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 Ó 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