On the calculation of cosmic-ray exposure ages of stone meteorites

Gcochlmlca

ct Cosmochlmlca

Acta. IY7b.

Vol. 40. pp. 749 to 7b?. Pergdmun

Press.

Prmtcd

m Great

Br!taw

On the calculation of cosmic-ray exposure ages of stone meteorites PHILIP J. CRESSY,JR. Earth Resources Branch, Goddard Space Flight Center, Greenbelt, Maryland 20771, U.S.A. and DONALD D. B~GARD Johnson Space Center, Houston, Texas 77058, U.S.A. (Received

26 May 1975; accepted

in retlisedfbrm

11 November

1975)

Abstract-Abundances of cosmic ray-produced noble gases and 26Al, including some new measurements, have been compiled for some 23 stone meteorites with exposure ages of <3 x 10s yr. Concentrations of cosmogenic He, Ne, and Ar in these meteorites have been corrected for differences in target element abundances by normalization to L-chondrite chemistry. Combined noble gas measurements in depth samples of the Keyes and St. Severin chondrites are utilized to derive equations for to an adopted normalizing the production rates of cosmogenic ‘He, 2’Ne, and ssAr in chondrites 26Al concentrations and the calcu‘average’ shielding: ‘*Ne/“Ne = 1.114. The measured unsaturated lated equilibrium 26Al for these meteorites are combined to estimate exposure ages. These exposure ages are statistically compared with chemistryand shielding-corrected concentrations of cosmogenic He, Ne, and Ar to derive absolute production rates for these nuclides. For L-chondrites, at ‘average’ shielding, these production rates (in 10-s cm3/g lo6 yr) are: 3He = 2.45, *‘Ne = 0.47, and 3sAr = 0.069, which are -25% higher than production rates used in the past. From these production rates and relative chemical correction factors, production rates for other classes of stone meteorites are derived.

INTRODUCTION

ONE OF the more important factors which pertain to our understanding of the history and relationship of solid objects in the solar system is the exposure chronology of meteorites to particle irradiation, especially cosmic rays. Cosmic-ray exposure ages are nearly always derived using the abundance of a nuclear reaction-produced noble gas isotope and an empirically or theoretically-derived production rate. One approach to determining exposure ages is to utilize a stable/radioactive nuclide pair (e.g. 3He/3H, zzNe/2ZNa, 21Ne/26Al, 38Ar/39Ar, s3Kr/siKr) and a cosmic-ray production ratio for the particular pair (e.g. ANDERS, 1963). This method has the disadvantage that the production ratios for many nuclide pairs are not accurately known as a function of meteorite chemistry and of the cosmic-ray energy spectrum in the meteorite. A second, more commonly used technique is to utilize average, empirical production rates for the stable noble gas nuclides. Derivation of these production rates initially requires an independent means of determining exposure ages of one or several meteorites. Traditionally such independent ages have been derived from a radioactive-stable nuclide pair, and are subjected to all the uncertainties associated with this technique. An independent method of determining absolute noble gas production rates based on the measured 26Al activity in low exposure age meteorites was o/

,

,I!‘

,I

reported by HERZOG and ANDERS (1971). Meteorites with exposure ages less than 3 x lo6 yr do not contain the full secular equilibrium activity of 26A1 (half life = 0.74 x lo6 yr). As the equilibrium activity of 26Al in several classes of meteorites can be determined with reasonable accuracy (within 10%; FUSE and ANDERS, 1969; CRESSY, 1971), the ratio of measured 26Al activity in these low exposure age meteorites to calculated 26Al equilibrium activity is a function of only the exposure time. HERZOG and

ANDERS(1971) calculated the exposure ages of several ordinary, carbonaceous, and enstatite chondrites and one eucrite whose ages were <2 x lo6 yr, and compared these to cosmogenic 3He and *‘Ne contents. These authors obtained production rates for 3He and *lNe which are 24% higher than older commonlyused values based on 3H/3He measurements in several meteorites (KIRSTEN et al., 1963). The meteorites used by HERZ~C and ANDERS (1971) represent a variety of chemical classes, and in some cases required sizeable chemistry corrections to 26Al, 3He, and *‘Ne to normalize to L-chondrite chemistry. Neither were any corrections made to the data to compensate for changes in production rates arising from shielding differences. There is also the possibility that meteorites of different classes may have possessed dissimilar orbits and thus different production rates. It is now apparent that all of the empirical production rates are substantially affected by variations in

749

PHILIP J. CRESSY. JR. and

750

meteorite chemistry, cosmic ray shielding, and possibly (CRESSY and RANCITELLI, 1974) meteorite orbit. Measurements of cosmogenic 3He, *lNe, “Ne. 38Ar, and 26A1 in different meteorite classes and separated mineral phases of a few meteorites now permit reasonably accurate, relative corrections to the production rates to compensate for chemical differences (BOCHSLER et trl.. 1969; FUSE and ANDERS, 1969: CRESSY,1971: BOGARDand CRESSY. 1973). Production rate corrections due to shielding differences can be made on the basis of the parameter 2’Ne/21Ne (EBERHARDT rt al.. 1966; WRIGHT et al., 1973; NYQUIST ef (I/., 1973; BOGARDPI trl., 1973; CRESSY, 1975; HERZOG and CRESSY. 1974). Using these chemistry and shielding corrections yields relative cosmic ray exposure ages of stone meteorites superior to those based on empirical constant production rates. The importance of accurately determining absolute noble gas production rates led us to re-examine production rates in meteorites with unsaturated “hAl activities. We utilized those meteorites reported in HERZOG and ANDERS (1971), plus several additional ones with low exposure ages. From these data, using relative elemental spallation production rates reported in earlier papers (BOGARD and CRESSY, 1973: CRESSY, 1971). we calculated absolute production rates for 3He. “Ne, and “8Ar in various classes of stone meteorites. EXPERIMENTAL Literature data on lbAl activities and cosmogenic “He and “Ne contents of a number of meteorites indicated exposure ages of <3 x lOh yr. However, for several of these meteorites only the 26Al or the noble gas data were available, or the data were old, incomplete, or contradicTable

I. New results,

DONALU D. BOGARII

tory. In these cases we analyzed samples for ‘“Al and/or noble gases. Seven meteorite specimens. weighing between 150 and 300 g, were analyzed non-destructively by gamma-gamma coincidence spectrometry. The low-level counting system consists of two 10.2 x 10.2-cm NaI(TI) detectors in an anti-coincidence mantle, shielded by 15 cm of steel on all sides and an additional 5 cm of lead on top. Coincident and non-coincident data arc accumulated simultaneously in a 4096-channel dual parameter pulse height analyzer. Typical backgrounds are: in the coincidence matrix. 0.05 count/min for 0.51 x 0.51 MeV and 0.51 x I.81 MeV. and O.OI for 0.51 x 2.32 MeV: in the non-coincident spectra, about one count;mln net in each detector for the 1.46-MeV ‘OK gamma ray. The potassium standard is a mixture of KCI m iron and dunite powder. Aluminium-26, adsorbed on 100 mesh iron powder, has a calibration uncertainty of 5”“. The powder standards are counted III epoxy-coated aluminum foil mock-ups to duplicate meteorite sample counting geometries. Other details of the counting system and procedures may be found in Ctu:ssv (1970. 1971). Twelve meteorites were analyzed for He, Ne and Ar ISOtopes by mass 5pectrometry. using general procedures given by B~CARD and CKESSY (1973) and WRIGHr et rr/. (1973). Instrument sensitivity was monitored with “He. “He. Ne. and Ar standards delivered by a gas pipette. and varied by less than +3”,, throughout the analyses. The large partial pressures of radiogenic ‘“Ar and low partial pressures of 3HAr in the spectrometer caused the Ar data for several of these meteorites to be of poorer quality than usual.

RESULTS New results on “Al specilic activltles and concentrations of K and of He. Ne and Ar isotopes are given in Table I. along with the meteorite class and sample source. Potassium concentrations are given in ppm. assuming terrestrial isotopic abundances. The standard deviations of the 2hA1 and K contents were compounded from sample and background counting statistics. Repeat noble gas analyses were made on St. Marks and Sena. with abun-

stable and radioactive

nuclides

’ Argon data margmal because of high pressure. * Possible hydrocarbon interference of up to 15”,;, on “‘Ar. A “Ne not blank-corrected. ’ Horace no. 2; incorrectly identified as ‘Ladder Creek’. Abundance uncertainties for most noble gas measurements are estimated as uncertainties &S~lO:~~. except for 38Ar and 40Ar, which have greater uncertainty: for ZhAl and K are derived from counting statistics. Sample sources: National Museum of Natural History, Arizona State University. American Meteorite Laboratory.

Cosmic-ray

Table

Meteorite

2. Cosmogenic 3

Be

2lN-3

exposure

and radiogenic SEA=

751

ages

noble gases,

“Ne/=Ne

10m8 cm3/g

31ie/2+ie

Fladiogenic

4OAr

K-AI

Age

109n-

Ap~ley Bridge

1.62

0.28

0.04

1.240a0.024

6.50

Bath FurM.ce

1.24

0.86

0.121

1.103 *0.01*

1.46

328

1.0

Bur-Gheluai

1.37

0.35

0.08

1.146*0.100

3.91

4376

4.1

CaVOUr

4469

3.7

2.91

0.83

0.70'

1.073 f0.001

3.51

2310

3.2

Eva

69.35

17.28

1.61'

1.098*0.006

5.17

4356

4.0

"LadderCreek"

25.23

1.199+0.005

1.64

3895

3.7

4075

3.9

3.31

0.51*

MaIOtas

w

0.98

0.15

Morland

1.19

0.27

0.06

1.094sl.014

4.41

4025

4.0

59.03

13.45

I.322

1.090 l0.006

4.39

3926

3.6

St.Marks

0.90 1.00

0.23 0.23

1.133*0.034 1.094*0.06*

3.91 4.36

3336 3210

3.6 3.7

SO"I?y

1.13

0.55

0.23

1.070 *0.01*

2.05

2074

2.9

Sena

1.98 2.10

0.30 0.35

0.07 0.07

1.168*0.036 1.177 l0.020

6.60 6.00

4441 5547

3.6 4.0

8.62 6.42

1.07 1.11

1.133

5.50 5.61

625

Pierceville

Bruderhelm internalstandards

47.4 41.2

w

1.123

w

(a) Not determinable because of large trapped component. See footnotes to Table 1. Uncertainties are estimated as +5-10x for ‘He and “Ne abundances. and f l&15% for j8Ar and 40Ar. Uncertainties in 22Ne/z1Ne are ” 20 of the measured ratios. dances generally agreeing within 15%. In addition, two analyses of a Bruderheim laboratory standard gave noble gas concentrations which agreed within 5% with the average of four earlier analyses reported by BOGARD and CRESSY (1973). Abundances of radiogenic ““Ar and cosmogenic 3He, ‘INe and 38Ar are given in Table 2. The blank-corrected 3He is taken to be entirely cosmogenic in origin. Cosmogenie “Ne, 22Ne and 38Ar have been calculated by assuming a two component mixture of air and cosmogenic gas with 20Ne/22Ne = 0.90, “Ne/‘lNe = 1.10, J6Ar/38Ar = 0.66, according to the technique given by BOGARD and CRESSY (1973). Radiogenic 40Ar concentrations have been calculated from measured 40Ar , corrected for air argon on the basis of 36Ar. Although the calculated K-Ar ages possess uncertainties of the order k 15%, they demonstrate that only for the meteorite Bath Furnace. has Ar loss been appreciable. Ratios of cosmogenic 3He/2’Ne also imply that Bath Furnace, Scurry, and perhaps Bur-Gheluai, have lost 3He. Except for these meteorites, the 3He/2’Ne and 22Ne/2’Ne data in Table 2 show a nearly linear correlation which is in good agreement with the trends observed in previous chondrite analyses (EBERHARDT et al., 1966; WRIGHT et a/., 1973; NYQUISTet al., 1973). This systematic variation of 3He/21Ne with “Ne/‘lNe has been attributed by all these investigators to differences in meteorite shielding. Thus, we adopt the “Ne/‘lNe ratio as a measure of relative cosmic-ray shielding. Several of the meteorites in these tables have isotope concentrations which merit further discussion regarding applicability to our objectives. This is found in an Appendix to this paper.

COSMIC-RAY

SHIELDING

From the ‘noble gas analyses of cores from the Keyes chondrite (WRIGHT et al., 1973) and the St. %verin amphoterite (SCHULTZ et al., 1973), it is clear that 3He, “Ne and probably 3sAr production rates are l&40’% lower near a meteorite’s surface than at depths of most meteorite samples. In our view, it is not only erroneous to speak of constant noble gas production rates, it is premature to seek ‘average’ rates without exploring means of correcting produc-

tion rates for cosmic-ray shielding. More importantly, being able to correct ‘average’ production rates for the cosmic-ray shielding experienced by a specific sample will permit more accurate determinations of cosmic-ray exposure ages. In this section we discuss in detail the derivation of such shielding factors. Visual inspection of the 3He and *lNe variations in cores of the Keyes L6 chondrite and stone A of the St. SCverin LL6 chondrite suggest that production rate variations with depth may be described by rather ratio for a restricsimple functions of the “Ne/“Ne ted range of meteoroid masses (SIGNER and NIER, 1962; NYQUIST et al., 1973). Ideally, these variations should be independently calculable from both the Keyes and the St. SCverin data. Unfortunately, although the cores are up to 60 cm long, most Keyes points fall in the “Ne/“Ne range of 1.08-1.11, with only a few points out to 1.135, whereas most of the St. SCverin data lie between 1.10 and 1.15, with one 22Ne/2’Ne value at 1.193. Although there is appreciable overlap in ‘shielding’, neither meteorite can be used singly to determine the noble gas production variation over the 22Ne/21Ne range 1.08 to 1.20. We have assumed both meteorites exhibit the same variations in 3He, “Ne and 3sAr production rates with cosmic-ray shielding, and have combined the data from the two meteorites, normalizing St. SCverin to Keyes to account for the difference in exposure ages and possible interlaboratory bias. In the following figures data from other laboratories for St. SCverin are also included, adjusted by the same factor used for the measurements of SCHULTZ et al. (1973); these data were not used in the calculations, however, since we cannot presume the same noble gas isotope calibration among the various laboratories. Figure 1 is a plot of *lNe against the spallation 22Ne/2’Ne ratio. The Keyes data were taken from

PHILIP J. (‘~1:ss‘r.JR. and DONALL)D. BOGAN)

752

ratio

11.0

9.0 ?Ne)

10

scm3/g

50 / 1 05

1 10

I

1 15 ?Ne.’

in kqcs

01

I.08 was

ILund

at ttx

center

01’

a 56 cm long core from which 5 cm has been ablated at each end (LORI~ and POL 1+,41’. 197.1). The minimum ratio of I. 10 in St. S&verin was found al the center of II 35 cm long core ~~hich has lost only I 3 cm by ablaLioll. Thus a “Ne, “NC ratio < I .0X would imply a radius apprcciahly greater than 30 cm. equivalent to ;I spherical mass greater than 500 kg: such low ratio\ and large IU:I~~SZS al-e rare among chondrites. so aga~ii our Inahihty In estrapolate the shielding trend 10 Io~cr “NC ‘!Ne ratios should not alTect most anal\\es. The equation for the cur;,c in Fig. I is: i7 I NC) = ” Ne)~,,

1 20

1 25

339.4

-

jj4.j

(“NC

“Q)

+

231.5 tz2Nes2!r\r$:

,.>

In ;I previous paper ( BO(;AIU)and CK~SSY. Fig. I. Spallogenic “Ne vs “Nei2’ Ne in the Keyes and 19731 we argued that the Bruderhem~ sample anal\lzed St. Skverin chondrites. Solid points are for Keyes (WRIGHT for elemental noble gas production rates represents et al., 1973). Open circles are St. St-verin data from average shielding conditions for chondrites. Thus we SCHULTZ et al. (1973): crosses represent St. Sttverin data choose our Bruderheim “Ne; ’ I NC ratio. I. 1 14.as the from FUNKHOUSERcft al. (1967). MART-I ct (11. (1969). MILLER and ZAHRINGER (1969). and ZLEIRINGER (1968). All ‘standard average’. permitting us to utilize the HruderSt. Sbverin “Ne data were multiplied by an empirical fac- heim elemental productions directI>. hormalizing the tor. 1.74. to compensate for differences in exposure ages ‘INe shielding equation to this neon isotope ratio and possible interlaboratory bias (see text). yields 9 ’ 'Ne ‘shielding titctor’. F(” Ne). = 3x.27 62.53 (“Ne”‘Ne) + X.1 (“Ne,“Ne)‘. A\,cr;tge chondritic ” Ne production rates should be multiplied WRIGHT rt ul. (1973). Over short ranges of the spallaby this factor to correct for non-average shielding tion neon ratio. “Ne variations app ear to be very conditions. In accord with the above discussion. nearly linear. From the Keyes data at “Nej”‘Ne F(2’Ne) is taken to be I.151 for “Nc,“Ne < 1.08. 2 1.099, we calculate 2’Ne to be 8.92 x lop8 cm3/g and 0.818 for “Ne:“Ne ‘:, 1.70. and should be used at “Ne/“Ne = 1.10. From St. Skverin data with due caution at such extreme ratios. (SCHULTZ et al., f973) in the “Nej”Ne range of The “RAr data scatter loo much for any reasonably 1.10-1.14, we calculate “Ne to be 5.13 x IO-* cm3/g reliable independent estimate of a normalization facat a neon isotope ratio of 1.lO. All St. Stverin 2’ Ne tor for the St. %vcrin data. Some of the scatter ol data were multiplied by 8.92j5.13 = 1.74. The Keyes “‘Ar data is certainly dut: to the ditliculty in measurand St. Skverin core data were correlated by uning the various Ar isotopes :lnd corrcctinp for nonweighted polynomial regression analysis using a total spallogenic 3XAr. but a major factor is most likclq of 36 data points. The calculated first order (linear) Therefore WC used the better than the 0.46 for a linear correlation. In view of the second order fit for “Ne. a sirnilal explained by this second order equation. Higher order relationship seems reasonable for “XAr: “‘Ne -“Ar equations do not provide a better fit, and have no does not show any obvious depth dependence. and u priori justification. both nuclides are produced by relatively low cncrgy In Fig. 1 the “Ne production is assumed to be nuclear interactions. The curve also provides a more invariant above 22Ne/2’Ne = 1.20. The calculated conservati\,e shielding correction than does a linear relationship curves upward in this region, being unat high “Nc/“Nz relationship to “XAr production constrained by data. In fact. we expect that “Ne proratios. The equation for the second order lit is: duction continues to decrease as the neon isotope (3XAr) = 41.82 - 67.19 (“Ne:‘2LNe) + 27.77 (“NC ratio increases above 1.20. Such a trend is suggested ‘lNei2. This reduces to a shielding Etctor. F(““At.1 = by observed variations in multiple analyses of Narel38.0 - 61.3 (‘2Ne,:‘2’Nc) + 25.2 (“Ne.‘“Ne)‘. For Ian and Pokhra, with 22Ne/21Ne between 1.174 and ‘“Nej2’Nc < 1.08 OI L 1.20. F(“‘Ar) is taken to bc 1.30 (HERZOG and CRESSY, 1974), and is consistent 1. I89 or 0.728. respect ivcly. with the “jA1 trend reported by these authors. HowFigure 2 is a plot 01’ .‘Hc vs “Ne!“Ne Ibr the ever, insufficient data are available to extend the calKeyes and St. Si-verin data. The ” Ne normalization culations to higher neon isotope ratios; since only factor of 1.74 resulted in ;I “Hc discontinuity between a very few meteorites have experienced such low the trends from the two cores. The St. SCverin data shielding, this deficiency is not serious. Similarly, we in Fig. 7 were adjusted by 1.80. E\-en this factor was have no data to constrain the calculated curve at insufticient to bring other St. S&verin data into agrcc22Ne/2’Ne ratios < 1.08. The minimum 2’Ne/2’Ne =

0.91.

Cosmic-ray

304 1.05

1.10

1.15 t =Ne12’Ne)

1 20

1.25

ID

Fig. 2. Spallogenic 3He vs “Ne/“Ne in Keyes and St. Siverin. Symbols are defined as in Fig. 1. St. Skverin ‘He data normalized to Keyes by a factor of 1.80 (see text). ment with the core trends; the normalized data of FUNKHOUSER rr u/. (1967) lie about 70; below the linear correlation line, although following the same trend. The 4”,, discrepancy between “Ne and ‘He normalizations for the data of SCHULTZ et ~1. (1973) is in agreement with the systematic “Ne differences noted between the Zurich and NASA/JSC laboratories. The equation for the least squares fit is: 3He = 115.3 - 60.29 (22Ne/21Ne), which reduces to a shielding factor F(3He) = 2.385 - 1.244 (22Ne/21Ne). Higher order correlations were tested, but did not improve the 0.63 *r2’ fit, and we had no independent justification for a higher order fit. The production of 3He from meteorite target elements requires higher particle energy than is needed for “Ne and 38Ar production. As in the 21Ne case, the data for Pokhra and Narellan (HLRZOG and CREsst:Y, 1974) indicate that this trend continues to high “Ne/“Ne ratios. Application of these shielding factors to other than ordinary chondrites requires some knowledge of the effect of target element composition on the spallation 22Ne/2’Ne ratio. It is clear from BOCHSLER er al. (1969) and BOGARD and CRESSY(1973) that this ratio is directly correlated with Si content and inversely with Mg. These relationships are explored in Fig. 3. a plot of spallogenic “Ne/“Ne against Mg/Si weight ratio. The solid points are taken from the Bruderheim mineral separates (BOGARD and CRESSY, 1973); ‘Pyroxene-2 from that paper has a “Ne/“Ne ratio of 1.120, not 1.102 as reported therein. For a separate from a given meteorite sample, the 22Ne/2’Ne ratio is essentially equal to the “Ne produced from Mg and Si divided by the “Ne from these targets: “Ne

[*‘NelM, + [‘2Ne]si

"Ne

CZ’Nehg

+

["Nelsi

Thus the variation of 22Ne/21Ne with Mg/Si is certainly a curve. The 22Ne/Z1Ne range among the Bruderheim silicate samples is 1.09-1.14, with Mg/Si between 0.5 and 1.2, insufficient to support a second order regression. The straight line in Fig. 3, “Ne/‘lNe

=

1.174

-

0.069

(Mg/Si),

is highly

corre-

exposure

753

ages

lated with the Bruderheim data (I’ = 0.97). but is useful only as a lower limit for Mg/Si
= 1.25 -

0.25 ‘MS3

+ 0.1 ‘Msl’ [Si]’

WI

.

r2 = 0.99.

Dividing this by 1.114, the whole-rock Bruderheim neon isotope ratio (BoGARt and CKESSY, 1973), gives the correction factor: (Fza 2,)c,,cn, = 1.122 -

0.224

LWI + 0.09g$. ~ CSil

1.25 i

I1.20

\

P I$ 1.15 z

1.10I-

1.05IO

Eu I I

DE II

Ho I 0.4

1

LL L HAu _ ml 0.8

lJ I 1.2

IMgl/CSil

Fig. 3. Spallation 22Ne/Z1Ne vs Mg/Si weight ratio in separated mineral fractions. Solid points are from Bruderheim (BOGARD and CRESSY, 1973), open circles from Elenovka and crosses from Otis (BOCHSLER et al., 1969). Bold face symbols represent whole-rock samples from which mineral separates were obtained. The regression lines correlating “Ne/“Ne with target element chemistry are described in the text. Average Mg/Si weight ratios for eucrites (Eu), howardites (Ho), diogenites (D), enstatite chondrites (E), LL, L and H chondrites, aubrites (Au), and ureilites (U) are indicated along the horizontal axis.

PHILIP J. CKESSY. JR. and

154 3. Shielding

Table

factors

for noble gas production

rates

chondrite-chemistry equivalents, obtained by dividing (22Ne/21Ne), ,. by (F,, 2,) I ,;,. Table 3 summarizes the shielding factors for 3He, 2’Ne and ‘“Ar. For H, L, and LL chondrites, average noble gas isotope production rates should be multiplied by the shielding factors calculated for specific spallation 22Ne/21Ne ratios in order to obtain the production rates applicable to a specific sample’s shielding condition. For other stone meteorites, the chondrite-equivalent 12Nei2’Ne calculated from the MgjSi weight ratio should be used instead of the measured neon isotope ratio. Indiscriminate ratioing of the shielding factors in Table 3 would result in a 3He/2’Ne variation with 2’Nei2’Ne which decreases above 22Nei2’Ne = 1.18: (‘He/2’Ne),,. = 6.1. This conflicts with the existence of 3He/2’Ne ratios > 7.5. and is certainly due to our assumed constant 2’Ne shielding factor at 22Ne/2’Ne > 1.20, and

= 2.365 - 1. 244(22Ne/21Ne)c

F(31m)

F(21Ne) = 38.27 - 62.53(22Ne/21Ne), +26. l(22Ne/21Ne)f 22 Ne/%e

F = 1.181 for

< 1.08

= 0.818 for 22Ne/21Ne 2 1.20

F(38Ar) = 38.0 - Fl. 3(22Ne/21Ne), +25. 2(22Ne/21Ne); 22 Ne/21Ne < 1.08

F = 1.189 for

22 N.d2’Ne

= 0.728 for

2 1.20

(22Ne/21Ne)c = (22Ne/21Ne)ub> /(Fz2,>, jchem

In order to use chondrite-derived 2’Ne/2’Ne shielding corrections for other stone meteorites, the measured neon ratio must first be converted to their Table 4. “Ne,

DONALI) D. BO(;AKI)

“He and “‘Al in meteorites

with short

exposure

ages ~..____

I Appley Brldgc

48

t2r

5” If7

9’

>1.46,IOb

0.28’ 0 3”

I Barwll

51

62 8f7

7’

3 Bath Furnace

53

t4c

66.8f7

6

4 khrt

19

t5ti

i4b

66 8W.6

1.78t0.36 I68f”

1” 67,

31 ,056,

“61 +a.,4

,a 17,

c

I”14

0.276 0 296

I

0.89”

I.012

0 85

L

“50,

0 85, 0 501

” 998

“311

5 Ladder Ocek

39.7 22.4 e

“31

36.1 ?6 5

0.37 p

0 371

0.34 p

0 341

“.3RCl

0.381 mean=“.351

mean = 39.3+2

6 shaw

2”

I

3

66.8?7

t2a

6

68.7 f8.2

7 Bur-Ghri”a,

30.4fl

8 culllrun

41 8?4.8 e

8C

0.95 f”

“9 CO.*“,

0.37 +a04

1.082

0 203

“8 (0.18,

0.35 c

0.922

“38”

,a 37,

0.55 k

0 933

55.9 17.0’

0 84 f”

61.5f7

L.22’0.26

0

,“.“7)

k

0 88”

0 998 0 998

0 22

I

0.65 I

9 Ma,otar

379t,4e

61.5 t7.0

l.“2f”.“6

(0.21)

0.98

L

0.933

I “5”

0.63 p 1” “orland

40.5 f-2.” c

61.5f7.0

1.I5to.10~0.26,

I I Scurry

31.4 f2.2 f

61.53.”

0.76f0.08

0 27 c

0.675 0.933

0 289

“171; WI5,

0.55

“589

0 696 mean = 0 643

0 182 E

0 933

0 589

12sena

33.3t2.0c

52.3f7.0’

1.“8+“.LI

,a 17,

0 325, c

“933

0 34s

13 Timocllin

17.9?1

.51.5+70

“.37?“.“3

(0.06,

017e

0 933

0 L8Z

(0.43,

".R m

"755

1.059

0 23, c

".821

"la"

3e

5".9?2ls

M.6f64

1.66%16

I5 St.Martr

35

62.9+6.6

0 *7+0.0* (0.16,

16COld BotheM

12.9to.9 a

51.5f5.6

"31f""3,"."5~

17 Mighei

35.6+2."g

53.3t5.7

14

Adhi

Kot

t*a

0.3 " oil:"

I l*?ll.*2Kl26, 0700

0 366 0.743

0715

0.61 " 18 MUrChlso" 19 t4ogwa

37 6

+4h

54.6?6.0

f3a

a.3t5

Ma

53.8k5.9 58.7%.6

1.43f0.12~0.36~

2" Pok"

31

2, Crornap

43.3t1.7 a

2

1.2,f0.24 (0.35,

0 14* "903

0.781 mean =0.845

"33h

0 802

0.412

0.14ro.08

"."55 0

0.705

0.078

0.92f0.28~0.32,

0.405 0

"791

0x.0+ 0

0.908

"79 r

"512 0.661

WI

=

I

0.87"

22canskkab

49.4k4.3 s

63.7*7 6'

1.5910.32 (0.54)

0.892 E

0.998

0.894

23WUowdple

45 8f2.i

f

61.5+-7."

1.46to.18,".40,

1.11"

0.933

1.189

"845f""SX

I 1,41* set=, 0“7R \e,=, 0.512 t0.032 se,=I II661 \ct=

I “37 ” 847

0.927 I ““7

References: a. HEYMANN and ANDERS (1967). b. CRESSY (1970) corrected for ‘hAl recalibration. c. This work. d. TOBAILEM ef al. (1971). e. HERZOC and ANDERS (1971). f. HEIMANN et (11. (1974). g. FUSE and ANDERS (1969). h. BOGARD et al. (1971). i. HEYMANN (1965). j. FIREMAN and GOEBEL (1970). k. HINENBERGER et ul. (1964a). 1. HINTENBERGER et (II. (1966). m. ZKHRINGER (1968). n. KIRSTEN et ul. (1963). o. MAZOR et al. (1970). p. Z~~HRINCER (1966). 9. NYQUIST et al. (1973). r. VINOGRAWV and ZADOROZHNYI (1964). s. HERZOG and CRESSY (1974). t. HERPERS et nl. (1969). U. TAYLOR and HEYMANN (1969). v. BOGARD er al. (1973). * (Z6A1),,,, corrected for shielding by F, = 3.2 - 2.0 (22Ne/Z’Ne), HERZOC; and CRESSY (1974). **The first uncertainty is from the measured 26A1 activity only; the value in parentheses is compounded from measurement and production rate uncertainties. t *‘Ne in Cold Bokkeveld and Sena are averages of two analyses; “Ne in Grosnaja by MAZOR el al. (1970) obtained from same sample as 26Al activity; ‘rNe in Appley Bridge reported herein obtained from same sample as 26A1 activity; “Ne in St. Marks reported herein probably obtained from the same sample as Z6A1 activity. : ‘He values in parentheses were not used in regression analysis: see text.

Cosmic-ray exposure ages perhaps to some extent to our extrapolated 3He variation at high neon isotope ratios. In fact, a 3He/Z1Ne-22Ne/21Ne correlation derived directly from the Keyes-St. Severin data (with a 1.034 ‘bias’ factor for St. Severin neon) yields 3He/2’Ne = 12.71 (Z2Ne/21Ne) - 8.77, very nearly that obtained from the Keyes data alone (WRIGHT et al., 1973).

Neon-21 production

rates

We now use shielding corrections for 21Ne production from Table 3, shielding corrections for 26A1 production from HERZ~G and CRESSY(1974), relative elemental ‘lNe production rates from BOGARD and CRESSY (1973), and 26A1 elemental production rates from CRESSY(1971), to calculate absolute 21Ne production rates. We follow the basic approach adopted by HERZCG and ANDERS (1971), plotting “Ne against short exposure age determined from 26A1 disequilibrium. HERZOG and ANDERS used 26Al production rates from FUSE and ANDERS (1969) and a “Ne elemental production equation which was essentially a compromise between the results of STAUFFER (1962) and HINTENBERGERet al. (1964b). For normal chondrites the two 26A1 production equations yield very similar results. Since we and Herzog and Anders normalize measured ‘lNe contents in other chondrites to L-chondrite chemical composition (specifically Bruderheim in this paper), the differences in the 21Ne equations should have no significant effect on the effective calculated average L-chondrite 21Ne production rates. In this paper we have included additional ‘short age’ meteorite data which were unavailable to Herzog and Anders, and have corrected 21Ne and 26A1 production for shielding, via the 22Ne/21Ne ratio, techniques also unavailable to those authors. A compilation of 26A1 and cosmogenic data is given in Table 4 for all known meteorites for which both parameters indicate a cosmic-ray exposure age significantly short compared with the 26A1 half-life (7.4 x 10’ yr). A few meteorites (Beenham, for example) have apparent short ages from 21Ne, but 26A1 unsaturation is too slight to be statistically useful in computing an age for our calculations. The third column gives the “Al equilibrium activity and uncertainty calculated from the target element production rates of CRESSY(1971) and either a reliable chemical analysis for each meteorite or the average composition for the meteorite class (compiled by the present authors from modern chemical analyses). The production rates marked by an asterisk have also been adjusted for shielding by the formula of HERZ~G and CRESSY (1974). The fourth column lists 26A1 exposure ages calculated from the equation: T= -(l/i) In [I (z”Al)/(26Al).,,,~], with i the 26A1 decay constant. The first listed uncertainties arise only from 26Al counting statistics; those in parentheses arise from both counting and production rate uncertainties. The age uncertainty, oT, is compounded in conventional fashion, using ff (In x) = adx.

755

The column labelled ‘P_!,,,,’ gives the ratio of elemental 21Ne production in each meteorite to that in Bruderheim (BOGARD and CRESSY, 1973). Thus 2’Ne/ P.!,,,,, is the “Ne content each meteorite would have if it had Bruderheim’s chemical composition. The F(2’Ne) column lists shielding factors using the 21Ne equation in Table 3 and the 22Ne/21Ne ratio given in the previous column. Where reliable neon ratios were unavailable for the sample measured for 26A1, a shielding factor of unity was assumed (this was tacitly assumed in all cases by HERZOG.~~~ ANDERS, 1971). The (‘rNe)* column contains “Ne contents for each meteorite normalized to Bruderheim’s chemistry and shielding, = C”NelC~(“Ne)l/(~,,,,,,,). Uncertainties in (2’Ne)* reflect the variation in actual “Ne measurements where no individual measurement can be associated with a given 26A1 analysis. The uncertainty attributed to Adhi Kot arises from attaching a + 0.05 error to the reported “Ne value of 0.8. 1.2.

I

I

AK l.O-

Wi 8.3 Ca

0.8-

BF

MI

Fig. 4. [“Ne], normalized to Bruderheim’s chemical composition and nominal average chondritic shielding (“Ne/“Ne = 1.114), vs cosmic-ray exposure age determined from 26A1unsaturation. Age error bars were derived from 26A1counting uncertainties only. Error bars for “Ne represent the variation of multiple analyses around an average value for cases where no single analysis is obviously applicable. Solid circles represent meteorites for which *“Al and “Ne measurements were made on the same or necessarily proximate samples. This condition was either known to be untrue or was indeterminable for meteorites represented by open circles. The slope of the weighted least squares line, 0.473 (X lo-” cm3/g IO6 yr), is the 2’Ne production rate for Bruderheim’s chemical composition and average shielding. The hash-mark labelled ‘0.466’ indicates the L-chondrite *rNe production rate reported by HERZOG and ANDERS(1971); ‘0.377’ is the previously accepted average L-chondrite rate; ‘0.267 is the L-chondrite rate proposed by FISHER(1972). The meteorite designations are readily identified with the names in Table 4.

PHILIP J. CK~SSY.JR. and

756

The Brudcr heim-normalized ‘I Ne contents of the meteorites in Table 4 are plotted in Fig. 4 against exposure age determined from 26A1 unsaturation. The closed circles represent “Ne and 2hAl data taken from the same or necessarily proxunate meteortte samples; for the open circles such a determination could not be made. Vertical error bars are uncertainties in (2’Ne)* from Table 4. The horizontal error bars are age uncertainties from 26Al counting statistics only, plotted in this way for the purpose of clarity. Appley Bridge is not plotted in Fig. 4: its “Ne content is totally inconsistent with its reported ‘(‘Al activity, even without correcting the latter for this sample’s cosmic-ray shielding. This and other samples which were omitted from subsequent regression analyses are discussed in the Appendix. The data were analyzed by a weighted least squares calculation, demanding that C2’Ne] = 0 at T = 0. Thus, P(21Ne) = [,I (2’Ne)* (T) ’ (o)-~];X (G) ‘. where o is the computed uncertainty in (“Ne)*/T. The uncertainties used in this calculation are those due to both “Al counting statistics and ‘hAl production rate uncertainties (i.e. the age uncertainties given in parentheses in Table 4). The uncertainty in P(“Ne) is computed from (Ccr-‘)) “I. The data were divided into several groups for computation (Table 5). The calculated “Ne production rates are remarkably consistent. indicating that no one of the identified sub-groups of ‘short age’ meteorites is dominating the rate calculation. Groups A and B, for which data were obtained from the same. or probably same, sample. were combined to give an average “Ne production rate. for Bruderheim’s chemical composition (DUKE c’r cl/., I961 : BAAIISGAARD et al., 1961) of 0.473 (kO.032) x 10~~’ cm”/g 10” yr, with a very satisfactory reduced chi square of 0.8. With an adjustment for minor chemistry diffcrences, this results in an average rate for L-chondrites of 0.470 (iO.032) x 10m8. This result is in excellent agreement with the 0.466 x lO_ 8 L-chondrite rate reported by HERZOG and ANDERS (1971). supporting their implicit assumption that shielding variations among their samples were randomly distributed about ‘average’ shielding. Applying our average rate to the 2’Ne elemental production equation obtained from Bruderheim (BOGARD and CRESSY. 1973) gives the following production rate equation. in IO- ’ cm3 (STP)/g IO” yr: /‘(“NC) = [F(“Ne)] (0.074X [Mg]

DONAI.I)

D.

BOGARI)

i 0.00467 LSi] + 0.003.3 LS] + O.(jOO93 [Ca] +0.000239 [FeNi]) where F(2’Ne) is the shielding factor and concentrations are in weight per cent. Visual mspection of Fig. 4 suggests that the high age data may deviate systematically to the left of the correlation line. Higher order equations were examined. but no statistical improvement in fit was observed. It should bc noted, however, that high ‘“Al ages arc especially sensitive to the choice of (‘“Al) Even correcting for target element composition and the 22Ne,2’Ne ratio (He~zoc; and CRESSY. 1974: DRESSY. 1975) may still result in a lo”,, or more overestimate of the *“Al production rate in a given sample, due apparently to our inability to properly allow for the effect of pre-atmospheric size. If, for example, (“‘Al) for Adhi Kot were lo”,, less than that given in Table 4, its ‘hAl age would be 3.2 x 1Oh yr. shifting the data point in Fig. 4 to within the calculated slope uncertainty. The tendency of data for certain meteorites (c.g. Adhi Kot and Barwell) to be similarly displaced from the average slopes in both Figs. 4 and 5 supports the possibility that errors in (“Al) arc responsible for these deviations. Although a non-linear correlation is not justified by our data, we should note that a general _ lo”,, reduction in cosmic-ray Rux experienced by ‘short age’ meteorites. reiativc to ordinary chondrites. would still be consistent with the observed “Ne 2hAl data. The data were also examined by linear regression analysis for the possibility that (“Ne) + 0 when T = 0. We conclude that a non-zero intercept is not supported by our data.

Relative elemental “‘Ar production rates were reported by BOGARII and CRESSY (1973). Absolute rates can not be calculated with any confidence by the technique adopted for “Ne. Since 3XAr contents arc nearly an order of magnitude lower for chondrites th?n L “‘Ne contents. 3XAr abundances in ‘short age’ meteorites arc generally good to one significant figure at best. evjen ignoring sampling problems. In addition. the correction to measured “‘Ar for atmospheric argon is uncertain because of the contribution to total ‘hAr from the decay of undersaturated cosmogenic ‘hC1 (r, 2 = 3.1 x 10’ yr) during the first million years of exposure. We have used four approaches to calculate an average 38Ar production

Table 5. Productton rates of * P(“5a) ?.ample Group from Table 4

Identity of Group

1W8 cm’ g-l myr-’

A

2,3,6,7,12,18,20.21,22

26A1and21Nemeasuredonsamesample

0.462 *il.042

B

5,8,13,14,17,19

26 Al and 21Ne on comparable, ii not identical samplee

0.488 10.049

C

2,3,7,12,22

21 Ne corrected for (22Ne/21Ne)Sp; same sample

0.471 SO. 061

D

5,6,8,13,14,21

Insufficient data for (22Ne/21Ne,Sp cox-~~tion

0.478 to.043

16,17,18,19,20

Cl and C2 chondrites only

O.466 10.063

E

A+B (15 samples)

0.473 kO.032

Cosmic-ray

all dependent on the derived “Ne rate. We describe below four factors (F3siZ,) by which we multiply the Bruderheim elemental 3*Ar production formula to obtain a rate equation. 1. Most simply, the 38Ar elemental production equation in Bruderheim (BOGARD and CRESSY, 1973) is normalized directly to the 0.473 “Ne rate, yielding F 38121 = 0.0547. 2. Bruderheim has been thoroughly analyzed for noble gas isotopes by a variety of laboratories. The average “Ne exposure age at average shielding is 18.9 x lo6 yr. Applying this age directly to the Bruderheim 38Ar equation gives F38,2, = 0.0529. 3. The 38Ar content from the Bruderheim equation is 1.26 x 10e8 cm3/g. The average 38Ar content for Bruderheim, at average shielding, from a number of laboratories, is 1.37 x 1O-8 cm3/g. This yields F38,21 = 1.37/(1.26 x 18.9) = 0.0575. 4. A survey of 2’Ne and 38Ar analyses of 35 L-chondrites with reliable 22Ne/21Ne ratios between 1.08 and 1.14 gives F38,21 = 0.0557. There are enough uncertainties in 38Ar analyses that we are not compelled to select any one of these approaches. The average F38,2, is 0.0552 (+0.0019), yielding the following 38Ar elemental production rate equation, in 10-s cm3/g 10h yr: P(38Ar) = [F(3BAr)](0.143[K] + O.O203[Ca] + O.O045[TiCrMn] +0.00118 [FeNi]), with F(38Ar) the shielding factor from Table 3: and concentrations in weight per cent. The average L-chondrite 38Ar production rate is 0.0685 (kO.0024) x 10m8 cm3/g lo6 yr. rate,

Helium-3

production

rates

In many ways 3He has more potential

than either *‘Ne or 38Ar for exposure age calculations, a fact long recognized by many investigators. Its production rate is considerably less sensitive to depth than are lower energy products such as *‘Ne and 38Ar, being only 10% lower at 22Ne/21Ne = 1.20 than at average shielding conditions. At average shielding the 3He/2’Ne ratio is near five, making 3He the most abundant cosmogenic gas nuclide. The agreement of 3He analyses among different laboratories appears superior to that for 21Ne or 38Ar. Although the eleare not mental contributions to 3He production known with any real confidence, 3He does not appear to be nearly as sensitive to chemistry variations as are other nuclides, making it also less susceptible to z:unpling problems. Unfortunately, 3He is demonstrably susceptible to diffusion loss, especially in feldspar minerals in meteorites (MEGRUE, 1966; HEYMANN et al., 1968; HINTENBERGER et al., 1966; and others). In addition, there is a suspicion that 3He may be sensitive to meteorite size to an extent not accountable by the hardness “Ne/*‘Ne irradiation index. The 3He/21Ne-22Ne/21Ne trends observed by SCHULTZ et al. (1973) and WRIGHT et al. (1973) in cores of St. Severin and Keyes, respectively, define very nearly the same relationship, but do not agree nearly as well

151

exposure

ages

with

trends

noted

by

EBERHARDT et

al.

(1966),

NYQUIST et al. (1973) and HERZOG and CRESSY (1974)

from analyses of many meteorites. With these qualifications in mind, we have approached the problem of an average 3He production rate from several directions. 1. HERZOG and ANDERS (1971) found an average L-chondrite 3He production rate of 2.48 x 1Om8 cm3/g lo6 yr, by least squares regression of 3He against 26A1 exposure age. Only seven of their fourteen short-age samples were useful for their correlation, as five carbonaceous chondrites and two ordinary chondrites showed serious helium loss. Four of their seven samples lie more than le from their correlation line, and the standard deviation derived only from the fit of the seven points to the line, ignoring age uncertainties, is f0.69 x 10h yr (28”,, relative). Thus their 2.48 value does not inspire much confidence. Of the 23 samples in Table 4, six are carbonaceous chondrites, and were omitted in our analysis. Bath Furnace, Cullison, Scurry, and St. Marks had 4He ~500 x 10m8 cm3/g, indicating extensive outgassing, and apparent 3He production rates were less

t3He)’ lo-*cd/g 3.0

Wi

-BF

v

OO

I

I 1.0 26 AL AGE,

I 2.0

I

3.0

106YEARS

Fig. 5. C3He], corrected to average shielding where possible by the 22Ne/21Ne ratio, vs Z6Al-determined cosmic-ray exposure age. Age error bars are from 26Al counting statistics only. Open circles denote meteorites for which extensive outgassing is inferred from low “He contents. The two meteorites identified by crosses (Belfast and Malotas) do not, as far as can be determined, meet the ‘same sample’ criterion. Solid points represent samples meeting the ‘same sample’ criterion and showing no evidence of serious gas loss. The slope of the weighted least squares line, 2.27 ( x 10-a cm3/g lo6 yr). is the chondritic ‘He production rate for average shielding. The line segments labelled 2.48, 2.0, and 1.4 represent, respectively, the L-chondrite rate reported by HERZOG and ANDERS (1971), the previously accepted L-chondrite rate, and the L-chondrite 3He production rate proposed by FISHER (1972).

PHILW J. CRESSY,JR. and

75x

than I.4 x IO ’ cm’/g 10” yr; these were also omitted from the analysis. Of the remaining 13. Belfast also showed extensive outgassing of 4He (88 x 10-’ cm3/g), but “He was definitely not iow. Appley Bridge and Morland have anomalous ‘(‘Al ages. and were discarded. Willowdale was not used because the neon data indicated very heavy shielding, for which shieldand ‘(jA1 ing corrections are more uncertain, and noble gases were not measured on the same sample. Data fqr the ten ‘useful’ samples along with five of the rejected analyses are given in Table 4 and plotted in Fig. 5. Shielding corrections, F(3He), are calculated from the equation in Table i. Data were not corrected for chemistry because. as HERZOG and ANDERS (1971) pointed out, any reasonable adjustments to L-chondrite chemistry would be small. certainly less than the uncertainties arising from the scatter of the data. The weighted least squares line ‘through’ the ten acceptable points is 3.27 f 0.18 x IO-’ cm”,/g IO” yr. and is shown in Fig. 5. Data arc plotted with age uncertainties from counting statistics only, although the regression analysis used statistics involving ‘“Al production rate uncertainties as well (as in the 2’Ne case). Four of the ten points fall > 10 from the correlation line; the uncertainty calculated from the deviations of individual points from the iine is kO.71. as large as that from Herzog and Anders and certainly not encouraging much confidence in the derived relationship. 2. We plotted “He contents against 2 ‘Ne age for 24 L-chondrites selected only for having reliable “Ne/*‘Ne ratios. The “He contents were corrected for shielding. and “Ne ages were corrected for shielding ;111d minor chcmistr~ \ariatiom. The average 3He Table 6. Awragc

production

L

U.410

II.466

If

0.441

0.434

0.311

DOYALU D. BWARD

productlon rate from this set of data is 2.41 + 0.23 X 10.-x cm3/g lo6 yr. 3. We calculated the ratio of C3He] vs 21Ne age for 59 L-chondrites having 3He contents > 25 x IO- 8 cm3/g. guarding against the possibility that 3He may be lost for some time after the exposure initiating event. Such losses should have minimal effects on the “He contents of long age meteorites. For this set, 3He and 21Ne could not always be corrected for shielding, but uncertainties should be well randomized in such a large set. The mean ‘He production rate is 2.49 & 0.22 x 1O-8 cm3/g 10h yr. 4. We calculated the average, shielding-normalized 3He contents of Bruderheim. Keyes, and St. Severin, and divided by their respective exposure ages from “Ne and 38Ar, yielding P(“He) = 2.62, 2.59, and 2.60 x IO-.* cm3/g IO6 yr respectively, with a mean of 2.60. The four ‘mean’ rates are comparable, and a grand mean of 2.44 (kO.14) x 10eR cm3/g lo6 yr is calculated for L-chondrites, in excellent agreement with the rates given by HERZOG and ANDERS (1971) and KRUGER and HEYMANN(1968). Table 6 summarizes average chondrite ‘He. “Nc and 38Ar spallation production rates from this work, and compares them with previously published rates. The L-chondrite production rate uncertainties given above are all less than lpj,,. In view of the less definable uncertainties involved in arriving at these production rates, we believe & lo”, is a reasonable figure to use for most stone meteorites, applied with caution to certain achondrites having highly nonchondritic target element composition. Relative uncertainties in a given production rate among the various meteorite classes are believed to be only a few per cent. (Vari-

rates for stone meteorites,

(a,

0.0685

0.0526

(a)

0.0121

E

0.422

0.423

Cl

0.301

0.291

0.212

0.0626 @)

c2

0.368

0.358

Cl.288 rb)

a.4

0.452

0.442

0.343.0.358

HO

0.334

0.427

E”

0.234

0.376

Au

0.681

0.654

0.0518

O.a419(b~

in 10~ ” cmZg

IO” yr

2.44

2.48

2.6

2.39

2.39

2.5

2.38

2.34

2.5

2.49

2.94

2.0

0.0618

0.0534

(b)

2.45

2.13

0.0101

0.0639

@)

2.42

2.a

0.341 (a,

0.121

0.114

(a,

2.52

2.64

0.261

0.114

0.182

(a,

2.52

2.64

2.8

2.64

2.76

2.9

(b,

,a)

0.023 4.453

(32,

”

0.621

0.574

D

0.513

0.523

N

Cl.303

0.366

0.285

(C,

“.

An

0.266

0.313

0.291

(C,

0.361

0.031

0.0348

(b)

2 61

2.51

2.53

2.64

0.233 (C)

2.50

2.55

0.354 (C)

2.58

2.60

0.035 251

2.8

2.8

P(“Ne) = O.O248[Mg] + O.O0467[Si] + O.o033[S] + O.O0093[Ca] O.O00239[FeNi]. P(38Ar) = O.l43[K] + O.O203[Ca] + O.O045[TiCrMn]

This work:

+

+

0.001 lS[FeNi]. + 0.0266 (100 - [TiCrMnFeNi]). P(lHe) = 0,0174[TiCrMnFeNi] Production rates for chondrites are estimated to be good to within &lop;,; uncertainties for some achondrites may be slightly higher (see text). * L-chondrite production rates as given by Herzog and Anders; others calculated from these using average of modern chemical analyses. (a) HEYMANN et trl. (1968).

(b) MAZORof (I/. (1970). (c) GANAPATHY and ANDERS(1069).

Cosmic-ray

ations in 3He production rates given in Table 6 are only marginally significant.) For chondrites, the 21Ne production rates reported herein are in good agreement with those derived from HERZOG and ANDERS (1971), and reinforce their conclusion that earlier ‘INe production rates based on an average 3He production of 2 x 10-s cm3/g lo6 are in error by about 25%. Because of the different elemental ‘lNe productions used by those authors (primarily the Mg/Si production ratio), however, the average production rates of some achondrite classes are markedly different. Herzog and Anders did not extend their study to 3*Ar; the present work suggests a similar 5 25% increases in chondrite 38Ar production rates over previous values, with little change in achondrite rates. No attempt was made to document the 3He production rates by class based on the previously-cited 3H and 3He data. Only the results of HERZOG and ANDERS (1971) and KRUGER and HEYMANN(1968) are compared. In general, the comparisons are good, although the agreement with Kruger and Heymann is probably fortuitous, since it is difficult to defend their choice of average chondrite 3H specific activity. In calculating meteorite class 3He production rates, we were faced with the problem of estimating elemental production rates for 3He. In a previous paper (BOGARD and CRESSY, 1973), we reported that the Bruderheim mineral fractions would not yield unique elemental production rates except those from FeNi and S. Yet we found no compelling reason to use previously estimated relative production rates (MAZOR et al., 1970, used by HERZOG and ANDERS, 1971, for example). The Bruderheim 3He data for the pyroxene and olivine fractions was re-evaluated as a simple metal (=TiCrMnFeNi), sulfur, and silicate (= 100% - CSTiCrMnFeNi) mixture. The contributions of sulfur and metal were determined to be 0.56, and 0.366 x 10e8 cm3/g, respectively; P(S)/P(TiCrMnFeNi) = 1.53, given in BOGARD and CRESSY (1973). Subtracting these contributions from the silicate fractions yields P, (silicate) = 0.56 f 0.06 x 10m8 cm3/g, identical to PI(S). Normalizing this to the average L-chondrite 3He production rate of 2.44 x 10m8 cm3/g lo6 yr results in the elemental production equation given at the bottom of Table 6. The resulting production rates using this equation differ only slightly from those obtained using the formula of Herzog and Anders, except for Cl and C2 chondrites, which have lost 3He and thus provide no test of the different elemental productions. Both elemental formulas provide better agreement with observed C3He]/ T(“Ne) ratios than do the production rates of KRUGER and HEYMANN(1968). The present 3He production equation does not include a specific oxygen factor, because such a factor proved impossible to calculate (BOGARD and CRESSY, 1973). Therefore, we prefer to use the Bruderheim two-component observational model, allowing P,(O) to be swallowed up in the ‘catch-all’ P, (silicate). Although we have determined average production

exposure ages

759

rates for three spallogenic noble gas nuclides, we have already mentioned hazards in the application of 38Ar (sample inhomogeneity, measurement problems) and 3He (gas loss) contents for exposure age calculations. Consequently, we recommend that, where discrepancies exist among ages calculated from these three nuclides, ‘INe age be preferred as least susceptible to error, although for Ca-rich anchondrites 38Ar is largely spallogenic with production rates of the same magnitude as those for “Ne. *lNe and 38Ar production

in eucrites and howardites

The production rates of 38Ar and *lNe are strongly dependent on [Cal and [Mg], respectively (BOGARD and CRESSY, 1973). Eucrites and howardites have Ca/Mg ratios of about 1.5 and 0.5, respectively, characteristically distinct from each other and from the -0.08 ratio in ordinary chondrites. A plot of 3sAr vs *lNe for these achondrites (Fig. 6) should test the relative assignments of Pzl (Mg) and P38 (Ca), and should permit classifications in the absence of a chemical analysis. Chemical composition varies appreciably within each class. In the cases of meteorites for which chemical analyses are available (solid symbols), production rates were calculated from the elemental equations in Table 6. The observed 38Ar and *‘Ne contents were multiplied by the appropriate ratio of average to calculated production rate, effectively normalizing noble gas isotope contents to average eucrite or howardite composition. The lines extending from the solid symbols in Fig. 6 describe the direction and extent of this normalization.

,

1

8

I

I

0.744'/

Eu /

/1

’

73

10

(” NelW8cd/g in calcium-rich achondrites. The lines are exnected 38Ar/21Ne snallation oroduction rate ratios for e&rites (circlks) and ‘howardite’s (squares) from Table 7. Open symbols denote meteorites for which no adequate chemical analyses are available. For solid symbols, chemical data are available; the lines extending from these points describe the extent to which measured data would be altered by normalizing the individual meteorites to the appropriate average chemical composition. Specific meteorite designations are discussed in the text. The noble gas data are primarily from HEYMANNet al. (1968); Z~~HRINGER (1968), and GANAPATHYand ANDERS (1969).

Fig. 6. [38Ar],,, vs [*‘Ne],,

760

PHII IP J.

C‘RLSSY.

JR. and DONAI.II D.

The slopes of the Eu and Ho lines were obtained from the average production rates in Table 6. The dashed line is the Eu production ratio predicted by the relative elemental equations given in MAZ~R rf trl. (1970). The eucrite ratio derived from the present work fits the data rather well especially after chemical normalization. The Ho line from Mazor rf rll. coincides with our line; both describe the trend of data, although with a good deal more scatter than in the eucrite case. Some discrepancies can probably bc attributed to unreliable chemical analyses and to the polymict brcccia nature of howardites. The samples identified by number in Fig. 6 have been variously classified as both cucritcs and howarditrs in the literature. Since the 38Ar “Ne trends reflect the Ca/Mg diffcrences hetwcen cucrites and howarditcs, the position of an unanalyzed sample’s data on this plot should provide a chemical classification. Several spccitic cases are summarized below: I. Medanitos: Ho (MASOK. 1967: HEYMANNc’f r/l.. IY6Y). ELI(DUKE and SILVER, 1967). A recent chemical analysis by H~ITCHISON and SYMES (1973) clearly shows eucritic composition. The unaltered data point falls slightly to the right of the eucrite line, reflecting the above-average Mg content. -. 7 Bialystok: Ho (DUKE and SILVER, 1967). Eu (MASON. 1967; H~YMANN ef al.. 1968). No chemical analysis is available, but the data point falls conspicuously near the eucrite line. 3. Nobleborough: Ho (DUKE and SILVER, 1967). Eu (MAXI&, 1967: HEYMANNef (II., 1968). The data point falls exactly on the eucrite line. 4. Petersburg: Ho (DUKE and SILVER, 1967), Eu (MASON, lY67; HEYMANN rt al.. 1968). The 1861 chemical analysis in UREY and CRAIG (1953) is eucritic: the unaltered data point lies nearer the eucritie line than the howardite line. 5. Jodzie: Ho (MASON, 1967: DUKE and SILVER, 1967: HEYMANNrt u/.. 1068). The noble gas data of GANAPAI‘HYand ANDERS (1969) lie very close to the eucrite line. 6. Binda: Ho (MASON, 1967) Eu (DUKE and SILVFK. lY67: GANAPATHY and ANDERS, 1969). The 1913 chemical analysis in UREY and CRAIG (1953) is howarditic. Even without chemical normalization, the Binda data lie very close to the howardite line. 7. Luotolax: Ho (DUKE and SILVER. 1967), Eu (MASON, 1967: HEYMANN rt ~1.. 1968). The chemical analysis in UREY and CRAIG (1953) is eucritic, while that by WIIK (1969) is howarditic. The measured “Ar/“Ne ratio (plotted) is consistent with a 50-50 mixture of eucritic and howarditic material. SUMMARY

Cosmic-ray

shielding corrections to ‘He, * ‘Ne and have been derived utilizing the spallation *‘Ne/“Ne ratio (Table 3). The dependence of this “Ne/“‘Ne ratio on the Mg/Si target element 3XAr production

BOGARD

weight ratio has also been described (Fig. 3). After taking shielding and target element composition into account, *lNe concentrations and 2hAl exposure ages for 15 ‘short-age’ meteorites were subjected to weighted least squares regression analysis, yielding average “Nc cosmic-ray production rates (Table 5). Indirect methods using new “Ne exposure ages led to average ‘He and ‘8Ar production rates (Table 6). These production rates procide a means for chemically distinguishing eucrites and howardites. The resulting shielding-corrected noble gas elemental production rate equations provide the firmest basis yet for estimating accurate cosmic-ray exposure ages of stone meteorites from these noble gas nuclides. .-lchnon/rdll?lr,~r\ We wish to thank L. SIMMSl’or assistance in the noble gas analyses, and R. CLARKE of the Smithsonian Institution. C. MOORE of Arizona University. and 0. MANUI:L of the University of Missouri, for providing samples for this stud). REFERENCES ANDEKS E. (1963) Meteorite ages. In Thr Moon. Mereorrres ~lrlrl Con~rts, (editors B. M. Middlehurst and C. P. Kuiper), pp. 402-495. Umversity of Chicago Press. BAAIJXAARD H., C’AMPBEt.L F. A.. FoLtNsBEE R. E. and CUMMINC; G. L. (1961) The Bruderheim meteorite. J. Grop$l\. Rex 66, 3574m 3579. BOTHSLER. P., EBF.RHARI>TP.. GEISS J. and GROGLERN. ( 1969) Rare-gas measurements in separate mineral phases of the Otis and Elenovka chondrites. In Mrteorirr Rrsrtrrc/~. (editor P. M. Millman), pp. X57- 874. D. Reidel. BOC;ARD D. 0.. CLARK R. S., KEITH J. E. and RE\NOLIIS M. A. (1971) Noble gases and radionuclides in Lost City and other recently fallen meteorites. J. Groph~ Re.t. 76, 4076&4083. BOGARLI D. D. and CREW P. J. (1073) Spallation production of “He. “Ne and 3XAr from target elements in the Bruderheim chondrite. Geochim. Cosmochirn. .4crtr 37, 527~ 546. BO(;AKI)D. D., RE~NOLIXM. A. and SIWMS L. A. (197.3) Noble gas concentrations and cosmic ray exposure ages of eight recently fallen chondrites. Groc’kim. Cosmoc~lzi~n. .4cttr 37, 2417~ 2433. CRESSYP. J. (1970) Multiparameter analysis of gamma radiation from the Barwell, St. Skverin and Tatlith meteorites. Geochirn. C‘o.snwchim. Acttr 34, 771 779. CRESSYP. J. (1971) The production rate of AIZh from target elements in the Bruderheim chondrite. Grochim. COS~IOchink. Acttr 35, 1283 1296. CRESSY P. J. and RAN~IWLLI L. A. (1974) The umque cosmic-ray history of the Malakal chondrite. Earth Plunpt. Sci. Left. 22, 275-283. CRESSY P. J. (1975) “‘Al in cores of the Keyes chondrite. J. Grop/qx Res. 80, 1551 -1554. DUKE M., MAYNES D. and BROWN H. (1961) The petrography and chemical composition of the Bruderheim meteorite. J. Geophys. Rex 66, 3557-3563. DUKE M. B. and SILVER L. T. (1967) Petrology of eucrites. howardites and mesosiderites. Geochim. Cosn~rhim. Actu 31, 1637.-1665. EBBRHARD,I P.. EUGSTER 0.. GEISS J. and MARTI K. (1966) Rare gas measurements in 30 stone meteorites. Z. Nuturfo,sch. 21a, 414~ 426. FIREMAN E. L. and Gwatt. R. (1970) Ar.” and At-“” in recently fallen meteorites and cosmic-ray variations. .I. (;rwp/~,‘\. Rcs. 75, 2 I IS 2124.

Cosmic-ray

FISHER D. E. (1972) Cosmogenic rare gas production rates in chondritic meteorites. Earth Planrt. Sci. Lett. 16, 391-395. FLJNKHOUSER J., KIRSTEN T. and SCHAEFFER 0. A. (1967) Light and heavy rare gases in four fragments of the St. I- Severin meteorite. Earth Planet. Sci. Lett. 2, 185-190. FUSE K. and ANDERS E. (1969) Aluminum-26 in meteorites-VI. Achondrites. Geochim. Cosmochim. Acta 33, 653-670.

GANAPATHY R. and ANDERS E. (1969) Ages of calcium-rich achondrites-Il. Howardites, nakhlites. and the Angra dos Reis angrite. Gcwchim. Cow~uchinr. Acfu 33. 775m 787.

HEIMANN M., PAREKH P. P. and HERR W. (1974) A comparative study on ZhAl and 53Mn in eighteen chondrites. Geochim. C’osmochim. Acta 38, 217-234. HERPERS U., HERR W. and WOLFLE R. (1969) Evaluation 2hAl and special trace eleof 53Mn by (n 3;’) activation ments in meteorites by y:coincidence techniques. In Meteorite Research, (editor P. Millman), pp. 387-396. D. Reidel. III RTOG G. F. and ANDERS E. (1971) Absolute scale for radiation ages of stony meteorites. Geocl?im. Cosmochim. Actu 35, 605-611. HERZOG G. F. and CRESSY P. J. (1974) Variability of the Alz6 production rate in ordinary chondrites. Geochim. Cosmochim. Acta 38, 1827-1841. HEYMANN D. (1965) Cosmogenic and radiogenic He, Ne, and Ar in amphoteric chondrites. J. Geophys. Res. 70, 3735-3743.

HEYMANN D. and ANDERS E. (1967) Meteorites with short cosmic ray exposure ages, as determined from their AIZh content. Geochim. Cosmochim. Acta 31, 1793-1809. HEYMANN D., MAZOR E. and ANDERS E. (1968) Ages of calcium-rich achondrites-I. Eucrites. Geochim. CosmoIhim. Acta 32, 1241-1268. HIKTINH KW K H.. KOENIGH.. SCHI:LI~ L. and WKNKI; H. ( lY64a) Radiogene, spallogene and primodiale Edelgas in Steinmeteoriten. 2. Naturforsch. 19a, 327-341. HINTENBERGER H. KGNIG H., SCHULTZ L., WKNKE H. and WLOTZKA F. (1964b) The relative production cross-sections for ‘He and “Ne from Mg, Si, S, and Fe in stone meteorites. Z. Naturforsch. 19a, 88-92. HINTENBERGER H., KONIG H., SCHULTZ L. and WKNKE H. (1965) Radiogene, spallogene und primordiale Edelgase in Steinmeteoriten Ill. Z. Naturforsch. ZOa, 983-989. HINTENBERGER H., SCHULTZ L. and W~NKE H. (1966) Messung der Diffusionverluste von radiogenen und spallogenen Edelgasen in Steinmeteoriten Il. Z. Naturforsch. 21a, 1147-l 159. HUTCHISON R. and SYMES R. F. (1973) More data on the eucrite series. (Abstr.) Meteoritics 8, 46. KIRSTEN T., KRANKOWSKY D. and ZKHRINGER J. (1963) Edelgas-und Kalium-Bestimmungen an einer grossern Zahl von Steinmeteoriten. Geochim. Cosmochim. Acta 27, 1342. KRUGER S. T. and HEYMANN D. (1968) Cosmic-rayproduced hydrogen 3 and helium 3 in stony meteorites. .I. Geophys. Res. 73, 47844787. LORIN J. C. and POUPEAU G. (1973) Track studies in Keyes and Saint-SCverin chondrites. (Abstr.) Meteoritics 8, 410-41 I.

MARTI K., SHEDLOVSKY J. P., LINDSTROM R., ARNOLD J. R. and BHANDARI N. (1969) Cosmic ray produced radionuclides and rare gases near the surface of St. Skverin meteorite. In Meteorite Research, (editor P. M. Millman), pp. 246-266. D. Reidel. MASON B. (1967) The Bununu meteorite, and a discussion of the pyroxene plagioclase achondrites. Geochim. Cosmo(,him. Acfcl 31, 107-l 15. MASOK B. (1Y6Y) The Ladder Creek, Horace, and Tribune meteorites (Greeley County, Kansas). Meteoritics 4, 240-243.

exposure

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MAZ~R E., HEYMANN D. and ANDERS E. (1970) Noble gases in carbonaceous chondrites. Geochim. Cosmochim. Acta 34, 781-824.

MEGRUE G. H. (1966) Rare-gas chronology of calcium rich achondrites. J. Geophys. Res. 71, 4021-4027. MUELLERH. W. and Z.&HRINGER J. (1969) Rare gases in stony meteorites. 1n Meteorite Rcwcrrch, (editor P. M. IMillman), pp. 845-856. D. Reidel. NYQUIST L., FUNK H., SCHULTZ L. and SIGNER P. (1973) He, Ne and Ar in chondritic Ni-Fe as irradiation hardness sensors. Geochim. Cosmochim. Actu 37, 1655-1685. PAREKH P. P., HEIMANN M. and HERR W. (1973) Role of Mn-53 in meteoritics. (Abstr.) Meteoritics 8, 63-64. SCHULTZ L., PHINNEY D. and SIGNER P. (1973) Depth dependence of spallogenic noble gases in the St. SCverin chondrite. (Abstr.) Meteoritics 8, 435-436. SK;MK P. and NIIK A. 0. (1962) The measurement and mterprelatlon 01 rare gas concentrations in iron meteorites. In Researches on Meteorites. (editor C. B. Moore), pp. 7-35. Wiley. STAUFFER H. (1962) On the production ratios of rare gas isotopes in stone meteorites. J. Geophys. Rex 67, 2023-2028. TAYLOR G. J. and HEYMANN D. (1969) Shock, reheating, and the gas retention ages of chondrites. Eurth Planet. Sci. Lett. 7, 151-161. TOBAILEM J., NORDEMANN D. and GRIEBINE T. (1971) Gamma emitters and exposure age of the Bovedy meteorite of the fall of April 25, 1969. Nature Phys. Sci. 229,

118-119.

UREY H. C. and CRAIG H. (1953) The composition of the stone meteorites and the origin of the meteorites. Geoc&m. Cosmochim. Acta 4, 3682. VINOGRADOV A. P. and ZADOROZHNYI I. K. (1964) Inert gases in stony meteorites. Geokhimiyu 7, 587-600. WIIK H. B. (1969) On regular discontinuities in the composition of meteorites. Comment Phys. Math. 34, 135-145. WRIGHT R. J., SIMMS L. A., REYNOLDS M. A. and BOGARD D. D. (1973) Depth variation of cosmogenic noble gases in the _ 150 kg Keyes chondrite. J. Geophys. Res. 78, 1308-1318. Z~HRINGER J. (1966) Die Chronologie der Chondriten auf Grund von Edelgasisotopen-Analysen. Meteoritika 27, 25-40. Z#HRINGI.K

J. (1968) Rare gases in stony

chim. Cosmochim.

Acta

meteorites.

Gm-

32, 209-237.

APPENDIX Several 4 deserve Table

of the meteorite samples further discussion.

listed in Tables

1 and

1

The ‘Ladder Creek’ sample analyzed for noble gases has obviously had a long cosmic-ray exposure, in contrast to the short age deduced from the ldw levels of 3He and ‘lNe reoorted bv NYQUIST et al. (1973). Z;~HRINGER (1966) and H&ENBER&ER e; al. (1964ai MACON (1969) reported that. from mineralogical evidence. Ladder Creek, Tribune. and Horace no. 2, all L6 chondrites found m the same area of Kansas, were most probably fragments of the same meteorite. Clarke (private communication) of the Smithsonian Institution identified our ‘Ladder Creek’ sample as part of Horace no. 2, which clearly should not be considered part of the Ladder Creek meteorite any longer. On the other hand, the noble gas analysis of Tribune by ZKHRINGER (1966) certainly supports its identity with Ladder Creek. The Pierceville meteorite was selected because of an old analysis by HINTENBERGER et al. (1965): 3He = 0.03, 2’Ne = 0.30 x lo-* cm3/g. Our 16AI and noble gas results

762

PHILIP J. CRESSY.

JR.

and DONAI.I) D.

mdicate a much longer exposure age. Either two different meteorites were analyzed, or the analysis of Hintenberger et al. reflects massive gas loss (aHe/*‘Ne = 0.1). Eva was examined because of the low ZhAI activity (42 dumikg) reported bv PAREKH et al. (1973). The noble gas i&top& were measured first, with the intention that “AI would be measured on the same sample if the cosmogenic noble gas contents supported a short exposure age. They clearly do not; the low 2hA1 activity is most likely due to low shielding. Cavour, an H6 chondrite, has an essentially saturated level of 26AI, suggesting an exposure age on the order of 3 x IO’ yr or more. Yet the 3He and *‘Ne data in Table 1 indicate an age of about I.5 x IOh yr. in which time ‘hAI should only have built up to around 45 dpm!kg. An analysis of the same sample by HINTENBERGERet (I/. ( 1965) shows the same He and Ne isotope concentrations. There appears to have been some radiogenic gas loss, and spallation 3He/Z’Ne falls somewhat below the “Ne/“Ne correlation lines of EBERHARDT et al. (1966) and WRIGHT cr trl. (1973). Cosmogenic gas loss is the most likely cause of the discrepancy, but the possibility of an unusual cosmic-ray flux can not be overlooked (see discussion of Morland). Noble gas analyses of Beenham by HINTENBER~~ERet (11.(1965) and ZAHRINGER (1966) indicated an exposure age on the order of 1.5 x 10h yr. The measured 2hAI activity in Table I is more than 907, of the expected equilibrium value, implying an age of greater than 2.5 x IOh yr. A recent analysis of this meteorite by NYQLJIST et ul. (1973) yields a “Ne content quite similar to those reported earlier. but a ‘“Ne/“Ne spallation ratio of 1.186, giving a shielding-corrected age of 2.3 x IOh yr (using techniques presented in this paper). Thus, although Beenham’s radiation age is technically short relative to the Z6AI half-life. it is too long to be useful in the present investigation. Tuhle 4 Several other meteorites were omitted entirely from the regression calculations, although they are plotted (except for Appley Bridge) in Fig. 4. Cold Bokkeveld fell as a shower in 1838. Although the noble gas data from several samples agree reasonably well, the relationship between the one sample counted for ZhAl and those analyzed for noble gases is unknown: the shielding factors could be very different. Scurrv. Belfast and Willowdale had 22NeiZ’Ne ratios < 1.07, indicating great shielding. There is no reason to believe the noble gas isotopes and 2hA1 were measured

BOGARII

on the same samples, and even if they were, there IS no reason to equate the shielding effects on “Ne and *‘Al even approximately at such great depths. Malotas fell as a shower in 1931. There IS no known relationship between the samples analyzed for “Ne and that measured for *‘Al. Since the “Ne results are grossly different, one should expect that ZhAl specific activities may also vary widely among the many fragments. The Malotas point in Fig. 4 includes the lower “Ne value, from ZHRINGER ( 1966). Although St. Marks appears to fulfill the ‘same-sample‘ criterion (see text), this sample was not used in the production rate computations only because. with its small age uncertainty and large production rate deviation. it would cxcrt a singularly large and unwarranted influence on the average rate, lowering it to 0.439 x 10mH cm’/g IOh yr. The rate from St. Marks alone is 0.322 + 0.059 x IO-‘. This is some 2.50 smaller than the average without St. Marks. a deviation that is not statistically acceptable. For example. usmg Chauvenet’s criterion for rejection of subpetted values a maximum deviation of only 2rr is permitted for 15 samples: a deviation of 2.50 IS not expected foi less than 40 measurements. Appley Bridge and Morland. although titting the ‘same sample’ criterion, lie nowhere near the other short age sample data. and can in fact also bc rejected by Chauvenet’s criterion. The “He and ““Ar contents of both imply no gas loss. The analyses of HFYMANN (1965) on Appley Bridge and HINTENBERGER(11u/. (1964a) on Morland agree rather well with our results. both in cosmogenic and in radiogemc gas isotopes. In the absence of outgassing, we conclude that the “Al contents of these meteorites refect exposure to a high cosmic-ray flux. almost certainly in the period following the collision which initiated cosmic-ray exposure. Any pre-collision contribution to measured ZhAl would require a near-surface location in that body. which would also result in inflated “Ne contents. CKFSSY and RAN(~TEI.I.I (1974) concluded from cosmogenic isotope data that the Malakal chondrite was exposed to a cosmicray Ilux some three times larger than that incident on most chondritcs. If Morland were cvposed to a similar Hux. its ‘hAl age would be 0.26 myr. This leads to a “Ne production rate by the high tlux of I.04 x IO s cm/g IO”. more than 2.1 times greater than expected for an H chondrite exposed to a normal Ilux. Similarly. the “He production rate is approximately twice that cxpccted and “aAr about three times. Similar treatment of the Appley Bridge data yields qualitatively similar results. hut the high “Ne,,“‘Ne ratio in this sample lies outside the region of confidence for shielding corrections.