Noble gas concentrations and cosmic ray exposure ages of eight recently fallen chondrites

Geochimica et Cosmochimica Acta, 1973,Vol. 37,pp. 2417to 2433. PergsmonPress. Printedin NorthernIreland

Noble gas concentrations and cosmic ray exposure ages of eight recently fallen chondrites D. D. BOGARD, NASA

M.

A.

REYNOLDS

and L. A. SIMMS*

Manned Spacecraft Center, Houston, Texas 77058, U.S.A.

(Received 2 January

1973; accepted in revised form 24 April 1973)

Ab&&--Abundances and isotopic compositions of He, Ne, Ar, and Xe have been measured in eight recently fallen chondrites. Ratios of concentrations of cosmic ray-produced sHe, srNe, szNe and 3*Ar indicate that all eight samples experienced less than overage cosmic ray shielding. sHe and srN0 exposure ages were calculated using shielding corrected chondritic production rates and the measured 22Ne/21Ne. Exposure ages calculated from 22Na/22Ne and 2BA1/21Ne ratios and constant relative production rates show a bias between the two ages due to variations in 22Na/26Al. Arguments are presented that this bias is due to irradiation hardness differences, and therefore the use of constant values for both the 22Na/22Ne and 26A1/21Ne production ratios is not permitted. Dwaleni, Swaziland, was found to be an unusual gas-rich chondrite with high concentrations of solar-derived He and Ne and planetary-type Xe. INTRODUCTION

AND EXPERIMENTAL

MEASUREMENTS of the abundances

of nuclear reaction-produced noble gases and radionuclides in meteorites can yield considerable information about the ages, origins and past histories of these solar system objects. Essentially all estimates of exposure times to galactic cosmic rays are based on measured concentrations of one or more cosmogenically produced noble gas isotopes. Although a large amount of data has been accumulated, many details of meteorite exposure histories are still not well-understood. Within the past few years, measurements of cosmogenic radionuclides in stone meteorites have become common enough to offer, in conjunction with noble gas data, the possibility of increased accuracy in the calculation of exposure ages. As several of the radionuclides of interest are short-lived, part of this information is detectable only in recently fallen meteorites, In order to calculate cosmic ray exposure ages and to complement radionuclide data obtained in other laboratories, we have determined abundances and isotopic compositions of the noble gases in several recently fallen stone meteorites. These noble gas measurements also serve to identify unusual characteristics that may occur for some of these meteorites, and thereby suggest additional studies. A portion of this data has already been reported (BOGARD et al., 1971; BOGARD et ab., 1972). Names and pertinent information for the investigated chondrites reported here are given in Table 1. All specimens used were small (~1 g) samples obtained through the generosity of R. Clarke, Jr. of the Smithsonian Institution. Recovered masses of all meteorites listed in Table 1, except Barwell and Kabo, were less than 10 kg. Dwaleni, Swaziland proved to be an unusual type rich in solar-derived noble gases (REYNOLDS et al., 1971). Further details of these meteorite falls may be found in reports by CLARKE (1971, 1972). RESULTS

Noble gas isotopic abundances were determined with an all metal, statically operated, mass spectrometer equipped with an electron multiplier. Techniques and extraction aonditions * Also Northrop Services, Inc., Houston, Texas 77068, U.S.A. 2417

D. D. BO~ARD,M. A. REYNOLDSend L. A. SJIBQJS

2418

Table 1. Recently fallen chondrites analyzed for noble gases Meteorite Malakal, Sudan Dwaleni, Swaziland Nejo, Ethiopia Kiffa, Mauritania Kabo (Gwarzo), Nigeria Wethersfield, Connecticut Barwell, England Trtthlith, Saudi Arabia

L H L H H L L L

Approx. wt. recovered (kg)

Fell

Type

“Aug. 12, Oct. 12, May 11, Oct. 23, Apr. 25, Apr. 8, Dec. 24, Oct. 5,

1970 1970 1970 1970 1971 1971 1965 1967

2 3.2 2.4 <2 210 0.36 47 2.5

used were standard ones and have been previously described (BOCUIZD and Cnxss~, 1972; Wamar et al., 1972). Instrument sensitivity was monitored with small quantities of the noble gases delivered from calibrated pipettes. Noble gas extraction blanks were monitored periodically. Noble gas isotopic abundances measured in these samples are given in Table 2. Multiple analyses were performed for 8ll semples. However, a few analyses were made on small mass samples and are consideredless accuratebecause of small spectrometersignals and larger bhmks. Uncertainties in measured concentrations due to variations in spectrometer sensitivity are 13 per cent for He, Ne, and &lS per cent Xe, based on variations in a large number of noble Table 2. Noble gas abundances (lo-* cm3STP/g) Relative sensitivity variations based on standard gas calibrationsm &3% for He, Ne and Ar, and &IS% for Xe. Gas concentrations have been corrected for mass discriminationand for system blanks Meteorite Malakal chondrule Dwaleni light phaee dark phase Nejo Kii% Kabo light phase dark phaee whole rock Wethersfield Barwell Tathlith

wt. (mg.) 3He 355 441 32.8

5.40 5.03 4.79

302 51.1 481 71.3 372 18.6 304 18.0 62,s 33.4 305 34.4 9.78 251 29.0 9.73 9.80 304 309 7.89 282 7.63 348 6.04 62.0 5.62 20.8 249 20.4 60

*He 72.9 41.0 98.1

“ONe SIN0 22Ne 2.02 2.11 -

1.55 157 158

e=Ar

1.85 0.68 1.85 0.60 1.68 <1.5*

8sAr

40Ar ls2Xe

0.30 0.30
268 0.062 283 139*

5160 114,080 790 700 1740 1710

23* 570 259 2.51 4.9* 5.11

7.89 9.79 1.61 8.17 51.1 21.2 2.62 3.03 0.69 2.58 2.95 0.65* 451 6.36 l-O* 5.07 5.94 1.49

1.31 5.74 0.39 0.44 1.04 I.05

5260 0.011 5526 0.046 4375 0.015 4220 5680 0.003 5430

949 1100 928 164 146 1066 860 1630 1670

1.5* 33.9 1.75 1.71 3.28 1.6* 2.27 53.8* 3.32

157 1.9 I.61 1.68 1.53 0.92 0.85 3.02 3.11

0.51 0.84 059 0.38 0.30 0.37 0.74 0.89

4815 5200 4775 860 0.022 383 5140 0.017 5950 0.020 -

1.76 5.19 1.88 1.86 1.95 0.96 1.14 3.35 3.80

1.23 3.34 1.70 1.08 OS1 0.82 1.28 2.4*

* Neon and argon abundances have been corrected for blanks exceeding 20% and are consequently less accurste. Blank corrections for most other concentrations of cosmogenic 21Ne and 3*Ar and radiogenic$He end 4oAr were less than a few per cent.

Noble gas concentrations and cosmic ray exposure ages

2419

gas standard analyses during the same time period. In addition, there is a 2-3 per cent uncertainty in the absolute calibration of our gas pipettes. Blank corrections were generally not significant for the main spallogenic isotopes *He, 21Ne, and 3sAr, but in several oases were appreciable on sONe, ssAr and 4OAr. Those cases of larger blank corrections are indicated in Table 2. Several meteorites show small excesses of s”Ne and 36Ar above those expected from spallation reactions; these excesses may be due to either adsorbed air or to trapped planetary gases. Kabo, Barwell, and Dwaleni obviously possess trapped gas, which in the case of Dwaleni is solar-like in composition. These trapped gases are discussed later. Isotopic abundances of Xe were measured for five meteorites and are reported in Table 3. Ratios of 12sXe/132Xewere measured for two additional meteorites. Exoept for mass 129, all measured Xe isotopic spectra are consistent with a mixture of trapped planetary Xe, cosmogenic Xe, and atmospheric Xe. ls*Xe shows varying excesses of the decay product of extinct lzeI, which for six of these meteorites ranges 5-20 x lo-l2 cm3 STP excess lzBXe/g (Table 4). These 12eXe excesses are typical of those determined for other ordinary chondrites (Z~~HRINQER,1968). Table 3. Isotopic composition of Xe measured in some recently fallen ohondrites (relative to lgeXe = 1.00) Uncertainties given are one sigma of multiple measurements. Instrumental discrimination corrections of 05 %) per mass relative to mass 132 have been applied Meteorite

‘24Xe

Dweleni light

-

Dwsleni dark

0.004 & 0.001 -

Nejo

Air NIER (1950) AVCC Enasm~ et d. (1967) Lunar surface

et al.

(1971)

0.004 fO.001 -

-

0.007 f 0.001 -

Malaksl

PODOSEK

-

-

Tathlith

1*6X0

0.0036

0.009 *0.001 -

1aaxe

=oxe

0.080 f 0.005 0.079 fO~OO1 0.077 f0.002 0.077 *0.001 0.084 f 0.005 0.082 f 0.002

1.134 f 0.006 l-140 f 0.007 1.154 *0.011 1.085 -&IO.008 1.059 fO*OO2 1.444 fO.016

0.0714

0.0034

0.0046 * 0.001

0.0041 f 0~00004

0.0820 f 0.0004

0.0048 & 0~0001

0.0043 fo~oool

0.0839 fo~ooos

1*oxe

wxe

1=X0

0.817 f 0.007 0.822 f 0.003 0.799 f 0.006 0.798 kO.004 0.819 f 0.002 0.805 f0.014

0.152

0.788

0.388

0.1605 &0~0011

0.817 f 0.003

0.382 fO.002

0.321 f 0*002

0.1649 &-to.0006

0.823 rto.005

0.371 f 0.002

0.3011 f0~0014

0.983

1.050 f 0.005

‘31Xe

0.159 f0.004 0.159 *0.001 0.157 f0.002 0.157 f 0.002 0.161 f 0.004 0.160 hO.001

0.383 ho.006 0.383 &O.OOl 0.384 &0.004 0.387 fO.005 0.385 fO.002 O-389 ~0~001

0.322 f 0.003 0.321 *eoo1 0.329 5 0.004 0.327 f0.003 0.324 fO.002 0.324 fO.004 0,330

Table 4. Radiogenic noble gases and gas-retention ages. a40Arand “He are f5-lo%, is * 15%. Ages possess an estimated analytical uncertainty of f 10-15% Chondrite

4oAr 10w6cm3/g)

lzgXe

K-Ar age* (109 Yr)

4He (IO+ cm3/g)

He age? (lo9 yr)

(1O-l2 cm3/g)

lngXe

Malakal Dwaleni (light) Nejo Kiffa Kabo

1.4 50 42 54 43

0.4 4.3 3.9 4.3 4.2

0.7 7.0 17.3 9.4

0.1 I.7 4,2 2.8

250 10 7 6 -

Wethersfield Barwell Tathlith

3 55 63

0.7 3.8 4.7

1.5 IO.5 15.5

0.4 2.3 3.1

10 20 9

* K abundances from CRESSY (1970), Rancitelli (personal communication, 1972), and Jarosewich (personal communication, 1972). t U abundances assumed: Type H = 12 ppb, Type L = 17 ppb (FISHER, 1972).

D. D. BOUARD, M. A. REYNOLDSand L. A. SIMMS*

2420

From the concentrationsof radiogenic*He and *sAr (Table 2), the 4”K--4”A.rand U, Th-4He gas retention times for these meteorites are estimated in Table 4. The K abundanoes utilized were measured in other labomtories and are generaSlytypical of ordinary ohondrites. Measured U and Th abundances were not available and we adopted type H and type L chondrite averages given by FISEER (1972). Although the ages given possessandytioal and sampling uncertainties of the order of 110-15 per cent, some obvious differences among meteorites exist. Six meteorites show K-Ar ages in the range 33-4.7 x lo9 yr, and probably are not distinguishable within ~ce~~~ti~. The *He s,ges for four of these samples are ~~e&ntly lower, implying some gas loss. Malakal and Wethersfield show very low Q@Arand 4He ages, indicating severe radiogenic gas loss. These two meteorites may have been involved in an event of episodic gas loss postulated to have occurred to several type L chondrites approximately 0.5 x lo9 yr ago (HEYXANN, 1967; FISHER, 1969). The Ucerct, Venezuela chondrite (fell January, 1970) also possesses low, nearly concordant 4OArand 4He gas retention ages (BOCA~Det I%?.,1971). As will be discussed later, there is also strong indication of loss of cosmogenically-produced 3He for Malakal and Wethersfield. Calculated concentrationsof the major spallation produced isotopes in these meteorites are given in Treble5. Abundances of sHe are the metbsuredvalues after small (gene~y less than 1 per cent) blank corrections have been applied. Concentrations of 21Ne, 22Ne and %Ar have Table 5. Calculated oosmognetic noble gases ( 10es cm3 STP/g). Gas concentrations have been calculated from the raw d&s, according to the method given in the text. Abundance uncertainties represent the f3% sensitivity variation or the spread in values for duplicate ancmlyses, whichever is greater Meteorite M&&i% Dwaleni Nejo Kiffs Ksbo Wethersfield Barwell Tathlith

3He

2rNe

3sAr

5-21 rto.19 50.1 11.5 18.3 kO.6 339 f1.0 9.79 10.29 7.79 &O-23 590 10.20 206 fO.6

l-55 +0.05 732 10.24 2.60 10.08 4.90 &O-40 l-60 fIo.05 162 50.05 0.89 Ito. 3-05 Ito.

0.216 &0.007 I.16 10.04 0.33 10.03 090 f0.05 0.330 f0~010 0.22 kO.02 0,265 10.02 0.55 50.03

3.36 &0.16 6.41 -10.27 7.04 &0.32 692 &to.60 6.11 i-026 4.80 l 0.20 6.62 1-0.37 6.75 $0.28

1.163 &O.OOS 1.14 40.02 I.15 +0*01 1.175 +0.015 1.155 10.015 1.12 +0.02 1.14 *o-o2 l-17 rto.01

been calculated from the non-blank corrected values under the assumption of a two component mixture of trapped plus spa&&ion gases (BO~ARDand CRESSY,1973). The trapped component was given an atmospheric isotopic composition; the cosmogenic component was taken to be 22Ne/2rNe = 1.10, 2’JNe/22Ne= 0.85, and s6Ar/3sAr = 0.66. Reasonable uncertainties in the exact spallation composition assumed generate <2 per cent ~ce~&inti~ in the calculated concentrations of cosmogenio 2xNe, “Ne &nd 3*Ar. System blanks possessed near-atmospheric values of the above isotope ratios. The concentrationsof Table 5 are calculated averages of the multiple whole rock analyses (Trtble 2), except that the larger samples analyzed for Kiffa, Barwell, and Tathlith were given twice the relative weight of the smaller samples, for reasons previously mentioned. Unee~ties given are either the 13 per cent spectrometer sensitivity varistion, or represent the range of calculated spallation concentrations obtained for the

Noble gas concentrations

and cosmic ray exposure ages

2421

multiple analyses, whichever is greater. For Dwaleni light phme, a 2 per cent correction was applied to the observed 3He to correct for small amounts of solar 3He (assuming the presence of 2 x 10B5 cm3 radiogenic 4He and trapped solar gas with 4He/eHe = ~,~~&-BLAcE, 1970). Also, for this sample, the cosmogenic 22Ne wae calculated assuming trapped 2eNe/*2Ne = 12.5, which introduces an additional indeterminant uncertainty in the calculated 22Ne/elNe.

COSMIC RAY EXPOSUREAGES

A standard method of calculating exposure times of meteorites to galactic cosmic rays, i.e. the time of their existence ss small objects of less than about two meters diameter, is to divide the measured concentration of a cosmogenic noble gas nuclide (usually 3He, 21Ne, or 3sAr) by an empirically-derived production rate for that nuclide. Normally, the production rate used is a mean value determined for many meteorites of a given chemical class, and therefore averaged over a range of shielding conditions and meteorite orbits. Another technique is to employ the concentrations of a radionuclide/stable nuclide pair measured for the sample (e.g. 3H/3He, 22Na/22Ne,3sAr/38Ar,*lKr / *OKr, etc.) and an assumed relative production rate for these nuclides. Reviews of some of these techniques and their limitations are given by ANDERS (1963), EUGSTERet al (1967a), HERZO~ and ANDERS (1971). A number of factors can create variations in production rates of cosmogenic nuclides among different meteorites. Relative differences in average production rates of stable noble gases which arise from chemical differences among the various classes of stone meteorites now appear to be reasonably well-defined, especially for ordinary chondrites (BOGARD and CRESSY, 1973). Variations in production rates arising from differences in shielding conditions are less well-known but definitely exist (EBERHARDT et al., 1966; NYQUIST, 1969; NYQTJIST et al., 1973). Possible effects of different meteorite orbits on production rates is highly uncertain, especially as orbital parameters are known for only two meteorites (FORMAN et al., 1971). In order to calculate exposure ages for the meteorites listed in Table 5 we shall now consider the effect of each of these factors: target chemistry differences, shielding differences and absolute production rate uncertainties. The production rate of 3He in ordinary chondrites has historically been taken to be 2-O x lo-* cm3 STP/g-106 yr, based on 3H and 3He measurements in a number of chondrites and an assumed 3H/3He total production ratio of O-5 (BE~EMANNet al., 1959; GEISS et al., 1960; GOEBEL and SCHMIDLIN,1960, KIRSTEN et al., 1963). From typical 3He/21Ne and 3He/38Ar ratios measured in ordinary chondrites, the average 21Ne and 3sAr production rates have been taken to be approximately 0.38 x IO-* cm3 21Ne/g-10s yr and O-054 x lo-* cm3 38Ar/g-106yr (HEYMANNand I@AZOR, 1967; HEYMANNet al., 1968). However, HERZO~ and ANDERS(1971) have utilized 26Al, 3He and 21Ne data on several stony meteorites with exposure ages <3 x lo6 yr to show that these older production rates are too low by ~24 per cent. In an extension of this work CRESSY and BO~ARD (1972) concluded that these older production rates may be too low by as much as 40 per cent. Therefore, we adopt production rates 30 per cent higher than those quoted above, or 3He = 2.6 x 10m8,21Ne = 0.49 x 10e8, 38Ar = 0.070 x 10-s (units cm3/g-106 yr) for L chondrites. The production rates used for average H chondrites are (units cm3/g-106 yr) 3He = 2.6 x lo-*, 2lNe = 0.46 x lo- 8, 38Ar = 0.074 x 1O-8(adopted from BOGARD 5

2422

D. D.

BOCURD, M.

A. REYNOLDSand L. A. SIMMS

and CRESSY, 1973). Although some uncertainty still exists in these absolute production rates, it affects only our calculated absolute exposure ages, and not the relative ages among meteorites. Production rate variations due to differences in shielding have not generally been considered in calculating exposure ages, although shielding effects can be substantially greater than target element affects for chondrites. Theoretical calculations and target experiments either are not directly applicable to noble gas data or give conflict~g predictions. Noble gas data on a large number of different chondrites show relative production rate differences for %Heand WNe of a factor of two or greater (EBERHARDT et al., 1966; NYQUIST et al., 1973). A direct measurement of changes in cosmogenic noble gas ooncentrations produced by shielding in a single chondrite showed variations in 21Ne and 3He of 28 per cent and 14 per cent, respectively, with larger preatmosphe~G variations indicated (WRIOHTet ai_, 1973). Clearly, failure to correct for these shielding differences can lead to sizeable errors in relative exposure ages, even for meteorites belonging to a single chemical class. The relationship between aHe/alNe and a2Ne/21Nefor recently fallen chondrites analyzed in our laboratory is shown in Fig. 1. The recently fallen Uoera chondrite falls outside the bounds of Fig. 1 because of low aHe/zrNe (BOOARDet cd., 1971). The solid liue and the dashed line are the relations~ps derived from the different ordinary chondrite analyses of EBERBARDT et aE. (1966) and NYQUIST (1969), respectively. The points labeled Keyes represent samples measured over a total core length of 56 cm from the Keyes L chondrite (WRIUH!V et ah, 1972) and define a very similar relationship. The correlation of 3He/21Ne with 22Ne/arNe has been attributed by all of these investigators to increasing ratios with decreasing cosmic

31 I.06 ’

I

I.06

I

1.10

I

I.12

t

I.14

I

Id6

I

I.18

I

I.20

22Ne ?‘Ne

Fig. 1. ~l&tio~~p of oosmogenic aHe/%% and 22Ne/21Ne for recently f&&n ehondrites. In addition to the meteorites of Table 1, the following axe 8180 plotted: (A) Allende, (H) Haverii, (LC) Lost City. The points labeled Keyes we different depths of the Keyes L chondrite (WRIU~ et al., 1973). The solid line is from chondrite data of EBERHARDT et al., (1966), and the dotted line is from NYQUIST (1969). The Kabo meteorite ia labeled G after its Gwarzo synonym (Table 1).

Noble gas concentrations

and cosmia ray exposure ages

2423

ray shielding. Previous chondrite data tend to show average values of 22Ne/21Ne= 1.11. Thus, all plotted data for recently fallen ohondrites except Allende indicate lower than average shielding for these meteorites, which is consistent with their small recovered masses (Table 1). However, the shielding model derived by NYQUIST (1969) on the basis of noble gas analyses of both metal and bulk phases of chondrites indicates that all of the meteorites of Fig. 1 possessed minimum preatmospheric masses of 100 kg. Such preatmospheric masses are several times the recovered masses, and imply either that appreciable mass was not recovered or that large atmospheric ablation losses occurred for most of these chondrites. Preatmospheric masses which are considerably larger than the recovered masses appear to occur commonly for chondrites (NYQUIST, 1969). We interpret those samples plotting below the trend lines as having experienced some gas loss. Thus, the SHe121Neand radiogenic 4He concentration for Malakal indicates that this meteorite has lost much of its He, and Wethersfield and Lost City may have lost small amounts of He. As none of the meteorites plotted in Fig. 1 exhibit ‘average’ cosmic ray shielding, some shielding correction must be applied to the average production rates previously mentioned before calculating exposure ages. We employ the measured 22Ne/21Ne ratios as the parameter for applying shielding corrections and adopt a value of 1.11 as the average of this ratio for ordinary chondrites. The various depth samples of the Keyes chondrite (WRIOHTet a,?., 1973) confirm the fractional change in 21Ne production rate with 22Ne$Ne suggested by different chondrite analyses (EBERHARDTet aE.,1966). We utilize this trend and the measured 22Ne/21Nefor the recently fallen chondrites to determine shielding correction factors relative to the average 22Ne/21Nevalue of 1.11 and the 21Neproduction rates given earlier for type H and L chondrites. For the Haverij ureilite, the L chondrite average 21Neproduction rate is multiplied by 1.39 to correct for target chemistry differences (HERZOGand ANDERS, 1971; BOUARDand CRESSY,1973) and a SHe production rate of 2.7 x 1O-8cm3g-lo6 yr is used (EBERHARDT et aE., 1965). From the SHe/21Nevalues measured for Keyes, the 3He produotion rate appears to possess a depth variation 0.55 times as great aa the variation in 21Neproduction rate. This value was used as a shielding correction on the average chondrite zHe production rate of 2.6 x lo-* cmS/g-lO* yr. Shielding corrections for 38Ar production rates were not determined for the Keyes meteorite data. However, a portion of the Keyes data and SHe/38Armeasurements on other meteorites (EBERHARDTet al., 1966) imply that some increase in 38Ar production rate occurs with moderate shielding. The calculated 3He, 21Ne,and 88Arcosmic ray exposure ages for these meteorites are graphically given in Fig. 2. The solid 45’ line represents the loci of all concordant ages. The open circles are exposure ages calculated with average production rates for the meteorite class; closed circles are calculated with shielding corrections applied to 3He and zlNe. No shielding corrections were attempted for 38Ar ages. These shielding corrections create greater concordancy in 3He and 21Neages except for Malakal and Wethersfield. However, the apparent SHe loss in these meteorites has caused a downward displacement of their plotted points in Fig. 2, and concordancy in ages has been destroyed. For some meteorites, the shielding correction increases the 21Neexposure age by 30 per cent. We consider these shielding-corrected

D. D. BOGARD, M. A. REYNOLDSmd L. A. Snms

2424

hk 0

I.0

8

8:; 0.6 D-5 t! 0.4 a 8 0.3 G 0.2 I -J? i+

2

3 4 567610

I5 20

2’Ne exposure age, 106yr

Fig. 2. 21Ne, 3He, and s*Ar exposure ages for recently fttllen chondrites. Open circles are mges c&ulsted with the following pr~uction rates for L chondrites (units 10-s cm* STP/g-106 yr): 3He = 2.6, =Ne = 0.49, =Ar = 0.070 (see text). Solid points represent ages corrected for shielding bssed on the observed 22Ne$Ne ratios and the Keyes snelyses (WRIQ~ et cbl.,1973). ages to be the best estimate obtainable from the data presented here and the cu~ent knowledge about the various production rates involved. The exposure ages shown in Fig. 2 for the three H chondrites and the five L ehondrites are all typical of the known range in ages of the two chondrite classes (ZXHRINGER, 1968). However, noble gas exposure ages for most chondrites have not, been corrected for shielding effects, and this fact may alter the histogram distribution of such ages exposure

(Z~HR~~ER,

1968). 22Na/22He AND 2sAl121BIeAC+ES

Concentrations of cosmogenic 21Ne and 22Ne in these meteorites may also be combined with measured activities of z2Na and zsAl to calculate cosmic ray exposure ages according to the equations (ANDERS, 1963): tN

ZlNe . uA.l z6Al . oNe

22Ne. uNa ’ rt 22Na (uNa + aNe) ’

where t is exposure age and o the production cross-section for individual nuclides. The decay rates of both radionuclides are generally considered to be in secular equilibrium with their cosmic ray production rates at, the time of meteorite fall. Short exposure ages and high energy solar flares, however, may alter this equilibrium. To calculate an exposure age, the 22Naj22Neand 26A.lj21Ne production ratios as a function of particle energy (shielding) must be assumed, as these ratios have not been directly measured for silicate systems. Likewise, it must be assumed that these production rates have not varied over the exposure time of

Noble gas concentrations and cosmic ray exposure ages

2425

the meteorite. Various investigators have utilized other techniques to deduce exposure ages, and have empirically derived 22Na/22Neand 26Al/21Neproduction rates for a few ohondrites (SP~NAOEL and SONNTAQ,1967 ; SPANNAQEL and HEUSSER, 1969; MARTI et al., 1969). However, these results show considerable variations. Two determined values of the 22Na/22Neproduction rstios were O-50 and O-30, while four production ratios of 2’JAl/21Neranged from 0.30 to 0.23. In calculating exposure ages for the Lost City and Ucera chondrites, BO~ARDet al. (1971) discussed earlier determinations of these total production ratios and adopted average values of 22Na/22Ne= O-40 and 2sAl/21Ne= O-26. These average vslues have been used in calculating 22Na/22Neand 26Al/21Necosmic ray exposure ages for the recently fallen chondrites in Table 1, and these ages are listed in Table 6 in Table 6. Measured 22Na/26Alratios and calculated 22Na/22Neand 26Al/21Neages. The 22Na/22Ne and 2sAl/s1Neages and the ratios of these ages given in the second column from the right have been calculated with the average, empirically-determinedproduction ratios shown. The age ratios given in the last column have been calculated with the production ratios 22Na/22Ne = 0.30, ssAl/slNe = 0.26 to give greater concordancy with 21Ne ages. The variable bias between the two types of ages reflects the ~40 y0 variation in ssNa/ssAl. Bar-wellvalues in parentheses have been corrected for lack of 26Al equilibrium. Analytical uncertainties in 2sNa/22Ne and 26Al/21Neratios are estimated at f lo-15 %

Moteorite Barwell Kit% Tathlith Dweleni Lost City Ucere Nej0 Wethersfield Kabo Mel&al

=Ns x 1.56 1.46 1.44 1.42 1.40 1.28 1.16 1.15 1.14 1.04

Ref.

l

t * t $ t t t t t

=NQ2Ne age (l@ Ye P. R. = 0.40 2.5 15.3 8.6 24.3 6.1 21 9.4 6.2 5.0 4.6

aaNe/=Ne age

aaA1/21Netvge (10s yr) P. R. = 0.26

P. R. = 0.40/0.26

2.2 12.3 6.9 19.6 4.8 15 6.1 4.1 3.2 2.7

1.12 (1.23) 1.24 1.25 1.24 1.26 1.40 1.55 1.51 1.56 1.71

*sAl/alNe age

=Na/N@

age

““AAI/=Ne age P. R. = 0.30/0.26 0.84 (0.92) 0.93 6.94 0.93 0.95 1.04 1.16 1.13 1.17 1.28

* CRESSY (1970; 1971). t Rencitelli, personal communiostion (1972). $ BOGARDet al. (1971).

order of decreasing 22Na/26Al. Measured activities of 22Naand z6A1used for Bar-well and Tathlith were determined by CRESSY (1970; 1971a), and activities measured in the other six chondrites are unpublished data kindly supplied by L. Rancitelli (personal communication, 1972). Uncertainties in the 22Na/22Neand 26Al/21Neages arising from analytical measurement uncertainties alone are generally about f lo15 per cent. The ratios of the 22Na/22Neage to the 26A1/21Neage for these chondrites (second column from the right in Table 6) are not constant. For all ten recently fallen chondrites, the 22Na/22Neages exceed the 26Al/21Neages by factors ranging from 1.12 for Barwell to 1.71 for Malakal. However, 26A1in Barwell is not in secular equilibrium, due to its low exposure age, and the (22Na/22Ne)/(26A1/21Ne) age ratio for this meteorite should be corrected upward to the second value of 1.23 shown in Table 6, As can be seen from the above equations, the variation in the ratio of these ages is due to the large variation in 22Na/26Al(Table 6). If we assume that a

D. D. BOGASD,M. A.

2426

REYNOLDS

tend L. A. SIMXS

aaNa/Ve total production ratio of 0.40 is correct for all meteorites, the 28Alj21Ne production ratio would have to vary between O-29 and 0.44 to permit these ages to be concordant. Alternatively, if a aeAl/arNeproduction ratio of O-26 is correct for all meteorites, the aaNa/2rNeproduction ratio would have to vary between O-36and 0.23 for concordant ages. Therefore, so single set of values of the 22Na/22Neand @JAl/aWe production ratios will give. concordant ages for all meteorites. This fact is contrary to the common assumption in calculating such ages. Obviously, care must be exercised in applying to a given meteorite a radioactive/stable production ratio derived from another meteorite. A probable explanation for the variation in 22Naj2~Al,and therefore, the bias in 2aNa[22Neand 2*Al~21Ne ages for these memories, is in the evidence that they have experienced a hard and variable cosmic ray spectrum as deduced from their aaNe/afNe ratios. The measured 22Na/26Alratio in chondrites appears to be variable by a factor of two, while evidence has been presented that 26Al activities vary by much less (FUSE and ANDERS,1969; CRESSY,1970, 1972; MAIXTIet al., 1969). Figure 3 demonstrates the relationship between 22Naand 26Al activities measured by several laboratories for a number of recent chondrite falls, including those of Table 6. Measured 22Naand 26A1activities in six different samples of the Saint Severin chondrite define a distinct trend which is also consistent with these activities measured in four samples of Bruderheim, six samples of Lost City, and several other recent chondrite falls. (Some older measurements of these radionuclides are probably not

0.8

I.0

1.2

1.4

I.6

2.0

*‘NdZ6Al

Fig.

3. &18tionship of 2aNa ad saAlactivitiesfor severalreoentlyftien chondrites.

The straight line through the St. Severin data demonstrates an increase in e2N8/ a*Alwith a decrease in irradiation hardness. Data have been compiled from the following sources: CRESSY, 1970, 1971a, X97lb; BOUARDet cd., 1971; WRIGLEY, 1971; FIEEMAN, 1967; HONDA and ARNOLD, 1964; S=DLOX%KY et cd., 1967;

ROWE et cd., 1963; I%ARTIet al., 1969; S~ssvrrao~~and SONNTAQ,1967; TOBAILEW et al., 1967; HEWERS et d., 1969; Rancitelli, personal communication, 1972,

Noble gas conoentratiom and cosmic ray exposure ages

2427

as accurate, and were not included in Fig. 3.) The Saint Severin data point with the lowest 2aNa value is actually a combination of the 22Na results of MARTI et aZ. (1969) (no *aAl data was given) and the aeAl results of CRESSY(1970) for different samples of the meteorites slice labeled DIII, and therefore possesses considerably more uncertainty in its a2Na/asAlvalue. As the line shown in Fig. 3 possesses a slope of l-05, most of the variation in zzNa/26Al is in the 22Na activity, and the arguments of the authors quoted above appear substantiated. As suggested by the Saint Severin data of Fig. 3, we assume that the variation in 22Na/26Alamong chondrites is largely due to shielding differences, with larger values of this ratio indicating larger shielding. There is also some indication that the 22Na/26Alratios are inversely correlated with the 22Ne/21Neratios, as might be expected if the variations in both are due predominantly to shielding. However, the relationship is not clearly defined, and more radionuclide and noble gas measurements are needed on the same samples of meteorites which exhibit a wide variation in both of these ratios. Unfortunately, noble gas data reported for Saint Severin were generally not measured on the same specimens as were radionuclides. However, the data of MARTIet al. (1969) and FUNKHOUSER et al. (1967) indicate a variation in cosmogenic ZlNe in Saint Severin of at least a factor of l-6, implying that the a6A1/21Neratio in this meteorite also varies considerably. It also will be interesting to compare ssNe/21Ne ratios with other shielding indicators such as 6oCo or 64Mn activities (CRESSY, 1972) when these data become available. The effect of solar cycle modulation and differences in orbital parameters on the meteorite data shown in Fig. 3 and consequently on calculated 22Na/22Neand s6Al/z1Neages is apparently not a major factor for most chondrites. Measurements on s7Ar/s0Ar demonstrate that the time of meteorite fall and orbital parameters may produce up to a factor of five variation in the production rate of 37Ar (half life 34 days) relative to 30Ar (269 yr) (DAMS et al., 1963; FIREMANand SPANNA~EL,1971a, b; FORMANet al., 1971; BEQEMANN, 1972). However, FIREMAN (1967) did not observe any appreciable variation in 22Na/26Alamong five chondrites which fell over a seven year period. (A sixth chondrite, Harleton, possess a lower 22Na/26A1,and is discussed below.) Differences in orbits among chondrites could be expected to affect the cosmic ray production rates of the stable noble gases and radionuclidea such as 22Na and 26Al. Comparisons of measured 26Al activities with calculated values have led CRESSY(1971; 1973) and WILKENINCJ et al. (1972) to postulate that the eucrite and ureilite meteorite classes may have possessed different orbits and lower production rate compared to chondritea. Those few chondritea in Fig. 3 which plot substantially off the trend line may do so because of the effect of different orbits. Harleton may be such a meteorite, as three different measurements of 22Na and 26Al (the two points labeled H in Fig. 3 and a third point which plots far off the Fig. at 22Na = 138, a2Na/26Al= 2.76) show essentially the same trend as the other data of Fig. 3, but displaced to higher values of 22Na/26Al. It is also conceivable that the insensitivity of 26Alproduction rate to shielding does not hold for very low shielding conditions and small values of s2Na/26Al(HERZOG, 1972).

It has been pointed out to us (Nyquist, private communication) that the 22Na/26Al values for these chondrites also show a trend with the time of year the meteorite fell

2428

D. D. BOCJARD, M. A. REYNOLDSmd L.

A. SIMMS

(Table 1). Thus, Kiffa, Tathlith and Dwahmi, which all fell in the month of October, possess Wa/26Al values of 1.44 & O-02, and Nejo, Wethersfield, and Kabo, which fell in the months of April and May, possess values of 1.15 f 0.01. Lost City and Ucera fell in January, and possess intermediate 22Naf26Al. Such a relationship suggests that these chondrites may have had similar orbits and that the 22Na/26A1 values are in part determined by whether the meteorite fell before or after passing its perihelion. However, the time of day of meteorite fall does not appear consistent with such in~~retation (see ~ETHERILL, 1969, for a discussion of meteorite orbits). Such an interpretation would also require a 22Naf26Alproduction ratio near or inside the Earth’s orbit which is several times the production ratio near aphelion. Therefore, we presume that variations in 22Na/26A1shown by different ohondrites of Table 6 are predominantly due to shielding differences, with smaller effects arising from differences in orbits and target chemistry variations. The apparent correlation of 22Na~26~ with the time of meteorite fall may, however, prove an insisting subject for future investigation. The strong bias between 22Na/22Neages and 26Al/alNe ages (Table 6) demonstrates that the averaged radioactive/stable production ratios previously assumed for the nuclide pairs are incorrect for these low shielded chondrites. We may then ask what production ratios for 22Na/22Neand 2*Al/21Newill give the greatest exposure age concordancy with the aHe and 21Neages of Fig. 2. The most concordant set of values appear to be aaNa/22Ne= 0.30 and 26Alf2lNe = 0.26. This is shown by Fig. 4 where the 22Na/22Neand 2sAl/21Neages are plotted against the shieldingcorrected 21Neages of Fig. 2. The three types of ages shown in Fig. 4. are concordant within -+20 per cent for all ten chondrites except the 21Ne age for Ucera and the 26Al~21Ne age for Malakal. However, both of these me~orites plot far off the trend

uk ’ 3 g

25-

0(22Na/2*Ne)P.R.~O~30

zo-

l(26Al/2’Ne)

e

15 -

2 g

IO-

2

8-

*‘Ne

l?R.=0*26

exposure age, 106yr

Fig. 4. 21Nk,22Ns/*2Ne and 26A1/21Nesges for recently faIlen chondrites. The We ragesme the shielding-correctedvltlueeof Fig. 2. The 22Na/22Neand 2aAl/zlNe ages hwe been normalized to the 21Ne ages by &opting production mtios of 0.30 and 0.26, respectively.

Noble gas concentrations and cosmic ray exposure ages

2429

line of Fig. 3, and factors other than shielding may have affected their production rates of 26A.land 2lNe (BOQARDet ai., 1971; P. J. Cressy, personal comm~ication, 1973). The a‘%a/22Ne and aaAl/21Neages of Fig. 4 have been normalized to the zlNe ages for production ratios of 0.30 and 0.26, respectively; however, these two ages still contain an internal bias due to shielding differences (Table 6). As stated previously, no single set of production ratios for the 22Na/a2Neand 26Al/21Nenuelide pairs can yield concordant ages for all of these chondrites. For the normalized production ratios chosen in Fig. 4, the greatest concordancy in ages is seen from the data in Table 6 to occur for measured 22Na/26AlCJ 1.35. If we adopt production ratios of 0.30 and 0.26 for the case of 22Na/aeAl= 1.35, shielding-corrected 22Na/22Ne and 26Al/elNeproduction ratios for other values of 22Na/26AIcan be derived. As seen from the age equations given earlier, any variation in 22Naj26Alwill imply a con~omitant change in the 22Na~22Ne or the 2BAl/2fNeproduction ratios. For example, a 22 per cent increase in 2sNa/26AIto 1.65 would require a 22 per cent increase in the 22Na/22Neproduction ratio relative to the 26A1/21Neproduction ratio in order to maintain concordant ages. It is not known which of these two production ratios is more sensitive to shielding changes in 22Na/26Al, and both may be sensitive. U~o~unately, the various ages in Fig.4 show too much scatter to permit resolution of this question. However, the fact that production of both 22Na and 22Ne are sensitive to shielding may mean that the 22Na/22Neproduction ratio does not change appreciably with depth (TRIVEDIand GOEL, 1969; WRIGHTet al., 1973). The derivation of the production rates for the alNe, 2zNa/22Ne,and 26Al/21Neages are interdependent, with the particular values used in Fig. 4 yielding the greatest con~ordancy of ages. Thus, the absolute values of the z2Na~22Neand 26Al/e1Ne production ratios listed in Table 6 can only be as accurate as the shielding-corrected 21Neproduction rates. However, the relative values of these production ratios as a function of 22Na/26A1 is independant of 2lNe. The commonly held view that 22Na/22Ne and 26A1/21Neages based on constant production ratios are inherently more accurate than those ages based on noble gas concentrations aIone is not supported by the results presented here, if proper shielding corrections are applied to the noble gas production rates. In order to calculate more reliable 22Na/22Neand *6Al/21Ne absolute exposure ages, it obviously is necessary to make direct measurements of these two production ratios in target experiments or additional measurements in meteorites of known shielding characteristics. Effects of irradiation hardness on the production ratios such as that indicated by Table 6 also requires consideration. Hopefully, measurements of additional eosmogenic nuclides in the recently fallen chondrites reported here will allow better definition of these and other radioactive/ stable production ratios. TRAPPEDGASSES Dwaleni, Swaziland, dark phase (Table 2) contains large concentrations of trapped He, Ne and Ar, while Kabo dark phase and Barwell show much smaller amounts of trapped Ne and Ar. REYNOLDSet al. (1971) concluded that Dwaleni ranked in the upper five or so meteorites in He and Ne concentrations and that the trapped gas was largely solar in composition. A number of stony meteorites

2430

D. D. BOUARD, M. A. REYNOLDS and

L. A. SIMMS

possess a light-dark structure, and a fraction of these have been found to contain large e~chments of the lighter noble gases, He, Ne and Ar in the dark phase. These gas abundances almost certainly had an origin in an early solar wind and were introduced into the dark phase of the meteorite either directly, via an irradiation or shook process, or by means of a ‘CARRIER’(FREDRIESSON and KEIL, 1963 ; MULLERand Z~HRINGER,1966; MAZOR and ANDERS,1967; W~~NXE,1965; SUESS et al., 1964). Thus, these gas-rich meteorites, along with lunar fines, offer a direct sampling of the relative elemental and isotopic abundances of solar wad-implanted noble gas elements. The calculated trapped solar gases in Dwaleni dark phase are given in Table 7. For comparison, noble gas data are also given for two other gas-rich meteorites Table 7. Trapped noble gas abundances in Dwaleni, Swaziland compared to those,in two other Ras-rich meteorites

DWASli

Jodzie* -200 mash St. Mesmia~

113,000 124,000 80,009

663

20.6

1173 48 166 ~6

0.045

0*00015

13.2

201

27.4

0.032 0.024

0~00029 @00032

12.7 11.6

10.5 480

24 30

+ MAZOX end AXCDEBS(1967). t HEYMANNand MAZOR (1966). Abundance unaerteinties for Dwaleni are estimated at f6

456 1500 ~250

% for He, Ne and A?, and f I6 % for Xe.

with somewhat similar trapped gas composition. Note that due to a calibration correction, the He and Ne (but not the Ar and Xe) abundances for Dwaleni are approxima~ly 30 per cent lower than those reported by REYNOLDSet ak. (1971). The *He concentration is the measured value in the dark phase minus an assumed 1 x 1O-6 ems radiogenic component (Table 4). The aoNe and 36Ar concentrations are the measured dark phase values minus a spallogenic component based on the oosmogenic abundances for Dwaleni light phase (Table 4) and spallation ratios of a”Nej21Ne= 0.9 and 36Ar/s*Ar = O-66. These corrections amounted to 0.9 per cent, I.2 per cent and 3.3 per cent for 4He, 2oNe and a6Ar, respectively. No correction was made to the 132Xeconcentration. The trapped solar 2@Ne/22Neratio in Dwaleni was obtained from an isotopic correlation plot of “ONe/aaNeagainst a1Ne/22Nefor the measured light and dark phase values of Table 2 (see BLACK, 1972, for a discussion of this technique). A line drawn through these two values also passes through a point representing typical cosmogenie Ne in ~hondrites (20Nej22Ne= 0.8, 21Ne/22Ne= O-87), and for an assumed trapped solar 21Ne/22Neof 0.03, yields a trapped solar 20Ne/22Ne.= 13.2 f 0.2. The trapped solar 2oNe/22Nevalue is not very sensitive to the trapped 21Ne/22Neassumed, and an upper limit value for 21Ne/22Neof 0.04 would only lower the trapped 20Ne/e2Neto 13.0. An analogous plot for the dark and light phases of Kabo also passes through typical spallation neon, but for an assumed trapped 21Ne/2*Ne= O-03, yields trapped 20Ne/22Ne= 9.7. Trapped neon in Kabo therefore does not appear to be the solar type. Trapped 3He/4He in Dwaleni is difficult to determine because of the large concentration of cosmogenic 3He present. If we subtract the cosmogenic 3He value of 50.1 x lo-* ems/g determined in the light phase from the measured 3He in the dark

Noble g8a concentrations and cosmic r8y exposure 8g0S

2431

phase (Table 2), a trapped 4He/8He ratio of 6.6 x 10s results. Both the trapped aONe/azNeand 4He/sHe determined for Dwaleni are appreciably larger than values determined for most other gas-rich meteorites. However, these values fall within the ‘gas-rich zone’ defined by BLACK (1970). The trapped 4HelZoNeand 20Ne/a6Ar for Dwaleni fall within the range of values found for other gas-rich meteorites. The Xe in both Dwaleni dark and light phases is predominantly of the planetary type rather than the solar type, as seen from the isotopic data given in Table 3. On the other hand, the Xe isotopic compositions for Barwell, Tathlith, Nejo and Malakal show displacement toward the atmospheric value, and Xe in these meteorites is probably a mixture of absorbed air and AVCC. The existence of larger amounts of planetary-type Xe in Dwaleni dark over Dwaleni light is consistent with the conclusions reached by MAZOR and ANDERS(1967) that both solar and planetarytype gases are covarient in gas-richmeteorites, and were probably introduced together via a carbonaceous chondrite carrier. In this context, it is interesting to note that Dwaleni dark phase also contains four times the concentration of excess radiogenic laOXe than Dwaleni light phase. Either Dwaleni dark phase also contains four times the iodine concentration of the light phase, or radiogenic lzOXe has been introduced into the meteorite along with the trapped Xe. Acknowledgements-We thank R. CLARKEof the U.S. National Museum for furnishing the meteorite samples 8nd L. RANCITELLI and E. JAROSEWICH for permitting us to use their unpublished data REFERENCES ANDERSE. (1963) Meteorite sges. The Moon, Meteorites and Corn& (editors B. M. Middlehurst and G. P. Kuiper), pp. 402-495. University of Chicago Press. BEUEMANNF., EBEREARDT P. and HESS D. C. (1959) 3He-3H Strahhmgtxdter eines Steinmeteoriten. 2. Naturforsch. 14a, 500-503. BEQEUNN F. (1972) Argon 37/8rgon 39 activity ratios in meteorites and the spatial constancy of the cosmic radiation. J. Gwphys. Res. 77, 3650-3659. BLANK D. C. (1970) Trapped helium-neon isotopic correlations in gas-rich meteorites and carbonaceouschondrites. awehim. Comchim. Acta 34,132-140. BLACKD. C. (1972) On the origins of trapped helium, neon and argon isotopic variations in meteorites-I. Gas-rich meteorites, lunar soil, 8nd breccia. cfeochim. Cosmochim. Acta 86, 347-376. BOU~RD D. D., CLARKR. S., KEITH J. E. and REYNOLDSM. A. (1971) Noble geses end radionuclidesin Lost City and other recently fallen meteorites. J. Gwphys. Res. 78, 40764083. BO~ARDD. D., GIBSONE. K., JR., MOORED. R., TTJRNER N. L. and WILKIN R. B. (1972) Noble gas and carbon abundances of the Haverb, Dingo Pup Donga, and North Haig ureilites. &ochim. Coemochim. Acta 37, 547-558. BO~ARDD. D. and CRE~SYP. J., JR. (1973) Spa&&ion production of sHe, zlNe and ssAr from target elements in the Bruderheim chondrite. Geoohim.Cosnzochim. Acta 37, 527-546. CLARKE R. S., JR. (1971) The meteoritic81bulletin, Meteor&a 6, 111-124. CLARKER. S., JR. (1972) The meteoritic81bulletin. Metwritics 7, 215-232. CRESSYP. J., JR. (1970) Multiparameter 8nitlysis of gamma radiation from the Barwell, St. Severin and Tathlith meteorites. Geochim. Coamochim. Acta 34, 771-780. CRESSYP. J., JR. (19718) The production rate of zsAl from target elements in the Bruderheim chondite. Geochim. Cosmochim. Acta 35, 1283-1296. CRESSYP. J., JR. (1971b) Cosmognic redionuclides in the Lost City and Ucera meteorites. J. Geophys. Res. 76, 4072-4075.

2432

D. D. BO~ARD,M. A. REYNOLDSand L. A. SIMYS

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Noble gas concentrations

and cosmic ray exposure ages

HEYMANN D., MAYORE. and ANDERS E. (1968) Ages of calcium-rich

2433

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