The origin of chondrules: experimental investigation of metastable liquids in the system Mg2SiO4-SiO2

Geachimica et Cosmochimlca Acta. 1976, Vol. 40, pp. 889 to 896. Pergamon Press. Printed inGreatBritain

The origin of chondrules: experimental investigation of metastable liquids in the system Mg2Si04Si02* MILTON BLANDER Chemical

Engineering

Division,

Argonne

National

Laboratory,

Argonne,

Illinois

60439, U.S.A.

H. N. PLANNERand K. KEIL Department

of Geology

and Institute of Meteoritics, University New Mexico 87131, U.S.A.

of New Mexico,

Albuquerque,

and L. S. NELXIN and N. L. RICHARDSON High Temperature

Sciences (Received

Division,

Sandia

Laboratories,

Albuquerque,

New Mexico

87115, U.S.A.

26 May 1975; accepted in revised form 18 December 1975)

Abstract-Laser-melted magnesium silicate droplets, supercooled 400-750°C below their equilibrium liquidus temperatures before crystallization, were examined to provide a comparison with meteoritic and lunar chondrules and to examine physicochemical parameters that may indicate the conditions of their formation. Internal textures of the spherules strikingly resemble textures observed in some chondrules. Definite trends in crystal morphology, crystal width, and texture were established with respect to nucleation temperature and bulk composition. Such trends provide a framework for determining the nucleation temperature of chondrules. The only phase to nucleate from the supercooled forsterite-enstatite normative melts was forsterite, which was present in more-than-normative amounts. Highly siliceous glass (-65 wt. y0 SiOJ was identified interstitially to the forsterite crystals in seven of the

spherules and is thought to be present in all. The presence of enstatite and the large proportion of crystals in some meteoritic chondrules implies that they were maintained at temperatures considerably in excess of 600°C at some point in their history. et al. (1972) shows that highly supercooled spherules of molten silicates indeed crystallize with textures that WE REPORTexperimental studies on the crystallization are strikingly similar to those in chondrules. The experimental conditions consisted of rapidly cooling inof metastable liquid magnesium silicate droplets to investigate the role of supercooling in the origin of dividual molten spherules to room temperature and chondrules. Chondrules are small spherical bodies, is believed to be similar to the conditions causing generally less than 2 mm in diameter, largely of silichondrule formation by small-scale thermal events, cate composition, found embedded in the groundmass e.g. impact splattering on the Moon. matrix of chondritic meteorites and in some lunar In the present paper, we demonstrate how the texbreccias and fines. They usually consist of various ture, crystal morphology, crystal size, and the parproportions of olivine, pyroxene, plagioclase, and ameters controlling nucleation of a crystalline phase glass, which display morphologies that give rise to might be utilized to provide a more quantitative a variety of internal textures. Minor amounts of approach for determining the conditions necessary to metallic nickel-iron, troilite, and other opaques also produce specific features of chondrules (BLANDER et occur. Because of their spherical shape, their mineraal., 1973). A laser beam was used to melt silicates logy, and composition, chondrules are generally conheld in a furnace. Experimental conditions were considered to have crystallized from individual droplets trolled so that (1) the spherules under study cooled of liquid. much more slowly than those of NELSONet al. (1972), FREDRIKSS~N (1963), KEIL and FREDRIKSSON (1964), and (2) the approximate temperature at which the KURAT (1967a, b) and BLANDERand KATZ (1967) conspherules began to crystallize could be ascertained. cluded that chondrules not only formed from isolated The information so gained should ultimately prove molten droplets, but that the melt existed in a metavaluable in judging which of the many postulated chondrule-forming processes (NELSONet al., 1972) are stable state considerably below the equilibrium liquidus before a crystalline phase nucleated. An ex- feasible. In addition, in the context of the constrained perimental study of this idea conducted by NELTON equilibrium theory (BLANDERand KATZ, 1967), information on the magnitude of supercooling possible is important in deducing lower limits on the nebular *This paper was presented at the 35th Annual Meeting gas pressure where chondrules and chondrites may of the Meteoritical Society in Chicago, November 16-18. have formed. 1972. INTRODUCTION

889

M. BLANDER

890 EXPERIMENTAL

PROCEDURE

The bulk composition of many chondrules 1s similar to the MgzSi04--SiOz system (e.g. KEN.. 1968). Therefore. a starting material of enstatite bulk composition was used. It was synthesized from crushed silica glass and roasted MgC03 which were of a minimum 99.90,:, purity. Small fragments were set on iridium wire loops at the focus of a continuous wave 250 W CO2 laser (Model 41. Coherent Radiation Laboratories, Palo Alto, California). The laser power was raised until the sample melted into a bead which flowed into the loop, filling it (BLANDER et trl., 1973). During this heating process, some silica vaporized. The furnace in which the silicates were laser-melted is a modified Marshall Model IS10 tubular furnace with a I?-in. o.d. The inner tube has a l-in. diameter and a IO-in. overall length of which 6in. were heated by multi-strand platinum~40”,, rhodium windings. Two h-mm viewing holes wcrc located at opposite sides of the furnace midpoint. The frozen spherules had been premelted on to a circular loop at the end of a 0.003- or 0.005-in. iridium wire made bq tightly winding the wire around an appropriately-sired drill shank. The free end of the wire was supported in a hole in an alumina rod which was placed in the furnace preset at the desired temperature. A low-power beam of the CO, laser (wavelength IO.6 pm) was focused on to a bead located at the center of the furnace. The laser power was then increased until the bead was heated to temperatures which were probably considerably more than 100’ above the liquidus. With such large superheatings, all crystal nuclei should be destroyed. The very high temperatures were evidenced by significant vaporization of silica. The laser was then turned off, and the bead cooled for periods ranging from I to 17 set before crystallization began. With the longer durations of cooling, the bead temperatures closely approach the furnace tempcrature. Crystallization was manifest by a readily observable ‘bhck’ or recalescence (i.e. brightening), the duration of which. in some experiments, was too fast to time visually and in others lasted as long as 5 sec. The spheres range in diameter from about 1 to 2.5 mm. and from the length of the blick and spherulc sizes we estimate that the slowest crystallization rate for a spherule averaged about 0.03 cm/set. We measured the ambient temperature of the furnace to )2’C in a thermocouple well which paralleled and was alongside the furnace tube. Temperature measurements in the thermocouple well were within I ‘C of a thermocouple whose bead was placed in the same position as the experimental beads. The dimensions of the resultant beads. the duration of time between laser turnoff and initiation of crystallization, and the apparent duration of the blick were also measured. The duration is approximate as we could view only one side of the spherule. Immediately following the recalescence. the sample was pulled from the furnace and quenched to room temperature. In addition to the above experiments. three magnesium silicate glass spherules, formed during the preparatory proccss of the experiment. were placed in furnaces with temperatures in the range of 80s924°C and allowed to devitrify for periods of 2.25 min-7.23 days.

ANALYTICAL

PROCEDURES

Fifty-two spherules which had cooled for two or more seconds were examined to determine their internal features (crystal morphology, crystal size. and texture), phase and bulk compositions (Table I). Polished thin sections of the spherules were studied microscopically and bulk compositions were determined by electron microprobe broad beam techniques (PHINZ et al.. 1971). The electron microprobe analyses were conducted using enstatite of known composition as a standard and were corrected for instrumental

CI td.

effects (drift, deadtime. background) according to KE.IL (1967) and for differential matrix effects as outlined by BENCF and ALREE (1968) and ALHE and RAY (1970). X-ray powder diffraction analysis was in some cases employed to identify the crystalline phase. NUCLEATION

TEMPERATURES

Laser melting of the spherules caused considerable vaporization of silica. The resulting spherules. which initially contained 59.9 wt. I’;, SiOZ. have compositions (homogeneous. except for three noted in Table I) in the range 43.9-59.7 wt. ‘lo SiO, and thus represent a broad range of both forsterite- and enstatite-normalive compositions. Despite this, the only detectable crystalline phase to nucleate from the supercooled melts was forsterite. The nucleation temperature for each spherule was estimated from an energy balance. in conjunction with the cooling times listed in Table I, assuming that energy loss at the surface of the spherule occurred by radiative heat loss only. and that the temperature within the spherule was approximately uniform at any given time. The accuracy of these estimates cannot be checked without detailed data on the optical properties of the melts. The change in temperature of the spherule corresponding to the rate of energy loss by radiation from its surface is given by

pVC,(dT/dz)

= - AtdTT” - 7’;).

(1)

where p, .4. I/. and c‘, arc the density. surface area, volume and specific heat of the spherule, respectively; E is the relative cmittance of the surface (taken as 0.8); ff is a universal constant (13.55 x 1O-~‘3caljsec cm2K4); 7 is time; and T is the absolute temperature with T, representing the ambient furnace temperature. If we assume I>, C, and the diameter. d, of the sphere to be essentially independent of temperature and composition. then equation (I) can be integrated to obtain the time interval. AT. to cool from an initial temperature, 7;. to a temperature. T

From equation (2), one may estimate nucleation temperatures. Uncertainties in the initial temperature affect the calculated nucleation temperature. However, this effect is small; for example, for an increase of 400°C in initial temperature, an increase of only 30°C is calculated for a l-mm spherule cooled for 2 sec. For spherules cooling longer, this error decreases. Estimated nucleation temperatures are within the range 985-134h’C and their distribution with respect to spherule bulk composition and corresponding ambient furnace temperature is shown in Fig. I. Because convective heat dissipation was not taken

The origin Table

of chondrules

891

I. Composition and crystallization conditions of the magnesium silicate ules, arranged in order of decreasing ambient furnace temperature

spher-

49.1

51.0

100.1

1346

12

52.4 54.2 50.5 49.6

48.2 45.9 49.8 50.5

100.6 100.1 100.3 100.1

1318 1295 1290 1271

17 15 6-7 4-m

1346 1318 1295 1300 1289

52.4 52.2 55.1 47.0 54.6

48.2 47.3 4k.2 53.0 45.2

100.6 99.5 99.3 100.0 99.8

1266 1259 1248 1240 1234

13 8 13 4 11

1268 1269 1249 1285 1237

49.8

50.5

46.0 46.6 45.8 45.1

1220 1191 1186 1186 1179

5

53.4 53.0 54.4 55.8

99.8 99.4 99.6 100.2 100.9

9 5 10 4

1231 1200 1207 1188 1233

50.4 53.0 53.8 53.5 52.0

49.4 47.7 46.1 45.4 48.0

99.8 100.7 99.9 98.92 100.0

1173 1173 1162 1162 1147

10 4-112 5 7-112 5

1180 1204 1177 1164 1189

54.4 48.9 51.1 43.9 54.4

45.2 50.4 48.8 56.2 45.8

99.6 99.3 99.9 100.1 100.2

1145 1143 1141 1140 1128

4-l/2 5 5 2 E-112

1177 1194 1167 1257 1131

52.6 47.9 46.4 48.7 46.5

46.8 51.4 54.7 51.8 53.6

99.4 99.3 101.1 100.5 100.1

1119 1118 1113 1110 1106

4 3

1 0.5

7-l/2 4

1174 1229 1291 1126 1165

48.6 51.5 46.4 49.8 48.4

50.0 48.0 54.1 50.5 51.5

98.62.3 99.5 100.5 100.3 99.5

1068 1066 1056 1054 1042

3-l/2 3 4 3 3-l/2

1154 1175 1099 1207 1106

2 3 1 3 2

1.47 1.49 1.14 1.78 1.18

E E

48.7 48.9 51.3 52.0 53.0

51.3 51.0 48.3 48.1 48.0

100.0 99.9 99.63 100.1 101.0

1003 1003 1003 1003 1003

3 4 4-l/4 5 5

1070 1070 1039 1033 1072

2 1 1 1 1

0.94 1.25 1.03 1.12 1.64

E E R E E

48.7 48.7 52.9 52.5 55.2

51.4 51.0 47.0 47.8 45.3

100.1 99.7 99.9 100.3 100.5

1002 1007. 997 989 989

4 4-l/2 5 4-112 16

1066 1062 1022 1036 989

1 2 1 1 2

1.23 1.35 1.07 1.18 1.03

8 R 8 E 8

53.9 54.1 53.1 49.3

45.6 45.9 46.9 49.9

99.53 100.0 100.0 99.2

986 982 973 970

5-114 10-l/4

1019 989 985 1004

l-112 2 l-112 1

1.21 1.41 1.44 0.98

8

57.8 56.8 59.7

41.3 43.3 40.7

99.1 100.1 100.4

924 800 800

2

,'-,I2

-

5

l-112 2 .3
4-5

2

:-1,* 3

2 <1
1 2

a35++ 1.6~10~~ 6.2x105+

1.69 1.82 1.87 1.82 1.55 2.55 2.34 2.25 (1.78)' 2.24

E RG E E E R RG RG 8 E

1.31 2.20 1.54 1.74 1.82

8 E E R E

2.13 1.53 1.34 1.23 1.82

E RG E

1.50 1.93 1.52 1.16 1.37

E

1.55 (1.64)' 1.47 1.74 1.49

(1.04)' (1.16)' (1.14)1

s RG

e 8 EC?) 8 E E E E E

EC?) E R

s 8 E

s 8 8

1 Average diameter measured from thin section. ’ Thin section surface contains numerous cracks and voids. 3 Center of spherule richer in MgO than along periphery; all other spherules appeared to be uniform. * Visual estimation. t Devitrification period (set). $ Total length of time in the furnace. The length of time at 924” is much less than this since the bead probably took more than a minute to heat to 900”. n.a. Not analyzed. fo Forsterite. The textural classifications in the last column are as follows: E derivative of the excentroradial texture, R radial texture, RG random grain texture, S spherulitic texture.

into account, the spherule nucleation temperatures were probably lower and closer to the ambient temperatures, T,. Ln terms of the degree of supercooling,

the molten spherules cooled within the range 4OG3OO”C below the equilibrium liquidus before nucleating the crystalline phase, forsterite.

M. BLANIXK rr trl

892

I

I

MgzSiOq55

50

I

45

MgS103

I

35

I

WEIGHT % MgO

Fig. I. Crystal morphologies and crystal width classes that correspond to the estimated nucleation temperatures of the spherules. Crystal morphologies: 0, bars: A. dendrites: 0. fibers; and Q, submicroscopic crystals. Crystal width

classes: open symbols (e.g. A), fine (>Z i(m): right half shaded (e.g. A). very fine (0.5-I pm): left half shaded (e.g. A), extremely fine (0.34.5 /cm); filled symbols (e.g. A). submicroscopic crystals (~0.3 pm). Short dashed lines separate general temperature-composition fields for morphologies correlated with nucleation temperature. Equilibrium phase diagram for a portion of the Mg,SiObSi02 system shown for reference (after BOWEN and ANDERSEN. 1914: GREIG, 1927; SMYTH. 1974). Dotted line illustrates the metastable extension of the forsterite liquidus. Abbreviations: Fo, forsterite; Pr, protoenstatite; En. orthoenstatite; Cr. cristobalite; Trid. trdymite: Qz. /J-quartz; and L. liquid. NUCLEATION

TEMPERATURES

AND

INTERNAL FEATURES The nucleation of forsterite from the cooling metastable molten droplet causes an increase in luminosity in both the visible and i.r. range and an increase in the temperature of the spherule which is far more rapid than the cooling rate before nucleation. This event, known as recalescence, is the result of the release of the heat of fusion as forsterite crystallizes from the melt. The recalescence is important in that most of the features observed in the spherule are directly the result of this event or the result of some subsequent thermal process acting on the resulting primary features. The temperature rise during recalescence is also important in that it can be so large that during the heating and subsequent cooling a range of textures and morphologies can form in a single spherule since crystallization occurs all during the large rise and fall in temperature. The initial rise in temperature is so large that when the bead temperature is not very far from the furnace temperature, textures and morphology will be largely governed by the magnitude of the temperature rise and not by the relatively low cooling rates immediately before recalescence.

The nucleation temperature establishes the initial conditions for the growth of forsterite from the melt. The rise and fall of the temperature of the spherule. as the recalescence and subsequent cooling progresses, could expectedly bring about a change in the internal features of the spherule. For example. the morphology of the crystalline phase formed late in this thermal event may differ significantly from that formed immediately after nucleation or at the temperature peak. Since the degree of supercooling is the major control on recalescencc. the variations of these features with the degree of supercooling might bc demonstrated by plotting them as a function of nucleation temperatures and of the bulk composition ol the spherule. Crystal morphologies displayed by the forsteritc in the spherules are skeletal bars. dendrites, fibers, and submicroscopic crystals. With increasing degree of supercooling for a given bulk composition the general systematic change in morphology is from a bar + dendrite association to dendrite + fiber. and fiber + submicroscopic associations. in that order. If then. with increasing magnitude of supercooling. the trend bars-dendrites -fibers submicroscopic crystals were followed, the observed association in a single spherule could be understood from the changes in temperature of the spherule during the recalescencc. When the crystal morphologies. corresponding to the greatest supercooling of each of the spherules. arc plotted. composition--temperature limits can be established for the various morphologies (Fig. 1). Different crystal morphologies fall into different areas on the temperatureecomposition plot. There are. however. four spherules that do not lit accordingly and it is suggested that either some observation with respect to their nucleation temperatures is incorrect or that the morphology corresponding to the greatest supcrcooling could he in a plane other than that of the thin section. The nucleation temperature of a sphcrule, therefore. can be determined from Fig. I within specific limits. if one knows the bulk composition and if the forstcritc crystal morphology corresponding to the greatest supercooling ia present in the thin section of the investigated spherulc. Another internal fenturc that appears to be a good indicator of nucleation temperature is the crystal size. Since the forsterite crystals are generally elongate. the crystal width, as an indication of crystal size, therefore would have the advantage of being least affected by its orientation with respect to the plane of the thin section. This approach has been used as a relative crystal size indicator with the approximation that the width of a dendrite be taken as the width of the dendrite body from which the dendrite extensions originate. Crystal size has been divided into four arbitrary groups: fine (->2 pm): very fine (0.5-2 pm); cxtremely fine ( - (.YP0.5 pm); submicroscopic ( <0.3 /lrn). From optical microscopy we observe that crystal size generally can be correlated with crystal morphology. i.e. crystal width generally decreases with greater

Fig. 2. Photomicrographs of thin sections of spherules exhibiting pamllel grain, radial and spherulitic textures. (a) Spherule 2X9 showing parallel grains. Furnace temperature 1054°C.Estimated nucleation temperature 1207°C; diameter 1.75 mm. (b) Spherule 23E7 showing parallel grains. Furnace temperature 1066°C.Estimated nucleation temperature 1175°C; diameter 1.29mm. (c) Spherule 24E7 exhibiting radial texture. The spherule is strained and cracked. Furnace temperature 1002°C.Approximate nucleation temperature 1062°C; diameter 1.31 mm. (d) Sphexule23El2 exhibiting radial texture. Note curved growth of radiating lines which appear to be crystals stacked in a curved succession. Furnace tem~turc 1042°C.~st~ated nucleation tem~rat~e 1106°C;fongest ~rn~sion 1.27 mm. fe) Spherute 24E9 exhibiting spherulitic texture, ‘Titereappear to be a iarge number of nucleation sites Ieading to many individual sphetulites which frlt fhe spIter&e and border each other. The spherule exhibits curved radiating lines analogous to those in Fig. 36. Furnace ~rn~mt~ 9gYC. Estimated nucleation t~~ra~re 1022X; longest dimension I.04 mm. ff) Glass spherute MB6 devitrified at 950°C for 20 sec. The texture of this spherule is very similar to 24E9 &own in Fig. 3e. There appear to be a number of spherulites with fibers radiating from many nucleation sites. The spherules listed in Table 1, which were devitriflcd at lower temperatures, had smaller spherulites which had not completely filled the spherule. (SiOl = 58.6x.); diameter 1.04 mm.

892

Fig. 3. Photomicrographs of thin sections of spherules taken with crossed nicols exhibiting excentroradial and random grain textures. (a) Slice on outer part of spherule EC showing excentroradial texture. The center slice of this spherule exhibited a parallel grain texture. Furnace temperature 1240°C. Estimated nucleation temperature 1285°C; longest dimension 1.35 mm. (b) Spherule 24E4 showing excentroradial texture. Furnace temperature 1003°C. Estimated nucleation temperature 1070°C; longest dimension 1.25 mm. (c) Spherule EG showing excentroradial texture. Furnace temperature 1173°C. Estimated nucleation temperature 1180°C; longest dimension 1.40 mm. (d) Spherule 24E14 showing excentroradial texture. Furnace temperature 970°C. Estimated nucleation temperature 1004°C; longest dimension 1.29 mm. (e) Spherule E4 showing random grain texture and some tendency for parallel growth. Furnace temperature 1173°C. Estimated nucleation temperature 1204°C; diameter 1.25 mm. (f) Spherule EK showing both random grain and parallel grains. Furnace temperature 1318°C. Estimated nucleation temperature 1318°C; diameter 1.81 mm.

The origin supercooling. This suggests that the smallest crystal width in a spherule is related to the degree of supercooling at the time of nucleation. If the crystal size classifications corresponding to the smallest crystal width in each spherule are plotted at their nucleation temperatures (Fig. l), specific limits of width can be established with respect to temperature and composition. Unfortunately, there is an apparent overlap of fine and very fine crystal size classes. Despite this, Fig. 1 represents an additional trend which helps to define nucleation temperature, provided that the bulk composition and the smallest crystal width present in the spherule can be ascertained. The texture of the spherule can also be correlated with nucleation temperature. Textures are classed as radial, excentroradial, spherulitic, parallel grains, or random grains (Figs. 2 and 3). The parallel growth texture is described here as long, approximately trapezoidal grains oriented parallel to each other or to sets of parallel oriented trapezoidal grains that are inclined to each other. Textures described as random grains consist of short, stubby grains that have no obvious crystallographic relationship to other grains within the spherule. Examination of grains displaying an excentroradial texture reveals that they are wedge-shaped with their two diverging sides radiating from the nucleation center. In three-dimensional perspective, a single grain within such a spherule would appear similar to that pictured in Fig. 4A, where the nucleation center lies at the back surface of the spherule at the intersection of planes 1 and 2. Planes passing through such a spherule with orientations shown in the dia-

of chondrules

893

gram produce two varieties of the excentroradial texture (Figs. 4B and 4C) and a third texture that is not excentroradial (Fig. 4D). Such a perspective is of more interest when considering additional wedgeshaped grains all radiating from the same nucleation center since certain sections through such a spherule produce textures that are remarkably similar to the parallel grain textures. We feel that most of those spherules classified as having parallel grain textures are the result of various thin section orientations with respect to a spherule containing excentroradiallyarranged, wedge-shaped grains. Textures classified as excentroradial or as parallel grains are reclassified here as varieties of excentroradial grains. It should be noted that the majority of the spherules produced from the experiments display textures of this type. When the textures of the spherules are superimposed upon nucleation temperature, it is found that only broad regions exist, one consisting essentially of spherules displaying a spherulitic texture, and one that contains the remaining textures (undifferentiated). Since the temperature limits have not been established for most of the textures and compositions, it cannot be stated with certainty that a spherule, e.g. with an excentroradial texture, nucleated below a temperature of 1300°C. In addition, since textures that are derivatives of excentroradiating grains occupy a wide range of nucleation temperatures, e.g. at least a 300°C interval at 50wt. ‘? MgO, texture alone is a poor indicator of the degree of supercooling achieved by the metastable melt before nucleation in this range.

__ !

ON UNDERSTANDING

THE PROPERTIES

OF CHONDRULES

_NUCLEATION CENTER 7

PLANE

I

PLANE

2

PLANE

3

Fig. 4. Derivation of textures from the excentroradial growth of forsterite crystals. (A) Diagram of a spherule containing a single wedge-shaped grain with its nucleation center on the back side of the spherule at the intersection of planes 1 and 2. Lines outlining the wedge-shaped grain are added to give three-dimensional perspective only. (B-D) Grain boundaries that correspond to thin section views along planes I-3, respectively.

Crystal morphology, crystal width, and texture provide trends with respect to nucleation temperature and bulk composition. Singularly, or in combination with each other, these features can be compared with Fig. I and Table 1 to establish limits to the nucleation temperature of a supercooled droplet or of chondrules of similar compositions. Alone, however, a feature such as the texture of a spherule or chondrule may not be a good indicator of nucleation temperature because of the range of conditions under which it can be produced. Knowledge of the limits on the nucleation temperature of a chondrule thus obtained provides information for testing various models of chondrule formation. There are chondrules that at least superficially resemble the spherules produced in this study. Because chondrules have variable composition and usually have compositions which differ from those in our experiments, definitive conclusions about all chondrules are not possible at present. However, there are some general conclusions which can be drawn from our observations and from theories of crystal nucleation and growth.

894

M. BLANDER rt al.

Once crystallization is initiated. heat is released because of the enthalpy difference between the metastable supercooled liquid and the crystalline solid. The rate of heat input at the crystal-liquid interface will be governed by the rate of growth of the crystals and the rate of growth depends on temperature. At the melting point, the growth rate, R, is zero. R increases with the supercooling, AT reaches a maximum and then decreases. In all of the measurements reported here, the supercooling is sufficiently great so that the rate of growth decreases with an increase of supercooling. The faster the growth rate, the more rapidly heat is generated during crystal growth and the shorter the blick. Spherules which cooled for shorter times and were hotter at the time of crystallization than those in Table 1 were observed with a photomultiplier and had much shorter ‘blicks’ than the spherules listed in Table 1. (The duration of the blick is also a function of the amount of material to crystallize.) De\ itrilied glasses have much slower growth rates at lower temperatures (~950 C). Thus, the greater the supercooling, the slower the release of the heat of crystallization for a given composition. The total amount of heat released is also governed by the amount of crystallization which appears to increase the lower the silica content. The heat released is dissipated during and after the recalescence. From equation (2), we can deduce that, for a given crystal nucleation temperature, heat per unit volume will be dissipated more slowly from larger drops, Therefore, the interiors of large droplets will rise faster to, and stay longer at, high temperatures than small droplets. Crystal shapes in large drops (which dissipate heat more slowly than small drops) or droplets which contain a large fraction of crystals (which have more heat to dissipate than those with a small fraction of crystals) should therefore be characteristic of higher temperatures than textures of smaller droplets or those droplets containing fewer crystals. Because of limitations in techniques, the influence of droplet size was not studied thoroughly. Whether nucleation is heterogeneous or homogeneous, the degree of supercooling at nucleation is smaller. on the average, for large droplets than for small ones (BEG. 1953; PRICE and GORNICK, 1967). Thus. since it is a function of temperature, chondrule textures should also be a function of size because of differences in nucleation temperature. In addition, composition will undoubtedly influence textures, both because of differences in heat released and differences in modes and rates of crystal growth. Superimposed on this is a spread in nucleation temperatures which arises because nucleation (both heterogeneous and homogeneous) is a chance event. We conclude that supercooled droplets should exhibit a variety of textures, as is observed in chondrites. Crystal textures like those of chondrules are produced from magnesium silicate melts which are supercooled as much as 750°C below their liquidus. The excentroradial spherules, although they supercooled

deeply, appear to have nucleated heterogeneously either on dust or on the wire support, and all of the spherules probably nucleated heterogeneously. Simrlar metastable liquids would probably have a longer lifetime when floating or falling freely in a gaseous, more dust-free environment and their lifetime would be dependent on the number of nuclei they encounter. If too few nuclei are encountered, nucleation would tend to be homogeneous. From the viewpoint of the constrained equilibrium theory for the formation of chondrites (BLANI~ERand KATZ, 1967) such large supercoolings support the feasibility of direct condensation to metastable supercooled droplets at low nebular pressures. Thus, olivine and olivine-pyroxene chondrules could have been condensed directly from the nebula at pressures in the range of 10-3~10-4 atm (BLANDERand KATZ, 1967).

The production of chrondrules by other mechanisms (e.g. impact melting; melting by an electrical discharge; explosive volcanism; etc.) cannot be excluded by our data. Our spherules did not crystallize enstatite, and pyroxene-bearing chondrules produced by these mechanisms would have to be kept hot enough to crystallize the pyroxenes to their present form. This places limitations on the subsequent history of the chondrule precursors. The fact that such large supertoolings were obtained while the droplet was supported on an iridium wire has important implications for nebular condensation. It illustrates that not all solids are effective as heterogeneous nuclei for crystallization of silicates and other oxides. In the nebula, the presence of metallic iron-nickel alloys (similar in structure to iridium) does not prevent the direct condensation or continued existence of metastable supercooled silicate liquids. Metal grains imbedded in chondrules do not appear to have been nucleation sites for the crystallization of silicates. Consequently, if metal is present before or after metastable liquids are formed, such liquids may still persist. The total absence of enstatite in ail spherules is also significant. It indicates that even silicates may not provide good nucleation sites for other silicates. Thus, even the presence of silicate or oxide dust does not guarantee that metastable supercooled liquids will not form and persist in a nebula. One possibility which arises from our observations is that more than the normative amount of olivine will crystallize from the melt. If a nucleation constraint is placed upon the crystallization of both enstatite and silica, but not upon olivine. then olivine can crystallize until it reaches the metastable extension of the olivine liquidus (HARKER, 1909). This metastable extension is illustrated in Fig. 1. If this constrained equilibrium crystallization of olivine occurs, then the residual glass would be of an enstatite + silica normative composition. Because of the small size of the areas of residual glass in most spherules, the glass compositions could be determined in only the seven spherules listed in Table 2. It averaged about 65.3 wt y,, silica

The origin of chondrules Table 2. Electron microprobe

895

analysis of the glass phase in seven spherules

Spherule Bulk Composition (wt. X)

Glass Cmposirfon Average

(wt. X) Estimated WCleatiW. Temperature ('0

Range

SiO2

MsO

Total

SiO*

w

Total

E5

55.8

45.1

100.9

65.6

34.7

100.3

65.2-66.0

33.4-35.2

1233

24Bll

53.9

45.6

99.5

65.2

35.0

100.2

65.0-65.4

X.4-35.5

1019

w

54.2

45.9

100.1

65.3

34.3

99.6

64.7-66.8

33.4-35.1

1295

ET

53.4

46.0

99.4

65.4

34.4

99.8

64.7-66.2

33.5-35.2

1200

2312

52.6

46.8

99.4

65.6

34.6

100.2

65.6-65.7

34.3-34.9'

2367

51.5

68.0

99.5

65.2

34.7

100.3

EG

50.4

49.4

99.8

65.2

34.6

99.8

Spherule

with very little variation. Since the silica content of the glass is greater than that of enstatite, more than the normative amount of olivine is present and equilibration of the spherules would require resorption of some of the olivine. The constrained equilibrium formation of forsterite does not appear to have reached the metastable liquidus shown in Fig. 1, probably because of the slow rate of crystallization from these glass compositions. The fact that the nucleation temperatures listed in Table 2 range from 1019 to 1295°C suggests that the composition (silica content) is more significant than temperature on the crystallization rate. Another observation can be made on the devitrified glasses listed in Table 1. The devitrification was manifested by incomplete spherulitic growth in all three cases. The sizes of the spherulites was the same in MB2 and MB5 (10.05 mm) and both were larger than the spherulites in MB1 (I 0.02 mm). Although MB5 was in a furnace maintained at 924°C for 135 set, the time at temperatures high enough for effective spherulitic growth (> SSO’C) is considerably shorter. This is supported by our observation that a thermocouple placed at the same position in the furnace barely had reached 924°C immediately before removing the spherule. From the relative lengths of time the similar spherules MB2 and MB5 were in the furnace, we can deduce that the rates of crystal growth differed by a factor of more than 4 x lo3 and probably more than 104. Such large temperature effects on the growth rate usually indicate that one is close to, or below, the glass transition temperature and that large temperature coefficients will also obtain at lower temperatures. Thus, one would expect that at much lower temperatures (e.g. at 55&6OO”C) significant crystal growth of even spherulites will not occur on even a cosmic time scale of, e.g. 10’4-1017 sec. This places an important limitation on possible modes of formation of chondrules. Chondrules which contain abundant crystals and which have compositions which tend to form glass on quenching would have to have been kept at temperatures not too far below the glass transition temperature. In laser spin-melting experiments, we have found that quenching of lasermelted droplets of MgSi03 to room temperature

SiO2

np0

1174 1175

64.5-65.7

33.8-35.4

us0

forms glass exclusively. Thus, crystalline enstatite chondrules, such as are found in enstatite chondrites, would appear to have remained at temperatures at least as high as 600°C on a cosmic time scale or at higher temperatures on a smaller time scale to even initiate crystallization. To form the textures and nucleate the enstatite observed in some chondrules (rather than forsterite), temperatures of the chondrules must have been considerably higher than 600°C and probably higher than 900°C.

CONCLUSIONS 1. Molten droplets of magnesium silicate mixtures supercooled as much as 750” and crystallized with textures similar to those of chondrules. 2. Supercooling is an important factor contributing to the large variety of textures and crystal sizes observed in chondrules. 3. Nucleation constraints on the crystallization of a stable phase (e.g. enstatite) can lead to the formation of more than the normative amounts of another crystalline phase (e.g. forsterite, HARKER, 1909). 4. If chondrules of crystalline enstatite were formed by a quench process, as, e.g. impact splattering, they must have been kept at temperatures considerably in excess of 600°C and probably in excess of 900°C at some point in their history. 5. If chondrules were formed by direct condensation to metastable supercooled liquids, they could have formed at nebular pressures as low as 10e310e4 atm.

Acknowledgements-We thank Mrs. G. H. KUCERA for devitrifying the three glass samples and Dr. C. H. DONALDSON for many incisive suggestions. This work is part of the thesis submitted to the University of New Mexico by

H. N. Planner in partial fulfillment of the requirements for a Master of Science Degree. This work is supported largely by the National Aeronautics and Space Administration, Work Order No. 13441 to the Chicago Office of ERDA (M. Blander, Principal Investigator), Grants NGL 32-004-063 and 32-004-064 (K. Keil, Principal Investigator), and partly by the U.S. Energy Research and Development Administration.

896

M. BLANDER et rrl. REFERENCES

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