The flash melting of chondrules: an experimental investigation into the melting history and physical nature of chondrule precursors

Geochimica et Cosmochimica Acta, Vol. 62, No. 15, pp. 2725–2735, 1998 Copyright © 1998 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/98 $19.00 1 .00

Pergamon

PII S0016-7037(98)00176-8

The flash melting of chondrules: An experimental investigation into the melting history and physical nature of chondrule precursors HAROLD C. CONNOLLY JR.,*,1 BRIAN D. JONES2 and ROGER H. HEWINS2 1

California Institute of Technology, Division of Geological and Planetary Sciences, Mail Code 100-23, Pasadena, California 91125, USA 2 Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08855-1179, USA (Received April 14, 1997; accepted in revised form May 8, 1998)

Abstract—Constraints placed on chondrule formation have largely been generated from experiments which use a long duration, below liquidus isothermal melting (minutes to hours) rather than a short duration, above liquidus flash melting event (seconds to minutes). In this paper we examine how a short duration, superliquidus heat pulse can produce chondrule textures. By incompletely melting material with a type of flash melting we show that the maximum temperature limit of chondrule formation was approximately 2100°C, almost 400°C higher than previously constrained. Previous experiments also have not studied the effect of variations in precursor grain size on the formation of chondrule textures. For this reason we simultaneously investigate the effect of variations in the grain size of a starting composition on the formation of chondrule textures. We show how MgO-rich (Type IA) chondrules and other fine-grained chondrules could only have been formed from the incomplete melting of a rather uniformly grain sized precursor of less than 63mm. Because fine-grained, MgO-rich chondrules have the some of the highest chondrule liquidus temperatures, we proposed that these types of textures define a minimum melting temperature for chondrule formation. Copyright © 1998 Elsevier Science Ltd erately volatile elements such as Na in synthetic chondrules is greatly enhanced compared with longer duration experiments such as those used by Hewins and Radomsky (1990) and Lofgren and Lanier (1990). In these early experiments (Hewins and Radomsky 1990; Lofgren and Lanier, 1990) synthetic chondrules were consistently depleted in Na by approximately 100% of their original bulk concentrations. Since chondrules contain Na (residing within their glassy mesostasis), a paradox between the thermal conditions of synthetic and natural chondrules appears to exist. Although Na can be retained in experimental chondrules by enriching the partial pressure of Na in furnace gases (Lewis et al., 1993), it is not clear that the required Na-enrichments to the chondrule forming environment(s) are possible. Therefore, if the retention of moderately volatile elements such as Na is an independent test of chondrule melting conditions (maximum temperature and melting duration), then the melting conditions experienced by chondrules may have been more like the flash melting conditions studied by Yu et al. (1996) and Yu and Hewins (1998). Another major unanswered question in the formation of chondrules is the nature of their precursors. Simply stated, what was the grain size of chondrule precursors (Grossman et al., 1988)? Key to understanding this issue is the study of relict grains within chondrules (Jones, 1996; Jones and Danielson, 1998; Nagahara, 1981; Rambaldi, 1981). Relict grains are crystals within chondrules interpreted to be survivors from previous generations of chondrule formation. These grains show that chondrule precursors were diverse, composed of a range of compositions (mainly olivines and pyroxenes) and grain sizes. Although the potential importance of experimentally studying chondrule precursors has been known for some time (Radomsky and Hewins, 1990), no systematic investigations have been performed. In this paper we investigate through experimental petrology

1. INTRODUCTION

Unequilibrated chondritic meteorites are accretionary rocks composed of various structural components. The most abundant of these components, chondrules (up to 80 vol% in ordinary chondrites), are small (millimeter sized), silicate-dominated igneous spheroids that have textures and mineral chemistries suggesting crystallization occurred in situ as isolated melt droplets (Jones and Scott, 1989; Jones, 1992). Although the exact environment of chondrule formation is unknown, chondrules are convincing argued (Taylor et al., 1983; Hewins, 1988; Grossman, 1988; Grossman et al., 1988) to have formed as free-floating objects within the protoplanetary nebula. Chondrules are thus the fossilized evidence of the processes that operated within our protoplanetary nebula and a study of their formation should provide constraints on nebular processes. Critical to unraveling the mystery of chondrule formation is understanding their melting histories. The fact that chondrule formation occurred on laboratory time scales has permitted researchers to constrain their peak formation temperature to approximately 1750°C if the duration of melting ranged from 15 min to a few hours (Hewins and Radomsky, 1990; Lofgren and Russell, 1986; Lofgren, 1989; Lofgren and Lanier, 1990; Radomsky and Hewins, 1990). Although these early results are applicable to current popular chondrule formation models (Boss, 1996), melting for shorter durations (seconds to a few minutes) from temperatures in excess of 1750°C have been suggested (Hewins and Radomsky, 1990; Lofgren and Lanier, 1990). Using a short duration, flash melting event, Yu et al. (1996) and Yu and Hewins (1998) showed that the retention of mod*Author to whom correspondence should be addressed (vorlon@ gps.caltech.edu). 2725

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H. C. Connolly Jr., B. D. Jones, and R. H. Hewins Table 1. Electron microprobe analysis of a glass made from the starting composition (Jones, 1991).

The olivine disappearance temperature was determined by Jones (1991) to be 1211°C.

the furnace is immediately cooled at 2800°C/h to 1211°C (the olivine disappearance temperature). From 1211°C to the quench temperature of 1000°C, the furnace was cooled at 500°C/h. The duration of each melting event was monitored to ensure consistency between experiments. The point of using a flash melting technique is to deliver as much heat into a system as possible in the shortest amount of time (Hewins and Connolly, 1996; Yu and Hewins, 1998). Our experiments are dynamic melting; therefore, we do not attempt to attain thermodynamic equilibration within our charges. As will be shown in the results section, our charges do melt, suggesting that melting from above liquidus temperature does facilitate heat transfer to the charges interior, consistent with the findings of Maharaj and Hewins (1995). Two series of experiments were performed. In series I each grain size material (20 – 44 mm, 45– 62 mm, 63–124 mm, and 125–249 mm) was melted at ten different initial melting temperatures. Series II experiments used the starting composition with a grain size of 20 –249 mm and four different initial melting temperatures. All charges were mounted in epoxy and then placed into the Rutgers JEOL 8600 electron microprobe for documenting textures with backscattered electron imaging. 2.1. Texture Terminology

the production of chondrule textures using a superliquidus, rapid melting event (a type of flash melting). We assume that chondrules were formed from the incomplete melting of crystalline mineral aggregates as free-floating objects within our protoplanetary nebula. Our experiments place a new maximum temperature limit on chondrule formation. In conjunction with our study of chondrule melting histories we conduct an investigation into the physical nature of chondrule precursors. We show how the grain size of starting materials affects the dynamic melting process and hence the type of chondrule texture produced. Our grain size study places new constraints on the physical nature of chondrule precursors. 2. EXPERIMENTAL TECHNIQUE Experiments were performed on a 1 atm Deltech, vertical, muffletube furnace. The oxygen fugacity was maintained at 0.5 log units below the Fe-FeO buffer curve with a mixture of high purity CO and CO2. Experimental temperature conditions were measured with a Type S thermocouple contained within the sample rod. Calibration (61°C) of the thermocouple to the melting point of pure gold (1064.5°C) was conducted approximately every ten experiments. One of the goals of our experiments was to explore the production of chondrule textures by melting crystalline materials using a flash or short during melting (superliquidus temperatures for short times). To achieve this objective within our furnace (maximum working temperature of 1625°C), required the manufacture of a very FeO-rich, low liquidus (1211°C as determined by Jones, 1991), analog chondrule composition. Although more FeO-rich than previously studied compositions, it is within the range determined for natural chondrules (Hewins and Radomsky, 1990) and, therefore, a valid analog composition. Our experimental starting compositions (Table 1) were prepared from 70% Fa100 slag material (by volume; 76% Fa100, 19% glass and dendrites and 4% magnetite), 10% orthopyroxene, 10% plagioclase (;An50), and 10% diopside (Jones, 1991). The minerals were all ground and sieved separately. Mineral separates were then combined to make starting materials of five different grain sizes: 20 – 44 mm, 45– 62 mm, 63–124 mm, 125–249 mm (Fig. 1), and 20 –249 mm (Fig. 2). Experiments were performed by pressing 60 mg of starting material into pellets and attaching these pellets to a Pt90/Rh10 hang wire using a modified hang wire technique of Donaldson et al. (1975). To achieve a controlled, rapid melting episode room temperature pellets are placed into a furnace heated to a superliquidus temperature. The superliquidus temperature achieved by the furnace is defined in this paper as the initial melting temperature (Table 2) which charges experienced. Once a pellet is lowered into the hot spot of the furnace,

The terminology used to describe the textures produced in these experiments is based on Donaldson (1976), McSween (1977), Scott and Taylor (1983), Jones and Scott (1989), and Jones (1990). Microporphyritic olivine (MPO) textures are defined as having euhedral to subhedral phenocrysts set in a glassy mesostasis with the majority of crystals having a length and width of 100mm or less. Porphyritic olivine (PO) textures are defined as euhedral phenocrysts with a length greater than 100mm set in a glassy mesostasis. Skeletal or hopper crystals may be present in PO, but they are not the dominant crystal form. Hopper-porphyritic olivine (HPO) texture is defined as dominated by hopper olivines greater than 100 mm. Elongate-hopper, porphyritic olivine (ELHPO) is defined as a texture dominated by hopper olivine crystals with an aspect ratio of at least 3 to 1 set in a glassy mesostasis. The majority of the crystals always have a length greater than 100 mm. Barred olivine (BO) is defined as two or more parallel plates of optically continuous olivine crystals set in a glassy mesostasis (Weisberg, 1987). All of the described textures have a glassy mesostasis which is a defining characteristic of our results. 3. EXPERIMENTAL RESULTS

3.1. Series I Experiments: Grain Size Separates Experimental results are listed in Table 2 and schematically presented in Fig. 3a,b. Four texture fields, as defined by the data, are presented in Fig. 3a: MPO (Fig. 4a,b), PO (Fig. 4c,d), BO (Fig. 4e,f), and glass. The PO field can be further divided (Fig. 3b) into textures closely resembling Type IIA chondrules PO (Fig. 4b); HPO (Fig. 5a); ELHPO (Fig. 5b); and even transition textures such as ELHPO to BO. Both the initial melting temperature and the grain size of the starting material affect the degree of melting and hence texture production. 3.2. Series II Experiments: Unsorted Grain Size Starting Composition Experiments were conducted using the same chemical composition as Series I experiments, but the grain size of the material was 20 –249 mm. The results of these experiments are listed in Table 3. The textures are the same as those reported above with the exception of MPO, which is absent in these experiments. Most charges within the PO field have HPO textures similar to the HPO texture produced in series I experiments. Results (Fig. 6) are comparable to those for series I

The flash melting of chondrules

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Fig. 1. Backscattered electron images of grain mounts for the four starting compositions used in Series I experiments. This figure clearly shows that the initial grain size of each composition is different. a: 125–249 mm; b: 65–124 mm; c: 45– 62 mm; d: 20 – 44 mm.

experiments with 63–124 mm material, except that the PO/BO boundary is at a higher temperature. 4. DISCUSSION: EXPERIMENTAL RESULTS

The data presented in this paper shows that (1) chondrule textures can be produced from a superliquidus, short duration melting event, (2) variation in the initial temperature conditions can produce variations in the textures produced, and (3) the grain size of the starting material affects the number, kind, and distribution of crystals and nuclei remaining after melting and hence the type of texture produced. We explore these findings below. 4.1. Texture Production: Variations in Melting Conditions for a Constant Grain Size

Fig. 2. Backscattered electron image of a grain mount from a composition with a grain size range from 20 –249 mm that was used for Series II experiments.

The data from series I and II experiments (Figs. 3, 6) show how the production of chondrule textures from a starting material with a limited range in grain size is a function of the imposed melting conditions. Although ideally we would have liked to have varied only the initial melting temperature for

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H. C. Connolly Jr., B. D. Jones, and R. H. Hewins Table 2. A list of experiments, initial grain size of starting composition, initial melting temperature and the texture produced.

each charge, the furnace design made this impossible. Charges experienced both a change in the initial melting temperature and in the duration of the melting event. How temperature and melting time independently affect the production of chondrule textures in the laboratory has been shown (Connolly and Hewins, 1991; Lofgren, 1983,1989; Lofgren and Russell, 1986; Lofgren and Lanier, 1990; Radomsky and Hewins, 1990; Hewins and Connolly, 1996; Lofgren, 1996). In these earlier experiments melting was isothermal for 15 min to a few hours, producing glass, barred, or radial textures from liquidus to above liquidus melting temperatures whereas porphyritic textures were produced from melting temperatures far below liquidus temperatures. The results of the current study show that textures similar to those produced in earlier studies are also produced from short duration melting at substantially superheated temperatures as predicted by Radomsky and Hewins (1990) and Lofgren and Lanier (1990). The progression of textures produced from MPO to PO to BO to GL as a function of greater initial superliquidus melting temperatures correlates to an increase in the degree of melting experienced by charges.

As in longer melting duration experiments, the production of textures in our partial melting experiments depends on heterogeneous nucleation (Lofgren, 1983, 1996). Crystal growth requires the existence of material within a melt after heating stops, thus reducing the kinetic barrier of nucleation (Dowty, 1980; Kirkpatrick, 1981; Lofgren, 1983; Cashman, 1990). When melting is relatively limited (high amounts of partial melting), more nuclei or embryos survive producing coarse textures (MPO or PO). However, the greater the melting, the smaller the nuclei or embryos which survive (both in size and number) and hence the greater the degree of undercooling needed to induce crystal growth, producing textures such as barred. 4.2. Texture Production: Variations in Grain Size of Starting Material for Constant Melting Conditions Along with showing how initial melting conditions affect the production of chondrule textures, our data also show the importance of the grain size of the starting material on texture

The flash melting of chondrules

Fig. 3. (a) Schematic diagram of initial melting temperature vs. the grain size of the starting material. The experimental data points plotted define the boundaries for the texture fields. The boundaries are plotted as dotted lines to express that the exact temperature of transition from one type of texture to the next is not precisely defined. (b) Schematic diagram of initial melting temperature vs. the grain size of the starting material for the PO field only.

production. Experiments with different initial grain sized starting material melted at identical conditions produced different textures. Figure 7 best illustrates this point. All of the experiments in Fig. 7 were melted with an initial temperature of 1225°C, but the grain size of the starting material varied. The results of the 20 – 44 mm and 45– 62 mm experiments are different from those of the 63–124 mm and 125–249 mm experiments. The finer the material, the more nuclei remain in the melt after melting. The fewer the nuclei present after melting, the fewer the number of crystals which can grow, but these same crystals can grow larger due to decreased competition between growing crystals for elements within the liquid. The porphyritic field in Fig. 3 occurs at lower peak temperatures for the 20 – 62 mm starting materials than the 63–249 mm starting materials. Compared with the 63–249 mm experiments, the 20 – 62 mm experiments are more completely melted at higher superliquidus temperatures, suggesting that the dissolution of small (20 – 62 mm) grains occurs quicker than larger (63–249 mm) grains (Greenwood and Hess, 1996). The porphyritic field is large for 63–124 mm indicating that, for the experimental conditions, PO textures are easily produced with this grain size range for the specific thermal conditions imposed. The transition from MPO to PO appears to be restricted to a narrow temperature range (approximately 1250°C) with no transition textures produced. The production of PO requires

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that relatively few nuclei remain in the melt after melting, whereas MPO texture requires many more nuclei present. The shift from PO to BO occurs over a wide temperature range and is represented by a progression of transition textures. As the initial melting temperature is increased toward temperatures that produced BO, HPO, and ELHPO and finally ELHPO/BO are produced. As the initial melting temperature approaches the BO field, more melting occurs within the charges, and smaller and smaller embryos remain requiring greater degrees of undercooling before crystal growth can begin. The BO field is smallest at the 20 – 44 mm grain size range and largest at the 125–249 mm grain size range, suggesting that dissolution of grains is faster in the finest grain size fraction. Barred textures are easily produced over a large temperature range (about 400°C) for the 125–249 mm grain size range, suggesting that dissolution of the largest grain size for our experimental conditions is not complete. Barred olivine produced from the incomplete melting of chondrule analog compositions depends on the presence of just a few embryos, ideally only one for a single-grouplet barred texture (Lofgren and Lanier, 1990; Hewins and Radomsky, 1990). BO production also requires a greater amount of undercooling than that experienced by other textures such as PO. Our experimental method appears to favor the production of BO textures over previous reported techniques, providing the proper melting and undercooling conditions for its development. Glass was also easily produced in all but our 125–249 mm material, confirming the conclusions of Radomsky and Hewins (1990) that glass production in the laboratory is easy. Glass charges are completely melted, lacking any form of nuclei when cooling began, thus providing no substrate for nucleation and crystal growth. 4.2. Series II Results In the series II experiments we used the same bulk composition as series I, but mixed the grain size range to represent all the sizes studied in series I. The data from series I experiments (Fig. 3) show that the experiments with the largest grain size (125–249 mm) produced barred texture up to 400°C above the olivine disappearance temperature and did not produce a glass field. However, in series II experiments the BO field extends only 100°C above the olivine disappearance temperature, with a glass field existing from temperatures above the BO field even though the starting material contains some grains ranging up to 249 mm. Total melting is produced faster in series II compared to series I experiments. It would appear that the dissolution rate of large grains was faster in the mixed grain-sized material. Although given longer melting times the results of the two series of experiments would likely merge, it is the kinetics of melting that govern these experiments. Grains can be superheated without melting (Greenwood and Hess, 1996; Hewins and Connolly, 1996). In our experiments the largest relict olivines have the longest dissolution times, suggesting that they are superheated the longest. Our results suggest that starting materials composed of all larger grains (series I, 125–249 mm) will completely melt at a slower rate than compositions with fewer large grains (series II, 20 –249 mm). Although true as a general

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Fig. 4. Backscattered electron images of charges that represent each of the main texture fields. All of these charges has the same initial grain sized starting composition (20 – 44 mm) but were melted from increasing high temperatures with A representing the lowest temperature and E representing the highest temperature. The production of texture is clearly a function of the initial melting conditions. (a) A section of charge FL104 showing MPO texture. (b) The exterior of charge FL104 showing MPO texture before sectioning. (c) A section of charge FL87 showing a PO texture. (d) The exterior of charge FL87 showing a PO texture before sectioning. (e) A section of charge FL88 showing a BO texture. (f) The exterior of charge FL88 showing a BO texture before sectioning.

The flash melting of chondrules

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Fig. 6. A schematic diagram of the results of Series II experiments. The texture field boundaries are plotted as dotted lines to express the uncertainty of the precise temperature that the texture changes.

5. DISCUSSION: NATURAL CHONDRULES

5.1. The Formation of Barred Olivine Chondrules: Upper Temperature Limits on Chondrule Melting

Fig. 5. (a) A backscattered electron image of a section of charge FL99 showing a HPO texture. (b) A backscattered electron image of a section of charge FL49 showing an ELHPO texture.

statement, the survival of individual grains is also influenced by chance features such as grain shape and the presences of fractures, which will produce slight variation in the production of chondrule textures. Table 3. A list of experimental charge numbers, initial temperatures and the type of texture produced for our mixed grain series II starting composition Charge # FL109 FL122 FL119 FL110 FL113 FL118 FL121 FL107 FL115 FL117 FL123 F120 FL108 FL111 FL116

Initial temperature (°C)

Texture

1300 1300 1300 1400 1400 1400 1400 1500 1500 1500 1500 1600 1600 1600 1600

HPO PO HPO BO BO BO BO GL GL GL GL GL GL GL GL

Barred textures represent a borderline between complete and incomplete melting. Because barred textures require such an extreme degree of melting they are interpreted as an indicator of the maximum melting temperatures experienced by chondrules (Hewins and Radomsky, 1990; Lofgren and Lanier, 1990; Radomsky and Hewins, 1990). Using the maximum calculated liquidus temperature for barred olivine chondrules, Hewins and Radomsky (1990) placed an upper limit on the melting temperature experienced by chondrules of approximately 1750°C. The data presented in this paper show that by using a flash heating technique chondrule analogs can be produced. Our results show that barred olivine textures can be formed as much as 400°C above the liquidus. Considering the most magnesian bulk chondrule compositions discussed by Hewins and Radomsky (1990), BO chondrules could have experienced temperatures that approach or exceed 2100°C, provided the precursors were coarse grained (see below discussion) and chondrule melting resembled the type of flash melting technique used in the experiments presented here. If chondrules experienced a slightly shorter melting event, barred olivine chondrules could have formed from even higher initial melting temperatures. Although barred olivine chondrules are rare (4% of the total surveyed population of chondrules, Gooding and Keil, 1981) their bulk compositions span the range of almost all observed compositions (Hewins and Radomsky, 1990). If chondrules were formed from melting conditions similar to that used by Lofgren and Lanier (1990) and Radomsky and Hewins (1990) and did not encounter any external nuclei during formation (Connolly and Hewins, 1996), each barred olivine chondrules would basically represent a different melting event that heated each one to its liquidus. Although this kind of scenario is not impossible, an advantage to melting chondrules by some type of flash melting is that the range of melting conditions responsible for producing barred olivine is much greater. It is not

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Fig. 7. Backscattered electron images of charges all melted from the same initial temperature (1225°C) but with different grain-sized starting material. The initial grain size of the starting material clearly affects the type of texture produced. (a) Charge FL105 with a 20 – 44 mm starting composition. (b) Charge J41 with a 45– 62 mm starting composition. (c) Charge J44 with a 63–124 mm starting material. (d) Charge FL139 with a 125–249 mm starting material.

limited to near-liquidus temperatures and, therefore, produces many different types of chondrule textures in one formation event, providing that some variability in chondrule bulk compositions, hence liquidus temperatures, existed. Of course, as will be discussed below, two chondrules could have had the same bulk compositions, experienced the same melting conditions, while the resulting textures (e.g., BO, PO) could have been different if the grain size of their precursors were different. 5.2. The Formation of Porphyritic Textures and Chondrule Melting Our data suggest that porphyritic chondrules could have been produced from a type of flash melting event. However, the distinction between porphyritic textures typically associated with chondrules (i.e., Jones and Scott, 1988; Jones, 1990), and the actual diversity or kinds of porphyritic textures is not discussed in the literature. In our results, the porphyritic olivine field is divided into different types of porphyritic textures. A large area within the PO field (Fig. 3a,b) is actually represented

by hopper crystal textures, not the classical porphyritic olivine texture (Jones, 1991; Nagahara, 1981; Scott and Taylor, 1983) commonly associated with chondrules. Although present in chondrites (Fig. 8; Klein et al., 1980; Tschermak, 1885; Wasson et al., 1995) the abundance of HPO and ELHPO textures is unknown. Previous studies have lumped many different types of porphyritic chondrules into one large textural class, divided mainly on their modal mineralogy (i.e., PO, PP, POP chondrules). This type of general classification may be useful, but some porphyritic textures such as barred olivine/porphyriticolivine-pyroxene (BO/POP) cannot be formed as incomplete melts like other porphyritic textures (Connolly and Hewins, 1996). Therefore, it is useful to consider the formation conditions of different kinds of porphyritic textures. Although Lofgren (1989) did produce some textures similar to our HPO textures, his and other isothermal melting experiments (Lofgren and Russell, 1986; Lofgren, 1989: Lofgren and Lanier, 1990; Radomsky and Hewins, 1990) appear not to have produced distinct temperature/time HPO and ELHPO fields between PO and BO. The production of HPO or ELHPO was

The flash melting of chondrules

Fig. 8. Backscattered electron images of chondrules with HPO/ ELHPO textures. (a) Carraweena AMNH 3902-1. (b) Semarkona AMNH 4244-2.

not completely random in our experiments. Approximately 65% of the porphyritic charges have a HPO and ELHPO texture, apparently much higher than found in chondrites (although no firm abundance data exists). Therefore, it appears that although our technique produced analog BO and MPO textures, it also produces too many HPO/ELHPO textured spheres. The reason for such a high abundance of these textures is due to the grain size of the starting material, with the finest starting materials producing the lowest fraction of HPO and ELHPO textures. We cannot, however, rule out that the cooling rate used in our experiment may also be too fast (Jones and Lofgren, 1993), and slower or nonlinear cooling rates should be explored in the future. 5.3. Chondrule Precursors and Minimum Melting Temperature The data presented in this paper show that the grain size of the starting material can affect the production of chondrule textures but it does not provide any firm constraints on the physical nature of type II (FeO-rich), type IAB, or type IB (divisions of FeO-poor) chondrule precursors other than they could have been completely crystalline. The lack of constraint for these chondrule types is not rewarding; however it is a conclusion from our work.

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In our experiments, MPO textures were only formed from a uniform grain size starting material of less than 63 mm coupled with low degrees of melting (compared to that experienced by porphyritic and barred spheres). Because MPO textures are indicative of the more reduced, MgO-rich type IA chondrules, we suggest that the precursors of type IA chondrules were, on average, uniformly less than 63 mm in size and that they experienced relatively low degrees of melting (Connolly and Hewins, 1996). Although some type IA chondrules have large dusty olivines (relict grains), the majority of the phenocrysts are small, requiring a uniformly fine-grained precursor assemblage. Using our data to extrapolate on the nature of precursors for other types of chondrules not discussed thus far, we further suggest that finer grained chondrules such as dark zoned chondrules and agglomeratic chondrules (Weisberg and Prinz, 1996; Zanda et al., 1997) had precursors that were on average uniformly finer grained than those of MPO chondrule precursors and that these chondrules experienced less melting than MPO, type IA chondrules. These finer-grained chondrules may be either the best analogs for unmelted chondrule precursors (Hewins et al., 1997) or represent the least recycled (i.e. experienced very few melting events) chondrules (Zanda et al., 1997). The formation of MPO or finer-grained chondrules by a flash melting event(s) similar to our experiments requires that initial melting was not more than a few 10°C above their liquidus, if that. Therefore, following the style of Hewins and Radomsky (1990) if all chondrules were formed via a flash melting event the maximum melting temperature of 1750°C suggested by Hewins and Radomksy (1990) becomes a minimum initial melting temperature which chondrules experienced. It should be noted that this temperature could be much higher if the duration of the event was shorter. It is impossible for the duration of the heating of fine-grained, MgO-rich chondrules to be much longer than in our experiments or volatile phases such as sulfide would be lost from the chondrules (Yu et al., 1995; Hewins et al., 1996), contrary to what is observed in some of these objects.

6. CONCLUSIONS

Chondrule textures can be formed as the products of incomplete (MPO, PO) to complete (BO) melting from rapid, superliquidus melting coupled with linear cooling rates. Our experimental technique (coarse starting material) produces barred olivine from a much larger range of melting conditions than previous melting techniques. If chondrules were flash melted, then the maximum melting temperature was at least 2100°C. The maximum melting temperature experienced by chondrules is no longer limited to the liquidus temperatures of barred chondrules. The production of MPO texture only from grain sizes less than 63 mm with low degrees of melting suggests that Type IA chondrule had precursors that were on average less than 63 mm. Fine-grained chondrules such as type IA, MPO chondrules and agglomeratic chondrules experienced the lowest amount of melting (compared with other porphyritic and nonporphyritic chondrule types) thus representing a minimum melting temperature of chondrule formation of 1750°C.

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H. C. Connolly Jr., B. D. Jones, and R. H. Hewins

Acknowledgments—We would like to thank Jeremy S. Delaney, Gary E. Lofgren, Richard D. Ash, Brigitte Zanda, and Claude Herzberg for numerous discussions and constructive comments. The quality of the paper was greatly improved by helpful reviews from G. E. Lofgren, S. S. Russell, and C. M. Alexander. We also thank Bill Seldon of the Rutgers Geology Museum for the gift of minerals for use in our starting composition. This research was funded by NASA grants NGW 9-35 to Roger H. Hewins, PI, NAGW 3391 training grant for Harold C. Connolly Jr. and NASA NAG5-4319 to D. S. Burnett, PI. California Institute of Technology Division contribution number 5813.

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