High time resolution by use of the 26 Al chronometer in the multistage formation of a CAI

Transcription

1 Earth and Planetary Science Letters 182 (2000) 15^29 High time resolution by use of the 26 Al chronometer in the multistage formation of a CAI Weibiao Hsu a, G.J. Wasserburg a; *, Gary R. Huss a;b a Lunatic Asylum, Division of Geological and Planetary Sciences, Mail Code , California Institute of Technology, Pasadena, CA 91125, USA b Department of Geology and Center for Meteorite Studies, Arizona State University, Temple, AZ 85287, USA Received 29 May 2000; received in revised form 17 July 2000; accepted 24 July 2000 Abstract The Allende calcium-rich inclusion (CAI) 5241 has been found to contain distinct initial 26 Al/ 27 Al in the three consecutive igneous zones that have been identified by extensive petrogenetic studies to have formed in three distinct crystallization events. The zones in order of sequence of formation from the petrologic observations are: (1) spinel-free islands (SFI) included in (2) a pyroxene^spinel-rich core (SRC) which in turn is included in (3) a melilite mantle (MM). The initial ( 26 Al/ 27 Al) values of these zones are respectively (4.6^5.0)U10 35, 4.3U10 35, and 3.3U It is argued that these distinct ( 26 Al/ 27 Al) 0 values are not the result of metamorphism but reflect the relative times of formation by crystallization from melts. Relative to the canonical value of ( 26 Al/ 27 Al) 0 =5U10 35, we find the following chronology: t SFI W0, t SRC W10 5 yr and t MM W4U10 5 yr. The three-layer CAI is inferred to have sampled a reservoir with an initial uniform 26 Al/ 27 Al ratio that decreased in value due to radioactive decay. From these observations we conclude that we have resolved time differences of a few hundred thousand years at the very early stages of formation of the solar system. The SFI, SRC, and MM zones reflect sequential addition of molten CAI material which crystallized rapidly without seriously metamorphosing the previously formed material. These additions took place over a time of about years. We believe that these sequential events are not compatible with condensation in a hot region of the solar nebula. It is proposed that a scenario involving stages of protoplanetary accretion and of melt generation in protoplanetary sites heated by 26 Al might provide a possible source for CAIs and chondrules. ß 2000 Elsevier Science B.V. All rights reserved. Keywords: calcium-aluminium inclusions; Al-26; chondrules; solar system 1. Introduction Calcium-rich inclusions (CAIs) are known to have originally contained 26 Al (d = 1.06U10 6 yr) * Corresponding author. Fax: ; gjw@gps.caltech.edu at the time of their formation [1^3]. The initial 26 A1/ 27 Al (( 26 A1/ 27 Al) 0 ) for many CAIs has been found to be 5U10 35, but in some cases it is well below this value. No higher values have ever been found and substantiated in CAIs. The major minerals in CAIs correspond to the principal oxide phases that would condense from a hot mass of gas of solar composition [4]. Coarse-grained CAIs comprise three main types [5]. Type B and com X / 00 / $ ^ see front matter ß 2000 Elsevier Science B.V. All rights reserved. PII: S X(00)

2 16 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 pact type A CAIs appear to have originated through crystallization of molten or partially molten droplets. `Flu y' type A CAIs appear to be aggregates of condensates from a gas phase [6]. Some coarse-grained CAIs contain vugs, voids and/or vesicles coated with microscopic wollastonite needles that were clearly deposited from a vapor [7]. Most type B CAIs are round or rounded objects but often are distorted and contain streaks or regions of very ne-grained black `matrix' material folded or included in their interiors. CAIs are present in essentially all classes of chondritic meteorites although their abundance and the mixture of di erent types varies among the chondrite classes [8^11]. The CAIs contain isotopically anomalous oxygen which varies among and within the various mineral phases [12,13]. Many CAIs have isotopically anomalous abundances for many heavy elements that are not due to the radioactive decay of short-lived nuclei, but rather are general isotopic anomalies re ecting di erent mixing ratios from diverse precursor stellar sources which were then melted in the solar system [14,15]. While these e ects are typically small (fractional isotopic shifts of 10 33^1034 ), they demonstrate that these materials are not completely homogenized early solar system material. Surviving material from the numerous protosolar sources that made up the original mix to form the bulk solar material has been found as rare presolar dust grains contained within the matrix of many chondritic meteorites [16]. In this paper, we report on the 26 Al^Mg isotopic systematics in di erent lithic units within a single CAI that represent distinct stages of formation. Among CAIs, ( 26 Al/ 27 Al) 0 values range from 5U10 35, which is found in a large number of CAIs from carbonaceous, ordinary, and enstatite chondrites [3,8^11], to (see [3]). Clear evidence for the presence of 26 Al has now been found in chondrules with ( 26 Al/ 27 Al) 0 up to V1U10 35 [8,10,17,18]. Some CAIs show low ( 26 Al/ 27 Al) 0 values overlapping with the range found for chondrules, while others show internal scatter of inferred ( 26 Al/ 27 Al) 0 values within a single CAI. Some of these samples have individual crystals with low values, while the majority of the Al-rich phases show values at or near the canonical value of 5U10 35 [19^22]. It has been argued that the redistribution of radiogenic Mg ( 26 Mg*) cannot, in general, be attributed to metamorphism within the parent planet of the chondrite containing the CAI. This is shown by the fact that there is a full range of 26 Al/ 27 Al values in di erent CAIs and chondrules in the same object [8,22]. As the susceptibility of these inclusions and chondrules to thermal metamorphism would be about the same, it follows that these meteorites must represent aggregates of material from diverse sources with distinctive metamorphic histories and formation ages that were nally assembled in the meteorite parent body now sampled. Rather strict bounds on subsequent thermal metamorphism have been established by determination of the Mg di usion coe cients in spinel, plagioclase and melilite [23^25]. While there are distinct di erences in di usion coe cients for di erent phases (e.g., hibonite is much more resistant to Mg redistribution and there may be di erences in behavior between A î kermanite and Gehlenite), it is not plausible to attribute the range in ( 26 Al/ 27 Al) 0 in di erent CAIs and chondrules in the same meteorite to in situ metamorphism. In all cases, the Al-rich phases are immediately adjacent to high Mg phases, which can readily provide normal Mg in any isotopic exchange. In the present study we have selected the Allende type B CAI USMN 5241 for investigation. El Goresy et al. [26] rst recognized that this CAI required a multistage formation history. This type B CAI has been subjected to extensive petrochemical studies [26^29]. While some discrepancies exist in interpretation, it is well established that there are three distinct petrographic units within this CAI. These are spinel-free islands (SFI), a spinel-rich core (SRC), and a melilite mantle (MM). From consideration of the petrology, mineral chemistry and phase equilibrium relationships, it is clear that the SFI are older CAI fragments or clasts that were later entrapped in a liquid of CAI composition that crystallized around them. These units were then engulfed in another liquid of mostly melilite composition that formed the mantle. This CAI also contains sul de inclusions with a wide assortment of ultra-refrac-

3 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 17 tory metals and oxides [27]. Meeker [29] has shown that it was subjected to a late stage of Na metasomatism that permeated the CAI through ne cracks. Detailed descriptions of the textures have been presented by El Goresy et al. [26], MacPherson et al. [28], and Meeker [29]. The reader can nd both fuller descriptions and extensive photomicrographs of this CAI. A beautiful Al concentration map by Meeker [29] was used as a guide to the present study. If the three distinct lithologies of this sample represent events su ciently separated in time, then we might expect this time sequence to be recorded in the Al^Mg isotopic systematics of the three lithologies. This would require that the three events, which involve high temperature melts, would have to have been of short enough duration to preserve such e ects. The assemblage must subsequently have also been kept at su ciently low temperatures to avoid signi cant mobilization of magnesium. 2. Experimental A polished thin section of the Allende CAI 5241 (Caltech MQM-999) was rst examined with an optical microscope and then studied with a JEOL JSM-35CF scanning electron microscope (SEM) equipped with a Tracor Northern energy dispersive X-ray analysis system. Backscattered electron images were taken of the interesting areas with the SEM. Magnesium isotopic ratios and Al/Mg ratios were measured with PAN- URGE, a modi ed CAMECA IMS-3f ion probe, using an O 3 primary beam. The details of the method have been described by Fahey et al. [30,31]. A mass-resolving power of 3000 was used, which is su cient to resolve hydrides and other interferences. The count rate of 24 Mg was kept below 2U10 5 s 31 to reduce uncertainties from deadtime corrections. The measurements were carried out in the mass sequence 24, 25, 26, and 27. The tabulated results of each analysis are the average data of 50^250 cycles through the above mass sequence. Standards of Burma spinel, San Carlos olivine, melilite glass, Miyakajima plagioclase, and Cr diopside were analyzed before and after each set of measurements, and their respective Al/Mg sensitivity factor ratios were determined from electron probe and ion probe data. Instrumental mass fractionation, which di ers among minerals of interest, was accounted for by comparing the measured 25 Mg/ 24 Mg ratios for mineral standards with the 25 Mg/ 24 Mg of terrestrial Mg ( [32]) and is given in x/amu by: v 25 Mg ˆ 25 Mg= 24 Mg meas 31 U1000 0:12663 The intrinsic isotopic mass fractionation for sample minerals (F Mg ) was found by subtracting the mean v 25 Mg for the appropriate standard mineral from the v 25 Mg measured for the sample. Instrumental fractionation for each mineral is reproducible to þ 2x, so F Mg values that exceed this uncertainty are considered signi cant. After correcting for instrumental mass fractionation using a linear law (i.e., assuming that the fractionation for 26 Mg/ 24 Mg is twice that of 25 Mg/ 24 Mg), the 26 Mg/ 24 Mg ratio obtained for several mineral standards was þ Excesses or de cits of 26 Mg remaining after correcting for mass fractionation are reported in x relative to terrestrial 26 Mg/ 24 Mg: N 26 Mg ˆ 26 Mg= 24 Mg meas 31 U1000 0:13932 Di erences in ionization e ciency between Al and Mg were accounted for by comparing the measured 27 Al / 24 Mg to the 27 Al/ 24 Mg ratio for each standard mineral. All data are reported with 2c uncertainties. 3. Petrography An Al concentration map [29] which provided detailed petrographic information about #5241 was used as a guide in our study. The outer part of this CAI is a 1 mm thick layer of nearly monomineralic MM in which large melilite crystals are oriented perpendicular to the outer surface (Fig. 1). Melilite in the mantle displays dis-

4 18 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 Fig. 1. Backscattered electron image of Allende 5241 CAI. This CAI consists of three distinct lithologies: melilite mantle (MM), spinel-rich core (SRC), and spinel-free islands (SFI). The core contains spinel, melilite, anorthite, and fassaite. Three SFI (#1, #3, and #4) are indicated. Black areas are spinels.

5 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 19 Fig. 2. A close-up, backscattered electron image of SFI #3. It has anorthite (An), melilite (Mel), and fassaite (Fa). Note that this SFI is rimmed by a layer (black) of densely packed spinel crystals. tinct reverse chemical zoning, with melilite toward the center of the crystals being more Mg-rich than that toward the rim [26,28]. The MM also contains a few spinel grains and numerous small fassaite inclusions. Interior to this mantle is the core, which is composed of two petrographically and chemically distinct units. The dominant unit is spinel-rich and consists of fassaite, melilite, anorthite, and spinel. Included within this unit are SFI (#1, #3, and #4) consisting of fassaite, melilite and anorthite. The SFI are rimmed by a layer of densely packed spinel crystals (Fig. 2) which appear black in the backscattered electron image. 4. Results The regions selected for study were: three separate SFI contained within the core (see Fig. 1); two regions of the SRC; and two areas of the MM. Measurements were carried out during three periods each separated by more than 2 months. All units were measured during the rst two periods. The results are in excellent agreement. In the third period, we analyzed spinel and pyroxene grains within the MM and in the SRC and measured another anorthite from SFI #4. Spinel grains and pyroxenes within the core and the MM were also studied to establish initial 26 Mg/ 24 Mg values. Fassaite and melilite in the three SFI have low 27 Al/ 24 Mg ( 6 2). Anorthite has high 27 Al/ 24 Mg (up to 360) and exhibits large 26 Mg excesses (up to 130x) that are well correlated with Al/Mg. The results for SFI #1 are shown in Fig. 3. It can be seen that there is an excellent correlation for the analyses. The plagioclase data were obtained on 10 locations in two individual crystals. Fassaite and melilite provide the low Al/Mg results. The slope corresponds to ( 26 Al/ 27 Al) 0 = (5.0 þ 0.1)U Results for SFI #3 are shown in Fig. 4. Data were taken on six locations in one individual anorthite crystal. Fassaite and melilite within the island were also analyzed. There is again an excellent correlation with a slope corresponding to ( 26 Al/ 27 Al) 0 = (4.7 þ 0.1)U Re-

6 20 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 Fig. 3. Al^26 Mg evolution diagram of SFI #1 with analyses of anorthite, fassaite, and melilite. Ten analyses were made in two anorthite crystals during two periods separated by more than 2 months. Anorthite shows a wide range of Al/Mg ratios and large 26 Mg excesses which are well correlated with reduced (M 2 = 1.5) Al/Mg. Fassaite and melilite have low Al/Mg ratios. Slope and initial value (relative to normal) are indicated. Fig. 4. Al^26 Mg evolution diagram of SFI #3. Six anorthite measurements were taken in a single crystal. It exhibits a moderate range of Al/Mg ratios (200^300) and clear 26 Mg excesses that are well correlated with its Al/Mg (reduced M 2 = 0.3).

7 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 21 Fig. 5. Al^26 Mg evolution diagram of SFI #4. Three anorthite crystals were analyzed during three periods each separated by more than 2 months. The high data point ( 27 Al/ 24 Mg = 290) was taken in the last period of experiments and is in precise accord with the previous data. The data give a reduced M 2 = 0.5. Fig. 6. Al^26 Mg evolution diagram for combined data from SFI #1, #3, and #4. The data generally plot on or slight below the reference line of ( 26 Al/ 27 Al) 0 =5U There is more scatter in the combined data than is present in that for the individual islands.

8 22 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 Fig. 7. Al^26 Mg evolution diagram of the SRC. Five anorthite crystals from di erent locations were analyzed. A very good correlation is again evident (reduced M 2 = 0.2). The data de ne ( 26 Al/ 27 Al) 0 = (4.3 þ 0.1)U10 35 which is well below the reference value of ( 26 Al/ 27 Al) 0 =5U Fig. 8. Al^26 Mg evolution diagram of the MM. The melilite with higher Al/Mg V20 is within a few Wm of the edge of the inclusion. Al/Mg of melilite decreases inward toward the SRC. The data are well correlated (reduced M 2 = 0.2) and have a slope far below that of the reference line.

9 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 23 Fig. 9. A histogram of 26 Mg/ 24 Mg in spinel from (a) the MM and (b) the SRC. The weighted mean of 12 measurements for spinel in the MM is 1.5 þ 1.1x (2c). It is slightly higher than that (0.4 þ 1.0x) of spinel in the SRC, but the two data sets overlap su ciently so that a shift in initial 26 Mg/ 24 Mg is not demonstrated with certainty. The terrestrial 26 Mg/ 24 Mg value was taken to be þ 8. sults for SFI #4 are shown in Fig. 5. Note that the high data point ( 27 Al/ 24 Mg = 290) was taken in the last set of experiments and lies precisely on the isochron obtained in the previous data set. The slope corresponds to ( 26 Al/ 27 Al) 0 = (4.6 þ 0.1)U The inferred initial ratios for SFI #3 and SFI #4 are indistinguishable, but may be distinct from that of SFI #1. All of the data on SFI #1, #3, and #4 are combined in Fig. 6. We have drawn a reference line of ( 26 Al/ 27 Al) 0 =5U10 35 which is the canonical value of Lee et al. [2] and the result found here for SFI #1. There is more scatter in the combined data than in the data for the individual islands. If this range in ratios re ects a time di erence between SFI #1 and SFI #3 and #4, this corresponds to 9U10 4 years. Results on the SRC are shown in Fig. 7. Several of the large plagioclase grains in the core were measured (Fig. 1), along with the spinel. A very good correlation is again evident. The data de ne ( 26 Al/ 27 Al) 0 = (4.3 þ 0.1)U The data on the core lie well below the reference line of ( 26 Al/ 27 Al) 0 =5U Data for the MM are shown in Fig. 8. The values of 27 Al/ 24 Mg in the melilite are not high but there is a su cient range to establish a clear correlation. Two large mantle melilite crystals (V1 mm) at di erent locations were analyzed. The edge of the melilite crystal close to the rim of the inclusion has a 27 Al/ 24 Mg of 20 and a clear 26 Mg excess (6.0 þ 0.7x). The 27 Al/ 24 Mg ratio of melilite and the 26 Mg excess decrease inward. The data are well correlated and have a slope of (3.3 þ 0.8)U10 35, far below that of the reference line. There is a hint of an elevated ( 26 Mg/ 24 Mg) 0 from the melilite data. In order to check this, an extensive series of measurements were made on spinels and small pyroxenes in the MM as well as a new set of data on spinels in the SRC. A histogram of these results is shown in Fig. 9. The weighted mean of 26 Mg/ 24 Mg for the 12 measurements (1.5 þ 1.1x) is indistinguishable from the intercept of the regression line in Fig. 8 (1.1 þ 0.3x). Comparison of the spinel data for the MM with those for the SRC shows an apparent slight di erence in the initial values of ( 26 Mg/ 24 Mg) 0 between the mantle and the core (Fig. 9), but the two data sets overlap su ciently so that a conclusive answer could not be obtained. 5. Discussion From the results presented above, it appears that there are distinct ( 26 Al/ 27 Al) 0 ratios associated with the SFI, the SRC that encloses them, and the overlying MM. The SFI have the highest ( 26 Al/ 27 Al) 0, close to or slightly below the canonical value. The SRC has a lower ratio. The MM has a signi cantly lower ( 26 Al/ 27 Al) 0 although there is a larger analytical uncertainty in the iso-

10 24 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 chron. The ( 26 Al/ 27 Al) 0 for the three distinct petrographic units are: for the three SFI (4.6 þ 0.1)U10 35, (4.7 þ 0.1)U10 35, and (5.0 þ 0.1)U ; for the SRC (4.3 þ 0.1)U10 35 ; and for the MM (3.3 þ 0.8)U This is in accord with the relative ages determined from petrographic considerations [26]. The values appear to re ect the decrease with time of 26 Al/ 27 Al due to radioactive decay from an initial state with a speci c 26 Al/ 27 Al ratio. A spatially heterogeneous distribution of 26 Al/ 27 Al is very unlikely to give this sequential result. The initial 26 Mg/ 24 Mg values of the SFI and SRC are not distinguishable from each other or from the terrestrial normal value. However, there is a hint of a possible increase above the standard value of the initial 26 Mg/ 24 Mg in the MM. Decay of 26 Al present at 26 Al/ 27 Al = 5U10 35 would shift the 26 Mg/ 24 Mg ratio of the bulk solar material by about 0.05x. To shift the initial 26 Mg/ 24 Mg of the source material for the mantle by N 26 MgW1x would require an Al/Mg ratio V12 times the solar value, which is close to the ratio of bulk CAIs. In terms of the origin of CAIs, clear evidence of shifts in ( 26 Mg/ 24 Mg) 0 would be important in identifying the re-melting of a high Al source for CAIs. This cannot be established from the existing data. The above observations on a single CAI require that CAI formation was not a series of events in a very narrow time interval but extended over a period of at least 4U10 5 yr. We note that this implies large di erences in the abundance of 41 Ca (d = 1.5U10 5 yr) over this time period [33]. It is quite plausible that the much larger range of 26 Al/ 27 Al found in the general body of data on CAIs is a result of formation times of CAIs over a time period of 2U10 6 yr. Arguments regarding this matter have been presented by [8,21,31]. In particular, Podosek et al. [21] have pointed out that the MMs appear to have formed later than the cores. However, in all previous discussions, the possibility of later metamorphism or partial recrystallization of the CAIs was considered to be so plausible that a strong inference on the time interval of the primary igneous CAI formation episodes was not addressed. Secondary metamorphism probably did not play an important role in the evolution of the Al^Mg systematics of Allende 5241 CAI. The three petrographic components exhibit excellent internal isochrons and do not show any evidence of disturbance. Mantle melilite within a few Wm of the edge of the CAI contains clear 26 Mg excess which is well correlated with Al/Mg. In addition, mantle melilite crystals are relatively large (V1 mm) compared to the anorthite grains (a few hundred Wm) in SFI and SRC. Di usion couple experiments show that the Mg di usion rate in melilite (A î kermanite) is indistinguishable from that in anorthite [24,25]. Although CAI melilite is more Al-rich, we expect that any thermal metamorphism would have a similar e ect on di usive Mg homogenization in both anorthite and melilite. Therefore, the di erent inferred ( 26 Al/ 27 Al) 0 of the three petrographic components of this CAI cannot be attributed to secondary metamorphism. It follows that the three zones in 5241 must have formed in three events covering a time interval of 4U10 5 yr. The results presented here and previous observations on CAIs and chondrules in a wide range of chondrite types require that the physical^chemical processes that produce CAIs were active over an extended time period. The observations require that: (1) liquid droplets of CAI composition formed and crystallized. These CAIs were then preserved at low temperatures for extended times; (2) later CAI-type liquid droplets were formed and collided with or merged with previously formed CAIs or CAI fragments and were again rapidly cooled; (3) this process was repeated over an extended time period; (4) chondrules formed later from liquid droplets. These chondrule droplets were much closer to solar composition for `non-volatile' elements; (5) CAIs, chondrules, fragments thereof, unprocessed presolar grains, metal fragments (droplets) were then aggregated together with materials that had been subjected to extensive alteration and were then accumulated into protoplanetary objects. These were again disrupted and re-aggregated into the nal meteorite parent body that was again broken up and presently sampled. The CAIs must represent a process, not an event. The focus of any discussion, therefore, must be tied to those mechanisms that could

11 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 25 form CAI liquids and cool them su ciently rapidly. It has long been clear that the mineral phases typical of CAIs could form as condensates (or sublimates) from a gas of solar composition (e.g., [34,35]). However, the requirement that compact type A and type B CAIs formed from liquid droplets has necessitated ongoing investigation of the equilibrium condensation model to explain the presence of liquids. It has been recently proposed that Ca^Al-rich liquids of approximately CAI composition can be formed by condensation from a hot region of nebular gas of solar composition, although special conditions are required [36,37]. In particular, with high dust enrichments, there appears to be a wide range of conditions for liquid drop formation for both CAI-type liquids and for those liquids approximating chondrules [37]. The models for formation of liquids are critically dependent on theoretical^ empirical relationships for the thermodynamic properties of the melts. In most solar nebula models, the major reactive volatile elements (H, C, O, S) for the most part, are only responsible for the oxidation states of the condensed species. Given the oxidation state, these major volatile elements do not play a signi cant role in determining the relative stability of the condensed oxide phases at high temperatures. Indeed, most of the results could be obtained without any consideration of the hydrogen and only use a total rock vapor for the condensation. The results depend on the C/O ratio and the gas^liquid solid relationship of the `condensable' species. There is a wide range in possible dust-to-gas ratios within the accumulating nebula. As argued by Wood [38] and Wood and Mor ll [39], the accretion disk would be greatly enhanced in dust and provide a region with high dust to gas ratios. It is conceivable that the dust would be so enriched in the disk that liquid droplets might form. However, the mechanism for causing melting and rapid cooling in local regions in the gas^dust accretion disk has always been problematic. There has long been a strong preference by most workers to directly connect CAIs and chondrule formation to nebular gases and nebular processes. The multistage formation of a CAI over a prolonged time, as shown here, makes a nebular disk model less attractive. We will here pursue an alternative approach that may be fruitful. Insofar as liquid droplets of CAI composition can form as condensates from a gas phase, then it is possible that they are nebular condensates, nebular condensates at a high dust/gas ratio, or condensates of gases formed by evaporation of solid matter with no direct connection with a gas-rich solar nebula. The temperature and pressure ranges in each of these potential regimes vary greatly, but the temperatures must always be well above V2000 K while the pressures may range widely (up to V10 4^105 dyne/cm 2 ). The preponderance of evidence from astronomical observations of accretion disks shows their temperatures to be rather low. Producing multistage CAIs over an extended time in the nebula is also a problem that has not been properly addressed. Repeated condensation by heating and cooling during the nebular phase can explain di erent times of CAI formation, but not the rapid cooling required or the repeated juxtaposition of freshly formed CAI droplets with much earlier formed and crystallized CAIs without melting of the previously formed CAIs (see also [40]). It is known that the rate of sweeping out of small objects ( 6 1 m) into the protosun due to gas drag during the nebular accretion phase is rapid (time scale of V10 5 yr). To preserve CAIs formed during this period, it appears to be required that small fragments be assembled into protoplanetary bodies of around 1 km in size (cf. [41]). This certainly applies to microscopic particles like circumstellar dust grains that are preserved in meteorites. It follows, independent of the formation mechanism of CAIs, that planetary accretion concomitant with CAI formation is an intrinsic part of any model relating to the stages of nebular accretion. In order to preserve some of the CAIs in asteroidal bodies, they must be rapidly accreted with other matter into much larger bodies. The fact that CAIs with a spread in ( 26 Al/ 27 Al) 0 are found in carbonaceous chondrites, ordinary chondrites and enstatite chondrites shows that these objects were either produced over an extended time from a single source and then widely distributed, or that CAI production involved similar processes that occurred in distinctly di erent regions. In either

12 26 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 case, the CAIs would have to be produced over time, aggregated and stored in protoplanets in an ongoing fashion. The case of a single source for CAIs which were then widely distributed throughout the solar system allows there to be contemporaneous regions, some with no 26 Al and some with 26 Al, resulting in a highly heterogeneous spatial distribution of this key nuclide within the solar system. This approach, while possible, has not provided any deeper insight into processes occurring in the early solar nebula. We now focus on a possible scenario of planetary processes that could play a role in CAI formation (cf. [24,42,43]). We consider an early protoplanet to be an agglomeration of presolar dust grains and possible nebular condensates. The exterior of such bodies is maintained at the rather low temperatures typical of the nebular disk [44,45]. It has been long recognized that planetary bodies of chondritic composition containing 26 Al/ 27 Al=5U10 35 of su cient size to provide insulation would be subject to extreme heating in their interiors (cf. [2,24,42,46]). If thermally insulated and mechanically con ned, they could, in principle, reach temperatures of up to V10 4 Kin their insulated interiors. Material in the heated interior of such a body would be evaporated, brought to high pressures and would then jet out, providing a source of gases that could form CAI droplets as rst condensates. Some of this matter would re-accrete and be mixed in with the cool debris in the outer surface layers of the small (a few to 100 km radius) protoplanet. This process could continue to occur for times of over 10 6 years and would repeatedly add hot new material to the exterior that would cool quickly. The cooling rates of the droplets would be controlled by the local environment, which would be at a rather high matter density, and by overall radiation into the nebula. Heating of intermediate zones of the protoplanet could provide gases rich in volatiles and at rather high densities, causing subsequent alteration of the earlier formed materials contained in the outer zones and in the debris pile of the outer layers. Such a sequence would provide a mechanism for generating multistage CAIs as discussed above, late stage volatile alteration, and the incorporation of ne matrix material engulfed in some CAIs. Bulk melting of the protoplanetary interiors would take place in di erent bodies at di erent times and melts would be preserved for several million years. Indeed, such 26 Al heating would supply a wide range of metamorphic histories within a single planet as has been argued elsewhere. Collisions of small planetary bodies with molten interiors could produce droplets to form chondrules. The presence of FeNi metal in chondrites and chondrules requires highly reducing circumstances. While FeNi metal can be readily produced by condensation in a hot nebular H-rich gas, it cannot occur with melted bulk material of the composition of CI chondrites because of the high O content. A planetary model of this sort to form chondrules with metallic FeNi would therefore require a starting material with oxidized Fe but which was very carbon-rich (C/O x 1/2). This material, upon heating, could produce large (e.g., planetary cores) and small metal segregates of `original' FeNi metal from the oxides. While this scenario has some attractive characteristics, there are many issues and problems that require attention. The thermal and mechanical evolution of protoplanets that have an extremely high internal heat source has not been addressed. The conditions being considered here are close to a `meltdown' conditions corresponding to a reactor or the burial of large amounts of extremely radioactive material. The pressures generated by con ned gas phases at high temperatures (V2000 K) could be very disruptive, exceeding the tensile strength of the material and the gravitational pressure. It is quite possible that these pressures would disrupt the body. The rst gas phase formed would be volatile-rich and produce some condensates. It is not clear where this early gas phase product is represented in the meteorite record (the nes?). Whether CAI material would be subsequently left as a residue or form as a condensate is not clear. The hibonite in the Blue Angel with ( 26 Al/ 27 Al) 0 V5U10 35 [47] appears to be a vapor sublimate, not the product of crystallization from a melt, while most other type B CAIs are clearly the result of crystallization of liquid droplets. These matters cannot be answered here and remain to be addressed. Nonetheless, we

13 W. Hsu et al. / Earth and Planetary Science Letters 182 (2000) 15^29 27 consider that a scenario involving protoplanetary bodies embedded in a nebular accretion disk model may permit insight into what has been an intractable problem. It would permit a wide range of metamorphic temperatures and chemical alteration of the original materials, with widely varying f O2. We note that for any scenario there does not appear to be a plausible mechanism for generating a melilite melt that is believed to be plastered on the outside of CAIs like #5241 [26]. A fundamental problem with all models remains with regard to the 16 O excesses coupled with normal 17 O/ 18 O that are typical of CAIs [48,49]. The matter hinges upon whether the 16 O excesses are due to the preservation of presolarnucleosynthetic materials or to non-mass-dependent fractionation e ects produced within the solar system as discovered by Thiemens and Heidenreich [50] and Thiemens [51] in laboratory experiments and as found in the Earth's upper atmosphere [52,53]. While there is still no clear explanation of the cause of these e ects, they are real in both the laboratory and nature. In a recent paper Hathorn and Marcus [54] have proposed a mechanism for the e ects found in ozone. However, there is no model relating to formation of condensed oxide phases. In the standard model that attributes the 16 O excess in CAIs to mixtures of di erent nucleosynthetic batches and exchange between them, we nd no supporting evidence in commensurate isotopic e ects in other elements for the same samples or in 16 O anomalies in presolar circumstellar oxide condensates. The model proposed by Clayton et al. [12] to explain the O isotope variation in CAIs, exchange between the CAI (initial composition N 17 OW 340x, N 18 OW340x) and the surrounding nebular gas, has been called into question by ion probe studies of the distribution of O isotopes in CAIs (e.g., [13]). The planetary scenario for CAIs would require that the 16 O enrichment is a consequence of non mass dependent fractionation e ects in condensates from a gas phase rich in O 2. Such an argument for self-shielding was rst proposed by Arrhenius et al. [55]. This has been examined by Navon and Wasserburg [56], who concluded that, while self-shielding at appropriate oxygen densities could indeed, in principle, provide a mechanism to produce the mass-independent isotope shifts in O, under nebular conditions the O 2 content is far too low. The `oxygen problem' remains an issue for any mechanism of CAI formation so far being considered and must explain both the apparently di erent oxygen reservoirs and the variability of the 16 O enrichment in di erent phases in a single inclusion. We have no explanation for these important observations. Acknowledgements This paper is dedicated to Brian Mason of the U.S. National Museum who graciously provided us this material and who has, as always, been a guiding, friendly beacon in many things meteoritic. We have pro ted greatly from insightful and positive reviews. The anonymous reviewer has improved the introduction, A. El Goresy has directed us to major petrogenetic issues and Ernst Zinner has given a meticulous and penetrating critique that was of great use in improving this report. This work was supported by NASA NAG (G.J.W.), NAG (G.R.H.), and by the MacArthur Endowment Fund. Division Contribution Number: 8724(1055).[AH] References [1] T. Lee, D.A. Papanastassiou, G.J. Wasserburg, Demonstration of 26 Mg excess in Allende and evidence for 26 Al, Geophys. Res. Lett. 3 (1976) 109^112. [2] T. Lee, D.A. Papanastassiou, G.J. Wasserburg, 26 Al in the early solar system: Fossil or fuel?, Astrophys. J. Lett. 211 (1977) L107^L110. [3] G.J. MacPherson, A.M. Davis, E.K. Zinner, The distribution of aluminum-26 in the early solar system ^ a reappraisal, Meteoritics 30 (1995) 365^386. [4] L. Grossman, Refractory inclusions in the Allende meteorite, Annu. Rev. Earth Planet. Sci. 8 (1980) 559^608. [5] G.J. MacPherson, D.A. Wark, J.T. Armstrong, Primitive material surviving in chondrites, in: J.F. Kerridge, M.S. Mathews (Ed.) Meteorites and the Early Solar System, University of Arizona Press, Tucson, AZ, 1988, pp. 746^807. [6] G.J. MacPherson, L. Grossman, `Flu y' type A Ca-, Alrich inclusions in the Allende meteorite, Geochim. Cosmochim. Acta 48 (1984) 29^46. [7] J.M. Allen, L. Grossman, A.M. Davis, I.D. Hutcheon,

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