Noble gases and nitrogen released from lunar soils by acid etching

Geochimica et Cosmochimica Acta, Vol. 59, No. 23, pp. 4983-4996, 1995 Copyright 0 1995 Else&r Science Ltd Printed in the USA. All rights reserved 00 I6-7037/95 $9.50 + .OO

Pergamon

0016-7037(95)00323-l

Noble gases and nitrogen released from lunar soils by acid etching PAUL

E. RIDER, ROBERT 0. PEPIN, and RICHARD H. BECKER

School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA (Received

May 28, 1994; accepted in revisedfom

August 15,

1995)

Abstract-A stepwise acid-etching technique similar to the closed system stepwise etching (CSSE) method developed at ETH Zurich was used to examine the solar wind reservoirs of lunar soil grains. Samples were treated with weak acids (H,O, H,SO,) to facilitate the release of the most shallowly implanted gases. Noble gas abundances and isotopic compositions, including Kr and Xe in some cases, and a few nitrogen data were obtained for mineral or grain-size separates of three lunar soils (plagioclase from 6005 1, pyroxene from 7508 1, and <25 pm bulk 79035 ) . The 6005 1 plagioclase grains, considered to be a possibly unique resource for determining the modem-day solar wind composition, show unusually low contents of solar wind He, Ne, and particularly Ar, but do not otherwise possess any characteristics clearly attributable to a modem-day solar wind exposure. Initial water and acid treatments of the grains, however, release an apparently pure SEP component. The 75081 pyroxene and the size separate of bulk 79035 both yield Kr and Xe compositions in initial etch steps that are characteristic of undiffused solar wind, significantly increasing the database for measurements of solar wind Kr and Xe where possible laboratory thermal diffusion and fractionation effects are not a concern. Pyroxene in particular appears to be a suitable alternative to ilmenite for the purpose of making measurements of this kind. Nitrogen release by acid etching is not at present quantitative, and while it appears possible to obtain reasonable isotopic ratios for solar wind N, we are unable to use the technique to determine solar nitrogen to noble gas ratios. Light noble gases in all three soil separates, other than the aforementioned behavior of 60051, appear to behave in accord with expectations based on acid-etching analyses performed by the Ztirich group. 1. INTRODUCTION

plantation profiles or mass-dependent fractionation in gas release. The use of both CSSE and pyrolysis/combustion techniques to measure trapped gases in lunar soils has resulted in a controversy over the interpretation of the various sets of experimental data. The different procedures yield results that have led to varying views of the origins and subsequent histories of the gases now present in these samples. These interpretative differences are well documented in the literature (e.g., Becker and Pepin, 1989; Becker, 1990; Kerridge et al., 1991; Geiss and Bochsler, 1991; Benkertet al., 1993; Nichols et al., 1994; and others). The Minnesota model contends that lunar noble gases originate from the solar wind but appear in soil grains as two separable components. The first consists of recently acquired, relatively unfractionated solar wind located in grain surfaces, the second of gas in deeper sites populated over time by inward diffusion accompanied by fractionation of the gases initially implanted in grain surfaces. The Ztlrich model also argues for the existence of a surficial solar wind component and a second, more deeply sited component. It suggests, however, that this second reservoir is populated by direct implantation of “solar energetic particles” (SBP) , corpuscular radiation mom energetic than solar wind but less energetic (and much more abundant) than solar flare particles. Each of these models is likely to be oversimplified. Becker ( 1990) has explored an interpretative middle grotmd between them, and a third proposed model combines features of both (Nichols et al., 1994). What appears to be clear at present is that a distinct, isotopically heavy SBP-like component, at least for the light noble gases, is required at some level of abundance. It is intuitively obvious that the CSSE technique is more likely than pyrolysis or even low-temperature combustion to

The determination of the composition of solar corpuscular radiation can provide important information about the original composition of the solar system. Solar wind and solar flares originate in the outer layers of the sun, one of the least altered and therefore, most primordial reservoirs of solar system material. One of the methods used to study these emanations is to measure the compositions of solar ions implanted in the surfaces of lunar regolitb mineral grains. Experiments of this nature can yield information not yet available by other means (e.g., direct measurement of the present-day corpuscular radiation) on elemental and isotopic abundances of solar noble gases and nitrogen. Measurements made on judiciously chosen samples can also provide insights into such questions as possible changes in the solar particle flux and composition over time. Three experimental procedures have generally been used to examine the composition of noble gases in lunar fines. The most common has been pyrolysis. Low temperature combustion (Prick et al., 1988; Becker and Pepin, 1989) can also release gases from minerals (ilmenite and iron) that are oxidizable. If one wishes to know the distribution of gases within the grains, however, even low-temperature pyrolysis and oxidation experiments are complicated by the possibility of diffusive redistribution of gases during the heating process. The research group at ETH, Zurich has developed an elegant acid etching technique (closed system stepwise etching (CSSE), Wieler et al., 1986; Signer et al., 1993) for releasing embedded solar noble gases from lunar soils, and has used it to study their siting and composition. The entire process occurs at or near room temperature and therefore eliminates from the experiment any effects of laboratory thermal alteration of im4983

P. E. Rider. R. 0. Pepin, and R. H. Becker

4984

yield data free of ambiguities imposed by possible laboratory thermal diffusion and fractionation effects. We therefore developed our own in-vacua chemical etching protocol, intending to supplement the data on noble gases obtained previously by the Zurich group with measurements of nitrogen, krypton, and xenon (which when we began this study had not been analyzed by CSSE). Precise measumments of relative abundances and isotopic compositions of unfractionated solar wind gases, in particular those of Kr, Xe, and N, are central to inferences of the mass distributions of these elements in the solar nebula and in primordial planetary atmospheres (Pepin, 1991) . Solar krypton and xenon compositions have been inferred from a compilation of select lunar and meteoritic experimental results (Pepin, 1991) . The experimental database for these gases was sparse, although Wieler et al. (1992, 1993) and Wieler and Baur ( 1994) have since added significantly to it with the first determinations of Kr and Xe compositions in lunar ilmenite grains by CSSE. The lunar nitrogen problem is also well documented in the literature (e.g., Kerridge, 1989, 1993): relative abundances well in excess of those expected from assessments of solar nitrogen to noble gas ratios (Cameron, 1982; Anders and Grevesse, 1989), an uncertain isotopic composition in the contemporary solar wind, and evidence for a profound secular change in this composition over solar history. Measurements of nitrogen associated with solar-wind noble gases are important in addressing this problem. We present here results from our initial series of experiments.

with eighty-five plagioclase grains from soil 71501 for, among other things, their “Ar contents and masses. One can thus calculate 76Ar surface densities for each of the grains. A typical well-mixed soil should show a wide distribution of SbAr surface densities reflecting both multiple generations of grain input into the soil and a stochastic variation in exposures of individual grains to the solar wind due to burial and reexcavation. This is indeed the case for 71501 (Fig. la). Grains from 60051, however, show a sharp spike at a very low “Ar concentration (Fig. lb), suggesting a short exposure history with little or no cycling through the lunar regolith. These data point to a single exposure episode, and, given the appearance of the collection site, a recent one. A sample of 6005 1 was size-separated by sieving in acetone. The 74-147 pm fraction was separated from magnetic material using a hand magnet, density separated in methylene iodide, and the p < 3.2 fraction handpicked for plagioclase grains. Grains were identified as either “cloudy” or “clear. ” “Cloudy” grains had a shocked or fractured appearance while “clear” grains appeared pristine. We believe the “clean” 6005 1 plagioelase grains described by Wieler et al. ( 1980) correspond to our “clear” grains. A 5.3 mg aliquot of clear grains and a 9.5 mg aliquot of an approximately 50150 mixture of “cloudy” and “clear” grains were analyzed. 2.2. Gas Extraction and Proees&ng procedures The gas extraction system we developed allows us to perform acidetching experiments similar to those of the Zurich group, however with several important modifications (Rider et al., 1992). Because our initial intention was to examine the shallowly implanted solar wind reservoir, we chose to etch our samples with weak acids:

2. ANALYTICAL PROCEDURES 2.1. Sample ?Wecth

and Reparation

Samples analyzed were chosen to assess the capabilities of our system and to address some questions that interested us. Soil 75081 is from the southwest rim of Camelot crater, from the upper 5 cm of the regolith and approximately 1 m from a basalt boulder. It contains appreciable amounts of ilmenite and pyroxene, the most retentive minerals for solar noble gases. Approximately 1 g of bulk soil was sieved in acetone. The 38-74 pm fraction was magnetically separated using a hand magnet and density separated in methylene iodide. Examination of the p > 3.2 separate showed that it consisted of -9t95% pyroxene and +--lo% ilmenite and other minerals. Samples of 10.4 and 31.9 mg were analyzed. 79035 is a breccia sample taken from the Van Serg ejecta blanket. Breccias collected from Van Serg crater contain a complex combination of components, including basalts, several types of glasses, breccia fragments, agglutinates, and feldspar. 79035 is highly friable and easily disaggregated into what appears to be a loosely consolidated soil. It is thought to have received its complement of solar wind gases l-2 Gyr ago (Thiemens and Clayton, 1980; Wieler et al., 1983) and it contains appreciable amounts of ilmenite. This breccia has been analyzed by many laboratories using several techniques (Becker and Pepin, 1989; Wieler et al. 1993; Nichols et al., 1994; Nier and Schlutter, 1994) and has played a pivotal role in the effort to understand the. origin and distribution of gases in lunar soil grains. We analyzed 10.5 mg of a <25 pm size separate of 79035. Soil 60051 was collected 50 m south-southeast of the ALSEP (Apollo Lunar Surface Experiments Package) control station. It is believed to be ejecta from a nearby subdued crater. The area from which the sample was gathered was repotted to have had an unusual appearance. The Apollo 16 crew described it as containing white, brecciated particles that were “caliche-like,” possibly indicating freshly exposed mineral grains spalled by impact from local rocks. Data from Wieler et al. ( 1980) provide further evidence for the special nature of the soil. They analyzed eighty-three individual 150200 pm plagioclase grains with a “clean” pristine appearance along

71501 Pk3gmme

5

0 25

20

r a 15 ii ij & fi 10 z’

60051 Plagioclase

5

0

0

25

50

15

100

saAr (10’’atoms/cm2) FIG. 1. Single grain data from Wieler et al. (1980). Shown is a histogram of 36Ar surface concentrations for 85 phtgioclase grains from soil 71501 (a) and a similar histogram for 83 “clean” plagioclase grams from soil 60051 (b). Surface concentrations were calculated from the measured gas concentrations and estimates of grain surface areas derived from the measured grain masses.

Noble gases and N released from lunar soils 0 2 4 6

PH 8 36Ar 10

n

12

5

10

14 15

Step Number FIG. 2. Light noble gas etch rate for a 16-37 pm sample of soil 67701 exposed to three weak acids (Rider et al., 1992). The etch rate is calculated as the percent of total release of “He, %e and MAr per min of exposure to acid. Also shown is the acid pH for each step.

H,O( ! ) foliowed by either H&O? or H2S03. This choice of acids eliminates the reagent nitrogen contamination that would be introduced by using, for example, HN03. The way in which the acids are produced in the vacuum system (described below) allows us to control their strength and thus their etching rates. Tire relative effectiveness of each reagent in releasing gases from lunar soils is strikingly illustrated in Fig. 2, in this case from a size separate of 67701 ana-

0 x

4985

lyxed to test experimental protocols. It should be noted that a disadvantage of our choice of acids is that we catmot completely release the implanted solar gases present in the grains by the etching process. Use of these acids allowed considerabie flexibility in system construction. The gas extraction system (Fig. 3) is made primarily of commercial stainless steel vacuum components, rather than requiring the specialized materials used by the Zurich group (Signer et al., 1993). It connects directly to the gas processing system illustrated in Fig. 1 of Frick et al. ( 1988). Samples are exposed to Hz0 and then, except for the initial tests where H&O3 was used, to H2SOj produced by dissolving gaseous So, into the water. The SOr is produced by in-vacua high-temperamre (500-700°C) dissociation of Na,SO+ Both the water aud SO2 are subjected to cleanup procedures to remove contaminant noble gases and nitrogen prior to sample analysis, the water by repeated vacuum distillations and the S@ by passing it over 600°C CuO and pumping away noncondensable gases after freezing it in a glass capillary. Two cleanup cycles are sufficient to remove N and noble gases horn the SO,. Samples are loaded into either a stainless steel or tungsten sample finger. The tungsten finger, used for the 31.9 mg 75081 sample and the grain-size separate of 79035, allows us to pyrolyze the residue remaining after etching to temperatures of -12OO“C (200-3CKPC hotter than the stainless steel finger.) Vapor-deposited tungsten tmns out, however, to be permeable to atmospheric nitrogen at high temperatures. Enclosing the finger in a vacuum jacket during the pyrolysis steps is a moderately effective way to minimize this problem. With such an arrangement, low-tempemtme pyrolyses ( <250°C) release negligible amo~ts of contamhmnt N2. High temperatures, however, continue to facilitate the release of atmospheric nitrogen, even after extensive outgassing of the tungsten finger prior to sample loading and analysis. Therefore use of the tungsten finger is precluded if nitrogen is to be measured at temperatures greater than a few hundred “C. The loaded sample finger is attached to the extraction line and samples are pumped, typically for 3-4 days while at room temperature, to remove adsorbed terrestrial gases. At the start of each run, water (usually 0.1-0.2 cc) is transferred to the sample finger by freezing. The water is warmed to room temperanue, and the sample

Nupm Valve QUm

0

Stainless

=

Pyrex

Steel CONVECTRON Gauge

Controlled Temperature Bath

To Gas Processing Line and Mass Spectrometer b

Water

Reservoir

Tungsten. SS SampleFinger

FIG. 3. System diagram for the Minnesota CSSE gas extraction line.

4986

P. E. Rider. R. 0. Pepin, and R. H. Becker

“washed.” Water washes and acid etches are terminated by immersion of the finger in liquid nitrogen. (The possibility that evolved Ar, Kr, or Xe were trapped in the ice thus formed was tested by condensing a sample of our calibration gas mixture into a sample finger containing water and then performing the analytical procedure. All the Ar, Kr, and Xe were recovered.) Evolved gases are processed and measured using procedures similar to those described in Prick et al. ( 1988), with one notable change. The charcoal separation finger has recently heen replaced by a finger containing sintered metal filter elements to facilitate cleaner separation of the heavy noble gases. Additional Hz0 treatments are performed for times ranging from a few hours to several days. These steps remove remaining adsorbed terrestrial contamination from the sample and sample finger walls. The number and duration of Hz0 treatments required varies from sample to sample. Water washes were usually performed until the *Ar/“6Ar ratio decmased to a value less than 10. It is evident from elemental and isotopic abundances that in addition to removing contamination, Hz0 also removed solar gases from the samples. Thus, we effectively used water as a weak acid to measure the most shallowly implanted solar gases. The ability of Hz0 to etch lunar soils is a potentially important result of our study because of its implications with regard to methods of sample preparation. Water-based heavy liquids like Clerici solution may in fact remove the most shallowly implanted solar (and/or lunar) ions. This may require revising the technique for separating ihnenite, which has been the mineral of choice for studies of shallowly implanted solar gases, from lunar samples. The freeze-thaw technique for d&aggregating meteorites may also have a similar effect on surface.-sited gases. After the water washes, SO, is condensed into the sample finger and HzSOs formed around the sample. The strength of the acid and duration of exposure are varied to control the degree of etching. pH values ranged from 1.8 to 1.0 for the samples measured in this study; etch times from a few minutes to several days. Most treatments were performed at room temperature; some, however, included heating to temperatures as high as 90°C (see data tables for details). When the acid is no longer effective, H20 and SO* are. distilled from the finger and the sample is then pyrolyxed in several steps. The first is usually at -250°C in order to release residual H,O and SO*. One or two intermediate steps at -600°C follow. High temperature pyrolysis at -950°C (for the stainless steel finger) or - 1200°C (for the tungsten finger) completes the analysis. The tungsten finger was covered with a quartz vacuum jacket during pyrolysis to limit its oxidation and, as mentioned above, to minimize nitrogen contamination. Pyrolyses of samples in the stainless steel finger were performed without the vacuum jacket. Pyrolysis almost certainly did not liberate all of the gases remaining in the samples. Estimates of the effectiveness of the pyrolyses can be made by considering the thermal release patterns of plagioclase and pyroxene separates from soil 71501 and of bulk 79035 (Frick et al., 1988). These indicate our maximum pyrolysis temperatures were sufficient to liberate 95-100% of the He and Ne, >90% of the Ar and N2, and >80% of the Rr and Xe. Incomplete gas release should not pose a problem with regard to the most recently implanted solar wind, which presumably resides in labile sites in radiationdamaged grain surfaces. However, significant amounts of more deeply sited Ar, Kr, and Xe will remain in the samples. This is particularly true for more retentive minerals like pyroxene and ilmenite. Thus, some SEP and/or diffused solar as well as cosmogenic species will be left behind in the sample residues after the final pyrolysis steps. 2.3. Blank Determiuatious Analytical blanks used to correct the data from the plagioclase separates were obtained from a set of “empty finger blanks” performed before samples were loaded into new fingers. H2SOY(pH - 1.1 for the tungsten finger, pH -2 for the stainless steel finger) was used to scour interior surfaces, typically for 15-20 h, and the gases produced were measured to determine the finger’s contributions to the background. This procedure was repeated until noble gas, particularly argon, and nitrogen abundances were negligible. The blanks used to correct the plagioclase data were selected from the later acid treatments that yielded small amounts of adsorbed air contaminants. For the pyroxene and bulk soil-size separates, the water

wash and acid etch blank was determined by performing the entire gas processing procedure with the sample loaded but without reagents in the sample finger (a “no reagent” blank). Blank measurements were performed at the beginning of each run, and amounts of contaminants (in the form of adsorbed gases) should decrease over the course of the analysis. Blank runs were also of sufficient length (approximately 1 day) to exceed the duration of most of the steps in the sample analysis. Therefore, these blanks should be safe upper limits for most steps. For those few etch steps whose durations exceeded blank accumulation times by significant amounts, gas amouuts liberated were ahnost always large enough to make the blank corrections negligible. For pyrolysis steps, repeat pyrolyses of similar duration and temperature to those Performed

during each experiment were used to blank correct the steps. These typically yielded gas amounts of the same order as the empty finger “acid etch” blanks, empty finger high temperaturepyrolysis blanks, and “no reagent” Ma&s: Typical “no reagent” bl&ks,~in cc STP, were: 4He = 2 X 10-s: 22Ne = 2 x lo-“: 16Ar = 3 X lo-“: 84Xr = 5 x lo-“; lJZXe = 2 X lo-‘“. The N, blank was about 0.25 ng. 3. RESULTS AND DISCUSSION Samples were selected with specific experimental objectives in mind: 60051 as a possible recorder of simple singlestage exposure to the recent solar wind, 75081 pyroxene to assess the ability of this mineral, compared with rarer and less readily separable ilmenite, to preserve an undiiffused record

of the heavier solar-wind noble gases; and unseparated 79035 to test the proposition that even glasses, agglutinates, and the various other mineral constituents of bulk soils and soil breccias may contain unaltered grain-surface reservoirs of at least solar-wind Kr and Xe which can be sampled by weak-acid etching. We find that, in general, light noble gases evolved from these samples display isotopic signatures in accord with previous acid-etch measurements reported by the Zurich laboratory, although elemental abundance patterns in many fractions reflect the low retentivity of these materials for He and Ne. In this section of the report we compare these and previous results, both to assess the utility and accuracy of our version of the CSSE extraction technique for noble gases and nitrogen and to discuss specific cases where the data are unique or interpretations may differ. Measurements in this study of Kr and Xe in 75ogl pyroxene and bulk 79035 have effectively doubled the database of heavy noble gas compositions determined by acid-etching. F’yroxene does indeed faithfully record their primary isotopic compositions, and the hypothesis that undiised solar-wind Xr and Xe reservoirs are present and resolvable in bulk sample constituents appears to be correct, at least for 79035. Discussion of their isotopic distributions, in the context of Zurich results from etched 71501 and 79035 ilmenites (Wieler and Baur, 1994) and of pyrolysis/combustion data obtained in several laboratories from a number of lunar and meteorite samples, is deferred to a companion paper by Pepin et al. (1995). 3.1. Evaluation of Anakytieal procedures Since we are introducing a new experimental system and analytical protocol, it is appropriate to provide evidence bearing on the validity of the data we have obtained. Usually, this would be in the form of a demonstration of analytical reproducibility on replicate samples. However, it is the nature of stepwise studies on mineral and grain-size separates that, given the small sample amounts usually available and the time

Noble gases and N released from lunar soils

involved in doing high-resolution measurements, analyses are rarely carried out on identical samples, either within or between laboratories. In the few instances when such experiments have been performed, differences such as variations in timing or temperatures of the various steps occurred for the sample aliquots, making comparisons difficult except for gas totals. The two splits of 75081 pyroxene analyzed in this study suffer from this problem. Table 1 presents the data for He, Ne, Ar, and N in the 10.4 mg split, analyzed in the stainless steel finger and therefore limited to a 950°C pyrolysis. Table 2a presents the equivalent data for the 31.9 mg split, pyrolyzed in the tungsten finger to 1200°C; Tables 2b and 2c present Xr and Xe data, respectively, for this split. To provide a validity check, total gases for the two splits should produce

4987

similar isotopic and elemental data, with any differences explainable in terms of the different ways the two aliquots were allalyzed. To some degree, this is the case. 4He and ‘*Ne abundances agree within error (‘*Ne abundances for the 10.4 mg and 3 1.9 mg samples are 2937 2 354 and 2571 2 M)l X low8 cc STP/g respectively). %r and ‘“‘Xe yields of 234 5 69 and 26.9 + 5.2 X lo-‘O cc STP/g for the 10.4 mg split are also in reasonable agreement, allowing for incomplete release of xenon from the smaller sample during pyrolysis. Total sample 2’Nel”Ne and “6Ar/“8Arratios for the 10.4 mg sample were isotopically heavier than those of the 31.9 mg sample, but again a possible explanation is the incomplete release of spallation gases from the 10.4 mg sample as a result of its lower maximum pyrolysis temperature. Incomplete release does not

Table 1. Helium, neon, argon and nitrogen abundances and isotopic ratios from 10.4 mg of 75081 pyroxene (38-74 j&r). The separate was analyzed in a stainless steel finger. Helium abundances are exnressed in units of 10-4 cc STFVn,neon abundances in units of lo-6 cc UP/g, and argon abundances in units of 10-g cc &‘/g. Nitrogen abundances are expressed in ppb. lo errors are listed below each measurement.* T = WC, ** T = 89’C. Step Demription

t4N

4He.

‘%Ie@He

%Ie

2%k-@Ne

2tNflNe

3aAr

%rPbr

1st Water Wash 70 min 2nd Water Wash 675 min 3rd Water Wash 1492 min 4th Water Wash 1232 min 5th Water Wash 802 min 6th Water Wash 11043 mill 7th Water Wash 15008 min 8th Water Wash ?min 1st Acid Etch 60 min: pH=1.7 2nd Acid Etch 230 min: pH=1.7 3rd Acid Etch 850 min: oH=1.7 4th Acid itch 850 ruin: pH=1.4 5th Acid titch 835 min: pH=1.3 6th Acid Etch 1405 min: pH=1.3 7th Acid Etch 150 min: pH=1.3* 8th Acid Etch 930 min: pH=1.3** 9th Acid Etch 39500 min: pH=1.3 1st Pyrolysid 60 min: T=206’C 2nd Pyrolysis 45 mitt: T=Zoo’C 3rd Pyrolysis 105 min: T=6oo’C 4th Pyrolysis 45 min: T=949”C 5th Pyrolysis 90 min: T=95o’C

354.8 f16.1 _______ _ _______________ ________

0.08 0.01 1.80 0.25 6.49 0.72 ________3.78 ________ 0.46 2.19 108.7 6.0 0.25 300.0 16.58 13.9 2.04 300.0 7.29 13.9 0.88 4.20 465.4 20.6 0.51 12.39 54.8 3.7 1.09 190.4 3.09 0.40 9.5 72.1 4.41 4.5 0.50 113.5 15.09 6.3 1.27 155.8 8.92 8.0 0.84 29.8 1.86 0.26 2.6 77.9 3.18 0.41 4.8 20.2 0.79 2.1 0.11 70.2 2.06 4.4 0.24 833.7 1.80 35.3 0.24 _______ 0.29 _______ 0.08 4547 182.5 184 15.7 845 1 128.9 340 11.1 5651 0.66 228 0.13

2465 182 1907 73 2358 84 2032 73 2475 93 2380 81 2754 98 3751 134 2448 112 2282 109 2446 117 2355 106 2446 112 2481 94 2406 91 2470 93 2638 308 2678 296 2294 140 2807 474 3520 562 1288 103

0.66 0.10 5.37 0.76 11.39 1.37 10.70 1.35 9.82 1.33 72.71’ 6.43 22.88 2.69 4.29 0.75 11.87 2.02 2.38 0.44 3.30 0.61 13.70 2.22 9.01 1.53 1.90 0.36 4.21 0.78 0.70 0.15 35.98 11.93 1.23 0.32 0.17 0.06 72.42 12.65 111.1 19.4 9.45 2.04

13.44 0.30 13.45 0.28 13.63 0.34 13.43 0.33 13.29 0.36 12.98 0.28 13.32 0.29 13.27 0.28 13.53 0.33 13.47 0.29 13.52 0.29 13.25 0.31 13.59 0.35 13.59 0.29 13.33 0.28 13.29 0.29 17.01 1.82 12.22 0.51 11.06 1.37 12.82 0.52 16.36 0.66 12.88 0.77

0.0330 0.0015 0.0327 0.0014 0.0326 0.0014 0.0325 0.0014 0.0326 0.0014 0.0324 0.0014 0.0327 0.0014 0.0320 0.0014 0.0329 0.0014 0.0331 0.0014 0.0330 0.0014 0.0331 0.0014 0.0332 0.0014 0.0326 0.0014 0.0328 0.0014 0.0331 0.0015 0.0321 0.0010 0.0324 0.0011 0.0324 0.0031 0.0316 0.0010 0.0350 0.0011 0.0532 0.0017

5.9 0.5 30.3 3.2 89.1 9.3 107.0 11.2 91.5 9.3 lM4 106 535.3 54.6 70.0 6.0 133.5 11.5 27.9 2.8 37.5 3.8 166.6 13.2 147.4 12.6 17.2 1.6 110.7 12.7 18.5 1.9 782.4 143.3 30.2 5.5 _______ _______

Sum Pyrolyses

19480 970

314.2 27.3

3051 508

194.4 25.4

14.63 0.61

0.0345 0.0011

1790 336

5.128 0.101

1.20 0.02

Total

21800 1090

408.4 37.5

2884 415

415.3 50.1

14.14 0.55

0.0335 0.0012

5235 739

5.301 0.081

1.20 0.03

252.4 46.2 1326 251 181.0 32.5

5.178 0.055 5.232 0.081 5.421 0.055 5.356 0.057 5.383 0.058 5.486 0.054 5.333 0.053 5.419 0.091 5.396 0.070 5.537 0.147 5.500 0.125 5.431 0.055 5.340 0.054 5.426 0.068 5.431 0.070 5.605 0.166 5.308 0.104 5.294 0.101 _______ --_--__ 5.279 0.107 5.099 0.100 5.111 0.101

%rP6Ar 28.19 2.41 10.51 0.16 4.37 0.07 4.59 0.09 1.71 0.03 0.90 0.01 0.84 0.01 1.37 0.06 0.39 0.01 1.87 0.14 1.61 0.09 1.18 0.01 0.98 0.01 1.57 0.04 1.02 0.03 z 0:53 0.01 2.39 0.07 _______ ______ 2.16 0.04 0.89 0.02 1.89 0.04

P. E. Rider, R. 0. Pepin, and R. H. Becker

4988

Table 2(a). Helium, neon, argon and nitrogen abundances and isotopic ratios from 3 1.9 mg of 7508 1 pyroxene (38-74 m). The separate was analyzed in a tungsten fhrger. H&urn abundances am expressed in units of lo-4 cc STP/g, neon abundances are expressed in units of 10-ecc SW/g, argon abundances are expressed in units of 10-s cc STP/g, nitrogen abundances are expressed as ppb. lo errors are listed below each measurement. * 60 minutes at WC, ** T=MO”C. 4He 4H$JHe 2%Je %lt@Ne 2tN&Ne %4r %rPur 4%irP6Ar Step~tion w 815N “Vaoor Wash” app;ox. 4 days 1st Water Wash 4Omin 2nd Water Wash 40min 3rd Water Wash 45 min 4th Water Wash 961 min 1st Acid Etch 2520 min: pH=l.3* 2nd Acid Etch 2280 min: pH=1.2 3rd Acid Etch 3900 min: pH=1.2 4th Acid Etch 960 min: pH=l.2** 1st Pyrolysis 130 min: T=3OtX 2nd Pyrolysis 60 min: T=5oo’C 3rd Pyrolysis 60 min: T=894”C 4th Pyrolysis 20 min: T=l2oo’C Sum pyrolyses

_________ ________-__---_ _______ ________-______ _______ ---____ ________________________

________ ______ _______ _______ _______ ________

2231 38 2206 37 2386 40 2640 45

2.58 0.20 2.42 0.19 2.78 0.22 8.38 0.65 90.32 7.06 6.79 0.53 2.13 0.17 1.31 0.10 4.21 0.33 30.26 2.37 137.4 10.7 32.64 2.56

13.33 0.13 13.35 0.13 13.34 0.13 13.37 0.13 12.82 0.13 12.77 0.13 12.31 0.12 13.01 0.13 13.10 0.13 12.77 0.13 12.44 0.13 11.18 0.12

0.0328 0.0003 0.0329 0.0003 0.0325 0.0004 0.0326 0.0003 0.0347 0.0003 0.0509 0.0005 0.0711 0.0007 0.0348 0.0004 0.0332 0.0003 0.0325 0.0003 0.0328 0.0003 0.0950 0.0010

22.0 1.3 20.4 1.3 24.8 1.5 59.4 3.6 2015 134 168.5 10.7 40.9 2.7 25.0 1.5 28.9 1.8 80.3 4.8 691 .O 59.4 590.1 51.7

5.406 0.070 5.361 0.028 5.377 0.027 5.381 0.027 5.335 0.027 5.096 0.030 4.988 0.105 5.306 0.027 5.482 0.029 5.555 0.069 5.139 0.026 4.177 0.021

9.37 0.10 2.23 0.04 1.61 0.03 3.69 0.04 1.24 0.01 3.00 0.03 47.65 0.98 47.26 0.48 4.22 0.09 5.82 0.07 3.49 0.04

294.9 18.3

2333 40

204.5 16.0

12.28 0.13

0.0437 0.0004

1390 118

4.705 0.026

6.08 0.06

388.0 24.1

2352 40

321.2 25.1

12.49 0.13

0.0410 0.0004

3766 275

5.071 0.025

4.01 0.05

32.3 f2.51

______ 0.31

4.98

17.0 1.6 cl650 85 273.0 15.0 40.4 3.0 3.5 0.4 15.2 1.4 1927 98 3193 162 11170 560

8.5 3.0 1.1 3.1 55.4 3.1

4.52 0.28 5.14 0.32 14.73 0.91 50.57 3.13 7.83 0.48 2.45 0.15 2.89 0.18 12.34 0.84 83.34 5.16 189.8 11.7 9.47 0.59

16310 807

38.7 3.1

18300 927

24.7 2.0

16.9 4.3 16.3 14.1

2264 23,‘: 39 2317 39 2280 38 2550 43 2252 39 2042 36 2445 44

Table 2(b). Krypton abundances and isotopic ratios from 3 1.9 mg of 7508 1 pyroxene. Abundances are expressed in units of 10-m ccSTF/g. The separate was analyzed in a tungsten finger. lo errors are listed below each measurement. * 60 minutes at WC, ** T=140°C. %r/‘% 83~r/@ti w~r/84~r 7Xr/%r 8%@4~r step Description %r “Vapor Wash” appkx. 4 days 1st Water Wash 40min 2nd Water Wash 40 min 3rd Water Wash 45 min 4th Water Wash 967 min 1st Acid Etch 2520 min: pH=l.3* 2nd Acid Etch 2280 min: pHrl.2 3rd Acid Etch 3900 min: pH=l.2 4th Acid Etch 960 min: pH=l.2** 1st Pyrolysis 130 min: T=3oo’C 2nd Pyrolysis 60 min: T=5oo’C 3rd Pyrolysis 60 min: T=894’C 4th Pyrolysis 20 min: T=12oo’C Sum

Pyrolyses

51.9 Lto.03 2.8 0.3 1.0 0.1 1.2 0.1 2.8 0.3 89.1 4.4 17.7 0.9 2.9 0.3 0.8 0.1 3.5 0.3 3.3 0.3 37.4 1.9 35.5 1.8

0.00601 0.00005 0.00630 0.00026 0.00700 0.00025 0.00620 0.00037 0.00630 0.00028 0.00640 0.00006 0.00650 0.00013 0.00730 0.00017 0.00610 0.00030 0.00660 0.00023 0.00660 0.00025 0.00670 0.00006 0.01590 0.00015

0.0392 0.0005 0.039 1 0.0008 0.0399 0.~0009 0.0404 0.0010 0.0405 0.0008 0.0409 0.0005 0.0403 0.0005 0.043 1 0.0009 0.0367 0.0011 0.0392 0.0006 0.0414 0.0008 0.0421 0.0005 0.0660 0.0008

0.2008 0.0005 0.2043 0.0013 0.2011 0.0027 0.1992 0.0034 0.1999 0.0013 0.203 1 0.0005 0.2030 0.0010 0.2089 0.0020 0.2026 0.0025 0.2029 0.0012 0.2041 0.0016 0.2038 0.0006 0.2379 0.0008

0.2009 0.0044 0.1995 0.0011 0.2033 0.0019 0.1992 0.0018 0.2009 0.0013 0.2023 0.0005 0.2043 0.0009 0.2134 0.0014 0.1984 0.0024 0.2003 0.0011 0.2049 0.0017 0.2052 0.0006 0.2556 0.0008

0.3073 0.0007 0.3032 0.0020 0.3050 0.0029 0.3057 0.0022 0.3089 0.0021 0.3052 0.0007 0.3085 0.0010 0.3061 0.0021 0.3009 0.0018 0.3049 0.0014 0.2993 0.0015 0.3053 0.0006 0.3043 0.0007

79.7 4.3

0.01080 0.00011

0.0526 0.0006

0.2190 0.0008

0.2274 0.0008

0.3046 0.0007

190.2 10.0

0.00826 0.00009

0.0457 0.0006

0.2098 0.0007

0.2132 0.0007

0.3053 0.0008

;$

Noble gases and N released from lunar soils

Table 2(c) Xenon abundances and isotopic ratios from 3 1.9 mg of 7508 1 pyroxene. Abundances are expressed in units of lO-‘Occ STP/g. The separate was analyzed in a tungsten finger. lo errors are listed below each measurement. * 60 minutes at WC, ** T=MO’C. Step Description

132~

“Vapor Wash” approx. 4 days 1st Water Wash 40 min 2nd Water Wash 40 min 3rd Water Wash 45 min 4th Water Wash 967 min 1st Acid Etch 2520 min: pH=1.3* 2nd Acid Etch 2280 min: pH=l.2 3rd Acid Etch 3900 min: pH=l.2 4th Acid Etch 960 min: pH=l.2** 1st Pyrolysis 130 min: T=3oo’C 2nd Pvrolvsis 60 I&: TLSOtYC 3rd Pyrolysis 60 mik -i=894'C 4th Pyrolysis 20 min: T=l2oo’C

30.78 f0.03 3.84 0.37 1.41 0.13 0.37 0.05 0.38 0.05 12.52 1.19 6.93 0.66 1.75 0.22 0.73 0.09 0.17 0.03 0.28 0.04 3.97 0.50 5.92 0.56

Sum Pyrolyses

Total

12%

126Xe

128Xe

129Xe 132% I: 1

13%e

0.00334 0.00009 0.00268 0.00030 ------------------------------------------------------------0.00480 0.00017 0.00545 0.00019 0.00432 0.00064 0.00431 0.00066 --------------------___________ ----------0.00420 0.00037 0.01863 0.00047

0.00331 0.00011 0.00337 0.00018 0.00282 0.00028 0.00485 0.00041 0.00432 0.00045 0.00526 0.00017 0.00618 0.00018 0.00679 0.00040 0.00647 0.00042 0.00490 0.00060 0.00487 0.00058 0.00560 0.00024 0.02795 0.00086

0.0707 0.0004 0.0746 0.0005 0.0734 0.0016 0.07 19 0.0012 0.0785 0.0023 0.0835 0.0009 0.0851 0.0006 0.0854 0.0022 0.0808 0.0029 0.0720 0.0023 0.0826 0.0030 0.0860 0.0011 0.1172 0.0013

0.987 0.002 0.981 0.005 0.977 0.005 0.998 0.007 0.990 0.007 1.034 0.004 1.037 0.005 1.015 0.008 1.007 0.011 0.972 0.012 1.009 0.013 1.047 0.005 1.036 0.005

0.1504 0.0007 0.1527 0.0014 0.1520 0.0021 0.1517 0.0019 0.1525 0.0037 0.1621 0.0011 0.1653 0.0015 0.1644 0.0015 0.1575 0.0019 0.1660 0.0036 0.1550 0.0051 0.1623 0.0012 0.1839 0.0016

0.7923 0.0016 0.7965 0.0042 0.7824 0.0043 0.7853 0.0038 0.8091 0.0054 0.8196 0.0033 0.8268 0.0021 0.8150 0.0048 0.8086 0.0068 0.7814 0.0088 0.7861 0.0109 0.8151 0.0050 0.9348 0.0031

0.3881 0.0010 0.3863 0.0024 0.3951 0.0049 0.4138 0.0050 0.4139 0.0049 0.3694 0.0018 0.3715 0.0016 0.3687 0.0028 0.3856 0.0023 0.3719 0.0066 0.3772 0.0045 0.3714 0.0026 0.3682 0.0017

0.3282 0.0013 0.3254 0.0026 0.3313 0.0026 0.3226 0.0029 0.3205 0.0041 0.3035 0.0017 0.3040 0.0015 0.3105 0.0021 0.3017 0.0035 0.3132 0.0056 0.3206 0.0053 0.2993 0.0019 0.3042 0.0020

10.34 1.13

0.01228 0.00041

0.01836 0.00061

0.1035 0.0013

1.039 0.005

0.1745 0.0016

0.8823 0.0041

0.3697 0.0022

0.3029 0.0021

32.27 3.29

0.00740 0.00029

0.00976 0.00033

0.0903 0.0011

1.035 0.005

0.1668 0.0014

0.8407 0.0037

0.3704 0.0021

0.3038 0.0019

explain the distinct 4He/“He and 2?Je/22Ne ratios and differing argon concentrations of the two 75081 splits, however. The differences are primarily due to anomalous results from the eighth acid etch and fourth pyrolysis steps of the 10.4 mg sample. Although we cannot account for the anomalous values, it should be noted that this sample was run while analytical procedures were still being refined. Elemental and isotopic compositions determined for the 31.9 mg sample are almost identical to those measured for a similar grain size separate of bulk 75081 (Hintenberger et al., 1974) _Therefore, other than a small number of clearly anomalous measurements, there appears to be consistency between the two splits and with a previous measurement of the bulk sample. Another check, this time for the heavy noble gases, is provided by a comparison of our gas release data with release patterns observed for other samples by CSSE. Table 3a shows our data for He, Ne, Ar, and N from sample 79035, Table 3b shows the Kr data, and Table 3c the Xe data for this sample. If we compare the elemental ratios ‘6Ar/‘“2Xe and 84Xr/‘“2Xe as a function of 36Ar release for the larger split of 75081 pyroxene (Table 2) with ilmenite data from soil 71501 (Wieler et al., 1992), or for the bulk 79035 (Table 3) with 79035 ilmenite data ( Wieler et al., 1993 ) , we find that although the various samples show differing excursions from flat patterns, the overall results are similar. (These comparisons are made separately because of a known enhancement factor of 2 in Xe relative to other noble gases in Apollo 17 breccias.) Figures 4 and 5 show the 36Ar/“2Xe and s4Kr/“2Xe comparisons for the 75081 pyroxene. Our earliest “water-wash” steps show large depletions of both Ar and Kr relative to Xe not seen in

131Xe

wk

1NXe

the ilmenite sample, but the xenon isotopic compositions of these initial treatments contirm that this is due to the removal of adsorbed atmospheric Xe (the intended function of the washes). Similar consistency is seen between the krypton and xenon isotopic data obtained here for 75081 pyroxene and 79035 bulk samples and data from the 71501 ilmenite. Although we defer a discussion of the significance of the krypton and xenon isotopic compositions to a companion paper (Pepin et al., 1995), we note that the isotopic compositions obtained for specific steps of interest, such as initial acid etches, and for the total gas releases from these two samples are fully consistent with interpretations developed in the companion paper, based on a variety of samples including prior results on meteorites as well as the CSSE data of the Zurich group (Wieler and Baur, 1994), and provide corroboration for those interpretations. In fact, one outcome of the present study is the indication that pyroxene is probably as good a mineral as ilmenite for determinations of elemental and isotopic characteristics of the heavy noble gases. Given that it is generally much more abundant than ilmenite, and may be easier to separate and purify without the use of aqueous-based heavy liquids, one might consider using pyroxenes more often in further studies of heavy solar species in the lunar regolith. 3.2. Nitrogen Although we have established to our satisfaction that our etching procedure works, a major disappointment is that we have not yet been successful in achieving one of our objec-

P. E. Rider, R. 0. Pepin, and R. H. Becker

4!990

Table 3(a). Nitrogen, helium, neon and argon abundances and isotopic ratios from 10.5 mg of bulk 79035 (C 25 pm). The sample was analyzed in a tuqsten fmger. Helium abwdances aa expressed in units of lo4 ccSTP/g, neon abundances in units of 106 ccSTPIg and argon abundances in units of 10-s ccSTP/g. Nit&en abundances in ppb. lo errors are listed t&low e&h rpeaswment. Step Description

‘4N

1st Water Wash 40 min 2nd Water Wash 40 min 3rd Water Wash 2407 min 1st Acid Etch 865 min: pH=l.3 2nd Acid Etch 730 min: pH=l.3 3rd Acid Etch 2525 min: pH=l.3 4th Acid Etch 3952 min: pH=l.3 1st Pyrolysis 20 min: T=25O”C 2nd Pyrolysis 40 min: T=25O”C 3rd Pyrolysis 45 min: T=6CO”C 4th Pyrolysis 60 min: T--9oo”C 5th Pyrolysis 45 min: T=ll35”C 6th Pyrolysis 55 min: T=ll35”C 7th Pyrolysis 55 min: T=l2OO”C 8th Pyrolysis 55 min: T=l2OO”C

137.6 _______ 1t8.96 _______ 116.0 -8.4 8.7 12.1 275.9 31.2 3.7 82.2 865.0 -155.0 46.7 6.6 78.6 -106.9 18.7 6.6 23.5 -100.8 3.1 23.2 8.7 ---------

6’5N

4He

4He&le

me

%k@Ne

0.418 0.014 0.977 0.033 20.60 0.74 959.6 35.2 25.16 0.89 24.73 0.87 18.24

0.97 0.04 1.43 0.06 36.67 1.46 3451 138 36.78 1.46 19.38 0.77 13.05 0.52 22.78 0.91 0.05 0.00 325.2 13.1 799.5 31.8 235.7 9.38 0.08 0.00 0.56 0.02 0.23 0.01

ztN&Ne

%k

3%t3%r

2.71 0.09 2.29 0.09 2.73 0.09 3.00 0.09 3.02 0.09 3.01 0.24 2.47 0.09 2.41 0.09 1.39 0.08 2.36 0.15 12.89 0.10 12.35 0.10

0.0331 0.0004 0.0327 0.0003 0.0325 0.0003 0.0325 0.0003 0.0360 0.0004 0.0341 0.0003 0.0330 0.0003 0.0322 0.0003 0.0344 0.0011 0.0331 0.0003 0.0333 0.0003 0.0400 0.0004 0.0483 0.0009 0.0357 0.0005 0.0372 0.0006

9.2 0.7 10.0 0.8 194.3 14.2 49200 4635 1291 98 499.8 37.0 315.9 23.4 1957 154 2.3 0.2 1045 81 12594 1056 4229 331 2.1 0.2 4.5 0.3 3.2 0.2

5.409 0.027 5.412 0.028 5.404 0.028 5.227 0.010 5.300 0.011 5.364 0.013 5.414 0.016 5.500 0.011 5.418 0.03 1 5.407 0.011 5.162 0.010 4.838 0.009 4.782 0.050 5.144 0.030 4.887 0.040

4%/3ur

91.i 7.5 96.2 7.6 595.7 33.2 48420 2424 65320 3270 2250 116 19670 987 42170 2113

-48.1 13.0 -38.3 8.7 -28.7 6.5 -15.4 3.1 9.6 0.4 3.7 3.2 3.1 3.4 1.5 3.0

47.04 1.67 0.143 0.005 416.2 15.2 2219 80 11 1.7 4.0 0.199 0.007 2.219 0.075 1.309 0.044

2428 42 2433 42 2485 13 2561 118 2997 13 2712 9 2542 12 2716 125 2540 30 2553 118 3276 151 3901 180 3254 84 3478 40 3579 45

Sum Pyrolyses

178600 90043

-0.1 2.1

2798 101

3153 147

1384 56

12.25 0.09

0.0344 0.0003

19840 1623

5.132 0.010

3.02 0.00

Total

179600 9000

-0.8 2.2

3847 139

2969 136

4943 197

12.58 0.09

0.0331 0.0003

71360 6433

5.204 0.010

2.61 0.01

1.8 _________ 0.64

13.34 0.10 13.24 0.10 13.21 0.09

40.33 0.28 5.48 0.12 3.39 0.02 2.38 0.01 3.09 0.00 4.43 0.01 4.81 0.01 6.86 0.01 35.71 0.39 6.76 0.01 2.47 0.00 1.73 0.00 33.25 0.62 70.55 0.51 149.1 1.40

71501 ilmenite ,.......,.......... ~ A/ i.. ..... ... .. .. ..j __ _ _ - _ _ _ 10000

3 T

1000 c

75081

pyroxene

1

75081

pyroxene

i

50 % =Ar released FIG. 4. “Ar/“*Xe for the 31.9 mg sample of pyroxene from 75081 plotted vs. percent 36Ar released. Equivalent data from measurements of a 71501 ilmenite separate (Wider et al., 1992) are also shown.

50

100

%*Ar released FIG. 5. %r/“*Xe for the 3 1.9 mg sample of pyroxene from 75081 plotted vs. percent %Ar released. EQuivaIent data from measurements of a 71501 ilmenite separate (Wieler et al., 1992) are also shown.

Noble

gases and N released from lunar soils

4991

Table 3(b). Krypton abundances and isotopic ratios from 10.5 mg of bulk 79035. Abtmclances are expressed in units of 1CksccSTP/g. The sample was analyzed in a tungsten linger. lo errors are listed below each measutement. StepDc8crintion SKI 7%#4Kr wr/wr *2Kr/J34Kf ‘33~r15’~r ~~KI@‘KI 1st Water Wash 40 min 2nd Water Wash 40 min 3rd water Wash 2407 min 1st Acid Etch 865 min: pH=1.3 2nd Acid Etch 730 min: pH=l.3 3rd Acid Etch 2525 min: pH=l.3 4th Acid Etch 3952 min: pH=1.3 1st Pyrolysis 20 min: T=25O”C 2nd Pyrolysis 40 min: T=25O”C 3rd Pyrolysis 45 min: T=600°C 4th Pyrolysis 60 min: TSOVC 5th Pyrolysis 45 min: T=ll35%! 6th Pyrolysis 55 min: T=1135”& 7th Pyrolysis 55 min: T=l2OO”C 8th Pyrolysis 55 min: T=12OO”C

0.17 *ti.oz 0.02 0.00 0.05 0.00 26.44 1.92 1.95 0.14 0.25 0.02 0.10 0.01 0.57 0.04 0.01

0.00581 0.00018 0.00524 0.00045 0.00724 0.00034 0.00637 0.00003 0.90641 0.00009 0.00656 0.00015 0.00626 0.0002 1 0.00633 0.00009 0.00530

:: 0:06 5.51 0.40 5.27 0.38 0.01 0.00 _______ _____________

0.00062 0.00653 0.00009 0.00659 0.00004 0.00795 0.00007 0.0083 1 0.00037 _______ _--__-_______

0.0395 0.0006 0.0426 0.0022 0.0398 0.0013 0.0406 0.0002 0.0406 0.0002 0.0411 0.0005 0.0411 0.0005 0.0415 0.0004 0.0387 0.0017 0.0421 0.0004 0.0416 0.0002 0.0442 0.0003 0.0422 0.0013 _______ _______

0.2061 0.0015 0.2037 0.003 1 0.2004 0.0027 0.2035 0.0007 0.2036 0.0007 0.2055 0.0012 0.2027 0.0022 0.2056 0.0008 0.1976 0.0034 0.2070 0.0009 0.2055 0.0007 0.2091 0.0007 0.2056 0.0040 _______ _______

_______

0.2008 0.0016 0.2085 0.0034 0.1975 0.0026 0.2026 0.0009 0.2024 0.0011 0.2024 0.0012 0.1997 0.0020 0.2041 0.0012 0.2098 0.0040 0.203 1 0.0010 0.2039 0.0010 0.2134 0.0010 0.2096 0.0030 _______ _______ _______ _______

0.3064 0.0024 0.3060 0.0045 0.3046 0.0025 0.3035 0.0011 0.3041 0.0012 0.3039 0.0017 0.305 1 0.0022 0.3009 0.0015 0.3021 0.0056 0.3008 0.0014 0.3030 0.0011 0.3074 0.0011 0.3000 0.0048 _______ _______

-__-___

Sum Pyrolyses

12.20 0.89

0.00716 0.00006

0.0421 0.0003

0.2012 0.0007

0.2080 0.0010

0.3046 0.0011

Total

41.19

0.00661

0.0412

0.2046

0.2041

0.3039

tives, the determination of nitrogen abundances and isotopic compositions along with those of the noble gases. Nitrogen data obtained in this study are included in Tables l-3. We have established that, at present, we cannot determine total nitrogen yields and isotopic compositions because our sample fingers either cannot be heated sufficiently for complete release (stainless steel), or are permeable to nitrogen when heated (tungsten). There is unambiguous evidence, however, that our acids release sample nitrogen during the etch process. Many Apollo 17 breccias, including 79035, contain isotopitally light nitrogen with compositions as low as - -200% (Clayton and Thiemens, 1980). The acid-etch steps of the 79035 <25 pm sample clearly show that light nitrogen is released (Table 3a). These result8 cannot represent anything but sample nitrogen because there are no sources of terrestrial nitrogen that have such low S-values. However, the 14NPAr ratios in these etch steps are an order of magnitude lower than “solar” values (Cameron, 1982; Anders and Grevesse, 1989) and two orders of magnitude lower than those seen in bulk 79035 (Becker and Pepin, 1989). Since there are no indications of excessive amounts of Ar, it appears that nitrogen extraction is incomplete in these steps. This situation has been observed in other samples as well. We also can not account for all of the nitrogen expected from our size separate of bulk 79035 even when pyrolyzed to 12OO”C,where we would in fact expect excesses of N due to contamination from the tungsten finger.

It is obvious our procedure for measuring nitrogen by acid etching requires further development. It is possible that the difficulty with nitrogen arises because the dominant form in which it exists in the grain surfaces is not as NZ but rather isolated N atoms, facilitating the formation of either NH: or NO; or NO, ions upon etching. These species may be trapped in the silicious residue produced by the etching, and may simply not be released at the pyrolysis temperatures used here. If so, it may be very difficult to convert the majority of the nitrogen extracted by etching into a form that we can further process, and 14N/%Ar ratios may not be measurable on a stepwise basis using this analytical technique. 3.3. Sample 60051: Modem-Day Solar Wind? Helium, Ne, and Ar results for the “clear” and “cloudy + clear” plagioclase separates from 60051 am presented in Tables 4 and 5, respectively. Neon isotopic data am shown in Fig. 6. The Ar data for the “clear” sample have obviously been compromised by atmospheric connknation, as the total Ar for thisseparatehasa40Ar/~~ratiOof56,comparedtoaratioof -2 for the second plagioclase sample. Ekkating the first water wash from consideration reduces the ovtxall 4oAr/“6Arratio of the “clear” sample to -8, but it is obvious from their 4oAr1”6Ar ratio8 that some of the other steps have also been somewhat contaminated.Wecancorrectthcindividualstepsforairargon (taking the trapped component to have a @APAr ratio of 1),

P. E. Rider, R. 0. Pepin, and R. H. Becker

4992

Table 3(c). Xenon abundances and isotopic ratios from 10.5 mg of bulk 79035. Abundances expressed in units of 10-8 cc STP/g. The sample was analyzed in a tungsten finger. la errors are listed below each measttrement. t26Xe t=Xe tBXe WKe 13tXe 13% tXXe step Description Wie tHXe 132xe = 1 1st Water Wash 40 min 2nd Water Wash 4Omin 3rd Water Wash 2407 min 1st Acid Etch 865 min: pH=1.3 2nd Acid Etch 730 min: pH=l.3 3rd Acid Etch 2525 min: pH=l.3 4th Acid Etch 3952 min: pH=l.3 1st Pyrolysis 20 min: T=25O”C 2nd Pyrolysis 40 min: T=25O”C 3rd Pyrolysis 45 min: T=6OO”C 4th Pyrolysis 60 min: T=9OO”C 5th Pyrolysis 45 min: T=ll35”C 6th Pyrolysis 55 min: T=ll35”C 7th Pyrolysis 55 min: T=l2OO”C 8th Pyrolysis 55 min: T=l26o”C

0.24 f0.01 0.07 0.00 0.02 0.00 5.84 0.25 0.91 0.04 0.21 0.01 0.07 0.00 0.07 0.00 _______ _______ 0.12 0.01 0.50 0.02 2.21 0.09 0.01 0.00

0.00443 0.00023 0.00488 0.00016 0.00568 0.00013 0.00487 0.00052 _______

0.00327 0.00009 0.00370 0.00017 0.00304 0.00040 0.00448 0.00004 0.00452 0.00011 0.00440 0.00011 0.00368 0.00023 0.00396 0.00017 _______ _______ 0.00429 0.00025 0.00452 0.00012 0.00663 0.00008 0.00375 0.00039 _______

_______ _______ ___ ____ _______

0.0724 0.0006 0.0716 0.0014 0.068 1 0.0013 0.0833 0.0005 0.0833 0.0006 0.0833 0.0008 0.0792 0.0017 0.0807 0.0011 _______ _-___-0.0846 0.0010 0.0834 0.0007 0.0872 0.0005 0.0692 0.0015 _______

0.9853 0.0063 0.9866 0.0073 0.9923 0.0107 1.0637 0.0057 1.0606 0.0060 1.0577 0.0063 1.0453 0.0094 1.0481 0.0078 _______ _______

0.1514 0.0015 0.1514 0.0020 0.1504 0.0033 0.1635 0.0013 0.1631 0.0014 0.1607 0.0019 0.1597 0.0020 0.1614 0.0028 _______ _______

0.7948 0.0032 0.7922 0.0074 0.7929 0.0086 0.8201 0.0027 0.8201 0.0029 0.8145 0.0036 0.8018 0.0044 0.8098 0.0080 _______ _______

0.3893 0.0024 0.3884 0.0024 0.3793 0.0038 0.3701 0.0014 0.3718 0.0018 0.3685 0.0020 0.3761 0.0029 0.3736 0.0032 _______ _______

0.3276 0.0029 0.3224 0.0038 0.3135 0.0036 0.2993 0.0022 0.3021 0.0024 0.3037 0.0029 0.3060 0.0026 0.3060 0.0035 _______ _______

1.0695 0.0087 1.0683 0.0071 1.0652 0.0058 1.0115 0.0120 _______ _______ _______ _______ _______

0.1660 0.0015 0.1630 0.0015 0.1667 0.0013 0.1517 0.0030 _______

0.8152 0.0066 0.8224 0.003 1 0.8335 0.0029 0.7978 0.0118

0.3741 0.0035 0.3629 0.0016 0.3717 0.0014 0.3958 0.0053 _______

0.3070 0.003 1 0.3009 0.0024 0.3020 0.0023 0.3348 0.0067 _______

_____________________ -_-____ ____ ___ _______ _______

2.91 0.12

0.00547 0.00015

0.00610 0.00010

0.0862 0.0006

0.0062

0.1658 0.0014

0.8302 0.0032

0.3704 0.0016

0.3022 0.0024

10.27 0.44

0.00489 0.00013

0.00490 0.00007

0.0837 O.OQ05

1.0611 0.0059

0.1637 0.0013

0.8219 0.0030

0.3710 0.0016

0.3014 0.0023

Sum Pyrolyses

Total

0.00387 0.00021 0.00356 0.00016 0.00436 0.00040 0.00471 0.00011 0.00468 0.00013 0.00496 0.00016 0.00386 0.00017 0.00470 0.00032 _______ _______

2OCR * 0 ,~,~‘,~,~‘~,,(‘,~,~‘~,~,‘~,~,‘~~~~’,”’ 0.00 0.10 0.20 0.30 0.40

0.50

0.60

0.70

0.80

21Ne/ =Ne FIG. 6. Neon three-isotope plot of the water wash, acid etch and integrated pyrolysis steps of the “clear”-grained [open diamond symbols] and “cloudy + clear”-grabed (filled diamond symbols) 6005 1 plagioclase separates. Also shown are the solar wind (Bcchsler and Geiss, 1977). Ztich SEP compositions (Benkert et al., 1993) and GCR composition (Wieler et al., 1986) (circle symbols). Error bars represent lo deviations.

1.0653

ad, assuming the other gaseshave not been effected, consider tlleconsequencesoftbecormcmddata.(Thesecorrected.‘6Ar dataamshowninTable4). The assumption that other gases are uncontaminated needs to be tested, because if one examines the data in Table 4 there is a very obvious connection between high 4oAr/“6Arratios and low ( SEP-like) zoNe/“Ne ratios in the water washes and early acid etches. We must ask whether the neon isotopic ratios are in fact SE%related, or whether they may be solarwind values contaminated by atmospheric Ne. Fortunately, there are two arguments against the contaminationbyjx#besis. First, the Ne/Ar ratio in air is such that, for tbe amounts of air Ar calculated in tbe various steps, Ne is a negligible contributor to all steps except the first. Even in that step, if the contaminant were adsorbed air, the Ne contribution could well be insignificant, because the Ne/Ar adsorbed ratio is expected to be much less than tbe actual air ratio. L&cot& the %Ar/ “Ar ratios in tbe presumably contaaojnated steps are lower titan in air so that corrected %ArPAr ratios are also suggestive of SEP (Benkert et al., 1993). ‘I’be argunesnts for Ne apply as well to He, so it is assumeo that He and Ne measurements can be taken at face value. Note tbat the SBP-like 9VePNe ratios are not tbe result of au over-correction for ‘+OAr++ since the effect of the interfering isotope is explicitly determ&d in all of our treatment steps from concurrem measurements of 4oAr+.

Noble gases and N released from lunar soils

4993

Table 4. Helium, neon and argon abundances and isotopic ratios from 5.3 mg of “clear” 6005 1 plagioclase (74-147 p). The separate was analyzed in a stainless steel finger. Gas abundances are expressed in units of 10-8cc STP/g. la errors arc listed below each measurement. Step Description 1st Water Wash 60 min 2nd Water Wash 228 min 3rd Water Wash 868 min 1st Acid Etch 143: pH=l.8 2nd Acid Etch 773 min: pH=l.8 3rd Acid Etch 92 min: pH=l.6 4th Acid Etch 94 min: pH=l.4 5th Acid Etch 255 min: pH=l.3 6th Acid Etch 267 min: pH=l.2 7th Acid Etch 752 min: Ph=l.O 8th Acid Etch 77677: pH=l.O 1st Pyrolysis 65 min: T=6oo’C 2nd Pyrolysis 61min: T=25O’C 1st Combustion 61 min: T=ZJo’C 3rd Pyrolysis 65 min: T=95o’C

4He 117

4He/3He

2oNe

*0Ne/ZZNe *lNe&Ne

36ArPur

0.0283 0.0014 0.0312 0.0011 0.0301 0.0011 0.0299 0.0011 0.0310 .0.0013 0.0289 0.0025 0.0308 0.0020 0.0313 0.0018 0.0371 0.0012 0.1171 0.0039 0.2636 0.0111 0.2260 0.0024 0.1650 0.0362 _______ _______ 0.0637 0.0024

153.8 1.6 2.76 0.03 1.16 0.01 5.17 0.05 1.32 0.01 4.85 0.05 15.91 0.16 29.30 0.29 259.3 2.6 116.7 1.2 33.94 2.00 244.5 38.2 0.09 0.03 0.76 0.07 29.83 0.38

5.257 0.054 5.257 0.066 5.123 0.088 5.132 0.058 5.544 0.097 5.520 0.066 5.399 0.056 5.453 0.055 5.344 0.053 4.914 0.049 1.730 0.019 5.325 0.053 4.939 1.124 1.531 0.047 1.414 0.016

289.9 1.6 241.0 7.4 35.74 1.41 138.7 2.5 181.7 3.2 19.43 0.44 5.82 0.14 3.21 0.07 1.40 0.01 1.15 0.02 18.64 0.83 4.63 0.05 -______ -______ 200.7 11.3 51.12 1.02

3.16 0.03 0.51 0.01 1.00 0.01 2.74 0.03 0.50 0.00 4.52 0.05 15.62 0.16 29.05 0.29 258.9 2.6 116.6 1.2 31.87 1.88 241.5 37.7 0.10 0.03 0.26 0.02 24.74 0.32

3;; 0.958 5.038 0.322 5.112 0.104 4.983 0.105 6.016 0.304 5.541 0.071 5.402 0.057 5.455 0.056 5.344 0.053 4.914 0.049 1.659 0.019 5.325 0.054 4.939 1.124 _______ _______ 1.229 0.015

%rPar

3% (COK)

3%13hr ( )

287 31 439 41 1671 125 179 24 398 38 948 75 937 74 5503 363 2167 154 770 43 26400 527 822 45 346 27 4175 103

8952 1658 4361 236 3111 142 2763 50 6887 531 3399 250 3449 155 3320 86 3145 36 3090 43 1769 42 2412 41 5162 211 973 1 767 3077 45

29.3 2.9 121.0 10.2 18.1 1.9 98.8 8.4 54.8 4.6 20.6 2.1 42.1 3.8 47.0 4.1 179.0 16.3 59.5 5.2 49.6 6.6 138.8 19.0 1.9 1.0 _______ _______ 19.9 3.2

Sum Pyrolyses

31740 702

2540 54

160.6 23.2

9.84 0.78

0.2076 0.0028

275.2 38.7

4.075 0.049

10.23 0.19

266.6 37.5

4.044 0.050

Total

45160 1690

2671 63

880.4 90.0

11.38 0.40

0.0922 0.0024

899.1 46.70

4.500 0.051

56.21 0.42

731.1 38.0

4.346 0.058

320

11.36 0.45 11.15 0.24 13.58 0.48 11.21 0.24 11.59 0.27 13.31 0.46 13.62 0.35 13.67 0.33 12.98 0.27 11.71 0.27 8.56 0.66 9.68 0.70 9.14 3.86 -____--

36Ar

11.26 1.05

gas concentrations measured in our ‘‘clear” sample are larger than their values, after factoring in the grain size difference, average 4He surface densities are essentially equal and average 36Ar surface densities agree to within a factor of two. Gas

Table 6 shows a comparison of our elemental abundance data for the two 6005 1 total samples (argon-corrected in the case of the “clear” sample) with the averaged values for the single grain measurements of Wieler et al. ( 1980). Although

Table 5. Helium, neon and argon abundances and isotopic ratios from 9.5 mg of “cloudywlear” 6005 1 plagioclase (74- 147 p). The separate was analyzed in a stainless steel finger. Gas abundances are expressed in units of 10-s cc STP/g. la errors are listed below e&h measurement. * T = 53’C. Step Description

4He

4H&He

1st Water Wash 63 min 2nd Water Wash 955 min 1st Acid Etch 61 min: oH=l.l 2nd Acid-Etch 1093 min: pH=l.l 3rd Acid Etch 927 min: pH= 1.0 4th Acid Etch 948 min: pH=l.O* 1st Pyrolysis

1531 f158 1023 114 22390 1858

11535 579 2499 62 2853 187

535 626 80 174 40 17230

42 2988 136

me

%&*Ne

*lNe@*Ne

36~r

36ArPur

@A&k 89.54 3.07 13.62 0.23 1.61 0.01 1.24 0.01 1.93 0.03 9.83 0.47 7.59 0.15 7.40 0.04

7977 682

12.78 0.30 13.38 0.29 12.20 0.12 12.12 0.13 10.95 0.32 12.06 0.52 9.07 0.09 11.34 0.12

0.0299 0.0014 0.0345 0.0013 0.0364 0.0004 0.0572 0.0018 0.1164 0.0044 0.0402 0.0029 0.2590 0.0078 0.0718 0.0022

1.6 0.1 5.7 0.2 627.9 16.5 243.8

300 1892 19 2722 84

27.2 2.3 46.0 3.7 638.7 43.3 111.8 8.3 12.7 1.1 6.7 0.7 198.5 16.4 248.6 19.4

;:: 0.7 4.6 0.2 26.4 0.8 41.6 1.7

5.636 0.076 5.135 0.062 5.351 0.053 5.183 0.052 4.683 0.049 4.800 0.054 3.934 0.059 2.239 0.025

Sum Pyrolyses

25200 2082

2094 40

447.1 35.8

10.21 0.11

0.1653 0.0047

68.0 2.5

2.689 0.038

7.47 0.08

Total

56920 4867

2474 115

1290 94

11.44 0.13

0.0895 0.0021

980.0 25.9

4.946 0.052

2.19 0.02

2nd Pyrolysis

P. E. Rider, R. 0. Pepin, and R. H. Becker

4994

concentrations are less in the “clear” sample than in the “cloudy + clear” sample by 20-30% for all three noble gases measured. This is consistent with the “cloudy” plagioclase having a more extensive solar wind exposure history. Other than this difference in gas concentrations, however, there is little in the elemental or isotopic compositions of the total gases to differentiate our two samples. In fact, other than having overall lower gas contents (when corrected for grain size) and being somewhat less spallogenic, the “clear” sample fits in with a series of plagioclase separates from the Apollo 16 deep drill core reported by Wieler et al. ( 1983), whose histories are presumably much different from that proposed for 6005 1. Thus, there is nothing in the total gas data that would mark this sample as particularly modern-day. There is some interesting information in the stepwise data for 60051. Specifically, as noted above, the earliest releases in both the series of water washes and series of acid etches show SEP-like neon (and apparently Ar as well). This is somewhat unexpected, especially if the sample consists of grains with a very short exposure history. It has been generally assumed that the SEP contribution to the total solar gas in a given lunar sample builds up over time from values of -0.1 to values of -0.5 (Wieler et al., 1986; Nichols et al., 1994), due to processes that remove grain surfaces and maintain the solar-wind component in more or less steady-state concentration ( “saturation”). If the solar-wind component has not yet saturated, the relative abundance of SEP should not have had a chance to increase, and moreover, there should not be a significant amount of SEP near grain surfaces to be etched in the early steps. In this context, the results of measurements performed on single plagioclase grains from this soil (A. 0. Nier and D. J. Schlutter, pers. commun.) using pulse heating (Nier and Schlutter, 1993a) are of interest. In previous studies (Nier and Schlutter, 1993b, 1994), ilmenite grains from breccia 79035 and soil 71501 were shown to yield a two-component noble gas system, one released at low temperatures with a solar wind-like isotopic composition, and the other released at higher temperatures with a *“Ne/*‘Ne composition similar to SEP. When 60051 plagioclase grains were measured there was no indication of a significant, isotopically heavy, deeply sited second component. These pulse heating results support the idea that the SEP component does not yet exist in this population of plagioclase grains to the degree seen in other samples, as one would expect if these grains are extremely young. TABLE 6.

1.0 r

75081 pyroxene 31.9 mg

%30Ar released

%%Ar reteaeed FIG. 7. %d6Ar vs. percent “Ar released for (a) both analyses of 75081 pyroxene (results from a CSSE experiment performed on a 7 1501 pyroxene separate reported in Wieler et al. (19t36) are shown for comparison) and (b) for the 60051 ‘*clear”-grained and “cloudy” + “clear”-grained samples. The ratios and the “6Ar abundances are corrected for air contamination, when appropriate, assuming a *Ar/ 3bAr ratio of 296 for air and 1 for the sampIe. Solar-wind value from

Bochsler and Geiss (1977). SEP value (same as SW) from Benkert et al. (1993).

Gas abundances are expressed in units of lo-” cc SIT/g. la errors are listed below each measurement. a&!

(Wieler et al. 1980)

26500 f11300

60051 “clear” grains (this work)

45200 f1700

60051 “cloudy +clear” grains (this work)

56900 It4900

60051 “clean” grains

71501 plagioclase grains (Wieler d al. 1980)

251000 f1640aO

me

3fi&

_____ _____

210 M20

880 SO

731 k38

1290 B4 --___ --___

980 3~26 1870 f1800

Benkert et al. ( 1993) also observed early release of SEP Ne, in an ilmenite sample. Their explanation, which invoked an easily etched phase that had lost its solar wind component, should not apply here, since the 60051 sample as a whole should be easily etchable. One would expect to find solar wind ions in sites as readily etchable as those supplying the SEPlike component. It is noteworthy that the (contamination-corrected) integrated zoNe/‘6Ar ratio for the first five steps equals that of solar wind or SEP (2”Ne/.16Ar = 45; Bochsler and Geiss, 1977; Benkert et al., 1993). If solar wind has been lost

Noble gases and N released from lunar soils from some phase, it must have undergone that loss in a way that physically removed the entire component (Frick et al., 1988; Nichols et al., 1994), rather than by diffusion of Ne, since otherwise the solar wind Ar component would be partially retained and the SEP Ne partially lost. No initial SEP-like component was seen in the second 60051 separate. A likely explanation is in the different experimental protocols employed for each sample. Weak acid (pH - 1.8) was used initially to analyze the “clear”grained sample, whereas strong acid (pH - 1.1) was used for the “cloudy + clear”-grained sample. Consequently acid etch #l removed -50% of the total *“Ne, most likely burying the SEP-like “surface” components in the overall release of solar wind Ne. The initial water wash appears to contain a mixture of solar-wind and SEP Ne, possibly reflecting the presence of solar-wind Ne in more radiation-damaged surfaces of the ‘‘cloudy” grains. 3.4. General Observations About the Light Gases Examining the 2we/“6Ar (Fig. 7a,b) and 4He/‘6Ar ratios (not shown) from the mineral separates in Tables 1,2,4, and 5 as a function of 36Ar release, there is a general tendency for the ratios to decrease initially, and then to increase. Many of the samples analyzed in the Ztirich laboratory show this behavior as well. Their explanation is that the upward trend at higher release represents the measurement of relatively unfractionated SEP, after release of a solar-wind component that has been elementally fractionated by some process. Our samples do not approach the SEP elemental ratios (equivalent to solar, according to Benkert et al., 1993) at the end of our analyses, perhaps because of insufficient resolution in the pyrolysis steps, or perhaps simply because these minerals, unlike ilmenite, are not sufficiently retentive for SEP He and Ne. Note that data from a 71501 pyroxene separate reported by Wieler et al. ( 1986) also showed decreasing ratios of He and Ne relative to Ar for argon releases approaching 100%. Isotopic ratios, particularly for neon, also show patterns similar to those seen previously by the Zurich group. Considering specifically the larger pyroxene sample and the ‘‘cloudy + clear” plagioclase, 4He/‘He and *we/**Ne ratios begin in the vicinity of the solar wind ratios and trend in the direction of mixing with SEP (or of mass fractionation) before becoming spallogenic. These data however, do not contribute to resolving the question of the presence of an SEP component as opposed to a fractionated solar-wind component, since it is apparently difficult to remove the entire solar particle reservoir of our samples under the most extreme conditions of etching used here. Other than the early releases of Ne from the “clear” plagioclase, there are no unambiguous examples of the SEP component in the etching data such as those observed by Benkert et al. ( 1993 ) . One pyrolysis step, the first 1135°C pyrolysis of bulk 79035, does however show the distinct SEP signature. The neon, argon, and helium isotopic compositions from this step are all distinctly SEP-like. This observation is consistent with the current conception of solar wind and SEP reservoirs in that one might expect to observe the more deeply sited gases if the previous pyrolyses were of sufficient length and duration to release the surficially sited solar wind gases. It is the

4995

best isotopic evidence we have found for the existence of a deeply sited SEP component. However, the elemental ratios of this step are fractionated with respect to expected values. Moreover, later pyrolyses, while admittedly yielding much less gas, look less SEP-like and more solar, inconsistent both with the idea that the solar reservoir had been emptied and with the possibility of mass fractionation. The best explanation for this behavior would be a trace phase in this bulk soil that was extremely retentive of its solar wind component (perhaps buried within agglutinatic material) and which finally released its gases in these final pyrolysis steps. 4. CONCLUSIONS Initial results of CSSE experiments performed on lunar soils using relatively weak etchants to probe the most shallowly implanted reservoir of solar wind ions appear to have promise for the noble gases. H2SOYis a sufficiently strong acid to release significant fractions (up to 35% in pyroxene and around 80% in a bulk soil) of the implanted solar gases in lunar regolith materials. Even water can release nonnegligible amounts of these solar gases. As a result, the use of aqueous liquids (including Clerici solution) in the preparation of lunar separates is contra-indicated. The patterns of noble gas abundances and isotopic compositions observed for the solar-wind component, and the SEP component to the degree we can measure it, in these experiments are similar to those seen previously in CSSE experiments using strong acids (Benkert et al., 1993; Wieler and Baur, 1994). In addition, it appears that for Ar, Kr, and Xe, results obtained from pyroxene separates are of comparable usefulness, as far as the preservation of solar wind signatures is concerned, to those obtained on the much less abundant ilmenite, which has been until now the mineral of choice. This is not true for He and Ne, however. Although this study was designed with the intention of measuring nitrogen in conjunction with the noble gases, this goal has not yet been achieved. Nitrogen release from the samples was considerably less than expected, possibly because the nitrogen is released in a nonvolatile form from etched samples. It is not yet possible to free the nitrogen by pyrolysis from etching residues, because a suitable material (low blank and capable of being heated to high temperature) has not been found for the fabrication of the sample finger. However, there is evidence from 79035 acid treatment steps that trapped solar nitrogen is released to some degree by this technique. A separate of “clear” plagioclase grains from soil 60051 was found to release what appears to be endmember SEP under very mild etching steps prior to the release of solar wind ions. This is an interesting observation, considering that the use of weak acids should select against the possible observation of SEP gases in favor of the solar wind component. We do not believe this to be an experimental artifact, nor is it likely to be due to the presence of highly etchable glass, since the sample is handpicked to be free of such material. Why we do see the SEP, and in such large quantity, we do not understand. We also appear to observe the SEP endmember in a pyrolysis step late in the analysis of a grain-size separate of lunar breccia 79035.

P. E. Rider, R. 0. Pepin, and R. H. Becker

4996

The “clear”

plagioolase

from soil 60051 was thought to

be a unique soil component with a short, modem-day solar

wind exposure. Noble gas analyses are consistent with a relatively short exposure but do not otherwise differ significantly from a suite of plagioclase separates from the Apollo 16 deep drill core. The question of the uniqueness of this soil now rests on planned nitrogen isotopic measurements by standard pyrolytic means. Acknowledgments-We thank R. Wieler, 0. Eugster, and an anonymous reviewer for their careful, thorough, and helpful reviews. We also thank Bob Oswald for preparing the mineral separates used in this study. This work was supported through NASA grants NAG 960 and NAGW-3336. Editorial handling: K. Marti REFERENCES Anders E. and Grevesse N. ( 1989) Abundances of the elements: Meteoritic and solar. Geochim. Cosmochim. Acta 53,197-214. Becker R. H. ( 1990) Model calculations of solar wind and SEP neon isotopic distributions in lunar regolitb grains. Lunar Planet. Sci. XXI, 56-57 (abstr.). Becker R. 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, 11351146. Benkert J.-P., Baur H., Signer P., and Wieler R. (1993) He, Ne, and Ar from the solar wind and solar energetic particles in lunar ilmenites and pyroxenes. J. Geophys. Res. (Planets) 98, 13,14713,162. Bochsler P. and Geiss J. ( 1977) Elemental abundances in the solar wind. Trans. Intl. Astron. Union XVI 3, 120- 123. Cameron A. G. W. ( 1982) Elemental and nuclidic abundances in the solar system. In Essays in Nuckar Astrophysics(ed. C. A. Barnes et al.), pp. 23-43. Cambridge Univ. Press. Clayton R. N. and Thiemens M. H. ( 1980) Lunar nitrogen: Evidence for a secular change in the solar wind. In The Ancient Sun (ed. R. 0. Pepin et al.), pp. 463-473. Pergamon. Frick U., Becker R. H., and Pepin R. 0. (1988) Solar wind record in the lunar regolith: Nitrogen and noble gases. Proc. 18th Lunar Planet. Sci. Con& 87-120. Geiss J. and Bochsler P. ( 1991) Long time variations in solar wind properties: Possible causes versus observations. In The Sun in Time (ed. C. P. Sonett et al.), pp. 98-l 17. Univ. Arizona Press. Hintenberger H., Weber H. W., and Schultz L. (1974) Solar, spalloge&, and radiogenic rare gases in Apollo 17 soils and breccias. Proc. 5th Lunar Sci. Con& 2005-2022. Kerridge J. F. ( 1989) What has caused the secular increase in solar nitrogen- 15? Science 245,480-486.

Kerridge J. F. ( 1993) Long-term compositional variation in solar corpuscular radiation: Evidence from nitrogen isotopes in the lunar regolith. Rev. Geophys. 31,423-437. Kerridge J. F., Signer P., Wieler R., Becker R. H., and Pepin R. 0. ( 1991) Long-term changes in composition of solar particles implanted in extraterresbial materials. In The Sun in Time (ed. C. P. Sonett et al.), pp. 389-412. Univ. Arizona Press. Nichols R. H., Jr., Hohenberg C. M., and Olinger C. T. (1994) Implanted solar helium, neon, and argon in individual lunar ilmenite grains: Surface effects and a temporal variation in the solar wind composition. Geochim Cosmochim. Acta 58, 1031- 1042. Nier A. 0. and Schlutter D. J. ( 1993a) Helium in interplanetary dust _ particles. Lunar Planet. Sci. XXIV, 1075-1076 (abstr.). Nier A. 0. and Schiutter D. J. (1993b) Extraction of He and Ne from individual lunar ilmenite grains by pulse heating. Meteoritics 29, 412 (abstr.). Nier A. 0. and Schlutter D. J. (1994) Helium and neon in lunar ilmenites of different antiquities. Meteoritics 29,662-673. Pepin R. 0. ( 1991) On the origins and early evolution of terrestrial planet atmospheres and meteoritic volatiles. Zcarus 92,2-79. Pepin R. O., Becker R. H., and Rider P. E. ( 1995) Xenon and krypton isotopes in extraterrestrial regolith soils and in the solar wind. Geochim. Cosmochim. Acta 59,4897-4822 (this issue). Rider P. E., Becker R. H., and Pepin R. 0. (1992) Measurement of solar wind noble gas composition in lunar soils by in vacua acid etching. Lunar Phznet Sci. Xron, 1149- 1150 (abstr.). Signer P., Baur H., and Wieler R. (1993) Closed system stepped etching: An alternative to stepped heating. In Proc. AZfred 0. Nier Symp. Znorg. Mass Spectrometty, Durango, New Mexico (ed. D. J. Rokop), Los Akunos Natl. Lab. Publ. LA-12522-C pp. 181202. Thiemens M. H. and Clayton R. N. ( 1980) Ancient solar wind in lunar microbreccias. Earth Planet. Sci. Zen. 47, 34-42. Wieler R. and Baur H ( 1994) Krypton and xenon from the solar wind and solar energetic particles in two lunar ihnenites of different antiquity. Meteoritics Y&570-580. Wieler R., Etique P., Signer P., and Poupeau G. ( 1980) Record of the solar corpuscular radiation in minerals from lunar soils: A comparative study of noble gases and tracks. Proc. ZZth Lunar Planet. Sci. Conf, 1369-1393. Wieler R., Etique P., Poupeau G., and Signer P. ( 1983) Decrease of the solar flare/solar wind flux ratio in the past several aeons deduced from solar neon and tracks in lunar soil plagmclases. Proc. 13th Lunar Planet. Sci. Conf, A713-A724. Wieler R., Baur H., and Signer P. (1986) Noble gases from solar energetic particles revealed by closed system stepwise etching of lunar soil minerals. Geochim. Cosmochim. Acta 50, 19972017. Wieler R., Baur H., and Signer P. (1992) Krypton and xenon from solar energetic particles in a lunar ilmenite. Lunar Planet. Sci. XXIII, 1525-1526 (abstr.). Wieler R., Baur H., and Signer P. (1993) A long-term change of the Ar/Kr/Xe fractionation in the solar corpuscular radiation. Lunar Planet. Sci. WrIV, 1519-1520 (abstr.).