JotrRNAL OF GEOPHYiiiICAL RKSEAllCH VOL. 68, No. 21 NOVEMBER I, 1963

The Helium Contents of Metallic Meteorites

CARL AUGUST BAUER Pennsylvania State University, University Park

Abstract. The Hes and He' concentrations were measured in 53 samples taken from 34 different metallic meteorites. Two meteorites were identified, Deep Springs and Clark County, that have considerably higher helium concentrations than any previously measured. The hexahedrites Edmonton and Braunau probably have the lowest Hea/He' ratio yet found, with the possible exception of selected samples from Washington County. The exposure ages rela­tive to Grant were estimated for 25 of the meteorites. The estimates for the 3 oldest are: Deep Springs, 1550; Pinon, 1140; and Clark County, 1050 m.y. The lowest age found in this study was 40 m.y. for Monahans. The helium concentrations in the meteorites of low nickel content are lower than those in the medium octahedrites. The hexahedrites. as a class, have especially low helium concentrations and low ratios, but this study did reveal two hexahedrites, Iredell and Cedartown, that have higher helium concentrations and normal ratios. It is sug­gested that the anomalous helium concentrations of the hexahedrites are attributable to the presence of a small concentration of background helium that contains little, if any, Hes.

Introduction. Most of the heliwn found in metallic meteorites was produced by the action of cosmic radiation before the meteoroid entered

the earth's atmosphere. The concentration of this cosmogenic helium is proportional to the in­

tensity of the cosmic radiation and to the length of time during which the meteoroid was exposed .

The Hes/He' ratio of the cosmogenic helium depends on the size of the meteoroid and the position of the measured sample within the meteoroid. Therefore, the measurement and study of the concentrations of He" and He' in meteorites malce it possible to learn much about the history of meteoroids and about the char­

acteristics of cosmic radiation. The interpreta­

tion of the measurements is complicated by the possibility that the intensity and energy spec­trum of cosmic radiation may vary with time and with position in space and by the possi­

bility that the size of a meteoroid may have de­

creased either continuously due to the slow wearing away of its surface by collisions with small particles, or discontinuously in larger steps by the occasional breaking off of large pieces by collisions with bodies of more nearly its own size. Also, the measured helium concentration may include some heliwn that is not of cosmogenic origin, such as radiogenic heliwn or primordial helium. The ultimate goal in work such as this is to make a detailed interpretation of the meas­ured concentrations of noble gases in meteorites

and, when they become available, in samples from the moon and planets.

The general purpose of the present investiga­

tion was to better determine the range of varia­tion in the concentrations of He" and He' in metallic meteorites. The concentrations of total helium have been determined previously for a rather large number of metallic meteorites, but the determinations of the separate concentra­tions of He' and He', and thus also the Hes/He' ratios, are still too few to reveal how they vary with differences in the other characteristics of the meteorites. Considerable effort was made to

secure samples of particular meteorites that would give crucial information on how the helium concentrations depend on other charac­teristics.

One specific aim of this investigation was to determine whether the different classes of metal­lic meteorites differ systematically in their helium concentrations and in the He8/He' ra­tios. In particular, nickel-rich ataxites and hexahedrites were sought for this investigation.

Another specific aim was to identify some

meteorites with unusual helium contents, in par­ticular those with unusually high helium concen­trations and those with unusual ratios. Such

meteorites can then be the subject of the more detailed and exhaustive investigations needed for the determination of ages and exposure history and for similar studies. The meteorites with un-

6043

6044 CARL AUGUST BAUER

usual helium concentrations are important be· cause they will generally give the most cmcial tests of the theoretical interpretations of the helium contents. For example, any meteorite with an unusually high ratio will give a valuable indication of the maximum possible value of that ratio in helium produced by cosmic radia· tion. Since the maximum value of this ratio occurs in the helimn that is produced by pure primary cosmic radiation, we expect to observe high ratios only in material that had a small amount of shielding and thus a small exposure to secondary cosmic radiation. Also, it is these meteorites that had a small amount of shielding, and thus the most intense exposure to cosmic radiation, that provide the best material for detecting radioactive cosmogenic nuclides that have been produced in small amounts. On the other hand, the helium· rich meteorites, which have had the greatest integrated exposure to cosmic radiation, provide the most promising material for detecting stable cosmogenic nu· clides that have been produced in only small amounts.

Several criteria were applied in this search for meteorites with unusually high helium con· centrations and/or high ReS/He' ratios. Since, at present, no criterion other than the exposure age determination itself is available for selecting meteorites that have been exposed to cosmic radiation for a long time, the most promising criteria to apply are ones that indicate that the material had a small amomlt of shielding in space, i.e. that it had a small meteoric mass. For this reason, meteorites were sought with as ma,ny as possible of the following characteristics: a small meteoritic mass, a complete individual spe­cimen, a well-developed fusion cmst and other evidences of freshness of fall, well·substantiated circumstances of find, well-investigated area of find, and favorable surrounding terrain so that there was a good probability of finding other individuals of the same fall had they existed.

The measurements. In this investigation, the helium contents were measured in a total of 53 samples taken from 34 different metallic meteor­ites. I made the measurements at the University of Minnesota in the laboratory of Dr. A. O. Nier. Most of the measurements were made dur­ing the summer of 1959 with the double-focusing mass spectrometer that was especially designed and constmcted for simultaneously measuring

the He" and He' in very small gas samples. The apparatus and methods used were essentially the same as those described by Hoffman and Nier [1958].

The masses of the samples were mostly in the range 0.10 to 0.30 g depending on whether the anticipated, or previously indicated, helium con. centration was large or small. Most of the sam. pies were in the form of one or two pieces of solid metal. Some helium measurements were made with samples in the form of fine filings and in the somewhat coarser fOTm of sawings. The present work did not reveal any systematic dif. ferences in the helium concentrations of the different forms of samples taken from adjacent positions in the same meteorite when the sam. pIes were clean. This result agrees with those of several previous investigators [Hoffman and Nier, 1960 ; Paneth, 1939] .

Table 1 gives the helium concentrations deter. mined in this investigation together with other pertinent information about each meteorite. For those meteorites for which more than one sample was measured, the adopted values of the He' and He' concentrations are the simple average of the individual measurements with the follow· ing exceptions. In four samples the He' concen· tration could not be determined because of ex·

perimental difficulties. In these four samples the following procedure was used to obtain the most useful adopted values because the measurement of only Re" for these samples gave no infol"llliL' tion on the Hes/He' ratio. The average ratio was calculated from the samples for which both He" and Re' were measured and was taken 3.9

the adopted value of this ratio. This adopted ratio was then used to obtain a calculated con· cent ration of He' in the sample for which only Re" was measured. The adopted amount of He' was then taken as the average of the individual amounts of He" including the calculated value for the sample for which the He' measurement was lacking. The total helium concentrations and ratios were calculated from the adopteD concentrations of He" and Re'. Most of the in­formation given in columns 6, 7, and 8 of Table 1 was taken from the catalog by Hey [1953]. For a few meteorites this information had to be taken from other sources (E. P. Henderson, private communication, 1958) [Lovering et al., 1957] .

Figure 1 shows that in the present study sev-

HELIUM CONTENTS OF METEORITES 6045 TABLE 1. The Measured Helium Concentrations of Metallic Meteorites

Concentration, cm3/g X 106 (STP) Known He3/He' Type %Ni Mass, kg Sourcet

Meteorite He3 He' He3 + He'

Braunau �2 0.0397 0.517

�1 0.0388 0.517

Adopted 0.0392 0.517 0.556 0.0759 H 5.21 39 E.P.H.

Cedartown 2.52 10.28 12.80 0.245 H 5.48 11.3 E.P.H.

Coahuila #1 0.0477 0.378

�2 0.0535 0.307

#la 0.0543

Adopted 0.0518 0.351 0.403 0.148 H 5.59 2064 E.P.H.

Coya Norte #1 0.252 1.79 �2 0.270 2.05

Adopted 0.261 1.92 2.18 0.136 H 5.51 17.9 H.H.N.

Edmonton (Can.) 0.00739 0.101 0.108 0.0732 H 7.34 E.P.H.

Irede!l* 4.63 16.22 20 . 85 0.285 H 5.51 1.5 E.P.H.

Mayodan �1 0.130 1.55

#4 0.140 1.45

Adopted 0.135 1.50 1.64 0.0900 H 5.48 15.4 H.H.N.

Negrillos 0.0967 0.452 0.549 0.214 H 5.32 28.5 E.P.H

Rio Loa 0.425 1.92 2.34 0.221 H 5.70 4 E.P.H.

Sandia Mts. #1 0.429 1. 90 #2 0.447

Adopted 0.438 l.n4 2.38 0.226 Hb 5.93 45.4 H.H.N.

Sierra Gorda #1 0.374 1.58

112 0.362 1.68

Adopted 0.368 1.63 2.00 0.226 H 5.58 22 E.P.H.

Union Chile #1 * 0.340 1.68 #2* 0.343 1.66

Adopted 0.342 1.67 2.01 0.204 H 5.63 22 E.P.H.

Santa Rosa 1.12 4.79 5.91 0.234 D2-Ob 6.83 9]3 H.H.N.

Tombigbee River #1 * 0.0693 0.488 0.557 0.142 D2 4.11 43.54 E.P.H.

Arlington 2.27 7.87 ]0.14 0.288 Om 8.60 8.95 P.W.G.

Clark Co. #1 10.97 43.37 #2 11.39

Adoptcd 11.18 44.20 55.38 0.253 Om 7.02 11.3 H.H.N.

Mercedit.as 5.65 20.36 26.01 0.278 Om 7.33 42.9 P.W.G.

:-'furireesboro 6.51 23.44 29.95 0.278 Om 8.6 P.W.G.

Oroville 6.70 22.57 29.27 0.297 Om 24.5 P.W.G.

Toluca #1 0.809 3.56 #2 0.658 2.68

Adopted 0.734 3.12 3.85 0.235 Om 8.31 18200 J.H.H.

Xiquipilco 0.302 1.36 1.66 0.222 Om 8.31 18200 Wards

Bethany ?0.00068 Of 8 16400 C.F. Jamestown 0.560 2.30 2.86 0.243 Of 9.75 4 P.W.G. Putnam Co. 3.61 12.47 16.08 0.289 Of 7.89 32.7 H.H.N.

Bristol 3.47 12.66 16.13 0.274 Off 8.15 20 H.H.N.

Ballinoo #la 0.124 1.20 #lb 0.105 0.971

Adopted 0.114 1.086 1.20 0.105 Off 9.87 42.2 P.W.G.

6046 CARL AUGUST BAUER

TABLE 1. (Continued)

Concentration, cms/g X lOG (STP) Known % Ni Mass, kg

Meteorite

Chinge Deep Springs #1

#2b #2c #3a

Adopted Hoba Monahans Pinon Smithland Tlacotepec #1

#2

Adopted Weaver Mts. #1

#2 #3 #4 #5*

Adopted

0.219 13.11 14.91 13.65 14.28

13.99 0.348 0.257 3.10 0.630 5.19 5.69

5.44 2.48 2.85 2.99 2.90 3.14

2.87

1.02 49.82 53.58 51.71 51.62

51.68 1.57 1.03

13.98 2.58

21.41 22.31

21.86 9.66

11.73 11.07 11.89

11.06

1.24

65.67 1.92 1.29

17.08 3.21

27.30

13.93

Hea/He' Type

0.215

0.271 0.222 0.250 0.222 0.244

0.249

0.260

16.71

13.44 17 10.88 16.58 16.42

16.23

18.03

80.

11.5 60000

27.9 17.85

5++

94.4

38.8

Sourcet

E.P.H.

H.H.N. W.J.L. H.H.N. H.H.N. C.F.

H.H.N.

L.F . B.

* Measured during the summer of 1960 with the 60° Nier-type mass spectrometer described by Signer and Nier [1960].

t The sources from which the meteorite samples were obtained, as indicated by the initials in the last column, are:

E.P.H.-Dr. E. P. Henderson, U. S. National Museum, Washingt.on 25, D. C. H.H.N.-Dr. H. H. Nininger, American Meteorite Museum, Sedona, Arizona P.W.G.-Dr. P. W. Gast, Geological Museum, Univ. of Minnesota, Minneapolis, Minnesota

C.F.-Dr. C. Frondel, Harvard Univ., Cambridge 38, Massachusetts W.J.L.-Dr. W. J. Luyten, Univ. of Minnesota, Minneapolis, Minnesota J.H.H.-Dr. J. H. Hoffman, U. S. Naval Research Lab., Washington 25, D. C. L.F.B.-Col. L. F. Brady, Museum of Northern Arizona, Flagstaff, Arizona Wards-Wards Natural Science Establishment

era! meteorites were identified as having unusual helium concentrations. Deep Springs and Clark County were found to have very much higher helium concentrations than any meteorite pre­viously measured and a number of meteorites (mostly hexahedrites) were found to have un­usually low Hea/He' ratios. These results were

given in a preliminary report of this investiga­tion [Bauer, 1960J. Deep Springs and Clark County have subsequently been subjected to an exhaustive study [Signer and Nier, 1962J.

The highest He"/He' ratio measured in this group of 34 meteorites was 0.297 for Oroville. Some investigators [Dalton et al., 1953; Reas­beck a:nd Maryne, 1955; Schaeffer, 1961J have found, among a much smaller number of mete-

orites, nine meteorites with ratios that exceed 0.30. Of these nine meteorites, only Toluca was

included in the present study, and its ratio was

determined as 0.235 instead of 0.34. Three of the nine meteorites were included in the study of Signer and Nier [1962J, and the measured ratios of all three are given there as less than 0.26. It is possible that these differences arise because the samples measured may have come from dif­ferent positions in these meteorites. However, we do not yet have sufficient comparisons be­tween different laboratories, and so it also seems

possible that some of the differences may be due

to experimental errors. This study revealed two meteorites, Edmon­

ton and Braunau, tha.t have lower ratios than

HELIUM CONTENTS OF METEORITES 6047 I I

,30 0 0 0

y 00 I

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.20

j-... CD :I: -�o ""

'" CD :I:

0 �O .10

-

-

-

I J

o 0

0

I

I

I

I

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I I -

0 -

0 -

I -

-

-

-

-

-

-

-

-

-' I 10 20 30 40 60 He4 Concentration in cma/g x lo·(ST'"

Fig. 1. The measured Hea/He' ratios plotted against the measured He' concentrations for all metallic meteorites for which measurements of both He' and He' concentrations were avail­able. Each distinct meteorite is shown only once. If it was measured in this investigation, the values were taken from Table 1 and are shown by large open circle. Small solid dots show the additional meteorites that were measured by other investigators. Those meteorites in which measurements have been made at different positions within the meteorite are shown either by a representative point or by a line that shows the range of individual measurements.

any meteorite previously measured, with the possible exception of selected samples of Wash­ington County.

Estimates of the exposure ages. Figure 2 shows the theoretical relationship between the HeajHe' ratio and the rate of He' production

in iron spheres relative to that in a meteoroid of infinitesimal mass. The specific curves shown in Figure 2 were drawn by adapting the calcula­tions of Signer and Nier [1960], but the curves would retain their general character for consid­erable variation in the observationally deter­mined parameters. The reader is referred to the paper by Signer and Nier for further details of their calculations and the assumptions on which they are based.

The He" and He' concentrations in the Grant meteorite have been thoroughly measured at many different internal positions, and the result­ing helium distributions have been subjected to detailed theoretical analysis [Hoffman and Nier, 1958; Signer and Nier, 1960 and 1961]. Since Grant has been thoroughly studied and since its helium concentration and distribution are favor­able for a theoretical interpretation, it is the best meteorite to use as a basis of comparison. The studies of Grant indicate that it had a

meteoric mass of about 2000 kg and, at a point

halfway from the center to the meteoric surface, it had a He' concentration of 19.80 X 10-0 cma/g (STP) and a Hea/He' ratio of 0.270. The curve (similar to the ones in Figure 2 but not shown there) drawn through the points halfway from the center to the meteoric surface indicates that the rate of He' production at this point in Grant was 1.18 relative to that in a meteoroid of in­finitesimal mass.

We notice in Figure 2 that the maximum rela­tive rate of He' production, which occurs near the center of a 100-kg mass, is only about 1.5, that is, 1.27 times the rate at the comparison point in Grant. This makes it possible to draw an important conclusion about the eJl.'Posure age of a helium-rich meteorite like Deep Springs relative to that of Grant. In making this com­

parison we must make the assumptions, which are usually made and which are used throughout this paper, that both meteoroids were exposed to cosmic radiation of the same intensity and

the same energy distribution. Some evidence has already been obtained that these assump­tions are true [Stoenner et al., 1960; Fireman and DeFelice, 1960; Arnold et al., 1962; Stauf­fer and Honda, 1962; Stoenner and Davis, 1962J. We can, however, reach an important conclusion about Deep Springs without any in-

6048 CARL AUGUST BAUER

. I O�---------------+----------------4---------------�

.OOO�� --�--�--L--.�5 --�--��L-�--�

I.-O--��--�--��

1 . 5

Relative Rate of He4 Production Fig. 2. The theoretical relationship between the Rea/Re' ratio and the rate of Re" produc­

tion relative to that in a meteorite of infinitesimal mass. The light dashed curves show the relationship along a radial line from the center to the surface in spherical meteorites with meteoric masses of 10', Hi', 10', and 1(}' kg. One heavy continuous curve connects the points representing the centers of these different masses and the other heavy continuous curve con­nects the points lying 0.6 of the way from the center to the surface of the pre-atmospheric bodies.

.

formation or assumptions about the mass of the meteoroid or the position of the measured sam­ple within it. Even if the Deep Springs sample came from near the center of a 100-kg mass, where the ma.ximum rate of He' production pre­vails, the rate of He' production would be only about 1.27 times that at the comparison point in Grant. However, the He' concentration in Deep Springs is much larger (2.6 times that of Grant), and we thus conclude that it has been exposed to cosmic radiation much longer than Grant.

For some of the meteorites in Table 1, it is possible to make rough estimates of their rela­tive exposure ages in a way in which a reason­able allowance is made for the different amounts of shielding. Of course, absolute exposure ages cannot be calculated from only the observed concentrations of Hea and He' at a single place in a meteorite, which is all that was measured in this investigation. Moreover, as shown in Figure 2, the observed ratio does not uniquely determine both the mass of the meteoroid and the radial position of the measured sample in the meteoroid; though if either one of these is

known the other one, and also the relative rate of He' production, can be read from Figure 2. Therefore, it is possible to estimate the relative rate of He' production, and thus also therelar tive exposure age, only to the extent that some means is available for estimating either the mass of the meteoroid or the position of the sample in the meteoroid.

Previous work [Hoffman and Nier, 1960; Signer and Nier, 1961] indicates that, except for the very large meteorites, the samples taken usually occupied positions less than 0.6 of the way from the center to surface of the meteoroid. As shown by Figure 2, theoretical calculations of the relative rate of cosmogenic helium production indicate that, in this inner portion of a mete­oroid of a particular mass, the variation of the Hea/He' ratio with radial position is quite small compared with the full range of values of this ratio from the center to the meteoric surface. Also, the measurements of the helium concen­trations at many internal positions in a few meteorites show directly that the ratio does not vary greatly in the meteorite and that the ratio

HELIUM CONTENTS OF METEORITES 6049 increases more and more rapidly with increas­ing distance from the center. The following

ranges of the observed ratio have been reported [Hoffman and Nier, 1958, 1959, 1960]: Grant, 0.26 to 0.30; Carbo, 0.238 to 0.240; Casas Grandes, 0.224 to 0.240; and Keen Mountain, nearly consta,nt at 0.279.

Since the evidence indicates that the meas­ured samples usually come from the inner por­tion of the meteoroid, the curve that connects the points lying 0.6 of the way from the center to the meteoric surface was drawn in Figure 2. We notice that in these inner portions of the meteoroids the Hes/He' ratios occupy a rather narrow band across the diagram, i.e., the region between the two heavy continuous curves. Therefore, if the sample measured came from this inner portion, the He"/He' ratio gives us an indication of the mass of the meteoroid and also of the relative rate of He' production.

Table 2 gives estimates of the relative ex­posure ages of the meteorites for which it may be hoped that the assumptions used in making

TABLE 2. Estimated Exposure Ages Relative to Grant

Meteorite

Cedartown

Iredell Negrill08 Rio Loa Sandia Mts. Sierra Gorda Union, Chile Santa Rosa Arlington Merceditas Murfreesboro

Oroville Toluca Xiquipilco Jamestown

Putnam Co. Bristol

Chinge Deep Springs Hoba Monahans Pilion Smithland Tlacotepec Weaver Mts.

Relative Rate of He4 Production

0.85 1.29

0.30

0.44 0 . 51

0.51 0.12

0.67

1.31

1.25

1.25

1.35

0.68

0.44 0.82

1.32

1.22

0.32

1.19

0.30

0.92

0.44 0.84 0.90

1.08

Relative Exposure

Age

0.72 0,75

0,090

0.26

0.23

0.19

0.83

0.43

0.36

0.97

1.12

1.00

0.27

0.18

0.17

0.56

0.62

0.19

2.59

0.31

0.067

1.89

0.18

1 . 45 0.61

Exposure Age, m.y.

430 450

54

160

140

110 500

260

220

580 670

600

160

110

100 340

370

110

1550

190

40

1140

110

870

370

the estimates are approximately true. A curve (similar to the heavy ones in Figure 2 but not shown there) was drawn through the points halfway from the center to the meteoric surface. This curve is representative of the inner por­tions of the meteoroid. The relative rate of He' production for each meteorite, corresponding to its observed Hes/He' ratio a.s given in Table 1, was read from the curve and entered in the second column of Table 2 (except for Hoba, which is discussed separately). The exposure ages relative to that of Grant were calculated by the relation

TM/Ta = RaHeM4jRJfHea4

where T is the exposure age, R is the relative rate of He' production, and He' is the observed He' concentration given in Table 1. The sub­scripts G and M denote Grant and the other meteorite, respectively. As described previously, the values at the corresponding point in Grant were taken as follows: He' concentration, 19.80 X 10-6 cm"/g(STP) ; HesjHe' ratio, 0.270; and the relative rate of He' production, 1.18. The exposure age in years is given in the last column of Table 2 and it was calculated on the basis that the exposure age of Grant is 600 m.y. [Hoffman and Nier, 1958; Signer and Nier, 1962].

No estimates are made for meteorites with a

ratio less than 0.21. As shown in Figure Z, the lowest ratio expected in cosmogenic helium is about 0.18, and it would occur near the center of a very large meteorite. Therefore, when the observed ratio is below this value we expect that the helium is not all of cosmogenic origin, and therefore the assumptions on which the estimates are made do not hold. Also, as shown in Figure 1, the meteorites with ratios that are less than 0.21 have only small amounts of helium and the determinations are therefore subject to relatively larger experimental errors.

Clark County is not included in Table 2 be­cause experimental difficulties prevented the recording of He' in one sample. In the only other sample measured the He' appears to be slightly high, and thus the Hes/He' ratio ap­pears too low. This is probably due to the fact that the only sample of tIllS meteorite that was

then available consisted of old sawings from which apparently clean shavings of metal had to be picked out individually with the use of a

6050 CARL AUGUST BAUER microscope. Because of this low determination of the ratio, the relative exposure age of Clark County was estimated as 2.3 times that of Wil­liamstown in a preliminary report on this work [Bauer, 1960]. The exposure age in years given there is too high, partly because the relative ex­posure age adopted there was too high, but primarily because the exposure age was calcu­lated on the basis of the age determined by Schaeffer [1960] for Williamstown, namely 2400 million years (a value now known to be too "large). Fortunately, Clark County has since been extensively investigated by Signer and Nier

[1962J. Their measurements confirm the excep­tionally high concentration of helium previously reported by me, but they show that the earlier measurement of the He' was slightly high. If the later measurements (He', 38.20 X 10-" cm"/g(STP); He'/He', 0.286) of Clark County by Signer and Nier are used to estimate the relative rate of He' production by the method described in this paper, the value obtained is 1.30 and the relative exposure age is 1.75 times that of Grant or 1050 m.y., which is essentially the same as their value of 1000 m.y., obtained from the more detailed analysis.

Three meteorites selected for this study have unusually long exposure ages: Deep Springs, 1550; Pinon, 1140; and Clark County, 1050

m.y. The small mass that is indicated for Deep Springs gives some support to the claim that it was observed to fall, as reported by Hey [1953], because it indicates that the known mass COIll­prises most of the meteorite. The lowest age estimat�d is 40 m.y. for Monahans.

The exposure ages estimated in this paper can be compared with those calculated in the more detailed analysis of Signer and Nier for five meteorites that are common to the two studies. The meteorites are (with the ages from Table 2 followed by the ages given by Signer and Nier in parentheses) Deep Springs, 1550 (1500); Clark County, 1050 (1000); Merceditas, 580 (600); Negrillos, 54 (30); and Rio Loa, 160 (150) .

As a further test of the reliability of the ex­posure ages estimated by the method described in this paper, I used the method to estimate the ages of all the meteorites (except Washington COlmty) for which exposure ages were given by Signer and Nier [1962] on the basis of their detailed measurements and calculations. 'This comparison is shown in Table 3. The estimated ages are based on their measured values of the Rea/He' ratios and their He' concentrations (as given in the table) so that the re<lults of the two methods, unaffected by differences in the measured values, can be compared. This com-

TABLE 3. Comparison of the Estimated Exposure Ages with the Exposure Ages That Were Calculated by Signer and Nier [1962]

He., Relative cm"/g (STP) Rate of He'

Meteorite He"/He' X 10' Production

Deep Springs 0.268 54.60 1.17 Clark Co. 0.286 38.20 1.20 Aroos 0.259 25.25 1.06 Merceditas 0.265 20.40 1.13 Charcas 0.264 20.40 1.12 Grant 0.270 19.80 1.18 Williamstown 0.258 18.00 1.05 Carbo 0.242 16.85 0.81 Keen Mt. 0.277 7.60 1.24 Odessa 0.249 4.15 0.90 Tocopilla 0.231 2.95 0.62 Sikhote Alin 0.244 4.90 0.83 Casas Grandes 0.238 5.60 0.73 Admire 0.231 2.70 0.62 Coya Norte 0.210 1.44 0.23 Rio Lao 0.226 1.40 0.51 Toluca 0.227 1.19 0 . 52 Negrillos 0.24 0.40 0 . 77

Estimated Relative Exposure Age,

Exposure Age m.y.

2.78 1670 1. 76 1060 1.42 850 1.08 650 1.09 650 1.00 600 1.02 610 1.24 740 0.37 220 0.27 160 0.28 170 0.35 210 0.46 270 0.26 160 0.37 220 0.16 100 0.14 80 0.031 20

Exposure Age by Signer and

Nier,

m.y.

1500 ± 100 1000 ± 100

800 ± 150 600 ± 150 600 ± 150 600 650 ± 100 600 ± 150 200 ± 50 450 ±300 250 ± 100 300 ±200 250 ± 100 150 ± 50 250 ± 200 150 ± 50 250 ± 150

30 ± 15

HELIUM CONTENTS OF METEORITES 6051 panson shows t�e usefulness, and the limita­

tions, of the estlmated ages. Only two of the

eighteen meteorites have estimated ages that

differ from the calculated ages by more than the

error estimated by Signer and Nier for their

calculated ages. It should be remembered that the estimated ages are based on the assumption

that the sample came from 0.5 of the way from

the center to the surface of the meteoroid, but

in the method used by Signer and Nier the

radial position of the sample is determined by

the measurements of the four nuclides, He', He',

Ne", and Ar"". Therefore, the largest discrep­

ancies are expected and found in those mete­

orites for which the radial position of the sample

is hrthest from the assumed position. The close

agreement indicates the usefulness of the esti­

mates of the relative exposure ages based on

measurements of only He' and He< in samples

taken from a single position within a meteorite .

Of course, the resulting estimates are no sub­stitute for carefully calculated ages based on

more extensive measurements and analysis;

however, they can be most helpful in quickly

revealing promising meteorites for the more ex­

haustive studies . On the other hand, estimates

made in this way do make some allowance for shielding and, therefore, should be considerably better than those based simply on an average rate of He' production [Urey, 1959J.

Another consideration that can be useful in applying Figure 2 in estimating relative ex­pOSlue ages is that a meteoroid had at least as much shielding as provided by its meteoritic mass. For the method of estimating the relative rate of He' production in this paper, this con­sideration becomes important for those mete­orites that have a mass that is greater than in­dicated by their observed Hes/He' ratio and the curve drawn through the points halfway from t.he center to the meteoric surface in Fig­ure 2. For example, the meteorite Hoba has a ma..',s o,f about 60,000 kg, and there is also evi­dence that a layer of considerable thickness has weathered away from the outside. The meas­

ured ratio is 0.222 for my sample, which came from the surface of the meteorite. If we assume, as before, that the measured sample came from halfway between the center and the meteoric surface, the mass corresponding to this ratio is only about 20,000 kg, or less than the pres­('ntly known mass. We also notice that a ratio

of 0.222 is itself characteristic of a point be­tween 0.6 and 0.7 of the way from the center to the surface of a 60,OOO-kg mass. Therefore,

for such a meteorite we must proceed in a

different way. It is reasonable to take the me­teoric mass as at least 10" kg to allow for the mass lost in coming through the atmosphere and for the mass that has weathered away sinee it reached the earth's surface. This implies that the present surface was about 0.8 of the way from the center to the surface of the meteoroid and that, before weathering, the initial mete­oritic surface was somewhat farther out. The observed ratio corresponds to a relative rate of He' production of 0.30 for a lQ'-kg mass, and this is the rate entered in Table 2 for Hoba.

Another consideration is useful in the ap­plication of Figure 2 to the study of meteorites; namely, the mass of the meteoroid must have been at least large enough to provide at its cen­ter the amount of shielding indicated by its measured Hes/He' ratio . For example, if the measured ratio in Pinon of only 0.222 is correct, we conclude from Figure 2 that the mass of the meteoroid was at least 5000 kg even if the meas­ured sample came from near the center . Since the known mass of Pinon is only 17.85 kg, we

expect that all of this meteorite has not been recovered . In this interpreta,tion, we assume that Pinon is not like the very unuSllal Washington County meteorite [Tilles, 1962] in which some of the helium is of a non cosmogenic origin.

The Arlington meteorite was selected for in­vest.igation because I believed that it would have an unuSllal helium content.. Arlington, found in 1894, is a single piece with a mass of about 8.95 kg. The fusion crust appears to be fresh, it has well-developed flight markings, and it appears to cover the entire surface except where it has been cut. For these reasons, it seems likely that the reeovered meteorite repre­sents a substantial portion of the meteoritic mass. Reasoning in this way, we conclude that the meteoric mass may also have been small. Arlington is also interesting for another reason; it has an unusual plate-like shape about 25 by :10 em by 2 to 5 em thick. At the time the flight pattern formed, the meteorite was evidently moving broadside through the atmosphere. Thus it presented an unusually large cross section for its mass as it penetrated the atmosphere, and it would ha:ve been decelerated more rapidly than

6052 CARL AUGUST BAUER

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F i n e s1 H I I

I

F i n e H U) Q)

I I

III I I I

I---l

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II OII-I----t

o 1 0 2 0 3 0 4 0 !S O 6 0 7 0 T01 a l H e l i u m C o n c e n 1 ra t i on i n cm"/g x 1 06 (STP)

Fig. 3. The concentrations of total helium in the different classes of metallic meteorites. Each distinct meteorite is plotted only once. If it was measured in this study, its helium con­centration taken from Table 1 is plotted in the lines numbered 1. Other meteorites which were measured by Paneth and his associates have their helium concentrations plotted in the lines numbered 2 [Arrol et al., 1942 ; Chackett et al., 1953 ; Dalton et al., 1953]. Still other meteorites that were not included in either of the previously mentioned programs have their helium con­centrations plotted in the lines numbered 3 [Hoffman and Nier, 1958, 1959, 1960 ; Signer and Nier, 1962] . In meteorites in which measurements were made at many different radial positions a horizontal line shows the range in the helium concentrations. The average helium concentra­tion of all entries for each class of meteorite is shown by a heavy vertical bar across all three lines.

a more rounded object. It seems possible that, if Arlington's geocentric velocity was not un­usually high, it would have suffered less mass

loss in the atmosphere than most meteorites .

Therefore, before the measurement was made, it appeared that a sample of Arlington would represent material from a point relatively far from the center of a small meteoric mass. For such a sample we would expect an unusually high Hd' /He' ratio and, if the exposure age was long, an unusually high helium concentration.

As shown in Table 1, Arlington did have the third highest Hes /He' ratio among the mete­orites included in this study, and in a sense this confirmed the reasoning about it . However, its

ratio was not exceptionally high. The ratio in­

dicates that the meteoroid had a small amount

of shielding and, therefore, its rather normal He' concentration indicates an exposure age of 220 m.y., less than that of Grant .

The helium concentrations in different classes of meteorites. Figure 3 shows the concentra­

tions of total helium in meteorites of the sepa­rate classifications. Since the concentration of total helium, Hes + He', is shown in this dia­gram, we can include the numerous meteorites in which this was measured in the classical work of Paneth and his associates, as well as the me­

teorites in which the two isotopes were meas­ured separately.

The average helium concentration and the distribution of the individual values are very

similar for the medium octahedrites and the nickel-rich ataxites. An examination of the esti-

HELIUM CONTENTS OF METEORITES 6053

mated exposure ages in Table 2 also indicates

that the nickel-rich ataxites are similar to the

medium octahedrites. This result does not agree

with those of some other investigators, who

group the nickel-rich ataxites with the hexahe­

drites and state that most nickel-rich ataxites

have exposure ages of less than 300 m .y. (see

page 293 of the review paper by Anders [1962] ) .

The present investigation has brought to light

no less than three nickel-rich ataxite9-Deep

Springs, Pinon, and Tlacotepec-that have ex­

posure ages greater than 800 m.y., and this

finding is in direct contradiction to any group­

ing together of nickel-rich ataxites and hexabe­

drites. We also notice in Figure 3 that of the

two meteorites with the largest helium concen­

trations-Deep Springs and Clark County-one

is a nickel-rich ataxite and the other is a

medium octabedrite.

There are still too few measurements of the

helium concentrations of finest and fine o ctabe­

drites to show their distribution clearly. Figure

3 shows that the average helium concentration

of these two classes of meteorites is lower than that of the medium octahedrites, but a special

search for helium-rich meteorites in these two

classes has not been made. This could easily ac­count for the difference in the average values

at this time. Among the four classes of nickel-poor mete­

orites, the average helium concentrations appear uniformly low. The measures now available sug­gest that the average helium concentration de­creases steadily along the following sequence : medium octabedrites, coarse o ctahedrites, coars­est octabedrites, and nickel-poor ataxites and hexabedrites. There are not yet enough meas­urements of the coarsest octabedrites and the nickel-poor ataxites to clearly define the dis­tribution or average of their helium concentra­tions, but the trend is suggestive . It clearly indicates a need for further investigation of this problem.

The helium concentrations have now been measured in a considerable number of hexabe­drites ; therefore, the results should give us a good indication of their distribution and their average value. I first noted their uniformly low helium concentrations some years ago [Bauer, 1949] , and this conclusion was confirmed in the preliminary report of the present investigation [Bauer, 1960] . In this investigation a special

effort was made to acquire samples of hexabe­clrites having high concentrations of helium. Although this search did not produce any hexa­hedrites comparable to the metallic meteorites that are richest in helium, it did succeed in identifying Iredell and Cedartown, which have unusually high helium concentrations in com­parison with most hexabedrites.

Among the hexahedrites included in Figure 3 a.re nine from the group known as the North Chilean hexahedrites. If the average helium concentration of the hexahedrites is computed counting the average value of aU the North Chilean hexabedrites as one, the new average for the class is 5 . 15 X 1 0-8 cm8/g (STP ) . Since this is not significantly different from the aver­age, 3 .77 X 1 0-< cm"/g (STP) , computed by in­cluding each of the North Chilean hexahedrites separately, I conclude that the low average value for the hexahedrites is real and that it is not due to the inclusion of many different pieces of the sa.me fall. In the preliminary report of this work it was also pointed out that many of the hexahedrites have unusually low He8/He4 ratios. This special problem is discussed sepa­rately in the next section .

The helium concentrations in hexahedrites. Figure 4 shows the measured He8/He' ratios plotted against the He' concentrations for 1 7 hexabedrites, 2 nickel-poor ataxites, and Bal­linoo . The two nickel-poor ataxites measured in this investigation, Santa Rosa and Tombigbee River, are included because these meteorites are related to hexahedrites and also because they have low ratios. Ballinoo is included because of its unusually low ratio . These last three me­teorites are shown in Figure 4 for comparison only and do not enter into the following dis­cussion .

As previously noted, the lowest predicted ratio in cosmogenic helium is about 0.18. Fig­ure 4 shows no less than seven hexahedrites with ratios below this value, and, since it is unlikely t.hat these are all due to experimental errors, it therefore appears that not all of their helium is of cosmogenic origin. It is significant that all of these meteorites with low ratios have relatively small helium concentrations. At one time it ap­peared that all hexahedrites had low helium concentrations, but later Keen Mountain and Sikhote Alin were found to have moderate he­lium concentrations . In this investigation Cedar-

6054 CARL AUGUST BAUER

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Fig. 4. The measured He"/He' ratios in hexahedrites, and related meteorites, plotted against their He' concentrations. Hexahedrites measured in this investigation are shown by large open circles. Hexahedrites measured in other studies are shown by small solid dots, ex­cept Sikh ate Alin which is shown by a line that connects the values found in different sam­ples. Two nickel-p oor ataxites and Ballinoo are shown by plus signs. The two curves represent the relation for variable amounts of cosmogenic helium m ixed with a constant amount of back­ground helium .

town and Iredell were found to have even hi gher helium concentrations. All four of these hexahe­drites with the larger amounts of helium have ratios well within the range possible in cosmo­genic helium.

It is unlikely that the low ratios can be at­tributed to atmospheric He' leaking into the system during the extraction and measurement of the helium. Since such a leakage would also cause the ratios to be abnormally low in those meteorites with a low helium concentration, it is important to consider the evidences that the low ratios are not caused by such a leakage. The primary purpose of this investigation was to survey the helium concentrations in as many m<>teorites as possible, rather than to obtain highly precise values for j ust a few. Therefore, blank runs were not made repeatedly and the many minor corrections were not as carefully investigated and determined as they would be in definitive work . However, the measurements: were made in such a way that two checks could be made on the <>ffects of atmospheric He' leak­ing into the system .

First, with onIy a few exceptions, each meas­

urement was made in such a way that it, was possible to observe the He' huildup in the source

region of the spectrometer due to atmospheric He' leaking through the Pyrex parts of the re­

circulation system . This buildup was typically about 2 X 10-9 cm'/min ( STP) . In the proce­dure used this atmospheric He' accumulated for less than two minutes and therefore was · gen­erally less than 4 X 1 0-9 em" (STP) at the time the meteoritic helium first produced a response of the recorder. For most of the meteorites this small accumulation of atmospheric He' was negligible c,ompared to that from the sample. For meteorites with extremely small helium con­centrations, like Edmonton, the reductions were made by extrapolating the tracings back to the time the valves to the evacuating pumps were closed and therefore the effect of atmospheric helium leaking into the recirculation system should have been largely eliminated. Conse­quently, none of the helium measurements re­

quired a correction for this leakage. A second check on the atmospheric He' leak­

ing into the system was possible because each sample was heated a second time for twenty minutes in exactly the same way as the first

time. The helium obtained from the second heating was generally only a few per cent of that obtained on the first heating and therefore

HELIUM CONTENTS OF METEORITES 6055 the atmospheric He' leaking into the system could be determined . The He"/He' ratio deter­;nined from the second heating was generally �.maller than from the :lirst heating . The smaller

ratio obtained from the second heating was at­tributed to atmospheric He' leaking into the extraction system during the twenty minutes while the sample was being heated. Therefore , in all the reductions the meteoritic He' from the second heating was calculated from the meas­ured amount of He" from the second heating multiplied by the He'/He' ratio determined from the first heating. The remaining amount of He' from the second heating was attributed to atmospheric He' leaking into the extraction

system and therefore was not included in calcu­

lating the measured amounts of helium in the meteorite. For most meteorites this correction was negligible. If we assume that the same amount of He' leaked into the system during the identical first heating, the ratio of the me­

teoritic helium is given by the relation

p = (H e13 - H e�3)/(H el� - He/) The subscripts 1 and 2 indicate the helium measured from the first and second heatings.

This correction for helium leaking into the ex­traction system during the first heating has not been made in calculating the helium concentra­tions and ratios given in Table 1 because I be­lieve that the experimental procedures and the results do not insure the reliability of such small corrections at this time. This correction is n�ligible for most of the meteorites. It is larg­est for those meteorites, like Edmonton , that haYa a very small concentration of helium. For Edmonton it changes the ratio from 0.073 to 0.1 17. For Braunau # 1 it changes the ratio from 0 .075 to 0 .088 .

The measured helium concentrations them­selves give some further evidence that the small ratios are not caused by atmospheric He' leak­ing into the system. N egrillos, Chinge, and :Monahans all have small helium concentrations and thus they should have shown the maximum effect of such leakage . However, their ratios are all greater than 0.214 . In comparison, Ballinoo, Mayodan, and Coya Norte have higher concen­trations of helium that should not be affected as much by such leakage, but they all have ab­normally low ratios. For these reasons the low ratios appear to be real.

The most p romising explanation of the dis­tribution of the hexahedrites in Figure 4 ap­

pears to be that these meteorites contain a

small amount of He' that is not of cosmogenic origin. This postulated noncosmogenic He' is hereafter referred to as background helium so

as not to imply that the He" and He' concentra­tions give any indication of its source, whether radiogenic, primordial, etc . If this hypothesis is true, we expect that in any such meteorite in which there is a low concentration of cosmo­genic helium ( either because of a large amount of shielding or a short exposure age) the back­ground He' will contribute an appreciable frac­tion of the total He' found in the meteorite and therefore the measured ratio will be low . In those meteorites in which there is a relatively large amount of cosmogenic helium, the back­

ground He' contributes only a negligible frac­tion of the total He' and the measured ratio is normal for cosmogenic helium.

The curves drawn in Figure 4 were calculated according to the relation

H e3/ H e� = p - ( A/H e4) Curve A represents this relation for p = 0.286 and A = 0 .0286 X 10-0, which are the values of p and A that give an approximate :lit to Edmonton and Iredell. Curve B represents this relation for p = 0.273 and A = 0.273 X 10 .... , which are the values that give an approximate :lit to Mayodan and Cedartown . All the hexahe­drites fall essentially in the region bounded by these two curves.

If the background helium is pure He' (as it will be if it is of radiogenic origin) , then, in terms of the hypothesis that the distribution shown in Figure 4 arises because different amounts of cosmogenic helium are mixed with small amounts of background He', the constants p and A are interpreted in the following way. p is the He"/He' ratio in the cosmogenic helium and A = pHeB', where He/ represents the con­centration of background He'. Curve A thus represents the mixture of cosmogenic helium with a ratio of 0.286 and background He' with a concentration of 0.1 X 10-0 cm3jg (STP) . Curve B represents the mixture of cosmogenic helium with a ratio of 0.273 and background He' with a concentration of 1 .0 X 10-0 cm'/g (STP) .

In fitting curves of this type to the points in Figure 4, we notice that the ra,tio in the cosmo-

6056 CARL AUGUST BAUER

genic helium is determined by the hexahedrites with the highest concentrations of helium, that is, the points in the upper right-hand region of the figure . Since the ratio in the cosmogenic helium depends upon the amount of shielding around the measured sample, and this would be different for the different meteorites, therefore, according to this interpretation, the vertical separation of the curves in the upper right-hand region is associated with this difference. We also notice that the value of A, and thus also of HeB', is determined by the horizontal position of hexa­

hedrites with low ratios, that is, the points in the lower left-hand region of the figure . Since the amount of background helium in the differ­ent hexahedrites would likely not be exactly the same, the horizonta1 separation of the curves in the lower left-hand region is associated with this difference.

If the background helium itself contains a

small amount of He" ( as it may if it is of pri­mordial origin) , then, in terms of the hypothesis that the distribution shown in Figure 4 arises because different amounts of cosmogenic helium are mixed with small amounts of this back­ground helium, the constants p and A aTe inter­preted in the following way. p is the He"/He' ratio in the cosmogenic helium and A = (p - u) Hen', where (1' is the ratio in the background helium and Hen' is the concentration of back­ground He'. In this case, in which the back­ground helium also contains some He", the distri­bution of points in Figure 4 does not make it possible to determine the value of u, though , of course, u must be smaller than the smallest measured ratio (0 .073) which was found for Edmonton. As an example, if we take u = 0 .05, we have the the following values for the other quantities. Curve A represents the mixture of cosmogenic helium with a ratio of 0.286 and background helium with a ratio of 0 .05 and a

He' concentration of 0.121 X 10-6 cm"/g (STP) . Curve B represents the mixture of cosmogenic helium with a ratio of 0.273 and background helium with a ratio of 0.05 and a He' concentra­tion of 1.224 X 10-6 cm"/g (STP) . The curves

A and B for this case are exactly the same as for the case in which the background helium is pure He', except that on the left end they stop at He"/He' = (1' rather than continuing down to zero. This corresponds to the fact that the meas­

ured ratio is never less than u, the ratio in the

TABLE 4. Values of the Parameters for Curves A and B in Figure 4

Curve A Curve B

p 0 . 286 0 . 273 A, em 3 /g (STP ) X 106 0 . 0286 0 . 273 HeB' for 0' = 0,

em3/g (STP ) X 106 0 . 1 1. 0 HeB4 for 0' = 0.05,

cm3/g (STP ) X 106 0 . 121 1 . 224

background helium , even if the concentration of cosmogenic helium decreases to zero .

For convenience the values of the parameters used in drawing curves A and B in Figure 4 are given in Table 4.

It is evident that, even if the hypothesis used i

here is otherwise correct, it is not possible tD determine from the He" and He' concentrations alone whether the background helium contains a small amount of He". The general hypot.hesis, however, is that the hexahedrites contain a small concentration of background helium which is ali, .

or mostly, He' and which noticeably affects the mea,sured ratios when the concentration of cos­mogenic helium is low. This general hypothesis gives a possible interpretation of the distribu­tion of points in Figure 4. Further testing of this hypothesis can best be done by the measurement and interpretation of the concentrations of other

nuclides . .

Acknowledgments. I am deeply indebted to a number of people whose efforts and interest made this investigation possible . Dr. A. O. Nier kindly offered me the privilege of working in his labora­tory during two summers . Dr. J. H. Hoffman p:t. tienUy taught me how to make the helium meas­urements with the mass spectrometer. Several men, as indicated in Table 1, were most helpful in providing the meteorite samples that were used in this study. Dr. E. P. Henderson , Dr. H. H. Nininger, Dr. P. W. Gast, and Dr. C. Frondel were especially helpful in providing many important samples at very critical times during the program.

This work was financed by a grant from the National Aeronautics and Space Administration.

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287-325, 1962 . Arnold, J. R., M . Honda, and D . Lal, Record of

cosmic-ray intensity in the meteorites, Re· searches on M e teontes, edited by C. B. Moore, 227 p p ., John Wiley & Sons, New York and London, 1962.

HELIUM CONTENTS OF METEORITES 6057 • .urol, W. J., R. B. Jacobi, and F. A. Paneth, Me­

teorites and the age of the solar system, Nature, 149 235-238, 1942 .

Ba'.le; C. A ., On the age and origin of meteorites,

Ph.D . thesis, Harvard University, 1949 .

Bauer, C. A., New measurements of the He-3 and

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341, 1960. . Chackett K. F., P. Reasbeck, and E . J . Wilson,

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Dalton, J. C., F. A . Paneth, P. Reasbeck, S . J . Thomson, and K . 1 . Mayne, C osmic-ray pro­duction of helium in meteorites and their ages, Nature, 172, 1168-1169, 1953 .

Fireman, E . L., and J. DeFelice, Argon 37, argon 39, and tritium ill; meteorites and the spatial constancy of cosmIc rays, J. Geophys. R es., 65, 3035-3041 , 1960 .

Hey, M. fl., Catalogue of Meteorites, 2nd ed., 432 pp ., British Museum, London, 1953.

Hoffman, J. H., and A . O . Nier , Production of helium in iron meteorites by the action of cos­mic rays, Phys. R e v ., 111S, 2112-21 17, 1 958 .

Hoffman, J. fl., and A . O. Nier, The cosmogenic He' and He' distribution in the meteorite Carbo, Geochim. Cosmochim. A cta, 17, 32-36, 1959.

Hoffman, J. H., and A. O. Nier, Cosmic-ray-pro­duced helium in the Keen M ountain and Casas Grandes meteorites, J. Geophys. R es., 65, 1063-1068, 1960.

Lovering, J. F., W. Nichip oruk, A. Chodos, and H. Brown, The distribution of gallium, germa­nium, cobalt, chromium, and copper in iron and stony-iron meteorites in relation to nickel con­tent and structure, Geochim . Cosmochim . Acta, 11, 263-278, 1957 .

Paneth, F. A ., The age of meteorites, Occasional Notices Roy. Ast1'On. Soc ., no. 5, 57-65, 1 939 .

Reasbeck, P ., and K. 1. Mayne, The ages and origin of meteorites, Nature, 176, 186-188, 1955 .

Schaeffer, O . A ., Cosmogenic rare gas contents of iron meteorites, Phys. To day, 13, 18-22, 1 960.

Schaeffer, O . A ., Cosmic ray ages of iron mete­orites, in Prob lems Related to Interplane tary Matter, Natl. A cad. Sci., Natl. Res. Council Publ . 845, 22-27, 1 961 .

Signer, P ., and A . O . Nier, The distribution of cosmic-ray-produced rare gases in iron mete­orites, J. Geophys. R es., 65, 2947-2964, 1960.

Signer, P., and A . O . Nier, . The distribution of rare gases in iron meteorites, in Prob lems R e lated t o Interplanetary Matter, N atl. A cad. Sci., N atl. R es. Council Pub l. 845, 31-44, 1961 .

Signer, P., and A. O . Nier, The measurement and interpretation of rare gas concentrations in iron meteorites, in R esearches on M e teoritef!, edited by C. B. M oore, 227 pp., John Wiley & S ons, New York and London, 1962 .

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Tilles, D ., Primordial gas in the Washington County mete orite, J. Geophys. Res., 67, 1687-1689, 1962.

Urey, H. C., Primary and secondary obj ects, J. Geophys. R es., 64, 1721-1737, 1959.

( Manuscript received June 21, 1 963 ; revised August 18, 1963.)