Gas retention and cosmic-ray exposure ages of a basalt fragment from Mare Fecunditatis

EARTH AND PLANETARY SCIENCE LETTERS 13 (1972) 375-383. NORTH-HOLLAND PUBLISHING COMPANY

GAS RETENTION

AND COSMIC-RAY EXPOSURE

BASALT FRAGMENT

AGES OF A

FROM MARE FECUNDITATIS*

J.C. HUNEKE, F.A. PODOSEK and G.J. WASSERBURG Lunatic Asylum, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91109, USA Received 1 December 1971

An 4°Ar-39Ar gas retention age and an 38Ar-37Ar cosmic ray exposure age have been determined on a total rock sample of the basalt fragment B-1 returned from Mare Fecunditatis by the kuna 16 mission. This sample shows large low-temperature loss of radiogenic 4°Ar but defines a reasonably good high-temperature plateau at 3.45 + 0.04 AE. This is presumed to date a period of igneous activity in Mare Fecunditatis. This activity is later than that at Mare Tranquillitatis but earlier than at Oceanus Procellarum and Mare lmbrium. The cosmic ray exposure age is 475 m.y.

1. Introduction The sample return from Mare Fecunditatis by the Luna 16 lunar probe offers the opportunity to determine a time of igneous activity in a major mare area on the eastern part of the lunar surface. Comparison with samples from Mare Tranquillitatis, Oceanus Procellarum, Fra Mauro and Mare Imbrium should assist the elicitation of the formation and flooding history o f many o f the dominant features of the near-side lunar surface. The time of the formation of a small lithic fragment from the Luna 16 sample return has been determined by the 4°Ar-39Ar technique, using stepwise degassing of the sample after conversion of a fraction of the 39K to 39At by a neutron irradiation [1 ]. The feasibility of obtaining meaningful 4 ° A r 39At ages on small igneous soil fragments has been established [ 2 - 4 ] , although there may be ambiguities inherent in applying the 4°Ar-39Ar techniques to unseparated samples [3, 5]. A cosmic-ray exposure age has also been determined from the correlation of spallogenic 38At with 37At produced by neutron reactions on Ca during the irradiation [3]. * Contribution No. 2098.

The sample analyzed was taken from a single basalt pebble (B-l) from the Luna 16 mission; this sample is described in detail in a companion paper [6]. Prior to analysis the whole pebble was immersed in acetone and washed in an ultrasonic cleaner to remove dust. Two chips totalling 9.3 mg were taken from the center of the pebble (as a slice of bread from the middle of the loaf) with a thin crust of glass on the periphery. These chips had the minimum exterior surface which we could obtain without risk of losing more sample. A serious concern in this investigation is the extent to which the thin glaze on the pebble represents significant heating or the addition of foreign material containing excess 4o Ar. The extent of shock damage may also be of importance [3, 7]. We have no a priori basis for judging these effects and will use the internal consistency o f the data in order to arrive at a final interpretation.

2. Experimental procedure Unless discussed below the experimental procedures used in this study are the same as those used previously by Turner et al. [3]. The sample was wrapped in high-purity A1 foil

376

Y.C. Huneke et aL, Age determinations o f basalt from Mare Fecunditatis Table 1 Concentration and isotopic composition of Ar in a neutron irradiated fragment~ of Luna 16 B-1. Temperature (a) [°C}

4°Ar(b) [10 -8 ccSTP/g]

36Ar/4°Ar

3~ Ar/4°Ar(C)

38Ar/4°Ar

39Ar/4°Ar

545

75 -+ 9

0.0320 + 0.0060

0.1546 ± 0.0190

0.01460 ± 0.00120

0.1013 ± 0.0032

635

259 ± 16

0.0394 + 0.0003

0.1100 -+ 0.0006

0.0118 ± 0.0003

0.0409 ± 0.0002

745

861 ± 44

0.0493 ± 0.0001

0,1864 ± 0.0003

0.0150 ± 0.0001

0.01559 ± 0.00006

850

823 -+ 42

0.0492 ± 0.0001

0.3172 ± 0.0003

0.0181 -+ 0.0001

0.01043 ± 0.00008

935

797 + 41

0.0334 -+ 0.0001

0.3696 -+ 0.0004

0.0167 ± 0.0002

0.01003 ± 0.00010

1040

224 ± 14

0.1012 + 0.0005

0.5089 ± 0.0009

0.0342 ± 0.0005

0.01023 ± 0.00027

1195

315 -+ 18

0.1011 ± 0.0002

2.1517 ± 0.0024

0.0846 + 0.0003

0.01067 ± 0.00017

1455

74 +_ 14

0.0789 ± 0.0018

2.719 ± 0.099

0.0972 ± 0.0013

0.01317 + 0.00065

! 650

18 ± 23

0.0431 -+ 0.0075

1.568 ± 0.024

0.0648 ± 0,0110

0.0083 ± 0.0038

(a) Temperatures above 800°C are obtained with an optical pyrometer, assuming a spectral emissivity of 0.7. Temperatures below 800°C were deduced from oven currents. (b) Errors do not include an additional uncertainty of up to 10% in pipette calibration. (c) Corrected back to the beginning of the neutron irradiation using X3v = 1.975 X 10 --2 day -1 and X39 = 7.2 X 10 -6 day -1 ~ Sample weight 0.0093 g.

and v a c u u m sealed along w i t h 5 m g o f Ni flux wire in a q u a r t z t u b e . Samples were irradiated for 6.9 days in t h e s h u t t l e t u b e facility o f the G e n e r a l Electric Test R e a c t o r , P l e a s a n t o n , California. T h e irradiation, d e s i g n a t e d LAV-1, also i n c l u d e d samples o f lunar r o c k 15 555 [ 5 ] , for w h i c h all e x p e r i m e n t a l p r o c e d u r e s were the same as r e p o r t e d here. T h e samples received a h i g h energy (El > 0.1 8 M e V ) n e u t r o n fluence o f 1 X 1019 c m -2 a n d a t h e r m a l n e u t r o n ( E < 0.17 eV) fluence o f 8 X 1019 cm -2 . A f t e r i r r a d i a t i o n the sample was r e m o v e d f r o m the q u a r t z t u b e a n d l o a d e d i n t o the e x t r a c t i o n syst e m in t h e same foil in w h i c h it was irradiated. Spect r o m e t r i c chlorine i n t e r f e r e n c e a n d e x t r a c t i o n system b l a n k s were d e t e r m i n e d in several b l a n k r u n s interspersed w i t h sample analyses. F o r this series we det e r m i n e asCl equivalent to 10 -~1 ccSTP Ar, HC1/C1 = 0.03, a n d 4°Ar e x t r a c t i o n b l a n k s of 4 X 10 -9 ccSTP

at 1650°C, 2.5 X 10 -9 ccSTP at 1 4 5 5 ° C a n d 1.5 X 10 -9 ccSTP at lower e x t r a c t i o n t e m p e r a t u r e s . T h e b l a n k s following the analysis o f B-I s h o w e d the e x t r a c t i o n to be c o m p l e t e . A h o r n b l e n d e ( h b 3 g r ) [3, 8] sample was used to m o n i t o r the 39K(n,p)a9Ar reaction. F o r this m o n i t o r 4°Ar/K--- ( 5 . 6 9 + 0 . 0 9 ) X I 0 -3 cc STP/g [3] a n d ( 4 ° A r , / a g A r , ) = 13,91 + 0.01. This gives a c o n v e r s i o n coefficient C39 (K) = 39Ar*/K = 0 . 4 0 9 +- 0 . 0 0 7 X 10 -3 ccSTP/g.

(1)

F r o m previous irradiations [3] in the same facility K / C a = ( 0 . 5 4 ± 0 . 0 2 ) 39Ar*/aVAr, so t h a t , for this i r r a d i a t i o n

(2)

377

J. C. Huneke et al., Age determinations o f basalt from Mare Fecunditatis

1000

Csv (Ca) ~ 37Ar/Ca = 0.221 + 0.009

X

I

I

~

1

I

10 -3 /

ccSTP/g.

/

x

~

\

\

\\\N

(3)

The flux received by the B-1 sample w a s f = 1.003 +- 0.005 times that of the monitor.

4oAr/ / /

.%00

/

'

#

,j

\

/

N

\

\ "\3'At

/ __

\ \,

3. Results The Ar isotopic composition of each release fraction is presented in table 1. The data have been corrected for extraction blanks, flux difference from the monitor, and decay of 3TAr and 39Ar during the irradiation and between irradiation and analysis, and the small contributions (generally ~ 1%) due to C1 and HC1 in the spectrometer. The error assigned to the 4°Ar amounts includes 5% spectrometer sensitivity variation and an error of 50% of the blank correction. The errors in the ratios are based on statistical errors in ratio measurement and do not include the error in the a°Ar blank corrections. Further corrections to the data are necessary to account for interference effects from neutron reactions on Ca and K. Ca interferences are subtracted taking (36Ar/37Ar)ca = 0.000305 [3],(3SAr/37Ar)ca = 0.0001 [9] and (39Al/37Ar)ca = 0.000732 [31. K interferences are subtracted taking (38Ar/39Ar,)K = (4°Ar/39Ar*)K = 0.01 [9]. Ca contributions to 4°Ar and K contributions to 36Ar and 3TAr are assumed negligible. The corrected amounts after subtraction of interferences are illustrated in fig. 1. The 39Ar is due to 39K(n,p) reactions and is denoted 39Ar*. The amount of P in this fragment has been determined by electron microprobe measurements using a defocussed electron beam; assuming this P and all C1 is contained in apatite, a C1 content of < 1 ppm is estimated [6]. The total amount of 38Ar from thermal neutron capture on 37C1 is thus 0.6% of the measured 3BAr and is negligible. The remaining stable isotopes are due to reactions on K, Ca, Ti and Fe induced by cosmic ray and solar flare irradiations (subsequently referred to as cosmogenic Arc) , trapped solar wind and possibly lunar atmosphere Art, and radiogenic 4°Ar* from the decay of 4oK. Since the 36mrand 3SAr abundances are so high, relative to "OAr (table 1), a detailed consideration of

o

0

'

/

b

A/

\ /

L

\\

"

"\

\ \ 3BAr

\ LUNA 16 B-1

\

\ \\%~.

\ o.1

& , l

500

, I '~L~!

I!

'

i

,

, ,I

1000 ' 1500 RELEASE TEMPERATURE (°C)

, , ,

'

Fig. 1, Differential Ar amounts released are shown as a function of release temperature, 37At and 39At, have been corrected for decay during and after irradiation. All stable isotopes have been corrected for contributions from neutron reactions on K and Ca, and 39Ar, has been corrected for contributions from neutron reactions on Ca.

spallogenic and trapped contributions to 4°Ar is required. 3.1. Trapped solar wind and lunar atmosphere A r There is considerable uncertainty in the proper correction for trapped 4oAr. Astrophysical considerations indicate that 4oAr/36Ar <{ 1 in the sun, but large excesses of trapped 4°Ar are observed in samples of lunar soil (4oAr/36Ar = 1.1) and breccia (4oAr/36Ar = 2.2) [10] which are commonly explained by ion reimplantation of 4°Ar into grain surfaces from a temporary lunar atmosphere [11 ]. The lowest (4oAr/36Ar)t ratio observed in individual Apollo 12 soil grains is 0.15 [12] ; this presumably constitutes an upper limit to the surface implantation composition. The total observed 36At con-

378

J. C Huneke et al., Age determinations of basalt fi'om Mare Fecund#at&

o.o;-

oc!

1195 °C

e.. #

".5.d

L U N A 16 B-1

935 °C 8 5 0 °C

002 ~ 01__ 0

0.2

0.4

0.6

0.8

39A r * / 3 7 A r

Fig. 2. The correhuion of cosmic-ray produced 38Arc with 39Ar, from (n,p) reactions on 39K. The intercept and the slope of the correlation provide measures of 38Arc (Ca) and 3SAte (K) produced by cosmic-ray interactions with Ca and K respectively. Departures from the correlation are caused by contributions from spallation reactions on Ti and Fe.

tent of the B-1 fragment could, however, be accounted for by a contamination of 0.1 mg of finer soil particles, (only 1% of the sample weigllt) either adhering to the surface or entrapped beneath or in the glass crust on the sample. Despite our precautions to minimize such contamination, this possibility cannot be excluded. We will therefore evaluate the data for (`mAr/36Ar)t = 0.5 -+ 0.5, covering the possible range. Making the corrections with the smaller value (`mAr/36Ar)t = 0.15 would raise the high-temperature plateau age by 0.03 AE. 3.2. Galactic and solar cosmic-ray produced A r There is little information about the relative contribution to `mAr from nuclear processes arising from irradiation by galactic and solar cosmic rays. For galactic cosmic-ray produced Ar in iron meteorites, IAmmerzahl and Zfihringer [13] have shown that (`mAr/38Ar)c = 0.2 and (36Ar/38Ar)c = 0.65. In lunar samples where the primary target for cosmic-ray produced Ar is Ca and, to a lesser extent Ti, the same (36Ar/~Ar)c ratio is observed (c.f. [9] ). It is expected that (4°ArffaAr)c ~ 0 for Ca since only the relatively rare heavier isotopes could contribute to 4°Ar. Using isotope correlations in Ar from neutron irradiated lunar rocks, Turner [2] has deduced a range of possible (4°Ar/38Ar)c values. From these calculations,

a ratio of (a°Ar/38Ar)c ~ 0.3 is deduced for (39Ar/37Ar)ca = 7.3 X 10 -4. For a small soil particle, production of Ar by solar flare irradiation on K and Ca must also be seriously considered. Yaniv et al. [14] have demonstrated that in many soil particles the solar flare products dominate the galactic cosmic-ray products. Assuming a cross section for the production of Ar from Ca and K of I00 mb over the energy range of interest (10 100 MeV) [cf. 15-17] and a solar flare particle flux of 50 particles/cm=/sec [18, 19], the total observed 36Ar in the fragment (2 X 10-6 ccSTP/g) could be produced in 2 X 10 a yrs. Such an exposure time to solar flare particles is common for soil fragments [ 2 0 - 2 3 ] . Thus the possibility of solar flare contributions to the 36Ar and 38Ar is not negligible. The isotopic composition of such contributions is unknown. Both 36Ar and 3SAr will be produced, and target abundances favor the production of 36Ar over 3aAr. Any reaction which would produce 4°Ar would also produce 3SAr or 3~'Ar at a much greater rate, so we expect (4°Ar/3SAr) ~ 0 for solar flare products. In so far as care was taken to avoid surface contamination, much of what is presumed to be trapped 36Ar and 3aAr may instead be due to solar flare interactions. Since we have no conclusive means of establishing whether galactic [(`mAr/38Ar)c ~< 0.3] or solar flare [(4°Ar/3SAr)c ~ 0] irradiations are dominant, we will make corrections for (4°Ar/38Ar)c = 0.15 -+ 0.15.

3.3. Separation o f cosmogenie and trapped A r In order to compute the 'mAr corrections (sections 3.1 and 3.2) and the cosmic-ray exposure age (section 4), the cosmogenic and trapped contributions to 36Ar and 3BAr are formally separated assuming (36Ar/38Ar)c = 0.65 and (36Ar/3SAr)t = 5.32. If an appreciable amount of 36Ar is due to solar flare irradiation, this procedure leads to an overcorrection for `mAr. Further, if (36Ar/38Ar)c > 0.65 for any solar flare produced At, the above procedure will overestimate the amount of trapped 36Ar, again leading to an overcorrection to `mAr. Nevertheless, further discussion will be based on the formally separated trapped and galactic cosmic-ray produced At, as the extent and isotopic composition of solar flare produced Ar is unknown.

379

J.C. Huneke et al., Age determinations o f basalt from Mare Feeunditatis

4. Cosmic-ray exposure age

RELEASE TEMPERATURE (°C)

The calculation of a cosmic-ray exposure age for a neutron irradiated sample has been detailed by Turner et al. [3]. Assuming only Ca and K as targets for the production of 38Arc,

1.0

~

~

~

~ LUNA 16 B-1

0.1

38Arc P38 (Ca) P38 (K) 39At , 37At - C37 (Ca) Texp + C-~9(K-) Texp 3TAr '

(4) I

0.01

where 38Arc and 39Ar* are calculated for each temperature fraction by the procedure outlined in section 3. Pas (Ca) and 1°38 (K) are the production rates of aSAr by cosmic rays on Ca and K, respectively and Tex p is the duration of exposure to cosmic rays. From this linear relationship between measured quantities we may determine the exposure age Texp from the ordinate intercept at 39,Ar/37Ar = 0 knowing the production rate of 38Arc from Ca. In addition the relative production rate Paa (K)/P3a (Ca) can be calculated from the ratio of the slope to the ordinate intercept. Contributions from spallation reactions on Ti and Fe cannot be corrected for and will cause data points to lie above the correlation line predicted by eq. (4). The 38Arc/37Ar and 39Ar*/aTAr ratios in the thermal release fractions from the Luna 16 fragment are plotted in fig. 2. The fractions dominated by the gas release from plagioclase, which has large amounts of Ca and essentially no K, Ti and Fe provide the best measure of the intercept. This release occurs in the 850°C and 935°C fractions. The linear relationship below this temperature is excellent, showing no effect of gas loss from the K-rich interstitial phases presumably responsible for much of the 4°Ar* loss (section 6). Using a lunar surface production rate P3s (Ca) = 1.4X 10 -8 ccSTP 38Ar/g Ca/my [3] from galactic cosmic ray irradiation, the exposure age corresponding to the intercept value of (38Arc/37Ar)o = 0.030 is Texp = 475 my. This age is similar to exposure ages calculated for bulk lunar soil and several rocks from the Apollo 1 1 return [cf. 24, 25] and is typical for individual soil particles studied by Yaniv et al. [14]. This age is an upper limit to near-surface exposure to galactic cosmic rays and allows for no significant

Electron microprobe analysis of 19 plagioclase grams 0.001

j~2---~

iJ 0

0.2

0.4 FRACTION

0.6

0.8

1.0

39Ar RELEASED

Fig. 3. The apparent K/Ca ratio of minerals contributing to each gas release is shown as a function of cumulative 39Ar release. An average K/Ca ratio for plagioclase from microprobe analyses is shown tbr comparison. contribution by solar flare particle irradiation. It is considerably shorter than the 4°Ar-39Ar gas retention age and reflects the fact that the fragment has been partially or completely shielded from cosmic rays during much of its lifetime. A history of uniform mixing throughout the regolith over the last 3.5 AE implies a regolith depth of ~ 1000 g/cm 2 . A relative production rate of Pas (K)/P38 (Ca) = 3 is deduced from the slope to ordinate intercept ratio in fig. 2. This is significantly higher than the ratio P3s (K)/P38 (Ca) = l -+ 0.5 determined for Apollo 14 sample 1 4 0 0 1 , 7 , 1 [4] and may well reflect a lower energy irradiation by solar flare particles. If solar flare irradiation does contribute significantly to 38Arc, the galactic cosmic-ray exposure age and equivalent mixing depth are correspondingly shorter.

5. K/Ca ratios The 39Ar* from 39K(n,p) reactions and the 3TAr from 4°Ca(n,a) reactions provide information about the K and Ca concentrations of the minerals contributing to each gas release fraction [3]. The apparent K/Ca ratio of each release fraction is a function of the relative amounts of 39Arand 3TArfrom each

J. C. Huneke et al.. Age determinations o¢' basalt from Mare Fecunditatis

380

RELEASE TEMPERATURE (°C)

~

~

o

,,I

~ z 0 Ld <[

o~ o

58 I

LUNA 16

B-1

i

J

I

I

I_

I

-

I--

~-

1

34 <:~ I-Z Ld

30

<[ O_ <~

26

' 0'2-

I_ 04

I

06

I 08

10

FRACTION 39Ar RELEASED Fig. 4. The a p p a r e n t age o f each release f r a c t i o n is s h o w n as a f u n c t i o n of c u m u l a t i v e 3 9 A t release.

of the mineral components contributing to the total gas release and can be a useful diagnostic tool in the interpretation of the Ar isotopic data. The K/Ca ratio [eq. (2)] as a function of 39Ar release is illustrated in fig. 3. The average K/Ca ratio from electron microprobe measurements on a number of plagioclase grains [6] is shown for comparison. The early gas release (< 60% 39Arrelease ) is evidently dominated by the extremely high-K interstitial phases responsible for most of the low temperature losses observed in lunar samples [2, 3, 26]. The very low K/Ca ratio at the highest temperature (> 90% loss) probably reflects release from K-poor pyroxene, and the intermediate ratios reflect release from plagioclase together with some contribution from the other phases. This pattern is apparent in the graph of 3 8 A r e / 3 7 A r VS 3 9 A r * / 3 7 A r (fig. 2). The K and Ca contents calculated from the measured 39Ar* concentration and the 37Ar/39Ar* ratio for the total gas release are 1320 + 150 ppm K and 8.1 + 1.0% Ca, respectively, in agreement with the more precise values measured by normal isotope dilution techniques on a "total" rock sample [6].

6. Formation age

After corrections for trapped and cosmogenic 4OAr (section 3) the apparent 4oAr*/K ratio of each release fraction is determined from the corresponding 4oAr*/a9Ar* ratio by

4OAr*/K = (4oAr*/39Ar*) f 639 (K).

(5)

The apparent age is obtained from

1

t=~-in

( 1 +),ek

4OK ] '

(6)

w h e r e × = × e +X~~ = 5 3" 0 5 × 10-1°/yr, 3,e = 0 5' 8 5 X 10 -1°/yr and 4°K/K = 0.0119%. The isotopic composition of K is assumed the same in all minerals of the sample as in the monitor. The 4oAr*/39Ar* ratio and apparent age of each fraction are presented in table 2 and the age plotted versus fractional 39Ar release in fig. 4. These figures incorporate a 50% uncertainty in the 4°Ar blank corrections. The spectrum of apparent ages reveals large low temperature losses of 4oAr prior to the irradiation but reaches a high-temperature plateau above 850°C. An average of the last five temperature fractions (excluding the 1650°C release), weighted according to gas amounts, yields a high temperature age of 3.45 -+ 0.04 AE. Interpretation of this age must recognize the following caveats: (1) There is a suggestion of a decrease in apparent age in the highest temperature fractions as observed in some Apollo 14 samples [3]. If this is real the best age estimate is the highest apparent age [3, 7] : 3.52 AE.

381

J.C. Huneke et aL, Age determinations of basalt from Mare Fecunditatis Table 2 Apparent ages of individual thermal release fractions from a neutron irradiated fragment of Luna 16 B-I. Temperature (a) [°C]

4°Ar,(b) [10 -8 ccSTP/g]

4°Ar,/39Ar,(b)

Apparent age AE [109 yr]

545

74 +- 9

9.7 +- 1.1

0.80 +-0.07

635

254 +- 17

24.0 _+ 0.9

1.57 -+0.04

745

841 +-48

63.0 +- 1.7

2.80 +-0.04

850

804 +-46

95.7 +- 2.5

3.43 -+0.04

935

785 +_43

100.8 +- 2.1

3.52 -+0.03

1040

213 +-18

96.4 + 6.6

3.45 +-0.11

1195

303 +-22

94.7 +- 4.7

3.42 +-0.08

1455

73 + 14

87.2 +- 17.0

3.29 +-0.30

(a) See footnote (a) to table 1. (b), denotes 4°Ar from the radioactive decay of 40 K and 39Ar deriving from 39K (n,p) reactions during the irradiation. (2) Well-defined plateaus differing by 0.1 AE have been observed in a whole rock sample and a plagioclase separate of lunar sample 15 555 [5]. This difference was ascribed to reheating of the sample and loss from the high-K interstitial phases. No mineral separates of B-1 have been analyzed and the possibility that a similar effect occurs cannot be excluded. (3) Unusual age spectra have been observed in the gas release from several Apollo 14 samples where the initial apparent ages were abnormally high and decreased throughout the 39 Ar release to abnormally low values [3]. In these cases, the apparent ages for the last 40% of 39Ar release have been less than the age calculated from the total Ar release and presumed to be a lower limit to the age. There is no evidence for such a pattern in the release from B-l, although the pattern may be masked by the large low temperature losses. With allowance for uncertainties in the interpretation of the age spectrum a range of ages from 3.3 AE to about 3.6 AE is possible under extreme assumptions. Agreement of the plateau age with an

Rb/Sr mineral isochron age of 3.42 -+ 0.17 AE obtained on the same fragment [27] engenders confidence that both techniques correctly date the formation time.

7. Discussion Since Shoemaker et al. [28] calculated that 95% of the material sampled at any particular location derives from within 100 kin, the fragment B-1 most probably originated in Mare Fecunditatis, although the possibility that it derives from other sites is not completely negligible. We note in this regard that three small igneous fragments from Apollo 14 soils were all clearly related to each other and to larger rocks found at that site [3]. On the other hand, one of the four soil fragments studied by Sutter et al. [4] is clearly not related by age to the other three or to larger rocks [29]. There is mounting evidence from visually estimated ages based on surface cratering fluxes [30, 31 ] and the radiometric ages of returned samples [2, 32]

382

J.C. Huneke et al., Age determinations o f basalt from Mare Feeunditatis

that the lava flows covering mare surfaces are neither simultaneous nor mare-wide. In the small area sampled by each of the Apollo returns, two or more rocks representing different flow units with different ages have been returned, indicating a complex local igneous activity. The age obtained for the igneous l'ragment B-1 must be interpreted in this context. Radiometric ages determined for samples from other lunar sites extend through the period of 3.15 4.05 AE [2, 32]. The delineation of this interval is important to an understanding of lunar evolution [32], and it is notable that, irrespective of an actual Mare Fecunditatis origin for the fragment B-l, the igneous activity dated by this fragment also lies within this interval. The extensive coverage of the lava flows spanning such a short time interval suggests that the flows are a result of lunar-wide conditions. While local lava flows may be triggered during this interval by isolated impact events, or localized by fractures near previous impact structures [33], the lava is not generated by these events. An age of 3.45 AE for this area of Mare Fecunditatis can also provide an additional control on visual techniques for estimating relative formation ages based on lunar cratering flux and surface erosion processes [30, 31 ]. These are valuable methods for extending the few absolute age measurements so far obtained to unsampled areas on the lunar surface. To date, however, these techniques do not have the finer resolution (either areal or temporal) necessary to separate many of the events observed just in the returned samples. On the other hand, it is not clear how representative the returned samples are of the larger surrounding area. By a model of crater erosion by a small impacts, Soderblom and Lebofskey [31] determine an integrated particle flux for a site in the western part of Mare Fecunditatis intermediate between the integrated fluxes at the Apollo 11 and the Apollo 12 sites. Although a similar measurement is not available for the actual Luna 16 site, this is certainly consistent with the B-1 chronology. From crater morphology criteria, Ronca [30] has determined a wide range of ages for different regions of Mare Fecunditatis with an increase in igneous activity in the mare area during the period 3.3 3.8 AE, an interval including the formation time of B-1, He also arrives at an age of 4.0 4.5 AE for the Luna 16 site, but with a strong

cautionary statement that the site is relatively inhomo. geneous within the area used for the age estimate. Again, cratering flux ages vary from site to site within the same mare, and care must be taken to establish both that the area dated by these techniques comprises a reasonably well defined unit and that the rock derives from this unit. There is, for example, an inconsistency between the absolute age of 3.31 -+ 0.04 AE for rock 15 555 [5] from the Apollo 15 return and an erosion model age of 3.5 + 0.1 AE for the same site [31] which can be attributed to failure of either of these criteria. The reliability and resolution of absolute age estimates available from cratering flux and erosion models can be more closely controlled only after a good correspondence between area and returned samples is established.

Acknowledgements We gratefully acknowledge the essential support of Pal Young and A1 Massey. We are indebted to the Academy of Sciences of the USSR for making samples returned by the Luna 16 probe available for general scientific study and to the Lunar Sample Analysis Planning Team for entrusting us with the analysis of the unique fragment B-1. This work was supported by NASA Contract NAS-9-8074 and NSF grant GP-19887.

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