Implanted solar helium, neon, and argon in individual lunar ilmenite grains: Surface effects and a temporal variation in the solar wind composition

Geochimica

et Cosmochimica

Pergamon

Acta, Vol. 58, No. 2, pp. 1031-1042, 1994 Copyright 0 1994 ElsevierScienceLtd Printed in the USA. All rights reserved 0016-7037/9<$6.00 + Ml

Implanted solar helium, neon, and argon in individual lunar ilmenite grains: Surface effects and a temporal variation in the solar wind composition NICHOLS R. H. JR., HOHENBERG C. M., and OLINGER C. T.*

McDonnell Center for the Space Sciences and Physics Department, Washington University, One Brookings Drive, St. Louis, MO 63 130, USA (Received

June 24, 1992; accepted in revised.ftirm

September

24, I993

)

Abstract-Solar implanted helium, neon, and argon were extracted from individual ilmenite grains from lunar soil 7 150 1 and soil breccia 79035 using laser vaporization and analyzed by static mass spectrometry. Clear differences were observed for these two grain populations, one from a contemporary soil and the other from an ancient soil exposed on the lunar surface approximately 1 Ga ago. The different trends between the *‘Ne/**Ne ratios and the He/Ne ratios do not simply reflect differences either in regolith gardening or in diffusive losses but rather suggest a greater relative helium abundance in the ancient solar wind by a factor of about 1.8. The majority of the grains are enriched in solar energetic particle (SEP) Ne relative to solar wind (SW) Ne in a manner that increases with surface exposure. The progressive enrichment in retained SEP Ne relative to SW Ne is explained by a combination of diffusion and nonfractionating losses of the less deeply implanted SW component. The neon isotopic differences observed in various analyses of ilmenite separates from these two soils and previously attributed to a secular variation of either the SW/SEP flux ratio or the SW Ne isotopic composition may alternatively be a natural consequence of greater SW losses which accompany an enhanced helium flux in the ancient solar wind. INTRODUCTION

heavy neon component is higher than that of the SW. A possible identification of this component might be conventional solar flare (SF) Ne, with 20Ne/22Ne ratios similar to those observed in some solar flares ( MEWALDT et al., 1984; NAUTIYAL et al., 1981, 1986; RAO et al., 1991; cf. BENKERT et al., 1993). However, the inferred abundance is several orders of magnitude higher than expected from conventional solar flares (WIELER et al., 1986). WIELER et al. (1986) attribute this more energetic heavy neon to implanted solar energetic particles (SEP), intermediate in energy and flux between the solar wind and the more energetic conventional solar flares. This component is inferred to have a 2oNe/22Ne ratio of 11.2 f 0.2 and a measured abundance that is up to 50% of the total implanted solar neon ( WIELER et al., 1986; BENKERT et al., 1993). Astronomic observations ( WIDING and FELDMAN, 1989), as well as theoretical analyses ( VON STEIGER and GEISS, 1989), strongly suggest that such a component is indeed plausible. Although the two lunar samples analyzed in the present work seem to have similar neon endmember isotopic compositions (SW Ne and SEP Ne), the relative contributions of these two components differ significantly between the samples (WIELER et al., 1983, 1986; BENKERT et al., 1988). Since these soils were exposed on the lunar surface at different epochs, WIELER et al. ( 1983,1986) and BENKERT et al. ( 1988) originally suggested that the compositional variation reflects a secular variation in the SW/SEP flux ratio. On the basis of more recent stepwise etching experiments, BENKERT et al. ( 1993) also suggest a possible secular change in the (20Ne/ 22Ne)sw isotopic ratio.

LUNAR SOILS PROVIDE records of the solar corpuscular radiation to which they have been exposed. By comparing radiation effects in soils exposed at different times it may be possible to delineate temporal variations in the composition of solar radiation ( KERRIDGE, 1975; PEPIN, 1980; c.f. KERRIDGE et al., 199 la). Noble gases extracted from two soils in particular have been extensively studied using closed-system stepped-etching ( WIELER et al., 1986; BENKERT, 1989; BENKERT et al., 1988, 1991, 1993) and stepped-combustion ( FRICK et al., 1988; BECKER and PEPIN, 1989). Both techniques reveal the apparent depth profile or depth-dependence of the implanted gases, but the results and the interpretations differ in detail. One of the samples (soil 7 150 1) was recovered from the present lunar surface as a contemporary soil and records a relatively recent exposure to solar radiation ( < 100 Ma; ARVIDSON et al., 1976; WIELER et al., 1983). The other sample (breccia 79035 ) is a loosely consolidated soil breccia ( FRICK et al., 1988) exposed on the surface about 1.O Ga ago (CLAYTON and THIEMENS, 1980; KERRIDGE et al., 1991b). Stepwise Closed System Etching: The Ziirich Group Stepped-etching experiments on lunar mineral separates ( WIELER et al., 1986) reveal a depth profile for the surfaceimplanted gases which indicates that at least two distinct solar components are present: a solar wind (SW) component implanted to depths of a few hundred Angstriims and an isotopically heavier component implanted from one to three orders of magnitude deeper ( ETIQUE, 198 1). This suggests that the characteristic implantation energy ofthe isotopically

Stepped Combustion: The Minnesota Group FRICK et al. ( 1988) and BECKER and PEPIN ( 1989) performed stepped-combustion experiments on aliquots from

* Pr~~eni addrexs: N-4, MS E54 1, Los Alamos National Laboratory. Los Alamos, NM 87545, USA 1031

R. H. Nichols Jr., C. M. Hohenberg.

1032

the same two ilmenite separates, lunar soil 7 150 1 and soil breccia 79035, studied by the Zurich group. The isotopic composition of neon released at successive steps shows a clear transition from isotopically lighter neon at the surface to an isotopically heavier component as the oxidation progressively liberates gas from deeper within the grains, yielding gas-release profiles that are similar to those observed in the Zurich stepped-etching experiments. BECKER and PEPIN ( 1989) and FRICK et al. ( 1988), however, attribute this depth-dependent composition to the superposition of unfractionated SW Ne with a reservoir of neon which has been modified both by diffusive loss from the grain surface resulting in an isotopically heavier neon and by diffusive migration into the interior of the grain. The *‘Ne/**Ne ratios obtained in such a model ranges from 12.4 to 13.2, depending upon the relative initial implantation depths of 20Ne and 22Ne ( FRICK et al., 1988 ). These ratios can be produced over a large range of gas losses (-60-99.9%) and do seem to mimic the presence of a second implanted component (i.e. SEP) with a lower 20Ne/2’Ne ratio. However, as noted by BECKER ( 1990), the diffusive fractionation model by itself cannot account for the lowest 2”Ne/ 22Ne ratios seen in the late etching steps of the Zurich group. Even if diffusive losses did occur, the fractionation model still requires the presence of an SEP component to account for the extremes of the isotopic compositions observed. The Minnesota interpretation thus differs from the Ztirich interpretation by relying on a larger contribution from inward diffused SW to account for the heavy neon observed in these samples. Both the stepped-combustion and the stepped-etching experiments demonstrate real differences between modern lunar soil and the ancient soil breccia. Although the two soils display qualitatively similar gas-release profiles, at nearly every extraction step the neon from soil breccia 79035 is isotopically heavier than that from the more recently exposed soil 7 I 50 1. The Minnesota group suggests that these differences may represent a secular variation in the isotopic composition of SW Ne (BECKERand PEPIN, 1989), rather than a secular variation in the SW/SEP flux ratio as proposed by the Zurich group (WIELER et al., 1983, 1986). Single Grain Analyses:

This Work

To examine the record of implanted gases in more detail, individual ilmenite grains from these same two lunar soils, modern soil 7 150 1 and ancient soil breccia 79035, were analyzed using a laser gas-extraction technique ( HOHENBERG et al., 1990). Ilmenite was chosen in all of these studies because it is the most noble-gas retentive lunar mineral and as a modest conductor it is minimally affected by track-producing solar flare ions that can otherwise damage the lattice and potentially enhance diffusive losses of implanted solar gases. This single grain work nicely complements the earlier studies by providing details of the implanted solar gas inventory that are not accessible in the study of bulk ilmenite separates. Stepped-etching and stepped-combustion techniques reveal trends in the implanted noble-gas compositions averaged over thousands of grains and are useful in assessing global properties such as average gas concentrations, average isotopic compositions, and representative depth profiles.

and C. T. Olinger

While the individual grain study loses some of this information, such as depth-dependence or thermal-release profiles, it has the unique advantage of providing the grain-specific exposure record. Each grain in a soil records its own unique exposure history which is accessible only through grain-bygrain analysis, and it is the extremes of these exposure conditions and the trends among them that yield valuable new insight. In addition, because the same mineral from both samples is studied. the only significant variable between different grains in a given soil population should be the specific exposure history of each grain. Differences between the two grain populations should therefore reflect variations either in the exposure history of each population or in the solar corpuscular radiation to which they were exposed. EXPERIMENTAL

PROCEDURE

AND RESULTS

Ilmenite separates from lunar regolith breccia 79035 and soil 7 150 1 were readily available as existing aliquots of the 90- 125 pm size fraction of ilmenites prepared by J.-P. Benkert, who graciously shared the samples (see BENKERT, 1989, and BENKERT et al.. 1993, for details of sample preparation). The grains were individually weighed using a Cahn-31 microbalance and placed in holes drilled into a stainless steel mount, which served as a sample holder for the laser extraction. Filled sample mounts were placed under a Pyrex viewpott within the ultra-high-vacuum sample system of a pulse-counting noble-gas mass spectrometer ( HOHENBERG, 1980). Modest vacuum bake-outs ( - 150°C) were achieved with heat lamps overnight, but the samples themselves were shielded from direct radiation during this time to prevent sample degassing. Very low blanks (typically below 2.5 x 10-‘0ccSTP4He, 2.5 X 10~‘3ccSTP2”Ne, and 5 X 10mr3 ccSTP 36Ar) are routinely achieved by this procedure. The mounted grains can be individually vaporized using a Nd:Y 4G CW laser whose output power is continuously variable from 0 to -70 W using a pair of water-cooled Clan polarizers for attenuation. The laser output is injected onto the axis of an optical microscope by a dichroic mirror and focused through the viewport onto the individual samples. The heating process. monitored in real time by a CCD camera mounted on the same microscope. showed the ilmenite grains rapidly going from solid to liquid and completely vaporizing. typically within two minutes. Processing, analysis, and calibration procedures ofthe sample gases were similar to those described by HOHENBERG et al. ( 1990) except that helium was analyzed after neon. followed by the analysis of argon which was separated from helium and neon using activated charcoal at liquid nitrogen temperature. Calibration for procedures which involve laser volatilization of extremely small particles is difficult using conventional methods. A more direct calibration, and the one used for this work, consists of laser volatilization of small fragments of material with known noble gas content. We selected two calibration standards: olivine crystals from the Springwater pallasite and metal from the Grant (BIB) iron meteorite. Both of these objects have long cosmic ray exposure ages and quantities ofspallation-produced ‘He. “Ne. and “Ar sufficiently large and uniform to be useful as calibration standards. While much data exists for Grant. as tabulated by SC‘HULTZ and RRUSE ( 1989). only one measurement has been published for Springwater olivine ( MEC;RUF, 1968). We consequently rely upon Grant metal for the primary calibration and in turn use this to independently verify the spallation noble gas content of Springwater olivine. a material used extensively as a standard for laser extraction. Several metal slivers from a sawing residue of Grant (sample Bar J, ~ 100 mm obtained from R. Wieler) and several olivine crystals from Springwater, ranging in mass from about 10 to about 100 pg. were loaded into the laser extraction cell, volatilized, and analyzed using the same procedures as those used for the lunar ilmenite grains. Our calibrations for Grant were made by using the average concentrations of3He (4.61. 4.65. 4.60) X 10mh ccSTP/g, “Ne (5.54, 5.40, 5.40) X IO-’ ccSTP/g. and ‘*Ar (28.4, 27.5, 29.0) X IO-* ccSTP/g measured in three Grant metal samples that bracketed the Bar J-100 position (Bar J: -47 mm. -104 mm, and -137 mm, respectively,

1033

Noble gases in lunar ilmenite grains Table 1. Comparison of the single grain analyses (this work, 90 - 125 pm), the stepped-etching analyses (Ziirich, 90 - 125 pm) and the stepped-combustion analyses (Minnesota, 100 - 150 pm) for lunar ilmenites from 7 1501 soil and 79035 breccia. 20Ne

36~r

x 10-6 cc/g

x 10-6 cc/g

-

13.7 (3) 13.7 (3)

34.5 (3) 25.9 (3.7) 27.6 (0.5) 33 (3) 31.6 (-) 42.7 (0.9)

13.02 (1) 12.95 (4) 13.06 (5) 12.71 (1) 12.57 (4) 12.68 (10)

Solar Wind?

_

_

Solar Windt 71501 a 71501 b 71501 c 79035 a 79035 b 79035 c

_

_

2255 (226) 894 (45) 1400 (180) 630 (94) 1470 (49) 644 (5) 2892 (290) 817 (40) 2510 (-) 782 (-) 4529 (34) 1197 (50)

4He/36Ar

2ONe/36Ar

570 (70) 550 (50)

15960 (5490) 25000 (6000)

28 (9) 45 (10)

252 (25) 222 (22) 228 (8) 354 (35) 321 (48) 378 (16)

6500 (650) 5400 (500) 5300 (200) 8764 (880) 7900 (1200) 10600 (242)

26 (3) 24 (5) 23 (1) 25 (3) 25 (4) 28 (1)

2ONe/22Ne 4He/2ONe

4He x 10-4 cc/g

t SW,: Geissefal. (1972).$SWb:cf AndersandCirevesse(1989),Benkertet 01.(1993).(seetext): a This (1989).CMinnesota:Becker and Pepin (1989).Fricket al. (1988)

as reported by GRAF et al., 1987; SIGNERand NIER, 1960). These concentrations are quite uniform, as are other reported values for Grant metal tabulated by SCHULTZand KRUSE ( 1989)) and yield average concentrations for our Grant calibration standard (Bar J, -100 mm)of4.62 X 10m6ccSTP/g3He, 5.45 X 10-sccSTP/g21Ne, and 28.4 X IO-’ ccSTP/g “Ar. Using these values and the measurements of four olivine grains from Springwater we obtain concentrations of 228 X IO-*, 73 X 10m8,and 0.83 X lo-* ccSTP/g for ‘He, *‘Ne, and 38Ar,respectively, for Springwater olivines. While the ‘He concentration is within 5% of the value determined by MEGRUE ( 1968), the *‘Ne and “Ar concentrations are 38% higher and 30% lower, respectively, than the values determined by MEGRUE( 1968). This is an important observation since Springwater olivines have been routinely and extensively used in calibration of other laser extractions ( HOHENBERGet al., 1990; OLINGERet al., 1990). Absolute calibration accuracy is estimated to be +lO% for helium, f5% for neon, and + 10% for argon with relative precision on a grain-to-grain basis far greater than this. The noble gas data for each ilmenite grain are given in Tables A I and A2, in the Appendix. These data are corrected for procedural blanks and for interferences due primarily to 4oAr++ and CO;’ at *we and **Ne, respectively, with cumulative corrections typically less than 1%. The data shown in each figure are also corrected for spallation effects which are also quite minor. A comparison of the average helium, neon, and argon compositions from this work with that of previous stepped-etching and stepped-combustion studies is given in Table 1.

work. b

Ziirich:Benken

This difference is consistent with previous results for bulk ilmenite separates from these samples ( WIELER et al., 1983; BECKER and PEPIN, 1989; BENKERT et al., 1993). Although it is clear that there is a difference between the average *‘Ne/ 22Ne ratio in the modern soil and the breccia, it is not at all apparent whether this difference is due to secular changes either in the neon isotopic composition or in the SW/SEP ratio, as has been suggested by others (cf. KERRIDGE et al.,

DISCUSSION Elemental and Isotopic Differences Between the Soil and the Breccia Figure 1 displays the 20Ne/22Ne ratios versus the 4He/22Ne ratios observed in the individual ilmenite grains from both the modern lunar soil 7 1501 and the ancient soil breccia 79035. Even though noble gases are implanted with uniform isotopic and elemental compositions the retained elemental and isotopic compositions evolve in a systematic way. In both modem and ancient soils the neon compositions become isotopically heavier as the 4He/22Ne ratios decrease, reflecting variations in the individual exposure histories. The more striking feature of Fig. 1, however, is the difference in the two fields of points delineated by the least-squares best-fit lines and by the average isotopic and elemental compositions (Table 1). The range of values and the average values for the *‘Ne/ **Ne ratios are shifted by -2-3s toward lower (heavier) values in grains from breccia 79035 relative to soil 7 150 1.

4000

6OC0

8ooO

10000

4He122Ne

FIG. I, 2oNe/2ZNeversus 4He/22Ne for soil 7 1501 and for breccia 79035. The different trends in the data from these samples suggest that each sample was exposed to a distinct solar wind composition, with the ancient solar wind higher in 4He (see text). As the He/Ne ratios decrease (due to helium loss) the SEP Ne contribution to the neon inventory increases. Unweighted least-square fits to the data are shown to delineate the trends and not to imply the presence of simple two-component mixing alone. In this and subsequent figures the following convention is used: ( I ) the present day solar wind compositions (Table I ) are indicated by SWa,b; (2) the averages of single grain (SS = St. Louis Soil 71501, SB = St. Louis Breccia 79035), stepped-etching (ZS = Ziirich Soil 7 150I, ZB = Ziirich Breccia 79035) and stepped-combustion (MS = Minnesota Soil 7 150 1, MB = Minnesota Breccia 79035) noble gas analyses are shown by the solid triangles and are given in Table I; and (3) unless shown, the uncertainties in the isotopic measurements are smaller than the plotted symbols and the uncertainties in the elemental ratios and gas concentrations are - 10%.

1034

R. H. Nichols Jr., C. M. Hohenberg, and C. T. Olinger

199 la), or whether it is due to some other process. Since SEP Ne is isotopically heavier than SW Ne, any process which enhances the retained SEP contribution relative to the SW contribution will result in isotopically heavier neon without requiring any secular changes in the implanted neon composition (addressed more fully below). A plot of “Ne/“Ne versus 4He/36Ar (Fig. 2) shows trends similar to those in Fig. I, but with slightly more scatter. Figures 3 and 4 show that the neon isotopic composition becomes progressively heavier and that the helium is progressively lost (relative to neon) with increasing exposure to the solar wind, indicated by the “Ne concentration. In an interesting contrast to Fig. 4. which shows clearly the systematic decrease in the 4He/2”Ne ratio with exposure, a plot of 20Ne/36Ar versus *‘Ne (Fig. 5 ) shows neither progressive fractionation of the Ne/Ar ratio as a function of exposure nor any difference in the 20Ne/3hAr ratios between the soil and the breccia grains. Whether there is any substantial systematic difference between the retained Ne/Ar ratios in the samples and the modern SW ratio is uncertain because the SW Ne/Ar composition is not very well constrained at the present time (Table 1). Secular Change in the Solar Wind Helium Abundance Figures I. 2, and 4 show higher average 4He/20Ne and 4He/3hAr ratios in the ancient soil (breccia 79035) than in the modern soil (7 1SO1 ), an observation shared by other workers (cf. KERRIDGE et al., 199 1a). These differences cannot be due solely to diffusive losses. If the differences in the helium compositions of these two populations were to be governed by diffusion, a larger helium depletion relative to the other gases would be expected in the ancient soil where more than a billion years is available for helium redistribution. Instead, the ancient soil has a higher average 4He concentration than the modern soil, while the neon and argon concentrations are comparable between the two and the average

78

0

8 0.001

l

71501 79035

A

Components

,I

I

0

1

l

l

I

0.002 [20Ne] cc STP/g

FIG. 3. 20Ne/22Ne versus “Ne concentration for soil 7 150 I and for breccia 79035. The grains with the highest neon concentrations, reflecting more exposure to the SW, clearly have the heaviest neon isotopic compositions. As shown in Fig. 5, however, neon is not systematically fractionated from argon indicating that the diffusive loss of neon was not a prevalent process. Thus, the trend shown here is consistent with the presence of an SEP component coupled with the non-element specific loss of gas from the grain surfaces, as might be caused by collective surface loss which preferentially removes SW relative to SEP contributions (see text).

20Ne/36Ar ratios are identical (Figs. I, 2, 4, and 5; Table I). Moreover, if the compositions of the lightly exposed modern soil grains in the upper right in Figs. 1 and 2 were representative of the SW source composition for both data sets then diffusive modification could not account for the elevated 4He/ 2oNe and 4He/36Ar ratios seen in the lightly exposed breccia

14.’

jli’ __+_,_______

L

_____.____________-________________.SWa,,______

_-

13.

500

0

.

.

-

I

I

.

B

z” q

0

13.

6

6

300-

12.St

0.000 10000

2cQco

3ciIoo

71501 79035

A

Components

0

200-

I””

OL 0

12.

l

El

00

“2 400-o

0

0

O.cill

0

.

0.002

0.003

+QNel cc STFVg

4ne136Al FIG. 2. “Ne/‘*Ne

versus 4He/36Ar for soil 7 150 1 and for breccia 79035. Analogous to Fig. I, the different trends in the data from the two samples suggest that each sample was exposed to a distinct solar wind composition, with the ancient solar wind higher in 4He (see text).

FIG. 4. 4He/20Ne versus *‘Ne concentration for soil 7 150 1 and for breccia 79035. The majority of the grains from both data sets have He/Ne ratios that are lower than the modem SW composition (GEISS et al., 1972). The single grains with the highest neon concentrations clearly have the lowest He/Ne compositions. Thus, the diffusive loss of helium increases with increasing surface exposure.

Noble gases in lunar ilmenite

0

0.002

[%el

0.003

CCSTFVg

for soil 7 150 I and FIG. 5. 20Ne/36Ar versus *‘Ne concentration for breccia 79035. No progressive loss of neon, relative to argon, with exposure is evident, unlike that for helium (Fig. 4). The Ne/Ar ratios scatter in a random way slightly below the range for the modern day solar composition (shaded area). The lack of a systematic trend in the Ne/Ar ratio and the presence of such a trend for the 2%e/22Ne ratio (Fig. 3) suggest that neon loss (and subsequent fractionation) is not responsible for the neon isotopic structure. The similarity in the Ne/Ar ratios between the modern soil and the soil breccia argue against any substantial difference in the Ne/Ar ratio in the ancient SW.

grains. There is no way to remove neon and argon from the breccia grains without a corresponding loss of at least as much helium. These observations suggest that the most important difference between the modern soil and the ancient soil is the relative helium abundance and that this difference is most likely due to an elemental difference in the incoming solar corpuscular radiation rather than diffusive modification of the gases following implantation. Although both SW and SEP contributions are present in the implanted gases, it is clear from all studies that the lowerenergy SW contribution dominates the incoming particle flux. Indeed, the lightly exposed grains from the modern soil in Figs. I and 2 plot close to the range of contemporary SW compositions ( GEKS et al., 1972; CERRUTI, 1974; cf. ANDERS and GREVESSE, 1989 ). We note here that the abscissa values of the SW compositions on Figs. 1 and 2 relative to the plotted data are less certain than the plotted data itself because the elemental composition of the solar wind is not known very well, as indicated by the large error bars and the range of reported values (Table 1). The specific position of the SW value with respect to the data is also a function of the relative sensitivity of the mass spectrometer for the plotted elements which even after calibration, is still subject to some systematic uncertainty. These considerations, however, do not affect comparisons among grains of a single soil nor between those of the two different soils, and the difference between the two data sets cannot be due to any procedural or instrumental factors because subsets of grains from both the soil and the breccia were analyzed in an alternating fashion. The lines shown in Figs. 1 and 2 are primarily drawn to delineate the trends in the data and not necessarily to suggest

grains

1035

that simple two-component mixing alone, e.g., between a SW component and a well-defined second component with heavy neon and depleted helium, is the dominant mechanism that defines the trend. The requirements for simple two component-mixing are probably not met due, in part, to the complex processes which can affect the mixtures of noble gas components located near the grain surfaces, especially in the gas-rich grains. Nevertheless, for both the modern soil and the ancient soil breccia, the locus of points defined by the more lightly exposed grains (upper-right portion Fig. 1) should represent the less modified endmember whose isotopic and elemental compositions are near that of the respective SW source compositions. The two populations do exhibit similar trends when considered individually, but, as previously mentioned, the two trend lines are clearly not the same (Figs. I and 2). The modern soil grains lie on a trend line between the lightly exposed grains, whose compositions are similar to the modern SW compositions, and the more heavily exposed grains, whose neon is isotopically heavier and helium quite deficient. (For the moment we defer discussion of the heavily exposed grains and the isotopically heavy neon endmember.) The most lightly exposed grains from soil breccia 79035 do lie to the right of the lightly exposed grains from the modern soil in Fig. 1. One-quarter of the ilmenite grains from the ancient soil have 4He/20Ne ratios exceeding that of the modern SW composition (Figs. 1 and 4). The same progressive behavior is seen in Fig. 4 where the data for each sample show helium loss with increasing exposure, but the data for the ancient soil are consistently displaced upward (toward higher He/Ne ratios). The only explanation that seems able to account for the major differences between the soil and the breccia is a secular change in the relative helium content of the solar wind. If the two trend lines of Figs. 1 and 2 correspond to mixing between relatively unmodified SW, at the upper right, and modified surface components, to the lower left, then the He/ Ne ratios of the two distinct SW endmembers can be determined from where these lines intersect the respective SW Z”Ne/22Ne values. The initial closed system etching steps of ilmenite separates from the modern soil (7 1501) and the ancient soil (breccia 79035) yield the same value of 13.7 for the 20Ne/22Ne ratio in both of these samples ( WIELER et al., 1993; BENKERT et al., 1993), suggesting that there has been no substantial change in the SW Ne isotopic composition. Using this value to define the isotopic composition of SW Ne for both epochs, we can locate the SW endmembers for each of the two trend lines, corresponding to the 4He/22Ne ratios of the two lines extrapolated to the SW 20Ne/22Ne value (Fig. 1). This suggests that helium was enhanced by about a factor of two ( 1.8) in the ancient SW compared with the modern SW. This enhancement is confirmed by the 4He/ *‘Ne ratios at the ordinate intercepts of Fig. 4, corresponding to minimal exposure for each data set. The 4He/36Ar ratios are also elevated by the same factor in the ancient SW, demonstrated by the intersections of the trend lines in Fig. 2 with the (20Ne/22Ne)sw value of 13.7. Therefore, we conclude that the He/Ne and the He/Ar ratios in the SW about 1 Ga ago were significantly higher than it is today, probably by a factor of about 2. No evidence for a similar variation in the Ne/Ar ratio is observed (Fig. 5) since there is no obvious distinction

1036

R. H. Nichols Jr., C. M. Hohenberg, and C. T. Olinger

between the data fields of these two soils. We note here that the helium enhancement in the ancient solar wind can perhaps best be directly observed by the analyses of single grains because stepwise pyrolysis, closed-system etching, and all other collective studies on bulk ilmenite separates are dominated by the noble gas compositions of the most gas-rich grains, which are clearly depleted in helium relative to the other gases (Figs. 1, 2, and 4). The lightly exposed grains provide the most insight into the true source compositions. Gas-Loss and Elemental Fractionation of Helium from Neon and Argon Despite the isotopic and elemental differences between the soil and the breccia, both data sets display qualitatively similar trends reflecting the progressive modification of the implanted solar gases. An important question is whether gas loss, the addition of an SEP component, or a combination of these mechanisms is primarily responsible for .the isotopic trends away from the SW endmembers, which plot from the upperright ends of the trend lines in Figs. 1 and 2 toward modified compositions at the lower-left. The decrease in the He/Ne and He/Ar ratios with increasingly heavy neon, shown in these figures, is probably not due simply to the addition of an SEP component with lower He/Ne and He/Ar ratios for two reasons. First, the noble gas elemental compositions of the SEP component have been reported by some workers to be essentially solar ( WIELER et al., 1993), although the pulsedheating experiments of NIER and SCHLUTTER ( 1993) provide preliminary evidence for non-solar He/Ne ratios in the SEP component. Second, the trends in the He/Ne and He/Ar ratios follow exposure (Fig. 4) which suggests that they are dose-related and may be driven by the diffusive loss of helium. While the diffusive loss of helium in lunar samples has been noted by other workers (cf. KERRIDGE et al., 199 la), and is clearly indicated by the present data, evidence for diffusive losses and the corresponding isotopic fractionation of neon in lunar ilmenites is not as apparent. It is harder to quantify losses of neon relative to argon since the 20Ne/36Ar ratio for the contemporary solar wind is not known very well, with values ranging from -28, from the Solar Wind Composition Experiment (SWCE; GEISS et al., 1972) to -45 (cf. ANDERS and GREVESSE, 1989; cf. BENKERT et al., 1993). Whereas the majority of the grains in both data sets display Ne/Ar ratios that could fall within the rather large uncertainty of the contemporary SW ratio if centered around a value of 28 (Fig. 5), the more recent value for the SW Ne/Ar ratio of -45 would suggest that some neon may indeed have been lost relative to argon in these grains. There is no obvious trend in Fig. 5 that would indicate a progressive and systematic neon loss (and corresponding isotopic fractionation) with increasing “Ne concentration (a measure of the exposure), such as that seen for helium (Fig. 4). There also appears to be a large amount of scatter in the observed Ne/Ar ratios which is well beyond analytic uncertainty. That there is no systematic decrease of the Ne/Ar ratios with exposure, such as that seen for the He/Ar ratios, seems to preclude diffusive loss as the dominant mechanism shaping the isotopic compositions of neon which do vary with exposure. Although conditions near the surfaces are quite complicated, some of

the major features in Figs. 4 and 5, and the differences between them, seem best explained by two different loss mechanisms both important for lunar ilmenites: a fractionating (diffusive) loss and a non-fractionating loss mechanism. However, the relative importance of these two mechanisms differs from gas-to-gas. The question is not whether the SW Ne and Ar in the outermost layers of the grains have been diffusively fractionated at all, but whether such effects dominate the neon and argon inventories of the individual grains. Diffusive neon losses may well have occurred, as evidenced by the generally depressed Ne/Ar ratios compared to the most recent SW Ne/ Ar ratio (Fig. 5 ). The initial closed-system acid-etching steps reveal mildly fractionated elemental ratios, but a normal (SW) Ne isotopic composition ( WIELER et al., 1993; BENKERT et al., 1993). This implies that the SW gases in the outermost layers may have indeed experienced some diffusive loss. In contrast, the data of BECKER and PEPIN ( 1989) show that the low-temperature extractions in stepwise pyrolysis from both 79035 and 7 150 1 have essentially solar elemental compositions which would suggest little diffusive modification of the most lightly bound contributions; this is also a conclusion supported by EBERHARDT et al. (1970) and FRICK et al. ( 1988) for other bulk ilmenites. The Ziirich group, however, has suggested that the initial BECKER and PEPIN ( 1989) pyrolysis steps may in fact represent an overprint of diffusion, favoring the light elements, during the low-temperature laboratory release thereby making the measured near-surface abundances appear elementally lighter than they actually are ( KERRIDGE et al., 199 1a). These initial releases, however, represent only a small fraction of the solar gases present and little difference exists for the total inventory for all three techniques. Stepped-combustion. stepped-etching, and single-grain volatilization yield Ne/Ar ratios for the total extractions that are essentially the same (Fig. S), which may or may not be that of unfractionated solar wind (depending the actual value of the SW Ne/Ar ratio). These observations reinforce the previous conclusion based upon the single grain data alone: no single process has occurred which has resulted in large and systematic fractionation of neon from argon in bulk ilmenite samples of either soil, nor in the individual ilmenite grains. If significant losses of neon and argon have occurred it is not by strictly diffusion but by another mechanism as well which does not lead to elemental or isotopic fractionation, i.e., a process that removes neon and argon from grain surfaces in a democratic, mass-independent, manner. Non-Element-Specific Losses of Neon and /or Argon from the Outer Layers of the Grains? In evaluating whether non-fractionating loss processes are at work it is important to consider the physical state of the most heavily exposed grain surfaces. The flux of solar wind hydrogen and helium at the lunar surface is known from the SWCE flown on a number of the Apollo missions ( GEISS et al., 1972). For a nominal hydrogen implantation depth of about 200 Angstrams ( TAMHANE and AGRAWAL, 1979), and if all the hydrogen were retained, there would be one implanted solar wind proton for each host lattice atom after

Noble gases in lunar ilmenite grains

-20 years of surface exposure. Siniilarly, one helium atom per host atom would be present after -300 years, yielding a “He surface concentration of about 5 X 10e3 ccSTP/cm’. A helium saturation time of 300 years was, in fact, inferred by ERERHARDT et al. ( 1970) from the observed surface concentration in another collection of lunar ilmenites. The more heavily exposed ilmenite grains from our particular populations (Tables A 1, A2) also have estimated average surface concentrations of 4He 2-5 X 10m3 ccSTP/cm2 (depending on the grain geometry assumed). Thus, the near-surface regions of the most gas-rich grains contain nearly one implanted SW He atom per host lattice atom, confirming that the helium is near saturation and plausibly subject to loss (as independently inferred from the elemental ratios considered above). Although this calculation clearly neglects the effects of inward diffusion of helium in the ilmenite, which would tend to reduce the helium concentration in the lattice, gas ion-probe data shows that little inward diffusion has actually occurred since the peak helium release is still at about 200 Angstriims (KIRSTEN, 1977; MOLLER et al., 1976). The average neon concentrations on the surfaces of the most heavily exposed grains are about 1.5-5 X 10m6 ccSTP/ cm2 which, based on the SW Ne fluxes from the SWCE (GEISS et al., 1972), correspond to integrated surface exposure times of 300- 1000 years. These times are shorter than the integrated surface residence times of individual grains deduced from SF tracks and cosmic ray effects (- 103-lo4 yr.; cf. GOSWAMI et al., 1984). Some regolith gardening models suggest that up to -76 of the particles may have been exposed on the surface for up to 100 times the saturation time scale of SW Ne (DURAUD et al., 1975; CROZAZ, 1972; GAUL~ et al., 1974). Therefore, it seems inescapable that many of the grains have been exposed on the lunar surface for times considerably longer than the saturation time for neon. An important question then is what happens to the mineral lattice under such exposure conditions. Although hydrogen is perhaps less effective than helium in causing radiation damage (cf. BORG et al., 1983), hydrogen is present in the SW at lo-20 times the abundance of helium and is also chemically active. Both the quantity of hydrogen and its chemical activity are potentially important. When the host ilmenite lattice becomes dominated by implanted alien species. which clearly occurs on the heavily exposed portions of grain surfaces, the structure ceases to be ilmenite. The implication here is that the most heavily exposed portions of grain surfaces must be highly disordered, chemically altered, and consequently quite prone to the effects of surface comminution by micro-impacts and micro-flaking (caused by extreme lattice disorder at the end of the hydrogen and helium implantation range). These processes are to be distinguished from atomic sputtering which can cause isotopic fractionation of the major lattice constituents on the grain surface by scattering (e.g. KERRIDGE and KAPLAN, 1978). Sputtering cannot reach the more deeply implanted solar wind species until the overlying material is sputtered away, and the atomic sputtering of mineral surfaces occurs at relatively low rates (co.05 A/yr.; cf. FRICK et al., 1988; KERRIDGE, 199 I ). It would take more than 4000 years to sputter away the 200 A SW implantation zone which is far longer than the inferred saturation times. Thus atomic sputtering

1037

could not seriously affect the SW implantation zone in the available SW saturation time. Micro-flaking (or “micro-blistering”; BORG et al., 1983), on the other hand, may occur when the population of the implanted species approaches or exceeds that of the host, resulting in the loss of both lattice definition and presumably the physical integrity of the grain surface. at which time collective loss of the surface material can occur. Preliminary TEM studies of ultra-microtomed thin-sections of several gas-rich ilmenite grains from the 7 1501 soil indeed show evidence for lattice disorder near the grain surfaces. We observe contrast speckling caused by diffuse scattering from radiation damaged regions that is most marked in the first several hundred AngstrBms, but extends to depths of up to 1000 Angstrams. Convergent beam diffraction studies also reveal near-surface diffuse scattering caused by radiation damage. In addition, Fe/Ti ratios are quite variable among the near-surface regions of different contrast (with atomic Fe/Ti changing by factors of more than 2 both above and below the value of unity measured on the grain interiors), indicating that chemical reduction of iron by SW protons and that surface restructuring of the ilmenite has occurred (T. Bernatowicz, pers. commun.). Thus, the chemical/physical state of grain surfaces, the measured surface concentrations of noble gases, and the systematics observed in Figs. 4 and 5 suggest that in addition to diffusion, a non-fractionating loss mechanism, probably surface micro-flaking, must be at work. We should also consider processes other than micro-flaking that tend to limit the accumulated noble gas surface concentrations. Grains exposed on the lunar surface are coated to varying degrees with fine-grained dust particles that can intercept a fraction of the low energy SW before it reaches the ilmenite surface itself. WIELER et al. ( 1983) noted that microscopic black glasses still contaminate the ilmenite surfaces even after processing in the laboratory. While these may be glass splashes from micro-impacts (cf. HARRING, 1980) that stuck sufficiently well to survive the handling, there may also have been dust bound to the surface of these grains by electrostatic or van der Waals forces. KERRIDGE and KAPLAN ( 1978) have also suggested that vapor-deposited material from meteorite impacts could produce layers of up to 200 Angstrbms thick on the surfaces of the grains. Surface coatings of silicate glasses are indeed observed in ultra-microtomed thin-sections of gas-rich ilmenite grains from the 7 I50 1 soil (T. Bernatowicz, pers. commun.). One significant difference between these two mechanisms (surface loss and surface shielding) is that a protective dust coating may be equally effective for mature and immature grains alike, whereas nonfractionating losses (erosion or micro-flaking from the nearsurface zone) are driven by processes that increase in effectiveness with exposure. The latter trend is observed in the data. The effect of retained glass splashes is limited by the fact that they occur only on localized portions of the grain surfaces, leaving much of the surface exposed to solar radiation. Although helium and neon diffuse much faster in glass than in ilmenite, such retained coatings would not substantially reduce the surface concentrations nor help enhance the SEP contribution because the bulk of the SW in these coatings is also measured. The present data suggest that non-fraction-

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R. H. Nichols Jr., C. M. Hohenberg, and C. T. Olinger

ating surface losses are more important than glass or dust coatings since the observed trends favor heavy neon (enhanced SEP/SW ratios) in the more heavily exposed grains. Under conditions driven by diffusion, the rate of diffusive loss of helium far exceeds that of neon, and both diffuse more readily than argon, so that the retained He/Ne ratios evolve more quickly than the Ne/Ar ratios. The reason that the observed He/Ne ratios are lower than the SW ratio, and correlated with exposure duration (Fig. 4), is at least in part due to the preferential diffusive loss of helium. Diffusive loss and the accompanied fractionation alone, however, cannot explain the heavier gas data, which require the presence of the second, non-fractionating loss mechanism, i.e., “micro-flaking”. If the timescale for diffusive loss of a SW He atom from the surface zone is much less than that for non-fractionating physical loss of the surface (micro-flaking), then the grain can suffer a progressive decrease in the He/Ne ratio since the major loss mechanism for helium is diffusion. If, on the other hand, the timescale for diffusive loss of a SW Ne atom from the surface generally exceeds that for surface removal by non-fractionating micro-flaking, a progressive trend in the Ne/Ar ratio may not be present since the major removal process for neon is non-fractionating. The variations in the He/Ne and Ne/Ar ratios that are present in Figs. 4 and 5 plausibly reflect the presence of these two competing processes. Diffusive losses tend to reduce both the He/Ne and the Ne/Ar ratios in the surface material but as exposure continues to the point of surface loss the material containing the most highly fractionated SW is removed, exposing a fresh surface and less fractionated bulk elemental ratios. The downward trends in the He/Ne and Ne/Ar ratios caused by diffusive losses are punctuated, at random intervals, by surface flaking, producing upward shifts toward the SW values. Superposition of these two effects, which are in opposite directions (one continuous and the other stochastic), may account for both the trends and the scatter in Figs. 4 and 5. Since diffusive losses are progressively greater for helium, neon, and argon, both the He/Ne and the Ne/Ar ratios observed in these grains should generally be less than solar. However, the extent of this loss is unclear for neon, due in part to the uncertainty in the modern SW Ne/Ar ratio. It should be pointed out, however, that the most recent estimate of the SW 20Ne/36Ar ratio of about 45 (cf. ANDERS and GREVESSE, 1989) is also the highest and would suggest a -43% average reduction in the retained Ne/Ar ratio for both the modern and ancient soil samples relative to modern SW solar. This is true for both the averages of the individual grains (this work) and the bulk ilmenites (Ziirich and Minnesota). Such a fractionation of neon from argon, which differ in mass by a factor of 2, is of the same order as that suggested by helium and neon (Fig. 4), which differ in mass by a factor of 4 and thus is seemingly in conflict with simple diffusion as the dominant neon loss mechanism. Neon Isotopic Trends Referring back to the isotopic differences between modern soil 7 150 I and ancient soil breccia 79035, we noted that ilmenites from the ancient soil contained neon that was 2-3% heavier than those from the modern soil. We also noted that the ancient solar wind was enhanced in helium, relative to

neon (and possibly argon), by about 180%. In addition, the quantity of solar helium is greater in the ancient soil (Table 1). If surface removal by a non-fractionating process, enhanced by a disordered lattice, is responsible for the isotopically heavy neon observed in these soils, then an enhanced helium flux (and possibly hydrogen) in the ancient solar wind may well enhance the rate of surface removal from the ancient soil. This enhancement in helium flux coupled with the reported 5-8X longer maturity of the breccia (cf. BENKERT et al., 1993) suggests an effective surface exposure for the breccia that is clearly longer than the effective exposure for the soil. The resulting higher rate of surface removal lowers the contribution of the near-surface (SW) species with respect to the deeper (SEP) species, resulting in heavier retained neon in the older soil. For this reason we cannot conclude as others have done (BECKERand PEPIN, 1989; WIELER et al., 1983: BENKERT et al., 1988) that the ancient solar corpuscular radiation must either have been isotopically heavier in SW Ne or had an SEP/SW ratio that was enhanced relative to the present. Observation of heavier neon in the older soil can simply mean a higher rate of surface removal due to an enhanced helium flux on that soil. We therefore concur with both Ziirich and Minnesota that the ancient breccia has heavier neon, but we do not agree that this necessarily implies secular change either in the SEP/SW flux ratio or in the neon isotopic composition. We have argued that processes other than diffusive loss are likely to be responsible for the elemental and isotopic structure of noble gases heavier than helium in lunar ilmenites. Although the scatter and lack of obvious trend in Fig. 5 and other considerations presented above argue against massive diffusive losses of neon, it is important to consider the degree of diffusive gas loss that would be required to account for the observed neon isotopic trends. Simple Rayleigh distillation predicts a maximum isotopic fractionation of 4% if most ofthe diffusive loss occurs during the implantation itself. Given the steep thermal gradient at the lunar surface, gas loss during implantation itself is more likely to occur than loss during residence in a relatively cool subsurface site. This yields a minimum retained (20Ne/2’Ne)SW ratio of 13.2. significantly higher than many of the ratios observed in the single grains. Diffusive loss subsequent to implantation can indeed produce lower ratios but it is a very inefficient way to enrich the heavy neon (e.g. BECKER, 1990). For instance, it would require the loss of more than 90% of the implanted neon to produce a 20Ne/2’Ne ratio as low as the heaviest neon composition observed in these ilmenite grains and it would require loss of more than 98% to produce a ‘“Ne / 22Ne ratio of 11.2, the lowest value observed in the stepped-etching experiments. Moreover, the breccia grain with the lowest 20Ne/22Ne ratio has a 2oNe/3hAr that is higher than the breccia average and there is no correlation between these values in either population (compare Figs. 3 and 5 ). The simple diffusive loss model introduced above ignores any addition of fresh source neon and inward diffusion, both of which tend to limit the range of isotopic compositions possible in the retained neon (BECKER and PEPIN, 1989; FRICK et al.. 1988), and make diffusion even less viable as the major mechanism shaping the Ne isotopic structure. Furthermore, the closed-system etching experiments of WIELER et al. ( 1986) show that the heavy neon component lies deeper

Noble gases in lunar ilmenite grains

below the surface, beyond the range of SW Ne. This, in itself, does not preclude the diffusion model since inward diffusions could occur. The fact that the heavy neon accounts for about one-half of the total neon implanted in the more heavily exposed ilmenite grains (as discussed below) and the fact that the lowest 20Ne/22Ne ratios are unreachable by diffusion (BECKER, 1990) argue convincingly that this component cannot be the tail end of a diffusively driven SW reservoir. Thus, as stated both by the Zurich and Minnesota groups, solid-state diffusion alone cannot explain the isotopic results observed for neon trapped in these minerals; a separate SEP component is required. When examined on a grain-by-grain basis in the present study, a dominant role for diffusion in establishing either the observed neon isotopic structure or the noble gas elemental ratios (other than helium) are no longer required. The Retained SEP/SW SEP/SW Flux Ratio

Ratio and Limits on the

Due to the presence of complex surface processes such as non-fractionating loss effects and the diffusive loss and redistribution of helium, the data as shown in Fig. 1 cannot be interpreted as simple mixing between two pure endmember components. Nonetheless, two distinct solar neon components, SW and SEP, are required to explain the data. The trend lines in Figs. 1 and 2 are populated by lightly exposed grains near the SW endmembers (which are different for the soil and the breccia) and heavily exposed grains, containing a larger contribution of isotopically heavy neon, which we will take to be a superposition of SW and SEP components, and elementally depleted helium. While the Ne/Ar ratios are below some estimates of the SW composition, they do not correlate either with exposure, with the progressive decrease in the 2oNe/2’Ne ratio, or with helium loss. We shall therefore assume for the present exercise that the SW Ne contribution to the heavy neon endmember is not appreciably isotopically fractionated, having a 2r’Ne/22Ne ratio of approximately 13.7. The other contribution comes from SEP Ne with a 20Ne/ “Ne ratio of 1 1.2 ( WIELER et al., 1986; BENKERT et al., 1993). The specific mixtures of these two neon components required to produce the neon isotopic ratios provide reasonable estimates for the contributions of SW Ne and SEP Ne retained in the individual ilmenites. The most heavily exposed grains in Figs. 1 and 2 exhibit a 20Ne/22Ne ratio of about 12.5. When partitioned between the SW and the SEP endmembers, the 12.5 value yields a retained SEPNe/SWNe ratio of - 1.O. Even fractionated SW with a 2oNe/22Ne ratio of 13.2 would yield a retained SEP/ SW ratio of 0.54. These values are clearly much higher than estimates for the incoming flux ratio (@(SEP,,)/@( SWN,) -0.02-0.07, WIELER et al., 1986) and high enough that the gas ion-probe analyses of KIRSTEN (1977) and MOLLER et al. ( 1976) might have shown a distinct SEP contribution at depths below the SW implantation depth if heavily exposed grains had been examined. The gas ion-probe data do suggest that the peak helium release from lunar ilmenites corresponds to an implantation depth of a few hundred Angstroms, typical of SW species, implying that the SW implantation profile has generally been preserved and that the SW contribution generally dominates the near-surface solar gas inventory. How-

1039

ever, the SEP component is clearly enhanced above its influx proportion in the most heavily exposed grains by mechanisms, such as those proposed above, that limit the accumulation of SW Ne. That the gas ion-probe data of KIRSTEN (1977) and MOLLER et al. (1976) do not show significant helium concentrations at a depth greater than the SW implantation depth may indicate either that more lightly exposed surfaces were selected, such as those plotting closer to the SW composition in Figs. I and 2, or that the range of SEP is sufficiently long that its contribution, spread over a large depth range, makes its specific presence undetectable in the gas-ion probe profile. The neon compositions of the lightly exposed grains are likely to represent mixtures of SW and SEP Ne that are more representative of the primary particulate influx from the sun because these grains have been least affected by the mechanisms that limit the accumulation of SW species. These grains, however, have compositions that are within error of the SW composition. The isotopic compositions of neon released in the initial etching steps ( BENKERT et al., 1993) and in the initial combustion steps (BECKER and PEPIN, 1989) of the modern soil 7 150 1 ilmenites are also indistinguishable from modern SW composition as measured in the capture foils of the Apollo SWCE ( GEISS et al., 1972). The foils will capture both SW and SEP unless the SEP component only accompanies flares, implying that the neon isotopic composition determined from the foils will be a superposition of SW and SEP contributions. Because the initial etching and combustion steps show neon compositions that are, within error, equivalent to the SW foil composition, we are led to conclude that the actual SEP/SW flux ratio must be lower than a few percent in the solar particle stream, whereas the most heavily exposed ilmenites have retained Ne SEP/SW ratios of about 1:l. CONCLUSIONS Differences in noble gas compositions are evident between the recently irradiated soil 7 150 1 and the earlier-exposed breccia 79035. In Figs. 1 and 2 the 7 150 1 data trend between a contemporary solar wind composition and a component with isotopically heavy neon and low He/Ne ratios (reflecting helium loss). The soil breccia, however, requires an ancient solar wind that is richer in helium by a factor of about 1.8. Although helium has been lost from these grains (Figs. 1, 2, and 4), the lack of observed trends in the Ne/Ar ratios and the great variability of those ratios argue that neon has not been systematically fractionated from argon and that the neon isotopic structure is probably driven by non-fractionating surface losses (micro-flaking) rather than by diffusive modification. Observation of isotopic differences in neon between the ancient and modern soils do not necessarily imply a secular change in either the neon isotopic composition or the SEPNe/SWN, flux ratio as previously reported since the elevated helium abundance in the ancient solar wind may well have increased the rate of surface loss, relative to the modern soil, thereby elevating the retained SEPN,/SWN, ratio. Acknowledgments-We

wish to thank Dr. J.-P. Benkert for graciously sharing his mineral separates with us, as well as Drs. R. Wieler, R. H. Becker, and A. 0. Nier for many helpful discussions. We also acknowledge R. Wieler and A. 0. Nier for their prompt help with the calibration of the Springwater and Grant standards. We thank Drs.

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R. H. Nichols Jr., C. M. Hohenberg, and C. T. Olinger

R. Wieler, R. H. Becker, and J. F. Kerridge for their insightful and thorough reviews. We also express our gratitude to K. Kehm for aid during sample measurements. This work was supported, in part, by NASA graduate student fellowships NGT-59854 and NGT-50194 and by NASA grant NAGW-3344. Editorial handling: K. Marti

REFERENCES ANDERSE. and GREVESSEN. (1989) Abundance of the elements: Meteoritic and solar. Geochim. Cosmochim. Acta 53, 197-2 14. ARVIDSONR. et al. ( 1976) Cosmic ray exposure ages of Apollo I7 samples and the age of Tycho. Proc. 7th Lunar Sci. ConJ, 28 l72832. BECKERR. H. ( 1990) Model calculations of solar wind and SEP neon isotopic distributions in lunar regolith grains. Lunar Planet. Sci. Conf XXI, 56-57 (abstr.). BECKERR. H. and PEPIN R. 0. (1989) Long-term changes in solar wind elemental and isotopic ratios: a comparison of two lunar ilmenites of different antiquities, Geochim. Cosmochim. Acta 53, 1135-l 146. BENKERTJ.-P. ( 1989) Solare Edelgase bei stufenweisem Atzen von Ilmeniten und Pyroxenen aus dem Mondregolith. Ph.D. thesis, ETH Zurich (Nr. 8812). pp. 127. BENKERTJ.-P.. BAUR H., PEDRONIA., WIELERR., and SIGNERP. ( 1988) Solar He, Ne and Ar in regolith minerals: All are mixtures of two components. Lunar Planet. Sci. Con/: XIX, 59-60 (abstr.). BENKERTJ.-P. et al. ( I99 1) Evolution of isotopic signatures in lunarregolith nitrogen: Noble gases and N in ilmenite grain-size fractions from regolith breccia 79035. Lamar Planet. Sci. Con{ XXII. 8586 (abstr.). BENKERTJ.-P., BALJRH., SIGNERP., and WIELERR. ( 1993) He, Ne and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes. J Geophvs. Res. 98, 13,147- 13.162. BORC;J. et al. ( 1983) A model for the accumulation of solar radiation damage effects in lunar dust grains, based on recent results concerning implantation and erosion effects. Proc. 13th Lunar Planet. Sri. Cont. in J. Geonhvs. Res. 88. A725-A730. CERUTTI H. ( 1974) Die ~Bestimmung des Argons im Sonnenwind aus Messungen an den Apollo-SWC-Folien. Ph.D. thesis, Univ. Bern. CLAYTONR. N. and THIEMENSM. H. (1980) Lunar nitrogen: evidence for secular change in the solar wind. Proc. Conf: Ancient Sun (ed. R. 0. PEPIN et al.), 463-473. CROZAZG. ( 1972) Proc. 3rd Lamar. Sci. Cor$. 29 17-293 I. DURAUDJ. P., LANGEVINY., and MAURETTEM. ( 1975) The simulated depth history of dust grains in the lunar regolith. Proc. 6th Lunar. Sci. Con/.‘, 2397-2415. EBERHARDT P. et al. ( 1970) Trapped solar wind noble gases in Apollo 12 lunar fines 1200I and Apollo I 1breccia 10046. Proc. 3rd Lunar Planet. Sci. Cont.‘, 1821-1856. ETIQ~JEP. ( I98 I ) L’utilisation des plagioclases du regolith lunaire comme detecteurs des gaz rares provant des rayonnements corpusculaires solaires. Ph.D. thesis, ETH Zurich. FRICKU., BECKERR. H., and PEPINR. 0. ( 1988) Solar wind record in the lunar regolith: Nitrogen and noble gases. Prac. 28th Lunar Planet. Sci. Conf, 87-l 19. GAULTD. E.. HORZ F.. BROWNLEE D. E., and HARTUNGJ. B. ( 1974) Mixing of the Lunar Regolith. Proc. 5th Lttnar Planet Sci. Con/;, 2365-2386. GEISSJ. et al. ( 1972) Solar wind composition experiment. In Apollo 16 Prelim. Sci. Report; 14-l to 14-10. NASA SP-315. GOSWAMIJ. N., LAL D., and WILKENINGL. L. (1984) Gas-rich meteorites: Probes for particle environment and dynamical processes in the inner solar system. Space Sci. Rev. 37, 1 I I- 159. GRAFT. et al. ( 1987) Depth Dependence of “Be and 26AIProduction Rates in the Iron Meteorite Grant. Nucl. Inst. Meth. Phyx Res. B29, 262-265. HARTUNC J. B. ( 1980) Lunar rock surfaces as detectors of solar processes. In Proc. Conf Ancient Snn (ed. R. 0. PEPINet al.), 227243. HOHENBERGC. M. ( 1980) High sensitivity pulse-counting mass

spectrometry system for noble gas analysis. Rev. Sci. Instrum. 51, 1075-1082. HOHENBERGC. M., NICHOLSR. H. JR., OLINGERC. T., and Gos. WAMIJ. N. (1990) Cosmogenic neon from individual grains of CM meteorites: Extremely long pre-compaction exposure histories or an enhanced early particle flux? Geochim. Cosmochim. Acta 54,2133-2140. KERRIDCEJ. F. ( 1975) Solar nitrogen: Evidence for a secular increase in the ratio of nitrogen-15 to nitrogen-14. Science 188, 162-164. KERRIDGEJ. F. ( I99 1) A re-evaluation of the solar wind sputtering rate on the lunar surface. Proc. 21.~ Lunur Planet. Sci. Conf, 30 I306. KERRIDGEJ. F. and KAPLAN1. R. ( 1978) Sputtering: Its relationship to isotopic fractionation on the lunar surface. Proc. 9th Lunar Planet. Sci. Con/:, l687- 1709. KERRIDGEJ. F. et al. ( I99 I a) Long term changes in composition of solar particles implanted in extraterrestrial materials. In The Sun in Time (ed. C. P. SONNETTet al.), pp. 389-412. University of Arizona Press. KERRIDCEJ. F., IOM J. S., KIM Y., and MARTIK. ( 199lb) Evolution of isotopic signatures in lunar-regolith nitrogen: Noble gases and grain-size fractions from regolith breccia 79035. Proc. Lunar Planet. Sci. Conf.’22, 2 15-224. KIRSTENT. ( 1977) Rare gases implanted in lunar fines, Phil. Trans. Rol,. Sot. London 285, 391-395. MEGRUE G. H. (1968) Rare Gas Chronology of Hypersthene Achondrites and Pallasites. J Geophvs. Res. 73, 2027-2033. MEWALDTR. A., SPALDINGJ. D., and STONEE. C. ( 1984) A highresolution study of the isotopes of solar flare nuclei. Ap J. 280, 892-901. MOLLERH. W. et al. ( 1976) Rare gas ion probe analysis of helium profiles in individual lunar soil particles. Proc. 7th Lanar Planet. Sci. Conf, 937-95 I NAUTIYALC. M., PADIAJ. T., RAO M. N., and VENKATESANT. R. ( I98 1) Solar flare neon: Clues from implanted noble gases in lunar soils and rocks. Proc. Lanar Planet. Sci. Conf: 12B, 627-637. NAUTIYALC. M., PADIAJ. T., RAO M. N., and VENKATESAN T. R. ( 1986) Solar flare neon composition and solar cosmic-ray exposure ages based on lunar mineral separates. Astrophvs. J. 301,465-470. NIER A. 0. and SCHLUTTERD. J. ( 1993) Extraction of He and Ne from individual lunar ilmenite grains by pulsed heating. Meteoritics 28, 4 I2 (abstr.). OLINGERC. T., MAURETTEM.. WALKERR. M., and HOHENBERG C. M. ( 1990) Neon measurements of individual Greenland sediment particles: Proof of an extraterrestrial origin and comparison with EDX and morphological analyses. Eurth Planet. Sci. Left. 100,77-93. PEPIN R. 0. (1980) Rare gases in the past and present solar wind. In Proc Cont. Ancient Sun (ed. R. 0. PEPINet al., 41 I-421. RAO M. N. et al. ( 199 I ) Composition of solar flare noble gases preserved in meteorite parent body regolith. J. Geophys. Rex; Space PhJ:sic.s 96, 1932 I - 19330. SCHULTZL. and KRUSEH. ( 1989) Helium. neon and argon in meteorites-A data compliation. Meteoritics 24, 155-l 72. SIGNERP. and NIER A. 0. ( 1960) The Distribution of Cosmic-Ray Produced Rare Gases in Iron Meteorites. J. Geophvs. Re.s. 65, 2947-2964. VON STEICERR. and GEISSJ. ( 1989) Supply of fractionated gases to the corona. Astron. Astroph)s. 225, 222-238. TAMHANEA. S. and ACRAWALJ. K. ( 1979) Diffusion of rare gases of solar wind origin from lunar fines as bubbles. Eurth Planet. Sci. Left. 42, 243-250. WIDING K. G. and FELDMANU. ( 1989) Abundance variations in the outer solar atmosphere observed in skylab spectroheliograms. Astrophys. J. 344, 1046- 1050. WIELERR., ETIQUEPH ., and SIGNERP. ( 1983) Decrease ofthe solar flare/solar wind flux ratio in the past several Aeons deduced from solar neon and tracks in lunar soil plagioclases. Proc. 13th Lanar Planet. Sci. Con/: and J. Geophys. Res. .rnppl. 88, A7 13-A724. WIELERR., BAURH., and SIGNERP. ( 1986) Noble gases from solar energetic particles revealed by closed system stepped etching of lunar soil minerals. Geachim. Cosmochim. Acta 50, 1997-2017. WIELERR.. BAURH., and SIGNERP. ( 1993) A long term change of the Ar/Kr/Xe fractionation in the solar corpuscular radiation. Lunar Planet Sci. Con/: XXIV. 15 19- 1520. (abstr.).

1041

Noble gases in lunar ilmenite grains APPENDIX

Table Al. He, Ne and Ar from 71501 ilmenite grainst~ Grain Mass L4Hel 31WdHe 120Ne1 2'Ne,%e 22N&Ne Xl00

10-6 ccSTP/g

_.___ 300.3

""75 0.002

XIWJ

“._“_

0x1( 0007

Xl00

.

747, 0029

202

33

405.7

0.075

99.4

7436 0.037

4.2

7.1

0003 0.272

0328 0.006

203

3.6

3 107 0121

10.359 0.224

204 205

4.2 27

,839

0.267 owl

7670 "028

2.7

7 132 0014

520

19 12

0.6

55.95 0.43

345

I7

41Y3.5

0.001 0.035

0255 0002

1351.6

0.25, 0.002

7840 0.016

56.5

I9 29

0.001 0036

917.3

0.255 0.002

7.817 0017

609

19.12 0.04 -

IO1

I5

39240

15110

0254 0.#2

7.821 0.013

81.0

1908 OW

102

2 X

llrn7

0253 O.001

7.873 0.014

650

19 I5

IO3

1,

294

213

4.1

46.9

2141

3.5

3794.6

215

l.,

3011X.2

0.001 0032

1096.9

7.631 0023

380

19

2465.0

0001 0.043

0.248 0002

216

843.3

7.6d5 0021

28.1

221

I7

3117.4

O.COI 0.033

0.245 ow3

1438.5

406&l

0.001 O.MZ

305.2

303

34

36300

om3 0.037 0001 0.035

13618

cO.Ool 0.049

305

15

677.1

0.001 0.044

306

2.2

4.489.3

OWd 0 034

311

2.9

2317.2

2.7

34404

313

28

3122.1

324

IX

5132

315

I8

316

0.034

369.3 1420.6 8592 92.8 11357 1649.6 362.4 1972 20827 155.9 1869.1 1323.5

973.4

2.0

ZO4Y.O

O.Nl 0035

9157

321

111

12.5

322

I.8

31418

323

2.4

3200.6

0158 0038 OWL 0.037

9.3 1195.1

27x33

0001

6X.2

0.248 0.031

7.750 0.014

45.1

0.246 0003 0.360 0010 0.249 o.w2 0.249 O.Ml

7.601 0.017 7.639 0.016 7.714 0013 7.714 0.013

28.2

24.7

5.8 42.1 59.1 17.4

0.256 0.004

7.503 0.018

0.270 ON5 0249 0002

7.521 0022 7703 0.012

0249 0002 0,249 O.OOl 0.251 Ooo2 0.253 0004 0253 ow2 0249 0003

7.926 0015 7.%dd 0013 7,703 0014 7.896 0.021

X3.8

7710 0.019 1.759 0.017

0905 0.041

8WJ 0.168

I.5

19.47 OW 19% 001 IPII 0.01 19.18 0.06 I908 005 19 II 005 26.41 0.08 19.w OM 19.09 0.04

,,I

I.8

II2

24

0251 0002 0.257 ON3

7535 0020

.-

-

7.485 003,

-

113

2.2

-

0245 0.002 0249 OM3

-

.-

-

-

0.245 0031 0.244 om2

7.599 0.020 7f2I 0021 7489 0.019 7.540 0.017

-

-

914.3

682.3

O.001 0.067

221.4

3051.8

0.002 o.Od2

1133.2

0.001 Ii4

2.7

1389.9

II?

I.2

1767 5

0048 O.Wl 0 04x

,216.X

2686.3

OWI 0.052

1081.7

_

-

600.1

0.001 0 095

,191

0.266 0005

1431 0027

-

-

2191.5

0.248 ON2 0248 0003 0.248 0003 0.252 0002 0.250 0.003

7.732 00,X

-

-

1454 0.018

-

-

7549 0.019 7.461 0019

-

-

II6

4.1

Ill

I6

II8

1.6

42109

oco2 0043

I19

30

IYY9.1

0.001 0048

1428.5

O.(K)1 Of45

16

1274.2

6.5

20.17 0.15

12,

2 I

1914.0

0001 0.056

65.4

I9.05 004

I22

21

7166.5

0001 0045

19.18 003 19.14 0.04 19.04 00s 19 19 0.05

123

26

1129.9

Oool O.Od8

312.8

124

20

2560.9

0.0, 0.046

769.4

39.2 472

64.6 47.0 31.0

0251 Oco3

7.577 OO21

20

%I.4

0241 0.0113

7727 0017

51.2

635.7

0.244 0.003 0245 0003 0252 0002 0247 0003 0248 0.002 0251 0.003

7382 0018 7426 00,X

I?,,

659 I

0249 "ccl3 0245 0003 "245 0002

7 305 0023 7503 0,019

0.258 OOW 0.245 0003 0249 ow3 0 249 0002

7343 0024 71183 0.022

,810."

I9

24931

O.WJl 0.055

19O4 0.05

127

2.1

2639.5

19.15 0.05

I28

l.2

3476.3

2551 0.30

129

25

1511.4

O.WI 0051 O.WI 0042 0001 0 OS4

19.17 0.05 19.16 0.04 19.08 0.04 19.m a.05

I30

2 I

2W7.3

0001 0042

I31

23

23390

132

25

22Y4.3

OCOI OO4d "WI OW6

I33

25

1686.0

"WI 0.054

379.8

19.12 O.lx 19.12 0.06 19.20 0.05 24.17 0.29

134

2 I

2275.4

O.Wl 0017

7482

I35

,Y

13390

136

2.0

2X290

I37

I.6

3078.6

7.816 0.014

717

24

2726.4

446

9675

0.248 0003

1.644 0.016

33.0

19.35 0.04 18.98 006 19.11 OO4

I40

7.611 0.017

2.0

o.w3

1893 OW 19.12 0.06 19.11 0.05

19

0252 0.003

18.8

19.33 005

353

126

0.258 0.002

72.1

17.2

3586 0.019 7.478 0.019

12s

12963

SO6 48.1

527.7 1101.2

O.Wl 0.067

1276.8

8305

523.7

120

1136.2

589

4468

19.27 0.05

2 l

565.5

342.3

2Y2, l

I39

10201

603.1

492.3

19.08 O.O$

6X9.2

2.2

-


0002 0.048

61.4

0001 0.046

342

7657 0021

0042
1277.6

1.167 0.012

24105

0001 0039 0001 0040

7806 0015 7736 0.027

II4 e6.7

12

0252 o.Oo2

IX

3374.1

-

-

1468.4

331

2292.5

7573 0027

IX829

"Ml2 0.055

29462

12X4.4

I9

0256 "co3 0256 0003

9966

I.8

0.001 0033

I.8

7.895 0.022 7.19, 0.017

110

I.3

138

2415%

341

0.2S8 0.007 0.251 0.002 0.249 ODOI 0.252 0.003

-

lo9

19.11 0.04

24

336

7.46X 0.012

19 IO 009 -

29.3

326

3178.5

0244 om2

59.4

21.9

615.5

20

7.760 0.012

7535 0.022 7.583 0.015 7.361 O&i2 7.684 0020

0001 0.062

335

0.250 0.002

21.4

2047.9

2494.1

-

-

7.493 0.018 7.854 0.016

I9

20

-

-

35.4

325

334

_

7 467 0020

7648 0.013

2?7O.I

2545

7593 0020

0247 0002 0247 ow3 0247 Ocm 0249 0.003 0252 0002 0.305 0010 0254 002

2X

1.6

0241 0002 0249 ow3

7.694 0.020 7155 0016

324

333

895.5

wdo O(Kf1 0.055 0.001

0.249 0002 025, owJ2

13X1.5

O.UI 0038

18967

-

I08

0.W 0037

24

-

-

19607

2515.4

332

-

l.Ml 0018

2784.8

4117

[email protected] 0.041 OMll 0.05, 0009 0038 0001 0.037 0001 0037

7415 0021

0249 0002

IY

O.Nl O.OdO

0.001 0.190

0248 0003

8535

25

<000l 0.001 0035 COOOI 0.035

526.0

I07

2.5

2X303

-

IWhO

236

27

7.8X0 (,."I8

22977

245

302

0259 0002

5464 I

20

23

O.Ool 0.055

-

-

29

20,00

175.1

-

10s

0.00, 0034

2419.5

7724 OOIX

IO6

3X967

30

0253 O.WZ

19.12 O.O4 19.M 0.06

2.6

2,

1152.5

29.08 0.14

233

301

-


I.0

1531.6

246

-

2925.1

109.9

OM)I 0036

O.Wl 0013

-

75RO 00,X

7 948 0 089

6X64

0002 0.038

7 616 O.OIR

0246 0002

7.637 0.085

2YS9.6

3407.3

0.247 ow2

II341

0252 0003

19 19 0.06

005

1565 1

0379 0.020

l*


20

005

2897.2

0.(x11 104

1970 009

001 0.042

2647.4

50

853.1

2000 0.05

9.8


004

0001 o.0d5 0.019 0.021

223

Xl00

1412.2

I999 008

224

38~r/36~r

10-6 CCSTP/S

0.003 O.Od3

7.7

11799

[%Arl

0.054

7.424 0.026


Xl00

574.0

0,258 0.005

IO.9

22Ne/=Ne

3167.0

264.4

7.564 0.017

Xl00

20

11091

0.271 003

2'Ne/2h

20

2073.6

17636

10-6 CCslwg

344

I8

533.1

[2ONel

343

346

OODI 0011

XlW

22.06 0.15

192X 005

O.Wl 0.030

-1HePHe

20.62 0.10

1.6

17.6

2091.0

312

IO-4 CCSWg

7.534 0.017

3.4

25

14Hc]

0.247 0.003

211

301

-

w

514.4

2789.1

3 I

x100

0001 OOdd

2.6

222

Continued. Grain Mass

1706.5

2%

35

10-6 CCSWg

38~r/36~r

0.111 0031

0.002 0027

212

136~~1

7188 IX589 455.1 12044 WI3

0001

Average 71501

2254 6

0.060 O.OO2 0043 O.Ool O.Odd 0.001 0.044 OSMII O&d "WI 0043 O.WI

0.047
270.3 935.6 11648 1145.9 321.4 10257

8944

16.3

L

22.3

18.96 0.04 ,892 008 18.81 0.09 lQ.lI 0.03

7723 0019 7429 0020

964 18.4

I9 I4 00s

1.615 0019

45.9

7624 0019

40.7

1449 0017

18.0

I8 99 00s 1892 0.04 1924 005

9.9 23.6 70 291

7596 0019

452

1655 0.019

37.2

0.253 0003 0.245 0002

7 542 0025 76lO 0018

15.4

0.249
7.671

34.5

0.003

_-EL-

365

,906 007 19tn OM ,940 0.13 18.95 OW I8 94 00s 1899 0.04 1931 008 18 89 004

I9 I6

1042

R. H. Nichols Jr., C. M. Hohenberg, and C. T. Olinger Table A2. He, Ne and AKfrom 79035 ilmenite grains?‘. Grain Maas [“lie] 3He/4He 12%el 2'Ne/2%e 22Ne/%e 13'%1

-

._

.”

ccSTP/g 101

3.8

IO180

102

39

1272,

cc.srPig 0.041 nix?,

3*~r/36~r

.” tcSTP/g

189 I

0.321 O.fXlS

7.750 0025

10.4

2272 00%

205.8

0.334

7.630 0.018

6.4

24.01 0.07

103

35

14 1

0222 0.015

I62

0.792 e021

8252 0106

1.3

38.18 0.19

1l.M

33

2026.6

3245

0.289 0003

7.637 0.025

13.1

21.45 0.06

I05

42

__

0.046 0,001

0254 001

7.86s 0.012

59.8

1949 004

lo6

31

2141.1

0.269

7.592

17.1

20.18

Ill$

3.1

5435.3

660

2036

1940.8 0.039

456.8

0001 0.035

1573.8

OOQI 5115.1

92.4

0.003

0.021

0.287 0002

8.087 0012

0.385 a.lxe

7.661 0,032

006 004

II2

36

113

40

2lO2.7

0037 0001

5207

0268 0.002

II4

54

I II I.3

0051 0001

179.0

0340 OMKi

401

4.1

24YI.R

O.M2
837.8

0302 O.W2

8 126 0013

548

2130 004

402

25

3979.4

0.037
1194.3

0258 0002

7814 0.015

42.4

19.75 0.05

403t

47

18792

0037
1140.1

0.273 0002

7826 0.012

536

404

25

3786.5

0035
2138.5

0273 0002

804l 0.013

1157

I9 73 0.04 19.47 OIM

406

4.1

1293.2

0045

516.5

301

29

13970

0054

2400

48

42124

0.035
12005

7715 0.031 7848 0014

11.5

412;

a277 0002 0340 0034 0257 0001

7.979 0.021

411

362

19.60 II04

4139

43

34574

0.037 O.Wl

1060.7

0.275 0001

8.174 0.012

56.8

19.99 004

4I4$

3.2

49760

0.034 OUOI

1863.5

a.260 Oa31

8.234 0014

63.4

1982 0.M

415t

23

59729

0037 0001

I2854

0257 0002

7.661 0.016

35.0

416

34

13743

0.042 O.MI

328.7

0299 0.001

421s

59

51210

1725.0

422

36

34544

0034 COO01 0043
7.873 0.016 7,854 0012 7.570 0.017

423

34

2403.2

0041 0.001

413.9

a275 0001 0.261 0.003 0.281 0.002

19.52 0.05 2026 005 19.56 004 20.56 0.06

424

43

965.3

0044 0.001

154.3

425

32

3492.5

0035
426

1.9

27215

411

30

IO&i8

432$

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