Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy

Cosmic-ray exposure ages of the ordinary chondrites and their significance for parent body stratigraphy JANE CRARB Department of Chemistry, The University

of Chicago,

Chicago,

IL 60637, U.S.A.

and LUD~LF SCHULTZ Max-Planck-lnstitut (Received

fiir Chemie, 65 Mainz, Federal Republic of Germany

16 October

1980; uccepted

in rerisedform

12 June 19X1)

Abstract “Ne cosmic-ray exposure ages have been calculated from literature data for 201 H. 203 L and 38 LL chondrites, corrected for shielding differences when possible. The distributions of exposure ages again show the familiar peaks at 4.5 and 20 Myr for the H’s, but no outstanding events for the L’s and LL’s. If the L-chondrite distribution is interpreted as a series of discrete events, then at least 6 peaks between 1 and 35 Myr are needed to model it. The observations, that every petrologic type occurs in every large peak and that even the higher petrologic types contain solar wind gases. suggest that the parent bodies have been fragmented and reassembled into a megabreccia. For the H chondrites. both large and small peaks contain about 15”’ ,,, solar-gas bearing meteorites, which could mean that surface material has been mixed to depths represented by the largest event, on the order of a kilometer. In contrast. only 39; of the L’s contain solar wind, which may be related to breakup of their parent planet Those L’s with especially low radiogenic He (U,Th-He ages < I AE) tend to have low exposure ages: their distribution may be biased by a subgroup that had orbits coupling short capture lifetimes ulth significant solar heating.

INTRODUCTION

the late 1950’s the light noble gases have been measured in many stony meteorites, in order to determine cosmic-ray and gas retention ages. A cosmic-ray exposure age measures the time between an impact that produced a meter-sized meteoroid and its fall on Earth. Several trends have emerged from this work. The distributions of exposure ages for chondrites are approximately continuous up to about 50 Myr, with larger events superimposed (ANDERS, 1964; HINTENRERGEK et a/., 1964, 1965 ; EBERHARDT and GEISS, 1964; WANKE, 1966; ZAHRINGER, 1968). The upper limit of 50Myr apparently is determined by collisional destruction (EBERHARDTand HESS, 1960; GAULT, 1969). Gas retention ages occasionally approach 4.5 AE, but often are <4AE, especially for the L-chondrites (KIRSTEN et al., 1963; ANDERS, 1964; HINTENBERC~ER c’r a/.. 1964, 1965; WANKE, 1966; HEYMANN, 1967). Some questions are still unanswered, however, even after some 300 analyses. While many classes of stony meteorites show statistically significant peaks in their exposure age distributions (e.g. H chondrites: ANDERS, 1964; HINTENBER<;ER et al., 1964; WANKE, 1966; Z;~HRINC~ER,1968; TANENBAUM,1967; aubrites: EBERHARIN et ul.. 1965a: and diogenites: HERZOC and CRESSY, 1977), the L’s, within experimental error, could be either a true continuum or a series of closely spaced peaks (TANENBAUM,1967). There also is disSINCE

agreement on the timing of the outgassing events that gave rise to the low gas retention ages for chondrites. Some authors favor heating during major impacts on the parent body (ANDERS, 1964; HEYMANN. 1967: TAYLOR and HEYMANN, 1969, 1971; B(K~ARI>elf rll.. 1976), while others suggest that outgassing took place mainly in the events that produced the individual meteoroids (HINTENBERGERet al., 1966; WANKE, 1966: WETHERILL, 1978). Several additional leads have turned up in these studies. All petrologic types occur in all major peaks (WLNKE, 1968) and there is a suggestion that type 5 is overabundant in the 4.5 Myr peak in the H distribution (ZAHRINGER, 1968). The meteorites containing solar wind gases are distributed similarly to the other chondrites (W~~NKE,1966). L chondrites with particularly low gas retention ages tend to have low cosmicray exposure ages (ANDERS, 1964; WANKII. 1966, 1968: SCHULTZ, 1976). These observations provide clues to the structure and stratigraphy of parent bodies, which recent impacts have sampled to only limited. 5 1 km depths (ANDERS, 1978). Since the data base has been continuous]) expanded, it seemed worthwhile to take another look at exposure ages, to see what they can tell us about chondrite parent bodies. The compilation of noble gas data by SCHULTZ and KRUSE (1978, 1979) contains more than loo0 analyses for 471 ordinary chondrites. The Cambridge compendium (MOTYI.~WSKI. 197X) lists petrologic types for 390 of these chondrites, in a significant advance over the 174 classified cases in

JAM CRAW and LWOI.F SVHULTZ

2152

(1968). In addition, we have checked the data for sample mixups and diffusion loss, resulting in the deletion of 29 meteorites. This work is an exrension of SCHUI.TZ(1976). For maximum accuracy, exposure ages should be corrected for shielding differences (GEM et N/., 1960). For example. spallogenic ’ 'Ne varies by 30’?,,between center and surface of the St S&rin (LL6) chondrite (SCFIULX and SKNER. 1976). Ideally, a production rate would be derived for each sample from measurements of both the noble gases and a related radionuclide. Ilnfortunately, there are no data on radionuclides for most of the samples. However. the 22Ne:‘* ‘Ne and “Hei ‘Ne ratios are also sensitive to shielding differences, and various authors have devised shielding corrections using these ratios (EBERNARDT et ul.. 1966: CKESSY and B(X;ARD, 1976; SVHUI.T~ and SIGNER. 1978). We have used such shielding corrections. to determine improved exposure ages on samples where previous’ty constant production rates were assumed. ZHRINGER

CALCULATIONS The

noble gas data are from the compilation by and KRUSE (1978. 1979). augmented by analyses from WI-REK and St-nn~rz (I980) and FIK~:M,~~et
The ’ ‘Ne exposure age is basically derived from: r,, _ Z’NeiP 21. where Pzl is the “Ne production rate. Unfortunately, P,, IS not constant. but caries somewhat. depending upon the shielding. However, the “Hei2’Ne and “Nei*‘Ne ratios are also sensitive to shielding. and their relationships with P1, can be used to correct for such shielding effects (EBERHARDT et ui.. 1966: Ny~utsr et ul., 1973: CRESSY and BOGARV. 1976: Sc~ur-rz and SIGNER, 1978). CRESSY and BWARD (1976) use the correlation in the Keyes and St Severin between P,, and “Ne/*‘Ne chondrites to derive a shielding correction. They split P,, into a constant production rate for a given chemical class and average shielding (22Ne.‘L’Ne = 1.114) times a shielding factor F = 38.27 - 62,53(“Ne/‘*Ne) + XI(“‘Ne:“‘Ne)‘. For extreme values of 2’Ne.‘Z’Ne they use constant values of F. i.e. F = 1.181 for “Ne:“Ne < 1.08 and F = 0.818 for “Ne:*‘Ne > 1.20. The production rates for average shielding, in units of 10. s ccSTP, g-Myr. are 0.441 for H-chondrites. 0.470 for L’s and 0.4X3 for LL’s. Scwcr.~z and SIC;NER (lY78) used rhe depth dependence of the “Hel”Ne ratio to derive a shielding correction. Their expression for the production rate. P,, = r(8.45 & 0.34) - -‘He”‘Ne]!(6,9 t 0.7) ccSTP!g-Myr.

is normalized

to the

production

rates

of (‘RI-V.‘, ;rnJ

BWARD (1976). and is based on dnt:) from SCverin, and St Mesmin chondr~te~.

thz Kqw.

S!

Use of the “He,;L’Ne ratio ha> t\$ro ad\ani:cg~\. ii :i more sensitive to shielding variations over a wider range <)I depths and it also corrects for compositional diEerencas. since the “He production rate is le\y dependent 01: target chemistry than the “Ne production rate. ‘He “\;C I\ ;tIso less affected when a trapped Ne component I\; preqent Accordingly. the ‘He J’ Ne ratio ~.a\ used i\ hcnc\~r pacesible. The ‘“Ne. “Nc ratio wab used fttr shielding C‘
“Ne

and

“Ne

concentration\ Mere ~OI-ICC~CCIor ,: using I solar isotopic compositlvn fol the solar-gas bearing meteorite\ (2ZNe!‘“Ne = o~J:. “‘Ne,“‘Ne = 0.0024: GEIS~, t973). and atmosnhrrtc I-I~(I(X trapped component.

the spallogenic “Ne amount is insensitive to [he thoice for composition, rarely differing by more than a fcH percent. depending upon which of the two compr~>ltion\ i, assumed. For the spallogenic component “Ne Zf’~c <_ 1.1 and “Nei20Ne = 1.0 were used IRO~;AKI) ;md ( ‘KIs>!, 1973). Meteorites were considered to hc solar-@ax hearing if they met atl of the following criteria: I!: Solar ‘“Ne > 5( IO ‘) ccSTP/g; (2) the isotopic compo\lrion 01 Ne falls above the line joining the atmospheric and hpallogenie compositions on a plot of Z”Nr’21Ne \‘r “NC “NC. (3) 4He in excess of the radiogenic contributfcrn. i4) ‘“Ne:““Ar > I, after correction for the spallogcntc conipooent. These requirements were adapted from W~\ssos I 19743.

a trapped

A spallogenic contribution was subtracted assuming spallogenic (4He/‘Hel = 4 (Hr:uarh\u.

Statistical

formulas

SIEGEL(1956).

in the 4.5 Myr.

and

restrictloos

were

iaLcn from frequencie% types and i~ilar-g:t\

The x2 test was used to compare H peak for petrologic

from ‘Hc. 19671

Cosmic-ray

exposure

bearing meteorites. All meteorites with ages between 3 and 6 Myr were included as part of the peak. The results are Insensitl\c to changes in the peak width of + 1 Myr. Kru\kal Wallis one-way analysis of variance was used to test the significance of the differences in distributions of petrologic types for the L and LL chondrites. In this test the mctcorites In a chemical class are ranked in order of their exposure ages and the sum of the ranks of a given
RESULTS

AND

2153

ages of chondrites

peak’s standard are small.

deviation,

but otherwise

the changes

Peaks in the L distribution There has been some debate over the reality of peaks in the L-chondrite distribution (GEISS PI (I/., 1960: KIRSTEN et al., 1963: AXDFRS. 1964: TANEN-

H

40 no

shielding correction

DISCUSSION

The distributions of exposure ages for the H. L, and chondrites are shown in Fig. 1. As in previous versions, there are marked contrasts among the three c&se\ (AXDFRS. 1964: ERERHARDTand GEISS, 1964; HIVI NBERGHR(‘I tr/.. 1964. 1965; WANKE, 1966; Scrn:~ ~7. 1976). The major feature for the H condrites is a peak at 4.5 Myr, containing 45:,; of this class. and perhaps another peak at about 20 Myr. On the other hand, the L’s and LL’s appear more as a continuous series of intermediate sized peaks. The cfTect of shielding corrections is shown in Fig. 2, on linear rather than logarithmic scales. Use of shielding corrections sharpens the 4.5 Myr peak of the H chondrites somewhat, reflected by an II:, decrease in the LL

20 I-

.f

0

LL

d

(38)

5I

’

IO

Exposure

(A)

“‘I

1

nn

20

30

40

Age

50

60

(m.y i

I-=+ ” I I,

“‘I

ZO-

LL no

shielding

correction ct 0

I

+

S

1’1’1

;: 0

30-

5m

P-Ill

with shielding correction nn

H

Z 20-

I

L

(201)

no shielding correction

R

c uwaJ&

IO

I

0.1

I

Exposure

1

IO 20 40 60

Age

0

(m.y.1

Fig. I. Hlstogt-ams of exposure ages for the ordinary chondrltes, gtven on a logarithmic scale (20 intervals per decade). 4 hhlelding correction has been applied when po.\\lhle. The solar-gas bearing meteorites (shaded squares) parailel rhe distribution for the other meteorites.

(B)

IO

20

30

Exposure

Age

40

50

(m.y 1

Fig. 2. The effect of shielding correction on the exposure age distributions. The differences are minor. with a few peaks a little sharper in the corrected version.

2154

.IAX CRABR and LUIX~LPS~HUI:rz

----- “I

BAUM, 1967). These

peaks have marginal statistical significance (TANENBAUM. 1967). which may imply either a true continuum of events (W~~HERILL. 1974) or a quasi-continuum formed by a number of randomly spaced, comparably-sized peaks (TAXNBAUM. 1967; SCHULTZ, 1976). To pursue this question, we must consider the resolution of the age histograms, i.e. the apparent width of a single, isochronous peak. It will be broadened by experimental error, pre-irradiation (EBERHARDTrt trl.. 1965a; FUSE and ANDERS, 1969; S~H~:I~T%et crl., 1972). later breakup of large ejecta. and uncorrected shielding differences, We can estimate this effective width from the large peak in the H distribution. by assuming that this peak represents one major meteoroidproducing event. This approach ignores any minor contribution from a continuum of small events, but is still a useful approximation. An enlargement of the peak is given in Fig. 3, with a gaussian curve superimposed on it. While the fit is not ideal, it suggests that this peak has a 1 CT error in the age of about 20”“. This seems reasonable, considering experimental errors and possible interlaboratory bias. Between I and 35 Myr, as few as 6 peaks with 1 (r widths of 207” could produce a quasi-continuous distribution. similar to that observed for the L’s, This gives only a lower limit to the actual number of events since, particularly at higher ages. the histograms do not have enough resolution to distinguish between closely-spaced peaks and a continuum of small events. We turn, then, to theoreticaf arguments. Two considerations suggest that. for a given class of stones. a small number of events produce most of the meteorites. First, for a typical bombarding flux, ejecta from large events dominate (CHAPMAN. 1978). This is supported by the second point, that a wide range of stony meteorite types show peaks in their exposure age distributions (H chondrites: C2- -MAZOR er ~1.. 1970; aubrites--EBERHARm rt u/.. 1965a; howardites~ANAEArHv and ANDERS. 1969; diogenites HERZOC; and CRESSY, 1977). There is no reason to suppose that the L chondrite body is exposed to ;I different bombarding flux. so it seems likely that some

0 4 8 Exposure Age (m.y ) Fig. 3. The major H peak shown on an expanded scale, with a gaussian curve corresponding to the calculated standard deviation. An error of about 209, seems appropriate for ages around 5 Myr.

1

6 “P-J--h”

0

IO

20 Exposure

,

30 Age

40

io

&?

im y.)

Fig. 4. Distributions of exposure ages separated by petrologic type. For the H chondrites, 5’s are overabundant and 6’s underabundant in the major peak. The different types are fairly evenly distributed for the 1. chondrites. LL’s appear to cluster unevenly, but the numbers are small and the differences are not statistically significant

of the clusters in the L distribution do mdecd represent discrete events. It may be that the 1. parent planet has not suffered any recent. very large Impacts. or that is has been fragmented to the point where there are few large targets left to hit. Petrologic

types

Among the impacts reflected m the cosmic-rdy age distributions of chondrites, even the largest sample depths of only about one kilometer (ANIIERS. 1978). Hence, they can serve as localized probes for the structure of these bodies, which are thought to have been on the order of 100 km in radius, w*ith a I,ryered arrangement of petrologic types (WOOD. 1967: MINSI-ER and ALLEGRE, 1979; PEI.LAS and STORLER, 1979; MIYAMOT~rr ul., 1980). Curiously, all petrologic types from 3 to 6 occur in the 4.5 and 20 Myr peaks for the H chondrites (Fig. 4; WKNKEZ,1968). This cannot represent the original layered structure, since a substantial thermal gradient over tens of kilometer> must have persisted between types 3 and 6 for the millions of years of metamorphism. A simple regolith cannot produce the mixture of types, since it is expected to be only hundreds of meters deep (HOUSEN et cl/.. 1979; DURAUD et cd., 1979). It seems likely that the entire body has been fragmented and mixed into a megabreccia, as was proposed by DAVIS and CHAPMAX (1977) and developed for petrologic types and

Cosmic-ray exposure ages of chondrites solar-gas bearing meteorites by ANDERS (1978) (Fig. 5a-d). By considering the distribution of exposure ages of the various petrologic types (Fig. 4), we can gain further insight into the near-surface stratigraphy of the H parent body. With less than half of the data now available, W~NKE (1968) saw no systematic variation of petrologic type with exposure age, while ZAHRINGER (1968) noted an excess of type 5 in the 4.5 Myr peak. The new data confirm ZBhringer’s observation. Table I gives the probabilities that the different types occur with the same frequency in and out of the peak. Shielding corrections and exact choice of peak width have minor effect on these values. The results of the statistical tests reflect that, in addition to the excess of type 5 in the peak. type 6 is underrepresented. Types 3 and 4 have distributions similar to the average. A likely explanation for the variation in distribution of the different types is that the megabreccia is not completely mixed laterally or vertically, but still retains traces of the initial layered structure. An alternative possibility. but without other evidence to sup-

21.55

TABLE 1. Partitioning of H-Chondrites With Respect to the 4.5 m.y. Peak

Type 3 4 z Solar-qas bearing

Probability* 374: 95": 1? 2" 92%

*The probabilities from 1’ tests that the given type and the sum of the other types are identically partitioned between the 4.5 m.y. peak and the rest of the distribution.

port it. is that there are several independent H-chondrite parent bodies, each with a different proportion of petrologic types. Results of mixing processes are often visible on the scale of individual meteorites. BINNS (1967) classified about 20”” of the British Museum’s collection of ordinary chondrites as xenolithic- emi.e.containing material of more than one petrologic type or chemical class. This probably reflects impact mixing of a pre-existing megabreccia (Fig. 5e).

Fig. 5. Schematic of the collisional evolution of chondrite parent bodies. (a) Initial layered structure of petrologic types, radius _ 100 km. (b) Small impacts generate some regolith on the surface, but do not mix the petrologic types. (c) A near catastrophic impact fragments the body but is not sufficient to disperse the pieces. (d) The gravitationally bound pieces collect into a ‘megabreccia’, with all petrologic types exposed at the surface. (e) Smaller impacts convert the surface layer mto a regolith that is hundreds of meters deep. About 207; of the ordinary chondrites contain more than one petrologic type, which indicates that the regolith is often mixed on a meter scale. The H-chondrites may be from the surface layer of such a body, judging from the common occurrence of solar-gas-bearing meteorites (143,). (f) For the L-chondrites, a collision -0.5 AE ago may have broken up the body. Both outgassing and more representative sampling of the body may be responsible for the scarcity of solar-gas bearing L’s (.i”,,).

2156

JANECKABBand LKJDOI.F SCHILTZ

W~LKENING (1979) has proposed a model where parent bodies are agglomerates of pIanetesimals of varying degrees of metamorphism, rather than the usual ‘onion skin’ layers of petrologic types. Regolith processes might be able to produce the mixtures of petrologic types on a kilometer scale, so this model, too. is consistent with the evidence from exposure ages. For the L chondrites one again sees a11 petrologic types coexisting at similar exposure ages, although the peaks are less well defined than for the H-chondrites. A Kruskal-Wallis one-way analysis of variance gives a XI”/, probability that the different petrologic types come from populations with identical distributions. This apparent homogeneity may reflect either a more thoroughly mixed parent body, or simply the poorer statistics in the smaller, more closely spaced peaks. For the LL chondrites no conclusion is possible because of the small numbers. Solar-yas bearing meteorites Another, shallower probe for the stratigraphy of parent bodies is the distribution of exposure ages for solar-wind irradiated meteorites. Since the range for solar wind is only about lOOOf%, portions of these meteorites must once have been on the very surface of their bodies. For the H chondrites, the distribution for solar-gas bearing chondrites parallels that of the other H’s (Fig. 1; WXNKE, 1966). A xZ test gives a probability of about 90”,, that the gas-bearing H’s occur with the same frequency in and out of the 4.5 Myr peak as do the others. This suggests that gasbearing material has been mixed at least to depths represented by the major peak--an the order of a kilometer (ANDERS, 1978). Two lines of evidence show that solar wind implantation is an on-going process on parent bodies. The first is the relatively young K--Ar age of 1.4 AE for a xenolithic clast in the solar-gas bearing St Mesmin (LL) chondrite, which gives an upper limit to the time since St Mesmin was compacted from a regolith (SCHULTZ and SIGNER, 1977). The second is the presence of all petrologic types among solar-gas bearing meteorites. If the parent body had been irradiated primarily before its conversion to megabreccia, then the outer portions of the body~--type 3 and perhaps 4-~--would contain most of the solar wind, contrary to observation. Regolith thickness on large (R 2 100 km) asteroids is estimated to reach hundreds of meters (HOUSEN et al., 1979; DURAUD et a/., 1979). Such a thickness is of the order needed to explain the similar frequency of gas-bearing meteorites in large and small events, assuming that H chondrites are derived from the surface layer of a body that still contains much of its original mass (Fig. Se). The common occurrence of gas bearing H’s (14”,< of total) suggests that there has been sufficient time for a substantial regolith to develop since the last megabrecciation. As an alternative hypothesis, the body may have broken up, for instance,

to supply Apollo and Amor asteroids. Here the gasbearing meteorites are relicts of earlier generations of regolith, mixed into the interior of the body hg megabreccia formation (ANDERS. 1978). Certainly the process that mixed the petrologic types should h;t\c redistributed the gas-bearing surface layer 10 \ome extent, although it is difficult to see how llux ir~kcr could have been more evenly mixed than the prtrologic types. Unfortunately. the c\idcncc fr:ml s&i.gas bearing meteorites only provrdes a IOHCTiirnil for the mixing depth, and cannot distinguish between the above possibilities. Variations in the amount of solar Ne ma) :riso 1~~ providing information about homogeneity ot ihe ~LII’face layer. Meteorites in the major H peak lend lil have comparatively low amounts of solar kc rNe,i: I) r~cixitnum of 15 in the peak ha\2 out ‘“Ne, < 50(10 -“) ccSTP/g. while only 2 of thr 16 outside of the peak fall below this limit. This dlsparitj probably reflects uneven mixing of asteroidal rcs tmpl\. that a catastrophic event broke up and heated much of the L parent planet, producing fragments too smali to develop significant new regohth (HEYMANS. 1967: TAYLOR and HEYMANN, 1969; 1971). Subsequent impacts sample a more representative fraction of rht: body, rather than just a gas-rich surface layer (Fig. 5f). Also, portions of the body may have list aii of their solar gases as well as most of their radiogenic gases. Two lines of evidence suggest that this rdea is plausible: solar wind is more loosely bound than the radiogenic gases and so would be preferentiali! lo\1

Cosmic-ray

exposure

1962: EBERHARDT ef nl.,196%~; MANUEL., 1967), and some meteorites have been reheated to 95O-125O.C (WOOD, 1967; HEYMANN, 1967; TAYLOR and HEYMANN, 1971; SMITH and GOLDSTEIN, 1977). of the severely outgassed L’s Indeed, none (4He < 250ccSTPig, 217;) contains detectable solar Ne. However, not all of the L chondrites have such short gas retenti~~n ages. The few solar-gas bearing L’s seem to belong to this lightly reheated material. as survivors of ancient regolith regions that were either farther from the ma_jor impact or had previously been spalled off the parent body as kilometer-sized fragments, The four solar-gas bearing L’s with Ar analyses have relatively high K-Ar ages (X2---4.1 AE, for X70 ppm K). although they still make up only 50;; of the L’s with similarly high K-Ar ages. (Z&RINGER,

it has been known for some time that L chondrites with low 4He contents, or other signs of shock and reheating. tend to have lower than average cosmic-ray exposure ages (ANDERS, 1964; WXNKE. 1966. 1968; SCB~ILTZ. 1976). This is once again demonstrated in Fig. 6 for the L’s with radiogenic “He < 2SCylO _ “) ccSTP,/g (nominal U,Th-He age < 1 AE). These meteorites contribute 50”” of the cases below 10 Myr but only 20”,, of those above it. A x2 test gives a probability much less than lTjc;,that this difference is merely statistical. Two classes of explanations are considered below: those that relate the short exposure ages to the magnitude of the outgassing event and those that attributed short exposure ages and loss of radiogenic He to solar heating. The prevalence of shock effects and low 40Ar-39Ar ages among the L chondrites suggests that their parent body was involved in at least one collision with a body of comparable size, around 500 Myr ago (ANIXRS. 1964; HEYMANN, 1967; TAYLOR and HEYMANIU,1969: BOGARII et cd., 1976). The shorter exposure ages of the L‘s with particularly low 4He

0

‘H~,>250~(~*)

ccSTP/g

4tie,<250(10-*)

ccSTP,‘g

IO

20 Exposure

Fig. 6. The L chondrites

30 Age

40

50

(m-y.)

with especially

low radiogenic

(U.Th He age < I AE) tend to have lower cosmic-ray than the remaining

L’s, Meteorites with K-Ar are also indicated.

4He

ages ages < 1 AE

ages of chondrites

'157

might mean that a large, severely reheated fragment was expelled to an orbit where its ejecta has a short lifetime for capture by the Earth. Indeed, the effect may be causally related, if severe shock was necessary to relocate a fragment from the asteroid belt to a more favorable orbit, as has been suggested for group III irons (JAW and LKPSCHUTZ.1969). Another possibility is a completely separate parent body for the low 4He chondrites. This seems unlikely since heating effects are so widespread. Only! about ZOY, of the L’s have 4He > 12OO(IO a)ccSTP:g (U,Th-He age 24AE). in contrast to the H chondrites where the fraction is greater than 40”,,. Separate parent bodies would require at least 2 heavily shocked L-chondrite bodies. This is an improbable circumstance, since bodies of similar size are much less likely to collide than bodies of very different sizes. in which case most of the material is only lightly shocked (TURNER, 1979). This conclusion is weakened somewhat by the observation of BOGAKDit (il. (1976) ages have expet-ithat the L’s with low “Ar3”Ar enced more than one outgassing event in the last aeon. A different explanation was favored by Wavrc~ (1966, 1968), HINTENBERGF~R rt d. (1966) and W~IHI.KILL (1978). They suggested that much of the outgassing took place in the event that ejected the meteorites from their parent body. In an extreme version of the above model. WANKE (1966) proposed that when these impacts are larger. they produce more outgassing and greater acceleration, leading to a shorter transit time to Earth. This version has difficulty explaming the lack of a similar effect for the H chondrites in the 4.5 Myr peak, and aiso requires several coincidences to explain the concordant radiogenic ages of around 500 Myr (ANDERS, 1978). Nonetheless. some outgassing is expected in the last event. TI.KNI:R (1979) calculated that a small proportion of the ages will be reset by the final impact, and BOGARD et trl. (1976) find a few very young “Ar”‘Ar ages. But these cases by no means extend to a majority of the L’s. We now turn to the possibility of solar heating. Could the short exposure ages simply be due to loss of spallogcnic Ne? Severe shock reduces the gram size of the silicates, making them more s~ts~~p(tbl~ to diffusiot~ loss (HEYMANN, 1967). Indeed. 3O”,, of the meteorites with “He < 250( IO ‘) ccSTP;p have 3He/2’Ne < 4, suggesting that many have lost at least He during the cosmic-ray era. But in our data selection. meteorites with ‘Hel”Ne < 2 were excluded. which should rule out severe diffusion loss of “Ne. The situation is not changed significantly when the few meteorites with 2 < (3He 2’Ne) < 3 are also omitted. According to HRYMASN (1967). samples of Arapahoe (shocked LS) that were heated in the laboratory lost negligible Ne as long as “He,“Nc > 3. So, while diffusion loss of Ne may have shifted the distribution slightly. it is unlikely to he the sole cause of the short exposure ages.

JANECRARBand LUD~LFS~HUI.YZ

21%

Loss of 4He by solar heating has been suggested for a small group of H chondrites (HINTENBERGERet al., 1966; WANKE, 1966). This explanation can be extended to the L’s with low 4He, requiring that these meteorites had orbits with both a small perihelion distance to provide solar heating and a small semimajor axis to give a greater overlap with the Earth’s orbit. There is some support for this hypothesis in the common occurrence of low 3He/2’Ne ratios for the meteorites with especially low 4He; compared to the rest of the L’s, low ratios are nearly twice as common. However, solar heating cannot be responsible for the low ‘He in all of the cases. Many of these meteorites have concordant U,Th-He and K-Ar ages, which is not expected from solar heating. Rather, the exposure age distribution may be biased by a subpopulation with short exposure ages and low We due to solar heating. Discordant radiogenic ages are more common at exposure ages below 10 Myr, which is consistent with this idea.

The L’s have two characteristics that tend to enhance solar heating. First, they have experienced one or more outgassing events that have had the 2-fold effect of lowering their 4He contents and making them particularly susceptible to further loss. The second factor is that blackened, shocked chondrites will have low albedos and will get hotter in a given orbit than unshocked meteorites. We conclude, then, that some solar heating has occurred for the L’s and that it can provide at least a partial explanation for the coupling of especialiy low 4He contents with short exposure ages. In addition, L-chondrites with low exposure ages may be from especially outgassed material, either by chance or by derivation from a reheated fragment that is now in a special orbit.

taneously to solar heating and bigh capture prohability by the Earth. Acknowledgements--We ANDERS,who contributed assistance to this paper. reviews by Dr P. SIGNER was supported in part by

are grateful to Profcssor~ t-. much discussion and cd&o& AIso appreciated arc t~~tl~~trul and Dr D. Boc,41ta. This work NASA C;rant NGL-14-001-010.

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meteorites.

Eurth

Planer.

Sci. trrt.

2, 23 2%

B~GARDD. D. and CRE~SYP. J. (1973) Spatlation produc. tion of 3He, “Ne. and ‘*Ar from target element< in the Bruderheim chondrite. Ge~chint. C~~,mac+in~.,fctci 37, 527- 546. BOGARD D. D., HUSAIN L. and WKIC~HTR. .I. (19%) B”Ar-3”Ar dating of collisional events in chondritc pat-ent bodies. 3. Geophp. Rex 81, 5664- 567X. CHAPMANC. R. (1978) Asteroid collisions, craters. regoliths, and lifetimes. In Asteroids: An Espkwtrticm ,A.s.se,~ mint (eds D. Morrison and W. C. Weils), pp. 145 157 U.S. Govt Printing Office (NASA CP 20531. CREW P. J. and B&ARD D.‘D. ((976) On the caicnlation of cosmic ray exposure ages of stone meteorites. C;C+ chim. Cosmochim.

Acta 40, 749-762.

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VIII,

224-226.

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bodies in the solar system. Lunzrr Plunrt. .%i, X, 32.1 325. Hess D. C. (1960) fIelium in ‘rtonc meteorites. Astrophy. J. 131, 38, -46. EBERHARDT P. and Gtzss J. (1964) Meteorite ciasbe~“‘r anJ radiation ages. In Isotopic and Cosmic (‘hrmisrr! irds H. EBERHARDT P. and

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CONCLUSIONS

EBERHARDT P., EIJGSTER 0. and GUSS J. (1965a) Radiatmtr

The following picture can be constructed from evidence provided by cosmic-ray and gas retention ages, At least once, perhaps more often, the H parent body has been nearly catastrophi~l~y fragmented, mixing the original layered structure of petrologic types into a megabreccia. The surface layer is a regolith that shows small variations in the abundances of petrologic types over kilometer distances yet is occasionally mixed on the scale of individual meteorites. The relatively high abundance of solar-gas bearing H’s compared to the L’s suggests that most of the H chondrites come from the surface layer on a body that still is large enough to have a substantial, active regolith. The L parent planet has aho been mixed into a megabreccia, In contrast to the H body, it most likefy was fragmented by a collision about 500 Myr ago that left abundant

traces

in shock

features,

low gas reten-

tion ages. and scarcity of solar-gas bearing meteorites. A small group of L chondrites with especially few radiopenic He may have had orbits that lead simul-

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neon. Z.

Nut~~~~rsch.

Tschermaks

Mineral.

Petroyr.

Mitt.

10,

535 551.

EBERHARDT P., EUGSXR O., GEISS J. and MARFI K Il96hj

Rare gas measurements in 30 stone meteorites. /. .Y&rr21A, 414426. FIREMAN E. L., RANCITELU L. A. and forsch.

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GEIS~ J. (1973) Solar wind comnosition

Cosmic-ray

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1159

ages of chondrites

Light noble gases in Nucl. Truck DerecI. 2, Gas-rich regolith

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as In

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APPENDIX Mereorires

\viTh suspected ga., loss

The following were omitted from the histograms and discussion because all analyses gave ‘He/*‘Ne < 2.5, and there was no evidence for large shielding effects: Ham-Ambapur Nagla, Archie. Cloves No. I. Cross Roads, Darmstadt, Doroninsk, Erofeevka, Favars, Gruver, Kimble County, Nikolaevka, Schaap-Kooi. Scurry. Seres, Sweetwater, Tulia, Ucera, Willowdale. L-Bath Furnace, Harrisonville. Tuan Tut. LL-- Savtschenskoje. Cavour, Morland, Malakal, and Appley Bridge were omitted due to

2160

JANE CRABB and LUD~LF SCHULTZ

discrepancies between their noble gas and radionuclide posure ages (see CRESSY and BOGARD, 1976).

ex-

Discrepunt anulyses In cases of sample mixups (gas amounts differing by 507, or more) the value most often reported was used in calculating the exposure age. Gas retention age was used as the selection criterion when there were only two analyses. The analysis was chosen which had the higher age for H’s and the lower one for L’s, In the few cases where there was no clear preference the meteorite was omitted completely. The followmg data were used for such discordant cases (full references and data are given by S~HUL.TZ and KRUSE. 1978): H chondrites Achilles: ROWE et al. (1965). Adrian: HERZOG and CREW (1974); ZAHRINOER (1968). Bath: HINTENBERCER et al. (1965); HEYMANN and ANDERS (1967); GANAPATHY and ANDERS (1973). Farley: HINTENBERGERet al. (1964). Menow: MAINZ (unpublished); SRINIVASAN(1977). Ochansk: EBERHARDT et al. (1969); GANAPATHY and ANDERS (1973); HINTENBERGER cr al. (1964); NYQUIST et al. (1973); Z~~HRINGER 1966). Ozona: HERZOC~and CRESSY (1974). Pultusk: BLACK (1972); EBERHARDT et (II. (1966); HINTENBERGERet ul. (1964); KIRSTEN et al. (1963). St Germain-du-Pinel: HERZOG (1973); NYQUIST et ul. (1973); SCHULTZ and SIGNER (1974).

Salles: GANAPATHY and ANDERS (1973). Zhovtnevyi: V~NOGRAWV and ZADOROZHNYI (1964). Timochin was omitted. L chondrites Canakkale: HEYMANN (1966); MAINZ (unpublished1 Grant County: HERZ~G and CRESY (1974). Kunashak: KIRSTEN et al. (1963); LEVSKY (lY72): VIVO~;R~DOV and ZAD~ROZHNYI (1964). Loop: MAINZ (unpublished). Ness County (1894): NYQUW et al. (1973) HERZO(; and CRESSY (1976); HINTENFIERGERer al. (1965); Z~HRINGEK (1966). New Concord: HINTENBERGERet al. (1964); first value from Z~~HRINGER(1966). Pervomaisky: VIN~GRAWV and ZADOROZHP~YI(1064). Pierceville: CRESSYand B~CARD (1976); SRINIVASAN(1977). Vouille: TAYLOR and HEYMANN (1969). Finney was omitted. LL Chondrirvs Chainpur: Bandong: Vavilovka

HEYMANN and MAZOR (1968). MUELLERand %HRINFER (1968). was omitted.

Paired fulls Accalana was considered as part of Caraweena (HEY1965). La Lande, Melrose. and Taiban(1) were grouped as one meteorite (HEYMANN, 1965). The three analyses of Springfield were treated as separate meteorites (HENNECKE and MANUEL, 1976). MANN,