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Meteorites with short cosmic-ray exposure ages, as determined from their Al26 content
Geochhnica etCosmochimica Acts,lQ67, Vol.31,pp.1793 to18QQ. Pergsmon PressLtd. Printed inNorthern Ireland
Meteorites with short cosmic-rayexposureages,88 determined
from
their
Al= content
DIETER HEYMANN* and EDWARD ANDERS Enrico Fermi Institute for Nuclear Studies and Departments of Chemistry and Geophysical Sciences, University of Chicago, Chicago, Illinois 60637 (Received 29 April 1967; accepted ira revised form 30 May 196’7) &&&The Al26 content was determined by Y-+J coincidence spectrometry in thirty-one stony meteorites: ten carbonaceous, four enstatite, three bronzite, five hypersthene and five amphoteric chondrites; one angrite, one euorite, one ureilite and one diogenite. In the following eight meteorites, the A126 content was low enough to permit determination of a cosmic-ray exposure age (million years): Ivuna 0.18 f 0.06, Cold Bokkeveld 0.32 f O-04,Nagoya O-14 f 0.08, Pollen 1-O + 0.6, Grosnaja 1.5 f 0.4, St. Marks 0.9 f 0.2, Shaw O-40 f O-07, Appley Bridge 1.5 f 0.5. Seres and Bath have gas contents suggesting ages < 1 lo6 yr, but their A12’I contents correspond to ages > 1.5 and 2 1.9 lo6 yr. These two discordant cases appear to be due to diffusion losses and a sample mixup, respectively. The Goalpara ureilite has only about 60% its expeoted Al26 content. If this result is attributed to shielding, Goalpara must have had a preatmospheric mass of 2 700 kg. The mean Al26 content found for sixteen samples of ordinary and enstatite chondrites is 63 dpm/kg, appreciably higher than the value of 47 dpm/kg determined by means of y-ray speotrometry (ROW et al., 1963). The discrepancy may be due to a calibration error in the latter method, and to sampling statistics. The present data scatter little from the mean, thus mdi. eating a constant Al26 production rate for most chondrites. Production rates of Al26 in stony meteorites have been estimated for the following target elements: Mg 85 & 110, Al 410 f 410, Si 210 & 110, S 100 & 90, Ca 280 + 130, Fe + Ni 16 f 0.6 dpm/kg. INTRODUCTION TYPES of information can be obtained from measurement of cosmic-ray produced AP in meteorites : the degree of shielding from cosmic rays, and the exposure age. If the cosmic-ray exposure age of the meteorite is long compared to the half-life of A126,O-74 lo6 yr, a steady state is reached in which Al26disintegrates at exactly the rate at which it is formed. The production rate depends on chemical composition and sample depth; if the former is constant, as in a given class of meteorites, the Al28 content can be used to infer the sample depth. However, low values must be expected not only in large, heavily shielded meteorites, but also in small ones, owing to escape of cosmic-ray secondaries. Aluminum-26, being only a few mass numbers removed from the principal target nuclei Ala’, Mgee, and SP, is made mainly by low-energy secondaries. If the cosmic-ray age of the meteorite is not much longer than the half-life of A126,a steady state will not be achieved. Instead, the Ala6level A will be smaller than the steady-state level A, by a factor 1 - e-%, where Ais the decay constant of APi, and t, the cosmic-ray exposure age : Two
A = A,(1
-
e--lt)
* Present address: Department of Geology, Rice University, Houston, Texas 77001. 1793
(I)
1794
DIETER
HEYMANN
and EDWAECII ANDERS
If A, is known, the cosmic-ray age can be calculated from the measured Al26content, A. This method is particularly useful in the range O-l lo6 yr, where cosmogenic noblegas contents are low, and mass spectrometric measurements hence are difficult. The existence of meteorites with short ages has become an important argument in discussions on the origin of meteorites (ANDERS,1962; ARNOLD,1965; C)PIK,1965). The first Alz6 measurements in meteorites were performed by EHMANNand KOHMAN(1958) by p-counting of chemically isolated aluminum fractions. Later measurements were done mainly by non-destructive techniques. A few meteorites, including two of very short age, were measured by y-y coincidence spectromet’ry (ANDERS, 1960, 1962; GFELLERand HOUTERMANS,1961; BISWAS et al., 1963; DE FELICEet al., 1963). This method measures the two coincident O-51l-MeV photons emitted in the annihilation of positrons from the decay of A126. The majority of measurements to date were done by y-ray spectrometry, using the 1X%-MeV y-ray of A126 (VAN DILLA, ARNOLD and ANDERSON,1960; ROWE, VAN DILLA and ANDERSON,1963). This method requires large samples ( ~1 kg), but has the advantage of permitting simultaneous measurement of potassium, via the 1*46-MeV y-ray of K40. The present study was again carried out by y-y coincidence spectrometry. Its main purpose was determination of accurate cosmic-ray ages for meteorites of low cosmogenic gas content. Ten such meteorites were measured. To provide an estimate of the saturation level of A126in various meteorite classes, twenty-one other meteorites of long cosmic-ray age were also measured. Preference was given to rare types, in order to take advantage of the high sensitivity of the method. Samples as small as 5 g oan be measured with a standard deviation of & 20%) and for samples of 10 g or larger, an accuracy of 5--10% is readily attainable. PROCEDURE Coulatera Two spectrometers previous description
were used.
One of these had remained virtually unchanged from its
(LIPSC~UTZ et al., 1965), except that the detectors now consisted of
6.4 x 3.8 cm ory&& with 1.3.cm N&I light pipes, mounted on EMI 9678 photomultipliers. The second one had evolved gradually from the configuration described by Vmrx and ANDICR~ (1962). During the period covered by our measurements (1962-1967) the vacuum-tube electronics was replwed by RIDL transistorized single-channel analyzers. Expander amplifiers were used to minimize fluctuations in channel width. The iron shield w&s replaced by e lo-cm lead shield. The lead had been salvaged from cm early 19th.century shipwreck, and had o very low content of Pbfis and other radionuclides. * Channel level and width were calibrated by an Near+source at frequent enough inter&s to keep the counting efficiency for 61 I-KeV coincidences constant to within 3 %. Single pieces weighing more then 20 g were wrapped in Saran pleetio film and were taped directly to the crystal. Smaller pieces were placed inside polystyrene boxes of o.d. 3P64mm, height 13-22 mm. Samples consisting of powder or multiple fragments were likewise counted in polystyrene or lucite boxes. For carbonaceous chondrites, the polystyrene was coated with a * It was obtained from Muller’s M&eel Maztschapij N.V., Prinsengracht 544, AmsterdamC. Lead of comparably low radioactivity has since been identified among commercial samples
(WELLER,,A_NDERSON and BARXER, 1966).
Meteorites with short cosmic-ray exposure 8ges
1796
thin, vepor-deposited layer of aluminum, to prevent conteminetion of the meteorite with phbstic. Total counting times ranged from 1 to 10 weeks. Background For background dete rmirmtion, dtite slabs or powder were counted in the s8me geometry 8s the Bemples. Nornmlly sample end beckground counts were 8lternated every 3-7 days. Often backgrounds re nmined st8ble over many months. In such oaeee, background d8t8 for 1-2 months preceding and following the Bemple count were included in the average. Typical backgrounds varied from about 9-10 counts/l000 min in 1963 to 6-7 counts/l000 min in 1967. Most of the deorerease ~8s due to the transfer of the counters into 8 subterranean counting laboratory. Occ8sion8hy, when interference from outside sources w8s suspected, 8 dunite blank was meseured concurrently on one spectrometer while the sample W8S being counted on the other. Sample and blank were alternated 8t regular intervals. Calibration The positron counting yield for each sample ~8s determined by me8ns of 8 mockup of identical shape, containing 8 known amount Of N8 1s. For meteorites counted 8s single pieces, mockups were prepared by the technique of ROWE et al. (1963). A mold of Al-foil (O-08mm thick) ~8s shaped around the meteorite and ~8s reinforced with 8 Coating of Rezolin epoxy resin. The mold wm then filled with 8 st8nd8rd mixture of nickel powder, conteining 6-10x NaWl of known specific aotivity. The peaked density of this powder ~88 in the range 3-5-3-9 g/ml, close to the density of ohondrites. The nomixml electron density of 2.88 x 108*/g8lSO agreed rather well with the value for chondrites, 2.92 x 10s8/g(ROWE, 1963). Samples consisting of powder or multiple fregments were always counted in polystyrene boxes, fllled sufficiently to approxinmte cylindrical geometry. For these semples, mockups were prepared by Wing boxes of the s8me size with Na*sCl or Ni-N&l mixture. Sometimes the density of the mockup differed slightly from that of the sample. In such cases, the counting yield of the s&mple, Y,. ~8s found by applying 8 self-absorption correction to the counting yield of the mockup, Y,,,: Y, = Yoel~-*lPh where m and a are the m8sses of mockup end sample; a, the cross-sectional are8; and ,u, the m8ss absorption coefficient for 610 keV y-y coincidences in the N&l-Ni mixture (0.098 oma/g, 8s determined from 8 series of Na%‘l-Ni mixtures of verying 11188s density). This correction treats the s8mple 8s 8 plane-p8r8llel slab, end thus is not quite appropriate for samples of irregulsr shape. However, the error introduced is a negligible second-order effect, since the correction itself r8rely exceeds 10 %. Two N8** standards were used for calibration, both from the Nation81 Bureau of Stctndards. (1) NBS No. 4922: /l+ activity = 64.3 x lo4 Bf set on 10/14/1964. (2) NBS No. 4922-D: /Zl+ ectivity = 656 x lo4 /?+/secon 6/30/1964. From these stand8rds, N&l of low specific activity ~8s prepared (300-1000 Bf/min per g) for use in mockups. Direct intercomperison of N&l samples from the two standards revealed a discrepancy by 8 f8ctor of I.010 f 0.007 between calculated and observed activity ratios. This discrepancy mey be largely due to volumetric dilution errors rether th8n to errors in the nominal activity of the standerds. All calibrations baeed on the older standard (No. 4922) were therefore adjusted by this factor, to meke them consistent with the more recent set of me8surements. Dec8y corrections were made on the basis of 8 half-life of 942.3 days for Narn. In some owes, two or three mockups of the s&mesample were prep8red, to test reproducibility. If sufficient c8re ~8s exercised, the results were consistently reproducible to 6% or better, but in 8 few c8ses where shape or mess of the sample were not too carefully dupliceted, discrepancies 8s large aa 10% were found. For most samples, the Statistical counting error exceeds the estimated error in oaiibration, and only the former is quoted in our table of results. In those few 088% where the calibration error w8s the larger of the two, the st8nd8rd deviation ~8s raised accordingly.
1796 Analyti
DIETER HEYMAKN and EDWARD ANDERS of data
For most meteorites the calculation of Alz8 content was completely straightforw~d. Since only about 84% of all Alee decays proceed by positron emission ~RI~HT~~E et al., 19591, the observed positron rate ww divided by 0.84 to obtain the disintegration rate. In the ease of Hamlet, a 1959 fall, a correction for 2.58-y Na22 was required. The sample (consisting of fusion cruet ahips) was counted five times during 1962, 1963 and 1966. The Na22 and Ala6 contents were found by a least-squares analysis of the decay curve. A small correction for Na22 was also needed for Murray, a 1950 fetI1counted in 1962. %bXULTS
The data are listed in Table 1. Meteorites have been grouped by olass, according to the classififictation scheme of VAN SCHMTJSand WOOD (1967). It is instructive to compare our data with those of other workers. Direct comparisons are possible in only a few cases, to be discussed later, since onr samples were de~bera~ly chosen to avoid dup~cation. However, we can compare the data for ordinary and enstatite chondrites as a group (Fig. 1). Meteorites known to have short cosmio-ray ages were omitted, and so were data with standard deviations greater than &20%. Apparently there is a systematic discrepancy between our data and those of ROWE etal. None of ROWE’Sdata fall above 60 dpm/kg, while twelve of our sixteen Table 1. Aluminum-26 content of stony meteorites
Meteorite*
Clase§ Souroe 11
Alais* .IvUna* Orgueil* Cold Bokkeveld* Mlrray*t Nogoye Pollen Felix Grosnaja ornans
Cl Cl Cl c2 c2 c2 c2 c3 c3 c3
Abee Daniel’s Kuil Pillistfer St. l&%&r I St. Marks II
E4 E6 E6 E5 E5
C F F U S
Bath* PlainviewS Seres
H4 H5 H
H w F
Auseon* MeKmney 1* MeKinney II* Shaw Taiban II Zavid
L5 L4 LP L6 L5 L6
w F H A MP G
F CT,GR M F U F JW F F F
Number
Me 1486
Me 1736, 7, 8 6, 10 Me 1680 Me 1330 Me 1732 Me 1766 Me 1500 Me 1646
92. 1216 Me 1866
62/l
Recovered mass (kg)
Sample ma08
6 0.7 12 -4 13 4 0.25 3 3 6
7.44 33.4 30.7 45.9 43.0 950 5-58 50-3 73.3 49.2
107 1 23 14 14
(g)
25.8 15.8 39.2 209 14.1
Al26 # (dpmk) 44 * 6 6.6 -_tI.4 41 * 3 12-9 f oa9 42 f 4 6zk3 31 j, 6 54 f 4 43.3 & 1.7 53 *4 63 60 52 35 31
f 7 k6 + 6 f 2 & 3
21 700 8.5
53.4 68.0 39.0
64 i 4 55 * 2 61 &6
50 >190 >lOO 3.7 25 93
43.5 21.8 25.2 70.5 11.2 71.9
62 68 71 20 59 68
f 5 i 4 +5 & 2 f 5 -+.3
1797
Meteorites with short cosmic-ray exposure ages Table 1. (continued)
Meteorite*
c1assg source II
Number
Appley Bridge Bandong Hamlet * Jelica I Jelica II Vavilovke
LL6 LL6 LL4 LL6 LL6 LL
u F
614 Me 1753, 4
F F F
Me 511 Me 1353 Me 1659
Angre doe Reis Goalpara* Rode Sioux Co. *
An U Di Eu
F F F A
Me 1368, 9 Me 764, 1771 Me 1376
Recovered m&&3 (kg) 15 11.5 3.7 34 34 16 1.5 2.7 o-4 4.1
Sample mass (g) 64.5 25.4 51.5 67.5 43.3 117-l 9-48 12.36 25.12 10.78
Alss# (dpm/kg) 48 67 64 66 68 58 113 38 64 99
f f f f f f
2 6 6** 5 4 3
19 & 4 f 4 f 5
* Meteorites counted a~ powder or fragments are marked with an asterisk. Plainview, McKinney, Shaw and T&ban are finds; all others are falls. The two McKinney semples were first counted 88 single pieces, and then crushed and recounted. No systematic differenceswere found. t The Murray result was publishedpreviously (ANGERS,1962), but has been revised slightly, from 41 f 4 dpm/kg to 42 f 4 dpmlkg. $ An earliercount on the same specimen gave 68 f 5 dpm/kg, possibly due to a crtlibration error (ANDERS,1960). $ Claesiflcationof chondrites according to VAN SCJXMUS and WOOD (1967). Achondrites: An = angrite; U = ureilite; Di = diogenite, Eu = eucrite. 11We are indebted to the following donors who furnishedsampleson a gift, loan, exchange,or purchase basis. A = American Meteorite Museum, Sedona, Arizona. C = Geol. Survey of Canada (Dr. K. R. Dewson). F = Field Museum, Chicago (Dr. E. Oleen). G = Gregory and Bottley, London. GR = Dr. G. W. Reed, Jr., Argonne N&l. Laboratory. H = Harvard Collection (Dr. C. Frondel) JW = Dr. John A. Wood, Smithsonian Astrophys. Obs., Cambridge, Mass. M = Mu&e d’H.istoireNaturelle de Montauban (Mr. A. Caveille) MP = Max-Planok Inst. f. Chemie, Maim (Dr. H. Wlinke). S = South African Museum, Capetown (Dr. A. W. Crompton). U = U.S. National Museum, Washington (Dr. R. S. Clarke). W = Warda Natural Science Establishment, Rochester, N.Y. # Low values due to short exposure age are printed in italics. They were not includedin Fig. 1, or in the average A, values in Table 4. * * Also Na%, 105 f 18 dpm/kg at time of fall.
values do. To bring these two distributions into agreement, a shift by some 20% would be required. Some thirty-seven measurements by other workers exist, and they, too, show a tendency to lie above 60 dpm/kg and thus match our distribution rather than ROWE’S. Most of the low results among the, thirty-seven are due to Cressy. They were determined on chemically isolated aluminum fractions. Some difficulties were encountered in the yield determinations, and it is conceivable that these results are afltlicted with a systematio error.
1798
DIETERHEYMANNand EDW_&RD ANDERS I
I
I
Al26 Content a<100 6
Rowe
et al, 1963
kg
I
of Chondrites
n >lOOkg
(~1
4 2 5 In 00 0 z
Heymonn
and Anders,
1967
( y -7 coinc.)
$2 E
$0
4 2 0
6b
50 Al26
dpm/
7b
kg
Fig. 1. Alas oontent of ordinrtryand enstatite chondrites. Meteorites with short ages, or errors greater than i- 20 % have been omitted. The date by ROWE et al. (1963) and ANDERSONet aE. (1964) were determinedby y-ray speotrometry (top). They lie systematicallylowerthan our values obtainedby ~9 coinoidenc spectrometry (middle), and data from other leboratories, determined by y”y coincidence spectrometry or /?-oounting (bottom). Date in the bottom part of the figure were taken from E-m and KOHW (1958), HONDA and ARNOLD (1061, 1964), CHA~RAS~RTTY(1961), BISWAS et al. (1963), CRESSY (1964), and FIREMAN ( 1967). An additional point for the meteorite Cranes lies off scale et 81 f 10 dpm/ kg (NORDEWANN et al., 1965). The soarcity of meteorites with Alas contents ~60 dpm observed in our work may in part be due to sampling stetistios, and to systematic errorsin some of the other methods.
The discrepancy
between ROWE’S and our results oannot be due to sampling alone. include a higher percentage of finds, which might be expected to be low beoause of terrestrial deoay or greater shielding. But the finds are rather evenly spread throughout his distribution, rather than being concentrated at the low end. Direct intercomparison of data from different laboratories is possible in only fifteen cases (Table 2). Agreement is excellent in five of these : A&i&s, Bruderheim, Murray, Plainview and Sioux Co. (Cressy’s low result for PlaInview oan probably be attributed to shielding. With a recovered mass of over 700 kg, shielding effects ROWE’S meteorites
Meteorites
with short cosmio-ray exposure ages
1799
Table 2. Interaomperiaonof Al” data Alaa content (dpm/kg) *
Meteorite Class
Rowe: et al. (1963) y
CRESSY (1964)
SFIEDL~VKSY HONDA et al. et al. (1961, 4) (1967) Y-Y B+ Al,% A&@,
BISWAS et al. (1963) Y-Y
FIRE(1967) Y-Y
This work Y-Y
Murray Felix
C2 c3
44 f 4 38 f 4
42 f 4 64 f 4
Abee
E4
61 f5
63 f 7
Aohilles Ehole Plainview Richardton Richardton
H H H6 H5 H5
60 f 6 33 *2 66 f 6 152&6 II 29 f 3
Bruderheim Herleton Holbrook Holbrook La Lande MoKi~ey Potter
L6 LB L6 L6 L5 L4 LB
57 f 2 43 f 3 58 f 6
44 f 6
49 f 5
69 f 5
54 f 5
52 f 3
Hamlet
LL4
52 f 7
Sioux Co.
Eu
90 f 12
50 f 5 70 * 7
44 f 4
54 f 3
38 f 3 46 f 3
50 f 4
68 * 3 68 f 6
60 &- 6 45 f 5 I69 f9 II 74 & 6
63 f 4 63 f 6
70 f 4
60 f 6
64 f 6 99 f 6
* Earlier results from the same laboratory were omitted, if they were superseded by leter determinations (ANDERS, 1960, 1962; CEAKRABARTTY, 1961; EHMANNand KOEMAN, 1968; VAN DILU et al., 1960).
must be expected for this meteorite.) Marginal disagreement (by about two standard deviations) is found for five meteorites : Abee, Hamlet, Harleton, Holbrook and Potter. Taken by itself, each of these cases is not statistioally significant, but since all five deviate in the same direction, the discrepanoy may well be real. Finally, a substantial disagreement (by more than two standard deviations) exists for Ehole, Felix, La Lande, and possibly Richardton. With the exception of Ehole, all of the discordant, high values in Table 2 were determined by yq~ coincidence counting, and the low values, by y-ray spectrometry. One must therefore consider the possibility that one or the other method is afflicted with a systematic error. Errors in y-y coincidence spectrometry
Calibration can hardly have been at fault. Three NBS Naaastandards measured on our instruments gave results agreeing to within 2% (Lrrf3omrrz et a+!., 1965). Although BISWAS et al. do not state the origin of their Na** standard, a serious error
1800
DIETER HEYMANN and EDWARD ANDERS
from this source is quite unlikely in view of the wide availability of accurately calibrated Naz2 standards. Differences in self-absorption of mockup and meteorite also cannot have been an important factor. The nominal electron density of the mockups, 2.88 x 10z3/g, was almost identical with that of an average chondrite, 2.92 x 10z3/g. ROWE et al. used mockups in the same range, 2.84-2.89 x 10z3/g. Moreover, most of our samples and mockups had a mass thickness of only l-4 g/cm2, less than the measured absorption half-thickness for the mockups. Since the overall absorption was small, any difference between sample and mockup must have been a second-order effect. Extraneous counts are another possible source of error. In addition to positrons from A12G,four other sources can contribute counts in the 510-510-KeV coinoidence channel : members of the U and Th series having y-rays of 0.5-0.6 MeV energy in coincidence with y-rays of the same or higher energy; single high-energy y-rays scattered from one crystal into the other; positrons from SC**,the daughter of cosmogenic Ti** (t,,, = 48 yr) ; and electron-positron pairs produced by high-energy y-rays. All of th ese effects were investigated in an early phase of this work and were found to be relatively minor (ANDERS,1960 and unpublished work). The combined error would vary from meteorite to meteorite, depending on geometry and the abundance of K, U, Th and Ti **, According to our estimates, we would expect it to remain well below 5% in all cases. This would account for only a small part of the discrepancy. Errors in y-ray spectrometry
ROWEet al. used the 1.83 MeV y-ray of Al26for determination of the Al% content. However, the calibration was done by an indirect method. Instead of including a 1.83 MeV y-ray emitter in their mockup they added a known amount of natural potassium, using the 1.46 MeV y-ray of K*O as a standard for both K*O and Alz6 in the meteorite. To compensate for the slight mismatch in energy, they applied small corrections for photofraction, peak width and self-absorption. This method gave excellent results for potassium, as shown by the close agreement of their data with literature values. It may, however, have given low results for Alz6, owing to the “summing” of the 1*83-MeV y-ray with O-51-MeV annihilation quanta. This possibility is discussed by ROWE in the accompanying note. A significant part of the discrepancy may also be due to sampling statistics, ROWE et al. having accidentally chosen a larger number of shielded meteorites than we did. DISCUSSION Depth effect
If meteorites with short ages are omitted (italicized values in Table 1) only very few low values remain that could be attributed to a depth effect or escape of secoadaries. The values for carbonaceous chondrites are low, but that is an effect of composition, not size. These meteorites are known to have low eontents of Si and Al, the principal target elements for production of Alz8. Only five of the remaining meteorites have Alaa contents lower than 60 dpm/kg : Pillistfer, Plainview, Taiban, Vavilovka and Goalpara. The first four are marginal
Mctcorites with short comic-ray exposure ages
1801
cases, lying less than two standard deviations below either our arbitrary limit of 60 dpm/kg or the mean AlB6content for each class. Only Goalpara seems to be a clear-cut cast of shielding. The expected Ala6content for a meteorite of this composition is 60 dpm/kg, according to the data in the next section. Yet its observed Ala6 content is only 38 f 4 dpm/kg. A short age can be excluded, since its NeZ1content is 8.55 x IO-* ml STP/g, corresponding to an exposure age of ~25 lo6 yr (STAUFFER, 1961). Consequently shielding must be assumed. Reduction of the activity by a factor of 06 requires shielding by ~70 g/cmz, or ~20 cm. It is interesting that FLEISCHERet al. (1967) have estimated a shielding depth of >38 cm for another Goalpara sample from its lack of heavy ion tracks. This corresponds to a minimum mass of 2750 kg. Goalpara requires some further comment. The post-atmospheric mass of this stone was only about 2.7 kg. It is a find, and thus one cannot rule out the possibility that it was but a single stone from a shower of much larger total mass. One must wonder, though, why no other stones from this hypothetical shower ever turned up. Two alternative explanations are : high geocentric velocity, leading to a high degree of ablation, or, a preterrestrial collision that reduced Goalpara from a meter-sized to a decimeter-sized object. Such a collision would have had to occur <2 x lo5 yr before the fall of the meteorite, to prevent buildup of AlZ6to saturation levels. A variant of the latter possibility would be residence in a near-surface region of its parent body. No clear-cut choice among these alternatives is possible at present. It is curious that no other cases of substantial shielding turned up in this study. As shown in Fig. 1, ROWE et al. found a much larger percentage of low values. Possibly this discrepancy is due to sampling. More measurements and direct cross-checks will be needed to settle this point. Production
rate of Alz6 as a function
of chemical composition
Aluminum-26 in stony meteorites is made mainly from Al and Si, with smaller contributions from Mg, S, Ca and Fe. The relative production rates of a oosmogenic nuclide from several target elements can be estimated, if data for meteorites of different composition are available (STATJFFER, 1962). This method assumes a constant degree of shielding for all meteorites. For this calculation we used all data in Table i except for nine italicized values (short ages) and Goalpara (shielding). Generally, the data for each class were averaged, and assigned a weight equal to the inverse square of the standard deviation. In addition, a value of 2.30 f 0.25 dpm/kg was taken for the Al26 content of small, slightly shielded irons (mean of Arispe, Duchesne, Norfork and Treysa; LIPSCWTZ et al., 1965). A nominal sulfur content of O.5o/owas assumed for the irons. A least-squares analysis of these data gave the following production rates, in dpm A126/kgtarget element: 85 f 110 Mg: s: lOOf Al: 410 f 410 Ca: 280 f 130 Si: 210 f 110 Fe + Ni: 1.8 f 0.6. Most of these numbers look reasonable, showing the expected declinewithdistance from the target nucleus. The low value for Mg reflects the fact that only Mgw, with an isotopic abundance of 11.17 %, is a significant source of Al”. The high value
1802
DIETER HEYNANN snd EDWARD ANDERS
for Ca is definitely wrong, however. Both the Ca and Al values depend largely on two calcium-rich aohondrites: Sioux Co. (Ca = 7.15%, Al = 6.18%) and Angra dos Reis (Ca = 17.54%; Al = 4.62%). Taken at face value, the higher Al% content of Angra would seem to imply significant production from Ca, but the statistical errors alone make this conclusion very uncertain, to say nothing of differential shielding. Apparently the least-squares analysis has malapportioned the AP activity between Al and Ca. More measurements on Ca-rich achondrites are obviously needed. The large errors of these numbers are deceptive. They reflect the difficulty of correctly apportioning the total A.P activity among six elements. But the errors are complementary among geochemically coherent elements (e.g. Ca and Al), and thus tend to cancel as long as these elements remain coherent. For meteorites within the compositional range covered in this work, calculated production rates should be as accurate as the A126measurements in Table 1 on which these production rates are based, i.e. to about 5-10%. This expectation is borne out by the data in Table 3. Significant discrepancies are found only for Murray and Roda. Table 3. Observed and caloulated A126 contents AlB6 (dpm/kg) Clrtss or name Observed Cl Mm~ay c3 Abee Daniel’s Kuil Pillistfer H &s d. Reis Roda. Sioux CO. Irons
41.4 41.8 54.2 63 60 52 60.0 113 65.0
& 2.5 rt 4.2 f 2.7 * 7 f 5 &6 f 2.6 *f 91.4
64 & 4 99 f 5 2.30 & 0.26
Cfblc. 41.8 49.8 57.5 58.4 59.9 58.9 58.8 115.9 63.6 72.8 97.6 2.31
Nbort cosntic-ray exposure ages
Table 4 lists all meteorites known or suspected to have a low age. Three sets of ages have been oalculated for each meteorite whenever data were available. The He* and Ne” ages are based on the production rates given by ANDIZRS (1964). The Ala6 age was calculated from equation (1). A, was taken from the last column of Table 3, with an assigned standard deviation of &lo%. The AlaSdata for Mighei and Ladder Creek were taken from ROWE et al. A, for these two meteorites was based on their value for Murray and an average of their chondrite data, ex&&ng cases of less than 44 dpm/kg. CarbonaceoPts chlondtit~. Short ages are remarkably common in this class. Considerable effort was expended to determine whether Ivuna and Cold Bokkeveld had identical ages. Since they belong to different subolasses (Cl and C2), identical ages would imply an origin in the same collision and hence, presumably, the same body. Such a finding would have interesting genetic implioations. However, the
1803
Meteorites with short oosmio-ray exposure ages
suggest distinctly different ages, 0.18 and 0.32 lO@yr,with only a small probability of conoordance. The value for Cold Bokkeveld is higher than the upper limit of
date
Table 4; Meteoritea with short ooanio-ray expomre ages
Meteorite
He* age Clam (10’ yr)
Ivuna
Cl
Cold Bokkeveld
c2
1.0
0-25 0.3
Ne” age (100 yr)
Al% Ref.
2.0 2.0 0.39 0.2 2.6 2.4 0.26 1.5 1.8 0.8 5.4 4-5
h j h
Mighei
c2
Nogoya Pollen Grosnaja St. Marks Bath
c2 C2 c3 E5 H4
“Pseudo-Bath” Seres Farmington Ladder Creek
L
1.7 0.6 0.5 6.0 5.1 0.8
H L5
O-06 0.03
1.2 0.48 0.06
g e k
L6
Shaw Appley Bridge
L6 LL6
0.95 O-78 0.45 O-88
0.97 0.84 O-58 O-79
k e c c
a. b. c. d. e.
DE FELIUEet al. (1963). HEYXANN (1964, unpublished). HEY (1965). HEend M.AZOR(1967). HINTENBEROER et al. (1964).
: g d h h : f
AP (dpmlkg)
(d$$kg)
6.6 f 1.4
42 f 4
O-18f 0.05
12-Qf 0.9
50 f 5
O-32f 0.04
26 f 3
44 f 4
1-Of 0.3
6+3
31 f6 43.3f 1.7 33 f 3 64 f 4
50 f 5
50 58 58 59
f f f f
5 6 6 6
Ref.
(l?r)
O-14f 1.0f 1.5f O-9 f 2
i
0.08 0.5 o-4 0.2 1.9
? 61 f 6 o-4 f 0.4 34 f 3
59 f 6 64 f 6 52 f 5
21.5 SO.2 1.1f O-4
20 f 2 48 f 2
64 f 6 64 f 6
o-40f o-07 1.5f 0.5
a i
f. HINTEN~ER~ERet al. (1965). KIRSTENet aZ. (1963). :: MAZORet aZ. (1967). i. ROWE et aZ. (1963). STAUT~R (1961). ;: Z&iRINQER(1966).
50.2 106yr previously given by ANDEM (1962). A recheck of the old counting data revealed a readout error, incurred on a day when the printer did not function, requiring the data to be read off a photograph of the spectrum displayed on a cathoderay tube. The revised value, 2.4 f 3-l dpm/kg, still lies appreciably below the new result, but the diflerence may be statistical. It is quite puzzling that the He* and ,Nealcontents of Ivuna suggest an age in the l-2 lo6 yr range, much higher than the Aim age of 0.18 lo6 yr. Shielding is not a likely explanation, since Ivuna seems to have been a small meteorite. The ages of Nagoya (C2) and Cold Bokkeveld (C2) are presumably identical. Their Hes and Neal ages agree fairly well. The Al% age of Nogoya is appreciably lower, but the Alaa content of this meteorite has a large error (6 f 3 dpm/kg), owing to the small sample size, and hence this value still lies within 2a of the Cold Bokkeveld result of 12.9 f O-9dpm/kg. Thus the two results could be identical. MAZOR et cd. (1967) have found three other C2 chondrites with ages near O-3 l@ yr. The situation for the remaining C2 chondrites, Pollen and Mighei, is less clear-cut. Their AIM ages are identical, but this agreement may not be real. The Aim age of
DIETERHEYMANNand EDWARDANDERS
1804
Pollen agrees within its rather large error with the He3 and Nezl ages. Pollen was a, very small meteorite (0.254 kg) in which a lower production rate of Neel and AIzs would be expected owing to the lower flux of secondaries. Probably the true age of this meteoritelies near 1.7 lo6 yr. The A126age of Mighei is based on a measurement by ROWE et al. and may possibly be afflicted with a systematic error. Unfortunately, the He3 and Ne21ages of this meteorite are strongly discordant and offer little guidance. Obviously Mighei will have to be reinvestigated more carefully. * The C3 chondrite Grosnaja has an A12*age agreeing rather well with its Ne21 age. The He3 age is appreciably lower, suggesting diffusion losses. Enstatite chondrites. The three ages of St. Marks are in tolerable agreement. &on&e choondrites. The “Bath” specimen measured by KIRSTEN et al. (1963) seems to have been a mislabeled sample of a hypersthene chondrite (J. Z&RINOER, private communication). An authentic sample of Bath, obtained from the Harvard Collection, gave a noble-gas content differing strongly from the “Bath” sample of Kirsten et al. (Table 5). The He3 and Ne21 content of authentic Bath suggest a cosmic-ray age near 5 lo8 yr, in marked disagreement with the earlier measurement on “Pseudo-Bath.” The A12s content is consistent with the 5 lo6 yr value. Table 5. Noble gases in Bath chondrito (lo-’ ml STP/g) Met.
Source
He3
“B&h” Bath Bath
Tiibingan Harvard Yale
1.5 12.0 10.2
He4 115 1250 1258
R-e21
Ar,=
A#”
Ref.
0.45 2.04 1.93
0.05 0.55
350 4500
1 2 3
1. KIRSTENet ~2. (1963). 2. HEYMANN(1964, unpublished). 3. HINTENBEROER et al. (1966).
The case of Seres is rather clear-cut. The He3 age (O-06 lo6 yr) is much lower than the Ne2r age (0.48 lo6 yr), suggesting severe diffusion losses. Apparently even the Ne21 age is far too low, since the Alw content of this meteorite seems to be at saturation level, corresponding to an age of 2 15 lo* yr WOOD(1967) has found evidence of
severe reheating in this meteorite, which may account for the gas loss. Hypersthene chondrites. Three meteorites of this olass have short exposure ages. Farmington is the most extreme example, with an age of ~0*02 lo8 yr confirmed by both Al2s and noble-gas measurements (I(IIZEITENet al., 1963 ; ANDEES, 1962 ; DE FELICE et al., 1963; SIGNER, 1963; ZXERIXGLER,1966). Sha.whasesomewhat atypical texture and composition (FREDXUKSSONand M&ON, 1967) but Ladder Creek is & normal hypersthene chondrite. Both of these meteorites were found by Dr. N~IN~ER. It is interesting that all three hypersthene chondrites with ages less than 1 lo6 yr have clearly different exposure ages. If these valnes are typioal, then the eollisions forming hypersthene chondrites would seem to he s-d only ~0.6 10’ yr apart. At this frequency, the age distribution above 1 lo6 yr must reaembb a continuum, in which only events of exceptional magnitude might &and out MI discrete * Note addedin roof. A 62-g Mighei sample from the Field Museum gave sn Also content of 35.6 f 2.0 dpm/kg, close to the value for Pollen. With A, = 60 & 6 dpm/kg, the AIW age becomes l-3 f 0.4 lo6 yr, decidedly lower t&n the N& ttge.
Meteorites with short cosmic-ray exposure ages
pdxs.
1800
It is not surprising that the reality of such peaks is still a m&ter of contro-
VerSy.
The AlaSage of AppIey Bridge is appreciably higher than AmpMric ~~~~~. might be lessened if A, for Apples Bridge the Hea and NeQ ages. This ~sa~ment were somewhat higher than 64 dpm/kg. 2t
Ring Asteroids :iShwl Life” (192)
-
Apollo Asteroids (187)
Cosmic-my
$0 6xpwm
age
d0
100
(my.1
Fig. 2. Observed cosmic-ray age distribution for chondritee, compared with theoretical ~t~butio~ o&ulated by &NOLD’S Monte c8IYlO method for three di&rent type8 of ptarentbodies. Nutnber in parentheses!.give8 mmlber of caees in each astqory. Both the Moon and the Apollo asteroids show the observed preponderwce of short agee between 0 and 10 IO6yr* but the Moon has 8 Ierge excess in the 0-Z 1U8yr interval.
As long as only two stony meteorites with short ages were known, they could be dismissed as s~tisti~ freaks. However, the number has now grown tu more than a dozen, and includes representatives of all ohondrite classes except the bronzites. ObviousIy, we are dealing here with a general eharac~~stic of most, if not all chondrite iclasses. A short exposure age implies an orbit of high o&lision probab~ty with the Es&h. 18
1806
DIETERHEYMANNand EDWARDANDERS
From the work Of OPIK (1951,1963,1965) and ARNOLD (1964, 1965) it is obvious that only objects initially in Earth-crossing orbits need to be considered. The mean collision time for objects in Earth-crossing orbits is of the order ~10~ yr ; thus capture of a meteorite in < IO6yr has a probability of only a few per cent. If an additional step were needed to achieve an Earth-crossing orbit (e.g. earthward deflection from a Mars-crossing orbit), the probability of completing both steps in ~10s yr becomes vanishingly
small.
ARNOLD has favored the Moon as a source of stony meteorites. from the Moon Earth-like
at slightly
more than escape velocity
orbits, with a very high probability
Objects expelled
would find themselves
of collision with the Earth.
in
This
de richesses, as far as meteorites with short ages are concerned. Monte Carlo calculations by ARNOLD show that a majority of such ejecta would strike the Earth within the first 2 lo6 yr. Since stony meteorites with such short ages are uncommon, ARNOLD proposed that no major impacts have occurred on the Moon in results in an embarras
the past 2 IO* yr, and that most short-lived objects from previous impacts were swept up by the Earth in prehistoric times. An alternative source of stony meteorites are the Apollo asteroids. are in Earth-crossing
compares the observed tributions.
age distribution
All computations
Evidently,
They already
orbits and can thus produce meteorites with short ages. Figure 2 for chondrites
with three calculated
dis-
were done with ARNOLD’S Monte Carlo program.*
the ring asteroids can be eliminated outright, since they do not provide
any ages shorter than 3 lo6 yr, and have a surplus of high ages. Both the Moon and Apollo asteroids give a much superior match to the observed distribution respects. meteorites
The Apollo with
short
asteroid distribution ages,
without
the
shows about ad
hoc
in both
the right proportion
of
assumption required for the
Moon. A more complete discussion of meteorite orbits and ages will be given elsewhere. Acknowledgmeltts-We thank JOHNL. BARKERJR., AGATHAFRIS, KIYONO FUSE and LEOSARD LEVIN for assistance with the counting. The regression computations were done by E. ALLEN PETERSON,KENNETHROBERTSand KEN Sowmsm. This work wae supported in part by the U.S. Atomic Energy Commission, Contract AT(ll-1)382, and the National Aoronautics and Space Administration, Grant NsG-366. One of the spectrometersused in this study had been built under NSF Grant G-14298. RKFERENCES ANDERSE. (1960) The record in the meteorite+--II. On the presenceof AlaBin meteorites and tektites. Beochim. Coamochim. Acta 19, 63-62. ANDERSE. (1962) Two meteorites of unusually short cosmio-ray exposure age. Science 188, 431-433. ANDERSE. (1964) Origin, age, and composition of meteorites. Space Sci. Rev. 3, 583-714. ANDERSE. and ARNOLDJ. R. (1966) Age of craters on Mars. Science149, 1494-1496. ARNOLDJ. R. (1964) The origin of meteorites as small bodies. I. In IuotipaC~VW?COW&C Chemdy (editors H. Craig, S. L. Miller and G. J. Wasserburg), pp. 347-364. North-Holland, ARNOLDJ. R. (1965) The origin of meteorites as small bodies. II: The model. Astrophys. J. 141, 1637-l 647. &NOLD J. R. (1966)The origin of meteorites as small bodies. III: anera considerations. Astrophya. J. 141,1548-1556. + We are indebted to Prof. ARNOLDfor making his data and program available to us. A brief summary of these data WOW p~viou~ly published by AND~ORS end ARNOLD(1966).
Meteoritea with short cosmic-my exposure qes
1807
BISWASM. M., MAYER-B~RICXEC. and GEI?rNERW. (1963) Cosmic-rmyproduced NL+ and Ape activities in chonclrites. In Earth Soknee and Meted& (editms J. G&s end E. D. Goldberg) pp. 207-218. North-Holland. See also: ~YEE-B~RICICE C., BIEWAEM. M. and GENTNERW. (1962) y-spektroskopischeUntersuchuugenan Stetieteoriten. 2. Natwformh. 17% 921-924. C%UKUEZAB~ M. (1960) Cosmic-my-induced radioactivity in meteorites-B~O, Al*6, and Co” in aerolites and tektites. In Nuclear Chem. Research at Carnegie Inst. of Tech&cgy, 1960-61, Progress Report, by T. P. Kohmsn and A. A. Caretto, Jr. C~ESSY P. J. JR. (1964) Cosmogenic rsdionuclides in stone meteorites. Ph.D. Dissertation, U.S. AEC Report NYO-8924. DE FELICEJ., F~IO G. G. and FIRE-E. L. (1963) Cosmic-my exposure age of the Farmington meteorite from radioactive isotopes. Science142,673-674. EH~N W. D. end KOIXMANT. P. (1958) Cosmic-ray induced rcdioectivities in meteoritesI. Chemical and radiometric procedures for &minium, beryllium, and cobalt; and II. Al*O, Beis, end Coaoin aerolites, siderites, and tektites. Geochim. Cocmzochim. Acta 14,340-363 end 364-379. FIREIKAN E. L. (1967) Radioectivities in meteorites end cosmic-ray v&&ions. Geochim. Cosmochim. Acta 81,1691-1700. FLEI~~HERR. L., PRICrC P. B., WAIXER R. M., MAUREYJYz M. and MORC+AN G. (1967) Tracks of heavy primary cosmic rays in meteorites. J. Cfeophye. Rec. V&366-366. FR~~DRIKESON K. end MASONB. (1967) The Shew meteorite. Geochim.Cosnzochdm. Acta 81, 1706-1709. GFELLERChr., HOUTER~S F. G., OESCHUER H. and SC~ARZ U. (1961) “ry Koinzidenzmessung zur zersMnmgsfreien Messung des Geheltes von Meteoriten an Positronenstrahlern und y-ektiven It&open. Helv. Phy8. Aota 84, 466469. HEYMANN D. (1965) Cosmogenic and radiogenic He, Ne, and Ar in amphoteric chondrites. J. cfeqhy8. Rea. 70, 3736-3743. HEYMANND. and Mazoa E. (1967) Light-dark structureend rare gas content of the cerbonaoeous . chondrita Nogoye. J. aeophy8. Rm. Z&2704-2707. HINTENBER~ H., K~NIQ H., SCHULTZL. and Wm H. (1964) Radiogene, spmllogeneund primordiale Edelgese in Steinmeteoriten. 2. Naturforsch. 1% 327341. HINTENBER~ERH., K(~NIUH., SCHULTZ L. and Wm H. (1966) Rediogene, spellogene und primordiale Edelgese in SteinmeteoritenIII. 2. Naturforcrch. ala, 983-989. HONDAM. and ARNOLDJ. R. (1964) Effects of cosmic rays on meteorites. Science 148,203-212. HONDA M., UMEMOTOS. and ARNOLDJ. R. (1961) Radioactive species produced by cosmic reys in Bruderheim and other stone meteorites. J. Geophy8. RIM. @3541-3646. KIRETENT., KRANKOWSEYsnd Z&mINTER J. (1963) Edelgcs- und Kelium-Bestimmungen an einer grosserenZehl von St&meteor&en. Qeochim. Coemoohim. Actu 27, 1342. LIPSCEUTZ M. E., SIQNERP. and ANDERSE. (1965) Cosmic-ray exposure ages of iron meteorites by the Neal/Alas method. J. Geophys. Re8. 70, 1473-1489. MAZORE., HED. and ANDERSE. (1967) Rare gases in carbonaceous chondrites and ureilites. Menuscript in preparation. NININQERH. H. and NININUERA. D. (1950) The Nininger CoZEectionof Meteoritea. Winslow, A&one. NORDE~N~ D., TOBAILEYJ. end SCHINEIZICR M. (1965) La redioectiviti de le meteorite Grenes (chute du 13 novembre 1964) mesun%epar spectrometriey. C. R. Acad. Sci. Pa+ m, 6665-6666. CPIK E. J. (1951) Collision probabilities with the planets and distribution of interplanetcrry matter. Proc. Roy. Irish Acad. MA, 165-199. C~rx E. J. (1963) Survival of comet nuclei and the asteroids. In Advancer, in Astronomy and Astrophysics (editor Z. Kopal), Vol. 2, pp. 219-262. Academic Press. CPIKE. J. (1965) The stray bodies in the solar system. Pert II. The cometary origin of meteorites. In Advancea in Aetronomy and Astrophy8ic8 Vol. 4, pp. 301-336. Academic Press. RI~ETMIRER. A., SIB~ANTON J. R. and Ko~lldl~~T. P. (1969) Disintegrationscheme of long-lived Ales. Phgle. Rev. 118,1069. ROWE M. W. (1963) Quantitative measurementof gamma-ray-emitting radionuclidesin meteorites. Los Alemos ScientificLab. Report LA-2765.
DIETERHEYMANNand EDWARDANDERS
1808
Rown M. W., VAN DILLA M. A. and ANDERSONE. C. (1963) On the radioactivity of stone meteorites. ffeochim. Cosmochim. Acta 27, 983-1001. SIIEDLOVSKY J. R., CRESSYP. J., JR., and KOHMANT. P. (1967) Cosmogenic radioactivities in the Peace River and Harleton chondrites, J. Geophys. Res. 72, in press. SIQNERP. (1963) Unpublished work. STAUFFERH. (1961) Primordial argon and neon in carbonaceous chondrites and ureilites. Geochim. Cosmochim.Acta 24, 70-82. STAUFFER H. (1962) On the productionratios of rare gas isotopes in stone meteorites. J. Geophya. Res. %7,2023-2028. VAN DILLA M. A., ARNOLDJ. R. and ANDERSONE. C. (1960) Spectrometric measurement of natural and cosmic-ray induced radioactivity in meteorites. Geochim. Cosmochim. Acta RO, 115-121. VAN SCHMXJS W. R. and WOODJ. A. (1967) A chemical-petrologicclassificationfor the chondritic meteorites. Geochim. Cosmoohim.Acta 31, 741-765. VISTE E. and ANDERSE. (1962) Cosmic-ray exposure history of tektites. J. Geophys. Res. 67, 2913-2919. WELLERR. I., ANDERSONE. C. and BARKERJ. I;., JR. (1965) Radioactive contamination of contemporary lead. Nature 206, 1211-1212. WOOD J. A. (1967) Chondrites: their metallic minerals, thermal histories, and parent planets. Icarus 6, l-49. Z~~HRIN~ER J. (1966) Chronology of chondriteswith rare gas isotopes. Metwrititilca 27, 25-40.
AppemIix-AP
determination in atone meteorites by yaw fiwdrome@
M. W. ROWE Miller Institute for Basic Research in Science and Department of Physics, University of California, Berkeley, California WE AI-ZE grateful to ProfessorE. ANGERS(personalcommunication) for callingour attention to a systematic error in the Alss measurements in stone meteorites by ROWEet al. (ROWEand VAN DILLA, 1961; ROWE et al., 1963; ANDERSONet aE., 1964; Rowe and MANUEL,1964). In these reports, the A126was calculated indirectly by comparing the 1+33-MeV Al26 photopeak with the Table 1. Results of two calibration techniques on measurement of meteoritic Ala6 (ROWE et al., 1963). The A126standard yields results unaffected by summing. A128via KC1 Standard* Meteorite A126via A12sStandard Bruderheim Ehole Hamlet Harleton Ladder Creek?
0.97 0.96 0.89 0.93 0.90
+ f f * f
0.04 0.04 0.04 0.04 0.04
0.93 & 0.04 * The average of the two calibrations was used to estimate the reported Al*6 content of these meteorites, except for Hamlet where only the KC1 standard was used. t The Ala* oontent of Ladder Creekwas determined by “/-o/ coincidenceas well; it agreed exactly with the value obtained (32 f 3 dpm/kg) via the AlaBstandard. A second sample of Ladder Creek calibrated with a KC3 standard gave a value of 36 f 3 dpm/kg.
_
Meteorites with short cosmic-ray exposure ages
1809
l-46-MeV photopeak of KM in a KC1 standard. Small corrections were applied to account for differencesin efficiency, photofraction, peak width, and self-8bsorption between the two y-ray energies. We neglected, however, to correct for the loss of the 1*83-MeV y-rays which summed with O-51-MeV y-rays. The O-51-MeV y-rays resulted from annihilation of positrons emitted by Ala* in coincidencewith the 1.83-MeV y-rays. When a 0*51-MeV y-ray and a l-83-MeV y-ray interact with the detector simultaneously, only a single event is recorded-a 2*34-MeV sum pulse. A pulse of less than 2.34-MeV occurs when one of the summing y-rays is degraded by Compton scattering. In either case, the number of events appearingin the 1.83-MeV photopeak is reduced, occurring instead in another energy region. Most of the spectra of Rower.et al. were taken in the energy region 0.2-2*0-MeV; only three published spectra extend to 2.34-MeV (Stannern: ROWE, 1963; Pinto Mountains and Norton County: VAN DILLAet al., 1960). These three spectra do show excess counts in the energy range 1%2.4-MeV and a slight hump at about 2*3-MeV. HEYMANNand ANDERS(1967) point out that the average Ala6 content in ordinary chondrites measured by them is about 20% higher than that of Rowx et ~2. (1963). Of the meteorites studied by Rows et al. (1963), five were calibrated with a uniformly mixed Alla6standard in addition to the usual KC1 standard. These data (summarizedin Table 1) permit an estimation of the magnitude of the summing correction. This evidencesuggeststhat a correctionof 7 f 4 % is necessary to compensate for the loss of 1+33-MeV y-rays by summing. Clearly, additional measurementson duplicate samples 8re desirableto elucidatethe remainingdiscrepancybetween the data of HEYMANNand ANDERS(1967) and ROWE et al. REFERENCES ANDERSONE. C., Rowx M. W. and UREY H. C. (1964) Potassium and aluminum-26 contents of three bronzite chondrites. J. Geophys. Res. 69, 564-565. HEYMANN D. and ANGERS E. (1967). Meteorites with short cosmic-ray exposure ages, as determined from their Alss content. Geochim. Coamochim.Acta, 31,1793-1809. ROWEM. W. (1963) Quantitative measurementsof gamma-ray emitting radionuclidesin meteorites. Los Alamos ScientificLaboratory Report LA-2765. ROWE:M. W. and l&xux~ 0. K. (1964). y-Radioactivity in the Fayetteville meteorite. J. f&o~hys. Res. 69, 1944-1945. ROWE M. W. and Vm DILLA M. A. (1961) On the radioactivity of the Bruderheim chondrite. J. Cfeophy8. Res. 66,3553-3556. Rown M. W., Vm DILLAM. A. and ANDERSON E. C. (1963) On the radioactivity of stone meteorites. Geochim.Coamochim. Acta 2’7, 983-1001. VAN DILL~LM. A., ARNOLD J. R. and ANDERSONE. C. (1960) Spectrometric measurement of natural and cosmic-ray induced radioactivity in meteorites. Qeochim. Coamochim. Acta 20, 115-121.
Meteorites with short cosmic-rayexposureages,88 determined
from
their
Al= content
DIETER HEYMANN* and EDWARD ANDERS Enrico Fermi Institute for Nuclear Studies and Departments of Chemistry and Geophysical Sciences, University of Chicago, Chicago, Illinois 60637 (Received 29 April 1967; accepted ira revised form 30 May 196’7) &&&The Al26 content was determined by Y-+J coincidence spectrometry in thirty-one stony meteorites: ten carbonaceous, four enstatite, three bronzite, five hypersthene and five amphoteric chondrites; one angrite, one euorite, one ureilite and one diogenite. In the following eight meteorites, the A126 content was low enough to permit determination of a cosmic-ray exposure age (million years): Ivuna 0.18 f 0.06, Cold Bokkeveld 0.32 f O-04,Nagoya O-14 f 0.08, Pollen 1-O + 0.6, Grosnaja 1.5 f 0.4, St. Marks 0.9 f 0.2, Shaw O-40 f O-07, Appley Bridge 1.5 f 0.5. Seres and Bath have gas contents suggesting ages < 1 lo6 yr, but their A12’I contents correspond to ages > 1.5 and 2 1.9 lo6 yr. These two discordant cases appear to be due to diffusion losses and a sample mixup, respectively. The Goalpara ureilite has only about 60% its expeoted Al26 content. If this result is attributed to shielding, Goalpara must have had a preatmospheric mass of 2 700 kg. The mean Al26 content found for sixteen samples of ordinary and enstatite chondrites is 63 dpm/kg, appreciably higher than the value of 47 dpm/kg determined by means of y-ray speotrometry (ROW et al., 1963). The discrepancy may be due to a calibration error in the latter method, and to sampling statistics. The present data scatter little from the mean, thus mdi. eating a constant Al26 production rate for most chondrites. Production rates of Al26 in stony meteorites have been estimated for the following target elements: Mg 85 & 110, Al 410 f 410, Si 210 & 110, S 100 & 90, Ca 280 + 130, Fe + Ni 16 f 0.6 dpm/kg. INTRODUCTION TYPES of information can be obtained from measurement of cosmic-ray produced AP in meteorites : the degree of shielding from cosmic rays, and the exposure age. If the cosmic-ray exposure age of the meteorite is long compared to the half-life of A126,O-74 lo6 yr, a steady state is reached in which Al26disintegrates at exactly the rate at which it is formed. The production rate depends on chemical composition and sample depth; if the former is constant, as in a given class of meteorites, the Al28 content can be used to infer the sample depth. However, low values must be expected not only in large, heavily shielded meteorites, but also in small ones, owing to escape of cosmic-ray secondaries. Aluminum-26, being only a few mass numbers removed from the principal target nuclei Ala’, Mgee, and SP, is made mainly by low-energy secondaries. If the cosmic-ray age of the meteorite is not much longer than the half-life of A126,a steady state will not be achieved. Instead, the Ala6level A will be smaller than the steady-state level A, by a factor 1 - e-%, where Ais the decay constant of APi, and t, the cosmic-ray exposure age : Two
A = A,(1
-
e--lt)
* Present address: Department of Geology, Rice University, Houston, Texas 77001. 1793
(I)
1794
DIETER
HEYMANN
and EDWAECII ANDERS
If A, is known, the cosmic-ray age can be calculated from the measured Al26content, A. This method is particularly useful in the range O-l lo6 yr, where cosmogenic noblegas contents are low, and mass spectrometric measurements hence are difficult. The existence of meteorites with short ages has become an important argument in discussions on the origin of meteorites (ANDERS,1962; ARNOLD,1965; C)PIK,1965). The first Alz6 measurements in meteorites were performed by EHMANNand KOHMAN(1958) by p-counting of chemically isolated aluminum fractions. Later measurements were done mainly by non-destructive techniques. A few meteorites, including two of very short age, were measured by y-y coincidence spectromet’ry (ANDERS, 1960, 1962; GFELLERand HOUTERMANS,1961; BISWAS et al., 1963; DE FELICEet al., 1963). This method measures the two coincident O-51l-MeV photons emitted in the annihilation of positrons from the decay of A126. The majority of measurements to date were done by y-ray spectrometry, using the 1X%-MeV y-ray of A126 (VAN DILLA, ARNOLD and ANDERSON,1960; ROWE, VAN DILLA and ANDERSON,1963). This method requires large samples ( ~1 kg), but has the advantage of permitting simultaneous measurement of potassium, via the 1*46-MeV y-ray of K40. The present study was again carried out by y-y coincidence spectrometry. Its main purpose was determination of accurate cosmic-ray ages for meteorites of low cosmogenic gas content. Ten such meteorites were measured. To provide an estimate of the saturation level of A126in various meteorite classes, twenty-one other meteorites of long cosmic-ray age were also measured. Preference was given to rare types, in order to take advantage of the high sensitivity of the method. Samples as small as 5 g oan be measured with a standard deviation of & 20%) and for samples of 10 g or larger, an accuracy of 5--10% is readily attainable. PROCEDURE Coulatera Two spectrometers previous description
were used.
One of these had remained virtually unchanged from its
(LIPSC~UTZ et al., 1965), except that the detectors now consisted of
6.4 x 3.8 cm ory&& with 1.3.cm N&I light pipes, mounted on EMI 9678 photomultipliers. The second one had evolved gradually from the configuration described by Vmrx and ANDICR~ (1962). During the period covered by our measurements (1962-1967) the vacuum-tube electronics was replwed by RIDL transistorized single-channel analyzers. Expander amplifiers were used to minimize fluctuations in channel width. The iron shield w&s replaced by e lo-cm lead shield. The lead had been salvaged from cm early 19th.century shipwreck, and had o very low content of Pbfis and other radionuclides. * Channel level and width were calibrated by an Near+source at frequent enough inter&s to keep the counting efficiency for 61 I-KeV coincidences constant to within 3 %. Single pieces weighing more then 20 g were wrapped in Saran pleetio film and were taped directly to the crystal. Smaller pieces were placed inside polystyrene boxes of o.d. 3P64mm, height 13-22 mm. Samples consisting of powder or multiple fragments were likewise counted in polystyrene or lucite boxes. For carbonaceous chondrites, the polystyrene was coated with a * It was obtained from Muller’s M&eel Maztschapij N.V., Prinsengracht 544, AmsterdamC. Lead of comparably low radioactivity has since been identified among commercial samples
(WELLER,,A_NDERSON and BARXER, 1966).
Meteorites with short cosmic-ray exposure 8ges
1796
thin, vepor-deposited layer of aluminum, to prevent conteminetion of the meteorite with phbstic. Total counting times ranged from 1 to 10 weeks. Background For background dete rmirmtion, dtite slabs or powder were counted in the s8me geometry 8s the Bemples. Nornmlly sample end beckground counts were 8lternated every 3-7 days. Often backgrounds re nmined st8ble over many months. In such oaeee, background d8t8 for 1-2 months preceding and following the Bemple count were included in the average. Typical backgrounds varied from about 9-10 counts/l000 min in 1963 to 6-7 counts/l000 min in 1967. Most of the deorerease ~8s due to the transfer of the counters into 8 subterranean counting laboratory. Occ8sion8hy, when interference from outside sources w8s suspected, 8 dunite blank was meseured concurrently on one spectrometer while the sample W8S being counted on the other. Sample and blank were alternated 8t regular intervals. Calibration The positron counting yield for each sample ~8s determined by me8ns of 8 mockup of identical shape, containing 8 known amount Of N8 1s. For meteorites counted 8s single pieces, mockups were prepared by the technique of ROWE et al. (1963). A mold of Al-foil (O-08mm thick) ~8s shaped around the meteorite and ~8s reinforced with 8 Coating of Rezolin epoxy resin. The mold wm then filled with 8 st8nd8rd mixture of nickel powder, conteining 6-10x NaWl of known specific aotivity. The peaked density of this powder ~88 in the range 3-5-3-9 g/ml, close to the density of ohondrites. The nomixml electron density of 2.88 x 108*/g8lSO agreed rather well with the value for chondrites, 2.92 x 10s8/g(ROWE, 1963). Samples consisting of powder or multiple fregments were always counted in polystyrene boxes, fllled sufficiently to approxinmte cylindrical geometry. For these semples, mockups were prepared by Wing boxes of the s8me size with Na*sCl or Ni-N&l mixture. Sometimes the density of the mockup differed slightly from that of the sample. In such cases, the counting yield of the s&mple, Y,. ~8s found by applying 8 self-absorption correction to the counting yield of the mockup, Y,,,: Y, = Yoel~-*lPh where m and a are the m8sses of mockup end sample; a, the cross-sectional are8; and ,u, the m8ss absorption coefficient for 610 keV y-y coincidences in the N&l-Ni mixture (0.098 oma/g, 8s determined from 8 series of Na%‘l-Ni mixtures of verying 11188s density). This correction treats the s8mple 8s 8 plane-p8r8llel slab, end thus is not quite appropriate for samples of irregulsr shape. However, the error introduced is a negligible second-order effect, since the correction itself r8rely exceeds 10 %. Two N8** standards were used for calibration, both from the Nation81 Bureau of Stctndards. (1) NBS No. 4922: /l+ activity = 64.3 x lo4 Bf set on 10/14/1964. (2) NBS No. 4922-D: /Zl+ ectivity = 656 x lo4 /?+/secon 6/30/1964. From these stand8rds, N&l of low specific activity ~8s prepared (300-1000 Bf/min per g) for use in mockups. Direct intercomperison of N&l samples from the two standards revealed a discrepancy by 8 f8ctor of I.010 f 0.007 between calculated and observed activity ratios. This discrepancy mey be largely due to volumetric dilution errors rether th8n to errors in the nominal activity of the standerds. All calibrations baeed on the older standard (No. 4922) were therefore adjusted by this factor, to meke them consistent with the more recent set of me8surements. Dec8y corrections were made on the basis of 8 half-life of 942.3 days for Narn. In some owes, two or three mockups of the s&mesample were prep8red, to test reproducibility. If sufficient c8re ~8s exercised, the results were consistently reproducible to 6% or better, but in 8 few c8ses where shape or mess of the sample were not too carefully dupliceted, discrepancies 8s large aa 10% were found. For most samples, the Statistical counting error exceeds the estimated error in oaiibration, and only the former is quoted in our table of results. In those few 088% where the calibration error w8s the larger of the two, the st8nd8rd deviation ~8s raised accordingly.
1796 Analyti
DIETER HEYMAKN and EDWARD ANDERS of data
For most meteorites the calculation of Alz8 content was completely straightforw~d. Since only about 84% of all Alee decays proceed by positron emission ~RI~HT~~E et al., 19591, the observed positron rate ww divided by 0.84 to obtain the disintegration rate. In the ease of Hamlet, a 1959 fall, a correction for 2.58-y Na22 was required. The sample (consisting of fusion cruet ahips) was counted five times during 1962, 1963 and 1966. The Na22 and Ala6 contents were found by a least-squares analysis of the decay curve. A small correction for Na22 was also needed for Murray, a 1950 fetI1counted in 1962. %bXULTS
The data are listed in Table 1. Meteorites have been grouped by olass, according to the classififictation scheme of VAN SCHMTJSand WOOD (1967). It is instructive to compare our data with those of other workers. Direct comparisons are possible in only a few cases, to be discussed later, since onr samples were de~bera~ly chosen to avoid dup~cation. However, we can compare the data for ordinary and enstatite chondrites as a group (Fig. 1). Meteorites known to have short cosmio-ray ages were omitted, and so were data with standard deviations greater than &20%. Apparently there is a systematic discrepancy between our data and those of ROWE etal. None of ROWE’Sdata fall above 60 dpm/kg, while twelve of our sixteen Table 1. Aluminum-26 content of stony meteorites
Meteorite*
Clase§ Souroe 11
Alais* .IvUna* Orgueil* Cold Bokkeveld* Mlrray*t Nogoye Pollen Felix Grosnaja ornans
Cl Cl Cl c2 c2 c2 c2 c3 c3 c3
Abee Daniel’s Kuil Pillistfer St. l&%&r I St. Marks II
E4 E6 E6 E5 E5
C F F U S
Bath* PlainviewS Seres
H4 H5 H
H w F
Auseon* MeKmney 1* MeKinney II* Shaw Taiban II Zavid
L5 L4 LP L6 L5 L6
w F H A MP G
F CT,GR M F U F JW F F F
Number
Me 1486
Me 1736, 7, 8 6, 10 Me 1680 Me 1330 Me 1732 Me 1766 Me 1500 Me 1646
92. 1216 Me 1866
62/l
Recovered mass (kg)
Sample ma08
6 0.7 12 -4 13 4 0.25 3 3 6
7.44 33.4 30.7 45.9 43.0 950 5-58 50-3 73.3 49.2
107 1 23 14 14
(g)
25.8 15.8 39.2 209 14.1
Al26 # (dpmk) 44 * 6 6.6 -_tI.4 41 * 3 12-9 f oa9 42 f 4 6zk3 31 j, 6 54 f 4 43.3 & 1.7 53 *4 63 60 52 35 31
f 7 k6 + 6 f 2 & 3
21 700 8.5
53.4 68.0 39.0
64 i 4 55 * 2 61 &6
50 >190 >lOO 3.7 25 93
43.5 21.8 25.2 70.5 11.2 71.9
62 68 71 20 59 68
f 5 i 4 +5 & 2 f 5 -+.3
1797
Meteorites with short cosmic-ray exposure ages Table 1. (continued)
Meteorite*
c1assg source II
Number
Appley Bridge Bandong Hamlet * Jelica I Jelica II Vavilovke
LL6 LL6 LL4 LL6 LL6 LL
u F
614 Me 1753, 4
F F F
Me 511 Me 1353 Me 1659
Angre doe Reis Goalpara* Rode Sioux Co. *
An U Di Eu
F F F A
Me 1368, 9 Me 764, 1771 Me 1376
Recovered m&&3 (kg) 15 11.5 3.7 34 34 16 1.5 2.7 o-4 4.1
Sample mass (g) 64.5 25.4 51.5 67.5 43.3 117-l 9-48 12.36 25.12 10.78
Alss# (dpm/kg) 48 67 64 66 68 58 113 38 64 99
f f f f f f
2 6 6** 5 4 3
19 & 4 f 4 f 5
* Meteorites counted a~ powder or fragments are marked with an asterisk. Plainview, McKinney, Shaw and T&ban are finds; all others are falls. The two McKinney semples were first counted 88 single pieces, and then crushed and recounted. No systematic differenceswere found. t The Murray result was publishedpreviously (ANGERS,1962), but has been revised slightly, from 41 f 4 dpm/kg to 42 f 4 dpmlkg. $ An earliercount on the same specimen gave 68 f 5 dpm/kg, possibly due to a crtlibration error (ANDERS,1960). $ Claesiflcationof chondrites according to VAN SCJXMUS and WOOD (1967). Achondrites: An = angrite; U = ureilite; Di = diogenite, Eu = eucrite. 11We are indebted to the following donors who furnishedsampleson a gift, loan, exchange,or purchase basis. A = American Meteorite Museum, Sedona, Arizona. C = Geol. Survey of Canada (Dr. K. R. Dewson). F = Field Museum, Chicago (Dr. E. Oleen). G = Gregory and Bottley, London. GR = Dr. G. W. Reed, Jr., Argonne N&l. Laboratory. H = Harvard Collection (Dr. C. Frondel) JW = Dr. John A. Wood, Smithsonian Astrophys. Obs., Cambridge, Mass. M = Mu&e d’H.istoireNaturelle de Montauban (Mr. A. Caveille) MP = Max-Planok Inst. f. Chemie, Maim (Dr. H. Wlinke). S = South African Museum, Capetown (Dr. A. W. Crompton). U = U.S. National Museum, Washington (Dr. R. S. Clarke). W = Warda Natural Science Establishment, Rochester, N.Y. # Low values due to short exposure age are printed in italics. They were not includedin Fig. 1, or in the average A, values in Table 4. * * Also Na%, 105 f 18 dpm/kg at time of fall.
values do. To bring these two distributions into agreement, a shift by some 20% would be required. Some thirty-seven measurements by other workers exist, and they, too, show a tendency to lie above 60 dpm/kg and thus match our distribution rather than ROWE’S. Most of the low results among the, thirty-seven are due to Cressy. They were determined on chemically isolated aluminum fractions. Some difficulties were encountered in the yield determinations, and it is conceivable that these results are afltlicted with a systematio error.
1798
DIETERHEYMANNand EDW_&RD ANDERS I
I
I
Al26 Content a<100 6
Rowe
et al, 1963
kg
I
of Chondrites
n >lOOkg
(~1
4 2 5 In 00 0 z
Heymonn
and Anders,
1967
( y -7 coinc.)
$2 E
$0
4 2 0
6b
50 Al26
dpm/
7b
kg
Fig. 1. Alas oontent of ordinrtryand enstatite chondrites. Meteorites with short ages, or errors greater than i- 20 % have been omitted. The date by ROWE et al. (1963) and ANDERSONet aE. (1964) were determinedby y-ray speotrometry (top). They lie systematicallylowerthan our values obtainedby ~9 coinoidenc spectrometry (middle), and data from other leboratories, determined by y”y coincidence spectrometry or /?-oounting (bottom). Date in the bottom part of the figure were taken from E-m and KOHW (1958), HONDA and ARNOLD (1061, 1964), CHA~RAS~RTTY(1961), BISWAS et al. (1963), CRESSY (1964), and FIREMAN ( 1967). An additional point for the meteorite Cranes lies off scale et 81 f 10 dpm/ kg (NORDEWANN et al., 1965). The soarcity of meteorites with Alas contents ~60 dpm observed in our work may in part be due to sampling stetistios, and to systematic errorsin some of the other methods.
The discrepancy
between ROWE’S and our results oannot be due to sampling alone. include a higher percentage of finds, which might be expected to be low beoause of terrestrial deoay or greater shielding. But the finds are rather evenly spread throughout his distribution, rather than being concentrated at the low end. Direct intercomparison of data from different laboratories is possible in only fifteen cases (Table 2). Agreement is excellent in five of these : A&i&s, Bruderheim, Murray, Plainview and Sioux Co. (Cressy’s low result for PlaInview oan probably be attributed to shielding. With a recovered mass of over 700 kg, shielding effects ROWE’S meteorites
Meteorites
with short cosmio-ray exposure ages
1799
Table 2. Interaomperiaonof Al” data Alaa content (dpm/kg) *
Meteorite Class
Rowe: et al. (1963) y
CRESSY (1964)
SFIEDL~VKSY HONDA et al. et al. (1961, 4) (1967) Y-Y B+ Al,% A&@,
BISWAS et al. (1963) Y-Y
FIRE(1967) Y-Y
This work Y-Y
Murray Felix
C2 c3
44 f 4 38 f 4
42 f 4 64 f 4
Abee
E4
61 f5
63 f 7
Aohilles Ehole Plainview Richardton Richardton
H H H6 H5 H5
60 f 6 33 *2 66 f 6 152&6 II 29 f 3
Bruderheim Herleton Holbrook Holbrook La Lande MoKi~ey Potter
L6 LB L6 L6 L5 L4 LB
57 f 2 43 f 3 58 f 6
44 f 6
49 f 5
69 f 5
54 f 5
52 f 3
Hamlet
LL4
52 f 7
Sioux Co.
Eu
90 f 12
50 f 5 70 * 7
44 f 4
54 f 3
38 f 3 46 f 3
50 f 4
68 * 3 68 f 6
60 &- 6 45 f 5 I69 f9 II 74 & 6
63 f 4 63 f 6
70 f 4
60 f 6
64 f 6 99 f 6
* Earlier results from the same laboratory were omitted, if they were superseded by leter determinations (ANDERS, 1960, 1962; CEAKRABARTTY, 1961; EHMANNand KOEMAN, 1968; VAN DILU et al., 1960).
must be expected for this meteorite.) Marginal disagreement (by about two standard deviations) is found for five meteorites : Abee, Hamlet, Harleton, Holbrook and Potter. Taken by itself, each of these cases is not statistioally significant, but since all five deviate in the same direction, the discrepanoy may well be real. Finally, a substantial disagreement (by more than two standard deviations) exists for Ehole, Felix, La Lande, and possibly Richardton. With the exception of Ehole, all of the discordant, high values in Table 2 were determined by yq~ coincidence counting, and the low values, by y-ray spectrometry. One must therefore consider the possibility that one or the other method is afflicted with a systematic error. Errors in y-y coincidence spectrometry
Calibration can hardly have been at fault. Three NBS Naaastandards measured on our instruments gave results agreeing to within 2% (Lrrf3omrrz et a+!., 1965). Although BISWAS et al. do not state the origin of their Na** standard, a serious error
1800
DIETER HEYMANN and EDWARD ANDERS
from this source is quite unlikely in view of the wide availability of accurately calibrated Naz2 standards. Differences in self-absorption of mockup and meteorite also cannot have been an important factor. The nominal electron density of the mockups, 2.88 x 10z3/g, was almost identical with that of an average chondrite, 2.92 x 10z3/g. ROWE et al. used mockups in the same range, 2.84-2.89 x 10z3/g. Moreover, most of our samples and mockups had a mass thickness of only l-4 g/cm2, less than the measured absorption half-thickness for the mockups. Since the overall absorption was small, any difference between sample and mockup must have been a second-order effect. Extraneous counts are another possible source of error. In addition to positrons from A12G,four other sources can contribute counts in the 510-510-KeV coinoidence channel : members of the U and Th series having y-rays of 0.5-0.6 MeV energy in coincidence with y-rays of the same or higher energy; single high-energy y-rays scattered from one crystal into the other; positrons from SC**,the daughter of cosmogenic Ti** (t,,, = 48 yr) ; and electron-positron pairs produced by high-energy y-rays. All of th ese effects were investigated in an early phase of this work and were found to be relatively minor (ANDERS,1960 and unpublished work). The combined error would vary from meteorite to meteorite, depending on geometry and the abundance of K, U, Th and Ti **, According to our estimates, we would expect it to remain well below 5% in all cases. This would account for only a small part of the discrepancy. Errors in y-ray spectrometry
ROWEet al. used the 1.83 MeV y-ray of Al26for determination of the Al% content. However, the calibration was done by an indirect method. Instead of including a 1.83 MeV y-ray emitter in their mockup they added a known amount of natural potassium, using the 1.46 MeV y-ray of K*O as a standard for both K*O and Alz6 in the meteorite. To compensate for the slight mismatch in energy, they applied small corrections for photofraction, peak width and self-absorption. This method gave excellent results for potassium, as shown by the close agreement of their data with literature values. It may, however, have given low results for Alz6, owing to the “summing” of the 1*83-MeV y-ray with O-51-MeV annihilation quanta. This possibility is discussed by ROWE in the accompanying note. A significant part of the discrepancy may also be due to sampling statistics, ROWE et al. having accidentally chosen a larger number of shielded meteorites than we did. DISCUSSION Depth effect
If meteorites with short ages are omitted (italicized values in Table 1) only very few low values remain that could be attributed to a depth effect or escape of secoadaries. The values for carbonaceous chondrites are low, but that is an effect of composition, not size. These meteorites are known to have low eontents of Si and Al, the principal target elements for production of Alz8. Only five of the remaining meteorites have Alaa contents lower than 60 dpm/kg : Pillistfer, Plainview, Taiban, Vavilovka and Goalpara. The first four are marginal
Mctcorites with short comic-ray exposure ages
1801
cases, lying less than two standard deviations below either our arbitrary limit of 60 dpm/kg or the mean AlB6content for each class. Only Goalpara seems to be a clear-cut cast of shielding. The expected Ala6content for a meteorite of this composition is 60 dpm/kg, according to the data in the next section. Yet its observed Ala6 content is only 38 f 4 dpm/kg. A short age can be excluded, since its NeZ1content is 8.55 x IO-* ml STP/g, corresponding to an exposure age of ~25 lo6 yr (STAUFFER, 1961). Consequently shielding must be assumed. Reduction of the activity by a factor of 06 requires shielding by ~70 g/cmz, or ~20 cm. It is interesting that FLEISCHERet al. (1967) have estimated a shielding depth of >38 cm for another Goalpara sample from its lack of heavy ion tracks. This corresponds to a minimum mass of 2750 kg. Goalpara requires some further comment. The post-atmospheric mass of this stone was only about 2.7 kg. It is a find, and thus one cannot rule out the possibility that it was but a single stone from a shower of much larger total mass. One must wonder, though, why no other stones from this hypothetical shower ever turned up. Two alternative explanations are : high geocentric velocity, leading to a high degree of ablation, or, a preterrestrial collision that reduced Goalpara from a meter-sized to a decimeter-sized object. Such a collision would have had to occur <2 x lo5 yr before the fall of the meteorite, to prevent buildup of AlZ6to saturation levels. A variant of the latter possibility would be residence in a near-surface region of its parent body. No clear-cut choice among these alternatives is possible at present. It is curious that no other cases of substantial shielding turned up in this study. As shown in Fig. 1, ROWE et al. found a much larger percentage of low values. Possibly this discrepancy is due to sampling. More measurements and direct cross-checks will be needed to settle this point. Production
rate of Alz6 as a function
of chemical composition
Aluminum-26 in stony meteorites is made mainly from Al and Si, with smaller contributions from Mg, S, Ca and Fe. The relative production rates of a oosmogenic nuclide from several target elements can be estimated, if data for meteorites of different composition are available (STATJFFER, 1962). This method assumes a constant degree of shielding for all meteorites. For this calculation we used all data in Table i except for nine italicized values (short ages) and Goalpara (shielding). Generally, the data for each class were averaged, and assigned a weight equal to the inverse square of the standard deviation. In addition, a value of 2.30 f 0.25 dpm/kg was taken for the Al26 content of small, slightly shielded irons (mean of Arispe, Duchesne, Norfork and Treysa; LIPSCWTZ et al., 1965). A nominal sulfur content of O.5o/owas assumed for the irons. A least-squares analysis of these data gave the following production rates, in dpm A126/kgtarget element: 85 f 110 Mg: s: lOOf Al: 410 f 410 Ca: 280 f 130 Si: 210 f 110 Fe + Ni: 1.8 f 0.6. Most of these numbers look reasonable, showing the expected declinewithdistance from the target nucleus. The low value for Mg reflects the fact that only Mgw, with an isotopic abundance of 11.17 %, is a significant source of Al”. The high value
1802
DIETER HEYNANN snd EDWARD ANDERS
for Ca is definitely wrong, however. Both the Ca and Al values depend largely on two calcium-rich aohondrites: Sioux Co. (Ca = 7.15%, Al = 6.18%) and Angra dos Reis (Ca = 17.54%; Al = 4.62%). Taken at face value, the higher Al% content of Angra would seem to imply significant production from Ca, but the statistical errors alone make this conclusion very uncertain, to say nothing of differential shielding. Apparently the least-squares analysis has malapportioned the AP activity between Al and Ca. More measurements on Ca-rich achondrites are obviously needed. The large errors of these numbers are deceptive. They reflect the difficulty of correctly apportioning the total A.P activity among six elements. But the errors are complementary among geochemically coherent elements (e.g. Ca and Al), and thus tend to cancel as long as these elements remain coherent. For meteorites within the compositional range covered in this work, calculated production rates should be as accurate as the A126measurements in Table 1 on which these production rates are based, i.e. to about 5-10%. This expectation is borne out by the data in Table 3. Significant discrepancies are found only for Murray and Roda. Table 3. Observed and caloulated A126 contents AlB6 (dpm/kg) Clrtss or name Observed Cl Mm~ay c3 Abee Daniel’s Kuil Pillistfer H &s d. Reis Roda. Sioux CO. Irons
41.4 41.8 54.2 63 60 52 60.0 113 65.0
& 2.5 rt 4.2 f 2.7 * 7 f 5 &6 f 2.6 *f 91.4
64 & 4 99 f 5 2.30 & 0.26
Cfblc. 41.8 49.8 57.5 58.4 59.9 58.9 58.8 115.9 63.6 72.8 97.6 2.31
Nbort cosntic-ray exposure ages
Table 4 lists all meteorites known or suspected to have a low age. Three sets of ages have been oalculated for each meteorite whenever data were available. The He* and Ne” ages are based on the production rates given by ANDIZRS (1964). The Ala6 age was calculated from equation (1). A, was taken from the last column of Table 3, with an assigned standard deviation of &lo%. The AlaSdata for Mighei and Ladder Creek were taken from ROWE et al. A, for these two meteorites was based on their value for Murray and an average of their chondrite data, ex&&ng cases of less than 44 dpm/kg. CarbonaceoPts chlondtit~. Short ages are remarkably common in this class. Considerable effort was expended to determine whether Ivuna and Cold Bokkeveld had identical ages. Since they belong to different subolasses (Cl and C2), identical ages would imply an origin in the same collision and hence, presumably, the same body. Such a finding would have interesting genetic implioations. However, the
1803
Meteorites with short oosmio-ray exposure ages
suggest distinctly different ages, 0.18 and 0.32 lO@yr,with only a small probability of conoordance. The value for Cold Bokkeveld is higher than the upper limit of
date
Table 4; Meteoritea with short ooanio-ray expomre ages
Meteorite
He* age Clam (10’ yr)
Ivuna
Cl
Cold Bokkeveld
c2
1.0
0-25 0.3
Ne” age (100 yr)
Al% Ref.
2.0 2.0 0.39 0.2 2.6 2.4 0.26 1.5 1.8 0.8 5.4 4-5
h j h
Mighei
c2
Nogoya Pollen Grosnaja St. Marks Bath
c2 C2 c3 E5 H4
“Pseudo-Bath” Seres Farmington Ladder Creek
L
1.7 0.6 0.5 6.0 5.1 0.8
H L5
O-06 0.03
1.2 0.48 0.06
g e k
L6
Shaw Appley Bridge
L6 LL6
0.95 O-78 0.45 O-88
0.97 0.84 O-58 O-79
k e c c
a. b. c. d. e.
DE FELIUEet al. (1963). HEYXANN (1964, unpublished). HEY (1965). HEend M.AZOR(1967). HINTENBEROER et al. (1964).
: g d h h : f
AP (dpmlkg)
(d$$kg)
6.6 f 1.4
42 f 4
O-18f 0.05
12-Qf 0.9
50 f 5
O-32f 0.04
26 f 3
44 f 4
1-Of 0.3
6+3
31 f6 43.3f 1.7 33 f 3 64 f 4
50 f 5
50 58 58 59
f f f f
5 6 6 6
Ref.
(l?r)
O-14f 1.0f 1.5f O-9 f 2
i
0.08 0.5 o-4 0.2 1.9
? 61 f 6 o-4 f 0.4 34 f 3
59 f 6 64 f 6 52 f 5
21.5 SO.2 1.1f O-4
20 f 2 48 f 2
64 f 6 64 f 6
o-40f o-07 1.5f 0.5
a i
f. HINTEN~ER~ERet al. (1965). KIRSTENet aZ. (1963). :: MAZORet aZ. (1967). i. ROWE et aZ. (1963). STAUT~R (1961). ;: Z&iRINQER(1966).
50.2 106yr previously given by ANDEM (1962). A recheck of the old counting data revealed a readout error, incurred on a day when the printer did not function, requiring the data to be read off a photograph of the spectrum displayed on a cathoderay tube. The revised value, 2.4 f 3-l dpm/kg, still lies appreciably below the new result, but the diflerence may be statistical. It is quite puzzling that the He* and ,Nealcontents of Ivuna suggest an age in the l-2 lo6 yr range, much higher than the Aim age of 0.18 lo6 yr. Shielding is not a likely explanation, since Ivuna seems to have been a small meteorite. The ages of Nagoya (C2) and Cold Bokkeveld (C2) are presumably identical. Their Hes and Neal ages agree fairly well. The Al% age of Nogoya is appreciably lower, but the Alaa content of this meteorite has a large error (6 f 3 dpm/kg), owing to the small sample size, and hence this value still lies within 2a of the Cold Bokkeveld result of 12.9 f O-9dpm/kg. Thus the two results could be identical. MAZOR et cd. (1967) have found three other C2 chondrites with ages near O-3 l@ yr. The situation for the remaining C2 chondrites, Pollen and Mighei, is less clear-cut. Their AIM ages are identical, but this agreement may not be real. The Aim age of
DIETERHEYMANNand EDWARDANDERS
1804
Pollen agrees within its rather large error with the He3 and Nezl ages. Pollen was a, very small meteorite (0.254 kg) in which a lower production rate of Neel and AIzs would be expected owing to the lower flux of secondaries. Probably the true age of this meteoritelies near 1.7 lo6 yr. The A126age of Mighei is based on a measurement by ROWE et al. and may possibly be afflicted with a systematic error. Unfortunately, the He3 and Ne21ages of this meteorite are strongly discordant and offer little guidance. Obviously Mighei will have to be reinvestigated more carefully. * The C3 chondrite Grosnaja has an A12*age agreeing rather well with its Ne21 age. The He3 age is appreciably lower, suggesting diffusion losses. Enstatite chondrites. The three ages of St. Marks are in tolerable agreement. &on&e choondrites. The “Bath” specimen measured by KIRSTEN et al. (1963) seems to have been a mislabeled sample of a hypersthene chondrite (J. Z&RINOER, private communication). An authentic sample of Bath, obtained from the Harvard Collection, gave a noble-gas content differing strongly from the “Bath” sample of Kirsten et al. (Table 5). The He3 and Ne21 content of authentic Bath suggest a cosmic-ray age near 5 lo8 yr, in marked disagreement with the earlier measurement on “Pseudo-Bath.” The A12s content is consistent with the 5 lo6 yr value. Table 5. Noble gases in Bath chondrito (lo-’ ml STP/g) Met.
Source
He3
“B&h” Bath Bath
Tiibingan Harvard Yale
1.5 12.0 10.2
He4 115 1250 1258
R-e21
Ar,=
A#”
Ref.
0.45 2.04 1.93
0.05 0.55
350 4500
1 2 3
1. KIRSTENet ~2. (1963). 2. HEYMANN(1964, unpublished). 3. HINTENBEROER et al. (1966).
The case of Seres is rather clear-cut. The He3 age (O-06 lo6 yr) is much lower than the Ne2r age (0.48 lo6 yr), suggesting severe diffusion losses. Apparently even the Ne21 age is far too low, since the Alw content of this meteorite seems to be at saturation level, corresponding to an age of 2 15 lo* yr WOOD(1967) has found evidence of
severe reheating in this meteorite, which may account for the gas loss. Hypersthene chondrites. Three meteorites of this olass have short exposure ages. Farmington is the most extreme example, with an age of ~0*02 lo8 yr confirmed by both Al2s and noble-gas measurements (I(IIZEITENet al., 1963 ; ANDEES, 1962 ; DE FELICE et al., 1963; SIGNER, 1963; ZXERIXGLER,1966). Sha.whasesomewhat atypical texture and composition (FREDXUKSSONand M&ON, 1967) but Ladder Creek is & normal hypersthene chondrite. Both of these meteorites were found by Dr. N~IN~ER. It is interesting that all three hypersthene chondrites with ages less than 1 lo6 yr have clearly different exposure ages. If these valnes are typioal, then the eollisions forming hypersthene chondrites would seem to he s-d only ~0.6 10’ yr apart. At this frequency, the age distribution above 1 lo6 yr must reaembb a continuum, in which only events of exceptional magnitude might &and out MI discrete * Note addedin roof. A 62-g Mighei sample from the Field Museum gave sn Also content of 35.6 f 2.0 dpm/kg, close to the value for Pollen. With A, = 60 & 6 dpm/kg, the AIW age becomes l-3 f 0.4 lo6 yr, decidedly lower t&n the N& ttge.
Meteorites with short cosmic-ray exposure ages
pdxs.
1800
It is not surprising that the reality of such peaks is still a m&ter of contro-
VerSy.
The AlaSage of AppIey Bridge is appreciably higher than AmpMric ~~~~~. might be lessened if A, for Apples Bridge the Hea and NeQ ages. This ~sa~ment were somewhat higher than 64 dpm/kg. 2t
Ring Asteroids :iShwl Life” (192)
-
Apollo Asteroids (187)
Cosmic-my
$0 6xpwm
age
d0
100
(my.1
Fig. 2. Observed cosmic-ray age distribution for chondritee, compared with theoretical ~t~butio~ o&ulated by &NOLD’S Monte c8IYlO method for three di&rent type8 of ptarentbodies. Nutnber in parentheses!.give8 mmlber of caees in each astqory. Both the Moon and the Apollo asteroids show the observed preponderwce of short agee between 0 and 10 IO6yr* but the Moon has 8 Ierge excess in the 0-Z 1U8yr interval.
As long as only two stony meteorites with short ages were known, they could be dismissed as s~tisti~ freaks. However, the number has now grown tu more than a dozen, and includes representatives of all ohondrite classes except the bronzites. ObviousIy, we are dealing here with a general eharac~~stic of most, if not all chondrite iclasses. A short exposure age implies an orbit of high o&lision probab~ty with the Es&h. 18
1806
DIETERHEYMANNand EDWARDANDERS
From the work Of OPIK (1951,1963,1965) and ARNOLD (1964, 1965) it is obvious that only objects initially in Earth-crossing orbits need to be considered. The mean collision time for objects in Earth-crossing orbits is of the order ~10~ yr ; thus capture of a meteorite in < IO6yr has a probability of only a few per cent. If an additional step were needed to achieve an Earth-crossing orbit (e.g. earthward deflection from a Mars-crossing orbit), the probability of completing both steps in ~10s yr becomes vanishingly
small.
ARNOLD has favored the Moon as a source of stony meteorites. from the Moon Earth-like
at slightly
more than escape velocity
orbits, with a very high probability
Objects expelled
would find themselves
of collision with the Earth.
in
This
de richesses, as far as meteorites with short ages are concerned. Monte Carlo calculations by ARNOLD show that a majority of such ejecta would strike the Earth within the first 2 lo6 yr. Since stony meteorites with such short ages are uncommon, ARNOLD proposed that no major impacts have occurred on the Moon in results in an embarras
the past 2 IO* yr, and that most short-lived objects from previous impacts were swept up by the Earth in prehistoric times. An alternative source of stony meteorites are the Apollo asteroids. are in Earth-crossing
compares the observed tributions.
age distribution
All computations
Evidently,
They already
orbits and can thus produce meteorites with short ages. Figure 2 for chondrites
with three calculated
dis-
were done with ARNOLD’S Monte Carlo program.*
the ring asteroids can be eliminated outright, since they do not provide
any ages shorter than 3 lo6 yr, and have a surplus of high ages. Both the Moon and Apollo asteroids give a much superior match to the observed distribution respects. meteorites
The Apollo with
short
asteroid distribution ages,
without
the
shows about ad
hoc
in both
the right proportion
of
assumption required for the
Moon. A more complete discussion of meteorite orbits and ages will be given elsewhere. Acknowledgmeltts-We thank JOHNL. BARKERJR., AGATHAFRIS, KIYONO FUSE and LEOSARD LEVIN for assistance with the counting. The regression computations were done by E. ALLEN PETERSON,KENNETHROBERTSand KEN Sowmsm. This work wae supported in part by the U.S. Atomic Energy Commission, Contract AT(ll-1)382, and the National Aoronautics and Space Administration, Grant NsG-366. One of the spectrometersused in this study had been built under NSF Grant G-14298. RKFERENCES ANDERSE. (1960) The record in the meteorite+--II. On the presenceof AlaBin meteorites and tektites. Beochim. Coamochim. Acta 19, 63-62. ANDERSE. (1962) Two meteorites of unusually short cosmio-ray exposure age. Science 188, 431-433. ANDERSE. (1964) Origin, age, and composition of meteorites. Space Sci. Rev. 3, 583-714. ANDERSE. and ARNOLDJ. R. (1966) Age of craters on Mars. Science149, 1494-1496. ARNOLDJ. R. (1964) The origin of meteorites as small bodies. I. In IuotipaC~VW?COW&C Chemdy (editors H. Craig, S. L. Miller and G. J. Wasserburg), pp. 347-364. North-Holland, ARNOLDJ. R. (1965) The origin of meteorites as small bodies. II: The model. Astrophys. J. 141, 1637-l 647. &NOLD J. R. (1966)The origin of meteorites as small bodies. III: anera considerations. Astrophya. J. 141,1548-1556. + We are indebted to Prof. ARNOLDfor making his data and program available to us. A brief summary of these data WOW p~viou~ly published by AND~ORS end ARNOLD(1966).
Meteoritea with short cosmic-my exposure qes
1807
BISWASM. M., MAYER-B~RICXEC. and GEI?rNERW. (1963) Cosmic-rmyproduced NL+ and Ape activities in chonclrites. In Earth Soknee and Meted& (editms J. G&s end E. D. Goldberg) pp. 207-218. North-Holland. See also: ~YEE-B~RICICE C., BIEWAEM. M. and GENTNERW. (1962) y-spektroskopischeUntersuchuugenan Stetieteoriten. 2. Natwformh. 17% 921-924. C%UKUEZAB~ M. (1960) Cosmic-my-induced radioactivity in meteorites-B~O, Al*6, and Co” in aerolites and tektites. In Nuclear Chem. Research at Carnegie Inst. of Tech&cgy, 1960-61, Progress Report, by T. P. Kohmsn and A. A. Caretto, Jr. C~ESSY P. J. JR. (1964) Cosmogenic rsdionuclides in stone meteorites. Ph.D. Dissertation, U.S. AEC Report NYO-8924. DE FELICEJ., F~IO G. G. and FIRE-E. L. (1963) Cosmic-my exposure age of the Farmington meteorite from radioactive isotopes. Science142,673-674. EH~N W. D. end KOIXMANT. P. (1958) Cosmic-ray induced rcdioectivities in meteoritesI. Chemical and radiometric procedures for &minium, beryllium, and cobalt; and II. Al*O, Beis, end Coaoin aerolites, siderites, and tektites. Geochim. Cocmzochim. Acta 14,340-363 end 364-379. FIREIKAN E. L. (1967) Radioectivities in meteorites end cosmic-ray v&&ions. Geochim. Cosmochim. Acta 81,1691-1700. FLEI~~HERR. L., PRICrC P. B., WAIXER R. M., MAUREYJYz M. and MORC+AN G. (1967) Tracks of heavy primary cosmic rays in meteorites. J. Cfeophye. Rec. V&366-366. FR~~DRIKESON K. end MASONB. (1967) The Shew meteorite. Geochim.Cosnzochdm. Acta 81, 1706-1709. GFELLERChr., HOUTER~S F. G., OESCHUER H. and SC~ARZ U. (1961) “ry Koinzidenzmessung zur zersMnmgsfreien Messung des Geheltes von Meteoriten an Positronenstrahlern und y-ektiven It&open. Helv. Phy8. Aota 84, 466469. HEYMANN D. (1965) Cosmogenic and radiogenic He, Ne, and Ar in amphoteric chondrites. J. cfeqhy8. Rea. 70, 3736-3743. HEYMANND. and Mazoa E. (1967) Light-dark structureend rare gas content of the cerbonaoeous . chondrita Nogoye. J. aeophy8. Rm. Z&2704-2707. HINTENBER~ H., K~NIQ H., SCHULTZL. and Wm H. (1964) Radiogene, spmllogeneund primordiale Edelgese in Steinmeteoriten. 2. Naturforsch. 1% 327341. HINTENBER~ERH., K(~NIUH., SCHULTZ L. and Wm H. (1966) Rediogene, spellogene und primordiale Edelgese in SteinmeteoritenIII. 2. Naturforcrch. ala, 983-989. HONDAM. and ARNOLDJ. R. (1964) Effects of cosmic rays on meteorites. Science 148,203-212. HONDA M., UMEMOTOS. and ARNOLDJ. R. (1961) Radioactive species produced by cosmic reys in Bruderheim and other stone meteorites. J. Geophy8. RIM. @3541-3646. KIRETENT., KRANKOWSEYsnd Z&mINTER J. (1963) Edelgcs- und Kelium-Bestimmungen an einer grosserenZehl von St&meteor&en. Qeochim. Coemoohim. Actu 27, 1342. LIPSCEUTZ M. E., SIQNERP. and ANDERSE. (1965) Cosmic-ray exposure ages of iron meteorites by the Neal/Alas method. J. Geophys. Re8. 70, 1473-1489. MAZORE., HED. and ANDERSE. (1967) Rare gases in carbonaceous chondrites and ureilites. Menuscript in preparation. NININQERH. H. and NININUERA. D. (1950) The Nininger CoZEectionof Meteoritea. Winslow, A&one. NORDE~N~ D., TOBAILEYJ. end SCHINEIZICR M. (1965) La redioectiviti de le meteorite Grenes (chute du 13 novembre 1964) mesun%epar spectrometriey. C. R. Acad. Sci. Pa+ m, 6665-6666. CPIK E. J. (1951) Collision probabilities with the planets and distribution of interplanetcrry matter. Proc. Roy. Irish Acad. MA, 165-199. C~rx E. J. (1963) Survival of comet nuclei and the asteroids. In Advancer, in Astronomy and Astrophysics (editor Z. Kopal), Vol. 2, pp. 219-262. Academic Press. CPIKE. J. (1965) The stray bodies in the solar system. Pert II. The cometary origin of meteorites. In Advancea in Aetronomy and Astrophy8ic8 Vol. 4, pp. 301-336. Academic Press. RI~ETMIRER. A., SIB~ANTON J. R. and Ko~lldl~~T. P. (1969) Disintegrationscheme of long-lived Ales. Phgle. Rev. 118,1069. ROWE M. W. (1963) Quantitative measurementof gamma-ray-emitting radionuclidesin meteorites. Los Alemos ScientificLab. Report LA-2765.
DIETERHEYMANNand EDWARDANDERS
1808
Rown M. W., VAN DILLA M. A. and ANDERSONE. C. (1963) On the radioactivity of stone meteorites. ffeochim. Cosmochim. Acta 27, 983-1001. SIIEDLOVSKY J. R., CRESSYP. J., JR., and KOHMANT. P. (1967) Cosmogenic radioactivities in the Peace River and Harleton chondrites, J. Geophys. Res. 72, in press. SIQNERP. (1963) Unpublished work. STAUFFERH. (1961) Primordial argon and neon in carbonaceous chondrites and ureilites. Geochim. Cosmochim.Acta 24, 70-82. STAUFFER H. (1962) On the productionratios of rare gas isotopes in stone meteorites. J. Geophya. Res. %7,2023-2028. VAN DILLA M. A., ARNOLDJ. R. and ANDERSONE. C. (1960) Spectrometric measurement of natural and cosmic-ray induced radioactivity in meteorites. Geochim. Cosmochim. Acta RO, 115-121. VAN SCHMXJS W. R. and WOODJ. A. (1967) A chemical-petrologicclassificationfor the chondritic meteorites. Geochim. Cosmoohim.Acta 31, 741-765. VISTE E. and ANDERSE. (1962) Cosmic-ray exposure history of tektites. J. Geophys. Res. 67, 2913-2919. WELLERR. I., ANDERSONE. C. and BARKERJ. I;., JR. (1965) Radioactive contamination of contemporary lead. Nature 206, 1211-1212. WOOD J. A. (1967) Chondrites: their metallic minerals, thermal histories, and parent planets. Icarus 6, l-49. Z~~HRIN~ER J. (1966) Chronology of chondriteswith rare gas isotopes. Metwrititilca 27, 25-40.
AppemIix-AP
determination in atone meteorites by yaw fiwdrome@
M. W. ROWE Miller Institute for Basic Research in Science and Department of Physics, University of California, Berkeley, California WE AI-ZE grateful to ProfessorE. ANGERS(personalcommunication) for callingour attention to a systematic error in the Alss measurements in stone meteorites by ROWEet al. (ROWEand VAN DILLA, 1961; ROWE et al., 1963; ANDERSONet aE., 1964; Rowe and MANUEL,1964). In these reports, the A126was calculated indirectly by comparing the 1+33-MeV Al26 photopeak with the Table 1. Results of two calibration techniques on measurement of meteoritic Ala6 (ROWE et al., 1963). The A126standard yields results unaffected by summing. A128via KC1 Standard* Meteorite A126via A12sStandard Bruderheim Ehole Hamlet Harleton Ladder Creek?
0.97 0.96 0.89 0.93 0.90
+ f f * f
0.04 0.04 0.04 0.04 0.04
0.93 & 0.04 * The average of the two calibrations was used to estimate the reported Al*6 content of these meteorites, except for Hamlet where only the KC1 standard was used. t The Ala* oontent of Ladder Creekwas determined by “/-o/ coincidenceas well; it agreed exactly with the value obtained (32 f 3 dpm/kg) via the AlaBstandard. A second sample of Ladder Creek calibrated with a KC3 standard gave a value of 36 f 3 dpm/kg.
_
Meteorites with short cosmic-ray exposure ages
1809
l-46-MeV photopeak of KM in a KC1 standard. Small corrections were applied to account for differencesin efficiency, photofraction, peak width, and self-8bsorption between the two y-ray energies. We neglected, however, to correct for the loss of the 1*83-MeV y-rays which summed with O-51-MeV y-rays. The O-51-MeV y-rays resulted from annihilation of positrons emitted by Ala* in coincidencewith the 1.83-MeV y-rays. When a 0*51-MeV y-ray and a l-83-MeV y-ray interact with the detector simultaneously, only a single event is recorded-a 2*34-MeV sum pulse. A pulse of less than 2.34-MeV occurs when one of the summing y-rays is degraded by Compton scattering. In either case, the number of events appearingin the 1.83-MeV photopeak is reduced, occurring instead in another energy region. Most of the spectra of Rower.et al. were taken in the energy region 0.2-2*0-MeV; only three published spectra extend to 2.34-MeV (Stannern: ROWE, 1963; Pinto Mountains and Norton County: VAN DILLAet al., 1960). These three spectra do show excess counts in the energy range 1%2.4-MeV and a slight hump at about 2*3-MeV. HEYMANNand ANDERS(1967) point out that the average Ala6 content in ordinary chondrites measured by them is about 20% higher than that of Rowx et ~2. (1963). Of the meteorites studied by Rows et al. (1963), five were calibrated with a uniformly mixed Alla6standard in addition to the usual KC1 standard. These data (summarizedin Table 1) permit an estimation of the magnitude of the summing correction. This evidencesuggeststhat a correctionof 7 f 4 % is necessary to compensate for the loss of 1+33-MeV y-rays by summing. Clearly, additional measurementson duplicate samples 8re desirableto elucidatethe remainingdiscrepancybetween the data of HEYMANNand ANDERS(1967) and ROWE et al. REFERENCES ANDERSONE. C., Rowx M. W. and UREY H. C. (1964) Potassium and aluminum-26 contents of three bronzite chondrites. J. Geophys. Res. 69, 564-565. HEYMANN D. and ANGERS E. (1967). Meteorites with short cosmic-ray exposure ages, as determined from their Alss content. Geochim. Coamochim.Acta, 31,1793-1809. ROWEM. W. (1963) Quantitative measurementsof gamma-ray emitting radionuclidesin meteorites. Los Alamos ScientificLaboratory Report LA-2765. ROWE:M. W. and l&xux~ 0. K. (1964). y-Radioactivity in the Fayetteville meteorite. J. f&o~hys. Res. 69, 1944-1945. ROWE M. W. and Vm DILLA M. A. (1961) On the radioactivity of the Bruderheim chondrite. J. Cfeophy8. Res. 66,3553-3556. Rown M. W., Vm DILLAM. A. and ANDERSON E. C. (1963) On the radioactivity of stone meteorites. Geochim.Coamochim. Acta 2’7, 983-1001. VAN DILL~LM. A., ARNOLD J. R. and ANDERSONE. C. (1960) Spectrometric measurement of natural and cosmic-ray induced radioactivity in meteorites. Qeochim. Coamochim. Acta 20, 115-121.