Transient heating and chondrule formation: Evidence from sodium lossin flash heating simulation experiments
YANG YU* and ROGER H. HEWINS
Department of Geological Sciences, Rutgers University, Piscataway, New Jersey 08855, USA
(Received May8, 1996;accepted in revised form September2, 1997)
Abstract—Flash heating simulation experiments have been performed to determine the effects of heatingtime, cooling rate, ambient gasfO2
, and the sample bulk composition on the extent of Na loss of silicate meltsand the implications for chondrule formation. The samples studied include type IA, IAB, and IIAB chondruleanalog compositions as well as other synthetic silicate materials. The following experimental conditions wereemployed: heating temperatures ranging from 1300 to 1750°C, heating times ranging from less than 1 min to3 h, initial cooling rates ranging from 480°C/h to over 5000°C/h, and ambient gasfO2
ranging from 0.5 to 4log units below the iron-wustite buffer.
Bulk chemical analyses of the experimental charges show that compared to prolonged isothermal heating,flash heating with heating times of less than one minute dramatically reduced Na loss. However, the amountof Na retained by the charge depends greatly on the cooling rate following the heating at the peak temperature,ambient oxygen fugacities, and chondrule compositions. Generally speaking, shorter heating time, highercooling rate, higherfO2
, and a relatively Si-rich and Mg-poor composition favor Na retention.For type IIAB chondrule compositions, experiments show that with anfO2
close to that indicated by naturalmineral assemblages (about 10211.5to 10212 atm or higher at 1400°C), the flash heating events can reproducechondrule textures and Na contents well. With peak heating temperatures of 1400–1750°C, 90% Na retentionfor type IIAB composition requires a heating time of less than one minute followed by initial cooling ratesfrom ;500°C/h to;5000°C/h. This result supports the flash heating model to form type II chondrules andimplies that if they were formed in a nebular environment, it was dust and/or H2O enriched.
Type I chondrule compositions lose Na more easily than type II chondrules under the same heatingconditions, probably because of their higher Mg/(Si1 Al) ratios. This, plus the lowerfO2
The abundance of chondrules in primitive meteorites suggestswidespread melting early in solar system history. The cause ofthis melting is unknown, but some form of flash heating isprobable (e.g., Boss, 1996). The cooling rate experienced bychondrules is uncertain but is estimated from crystallizationexperiments as several tens to thousands of degrees per hour(Lofgren, 1989; Radomsky and Hewins, 1990; Yu and Hewins,1995). The nature of chondrule precursors is also uncertain.They might be nebular condensates aggregated at differenttemperatures and transformed to chondrules by essentiallyclose-system melting (Grossman, 1988). Alternatively, theycould be exclusively low temperature condensates melted un-der open-system conditions so as to generate the spectrum ofchondrule compositions (Sears et al., 1996; Huang et al., 1996).The extent to which moderately volatile elements such as Naare lost from chondrule melts is potentially very informativeabout chondrule precursors and heating-cooling conditions(Tsuchiyama et al., 1981; Grossman, 1988; Lewis et al., 1993;
Huang et al., 1996). As the experimental basis for discussingNa behavior is relatively limited, especially for flash heating,we here present new results on Na loss from melts of variouschondrule compositions under a variety of conditions.
Sodium has an evaporation (or condensation) temperature ofabout 700°C in a system of solar composition (Larimer, 1988),or somewhat higher depending on the elemental abundance ofthe nebular environment (Wood and Hashimoto, 1993), but stilllower than the chondrule-forming temperature range (1400–1800°C, e.g., Tsuchiyama and Nagahara, 1981; Planner andKeil, 1982; Lofgren, 1989; and Hewins and Connolly, 1996).Yet chondrules are not universally low in Na. Some of thenatural chondrules, especially FeO-rich (type II) chondrules,have Na contents comparable to or even slightly higher than thesolar abundance (Grossman and Wasson, 1983; Grossman,1988; Hewins, 1991; and Jones, 1994), even though manyisothermal experiments performed at close to chondrule-form-ing temperatures have demonstrated various degrees of Na lossfrom the experimental charges (e.g., Gibson and Hubbard,1972; Gooding and Muenow, 1976; Tsuchiyama et al., 1981;and Shimaoka and Nakamura, 1991). Such a seemingly con-tradictory behavior has stimulated and become the focus of the* Author to whom correspondence should be addressed.
ongoing debate about the chondrule-forming processes and thenebular environment.
Various hypotheses have been proposed to address the prob-lem of the survival of Na in chondrule melts, including flashheating. If the heating events forming chondrules were ex-tremely brief, e.g., about 20 s for nebular shock waves (Hoodand Horanyi, 1993; Hood and Kring, 1996), Na and othervolatile elements would have had very little time to evaporate(e.g., Grossman, 1988). Another possibility is an ambient gaswith high Na partial pressure (Lewis et al., 1993), although thesource of this Na is not well understood. Late entry of Na intonear-surface glass of Na- and FeO-poor (type I) chondrulesafter chondrule formation (Huang et al., 1994; Matsunami etal., 1993; Grossman, 1996a) has been observed but seemsunlikely to explain the uniformly Na-rich glass of type IIchondrules.
Flash melting, loosely defined as due to heating for less than100 s, is common to most recently considered chondrule for-mation models (Boss, 1996). It is, therefore, appropriate tostudy Na loss with flash heating, even though the energysources that could produce such instantaneous heating pulseshave not been unambiguously identified (Boss, 1996). Heatingtime/temperature is, of course, not the only factor affecting Nacontents of chondrules. Sodium loss from chondrule melts afterflash heating can be influenced by cooling rate, presumablycontrolled by local environmental factors, byfO2
throughout theheating-cooling cycle, and even by the structure and composi-tion of chondrule materials themselves. To some extent, deter-mination of these data is crucial to any further discussion of theflash heating model.
Currently the majority of the data on Na loss at high tem-peratures were obtained from isothermal heating experiments(e.g., Tsuchiyama et al., 1981). Except for limited data fromflash heating experiments (Connolly et al., 1993), the onlyattempts to quantitatively constrain the amount of Na loss inchondrule heating events were made by Grossman (1988) andHuang et al. (1996), who calculated the Na loss at differentfO2
based on the isothermal experimental data of Tsuchiyama et al.(1981) and the assumption of linear cooling rate. However,laboratory data were limited to superliquidus temperatures, andit is unclear to what extent the isothermal heating data, withloss rates measured in complete melts, are applicable to theflash heating and subsequent crystallization of chondrules.
This study evaluates the flash heating model by experimen-tally simulating the flash heating processes and measuring theNa loss from chondrule analog materials under various condi-tions, mainly peak heating temperature, cooling rate, andfO2
.We also vary bulk composition to cover the possibility thatdifferent chondrule types had very different precursor compo-sitions (Grossman, 1988). If all chondrule types instead had asingle precursor, which was modified by evaporative loss dur-ing melting (Sears et al., 1996), the experiments with the typeII composition are more relevant to chondrule formation con-ditions. In our opinion, there is strong evidence for loss of NaK, Fe, and S from natural chondrules, but also differences intheir precursor compositions (Hewins et al., 1997). Our exper-iments show strong dependence of the amount of Na loss on allthe factors studied.
2. SAMPLE DESCRIPTION AND EXPERIMENTALPROCEDURES
Starting materials with chondrule-like compositions include syn-thetic glass or mixtures of minerals. The synthetic glass is the one usedby Radomsky and Hewins (1990), made by fusing mixed commercialoxides and carbonates. The composition is similar to that of type IIABchondrules (Jones, 1996). The mineral mixtures were made by mixingdifferent proportions of natural San Carlos olivine (Fo88), orthopyrox-ene (En87), labradorite (An74), and albite, to approximate the type IA,IAB, and IIAB natural chondrule compositions (Jones, 1994). One ofthe samples of type IIAB glass material was doped with 10% graphite.Beside the chondrule analog compositions mentioned above, twogroups of nonchondrule compositions were also made as references.One of them comprises mixtures with different proportions of albiteand labradorite and the other comprises mixtures of albite and SanCarlos olivine. Table 1 shows the detailed mixing ratios and bulkchemical compositions of the starting materials, along with the calcu-lated liquidus temperatures for each bulk composition based on Her-zberg (1979). All the starting materials were in powder form with grainsizes between 20 and 150mm. The experimental charges were preparedby weighing about 60 mg of the powder and pressing them into pellets;3 mm in diameter.
Most of the dynamic crystallization experiments were conductedwith a DELTECH DT-31-VT-OS-C vertical muffle tube furnace atRutgers University. Where temperatures higher than 1650°C wereneeded, the experiments were performed at the JSC-ExperimentalPetrology Lab with an Astro furnace. The sample pellet was suspendedon a loop of Pt wire and placed within the furnace hot spot duringexperiments. A Pt-Pt90/Rh10 thermocouple was placed 2 mm abovethe sample. The thermocouple was frequently calibrated with the melt-ing points of Au (1064°C) and Pd (1554°C), and the uncertainty of thetemperature measurement for the sample was estimated to be65°C.The fO2
of the furnace atmosphere was controlled by mixing H2/CO2 orCO/CO2 gases in different proportions. ThefO2
was thus set at either0.5, 2, or 4 log units below the value for the Fe-FeO buffer at 1400°C.The gasfO2
variation with temperature is approximately parallel to theFe-FeO buffer, and thefO2
of these gas mixtures are called IW-0.5,IW-2, and IW-4, respectively, hereafter. An yttrium-stabilized zirconiasensor was used to monitor thefO2
. Such sensors suffer from dramat-ically increased electric conduction at lowfO2
and high temperatures.Therefore, forfO2
lower than IW-0.5, the sensor was used at a lowertemperature of 1100°C to determine the gas mixing ratio and then theextrapolatedfO2
at 1400°C was calculated from this ratio.The experimental procedures include flash heating simulation exper-
iments and isothermal heating experiments. For flash heating simula-tions, the sample was inserted into the preheated furnace. After oneminute from the sample insertion, the furnace was cooled at thepre-programmed cooling rates and finally quenched. One minute heat-ing is slightly longer than what is indicated for nebular shocks (Hoodand Kring, 1996), but Na loss in isothermal experiments is extremelysmall in the first minute (Tsuchiyama et al., 1981 and this study). Theone minute time lag is what is needed for the thermocouple to registerthe desired temperature. According to an independent testing (Maharajand Hewins, 1995), the sample interior takes less time to reach theambient temperature. The maximum heating temperatures were chosenso that chondrule textures can be reproduced, ranging from 1470° to1750°C. Tsuchiyama et al. (1981) applied their isothermal Na loss datato chondrules in a calculation assuming a natural Stefan-Boltzmanncooling history. We measured Na loss on charges actually cooled in thisway: the furnace was either programmed to follow six consecutivelinear cooling intervals with decreasing cooling rates or the powersupply to the furnace was turned off. The maximum cooling rate wasalways at the initial stage of cooling, and it ranged from;5000°C/h to480°C/h for different cooling paths. Whenever the phrase cooling rateis used in this paper, it will always mean the cooling rate when coolingstarts, or initial (maximum) cooling rate. The cooling curves used in theexperiments are shown in Fig. 1.
For most runs, we chose to use fixed furnace time (35 mins) for eachcharge. Consequently the charges run with different cooling rates werequenched at different temperatures (ranging from 1000°C to 1350°C).This is not a problem because we were trying to find the conditions thatcause minimum Na losses, and the runs with higher quenching tem-
160 Y. Yu and R. H. Hewins
perature were always associated with slow cooling, in which case theamount of Na loss is already too high when compared to chondrules(see discussions below).
Some of the charges were not cooled in the furnace but were takenout of the furnace immediately after they reached the desired temper-ature and cooled in air all the way to room temperature. The thermo-
couple reading indicated an initial cooling rate of several hundreddegrees per second (or;106°C/h). Such a heating/quenching cycle wasrepeated for one of the charges thirty times with a heating temperatureabout 100°C lower than the sample liquidus. Experiments of this naturewill be referred to as flash heating/quenching experiments thereafter.
In order to see how flowing furnace gas can affect the amount of Na
Note: IA, IAB, and IIAB are type IA, type IAB, type IIAB chondrule analog materials respectively.[1] Glass made from fusing reagent grade oxides and carbonates.[2] Ol: Mg/(Mg 1 Fe) 5 0.88; Opx: Mg/(Mg1 Fe) 5 0.87; Ab: An1; Lab: An74.T (liq) 5 liquidus temperature. Liquidus temperature is based on Herzberg (1979) and calculated only for chondrule analog compositions.
Fig. 1. Typical sample thermal histories employed in the experiments. Only the curves with maximum heatingtemperatures higher than sample liquidus are shown. For other peak temperature runs, the cooling curves are similar.
161Origin of chondrules
loss from the charge, some of the charges were run with static furnacegas. This was achieved by closing both the gas inlet and outlet endsafter the desiredfO2
was established. Independent tests show that thegasfO2
is relatively stable within the first 30 min of stopping the gasflow: thefO2
increases by 0.1 log unit for an initialfO2of IW-0.5 and by
0.5 log unit for an initialfO2of IW-2.
The isothermal heating was accomplished by inserting the sampleinto the hot furnace, keeping the sample at the desired temperature fora specific length of time, followed by quenching. Two groups ofisothermal experiments were performed: (1) 1530°C, 10 min isother-mal runs for all starting materials (chondrule analogs and albite-labradorite and albite-olivine mixtures. The main purposes were tosearch for any composition gradients in totally fused samples heated forshort times. (2) 1300°C, 3 h isothermal runs for type IIAB startingcomposition only. The purpose was to determine the Na loss behaviorwhen the temperature was well below the sample liquidus and numer-ous mineral crystals were still present in the melt.
All the charges were quenched in air, and the final charges werespherical with a diameter close to 3 mm. Some of the experiments wereduplicate runs, and the second charge was used for microprobe studies.Duplicate runs were also performed for selected experimental condi-tions to check the consistency of the Na loss results.
The Na analyses for bulk samples were accomplished by multichan-nel DCP-AES (Direct current plasma atomic emission spectrometry).The sample preparation procedure followed Feigenson and Carr (1985).The bulk experimental charge was carefully crushed with a Spexmixer/mill to 200 mesh, mixed with LiBO2, fluxed, and dissolved inHNO3 solution. The diluted solution was analyzed by DCP-AES withthe instrument optimized at Na. The standards used include USGSBHVO-1, BIR-1, and PCC-1, as well as a working standard OCC (theglassy type IIAB starting material). These standards were used both forcalibration purposes and as control samples to be analyzed togetherwith the experimental charges.
Electron microprobe analyses were performed on selected charges tolook for any possible Na concentration gradient across the charges aswell as to examine the petrographic textures produced. The quantitativeanalyses were carried out on a JEOL 8600 electron microprobe. Theaccelerating voltage was 15 kV; the beam current was 15–20 nA. Arastered 5mm beam and short counting times from 2 to 5 s for Na wereused to minimize the Na loss during analyses.
The amount of Na retained in the final charge after the heatingexperiments is expressed as Na/Nao, where Na5 the Na content of thecharge after heating, Nao 5 original Na content.
3. EXPERIMENTAL RESULTS
The precision of the DCP-AES measurements was tested byrepeatedly analyzing the same standard solution, and the resultsshow that the 2s errors were less than 2% for Na measurementsand were within 3% for the rest of elements with concentrationshigher than 1%. The accuracy of the DCP analyses was con-firmed by comparing our analytical results for standards withtheir reference values, and they agree within analytical uncer-tainties.
The consistency of the Na loss results for a given experi-mental condition was tested by multiple furnace runs. Severalexperimental conditions were chosen so that up to four samplesof the same composition were run independently under eachcondition. The results from these runs are consistent and re-producible with reasonable accuracy. Two examples of theDCP analyses for such replicate experiments are listed inTable 2.
All the DCP analytical results for Na are listed in Table 3 and4. Compared to the original composition, other major compo-nents of the charge beside Na also changed after heating.Usually Fe decreases due to its migration into the Pt supportwire. Analyses of Pt wires show that for experiments performedat IW-0.5, less than 3% of total Fe in the charge was lost to Pt.
Comparison experiments using Fe-coated Pt wires showed un-detectable effect on Na loss results. Potassium probably be-haves similarly to Na, but the change detected may not besignificant because of the low K content of our samples. Otherelements show slight increase because of the alkali and Felosses.
Electron microprobe analyses were performed on some ofthe isothermally heated charges. These charges were isother-mally heated at temperatures 30–50°C above their liquidus for10 min to ensure total melting. Back scattered electron imagesshow no sign of relict or melt-grown crystals. No apparentdiffusion profiles associated with Na loss are evident.
3.1. Flash Heating Experiments
Flash heating/controlled cooling experiments producedcharges with different glassy or crystalline textures. In general,a charge heated to a maximum temperature above its liquiduswill form glass or barred olivine (BO) texture, the latter beingmore common with fast cooling rate runs as opposed to slowcooling rate runs, because melting and destruction of nucleicontinue during the earliest (superliquidus) part of the coolinghistory. A lower maximum heating temperature (below thesample liquidus) will form either porphyritic olivine (PO), witholivines normally zoned from Fo84–90 (core) to Fo60–75 (rim)(see Yu and Hewins, 1995), or relict-olivine-grain textures, i.e.,samples resembling clast-laden impact melt rocks. Flash heat-ing/quenching experiments also produced charges with micro-porphyritic texture, but in contrast to the controlled coolingruns, grain size seldom exceeds 10mm. Multiple heating/quenching experiments grew olivine larger, but the olivinezoning is extremely limited compared to natural chondrules.Figure 2 shows examples of some of the textures producedfrom our experiments. The dependence of texture on initialtemperature and cooling rate agrees well with previous exper-iments (Tsuchiyama and Nagahara, 1981; Lofgren, 1989; Ra-domsky and Hewins, 1990; Yu et al., 1995; and Hewins andConnolly, 1996).
There are four variables in our flash heating experiments:
Table 2. Chemical analyses of charges from duplicate experiments.
heating temperature, cooling rate,fO2, and starting composition
of sample materials, each having its own effect on the Na/Nao
of the charge, though the extent of the effect from each variablealso depends on the other three variables.
Temperature is one of the key factors that affects Na evap-oration rate in previous isothermal experiments (e.g. Tsu-chiyama et al., 1981). Higher temperature tends to increase theNa loss, and this is generally the case for flash heating as well.However, under flash heating conditions, the actual effect ofchanging peak temperature depends greatly on cooling rates: ifthe samples descend rapidly to low temperature, the Na content
is relatively insensitive to peak temperature. In fact, as shownin Fig. 3, with an initial cooling rate of 5000°C/hr or higher, thedifference in Na loss from a sample heated to 1530°C and asample heated to 1750°C is negligible.
Cooling rate has a significant influence on Na evaporation ofheated charges. This is not surprising since the cooling ratecontrols the length of time a charge can stay at elevatedtemperature. The data in Fig. 3 also show that given the samepeak heating temperature, the amount of Na loss at lowercooling rate is far higher than that at higher cooling rate. Withan fO2
of IW-0.5, for a type IIAB chondrule composition flashheated to 1530°C (slightly above its liquidus) and cooled at amaximum cooling rate of 480°C/h, the Na/Nao is 0.73. How-ever, with a maximum cooling rate of 5000°C/h, Na/Nao is0.94. Although the quench temperature for the 480°C/h runs isartificially high (1350°C), the results show Na loss alreadygreater than appropriate for type II chondrules. The ability ofhigh cooling rate to hamper the Na loss from the charge is bestdemonstrated by the multiple flash heating/quenching experi-ments. Because the cooling following the heating was ex-tremely fast (;105 degrees per hour), a charge which experi-enced even thirty such heating/quenching cycles still has aNa/Nao of 0.97.
Low fO2enhances Na loss from a heated charge. Though this
has been demonstrated by previous isothermal experiments(Tsuchiyama et al., 1981), it was unclear whether the flashheating process could undermine such an effect. Our experi-mental results indicate that even though short heating time andhigh cooling rate reduced the time a charge spent at elevatedtemperatures, the effect offO2
on Na loss is still highly visible.Figure 4 shows the Na loss data for a type IIAB composition atvarious fO2
. The charges in the diagram were flash heated tovarious peak temperatures and cooled at 2400°C/h. ThefO2
effect can be seen for all initial temperatures, though it isstronger in higher temperature runs. A typical example isshown by a charge cooled from a peak temperature of 1530°C:the Na/Nao is about 0.90 at anfO2
of IW-0.5, but decreased toabout 0.40% at anfO2
of about IW-4. An interesting thing tonote here is that thefO2
in ambient gas could fail to control theNa loss behavior, if the spherule with its intrinsicfO2
has any
Table 4. Results of isothermal heating experiments.
Sample fO2T (°C) Heating Time Na/Na(o)
Type IAB IW-0.5 1530 10 min 0.53Type IA IW-0.5 1530 10 min 0.31Type IIAB IW-0.5 1530 10 min 0.70(0.69)
Note: Type IIAB starting materials include synthetic glass andmineral mixtures (in parenthesis).fO2
is expressed as log unit relative toFe-FeO buffer at 1400°C. A25L75 means Albite 25%1 Labradorite75%, A85O15 means Albite 85%1 San Carlos olivine 15%, and so on.
Note: Type IIAB starting materials include synthetic glass andmineral mixtures (in parentheses).fO2
is expressed as log unit relativeto Fe-FeO buffer at 1400°C.
[1] run in static furnace gas.[2] dopped with 10% graphite.[3] average of several independent runs.[4] flash heating/air quenching run. The charge was subjected to 30
heating/quenching cycles.‘‘—’’ means same as above.
163Origin of chondrules
buffering capacity. A charge of type IIAB composition dopedwith 10% graphite (ambient gasfO2
5 IW-0.5), when cooled atan initial cooling rate of 2400°C/h, lost twice as much Na as theone without the graphite (Table 3), probably because of thereaction:
Different starting compositions follow similar Na-loss pro-files as a function of cooling rate,fO2
, etc. However, when otherconditions remain the same, type IIAB composition shows thehighest Na/Nao, followed by type IAB composition, then typeIA (Fig. 5). It should be noted that the 1530°C peak heatingtemperature shown in the diagram is above the liquidus for typeIIAB composition, but below the liquidus for types IAB andIA. The type IIAB charge shows barred olivine texture,
whereas the two type I composition charges show numerousolivine relicts, indicating far lower degree of melting. If oneassumes that Na loss rate is inversely proportional to theamount of crystals present in the melt (Grossman, 1988), theresult should be the opposite of what is observed. Apparentlysuch difference in Na/Nao among different compositions haslittle to do with the relative proportion of the melt in eachcharge but is largely due to the properties of the melt. There aretwo kinds of charges with type IIAB chondrule composition inFig. 5: one had glassy starting material, the other had crystal-line starting material. There is a slight difference in Na/Nao
between the two, but such difference is likely due to the minorcomposition differences, since the latter has slightly higher Si,Al,and Na, and lower Fe, Mg, and Ca (Table 1, and seediscussion later).
Fig. 2. Backscattered electron images of selected charges. (a) Type IIAB chondrule analog composition with barredolivine texture (peak temperature5 1530°C, initial cooling rate5 2400°C/h,fO2
5 IW-0.5). Scale bar5 100mm. (b) TypeIIAB chondrule analog composition with prophyritic texture (peak temperature5 1470°C, initial cooling rate5 2400°C/h,fO2
5 IW-0.5). Normal zoning is apparent in olivine phenocrysts. Scale bar5 100 mm. (c) Type IA chondrule analogcomposition with relict olivine texture (peak temperature5 1500°C, initial cooling rate5 2400°C/h,fO2
5 IW-0.5). Someof the olivines show typical reverse zoning pattern: the core has the unmelted relict San Carlos olivine composition ofFo86–88, whereas the rim is Mg-rich, with composition of Fo92–96. Scale bar5 100 mm. (d) Type IIAB chondrule analogcomposition after repeated flash heating/quenching cycles (The charge was first heated to 1470°C, then immediatelyquenched in air. The procedure was repeated thirty times with a heating temperature of 1400°C.fO2
5 IW-0.5). Thecompositional zoning in olivines is very limited (less than 2% in most cases). Scale bar5 10 mm.
164 Y. Yu and R. H. Hewins
High temperature heating experiments could cause some Feloss to the supporting Pt wire. However, comparison experi-ments using Fe-plated Pt wires do not show statistically mean-ingful difference in Na loss from the charge. Beside Na and Fe,we have been unable to detect other compositional changescaused by heating experiments, such as Si. This is due to thenature of our flash heating experiments: the heating time andtemperature are much lower than that required for significant Siloss (Hashimoto, 1983).
The data shown in Fig. 3 through Fig. 5 were all obtainedfrom charges run with constantly flowing furnace gas. Figure 6compares the Na loss results between the normal flowing gasruns and static gas runs. Flowing furnace gas slightly increasesthe amount of Na loss from the charges, especially at lowerfO2
.The difference in Na loss is negligible atfO2
of IW-0.5, but atfO2
of IW-2, the charges run in flowing gas systematically lost6–7% more Na than those run in static gas. It should be pointedout that even with no gas flowing through, there is still gasmovement because of convection within the furnace chamber.
3.2. Isothermal Heating Experiments
Table 4 summarizes the results of the isothermal experi-ments. For the 1530°C, 10 min isothermal runs, charges withtype IAB and IA starting composition show abundant olivinerelicts, whereas the type IIAB charges are totally glassy with novisible relict crystals. DCP analyses show that Na/Nao variesamong the three compositions with a trend similar to thoseobserved from flash heating experiments (Fig. 5). An evenclearer picture is seen in albite-olivine mixtures with differentmixing ratios. The final charges of those mixtures are all glassy
Fig. 3. Cooling rate and temperature effect on Na loss during flashheating experiments. The temperatures shown are peak heating tem-peratures. With other conditions similar, higher cooling rates tend topreserve more Na in the charge. The higher the peak heating temper-ature, the more evident the effect of cooling rate.
Fig. 4. Oxygen fugacity effect on Na loss during flash heatingexperiments. The temperatures shown are peak heating temperatures.Sodium loss is greater at lowerfO2
. Similar to the cooling rate effect,fO2effect is stronger at higher peak temperatures. The straight lines con-necting the points are visual fitting lines.
Fig. 5. Composition effect on Na loss during flash heating andisothermal heating experiments. The amount of Na loss follows theorder of type IA. type IAB . type IIAB in all experimental condi-tions. Type IIAB glassy starting material (designated by *) has aslightly higher Na loss rate than that of type IIAB mineral mixturematerial.
Fig. 6. Effect of flowing furnace gas on the Na loss rate during flashheating experiments. Sodium loss is slightly faster in flowing furnacegas than in static furnace gas. This effect is more evident at relativelylow fO2
.
165Origin of chondrules
with no visible olivine relicts, and Na/Nao decreases withincreasing proportions of olivine in the starting material. Forthe albite-labradorite mixtures, on the other hand, no evidentdifferences in Na/Nao was observed among different albite andlabradorite mixing ratios.
The 1300°C, 3 h isothermal experiments on type IIAB start-ing material were performed at differentfO2
namely IW-0.5 andIW-2. The charges were about 50% crystalline because thetemperature was 200°C below the liquidus, but they still suf-fered significant Na losses under both conditions: 22% atIW-0.5 and 48% at IW-2.
4. DISCUSSION
Most chondrules in unequilibrated chondrites fall into twomajor categories: relatively refractory, FeO-poor chondrules(type I in McSween, 1977 and Jones and Scott, 1989) andrelatively FeO-rich chondrules (type II). Statistically, type Ichondrules are generally depleted in Na, whereas type II chon-drules exhibit high Na/Si ratios close to or even slightly higherthan those of CI chondrites. Type II chondrules seem to havelost little, if any, Na during the high temperature chondrule-forming events; the question is why. Type I chondrules eitherlost very little Na when formed and the low Na content reflectsthe pre-chondrule fractionation of precursor materials (e.g.,Wasson and Chou, 1974; Grossman, 1988; Hewins, 1991;Jones, 1994), or they suffered mild volatile loss during chon-drule formation and the low Na content provides evidence forit (e.g., Sears et al., 1996; Hewins et al., 1996), or Na loss wasextensive but some Na later reentered through late-stage nebulaor parent-body processes (Matsunami et al., 1993). In thefollowing discussion, we consider the possible scenarios toform chondrules, mainly involving the flash heating processand the nebular environment in which chondrules were formed,based on the Na loss experimental results. Since it is irrelevantto our experimental goals and conditions, we will not discussany of the secondary Na reentry processes at low temperatures.
4.1. Conditions Required to Form Type II Chondrules
The high Na concentrations in type II chondrules couldconceivably be maintained in many different ways, but if theyformed in the solar nebula, very special combinations of cir-cumstances are required. One possibility is that the nebular Napartial pressure was high in the type II chondrule-formingregions, suppressing the Na vaporization from the chondrulematerials. This effect was demonstrated by experimental study(Lewis et al., 1993), but the difficulty with this approach is thatto keep all Na in the chondrule materials requires a Na partialpressure at least 3 orders of magnitude higher than the nebularvalue (before Na condensation). Equilibrium thermodynamiccalculations following Wood and Hashimoto (1993) show thateven if the nebula was 1000 times enriched in dust, the requiredpartial pressure of Na can barely be achieved by total evapo-ration of Na from all this dust. Thus unless the source for theabnormally high Na pressure can be identified, it seems un-likely that there was an equilibrium vapor-liquid control main-taining the high Na concentration of type II chondrules, thoughit cannot be ruled out that the presence of some Na in the gascould assist other mechanisms which lead to Na retention.
Alternatively, it might be assumed that type II chondruleslost Na during their formation but, after local nebular gascooled down, some of the evaporated Na recondensed backonto chondrules (Huang et al., 1994). Though Na metasoma-tism could have occurred in the nebula or in the parent body,and there have been reports of type I chondrule glass that isenriched in Na and other moderately volatile elements near thechondrule surface (Matsunami et al., 1993; Grossman,1996a,b), the major problem for this model is the lack ofevidence that it applies to type II chondrules. They appearhomogeneous in glass and crystal compositions, except for theeffects of fractional crystallization, and it seems improbablethat any metasomatism would always be 100% complete.
A simpler approach to the type II chondrule Na probleminvolves flash heating and suggests that the heating eventsforming chondrules were extremely brief so that chondruleshad little or no time to lose Na (Grossman, 1988). This does notrequire the assumption of any unusual conditions, and manypossible models have been proposed to explain the mechanismsthat could generate such high energy in a very short time, e.g.,nebular shock waves, or, less likely, lightning discharges (Boss,1996). The heating duration necessary to form chondrules isunclear, but literally lightning-fast heating and cooling (flashheating followed by quenching) is an unlikely scenario sinceeven though Na could be preserved, chondrule textures cannotbe produced this way (Yu et al., 1995). A more extendedthermal history is needed to produce suitable crystals and willlead to Na loss unless the environment is non-canonical nebu-lar, or not nebular at all. The results of Na loss experimentsobtained from this study provide some clues to a nebularenvironment that controlsfO2
and the chondrule cooling rate,and they certainly favor a short heating duration.
The heating time in current flash heating models is not verywell defined but is suggested to be far less than 1 min (Hoodand Kring, 1996). The heating time used in our experiments isdefined as the time between inserting the sample into thefurnace and starting the cooling process, which was 1 min,though the time a charge spent at peak temperature was prob-ably 30–40 s (Maharaj and Hewins, 1995). Results show thatfor the type IIAB composition, if a charge was immediatelyquenched following the 1 min heating the Na loss is undetect-able by DCP analysis. By comparison, a charge isothermallyheated for 10 min at peak temperature lost;30% of Na.Therefore, if we consider only the Na loss from chondrulematerials, 1 min can be taken as the upper bound for the heatingduration in any flash heating model.
If Na loss is negligible during heating, then the rest of theprocess, namely the cooling stage, is critical in determining theamount of Na lost from chondrules. Quenching in air produceda maximum cooling rate of 106°C/h, and the Na loss was lessthan 3% of total Na in the charge even with thirty repetitions ofthe flash heating/quenching cycle. These experiments prove theviewpoint that chondrules could preserve volatile contents ifthey were formed by repeated small scale flash heating eventswith extremely fast cooling (106°C/h or higher, Wasson, 1996),except that the cooling rates in the quenching experiments aretoo high to produce chondrule textures. The ideal linear coolingrates for type II chondrule textures and mineral compositionsare approximately 100–1000°C/h (Tsuchiyama and Nagahara,1981; Lofgren, 1989; and Radomsky and Hewins, 1990; Alex-
166 Y. Yu and R. H. Hewins
ander and Wang, 1997). Our experimental results demonstratethat quenching is not necessary, and a cooling rate of severalhundred to several thousand degree per hour can still preservesignificant amounts of Na. Considering S as well as Na, wepropose 2500–5000°C/h decaying to 350–500°C/h for chon-drule cooling.
The cooling rates referred to above are all initial (or maxi-mum) cooling rates of specific cooling curves. Our data showthat the cooling rate at relatively lower temperature (close tothe quenching temperature) is also important in affecting Na/Nao. Actually, prolonged heating (for 3 h) at 1300°C stillcauses extensive Na loss from the charge (Na/Nao 0.78 atIW-0.5 and 0.52 at IW-2). The studies on exsolution lamellaein clinopyroxene and plagioclase by Weinbruch and Mu¨ller(1995) suggest a cooling rate of;10°C/h at the temperaturerange of 1200–1300°C, far lower than those used in our ex-periments at comparable temperatures in any of the coolingcurves. The cooling rates were measured on different kinds ofobjects: glass-free, granular type I chondrules in Allende byWeinbruch and Mu¨ller (1995) and type IIA chondrules forolivine zonation studies. It is possible, however, that type I andII chondrules experienced different thermal histories, since ourdata suggest that type II could not have Na/Nao . 0.9 for suchslow cooling. Further comparisons of olivine zonation andpyroxene exsolution are needed, on the same chondrules, asthis approach has the potential to constrain the lower temper-ature part of the cooling curves.
Sodium retention in type II chondrules requires relativelyhigh fO2
, even with flash heating. This is not surprising becausethe Na loss from chondrules involves a reduction processes.According to the calculation of Grossman (1988) based onisothermal heating data, retention of 90% of the Na with alinear cooling rate of 2000°C/h from liquidus temperaturesrequires anfO2
of 1027–1028 atm at a temperature of 1500°C.This is 7–8 orders of magnitude higher than the canonicalnebula fugacity value (;10216 atm, Wood and Hashimoto,1993). According to our experimental results, retention of 90%Na with an initial cooling rate of 2400°C/h at 1500°C requiresan fO2
of 10210 atm. Strictly speaking our experimental resultsindicate even lowerfO2
if linear 2000°C/h cooling rate isassumed. Nevertheless, although thefO2
inferred from our ex-periments is about 2 orders of magnitude lower than Gross-man’s calculation, it is still higher than the canonical nebulavalue but is consistent with thefO2
estimate for natural chon-drules.
Based on the iron-quartz-fayalite buffer adjusted for olivinecompositions (Kring, 1988; Johnson, 1986), thefO2
calculatedfor natural type I (Jones and Scott, 1989) chondrules is in therange of IW-3 to IW-4 or 10213 to 10214 atm at 1400°C. Suchvalues were shown to be in agreement with those calculated byother techniques (Zanda et al., 1994). These chondrules con-tain, as required for the calculation (Kring, 1988), olivine,Si-rich glass, and Fe-rich metal (which is also Si- and Cr-bearing). The calculatedfO2
for type II chondrules in Se-markona is IW-1.5 or 10211.5 at 1400°C. However, type IIchondrules do not normally contain kamacite, or magnetite, butonly minor taenite produced relatively late by S loss fromsulfide melt (Zanda et al., 1994; Hewins et al, 1997). They werenot buffered by a QFI-type assemblage during initial olivine
crystallization, and thefO2calculated for type II chondrules is,
therefore, only a lower limit to the true value.Figure 7 shows the conditions of peak temperature, oxygen
fugacity, and initial cooling rate (on a Stefan-Boltzmann cool-ing curve) needed for a type IIAB composition to retain 90%Na during a flash heating episode. The 1400°C lower limit forheating temperatures was chosen based on minimum degrees ofmelting in previous chondrule texture experiments (Hewins andConnolly, 1996), which led to estimates of typical temperaturesfor chondrules of all compositions of 1500–1850°C. The cool-ing rate required is a function of bothfO2
and peak heatingtemperature: with anfO2
of 0.5 log unit below IW (;10210 atmat 1400°C), the cooling rate can be as low as several hundreddegrees per hour if heated to 1400°C peak temperature, orseveral thousand degree per hour if it reached a higher peaktemperature. With anfO2
of more than 2 log units below IW(,10212 atm at 1400°C), the cooling rates would have to behigher and much more restricted, at least higher than 3000°C/heven at peak temperature of 1400°C. Figure 7 shows that topreserve type II Na concentrations, not only must cooling berapid, but the gas must be distinctly nonsolar. Plotting our bestestimates of chondrule initial temperatures and cooling ratesproposed previously (Hewins and Connolly, 1996; Yu andHewins, 1996) on Fig. 7, we see that thefO2
should be about 1/2log units below IW. With lower cooling rates advocated byothers (Weinbruch and Mu´ller, 1995; Jones and Lofgren, 1993)the fO2
would have to be even higher, but it could still beconsistent with the absence of primary kamacite or magnetite intype II chondrules.
The above Na-loss experimental results suggest that the flashheating model works reasonably well for type II chondrules,though it is subject to certain restrictions. To obtain the neces-sary degrees of melting, the heating temperatures were proba-bly above 1400°C. To retain Na and produce chondrule tex-tures, the heating lasted for less than 1 min, and the initialcooling rates were in the range of several thousand degrees perhour unless thefO2
was unusually high, in which case thecooling rate range can be lowered to several hundred degrees
Fig. 7. Flash heating conditions and ambient gasfO2required to
maintain 90% of Na in type IIAB chondrules. IW-0.5 is equivalent toan fO2
of 5 310210 atm at 1500°C, and IW-2 is equivalent to anfO2of
2 3 10211 atm at 1500°C. The rectangle in the diagram indicates thetype II chondrule forming conditions favored by Yu and Hewins (1996)and Hewins and Connolly (1996).
167Origin of chondrules
per hour. These parameters may need further refinement due tothe fact that the experiments were conducted at 1 atm, and thesize of the charge was larger than most actual chondrule sizes.In isothermal experiments, decreasing the mass of the chargeby a factor of ten doubles the amount of Na loss (Tsushiyamaet al., 1981). We take these conditions as the minimum require-ments for forming type II chondrules, and there are perhapsseveral possibilities for how these requirements could be sat-isfied.
Cooling rates of several thousand degrees per hour or lowerare too low for free radiation into open space. However, if thenebula was locally enriched in dust and chondrule material,then such cooling rates would be possible in these regions.According to Sahagian and Hewins (1992; and per. commun.),a sheet of chondrules a few hundred km thick embedded withina much thicker concentration of cold chondrules (or precursoraggregates) and fine dust can result in cooling curves compa-rable to those used in our experiments. High concentrations ofdust could also generate higherfO2
if the dust was partiallyevaporated at elevated temperatures (Wood, 1985; Wood andHashimoto, 1993). The Wood-Hashimoto code (Wood andHashimoto, 1993) is the only readily available way to calculateoxygen fugacities relevant to Na evaporation. Calculationsusing their method indicate that thefO2
equivalent to that fortype II chondrules at chondrule-forming temperatures could beachieved by enriching dust 1000 times relative to solar abun-dance and evaporating the dust (Fig. 8). However, higherfO2
observed for type II chondrules might alternatively be due tohigher concentration of H2O (ice). Figure 8 shows that similarfO2
could be achieved by enriching H2O to 100 times solarabundance. It should be pointed out that the Wood and Hashi-moto (1993) method is based on the assumption of equilibriumand does not consider the nonideality of the silicate melt (Ebeland Grossman, 1997), and, therefore, the calculation results canonly be taken as a rough guide.
Enriched dust seems to be a necessary condition both phys-ically and chemically for the cooling of type II chondrules(Hewins, 1989). This assumes that heating and cooling oc-curred as two consecutive steps. An alternative is that the heatsource was long-lived and slowly faded, and this fading, notdust blanketing, controlled the radiative cooling (Eisenhour,pers. commun., 1995). However, we prefer a flash heatingevent, as reviewed by Boss (1996). A paradox in the dustenrichment approach is that to retain Na in chondrule melts weappear to have to evaporate much Na, etc., from dust. Onecould postulate that a small fraction of the dust occurs as largeaggregates, and large dust aggregates melt and retain Na, whilesmall dust grains evaporate. The problem with this, if evapo-rating fine dust alone generates high enoughfO2
, is that it wouldbe difficult to transport dust grains to the midplane (Cuzzi et al.,1996), where the most massive concentration of dust would beexpected, unless dust can be entrained by the nebular shocks(Boss, 1996). Because of turbulence, dust could not settle toform a midplane concentration until after aggregates had been
Fig. 8. NebulafO2calculated using PHEQ program (Wood and Hashimoto, 1993). The assumptions on the system
compositions are labeled on each curve. The range offO2for natural type I and type II chondrules are from Jones and Scott
(1989) and Jones (1990).
168 Y. Yu and R. H. Hewins
densified by chondrule formation (Cuzzi et al., 1996). A furtherproblem is that though large particles, silicate and/or ice, mightbe concentrated in intereddy clumps (Cuzzi et al., 1996), theseclumps may be too small to generate a suitable chondrulecooling environment. To find an external control for chondruleNa concentrations and redox state, full-scale calculations of thetemperature, pressure, and composition of the gas generatedeither by such clumps after the passage of nebular shock wavesor by planetesimal-cometesimal collisions (Kitamura andTsuchiyama, 1996) are needed.
Of course there are other possibilities for controlling Na lossor retention. Several features of reduced (type I) chondrules areduplicated by graphite incorporation rather than gas buffering(Connolly et al., 1994). Our own experiments show that addinggraphite to the sample significantly increases the Na loss rate.This implies thefO2
controlling chondrule Na concentrationscould also be internally buffered by the precursor materials orcontrolled by reactions between them. If chondrule precursorscontained varying amounts of low temperature condensatessuch as organic compounds, magnetite, and ice, with the typeIIs containing more oxidizing combinations, the low pressurenebular gas could have had far less effect on Na loss duringmelting. Unfortunately there is no obvious way to demonstratethat chondrule precursors contained such phases.
We note that in the past we have appealed to either ambientvapor (Hewins, 1989) or flash heating (e.g., Yu et al., 1995) tocontrol chondrule volatile concentrations. Preliminary lowpressure experiments (Yu and Hewins, 1997) show higher ratesof Na loss than at 1 atm, as expected. Though flash heating isprobably important in restricting volatile loss, a high pressureambient gas would on the other hand provide ideal conditions.Certainly the pressures and compositions of transient cloudsdue to heating by nebular shock waves or collisions should benumerically evaluated.
4.2. Type I Chondrules and Evaporation
Any discussion of the Na concentrations of type I chondrulesdepends critically on assumptions about their origin. Chon-drules could have been formed by closed-system melting ofmaterial fractionated by nebular processes (e.g., Grossman,1988 and Hewins, 1991), or by open-system melting of undif-ferentiated, virtually solar composition solids (Sears et al.,1996). In their picture of chondrule formation, Sears et al.(1996) proposed that type I chondrules were derived frommaterial similar to type II chondrules and that heating in anenvironment with high oxygen fugacity caused little volatileloss or reduction, giving type II chondrules, while heating withlower oxygen fugacities caused extensive Na loss and FeOreduction to Fe metal, giving type IAB chondrule composi-tions. The evaporative volatile loss from type I chondrules issupported by a continuous negative correlation trend betweengrain size and abundance of volatiles observed in Semarkonatype I chondrules (Hewins et al., 1996). However, the finestgrained chondrules, which are closest in composition to theirprecursors, have high S, etc., but are FeO-poor, indicating thatnot all the features of type I chondrules are the results ofevaporation, and we believe that both nebular fractionation andopen-system melting played a role in forming chondrules. TheNa loss experimental results obtained from this study have
provided a quantitative constraint on the extent of Na lossunder different heating conditions. We have attempted to cal-culate the initial Na content of the precursor materials for typeI chondrules using these results, and they provide further sup-port for this argument.
Unlike type II chondrules, it would have been difficult fortype I chondrules to retain most of the original Na content. Thisis because relative to type II chondrules, type I chondrules wereformed in an environment with much lowerfO2
, were probablyformed at higher peak heating temperatures, and their meltproperties seem to favor Na loss. Type I chondrules are farmore reduced than type II chondrules, as documented above.Whether type I chondrules experienced higher overall heatingtemperature than type II chondrules is debatable, but since typeI chondrules generally have higher liquidus temperatures, theheating temperatures for some of them might have very wellexceeded those for type II chondrules with similar petrographictextures (e.g., porphyritic chondrules formed at temperaturesnear their liquidus temperatures, see Hewins and Radomsky,1990; Hewins and Connolly, 1996). As for compositional ef-fects, we have observed that some factor intrinsic to the sam-ples results in very different extents of Na loss for the differentcompositions under identical physical conditions. This is prob-ably related to melt structure (degree of polymerization), sincethe more Mg-rich, Si-poor compositions (e.g., type I chondrulematerials) lose more Na than the others (e.g., type IIAB chon-drule material; Fig. 9). It might be influenced by intrinsicoxygen fugacity of the silicates, if the ambient gas cannotthoroughly interact with the samples, though this effect isprobably minor because the composition effect is observed fordifferent compositions with identical FeO/MgO ratios, e.g.,albite-olivine mixtures (Table 4).
One way to compensate for the high Na losses would be toincrease the cooling rate. However, this does not seem to beplausible because the cooling rate required would be too high.Figure 7 is constructed with data of type II chondrule compo-sitions. It shows that with anfO2
lower than IW-2, the initialcooling rates would have to be 8,000°C/h or higher in order toretain 90% of Na for a type II chondrule. For type I chondrules,the cooling rate required for the same level of Na retentioncould be orders of magnitude higher because of the much lowerfO2
and the composition effect mentioned above. This is unre-alistic because the cooling rates this high won’t permit enoughcrystal growth to reproduce the chondrule textures (Yu et al.,1995).
Is evaporation the only cause for the low Na content ob-served in natural type I chondrules? We have tried to use theinformation gained from this study, namely thefO2
, thermalhistory, and melt composition effect on Na loss, to reconstructthe initial Na content of the precursor materials for differenttypes of natural chondrules. In our calculation, we used theexperimental results listed in Table 3, thefO2
values and liqui-dus temperatures calculated for the chondrules in the Se-markona chondrite (Jones and Scott, 1989; Jones, 1990). InSemarkona, the type I chondrules have mostly 0.07–0.65% andthe type IIs mostly 1–2% Na2O. The results show that eventhough the initial Na concentration of some type I chondrulesoverlap with the ranges for type II chondrules, the type Is ingeneral had lower initial Na (approximately 1 wt% Na2O, seeFig. 10). Four of the type I chondrules of Jones and Scott
169Origin of chondrules
(1989) show anomalously high initial Na values, probablyindicating Na metasomatism (Grossman, 1996a). Ignoringthese points we see a negative Na-Mg/(SiO2 1 Al2O3) corre-lation (Fig. 10). This suggests that factors other than evapora-tion, including chemically heterogeneous precursors for differ-ent chondrule compositions, also had an influence on the Naconcentrations of natural chondrules.
There are other possible factors which might explain thedifferences in Na contents between types I and II chondrules,which we are currently unable to model. These include thescenario that type I chondrules experienced lower ambient gaspressures or longer heating times than type II, as suggested byLibourel and Chaussidon (1995) and Sears et al. (1996). Lowerambient pressure may lead to higher Na loss rate (Yu andHewins, 1997), but it is important to see to what extent flashheating can reduce Na loss in vacuum and how the results will
compare with the 1 atm flowing furnace gas runs before such ascenario can be evaluated.
The difference in moderately volatile elements between typeI and type II could be in part due to the difference in oxygenfugacity and time-temperature conditions during heating. IffO2
was solely controlled by ambient gas, a nonnebular gas com-position is implied, especially for type II chondrules. Alterna-tively, their precursor materials could have different propor-tions of low temperature condensates (organics, magnetite, orice), as well as differences in olivine compositions, whichwould have had a large influence on intrinsic oxygen fugacityand hence on Na loss. The paradox of the just-right heatingmechanism, which raises all chondrules approximately to theirrespective liquidus temperatures, is well known. If the heatingevents forming both types of chondrules were the same orsimilar, and a range of intensities was involved, one can imag-
Fig. 9. Factors that may explain the composition effect on Na loss. In each diagram, Mg/(Si1Al) ratio, viscosity, andoriginal Na content of the melt are plotted against the starting composition of the charge, which is arranged from left to rightin the order of increasing Na loss under similar heating conditions (except in (d), where no obvious difference in Na lossamong different compositions is observed). (a) Chondrule analog compositions used in this study, where IIAB stands fortype IIAB chondrule composition, etc. mm5 mineral mixture; gl5 glass. (b) Compositions used in Tsuchiyama et al.(1981). (c) Albite and San Carlos olivine mixtures. (d) Albite and labradorite mixtures. All the viscosities were calculatedfor the temperature of 1500°C. sodium loss increases with increasing Mg/Si1Al ratio. However, the effects of melt viscosityand the original Na concentration are not as clear, especially in the melt with plagioclase composition (d). In addition, Naloss is not positively correlated with the Na content in the melt, as assumed in the calculation of Tsuchiyama et al. (1981).
170 Y. Yu and R. H. Hewins
ine evaporation driving the chondrule melts up a liquidussurface and producing the more refractory chondrule composi-tions (Sears et al., 1996). This scenario is consistent with ourobservations on Na, but it also requires Si loss, the evidence ofwhich is far from clear (Hewins et al., 1997).
5. CONCLUSIONS
We found that in order to keep Na in FeO-rich type IIchondrules, flash heating is needed, and the cooling rates needto be far higher than those currently favored. On the other hand,Na loss from FeO-poor type I chondrules is much easier and isdifficult to prevent efficiently even by flash heating.
Flash heating experiments have shown that although tran-sient heating events can dramatically reduce Na loss fromchondrule analog compositions, other factors, such as coolingrate,fO2
, and chondrule compositions, also play important rolesin determining the chondrule properties. Generally speaking,shorter heating time, higher cooling rate, higherfO2
, and arelatively Si-rich and Mg-poor composition favor Na retention.
For type II chondrule compositions, the relatively high Nacontents can be reproduced well with flash heating experi-ments. At the peak heating temperatures of 1400–1750°C, 90%of Na retention for type IIAB composition requires a heatingtime of less than one minute following by initial cooling ratesfrom ;500°C/h to;5000°C/h, and anfO2
ranging from 10210
to 10212 atm (1400°C). If chondrules formed in the solarnebula, such a combination of conditions implies a dust and/orH2O enriched nebular environment.
Type I chondrule compositions lose Na more easily than typeII chondrules because of the lowerfO2
environment comparedto that for type II chondrules and their higher Mg/Si1Al ratios.It is, therefore, difficult for type I chondrules to retain most oftheir original Na content during their formation, even if theheating was transient in nature.
The differences in Na contents between type I and type IIchondrule compositions could be partially due to the ease withwhich type I chondrules lose Na, but the differences in other
properties, such as refractory elements as well asfO2, imply that
they probably were formed in different settings in the nebulawith somewhat different precursor materials.
Acknowledgments—We thank L. C. Patino, M. J. Carr, and J. S.Delaney for their help in DCP-AES and electron microprobe analyses,G. Lofgren for access to JSC-experimental petrology lab Astro furnacefacilities, H. C. Connolly, Jr. for performing some of the furnace work,and J. A. Wood for providing PHEQ program. We are indebted to J.Grossman, D. Mittelfehldt, H. Nagahara, and D. Sears for detailedreviews which led to considerable improvements in the paper. Thisstudy was supported by NASA (OSS NAGW-2263 and PMG NAGW-3391).
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