Experimental Constraints on Chondrule Formation

Chondrule textures depend on the extent of melting of the chondrule precursor material when cooling starts. If viable nuclei remain in the melt, crystallization begins immediately, producing crystals with shapes that approach equilibrium. If not, crystallization does not occur until the melt is supersaturated, resulting in more rapid growth rates and the formation of skeletal or dendritic crystals. A chondrule texture thus indicates whether nuclei were destroyed, which implies a melting temperature above the liquidus temperature for its particular composition. The presence or absence of skeletal or dendritic crystals in chondrules can be used to constrain their peak temperatures, which range from 1400-1850oC. Heating times of less than a second result in aggregates of starting materials coated with glass, resembling agglutinates rather than objects with typical chondrule textures, suggesting that heating times are longer. Chondrule textures can be duplicated with a very wide range of cooling rates, but if olivine zoning is to be matched the cooling rate should be within the range 101000°C/hr. The size of overgrowths on relict grains cannot be used to infer cooling rates. Chondrules melted in a canonical nebular gas lose sulfur and alkalis in minutes, while iron loss from the silicate melt continues over many hours. Mass loss and isotopic fractionation can be suppressed if the partial pressures of the species of interest are high enough in the ambient gas. Chondrule bulk and mineral composition arrays can be reproduced to a large extent by evaporation. However, condensation of SiO into the melt can simulate the zonation in some chondrules, with pyroxene and a silica polymorph near the rims. The partial equilibration of chondrule melt with noncanonical nebular gas would require heating for time periods of hours. 1. General Introduction Chondrules, tiny spherules containing olivine, pyroxene, metal, sulfide and glass, with igneous textures, are the most abundant components in most chondritic meteorites, accounting for up to 80% of the rock (e.g., Zanda 2004; Jones et al. this volume). Chondritic bodies are in turn abundant throughout the main asteroid belt. Chondrules therefore document widespread heating in the early inner solar system. The central question of whether the heating mechanism was an astrophysical process or a planetary process is still not totally resolved, though heating by shock waves in the protoplanetary disk (e.g., Desch & Connolly 2002; Desch et al. this volume) is currently one of the most favored mechanisms. Chondrules might have been formed by condensation of gas (Varela et al. 2002), by agglomeration of a mist of microdroplets and dust (Wood 1996), or by melting of rock or of dustballs (e.g., Jones et al. this volume). There is evidence for formation of small cryptocrystalline chondrules in CB and CH chondrites by direct condensation of liquids (Krot et al. 2001), and of condensation of SiO into Mg-rich chondrule melts (Krot et al. 2003; Libourel et al. 2003). In the case of formation of chondrules from solids, the precursors are widely assumed to be nebular condensates, i.e. finegrained mineral dust, either of fractionated or of CI composition, though such material cannot easily be converted into typical chondrules (e.g., Hewins & Fox 2004; Jones et al. this volume). Some larger grains were probably present, either relicts of fragmented chondrules of different composition or of annealed condensates or refractory inclusions (Jones 1996a; Jones et al. 2004; Yurimoto & Wasson 2002). The disExperimental Constraints on Chondrule Formation 3 persion of oxygen isotopic compositions of olivine and bulk chondrule suggests a history of exchange of primitive material with nebular gas. Chondrules have a wide range of bulk compositions, inherited from precursors or explicable by evaporation and condensation, and a spectrum of textures controlled by the nature of their precursors and their thermal histories (e.g., Grossman 1988; Sears 1996; Hewins 1997; Connolly & Desch 2004; Zanda 2004; Jones et al. this volume). Since experimental petrology cannot be used to constrain the origins of chondrules, unless this diversity is considered, we here briefly define the key terms and concepts used in describing them. Chondrule compositional and textural types are described in McSween (1977), Scott & Taylor (1983), Jones (1996b, and references therein), Hewins (1997), Hewins et al. (1997) and Jones et al. (this volume). Chondrule classification based on their work is as follows. Type I are FeO-poor (unless metamorphosed) and Fe-metalbearing, and type II are FeO-rich. Type I chondrules are dominant in carbonaceous chondrites and type II chondrules are dominant in ordinary chondrites (McSween 1977; Zanda 2004). Each type is subdivided into A, AB and B according to the olivine/pyroxene ratio (Scott and Taylor, 1983), which depends on SiO2 content and thermal history. The textures are indicated by abbreviations such as P porphyritic, MP microporphyritic, B barred and R radiating, coupled with O and P at the end for olivine and pyroxene. Thus IA MPO, IAB POP, and IIB RP constitute fairly complete descriptions of chondrules (Hewins 1997). Chondrule textures are illustrated in Figures 1 and 2. Porphyritic olivine chondrules vary considerably in grain size, with type IIA generally being porphyritic, but type IA generally microporphyritic. Even finergrained chondrules have been described variously as dark-zoned, granular and agglomeratic (Weisberg & Prinz 1996). Such chondrules can be seen in backscattered electron (BSE) images to be porphyritic on a very fine scale, or cryptoporphyritic, or else porphyritic only in patches interstitial to relict material (protoporphyritic). Crystal number densities yield nominal grain sizes of 100, 40, 10 and 5 μm as the transitions between BO, PO, MPO, cryptoporphyritic and protoporphyritic textures (Hewins et al. 1997; Hewins & Fox 2004). Experiments aimed at producing chondrule-like objects in the laboratory have a history of over thirty years (e.g., Nelson et al. 1972). They have moved from simply producing melt droplets, and then crystallizing them, to exploring melt-solid and melt-gas interaction. Given the great diversity of chondrules, laboratory experiments are invaluable in yielding information on chondrule formation process(es) and for deciphering their initial conditions of formation together with their thermal history. In addition, they provide some critical parameters for astrophysical models of the solar system and of nebular disk evolution in particular (partial pressures, temperature, time, opacity, etc). The early work is best summed up in Lofgren (1996), where the key role of melting history and heterogeneous nucleation in controlling chondrule textures is emphasized. In this paper, we aim particularly at integrating the results of chondrule simulation experiments performed since Lofgren (1996) into the existing framework of chondrule studies. We also concentrate our attention on the formation of chondrules with porphyritic textures, because these are the most abundant.