Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon from Luna 16, 20 and 24 samples

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1 Meteoritics & Planetary Science 36, (2001) Available online at Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon from Luna 16, 20 and 24 samples B. A. COHEN1, G. A. SNYDER1, C. M. HALL2, L. A. TAYLOR1 AND M. A. NAZAROV3 1Planetary Geosciences Institute, University of Tennessee, Knoxville, Tennessee 37996, USA 2Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA 3Vernadsky Institute of Geochemistry, Moscow , Russia *Correspondence author's address: (Received 2001 February 9; accepted in revised form 2001 June 29) Abstract Eighteen new lithic fragments from the Soviet Luna missions have been analyzed with electron microprobe and 40 Ar- 39 Ar methods. Luna 16 basalt fragments have aluminous compositions consistent with previous analyses, but have two distinct sets of well-constrained ages (3347 ~ 24 Ma, 3421 ~ 30 Ma). These data, combined with other Luna 16 basalt ages, imply that there were multiple volcanic events filling Mare Fecunditatis. The returned basalt fragments have relatively old cosmicray exposure (CRE) ages and may have been recovered from the ejecta blanket of a young (1 Ga), nearby crater. A suite of highlands rocks (troctolites and gabbros) is represented in the new Luna 20 fragments. One fragment is the most compositionally primitive (Mg# = 91 92) spinel troctolite yet found. Both troctolites have apparent crystallization ages of 4.19 Ga; other rocks in the suite have progressively younger ages and lower Mg#s. The age and composition progression suggests that these rocks may have crystallized from a single source magma, or from similar sources mobilized at the same time. Within the new Luna 24 basalt fragments is a quench-textured olivine vitrophyre with the most primitive composition yet analyzed for a Luna 24 basalt, and several much more evolved olivine-bearing basalts. Both new and previously studied Luna 24 very low-ti (VLT) basalt fragments have a unimodal age distribution (3273 ~ 83 Ma), indicating that most returned samples come from a single extrusive episode within Mare Crisium much later than the Apollo 17 VLT basalts ( Ga). INTRODUCTION The Soviet Luna 16, 20, and 24 missions are so far the only autonomous missions to have returned samples from another planetary body. These robotic missions returned drill-core samples of the lunar surface from northeastern Mare Fecunditatis, the highlands just south of the Crisium basin, and southern Mare Crisium, respectively. Our aim in this study is to relate the chemistry of the rocks from these sites to the source and evolution of their parent magmas, and to determine the dates of their emplacement. The Luna 16 and 24 samples provide information on the nature and age of the basalt flows that filled Mare Fecunditatis and Mare Crisium. Although they are geographically close to each other, the two basins are filled with very different material. The Luna 24 basalts contain high aluminum but very low titanium (VLT) (<1.5 wt% TiO 2 ), similar to the Apollo 17 VLT basalts. In contrast, the Luna 16 basalts are high in aluminum (>11 wt% Al 2 O 3 ) and chemically similar to the Apollo 14 aluminous basalts. Mare Fecunditatis is a relatively old and degraded multiring basin that has been filled by lava flows. Luna 16 sampled surficial mare material between the two Copernican-age craters Langrenus and Taruntius. Basalts comprise most of the coarse lithic fraction of the core sample, with some anorthositic and noritic fragments transported to the site by younger impact events (McCauley and Scott, 1972). As a group, the Luna 16 basalts contain more TiO 2 ( 5 wt%) than the low-ti, aluminous Apollo 14 basalts (Taylor et al., 1991). The Apollo 14 basalts may be the oldest basalts known so far (Taylor et al., 1983; Taylor, 1982), though their origin is disputed (Snyder and Taylor, 1996; Warren et al., 1997). Mare Crisium is known to contain a lithologic treasure trove including some of the most evolved basalts on the Moon. At least four different flows can be stratigraphically mapped within this old basin (Head et al., 1978), and these distinctions are borne out in the Fe and Ti contents as mapped by the Clementine orbiter (Bussey and Spudis, 2000). The Luna 24 landing site was in a patch of low-ti, Fe-rich basalt surrounded by stratigraphically younger basalts (Head et al., 1978). The majority of Luna 24 basalt fragments are high-al, VLT basalts with similar chemical characteristics to the Apollo 15 green glass, though the basalts are much less primitive. Prelude preprint MS# Meteoritical Society, Printed in USA.

2 1346 Cohen et al. Luna 20 is the only highlands site on the eastern limb of the Moon where samples have been collected; however, the collection has received proportionately little attention in the west. Because it is a highlands site, the lithic fragments are predominantly anorthosites, as well as granulites, Mg-suite rocks, and impact melt rocks, at least some of which may be derived from nearby Mare Crisium (Podosek et al., 1973; Swindle et al., 1991). We have studied the petrology, mineral-chemistry, and 40Ar-39 Ar ages of 18 "new" fragments from soil returned by the Luna 16, 20, and 24 missions. Of the six fragments from Luna 16, ranging in mass from 5.5 to mg, four are finegrained basalts and two are regolith agglutinates. Six fragments from Luna 24, ranging in mass from 2.1 to 9.9 mg, are finegrained VLT basalts. Six fragments from Luna 20, ranging in mass from 1.25 to 7.55 mg, are Mg-suite plutonic rocks: two troctolites, two gabbros, and two single anorthite grains. SAMPLE PREPARATION AND ANALYSIS METHODS Eighteen fragments were received from the Vernadsky Institute in Moscow, Russia. Of these, sixteen were basalts or other igneous rocks, and two (2002 and 2004A) were single anorthite grains. Chips of allocated fragments that were large enough to split were thin-sectioned for electron microprobe (EMP) analysis. The Cameca SX50 electron microprobe at the University of Tennessee was used for all analyses. Minerals were used for calibration, and analysis conditions were 15 kev and 20 na, with a 20 s collection time for all elements. Petrographic descriptions and mineral chemistry of the fragments are included in the next section. Typical textures are shown in backscattered electron (BSE) images in Fig. 1. The remaining fragments, ranging in mass from 0.2 to 2.7 mg (Table 1), were irradiated for 350 h in location L-67 of the Phoenix-Ford Memorial reactor at the University of Michigan, producing a J-factor of The standard mineral 3gr hornblende (1071 Ma; Roddick, 1983) was simultaneously irradiated and analyzed. Stepped-heating experiments were performed using a continuous-wave laser as described in Snyder et al. (1996). The laser power varied from 50 to 4000 mw over each experiment. The temperature at each step was not measured; however, it scales with power (ap0.25), and the maximum temperature achieved (at 4000 mw) is #C. System blanks at masses 36 to 40 were, in order, , , , and cc STP. All data was corrected for blanks, mass discrimination, K and Ca interference, and decay since irradiation. Additionally, data were corrected for cosmic-ray spallation by deconvolving the 38Ar/36Ar signatures from cosmic ray-spallation (36Ar/38Ar = 0.66; Hohenberg et al., 1978) and terrestrial atmosphere (36Ar/38Ar = 5.32). An initial 40Ar/36Ar ratio, (40Ar/36Ar)i, was calculated from the isochron intercept for the high-temperature FIG. 1. BSE images of representative Luna 16, 20, and 24 samples: (a) 1609A basalt; (b) 1609B basalt; (c) 2003 troctolite; (d) 2004C troctolite; (e) 2004D gabbro; (f) 24067,3-6A basalt. The scale bar in all images is 100 lm.

3 Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon 1347 TABLE 1. Luna 16, 20, and 24 sample numbers, types, masses, (40Ar/36Ar) i, and whole-rock gas ages. Sample Type Mass (mg) (40Ar/36Ar) i Whole-rock gas age (Ma) basalt A basalt ~ ~ B basalt ~ ~ C glassy fragment ~ 0.06 > x > 1.78 ~ ~ A glassy fragment ~ ~ B basalt ~ ~ anorthite ~ ~ spinel troctolite ~ ~ A anorthite ~ ~ B gabbro ~ ~ C troctolite ~ ~ D gabbro ~ ~ ,3-6A basalt ~ ~ ,3-6B basalt ~ ~ ,4-6L basalt ~ ~ ,4-6A coarse basalt ~ ~ ,4-6B coarse basalt ~ ~ ,4-6C basalt ~ ~ 10 Luna 16 bulk soil ~ 0.003* Luna 16 bulk soil ~ 0.04 Luna 16 bulk soil ~ 0.50 Luna 20 bulk soil 2.01 Luna 20 bulk soil ~ 0.02 Luna 24 bulk soil \0.78 ~ 0.14#00 *Kaiser (1972). Eugster et al. (1973). Agrawal et al. (1974). Eugster et al. (1975). #Average and standard deviation of Bogard and Hirsch (1978). steps and subtracted from the data. The calculated (40Ar/36Ar) i values are tabulated in Table 1 and are generally comparable to (40Ar/36Ar) i values determined for the bulk soils at each site (Agrawal et al., 1974; Bogard and Hirsch, 1978; Eugster et al., 1973, 1975; Kaiser, 1972). As shown in Fig. 2, the lowest temperature steps usually did not fall on the isochron and have 40Ar/36Ar ratios that do not conform to the intercept value. These steps usually have chronologically meaningless apparent ages in the argon-release spectra. The whole-gas ages for each sample are also tabulated in Table 1. Plateaus were determined as a series of steps where the apparent age of each step was within 2r of the previous step and where a general upward or downward trend was not apparent. The steps used to compute the plateau ages are indicated on the argon release plots and usually make up more than 50% of the total 39Ar release; those cases that differ are discussed individually. The plateau age was computed by adding the volumes of 40Ar and 39Ar released over the plateau steps and computing an integrated age. The error quoted in the plateau age is 2r. The K/Ca ratio was computed from the 39Ar/37Ar ratio in each gas-release step and is plotted above the argon release spectra for comparison. Note that the scale on all FIG. 2. Isochron correlation diagram for sample 1609B (basalt). The high-temperature steps are used to fit a line whose intercept defines the initial 36 Ar/40Ar ratio.

4 1348 Cohen et al. argon-release plots is the same for apparent age but not for K/Ca, and that the K/Ca ratio has been stretched by a factor of 100. Luna 16 MINERAL CHEMISTRY AND PETROLOGY The six Luna 16 fragments are of two types: fine-grained basalts (1609A, 1609B, 1635B, ) and glassy, vesicular particles (1609C, 1635A). The most conspicuous aspect of the basalts is the presence of large ( lm in length) laths of ilmenite that comprise 10% of the samples. Plagioclases have Ca-rich cores; pyroxenes are also zoned to more Fe-rich rims. The zoning is typical of rapid crystallization in an evolving melt. The glassy particles have vesicles, a "porphyritic" texture, and the appearance of uniformity in the matrix. Spinels and ilmenites are relatively rare. Pyroxene, olivine, and spinel compositions are shown in Fig. 3. FIG. 3. Luna 16 mineral compositions: (a) pyroxene; (b) olivine; (c) spinel.

5 Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon A Basalt This sample is fine-grained (Fig. 1a). Both ilmenite and plagioclase are typically acicular and similar in their longest dimensions ( lm); plagioclase core compositions vary from An 90 to An 95 with rims being up to six An units lower. Pyroxenes and olivines are subhedral to anhedral and vary in size from 150 to 300 lm. Pyroxenes are nearly exclusively augites with Fe-rich and Ca-poor rims. Olivines also vary little. Spinels are minor, fine-grained, and blocky. 1609B Basalt This sample is similar to 1609A (Fig. 1b), but is distinctive in containing a large (350 lm) subhedral olivine phenocryst (Fo ), which is strongly zoned to Fo 32 at the rim. Finer grained ( lm) olivines vary in composition from Fo 46 to Fo 68. Grain sizes of particular minerals are identical to 1609A. Pyroxene compositions are, again, nearly exclusively augite with strong zonation to more Fe-rich rims. Plagioclase varies in composition from An 80 to An 92. Fine-grained chromites generally contain 25 30% Cr 2 O 3 but show a range in compositions. Rims on large grains are much lower in Cr, Al, and Mg, and higher in Ti. 1635B Basalt This sample consists of approximately equal proportions of plagioclase and mafic phases and is slightly coarser-grained than 1609A or 1609B, indicating slower cooling. Ilmenite laths vary in composition from Mg# (100 atomic Mg/Mg + Fe) = 1 9 to Cr# (100 atomic Cr/Cr + Al) = Much finer grained and blocky Cr-spinels are minor and scattered throughout. The plagioclase occurs both as blocky grains and as more acicular interstitial material and varies in composition from An 78 to An 92. The mafic phases include two pyroxenes, dominantly augite and less prevalent pigeonite. Larger olivine grains (possibly phenocrysts) typically have compositions from Fo 66 to Fo 58. Rims on these olivines are more Fe-poor, often by up to 5 8 Fo units. Finer-grained olivine, adjacent to oxides and associated with mesostasis may have compositions as Fe-rich as Fo The mineral chemistry of this sample is consistent with that of a mare basalt Basalt Plagioclase varies widely in composition from An 79 to An 92. Uncommon olivine grains range from Fo 49 Fo 59. Pyroxenes occur as principally two varieties, augite and uncommon pigeonite, with occasional ferroaugite grains. Pyroxenes are often zoned to more Fe-enriched rims. Ilmenite laths vary in composition from Mg# = 1 9 and Cr# = Finer-grained spinels have a range in compositions. 1609C Glassy Fragment This sample is a mostly fine grained, glassy sample with numerous vesicles. In general, the glass is relatively uniform in composition: SiO 2 = wt%; Al 2 O 3 = wt%; MgO = wt%; CaO = wt%; FeO = wt%. One area of glass ( lm) appears pseudomorphous (equant and blocky) after feldspar and is uniform in composition: SiO 2 = 45.2 (2) wt%; Al 2 O 3 = 19.5 (2) wt%; MgO = 8.46 (6) wt%; CaO = 13.2 (1) wt%; FeO = 10.8 (1) wt%; Na 2 O = (16) wt%. Ilmenites (Mg# = 1 4; Cr# = 44 78) occur as <50 lm angular grains and spinels were not seen. Plagioclase phenocrysts are angular ( lm) and vary in composition from An 86 to An 96, although most grains are An in composition. Pyroxenes are typically lm in size and vary from pigeonite to augite in composition. A rare Fe-enriched pigeonite was also analyzed. Olivines occur as small (generally <50 lm) grains that vary from Fo 56 to Fo 65 in composition. However, several olivine grains are more Fe-rich (Fo 41, Fo 17, Fo 8 ). 1635A Glassy Fragment Phenocrysts consist of mostly pyroxene and olivine and occasional plagioclase. Both pigeonite and augite occur. Olivine phenocrysts vary widely in composition from Fo 36 to Fo 71. Plagioclase phenocrysts vary in composition from An 81 to An 95. Finer grained K-feldspar is minor, associated with the fine-grained, glassy matrix, and yields a composition of Ab 7 Or 75 An 18. Ilmenite (Mg# = 5 18; Cr# = 18 88) is typically anhedral and relatively coarse-grained. Spinels (Cr# = 56) are rare and fine-grained. Luna 20 Of the six Luna 20 fragments, two are troctolites, two are gabbros, and two are single anorthite grains. Due to their small total-sample size, the anorthite grains were not analyzed by EMP. The pyroxene compositions within the gabbros and the spinel and olivine compositions of the gabbros and troctolites are plotted in Fig C Troctolite This sample is a porous troctolite (Fig. 1d) consisting of unzoned plagioclase (An ) and finer grained globular olivine (Fo ) Spinel Troctolite This sample (Fig. 1c) consists mostly of plagioclase and small (<60 lm) grains of olivine and spinel. This sample is unique in having extremely primitive olivine (Fo ), plagioclase (An ), and spinel. A rare Zn-rich spinel was also analyzed. FeNi metal is also present (Co = wt%; Ni = wt%). 2004B Anorthositic Gabbro This sample consists of mostly anhedral plagioclase ( lm) (An ). Olivine and pyroxene occur as interstitial grains <70 lm. Olivine varies in composition from Fo 83 to Fo 73. Pyroxenes occur as pigeonite and augite. Spinels occur as tiny grains (<30 lm) but also as larger grains (up to 100 lm) and exhibit two distinct compositions: Mg-Al chromite and true aluminous spinel. FeNi metal (Co = wt%; Ni = wt%) is also present, especially as one large (100 lm) grain. 2004D Gabbro This sample (Fig. 1e) is conspicuous in having very coarse-grained ( lm) pyroxene in combination with acicular plagioclase (<200 lm in longest dimension). The uniform high-ca plagioclase composition (An ) is indicative of a highlands affinity. The coarse pyroxenes vary in composition from pigeonite to augite, both zoned to more Fe-rich compositions. A traverse across one of the largest pyroxene grains indicated zonation from a pigeonite core to a ferroaugite rim. Some finer grained pyroxenes approach hedenbergite and pyroxferroite in composition.

6 1350 Cohen et al. FIG. 4. Luna 20 mineral compositions: (a) pyroxene; (b) olivine; (c) spinel. Minor, euhedral to subhedral olivine ( lm) is associated with plagioclase and is exclusively fayalitic in composition (Fo 1 2 ). These may represent part of the breakdown assemblage of pyroxferroite. The pyroxenes and fayalite compositions are more typical of mare basalt than highlands gabbro. Luna 24 All six fragments from Luna 24 are basalts. Based on textures and grain size, these fragments can be subdivided into fine-grained and coarse-grained (gabbroic) varieties. The pyroxene, olivine, and spinel compositions are plotted in Fig. 5.

7 Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon 1351 FIG. 5. Luna 24 mineral compositions: (a) pyroxene; (b) olivine; (c) spinel ,3-6A Fine-Grained Quench Basalt This fragment is conspicuous in exhibiting a very fine-grained (mostly <50 lm) texture, including acicular plagioclase, and fine-grained pyroxenes, rare olivines, and an occasional olivine phenocryst (Fig. 1f). Plagioclases are exclusively high-ca (An 96 to An 91 ). A relatively large (150 lm), euhedral olivine phenocryst is zoned with a core composition of Fo and a rim composition of Fo 60. Other smaller grains typically fall in the composition range of Fo 63 to Fo 50 with occasional grains as low as Fo 35. Pyroxenes are exclusively augites. Glass compositions are roughly mafic (SiO 2 = wt%; TiO 2 = wt%; Al 2 O 3 = wt%; MgO = wt%; CaO = wt%; FeO = wt%; Na 2 O = wt%). Minor, fine-grained (<40 lm) Cr-spinels exhibit uniform compositions ,3-6B Fine-Grained Olivine Basalt This sample is very similar to 24143,4-6L. Both samples have a greater proportion of oxides (ilmenite and Ti-rich spinel) than other Luna 24 samples. Plagioclase (An ) is typically acicular to tabular

8 1352 Cohen et al. (typically <150 lm in longest dimension) with a much lower aspect ratio than the plagioclase in quench basalt 24067,3-6A. Olivines and pyroxenes are similar and uniform in size ( lm). Olivines exhibit a narrow range in composition. Pyroxenes occur as both pigeonites and augites. Ilmenites (Mg# = 4; Cr# = 80 85) and spinels (Mg# = 1 2; Cr# = 66 68) are uniform in composition ,4-6L Olivine Basalt This sample is a fine-grained olivine basalt that is very similar in composition to 24067,3-6B. However, pyroxenes in this sample are exclusively pigeonitic. Plagioclase compositions vary little from An 94 to An 91. Olivine compositions are also uniform. Fe-Ti-rich spinels occur both as small (<30 lm) grains and large (up to 200 lm) sieve-textured grains ,4-6B Coarse-Grained Basalt This sample consists principally of a few large ( lm), euhedral, tabular, plagioclase grains which are typically high-ca in composition (An ). Finer interstitial grains are lower in Ca (An ). Pyroxenes (both pigeonite and augite) are typically euhedral and coarse-grained ( lm). Rare, fine-grained fayalitic (Fo 7 8 ) olivines appear associated with ilmenite (Mg# = 1; Cr# = 55 69) as the last crystallizing material ,4-6C Ilmenite-Rich Basalt This sample is finer grained than 24184,4-6B, but still coarser grained than the "finegrained" basalts. Plagioclases (An ) are typically acicular ( lm in length) to tabular and often occur in radiating "clots". Pyroxenes and olivine ( lm) often poikilitically enclose plagioclase. Olivines vary widely in composition from Fo 3 to Fo 54. Pyroxenes occur as pigeonites, low-fe augites, and ferroaugites. Rims on pyroxene grains are more Fe-enriched. Silica occurs in the mesostasis. Ilmenites (Mg# = 1, Cr# = 42 46) and spinels are finer grained (<50 lm) and associated with mesostasis and plagioclase. Two different spinels occur: Ti-rich and chromite. Luna 16 ARGON-40 ARGON-39 SYSTEMATICS Argon-release spectra and K/Ca ratios for all six fragments from Luna 16 are shown in Fig. 6 and ages are reported in Table 2. Each heating step is shown with a 1r uncertainty, though the plateau ages are reported with a 2r uncertainty. In this section, the interpretation of the release patterns is discussed. 1609A Basalt This sample is a fine-grained basalt. The presence of an incompatible-element-enriched, glassy mesostasis is suggested by the high K/Ca ratios in the lowtemperature steps. A plateau is reached over 38% of the 39 Ar release in the high-temperature steps, yielding an apparent age of 3155 ~ 4 Ma. The amount of degassing is estimated by comparing the 40 Ar/ 39 Ar ratio of the total sample to that of the plateau (plus higher temperature) steps. By this method, this sample appears to have lost 60% of its initial 40 Ar in an event at 1000 Ma. Such a large amount of degassing has the effect of decreasing the apparent age of the disturbed sites (lowtemperature release). The effect on the apparent plateau age is discussed later. TABLE 2. Cosmic ray exposure ages and depths of Luna rocks. Sample Type 38Ar/Ca* CRE age* P Fe /P Ca Estimated depth (cc STP/g) (Ma) (cm) 1609A basalt <5 1609B basalt > B basalt < basalt <5 1609C glassy fragment A glassy fragment C troctolite troctolite B gabbro D gabbro anorthite A anorthite ,3-6A basalt ,3-6B basalt ,4-6A basalt ,4-6L basalt < ,4-6B basalt ,4-6C basalt *Excluding Fe-produced 38Ar and high-temperature steps assumed to release 38Ar from pyroxene, as discussed in text. Using nominal production rate of cc STP/g Ca/Ma.

9 Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon 1353 FIG. 6. Argon-release spectra and K/Ca ratios for Luna 16 samples. The steps used in the calculation of sample age are indicated by the underbar.

10 1354 Cohen et al. 1609B Basalt This sample is similar to 1609A, but is distinctive in containing a large (350 lm) subhedral olivine phenocryst. The argon-release pattern of this sample is also similar to 1609A. The high K/Ca ratios indicate a K-rich glassy mesostasis that releases argon in the low-temperature steps and was disturbed at 1000 Ma. This disturbance caused 60% of the 40 Ar to be lost from the sample. A high-temperature plateau is reached over 43% of the 39 Ar release, yielding an apparent crystallization age for this sample of 3347 ~ 4 Ma. 1635B Basalt This sample is slightly coarser grained than 1609A or 1609B, indicating slower cooling. It has a higher proportion of K-rich mesostasis present than do 1609A or 1609B, as evidenced by its higher K/Ca ratios in the lowtemperature steps. It too was disturbed at 1000 Ma, causing 40Ar release, but was more intensely affected, probably because of the larger amount of glass present. Thus, only a minimum age is observable at the maximum of the curve in the argonrelease spectrum, at 3421 ~ 7 Ma Basalt This sample is essentially identical to 1635B in its argon release and K/Ca spectra. It experienced an extreme disturbance at 1000 Ma and now only yields a minimum crystallization age of 3426 ~ 6 Ma, identical within error to 1635B. 1609C Glassy Fragment This glassy sample has numerous vesicles and mineral fragments and the argon spectrum is problematic. The K/Ca ratio mimics that in the basaltic samples, but an order of magnitude less K appears to be present. The anomalously high apparent ages in the low-temperature steps is probably a result of experimental recoil, where 39 Ar is redistributed within the sample during high-energy conversion in the reactor, and sometimes completely lost from the lowretentivity sites (McDougall and Harrison, 1999). The downturn in apparent age in the highest temperature steps also reflects recoil redistribution. In the low-temperature steps, a negative apparent age is derived. This is likely the result of an overcorrection for 40 Ar contributed from the lunar environment, based on the amount of 36 Ar in the sample, caused either by 36Ar recoil effects or an inappropriate initial 40 Ar/ 36 Ar ratio. The remainder of the spectrum has a generally flat but ragged character. The age calculated from this part of the spectrum is 4274 ~ 6 Ma. 1635A Glassy Basalt This sample is also glassy and vesicular. The argon release spectrum is as problematic as that in 1609C and the contribution of excess 36 Ar is apparent. However, the calculated age over the flat part of the spectrum is much different: 2753 ~ 4 Ma. Excluding steps with extremely high or low apparent ages does not significantly change this age or error since only a small amount of 39 Ar is contributed by each step. Luna 20 Of the six Luna 20 fragments, two are troctolites, two are gabbros, and two are single anorthite grains. Figure 7 plots the argon release spectra and K/Ca ratios of all six samples and ages are reported in Table 3. Each heating step is shown with a 1r uncertainty, though the plateau ages are reported with a 2r uncertainty. In this section, the interpretation of the release patterns is discussed. 2004C Troctolite This sample is a porous troctolite. Though the K/Ca ratio in this sample is nearly constant in all heating steps, high apparent ages are exhibited in the lowtemperature releases. This effect is a recoil artifact, where the generated 39 Ar has enough energy to escape the K-rich phase. Usually, the 39 Ar implants itself into a neighboring Ca-rich phase and is outgassed at high temperatures, leading to low apparent ages, but that is not the case in this sample. The 39 Ar may have been lost from the low-retentivity sites altogether, consistent with the fine-grained, porous nature of the TABLE 3. 40Ar-39Ar and CRE ages of Luna 16 rocks. Sample Type Age (Ga) ~ 2r CRE age (Ma) Reference 1635A glassy fragment ~ This work 1609A basalt ~ This work 1609B basalt ~ This work basalt 3.35 ~ Cadogan and Turner (1977) basalt 3.39 ~ 0.05 n/a Fernandes et al. (2000) 1635B basalt ~ 0.007* 649 This work basalt ~ 0.006* 582 This work basalt ~ Huneke et al. (1972) basalt 3.50 ~ Cadogan and Turner (1977) 1609C glassy fragment ~ This work Luna 16 bulk soil 900 Kaiser (1972) Luna 16 bulk soil 360 Eugster et al. (1973) Luna 16 bulk soil 890 Agrawal et al. (1974) *Minimum age. Recalculated using decay constants of Steiger and Jäger (1977). n/a = not available.

11 Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon 1355 FIG. 7. Argon-release spectra and K/Ca ratios for Luna 20 samples. The steps used in the calculation of sample age are indicated by the underbar.

12 1356 Cohen et al. plagioclase. A well-defined plateau extends over 89% of the 39Ar release, yielding an age of 4191 ~ 22 Ma Spinel Troctolite The argon release spectrum for this sample is enigmatic. A highly degassed, K-rich phase is suggested in the low-temperature steps, but no such phase was found during EMP analysis. Recoil effects or uncorrected 36 Ar may account for the unreasonably high initial apparent ages. Even if a satisfactory explanation of the low-temperature steps is lacking, a clear plateau exists over 74% of the 39 Ar release, with an age of 4189 ~ 8 Ma. 2004B Anorthositic Gabbro This sample consists of anhedral plagioclase with interstitial olivine and pyroxene grains. It has a remarkably homogeneous K/Ca ratio, indicating that degassing is probably all from a single phase, plagioclase. Therefore, the high initial ages are probably due to recoil loss of 39 Ar. The subsequent step-up in apparent age points to diffusive 40 Ar loss at an extrapolated time of 2 Ga. However, the apparent plateau over 43% of 39 Ar loss has an old age of 4114 ~ 25 Ma. If true diffusive loss occurred, this plateau must be regarded only as a minimum age. 2004D Gabbro This sample consists of coarse-grained pyroxene with acicular plagioclase. The argon release profile shows signs of recoil or uncorrected 36 Ar in the lowest temperature steps. A K-rich phase is implied by the high K/Ca ratios in the low-temperature steps as well. In contrast to the Luna 16 and 24 basalts, this K-rich phase has not been appreciably disturbed, instead contributing with the more Ca-rich phases to a plateau over 64% of 39 Ar release. The age of 4021 ~ 10 Ma is significantly younger than the 2004B gabbro and 2004A Anorthite These samples are single grains of anorthite, possibly coarse grains from other rocks at the Luna 20 site. Neither was analyzed by EMP. The 2002 anorthite (Fig. 7e) is nearly identical to the 2004B gabbro in diffusive profile after the first 20% of 39 Ar released (Fig. 7c), with loss at 2 Ga, and a plateau over 52% of 39 Ar loss yielding an age of 4114 ~ 10 Ma. The 2004A anorthite has a significantly higher age of 4265 ~ 5 Ma over 61% of its 39 Ar release, accompanied by recoil and diffusion effects in the lowtemperature steps. Extrapolation of the diffusion profile between 10% and 40% of 39 Ar released points to a disturbance age of 4 Ga. Luna 24 All six fragments from Luna 24 are basalts. Their argon release spectra and K/Ca ratios are shown in Fig. 8 and ages are reported in Table 4. Each heating step is shown with a 1r uncertainty, though the plateau ages are reported with a 2r uncertainty. In this section, the interpretation of the release patterns is discussed ,3-6A Fine-Grained Quench Basalt This sample has a very fine-grained (mostly <50 lm), acicular texture. The argon release spectrum is typical of lunar basalts and qualitatively similar to the Luna 16 basalts in having a high-k mesostasis that outgasses at low temperatures. However, this TABLE 4. 40Ar-39Ar and CRE ages of Luna 20 rocks. Sample Type Age (Ga) ~ 2r CRE age (Ma) Reference impact melt 0.38 ~ Swindle et al. (1991) impact melt ~ Swindle et al. (1991) impact melt 3.75 ~ Swindle et al. (1991) recrystallized breccia ~ 0.04* Podosek et al. (1973) impact melt ~ Swindle et al. (1991) impact melt ~ Swindle et al. (1991) metaclastic rock 3.90 ~ Cadogan and Turner (1977) 2004D gabbro ~ This work impact melt ~ Swindle et al. (1991) highland basalt 4.1 n/a Huneke and Wasserburg (1979) 2004B gabbro ~ This work 2002 anorthite grain ~ This work 2003 spinel troctolite ~ This work 2004C troctolite ~ This work 2004A anorthite grain ~ This work anorthite grains 4.3 ~ Cadogan and Turner (1977) anorthite 4.36 n/a Huneke and Wasserburg (1979) Luna 20 bulk soil 351 Agrawal et al. (1974) Luna 20 bulk soil 355 Eugster et al. (1975) *Recalculated using decay constants of Steiger and Jäger (1977). Mid-temperature degassing age. Based on 38Ar. n/a = not available.

13 Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon 1357 FIG. 8. Argon-release spectra and K/Ca ratios for Luna 24 samples. The steps used in the calculation of sample age are indicated by the underbar.

14 1358 Cohen et al. sample has much less K in the mesostasis than the Luna 16 basalts and the degassing of the mesostasis is masked by high apparent ages in the lowest temperature steps. This effect is probably due to uncorrected 36 Ar rather than recoil effects, because a corresponding downturn in apparent age is not seen in the highest temperature release. A well-defined plateau exists over 68% of the 39 Ar release in the high-temperature steps, yielding an age of 3224 ~ 7 Ma ,3-6B Fine-Grained Olivine Basalt This sample is similar to 24143,4-6L in having a greater proportion of oxides (ilmenite and Ti-rich spinel) than other samples. The argon-release spectrum shows evidence for a large amount of uncorrected 36 Ar in the low-temperature steps but a well-defined high-temperature plateau. The K/Ca ratio shows little evidence of a glassy mesostasis and correspondingly little degassing. Recoil effects are not evident in the highest temperature steps. An age of 3279 ~ 8 Ma is derived from a plateau over 83% of the 39 Ar release ,4-6A Gabbro While EMP analyses were not performed on this fragment, its argon release spectrum is nearly identical to that of 24067,3-6B. A plateau over 61% of 39 Ar release yields an apparent age of 3277 ~ 6 Ma ,4-6L Olivine Basalt This sample is similar to 24067,3-6B in composition and argon-release spectrum. The K/Ca ratio does not indicate the presence of a K-rich mesostasis, and recoil effects are not prominent in the highest temperature steps. The high apparent ages in the low-temperature steps are probably contributed from 36 Ar. A plateau over 65% of the 39Ar release gives an age of 3274 ~ 14 Ma ,4-6B Coarse-Grained Basalt The predominance of well-crystallized, large plagioclase grains gives this sample a uniform K/Ca ratio and flat argon release spectrum, yielding an age of 3377 ~ 11 Ma from 100% of the 39 Ar released ,4-6C Ilmenite-Rich Basalt This sample is finer grained than 24184,4-6B, but still coarser grained than the "finegrained" basalts. The Si-rich glassy mesostasis contributes to argon degassing in the low-temperature steps, but the small size of this fragment (0.2 mg) limited the number of heating steps, and therefore, the resolution of the K/Ca spectrum. Though one heating step degassed >50% of the 39 Ar, its apparent age is in line with the subsequent steps and the plateau is made up of 92% of the 39 Ar release. The apparent plateau age is 3329 ~ 20 Ma. COSMIC-RAY EXPOSURE AGES Cosmogenic 38 Ar (as well as 36 Ar and 37 Ar) is produced by spallation of Ca, K, Ti, and Fe by cosmic rays in the top 100 cm of lunar soil in a typical surface production ratio of 1:1:0.1:0.05 (Hohenberg et al., 1978), where 38 Ar/ 36 Ar = 1.54 and 37 Ar/ 36 Ar >0. Trapped 38 Ar is implanted into the soil by the solar wind, with 38 Ar/ 36 Ar = 0.18 and 37 Ar/ 36 Ar = 0. Threeisotope correlation plots of argon data, corrected only for blanks, decay time, and interfering reactions, show that the 38 Ar in these samples is a mixture of cosmogenic and trapped components (Fig. 9), with no contribution to 38 Ar from a Cl-rich FIG Ar/37Ar/36Ar isochron correlation plots for Luna samples: (a) Luna 16; (b) Luna 20; and (c) Luna 24. The majority of heating steps in all samples released 38 Ar and 37 Ar from a mixture of cosmogenic ( 38 Ar/36Ar = 1.54) and solar wind (38Ar/36Ar = 0.18) sources. The slope of the mixing line corresponds to the cosmic-ray exposure age of each sample.

15 Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon 1359 source (which would be manifested by 38 Ar/ 36 Ar > 1.54). The low bulk K and Ti contents contribute a negligible amount of 38Ar, but the contribution from Fe may be up to a few percent in the basalts and gabbros. The slope of the correlation in the three-isotope plots, 38Ar/ 37 Ar, can be directly related to Ca-derived 38 Ar (Turner et al., 1971; Turner, 1972): 38 Ar Ca 38 Ar = 1.13 n α J 37 Ar where 1.13 relates the 38 Ar/ 37 Ar ratio to the cosmogenic-only component, n is the fraction of Ca-only derived 38 Ar (taken to be 97% in the basalts and gabbros; 100% in the troctolites and anorthites), a is the proportionality factor between 39 Ar/ 37 Ar and K/Ca in the flux monitor (0.545), J is an irradiation parameter ( ), and incorporates constants of K-decay and unit conversions to make 38 Ar/Ca into units of cc STP/g (Table 2). The 38 Ar contributed from Fe and Ca in pyroxenes is released in the highest temperature steps (most retentive sites), which generally plot slightly above the correlation. These points were excluded from the fit to determine cosmic-ray exposure (CRE) age but were considered in estimates of the stratigraphic depth, as discussed later. The CRE age is calculated by multiplying the 38 Ar/Ca ratio by the production function of 38 Ar from Ca (P 38 ) on the lunar surface. A value for P 38 of cc STP/g Ca/Ma (Turner et al., 1971) is the most common in determining nominal CRE ages and is used here in order to compare our calculated values to those of other samples. In most cases, the calculated CRE ages of these samples are consistent with previously determined CRE ages and the CRE age of the bulk soil (Tables 3, 4 and 5). The assumed value of P 38 is a large uncertainty. Measurements in lunar soils and meteorites (Eugster and Michel, 1995; Hohenberg et al., 1978) have provided empirical constraints on P 38 vs. depth but the stratigraphic depth of the Luna particles is not well-known. The production function actually depends both on depth of irradiation and the target element. Spallationproduced 38 Ar from Ca, P 38 (Ca), initially increases with depth, as the spallation process produces a cascade of secondary TABLE 5. 40Ar-39Ar and CRE ages of Luna 24 rocks. Sample Type Age (Ga) ~ 2σ CRE age (Ma) Reference dolerite 2.30 ~ Shanin et al. (1981) dolerite 2.40 ~ Shanin et al. (1981) coarse-grained basalt 2.93 ~ Burgess and Turner (1998) coarse-grained basalt 3.16 ~ Burgess and Turner (1998) fine-grained basalt 3.20 ~ Burgess and Turner (1998) coarse-grained basalt 3.20 ~ Burgess and Turner (1998) metabasalt 3.22 ~ Burgess and Turner (1998) 24067,3-6A fine-grained basalt ~ This work coarse-grained basalt 3.25 ~ Fugzan et al. (1986) metabasalt 3.26 ~ Schaeffer et al. (1978) 24143,4-6L fine-grained basalt ~ This work 24067,3-6B coarse-grained basalt ~ This work 24184,4-6A fine-grained basalt ~ This work metabasalt 3.29 ~ Burgess and Turner (1998) coarse-grained basalt ~ 0.05* 215 Wasserburg et al. (1978) 24184,4-6C coarse-grained basalt ~ This work coarse-grained basalt 3.33 ~ Schaeffer et al. (1978) coarse-grained basalt 3.34 ~ Fugzan et al. (1986) coarse-grained basalt 3.36 ~ Fugzan et al. (1986) metabasalt 3.36 ~ Hennessy and Turner (1980) feldspar from basalt 3.37 ~ Hennessy and Turner (1980) 24184,4-6B coarse-grained basalt ~ This work fine-grained basalt 3.39 ~ Fugzan et al. (1986) coarse-grained basalt 3.43 ~ Fugzan et al. (1986) fine-grained basalt 3.49 ~ Fugzan et al. (1986) fine-grained basalt 3.52 ~ Fugzan et al. (1986) lithic fragment ~ Stettler and Albarede (1978) cataclastic rock 3.75 ~ Fugzan et al. (1986) Luna 24 bulk soil > Bogard and Hirsch (1978) *Concordant Sm-Nd and 40 Ar- 39 Ar ages on single fragment. Recalculated using decay constants of Steiger and Jäger (1977).

16 1360 Cohen et al. particles, but declines with shielding at deeper levels. Because more energetic cosmic rays are required to produce 38 Ar from Fe, P 38 (Fe) function declines rapidly with depth. The ratio of P 38 from Fe and Ca has been used (Hennessy and Turner, 1980) to estimate the depth at which particles received most of their exposure. A similar calculation was performed for the samples in this study, where a separate slope was fit to the high-t steps in the 38 Ar/ 37 Ar/ 36 Ar correlation plots, a high-temperature 38 Ar/Ca ratio found and assumed to be from pyroxene, the Fe/Ca ratio for the sample found from the pyroxene mineralogy (see Mineral Chemistry and Petrology; 1.52 for Luna 16, 2.2 for Luna 20 and 24), and the P 38 (Fe)/P 38 (Ca) calculated. This ratio was compared with Fig. 5 in (Hennessy and Turner, 1980) to estimate a soil depth for four Luna 16 basalts and two Luna 24 basalts (Table 2). The CRE calculations further elucidate some behaviors in the 40 Ar/ 39 Ar spectra. In nearly every sample, the first few heating steps (up to 750 #C) had anomalously high 40 Ar/ 39 Ar ratios. These steps also had anomalously high 38 Ar/ 36 Ar ratios and low 37 Ar/ 36 Ar ratios. The excess amount of 38 Ar and 40 Ar in the lowest temperature steps is nearly certainly adsorbed terrestrial atmosphere, contributing argon directly and/or through addition of adsorbed Cl. DISCUSSION Volcanism in Mare Fecunditatis In contrast to the very low-ti Luna 24 basalts, the four Luna 16 basalts studied contain abundant ilmenite. However, as pointed out by Semenova et al. (1991), these basalts are not related to high-ti basalts from Apollo 11 or 17 by either nearsurface fractionation or melting of similar source regions. Compared to other mare basalts, the Luna 16 basalts have higher Al contents. However, the Al contents are not elevated to the levels suggested by Kurat et al. (1976), as determined by suspect defocused beam methods. Thus, the need for a source with affinities to ferroan anorthosites is not warranted. Due to the high normative olivine content of Luna 16 basalts, it is thought that the source for the Luna 16 basalts is relatively deep ( km) and similar in composition to that of the low-ti basalts (Steele and Smith, 1972). The argon release patterns in the basaltic fragments are typical of lunar basalts (McDougall and Harrison, 1999). The first degassing is from a very high-k phase, interpreted as a K-rich, glassy mesostasis. The K/Ca ratio reaches a low, constant value at about the same heating step as the beginning of the 39 Ar release plateau, indicating that there is an argon contribution from undisturbed, crystalline plagioclase. Though the mesostasis was disturbed, the high-temperature plateaus in samples 1609A and 1609B probably indicate the crystallization ages for these basalts. In samples 1635B and , more severe 40 Ar loss occurred, and only minimum ages are indicated. The low-temperature steps indicate a disturbance at 1 Ga, which caused 40 Ar loss from each basalt sample. The loss of 40Ar from all four samples at 1 Ga indicates that these samples were subjected to a thermal event at this time. Comparison of the extent of calculated 40 Ar loss with diffusion models suggests that the apparent plateau age in these samples is 40 60% younger than the true age (McDougall and Harrison, 1999), but this results in a model age older than the age of the solar system. It is more likely that these samples are not singlesource diffusion domains and that argon loss occurred primarily from an easily-degassed phase. Following Turner (1971), Arrhenius plots of the argon data, corrected only for blanks, decay time, and interfering reactions, were used to further understand the mechanism of argon loss in these samples. An effective diffusion parameter, D/a 2, was calculated for 39 Ar loss in each step: D π = F 2 a 36 2 i ti where F i is the cumulative fractional loss of 39 Ar in step i, and t i is the cumulative heating time experienced by the sample. The diffusion parameter D/a 2 in each heating step is also related to the temperature T (estimated from the laser power using a Stefan Boltzmann relationship): D D E = exp 2 2 a a RT i where E is the activation energy for the site and R is the gas constant. A plot of D/a 2 vs. 1000/T should show each effective diffusion domain by a constant slope proportional to E. In the Luna 16 basalts (Fig. 10), two effective diffusion domains are evident, one at low temperatures (\700 #C) and one at higher temperatures. This corroborates the inference that 39 Ar loss at low temperatures is from a high-k glassy mesostasis with different diffusion parameters than the crystalline plagioclase, and that the apparently high degree of 40 Ar loss is primarily a result of degassing of this phase and not the entire sample. The activation energy of the low-temperature domain is similar in all cases, 18 kcal mol 1 K 1. This value is quite different from the 29.3 kcal mol 1 K 1 suggested by Turner (1971) for 39 Ar loss due to solar heating at the surface of the Moon. The proximity of two Copernican-age craters, Langrenus and Taruntius, to the Luna 16 site suggests the basalt fragments may have been ejected from a young, nearby crater. Simple shock propagation through a crystalline target fails to cause complete Ar loss (Deutsch and Schärer, 1994), even at pressures high enough to convert plagioclase into glass. On the other hand, shock or burial in an ejecta blanket may be sufficient to degas a glassy mesostasis. The four crystalline basalt ages reported here, along with four additional basalt fragment ages (Cadogan and Turner, 1977; Fernandes et al., 2000; Huneke et al., 1972), and their CRE ages, are compiled in Table 3. Each reported age can be represented as a Gaussian curve, where the width of each Gaussian is proportional to the uncertainty in the age 0

17 Argon-40 argon-39 chronology and petrogenesis along the eastern limb of the Moon 1361 FIG. 10. Arrhenius plot for Luna 16 basalts. A linear fit was calculated for the low-temperature (<700 #C) heating steps (solid symbols). The slope of the line corresponds to the activation energy (E) of the lowtemperature diffusion domain. determination, and a unit area exists underneath each Gaussian. Adding the individual Gaussians produces an ideogram, or a histogram that accounts for measurement uncertainty (Fig. 11). Two normal distributions were fit to the peaks in Fig. 11 and are plotted in dashed lines. The two peaks in the ideogram are made up of (1) two basalts with a combined age of 3347 ~ 24 Ma and (2) five basalts with a combined age of 3421 ~ 30 Ma. Both groups are made up of samples dated in multiple laboratories, strengthening the case that these samples represent two different flows. The youngest basaltic fragment yields an age of 3.15 Ga, distinctly younger than the others and possibly representing a third flow. Because the basalts from this region have a distinct composition, exotic basalt fragments (emplaced by ejecta processes from some other region) would be recognized and do not seem to be represented in this collection thus far. Therefore, all three basalt age clusters probably represent volcanism in Mare Fecunditatis from similar sources. The volcanic period appears to have spanned 300 Ma (3.42 to 3.15 Ga), although discontinuously. The Luna 16 basalt samples have CRE ages contemporaneous or younger than the 1 Ga 40 Ar loss event. No equivalent stepwise loss of 38 Ar is evident, suggesting that exposure followed the 40 Ar loss event. The relatively old CRE ages and shallow estimated depths of the Luna 16 basalts are consistent with the suggestion that these samples were affected by a young (1 Ga) crater, which degassed their easily-disturbed mesostasis and threw them onto the lunar surface. The Luna 16 basalts with younger CRE ages were probably buried deeper FIG. 11. Ideogram of crystallization ages in Luna 16 basalts. The solid curve is a histogram of Gaussian distributions representing each sample's age and uncertainty. The dashed curves are normal distributions calculated to fit the ideogram peaks, showing that two basalts have a combined age of 3347 ~ 24 Ma and five basalts have a combined age of 3421 ~ 30 Ma. Data sources are summarized in Table 2. in the ejecta blanket and gardened to the near-surface at a later time. Three of the four Luna 16 basalts were very near the surface (<5 cm) when they received the bulk of their cosmicray exposure. Coupled with their relatively old CRE ages, this implies that the Luna 16 regolith is very thin and that not much unexposed material is available to be gardened. The two glassy fragments in this study exhibit irregular release patterns, where recoil effects and the contribution of a high initial amount of 36 Ar are evident in both samples. The absence of skeletal crystals in the glass and the presence of many minute, scattered grains of FeNi metal in these samples makes it probable that these samples are regolith agglutinates rather than primary basalts. The raggedness of the "plateau" steps probably reflects K or Ar redistribution in the glassy phase and uneven release from the embedded grains. In neither sample does the calculated "plateau" age match the crystallization ages of the basaltic fragments. In sample 1635A, a young age of 2.8 Ga is recorded, while an interestingly old age of 4.3 Ga seems to be recorded in sample 1609C. However, the argon release in an agglutinitic sample reflects contributions from regolith fragments of all ages, and the combined spectrum is virtually meaningless in terms of sample age. It is likely that the difference in ages is due to varying proportions of old anorthositic material. This is in contrast to impact or volcanic glass, where clast-free, completely melted samples are known to yield well-defined Ar plateaus (Culler et al., 2000; Huneke,