Solar and cosmogenic argon in dated lunar impact spherules

Geochimica et Cosmochimica Acta 71 (2007) 1624–1635 www.elsevier.com/locate/gca

Solar and cosmogenic argon in dated lunar impact spherules Jonathan Levine a

a,*

, Paul R. Renne

b,c

, Richard A. Muller

d,e

Chicago Center for Cosmochemistry and Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, USA b Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA c Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA d Department of Physics, University of California, Berkeley, CA 94720, USA e Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Received 11 August 2006; accepted in revised form 13 November 2006; available online 12 January 2007

Abstract We have studied lunar impact spherules from the Apollo 12 and Apollo 14 landing sites, examining the isotopic composition of argon released by stepwise heating. Elsewhere, we reported the formation ages of these spherules, determined by the 40 Ar/39Ar isochron method. Here, we discuss solar and cosmogenic argon from the same spherules, separating these two components by correlating their partial releases with the releases of calcium-derived 37Ar on a ‘‘cosmochron’’ diagram. We use the abundances of cosmogenic argon to derive a cosmic ray exposure age for each spherule, and demonstrate that single scoops of lunar soil contain spherules which have experienced very different histories of exposure and burial. The solar argon is seen to be separated into isotopically lighter and heavier fractions, which presumably were implanted to different depths in the spherules. The abundance of the isotopically heavy solar argon is too great to explain as a minor constituent of the solar particle flux, such as the suprathermal tail of the solar wind. The fact that the spherules have been individually dated allows us to look for possible variations in the solar wind as a function of time, over the history of the Solar System. However, the isotopic composition and fluence of solar argon preserved in the lunar spherules appear to be independent of formation age. We believe that most of the spherules are saturated with solar argon, having reached a condition in which implantation by the solar wind is offset by losses from solar-wind sputtering and diffusion.  2007 Elsevier Inc. All rights reserved.

1. INTRODUCTION Lunar impact spherules are nearly spherical droplets of glass found in lunar soil, formed from rock either melted or vaporized during meteoroid impacts on the Moon. A single scoop of lunar soil contains spherules from many different impacts, whose ages permit an estimate of the relative flux of impactors to the Moon (and the Earth) at different epochs in the history of the Solar System (Muller, 1993). In two previous papers (Culler et al., 2000; Levine et al., 2005), we used the 40Ar/39Ar isochron technique to determine the formation ages of 81 spherules from an Apollo 12 soil sample and 109 spherules from an Apollo 14 sample. Nearly all of these spherules were of impact origin;

*

Corresponding author. E-mail address: [email protected] (J. Levine).

0016-7037/$ - see front matter  2007 Elsevier Inc. All rights reserved. doi:10.1016/j.gca.2006.11.034

though lunar spherules can also be formed in volcanic eruptions, we argued that most of our spherules formed in impacts, using geochemical criteria (e.g., Delano and Livi, 1981) and, for those spherules with ages younger than 3000 Ma, the fact that they formed after the end of lunar volcanism. An observed excess of spherules younger than 400 million years old in each sample could indicate an increase in the production rate of lunar craters since that time, but alternative explanations, including geological sampling biases, cannot be ruled out. In addition to their radiogenic 40Ar, which allows the determination of formation ages, lunar spherules contain 36 Ar and 38Ar implanted by solar corpuscular radiation and created in situ by cosmogenic spallation of heavier nuclides, especially 40Ca. Noble gases of inferred solar origin were recognized in lunar materials from the earliest examinations of Apollo mission samples (Lunar Sample Preliminary Examination Team, 1969, 1970). Eberhardt et al. (1970)

Solar and cosmogenic argon in lunar spherules

showed by acid etching that most of the noble gases in lunar fines were concentrated in the outer 200 nm of the particles, favoring implantation of the noble gases by the solar wind. Pepin et al. (1970) and Hohenberg et al. (1970) performed stepwise heating experiments on lunar samples, in which they first identified cosmogenic noble gases in lunar materials, and also noted that the solar noble gases released in successive heating steps tended to become isotopically heavier. Additional measurements over the last 35 years have continued to observe this separation between isotopically lighter solar noble gases, released early in each experiment, and isotopically heavier gases released afterward. Evidently the isotopically heavier solar gases are released from greater depths in the specimens than is the isotopically lighter fraction. Two broad classes of explanations have been offered for this behavior: either the isotopically heavy noble gases represent a distinct regime of the solar wind which is implanted into lunar grains with relatively high energy (e.g., Black, 1972; Wieler et al., 1986), or an isotopically homogeneous solar wind is fractionated upon implantation, with heavier species impinging on lunar grains with higher average energies and thus penetrating more deeply than lighter isotopes (e.g., Hohenberg et al., 1970; Mewaldt et al., 2001). New measurements by Grimberg et al. (2006a,b,c) of Genesis mission samples favor the latter alternative. Here we present the first detailed analysis of solar and cosmogenic argon from individual dated lunar impact spherules. Our work cannot discriminate between the two explanations of separated isotopically light and heavy solar noble gases, but we instead discuss the implications of our observations for either model. We report the isotopic composition of the implanted solar argon, and we present data concerning the exposure of the spherules to solar and cosmic radiation, which reflect the history of overturning or ‘‘gardening’’ of the lunar soil. 2. METHODS Individual glass spherules were hand-picked from sieved fractions of lunar soils 12023 and 14163. The former sample (see Levine et al., 2005) was collected by the Apollo 12 astronauts from a 20 cm deep trench which they dug into the ejecta of Sharp Crater (diameter 14 m). The latter sample (Culler et al., 2000) was from the Apollo 14 mission, and represents ejecta from Cone Crater (diameter 300 m). The spherules ranged in size from 100–600 lm in diameter. The Apollo 12 spherules were examined by secondary electron microscopy to determine the abundances of major elements. In preparation for argon isotopic analysis, each suite of spherules, along with appropriate mineral and glass standards, was irradiated with fast neutrons, creating short-lived 39Ar and 37Ar from potassium and calcium (Merrihue and Turner, 1966; Turner et al., 1971). The Apollo 14 spherules (Culler et al., 2000) were irradiated for 100 h in the cadmium-shielded CLICIT facility of the Oregon State University TRIGA reactor. The fluence of fast neutrons received by the spherules may be characterized by the parameter J = 0.0261 ± 0.0001,

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where J is the fraction of 39K atoms transmuted to Ar (by the 39K(n,p)39Ar reaction), normalized by the branching ratio of 40K decay to 40Ar and the ratio of potassium isotopes 40K/39K. Similarly, the Apollo 12 spherules were irradiated for 207 hours in a cadmium-lined vessel in the core of the McMaster Research Reactor, in Hamilton, Ontario. The J parameter for Apollo 12 spherules varied between 0.0447 and 0.0458, depending on the position of each spherule in the reactor, with an uncertainty of 0.0003 (one standard deviation). The fast neutrons from the reactors initiated the reactions 39K(n,p)39Ar and 40Ca(n,a)37Ar in the specimens. Before neutron irradiation, the concentrations of 39Ar and 37 Ar were assumed to be negligible; though both nuclides may be produced cosmogenically on the Moon, the saturation concentration of 39Ar (half-life 269 years) should be low enough—and the decay of 37Ar (half-life 35 days) should be fast enough—for their abundances to be below our detection limits. The reactor-produced argon isotopes are therefore quantitative proxies for 40K and 40Ca, the respective parent nuclides of radiogenic 40Ar and most cosmogenic 36Ar and 38Ar. The irradiated spherules were degassed of their argon by automated step-heating with infrared lasers (an argon ion laser in the case of the Apollo 14 spherules and a CO2 laser for the Apollo 12 spherules). Heating took place under ultra high vacuum (pressure 109 torr), inside an extraction line leading to a MAP 215 noble gas mass spectrometer at the Berkeley Geochronology Center. The argon from each spherule was degassed and analyzed in 4–30 steps, with 6–8 steps being most common. Instrumental mass discrimination was monitored by regular analysis of air aliquots from an online pipette system, and a power-law correction was applied. Typical values of the discrimination D were 1.006 ± 0.002 per atomic mass unit for the Apollo 14 spherules (based on 231 air aliquots) and 1.004 ± 0.001 per atomic mass unit for the Apollo 12 spherules (202 air aliquots). Measured releases of argon isotopes were also corrected for procedural backgrounds, measured after every 3 heating steps of spherules or mineral standards. The instrumental blank level was 1–2 · 1015 moles of 40Ar, and 1017 moles of 36–39Ar. Further correction was made for the effects of interfering argon-producing nuclear reactions which occurred during irradiation. These reactions were monitored by co-irradiation of glasses and of the mineral standards hornblende Hb3gr (Turner et al., 1971; Renne, 2000) and Fish Canyon sanidine (Renne et al., 1998) with each suite of spherules. In particular, we were able to make corrections for small amounts of 37Ar produced from reactions on potassium and 39Ar from reactions on calcium. Thus, the corrected measurements of 37Ar represent only the calcium contents of the spherules, and 39Ar measurements likewise represent only their potassium contents. The interference corrections used for the Apollo 14 spherules were derived from many measurements in the Oregon State TRIGA reactor over a decade, and are summarized by Renne et al. (2005). 39

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Fig. 1 shows a correlation diagram of 38Ar/36Ar and Ar/36Ar ratios for a typical spherule. The measured partial releases of argon from this spherule lie along a straight line in this space, which we interpret as a mixing line between a component with 38Ar and 36Ar that are unaccompanied by 37Ar, and a component with all three isotopes in a certain proportion. We identify the former, calciumfree component as implanted solar argon, and the latter component, which contains 38Ar and 36Ar in proportion to the abundance of calcium, with cosmogenic argon. The first heating steps release the most nearly pure solar argon, implying that this component is derived from near the spherule surface, and successive heating steps contain progressively greater admixtures of cosmogenic argon. However, in most spherules, the total amount of cosmogenic argon is not more than 5–10% of the abundance of solar argon. The correlation diagram in Fig. 1 is analogous to an argon isochron diagram, in which 40Ar/36Ar and 39Ar/36Ar ratios are plotted. Just as the slope of an isochron line permits the derivation of a formation age, the slope of the mixing line in Fig. 1 permits the determination of the cosmic ray exposure age, a measure of the cosmic radiation dose received by the sample since its formation. We therefore call this correlation diagram a ‘‘cosmochron diagram,’’ and a mixing line in this diagram a ‘‘cosmochron.’’

3. ANALYSIS AND RESULTS

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3.1. Component identification Measuring the reactor-produced isotopes 37,39Ar along with the stable isotopes 36,38,40Ar in multiple release steps allows us to identify how much of each stable isotope is associated with potassium or calcium (and therefore could have formed in situ by radioactive decay or spallation) and how much is uncorrelated with these elements (and therefore must have been implanted or trapped in the specimen). The identification of radiogenic and non-radiogenic 40Ar components by correlating partial releases of 40Ar with those of potassium-derived 39 Ar is at the heart of 40Ar/39Ar dating (Merrihue and Turner, 1966). Similarly, Turner et al. (1971) introduced the idea of identifying cosmogenic and non-cosmogenic components of 38Ar by correlating partial releases with those of the calcium proxy 37Ar. As noted by Turner et al. (1971), calcium is the principal parent of cosmogenic argon in lunar materials because of its high abundance and the relatively large cross sections for the appropriate spallation reactions; however, small quantities of cosmogenic argon are created from potassium and heavier elements. 0.28

Cosmic ray exposure age 410 ± 36 Ma Intercept 0.19054 ± 0.00060 MSWD = 0.652 prob = 0.6588 n = 7 (out of 7)

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Fig. 1. Cosmochron diagram for Apollo 12 spherule H32. The correlation of 38Ar/36Ar and 37Ar/36Ar ratios shows that all the partial releases of argon from this spherule lie along a mixing line between a component with 37Ar = 0 (i.e., not associated with any calcium in the spherule) and a component in which 38Ar and 36Ar are released in proportion to 37Ar. The 37Ar (half-life 35 days) is a proxy for the presence of 40Ca in the spherule, because it was created by 40Ca(n,a)37Ar reactions when the sample was irradiated by fast neutrons in a reactor. 40Ca is also the dominant source of spallation 38Ar and 36Ar within the spherule; therefore we identify the component in which 38Ar, 36Ar, and 37Ar are released in a fixed proportion with cosmogenic argon. The other component, free of 37Ar, must be implanted solar argon, whose 38Ar/36Ar ratio may be found from the intercept of the best fitting line with the vertical axis. The slope of the best fitting line is proportional to the dose of cosmic radiation received by the spherule, which we characterize by a cosmic ray exposure age. MSWD, mean square weighted deviation of the measurements from the best-fit line. Prob, probability that residuals can be explained by measurement errors alone. For approximately 75% of the spherules, all partial releases of argon lie along straight lines on cosmochron diagrams, as in this case. Numbers next to each measurement denote the sequence of heating steps; 1r error ellipses are shown.

Solar and cosmogenic argon in lunar spherules

A correlation similar to that in Fig. 1 was noted by Eberhardt et al. (1970), who plotted 36Ar/38Ar against 1/38Ar ratios for bulk analyses of ilmenite grains of different sizes. An advantage of the cosmochron diagram is that the cosmogenic 38Ar is correlated in an obvious way with its dominant source nuclide, 40Ca, represented by its reactor-produced proxy 37Ar. We have neglected the small contributions of cosmogenic argon in lunar samples that are due to spallation of potassium, whose abundance is usually quite low, and heavier elements such as titanium and iron (e.g., Hohenberg et al., 1978). This is warranted insofar as we observe statistically good correlations between the 38Ar/36Ar and 37 Ar/36Ar ratios, even though most spherules are chemically heterogeneous (Levine and Rohde, 2006). Spallation of targets other than calcium would tend to make the cosmic ray exposure ages we report too high by up to 10%. The behavior shown in Fig. 1, with all heating steps from the specimen yielding isotope ratios that lie along a single straight line, is shared by most of the spherules. (Partial argon releases that were comparable to instrumental blanks were excluded from this analysis, because their large measurement uncertainties artificially enhance the agreement between the data and the cosmochron lines fit to them.) Seventy percent of the Apollo 12 spherules and 85% of the Apollo 14 spherules gave statistically acceptable cosmochron lines through all the measurements, with v2 per degree of freedom (equivalently, mean square weighted deviance or MSWD) small enough that mixing of only two components suffices to explain the data with 95% confidence. By contrast, only about half the spherules yielded straight-line isochrons (Levine et al., 2005), implying that more than two isotopically distinct sources of 40Ar were present in many spherules. Furthermore, many of the isochron lines were identified only after excluding one or more heating steps for suspected atmospheric contamination, partial loss of argon in a post-formation thermal event on the Moon, or incomplete degassing of a clast hidden within the spherule, explanations which are not demanded by the cosmochron analysis. The cosmogenic isotopes 38Ar and 36Ar, along with calcium-derived 37Ar, evidently constitute a more simply mixed system than do 40Ar and potassium-derived 39Ar. We can offer some possible explanations, none of them completely satisfactory, for the observation that straightline cosmochrons are more common than straight-line isochrons. First, it is conceivable that spherules include cosmogenic 40Ar, which could cause deflections from an isochron line. However, estimates of production rates by Hohenberg et al. (1978) suggest that any cosmogenic 40 Ar should be negligible compared with radiogenic 40 Ar. Another possibility is that many spherules form by the fusion of adjacent grains in meteorite impacts, and those without straight-line isochrons were not completely degassed as they formed. If multiple grains, formed at different times and each with its own endowment of radiogenic 40Ar, were fused into a single spherule, it would be impossible to define an isochron age for such a specimen. However, if all the parent grains previously resided beneath the penetration depth of cosmic rays (or, by coincidence, shared a common exposure history to cosmic rays), then the composite spherule would nevertheless have a sin-

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gle cosmic exposure age, and we would observe a straightline cosmochron. An origin for spherule parent materials beneath the cosmic ray penetration depth is plausible, according to Ho¨rz (2000), who suggests that spherules are formed from impacts into lunar bedrock, which is typically shielded by 10 m of overlying regolith. Nevertheless, the hypothesis of spherule formation from bedrock rather than from soils awaits experimental or observational confirmation. A third possible explanation for the relative scarcity of straight-line isochrons is that the distribution of implanted 40 Ar, which comes from atoms in the lunar exosphere that are ionized and accelerated by the solar wind (Manka and Michel, 1970), overlaps poorly with solar 36Ar in the spherules. A perfect isochron line is expected only if all the 40Ar in the specimen comes from two components, one of which is sited together with potassium-derived 39Ar (Merrihue and Turner, 1966), and the second with another identifiable isotopic component, which is used to normalize the isotopic ratios on both axes of the isochron diagram. For terrestrial samples, 36Ar from the atmosphere is used for this purpose. For extraterrestrial samples, the best candidate for this second component is solar argon (represented by the solar 36 Ar, which is more abundant than solar 38Ar), since it is implanted at the same time and from the same direction as the parentless 40Ar. However the implantation energy of solar argon (1 keV/amu) is much larger than the implantation energy in the Manka and Michel mechanism (0.03 keV/amu; Manka and Michel, 1970), so the solar wind atoms may reach greater depths than the parentless 40 Ar. Then, even slight abrasion or sputtering of spherules could make the 40Ar/36Ar ratio vary along their surfaces, rendering ill-defined one of the two isotopic components whose mixing gives rise to the isochron line. The fact that roughly half of all spherules we studied did not yield statistically acceptable isochron ages is important for the interpretation of our geochronological data in Culler et al. (2000) and Levine et al. (2005). In this paper, however, our focus is instead on the abundant examples of straight-line cosmochrons, and what information we can glean from them about the cosmogenic and solar argon found in lunar impact spherules. For spherules that have also yielded isochron ages, the geochronological data contextualize what we learn about exposure to cosmic and solar radiation. 3.2. Cosmogenic component A straight cosmochron line provides useful information from both its slope and its intercept with the vertical axis. If the amounts of 38Ar and 36Ar released in the nth heating step on a given spherule are represented by unknown mixtures of cosmogenic and solar argon, as for example by the expression 38 Arn ¼ 38 Arncosmo þ 38 Arnsolar , then we may relate the measured amounts of 36,37,38Ar by 38   38  38   38 Arn 37 Arn Ar Ar Ar ¼ 1  36 Arn 36 Arn 37 Ar 36 Ar 36 Ar solar cosmo 38  cosmo  Ar þ : ð1Þ 36 Ar solar

J. Levine et al. / Geochimica et Cosmochimica Acta 71 (2007) 1624–1635

Eq. (1) supposes that the solar component contains 36Ar and 38Ar in a fixed proportion, and that the cosmogenic component includes a fixed proportion of the three isotopes 36,37,38 Ar. The quantities in curly braces depend on the isotopic compositions of the mixing components, so they should not vary between successive heating steps if the two end-members are isotopically homogeneous. The constancy of the quantities in braces is demonstrated if the partial releases from a specimen plot along a single line on the cosmochron diagram. Then, the intercept of the cosmochron line determines the 38Ar/36Ar ratio of the implanted solar component. By contrast, the 38Ar/36Ar ratio of the cosmogenic component is not determined from these measurements, but must be assumed in order to use the slope of the line to calculate the accumulated cosmic radiation dose, which is encapsulated in the factor ð38 Ar=37 ArÞcosmo . This factor can be translated to other units, such as cosmogenic 38Ar per gram of Ca, for example, by measuring the 37 Ar produced during the neutron irradiation of a known quantity of calcium; we used hornblende Hb3gr (Turner

et al., 1971; Roddick, 1983; Renne, 2000) for this purpose. The 38Ar/36Ar ratio of the cosmogenic component should vary weakly with sample chemistry and shielding on the Moon. Estimates of spallation cross sections and primary particle penetration depths differ slightly as well (Turner et al., 1971; Huneke et al., 1972; Hohenberg et al., 1978; Eugster and Michel, 1995). We assumed a cosmogenic 38 Ar/36Ar ratio of 1.6 (Turner et al., 1971), and assigned this value a rather large uncertainty of 0.1 to encompass the various estimates of this ratio. We express the cosmic radiation dose received by a spherule in terms of an effective cosmic ray exposure age, adopting the nominal production rate of spallation 38Ar used by Turner et al. (1971) of 1.4 · 108 cm3 (STP) per gram of calcium per million years. Fig. 2 shows the cosmic ray exposure ages of those spherules for which an isochron formation age was found (Levine et al., 2005). We emphasize that no single production rate value is strictly applicable to all lunar samples, as this rate decreases as a function of shielding (i.e., depth) within the lunar soil. A spherule

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Fig. 2. Cosmic ray exposures of lunar spherules as a function of formation age. The cosmic ray exposures of Apollo 12 (top) and Apollo 14 (bottom) spherules are represented either as the accumulated quantity of cosmogenic 38Ar per gram of calcium (left axis) or, equivalently, as a cosmic ray exposure age (right axis), using the normal production rate of cosmogenic 38Ar from Turner et al. (1971). Only those spherules for which a formation age was determined by the 40Ar/39Ar isochron technique are shown. Error bars denote 1r. The physical limit that spherules cannot have been exposed for longer than they have existed is illustrated with a dashed diagonal line. Though each panel represents a set of spherules taken from a single scoop of soil, the cosmic ray exposure ages do not increase steadily with formation age, but rather fill a large fraction of the permitted space. This implies a complicated history of burial, exhumation, and mixing for the spherules from each soil sample. One spherule from each sample (open boxes) lies far beyond the physical limit, and we therefore believe that the isochron ages assigned by Levine et al. (2005) for these spherules are too young.

Solar and cosmogenic argon in lunar spherules

with an exposure age approximately equal to its formation age must have resided close to the lunar surface for most of the time since it was formed; a much younger exposure age implies a history of burial, at depths comparable to or greater than the penetration depth of primary galactic cosmic rays (1 m; Reedy et al., 1983). Fig. 2 shows that most of our impact spherules were buried for significant periods of time, even though they all formed near the lunar surface, were implanted with solar argon while residing at the surface, and were collected near the surface. Furthermore, different spherules from single scoops of soil have very different cosmic ray exposure ages, and this observation requires a complicated history of mixing and stirring for each soil sample. A model in which spherules are buried beneath a progressively thicker cover of soil, without overturning or exhumation, would predict cosmic ray exposure ages that increase smoothly as a function of formation age, but this behavior is not observed. As seen in Fig. 2, spherules with the oldest formation ages include those with very old—and very young—cosmic ray exposure ages. More complicated models of lunar ‘‘gardening,’’ the mixing and overturning of layers of the lunar surface by meteoroid impacts of all sizes, are needed to explain our observation that spherules experience quite different histories of burial before coming to rest near one another in the aftermath of the latest impact. The Monte Carlo gardening model of Arnold (1975) demonstrates that different soils can have quite different gardening histories, but the model would need to be extended to the scale of individual soil particles in order to explicitly examine the distribution of cosmic ray exposure ages expected for an ensemble of spherules. Obviously, the exposure age of a spherule cannot not exceed its formation age; this physical limit is represented by the dashed lines in Fig. 2. A small number of Apollo 12 spherules (6 out of 81), especially very young specimens which lie near the origin in the top panel of Fig. 2, appear to violate this limit by more than twice the measurement uncertainties. Two considerations may mitigate this. First, the assumed production rate of cosmogenic 38Ar from Turner et al. (1971) could be too low. Other estimates of this value, however, are a factor of 2 lower still (Hohenberg et al. (1978)). Second, our estimated exposure ages would be too large if some fraction of the cosmogenic 38 Ar were generated by spallation of potassium, titanium, or iron rather than calcium (Turner et al., 1971; Hohenberg et al., 1978), which together might account for up to 10% of the total. Both of these considerations demand caution in interpreting the cosmic ray exposure ages on an absolute scale, as would any real fluctuations in the production rate of cosmogenic argon. Nevertheless, our inferences about soil mixing and spherule burial depend only on the relative exposure ages, and are robust to different assumed production rates of cosmogenic 38Ar. Turner et al. (1971) examined rocks and breccia fragments collected near Cone Crater at the Apollo 14 landing site, and found cosmic ray exposure ages that cluster around 26 Ma, which they inferred was the age of the Cone impact. By contrast, there is no evidence for an excess of Apollo 14 spherules with this exposure age. Though the Cone impact presumably exhumed spherules as well as

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rocks from various depths in the soil, spherules may be different from the rocks and breccias examined by Turner et al. (1971), in that the spherules had been formed near the lunar surface, where they would have received some cosmic radiation dose immediately after formation. Such an extra dose would make the exposure ages of spherules older, by variable amounts, than the age of the impact that most recently brought the spherules to the surface. The rock fragments brought to the surface by the Cone impact, by contrast, may have received little cosmic radiation until they were exhumed. There are two spherules in Fig. 2 (one from each soil sample; these are illustrated with open symbols in the Figure) whose exposure ages are much older than their formation ages; these discrepancies are too large to be explained by an incorrect production rate of cosmogenic 38Ar or by an unaccounted-for contribution from spallation of other target nuclides. It is much more likely that the formation ages assigned to these spherules by Levine et al. (2005) were too young. Both isochrons had evidence for partial argon loss on the Moon, and this could have corrupted the age determination. One value of the cosmochron analysis is that it can identify implausible results of 40Ar/39Ar isochron dating. 3.3. Solar component The intercepts of the cosmochron lines, which represent the 38Ar/36Ar ratios of solar argon preserved in the spherules, are plotted in Fig. 3. The intercept values from both sets of spherules lie in the range 0:185 6 ð38 Ar=36 ArÞsolar 6 0:2, with nearly all the values being isotopically heavier than 0.188, which is the 38Ar/36Ar ratio of the terrestrial atmosphere (Nier, 1950) and most or all unfractionated terrestrial rocks (e.g., Renne et al., 2001). It seems reasonable to expect that some of the solar argon acquired by the oldest spherules would have been implanted billions of years ago. If so, any secular variations in the 38Ar/36Ar ratio of solar particles could, in principle, be observed among the lunar spherules. Secular variations in the isotopic composition of solar helium and neon have been reported in lunar and meteoritic samples (Benkert et al., 1988; Becker and Pepin, 1989); Heber et al. (2003) argues that these are a consequence of grain erosion, rather than a change in the solar output. Nevertheless, as seen in Fig. 3, we find no evidence for any systematic difference in 38Ar/36Ar ratios between the oldest and youngest spherules. Until very recently, most investigations of solar noble gases in lunar samples have identified two isotopically distinct implanted components (see review by Wieler, 1998). These two components are separated most explicitly in stepwise etching experiments (Wieler et al., 1986), which, in principle, release noble gases from progressively greater depths into a given specimen, and therefore provide a sense of any spatial variations in the argon isotopic abundances. An isotopically lighter component is identified with the lowenergy solar wind, and an isotopically heavier component, which is evidently implanted more deeply in lunar grains, has been called solar energetic particles, or SEP (Wieler et al., 1986). Benkert et al. (1993) determined the 38Ar/36Ar

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Fig. 3. 38Ar/36Ar ratios of the solar component retained in lunar spherules. The solar 38Ar/36Ar ratios are shown for the 81 Apollo 12 spherules (filled triangles) and 109 Apollo 14 spherules (open squares) whose isochron ages were defined by Levine et al. (2005). Error bars represent 1r. The dashed line is the 38Ar/36Ar ratio of the terrestrial atmosphere (Nier, 1950), shown for comparison. If any isotopically light solar argon has been lost by diffusion or sputtering by the solar wind, the 38Ar/36Ar ratio of the implanted component preserved in the spherules will be isotopically heavier than the solar wind value. There is no evidence in these data for a secular change in the isotopic composition of the retained solar argon.

ratios of the two components to be 0.196–0.206 for the SEP component, and 0.182–0.185 for the normal solar wind. However, Palma et al. (2002) prefer an isotopically lighter solar wind, with a 38Ar/36Ar ratio of 0.172 ± 0.002. The nature of the SEP component has long been enigmatic: Black (1972) suggested that it was implanted by solar flares, but Wieler et al. (1986) proposed instead that SEP gases represent suprathermal solar ions. On the other hand, Hohenberg et al. (1970) inferred that noble gas isotopes were fractionated by deeper implantation of the heavier isotopes into solid matter, based on the idea that heavier species are more energetic in the uniform-velocity solar wind. This behavior was modeled in detail by Mewaldt et al. (2001), and seems to be favored by the first examinations of Genesis mission samples (Grimberg et al., 2006a,b,c). Our data are unable to determine whether SEP noble gases are a true ‘‘component,’’ in that they have a source that is energetically or temporally different from ‘‘normal’’ solar wind, or whether an isotopically homogeneous flux of solar noble gases becomes fractionated by differential penetration into solid matter. As of this writing, the consensus on this issue is changing rapidly, due largely to new data from Genesis mission samples (Grimberg et al., 2006a,b,c). Our data must be accommodated by either paradigm, and the implications of our work for the two models are somewhat different. In what follows, we attempt to speak as generally as possible about our observations, as most of our inferences are robust to whether the ‘‘two-component’’ or the ‘‘one-component’’ understanding of solar noble gases proves correct.

The observation which both of these proposals seek to explain, namely that isotopically lighter and heavier solar noble gases are released separately from extraterrestrial specimens, is something that we observe unambiguously in the lunar spherules. Figs. 4 and 5 respectively illustrate how this separation is seen in data from an individual spherule and from the ensemble of spherules. Fig. 4 is a cosmochron diagram in which the first partial release of argon is isotopically lighter than the trend defined by all the remaining measurements, a feature which is common to roughly one-quarter of all the spherules. In Fig. 4, all partial releases except the first appear to define a mixing line between a cosmogenic component, with 38Ar and 36 Ar that correlate with 37Ar, and a 37Ar-free component whose 38Ar/36Ar ratio is 0.1897 ± 0.0014. The first datum in Fig. 4, which represents argon released from closest to the spherule surface, is so far from the mixing line as to demand a separate explanation. Though it is possible in principle that terrestrial atmospheric argon might be adsorbed loosely to the spherule surface, making the 38 Ar/36Ar ratio of the first partial release isotopically lighter, the observed 38Ar/36Ar ratio from the first heating step of this spherule is too light to be explained by any amount of atmospheric contamination. Kinetic isotopic fractionation during stepwise heating might, in principle, cause the first partial releases to appear isotopically light, but this effect is at least 10 times too small to account for the first measurement in this case. We therefore infer from Fig. 4 that the solar wind must itself be responsible for the light 38Ar/36Ar ratios observed in first heating steps. On

Solar and cosmogenic argon in lunar spherules

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0.25 Cosmic ray exposure age 235 ± 40 Ma Intercept 0.1897 ± 0.0014 MSWD = 0.614 Prob = 0.7679 n = 10 (out of 16)

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0.23

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Fig. 4. Cosmochron diagram for Apollo 12 spherule K37. Measurements, shown as 1r error ellipses, are labeled according to the sequence of heating steps attempted. For approximately 75% of the spherules, all measurements lie along a single line on a cosmochron diagram. Most of the remaining 25% of spherules have cosmochron diagrams like this one, with the first datum significantly below (i.e., isotopically lighter than) the trend. The release pattern illustrated here compares with observations of neon isotopes from lunar mineral grains, reviewed by Wieler (1998). The best fitting line shown here (and the summary of its properties at the top of the figure) ignores the first datum. Also excluded are five heating steps that released argon at levels consistent with instrumental blanks (not shown). MSWD, mean square weighted deviations of the ten fit measurements from the line. Prob, probability that the residuals can be explained by measurement errors alone.

Number of spherules

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120

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3rd heating steps

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100

100

80

80

60

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40

40

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-3 -2 -1 isotopically lighter

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1 2 3 4 isotopically heavier

Standard deviation units Fig. 5. Histograms of measured 38Ar/36Ar relative to cosmochron lines. Data represented here are for the 126 Apollo 12 spherules and 153 Apollo 14 spherules for which statistically acceptable cosmochrons are defined from all the measured argon releases, as in Fig. 1. If the releases from a lunar spherule were perfectly described by the mixing of cosmogenic argon with argon from an isotopically homogeneous solar component, the measured data would be normally distributed about the best fitting lines. This situation is approximated by the distribution of the third partial releases from the spherules (a). However, there is a strong tendency for the first partial releases (b) to be isotopically lighter than the trend defined by the cosmochron lines, indicating that the solar argon is composed of isotopically lighter argon that is released from nearer to the spherule surfaces, and isotopically heavier argon from greater depth.

the basis of similar observations, and assuming that the first releases of solar noble gases were unfractionated, Benkert et al. (1993), Pepin et al. (1999), and Palma et al. (2002) concluded that the solar wind is isotopically lighter in 38Ar/36Ar than the terrestrial atmosphere. However, if the solar wind fractionates isotopes as a function

of penetration depth, then the observed isotopic abundances in any heating step could be heavier or lighter than the true composition of the solar wind, depending on the range of depths being sampled and on any surface erosion the sample suffered before it was analyzed (Wieler et al., 1986; Wieler et al., 2006).

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Though it is most common for the cosmochrons of lunar impact spherules to show binary mixing between two welldefined components (as in Fig. 1), the ensemble of spherules shows evidence, in the aggregate, for isotopically heterogeneous solar argon. We illustrate this in Fig. 5. For each spherule, we calculated the deviation of the first-released argon from the best fitting cosmochron line, in units of the measurement uncertainty, and Fig. 5 presents a histogram of the results. If the solar argon released from each spherule were isotopically homogeneous, we would expect that the histogram in Fig. 5 would be normally distributed about zero, with the first partial release being isotopically lighter than the trend line in a random half of the spherules, and isotopically heavier in the other half. Such behavior is observed for measurements later than the first on each spherule, such as the third partial releases, which are shown in Fig. 5a. However, the distribution of first partial releases, shown in Fig. 5b, is strongly skewed toward isotopically lighter values: the first measurements from 85% of the spherules are isotopically lighter than the cosmochron line. The simplest explanation for this distribution is that, even where isotopically light and heavy solar argon cannot be resolved in individual heating steps, most spherules nevertheless released isotopically light solar argon that had been implanted nearer the surface and isotopically heavier solar argon from greater depths. Because the isotopically light fraction is implanted closer to the surface, it is conceivable that imperfect retention of solar argon, perhaps due to diffusion or to sputtering by the solar wind itself, could leave the retained argon enriched in the isotopically heavy fraction. Our data do not require the loss of any solar argon, but neither can we exclude the possibility. Diffusivities of most species at ambient lunar temperatures (100 C on the dayside; Saari and Shorthill, 1972) are difficult to measure because they are so slow, but extrapolation from measurements made on glass at temperatures as low as 400 C (Hazelton et al., 2003) suggests that diffusive loss of solar argon could be a significant problem. In particular, the diffusivities measured by Hazelton et al. (2003) imply that if a spherule were held at 100 C for 1 Ma, the outer 600 nm could be degassed, liberating nearly the entire endowment of solar argon. A second possibility is that the isotopically light solar argon, having been implanted closer to the surface, is preferentially sputtered away by continued exposure to the solar wind. We shall more fully consider the effects of solar-wind sputtering below. Though loss of some solar argon is possible, our data argue strongly against models in which all but the high-energy tail of the solar wind is lost from lunar samples. Mewaldt et al. (2001) proposed that the isotopically heavy fraction of solar noble gases retained by lunar samples represents the suprathermal tail of the total solar argon flux, accounting for only a few parts per million of the total solar flux of argon. Using the observed shape of the energy distribution of the solar wind, Mewaldt et al. (2001) calculated the depths of penetration of different species into solid matter, and obtained agreement between noble gas isotope ratios calculated over a range of depths and the measured SEP component from lunar samples. However, the quantity of

solar argon preserved in lunar spherules argues against its being derived from a volumetrically minor fraction of the total flux. From foils exposed during the Apollo missions, Geiss et al. (1972) measured the total solar argon flux to be 500 pmol 36Ar/mm2 Ma. From this measurement and the energy spectrum inferred from Mewaldt et al. (2001), we deduce a suprathermal solar flux of 1.5 · 102 pmol 36 Ar/mm2Ma. Fig. 6 shows that the fluence of solar argon retained by most spherules was 10 pmol of 36Ar per mm2. To accumulate this much solar argon from the suprathermal tail of the solar flux would require residence times in the top monolayer of lunar soil grains of hundreds of millions of years. There are two reasons why this is unreasonably long: first, the solar wind penetrates only tens of nm into lunar grains (Eberhardt et al., 1970), and therefore irradiates only the top part in 108 of the  10 m thick lunar regolith. The fact that only a tiny fraction of the soil is irradiated at any given time, coupled with the observation of solar argon in spherules (and mineral grains) from various depths in the regolith, implies that grains must be cycled through the lunar surface in much less time than 10% of the age of the Moon. Second, we argued above that spherules would have lost most of their solar argon by diffusion if they remained in direct sunlight for even 1 Ma, implying that spherules could never hold much more argon than they could receive in 1 Ma. We therefore infer that the isotopically heavy argon that we observe in spherules is not a mere 3 · 105 of the solar flux (Mewaldt et al., 2001), but that it probably represents a much more considerable fraction of the solar wind, as favored by Hohenberg et al. (1970). If so, the duration of exposure required to implant 10 pmol of 36Ar per mm2 into a spherule could be as low as 104 years. We believe this is a more plausible estimate of the residence time of a lunar soil grain in the top monolayer of the regolith. Fig. 6 also shows that, regardless of age, most spherules have retained not more than 25 pmol of solar 36 Ar per mm2. Spherules younger than 100 Ma old evidently intercepted this much solar argon over their lifetimes, but even spherules >3000 Ma old did not retain much more than this quantity of solar argon. We consider two possible explanations for this observation. First, spherules might receive solar argon in the immediate aftermath of the impact that creates them, and then, once they are buried, never return to the very top of the lunar soil. This would require that impacts distinguish between spherules they create and older spherules that are ejected cold and therefore neither degassed nor re-melted. There would need to be a strong tendency to deposit nascent spherules in the top monolayer of the soil, but a strong tendency against depositing older spherules there. This explanation is also supported by apparent anisotropy in the distribution of solar argon in spherule surfaces (Levine and Rohde, 2006). A second explanation for the appearance of Fig. 6 is that our spherules have been exposed to the solar wind for long enough to become saturated, with inputs of new solar argon offset by the sputtering of previously implanted argon or by diffusive losses. Jull and Pillinger (1977) calculated the saturation concentration of various species from the solar

Solar and cosmogenic argon in lunar spherules

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40 35 30 25 20 15 10 5 0 5000

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3000 40

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Fig. 6. Fluence of solar Ar retained by dated Apollo 12 spherules. The amount of solar argon released from 79 of the 81 dated Apollo 12 spherules are shown, normalized by the area each spherule would have projected to incident solar radiation (error bars denote 1r and ignore uncertainty in the spherule sizes). Two of the dated spherules contained evidence of clasts which were not completely degassed when the spherules were formed, and are therefore excluded because the total quantity of solar argon could not be unambiguously determined. The range of solar argon fluences retained by the oldest and youngest spherules is approximately the same, suggesting that spherules become saturated with solar argon, with implantation offset by sputtering or diffusion. The saturation fluence of 10–25 pmol/mm2 is approximately 10 times greater than the value calculated by Jull and Pillinger (1977).

wind, which must depend upon the abundance of each species in the solar wind and upon target material properties such as grain size, composition, and density. For particles much larger than 1 lm, the saturation concentration falls in inverse proportion to grain size, as expected for implantation into a thin surface layer. Jull and Pillinger (1977) estimated a saturation fluence of solar 36Ar of 1.7 pmol/mm2 for particles of Apollo 11 bulk soil composition and radius 100 lm, which is about 10 times smaller than the observed fluence of solar 36Ar retained by our spherules. Some of this factor-of-ten discrepancy could be due to easier penetration of glass by incident solar particles than the target material considered by Jull and Pillinger (1977). Based on our measured fluences being above the order of magnitude suggested by Jull and Pillinger (1977) and the insensitivity of our measured fluences to spherule age, we infer that most of the spherules are saturated with solar 36Ar, and that the saturation fluence is likely in the range 10–25 pmol/mm2. The saturation fluence may easily vary from one spherule to another based on what fraction of the surface was exposed to the solar wind. Spherules with slightly less solar argon might have been partially shielded on the lunar surface. On the other hand, spherules with considerably greater fluences of retained solar argon might have been exposed to the solar wind in multiple episodes, with different orientations. That spherules with such high fluences are seen to be rare in Fig. 6 supports our suggestion that buried spherules are unlikely to be returned to the top of the regolith by subsequent impacts. If the spherules are indeed saturated with solar argon, we cannot use the quantity of solar argon retained in the spherules to measure their residence times at

the lunar surface, except to say that they are longer than 104 years. 4. CONCLUSIONS It is hoped that detailed analysis of the solar material implanted into Genesis mission samples will reveal the elemental and isotopic composition of the solar wind with unprecedented precision and accuracy, and will clarify the relationship between the isotopically light and heavy solar noble gases. That the first results from Grimberg et al. (2006a,b,c) are forcing a major rethinking of the nature of the noble gases in the solar wind is an indication of the high value of the mission and of the data obtained from it. However, even if the present-day composition of the solar corpuscular radiation could be determined perfectly, regolith samples from the Moon and meteorites are still valuable for their unique inventory of fossil solar wind from different times throughout the history of the Solar System. Individual, dated lunar impact spherules are especially important in this context, because they offer a more finely resolved view of the past than do mineral separates from bulk soils (e.g., Wieler et al., 1986; Pepin et al., 1999; Hashizume et al., 2002) whose ages, in the aggregate, are estimated by various maturity indicators. As we see from the distribution of cosmic ray exposure ages in Fig. 2, individual spherules in a single soil sample have experienced greatly different histories of exposure and burial, so one cannot suppose that the maturity estimated for a bulk soil reflects a property shared by all the grains. This complexity, however, is an advantage for probing the meteoroid bombardment

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history of the Moon by measuring the 40Ar/39Ar ages of spherules: thorough processing of spherules through the upper few meters of the lunar regolith make it more likely that formation ages recorded in a single scoop of soil represent the ages of distinct meteoroid impacts in lunar history. In common with many experiments over the past 35 years, our measurements show that the solar argon implanted in extraterrestrial specimens contains separate isotopically lighter and heavier fractions, with the isotopically lighter argon implanted less deeply. Figs. 4 and 5 illustrate the tendency of the earliest heating steps to release the most isotopically light solar argon. If the isotopically heavy argon represents only a small fraction of the solar argon flux (e.g., Mewaldt et al., 2001), then its abundance in the spherules implies an unacceptably long residence time for each spherule in the top monolayer of the lunar soil, on the order of hundreds of millions of years. Such a slow rate of gardening of the lunar soil is inconsistent with the observation that solar noble gases are found in nearly all the lunar samples ever investigated. Moreover, a spherule could not be held in direct sunlight on the Moon for this long before solar wind implantation is offset by losses due to diffusion. Therefore, we disfavor models in which the isotopically heavy solar argon is only a minor constituent of the total solar particle flux. On the other hand, if an isotopically homogeneous solar wind becomes isotopically fractionated as it penetrates solid matter (Hohenberg et al., 1970; Grimberg et al., 2006a,b,c), the isotopically heavy fraction could easily represent a much larger portion of the total flux, so the implied residence time in the top monolayer of the regolith could be as low as 104 years. The uncertainty over what the isotopically heavy and light solar argon represent prevents us from using our data to determine the 38Ar/36Ar ratio of the solar wind. Of particular concern in this regard is the possible loss of an unknown portion of the isotopically light solar argon implanted in each spherule. Such loss could occur by diffusion at lunar dayside surface temperatures, over geologically short times (<1 Ma), or by preferential sputtering of the outermost layers by continued exposure to the solar wind. Bearing in mind this caveat, the 38Ar/36Ar ratio of solar argon retained by the spherules (Fig. 3) is independent of age. Not only is the isotopic ratio of retained solar argon insensitive to spherule age, but so too is its fluence, illustrated in Fig. 6. The spherules retained 10 pmol of solar 36Ar per mm2 of the surface they could have projected to space, which is roughly one order of magnitude larger than the saturation density of solar argon calculated by Jull and Pillinger (1977). We therefore conclude that the spherules become saturated with solar argon quickly, perhaps in 104 years, and that any further exposure to the solar wind either sputters away as much solar argon as it implants, or heats the specimen enough to liberate this quantity by diffusion. Our argon data were acquired by laser step-heating, but there are advantages to the closed-system stepwise etching system developed by Wieler et al. (1986) and also used by Becker et al. (1996). Stepwise heating blurs the initial distribution of gas in a specimen, but stepwise etching more closely approximates the removal of successively deeper layers. The recovery of the spatial distribution of argon in

lunar spherules from stepwise heating data has proved difficult or impossible (Levine and Rohde, 2006), but might be simpler from stepwise etching data. In particular, etching offers the best opportunity to probe the separation between the isotopically light and heavy fractions of solar noble gases (Grimberg et al., 2006c), and then to examine more pure releases of cosmogenic argon. In analyzing data from our step heating experiments, we have had to assume the isotopic composition of the cosmogenic component, but an etching experiment could measure it directly, and thereby constrain the relative spallation cross sections of different target nuclides. Most of the stepwise etching measurements made so far are of neon isotopes, but argon isotopes offer potential advantages if irradiated samples could be examined. Reactor-produced 39Ar and 37Ar allow for the deconvolution of implanted, cosmogenic, and radiogenic components, and measurement of radiogenic 40Ar permits the determination of a formation age in the same experiment as solar and cosmogenic argon are measured. Examination of solar and cosmogenic argon in individually dated soil grains provides a much more detailed view of the geological history of the Moon than has otherwise been attempted. ACKNOWLEDGMENTS We are grateful to the Ann and Gordon Getty Foundation and to the Folger Foundation for their support of our research. We thank T. Becker for his expert assistance in the argon laboratory at BGC. This manuscript benefited from constructive reviews by T. Swindle and R. Wieler, and from the editorial oversight of G. Herzog.

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