptible to foundering. magma ocean interior Figure 4 shows the effects of a planetesimal’s radius on its predicted melting behavior combined with the effects of the timing of its accretion discussed above (Fig. 1b). Four fields, numbered (1) to (4), correspond to the four accretion time intervals shown in Fig. 1b. Fig. 3. Cartoon showing the internal structure of the 50 km Planetesimals starting in field (1) would have become radius planetesimal exemplified in Fig. 2 at t=2 Myr. The core is assumed to be fully formed, but it is possible that small extensively molten before t = 1.5 Myr, with very thin droplets of metal may have been held in suspension by crusts as depicted in Figs. 2 and 3. Such planetesimals, we turbulent convection (symbolized by curved arrows) in the suggest below, would have been potential sources of magma ocean. The base of the crust is arbitrarily taken as the chondrules by impact splashing. Planetesimals starting in level at which the interior is 50% molten; the lower crust, with field (2) would also have undergone extensive internal less than 50% melting, is deemed to be rigid. The 2 m of dust shown on the surface is predicted by instantaneous accretion; in melting, but beneath a thicker insulating crust than for reality, continuous accretion probably led to a considerable field (1), and with a longer heating period, generally thickness of cool, loose, dusty debris, particularly after about becoming molten from t approximately 1.5 Myr up to t t = 2 Myr when 26Al heating was very weak. approximately 5 Myr depending on the time of accretion. Planetesimals starting in field (3) would have melted only In a comparable thermal model, Moskovitz and Gaidos partially, and they probably would have remained rigid. It (2011) predicted similar melting behavior. is possible that the melt fraction (basalt magma) would What if the radius had been larger? Going up in size, have migrated upward, and that these planetesimals if the radius had been doubled, and was 100 km instead included the parent asteroids for primitive achondrites like 50 km, the ratio of internal heat production to surface the lodranites and ureilites. Planetesimals from field (4) area would also have doubled, giving twice the heat flow would never have melted, although they may have become 2176 I. S. Sanders and E. R. D. Scott

100 high melt fractions almost certainly did develop within some young planetesimals. They were inferred by Taylor 1 2 3 4 et al. (1993) who argued that the IVB iron meteorites 80 interior will melt interior will melt interior will interior will completely completely partially melt remain solid crystallized from liquid iron that contained <1 wt.% before t = 1.5 Myr after t = 1.5 Myr after under thin crust under thicker t = 2.5 Myr sulfur (Goldstein et al. 2009) at >1770 K, a temperature crust at which primitive silicate material would have been well 60 over 50% molten. Taylor et al. (1993) also noted that, if pallasite olivine represents unmelted residue, then the 40 silicate melt fraction must have been between 70% and 90%. Keil et al. (1989) argued that the unusual texture of the Shallowater aubrite indicates a molten enstatite Planetesimal radius (km) 20 magma ocean at 1850 K. So while the Hevey and Sanders model is necessarily simplified, the evidence for magma oceans with high melt fractions suggests that the 0 0 0.5 1 1.5 2 2.5 model is not wildly wrong. The issue of precisely how Time of accretion (Myr after CAIs) global magma oceans were created is a matter for future Fig. 4. Plot showing the eventual outcome of heating by 26Al investigation; for now, we merely speculate that the decay in planetesimals as a function of radius and time of mechanism may possibly have been linked to gradual instantaneous cold accretion. The boundaries that delineate the accretion with the newly added material at an early stage four different fields are interpolated from Hevey and Sanders (e.g., t < 1.5 Myr) continually ‘‘dissolving’’ in any (2006, fig. 6) and Fig. 1b. Field (1) delimits planetesimals that highly radioactive rising basalt magma (see Kleine et al. will become substantially molten beneath a thin (e.g., <1 km) insulating crust before t approximately 1.5 Myr. Such 2012), or it was perhaps linked to the onset of convection planetesimals, we argue, will be prime candidates for bursting before significant melting had occurred, facilitated by a into chondrule spray if disrupted by impact between possible substantial reduction in bulk viscosity (Scho¨ lling t approximately 1.5 and t approximately 2.5 Myr. and Breuer 2009). Regardless of the details of the melting mechanism, we suspect that substantially molten heated and metamorphosed. They would have become interiors were the norm rather than the exception in chondrite parent bodies. Figure 4 can be regarded as a planetesimals that accreted within field (1) of Fig. 4. refinement of the related, but oversimplified and rather misleading, two-field diagram presented by Hevey and The Prevalence of Early Molten Planetesimals Sanders (2006, fig. 6) in which planetesimals were shown either to have melted or not melted. It also bears We suggest that early accretion leading to extensive similarities to fig. 5 of Moskovitz and Gaidos (2011), melting and core formation before t approximately although the latter has later accretion times for given 1.5 Myr was widespread in the protoplanetary disk. outcomes because it assumes 4 MeV, and not 3.1 MeV, as Between 100 and 150 separate parent bodies appear to be the decay energy per atom of 26Al. represented by the meteorites in the world’s collections, The thermal model of Hevey and Sanders (2006) one body for each meteorite group plus many more assumes that the internal 26Al heat source remains evenly represented by ungrouped meteorites (Meibom and Clark distributed at all stages during heating and cooling. 1999; Burbine et al. 2002). Only about one in five of these However, some authors question this assumption, arguing inferred bodies is chondritic. The rest of them, some four that basalt migrates rapidly upward as soon as it is of every five, melted and became differentiated bodies. If generated (e.g., Moskovitz and Gaidos 2011). As nearly all the low e182W measured in magmatic iron meteorites is aluminum enters the basaltic melt fraction, the removal of representative of the timing of melting in general, then it is such melt would also remove the heat source, forestall conceivable, indeed likely, that most of the material in the further internal heating, and invalidate the pattern of inner solar system existed as molten, or partially molten, melting shown in Fig. 2. Wilson and Goodrich (2012) even planetesimals by the close of the first 1.5 Myr. suggested that basalt migration away from the zone of partial melting was so rapid that ‘‘high degrees of mantle THE PRODUCTION OF CHONDRULES melting never occurred in any asteroids.’’ While we acknowledge that basalt was removed in The Case for Making Chondrules from Molten the case of the ureilite parent body, which we believe to Planetesimals have accreted late, in field 3 of Fig. 4, we cannot accept that basalt migrated from its source in all cases of The above discussion points to a young solar nebula planetesimal melting. Global magma oceans and very populated by largely molten spheres of primitive magma Origin of chondrules and chondrites 2177 undergoing turbulent convection at near-liquidus include the chondrules in the CB chondrites, believed to be temperatures, each enclosed by a thin outer shell of rigid, condensates from a giant impact plume about 4 Myr after thermally conducting crust, and coated by a layer of CAIs (Rubin et al. 2003; Krot et al. 2005). unconsolidated dust (Fig. 3). Molten metal possibly We consider in the following paragraphs how the formed suspended globules that would eventually predictions of the ‘‘splashing’’ scenario might be segregate as cores. As, over time, these planetesimals reconciled with what is known or can be inferred about must have been increasing in size and decreasing in the formation of chondrules. We initially reflect on the number as a result of mutual collisions and mergers (the following properties of chondrules: chondrule ages; necessary early steps along the stochastic road to planet chemical compositions, ‘‘peak’’ temperatures and formation), we deduce that many of the collisions would abundances; chondrule sizes; mutually indented shapes; have launched huge plumes of molten droplets cooling rates; relict grains; igneous rims; and metal (chondrule spray), mixed with loose dust from the inclusions. planetesimal surfaces, into the disk. Incidentally, while such collision plumes have commonly been described in Chondrule Ages Are Mostly 1.5–2.5 Myr After CAIs the literature as a ‘‘planetary’’ setting for chondrule A strength of the splashing model is that it can formation, this is misleading. A plume might more explain why chondrules are mostly between 1.5 and appropriately be regarded as a special case of a nebular 2.5 Myr younger than CAIs (Connelly et al. 2008; Kleine setting, albeit a rather local and ephemeral one. et al. 2008; Kita and Ushikubo 2012). 26Al-induced This scenario for chondrule formation, which is a planetesimal meltdown means that splash-generated clear alternative to the conventional shock-melting of chondrules would have been made from as soon as the dust-clumps, is the main subject of this article and we first planetesimals had melted, i.e., from about will now explore it in detail, amplifying and developing t=0.3 Myr onward. However, as explained above, we the case for it made recently by Asphaug et al. (2011). suggest that most chondrules made before t=1.5 Myr These authors noted that most collisions would have were destroyed because they accreted promptly and been oblique glancing blows in which much of the became buried inside planetesimals that were still highly ‘‘overlapping’’ portion of the smaller body (the radioactive and destined to melt. projectile) would have been engulfed by the larger body Those few chondrules that survive from before (the target), while the ‘‘overshooting’’ portion would t=1.5 Myr, such as some very old ones in Allende have undergone catastrophic decompression and (Connelly et al. 2011), may fortuitously have become expanded slowly down-range as a huge fan-shaped embedded in bodies that were too small to retain heat, or plume of droplets and debris. embedded in the cool outer layers of larger bodies Asphaug et al. (2011) emphasized that encounter (LaTourrette and Wasserburg 1998), or perhaps in velocities between merging planetesimals at the time of bodies where low temperatures were buffered by a large chondrule formation would generally have been close to, amount of ice. As these kinds of low-temperature setting or less than, the combined escape velocity of the merging would perhaps also have been necessary for the pair, namely tens of meters to a few hundred meters per preservation of CAIs, which date from t = 0, it may be second (the escape velocity in meters per second is no coincidence that the oldest chondrules appear to numerically about the same as the planetesimal’s radius survive in meteorites (i.e., CV chondrites) with a large in kilometers). At these very low velocities, no shock abundance of CAIs. melting would have occurred; almost the entire enthalpy Why would chondrule production have declined for melting would already have been generated by the rapidly after about t = 2.5 Myr? Chondrule production decay of 26Al and stored as magma in the planetesimals. by splashing would have continued for as long as molten The kinetic energy of impact would have caused planetesimals with thin crusts were colliding and mechanical disruption, but would have been negligible merging. By t approximately 2.5 Myr, heat loss by compared with 26Al decay as a cause of melting. conduction would have far exceeded internal heat The splashing model is, thus, quite different from, and generation by 26Al decay, and crusts would probably must not be confused with, the production of impact melt have been growing thicker, cooler, and mechanically spherules by energetic, high-velocity collisions. Such melt stronger as their underlying magma oceans crystallized droplets include crystal-bearing lunar spherules, evidently (Fig. 2b). In addition, following many collisions and the made during excavation of the huge impact basins on the production of a correspondingly large volume of Moon (Symes et al. 1998; Ruzicka et al. 2000), and chondrule-rich debris, thick accumulations of this debris terrestrial impact spherules of which those from the may have built up on the surfaces of remaining intact Eltanin site include beautiful coalesced droplets similar to planetesimals, rendering those planetesimals less compound chondrules (Kyte et al. 2010). They could also susceptible to bursting open on impact. In this light, the 2178 I. S. Sanders and E. R. D. Scott duration of chondrule formation between t=1.5 Myr The Inferred Time for Cooling to the Solidus Was and t=2.5 Myr would appear to coincide with a period Typically Several Hours of transition from a disk populated mainly by molten The successful experimental replication of chondrule bodies of various sizes, to a disk with fewer, larger textures under controlled rates of cooling shows that bodies (planetary embryos, perhaps) alongside a suite of near-liquidus droplets probably cooled and solidified newly accreted ‘‘second generation’’ chondritic bodies. over a matter of hours, and not seconds, nor days We note that chondrule production did not end (Lofgren 1989; Hewins and Radomsky 1990; Radomsky abruptly at t = 2.5 Myr; many CR chondrules have ages and Hewins 1990). This timescale was also inferred from of around 3 Myr after CAIs (Kita and Ushikubo 2012), a study of zoning in metal grains in CR chondrites based on their very low initial 26Al/27Al ratios, and their (Humayun 2012), and it is in broad agreement with the Pb-Pb ages. kind of time needed for an impact plume to expand and cool (Asphaug et al. 2011). Chondrules Have ‘‘Primitive’’ Chemistry, They Cooled From Near-Liquidus Temperatures, and They Are Chondrules in Some Meteorites Have Mutually Abundant Indented Shapes These three classic features of chondrules are clearly Hutchison and Bevan (1983) noted that some compatible with the splashing model. The essentially chondrules in Tieschitz (H3) are apparently molded unfractionated ‘‘primitive’’ chemistry of most chondrules, against neighboring chondrules, indicating that they were including their flat rare-earth element profiles (Jones still hot and plastic, if not liquid, when they came into et al. 2005), is consistent with the high degree of melting contact. Holme´n and Wood (1986) reported similar we envisage in molten planetesimal interiors prior to their textures in other chondrites. This remarkable feature was disruption (Fig. 3). The high subliquidus temperatures also described by others (Sanders and Hill 1994; from which most chondrules cooled (inferred from Hutchison 1996; Zanda 2004), but it has received only their igneous textures) is consistent with their derivation limited attention, perhaps because of arguments that the from hot convecting magma oceans where the deformed shapes could have resulted from the temperature was buffered just below the liquidus by compaction of chondrules into voids by shock (Sneyd steady radioactive heating off-set by efficient convective et al. 1988; Scott et al. 1992). and conductive heat loss. The high volume fraction of Recently, Metzler (2011, 2012) described spectacular chondrules, amounting to 80% or more by volume in examples of unshocked, mutually molded chondrules in some ordinary chondrites, is seen as a simple consequence large clusters, which he called ‘‘cluster chondrites.’’ They of the dominance of the molten planetesimals that occur as clasts up to 10 cm across in ordinary chondrite supplied those chondrules. breccias. He noted ‘‘this rock type consists of a mixture of deformed and undeformed chondrules and is Chondrule Sizes Are Mostly in the Range 0.1–2 mm characterized by low abundances of inter-chondrule Across matrix, low abundances of distinct chondrule fragments, The sizes of most chondrules are consistent with and restricted variations of chondrule sizes.’’ Hewins droplet sizes expected from the spraying or spattering of et al. (2012) have since reported similar clustering of larger volumes of magma. They fall in the range chondrules in Semarkona. observed widely in naturally formed droplets of magma Cluster chondrites are consistent with the splashing such as those, known as Pele’s tears, produced in basalt model. They suggest that molten chondrules in dense lava fountains at Kilauea volcano on Hawai’i. Other swarms, as would be expected in an impact plume, examples include the famous orange glass spherules at aggregated either into ‘‘sticky clusters’’ at least 10 cm the Apollo 17 site on the Moon (also presumed to have across, like giant compound chondrules made from formed in a lava fountain), and droplets produced by many thousands of individuals, or they accreted rapidly energetic impacts and shock melting, such as the crystal- to the surface of the target body, forming a blanket of bearing lunar spherules, and terrestrial impact-melt molded chondrule rock in the manner envisaged by spherules mentioned above. Also, chondrule-sized Asphaug et al. (2011). In the latter case, if the target droplets were produced experimentally by the spattering body were to have become the projectile in a later of melt in a solar furnace (King 1983). Finally, in their collision, then fragments of this welded chondrule model, Asphaug et al. (2011) showed that chondrule- blanket would have been launched and may eventually sized droplets result from equating the total surface have come to reside in the kind of chondritic breccia energy of droplets in the expanding plume with the described by Metzler. The accretion of hot chondrules energy associated with the catastrophic disruption and directly onto a planetesimal surface is suggested by the decompression of the interior of the molten planetesimal. parallel orientation of flattened molded chondrules seen, Origin of chondrules and chondrites 2179 for example, in the Bovedy (L3) chondrite (Sanders and Some Chondrules Contain Blebs of Metal Hill 1994). Small blebs of metal inside chondrules (e.g., Wasson The good size sorting observed by Metzler is and Rubin 2010) may have origins that are consistent significant. Size sorting has been observed widely in with the splashing model. Tiny globules of metal may chondrites, but its origin remains unclear. In the case of have been kept in suspension in the magma ocean by cluster chondrites, it appears that the sorting happened turbulent convection (Fig. 3). Some of these globules of locally, and very rapidly, during the brief interval between metal may have rained down late into the magma ocean the formation and accretion of a batch of chondrules. as the overlying crust, including any late-accreted material, was melted from below and thinned down Many Chondrules Contain So-Called Relict Grains (Fig. 2). Another possible explanation for metal inside (Xenocrysts) chondrules is that miniscule droplets of metal spray may Xenocrysts (foreign crystals) within a chondrule are simply have become engulfed, rather like xenocrysts, recognized because they are chemically out of equilibrium within silicate droplets in the expanding impact plume. with other crystals in the same chondrule and may also have anomalous oxygen isotopic compositions (Jones Problems with Making Chondrules from Clumps of Dust et al. 2005; Ushikubo et al. 2012). They were first reported by Nagahara (1981) and Rambaldi (1981). Nagahara We now consider a number of additional features of noted that these peculiar crystals imply that the chondrule chondrules that appear to pose difficulties for the host did not condense from a cooling vapor, so must have conventional view that chondrules began as clumps of been melted, and thus xenocrysts became dubbed ‘‘relict dust. We discuss sodium in chondrule olivine, oxidized grains’’ and were widely assumed to provide evidence for iron in chondrules, chondrule diversity, abnormally large precursor dust-clumps. crystals in chondrules, giant chondrules, the scarcity of Connolly and Hewins (1995) showed experimentally dust-clumps, and the cause of melting. In all cases, we that chondrule liquids tend to wet and swallow up dust show that the features can be explained in the context of grains that impinge on them. We suggest, therefore, that collision and splashing. xenocrysts were trapped and engulfed by melt droplets in flight, having been launched into the impact plume from Sodium Has Surprisingly High Concentrations in the loose surficial regolith. In this context, perhaps Olivine Crystals Within Chondrules smaller dust particles in the plume may similarly have Alexander et al. (2008) reported high concentrations been engulfed, and so become the seed crystals deemed of Na in chondrule olivine which, together with an absence necessary for the development of porphyritic textures in of isotopically mass fractionated isotopes of alkalis, point chondrules, as such seed crystals may not have been to high gas pressures and very closely spaced chondrules ubiquitous in the convecting magma prior to collision. (high chondrule densities) in enormous clouds measuring hundreds to several thousand kilometers across. They Some Chondrules Appear to Have Been Melted More inferred that these large chondrule clouds were generated Than Once by shock-melting of equally large clouds of precursor Textural evidence in chondrules for ‘‘multiple re- dust-clumps that would quickly have collapsed under their heating’’ events, such as igneous rims, has been widely own gravity and become new planetesimals. reported (e.g., Rubin and Krot 1996). In the context of We question the feasibility of their scenario. collision plumes, much of the re-launched loose debris Conventional dust-heating models do not explain how will have been derived from older chondrules. Thus, nebular processes could have concentrated the dust- chondrules which bear evidence of having been through clumps to the required extent, nor how nebular shock a few distinct heating and melting phases, may simply heating, or any other external heat source, could have have been caught up in collision plumes more than once. heated and melted such a huge mass of dust (equivalent Their coarse-grained rims may either be the enveloping to an entire planetesimal) in what was effectively a single mantles of compound chondrules, or simply be the result event. In addition, the energy would need to have been of heating of former dusty rims while suspended within delivered at just the right moment—after the cloud had the incandescent plume. Also, it is possible that, with become gravitationally unstable, but before its collapse turbulent motion in the expanding plume, an individual was completed. Finally, it is difficult to see how cold chondrule may have moved from a hot to a cooler region primitive matrix dust could have been mixed in with the and back again. Incidentally, we note that crystal- hot chondrules prior to accretion. bearing lunar spherules, which originated in impact By contrast, a molten planetesimal collision would plumes, in some cases also show chondrule-like features have created an enormous (planetesimal-scale) transient that suggest re-heating (Ruzicka et al. 2000). dense cloud of droplets. The droplets would have been 2180 I. S. Sanders and E. R. D. Scott immersed in their own Na-saturated vapor and therefore On the other hand, the production of type II capable of retaining high Na concentrations during chondrules by collision and splashing of molten olivine crystallization, at least during the early stages of planetesimals poses no problems, provided of course that plume growth. Dispersal of droplets after solidification the planetesimals were made of FeO-bearing magma. We in the cooling, expanding cloud would have allowed return to this issue in the final section of the article. mixing of chondrules from different impacts and addition of primordial dust containing presolar grains. Chondrules Are Chemically Diverse The dust could have come directly from the nebula or it While chondrules have primitive chemistry in a broad could have been derived as recycled dust from the poorly sense, they are not identical, and they vary particularly in consolidated regolith of the disrupted planetesimals. their contents of olivine and pyroxene. Their chemical In a parallel study to that of Alexander et al. (2008), diversity presents problems for the dust-clump hypothesis. Hewins et al. (2012) reported similarly high levels of Na A millimeter-sized chondrule precursor clump might in chondrule olivine, and also reported Na in melt typically have contained between 106 and 109 dust grains inclusions in the olivine, in the mesostasis, and in bulk between 1 and 10 microns across. In a well-mixed disk, chondrules, all in Semarkona (LL 3.0). They inferred therefore, all dust-clumps might be expected to have partial evaporative loss of Na from the molten sampled much the same statistically representative chondrules, followed by its later re-condensation and its selection of available grains and so shared the same incorporation in chondrule mesostases. Unlike primitive chemical composition. Thus, olivine-rich and Alexander et al. (2008), they were not committed to the pyroxene-rich chondrules are very unlikely to have formed idea of conventional shock melting, and suggested as an from clumps of micrometer-sized dust grains because alternative that chondrules may have formed in ‘‘debris nebular processes cannot conceivably have sorted the dust clouds after protoplanetary collisions,’’ but they did not into millimeter-sized monomineralic aggregates. elaborate on how the melt droplets formed. We suggest, in the context of the splashing model, Chondrules in enstatite chondrites are not only Na- that magmatic processes in molten planetesimals prior to rich but contain evidence for sulfidation of silicates and collision could have generated liquids ranging from metal-sulfide nodules (e.g., Rubin 1983). Lehner et al. olivine-rich to pyroxene-rich. With high degrees of (2011) and Petaev et al. (2012) propose that ordinary melting, olivine would have been the sole liquidus phase. ferromagnesian chondrules reacted with an S-rich and H- Olivine has an atomic Si ⁄ Mg ratio of 0.5, which is less poor gas above 1000C. Such conditions, we suggest, would than solar (0.9), so its removal by crystal settling from a have been more easily created within an impact-generated primitive unfractionated magma ocean would have plume rather than within the conventional solar nebula. driven the residual melt toward pyroxenitic compositions for which the Si ⁄ Mg ratio is 1. Chondrules Contain Iron as FeO Mostefaoui et al. (2002) observed that olivine-rich Type II chondrules contain significant levels of FeO. chondrules (lower Si ⁄ Mg) were made, in general, earlier Even type I chondrules are rarely completely free of it. than pyroxene-rich chondrules (higher Si ⁄ Mg). Tachibana FeO poses a serious problem for the production of et al. (2003) and Kita et al. (2005) confirmed this chondrules from dust-clumps in the nebula because the correlation between Si ⁄ Mg and age with more precise stabilization of FeO in chondrule melts requires ambient measurements and an extended data set. Tachibana et al. gas that is several orders of magnitude more oxidizing (2003) attempted to explain the correlation in terms of than the standard hydrogen-rich nebula. In an ongoing conventional flash-melting of dust balls in an open system effort to resolve this conundrum, Fedkin and Grossman over a number of cycles. With each cycle, differential (2006, 2010), Fedkin et al. (2012), and Grossman et al. evaporation of Si relative to less-volatile Mg meant that (2012) reaffirmed the view of Alexander et al. (2008) that successive generations of dust-grain precursors became chondrules were created in close proximity to each other progressively enriched in condensates with higher Si ⁄ Mg. (to retard evaporation of volatile elements like Na), but While we agree that evaporation and condensation also concluded that chondrules were enveloped by processes in the nebula may have been important, we oxidizing, H2O-bearing, gas to stabilize FeO. They cannot see how, in physical terms, the chondrules (with tentatively suggested that such a setting might have existed low Si ⁄ Mg) were removed from the system following in the aftermath of collisions involving icy planetesimals. each cycle. We prefer to explain the correlation in terms This setting for chondrule formation is difficult to of olivine crystal fractionation. We suggest that over reconcile with the shock melting of dust-clumps because time the Si ⁄ Mg ratio of the magma oceans in many the timing of the heating event would need to have planetesimals increased ‘‘in step’’ at roughly the same coincided precisely with the brief, ephemeral existence of rate, and so did the Si ⁄ Mg of the chondrules made from the H2O-enriched collision plume. those magma oceans by collision and splashing. Origin of chondrules and chondrites 2181

In support of this explanation, we draw attention to have compositions and textures like those of normal evidence in differentiated meteorites that magmatic millimeter-sized chondrules (Binns 1967; Prinz et al. fractionation driven by crystal settling probably did occur 1988; Hill 1993; Bridges and Hutchison 1997; Ruzicka during the first few million years. Baker et al. (2012) et al. 1998). Macrochondrules would be difficult to form reported a deficit in d26Mg in main group pallasites that by a nebular flash-melting process because of the need to corresponds to separation of crystals from liquid at around transfer heat rapidly to the center of what would 1 Myr after CAIs. A similar result was obtained from the presumably have been a large (e.g., golf-ball sized), Eagle Station pallasite by Villeneuve et al. (2011). On the porous, and thermally insulating dust-clump without HED, parent body crystal fractionation, albeit of glazing or vaporizing the outside first. Also, as the rise in pyroxene rather than olivine, led to igneous differentiation temperature would generally be proportional to the ratio over 1 or 2 Myr, reflected in a steady increase in d26Mg of the surface area to the mass of a dust-clump, then a going from the most primitive diogenites to cumulate flash-melting event that delivers just the right amount of eucrites (Schiller et al. 2011). energy to make millimeter-sized chondrules would barely affect the temperature of a centimeter-sized clump. We Some Chondrules Contain Abnormally Large and prefer to interpret macrochondrules simply as large blobs Oscillatory-Zoned Crystals of magma that failed to be shaken and dispersed into Some single olivine crystals in chondrules are normal-sized chondrule droplets in the collision plume. unusually large and may occupy >90 vol.% of the chondrule. They seem unlikely to have crystallized from Dust-Clumps Are Rarely Observed in Chondrites chondrule-sized melt droplets and more probably If chondrules had formed from precursor dust- originated in much larger volumes of melt. Large clumps, we might expect to find small lumps of such rounded olivine grains that occupy almost all the material in the interchondrule matrix. Although there is chondrule (e.g., fig. 1e in Jones et al. 2000) are especially evidence for them in some meteorites (Rubin 2011, pp. difficult to form by conventional mechanisms involving 553 and 554), millimeter-sized aggregates that might dust-clumps. However, rounded crystals may form in represent plausible precursor dust-clumps are really convecting magma under conditions that periodically rather rare (e.g., Scott and Krot 2007). Moreover, matrix require dissolution as well as growth. In some terrestrial is typically more FeO-rich than chondrules, so it is an settings, phenocrysts develop rounded and embayed inappropriate starting material. shapes, apparently for this reason. Some authors have reported dust-rich chondrules Olivine grains are also found as large (<1 mm) which they interpret as incipiently melted products of isolated single crystals in chondrite matrices and their nebular flash-heating (Nettles et al. 2001; Ruzicka et al. origin has not been satisfactorily explained. Steele (1989) 2012a). Certainly, these objects display clear evidence of and Weinbruch et al. (2000) argued that they are nebular low levels of intergrain melt, but we suggest that they are condensates, but Jones et al. (2000) found traces of low-Ca not necessarily products of dust-clump melting. We can pyroxene and mesostasis attached to large rounded forsterite envisage three possible ways that they might have been crystals, and inferred that they formed in chondrules. We generated in the collision and splashing scenario. First, suggest that the large isolated crystals in the matrix, like those they may be fragments broken from the incipiently in chondrules, were in suspension in asteroidal magma melted lower rigid crust (Fig. 3) of the disrupted oceans at the time of collision and disruption. projectile. Second, they may be some sort of welded Oscillatory chemical zoning is present in some large accretionary lapilli that grew in the impact plume of crystals of olivine and pyroxene in both chondrules and droplets and dust; this is how we would interpret chondrite matrices (Steele 1995; Jones and Carey 2006; chondrule Beg-6 reported by Ruzicka et al. (2012a). McCanta et al. 2009; Blinova et al. 2011). Oscillatory Third, they may be melt droplets that became choked in zoning in some elements such as phosphorus (McCanta flight with dust grains, which is perhaps just a special et al. 2009) may result from disequilibrium crystallization, case of the second example. In both the second and third but oscillatory zoning in Fe and Mg (Blinova et al. 2011) is examples, the principal source of dust would have been hard to explain by crystal growth in a droplet, and may the loose accumulation of regolith covering the colliding perhaps be attributed to variations in the environment of a planetesimals. growing crystal that was suspended in convecting magma (Shore and Fowler 1996). A Plausible Heat Source for Melting Dust-Clumps Remains Elusive Macrochondrules Exist The identification of a heat source capable of Some chondrules, called macrochondrules or mega- melting nebular dust-clumps on the required scale has chondrules, are more than one centimeter across, yet proved to be somewhat intractable. The currently 2182 I. S. Sanders and E. R. D. Scott favored nebular shock-melting model (Desch and planetesimals themselves were compositionally bimodal, Connolly 2002; Boss and Durisen 2005; Morris and being either volatile-depleted and reduced with most of Desch 2010) claims to provide an appropriate thermal their iron in metal, or volatile-bearing with much of their history for the inferred chondrule cooling rates, and iron as FeO. Morris et al. (2012) similarly suggest that bow shocks around planetary embryos may provide a plausible What Was the Planetesimal Source for Type I thermal regime for making chondrules. Nevertheless, as (FeO-Poor) Chondrules? discussed here, shock melting is difficult to reconcile with We tentatively suggest that type I chondrules came such observations as chondrule diversity, macrochondrules, from molten parent bodies which were the same as, or and the retention of Na and FeO in chondrules. Also, similar to, the parent bodies of certain iron meteorites. shock melting should lead to significant isotopic Many iron meteorites are extremely depleted in fractionation effects, but these are not observed. For moderately volatile siderophile elements, and their parent example, Fedkin et al. (2012) calculated that at enormously planetesimals had generally melted within the first inflated PH2O (·550, to maintain iron as FeO rather than million years (Burkhardt et al. 2008). As Bland and metal) and with huge dust enrichment (·600, to retard Ciesla (2010) noted, they may have acquired their evaporation), all iron would evaporate during shock-wave volatile-depleted chemical signature by accreting early heating, that olivine would grow in the unevaporated Mg- from partially condensed matter in the still-hot inner rich melt droplets before re-condensation, and large region of the infant disk, perhaps close to 1 AU (Bottke isotopic fractionation effects between olivine and glass et al. 2006) where the ambient gas would have been would be preserved. As such isotopic fractionation is not hydrogen-rich and reducing. seen, they concluded that chondrules were unlikely to In this light, we wonder whether the giant hit-and- have formed in nebular shocks. run collision postulated by Yang et al. (2007) to account Turning briefly to other postulated sources of rapid for the IVA iron meteorite parent body may have nebular heating, the once fashionable X-wind model created an enormous cloud of type I chondrules. That (Shu et al. 1997, 2001) in which proximity to the collision, it has been argued, left in its path a string of protosun is linked to melting, seems beset with secondary planetesimals, one of which (the IVA body) insurmountable problems and has now been largely was purportedly a sphere of molten metal perhaps discredited (Desch et al. 2010). Electrical discharge 300 km in diameter covered by a veneer of silicate rock. heating (Love et al. 1995), electromagnetic radiative One particular IVA meteorite, Muonionalusta, has a heating (Eisenhour et al. 1994), gamma-ray bursts Pb-Pb age of 4565.3 ± 0.1 Myr (Blichert-Toft et al. (McBreen and Hanlon 1999), and heating in protostellar 2010), suggesting that the metal was solid by that time jets (Liffman and Brown 1996), along with other heat and that the postulated giant collision happened sources mentioned in a review by Rubin (2000), all have somewhat earlier, consistent with the age of chondrules. serious shortcomings and are not considered viable. We note that the published age for Muonionalusta may The splashing model, in total contrast, has a clearly be about 1 Myr too old, as it is based on an assumed understood and quantifiable heat source in 26Al, and 238U ⁄ 235U ratio of 137.88 that is probably too high obviates the need to find a mechanism for the rapid (Connelly et al. 2011). Nevertheless, the uncertainty in heating of dust in the nebula. its age does not affect our contention that the postulated giant IVA collision could have been synchronous with CHONDRULES AND THE EVOLUTION OF chondrule formation. PLANETESIMALS The idea that volatile depletion is a signature of very early planetesimal accretion from a hot partially We now explore in the context of the splashing condensed nebula is supported not only by certain iron model, the nature of the planetesimals that preceded the meteorites but also by the basaltic achondrites. The period of chondrule formation and the properties of the HED meteorites and the angrites are both highly chondrite parent bodies that postdated chondrule depleted in volatile elements, and recent revision of their formation. Sr isotope systematics (Hans et al. 2011; Kleine et al. 2012; Moynier et al. 2012) suggests that their parent The Nature of the Postulated ‘‘Precursor’’ Planetesimals bodies accreted, like the iron meteorite parent bodies, to Chondrules close to the time of formation of CAIs. An earlier proposal by Halliday and Porcelli (2001) that volatile If the molten planetesimal splashing hypothesis is depletion in the angrites and HEDs might have resulted correct, then type I (FeO-poor) and type II (FeO-rich) from giant impacts 2 or 3 Myr after CAIs now seems less chondrules suggest, respectively, that the precursor likely. Origin of chondrules and chondrites 2183

On the same theme, Scott and Sanders (2009) appealed to a reservoir of volatile-depleted bodies dating 3 from the time of CAI formation as the refractory Mn- R poor end-member of a mixing line with Mn-rich CI-like dust to explain the whole-rock Mn-Cr isochron for 2

carbonaceous chondrites. % O ( ) 17 LL What Was the Planetesimal Source of Type II (FeO- L 1 rich) Chondrules? H We suggest that, in contrast to the reduced and volatile-depleted planetesimals that gave rise to type I EL chondrules, type II chondrules came from planetesimals EH 0 that were initially made from aggregates of dust and 010203040 26 water-ice. On becoming heated by Al, the ice would FeO/(FeO + MgO) in silicate (mol %) have melted to water, which would then have reacted, Fig. 5. Plot of mean molar FeO ⁄ (FeO + MgO) in equilibrated while still at a low temperature, with anhydrous silicate silicates of five chondrite groups against mean D17O of those grains and metal to produce minerals such as serpentine groups. and magnetite. On further heating, this oxidized suite of minerals would have remained oxidized as it became dehydrated and finally melted. disk is thought to have been isotopically very heavy. The accretion of water-ice would have required a Values of d17Oandd18Ocloseto+180&, i.e., on a slope 1 cold nebular setting. This setting may have developed line passing through Earth, were measured in tiny grains later than the time of accretion of the volatile-poor type of a nanoscale-mixed magnetite-sulfide phase scattered in I planetesimals, after the initially hot nebula had cooled the matrix of the pristine carbonaceous chondrite Acfer sufficiently for water-ice to condense, or perhaps it 094 (Sakamoto et al. 2007). These grains show just how existed farther from the Sun, where it had always been enriched in the heavy isotopes of oxygen water-ice may suitably cold. In the latter case, subsequent orbital conceivably have been. If water-ice supplied the oxygen migration presumably brought oxidized and reduced that stabilized iron as FeO in meteorites, as we propose, planetesimals close together, because type I and type II then the value of D17O in a meteorite might be expected chondrules are found side by side in many meteorites. to correlate with its degree of oxidation. We note that there are limits to the amount of water- Such a correlation is observed. A progressive increase ice that could have been incorporated into a planetesimal in oxidation state, expressed as molar FeO ⁄ (FeO + MgO) if the planetesimal were later to have melted. The latent in equilibrated pyroxene or olivine, going from the E heat absorbed by vaporizing ice is a massive 2.6 kJ g)1, chondrites, through the H, L, and LL chondrites to the R and the specific heat of steam is 2 J g)1 K)1. Thus, with chondrites, correlates with increasing D17O (Fig. 5). These only 6.6 kJ g)1 available in the dust, a planetesimal equilibrated compositions are the average of a range of accreting at t=0 from equal masses of ice and dust FeO ⁄ (FeO + MgO) and D17O values that would have could not have melted in time to make chondrules by existed in the chondrules and matrix of the original splashing, and perhaps it never would have melted. unequilibrated assemblage in each case. However, somewhat lower mass fractions of ice could The correlation line of Fig. 5 does not extend to have led to different levels of oxidation of iron as well as bulk carbonaceous chondrites, which have negative permitting melting in time to make chondrules. values of D17O. Nevertheless, chondrules within Planetesimals would have grown over a period of individual carbonaceous chondrites have recently been time, both from gradual accumulation of nebular dust, found to show a good correlation between D17Oand and from mergers with other planetesimals. Thus, they FeO ⁄ (FeO + MgO). The correlation has been observed would have been composite bodies that incorporated in chondrules in CO chondrites (Tenner et al. 2011), CR both reduced and oxidized iron prior to their eventual chondrites (Schrader et al. 2011; Tenner et al. 2012), and meltdown. This might explain why the distinction in Acfer 094 (Ushikubo et al. 2012). Again, this between type I and type II chondrules based on their correlation appears to corroborate the hypothesis that Fe ⁄ (FeO + MgO) values, particularly in ordinary oxidation was induced by H2O derived from water-ice chondrites, is not always sharp. with high D17O. Independent evidence suggesting that water-ice was a The correlation between oxidation state and D17Ois principal cause of oxidized iron in meteorites comes from not confined to chondrites and chondrules. In ureilites, a consideration of D17O in water-ice. Water-ice in the FeO ⁄ (FeO + MgO) in olivine correlates with D17O 2184 I. S. Sanders and E. R. D. Scott

(Clayton and Mayeda 1996; fig. 6 of Goodrich and As a possible solution to the puzzle, we imagine that Delaney 2000). Explaining its cause has proven difficult; the materials in each reservoir were stored for most of Goodrich and Wilson (2011) inferred that the correlation the time, not in the nebula as dispersed dust, chondrules, was somehow inherited from the state of the parent body and other tiny objects that could easily have been mixed prior to melt extraction. In line with that view, we radially, but as loose or poorly consolidated outer layers speculate here that the ureilite parent body accreted in a on planetesimals whose orbital radii remained more or heterogeneous way, with local internal variation in its less constant. Thus, we envisage each reservoir as a content of water-ice. We envisage that the higher the circum-solar annulus whose planetesimal population was ice content in a given subvolume of the parent body, the main carrier of its unique chemical and isotopic the higher would have been the final values of peculiarities. We imagine, over time, a succession of FeO ⁄ (FeO + MgO) and of D17O in that subvolume, and planetesimal collisions and mergers within an individual we suspect that these values would have persisted in annulus, such that each impact plume injected into its the solid residue following melt extraction. The same local zone a fresh batch of chondrules, along with older process appears also to have operated in the recycled debris from the regoliths and crusts, and that acapulcoite ⁄ lodranite parent body, where a similar the whole mixture was promptly accreted to new, or correlation is observed (McCoy et al. 1997; Greenwood existing, planetesimals before there was time for it to et al. 2012). have been dispersed throughout the disk. In this way, the distinctive chemical, petrographic, and isotopic traits of The Accretion of Chondritic Asteroids each ‘‘reservoir’’ would have been kept largely intact and eventually preserved in each individual chondrite parent Finally, in the context of the collision and splashing body (=chondrite group). Also, as each annulus would model, we speculate on the origin of a number of well- have remained an essentially closed chemical system, the known features of chondrites that, hitherto, have largely scrambling of materials within it could not have erased defied a satisfactory explanation, and we discuss their its initial near-solar chemistry despite more than 2.5 Myr wider implications. These features include: (1) the of processing. distinctive characteristics of each chondrite group, (2) the Undoubtedly, there would have been some exchange so-called ‘‘complementarity’’ between chondrules and of dust and chondrules between neighboring annular matrix, (3) the depletion of metal in most chondrites, and reservoirs, and the degree of intermixing may well have (4) the huge mismatch between the large number of increased with the passage of time as planetesimals meteorites that are chondritic, and the small number of became fewer and larger, and underwent orbital parent bodies that are chondritic. migration. It is perhaps for this reason that particular kinds of chondrule are not always confined to single Why Is Each Chondrite Group Distinctive? groups, but may be present in a number of groups (e.g., Each of the 15 chondrite groups is thought to come H, L, and LL), albeit in different proportions in each. from its own separate parent body, characterized by a How many of our imagined chondrule-forming unique suite of chondrules and other components, and collision events are recorded in an individual chondrite? having distinctive petrographic, chemical, and isotopic Statistical peaks in the distribution of chondrule ages features (e.g., Rubin 2000, 2010; Jones et al. 2005; Scott (Villeneuve et al. 2009) and the clustering of oxygen and Krot 2007). To account for each group’s properties, isotope compositions in Mg-rich olivine in chondrules Jones (2012) inferred that ‘‘multiple reservoirs of (Libourel and Chaussidon 2011) hint that the number chondrite components were present in the protoplanetary may have been in single figures. disk, and that these were separated spatially, temporally, or both, such that limited mixing occurred between the Why Do Chondrules and Matrix Display Compositional separate reservoirs’’ before and during accretion of the ‘‘Complementarity’’? respective chondrite parent bodies. Aware that turbulent In some carbonaceous chondrites, particularly in mixing and radial drift in the disk are likely to have those of the CR group, the chondrules and matrix are quickly destroyed a reservoir’s identity and led to the chemically quite distinct from each other, yet the two widespread homogenization of disk materials, Jones complement each other, such that when mixed together, (2012) states that ‘‘the problem of maintaining such a they have almost perfect solar, i.e., CI-like, element separation over an extended time period is currently one ratios (Hezel and Palme 2010). This remarkable of the biggest conundrums associated with our overall relationship, dubbed ‘‘complementarity,’’ suggests that picture of the early history of the solar system.’’ Wood chondrules and matrix were formed from a single volume (1988) also acknowledged this problem. of starting material with solar-composition. Hezel and Origin of chondrules and chondrites 2185

Palme (2010) envisage a local part of the nebula where debris plume of droplets and dust depleted in metal the chemically distinct refractory chondrules and volatile-rich dust were created from the same batch of primitive dust, and then promptly recombined and added to the chondrite parent body. With this kind of local, projectile’s self-contained process in mind, Palme et al. (2011) metal core claimed that ‘‘complementarity’’ rules out the production of chondrules from molten planetesimals. However, we suggest that ‘‘complementarity’’ need Fig. 6. Cartoon showing a cross-section through a colliding not have been the outcome of local processing, and that pair of planetesimals following an oblique impact during it can be reconciled with ‘‘splashing.’’ We imagine that accretion, based on results of Asphaug et al. (2011). The iron each evolving annulus of planetesimals and dust core of the impactor is shown largely embedded in the target body, leaving the ejecta plume depleted in iron metal and other remained an essentially closed chemical system, as siderophile elements. outlined above. With each collision and shower of hot chondrule droplets, evaporation and recondensation would have led to a net transfer of the more volatile Mars, like the Moon, may be depleted in metal as its elements from the chondrules to the surrounding fine density is low relative to that of the Earth, even after dust. If the chondrules and dust remained within their allowing for differences in the internal pressures of the two host annulus then, sooner or later, they would have been planets. This suggests that Mars possibly grew from reunited. In this way, even with many successive phases planetesimals that were predominantly depleted in of collision and re-accretion, a complementary siderophile elements, like the L and LL chondrites. We relationship between chondrules and matrix would have speculate that such depletion of metal may be linked to the been preserved. recent explanation for the small size of Mars proposed by Walsh et al. (2011). These authors postulated that Jupiter Why Are Some Chondrite Groups Strongly Depleted in and Saturn migrated toward the Sun until their orbits Siderophile Elements? reached a 3:2 mean-motion resonance and then migrated Chondrite groups other than the H, EH, and out to near their present locations. This so-called ‘‘Grand CH ⁄ CB groups are depleted to varying degrees in Fe, Ni, Tack’’ would have cleared the disk of most planetesimals and other siderophile elements relative to the CI as far in as 1.5 AU, leaving in its wake a mere smattering chondrites (e.g., fig. 2 in Krot et al. 2003). The depletion of scattered planetesimals from which Mars could grow. is particularly marked in the L and LL groups; in the LL We speculate that the residual, thinned out suite of group, Mg-normalized molar siderophile element planetesimals may have been biased toward second- concentrations are at roughly half their CI levels (Krot generation chondritic bodies with siderophile deficits. et al. 2003). We speculate here that this well-known metal Why Are Five out of Six Meteorite Falls Chondrites, depletion is a simple consequence of planetesimal When Only One in Five Parent Bodies Is Chondritic? mergers. In a typical oblique-impact merger, as Chondritic meteorites are common, accounting for portrayed by Asphaug et al. (2011), the molten metallic between 80% and 85% of observed falls, yet as we noted core of the projectile body would have become earlier, they appear to have been sourced from only about substantially embedded inside the larger target body, 20% of the 100–150 inferred meteorite parent bodies. This while the ejecta plume would have been dominated by disparity has led to considerable debate on the material from the projectile’s silicate mantle (Fig. 6). If, composition of asteroids (Burbine et al. 2002). We suspect with successive collisions, the metal repeatedly showed that the high abundance of chondrites among meteorites is this preference for joining the larger body of the merging a true reflection of their actual abundance in the asteroid pair, then the ejecta would have become progressively belt, and we postulate that the large number of depleted in metal and enriched in silicate, and so would differentiated parent bodies that have been sampled is no the chondrite parent bodies constructed from that ejecta. indication of the abundance of these bodies in the asteroid The corollary is that the target bodies, and ultimately the belt today, but is a legacy of the situation that existed in planets, would have a proportionately higher fraction of the infant solar system before chondrule formation. metal than the solar average. The process we envisage is The ‘‘splashing’’ hypothesis contends that as the the same, albeit on a much smaller scale, as that early molten bodies merged together and grew in size, envisaged for the formation of the Moon by giant eventually to produce planets, they released swarms of impact, which led to a large deficit of iron within the chondrules and dust, which aggregated to make Moon and an increase of iron in the Earth’s core. chondritic asteroids. The latter, second-generation 2186 I. S. Sanders and E. R. D. Scott bodies, it appears, came to dominate the tiny mass of bimodal division of chondrules into types I and II material that escaped being subsumed into planets, and reflects a bimodal chemical division of molten they now reside, somewhat battered and brecciated, in planetesimals, such that reduced refractory planetesimals the asteroid belt. Of the original molten bodies, perhaps supplied type I chondrules and may also have been the Vesta alone survives intact, while the many others are source of many iron meteorites, while oxidized volatile- represented only by fragments of iron, stony iron, and bearing planetesimals supplied type II chondrules and rare achondritic material not from Vesta, as either probably incorporated varying amounts of water-ice, isolated pieces or assembled into asteroidal rubble piles. reflected in the correlation between D17O values and We imagine that by t approximately 2.5 Myr, when 26Al FeO ⁄ (FeO + MgO) in some groups of meteorite. had lost its potency and chondrule formation was in Finally, we propose that at the close of the ‘‘meltdown rapid decline, most of the original molten planetesimals era,’’ chondritic planetesimals began to appear. We had already merged into larger bodies and disappeared. imagine that each chondritic parent planetesimal is a As John Wood (2000) so aptly put it, the beginning was mixture of chondrules and debris launched and re- ‘‘swift and violent.’’ accreted more than once in a discrete chemically restricted annulus in the disk, to account for its unique SUMMARY AND FURTHER WORK traits, its broad solar composition, its level of metal depletion and, in some cases, its ‘‘complementarity’’ We find it remarkable that the chronological between chondrules and matrix. evidence for the timing of early planetesimal meltdown So where do we go next in the quest to test and (by t approximately 0.3 Myr) and the timing of develop the scenario presented here? chondrite accretion (after t approximately 2.5 Myr) Conventional ideas about chondrule formation are coincide so well with the timing of these processes predicated on the intuitive, but now largely discredited, calculated from the inferred 26Al heat source in nebular belief that chondrites are samples of the very first dust. This coincidence, along with the evidence in our planetesimals to have formed. We submit that this meteorite collections for a very large number of early conceptual framework has contributed a great deal to molten bodies, has led us to envisage the inner solar the understanding of disk processes, and that we would system during its first 2 Myr as being populated with a be presumptuous to claim that all chondrules were made great abundance of substantially molten planetesimals. by ‘‘splashing.’’ After all, some chondrule-like objects in We might name those first approximately 2 Myr, from meteorites (CAIs in particular) are widely believed to which so little tangible evidence survives, the solar originate in a high-temperature nebular setting that was system’s ‘‘meltdown era.’’ As planetary embryos were not an impact plume. Nevertheless, we believe that probably already forming during this period, the molten meteorite chronology and the likelihood of 26Al heating planetesimals must have been continuously colliding and have brought about a new paradigm, which holds more merging, becoming fewer in number, and growing larger promise than the conventional view for understanding in size. Within this conceptual framework for the young the evolution of the nascent disk in general, and the disk, chondrule production from the ‘‘splashing’’ of origin of chondrules in particular. molten planetesimals seems an inevitable consequence, To move forward, we need thermal and petrological with new generations of chondritic planetesimals being models of planetesimal evolution that can accommodate spawned from the debris ejected and dispersed during collision and accretion history as well as different mergers. mechanisms of heat loss from magma oceans, not just We believe that the ‘‘splashing’’ hypothesis can be during the heating stage but also during cooling. We reconciled with much of what we understand about need more sophisticated models of impact plumes, to try chondrules, including their ages, chemical compositions, to understand how particles within them would have peak temperatures, abundances, sizes, cooling rates, evolved thermally and spatially under a range of collision indented shapes, ‘‘relict’’ grains, igneous rims, blebs of conditions including different impact angles, encounter metal, retention of Na, presence of FeO, diversity, and velocities, planetesimal sizes, planetesimal internal large phenocrysts, as well as other issues that constrain structures, and regolith thicknesses. We need further chondrule origins such as the formation of precise chronology for chondrules and differentiated macrochondrules, the scarcity of dust-clumps, and the meteorites to strengthen the evidence for meltdown need for a feasible heat source. However, we contend before chondrule formation. In particular, the possibility that several of these chondrule properties, most notably that the 26Al-26Mg chronometer may be flawed (Larsen the concentration of Na in olivine, challenge the long- et al. 2011), and that the 26Al heat source may have been standing conventional interpretation of chondrules as much weaker than assumed, needs thorough shock-melted dust-clumps. We speculate that the investigation. Also, we need to check our model’s Origin of chondrules and chondrites 2187 prediction that type II chondrules are younger than the Baker J. A., Schiller M., and Bizzarro M. 2012. 26Al–26Mg oldest type I chondrules. deficit dating ultramafic meteorites and silicate planetesimal On a broader front, we need to better understand the differentiation in the early solar system? Geochimica et Cosmochimica Acta 77:415–431. dichotomy of chondrites and why virtually all Binns R. A. 1967. An exceptionally large chondrule in the differentiated meteorites are derived from materials that Parnallee meteorite. Mineralogical Magazine 36:319–324. isotopically resemble ordinary and enstatite chondrites, Bland P. A. and Ciesla F. J. 2010. The impact of nebular with so few genetically related to carbonaceous chondrites evolution on volatile depletion trends observed in (Warren 2011). We also need to investigate whether our differentiated objects (abstract #1817). 41st Lunar and Planetary Science Conference. CD-ROM. model can shed light on the scarcity of meteorite breccias Blichert-Toft J., Moynier F., Lee C.-T. A., Telouk P., and carrying both chondritic and differentiated components, Albare` de F. 2010. The early formation of the IVA iron whether it can explain why the cooling rates of magmatic meteorite parent body. Earth and Planetary Science Letters iron meteorites are surprisingly high, or why basaltic 296:469–480. meteorites did not crystallize at the same time as iron cores Blinova A., Stern R., and Herd C. D. K. 2011. In situ SIMS oxygen isotope measurements of zoned olivines in the segregated (e.g., Kleine et al. 2012), and whether it can tell Tagish Lake meteorite chondrules (abstract). Meteoritics & us anything about the virtual absence of meteorites made Planetary Science 46:A22. from olivine rock (i.e., from planetesimal mantles) that Boss A. P. and Durisen R. H. 2005. Sources of shock waves in might complement irons, stony-irons, and basaltic the protoplanetary disk. In Chondrites and the achondrites. Finally, all the above avenues of potential protoplanetary disk, edited by Krot A. N., Scott E. R. D., and Reipurth B. Astronomical Society of the Pacific future enquiry should intersect the paths being followed in Conference Series 341. San Francisco: Astronomical the pursuit of numerical models of planetesimal accretion Society of the Pacific. pp. 821–838. and orbital evolution (e.g., Bottke et al. 2006; Johansen Bottke W. F., Nesvorny D., Grimm R. E., Morbidelli A., and et al. 2007; Walsh et al. 2011). We are optimistic that all O’Brien D. P. 2006. Iron meteorites as remnants of approaches will eventually converge on an agreed, self- planetesimals formed in the terrestrial planet region. Nature 439:821–824. consistent picture of the first critical steps of planetary Bridges J. C. and Hutchison R. 1997. A survey of clasts and development in the solar system. large chondrules in ordinary chondrites. Meteoritics & Planetary Science 32:389–394. Acknowledgements—We are grateful to Stuart Agrell who Burbine T. H., McCoy T. J., Meibom A., Gladman B., and inspired us both as students, to Bob Hutchison who was Keil K. 2002. Meteoritic parent bodies: Their number and identification. In Asteroids III, edited by Bottke W. F., steadfast in his conviction that differentiated planetary Paolicchi P., Binzel R. P., and Cellino A. Tucson, Arizona: bodies existed before chondrites, and to John Wood The University of Arizona Press. pp. 653–667. despite his conviction that they did not. We are indebted Burkhardt C., Kleine T., Bourdon B., Palme H., Zipfel J., to Klaus Keil, whom we honored at the Workshop on the Friedrich J. M., and Ebel D. S. 2008. Hf–W mineral Formation of the First Solids in the Solar System, for his isochron for Ca,Al-rich inclusions: Age of the solar system and the timing of core formation in friendship and distinguished leadership in meteoritics and planetesimals. Geochimica et Cosmochimica Acta 72:6177– cosmochemistry. We acknowledge financial support from 6197. Trinity College Dublin (IS) and the NASA Burkhardt C., Kleine T., Dauphas N., and Wieler R. 2012. Cosmochemistry program (ES). 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