Cosmogenic neon from individual grains of CM meteorites: Extremely long pre-compaction exposure histories or an enhanced early particle flux

Acfa Vol. 54, pp. 2 133-2 140 Geochrmica et Cosmmhimica Copyright 0 1990 Pergamon Press Printed in U.S.A.

0016-7037/90/$3.00 + .OO

plc.

Cosmogenic neon from individual grains of CM meteorites: Extremely long pre-compaction exposure histories or an enhanced early particle flux CHARLESM. HOHENBERG,’ ROBERT H. NICHOLSJR., ’ CHAD T. OLINGER,’ and J. N. GOSWAMI’ ‘McDonnell Center for Space Sciences and Physics Department, Washington University, St. Louis, MO 63130, USA ‘Physical Research Laboratory, Navarangpura, Ahmedebad 380 009, India (Received October 5, 1989; acceptedin revisedformApril 23, 1990)

Abstract-Meteoritic grains which contain solar flare VH ion tracks have clearly been individually exposed to energetic particles prior to assembly. In order to observe the effects of irradiation during the precompaction era, spallation-produced neon has been measured in individual grains, selected by the presence of solar flare VH tracks, from the CM regolith breccias Murchison, Murray, and Cold Bokkeveld. The presence of pre-compaction spallation neon correlates well with the presence of solar flare VH tracks (Z > 20) and, in this study, detection of SF tracks is the critical parameter used to identify those grains where pre-compaction spallation effects are likely to be present. Only a few percent of the grains (at most) that do not contain solar flare VH tracks contain amounts of cosmogenic Ne larger than would be produced during the conventional cosmic-ray exposure age (and for them the excess is only marginal), whereas most of the grains with solar flare VH tracks contain spallation-produced Ne in significant excess of that due to the nominal cosmic-ray exposure. The magnitude of this excess, which clearly must have been produced prior to compaction, provides evidence for extensive energetic particle exposure during the pre-compaction era. If a contemporary energetic particle complex is assumed (galactic and solar cosmic rays: GCR and SCR), and if production is taken at the maximum present rates, minimum GCR pre-compaction exposure times can be found. The most heavily irradiated grains from Murray and Murchison would require a minimum GCR regolith exposure time of 145 Ma to accumulate the observed cosmogenic Ne. This is the lower limit because it is computed using the peak production rates from the GCR cascade, which occur at roughly 60 g/cm*, and it requires that the grain spent its entire regolith residence time at that optimum depth. Studies of compaction constraints for CI and CM meteorites suggest that such long regolith residence times may be unlikely. The alternative to such long periods of parent body regolith activity is increased production rates in the early solar system from an enhanced energetic particle environment. INTRODUCTION

by more than an order of magnitude. Since the grains reside side-by-side in the meteorite, the excess cosmogenic Ne must have accumulated during the pre-compaction era. The precompaction spallation effects observed were quite large, but equally important was the variation observed among different aliquants of irradiated grains which suggested that there may be large variations among the individual grains in each set. If such were the case, even larger pre-compaction spallation effects might be found if the grains could be studied individually. The size of pre-compaction spallation effects observed in this early study, and the probability that even larger effects might be present in individual grains, led CAFFEEet al. ( 1987) to conclude that an enhanced particle flux in the early solar system might well be required. The alternative explanation, extremely long pre-compaction regolith exposure times, seemed to violate most of the current models for meteorite parent body evolution (cf. HOUSENand WILKENING, 1982), and a lively debate has emerged as to which explanation is the more viable (cf. WIELER et al., 1989). The most definitive information on the ancient energetic particle environment comes from mineral grains extracted from carbonaceous chondrites (rather than the earlier studies which included Kapoeta, Fayetteville, and Pesyanoe) for two reasons. Carbonaceous chondrites generally have short conventional cosmic-ray exposure ages, typically a few million years or less, so those grains with pre-compaction spallation

STUDIESOF MINERALGRAINS from gas-rich meteorites provide individual records of exposure to energetic particles, some of which occurred prior to assembly of the meteorite. Such effects range from spallation reactions in the uppermost few meters of the parent body regolith to solar flare VH tracks in the top few hundred microns (WALKER, 1980). The presence of solar flare particle tracks in such grains indicates that the grain was exposed at the very surface of the parent body regolith. Therefore, selection of grains on the basis of etchable solar flare tracks assures at least some near-surface exposure during the pre-compaction era. This is an important selection criterion since typically only a few percent of meteoritic grains show such effects (GOSWAMI et al., 1984). CAFFEE et al. ( 1983) first demonstrated the direct correlation between the presence of tracks and pre-compaction spallation Ne. The extremely large concentrations of cosmogenic Ne found in the track-rich grains indicated extensive energetic particle exposure prior to final compaction of the meteorite mass. Due to sensitivity limitations, CAFFEE et al. ( 1983) measured sets of individually selected grains (typically 10 grains). In no case did the unirradiated sets contain amounts of cosmogenic Ne in excess of that accumulated during the conventional cosmic-ray exposure age of the meteorite, while the SF-irradiated grains contained quantities exceeding this 2133

C. M. Hohenberg et al.

2134

effects are extremely obvious, providing unambigious

data on the correlation between SF tracks and pre-compaction spallation. More important, however, are the independent constraints that can be placed on the evolution and the duration of the pre-compaction era of carbonaceous chondrites. Large pre-compaction spallation effects have clearly been observed in grains from Kapoeta and Pesyanoe. Some of the Kapoeta grains would require GCR exposures of over 600 Ma in the parent body regolith to accumulate the observed dose of cosmogenic Ne (OLINGERet al., 1987, 1988). However, these differentiated objects do show evidence for extended surface processing and even contain clasts with ArAr ages as young as 3.9 Ga (RAJAN et al., 1979), so such long exposures cannot be effectively ruled out. This work deals exclusively with the CM chondrites where evidence for extended exposure times will, at least, require us to revise our understanding of regolith dynamics and the time scales involved in the evolution of the CM parent bodies. When more rigid constraints can be placed upon the compaction times (which are quite short by all current models), these objects will most effectively define the energetic particle environment during the pre-compaction era. This study reports on Ne extracted from individual meteoritic grains by laser volatilization. Use of laser extraction improves both the Ne sensitivity (by reducing the volume) and the Ne blank (by greatly reducing the heated surfaces), making possible the analyses of single meteoritic grains down to the sub-microgram size. Chemical etching is used to select grains exposed to solar flare VH particles; track-free grains provide the complimentary data also important in this investigation.

olivines which are homogeneous in the appropriate target elements: Na, Mg, Al, Si, Ca, and Fe. Several large fragments, to be used as reference standards, were loaded into Pt boats for conventional heating in a W coil (HOHENBERG,1980). Smaller fragments (20-100 pg) were individually weighed and placed in extraction cells for laser volatilization. Figure 1 shows the results of this calibration, indicating that both lasers achieve near complete extraction. It is unclear whether the few percent deviation is a real effect or a systematic error introduced by the microbalance. In either case the uncertainty in concentration for both calibrations is shown by the error bars in Fig. 1. The calibration samples were considerably larger than most of the meteorite grains, and uncertainty in concentration for the actual sample runs is largely dominated by the technical difficulty in weighing the small masses, typically only a few micrograms. The uncertainty in absolute accuracy is less than 10% for sample masses in excess of 3 fig and less than 20% for masses between 1 and 3 Fg. Within a suite of grains the precision (reproducibility and their relative mass) is considerably better than this. Samples of Murchison, Murray, and Cold Bokkeveld were first disaggregated by gentle crushing and by high-powered ultrasonic agitation. Individual olivine grains were then visually identified, manually extracted, mounted in epoxy, and polished. WN etchant, described by KRISHNASWAMIet al. ( 1971) was then used to reveal grains with solar flare tracks (typically a few percent). Track densities were measured using optical microscopy, and each irradiated grain was photodocumented. EDX spectra were measured for each grain studied (using a JEOL 540 SEM and Tracer Northern 5400 X-rav system)‘in oGer to confirm the mineralogy and obtain the tatget chemistry. Each irradiated grain was then removed from the epoxy, weighed using a Cahn C-3 I microbalance, and placed in an individual extraction cell for laser volatilization. Once the irradiated grains were removed from their mounts, the unirradiated grains were identified. In order to avoid erroneous results, each mount was re-etched to reveal any tracks not developed earlier. After this double etching, grains that were unambiguously observed to be track-free were selected and were then treated exactly as the irradiated grains. Blanks were run before each set of grains and typically after every third or fourth run. Procedural blanks at mass 21 remained quite low (typically 5 X 10 -I6 cc STP) throughout all of the runs and were

EXPERIMENTAL PROCEDURE The laser extraction system utilized, at different times, two different continuous wave (CW) lasers of roughly similar performance: an Ar-ion laser and a Nd:YAG laser, with 20 and 70 watts of deliverable power, respectively. The laser output passes through a beam expander and into the microscope body, reflected 90” by a dichroic mirror onto the optic axis of the microscope where it passes through the objective and is focused through the Pyrex vacuum viewport onto the sample. Aluminum or stainless steel SEM stubs, drilled with a matrix of 30 or so cells about 1 mm diameter by 1 mm deep serve as sample holders. Visible light from the sample passes through the dicbroic mirror undeflected and into a video camera for real-time observation on a TV monitor. Control of the laser power delivered to the sample is provided by two water-cooled Glan polarizers whose relative polarization axes are continuously variable. Each grain is heated slowly in its extraction cell through melting to total volatilization. After volatilization, the extracted gases are purified with a Ti flash getter, the heavier noble gases are removed with a charcoal finger held at liquid N temperature, and the Ne is analyzed in a pulsecounting high-sensitivity mass spectrometer ( HOHENBERG,1980). During mass spectrometry, two additional cold fingers suppress Ar and CO?, which cause interference at masses 20 and 22, respectively. Pulsed lasers, used in previous studies, have been shown to yield incomplete and variable gas extraction ( OLINGER et al., 1988). This is thought to be due to the high-energy pulse passing through the relatively transparent grain and into the optically dense substrate, where it explosively forms a crater. The expanding shock apparently fractures the grain into sub-micron fragments, but it does not extract all of the volatiles. This problem led to the utilization of the CW laser, which does deliver near-complete extraction, as shown below. Performance of the CW system was verified using olivine fragments from the Springwater pallasite, a meteorite with a long cosmic-ray exposure age (large amounts of spallation-produced “Ne) and large

90

.

Nd:YAG

0

Argon-Ion

92

Laser Laser

94

96

911

100

102

% Extraction FIG. 1. Laser extraction efficiency calibrations. Nd:YAG (filled circles) and argon-ion (open circles) laser extractions are displayed as percentages of gas extracted from individual grains relative to conventionally heated samples. Slight deviation from apparent 100% extractions may reflect balance non-linearity over the range of sample masses which span three orders of magnitude, may represent slight gas release from the smaller grains during the fragmentation process, or could be a real effect. Whatever the explanation, it does not affect any conclusions here.

Cosmogenic Ne in carbonaceous chondrites

never a factor in any of these analyses {typically a few percent of the signal for the unirradiated Murchison grains and less than 0.1% for most of the irradiated grains). Although BGGARD et al. ( 197 1) demonstrated the presence of solar gases in Murchison, the grains we analysed generally contain little solar Ne, allowing easy resolution of the cosmogenic *lNe. Some of the solar wind Ne may have been removed during the etching, even from surfaces that extend into the epoxy, but the bulk is probably carried by fine-grained matrix material, For most of the olivine grains in this study the amount of cosmogenic “Ne is essentially the entire signal at mass 21 since the cont~bution from the system blank is negligible and the cosmogenic component completely dominates that isotope. Even though corrections for solar Ne were made, based on the measured ‘%e and *‘Ne, they were generally negligible. When solar Ne was present in significant amounts, the cont~butions from doubly charged 4oAr and CO2 at masses 20 and 22 were not significant, so the quantity of cosmogenic *‘Ne could be determined with precision in all cases. Using the target chemistry obtained for each grain by EDX analysis and the ~~j~u~ production rates for an optimized depth calculated by REEDY et al. ( 1979) and ROSENBERG et al. ( 1978), minimum pre-compaction exposure times for contemporary GCR irradiation can be found for each track-rich grain. These are listed individually in Table 1 and displayed collectively as histograms for Murchison, Murray, and Cold Bokkeveld in Fig. 2. In both the figure and the table the apparent “age” refers to the time that would be required to accumulate the excess (over that produced during the conventional cosmic-ray exposure age) spallation Ne observed in each grain if it were exposed in a parent body regolith (2n geometry) to contemporary galactic cosmic rays for the entire time at the optimum depth (for maximum pr~uction rate). These values are therefore lower limits for the true regolith residence time of each grain, and the maximum value observed for each meteorite then becomes the lower limit for the duration of regolith activity on that meteorite parent body. Similar minimum pre-compaction exposure times would be obtained iFm~imum contem~r~ SCR production rates (at 3 AU) were assumed (CAFFEE et al., 1983), although it is difficult to imagine a regolith setting under current conditions in which SCR spallation effects would dominate GCR effects. Complimentary data for unirradiated (track-free) grains are shown in Fig. 3. These exposure ages are calculated assuming meteoritic production rates for 4~ geometry (REEDY et al., 1979). Peaks in the histograms for Murchison, Murray, and Cold Bokkeveld at 1.93 rt: 0.31 Ma, 6.08 k 0.65 Ma, and 0.35 + 0.09 Ma, respectively, correspond well with the documented conventional cosmic-ray exposure ages of these meteorites (c&SWAMI et al., 1984)) especially considering that the stated errors are from the measured cosmogenic 2’Ne alone and do not consider uncertainty in meteorite size and sample depth. It may be that a small fraction of the track-free grains could require longer exposures than can be accounted for by their nominal cosmicray exposure age, indi~t~n8 that some pre-compaction spallation may not be accompanied by solar flare tracks, but the effect does not appear to be statistically significant. Figure 4, with both the trackfree and the low end of the track-rich Murchison grains (open boxes), shows that the quality of the correspondence between solar flare tracks and pre-compaction spallation effects is quite good. Only one out of 44 grains from Murchison without solar flare tracks is more than 20 from the nominal exposure age (Figs. 3a and 41, and for Murray it is 2 out of 23 (Fig. 3b). Moreover, the fraction of grains showing pre-compaction exposure may vary within a given meteorite. Go. SWAMI et al. ( 1984) observed solar flare effects in only two percent of the olivines extracted from Cold Bokkeveld. Figure 3c shows that about 20% of the unetched Cold Bokkeveld grains studied here have pre-compaction spallation Ne. Presumably the fraction of preexposed grains varies within the host meteorite.

Table 1. Irmdiated Gnin Dala: Murcbison, Mumy, Cold Bokkeveld

mch3i5 mchSc8

mchla9 mch2g6 mchlfl mch3eS mchl g3

able on a grain-by-grain

basis which are not readily apparent

in the previous work on sets of grains ( CAFFEE et al., 1983)

nor selected samples

of bulk meteoritic

al., 1989; RAJAN and LUGMAIR,

material

1988).

( WIELER et

This information

13.1 34.1

20.9 28.4 14.9 2.7 53.0 19.4 28.7 4.S 27.6 5.4 5.3 14.2 4.0 12.0 28.5

2.07 2.57 2.91 2.09 3.26 2.49 1.98 2.43 3.18 2.84 2.82 2.53 4.49 4.11 4.31 5.91 6.02 7.10 9.95 10.74 8.43 12.61 14.50 17.05 21.31 27.98 32.63 35.68 29.45 28.80

(Z) (3.7) (8.0) (3.4) 13.0 4.5 (1.7) 10.0 7.5 (28.0) 20.0 5.2 19.0 28.0 50.0 (1.0) 1.8 21.0 rlOO.0 50.0 (3.7) (4.9)

10.4 6.0 14.9 5.8 8.6 16.1 10.2 9.8 7.4 7.6

2.90 2.84 7.03 13.21 3.45 13.46 3.74 16.47 29.50 42.12

12.24 12.32 22.42 43.89 44.32 45.24 46.43 53.82 fll.11 138.33

1.9 1.6 10.0 a.5 $ 26.0 $ # 7.7 10.0

3.9 1.4 1.9 9. I 3.0 6.6 12.9 2.8 0.9 6.8 9.1 3.8 3.2 2.1 8.1

I.54 2.42 2.49 3.53 3.79 3.11 4.06 4.13 4.36 1.12 4.53 6.83 7.89 a.59 9.91

7.68 7.82 IO.40 11.57 12.41 12.99 13.29 13.59 14.40 14.80 15.59 22.18 25.95 27.67 45.50

4.7 5.6 38.0 10.0 15.0 1.8 3.2 4.0 %! t: 3.0 2.8 24.0 4.7 $$

18.1 6.3 4.7 24.7 89.1 6.1

mchlhz mch3t7 mch2b2 mch3f2

15.3 2.1 4.6 10.1

mch-5387-S lc3 mch-805-S 2cS

66.7

mehIt

11.8 58.7

mch-805-4 2~4 mchlfj mch 1k7 mch-805-4 5~2 mchSk2 mchSd1 m&531 mch2c8 mch-805-Z 8~2

1.6 4.7 21.2 3.4 24.2 13.0 65.9 3.3 2.0

mch-I7CL3OOA4c7 20.6 m&Se3 24.3 mch-17~~18) 1~6 mch-l-170-3OOBLas&7 mchl j7 mch-805-6 6cS mch-805-l 4~8 mch-17Q3OO(t8) 6c9 mch-805-I 5~7 inch-SOS-4 5~5 mch3dS mch-805-4 6~3 mch-17W3C0(18) Sc7 mch-SOS-12~2 mch-SOS-2 Se4 m&S34 mchli5

0.75 0.82 0.84 0.85 0.87 0.95 0.91 0.87 0.73 1.89

3.71 3.95 3.97 4.u2 4.08 4.14 4.17 4.21 5.45 7.83 8.08 a.55 9.3S 9.87 10.50 10.98 10.99 11.41 11.57 12.59 13.12 13.62 15.20 15&i 18.19 18.62 24.07 24.77 30.15 33.06 33.77 37.38 41.62 48.90 76.98 87.52 96.15 107.87 11a.74 128.04 143.54

mch2cl

1.58

/::S;

(0.5) (0.3) (0.7) (94.0) (2.9) (19.0) (27.0) (13.0) 4.6 24.0 (70.0) 28.0 (0.5) (4.9)

MURRAY tmy-bg-7 mry-bg-4 mrv-be-3 m&b&7 mty-bg-7

6~7 last-c6 706 801 lc7 miy-bg-72d mty-bg-7 8& mzy-bg-6 Se-last mry-bg-7 2~2 mry-bg-2 lc-last COLD BOKKEVELD cb-799-4 3c-last

cb-799-2 ‘lolast cb-799-2a 4c2 cb-799-4 4cs cb-799-la 60 cb-799-4 5~6 cb-799-4 Sc3 cb-799-2 6~2 cb-799 3cS cb-13~6 cb-799-4 2c-last cb-799-4 3c3 cb-799-la Sc6 cb-799 2~2

It is clear from this study that details of exposure are avail-

2x Age Track Densityt (lo%m-2)

Sample

cb-799-2 Se9

DISCUSSION

2135

* Praluction rates are from Reedycf al. (1979). t Minimum track density; track cowling done by St. Louis group shown in parentheses. $ NO track development; grains aren‘t olivine. It Poor track developea $$ Unetched grains.

C. M. Hohenberg

et

ai. 20 -

-1

MURCHISON 16

(irradiated)

~CHISON

-

i?!

(unirredialed)

g12 !

” B-

MURRAY

MURRAY

(irradiated)

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(unlmdirted)

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(irradiated)

il.

s

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40 ZnEx~os&

COLD BOKKEVELD

I

(unetched)

,

7s Age

110 (Ma)

145

FIG.2. Model 2n exposure ages for a) irradiated Murchison olivines, b) irradiated Murray ohvines, and c) irradiated (closed boxes) and unetched (open boxes) Cold Bokkeveld olivines. M~imum “Ne production rates are assumed for the entire exposure, so these are

minimum GCR exposure ages. Two grains in the 45 Ma bin for Murray and one grain in the 15 Ma bin for Cold Bokkeveld were not otivines, preventing the etchant from revealing any tracks they may have contained. These three grains were included with the trackrich set on the basis of obvious pre-compaction spallation Ne.

must be included in the interpretation and modeling of parent body regolith dynamics and in the energetic particle complex to which these grains were exposed. Earlier work reported by CAFFEE et al. ( 1986) on sets of grains, individuaily selected as being track-rich or track-free, established that pre~om~~ion energetic particle inaction was quite extensive. Equally important was the large variation (factor of 3.7) in the required pre-compaction particle fluence between two different sets of track-rich Murchison olivines which su=ested that some individual track-rich grains would probably require substantially larger particle doses than the ensemble averages, This provided the motivation for the individual grain laser volatilization reported here. To put things into perspective, we have found more heavily irradiated grains and they do require substantially higher particle fluences than required by the ensemble of grains previously reported. The histograms of the irradiated grains from Murray and Murchison clearly show that the most heavily irradiated grains were exposed to large doses of energetic particles. Conventional models for pre-compaction irradiation would have such grains receive their pre-compaction dose from galactic cosmic rays while residing in the regolith of the parent bodies. However, conventional models for meteorite evolution have predicted rather short times for regolith activity, typically a few million years or less (cf. HOUSEN and WILKENING, 1982), clearly incompatible with the measurements reported here. Either these regolith models significantly understate the duration of regolith activity on the parent body, or a more active energetic particle environment characterized the early solar

?..I 10

D123456789 4 x Exposwe Age (Ma)

FIG. 3. 4r exposure ages for a) unrelated Mu~h~son olivines, b) uni~adiat~ Murray ohvines, and c) unetched Cold Bokkeveld olivines. The peaks are interpreted as the conventional cosmic-my exposure ages of 1.93 t 0.3 1 Ma, 6.08 i. 0.65 Ma, and 0.34 2 0.09 Ma for Murchison, Murray, and Cold Bokkeveld, respectively (see text). Deviations of grams to the right of the peaks possibly indicates some pre-compaction irradiation for these grams. The background track densities for the unirradiated grains were on the order of IO4 cmm2.

system. The following discussion presents the implications of a conventional (GCR) regolith exposure during the precompaction era and compares them with independent constraints on the compaction time of carbona~ous meteorites. The question of whether these are in potential conflict brings to focus the other alternative, an enhanced early particle flux. 12

I

I

‘O

’

n

I

l-4

10 -

’

MURCHISON

FIG. 4.4~ exposure ages of unirradiated Murchison olivines (closed boxes) and irradiated Murchison ohvines with 27r ages <: 10 Ma (open boxes). Note that there is little overlap between the irradiated and the unirradiated sets.

Cosmogenic Ne in carbonaceous chondrites Grains containing the most pre-compaction spallationproduced Ne set the most rigid constraints for the duration of regolith activity for the conventional (GCR) model. Using the individual target chemistry and the de~th~ependent production rates for each target nuclide from REEDY et al. ( 1979) and HOHENBERG et al. ( 1978), model (one-stage) exposure ages can be obtained for each grain. We can establish the lower limit for the duration of regolith activity on the parent body, for a GCR irradiation scenario, by assigning the grains most rich in eosmogenic Ne to the depth corresponding to maximum GCR production rate ( -60 g/cm’) and requiring that they spend their entire regolith history at this depth. These constraints lead to ~~n~~u~ regolith times of 145 Ma for both Murchison and Murray, and 45 Ma for Cold Bokkeveld (Fig. 2). However, comparisons between the measured solar flare track densities and the cosmogenic “Ne (Table 1 and Fig. 5) show that exposure cannot be a simple single-stage process. First, solar flare tracks must have been acquired at the very surface, not at the depth of optimum GCR production, and second, there is no correlation between track density and pi-compaction spaliation Ne concentration, It is therefore impossible for any one of these grains to have remained at a given (optimum) depth for the entire time, so the actual average spallation-production rate must have been lower than the m~imum value assumed. Therefore, the actual regolith exposure times must be longer than the calculated values since some vertical mixing clearly took place. Even though there is no correlation between the density of solar flare tracks and the quantity of pre-compaction spallation Ne, a good correlation exists between the presence of the two effects. Since the range of solar flare VH nucleii is a few hundred microns at most, and the range of GCR spallation reactions is a few meters, it may seem surprising that the two effects are so closely coupled: track-free grains seldom have pre-compaction spallation neon; track-rich grains nearly always do. It is easy to see why the latter is true (a grain that is within 100 p of the surface is clearly within the spaliation

w’

‘i

I

FIG. 5. Model exposure age vs. minimum VH track density. To eliminate possible inter-laboratory differences, only grains in which track counting was done at PRL are displayed.

2137

zone), but it is more difficult to understand the former unless most track-free grains come from beneath the nuclear-active zone, which is a few meters for the GCR case. What seems to be required is a two~om~nent regolith, one mature and one quite immature, as suggested by WIELER et al. ( 1989). The close coupling between SF tracks and cosmogenic Ne suggests, within the context of a conventional GCR exposure model, that there is very little mixing between the two components since only a few percent of the track-free grains (at most) have been in the uppermost few meters during regolith activity, which must have lasted for at least 150 Ma in the GCR model. What may be equally significant, however, is where the coupling fails: some (-20%) of the grains shown in Fig. 4 that are track-rich do not contain measurable quantities of pre-compaction spallation Ne (the quantity of cosmogemc’ *‘Ne is within 2a of that expected from sp~iation during the conventional cosmic-ray exposure), suggesting that they received tracks at the surface of the regolith and were subsequently buried by at least several meters of regolith within a few hundred thousand years. This suggests substantial mixing between the mature and immature components of such a two-component regolith. Whether these two observations can be fit by a self-consistent model for parent body regolith dynamics remains to be seen. These data, especially those for Mur~hison and Murray, which are based on 50 track-rich grains, represent a sufficient numerical sample that the distribution of apparent exposure ages may be statistically meaningful and lead to a more realistic estimate for the duration of regolith activity on the parent body. Comparing the full-width, half-maximum, of this distribution with a similar distribution of apparent exposure ages obtained by the Heidelberg group on lunar soils 10084 and 14003 ( LANGEVINand MAUR~TTE, 1976; WRSTEN et al., 1972), regoliths of known age, suggests that a more realistic value for the regolith exposure of the Murchison/Murray parent body might be about 300 Ma, twice the observed minimum GCR model age (NICHOLSet al., 1989). Regardless of the source of energetic particles, or the duration of the pre-compaction era, the distribution of apparent pre-compaction spailation eff&s among grains from the three CM meteorites can also provide info~ation about their respective heritage. The distribution of apparent exposure ages of individual grains from Murchison appears to be quite similar to that of Murray and different from that of Cold Bokkeveld. Model ages of irradiated grains from Cold Bokkeveld are compatible with parent body regolith activity lasting only tens of millions of years, an order of magnitude less than that for Murchison and Murray. While statistics are clearly not as good for Murray and Cold Bokkeveld, if real, these observations suggest common parent body processing for Murchison and Murray, distinct from that of Cold Bokkeveld. This reflects different parent bodies. spatial/temporal separations between the Cold Bokkeveld and the Mur~hi~n~ Murray source areas, or simply poor statistics in the case of irradiated Cold Bokkeveld grains. Are pre-compaction exposure ages of hundreds of millions of years viable with what we know, or what we think we know, about the formation and evolution of carbonaceous chondrites? Before we can answer this question we must establish what is really known about their history and evaluate

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any independent constraints on their formation time that may be imposed. The minimum pm-compaction exposure times of I50 Ma are themselves already long compared to times inferred from earlier studies of the irradiation features of gas-rich meteorites and from Monte-Carlo modelling of the evolution of asteroidal-sized parent bodies. All of these estimates lie in the I to 10 Ma range (e.g., POUPEAUet al., 1974; ANDERS, 1975; PRICE et al., 1975; GOSWAMI et al., 1976; HOUSEN et al., 1979; LANGEVINand MAURETTE, 1980; HOUSEN and WILKENING, 1982). Such short time scales are the direct result of Monte Carlo calculations that consider the low gravity, the strengths, and the probable flux of meteoroids impacting asteroidal-sized objects in the early solar system. These models all tend toward a fast accretionary rather than a mixing regolith, which limits GCR exposure in the near-surface regions. To the extent that these models integrate the chemical and petrographic structures of meteoritic breccias, they all can provide useful constraints, particularly with regard to origin, post-formational alteration, and parent body size. The duration of regolith activity that is predicted in some of these models is generally corroborated by the observed regolith matu~ty features (e.g., solar flare tracks, a~utinates, grainsize distributions, cosmogenic gases, etc.) that apply only to either the outermost part of the regolith (solar flare tracks, agglutinates, etc.) or the average effect in bulk material. For instance, regolith exposures times of some tens of millions of years were obtained for bulk samples Kapoeta and Fayetteville by RAJAN and LUGMAIR ( 1988) and by WIELER et al. ( 1989). However, it is difficult to deduce the age of a regolith from the exposure ages of bulk samples (as has been adequately demonstrated in the case of lunar soils where exposure ages are typically a few hundred million years in a host regolith whose age is 3.3 Ga). Bulk samples average the effects of heavily and lightly irradiated material, white the most heavily irradiated grains clearly establish a lower limit for actual regolith age. Estimation of regolith ages from tracks is also extremely difficult since the relationship between the observed track densities and the actual duration of regolith activity f exposure time within the nuclear-active zone of the regolith) is clearfy dependent upon the mixing processes. In fact, it is really not possible to use the surface exposure time (e.g., from solar flare track densities) to obtain the integrated exposure time to a depth of a few meters without first knowing more about the details of the mixing that is obviously present in the regolith (GAULT et al., 1974; HORZ et al., 1975; DURAUD et al., 1975 f . Regolith mixing clearly depends critically upon the meteoriod complex, gravity, and the mechanical properties ofthe parent bodies, none ofwhich are very well constrained. Periods of stable regolith mixing over hundreds of millions ofyears on meteorite parent bodies do not seem to be viabie with the current generation of models, but, without additional info~ation or chronomet~c constraints, it is not possible to evaluate their validity. There are, however, some qualitative arguments that independently suggest brief surface exposures. These arguments bear closer scrutiny. Anisotropic solar flare track dist~butions, observed in about 80% of the grains, led GOSWAMI and MACBOUGAH_

( 1983) to suggest that these meteoritic grains were irradiated on the surfaces of parent bodies in simple single-stage exposures. Yet, the measured track densities would require more than I 04- 10 ’ a to accumulate at the current solar tlam average rate. One may question the plausibility that these track-rich grains remained on the surface for 10 4- 10 ’ a without rolling over, especially when considered with the enhanced flux of meteoroids expected during the early evolutional stages of meteorite parent regoliths. While it is difficult to use this observation to quantitatively constrain surface residence, it might suggest that irradiated grains obtained their solar flare VH tracks in a few large flares, rather than a continuous irradiation by flares of contemporary intensity over lo4 to IO5 a. However, PRICE et al. f 1975) do show that in lunar core 60003, clearly a modern regolith irradiated by the contemporary particle complex, 75% of the grains containing z=-10’ tracks/cm2 are also aniso~opically i~adiated, and many of these show gradients indicating solar flare origin. Thus, single-stage irradiation leading to anisotropic solar flare track distribution does not necessarily reflect (as one might think) short GCR exposure times. Attempts have been made to date the compaction time of carbonaceous chond~t~s. This, of course, can set inde~ndent limits on the time available for exposure in the parent body regolith. The first serious effort to constrain the compaction time of CM meteorites was made by MACDOUGA~Land KoTHAN (1976) who studied fission tracks in olivines (U free) adjacent to matrix in the CM meteorites Murchison, Murray, and Mighei. Densities in excess of that which could be assigned to U fission were observed, and the excess track density was attributed to extinct ‘&Pu. The fact that fission track excesses were detected at all sets a lower limit of about 4.2 Ga for the association of olivine and matrix which, by itself, is only marginally compatible with duration of regolith activity required by the conventions GCR model for pre-compaction irradiation. More restrictive limits can be established if the specific 244Pu content of the adjacent matrix can be inferred from the average U in the matrix. Measurements of U dist~butions by MACD~UGALLand KOTHARI ( 1976) and independent studies of the Nd distribution in Murchison matrix (D. WOOLUM, per-s. comm., 1989) seem to confirm U and Nd uniformity. Using the most likely initial Pu/U ratio of 0.007 observed in St. Severin (HUDSON et al., I989), grain-matrix association seems to have occurred earlier than 4.5 Ga ago for Murray and earlier than 4.4 Ga ago for Murchison, leaving less than 100 Ma for free regolith activity for these particular grain-matrix assemblages ( CAFFEE and MACDOUGALL,1988 ) . The association of olivines with matrix does not, in itself, constitute proof of compaction. Alternatives to the interpretation that such grain-matrix contact means final sofidification include the possibility that the fission track record simply dates the intimate contact between an olivine and “matrix” either as loose entities or clods in the regolith, or that compaction is not a simple, one-stage process and dates obtained represent different stages of more complex brecciation. In fact, there does appear to be some structure in the distribution of apparent track ages among different Murchison grains measured by MACWUCALL and KOTHARI ( 19’76), Although

Cosmogenic Ne in carbonaceous chondrites this attempt to constrain the compaction time of the CM meteorites may be onen to interpretation, isotope chronometry on CI meteorites supports the time scale implied. MACDOUGALLet al. ( 1984) and MACDOUGALLand LUGMAIR( 1989) have studied the Rb/Sr systematics of carbonate veins from the CI chondrite Orgueil. These veins are the products of aqueous alteration ( MCSWEEN, 1987). Some veins are fragmented, but others run continuously through the meteorite, indicating that these could not have formed prior to brecciation. The measurement of Sr isotopes and the Rb/Sr ratio in this material constrains the time of fractionation (i.e., the separation of Rb-poor carbonates from a Rb-rich source, like the matrix). Model ages for this material, based upon the evolution of Sr isotopes from ALL (initial Sr derived from Allende refractory inclusions) until fractionation from Rb with carbonate formation, suggest rapid compaction. In fact, the dolomites require formation within 10 Ma of the major solar system differentiation events 4.56 Ga ago ( MACDOUGALLand LUGMAIR, 1989). This clearly leaves insufficient time for extended regolith processing prior to the compaction of the CI parent body, and, if this had been Murchison or Murray, instead of Orgueil, one would have a strong argument against the pre-compaction regolith histories required by the conventional GCR model. At the present time, however, we can only work to obtain suitable constraints for Murchison, or pre-compaction spallation measurements on Orgueil, both of which are difficult tasks. If isotope chronometry or other constraints prohibit the required GCR exposure times, the alternative, discussed by CAFFEE (1986) and CAFFEE et al. (1987), is an enhanced energetic particle environment in the early solar system. Since the galaxy was middle-aged 4.6 Ga ago, there is no particular reason to expect an enhanced galactic particle flux that coincided with the early evolution of our solar system. However, the Sun was going through its birth process at the time and virtually all solar mass type F stars go through a T-Tauri phase enroute to the main sequence ( FEIGELSON,1982; FEIGELSON et al., 1989; HUDSON, 1978; WALTER and BARRY, 1989). Therefore, if an enhanced particle environment is required in the early solar system, an active early Sun is more likely than an increase in galactic cosmic rays. Even if the early solar nebula were too dense to allow particles to penetrate very far, accretion of solid material removes the shielding dust, and T-Tauri winds appear to remove gas from the nebula, allowing the direct irradiation of planetesimal surfaces. The T-Tauri phase itself, and the subsequent Naked T-Tauri phase, which occurs after the accretion disk is dissipated, is rather brief, lasting 106-10’ a. Whether this time period coincides with parent body activity is not essential since the post T-Tauri solar activity remains enhanced over that of a middle-aged main sequence star for hundreds of millions of years ( MICELAet al., 1985; FEIGELSON and Knrss, 1989). If flare protons from an active early Sun are, in fact, responsible for the pre-compaction effects in these meteorites, it would require a minimum fluence of 1 x 10 I7 p/cm2 (of energy greater than 10 MeV) for a grain irradiated entirely on the surface, increasing to 4 X 10” for a grain irradiated at I g/cm2 average shielding, for energy spectra similar to

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contemporary solar flares. These estimates are clearly flawed since we do not know the energy spectra, and any regolith mixing will increase the proton budget necessary for the observed effects. To summarize, the conventional GCR explanation for the observed pre-compaction spallation effects requires an active regolith exposure on the Murchison / Murray parent body for at least 145 Ma. The doses presently observed are not so extreme as to require an active Sun in the early solar system. Long regolith exposure ages (several hundred million years) remain an alternative as long as they are consistent with independent constraints on the history and evolution of the CM parent body(s). Further regolith modeling and isotope chronometry constraining compaction of the CM meteorites may distinguish between these two possibilities. Until then the jury is still out. Acknowledgments--The

authors thank R. Wieler, K. Marti, and an anonymous reviewer for their constructive comments on this manu-

script. E. Koenig and B. Wilcox were especially helpful in the preparation of this work. We also thank D. Garrison for his contribution to the design of our laser extraction system. This work was supported

in part by NASA grants NAG 9-7 and NGT-50 194. Editorial handling:

K. Marti

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