Cosmic-ray exposure history at the Apollo 16 and other lunar sites: lunar surface dynamics

Geochlmica et Cosmochimics Acta,1974,Vol. 38, pp. 1626to 1642. PergamonPress.Printedin Northern Ireland

Cosmic-ray exposure history at the Apollo 16 and other lunar sites : lunar surface dynamic C. J. &~ORC+AXand C. E. RALSTOX for Space Physics, Washington University, St. Louis, Missouri 63130, U.S.A.

R. 5. DROZD, C. M. HOHE~BER~, Laboratory

(Received 2 January 1974; accepted i?t revised form 30 April 1974) Abs~~t~oncord~t *rK-Kr exposure ages for four station 11 breccias indicate an age of 50.3 f 08 m.y. for North Ray Crater. Ray structures visible from orbital photography suggest that stations 8 and 9 should contain a substantial amount of South Ray ejecta. Concordant s%r-Kr exposure ages at these sites indicate an age for South Ray Crater of 2.04 & 0.08 m.y. Surface effects (tracks, surface angularities, and micro-crater populations) show good general agreement with this young an age, but discrepancies on a sample-by-sample basis seem to indicate that extensive pre-surface irradiations must have occurred. A detailed pm-surface exposure history is derived for the parent boulder of samples 69935 and 69955. It is suggested that secondary impacts play a major role in near-surface regolithic stirring. Widespread pre-surfsee irradiation would in fact be expected if most of the newly excavated material had been transported to the surface by secondary impacts rather than by the South Ray event itself.

IN A PREVIOUS publication (BE~~~~~ et cd., 1973b) we reported data which tentatively established the age of North Ray Crater as 50 m.y, and the age of South Ray Crater as 2 m.y. This work reports slKr-Kr exposure ages for an expanded number of samples confirming the earlier conclusions. In addition, interoompa~sons with other methods provide new insights into lunar surface processes ; intercomparisons with conventional rare gas methods reveal difficulties for conventional techniques due to variations in the shielding conditions on the lunar surface. Uncertainties in the inte~retation of rare gas exposure ages are chiefly due to the variability of shielding, the possibility of complex exposure histories, and modifications due to surface erosion processes. The 8Kr-Kr method largely eliminates shielding uncertainties, but the probability of surface erosion and multiple exposures can raise serious questions for the interpretation of long exposure ages and exposure ages where complex histories are likely. Lunar surface features can be dated with certainty only when a simple history can be demonstrated or when multiple samples yield concordant results.

%K.r-&

DATINU

Krypton spallation spectra can be obtained from the krypton isotopic abundances (Table 1) by the following process. The amount of solar krypton is first estimated by correcting the ?Kr for fission effects, assuming %ssUspontaneous fission md a 3.95 b.y. decay time ~TERA et cd., I973), where uranium concentrations (Table 2) are determined from an aliquot of each sample using the technique of GEISLERet al. (in preparation). The remaining s6Kr is, at this stage, assumed to be of solar origin; consequently, solar krypton contributions at each isotope can be removed leaving only cosmogenic krypton. Successive iterations &rem&e over the entire procedure to correct for a small spallation yield at *%r (86Kr/*sKr in spallation is estimated to be 0.015 (MARTI and LUGMAIR, 1971) so this is a relatively minor correction). The resulting krypton spalletion spectra, derived using the BEOC 12 composition (EBERHARDT et aZ., 1972) to represent the solar component, are shown in Table 2. 1625

1626

R. J. DROZD, C. M, HOHEXBERU, C. J. MOWSANand C. E. RALSTOX Table 1. Krypton from lunar samples

P41

Sample

(cm3 STP/ g x lo-“)

674SS

7.0

67915 1500%

7.1

Total

9.7

679SS

16.1

68115

9.2

6881S

10.0

6993Sj’ 1650%

12.2

Total

18.6

699SS

5.6

68415

28.7

62235

76.2

15475

43.9

15595 1100”c

6.3

Total

11.1

14310

76.0

14306-L* 1200°C

26.0

Total

333-6

78

80

f

14.11 0.09

&

f

9.53 0.11

&

7.02 & 0.08

81 84 z

&

78.85 0.15

zt

0.30

35.85 o-45 27.55 & 0.33

0.241 io.012

f

59.64 o-47

f

74.11 0.76

21.S8 & 0.08

-

f

49.46 0.35

f

59.98 0.56

f

24.13 0.07

&

39.64 0.06

f

47-00 0.11

f

2S.92 0.04

f

6.010 0.039 6,564 0.047

0.298 *o.oos 0,477 * 0.080

1.024 & 0,022

*

5.849 0.040

0.899 0~018

f

5,297 0.032

O-217 -&O.Oll -

1,768 & 0.033

f

7.584 0.065

f

8.555 0.033

f

f

25.293 0.036

f

1,118 0.005 1.562 0,014

86

0.335 & 0.009

0.114 kO.005

5497 0.012

83

49.81 0.16

19.020 0.062

f

82 100

f

99.83

17.23 0.11

f

22.56 0.08 23.68 0.07

&

22.44 0.07

f

21.72 0.06

23.73 0.11 22-45 & 0.09

0.320 50.012

&

24.68 0.11

&

26.50 0.14

29.27 & 0.11

38.083 0.014

0.110 &O.OOS

&

58.21 0.11

f

69.49 0.09

f

&

68.563 0.032

0.161 kO.003

f

30.463 0,032

f

90.74 o-11

0.0696 *o-o012

f

18.41 0.24

&

54.79 0.74

&

91.43 1.06

&

f

11.52 0.12

f

34.62 0.36

O-167 &0.018 -

&

62.95 0.53

&

f

33.962 0.033

&

120.61 0.08

0.123 kO.004

*

153.80 0.11

&

36.23 1.36

0.449 f 0.039

&

63.64 1.23

f

76.21 1.73

27.14 + 0.13

f

13.66 0.20

-

jt

34.00 0.24

*

36.69 0.34

f

f f

*

19.29 + 0.41 i

6.60 0.07

f

f

113.67 0.09 137.96 & O-06

* f

23.05 0.10 24.54 0.12

f

+

30.48 0.07 30.S6 f 0.07 f

*

29.86 0.13

29.99 5 0.10

21.83 0.05

149.09 0.12 182-48 i o-17

4.06 & 0.02

117.97 1.69

13.93 & 0.35

f

79.19 0.84 187.27 f 0.10

16.84 & 0.02

f

20.39 0.78

13.41 & 0.01

29.50 0.06

Rare gases were extracted, ptied and analyzed m&88 spectrometrically in the manner deeoribed by HOHENBEFNet aE. (1967) and DROZD et al. (1972). Precision of the gas quentitiee ie better than 16 ‘A. Temperatures and tot&, where appearing, indicate temperature runs and give, respectively, the most spallogenio temperature fraction and the weighted average over all temperature fractions; otherwise the data are from 1500% melts. L and D refer to the Light and Dark fraction of 14306. * RACUX (1972). t Extra&ion from 69936 may be inoomplete due to partial melting of the sample.

Comio-ray

1627

exposure history at the Apollo 16 and other Inner sites Table 1 (Cont.)

SlUnple

E841 (cm8 STP/ g x X0-“)

14306-D* 1200% Total

14311* 1200°c

17.1

Total

499

14301* 1200°c

76.8

104.3

Total

9753

23.65 0.26

f

956 + O-05

2455.0

14318 1200°C

0.16

9.64 +. 0.07

2.1 17.2

Total

i

3050

Total

80

81 84 z

&

52.77 0.66

0850 j, 0.044

f

22.55 0.19

-

22.96

50.2

14171 1200°c

78

& f

64.67 0.29 24.21 0~09

82

83

86

100

0.810 f0*025 -

85.44 ~fi 0.92

f

1038 1.4

f

26.57 0.07

&

44.06 0.18

f 5086 + 0.39

f

28.63 0.03

&

89.07 0.48

&

109.55 0.85

f

26.26 0.16

*

4811 0.13

f

66.23 0.20

f

27.75 0.04

f

55.63 0.09

&

162.04 0.18

f

222.47 0.40

f

287.28 O-62

6.88 & 0.04

i

41.20 0.14

f

113.72 0.62

+

171.10 0‘55

&

220.63 0.76

f

i:

2580 0.04

f

2860 0.04

30.00 & 0.04

*

21.82 0.04

*

22.18 0.02

3086 & 0.02

+

2187 0.01

*

22*08 0.03

*

30.61 0,02

f

21.42 0.01

*

21.76 0.01

f

30.34 0.01

f

2.285 0.005

f

8.020 0.009

i

0.908 0.001

&

4.855 0.003

*

1.038 0.007

5-035 &- 0~009

0.955 + 0.001

f

o-0149 & 0.0043

0.0114 f0.0005

4.912 0.004

-

10.11 0.11

Table 2 Krypton spell&ion spectra (83 = 100) 78

Ssmple 67095

80

81

WI

82

84

P3lIP33

(wm)

49.1 & 0.5

0,618 &to.001

1*18* f0.03

*

208 02

f

51.9 0.2

f

0,373 0.007

f

f

15.4 0.2

53.4 &- 0.6

f

0.372 0.011

76.6 & 0.8

*

54*1 08

0.618 *to.005

0.064 kO.016

f

15.1 0.3

f

54.9 1.1

f

0,372 0.018

76.2 & 1.3

53.9 + 0.9

0.622 f0.008

0.391 rto.039

67455 67916

Spalletion spectm for applicable (see Table 1). described in text. PSI/P,, bracketed spectrum ia the

78-l 0.1

krypton based on date for most spallogenic temperature where Nmnbem in parentheses are ratios corrected for slow neutrons as for these samples are based on the corrected spectra. For 69935 the ‘soft’ component (BEERWAXX et al., 1973b).

* RANCITELIJ et al. (1973). t PIII/P,, CdCdSfXd using equation (1) from LW~MAIR end %hBTI (1972)+ $ 14301 and 14318 heve lerge amounts of fission xenon from z44Pu (Dnop, et al., 1972; BE~~~WANNet al., 1973a). The method of correcting krypton for this effect is described in text.

1628

R. J. DROZD, C. M. HOHENBERQ, Table Krypton 78

Sample 67955 68115

68815 69935

69955 68415

62235 15475 15595 14310 14306-L 14306-D 14171 14311 14301$ 14318$

spallation

spectra

52.2 0.3

0.367 &0.015

*

9.55 0.47

f

f

17.0 0.6

f

*

21.1 0.7

f

58.7 2.0

10.0 & 0.4

f

10.9 0.6

*

55.7 1.6

+

69.1 2.7 (56.2)

WI 53.0 * 1.6

14.6 0.1

+

17.9 & 0.1

f

16.7 0.1

16.9 & 0.4

f

*

(83 =

81

+

*

5.27 0.24

101 4.38 f 0.25

RALSTON

2 (Co&.)

80

16.5 & 0.1

[ll 16.4 f 0.6

C. J. MORGAN and C. E.

100)

WI

82 75.8 0.4 85.9 4.2 (81.9)

84

Ps1IPss

(ppm)

*

54.6 0.5

0.608 kO.003

0.018 f 0.004

+

24.9 6.8

0.656 *0.033

0.50 ho.05

18

0.671 *0.01s

0.782 *o.os2

19

*

82.5 3.1

f5

*

65.0 4.0

*11

0.573 *0.025

0.754 *0.075

[461 75.6 * 2.7

[351 61.1 + 5.4

0.611 ho.015

0.052 j10.013

79.8 & 0.4 (75.3)

*

56.3 0.5

0.601 f0.002

0.298 10.030

f

0.197 0.013

47.8 0.1

*

0.115 0.002

+

74.7 0.2

f

38.1 0.1

0.581 10.001

2.52 &0.25

*

49.7 0.4

f

0.038 0.001

75.6 & 0.5

*

54.8 0.4

0.595 &0.003

0.19 ho.02

+

48.7 0.7

*

0.162 0.018

&

75.5 1.0

f

51.3 0.7

0.589 + 0.004

0.208 + 0.029

63.6 0.3 (51.3)

f

18.7 0.1

l 0.1

f

0.068 0.002

+

81.5 0.1

*

38.1 0.1

O.SOlf & 0.002

2.90 kO.30

f

31.8 1.2

55.8 &- 2.9

+

0.761 0.067

f

78.6 3.2

*

24.1 1.1

0.638 *0.020

3.46 *0.35

+

25.8 0.4

*

56.8 0.9

f

0.835 0.041

+

79.0 1.7

*

20.1 0.4

0.645 10.010

3.46 +0.35

*

23.6 0.2

*

53.4 0.4

f

0.775 0.020

*

78.7 0.7

*

30.3 0.5

0.627 10.004

5.43 *0.54

+

19.4 0.2

+

53.0 0.5

0.029 & 0.001

*

77.5 0.7

f

34.6 0.3

0.620 ho.004

8.39 10.84

+

18.7 0.3

46.6 -& 0.5

f

0.164 0.048

f

69.9 1.0

0.553 *0.010

3.63 ho.26

*

21.0 0.5

f

55.1 1.1

+

0.531 0.023

+

88.3 1.7

0.681 ,to.o19

3.80 ho.40

66.0

=35 +15 =35 115

The gas-rich breccias 14301 and 14318 present somewhat of a special problem in that they contain large quantities of fission xenon due to 244Pu (DROZD et al., 1972; BEERMANN et al., 1973a). Fission krypton from 244Pu decay is, of course, also present in these breccias, a fact that becomes apparent when fission corrections are attempted in the normal way. The presence of plutonogenic krypton results in the assumption of too large a solar component which, when subtracted from the krypton composite spectrum, yields an erroneous spallation spectrum. The effect is most obvious at s4Kr where negative spallation yields sometimes result. It can be noted (Table 2) that (*4Kr/ssKr) spsu normally lies in the range of 0.2-0.5. In order to correct for the presence of 244Pu f&ion krypton, the amount of the total flssion correction is adjusted until the (*41(r1ssKr)Bpau falls within the normal range of values. Variation of this parameter over the extremes of the observed range produces only small ( - 3 per cent) changes in the other spallation

Cosmic-ray exposure history at the Apollo 16 and other lunar sites

1629

ratios. The resultant spallation spectra, derived for the (84Kr/*3Kr)8p&ll~0,35 rfI 0.15, are reported in Table 2, where the additional uncertainties due to this procedure are compounded with the norms,1s~ti~ie~ errors. For a simple exposure history (one-stage removal from ~btotally shielded to an exposed location), the %Kr-Kr exposure age is given by:

where 7 is the O-303 x IO6yr me&n life of siKr and PSI/Pi is the ratio of production rates for SKr and “Kr, a stable spa&&ion product. The exponential term, which need be considered only for the youngest of exposure ages, represents tbcorrection for non-equilibrium. If resonsnce neutron reactions are small compared with spallation, then the ratio of production rates is given by (LUGMAIR and MARTI, 1971):

If neutron effects are present, an alternative treatment is necessary. The most important neutron component in krypton is normally due to slow neutron captures on 79Brand 81Brwhich produce S°Kr and *2Kr. By means of intercomparisons between samples of similar chemistry, excess *“Kr and 8sKr from these neutron capture reactions can be detected and corrected for. In the absence of neutron effects the spallation yields at *‘?Kr and szKr csn each be described by a linear rel&tionship with the relative 7BKr and s%Krsp~l&tion yields. For example, the spa&&ion ( *°KrjssKr),,,,, ratio is given by:

where Ai%fis 3, the mass difference between *OKr and 83Kr. The parameter P depends upon the chemistry and is derived empirically. This technique is similar in principle to the treatment of LVG~IR and MARTI (1972) but the diversity of the Apollo 16 samples demands th& chemistry be treated on an individual sample, rather than a mission-by-mission, basis. The entries in parentheses of Table 2 represent true spallation yields, hsving been corrected for neutroncapture effects. Table 3 presents the slKr exposure ages which are based upon the neutron corrected data where applicable. NEON AND ARGOR EXPOWRE

AGES

The accumulation of spallation-produced Ne and Ar provide other means for ce;lcula;ting cosmic ray exposure ages. However, ages computed in this w&y are only as reliable as the appropriate spallation production rates and these can vary substantially from sample to sample. Neon and argon spallation ages (Table 3) are based upon the total quantities of spallogenic 21Ne and S8Ar measured in eaoh sample. Estim&es of the production rates are taken from a suitable suite of calibration samples. For this we use the results of BOGARDet ~2. (1971) who find production rates of (0564 f 0.096) x lo-* cm3 STP 21Nejg(Si & Mg)/m.y. and (1.69 $ 0.47) x lo-* ems STP S*Ar/g(Ca)/m.y. from a group of 48 Apollo 11 and 12 samples. Target abundances (Si, Mg and Ca) are obtained from the Apollo 16 Prelimina~ Science Report (1972), RHODES (1972), and ROSE et al. (1973). For samples with no reported chemistry, abundance estimates are based upon rocks of similar type and are so labeled in the table. Errors associated with uncertainties in the assumed spallation production rates (17 per cent for neon and 28 per cent for argon) and ~cert~inties in the me~urement of rare gas quantities (typically 10 per cent) resuIt in a minimum uncertainty of 20 per cent and 30 per cent for neon and argon exposure ages, respectively. In addition to these errors, there is the probability that systematio differences in shielding will introduce systematic biases into the inferred exposure ages unless careful choices are made for the calibration samples. For example, production rates

1630

R. J. DROZD, C. M. HOEENBERU,C. J. MOWAN and C. E. RALSTON Table 3. Spallation ages Sample

SurfaCe feature

67095

N.R.

67455

N.R.

67915

N.R.

67955

N.R.

68115

S.R.

68815

S.R.

69935

S.R.

69955

62235 15475 15595 14310 C.C.

14171

C.C.

14301 14318

-

-

50.2 18

&

50.3 1.4

&

17.3 4.1

f

38.0 13.0

f

50.6 1.5

f

21.0* 4.9

f

26.0* 10.0

f

50.1 1.6

f

17.9 4.2

f

32.0 12.0

f

2.08 0.14

f

f

2.04 0.09

f

1.75* 0.41 1.21 0.29

f

1.40* 0.33

f

2.13* 0.51

f

1.99t 0.16 4.23 0.21

f

1.63* 0.67

f

2.18 0.98

&

4.0* 1.7 -

f

92.5 5.9

+

32.5 7.8

f

113.0 42.0

f

153.3 2.9

f

104.0 24.0

f

163.0 54.0

f

473.0 9.0

f

336.0 79.0

543.0 &183.0

f

llO.O$ 9.0

f

112.0$ 26.0

*

2680 9.0 24.4$ + 1.1

f

14306

ssAr

slN0

f

*

68415

14311

s’Kr-Kr

f

113.0 26.0 -

f

105.0$ 34.0 173.0 55.0 -

-

-

SSl.O$ f 51.0

-

-

102.08 21.0 38.8 * 1.3

-

-

-

-

*

f

24.5 0.7

Spallation ages are in million years. Surface features dated by samples are given when possible: N.R., North Ray Crater; S.R., South Ray Crater; C.C., Cone Crater. 69955 contains spallation gases apparently produced under earlier pre-surfaceirradiationconditions (see text). The 14306 age is the average of both light and dark portions. Ages for those samples on which stepwiseheating experiments were done are based on weighted average of the most spallogenictemperatures. * For these samples, major element chemistry is estimated from samples of similar rock type (Apollo 16 Preliminary Science Report, 1972). t s1Kr-7sKr are, computed as describedin text and in BEHILMllrrnet al. (1973b). $ BEHR~ et al. (1972). $ CnozAz et al. (1972).

Cosmic-ray

exposurehistoryat the Apollo 16 8nd other lunar sites

1631

08x1 change by 8 factor of 2 for 8 veriation of 80 g/cm2 in shielding (REEDY and ARNOLD, 1972). Large uncertainties (perhaps 8s much 8s 8 factor of 2) must therefore be expected from the method; 8 f8ct that is overlooked by people who t8ke these spallation ages 8t face v8lue and even by some workers in the field. AGES OF NORTH AND SOUTH RAY

CIIUTERS

The *KKr-Kr exposure ages for 67095,67455,67915 and 67955 (Table 3), collected from the rim of North Ray Crater, cluster very tightly with a weighted average of 60.3 & 0.8m.y. This age, clearly to be associated with the formation of North Ray Crater, is in excellent agreement with *‘Kr-Kr results of M&T1 et aZ. (1973) who obtain an average of 48.9 f 1.7 m.y. for rocks 67015, 67075 and 67915. Two samples 68115 and 68815, collected from station 8 which is apparently located on a bright ray from South Ray Crater (Apollo 16 Preliminary Science Report, 1972), have *lKr-Kr exposure ages of 2.08 and 2.04 m.y. (Table 3). A third sample, 69935, when corrected for neutron effects in the manner described by BEFIRMANN et al. (1973b), has an apparent surface residence time of 1.99 m.y., suggesting that a common event may have transported these three samples to the surface. The general geologic setting of stations 8 and 9, being only several km from South Ray Crater and visibly part of the bright ray structure, makes it very likely that these rocks are associated with that event. The fresh appearance of South Ray Crater when compared with North Ray Crater (50 m.y.) and Cone Crater (24 m.y.), the fact that South Ray ejecta appears to overlie North Ray material, and the preponderance of fresh appearing angular fragments and the distribution of these fragments over the Apollo 16 site (Apollo 16 Preliminary Science Report, 1972) sre all consistent with a very young age for South Ray Crater. However, before accepting 2 m.y. as the age of South Ray it is necessary to consider other possible sources of fresh material. Baby Ray is also a young crater. Ejecta from Baby Ray overlies South Ray ejecta implying that Baby Ray is in fact the younger of the two. In order to exrtmine the possibility that Baby Ray is responsible for the young concordant exposure ages, we have computed the expected ejectrt thickness at the various sites from each of the major cratering events. Table 4 gives the average ejecta thickness expected at the various stations Table 4. Average ejecta thickness (mm)* At: North Ray St&ion 2 Station 8 Station 9 Station 11

12.5 3.4 3.9 - 13,000

Due to: South R8y 1.7 6.6 5.0 0.3

Baby Ray 0.03 0.33 0.19 0.003

* Computed from the formula T = 0.14RO”’

(v/R)-~

(MCGETCEINet al., 19’73), where T is the 8verage eject8 thickness, R the cmter radius, and r the distsnce from the crater center. All messurements 8re in meters.

1632

R. J. DZOZD,C. M. HOEEXBERO, CAJ. MORGAN

ad

C.

E. RALSTON

due to North Ray, South Ray and Baby Ray craters estimated from the semiempirical relationship given by MCGETCFXIN et al. (1973) : T = 0.14RO.74(r/R)-3, where T is the average ejecta thickness, R the crater radius and T the range from the crater center (ail measured in meters). Clearly such a formula cannot be correct in detail since it considers neither structure in the ejecta blanket nor re-dis~bution of regolithic material by the secondary impacts. It can, however, be of service evaluating the probability that a given sample or surface feature is associated with a particular creating event. It is apparent from Table 4 that the average ejecta thickness at stations 8 and 9 due to the South Ray event should be roughly 20 times that expected from Baby Ray, assuming no ~anu~arity in the ejecta blanket. Photometric evidence for a ray from South Ray Crater overlying these stations (ApolIo 16 ~r~li~~~ar~ Science Report, 1972) further enhances the probability that the South Ray event should dominate stations 8 and 9. We therefore believe that the weighted average exposure age of 68115, 68815 and 69935 (2.04 & 0.08 m.y.) probably dates the formation of South Ray Crater. MORRISONet al. (1973) and NETJKUMet al. (1973) suggest that a number of Apollo 16 rock surfaces are not in micro-cratering equilibrium. Rocks 60315, 62235, 62295, 68415, 68416 and 69935 have surfaces whose pit distributions have been used to deduce ages of 2-3 m.y. by these authors. ~tho~gh this interpretation has recently been brought into serious question (H~Rz et al., 1973), it is in good (but perhaps fortuitous) agreement with the age of South Ray Crater as inferred from our *?Kr-Kr data. If true, it would indicate that fresh rock surfaces have been exposed by the South Ray event. reveals that only one sample (69935) data has a sKr-Kr exposure age in in~cating either erroneous surface

A more detailed

intercomparison,

however,

of the three in that group for which we have agreement with this surface age (Table 3), ages or sub-surface i~adiations. We have

independent evidence that one of these samples (62235) could not have spent its entire 153 m.y. apparent exposure age in its recent surface location. It was collected from the rim of Buster which is a young crater, described as a possible South Ray secondary (Apollo 16 Preliminary Science Report, 1972). Therefore a simple surface residence is ruled out for this sample and sub-surface irradiation must be considered likely. Even though the relevance of impact pit ~stributions for estimating the surface exposure ages of these rocks has been questioned (H~Rz et al., 1973), such young surface ages tend to be supported by other evidence. Angularity and surface textures of all of these samples support the limited (few m.y.) surface residence time indicated by apparent impact disequilibrium (SWANN, Apollo Lunar Geology Investigation Team, 1972). Cosmic ray particle track data (YUIXAS, 1973) for 68415

and 62235 also indicate a very restricted (few m.y.) surface dwell time. If these observations are correct they stand as a serious problem for interpreting the sIKr-Kr ages of 62235 (153 m.y.) and 68415 (92 m.y.) as crater ages. The implication is of course that these two rocks must have accumulated a large part of their spallation krypton in lightly shielded but sub-surface locations, violating the simple exposure

Cosmic-ray exposure history at the Apollo 16 and other lunar sites

1633

assumption necessary to assign the slKr-Kr ages to a specific surface feature. Presurface irradiation seems also to characterize the exposure of samples 69935 and 69955 (developed in the next section). exposure ages among various station In summary, concordancy of 8Kr-Kr 11 rocks establishes an age of 50.3 & 08 my. for North Ray Crater. ‘FK-Kr data, with substantial supportive evidence from other sources, indicates an age of 2.04 f 0.08 m.y. for South Ray Crater. Of equal importance however is evidence suggesting that sub-surface irradiation is a general feature for much of the material excavated at the time of the South Ray event. LUNAR SURFACE DYNAMICS We have previously reported (BEHRMANN et al., 1973b) that sample 69935 contains a cosmogenic krypton component produced under irradiation conditions This conclusion was reached by inconsistent with a simple surface exposure. comparing the spallation spectrum of 69935 with that of 68815, a rock with a similar Sr/Zr ratio, similar geometry, and similar recent exposure configuration. Differences in these two spallation spectra seem to be due to an additional component in 69935 which was obtained during irradiation under much greater shielding than possible at the surface site. This ‘soft’ component is indicated by brackets in Table 2. We further observed that the e1Kr/78Kr spallation ratio is the same for 68815 and 69935 to within 4 per cent. Since 78Kr is the product of high energy spallation reactions and slKr is in equilibrium under the recent surface (high energy) exposure conditions, we concluded that the two rocks probably have had a similar surface residence time and that the previous exposure of 69935 must have been primarily characterized by relatively low energy neutron reactions. A more direct way to show this is in the form of an %Kr-‘8Kr exposure age for 69935 (BEHRMANN et al., 1973b), which refers to the duration of exposure to the more energetic particles. This agrees quite well with the surface exposure ages of 68115 and 68815 (Table 3). We have now examined 69955, a sister to sample 69935 (69935 came from the top of a station 9 boulder, 69955 came from the bottom of the same boulder). Lack of agreement between the two *lKr-Kr exposure ages (Table 3) is further evidence for the multiplicity of shielding conditions that must have characterized the exposure of this boulder. The krypton spallation spectrum of 69955, however, shows no obvious abnormalities that would lead one to doubt the apparent 4-2 m.y. BIKr-Kr exposure age, a number which is clearly incompatible with either the direct or the neutron corrected exposure age of 69935 (BEHRMANN et al. 1973b, and Table 3). This demonstrates first that complex exposure conditions can exist even for a sample with an integrated surface residence of only a few million years. Secondly, it demonstrates that the dating of surface features by means of exposure ages is sometimes questionable unless concordancy can be found among a number of different samples associated with that particular feature or event. In this case, the event dated is the excavation of a boulder and in this case concordsncy is elusive. Since 69935 and 69955 were part of the same boulder, the exposure history of each must be related. We can attempt to reconstruct this history by an independent treatment of the spallation records contained in each sample subject to the

1634

R. J. DROZD, C. M. HOEEINBERQ,C. J. MORGAN and C. E. RUSTON

0

EXPOSURE

TIME

AT DEPTH

(m.y.1

Fig. 1. Shieldingdepth vs the pm-surface irradiationtime requiredto accumulate the observed quantity of neutron-produced *OKr in 69936 using the neutron systematics of LINUENFELTERet al. (1972). The bromine abundance is assumed to be 830 ppb, similar to that in 68815 (lI(R;6HENB&XL et al., 1973). Possible errors in either the bromine content or in the quantity of neutron-produced @?Kr will &ift the curve in the direction of the arrow with a relatively small effect on the pre-surface burial depth. A pre-surface exposure time for the parent boulder of 2 m.y. is estimated from epallogenic krypton in 69965, indicated by the dashed line, and provides the pre-surface burial depth of about 360 g/cm2.

imposed by their physical proximity. We estimate that, during the heavily shielded period of exposure for 69935, 9 x 1O-13(-J$O per cent) cm3 STP/g of neutron-produced *OKrwas accumulated (Table 2 and BEERMANNet al., 1973b). The work of LINGENFELTER et al. (1972) provides an estimate of the rate of *‘JKr production from bromine as a function of shielding thickness. This can be graphically displayed for 69935 as a plot of shielding thickness va irradiation time required to produce the observed neutron effects (Fig. 1). The bromine content of 69936 is assumed to be similar to that measured in 68816 (830 ppb, KRXHENBQHL et al., 1973). Spallation krypton accumulated by 69936 during the more shielded portion of exposure is dominated by low-energy reaction products. This seems to imply a r&her restricted range of possible pre-surface irradiation depths, 300-1000 g/cm2 (BEHRMANN et al., 1973b). If the range of allowable pre-surface exposure times can be restricted in some way, a more precise burial depth can be obtained from the curve in Fig. 1. Since the slope is very flat in the 300-1000 g/cm2 region, such treatment is very insensitive to uncertainties in the abundances of either bromine or neutron-produced krypton (variations in these quantities shift the curve in the direction of the arrow). Relatively minor uncertainties in the pre-surface burial depth should result even if there sre relatively major uncertainties in the presurface exposure time end the abundances of bromine and neutron-produced krypton. YUHAS (1973) observes no solar flare tracks in crystals from 69956, indicating that the present orientation of the boulder (Fig. 2b) has been maintained during the entire surface residency. In fact the 69955 end apparently has never been exposed directly to the sun. We know that the 81Kr abundance is in equilibrium constraints

Cosmic-ray exposure history at the Apollo 16 and other lunar sites

1836

I 2470g/cm2 I r350glcm 9965

IQ 69935

T=Z.t

m.y.

T820m.y.

b a Fig. 2. &hem&& diagram of the exposure history for the parent boulder of 69935 rtnd 69955. (a) Conf$uration during the pre-surface portion of the exposure history. (b) Position as observed at station 9. Presumably the boulder was excavated by a secondary impact from the South Ray event.

under surfccce exposure conditions. The 4-2 m.y. apparent age (Table 3) and the 2 m.y. surface age (from 69935 data) suggest that approximately one-half of the spallation krypton accumula~ elsewhere. The apparent ratio of production rates P,,/P8s for this rock is typical of other Apollo 16 rooks (see Table 2), indicating that exposure conditions during the two periods of irradiation were not vastly different (referring primarily to the type and energy spectra of the nuclear-active particles present). A minimum pre-surface burial depth for 69955 can be set by the lack of solar flare tracks ( > 1 cm) ; a maximum depth can be set by the absence of prominent

low-energy neutron effects. With a bromine content of 130 ppb ~~~~~~ al., 1973) neutron effects should become observable as shielding approaches 300 g/cm2 (BEHRMANNet cd., 1973b). We therefore estimate (to within a factor of 2) that the pre-surface shielding of 69955 was similar to the shielding conditions existing at the surface site (180 g~cmz). This of course implies a similar krypton production rate [to within a factor of two (REEDYand ARNOLD,1972)] and therefore pre-surface exposure for a period of 22: m.y. From the dashed lines of Fig. I it can be seen that thisint~nimpIies apre-surface exposure depthof 350 f 100 g/cm2 for 69935. Furthermore, the presence of neutron effects in 69935 and the lack of them in 69955 suggests that the 69955 end of the boulder was up during the subsurface period (Fig. 2A). Since the boulder is 60 cm in diameter, 69955 would then have been shielded by about 170 g/cm2, corroborating the original estimate of 180 g/cm2 shielding. To summarize the exposure history for this boulder, the following account seems to emerge. Initially the boulder was buried and inverted from its recent oonfiguration with 69935 at the bottom and 69966 at the top (Fig. 2a). 69936 was shielded by about 360 & 100 g/cm2, implying about 170 g/cm2 shielding for 69966. It was irradiated under these conditions for about 22: m.y. until transported to the surface at the time of the South Ray event, where it has remained in its present (Fig. 2b) orientation for the last 2 m.y. This account is consistent with the presurface irradiation history previously deduced for the boulder (B-N et al., 1973b) though we oan now be more precise due to the constraint imposed by the additional data of 69955. The duration of the sub-surface irradiation is still the et

1630

R. J. Daozn, C. M. HOHENBERO, C. J. MOROANand C. E. RALSTON

An apparent 180 degree flip that occurred most uncertain of the parameters. upon ejection seems to be a common feature of cratering dynamics (SHOEE~AKER, 1963). None of the station 11 samples (from the rim of North Ray Crater) show any signs of pre-surface irradiations of any kind, suggesting the simple relocation of these samples from completely shielded sites to surface sites in a single excavationpresumably by the cratering event itself. Exposure ages for the four samples from this station cluster tightly and are interpreted as dating crater formation. On the other hand, it appears that extensive pre-surface irradiation is characteristic of material associated with South Ray Crater. In addition to the pre-surface history deduced for the parent boulder of 69935 and 69955, lithic fragments, coarse fines and soils collected from areas thought to be dominated by South Ray ejecta also appear to have preserved a record of irradiations received prior to the formation of South Ray Crater (SCHAEFFER and HUSAIN, 1973; BEHRMANN et al., 1973b; FLEISCHER et al., 1973; WALTON et al., 1973). It seems unlikely that so large a fraction of material ejected by an impact of this magnitude could have been previously irradiated. Track studies were reported on nine soil samples from the Apollo 16 site by BEHRMANN et al. (1973b). The results can be summarized as follows. Normal soil samples in the vicinity of North Ray Crater are well irradiated, consistent with a 50 m.y. crater age. Soils and coarse fines from stations 8 and 9, presumably blanketed by South Ray ejecta, contain little, if any, lightly irradiated material. Not one crystal* out of 274 studied showed a track density of less than 10’ t/cm2. Taking a nominal stirring depth of 5 cm, a freshly ejected crystal should have accumulated only 4 x lo5 t/cm2 in 2 m.y. The majority of the crystals in these soils contain track densities in excess of lo8 t/cm2 which classifies them as mature (CROZAZ et al., 1972), well-irradiated soils, inconsistent with a fresh exposure commencing 2 m.y. ago. Soil 69961, removed from beneath the parent boulder of 69955 and 69935 (presumable emplaced 2 m.y. ago), should sample pre-South Ray material but is not noticeably different in track density from sample 68501 which was collected in an area exposed to South Ray ejecta. Apollo 16 soils, including those at stations 8 and 9, have been found to have the high percentage of glassy agglutinates typical of a mature soil (ADAMS and MCCORD, 1973). Similar results were found by HEIKEN et al. (1973) for 68501. In contrast these same workers find low percentages of glassy agglutinates in soil samples collected from stations near North Ray Crater, suggesting that the material redistribution by the South Ray event is, in fact, more mature than North Ray ejecta. Even if we do not consider the results reported here, it can be shown that the South Ray Crater must be younger than North Ray because its ejecta blanket overlies North Ray material (Apollo 16 Preliminary Science Report, 1972). SCHAEFFERand HUSAIN (1973) report 38Ar-37Ar exposure ages of approximately 130 m.y. for six fragments from 68500. WALTON et al. (1973) measure a spallation age of 170 m.y. for soil 68841, both very much older than South Ray Crater. On * FLEISCHER et al. (1973) report finding a few crystals with low track densities, but these represent only a small fraction of the material and are found in sample 69961 (under the parent boulder of 69935 and 69955) as well.

Cosmic-ray exposure history at the Apollo 16 end other lunar titaa

1637

the other hand, SCHAEFFER and HUSAI~IN(1973) obtain ages of 30-55 m.y. and WALTONet al. (1973) obtain ages of 50-60 m.y. for coarse fines from the rim of North Ray Crater, in approximate agreement with the age of North Ray Crater inferred by *%r-Kr dating. Since the age of South Ray Crater is orders of magnitude less than would have been inferred from the soils and coarse fines, an important datum can be established here. Either the so-called South Ray soils contain, in fact, very little South Ray material, or the South Ray fine material consisted of B well-irradiated, mature regolith that was ejected on to the surrounding plains with little loss of particle tracks or rare gases. We have found no compelling evidence to show that the soils and coarse fines in question actually contain South Ray ejecta; if they do, an improbably large fraction of the material ejected by South Ray material must have been previously exposed to large fluxes of energetic solar and galactic particles. In addition, they must have preserved the rare gas and particle track record of this exposure through the hypervelocity cratering event. The cosmic ray exposure history and surface residence times of the station 8 and 9 boulders provide e, similar story in that the majority have *Xr-Kr exposure ages in excess of the surface residence times (as inferred from the track, impact pit and multiple sampling techniques previously discussed). One very attractive possibility is that the simultaneous transport of material to the surface 2 m.y. ago may not have been due to ejection from the South Ray Crater itself but to redistribution in the pre-existing regolith by the secondary impacts. The distance from the South Ray Crater to stations 8 and 9 are 3.5 and 3.9 km, respectively. This implies a minimum projectile velocity of 80 m/set for impact at these stations assuming a ballistic trajectory from the South Ray site. Although laboratory simulation studies have been more complete for hypervelocity impacts than for low velocity impacts, a certain amount of data is available. ELSTON and SCOTT (1971) simulated the arriving Allende stones by aircraft drop of basalt and sandstone projectiles which impact at about 100 m/set. These projectiles arrived nearly vertically and displaced an amount of target material equal to about ten times the projectile mass. Incoming South Ray ejecta at stations 8 and 9 impacted with angles typically ranging from 30 to 50 degrees and about 100 m/set. On the basis of this evidence, it is reasonable to expect that at least an order of magnitude more material was excavated from the secondary craters than was displaced by the South Ray impact. Therefore most of the material brought to the surface by the South Ray event should be local regolithic material which has been overturned by the secondary impacts. If this is true one would expect to find evidence of pre-surface irradiations in some of these samples due to the fact that many of the rocks excavated would have previously been at near-surface sites. This is consistent with observation : all of the rocks with significant pre-surface exposure (68415, 69935, 69955 and 62235) were in fact collected from the rims or adjacent to meter-sized, presumably secondary, craters (APOLLO LUNAR GEOLOGYINVESTIGATION TEAM, 1972). The following additional observations can be made. Fresh surfaces would presumably be exposed to the sun at the post-South Ray (surface sites, which is consistent with the particle track results and surface morphology of these rocks. 10

1638

R. J. DROZD,C. M. HOHENBERQ, C. J. MORUNand C. E. RALSTON

The rather shallow (6 mm) average ejecta thickness expected at stations 8 and 9 from South Ray (Table 4) would probably be more or less completely mixed with the pre-existing regolith due to secondary impact stirring. Direct ejecta might then be expected to represent only 10 per cent or less of the surface fragments which is more in line with the track results on the soils, the apparent soil maturity from agglutinate content, and the rare gas exposure ages of the coarse ties. No evidence is found for complex exposure histories in the vicinity of North Ray Grater. None of the station 11 rocks reported here had multiple exposures. SCHAEFFER and HUSAIIN (1973), WALTONet al. (1973) and BEE~FLMANN et al. (1973b) report rare gas exposure ages and cosmic-ray track densities for coarse fines and soils in good agreement with the rock exposure ages. If secondary craters are responsible for the bulk of lunar surface gardening, one might wonder why multiple exposures characteristic of near-surface mixing do not seem to be present for the North Ray samples. Table 4 shows that the average depth of the North Ray ejecta blanket will be roughly 13 m at station 11. This means that material overturned by the secondaries will be North Ray ejecta itself and consequently will have no previous irradiation history. Only when the average ejecta blanket is thinner than the penetration depth of secondary craters can pre-existing (and presumably pre-irradiated) regolithic material be brought to the surface of these secondary impacts. DISCUSSION In order to evaluate the role of secondary impacts in regolithic dynamics a detailed photographic assessment of the Apollo 16 site is currently being planned. Secondary craters formed by oblique, low velocity impacts should produce some distinguishing features, The craters should tend to be elliptical with the major axes in the direction of the primary crater. Ejecta from the secondary craters should be more concentrated on the down-range flank and the projectile should fragment, with debris concentrated on the down-range crater wall or the down-range crater flank for the lower angle impacts (GAULT,1973). All existing methods for obtaining cosmic ray exposure ages have certain limitations. In the slKr-Kr method the production rate is inferred from the equilibrium concentration of 210,000 year s1Kr and therefore reflects exposure conditions characteristic of the last few hundred thousand years. For this reason, the interpretation of long exposure ages is uncertain. Older rocks are likely to have had complex irradiation histories; it is improbable that the same production rate characterized each phase of the exposure. Since the sample is presumably collected from the surface, where production rates are near maximum, the apparent exposure age usually underestimates the true integrated exposure time when sub-surface irradiation has occurred. It is also evident from the extent of pre-irradiation among material apparently excavated by South Ray secondaries that young ages do not necessarily exclude complex exposure histories. Therefore the age of a lunar feature can be established with confidence only when concordant results are obtained from a number of samples. Although this is particularly important for older features, sample 69955is an example of a young sample where misinterpretation could easily have occurred. The typical krypton spallation spectrum of this sample (Table 2) provided no hint of the complex exposure conditions that existed for the boulder.

Cosmic-ray exposure history at the Apollo 16 and other luntsr sites

1639

Another complicating factor for older exposure ages is surface erosion which is progressing at a rate of about O-3-3 mm1m.y. (CROzAz et al.,1971; &m!E et d., 1971; ~ROZAZ et aE.,1972; HART et al.,1972). Roughly 300 g/cm2 would therefore have been eroded away during the 661 m.y. apparent age of 14311 if it had been sitting on the surface during the entire period. The change in shielding alone would have resulted in as much as an order of magnitude change in spallation production rates (REEDY and ARNOLD, 1972) and a grossly underestimated exposure age. The message here is that long exposure ages are to be treated with caution. Long exposure ages are likely to be wrong even for those bo~ders.~th simple exposure histories. &lost samples not physically associated with distinct large-scale lunar features and with long exposure ages probably accumulated the bulk of their cosmogenic rare gases through multiple, complex, or variably shielded exposures. The apparent exposure ages are consequently rather ambiguously related to the exposure history. Those samples with long exposure ages which, by chance, had rather simple exposures also provide erroneous values for the exposure ages due to surface erosion. In both cases, the true exposure age is likely to be longer than the apparent one. Even though these ambiguities may sometimes exist, ?Kr--Kr techniques have far less serious problems than those methods which only assay the accum~ation of eosmogenic rare gases with no direct assessment of production rates. The a*A_rages in Table 3 agree quite well with those obtained by KIESTEN etal. (1973a) who report ages of 33 m.y., 28 m.y. and 87 m.y. for 67455, 67915 and 68415, respectively. Although the interlaboratory agreement is encouraging, the ages show rather large systematic differences from the 61Kr-Kr results. In computing argon and neon exposure ages, prior knowledge of production rates is required. Only the accumulated spallation products and, in the case of the s8Ar-37Armethod, the major target abundances are measured. In order to provide accurate ages the shielding conditions for the ~libration samples and the samples studied must be identioal, Production rates, however, can change by a factor of 2 for each 80 g/cm” change in shielding (REEDY and ARNOLD,1972). Probably due, for the most part, to this shielding effect, there is a large amount of scatter in the data used to estimate the target dependent production rates (BOGARD et al., 1971). Errors associated with uncertainties in the production rates alone result in a minimum uncertainty for ssAr ages of 28 per cent. In addition, systematic differences in the shielding between the calibration samples and the samples dated can produce systematic biases in the computed ages. For instance, the SKr-Kr ages of the North Ray samples are 60 m.y.; those computed from the accumulation of cosmogenic SaAr are about 30 m.y. Coming from large boulders, one might expect an argon p~du~ion rate somewhat lower in these samples than found in the suite of calibration samples (near surface chips from Apollo 11 and 12). The result is, of course, that the inferred exposure ages are too low. This source of error is too often ignored and the relatively small errors sometimes reported in the literature seem to reflect this oversight (see, for instance; HUSAINet al., 1972; HUSAIN, 1972; SCELAEFFER and HUSAJX, 1973; KYUXSTEN et al., 1973a, 1973b). Exposure ages which are computed by those techniques which assume prior knowIedge of production rates without first taking into account the partioular shielding conditions should be viewed with skepticism.

1640

R. J. DROZD, C. M. HOEENBERQ, C. J. MORUAN and C. E. RALSTON

The computed krypton spallation spectra of the gas-rich (after BEE~RIKANN brecciae are particularly sensitive to the isotopic composition assumed for solar krypton. ZForexample, if SUCOR (PODOSEK et aZ., 1971) is assumed to represent solar krypton in 14318, [83],,,, = 24.2 x lo-r1 cmS STP/g, Psl/Pss = 0.684, and the apparent exposure age is 51-l m-y. If BEOC 12 (EBERHARDT et al., = O-691 (Table 2), 1972) is assumed, [83],,s = 20.0 x lo-i1 cm3 STP/g, Psi/P,, and the age is 38.8 m.y. (Table 3). There is, in fact, some evidence that the trapped component in 14318 is not solar (SRINIVASAN, 1973). For samples with a higher proportion of cosmogenic krypton (all samples in Table 2 except the gas-rich breccias, 14301 and 14318), uncertainties in the composition of solar krypton are relatively unimportant.* There are some samples for which it is possible to obtain spallation spectra essentially free of uncertainties due to fission and solar corrections. The crystalline rock 15475, low in solar gases, has such a large proportion of spallation krypton that, for all practical purposes, solar and fission corrections need not be made. The same is true for the 1200°C fraction of 14311 and nearly true for 62235. et al., 1973a)

AcL7aowledgement.s--Uranium determinations in this work were made by J. SHIRCK. We gratefully acknowledge many helpful discussions with R. M. WALKER and D. YUHAS. We thank B. DROZD for help with the drafting; we thank M. HOHENBERGand H. KETTERERfor manuscript preparation. This work was supported in part by NASA Grant NGL 26-008-065. REFERENCES ADAMS J. B. and MCCORD T. B. (1973) Vitrification darkening in the lunar highlands and identification of Descartes material at the Apollo 16 site. Proc. Fourth Lunar Sci. Colbf., Ceochim. Cosmochim. Acta Sup@ 4, Vol. 3, pp. 163-177. Pergamon Press. APOLLO LUNAR GEOLOUY INVESTIGATIONTEAM (1972) U.S. Geological Survey Interagency Report: Aetrogeology 5 1. Apollo 16 Preliminary Science Report (1972) NASA SP-315. BEHR~ V., CROZAZ G., DROZD R., HOHENBERGC. M., RALSTON C., WALTER R. M. and YTJHASD. (1972) Rare gas and particle track studies of Apollo 15 samples: Hedley Rille and special soils. The Apollo 15 Lunar Sample%, (editors J. W. Chamberlain and C. Watkins), pp. 329-332. Lunar Science Institute, Houston. BEHRMANNC. J., DROZD R. J. and HOKENBERGC. M. (1973a) Extinct lunar radioactivities: xenon from z44Pu and lzgI in Apollo 14 breccias. Earth Planet. Sci. Lett. 17, 446-455. BEHRMANNC., CROZA~G., DROZD R., HOHENBERUC., RALSTONC., WALKER R. and YWAS D. (1973b) Cosmic ray exposure history of North Ray and South Ray material. Proc. Fourth Lunar Sci. Conf., Cfeochim. Coemochim. Acta Suppl. 4, Vol. 2, pp 1957-1974. Pergamon Press. BOQARD D. D., F~NIXHOUSERJ. G., SCHAEFFER0. A. and Z~~IN~ER J. (1971) Noble gas abundances in lunar material--cosmic ray spallation products and radiation ages from the Sea of Tranquillity and Ocean of Storms. J. Qeophys. Res. 78, 2757-2779. CRO~L~G., WALKER R. and WOOLU~ D. (1971) Nuclear track studies of dynamic surface processes on the moon and the constancy of solar activity. Proc. Second Lunar Sci. Conf., Beochim. Coamochim. Acta Suppl. 2, Vol. 3, pp. 2543-2558. M.I.T. Press. CROZAZ G., DROZD R., HOHENBERGC. M., HOYT H. P. JR., RAOAN D., WALKER R. M. and YTJHASD. (1972) Solar flare and galactic cosmic ray studies of Apollo 14 and 15 samples. Proc. Third Lunar Sci. Conf., Qeochim. Cosmochim. Acta Suppl. 3, Vol. 3, pp. 2917-2931. M.I.T. Press. * Using SUCOR one obtains an age of 2.06 m.y. for 68815, which is almost identical to that computed by using BEOC 12 (Table 3).

Cosmic-ray exposure history at the Apollo 16 and other lunar sites

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