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40Ar/39Ar ages of lunar impact glasses: Relationsh

University at Albany, State University of New York Scholars Archive Atmospheric and Environmental Science Faculty Scholarship Atmospheric and Environmental Sciences 11-2015 40Ar/39Ar ages of lunar impact glasses: Relationships among Ar diffusivity, chemical composition, shape, and size John W. Delano PhD University at Albany, State University of New York, jdelano@albany.edu Nicolle Zellner Albion College, nzellner@albion.edu






Follow this and additional works at: http://scholarsarchive.library.albany.edu/cas_daes_scholar Part of the Cosmochemistry Commons, and the Geochemistry Commons This Article is brought to you for free and open access by the Atmospheric and Environmental Sciences at Scholars Archive. It has been accepted for inclusion in Atmospheric and Environmental Science Faculty Scholarship by an authorized administrator of Scholars Archive. For more information, please contact scholarsarchive@albany.edu. Recommended Citation Delano, John W. PhD and Zellner, Nicolle, "40Ar/39Ar ages of lunar impact glasses: Relationships among Ar diffusivity, chemical composition, shape, and size" (2015). Atmospheric and Environmental Science Faculty Scholarship. 4. http://scholarsarchive.library.albany.edu/cas_daes_scholar/4 40Ar/39Ar ages of lunar impact glasses: Relationships among Ar diffusivity, chemical composition, shape, and size N.E.B. Zellner a,⇑ , J.W. Delano b a Department of Physics, Albion College, Albion, MI 49224, USA b New York Center for Astrobiology, Department of Atmospheric and Environmental Sciences, University at Albany (SUNY), Albany, NY 12222, USA Received 18 November 2013; accepted in revised form 7 April 2015; available online 16 April 2015 Abstract Lunar impact glasses, which are quenched melts produced during cratering events on the Moon, have the potential to provide not only compositional information about both the local and regional geology of the Moon but also information about the impact flux over time. We present in this paper the results of 73 new 40Ar/39Ar analyses of well-characterized, inclusion-free lunar impact glasses and demonstrate that size, shape, chemical composition, fraction of radiogenic 40Ar retained, and cosmic ray exposure (CRE) ages are important for 40Ar/39Ar investigations of these samples. Specifically, analyses of lunar impact glasses from the Apollo 14, 16, and 17 landing sites indicate that retention of radiogenic 40Ar is a strong function of post-formation thermal history in the lunar regolith, size, and chemical composition. This is because the Ar diffusion coefficient (at a constant temperature) is estimated to decrease by 3–4 orders of magnitude with an increasing fraction of non-bridging oxygens, X(NBO), over the compositional range of most lunar impact glasses with compositions from feldspathic to basaltic. Based on these relationships, lunar impact glasses with compositions and sizes sufficient to have retained 90% of their radiogenic Ar during 750 Ma of cosmic ray exposure at time-integrated temperatures of up to 290 K have been identified and are likely to have yielded reliable 40Ar/39Ar ages of formation. Additionally, 50% of the identified impact glass spheres have formation ages of 6500 Ma, while 75% of the identified lunar impact glass shards and spheres have ages of formation 62000 Ma. Higher thermal stresses in lunar impact glasses quenched from hyperliquidus temperatures are considered the likely cause of poor survival of impact glass spheres, as well as the decreasing frequency of lunar impact glasses in general with increasing age. The observed age-frequency distribution of lunar impact glasses may reflect two processes: (i) diminished preservation due to spontaneous shattering with age; and (ii) preservation of a remnant population of impact glasses from the tail end of the terminal lunar bombardment having 40Ar/39Ar ages up to 3800 Ma. A protocol is described for selecting and analyzing lunar impact glasses. 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). 1. INTRODUCTION The Moon provides the most complete history of impact events in the inner Solar System since its formation 4500 million years ago (e.g., Neukum et al., 2001; Sto¨ffler and Ryder, 2001; Sto¨ffler et al., 2006; LeFeuvre and Wieczorek, 2011; Morbidelli et al., 2012; Fassett and Minton, 2013; Kirchoff et al., 2013). Since the Moon and Earth are close together in space, if properly interpreted, the Moon’s impact record can be used to gain insights into how the Earth has been influenced by impacting events over billions of years. The timing of impacts on the Moon, however, is not well understood and is important for several reasons (NRC, 2007). Since lunar impact glasses are droplets of melt produced by energetic cratering events and quenched during ballistic http://dx.doi.org/10.1016/j.gca.2015.04.013 0016-7037/ 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). ⇑ Corresponding author. www.elsevier.com/locate/gca Available online at www.sciencedirect.com ScienceDirect Geochimica et Cosmochimica Acta 161 (2015) 203–218 flight away from the target, their isotopic ages have the potential to provide constraints on the impact flux during the last several billion years, if the data are interpreted correctly. The impact flux can then be used to address the persistent question of whether or not there was a lunar cataclysm at around 3900 Ma (Tera et al., 1974) and what its relationship to the late heavy bombardment (LHB; e.g., Ryder et al., 2000) may be. Other questions about the impact flux can also be addressed. In addition, impact glasses sample widespread and random locations on the Moon making them a powerful tool for geochemical exploration of the Moon’s crustal composition (Delano, 1991; Zellner et al., 2002), even though the location of impact ejection may not be known. Additionally, the compositions of glasses collected at a specific site can tell us about the geographic, and stratigraphic, character of that site, when well-established criteria for confidently distinguishing lunar impact-generated glasses from lunar volcanic glasses (Delano, 1986) are applied. In the past decade or so, impact glasses have been increasingly used as tools to address the impact flux. Culler et al. (2000) studied 155 spherical glasses from the Apollo 14 landing site and interpreted the results in the context of both global lunar impacts and delivery of biomolecules to the Earth’s surface. In particular, they interpreted their 40Ar/39Ar isotopic data on those glass spheres (without having attempted to distinguish between impact glasses and volcanic glasses) as evidence for (i) an increased impact flux around 3900 Ma (the purported “cataclysm”) and (ii) a factor of 3.7 ± 1.2 increase in the last 400 Ma (Culler et al., 2000; Muller et al., 2001; Muller, 2002). In order to distinguish between impact and volcanic glasses, Levine et al. (2005) chemically analyzed the surfaces of spherical glasses from the Apollo 12 landing site and obtained 40Ar/39Ar ages on 81 lunar impact glasses. Although they also concluded that the age-distribution of their impact glass spheres was consistent with an apparent increase in the recent impact flux, Levine et al. (2005) suggested that local, young cratering events could be causing young spherical impact glasses to be disproportionately represented. While interesting, these studies were incomplete in the following ways: (i) chemical compositions of the glasses were not determined (Culler et al., 2000), (ii) glasses of volcanic origin were not excluded from the data-set (Culler et al., 2000), and (iii) xenocryst-free, homogenous impact glasses were not solely used (Levine et al., 2005). Since Culler et al. (2000) did not provide descriptions of their glass spheres, item ‘iii’ may also apply to that investigation. The first and second concerns are important because it is not relevant to include the isotopic ages of lunar volcanic glasses when reporting an impact flux. For example, Delano (1988) reported that nearly 50% of the glasses in the youngest regolith breccia, 14307, studied at the Apollo 14 site (i.e., most similar to the current regolith) were of volcanic origin. In addition, since those volcanic glasses were more frequently spherical in shape than were the impact glasses, it is plausible that Culler et al. (2000) had a significant proportion of volcanic ages among their reported ages. The third concern is important because inherited Ar from undegassed crystalline inclusions can affect the reported 40Ar/39Ar formation age of a glass (Huneke et al., 1974; Jourdan, 2012), thereby contaminating the inferred age-distribution of lunar impact events. Finally, both groups assumed that each impact glass was formed in its own discrete impact event and thus that multiple glasses could not be formed in the same impact event. We have obtained geochemical and chronological data on almost 100 xenocryst-free, homogeneous (or nearly so) impact glasses from the Apollo 14, 16, and 17 landing sites and with subsets of these 100 samples, we have demonstrated the efficacy of interpreting these data together to understand the history of the sample(s). For example, Delano et al. (2007) showed that four glass shards (i.e., fragments, not spheres) with the same composition (‘low-Mg high-K Fra Mauro’ (‘lmHKFM’) glasses of Delano et al., 2007; ‘basaltic-andesite’ glasses of Zeigler et al., 2006 and Korotev et al., 2010) from the Apollo 16 landing site were formed at the same time, in one event (and not four). Therefore, the approach of interpreting the age data in the context of the compositional data allows for a better interpretation of the impact flux, so that it is not artificially inflated. This study additionally reported that spherical glasses are more likely to possess the local regolith composition, while non-spherical glasses (i.e., shards, fragments) are more likely to possess a non-local composition. Zellner et al. (2009a,b) combined geochemistry, age, and shape to interpret the ages and provenance of impact glasses from several Apollo landing sites. Impact ages of 12 individual glasses from the Apollo 17 landing site (Zellner et al., 2009a) revealed that only nine impact events may have been involved, depending on the compositional grouping selected. A clustering of 40Ar/39Ar ages at 800 Ma (Zellner et al., 2009b) was observed in nine glasses from the Apollo 14, 16 and 17 landing sites, as well as in glasses from the Apollo 12 landing site (Levine et al., 2005), and at least seven separate impact events appear to have been involved in generating those glasses (Zellner et al., 2009b). Glasses from the Apollo 16 landing site were investigated by Hui et al. (2010), who specifically selected low-K glasses, classified as spherules with various shapes, in order to address the local impact flux at the Apollo 16 landing site. About 130 glasses from a sample of Apollo 16 regolith were analyzed for major and minor elements, and 30 of them (unpolished, to preserve sample-mass and the argon) had their 40Ar/39Ar ages determined. Some of those glasses appear to be neither homogeneous nor xenocryst-free (see Fig. 3 in Hui et al. (2010)). In order to distinguish among specific impact events, Hui et al. (2010) reported majorand minor-element compositions in addition to the 40Ar/39Ar ages for the impact glasses. Norman et al. (2012) suggested that in excess of 30% of glasses in a sample set could have been formed during the same impact event (i.e., glasses with the same composition and age). Even after accounting for multiple glasses formed in the same event, Hui et al. (2010) reported a high proportion of glasses (i.e., ‘spherules’) with ages <500 ma="" which="" they="" interpreted="" as="" being="" due="" to="" an="" increase="" in="" the="" recent="" impact="" flux="" 500="" though="" reported="" that="" regolith="" dynamics="" or="" surface="" collection="" could="" also="" be="" a="" possible="" explanation="" 204="" n="" e="" b="" zellner="" j="" w="" delano="" geochimica="" et="" cosmochimica="" acta="" 161="" 2015="" 203="" 218="" important="" result="" of="" detailed="" study="" was="" observation="" exterior="" i="" rind="" glass="" has="" composition="" is="" different="" from="" bulk="" may="" become="" useful="" constraint="" for="" inferring="" provenance="" s="" origin="" described="" below="" most="" recently="" norman="" al="" 2012="" chemical="" compositions="" 207pb="" 206pb="" model="" ages="" and="" u="" th="" pb="" spherical="" glasses="" volcanic="" origins="" apollo="" 17="" landing="" site="" had="" were="" broadly="" consistent="" with="" those="" known="" episodes="" lunar="" mare="" volcanism="" compositionally="" similar="" local="" consists="" largely="" mixture="" highland="" rock="" basalts="" defined="" by="" rhodes="" 1974="" many="" 6500="" suggested="" these="" locally="" derived="" produced="" small="" impacts="" during="" rather="" than="" global="" here="" we="" present="" new="" measurements="" improved="" interpretations="" 40ar="" 39ar="" on="" almost="" 100="" samples="" 14="" 16="" sites="" using="" conservative="" yet="" rigorous="" approaches="" better="" understand="" how="" argon="" diffusion="" affects="" sample="" age="" describe="" selection="" analysis="" methodologies="" involving="" size="" shape="" methods="" will="" allow="" investigators="" choose="" are="" likely="" yield="" reliable="" apparent="" so="" true="" representation="" impactors="" earth="" moon="" system="" revealed="" resultant="" offered="" 2="" characterization="" 1="" clean="" single="" phase="" not="" agglutinates="" prime="" analyses="" investigate="" rate="" over="" time="" because="" heated="" hyperliquidus="" temperatures="" melting="" event="" have="" been="" totally="" degassed="" quenched="" when="" analyzed="" contains="" maximum="" three="" ar-isotopic="" components:="" solar="" wind="" cosmogenic="" nuclides="" radiogenic="" previously="" 2002="" 2007="" 2009a="" current="20" crystalline="" nature="" devitrified="" ii="" contain="" neither="" unmelted="" mineral="" grains="" xenocrysts="" nor="" clasts="" xenoliths="" iii="" do="" possess="" crusty="" dusty="" outer="" rims="" iv="" demonstrably="" 1986="" geochemical="" data="" entire="" set="" both="" can="" found="" appendix="" propose="" following="" section="" while="" criteria="" mentioned="" above="" necessary="" investigations="" sufficient="" since="" extent="" diffusive="" loss="" residence="" near-surface="" duration="" magnitude="" diurnal="" temperature="" cycles="" fig="" related="" 3="" discussed="" 4="" it="" too="" must="" considered="" preparation="" selected="selected" listed="" previous="" individually="" mounted="" within="" container="" crystalbond="" adhesive="" each="" ground="" polished="" expose="" portion="" microbeam="" essential="" maximally="" preserve="" isotopic="" generally="" 650="" lm="" 1a="" planar="" electron="" microprobe="" determine="" photomicrograph="" provides="" record="" often="" helpful="" later="" stages="" manuscript="" transmitted="" light="" photomicrographs="" note="" free="" inclusions="" green="" sphere="" 160="" across="" brown="" shard="" 324="" high-ti="" exotic="" 1975="" zeigler="" 2003="" shown="" shows="" determining="" dark="" inner="" ring="" boundary="" between="" according="" minimum="" required="" text="" cre="" 750="" time-averaged="" 290="" k="" x="" nbo="" 0="" would="" whereas="" 33="" exceed="" 205="" used="" jeol="" 733="" department="" environmental="" sciences="" at="" rensselaer="" polytechnic="" institute="" troy="" ny="" major-element="" all="" isotopically="" dated="" operating="" conditions="" following:="" beam="" nanoamps="" diameter="20" count-time="" per="" element="" five="" wavelength="" dispersive="" spectrometers="60" including="" peak="" backgrounds="" measurement="" uncertainty="" amount="" exposed="" 5="" min="" effort="" constrain="" source="" regions="" show="" ratios="" major="" elements="" such="" mgo="" al2o3="" vs="" cao="" g="" 2006="" k2o="" proxy="" korotev="" 1998="" refractory="" 2009b="" addition="" helping="" establish="" relationships="" among="" paired="" distinguishing="" having="" picritic="" basaltic="" noritic="" feldspathic="" along="" whose="" first="" herein="" irradiated="" 300="" h="" phoenix="" ford="" reactor="" university="" michigan="" factors="" irradiation="" 05776="" 00030="" 07857="" 00048="" two="" separate="" irradiations="" fraction="" just="" 80="" same="" this="" 019875="" 0000363="" 0197070="" 0000604="" 019644="" 0000411="" depending="" location="" disk="" included="" mmhb-1="" hornblende="" 520="" but="" see="" jourdan="" renne="" concerns="" about="" monitor="" neutron="" fluence="" caf2="" salt="" correct="" reactor-produced="" interferences="" k2so4="" measure="" released="" ar="" measured="" vg5400="" mass="" spectrometer="" arizona="" tucson="" series="" extractions="" until="" counts="" peaked="" then="" decreased="" background="" levels="" appendices="" c="" d="" corrections="" blanks="" radioactive="" decay="" reactor-induced="" cosmic-ray="" spallation="" several="" spherules="" 15="" 15426="" 1979="" steele="" 1992="" well-defined="" 3340="" podosek="" huneke="" 1973="" 200="" ppm="" working="" standards="" reduced="" correlated="" errors="" isac="" hudson="" 1981="" deino="" software="" 2001="" weirich="" 2011="" constant="" steiger="" ja="" ger="" 1977="" 2010="" reduction="" table="" plateau="" more="" consecutive="" steps="" weighted="" average="" step="" one="" uncertainties="" calculated="" averages="" based="" least="" 2r="" quality="" assessment="" basis="" other="" ryder="" 1996="" hui="" stated="" studies="" report="" results="" high-k="" our="" group="" 73="" ones="" culler="" 2000="" levine="" 2005="" sizes="" shapes="" formation="" material="" soil="" surveys="" reid="" 1972a="" extracted="" regoliths="" collected="" spheres="" range="" 625="" keller="" mckay="" 6="" mm="" however="" what="" kind="" compositional="" clusters="" usually="" reflecting="" rocks="" target="" contrast="" observed="" impact-generated="" commonly="" necessarily="" few="" individual="" weakening="" claim="" ho="" rz="" cintala="" 1997="" there="" paucity="" theoretical="" modeling="" wu="" nnemann="" 2008="" showing="" porous="" target-materials="" generate="" higher="" melt="" volumes="" non-porous="" targets="" given="" energy="" uppermost="" 3-m="" porosities="" 37="" mitchell="" 1972="" densities="" 8="" cm3="" 206="" isotope="" figs="" 7="" young="" 250="" nd="" means="" determined="" nr="" release="" patterns="" good="" if="" data-mce-fragment="1">50% 39Ar was used in the age and most of the steps were concordant; “fair” if some of the steps were concordant; and “poor” if none of the steps were concordant. Sample # 40/36Ar 40/36Ar (1r error) Age (Ma) ±2r (Ma) # steps, %39Ar used in age Notes on Age Assessment of Age Size (mm) Shape Ref. Apollo 14 14259,624 7 1.277 0.696 45 12 5, 93.8% Plateau Good 346 Shard 7 10 0.363 0.004 1624 140 6, 100% Weighted Fair 404 Shard 8 31 0.355 0.003 1300 200 4, 57.1% Weighted Fair 233 Shard 8 53 2.650 0.569 3687 26 4, 100% Plateau Good 225 Shard 8 66 3429 16799 1037 32 1, 100% 1-step Good 115 Dumbbell 7 88 0.748 0.162 116 66 4, 89.1% Plateau Good 332 Sphere 7 100 3.660 0.420 783 76 2, 100% Weighted Good 332 Shard 8 168 0.477 0.197 451 228 3, 84.4% Plateau Good 144 Sphere 7 170 8 8 3573 24 4, 100% Weighted good 234 Shard 8 Apollo 16 64501,225 183 8 8 3808 393 5, 53.8% Weighted Good 215 Shard 7 185 0.444 0.019 3781 18 6, 94% Weighted Good 359 Shard 2,8 1873 6.678 0.986 3673 20 4, 100% Plateau Good 198 Shard 8 195 3.293 0.004 686 10 3, 90.1% Plateau Good 350 Shard 8 202 0.470 0.004 1530 70 3, 98.5% Weighted Good 404 Shard 8 223 7.554 0.145 3785 10 7, 90.8% Plateau Good 395 Shard 2,8 239 1.300 0.006 778 18 6, 79.3% Plateau Good 292 Shard 1,8 Apollo 17 71501,262 292 0.123 0.011 2500 1500 4, 94% Weighted Good 225 Shard 3,8 293 0.71 0.1 3740 50 3, 91.7% Weighted Good 287 Shard 3,8 301 0.102 0.002 102 20 1, 62% 1-step Fair 359 Sphere 3,8 304 0.399 0.011 1540 140 3, 97.9% Weighted Good 250 Shard 3,8 311 0.275 0.005 774 114 3, 96.2% Plateau Good 188 Sphere 1,3,8 322 0.413 0.004 1289 415 3, 100% Weighted Good 274 Shard 3,8 329 9.599 0.022 <1500 n="" a="" fair="" 225="" shard="" 8="" 349="" 0="" 526="" 003="" 1650="" 400="" 4="" 99="" 5="" weighted="" 242="" 3="" 352="" 282="" 004="" 1400="" 300="" 88="" 176="" sphere="" 360="" 025="" 018="" young="" nd="" 369="" 343="" 073="" 3630="" 40="" 87="" 2="" good="" 350="" apollo="" 16="" 66041="" 127="" 427="" 464="" 233="" 361="" 10="" 1="" 84="" step="" 574="" 437="" 103="" 2786="" 64="" 90="" 6="" 1-step="" 558="" 438="" 452="" 074="" 257="" 22="" plateau="" 327="" 443="" 949="" 087="" 510="" 94="" 7="" 413="" 455="" 050="" 014="" 988="" 44="" 53="" 345="" 465="" 418="" 222="" 4244="" 650="" 83="" 287="" 469="" 851="" 008="" 559="" 55="" 247="" 471="" 539="" 478="" 058="" 027="" 699="" 89="" 269="" 491="" 962="" 1100="" 200="" 77="" 493="" 262="" 567="" 404="" 100="" 93="" 292="" 520="" 3505="" 36="" 211="" 028="" 914="" 188="" 215="" chipped="" 530="" 407="" 007="" 948="" 54="" 60="" 368="" 531="" 645="" 685="" 86="" 229="" 533="" 304="" 140="" 260="" 540="" 24="" 767="" 502="" 2533="" 68="" 301="" 542="" 079="" 273="" 593="" 15434="" 28="" nr="" 1647="" 11="" 17="" 96="" 6000="" ls1="" 21="" 142="" 26="" 606="" 33="" 3717="" 482="" 82="" 322="" references="" zellner="" et="" al="" 2009b="" delano="" 2007="" 2009a="" this="" glass="" was="" previously="" described="" as="" vlt="" volcanic="" however="" reexamination="" of="" its="" composition="" shows="" that="" it="" is="" an="" impact="" onto="" mare="" basalt="" target="" the="" has="" mgo="" al2o3="" ratio="" lower="" than="" any="" known="" glasses="" ryder="" 1996="" hui="" 2011="" renne="" 2010="" steiger="" and="" ja="" ger="" 1977="" e="" b="" j="" w="" geochimica="" cosmochimica="" acta="" 161="" 2015="" 203="" 218="" 207="" crater="" size="" sizes="" craters="" produce="" lunar="" are="" unknown="" but="" they="" can="" provide="" insight="" into="" impactor="" created="" each="" resultant="" shape="" one="" thought="" formed="" only="" in="" cratering="" events="" km="" diameter="" g="" ho="" rz="" cintala="" 1997="" norman="" 2012="" micrometeorite="" impacts="" particular="" seem="" unlikely="" to="" generate="" significantly="" large="" volumes="" 107="" lm3="" for="" 400-lm="" spherule="" type="" which="" here="" other="" investigators="" prefer="" range="" m="" data-mce-fragment="1">100 km), especially if the glass composition is clearly exotic to the local regolith in which it was found (e.g., Delano, 1991; Symes et al., 1998; Zeigler et al., 2006; Delano et al., 2007; Korotev et al., 2010). Korotev et al. (2010) found that 75% of the impact glass in the Apollo 16 regolith is compositionally different from any mixture of rocks from which the regolith is mainly composed. Therefore, those impact glasses have been interpreted as being exotic to the Apollo 16 region and probably were formed by, and ballistically transported from, cratering events P100 km from the landing site (Zeigler et al., 2006; Delano et al., 2007; Korotev et al., 2010); Delano et al. (2007) found the majority of those exotic glasses to be non-spherical (i.e., shards). The shapes of the glasses reported herein have been used to suggest source terrain(s) as well as likelihood to report true 40Ar/39Ar ages. 4. RESULTS 4.1. Chemical composition and size: Implications for interpreting 40Ar/39Ar ages in lunar impact glasses All previous investigators (e.g., Culler et al., 2000; Levine et al., 2005; Delano et al., 2007; Zellner et al., 2009a,b) have implicitly assumed that lunar impact glasses are highly retentive of radiogenic 40Ar during prolonged residence in the shallow lunar regolith that is subjected to diurnal temperature variations. However, the rate of Ar diffusion was experimentally measured by Gombosi et al. (2015) in three large (1.6 mm diameter), inclusion-free, lunar impact glass spherules having uniform chemical compositions similar to that of the average Apollo 16 regolith with an X(NBO) value of 0.18. That investigation showed that significant loss of radiogenic 40Ar would occur during some exposure histories, such as 75% loss from a 400-lm diameter glass spherule residing at <2-cm depth="" below="" the="" lunar="" surface="" for="" 40="" ma="" fig="" 2="" shows="" range="" of="" diurnal="" temperature="" variations="" in="" regolith="" near="" moon="" s="" equator="" magnitude="" diminishes="" with="" to="" a="" nearly="" constant="" 260="" k="" 15="" c="" at="" 60="" cm="" langseth="" et="" al="" 1976="" lawson="" and="" jakosky="" 1999="" vasavada="" 2012="" diffusivity="" radiogenic="" 40ar="" depends="" on="" chemical="" composition="" melt="" structure="" which="" can="" be="" parameterized="" using="" fraction="" non-bridging="" oxygens="" x="" nbo="" mysen="" richet="" 2005="" lee="" 2011="" as="" shown="" eq="" 1="" given="" ar="" glass="" is="" inversely="" proportional="" its="" value="" :="" xnc="" xfc="" where="" oxide="" cations="" having="" network-modifying="" charge-balancing="" roles="" e="" g="" feo="" mno="" mgo="" cao="" na2o="" k2o="" network-forming="" other="" than="" si="" tio2="" al2o3="" references="" therein="" since="" cr2="" known="" dominant="" valence="" state="" cr="" materials="" smith="" 1974="" sutton="" 1993="" cro="" was="" included="" an="" additional="" component="" albeit="" minor="" one="" term="" titanium="" abundant="" some="" assumed="" contribute="" entirely="" farges="" 1996="" estimate="" temperature-="" time-integrated="" ar-diffusion="" coefficient="" glasses="" function="" it="" that="" main="" process="" causing="" loss="" thermal="" diffusion="" during="" cre="" cosmic="" ray="" exposure="" shallow="" rather="" episodic="" shock="" events="" spheres="" uniform="" abundances="" total="" lost="" f="fraction" residence="" determined="" by="" step-heating="" equation="" mcdougall="" harrison="" used="" coeffi-="" cient="" d="" huneke="" 1978="" radii="" ages="" below:="" a2="" p2t="" 2p="" p2="" 3="" ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi="" p="" r="" 6="" 0:85="" here="" sphere="" t="" seconds="" spent="" when="" diffusive="" occurred="" recorded="" age="" cycles="" occur="" upper="" cycle="" 80="" absolute="" temperatures="" cycling="" decrease="" increasing="" latitude="" time="" sample="" has="" resided="" within="" few="" meters="" eugster="" 2003="" 208="" n="" b="" zellner="" j="" w="" delano="" geochimica="" cosmochimica="" acta="" 161="" 2015="" 203="" 218="" post-formation="" history="" burial="" if="" been="" calculated="" based="" spallation="" production="" rates="" then="" actual="" m="" would="" greater="" podosek="" 1973="" limit="" decreases="" results="" are="" contours="" appropriate="" uppermost="" while="" rate="" 0="" 18="" 19="" gombosi="" dependence="" over="" observed="" inferred="" trend="" defined="" represented="" log="" several="" described="" associated="" each="" contour="" from="" had="" diameters="" ranging="" lm="" data-mce-fragment="1">1400 lm and CRE ages ranging from 30 Ma to 300 Ma. The two main goals of Fig. 3 are to (i) estimate the diffusivity of 40Ar in lunar glasses as a function of chemical composition, X(NBO), and to (ii) use that information to guide the selection of lunar glasses for 40Ar/39Ar dating in order to find those that have experienced minimal loss of 40Ar. The strategy for this estimation is based on using lunar glasses of known dimensions, CRE age, fraction of 40Ar lost, chemical composition, and shape (sphere or shard) to estimate the temperature-, time-integrated Ar diffusion coeffi- cient, represented here by log D(T, t). In generating the model illustrated in Fig. 3, it was assumed that diffusive loss of 40Ar from the glasses occurred as a result of their having resided within the thermal regime of the upper 1–2 m of the lunar regolith for a time recorded by their CRE ages, i.e., t in the diffusion equation (Eq. 2 above). The samples plotted in Fig. 3 are described in the following paragraphs. With additional Ar-isotopic data on actual lunar glasses and additional experimental work on Ar diffusion in lunar glasses (and compositional analogues), especially at high values of X(NBO) 0.50–0.60, the slope of the isotherms will become better constrained. 4.1.1. Apollo 16 impact glass (61502,13,3) Ar diffusion in this glass sphere (chemically homogeneous and clast-free) with radius 735 lm was reported by Gombosi et al. (2015). The chemical composition is similar to that of the local Apollo 16 regolith with X(NBO) = 0.187 (solid circles in Fig. 3). The values for log D(T, t) for this glass as a function of temperature, which were calculated using the experimental results of Gombosi et al. (2015), are shown by the nine points at X(NBO) = 0.187 in Fig. 3. Those points tightly constrain Ar diffusivity at the low end of the X(NBO) range observed in lunar glasses. 4.1.2. Apollo 15 volcanic green glass (15426) Spheres (chemically homogeneous and clast-free) of this low-Ti picritic glass (e.g., Delano, 1979; Steele et al., 1992) have an 40Ar/39Ar age of 3.38 ± 0.06 Gy (Podosek and Huneke, 1973) and a CRE age 300 My (Podosek and Huneke, 1973; Spangler et al., 1984). The dominant compositional group (‘A’ of Delano, 1979) among this suite of picritic volcanic glasses has X(NBO) = 0.598 (open star in Fig. 3). Podosek and Huneke (1973) analyzed green glass spheres with diameters ranging from 250 lm to 750 lm, and used 400 lm for much of their discussion. Using a radius = 200 lm, CRE age = 300 My, and fraction of 40Ar lost = 0.02 ± 0.01 (Podosek and Huneke, 1973), the log D(T, t) = 23.5 to 24.4. With this range, Fig. 3 shows that green glass spheres with diameters of at least 65– 185 lm would have lost 610% of their 40Ar in 750 My with that range of log D(T, t). 4.1.3. Apollo 17 volcanic orange glass (74220) Spheres (chemically homogeneous and clast-free) of this high-Ti picritic glass (e.g., Heiken et al., 1974; Delano, 1986) have an 40Ar/39Ar age of 3.60 ± 0.04 Gy (Huneke, 1978) and a CRE age 30 My (Huneke, 1978; Eugster et al., 1979). Using a sphere with radius = 40 lm based on the mass of individual glasses analyzed by Huneke Fig. 3. Values for the temperature-, time-integrated Ar diffusion coefficient, log D(T, t), in lunar glasses have been determined using their measured diameters, chemical compositions, CRE ages, and % Ar lost by thermal diffusion during their residence time in the shallow lunar regolith. The lunar glasses encompass a large range of (i) chemical composition, (ii) CRE ages, and (iii) 40Ar/39Ar ages. The results show a strong compositional dependence on log D(T, t) using the fraction of non-bridging oxygens, X(NBO). The minimum sizes of glasses required to retain at least 90% of their radiogenic 40Ar during CRE ages of 750 Ma for a range of temperatures and compositions are shown on the right side. All of the glasses have dimensions far in excess of the minimum sizes required for their compositions and CRE ages. As described in the text, the solid circles represent an Apollo 16 impact glass (61502,13,3); the open star represents an Apollo 15 volcanic green glass (15426); the open square represents an Apollo 17 volcanic orange glass (74220); the solid star represents an Apollo 17 impact glass (C6/301, 71501); and the open triangle represents an Apollo 16 impact glass (G3/225, 64501). Uncertainties on log D(T, t) for the lunar glasses, which are controlled by uncertainties in the CRE ages, are similar to the height of the symbols. The lunar volcanic glasses are not plotted in the subsequent figures involving lunar impact glasses exclusively. N.E.B. Zellner, J.W. Delano / Geochimica et Cosmochimica Acta 161 (2015) 203–218 209 (1978), X(NBO) = 0.505 (Delano, 1986), CRE age = 30 My, and estimated fraction of 40Ar lost 0.03–0.07, the value of log D(T, t) = 23.1 to 23.9 (open square in Fig. 3). With this range of log D(T, t) values, Fig. 3 shows that orange glass spheres with diameters of at least 120 lm–280 lm would have lost 610% of their 40Ar in 750 My with that range of log D(T, t). 4.1.4. Apollo 17 impact glass, C6/301 (71501) This sphere (chemically homogeneous and clast-free) is a light green glass with X(NBO) = 0.248, CRE age = 75 ± 10 My, 40Ar/39Ar age = 102 ± 20 My, diameter = 360 lm, and fraction of 40Ar lost = 0.24. These characteristics yielded log D(T, t) = 21.0 to 21.2 (solid star in Fig. 3), showing that a glass with this composition would require a minimum diameter of 2700 lm–3100 lm to have lost 610% of its 40Ar in 750 My with that range of log D(T, t). 4.1.5. Apollo 16 impact glass, G3/225 (64501) This angular shard (chemically homogeneous and clast-free), a brown glass with X(NBO) = 0.201, CRE age = 145 ± 20 My, 40Ar/39Ar age = 3739 ± 20 My, and average dimension = 184 lm, was reported by Delano et al. (2007). This glass (open triangle in Fig. 3), which belongs to a distinctive suite of impact glasses at the Apollo 16 site (Zeigler et al., 2006; Delano et al., 2007), had lost 61% of its 40Ar. These characteristics yielded log D(T, t) = 24.7 to 25.4. Fig. 3 shows that a glass with this composition would require dimensions of only 20 lm– 40 lm to have lost 610% of its 40Ar in 750 My with that range of log D(T, t). The implication for this glass is that it had spent most of its CRE history at low temperatures, insulated from diurnal temperature variations by the overlying regolith. 4.2. Interpreting 40Ar/39Ar Data We do not know whether the data for the lunar glasses shown in Fig. 3 are typical for the regolith-gardening process since the end of the late heavy bombardment. However, the slope of the isotherms (Fig. 3) suggests that the Apollo 15 green volcanic glass, Apollo 17 orange volcanic glass, and impact glass sphere C6/301 all resided at comparably shallow depths in the lunar regolith during their temperature-, time-integrated CRE histories. Those three glasses retained >75% of their radiogenic 40Ar to yield reliable ages. Glass shard G3/225 resided at a greater depth (i.e., cooler) in the lunar regolith that allowed this glass to retain P99% of its 40Ar, and a reliable 40Ar/39Ar age. With this model of argon diffusivity as a guide, the current investigation revisits 40Ar/39Ar ages on 22 lunar impact glasses (Delano et al., 2007; Zellner et al., 2009a,b) and introduces ages for 73 new ones from the Apollo 14, 16, and 17 landing sites (Table 1, Appendices B and C). These glasses were not only free of exotic components, such as unmelted crystals and lithic fragments derived from the impacted target, but also had known sizes, shapes, and chemical compositions (Section 2.1). After laser step-heating on 98 impact glasses, 85 yielded 40Ar/39Ar ages (single-step, plateau, or weighted), 10 yielded indeterminate “young” ages, and three yielded no ages. In an effort to distinguish those impact glasses that have a stronger likelihood of having retained a reliable 40Ar/39Ar age of impact formation from those that did not, the minimum size associated with an exposure scenario of a 750-Ma CRE history (Fig. 3) has been applied as a selection criterion to those 95 impact glasses that yielded ages, as well as to impact glasses from other studies (e.g., Ryder et al., 1996; Hui, 2011). Evaluative assessments for each age determination are given in Table 1 and Appendix B, where argon release patterns were deemed “good” if >50% 39Ar was used in the age and most of the steps were concordant; “fair” if some of the steps were concordant; and “poor” if none of the steps were concordant. Only ages determined to be “good” or “fair” are included in the following figures, except where small size excludes the sample. Figs. 4 and 5 illustrate which of these impact glasses were large enough to have retained at least 90% of their radiogenic 40Ar during that model exposure history, and which ones were not of sufficient size. As noted in Delano et al. (2007), the shapes of the lunar impact glasses have been described as being either spherical (Fig. 4) or broken shards (Fig. 5). Among the glass spheres (Fig. 4), only 40% are likely to have accurately recorded their ages of impact formation. Fig. 4 shows that most of the impact glass spheres that did not satisfy the minimum required size to have retained at least 90% of their radiogenic 40Ar during a 750-Ma exposure age have chemical compositions with X(NBO) < 0.25 (open symbols in Fig. 4). Those lunar glasses have lunar highlands feldspathic compositions with higher Ar diffusivities at a given temperature than more mafic glasses with higher X(NBO) values and lower Ar diffusivities (Fig. 3). Fig. 4. Lunar glass spheres that have been analyzed by Ryder et al. (1996), Zellner et al. (2009a,b), and Hui (2011) with known chemical compositions, dimensions, and 40Ar/39Ar ages have been plotted, along with spheres from this study (Table 1, Appendices A and B). Glass spheres having sufficient sizes that could have retained at least 90% of their radiogenic 40Ar following 750 Ma in the shallow lunar regolith at a time-integrated temperature of up to 290 K (Fig. 3) are indicated by solid symbols. Glass spheres that would have been too small to have retained at least 90% of their radiogenic 40Ar during that temperature, time history are shown by open symbols. 210 N.E.B. Zellner, J.W. Delano / Geochimica et Cosmochimica Acta 161 (2015) 203–218 When the minimum size criterion for the same exposure scenario was applied to the impact glass shards (Fig. 5), 40% of those analyzed impact glasses were found to not satisfy the minimum required size criterion. As expected, most of the impact glass shards that were found to be too small and were likely to have lost 40Ar* had X(NBO) < 0.25 (Fig. 5). Fig. 6 is a compilation of the impact glass spheres (Fig. 4) and shards (Fig. 5) that exceeded the minimum size requirement for the model exposure history. Consequently, the impact glasses in Fig. 6 are considered likely to have yielded reliable 40Ar/39Ar ages. Fig. 7 shows a histogram of the resulting age-frequency distribution of those impact glasses that satisfied the minimum required size criterion. 5. DISCUSSION 5.1. Implications for 40Ar/39Ar dating of lunar impact glasses The chemical compositions not only distinguish between impact-generated glasses and volcanic glasses (an essential distinction if impact flux is the focus of an investigation; Delano, 1986) but also allow the X(NBO) to be determined. By knowing X(NBO), the minimum size of glass required to yield an accurate 40Ar/39Ar age of impact melting can be estimated (Fig. 3). For example, lunar impact glasses with X(NBO) 6 0.25 are dominantly feldspathic highlands compositions (e.g., anorthosite–norite–troctolite; Korotev, 2005; Prettyman et al., 2006; Taylor, 2009; Wu et al., 2012) and are thus most susceptible to diffusive loss of radiogenic 40Ar during extended residence in the shallow (<2-cm depth="" gombosi="" et="" al="" 2015="" regolith="" during="" diurnal="" temperature="" variations="" figs="" 2="" and="" 3="" the="" effect="" of="" greater="" diffusion="" for="" glasses="" with="" low="" x="" nbo="" values="" is="" clearly="" evident="" in="" 4="" 5="" where="" majority="" impact="" 0="" 25="" did="" not="" satisfy="" minimum="" size="" criterion="" hence="" were="" likely="" to="" have="" yielded="" apparent="" rather="" than="" true="" 40ar="" 39ar="" ages="" contrast="" lunar="" picritic="" volcanic="" 39="" 60="" e="" g="" apollo="" 15="" green="" a="" yellow="0.524;" 17="" orange="0.505;" refer="" delano="" 1986="" known="" varieties="" diameters="" often="" 250="" lm="" yield="" eruption="" 3300="" 3700="" ma="" husain="" schaeffer="" 1973="" podosek="" huneke="" 1978="" spangler="" 1984="" that="" consistently="" overlap="" 87rb="" 87sr="" or="" 147sm="" 143nd="" local="" crystalline="" mare="" basalts="" papanastassiou="" 1977="" nyquist="" fig="" glass="" shards="" been="" analyzed="" by="" 2007="" zellner="" 2009a="" b="" chemical="" compositions="" dimensions="" plotted="" along="" from="" this="" study="" table="" 1="" appendices="" having="" sufficient="" sizes="" could="" retained="" at="" least="" 90="" their="" radiogenic="" following="" 750="" shallow="" time-integrated="" up="" 290="" k="" are="" indicated="" partially="" filled="" boxes="" would="" too="" small="" time="" history="" shown="" open="" symbols="" 6="" compilation="" spheres="" solid="" circles="" see="" residence="" appendix="" uncertainties="" age="" larger="" 7="" age-frequency="" distribution="" unshaded="" bins="" shaded="" exceed="" required="" these="" accurate="" events="" generated="" melts="" number="" within="" each="" bin="" n="" j="" w="" geochimica="" cosmochimica="" acta="" 161="" 203="" 218="" 211="" shih="" 1992="" empirical="" observation="" provides="" strong="" additional="" evidence="" observed="" relationship="" ar="" diffusivity="" decreases="" sharply="" increasing="" while="" as="" function="" has="" estimated="" stringent="" exposure="" experimental="" work="" on="" lunar-relevant="" preferably="" actual="" needed="" better="" define="" young="" 500="" increased="" cratering="" rate="" vs="" thermal="" strain="" when="" analyzing="" single="" landing="" site="" shapes="" spherules="" exotic="" become="" especially="" important="" criteria="" consider="" developing="" hypotheses="" about="" global="" flux="" over="" basis="" 14="" culler="" 2000="" muller="" 2001="" concluded="" factor="" last="" 12="" levine="" 2005="" 16="" hui="" 2010="" sites="" u="" th="" pb="" norman="" 2012="" also="" interpreted="" being="" consistent="" an="" terrestrial="" records="" used="" possible="" factor-of-two="" increase="" shoemaker="" 1983="" grieve="" 1994="" mcewen="" 1997="" issue="" remains="" unresolved="" grier="" bland="" although="" results="" current="" investigation="" show="" frequency="" alternative="" explanation="" offered="" we="" hypothesize="" intrinsically="" prone="" breaking="" into="" geologically="" short="" lifespans="" support="" notion="" comes="" differential="" analysis="" showing="" contain="" high="" stresses="" ulrich="" 1974="" exotherms="" caused="" rapid="" quenching="" hyperliquidus="" temperatures="" make="" impact-generated="" susceptible="" broadly="" analogous="" inexpensive="" glassware="" fractures="" spontaneously="" laboratory="" because="" induced="" manufacturing="" process="" effectively="" removed="" subsequent="" annealing="" consequently="" be="" expected="" short-lived="" if="" correct="" occurrence="" reported="" previous="" workers="" need="" require="" substantial="" preponderance="" impact-produced="" range="" unlike="" lower="" cause="" those="" less="" possibly="" related="" annealed="" warm="" pyroclastic="" deposit="" near-liquidus="" arndt="" curves="" relative="" plots="" referred="" some="" literature="" ideograms="" frequently="" illustrate="" meteorites="" asteroidal="" can="" influenced="" one="" two="" samples="" well-defined="" spikes="" point="" misleadingly="" enhanced="" 8a="" shows="" plot="" 100="" here="" multiple="multiple" seen="" data="" younger="" 8b="" other="" hand="" 47="" satisfied="" our="" elimination="" thus="" lost="" appreciable="" fraction="" significantly="" 1000="" compare="" which="" 64="" respectively="" signal-to-noise="" ratio="" overall="" since="" most="" found="" vulnerable="" diffusive="" loss="" associated="" it="" surprising="" probability="" propose="" plausible="" currently="" available="" among="" depending="" value="" different="" appearance="" any="" investigators="" 2009b="" 2011="" specifically="" though="" there="" no="" indication="" obvious="" recent="" peaks="" representing="" careful="" comparison="" composition="" just="" uncertainty="" arrows="" represent="" similar="" episodes="" well="" documented="" elsewhere="" schmitz="" 2003="" 800="" swindle="" 2009="" but="" improved="" diminished="" preservation="" application="" displays="" prominent="" decline="" all="" 3500="" 212="" specific="" goes="" beyond="" due="" half-life="" trend="" hypothesized="" suspect="" more="" gradually="" shatter="" smaller="" pieces="" impact-gardening="" 8="" occurring="" prior="" applied="" indicate="" three="" recorded="" implying="" regional="" production="" identify="" others="" may="" statistically="" significant="" both="" figures="" ryder="" 1996="" scale="" y-axis="" same="" nd="" included="" either="" figure="" 213="" data-mce-fragment="1">3500 Ma Fig. 7 shows 10 shards and spheres with ages of formation that are >3500 Ma forming a distinct age-frequency peak. These old impact glasses have been identified at the Apollo 14, 16, and 17 landing sites. The large compositional range (X(NBO) = 0.21–0.38) among these impact glasses (Fig. 6) and the occurrence of three peaks in Fig. 8b suggest that they are products of multiple impact melting events into compositionally diverse regions. While Culler et al. (2000) and Muller et al. (2001) also reported several peaks within that interval, it is well known from lunar sample analysis (Papanastassiou et al., 1977; Turner, 1977; Huneke, 1978; Nyquist and Shih, 1992) and photogeology (Wilhelms and McCauley, 1971; Head, 1976; Hiesinger et al., 2000) that the Moon was undergoing extensive volcanism during that time in the form of crystalline mare basalts and picritic volcanic glasses. Therefore, in order to determine cratering rates, it is essential to distinguish between lunar volcanic glasses and lunar impact glasses, so that data from impact glasses only are plotted (as in Figs. 6 and 7). Culler et al. (2000) and Muller et al. (2001), for example, did not chemically analyze their glasses prior to 40Ar/39Ar dating, but rather assumed that volcanic glasses were not a significant component in their suite of Apollo 14 glasses. Delano (1988) observed that volcanic glasses were common (50%) among the hundreds of glasses analyzed in Apollo 14 regolith breccias. Although a lower percentage of volcanic glasses was reported in Apollo 14 regoliths by the Apollo Soil Survey (1971) and Reid et al. (1973), the assumption by Culler et al. (2000) was flawed at some level. Consequently, the data reported by Culler et al. (2000) and Muller et al. (2001) are likely to be contaminated to some extent by ages of volcanic glasses, whereas the peaks in the current investigation within the age-interval 3500–3800 Ma (Fig. 8a and b) are composed exclusively of ages from lunar impact glasses. Among the >3500 Ma impact glasses in Figs. 7 and 8b are the lmHKFM impact glasses (Delano et al., 2007), also known as ‘basaltic andesitic’ (‘BA’) glasses (Zeigler et al., 2006; Korotev et al., 2010). Those impact glasses, which are found most frequently at the Apollo 16 landing site, have a chemical composition that is exotic to the Apollo 16 site (Zeigler et al., 2006; Delano et al., 2007; Korotev et al., 2010) with X(NBO) = 0.21–0.24, and 40Ar/39Ar ages of 3730 ± 40 Ma (Delano et al., 2007). A potential source-crater of these lmHKFM glasses could be either Robertson (90 km diameter) or McLaughlin (80 km diameter), both of which occur in the Procellarum-KREEP terrain (as inferred by Zeigler et al. (2006) and Korotev et al. (2010)) and have ages of 3700 ± 100 Ma (Kirchoff et al., 2013). If, as previously discussed in Section 5.2.1, the gradual decline in the occurrence of lunar impact glasses with time is due largely to spontaneous shattering due to thermal strain, then the prominent occurrence of impact glasses with 40Ar/39Ar ages of 3500–3800 Ma (Figs. 7 and 8b) requires an additional perspective. We suggest that those impact glasses could represent the lingering remnants of an initially large population of impact glasses generated during the tail end of the late heavy bombardment. The absence of lunar impact glasses with 40Ar/39Ar ages >3900 Ma could reflect (i) an increased rate of shattering of glasses during vigorous gardening of the regolith during the late heavy bombardment, as well as (ii) higher rates of diffusive Ar loss from impact glasses when the regolith had a steeper thermal gradient than the present one (Fig. 2). Since the lunar highlands surface has been dominated by feldspathic materials with X(NBO) 6 0.25 throughout most of the Moon’s history, impact glasses derived from fusion of feldspathic highlands materials would have to be large (>1 cm; Fig. 3) in order to preserve 40Ar/39Ar ages >3900 Ma (e.g., Imbrium impact event at 3934 ± 3 Ma; Merle et al., 2014). If, in addition, the lunar regolith was warmer at >3900 Ma (Nemchin et al., 2009), then the minimum required size of feldspathic impact glass with X(NBO) 6 0.25 would likely be >>1 cm (Fig. 3) in order to yield reliable 40Ar/39Ar ages >3900 Ma. Since no such impact glasses have yet been identified in the current suite of lunar samples, lunar feldspathic impact glasses with 40Ar/39Ar ages >3900 Ma are likely to be exceptionally rare. Thus, 40Ar/39Ar dating of feldspathic lunar impact glasses is not likely to provide much information about very old episodes of lunar bombardment. Alternatively, if large impact basins, such as South Pole-Aitken, melted mafic lithologies (Pieters et al., 2001, 2010; Hand, 2008; Hurwitz and Kring, 2014) and produced glasses, then such impact glasses would have high values of X(NBO) and low Ar diffusivities compared to feldspathic glasses (Fig. 3). Such as-yet-undiscovered impact glasses would have the potential of yielding reliable 40Ar/39Ar ages for impact events at >3900 Ma. 5.4. Lunar impact glasses and biomolecular clocks With careful attention to chemical composition, size of sample, and exposure history, lunar impact glasses should be capable of providing important information about the bombardment history of the Earth–Moon system during at least the last 3800 Ma. If, in addition to the Cretaceous/Tertiary mass extinction event (Alvarez et al., 1980), any other major biological events in Earth’s biological history have been influenced by brief episodes of increased bombardment, then an important link might ultimately be found between the ages of lunar impact glasses and the timing of biological events inferred from biomolecular clocks (Hedges and Kumar, 2009; Knoll, 2014). With improved accuracy in the dating of lunar impact glasses and calibration of biomolecular clocks, the Moon may ultimately be recognized as a ‘witness plate’ for biologically important events (Delano et al., 2010). 5.5. Reporting data To allow the independent assessment of the quality of lunar impact glass data, future investigations should include morphological information (e.g., color, shape, size), geochemical composition (including analytical uncertainty in the measurements and X(NBO)), 40Ar/39Ar data (including 2r-uncertainty in the ages), and an evaluation of whether or not the data set includes multiple glasses that 214 N.E.B. Zellner, J.W. Delano / Geochimica et Cosmochimica Acta 161 (2015) 203–218 may have formed in the same impact event (Fig. 8b). In addition, when available, CRE ages, and inferred D(T, t) of the glass would be useful for application of the minimum size criterion for the measured exposure history; otherwise, an assumed exposure history, as described in the current study, would be required. Compositional data, including X(NBO) values, and ages for all of the glasses described herein are included in Table 1 and Appendices A, B and C. 6. CONCLUSIONS We have analyzed 100 inclusion-free lunar impact glasses and provide geochemical and chronological data on 73 of them for the first time. Our findings are as follow: (i) Size, shape, chemical composition, and rates of diffusive loss of radiogenic 40Ar are important for interpreting 40Ar/39Ar ages of lunar impact glasses. (ii) The age-distribution of lunar impact glass spherules (Fig. 4) is dominated by ages <1000 ma="" in="" contrast="" to="" ancient="" lunar="" volcanic="" glasses="" that="" commonly="" occur="" as="" spherules="" impact="" glass="" may="" be="" prone="" shattering="" into="" angular="" shards="" during="" gardening="" of="" the="" regolith="" due="" thermal="" stresses="" those="" acquired="" quenching="" from="" hyperliquidus="" temperatures="" if="" this="" inference="" is="" correct="" 40ar="" 39ar="" age-distributions="" would="" intrinsically="" biased="" toward="" young="" ages="" and="" point="" misleadingly="" a="" recent="" increase="" flux="" iii="" accuracy="" related="" size="" chemical="" composition="" based="" on="" empirical="" results="" study="" experimental="" gombosi="" et="" al="" 2015="" retention="" radiogenic="" hence="" reliability="" increases="" with="" physical="" increasing="" x="" nbo="" values="" sample="" iv="" age="" distribution="" all="" fig="" 7="" 8b="" reflect="" two="" distinct="" processes:="" diminished="" preservation="" caused="" by="" smaller="" pieces="" remnant="" population="" data-mce-fragment="1">3500 Ma that survived from the tail end of the late heavy bombardment. ACKNOWLEDGMENTS The authors thank Tim Swindle for his insight and guidance in interpreting 40Ar/39Ar ages and Clark Isachsen, Eric Olsen, and Fernando Barra for assistance with obtaining the age data. Useful comments by Ryan Zeigler, Greg Herzog, and an anonymous reviewer helped to improve the manuscript from an earlier version. Work by NEBZ was funded by the NASA LASER program grant #09-LASER09-0038 and by NSF Division of Astronomical Sciences grant #1008819. Work by JWD was supported by NASA Astrobiology grant #1079329-1-50310. APPENDIX. SUPPLEMENTARY DATA Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.gca.2015.04.013. REFERENCES Alvarez L. W., Alvarez W., Asaro F. and Michel H. V. (1980) Extraterrestrial cause for the Cretaceous Tertiary Extinction. Science 208, 1095. Apollo Soil Survey (1971) Apollo 14 – nature and origin of rock types in soil from the Fra Mauro Formation. Earth Planet. Sci. Lett. 12, 49–54. 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