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Experimental Constraints on Chondrule Formation

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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 1400-1850×C. 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 10- 1000×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.
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Chondrites and the Protoplanetary Disk
ASP Conference Series, Vol. 341, 2005
A. N. Krot, E. R. D. Scott, & B. Reipurth, eds.
286
Experimental Constraints on Chondrule Formation
R. H. Hewins
CP 52 LEME, Muséum National d'Histoire Naturelle, 57 rue Cuvier, 75321 Paris
CEDEX 05, France
Laboratoire Magmas et Volcans,OPGC - Université Blaise Pascal, 5 rue Kessler,
63038 Clermont-Ferrand cedex, France
Geological Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854,
USA
H. C. Connolly, Jr.
Physical Sciences, Kingsborough College and the Graduate School of the City
University of New York, 2001 Oriental Blvd., Brooklyn, NY 10024, USA
Earth and Planetary Sciences, American Museum of Natural History, Central
Park West, New York, NY 11024, USA
G. E. Lofgren
Astromaterials Acquisition and Curation, NASA Johnson Space Center, Houston,
TX 77058, USA
G. Libourel
Centre de Recherches Pétrographiques et Géochimiques, CRPG-CNRS, UPR
2300, 15 Rue Notre-Dame des Pauvres, BP 20, 54501 Vandoeuvre les Nancy,
France
Ecole Nationale Supérieure de ologie, ENSG-INPL, BP40, 54501 Vandoeuvre-
les-Nancy, France
Experimental Constraints on Chondrule Formation 287
Abstract. Chondrule textures depend on the extent of melting of the chondrule precur-
sor material when cooling starts. If viable nuclei remain in the melt, crystallization be-
gins 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 tem-
peratures, 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 10-
1000°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 iso-
topic 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 non-
canonical 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 meteor-
ites, accounting for up to 80% of the rock (e.g., Zanda 2004; Jones et al. this vol-
ume). 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. fine-
grained mineral dust, either of fractionated or of CI composition, though such mate-
rial 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 refrac-
288 Hewins et al.
tory inclusions (Jones 1996a; Jones et al. 2004; Yurimoto & Wasson 2002). The dis-
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-metal-
bearing, 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 oli-
vine/pyroxene ratio (Scott & Taylor 1983), which depends on SiO2 content and ther-
mal 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 oli-
vine and pyroxene. Thus IA MPO, IAB POP, and IIB RP constitute fairly complete
descriptions of chondrules (Hewins 1997). Chondrule textures are illustrated in Fig-
ures 1 and 2.
Porphyritic olivine chondrules vary considerably in grain size, with type IIA
generally being porphyritic, but type IA generally microporphyritic. Even finer-
grained chondrules have been described variously as dark-zoned, granular and ag-
glomeratic (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). Crys-
tal number densities yield nominal grain sizes of 100, 40, 10 and 5 Pm as the transi-
tions 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, tempera-
ture, 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.
Experimental Constraints on Chondrule Formation 289
Figure 1. Olivine- and pyroxene-rich chondrules. (a) Barred olivine chondrule, PPL, oli-
vine light grey, glass medium grey. (b) Type IIA PO chondrule, XPL, euhedral olivine
phenocrysts in glass (black). (c) Metal-rich type IA MPO chondrule, XPL, olivine white
and grey, metal black. (d) Type IA MPO chondrule, XPL. (e) Type IA POP, XPL, oli-
vine white and grey in center, pyroxene grey near margin. (f) Type I(A)B P(O)P, XPL,
olivine grey and white, pyroxene showing twinning. PPL = plane polarized light; XPL =
cross polarized light. Average diameter ~700 Pm. (c) – (f) courtesy of B. Zanda.
a
a
a
c
290 Hewins et al.
Figure 2. (a) Cryptocrystalline chondrule from the CB chondrite Hammadah al Hamra
237, BSE. (b) 500-Pm thick igneous rim on 1 mm BO/PO chondrule in the CV chondrite
Efremovka, Mg X-ray image. (c) 750-Pm diameter Type IAB chondrule in Chainpur
with dusty olivine relict grains, PPL. (d) Chondrule in the CR chondrite Renazzo 1.5 mm
long consisting of an aggregate of coarser grained and finer grained domains, BSE, oli-
vine black, metal white. PPL = ordinary light, BSE = back-scattered electrons. (a) and (b)
courtesy of A. Krot.
Ideally, by comparing the charges prepared in these experiments with natural
chondrules, we can define the peak temperature, heating time and cooling rate ex-
perienced by most chondrules. However, there are many complications in inferring
the formation conditions of chondrules: there may be several thermal histories that
produce similar textures; differences in precursor properties may influence chon-
drules; chondrule melts may collide with dust grains, or experience evaporation
and/or condensation. Chondrule properties are intimately related to the pair of pa-
rameters peak temperature and duration of the heating pulse. In particular, long heat-
ing time and high peak temperature both contribute to dissolution of crystals and the
elimination of nuclei, meaning that it is difficult to define either heating time or peak
Experimental Constraints on Chondrule Formation 291
temperature uniquely. Recent experiments also demonstrate effects due to evapora-
tion or condensation during melting and solidification, and mixing of objects during
the heating process. However, note again that the results for each set of experiments
need not be definitive for natural chondrules. They give formation conditions for
chondrules only as formed by a chosen chondrule model, e.g., partial melting of
coarse solid precursors, dust seeding of total melts, condensation, etc., for the particu-
lar physical conditions (including gas compositions and pressures) of the experi-
ments.
2. Texture Simulation Experiments and Thermal History
Most of the experiments simulating chondrules have been based on the assumption
that they formed by melting of an aggregate of solid grains, with total pressure in
their formation environment of little importance in producing textures, and that they
experienced virtually no gain or loss of elements from or to the ambient nebular envi-
ronment. Typically experiments used pressed pellets attached to Pt wires, heated at 1
atm pressure, but we also discuss here experiments in which small quantities of solids
injected into the furnace were incorporated into a droplet or large amounts agglomer-
ated together during the course of cooling. The oxidation state of the charges was
controlled by gas mixing. [The oxygen fugacity (fO2) of an experiment is given in
terms of log(fO2) relative to the iron-wustite (IW) buffer curve, a temperature de-
pendent parameter that defines equilibrium between Fe0 and Fe2+. Therefore “IW-1”
means that the oxygen fugacity is one log unit below the IW buffer at the temperature
given.] Loss of alkali elements by evaporation and Fe to the support wire were sig-
nificant in many cases. Although the experimental charges were not closed systems,
this has no obvious effect on textures, and the results have been applied to the closed
system model of chondrule formation. However, Fe loss can influence olivine zoning
and thus complicate interpretation of cooling rates (Weinbruch et al. 1998). Experi-
ments specifically designed to study interactions between the chondrule melt and a
gas phase are discussed in section 3.
2.1. Textures
The hierarchy of crystal morphologies with respect to extent of melting has been well
established, as well as the importance of heterogeneous nucleation (e.g., Lofgren &
Russell 1986; Lofgren 1989, 1996; Radomsky & Hewins 1990; Faure et al. 2003). A
silicate liquid which is totally melted and superheated (heated above its liquidus tem-
perature, or temperature above which at equilibrium no crystals are present) contains
some memory of crystalline material in the form of embryos, which may graduate to
viable nuclei a long way below the liquidus temperature during cooling, unless this is
so fast that a glass is formed. The ensuing crystal growth is rapid because the super-
cooled liquid is far from equilibrium (supersaturated), and therefore the crystals are
skeletal, dendritic or spherulitic as the growth rate of the crystals increases with the
degree of supercooling (Lofgren 1996). Alternatively, a melt may contain relict
grains or viable nuclei on which growth can commence immediately when heating
and dissolution stop, giving rise to crystals that have equilibrium shapes and for most
phases are equant. The size of the crystals depends on the number density of nuclei or
relict grains. The sequence of textures in chondrules depends on the temperature and