Noble gases from solar energetic particles revealed by closed system stepwise etching of lunar soil minerals

Gwchrmrca I Cosmcxhtmtca Acta Vol. 0 Perpamon Journals Ltd. 1986. Printed

00 16-703?/86/s3.00

50, pp. 1991-2017 in U.S.A.

+ .oO

Noble gases from solar energetic particles revealed by closed system stepwise etching of lunar soil minerals RAINER WIELER, HEINRKH BAUR and PETER SIGNER ETH-Ziirich, Institut fur Kristallographie und Petrographie, NO C 6 I CH-8092 Ziirich, Switzerland

(Received January 15, 1986; accepted in revisedform June 6, 1986)

Abstract-He, Ne, and Ar abundan~s and isotopic ratios in plagiociase and pyroxene separates from lunar soils were determined using a closed system stepwise etching technique. This method of noble gas release allows one to separate solar wind (SW) noble gases from those implanted as solar energetic particles (SEP). SEP-Ne with *%e/“Ne = 11.3 & 0.3 is present in all samples studied. The abundances of SEP-Ne are 2-4 orders of magnitude too high to be explained exclusively as implanted solar flare gas. The major part of SEP-Ne possibly originates from solar “suprathermal ions” with energies < 0. I MeVfamu. The isotopic composition of Ne in these lower energy SEP is, however, probably identical to that of real tlare Ne. The suggestionthat SEP-Ne might have the same isotopic composition as planetary Ne and thus possibly represent an unfractionated sample of solar Ne is not tenable. SW-Ne retained in plagieclase and pyroxene is less fractionated than has been deduced by total fusion analyses. Ne-B is a mixture of SW-Ne and SEP-Ne rather than fractionated SW-Ne. In contrast to SEP-Ne. SEP-Ar has probably a very similar composition as SW-Ar. 1. INTRODUCTION

was more specifically associated with the low energy portion (- 1 MeV/amu) of the SF-Ne spectrum THE ISOTOPICCOM~SIT~ON of the elements in the sun (BLACK, 1983). Ne-C is released at higher extraction is fundamental to studies on the origin of the elements, temperatures than is SW-Ne. ft was reported to have the formation of the solar system and processes in the a ?Ne/**Ne ratio of 10.6 f 0.3 and is thus isotopically sun. The information on isotopic abundances of noble heavier than SW-Ne as observed in the Solar Wind gases in the sun stems predominantly from studies of Composition (SWC) experiment fZeNe/22Ne = 13.7 solar particles trapped outside the terrestrial atmo+ 0.3, GEISS et al., 1972). Black’s interpretation was sphere, especially in lunar and meteoritic samples. not widely accepted originally, but was met with new SUESS et al. (1964) and SIGNER ( 1964) first suggested interest when the abundances and isotopic composition that the large quantities of noble gases in the so-called of several elements in the solar flare radiation could gas-rich meteorites might have been implanted by the be determined by satellite borne charged particle desolar wind (SW).’ Subsequently, numerous studies on tectors. These instruments measured ions in the - 10 trapped SW gases have been reported (e.g. EBERHARDT MeV/amu region. The 2%e/22Ne ratios observed were et al., 1965, 1970; HEYMANNand YANIV, 1970; HINdistinctly lower than the respective SW value, namely TENBERGERet ai., 1970). Besides implanted solar wind 7.71::: (DIETRICH and SIMPSON, 1979) and 9.1 F 1.9 particles, effects caused by solar particles with much (MEWALDTet al., 1984), respectively. DIETRICH and higher energies are also well studied in extraterrestrial SIMPSON(1979) and MEWALDT et al. (198 1) noted that material, namely solar flare tracks, i.e. lattice damage solar flare Ne measured by satellites has within error caused by heavy particles (2 ;r 20) accelerated in solar limits the same isotopic composition as Ne-A (r’Ne/ flares (SF). The SW particles have energies of a few 22Ne - 8.2, BLACK, 1972) a component, also called hundred eV/amu and thus are implanted only several “planetary Ne”, which is found in many chondrites hundred A into solid matter. SF particles, on the other (SIGNER and SUES& 1963). This similarity led both hand, are accelerated to energies up to about 100 MeV/ groups of authors to suggest that SF-Ne or planetary amu and SF tracks are therefore found at depths of up Ne might represent the composition of bulk solar Ne. to several mm. The ffux of SF particles is much lower In this case, SW-Ne would be a fractionated sample of than that of SW particles, such that the implanted SF solar Ne. If such a close relationship between bulk solar noble gas ions were thought to be completely swamped Ne, solar flare Ne and planetary Ne could be substanby the much larger SW gas amounts. Neve~hele~, tiated, this would severely affect our understanding of BLACK(1972) attributed for the first time a trapped the origin and trapping mechanisms of planetary noble Ne component-which had already been found in gas gases. It is thus of crucial importance to determine the rich meteorites by BLACKand PEPIN (1969)-as imisotopic abundance of SF-Ne more accurately than it planted SF gas. Later, this component, lahelled Ne-C, is possible to date by satellite borne instruments. A promising approach towards this goal is the study of material which has been exposed for a long time to ’ Acronyms used in this paper include: CSSE: Closed System the solar corpuscular radiation and from which the Stepped Etching; GCR: Galactic Cosmic Rays; SCR: Solar abundant amounts of noble gases implanted by the Cosmic Rays; SEP: Solar Energetic Particles: SF: Solar Flare; solar wind have been removed. Samples from the lunar SW: Solar Wind. 1997

I998

R. Wieler. H. Baur and P. Signer

regolith have high concentrations of trapped solar noble gases and are thus ideally suited for such studies. In a previous study, our group therefore analyzed several plagioclase fractions of a lunar soil etched to different depths to remove the outermost solar wind gas bearing grain surface layers (ETIQUE ef al., I98 1). We found a Ne component with 20Ne/22Ne = 11.3 t 0.3 in up to some 30 pm depth. The concentrations of this gas, which has a similar composition as Ne-C. decrease with increasing etching depth and we concluded that it rep resents, at least in part. SF-Ne. Subsequently, NAUTIYAL~~al. (198 1) reported similar results, while YANIV and MARTI (198 1) interpreted 3He excesses near the surface of a lunar rock as being of SF origin. In this first etching study, we could not exclude that the ” 11.3-Ne” might mimic a pure gas component as the result of a mixture in constant proportion between SW-Ne and SF-Ne, where the latter could have an actual 20Ne/22Ne ratio as measured by the satellite experiments. To clarify this point and to reach a more thorough understanding of the different noble gas components in extraterrestrial materials, we have now developed a Closed System Stepped Etching technique (CSSE), comparable to the one applied by VILCSEK and W~~NKE(1965). The technique allows us to measure directly the He, Ne, and Ar released by the acid attack in each step. Advantages of the closed system etching technique. In the experiments reported by ETIQIJE et al. (I 98 I ). a separate aliquot sample was required for each determination of the noble gases remaining after different degrees of etching. The solar wind portion was thereby lost. The CSSE procedure not only eliminates problems associated with aliquoting small samples. but also allows analysis of the gases released in each step from various depths or phases. The possibility of determining elemental and isotopic composition of the noble gases sited in all etched parts of the grains clearly allows an improved interpretation of the data. Although the notion of sampling progressively deeper grain layers is an oversimplification at least for pyroxene (see section 4.2.) it is shown in the following that the CSSE technique is nevertheless capable of efficiently separating solar noble gas components implanted to different depths. Furthermore, the gas release by etching is not expected to cause severe elemental or isotopic fractionation. Moreover, neither phase transitions nor solid state reactions affect the gas release. These are important advantages over stepwise heating experiments. In this paper we present a comprehensive discussion of CSSE determinations of He, Ne, and Ar in six mineral separates from three lunar soils. Preliminary reports of this study are given by WIELER et al. ( 1984, 1985). The main result is that I‘ 1I .3-Ne” is present in all these samples and that it results from solar ions implanted with energies above those of the solar wind. We will call this component SEP-Ne (Solar Energetic Particles). Anticipating section 5. we note here that possibly the major part of SEP-Ne was not ejected during solar flares. In this paper, “Solar Energetic Particles“

and “Solar Flare Particles” are therefore I~OIused as synonymous terms. 2. EXPERIMENTAL PROCEDLJRI The CSSE extraction line is shown in Fig. I, it 1smade or supremax ( 1720) glass except for two Pyrex glass valves i ~1. B) with teflon pistons. These valves are operated in an outer vacuum of about 10-l torr to keep the leak rate of air mn; the system negligible. The nitric acid in the break-offampoule had previously been purified from atmosphenc noble gases by repeated vacuum distillation and subsequent removal of the gas phase. Thereby, the initially I4 n HNOj presumabl! was diluted, as is indicated by the etching times during the sample runs (see Table 2). which were several times longer than was expected from preliminary etching experiments with untreated HNOJ in air. In the first 3-5 etching steps the samples were exposed YO acid vapour only. Afterwards, the acid was distilled into thcL sample tube. In some instances. the sample storage volume “a” was heated up to 70°C to enhance the ctcbing speed Before expanding the gases evolved into volume “b” for 3 first purification step, the acid was cooled by a water-ice mix ture to reduce the vapour pressure of the HNOI. ‘Theefficienr removal of active gases by exposure to CaO was tollowed h> purification on conventional TiZr and AlZr gettcn in a stat n less steel line. To prevent losses of Ar from the .samples. n(~ cool traps were used. Volume “b” is about 7 times larger than volume “a”: thus about 15% of the gas released in one step was actually recorded in the following steps. No correction for this effect is applied to the data. Etching times were increased from about one hour up trfour days per step. Before each sample analysis the wholr system was baked two weeks at 110°C. wherebl the CaO ir, volume “b” was held at 250”. The etching time per step, the moment 01’distilling the liquid acid into the sample tube and the temperature of the acid were chosen to obtain reliable %e/22Ne ratios in as man\

C I

break

volume

‘a’

L5 off

\

I---

1

\ Hh03

I

a\ sa’mple

pump

f&her purification and mass spectrometer

FIG. I. Schematic representation 01. the < IoscJ System Stepwise Etching (CSSE) device. Stainless steel tubes arc marked by heavy lines, the rest of the device is made of SW premax (1720)glass, with exception of valves A and B (Pyrex glass with teflon cocks). A housing around these valves allows, an external pressure on the valves of about !O ’ !:7rr

Lunar soil noble gases steps as possible. In Table 2, these parameters are given for each step. After two to four weeks of etching, the gas amounts became too low for a precise analysis of the Ne isotopic abundance. The residual sample was then washed with ethanol. The noble gases in the plagioclase residues were subsequently analyzed by total fusion at 18OO’C. Since the residue of the 7 150 I pyroxene was known to contain still a large fraction of the noble gases, it was further studied by stepwise heating. Temperatures of the pyrolysis steps are also given in Table 2. As a control, an aliquot of each CSSE sampte was analyzed by total fusion at 1800°C. These samples are in the following termed “total fusion ahquots”. All mass spectrometric noble gas anaiyses were performed essentially as described by SIGNER 6-fal. (1977). Background and blank. An “acid blank” run, consisting of a procedure identical to a sample run but without sample was carried out. In addition, “dry blank” steps of different duration were performed in each sample run before opening the break off. These blank experiments are summarized below: 1) In the acid blank (as well as the sample runs) the background at masses 2, 14, and 16 was enhanced two to fourfold over the corresponding values in the dry blank steps. 2) In the first acid blank step, the following noble gas amounts were released (lo-* cm3 STP): “He: 27,

2aNe: 4.6,

36Ar:0.027,

*Ar: 8.

These values are of the order of the gas amounts measured in a sample step. Thus, prior to al1 sample runs, the gases accumulated in the acid ampoule were removed by repeated expansion into volume “b” and subsequent pumping. This procedure was applied immediately after destruction of the break off and lasted for less than two minutes. The HNOJ was thereby cooled to O”C, to minimize uncontrolled etching of the sample. Based on preliminary experiments, the noble gas amounts lost during these two minutes are estimated to be less than 2% of those measured in the first step. 3) In the acid blank steps folIowing the initial one, the noble gas amounts decreased rapidly; after the second step they were comparable to the values of the dry blank steps. This shows that noble gas leaching out of the supremax glass is negligible. Neither the distillation nor the heating of the HNOs led to enhanced acid blank amounts. 4) Ar blank amounts are nearly independent of the duration ofa step, indicating that they are accumulated predominantly by degassing of the teflon while valve B is closed. For step durations longer than about 10 hours, He and Ne blank amounts are roughly proportional to the collection time, presumably because they are mainly caused by diffusion through those parts of valve A (Pyrex) always connected to volume “a”. Typical acid blank amounts are (in lo-’ cm3 STP): step time

4He

20Ne

36Ar

“Ar

Ih 48 h

0.25 3

0.007 0.05

0.007 0.008

2 2.5

As judged from the blank experiments discussed above, individual blank corrections should be correct within +20%. Note, however, that there is evidence for an underestimation of the Ar blank by a factor of about 2 (see section 4.1). 3. SAMPLES

STUDIED

Six mineral separates of three different lunar soils were studied. The analysis of sized mineral separates offers important advantages over the study of bulk soils. Mineral separates are more homogeneous than bulk samples. They are thus etched more uniformly and offer better chances for the separation of gas components in different sites. Furthermore, as discussed by WIELERet nl. (19801, a mineral population of a grain size around 200 rrn acquired its implanted gases during a time span on the order of 10-100 miihon years. Thus. the

I999

time and duration of the exposure of a sample are more constrained for a mineral separate than for a bulk soil. In bulk samples, the gas rich microbreccias and agglutinates may have acquired solar gases possibly over billions of years (cf: B0RCi ef al., 1976). Plagioclase and pyroxene are known to have differing retention characteristics for the light SW noble gases He and Ne. On the basis of the ‘He/%Ar and %Ie/36Ar ratios of a large number of mineral separates from different soils, SIGNER et ai. (1977) showed, that SW-He and SW-Ne are retained about an order of magnitude more efficiently in pyroxene than in piagioclase. In this study we focussed on the analysis of plagioclase separates because this mineral is easily etchable in nitric acid. To minimize ~n~mination with material more retentive for SW-Ne (besides pyroxene also olivine and especially ilmenite), Apollo- 16 highland samples were preferred. A pyroxene and a plagioclase separate of mare soi 7 150 1 complemented the study. The six samples analysed are listed in Table 1. The plagioclam samples were prepared as the nonmagnetic fraction of the sieved bulk soil, using a Fmntz magnetic separator operated at 0.9 Amp. The two 64421 samples were aliquots. One of the 655 11separates was finally obtained by handpicking the cleanest grains from the nonmagnetic fraction. The 7 1501 pyroxene separate was also prepared by handpicking from a fraction enriched in this mineral by means of the magnetic separator. Handpic~ng yielded separates less conmminated by small r&I particles. Nevertheless, minute plagiociase fmgments adhering on part of the grains of the pyroxene separate were observed. The residues of plagioclase samples treated in hot HN03 (6442 1Nr 1;7 1501) appeared flaky with virtually no “original” grains left. The majority of the crystals of the etching residues of the plagioclase samples never etched in hot acid had not changed their appearance, although the grain size of the particles was significantly reduoed. Some tiny fragments present in the residue indicate that a few grains may have been fragmented, possibly by preferential etching along fractures. SEM pictures of plagioclase grains etched at room temperature showed typical etch pits on an overall rather smooth surface. No deep cavities or cracks were observed. The pyroxene etching residue appeared essentially unaltered. The SEM revealed clear etching features on only a few percent of the grain surface. The etching residue of the 6442 I plagioclase Nr. 2 was sub divided into two parts (see Table 2). The two parts were sep arated during the backdistillation of the HNOs after the end of the CSSE analysis. “High” denotes the fraction adhering on the upper part of the sample tube, “low” the fraction remaining at the bottom of the tube. The residue “low” appeared less altered than the residue “high”. The two fractions did

TABLE 1.Samples analyzed by CSSE Weight

separation sampte Nr.

mode

nonm*

It8

64421, 1. Ahq.

GtiR

size

loss by Weight

etchmg

frmf

fm3)

(W)

IOO-300

75.2

55

100-300

54.5

40

0.9A

64421. 118 2. Aliq.

lW”mag.

65511, 13

handpicked

150-250

II.5

30

plagi* clas?

nonma&

IOO-I50

19.2

50

Plagio-

iI0”lIIqg.

IGO-so0

9.0

IS

100-500

8.5

<7

65511, 13

7lSOl.

139

0.9A

clax 71501,139

pymxene

0.9A

0.9A handpicked

R. Wieler, H. Lkwr and P. Signer

2ooo

Table 2: Noble Gas Data of 5 Lunar Mineral SeparatesAnalyzedby CSSE 4He

*'Ne

36Ar

4/3

20/22

2272'

36/38

680 2120 940 (;;;I

6:'; 468:l 467.1 24.1 102.8 31.4 17.8 6.4 12.4 8.1

2150 2660 2360

800 1210 1110

24:; 63.3 40.7 10.0 74.2 57.5 56.3 24.1 36.5 22.8 15.8 13.8

::;

_-

12.68 12.27 11.39 11.02 9.98 9.31 8.51 7.90 8.08 8.14 9.56 10.50

30.0; 25.01 14.39 9.65 7.34 5.58 4.43 3.73 3.78 3.76 5.33 9.61

5.40 5.39 5.34 5.19 4.86 3.44 1.89 1.05 .a3 1.10 1.41 2.26 3.08

2300

69.9

13.3

2290

12.19

25.02

5.26

32.00

0.1

0.1

sum etching steps + residue

10990

509.1

1226

2460

10.02

6.64

4.24

(1.09)

6.3

69.9

aliquot (total fusion)

30830

1000

1843

2330

11.12

9.27

4.57

1.74

7.1

70.3

2.5 14.1 114.8 949.4 261.7 36.8

5.36 5.41 5.39 5.34 4.88 2.81 1.11 .83 .75 .74 .76

0

2400 __ ----

35.88 30.71 30.27 23.80 12.25 8.32 5.90 4.98 4.30 3.81 3.56

146.0 9.40 2.80

:*; 6:5 7.3 5.0

12.91 12.88 12.72 12.13 11.09 10.10 9.40 a.97 a.43 a.15 7.89

0

24;; 2610 2660 2410 2210

4187 y; (15.0) --__ __

0.5 0.2 0.7 0.8 0.8 0.9 0.7

5.4 7.1 7.8 7.7 8.5 9.6 6.5

'2.3 24.0

2310 2370

10.20 10.25

6.33 6.47

2.32 1.66

144 8.87

0.5 1.7

3.4 11.3

Sample

etching cond.

step Nr.

64421 plagioclase nornag. fraction Mr. 1

: 43 5 6 7 8 9 10 11 12 13

.5h lh 4h 15h 9h 40h 48h 72h 96h 7h 17h 48h 96h

vap vap vap vap liq liq liq liq liq liq 60C liq 65C liq 70C liq 70C

etching residue

64421 plagioclase nomeg.

1

fraction

:

lh lh 4h 15h 24h 24h 72h 48h 48h 72h 72h

vap vap vap vap vap vap vap liq liq liq liq

270 300 0;;'

170 800 2030 3190

2180 2100 2600 2o;o 2300 2950

40136

*'Ne 38Ar (cosmoa.1

__ i

__ __ __ __ ___

0.1 0.1 0.8 0.9 1.3 0.7 1.1 0.7 :::

__ (0

20)

4:o 0.6 12.3 12.2 15.5 7.4 10.2 4.8

;:62

I liool (‘60)

2x 7418 113.4 50.3 54.5 45.2 40.5 31.9 29.8 20.3

1630 5460

43.0 133.9

sum etching steps + residue

13560

588.0

1468

2370

10.42

7.57

4.47

(1.90)

6.9

67.7

aliquot (total fusion)

30830

1000

1843

2330

11.12

9.27

4.57

1.74

7.1

70.3

5.49

9.85

(0.:)

Nr. 2

4 5 6 7 8 9 10 11

etching residue "high" etching residue "low"

6miZ plaglo-

: 3 4 5 6 7 8 9

.8h 4h 15h 24h 30h 48h 48h 48h 48h

vap vao vab liq liq liq liq liq liq

930 390 300 230

9930 5170 1850

187.2

50.4 227.3 236.7 114.7 25.6 15.6 11.3

2330 12.87 2560 12.44 2360 11.10 (2000) 9.41 -7.46 -5.41 -4.2' __ 3.50 __ 3.11

29.96

: 0" (0.1) (0.4)

0

I%/ (2'0)

186.5 119.4 154.4 79.1 57.1 35.5 22.1 16.8

7250

'47.7

53.0

1870

5.64

2.26

1.69

11.90

sum etching steps t residue

26780

1006

750.1

2370

8.10

3.70

3.35

(5.10) 30.7

aliquot (total fusion)

30710

812.0

765.5

1980

8.80

3.60

3.75

3.52

23.6

39640 10530 7200

914.4 339.5 290.2

238.2 247.8 433.0

2480 2990 3220

12.85 12.60 12.03

29.70 26.85 21.10

5.33 5.40

5.26 3.35

(0.;) (0.;) 0.4 1.9

3860 1620 700 320 620 1640 390 n.d.

272.6 164.5 52.3 22.8 117.7 192.8 84.1 106.7

558.5 274.5 43.4 14.2 98.0 155.6 37.0 33.2 n.d. 12.3 12.3

2490 11.35 1640 10.55 390 10.41 310 10.72 730 9.61 450 9.94 450 8.81 n-d. 8.19 (540) 7.12 -7.02 -6.85 6.37 5.89

14.34 9.81 11.59 23.12 7.11 8.63 5.00 3.97 3.20 3.11 3.06 2.69 2.49

;.;'o 4:96 4.63 4.18 4.28 4.36 3.12 2.23 n.d. 1.63 1.65 1.93 2.60

;.;; 2:89 ------

handpicked

etching residue

65511 clase

plagio-

1

V-

fraction

lh vap 3h vap

: 10h 16h 6a 24h 6b 24h 6c 24h 6 sum 7 15h 8 24h 9 24h 10 24h 11 40h 12 24h 13 24h

vap vap 40C vap 70C vap 72h liq liq liq liq liq liq liq

(;:z; (360)

I;;;\ ;;;;{

(720)

2i.i 47:* 22.0 10.9

;:;

25.15 14.75 7.47 3.69 2.40 2.00 1.84 1.75

5.40 5.26 4.28 2.05 1.17 .98 .97 .90

2.20 (0.4) (1.:) 2.98 (3.60) 1.7 6.3 -2.6 8.8 -4.2 11.8 -_ 4.1 10.7 _3.3 a.3 -3.0 7.1

n.d: -----

11.1

24.3 78.6 70.3

i? 0:3

2::

1.4 1.7 1.7 3.0 2.3 1.9 2.1 1.2 0.7

514 9.4 5.7 9.9 n.d. 6.0 5.9 3.5 2.2

etching residue

17500

339.5

68.8

1880

9.15

4.37

2.35

16.80

7.6

18.7

sum etching steps t residue

84400

2886

2090

2140

10.79

8.22

4.64

4.90

25.1

71.7

108160

3405

2229

2320

10.73

8.01

4.50

3.57

30.8

93.1

aliquot (total fusion)

2001

Lunar soil noble gases TABLE 2. (Continued)

Sample

step Nr.

7'501 pymxene handpicked

1 : 4

5

6

etching cond. lh 6h llh 9h 17h 8h

vap vap vap 1'0 ii; liq 50C

15h liq 50C 48h lia 50C 9 10 11 12

48h lid 65C 48h liq 75C 70h liq 70C 24h liq 70C

sum etching steps etching residue stepwise heating

1 2 3 2 6

5ooc 7ooc 9ooc 105oc 13ooc 18OOC

413

20122

22/21

4He

*‘Ne

5380 27460 9170 6810 4620 13580 12810 11440 16350 15040 8680 __

115.0 671.8 171.5 349.9 156.2 1089 349.0 163.0 304.0

11.4

2290 13.26 31.10 2730 13.12 31.06 4x 2720 12.93 29.45 41:5 2230 13.07 28.69 69.0 1880 12.88 26.37 36.0 1930 12.88 29.21 101.0 2120 '2.97 29.81 28.5 2480 '2.52 30.50 10.2 14.2 (3400) 11.93 31.93 -- 11.62 32.76 14.9 -- 11.19 33.45 4.5 -- (11.34)(32.21) (.'I

131800

3860

373.6

2870

12.66

2940 167700 5250 150

100.5 5330 5180

785:3 28.8 (.2)

240.6 33.3

(2700) 1430 100 (100) __

15290

966.2 809.4

(iii

sum etching + heating steps

307900

aliquot (total fusion)

285700

325.6 155.1

36Ar

36/38

40/36

2'Ne 38Ar (cosmos.)

5.41 5.44 5.43 5.40 5.37 5.32 5.22 5.02 5.22 5.32 5.35

--__ -__ __ __ -_ __

:

:

30.21

5.35

__

0.6

0.5

13.56 12.92 12.02 10.26 2.27

31.29 31.23 30.65 6.67 1.35

5.78 5.61 5.50 4.94 2.20

0 :

0 0

9.4 9.3 __

4.: 10.2 __

1400

12.28

21.26

5.04

--

19.3

15.4

1270

12.41

19.48

4.98

.50

21.2

14.2

(.2)

__

__

__ __ __

(8.00) 2.34 .26 (.08) __ __

i

0

0 0 0

0.1 0.3 0.1 0 0 0

z 0 0.2 0.2 0.1 0

Noble gas concentrations (in lo-’ cm3 STP/g) and isotopic ratios of etching steps of 5 of the 6 samples discussed in this work. The data of the total fusion/stepwise heating of the etching residues as well as of the total fusion of the unetched aliquots are also given. All concentrations relate to original sample weight. Cosmogenic *‘Ne and ‘*Ar concentrations were calculated as described in section 4.7. In column 3 the time of each step is given, vup/liq indicates whether the sample was in contact with HN03 vapour or liquid HN03. The temperature of the sample/acid volume (Centigrade) is also given, if different from room temperature. During steps 6a and 6b of the nonmagn. plagioclase separate of soil 655 I 1, only the sample was heated, the acid thereby remaining at room temperature. This resulted in a strongly reduced etching efficiency, as is best seen by the amounts of cosmogenic gases. Steps 6a, 6b, and 6c were therefore combined. Errors: Ne, 36Ar and ‘*Ar concentrations are estimated to be correct within 5%, the corresponding isotopic ratios to within l%, except for the most gas poor steps (*“Ne < 20. IO-’ , %Ar < 5 - lo-* cm3 STP/g), for which these errors amount up to 10% and 290, respectively. The same values apply to 4He and ‘He/‘He for the steps with etching times < 12 hours. In the longer steps, due to the large He and @Ar blanks, the errors of 4He, ‘He/‘He and “Ar/36Ar are some 10% for numbers not in parentheses and some 20% for numbers in parentheses. Data not given could not be reliably determined. Etching residues “high” and “low” for 6442 1 plagioclase Nr. 2 are explained in section 3.

not, however, offer additional insights, and the data of the two samples is thus not shown separately in Figs. 2d and 7. 4. RESULTS The detailed results from analyses of five samples are presented in Table 2. For the sixth sample, the 7 1501 plagioclase, only the Ne isotopic ratios are given in the inset of Fig. 2d. The data are corrected for extraction blanks, mass discrimination and CO*++ contributions on the ‘*Ne peak. Additional explanations and error estimates are given in the table caption. Since the preliminary publication of the 655 11 data (WIELER et al., 1984, 1985) a new noble gas mixture for the sensitivity calibration of the mass spectrometers has been prepared. All data were recalibrated which explains the small differences in 36Ar concentrations and elemental ratios between the preliminary publications and this work. Furthermore, the C02++ correction has been reevaluated, leading to slightly altered positions of a few points in Fig. 2b compared to Fig. 1 in WIELER et al. (1984). 4.1. CSSE versus total fusion of aliquot samples

Lunar soil mineral separates of a few mg weight and around 200 pm grain size range can hardly be aliquoted with respect

to their noble gas concentrations and elemental ratios to better than some 15% (e.g. SIGNER et al., 1977). This is the consequence of the variations ofthe gas concentrations in individual grains from the same soil by up to 2 orders of magnitude (WIELER et al., 1980). Comparison of the sum of the gas amounts released in all etching steps including the etching residue with the corresponding data of the respective aliquots measured by total fusion shows the following: For the separates of soils 655 11 and 7 150 1, the elemental ratios and all noble gas concentrations except “Ar agree within 20%. This reproducibility is close to that achieved in the above mentioned conventional analyses. The isotopic ratios except 40Ar/36Ar agree to better than 12%, which is also within the range observed bv SIGNER et al. (19771. The 40Ar/MAr ratios mav indicate an underestimation of the “‘Ar CSSE blank by roughly a factor of two. Fortunately, this *OAruncertainty does not affect any conclusion presented in this paper. The good agreement of CSSE and total fusion data for all other concentrations and isotopic ratios shows, on the other hand, that the blank values used for correction of the CSSE data were correct, and that no gas losses occurred in the CSSE of the 655 11 and 71501 samples. The two aliquots of the 64421 plagioclase separate were the last samples analyzed. For both of them, the sum of the solar gases released in the etching and the total fusion of the residue shows deficits (relative to the gas concentrations found in the total fusion alisuotl of about 60%. 40%. and 20% for He, Ne, and Ar, respectively. Cosmogenic gases do not reveal such a deficit. Furthermore, the extremely low gas amounts released in the initial etching step in both of these samples

2002

R. Wieler, H. Baur and P. Signer

8

4

0

64421

Plag.

.2

No

2

.4

\\

\I

6

21Ne/22Ne FIG. 2. Ne three-isotope plots of 6 plagioclase fractions of three Apollo-16 and one Apollo-l 7 soils. Data m Fig. La are from ETIQUEet ~1. (198 I), the other data are from the CSSE experiments presented in this work. Error bars are smaller than symbol size. Best fit lines through 6 I50 1 grain size suite and etched sample data points (broken and heavy line in Fig. 2a, respectively, as well as best fit lines through the CSSE points in Figs. 2b-d. are given. Points 6-9 were used to calculate the fit line in Fig. 2c, the points used for fitting the other lines are readily recognized in the respective figure. Numbers close to data points in Figs. 2b-d indicate CSSE step number, when no numbers are given, step numbers increase with decreasing 2oNefUNe ratio. res = etching residue, total fusion analysis; t.f. = ahquot of CSSE sample, total fusion analysis; Z = weighted average of CSSE steps plus residue. In Fig. 2a, several Ne components pertinent here are indicated: (SWC, SF,, Ne-C, see introduction); 1 I .3 indicates the trapped component with r%e/22Ne = I 1.3obtained by extrapolation of the heavy line in Fig. 2a, this component is indicated by an asterisk in the other panels of the figure too; GCR represents cosmogenic Ne in a plagioclasc fraction of lunar rock 76535 (LUGMAIRef al, 1976): Ne-B is discussed in section 5. I.

Lunar soil noble gases are unusual. We attribute these observations to a pre-etching of the samples prior to the start of the analysis. This was presumably caused by traces of nitric acid remaining in the system from the preceding experiment. The deficit was recognized after the analysis of the 6442 1 plagioclase Nr. 1. Therefore. before loading the second aliquot of this sample, the system was baked for several hours to remove any nitric acid. Obviously, this procedure had only limited success. It thus appears likely that the CaO, used in all previous runs, accumulated nitric acid to a degree where the equilibrium vapour pressure is sufficient to pre-etch a new sample during the two weeks of baking the system prior to the analysis. The 36Ar amount in step 5 of the 6442 1 plagioclase Nr. 1 is too low to be consistent with the fact discussed in section 2, namely that about 15% of the gas released in one step shows up in the following step only, as a consequence of the ratio of the volumes a and b (see Fig. I). The most plausible explanation for this deficit is an improperly closed valve (prime candidates are valves A and B in Fig. I). Unfortunately, this cannot be verified a posteriori. It seems, however, very unlikely that the low gas amounts in step 5 indicate an unrecognized leak which may have affected all steps of the sample in consideration and may even explain the gas losses occurred for both 6442 I separates. Leak tests of all crucial valves against air pressure as well as calibration gas measurements do not indicate such a possibility. We are therefore confident that despite the unexplained gas loss in step 5 of 6442 1 plagioclase Nr. 1, the data of the more retentive part of the trapped gases in both 6442 I samples are valid. 4.2. Neon isotopic release patterns 4.2.1. Pfagioclase. The Ne isotopic data of all plagioclase separates studied by CSSE are shown in Figs. 2b-2d. In Fig. 2a the results of the study on etched plagioclase grain size fractions of soil 6 150 1 (ETIQUE et af., 198 1) are shown because of their importance for the understanding of the CSSE experiments. Also given in Fig. 2a are the Ne components pertinent for the following discussion. First, we summarize the main features of the data reported by ETIQUE et al. ( 198 1). Then, the CSSE data are discussed in the framework of the conclusions obtained there. In the three-isotope plots shown in Fig. 2, data points of a mixture of two gas components falI along a straight line between the two points representing the parent components. We note that the data points of the grain size suite of the unetched samples in Fig. 2a scatter around the dashed straight line with a correlation coefficient of 0.9 1. Interpreted as mixing line of a solar and a cosmogenic component, the extrapolation of this line yields a 20Ne/22Ne ratio for the solar component of 12.0 + 0.2 and a 2’Ne/22Ne ratio for the cosmogenic component of 1.1 + 0.1. The latter value is significantly above 2’Ne/22Ne ratios for cosmogenic Ne in lunar plagioclase and has never been observed before. The data points of the aliquots etched to various degrees all plot very well (with a correlation coefficient of 0.998) on a straight line which is significantly different from that defined by the unetched samples: On the left hand side, this line defines a 2oNe/22Ne ratio of 11.3 f 0.3 at a 2’Ne/22Ne value of 0.031 (the SW-value. EBERHARDT et al., 1972). On the right hand side, it passes close to the point labelled “GCR”. This point, given

2003

in all four panels of Fig. 2, represents GCR produced Ne in plagioclase of lunar troctolite 76535 at a shielding of -75 g/cm* (LUGMAIR etal., 1976). Note that this particular GCR-Ne composition is intended as a reference point only. GCR-Ne in the CSSE samples does not necessarily need to have exactly the same composition. The value of I 1.3 + 0.3 is similar to that of Ne-C but significantly below the SW ratio or that of 12.0 obtained by extrapolation of the data of the unetched samples. Taken at face value, the unexpected data pattern in Fig. 2a implies the presence of four components, two of which are swamped in the unetched grains and become detectable only in the etched samples. However, ETIQUE et al. (I 98 1) and ETIQUE ( 198 1) explained these data as follows: First, the straight line defined by the data points of the etched samples is the result of a mixture of GCR-Ne and a new type of trapped Ne, the latter residing within the crystals at depths of up to at least 35 pm. This new type of trapped Ne was viewed as either pure solar flare Ne with 2%Je/22Ne = 11.3 or as a mixture of SF-Ne and SW-Ne diffused into the grains. In the latter case, the pure flare component would have a “‘Ne/“Ne ratio lower than 1 1.3. To mimic a single gas component, the mixing ratio would need to be the same in all etched fractions, i.e. at any depth within the grains. Such a constant mixing ratio between SW-Ne and SF-Ne could result if migrated SW-Ne were preferentially retained in lattice defects produced by heavy flare particles. This second alternative had to be considered because the satellite borne instruments indicated SF-Ne to have “Ne/“Ne values around 8-9. Second, the position of the data points of the unetched samples were explained by an addition of SW-Ne, whereby the concentration ratio of retained SW-Ne to trapped Ne with 2oNe/22Ne = 11.3 varies more or less smoothly with grain size. Thus, the resulting linear array is an artifact and the extrapolations to either side have no physical meaning. After the summary of the etching experiment performed by ETIQUE et al. (198 I), we turn now to the discussion of the data obtained by the Closed System Stepwise Etching technique (CSSE). The data points of all four Apollo-16 highland separates follow an identical pattern (Figs. 2b-d). Three groups of points can be distinguished: 1) In the first step, Ne with *‘Ne/**Ne around 12.9 is released (the somewhat lower value of the 64421 plagioclase Nr. 1 is explained by the pre-etching of this sample). In the following steps this ratio decreases steadily. 2) The data points of the Ne released after the first few etching steps group along straight lines with correlation coefficients of 0.998-0.999. These lines are all similar to that defined in Fig. 2a by the etched plagioclase grain size suite of soil 6 1501. With increasing step number, the data points plot closer to the GCR point. Usually, the first step leading to a data point on

R. Wieler, H. Baur and P. Signer

2004

the correlation tine coincides with the onset of the more

harsh treatment of the samples by liquid acid. 3) The data points of the last etching steps of plagioclase No. 1 of soil 64421 where the acid had been heated, as well as all the data points from residue extractions, plot slightly above the respective correlation lines and again closer to solar Ne. For clarity. the step numbers of the data points of the 64421 plagioclase Nr. 1 are given in Fig. 2c. Note that the correlation line in this figure is drawn through points 6-9 only. The three groups of data points reveal the following features: 1,1 Initial gas release. The ‘%Je/“Ne ratios around 12.9m the first steps of each sample are close to the highest values observed in total extraction analyses of lunar ilmenites (- 13.0, EBERHARDT ezal.. 1972; WIELERet al., 1983). Ilmenite is the most retentive mineral for SW-Ne in lunar soils. In contrast. the 20Ne/22Neratios of solar gases obtained by total fusion analyses of Apollo-16 plagioclases range between 12. I and 12.8 after correction for cosmogenic contributions (WIELER et al., 1983). Particularly, the total fusion aliquots of the 655 11 and 6442 1 plagioclase samples yield ratios of 12.2 and 12.6, respectively. 2) Data along the mixing line. All Apollo-16 plagioclase separates studied by CSSE display data points on similar correlation lines. The 5 lines intersect the straight line through the origin and the SW point at the following %e/22Ne ratios 61501

i i.3 t 0.3

64421 Nr. I

II.6 t 0.2

64421 Nr. 2

11.3to.i

655 11 handpicked

10.9 i 0.1

655 I 1 nonmag. fraction

11.2?I 0.1

The mean of these values is 11.3 & 0.3. This component. first found in piagioclase of soil 6 I50 1, is present also in all Apollo16 plagioclase samples studied by CSSE. Until we discuss the nature of this component in section 4.5. we will call it “I I .3Ne”. The component at the lower right end of the correlation lines is cosmogenic Ne. In all plagioclase separates studied, it has a *‘Ne/‘*Ne ratio within 4% of the reference value representing CCR-Ne in a plagioclase fraction separated from the lunar troctolite 76535 by LUGMAIR eral. (1976). This indicates that the mean GCR irradiation depths of soil minerals studied here were comparable to the 75 p/cm* estimated for the 76535 sample, provided that SCR produced Ne, with its lower 2’Ne/22Ne ratio, accounts for only a negligible fraction of the total cosmogenic Ne in these samples. This has shown to be the case for the 61501 plagioclase separates. in which SCR-Ne amounts are less than 5% of the GCR-Ne amounts (ETIQUE, 1981). 3) Finalgas release. The data points of the steps where the HNO3 was heated, as well as those from the etching residues are explained by the release of a mixture of 11.3-Ne with trapped SW gases still retained in acid resistant fragments of SW-Ne retentive minerals contaminating the plagioclase sep arate. In the following section we show that pyroxene releases a major portion of its solar gases but in hot HNOj The position of the data points of the etching residues might also, at least in part, be explained by the release of some 11.3-Ne diffused into the grains, such that enhanced “%e/“Ne ratios result near the grain center (R. H. BECKER,pers. commun.). This cannot be ruled out rigorously. However. the position of the data points of the “hot” steps of 6442 1 plagioclase Nr. I (steps

Nr. 10-I 3) clearly indicate that traces of SW-tie must still have been present in ah etching residues not treated in hot acid. The scatter of the data points of the plagiochtsc separate ot mare soil 71501 (Fig. 2d, inset) can be explained by such contamination too. In contrast to the Apollo-16 separates. this sample continued to release SW-Ne after several steps of etching in liquid HNOr at room temperature. As Fig. 2d illustrates, not even heating the HNOr in order to increase th< etching efficiency produced an unambiguous pattern. Thus. the contamination of the plagioclase separate of the Apollo17 mare soil with pyroxene and ilmenite prevents a clear-cut separation of SW-Ne and 1I .3-Ne. On the other hand, Figs 2c and 2d clearly demonstrate that the 11.?-Necomponent is still perfectly revealed in the 64421 separates despite the accidental pre-etching of these samples.

4.2.2. 71501 Pyroxene. The Ne isotopic data of the pyroxene separate are given in Fig. 3. The first etching step released Ne with a 20Ne/22Ne ratio of 13.3 2 0. ! consistent with the SW value of 13.7 2 0.3. The ‘“NeJ 22Ne ratios of steps 2-7 cluster around i 3. They arc, as already seen above for the plagioclases. stgnificantly higher than the range of 12.3- 12.8 for solar Ne inferred from total fusion analyses of lunar pyroxenes (WIELER pt al., 1983). The points of all following etching steps m Fig. i plot on a very steep straight line with positive slope, Evidently, no recognizable amounts of cosmogenic Ne are released from the pyroxene by etching, indicating that only a very small fraction of the grain volumes were attacked by the HN03. Most remarkable are the lowest 20Ne/22Ne ratios observed in steps Nr. 11 and 12. The Ne released in these last two etching steps with measurable amounts of noble gases is within error limits identical to the 11.3-Ne found in plagioclase. After step 12, the release of noble gases by etching ceased rather abruptly, such that no reliable data could be obtained any more. Comparison with the total fu sion analysis of the aliquot showed that the etched sample still had to contain a large fraction of its trapped gases. Therefore, the etching residue was further annlyzed by stepwise heating. The data points of the first three heating steps closely follow the path of the points obtained by etching, whereby the first heating step i-t’.. leased Ne isotopically similar to the SW value. Pyrolysis steps Nr. 4 and 5 released a mixture of i ! .3-Ne and cosmogenic Ne, with a possible addition of traces oi SW-Ne. The 11.3-Ne component has thus unambiguously been found not only in all studied plagioclase separates. but also in the 7 150 I pyroxenc. 4.3. Elemental

release patterns

The host material of the I 1.3-Ne may also be ehpetted to conserve noble gas element abundances differing from those of the retained solar wind. CSSE offers an important improvement in studying this question. because in the first etching study by ETIQUEUIal. ( 198 1) it was not possible to infer reliable element ratios fog the noble gas fractions lost by etching. In Fig. 4 the 4He/36Ar and “Ne/‘“Ar r;tnos 3~ ::

2005

Lunar soil noble gases

1 2

1304

27b

08

-

/*f

12&o 6_

71501

Pyx.

*‘Ne/**Ne FIG. 3. Ne three-isotope plot of CSSE data and stepwise heating data of 71501 pyroxene. Etching and heating step numbers are given close to respective data points. The asterisk indicates the trapped Ne component inferred by the data of the etched 61501 plagioclase (see Fig. 2a). SWC = Solar Wind Composition (GEM et al., 1972). GCR represents cosmogenic Ne in a olivine fraction of lunar rock 76535 (LUGMAIRet al.. 1976); t.f. = aliquot of CSSE sample, total fusion analysis; Z = weighted average of etching plus heating steps.

following etching steps plot on the mixing lines in Fig. 2. The He amounts released after the third steps were less than a factor of two above the blank level. The apparent increase of the 4He/36Ar ratio between steps 5 and 9 of the 655 11 separate must therefore be viewed with caution. The 4He measured in the etching residue is most probably not solely of radiogenic origin. Radiogenic 4He accumulated in plagioclase during 4 billion years should range between 3000 and 7000 * lo-’ cm3 STP/g; assuming a U concentration of 20-50 ppb (cf BOGARD~~al., 1975, and references therein), a Th/ 36Ar and 2’%e/36Ar ratios in the initial steps. The values U ratio of 4 and no diffusional losses. For the 655 I 1 in 655 11 are about a factor of 100 and 10, respectively, separate, the 4He concentration measured in the etchbelow the abundance ratios in the solar wind (CERU~I. ing residue is a factor of 5 higher than expected from 1974). Due to the pre-etching, the respective values in 6442 1 are of limited interest. The decrease of the 4He/ the above assumptions. (For this estimate, the effective He concentration in the etching residue is used, and 36Ar and 2’%e/36Ar ratios in the following two steps not the value in Table 2, which is normalized to the probably reflects the larger implantation depth of the heavier SW elements (TAMHANE and AGRAWAL, total weight of the unetched sample.) It cannot rigorously be excluded that some of this excess is SW-He 1979). Another explanation is that the most recently migrated from the outermost surface into the interior implanted and thus least fractionated SW gases are of the grains. However, sufficient SW-Ne is released released first (FRICK et al., 1986). The low ratios observed in step 3 of the 655 11 separate and in step 4 of from acid resistant contaminants in the etching residue to also explain the excess He, without need to invoke the 6442 I sample are taken to indicate that all directly SW-He migration. implanted SW-He and SW-Ne is removed efficiently Besides the measured element abundances, 2oNe/ at this state. This interpretation is supported by the Ne 36Ar ratios for the solar gases, corrected for cosmogenic isotopic data in as much as essentially all points of the

function of the cumulative release of 4He and *‘Ne, respectively, are shown for two plagioclase separates and the pyroxene sample. Because of the gas loss by the pre-etching, the abscissa values for 6442 1 are normalized to the concentrations measured in the total fusion aliquot. All release patterns have a similar general appearance, displaying a minimum in both the 4He/36Ar and 2oNe/36Ar ratios after the first few steps of relatively mild etching. 4.3.1. Plagioclase. The low retentivity of SW-He and SW-Ne in plagioclase is reflected by the low 4He/

2006

R. Wieler. ii. Baur and P. Signer 1

200

8

1

I t

,____.

I

t 1200

1 Ig‘$OT_

_

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1 _

_

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80 i

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% 4He rei. 1 2 E

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pi 800

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,

t

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100

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20Ne released

%

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released

FIG. 4. 4He/36Arvs. cumulative fraction of ‘He released and 2%e/36Ar vs. cumulative fraction of -“ha released for two of the plagioclase and the pyroxene sample. Values off scale for stepwise heating data of the

7 1501 etching residue are indicated. In the lower left panel, the solar ratios (i.e. ratios corrected for cosmogenic contributions) are indicated too. Data of etching residues are marked by res. S and t.f. have tht- samt significance as in Fig. 2.

cont~butions (see section 4.7), are given in Fig. 4. Measured and corrected ratios both exhibit a pronounced “V” shaped pattern. The highest corrected 2%Je/36Ar ratio in sample 6442 1 is more than two orders of magnitude above the minimum value in step 4. Such a bimodal gas release pattern can easily be attributed to the release of two distinct components of solar type noble gases. The component implanted to larger depths resides in grain regions which are less severely radiation damaged than the SW bearing surface layer, leading to a less pronounced depletion of Ne by diffusive losses. Considering that even cosmogenie Ne is lost to some 30-70% from lunar soil plagioclase (FRICK et al., 1975: SIGNERrt ul., t 977). one realizes, however, that substantial tosses of any trapped Ne must have occurred also. Indeed, the highest values for the solar 2%e/36Ar ratios found in the two plagioclase separates are around 6 and 25, respectively, clearly lower than the “solar system” value of 37 (ANDERS and EBIHARA, 1982). Thus, the elemental release patterns of the CSSE plagioclase separates are fully in agreement with the association of I I .3-Ne with solar particles implanted with energies above those of the solar wind. 43.2. Pyroxene. The V shaped pattern of the etching steps in both right hand panels of Fig. 4 is very pronounced. The different retentivity of pyroxene and

plagiodase for light SW gases is manifests by the about fwefotd different ordinate values of the initially released fractions on left- and right-hand side of Fig. 4, respectively. Remarkable is the t@Ne/36Ar ratio of the last etching step which yielded sufficient gas for a precise analysis (step 1I). It is within the limits oferror identical to the “solar system” value of 37 and the solar wind value of 45 f IO (~ERUTTI, 1974). Note that this 1I th step also released pure 11.3.Ne. About two thirds of the trapped noble gases appeared but in the pyrolysis. including a large fraction of the SW-Ne. Tht: 20Ne/3hi4r ratio of 170 observed in the second pyrolysis step cleari!indicates that this mode of gas release causes severe element fractionation. in summary, the He/Ar and Ne/Ar release patterns of both mineral types reinforce the conclusions drawn from the Ne isotopic data, that two distinct solar gas components are preserved in mineral grains from lunar soils. The more deeply sited one apparently is betteer retained than the surficially implanted SW. 1n the following two sections, both these components are dlscussed in detail. 4.4. Solar wind Ne In section 4.2. it was shown that the inttld sreps release from both mineral types Ne with n ““Ne,/“*Nc

Lunar soil nobIe gases

ratio similar to the value found in ilmenites. However, in view of the strong and variable element fmctionation of the SW gas released early in the etching process, the question about the significance of the observed isotopic ratios arises. Isotope fractionation might occur as a result of two processes, namely: i) the gas extraction technique used, and ii) different depth distributions for different isotopes caused either by the implantation process or by diffusive redistribution of implanted gases. Concerning the first edibility, major Ne isotopic fractionation in initial release fractions of stepwise heating experiments of iunar soil samples is indicated by 2”Ne/22Ne ratios of up to 15 (PEPIN et al., 1970). The CSSE technique is, however, a priori expected to cause no or at least considerably less fractionation than stepwise pyrolysis or also stepwise combustion, because in CSSE diffusion of the releasing gases should play but a minor role. The 36Ar/3*Ar release pattern of the pyroxene sample, discussed in more detail below (see Fig. 5, section 4.6), strongly supports this expectation, inasmuch as the first five etching steps released Ar with virtually identical ~Ar~3gAr ratios, and thus no isotopic fractionation by the etching of pyroxene is indicated. Unfortunately, this reasoning cannot be applied to the

I

~ 5.8 -

I

st heating

I

I

1

step

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PYX

5.6-

---_-__

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Ar data of the plagioclase samples due to the influence of cosmogenic AK,but we see no reason why it should not be extended to this mineral too. Concerning fractionation effects caused by different implantation depths, we note that the mean depth of directly implanted SW-He is expected to be nearly a factor of two smaller than that of SW-Ar (TAMHANE and AGRAWAL, 1979). This could readiiy explain the observed decrease of the HefAr (and Ne/Ar) ratios in the first etching steps. For different isotopes of one element, the situation is less clear. Range calculations by LINDHARDet al. (1963) indicate a larger penetration depth of the heavier isotope, provided that light and heavy isotopes have identical velocities, as is commonly assumed for the solar wind. However, no data on that topic for elements in the mass range of Ne are known to us. Furthermore, the variations of the angle of incidence would smear out different penetration depths to some extent. Basically it is not possible to derive simultaneously both the isotopic composition of the retained SW gases and the depth dist~butions of each isotope from the CSSE data. Nevertheless, our data speak against severely different geometric distribution of different SW noble gas isotopes within the grains. Again, the argumentation relies on the pyroxene data only, because cosmogenic gases do not interfere there. The observation used above to rule out an isotopic fractionation of SW gases caused by the etching applies here too: The virtually identical 36Ar/3*Ar ratios of the first five etching steps of the 7 150 1 pyroxene also argue against different depth distributions of different SW-Ar isotopes. Furthermore, the Ne data points of six etching steps following the initial one cluster around *@Ne/=Ne ratios of 13.0. Such a release pattern is not expected if the different Ne isotopes have different depth profiles within the host material attacked. This conclusion agrees with the arguments given by BLACK (1972) against different depth distributions of different isotopes of one element. In summary, it seems reasonable to conclude that the Ne released in the initial step is not appreciably affected by either of the two processes considered. The Ne composition of the first step can thus be taken as representative to the SW-Ne retained. Lunar ilmenite retains SW-Ne between 10 and 20 times more efficiently than plagioclase (SIGNER et al., 1977). Therefore, I 1.3-Ne should play a minor role in this mineral, and thus, the *~e/‘*Ne ratios around 13 for the trapped Ne (EBERHARDTet al., 1972; WIELER et al., 1983) should be representative for SW-Ne in ilmenite. The 20Ne/22Ne ratios of 12.9 measured in the first step of the plagioclase separates indicates that the retained SW-Ne in this mineral is but slightly more fractionated than in ilmenite. Actually, 12.9 should be considered as a lower limit for the *“Ne/‘*Ne ratio of SW-Ne retained in plagioclase, because some 11.3-Ne may be admixed even in the first release fraction. Concerning the 7 150 1 pyroxene, the wef22Ne ratio of 13.3 f 0.1 recorded in etching step Nr. I is identical

1

5.0 -

65511 Plagf handpicked

1

L L IJL i.

j

00 % 36Ar

released

FIG. 5. 36Ar/38Arratio vs. cumulative fraction of 36Ar released of CSSE data of one plagioclaseand the pyroxene sample. Total fusion and stepwise heating data, respectively, of both etching residues are also given. Note break in ordinate scale. Errors in the &Ar/‘*Arratio are about 0.75% for steps releasing more than 2% of the total %Arand 1.5% for the less gas rich steps.

2007

2008

R. Wieler. H. Baur and P. Signei

within error limits to the value found in the Solar Wind Composition experiment. This indicates that the retained SW-Ne in this mineral may even be unfractionated. A note of caution is in order here: FRKK and PEPIN ( 198 1) proposed that the most recently implanted SWNe may be less fractionated than the gas implanted earlier. This gas may furthermore be released first in heating or etching experiments. Although no evidence for such an effect is apparent from our data, we cannot strictly exclude the possibility that only part of the retained SW-Ne has conserved a *“Ne/“Ne ratio = 12.9. Summarizing this section, the gas released in the initial CSSE steps indicates that at least part ofthe SWNe retained in lunar soil minerals is isotopically less fractionated than is inferred from total fusion data. Presumably no severe isotope fractionation occurs either by the CSSE release technique or by isotope dependent implantation depths. Conversely, no reservoirs of elementally unfmctionated solar wind He, Ne. and Ar were revealed by CSSE in any of the samples studied. 4.5. SEP-Ne The most important result of this study is the recognition of the presence of a trapped Ne component with 2”Ne/22Ne - 11.3 in all separates studied. The line of argumentation which led ETIQUE ef al. (198 1) and ETIQUE (198 1) to conclude that 11.3-Ne is implanted SF-Ne, possibly mixed with SW-Ne, is the following: This gas is found in depths of up to some 35 pm nominal depth below the grain surfaces. Its concentration decreases with increasing etching depth. Diffusive migration of SW-Ne from the grain surface towards the center cannot be the sole reason for this gas distribution. As mentioned before, these authors considered the possibility that 11.3-Ne is a mixture between migrated SW-Ne and SF-Ne, in order to explain the fact that the satellite measurements available at that time yielded %le/**Ne ratios for SF-Ne of about 7.7 only. The CSSE data base now allows a refined argument for 11.3-Ne being indeed a distinct trapped solar gas component and not a mixture. (For the reasons discussed in section 5.4, we will call this component SEPNe rather than SF-Ne.) To arrive at this conclusion. the following alternative explanations have to be ruled out: a) As propsed by ETIQUEet al. (198l), I 1.3-Ne is a mixture in constant proportion of SEP-Ne and SW-Ne, whereby SWNe migrated from the grain surface towards the grain interior. b) 11.3-Neresults from a mixture in constant proportions of SEP-Ne with SW-Ne, whereby the SW gas stems from grain surface spots not totally degassed in the initial steps of mild etching. Traces of pyroxene and ilmenite may provide such retentive sites. In both alternatives a) and b), the 2”Ne/22Ne ratio of the pure SEP component would be lower than I 1.3. c) The 11.3component is SEP-Ne which, however, has the same isotopic composition as SW-Ne. The emerging %e/ **Ne ratio (about 20% lower than the true value) is an artifact

caused by a considerably larger mean penetration depth ol the heavy isotope. Penetration depths varying with isotope mass may be conceivable for SEP-ions, despite the fact that in the preceding section we considered this effect unlikely to occur for SW-ions. Alternative b) has to be considered because the data pomts of the etching residues fall slightly above the correlation lines in Figs. 2b and c. indicating that the residues still contained some SW-Ne. Furthermore, the data of the plagioclase fraction of mare soil 71501 prove that severe contamination of the plagioclase with more retentive minerals indeed can occm (see section 4.2.1). For Apollo-16 highland samples. contam ination is. however. considered as a minor problem: -Apollo-l6 highland soils contain very low amounts of SW retentive minerals. -In all four Apollo- 16 plagioclase separates analyzed by CSSE the actual %e/**Ne ratio of the component called hem “ 11.3-Ne” is indeed very similar to the value of I 1.3deduced from the 61501 plagioclase fractions. in spite of the latter sample having been etched by a more aggressive HF/H2S0, mixture. Furthermore, the two 655 1 I separates were contaminated to very different degrees but contain nevertheless nearly identical ” 11.3-Ne”. ---In the Apollo-16 plagioclase separates. the fraction of the total SW-Ne still present in the etching residue amounts to less than 20% of the total 11.3-Ne contained in the separates (calculated by assuming the compositions of the two trapped, retained components as stated in section 5.3). The residual SW gas probably is residing in sites not readily etchable by HNOl. The fact that the SW-Ne contained ir! these retentive sites amounts to only about one fifth of the 11.3-Ne released in the CSSE suggests the presence of a: most a minor contribution ofSW-Ne to the I I 3 compment in the late etching steps. ---In steps Nr. 10-I 3 of the plagioclase separate 6J43 I Nr. ! the HN09 was held at 70” to increase the etching speed. Some of the respective data points fall slightlv above the correlation line, indicating traces of SW-Ne which was not released by the etching with cold acid. Most remarkably. the plagioclase grains themselves must have been nearly completely dissolved in steps 10 and 11.This is deduced by the amounts of cosmogenic “Ne and “Ar released in steps lo- 13. It thus follows that essentially all the remaining SEP-Ne contained actually in the plagioclase grains and not in contaminants was also released in steps 10 and 1 1 No evidence for a %le/“Ne ratio of the trapped componem in these steps below I 1.3 is seen. Taking all the above observations and arguments together leads us to conclude that a constant mixing ratio of SW-Ne and SEP-Ne in all CSSE steps of u sample would be fortuitous. This conclusion is further supported by the observation that the mixmg ratio would have to be very similar for all five plagioclasc separates originating from three different lunar soils,: Assuming the 2r’Ne/22Neratios between 10.9 and I i .6 to be caused by mixing of SW-Ne and SEP-Ne with 20Ne/22Ne = 13.0 and 7.8, respectively. would requrrc constant mixing ratios to within 230%. The Same reasoning can be applied to alternative a): As discussed in section 4.3, the elemental release pattern shows that the samples probably have lost variable fractions of their SEP-Ne, whereby different regions of the grains must have been affected differently. This cannot be reconciled with the requirement of constant mixing proportions of the two trapped Ne components in all etching steps.

Lunar soil noble gases Alternative c) is discarded for the following reasons: -A

larger mean penetration depth of the heavy SEPNe isotope would lead to a variation of the “‘Ne/ ‘*Ne ratio with depth: A depletion of the light isotope in large depths would be compensated by an enhanced 20Ne/22Ne ratio near the surface. Notwithstanding, a distinct and identical 11.3-Ne component is released from all dissolved grain portions, from all etching residues measured by ETIQUE et al. ( 198 1), and fu~he~o~ in all high temperature extraction steps ofthe pyroxene etching residue. In the scenario discussed here, this would imply that all 1 I .3-Ne originates from the high energy tail of SEPNe. In particular, this would apply already to the I 1.3-Ne released immediately after the mild etching which removed the SW-Ne. The complementary SEP-Ne fraction with its apparent ‘“Ne/**Ne ratio larger than the SW value would then be released in the first etching steps, hidden by the SW-Ne. Such a depth distribution of SEP-Ne seems inconceivable, since the amounts of the trapped Ne released as pure I 1.3-Ne (i.e. with ~~-~ints on mixing lines in Fig. 2) already account for up to at least 35% of the total Ne in plagioclase (see section 5.3). -This explanation conflicts with the observed inverse correlation between 20Ne/22Ne and 2”Ne/36Ar in unetched plagioclase separates from many different lunar soils observed by WIELER et al. (1983). This correlation was interpreted as variable mixing ratios between retained SW- and SEP-gas, assuming the two components to have different element- and Neisotope abundances. -Different penetration depths for different isotopes should also affect Ar. As the relative mass difference of 36Ar and “Ar is only about half of that between “Ne and Z2Ne, the expected decrease of the 36Arj ‘*Ar ratio with progressing etching would be expected to be on the order of 10%. However, the pyroxene data limit such variations to less than about 3% (see next section). The 7 150 1 pyroxene data offer additional evidence that 11.3-Ne is a distinct component of solar origin. Besides the similarity of the Ne released in the last etching steps of the pyroxene (steps I 1 and 12) with the 11.3-Ne deduced from the plagioclase data, also the 20Nef”Ar ratio of step 11 of about 34 is remarkably close to the “solar system” value of 37 (ANDERSand EBIHARA, 1982), and also similar to the ratio of 43 determined by satellite borne instruments for Ne/Ar in solar flares (BRENEMANNand STONE,1985). Note that such a comparison is meaningful only if the Ne/ Ar fractionation which might occur as the result of element dependent implantation depths is not too severe for SEP gases. We have no argument that the implantation depths for SEP-Ne and SEP-Ar should be very similar. However, the measured value in step 11 would also be reconcilable with the two “solar” ratios used for comparison by assuming a somewhat lower

2009

penetration depth of the lighter SEP element. It seems thus a reasonable hypothesis that steps 11 and 12 of the 7 150 1 pyroxene released essentially pure SEP-Ne and Ar. How can this claim be reconciled with the presence of large amounts of SW gases in the etching residue of the pyroxene? As pointed out above, no cosmogenic Ne, released by the etching of the pyroxene, is detectable and the gas amounts liberated by the acid decreased strongly after step 12. Moreover, the weight loss of the pyroxene separate caused by the etching is much smaller than that of the plagioclase samples (Table 1). These observations indicate that probably only the most severely radiation damaged sites were etched. This is in fact confirmed by an SEM survey of the etching residue, which shows that only a few percent of the surface area of the pyroxene grains show evidence of being etched. WIELERet al. ( 1980) argued that a few percent of the grain surface area can easily contain a large fraction of the trapped gases. Thus, selective etching of such sites can indeed account for the large fraction of the SW gases retained during the etching as well as for the 11.3-Ne detected in steps 4 and 5 of the subsequent stepwise heating of the residue. The above considerations show that all the effects proposed to explain the 11.3-Ne as an artifact meet with severe problems or demand fortuitous coincidences. We are thus led to conclude that 11.3-Ne is indeed a distinct component, namely pure SEP-Ne. It is presumably trapped in all lunar regolith material together with SW gases. The retained SEP-Ne has, according to our studies, a 2oNe/22Ne ratio of 11.3 + 0.3. It should not be noticeably fractionated relative to the impingent SEP radiation. This is concluded from the apparently good retention of SEP Ne in pyroxene, manifested by the nearly solar 2%e/36Ar ratio of step Il. as well as from the similar composition of SEP Ne retained in two types of minerals with different diffusivity for Ne. SEP-Ne in lunar minerals has a similar 2oNe/22Ne ratio as Ne-C found in meteorites, which BLACKS1972) attributed to be of solar flare origin. The positive slope of the linear array formed by the CSSE data points of 7 1501 pyroxene (Fig. 3) indicates that the 2’Ne/22Ne ratio of SEP-Ne is also slightly lower than the SW value. Assuming that no cosmogenic Ne was released in step 11, the SEP-Ne composition is 2oNe/2’Ne/2~Ne = 1 I .3 + 0.3:0.030 + 0.00 I : I. Taking the first and the Iast step to represent pure SW-Ne and pure SEP-Ne, respectively, the 2rNe/22Ne ratios of the two components (SW-Ne - 0.032, SEPNe - 0.030) would differ by about 6%, roughly half the difference of the corresponding 20Ne/22Ne ratios. SEP-Ne may thus be related to SW-Ne by a fractionation depending linearly on the mass difference between the isotopes. Should a small cosmogenic contribution have occurred, the value of 0.030 would be an upper limit only for the 2iNe/22Ne ratio of SEP-Ne. In any case, this value is lower than the 2’Ne/ZZNe ratio

2010

R. Wieler. H. Baur and P. Signer

of 0.042 + 0.003 inferred by BLACK(1972) for Ne-C. Note, however, that BLACK (1972) had to derive his value by correcting data of Fayetteville and Holman Island for cosmogenic gases, a difficult procedure to do for gas-rich meteorites. 4.6. He and Ar isotopes, SEP-Ar The “Ar/“Ar ratios as a function of the cumulattve fraction of 36Ar released are given in Fig. 5 for one plagioclase and the pyroxene separate. No 3He/4He ratios are given, because in all steps where 4He was not predominantly of SW origin, the observed He amounts were severely disturbed by blank contributions. This precludes any inferences on SEP-He. Also the 40Ar data are not reliable due to blank problems (see section 4.1). Initial gas release. All samples show very similar “Ar/“Ar ratios between 5.4 and 5.5 in the initial release fraction. The pyroxene even released Ar in the first five steps with virtually identical 36Ar/38Ar values of 5.4. As pointed out in section 4.4, this provides an additional argument against isotopic fractionation of SW gases due to the CSSE release technique. The values observed in the initial steps are all slightly higher than the atmospheric ratio of 5.35 (NIER, 1950) and the ratio of 5.33 + 0.03 determined for SW-Ar (EBERHARDTet al., 1972). This is in agreement with the report by FRICK and PEPIN (198 1) of a 36Ar/3sAr ratio in the modem solar wind slightly above the atmospheric value. Detection of SEP-Ar by three-isotopic correlation diagrams, analogous to Ne, is impossible. However, because only very minor amounts of cosmogenic Ne (and thus by inference also cosmogenic Ar) are released by CSSE of the 7 1501 pyroxene, information on the composition of SEP-Ar can be obtained from the data of those etching steps of the pyroxene experiment where the SEP contribution dominates the SW portion. This requirement is best fulfilled for step I 1, where, according to the previous section, possibly rather pure SEP-Ne and SEP-Ar are released. With the exception of step 8, all steps where hot acid was used, released Ar with 36Ar/38Ar ratios between 5.2 and 5.35 (Table 2), values only slightly below those of the first steps which released predominantly SW gases. This observation taken at face value may be interpreted to show SEP-Ar to have a 36Ar/38Ar ratio about I-3% below the SW value. However, in view of step 8, a careful examination is in order. A probable explanation for its 36Ar/38Ar ratio of 5.02 +- 0.1 is the following: This step was the first one where hot HNO, was applied for a long time (48 hours). According to the observations on the etching of plagioclase (section 4.2.1), it is therefore probable that the minute quantities of plagioclase contaminating the pyroxene separate were completely dissolved in this step. In fact, less than 1% of 7 1501 plagioclase would contribute sufficient cosmogenic Ar to lower the measured 36Ar/38Ar ratio from 5.35 to 5.02, without affecting the position of point 8 in Fig. 3 noticeably. Possibly. traces of cos-

mogenic Ar from plagioclase may also have been freed in the other steps where the acid was hot. causing thereby the 36Ar/3sAr ratios slightly below the values observed in the first steps of the pyroxene run. Thus. the presently available data allow no final conclusion whether the 36ArJ38Arratios of SEP-Ar and SW-Ar arc different. Provided that in step 1 1 we indeed observed SEP-Ar without a considerable SW-Ar contributton. such a difference, if it exists. is smaller than YL. Wt, see no evidence for a 36Ar/38Ar of the SEP component as low as 4.1 * 0.8 as proposed by BLACK( 1973). even in view of the value of 5.02 in step 8. It is difficult to see how this ratio can be attributed to the release oi SEP-Ar with such a heavy composition. because this component definitely would have to show up in the later etching steps lo-12 also. Note. however. that Black’s value was based mainly on data from gas-rich meteorites. The observation that SW-Ar, planetary Ar. and atmospheric Ar all have within 2-3s identical 36Ar/3”At ratios is now to be extended to include SEP-Ar. In :I remarkable contrast, each of the respective Ne components has a distinct composition. The reasons for this fundamental divergence remain open. 4.7. The cosmogenic component The release pattern of the cosmogenic noble gases (distributed throughout the volume of the grains) ma) be used to indicate the actual geometric sites whence the observed trapped and cosmogenic gases originate. Figure 6 shows the “Ne/“Ar ratio of the cosmogemc gases as a function of the cumulative release of cos mogenic “Ar for two plagioclase samples. The other

Eb5t

m---l 4 GCR. 70gIcm2

ies

-i

handpicked 4

cu -

.2

0

20

40 %

60

38Arcos

80

100

released

FIG. 6. Ratio of cosmogenic Z’Ne/3”ArVS.cumulative frac.. tion of cosmogenic “Ar rebsed of CSSE data and etching residues of two plagio&se samples. The amounts of cosmogenie gases are calculated as described in section 4.7. The asterisk marks the (21Ne/3*Ar)CCR ratio for lunar plagioctase at 70 g/cm*. (See section 4.7.)

2011

Lunar soil noMe gases

Apollo- 16 plagioclase separates display a similar pattern. The concentration of cosmogenic 2’Ne and “Ar were calculated for the steps where they were not too severely masked by trapped gases. Essentially, these steps are those releasing predominantly SEP gases besides the cosmogenic component. We assumed therefore the following isotopic composition of the trapped and cosmogenic components: (2’Ne/22Ne), = 0.030 * 0.001 (section 4.5) (2’Ne/22Ne), = 0.77 i 0.04 (36Ar,‘3SAr),, = 5.35

+.05

-.I0

(36Ar/38Ar),,,,= 0.63 + .02. The error in the composition of the trapped Ar takes into account that the 36Ar/38Arratio of SEP-Ar is possibly slightly lower than the SW value (section 4.6.2). The resulting errors of the Z’Ne/3SArC,ratios are displayed in Fig. 6. No errors are given in Table 2. The first etching steps, which are not indicated in Fig. 6, are estimated to have released some 3-58 of the total cosmogenic 38Ar only. Both samples show a clear increase of the “Ne/ 3xArcos ratio with progressive etching. This indicates

previous losses of cosmogenic Ne, whereby the material etched first had suffered more severe losses than materidl etched later and the etching residue. This interpretation is in accord with the generally observed deficits of cosmogenic Ne in lunar plagioclase of up to 70% (SIGNER et al., 1977). The relative difference between the *‘Ne/‘*Ar ratio in the first etching step and in the etching residue is larger for sample 4442 1. This is to be expected from the very low 2’Ne/38Ar, ratio of -0.1 of the total fusion aliquot, showing that this sample suffered the most severe loss of cosmogenic Ne of all plagioclase separates studied in our laboratory. Even the etching residue of 64421 has a 2’Ne/38Ar, ratio of only 0.15. This value is still low compared to the production ratio of 0.5 at a shielding of 70 g/cm2, as calculated by ETIQUE (1981) using the elemental production rates given by HOHENBERGet al. ( 1978). Conversely, the 2’Ne/38Ar, ratio of 0.46 in the residue of 655 11 indicates that the cores of these grains were barely affected by losses of cosmogenic Ne. The apparent plateau of the 64421 plagioclase data in Fig. 6 probably has no significance: it is not observed in the ahquot sample (figure not given). We take the increase of 2iNe/38Ar,, to show that in CSSE the ptagioclase grains were etched in a manner comparable to that by which diffusion affects noble gases in the grains. This is an indication that the etching proceeded in an onion shell fashion. The nominal etching depths calculated in section 5.4 by means of the cosmogenic 38Ar amounts

should therefore

be meaningful.

5. DISCUSSION 5.1. Ne-B, fractionated SlV-Ne?

The interpretation of Ne-I3 needs reconsideration in view of this study. Ne-B, found in gas rich meteorites (BLACK and PEPIN, 1969), has a MNe/22Ne ratio of

about 12.5. It is often considered to be SW-Ne, fractionated in trapping and retention of the solar wind (e.g. review by PODOSEK, 1978). BLACK ( 1972) pro-

posed alternatively

that Ne-B might be a mixture of

SW-Ne and Ne-C. This proposition is now strongly supported by the CSSE data: The SW-Ne retained in

lunar minerals has a higher ‘?Ve/*‘Ne ratio than NeB, whereas the 2oNe/ZZNeratios of the total solar Ne, as inferred by data of total fusion analyses range from 12.2 to 12.6 for the samples measured here (see section 4.2.1). The latter values are similar to Ne-B and are also derived in a comparable manner. However. the CSSE experiments clearly proved these ratios to result from a mixture of SW-Ne and SEP-Ne. It is thus not appropriate to call Ne-B a distinct “component”. 5.2. SEP-Ne in lunar soils-SEP-Ne by satellite measurements The values of the 2oNe/22Ne ratio in SEP-Ne presently avaiiable are compiled in Table 3. Five out of the six values are lower than the SW ratio, and given the large error of the analysis by SIMPSONet al. f 1984), this statement may even hold for that entry. The conclusion that SEP-Ne is isotopically heavier than SWNe is therefore strongly implied by all available data. Considering the large statistical uncertainties of the satellite data, it is unclear whether the differences between the *‘Ne/*‘Ne ratios measured in contemporary solar flares and the lunar soil value. representing an average over the past few hundred million years (c[ WIELER et al., 1983), are real. The issue of a possible dependence of this composition on the energy of the particles is discussed in section 5.4. Here we address the suggestion by DIETRICH and SIMPSON( 1979) and MEWALDTet al. (198 I) that SEP-Ne may be identical in composition to planetary Ne. At least for the low energy part of the SEP spectrum which yields the bulk of the SEP particles detected in our experiments this notion is not tenable. In fact, SIMPSONet al. (1984) also point out that all satellite data now available taken

TABLE3. Determinations of *“Ne/22Neratios of SEP-Ne (qe/ UN&$?

Datasource

References

10.6 + 0.3

meteorites

Black ( 1972)

II.3 + 0.3

lunar soils

Etique et al. (1981): this work

I I.8 t

lunar soils

Nautiyal er a/. (198 I)

+ 2.3 1.5

charged panicle det.

Dietrich and Simpson (1979)

9.2+ l.9 - 1.8

charged particle det.

Mewaldt o al (1983)

+ 10.3 16.7 - 7.2

charged particle det.

Simpson et nt. (I9841

‘.7-

0.3

7012

R. Wieler. H. Baur and P. Signer

together suggest that the mean SEP-Ne composition is similar to Ne-C. Whether SW-Ne or SEP-Ne is closer in composition to the Ne in the source region on the sun cannot be decided by the CSSE experiments. As shown in section 4.5, our data just indicate the mass fractionation between the two components depends approximately linearly on the mass difference of the isotopes. Furthermore, to date, no conclusions regarding possible secufar variations of the isotopic compositions of SEP noble gases are possible. CSSE analyses of samples with large antiquities, namely old soil breccias and possibly drill core samples, are planned to elucidate this problem. These studies are also expected to bear on the hypothesis of a secular decrease of the SEP/SW flux ratio in the past 1-3 billion years (WIELERA al.. 1983).

SW-Ne. 5.4. SEP/SA’jlu ratio; energy spectrwu oi SEP- Ne implanted in lunar samples

5.3. Concentrations c$SEP noble gases in lunar samples We now turn to the question of the abundance of‘ SEP noble gases in tunar soil minerals. First, we estimate the amounts of SEP-Ne in the samples, by assuming the isotopic comp~tion of the three pertinent components as given in Table 4. All values in the table are derived from the CSSE data of the respective samples. With these assumptions we find between 30% (71501 pyroxene) and 50% (655 11 plagioclase handpicked) of the total trapped *‘Ne in each mineral to be of SEP origin. If part of the retained SW-Ne had a *ONe/‘zNeratio lower than the assumed values (c;f:section 4.4), these estimates would be somewhat too high. A lower limit for the SEP-Ne contribution in plagioclase can be derived by assuming all the trapped Nr from the steps with data points plotting above the correlation tines in Fig. 2 to be of SW origin. Even according to this conservative estimate, between 20 and 35% of the retained trapped *‘Ne is contributed by SEP. These fractional abundances are consistent with data from gas-rich meteorites (BLACK,1972). Clearly, SEP-Ne retained in lunar plagioclase and pyroxene is not merely a minor component. Even the value of 20% is several orders of magnitude higher than any estimate for the ratio of solar flare/solar wind particle fluxes. This feature is discussed in the next section. The unexpected prominence of SEP-Ne has implications for the inte~re~tion of trapped gases in lunar soil samples. Two examples are discussed here: TABLE 4. Assumed compositions of retained Ne components used to calculate amounts of each trapped component ---_l__“-~__

Pyroxene

Plagioclase

SW: 29Ue/22Ne

13.3

17.\,

SEP: 2oNe/2ZNe

Ii.3

as given in section 4.2. I for each sample

O.S5/0.95/ 1

0.76/0.72i I

cosmog: Z”Ne/ “Ne/*‘Ne

a) We propose to explain the variatron of the “‘Ne: *‘Ne ratio of implanted Ne obsened in extrarerrestriai samples of different antiquity by a secular- decrease ill the SEP/SW flux ratio (WIELER ei ~1, 1YMt rather than by a secular increase of the *“Ne/*‘Ne ratio in the solar wind (PEPIN. 1980). b) The noble gas data of grain size surtes cannot he interpreted as two component mixtures. This became evident from the straight Iine fitted through the data points of the unetched plagioclax grain sirt, sepawtcs of soil 6 150 1 which does not point towards a reasonabie “Ne/22Ne ratio of the GCR component (section 4.2 Fig. Za). It has become clear now that this hnc also does not pass through the point representing I-cbvx~

-,

The finding that up to 50% of the total trapped and retained Ne are SEP-Ne is certainly astonishing: The average SW proton flux is -2.4 +fO*/fcmL . set - 4~ 8 while that of solar flare protons above It) I~eVfamu is only about ?O/(cm’* sec.4%) {N~s~I~~~‘M~d ni.. 1977). This yields a flux ratio around 3 - 109 which is about six orders of magnitude higher than the concen tration ratio of retained SW-Ne and SEP-Ne. The question arises as to what causes this drastic difference. One effect to be considered is certainly the incomplete retention of SW-Ne in lunar soil minerals. SWNe retentivities of only some 2-5% for plagioclase and of lo-40% for pyroxene, respectively, are deduced by the NejAr ratios of a large number of mineral separates (SIGNER a al.. 1977; WIELERd a!.. $983). The retentivities for SEP-Ne are higher: This com~nent is apparently quanti~tively retained in pyroxene (section 4.5) and probably retained at the 20-50% level in plagioclase. The latter ftgures are deduced from the solar Ne/Ar ratios of the last CSSE steps of the plagioclase samples shown in Fig. 4, by assuming SEP-Ar to be retained quantitatively. The different retentivities for SW-Ne and SEP-Ne estimated here indicates now that an overabundance of the latter component of at best a factor of ten can be explained in this manner. Note that saturation of SW-gas due to surface sputtering caused by solar wind ions cannot be used to explain an enhancement ofthe SOP-Ne/SW-Ne concentration ratio. WIELER ef al. (I 980) showed ~turati~~n not to occur to a large extent in mineral populations of the grain size studied here, due to their limited lifetime as primary particles. Besides of the low retentivity of SW-Ne. one must consider the possibility of a very large flux of SEP particles with energies so low as not to be recorded nor” mally by charged particle detectors. In the following we compare the flux and energy spectrum of-solar flare particles as derived from track data with estimates of the SEP-Ne flux and spectrum deduced from the CSSE data. Thereby, the terms “solar flare (SF) particles” and “sotar energetic particles“ (SEP) wilf noi be used

Lunar soil noble gases as synunyms, to account for the possibility discussed below, that SEP noble gases trapped in extraterrestrial samples were to a large extent implanted with energies down to a few ten KeV/amu only. Such “suprathermal ions” are not necessarily associated with solar flares. Energy spectra of solar ilare track producing Fe group par&&s derived from data obtained on funar rocks are given by HUTCHEON d af. f1974) and by BLANFORDECat. I1975). From the tatter spectrum we calculate a mean integral flux of SF iron group particles with energies v 0.1 MeV/amu:

The spectrum by HUTCHEONel al. f f 974) is given for energies larger than 1 MeV/amu only. If we extrapolate to 0.1 MeV/amu, which seems appropriate in view of the spectrum given by BLANFQRDet al. (1975), the integral flux is about ten times larger, namely @(SF& = 1/(cm2 - set). With a H/Fe ratio in the SW of 2 * f04 (GEISS and BOCHSLER,19SS), we obtain thus for solar flare particles > 0. I MeVfamu:

To derive a flux ratio ~~SW~~)/~~SEP~~) by means of the CSSE data, we consider the 7 I 50 1 pyroxene only, because we can assume that this mineral quantitatively retains SEP-Ne (see section 4.5). For this consideration, the following additional assumptions have to be made: 1) 20-352 of the trapped Ne in the pyroxene is of SEP origin (section 5.2). 2) SW-Ar is well retained in all lunar soil minerals (SIGNER et al., 1977; WIELER et al., 1983). 3) The fraction of SW-Ne retained in the pyroxene is IO-20%, as deduced from the 20Ne/36Arratios in the first etching steps and by using assumption 2. 4) SEP.Ne and SEP-Ar are both well retained. This assumption implies that the Ne/Ar ratio in SEP is similar to the “solar system” value (section 4.5.). 5) Effective exposure times of the grains for SW and SEP particles are identical. With these assumptions,

we deduce a flux ratio:

This ratio is between two and almost four orders of magnitude lower than the range of values deduced by the track data. (This comparison impI~~it~y assumes identica1 Ne/Fe ratios in SEP, SF and SW). However. two effects have to be considered: Firstly, assumption 5) above is questionable; the effective exposure times for SEP ions may well exceed the SW exposure time. Second/y, SEP ions implanted with energies c 0.1 MeV~amu are recorded in CSSE experiments, but neither by the satellite borne detectors nor in the above track studies. With respect to the first effect, we note that identical SW and SEP exposure times for the grains imply all

1013

SEP gas detected to have been implanted during exposure of a grain on the very surface of the regolith. A cover of the grains by dust in the submicron range would, however, effectively shield the minerals from the SW but only partially from the SEP. A fine dust cover caused by adhesive forces was discussed by HoUsLEY(l980).

Concerning the flux of particles with energies below 0. I MeV/amu, little data is available. “Suprathermal” ions in the 5-50 KeV/amu range, arriving predominantly from near solar directions were detected by FRANK (1970) on two occasions. Both observations were followed by a smaii geomagnetic storm. which indicates that these ions are possibly of solar origin, induced by a small scale solar activity. By means of track detectors exposed during the Apollo- 17 mission, WOODSet al. ( 1973) and PRICEand CHAN( 1973) identified suprathermal ions emitted by the quiet sun. The threshold energy of the detectors used by WOODS et ai. f f 973) was about 20 KeVfamu, The fluence of these “quiet sun ions” seems to be negligible compared to the number of particles emitted during solar flares. The average flux of the ions detected by FRANK ( 1970). however, although highly uncertain, might exceed that of solar partides with energies > 0. I MeVtamu by orders of m~nitude fcf: FLELSCWER ef al., 1975). Thus it seems possible that these ions contribute the major fraction of the SEP noble gases found in lunar samples. A possible manifestation of these suprathermal ions in lunar soils was observed by BORG et al. ( 197 1). According to these authors, very high track densities f>fO” traeks/cm2) in the oute~ost f pm of lunar dust grains are probably caused by solar ions in the energy range as reported by FRANK (1970). However, this question is not settled fcf: ~AURETTE and PRICE. 1975). Of course, the two effects discussed above are correlated: The more low eneay SEP ions are trapped by the grains, the less is the difference between the SW and the SEP exposure times. For a more quantitative assessment of the possible influence of suprathermal ions, the energy spectrum of the SEP particles detected in lunar samples must be known. An attempt to derive this spectrum is made by means of Fig. 7. This figure shows the concentration of SEP-Ne in arbitrary units as a function of nominal depth for the handpicked 65511 plagioclase separate. The depth was deduced from concentmtions of vofume correlated, cosmogenic “Ar by assuming spherical grains of 200 pm diameter and isotropic etching. Note that the Ne concentration profile in Fig. 7 is significant only if the assumption of isotropic etching is reasonably correct. We choose the handpicked plagioclase separate for the following reasons: i) it consisted of the cleanest grains of all samples studied, ii) it had a relatively narrow grain size range, iii> the cosmogenic 21Ne,f38Arratio increased monotonically with the step number (Fig. 6, section 4.7). indeed indicating isotropic etching. Figure 7 also gives the track density in a lunar rock sample in arbitrary units as a function of depth (BLANFORDet al., 1975).

2014

R. Wieler. H. Baur and P. Signer I

I 5

1-3 \

\

65511 Plag handpicked

lo

depth

(pm)

100

FIG. 7. Concentration of SEP-Ne (in arbitrary units) as a function of the nominal etching depth of grains for the handpicked plagioclase sample of soil 655 1 I. The SEP-Ne concentration is calculated as the measured SEP-Ne amount in a step divided by the amount of cosmogenic ‘*Ar in this step (the latter quantity is assumed to be proportional to the grain volume etched in the step). The nominal etching depth again is calculated with the amount of cosmogenic 38Ar per step. assuming all grains to have diameters of 200 pm. Also given is the track density gradient measured on lunar rock 64455 by BLANFORDet al. (1975) (in arbitrary units), and furthermore, the average values of the ratio D5rm/Dr measured by WIELERet al. (1980) for three soils is indicated, again in arbitrary units. Thereby, Dr *,,,and DCare the track densities at 5 pm below the surface of a grain (at an arbitrary position) and at the center of the grain, respectively.

This line is intended as a reference only and should not be directly compared with the SEP-Ne depth profile, because the track density gradient in a rock exposed in fixed orientation does not reflect the turnovers. depth variations and break ups experienced by soil grains. These processes cause a significant flattening of the track density gradients. A more meaningful comparison should invoke track density gradients as measured in mineral grains from soils. WIELER et al. ( 1980) gave the ratio Dsrm/Dcfor a large number of plagioclase grains of three lunar soils. D5rmand 0, are thereby the track densities 5 pm below the grain surface on both sides of a randomly chosen diameter, and in the grain center, respectively. The average values of D5(rm. normalized to DC,are indicated in Fig. 7 for the three soils measured. The SEP-Ne gradient in 65511 is steeper than the track density “gradients” in all three soils. This indicates an overabundance of low energy SEPNe relative to the energy spectrum of the track producing particles. If correct, this observation support5 the notion that a large fraction of the SEP-Ne detected in lunar samples was implanted with energies below those of the track producing particles. On the other hand, Fig. 7 also indicates that part of the SEP-Ne is found at depths of up to several ten microns, implying implantation energies in the range of several MeV Jamu.

More quantitative statements concemmg the energ! spectrum of SEP-Ne implanted into lunar samples arc not possible because of the unknown irradiation histories of the grains and the uncertainty introduced b! the assumption of homogeneous etchmg. It has furthermore been suggested CR. H. BECKER. pers. corn mun.). that diffusive losses and possible redistributions of the various noble gas components may Icad tcl both elemental and also isotopic fractionation effects which could be difficult to assess quantitatively. Especially iti view of the leaky character of plagioclasc for iight noblt gases it is indeed possible that one or x\eraI of x5sumptions I-4, which were used above tt* de-i-~\etlx flux ratio for +(SW,,)/+(SEP,,) by means ot’the CSSI data. may have to be revised. Furthermore. the qual. itative depth profile of SEP-Ne given in FIN. 7 might have been affected by diffusive redistribution of SEPNe. However, considering the complexities oidiflusion in irradiated grains. we see no way to obtain mart quantitative information from our data. I hr analysis of retentive phases, i.e. ilmenite separates and possibl\ metal particles, is the most promising approach to clucidating this problem. Therefore. we are currently dcveloping a CSSE line which will make possible the UK of more aggressive etching agents like HI, The reservations expressed in the previous paragraph do. however, not affect the basic conclusion of this paper: SEP-Ne is a prominent component in lunar soii minerals. Its large abundance can reasonably be attributed to a major contribution of ions implanted with lower energies than those typical for solar flares. Thus. the bulk of the SEP noble gases in extraterrestrial samples may not have originated in solar flares but rather in events representing smaller scale solar activity. This suggestion speaks in favour of the interpretation by BORGet al. ( 197 1) concerning the very high track densities in the outermost 1 ctrn of lunar dust grains. Note that the above interpretation, in conjunction with the data shown in Fig. 2. implies identical ‘“Nc! ‘*Ne ratios of about I 1.3 for suprathermal NC-ions as well as the more energetic and more deeply implanted solar flare Ne. This indicates that the selection processes leading to the acceleration of SEP particles of all cnergies are closely related, irrespective of whether a flare or a smaller event produces the particles The shift from the SW-Ne isotopic composition to the SEP-Nc value seems to occur at an energy only slightly abovt* the SW value, somewhere between 0.5 and a few KeVi amu. 6. CONCLUSIONS Closed System Stepwise Etching of plagmclase and pyroxene separates of lunar soils by HNO, combined with on-line mass-spectrometric analyses 01’released He, Ne, and Ar is feasible and permits refined investigations of trapped and cosmogenic noble gases. This technique has potential for further applications, especially if a system allowing the use of more aggressive

Lunar soil noble gases

etching agents can be designed and if the blanks can be lowered. Gas release by etching causes considerably less isotopic and especially elemental fractionation than is the case for gas released by stepwise heating. The main results of this study of mineral separates from lunar soils 4442 1,655 11, and 7 150 1 by means of closed system stepwise etching are: 1) The Ne component with a %e/‘*Ne ratio of 1i -3, first found by ETIQUE(21nl. (198 1) in plagioclase grains of soil 6 1501. is present (besides the solar wind Ne) in all Apollo- 16 plagioclase separates studied as well as in the one pyroxene sample analyzed. This gas is SEP-Ne &SolarEnemetic Particles~. 2j SEF-Ne is found in depths oi up to 30;m. Probably the more deep@ implanted part of it stems from ions accelerated in solar tlares. However, SEP-Ne concentrations are much too high to be associated solely with solar flare particles. In the samples investigated here, SEP-Ne accounts for at least 20% and possibly up to 50% of the total trapped Ne. The ratio of retained SEP-Ne/SW-Ne is thus some two to four orders of magnitude larger than the ratio of the flux of iron group particles in the solar wind to the flux of solar flare iron group ions with energies above 0.1 MeVfamu. Possible reasons for this overabundance of SEP-Ne are: i) Low retentivity of the minerals for SW-Ne. ii) Longer effective SEP ideation time due to partial SW shielding of the grains by fine dust. iii) Our experiments also record SEP ions with energies below 0. I MeV/amu. Possibly, a large &action of the SEP partides found in the CSSE experiments have an identical source as the “suprathermal ions” found by FRANK( 1970). These particles are not ejected by solar flares but are presumably associated with a smaller scale solar activity. 3) At present, our best estimate for the long term average of the isotopic composition of SEP-Ne is:

2015

meteorites. Our data suggest that the observation of a nearly identical MAr/3aArratio of atmospheric Ar, planetary Ar and SW-Ar can be extended to include SEP-Ar also. This is in remarkable contrast to the respective Ne components which all differ significantly from each other. 9) SW-Ne retained in lunar plagioclase and pyroxene is less fractionated than it has been inferred from data of total fusion analyses. In ~1~~~. the retained SW-Ne has a ‘@Ne/ “Ne ratio of - 12.9, which is only marginally lower than the values found in ilmenite, despite the much larger diffisional losses of SW-Ne from the former mineral. As suggested by BLACK(1972), Ne-B is probably a mixture of SW-Ne and SEP-Ne, rather than the distinct com~nent “retained, fractionated SW-Ne”. $0) The cosmogenic f2’Ne/22Ne) ratio of ail pfagioctase sampies is similar, indicating a mean shielding of -75 g/cm” for all separates. The ratio of cosmogenic (“Nef’“Ar), in plagioclase increases monotonically with etching time. This is explained by losses of cosmogenic Ne. affecting predominantly the outer portions of the grains. The release pattern of the cosmogenic gases indicates that the plagioclase grains were etched more or less in an onion shell manner.

The experiments described here are in certain aspects complementary to solar flare Investigations by satellite borne instruments. The satellite data provide information on short term ~uctuations of the abundance of solar flare ions in a well defined energy interval. whereas studies of extraterrestrial material yield vahtes averaged over large and not well known intervals in time and energy. Studies of the latter type, however, constitute one of the very few means to obtain experimental data on the evolutionary history of the sun. The charged particle detectors on satellites are able to determine the abundances of a number of elements 2oNe/22Ne= 11.3 rt 0.3 up to the iron group and the main isotopic ratios of a 2’Ne/22Ne I 0.030. considerable number of these elements can also be The upper limit for the 2’Ne/22Ne ratio relies on the data of measured. All these data have, however, a rather limone pyroxene sampie only and needs confi~ation. Little, if ited precision, In extraterrestrial material, only noble any, isotopic fractionation of the retained SEP-Ne relative to gases of SEP origin are detectable, but these measurethe impingent radiation is expected. ments yield in favorable cases isotopic ratios of high 4) The “Neiz2Ne ratio of SEP-Ne reported here is close to that of IO.6 r 0.3 reported for Ne-C, a ~rn~nent found in precision. meteorites and attributed to solar Bare Ne (BLACK, 1972). Our work in progress concentrates on the investiHowever, our CSSE value of 0.030 for the “‘Ne/*“Ne ratio of gation of samples with large antiquities, especially lunar SEP-Ne is lower than the 21Ne/22Neratio 0.042 inferred for soil breccias, whereby the main effort will be devoted Ne-C by BLACK( 1972). to the analysis of ilmenites. A further goal is the analysis 5) The Z??Ie/ZZNeratio of SEP-Ne does not differ with imof gas rich meteorites by the etching technique. These plantation depth. This indicates that the selection mechanisms leading to supratbermal ions, on the one hand, and to solar studies are expected to rule on the hypothesis of a secflare ions. on the other, are closely related. ular decrease of the SEPlSW flux ratio (WIELER et a!., 6) The first measurements of Ne in contemporary solar 1983) as we11 as on a possible time variation of the Bares by satellite borne instruments indicated we/Z%e ratios isotopic composition of SEP-Ne. Such a variability may lower than those found for SEP-Ne in extraterrestrial samptes. This would have been in conflict with our preceding conclube indicated by the “Ne/“Ne ratio of Ne-C, which is sion. The larger satellite data base now available indicates. slightly lower than 11.3. Etching experiments may rehowever, that the two differently determined compositions veal whether this difference is not an artifact caused may well agree on average. by a supe~osition of small quantities of ptanetary Ne. 7) Based on the first satellite data, DJETRtCHand SIMPSON (1979) and MEWALDT et al. (1981) suggested that SEP-Ne measured by the satellite instruments might be isotopically identical to planetary Ne (Ne-A). According to these authors, this could imply that bulk solar Ne has the composition of Ne-A (20Ne/22Ne- 8). The isotopic composition of SEP.Ne presented here is in conflict with this proposition. 8) Our data indicate a %Arf3*Ar ratio of SEP-Ar which is at best I-3% lower than the ratio of solar wind Ar. We cannot confirm the value of 4. I * 0.8 for this ratio in the flare component derived by BLACKf 197.2) from analyses of gas-rich

Acknowledgemenrs-This study was stimulated by the Ph.D. thesis work of Ph. Etique. We are grateful to Th. Graf for preparing a new calibration gas mixture. Valuable reviews were provided by D. C. Black and C. M. Hohenberg. ln addition, helpful comments were communicated by R. H. Becker and L. Schultz. This work was supported by the Swiss National Science Foundation. Editorja~ ~und~~~g: K. Marti

R Wieler, H. Baur and P. Signer

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