Examining the compositions of impactors striking the Moon using Apollo impact melt coats and anorthositic regolith breccia meteorites

Journal Pre-proofs Examining the compositions of impactors striking the Moon using Apollo impact melt coats and anorthositic regolith breccia meteorites Eleanor C. McIntosh, James M.D. Day, Yang Liu, Courtney Jiskoot PII: DOI: Reference:

S0016-7037(20)30074-0 https://doi.org/10.1016/j.gca.2020.01.051 GCA 11631

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Geochimica et Cosmochimica Acta

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14 August 2019 25 January 2020

Please cite this article as: McIntosh, E.C., Day, J.M.D., Liu, Y., Jiskoot, C., Examining the compositions of impactors striking the Moon using Apollo impact melt coats and anorthositic regolith breccia meteorites, Geochimica et Cosmochimica Acta (2020), doi: https://doi.org/10.1016/j.gca.2020.01.051

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McIntosh et al.

Lunar Impactor Compositions

Examining the compositions of impactors striking the Moon using Apollo impact melt coats and anorthositic regolith breccia meteorites Eleanor C. McIntosh1, James M.D. Day1, Yang Liu2, Courtney Jiskoot1 1Scripps

Institution of Oceanography, University of California San Diego, La Jolla, CA

92093, USA 2Jet

Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109,

USA *Corresponding author: [email protected] Abstract: 374 words Main text: 8525 words Figures: 8 Tables: 3 Submitted to : Geochimica et Cosmochimica Acta Supplementary Materials, including Supplementary Figures and Tables, accompany this manuscript.

Keywords: Moon; impact melt coats; anorthositic regolith breccias; highly siderophile elements; Os isotopes; impactors Abstract: Impactors striking the Moon since the formation of its crust have left an indelible imprint on the lunar surface, in the guise of craters and associated impact rocks. The lunar crust 1

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has low intrinsic abundances of the highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au), at greater than 3000 times lower than in chondrite meteorites. Consequently, during impact, bolides with chondritic or differentiated iron-rich compositions should impart elevated HSE signatures to the lunar crust. Here we examine glassy lunar impact melt coats (IMC) from the outside rims of Apollo 16 cataclastic anorthosites (60015, 65325) and breccias (65035), as well as both fragments and powders of Antarctic anorthositic regolith breccia (ARB) meteorites (Miller Range 090034/36/70/75 and MacAlpine Hills 88105) for their petrography, mineral chemistry and bulk-rock compositions. The HSE concentrations for IMC range from ~0.001 to 0.1 × CI chondrite, with measured

187Os/188Os

between 0.1189 and 0.1366. Anorthositic regolith breccia

meteorites, which have components with 2.6-4.1 Ga ages, have similar HSE concentrations to IMC, but typically have lower 187Os/188Os (0.1164-0.1284). These latter Os ratios are generally less radiogenic than those measured in ~3.8-3.9 Ga Apollo impact melt breccias. The Apollo 16 IMC are not well-dated, but their KREEPy trace-element signatures and associated ages of 3.7 to 3.8 Ga for Apollo 16 glasses might imply, at least in part, an origin from the Imbrium or Serenitatis basin-forming impacts. Within the IMC, metal-schreibersite-troilite assemblages record significant inter-element HSE fractionation which is also reflected in bulk HSE patterns for both IMC and ARB meteorites. Variations in relative and absolute HSE compositions directly reflect the control of metal and sulfide segregation within and between impact melt and breccia lithologies. Collectively, IMC and ARB meteorites exhibit approximately 50% of the variation in Ru/Ir and

187Os/188Os

observed in lunar impact melt breccias. These results

imply that significant variations in inter-element compositions can occur during impact brecciation and melting and so some impact melt rock HSE compositions may not record the compositions of impactors that struck the Moon with fidelity. Nonetheless, the generally low

187Re/188Os

of lunar impact melt rocks means that osmium isotope ratios

provide evidence for impact composition, and a change from ordinary to carbonaceouslike impactors either with time - or location - striking the Moon. 1. Introduction

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Collisional impact in the Solar System is a unifying geological process that has acted on all planetary bodies (e.g., Melosh, 1989). Impact processes ultimately lead to the progressive accretion of mass and influence the chemical make-up of planets, including the delivery of volatile elements (Morbidelli et al., 2000; Robert, 2001; Albarède, 2009). Later impacts to differentiated planetary bodies after initial accretion and core-mantlecrust separation can generate surface melt rocks and breccias that potentially reveal the composition of the impactors themselves. On Earth, preservation of impact craters and associated melt rocks and impact melt breccias (IMB) is generally poor due to plate tectonic processes and weathering, but has nonetheless yielded important information on the composition of the impactors that formed them (e.g., Goderis et al., 2012; Koeberl, 2013). In contrast, the airless conditions and limited volcano-tectonic activity of the Moon allow inferences about impact processes since the formation of the lunar crust from as early as ~4.4-4.2 Ga (e.g., Norman, 2009). In particular, the relative and absolute abundances of the highly siderophile elements (HSE: Re, Os, Ir, Ru, Pt, Rh, Pd, Au) and 187Os/188Os

have been utilized as tracers for assessing the compositions of impactors

striking the Moon (e.g., Gros et al., 1976; Hertogen et al., 1977; Anders, 1978; Higuchi & Morgan, 1975; Norman et al., 2002; Puchtel et al., 2008; Fischer-Gödde and Becker, 2012; Sharp et al., 2014; Liu et al., 2015; Gleißner and Becker, 2017; Day et al., 2017a; Gleißner and Becker, 2019).

The principle of using the HSE to examine impactor compositions is that, during core formation, these elements are effectively stripped from the silicate portions of differentiated parent bodies due to high liquid metal/liquid silicate concentration ratios (D

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values = 103 to 106 (e.g., Kimura et al., 1974; Borisov and Palme, 1997; Mann et al., 2012). Any later contributions by impactors after metal-silicate equilibrium is known as ‘late accretion’ and can lead to enrichment in the HSE and other siderophile elements (e.g., Mo and W; see Day, 2016; Day et al., 2016). Chondritic relative abundances of the HSE in the Earth, Moon and Mars’ silicate shells are higher than predicted from metalsilicate equilibrium, and so are most readily explained by this process (Chou 1978; Morgan, 1985; Becker et al., 2006; Day et al., 2007; Brandon et al., 2012; Day and Walker, 2015; Day et al., 2017b; Tait and Day, 2018).

In the Moon, pristine crustal rocks have extremely low bulk-rock HSE concentrations, with non-chondritic inter-element ratios (≤3 × 10-4 × CI Chondrite; ˂0.130 ppb Ir) (Gros et al., 1976; Hertogen et al., 1977; Warren and Wasson, 1977; Ebihara et al., 1992; Day et al., 2010). For example, non-chondritic ratios in pristine lunar crustal rocks include high Pd/Ir (≥ 22), and Re/Ir (≥ 0.2) (Day et al., 2010 and references therein). Impactors striking the Moon are likely to have had HSE concentrations similar to those in chondrite meteorites (Chondrites = 235 to 1008 ppb Ir; Horan et al., 2003; van Acken et al., 2011; Fischer-Gödde et al., 2010; Day et al., 2016). The ~3,000 to 50,000 difference in HSE abundances makes these elements sensitive indicators of impact contamination. Even potential cometary impactors, which are made up of ≥90% ices, should have HSE abundances some 30 to 500 times greater than in the pristine lunar crust due to their probable formation from early nebular materials, similar to chondrites (Yamamoto, 1985).

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In general, IMB have compositions consistent with chondritic or iron meteorite impactors (Higuchi and Morgan, 1975; Hertogen et al., 1977; Warren et al., 1989; Korotev, 1994; Norman et al., 2002; Puchtel et al., 2008; Fischer-Gödde and Becker, 2012; Sharp et al., 2014; Liu et al., 2015; Gleißner and Becker, 2017, 2019). Two primary methods have been used to analyze the HSE: homogenized powders (e.g., Norman et al., 2002) and analysis of individual sub-fragments (e.g., Puchtel et al., 2008; Fischer-Gödde and Becker, 2012). Both methods provide complementary information. Homogenized powders can be used to assess a range of elemental and isotopic information. Sub-fragments enable investigation of the distribution of the HSE within IMB due of inefficient mixing and ‘nugget effects’ (McDonald et al., 2001; Puchtel et al., 2008; Sharp et al., 2014; Liu et al., 2015), including small-scale fractionation between different HSE carrier phases (Gleißner and Becker, 2017).

Studies of HSE compositions in Apollo IMB have found super-chondritic 187Os/188Os

and high Pt/Ir, Pd/Ir, and Ru/Ir that differ significantly from the present-day

population of chondrite meteorites sampled on Earth (Norman et al., 2002; Puchtel et al., 2008; Sharp et al., 2014; Liu et al., 2015). Exceptions are some Apollo 16 impact melt rocks which have chondritic HSE abundances and 187Os/188Os (Liu et al., 2015; Gleißner and Becker, 2017). The canonical interpretation for super-chondritic 187Os/188Os and high Pt/Ir, Pd/Ir and Ru/Ir in some Apollo IMB is that the impactors striking the Moon before ~3.5 Ga were of a different composition to the modern-day population of chondrites delivered to the inner Solar System (Norman et al., 2002; Puchtel et al., 2008; Sharp et al., 2014; Liu et al., 2015). Alternatively, these variations could reflect mixtures of

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chondritic and differentiated impactors (Fischer-Gödde and Becker, 2012), or could occur due to HSE fractionation in melt-sheets that formed the IMB (Day et al., 2016).

To further examine the geochemical compositions of lunar impact melt rocks, we have studied impact melt coats (IMC) on Apollo 16 rocks, and lunar anorthositic regolith breccia (ARB) meteorites. The IMC, also referred to in the literature as ‘impact melt splashes’, are small-scale features found on the exterior of some Apollo lunar crustal rocks. They have been interpreted to form during impact processes when melts were ‘splashed’ and rapidly quenched, creating a glass coating on the outside of the host rock (Morris et al., 1986). Despite their diminutive volumes, IMC provide a direct chemical link to impactors striking the Moon, are unlikely to have witnessed any potential largescale differentiation processes and, although not expressly dated, are likely to be the same age, or younger, than IMB (Delano et al., 2007). Lunar ARB meteorites are formed by multiple impactors striking the Moon. These meteorites allow study of parts of the Moon that were not sampled during the Apollo missions (Warren and Kallemeyn, 1991; Warren, 1994; Gallant et al., 2009) and also provide information on impact events away from the Procellarum KREEP Terrane, as sampled by Apollo IMB. The dated components of ARB that we examined are between 2.6 and 4.1 Ga (Bogard et al., 2000; Park et al., 2013; Supplementary Information).

2. Samples 2.1

Impact melt coats (IMC)

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We analyzed impact melt glass coatings from 60015, 65035 and 65325, and interior ferroan anorthosite material from Apollo samples 60015, 65035, 65325 and 62255. Sample 60015 is a cataclastic anorthosite with a surficial IMC that was taken from the Lunar Module/Apollo Lunar Surface Experiments Package on the Cayley Plains. The interior of 60015 is highly-shocked and composed of ~98% plagioclase (An96.5-97.1) (Dixon and Papike, 1975). The sample includes shocked pyroxene grains, ilmenite, Cr spinel and troilite (Dixon and Papike, 1975). Sample 62255 is from Station 2 on the Cayley Plains and is an anorthosite (An92-96) with IMC (Ryder and Norman, 1980). Sample 65035 is a dimict cataclastic anorthosite breccia with an IMC (James and Lindstrom, 1991) collected from Station 5, near Stone Mountain (Descartes Formation). It contains plagioclase (An96-97) and pyroxene (Wo2En63; Ryder and Norman, 1980). Sample 65325 was collected from Station 5 and is a cataclastic anorthosite that has a coat of glass on its surface. Nearly 99% of the interior is plagioclase (An97), while the other 1% is orthopyroxene (Wo2En63) (Warren and Wasson, 1977). The glass coating has been shown to have a KREEPy (potassium, rare earth elements, phosphorous) trace element component (Morris et al., 1986), however the interior anorthosite is considered pristine and free from meteoritic contamination (Warren, 1993).

2.2

Lunar anorthositic regolith breccia (ARB) meteorites Lunar meteorites are derived from the uppermost portion of the lunar crust

(Warren, 1994; Basilevsky et al., 2010). We analyzed ARB meteorites MAC 88105, MIL 090034, MIL 090036, MIL 090070, and MIL 090075. Samples MIL 090034/70/75 are considered to be fall-paired due to their similar compositions, textures, and proximity to

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each other at Antarctic collection field sites (Korotev et al., 2011; Zeigler et al., 2012). Meteorites MIL 090036, MIL 090070, MIL 090075, and MIL 090034 are composed of mineral fragments, lithic clasts, impact melt, and regolith clasts hosted in a fine-grain to glassy matrix (Liu et al., 2011; Calzada-Diaz et al., 2017; Martin et al., 2017). Miller Range 090036 is distinct due to the elevated incompatible trace-element abundances (Korotev et al., 2011). These samples are ARB with abundant feldspathic clasts, regolith components, and low bulk rock iron content (3.4-5 wt. % FeO) (Korotev et al., 2011; Liu et al., 2011; Zeigler et al., 2012; Calzada-Diaz et al., 2017). They also contain impact melt veins and the meteorites are considered to have experienced multiple impact events (Martin et al., 2017). Different thin-sections of individual ARB vary greatly from one another in terms of clast size and textures. Martin et al. (2017) found that MIL 090034, 27 and MIL 090075, 21 contain fragments of “clast-rich” feldspathic impact melt breccias that are less than 33 µm in size. Glassy veins separate these fragments and cut across the impact melt breccia clasts and melt veins (Martin et al., 2017). Miller Range 090034, 8 and ,10 contain a gabbroic anorthosite clast of >2 mm that consists of olivine and pyroxene fragments within a fine-grained matrix. Both sections of MIL 090075 (7 and 8) lack the well-defined clasts in MIL 090075, 21 of Martin et al. (2017). Miller Range 090070, 25 is a singular feldspathic clast that contains melt veins. Mineral fragments in this rock include anorthite, pyroxene, olivine, and pink spinel (Liu et al., 2011). Miller Range 090034 and 090075 were found to contain igneous lithic clasts with olivine and pyroxene composition intergrown with plagioclase. Miller Range 090036 has troctolite and noritic anorthosite clasts, in addition to impact melt and regolith breccias.

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Compared to MIL 090034, MIL 090070, and MIL 090075, MIL 090036 has a greater amount of SiO2 and K-rich phases (Liu et al., 2011).

Meteorite MAC 88105 is a feldspathic polymict regolith breccia that has been described in detail previously (Jolliff et al., 1991; Lindstrom et al., 1991; Neal et al., 1991; Warren and Kallemeyn, 1991; Koeberl, 2013). It contains grey fusion crust on the surface, impact melt breccia clasts, and anorthositic clasts and is paired with MAC 88104 (Jolliff et al., 1991; Lindstrom et al., 1991; Neal et al., 1991; Warren and Kallemeyn, 1991; Koeberl, 2013). The interior of the sample is grey and fine grained or glassy in some areas. The bulk composition of MAC 88105 is similar to that of present-day regolith materials of the Outer-Feldspathic Highlands Terrane (Joy et al., 2010). The age distribution of MAC 88105 exhibits two peaks. One group of clasts of MAC 88105 has an age of 3.79 ±0.14 Ga, while the other group of clasts has an age of ~3.2 Ga (Cohen et al., 2005).

3. Methods 3.1

Sample Preparation Preparation for IMC was minimal, involving disaggregation of the glass material

from ferroan anorthosite using a dedicated alumina mortar and pestle and making pure 12 to 48 mg ‘glass shard’ aliquots. For the ARB meteorites, a homogenous 500-1000 mg sample powder, used for major- and trace-element, and HSE abundance and

187Os/188Os

determination, was prepared using a dedicated agate mortar and pestle. Whole sub-

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fragments, ranging in size from 50 to 204 mg were also prepared for Re-Os isotope and HSE abundance analyses.

For mineral-scale studies, polished thin sections of 60015(,116 ,805), 62255(,197), 65035(,176), and 65325(,18) were examined using a transmitted/reflected light microscope. Plane polarized, cross polarized, and reflected light images of these thin sections were made and metal/sulfide grains and mineralogy were noted prior to quantitative analysis performed on 60015, 62255 and 65325. For the ARB meteorites, thin sections of MIL 090034(,10), MIL 090036(,9), MIL 090070(,9), and MIL 090075(,8) were examined using a transmitted/reflected light microscope. Backscatter electron images of lithic clasts were also obtained during the quantitative analyses.

3.2

Major-element mineral compositions Mineral grains and glass within polished-sections were examined for major- and

minor-element compositions at the University of Tennessee using a Cameca SX-100 electron microprobe (EMP). An acceleration potential of 15 keV and a 1 m spot size were used. Beam currents were 10 nA for plagioclase grains, 20 nA for metal and sulfide grains, and 15 nA for glass. Detection limits for metal and sulfide grains were 0.03 wt.% for Mg, Si, P, Al and Ti, 0.04 wt.% for S, Fe, Cr, Mn, and Mg, and 0.05 wt.% for Co and Ni. Detection limits for silicate grains were 0.03 wt.% for SiO2, TiO2, Al2O3, MgO, CaO, and Na2O, 0.04 wt.% for K2O, V2O3, and Cr2O3, 0.05 wt. % for Cr2O3, MnO, FeO, and P2O5, and 0.06 wt% for NiO.

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Highly siderophile element abundances in mineral phases Abundances of the HSE in metal, schreibersite and sulfide phases were

determined for 60015,805, 60015,116, 65035,176, and 65325,18 using a New Wave Research UP213 (213 nm) laser-ablation system coupled to a Thermo Scientific iCAPQc inductively coupled plasma-mass spectrometer (ICP-MS) at the Scripps Isotope Geochemistry Laboratory (SIGL). Monitored masses were 59Co, 60Ni, 63Cu, 95Mo, 103Rh, 105Pd, 182W, 185Re, 189Os, 193Ir, 195Pt,

and

197Au.

101Ru,

Spot analyses were done with a

100 m beam diameter, a laser repetition rate of 5 Hz, and a photon fluence of 3.5 J/cm2. Ablation took place in a 3 cm3 cell. The cell was flushed with He-gas and was mixed with an Ar carrier-gas flow of ~1L/min. We made every effort to analyze homogeneous phases, rather than amalgamations of phases. This meant that larger phase assemblages were measured in samples. Data collection lasted for 60 seconds for each analysis with 20 second of background and 40 seconds of laser ablation. The washout time was set for 120 seconds. Standards used for calibration and as unknowns were Hoba, Filomena, Coahuila and MASS-1 (see Day et al., 2018), and Fe was used as for internal standardization. The HSE abundance determinations were better than 7% for repeat measurements of minerals in the IMC and for standard reference materials.

3.4

Bulk-rock major- and trace-element analyses Major- and trace-element abundances were determined on MAC 88105, MIL

090036, MIL 090034/70/75, Apollo 60015, 62255, and 65325 using pressure-assisted digestion in Teflon Parr bomb vessels at the SIGL. Total analytical blanks, standards and sample powders were digested at 170℃ in Optima grade concentrated 27.5M HF and

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15.7M HNO3 for >72 hours in a convection oven. Resulting solutions were dried down and taken up twice using 15.7M HNO3 to remove fluorides, and subsequently doped with In to monitor instrument drift prior to dilution using 2% HNO3. Major- and trace elements were measured using a ThermoScientific iCAPQc ICP-MS in normal mode using methods described in Day et al. (2017b) and Tait & Day (2018). Measurements of standard reference materials BHVO-2 and BCR-2 as unknowns were within 1% of recommended values, with external precision of better than 3%, with the exception of Zn (4%), Se, Rb (6%), B (8%), Cs, Pb (10%), Mo (13%) and Te (14%).

3.5

Rubidium-strontium isotope analysis Apollo 16 IMC and ferroan anorthosite were measured for their rubidium-

strontium isotope systematics. After digestion using methods described in section 3.4, 5% aliquots were taken for Rb and Sr concentration determination using the iCAPQc ICPMS, with the remaining 95% solution being used for column separation of Sr using Sr specific resin, as described in Moynier et al. (2012). Strontium isotopic ratios were determined using a Thermo Scientific Triton thermal ionization mass spectrometer (TIMS) in positive ion mode at the SIGL. The NIST SRM 987 solution and BHVO-2g were also measured with samples. NST SRM 987 yielded an average of 0.710242  0.000012 and BHVO-2g yielded an average of

87Sr/86Sr

87Sr/86Sr

=

= 0.703466 

0.000002 (2SE) with Rb/Sr ratio determination for BHVO-2 of better than 1%.

3.6

Rhenium-osmium isotope and HSE abundance analysis

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Osmium isotope and highly siderophile element abundance analyses were performed at the SIGL. Samples were powdered and broken into fragments using an alumina mortar and pestle in a clean laboratory environment. Powders, fragments and blanks were digested in sealed borosilicate Carius tubes with an isotopically enriched spike (99Ru, 106Pd, 185Re, 190Os, 191Ir, 194Pt) and a 1:2 mixture of Teflon distilled HCl and HNO3 that was purged of Os by treatment with H2O2. Samples were digested in an oven for 72 hours at a temperature of 270 C. Osmium was triply extracted using CCl4 and then back-extracted into HBr (Cohen and Waters, 1996). Osmium was further purified by double micro-distillation (Birck et al., 1997). The remaining HSE were purified from residual solutions using anion exchange separation techniques (Day et al., 2016).

Osmium isotope compositions were measured on a Triton TIMS in negative mode at the SIGL. Precision for

187Os/188Os

was determined by measurement of UMCP

Johnson-Matthey standard and was better than 0.2% (2 SD; 0.11380  20; n = 11). Osmium measurements were corrected for oxide interferences, instrumental mass fractionation (assuming the exponential law) using

192Os/188Os

= 3.08271, the spike

addition, and the total analytical blank. Rhenium, Pd, Pt, Ru, and Ir were measured using a Cetac Aridus II desolvating nebulizer coupled to an iCAPQc ICP-MS. Data were corrected offline for mass fractionation using multiple measurements of an ‘isotopically natural’ standard solution throughout the run. The external reproducibility on HSE analyses was better than 0.25% on 0.5 ppb solutions and all reported values are blank corrected (see Table 1). Total procedural blanks analyzed with samples had 187Os/188Os = 0.230  0.095 with quantities (in picograms) of 3  3 [Re], 45  7 [Pd], 10  2 [Pt], 12 

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12 [Ru], 12  6 [Ir], and 3  4 [Os] (n = 5). Blank percentages ranged from very low blank additions (<0.5%) to up to 70% and 50% for Pd and Ru in some samples, respectively. Uncertainties on concentration measurements were therefore calculated by propagating the uncertainties from external reproducibility on standard measurements, together with associated blank percentages and are reported in Table 1.

4. Results We report bulk-rock highly siderophile element (HSE) abundances and measured 187Os/188Os

(Table 1), mineral grain HSE abundance analyses (Table 2) and whole-rock

trace element abundances (Table 3). Presented in the supplementary information are major element compositions for glass and silicate or metal, sulfide and schreibersite grains in impact melt coats (IMC) (Tables S1-3), silicate grain compositions in lunar anorthositic regolith breccia (ARB) meteorites (Table S4), standard data for trace element analysis (Table S5), Ar-Ar ages reported for ARM meteorites in the literature (Table S6), blank contributions to the HSE (Table S7), and interelement HSE ratios for IMC and ARB (Table S8). 4.1

Petrography, mineral and glass chemistry

4.1.1

Impact melt coats Impact melt coats (IMC) on 60015, 62255, 65035 and 65325 are dark gray to

black coats of glass with sharp contact to the interior cataclastic anorthosite (Figure 1). The IMC contain rounded (60015, 62255, 65325) to sub-rounded (65325) sulfide-metal assemblages with individual assemblages ranging from ~10 m to ~150 m in maximum diameter. At least three phases are observed in the largest of these assemblages in

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reflected light. The IMC on 60015 and 65035 are relatively thick, containing multiple vesicles and fragments of included cataclastic anorthosite material. The largest metal and sulfide grains tend to appear near the contact between the anorthosite and the glass impact melt coat. The IMC on 65325 is a thin veneer of glassy material that also contains vesicles and sulfide-metal assemblages.

Glass compositions for IMC range from dominantly Mg-poor (<0.1 wt.% MgO), CaO- (>19 wt.%) and Al2O3-rich (>34 wt.%) to Mg-rich (>11 wt.%) compositions, reflecting the localized melting of major mineral components (e.g., feldspar and orthopyroxene/olivine) in the associated crustal rocks (Table S1). A previous study on IMC reported a range of FeO (2.8-6.9 wt.%) and MgO (2.1-9.4 wt.%) contents (See et al., 1986), but did not report MgO wt.% in glass with less than <0.1 MgO wt.% as found in this study. Plagioclase grains in and around the IMC are all nearly pure anorthite (60015 = An95.7-96.4; 62255 = An95.1-96.8; 65325 = An95.8-96.2), while orthopyroxene grains in 60015 and olivine in 65325 are relatively ferroan (Mg# = 47-64) (Table S2). These values match with literature values for ferroan anorthosite material in the samples (An96-97; mafic phases Mg# 60-70; Dixon & Papike, 1973; Warren & Wasson, 1978; Ryder & Norman, 1980), although the orthopyroxene in the IMC of 60015 is more ferroan than the same minerals in the ferroan anorthosite and olivine has never previously been reported in 65325. Schreibersite grain major element compositions in 60015 typically have higher Ni and Co (>25 wt.% and 0.6 wt.%, respectively) contents than associated FeNi metal (typically >25 wt.% Ni, <0.6 wt.% Co) (Table S3; Figure S1). Schreibersite in 62255 typically contains greater than 20 wt.% Ni and greater than 0.6 wt.% Co. Blebs of FeNi

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metal have Ni and Co contents of greater than 15 wt. % and 0.6 wt. % respectively, while FeS exhibits lower Ni and Co contents (<10 wt. %, <0.4 wt.%). Schreibersite and FeNiP where FeNiP appears to be a single phase - in 65325 have overlapping Co and Ni contents. The schreibersite grain measured has ~15 wt.% Ni and ~0.5 wt.% Co. For 65325, FeNiP blebs have ~5 to 16 wt.% Ni, while Co ranges from 0.2 to 0.6 wt.%.

Highly siderophile element abundances of the troilite-schreibersite-metal assemblages in IMC are reported in Table 2 and Figure 2. Examples of differentiated FeNi metal-schreibersite-sulfide assemblages in impact melt coat 60015,116 are shown in Figure 1. Host phases of the HSE tended to be more abundant closer to the contact with brecciated anorthosite. Metal and schreibersite in 60015 have broadly flat CI-chondrite relative HSE patterns at ~1-10 × CI chondrite abundances, with the exception of a single metal grain with low Pd (~4 × CI) but high Pt, Ru, Ir, Os and Re (~100 × CI). Metal grains for 60015 show both relatively flat patterns for Pd, Pt, Ru, Ir and Os, corresponding to distinct chemistry in some fragments, while other metal grains display significantly more fractionated HSE patterns.

For 65035 and 65325 inter-element fractionations of the HSE are more pronounced within metal and schreibersite phases with high Re/Os in some grains and enrichments in Ru, Pt, Pd. The metal in 65035 shows a range in Ir values (~0.1-8 × CI). Schreibersite in 65035 has chondritic Re and Ir abundances, while Ru, Pt, and Pd values fall below that of CI-chondrites (0.1-1 × CI). The metal in 65325 exhibits relatively flat chondritic Re, Os, Ir, and Pt abundances with a slight enrichment in Ru, and a depletion

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in Pd. Metal and schreibersite in 65035 exhibits lower Ru compared to the metal and schreibersite in 65325. The schreibersite in 65035 has ~10-100 times lower abundances of Pt and Ru than in 60015 and 65325.

4.1.2

Miller Range 090034/36/70/75 anorthositic regolith breccia (ARB) meteorites Major minerals include large plagioclase (>0.5 mm), pyroxene and olivine grains

(<0.5 mm). Ilmenite commonly occurs as individual fragments (≤10 μm) or clusters and also as minute grains (<1 μm) associated with silica or impact-formed glass. Both chromite and pink spinel were observed in all sections. There are no large metal grains, although micron to sub-micron Fe/FeNi grains occur in the matrix and in regolith breccias in <1 vol.%. Phosphates occur occasionally in lithic clasts.

Olivine fragments are commonly small and subhedral (<100 µm in largest dimension) in all samples. Most of these fragments display intra-grain and inter-grain homogeneity with a composition of ~Fo60 (Table S4). Some olivine fragments in MIL 090036 contains higher Mg contents with Fo79 to Fo91. In MIL 090034, 10, a few olivine fragments examined are mantled by low-Ca pyroxene (En62Wo5). Most olivine fragments in direct contact with the matrix contain thin Fe-rich rims of ~3 μm thickness. In several troctolite clasts in MIL 090036 and 090075, olivine grains are Mg-rich with Fo72 to Fo83.

Common occurrences of pyroxene in all sections are fragments with exsolution lamellae. Some pyroxene fragments contain two sets of lamellae of <1 μm and 2-5 μm width, respectively. The lamellae can be pigeonite or augite compositions, whereas the 17

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host is augite or pigeonite, respectively. Pyroxene of enstatite compositions is commonly present at small (<50 µm) subhedral to anhedral fragments (En61-85Wo3) in all samples, but was found intergrown with pigeonite in one large fragment in MIL 090034 (En75Wo4) and in one large fragment (or multiple grains, >100 µm, En82Wo3) in contact with an olivine (Fo66) in MIL 090075, 8. These enstatite pyroxenes typically contain thin Fe-rich rims. The most Fe-rich pyroxene was observed in a pigeonite fragment (En29Wo6) with augite lamellae (En26Wo38) in MIL 090036.

Anorthite in lithic clasts or as mineral fragments has a fairly uniform composition of An91-98, similar to those in MIL 090034/70/75 (Calzada-Diaz et al., 2017; Martin et al., 2017). The orthoclase in a small lithic clast in MIL 090036 has a composition of An7Or88 with 1 wt.% BaO. Pink spinel occurs in all sections, which contain Al# [= molar Al/(Al + Cr + 2Ti)] of 81-97, Cr# of 16 to 3, and Mg# of 50-89. Some pink spinel grains also contain a thin Fe-rich rim of ~1 μm. Chromite grains contain a significant hercynite component (Al# = 32, Cr# = 62, Mg# = 23). Impact melt pockets and melt veins in MIL 090070, 9 are of feldspathic composition. The glass vein in MIL 090070, 9 contains 0.230.35 wt.% P2O5 with 0.8-4.5 wt.% FeO and 2-4 wt.% MgO. Matrix glass in MIL 090075 is also feldspathic with 1.5 wt.% FeO and 1.5-2 wt.% MgO. The silica- and K-rich glass associated with a small clast in MIL 090036, 9 contains ~74 wt.% SiO2 with up to 5 wt.% K2O.All samples contain cristobalite, ilmenite, chromite and troilite. MIL 090036 also contains large fragments of ilmenite (90 μm), rutile (up to 60 μm), zirconolite ([Ca, Ce] Zr[Ti, Nb, Fe+++]2O7; ~15 μm), baddeleyite (~20 μm), and zircon (~40 by 200 μm), which are indicative of more evolved melt sources.

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4.2 Trace-element chemistry of IMC and ARB meteorites Trace-element compositions of impact melt coats (IMC), lunar anorthositic regolith breccia (ARB) meteorites, and ferroan anorthosites (FAN) are distinct from one another (Figure 3; Table 3). All three materials show depletions in Rb, Pb, and a slight depletion in Ti. The trace-element compositions of the FAN contrast with those of the corresponding IMC. For example, FAN 62255 has the lowest abundance of incompatible trace elements, whereas IMC contain higher abundances of incompatible trace elements with IMC 65325 containing the highest abundance of trace elements. The incompatible trace element data for IMC 65035 and 60015 are broadly similar to that of impact melt splashes measured by See et al. (1986). The elevated abundances of trace elements, in particular the rare earth elements (REE), are similar to that of ‘KREEP’ (e.g., Warren, 2003). Lunar ARB meteorites MIL 090034/090070/090075 and MAC 88105 have lithophile element abundances of ~10 × CI-Chondrite, with the exception of Sc and V (Figure 3). Sample MIL 090036 has an elevated lithophile element pattern compared to other ARB.

Impact melt coats, ARB meteorites, and FAN samples have distinct Ni, Co, and Cr abundances from one another. Samples 62255 FAN, and 65325 FAN range from 4.35 to 27 ppm Ni, while FAN sample 65035 has a much higher abundance of Ni at 383 ppm. Lunar ARB meteorites have elevated abundances of Ni, Co, and Cr compared to FAN samples. Nickel in the ARB meteorites range from 36 to 140 ppm, while Co in these samples ranges from 8 to 16.6 ppm. Chromium abundances in ARB range from 420 to

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589 ppm. Impact melt coats contain the highest abundances of Cr, Ni, and Co. Sample 65325 FAN and IMC contain elevated abundances of Pb compared to other samples analyzed. Sample 65325 IMC contains 2.6 ppm Pb, while 65325 FAN contains 3.1 ppm Pb. All other IMC, ARB, and FAN samples contain less than 0.9 ppm Pb.

Samples 65325, 62255, and 65035 all have positive Eu anomalies ( .

Eu/Eu*

is

positive in FANs and negative in IMC. For example, Eu anomalies for FAN 65325 and 65035 are 29 and 5.3, respectively; their respective IMC have negative anomalies of 0.4 and 0.8. The scale of the Eu anomalies in IMC (0.6 ± 0.2) is smaller than that of ARB (2.1 ± 0.7). Compared to FAN, ARB meteorites have lower Eu anomalies, where FAN samples have an average Eu/Eu* anomaly of 25 ± 13. The Eu/Eu* of FAN samples also has a larger range (5.3 to 33) than that of ARB (0.8 – 2.7).

Incompatible trace element data for MIL 090036 and MIL 090034/70/75 support a heterogenous lunar terrane input. Miller Range 090036 has higher abundances of trace elements that are more similar to IMC compared to MIL 090034/70/75. This sample also has a slight negative Eu anomaly, while ARB meteorites MIL 090034/70/75 have a positive Eu anomaly. These observations are consistent with MIL 090036 being more clast rich with more mafic components. Miller Range 090034/70/75 have lower HSE abundances than that of MIL 090036, which suggest more limited impact contamination. In addition to variable Eu anomalies, La/Yb can be used to differentiate between FAN

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(>3.6), impact melt coat samples (3.20 ±0.07) and anorthositic regolith breccia meteorites (2.8 ±0.4), indicating variably fractionated REE patterns in samples.

4.3 Rubidium-strontium isotope systematics of IMC Samples 65035 IMC, 65035 FAN, 60015 IMC, 65325 IMC, and 65325 FAN were analyzed for Rb-Sr isotope systematics to confirm similarity to previously published Apollo materials. The impact melt coats have higher

87Sr/86Sr

ratios (0.701013 to

0.704414) than FAN samples (0.699232 to 0.699537), and IMC also have high 87Rb/86Sr (0.0299 to 0.0963) compared to FAN samples (0.0080 to 0.0164; Table 3; Figure S2). When compared to previously analyzed impact melt rocks, IMC and FAN samples are within range of previous data (Reimold et al., 1985; Norman et al., 2016). In detail, two of our FAN data fall within the lower bounds of previous

87Sr/86Sr

ratios for FAN

(Reimold et al., 1985; Norman et al., 2016).

4.4 Rhenium-osmium isotope systematics and highly siderophile element abundances 4.4.1 Apollo 16 impact melt coats The highly siderophile element (HSE) concentrations for IMC samples range from ~0.001 to 0.1 times CI chondrite values (Figure 4), with

187Os/188Os

between

0.1283 and 0.1302, with two exceptions: glass fragments analyses of 65035 (187Os/188Os = 0.1189) and 60015 (187Os/188Os = 0.1366) (Figure 5). The IMC have HSE contents and 187Os/188Os

similar to Apollo 14, 15, 16 and 17 impact melt rocks and fragmental matrix

breccias (Puchtel et al., 2008; Fischer-Gödde & Becker, 2012; Sharp et al., 2015; Liu et al., 2015; Gleißner and Becker, 2017; Gleißner and Becker, 2019). IMC 65325 has a

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broadly flat pattern, with HSE abundances ~0.1 that of CI chondritic values, except for one analysis of 65325 that has an Os abundance of ~0.01 × CI. IMC 60015 has a broadly flat pattern, but one analysis shows a depletion in Re, while another analysis of 60015 has depletions in both Re and Pd. A single analysis of IMC 65035 exhibits a Re depletion, and one analysis of 65035 has Ru, Ir, and Os depletions. The IMC on 65035 has broadly chondritic Pd and Pt, which is similar to some analyses of impact melt rocks reported by Gleißner and Becker (2017).

4.4.2 Lunar anorthositic regolith breccia meteorites Anorthositic regolith breccia meteorites have HSE concentrations that range from approximately 0.001 to 0.1 times CI chondritic values and most data fall within the range of data reported for impact melt breccias previously, most especially feldspathic granulite and fragmental matrix breccias (Puchtel et al. 2008; Fischer-Gödde & Becker 2012; Sharp et al. 2014; Liu et al. 2015; Gleißner and Becker, 2017; Gleißner and Becker, 2019; Figure 4). Many ARB meteorites exhibit relatively flat patterns. An exception is one fragment of MIL 090075, which has elevated Re, Os, and Ir. One fragment of MIL 090036 has elevated Os, Ir, Ru, Pt, and Pd abundances compared to other ARB. Except for a depletion in Ir, MIL 090034 has a flat pattern. Most of the absolute concentrations of the ARB meteorites are within the range of available data for IMB (Puchtel et al. 2008; Fischer-Gödde & Becker 2012; Sharp et al. 2014; Liu et al. 2015; Gleißner and Becker, 2017; Gleißner and Becker, 2019), but ARB meteorites have different observed HSE patterns. For example, there is a Pd depletion in MIL 090075 and 090034 that is similar to that observed in feldspathic granulite breccias (Fischer-Gödde & Becker 2012).

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Analyzes of IMC and ARB meteorites show heterogeneity in their (Pt, Pd, Ru, Os)/Ir ratios. Analyzes of 65035 exhibit the most variation (Pd/Ir = 1.3 to 16, Pt/Ir = 2 to 21, Ru/Ir = 0.7 to 1.9). Analyzes of 65325 (Pd/Ir = 1.3 to 3, Pt/Ir = 2.2 to 2.5, Ru/Ir = 1.9 to 2.2) and 60015 (Pd/Ir = 0.1 to 1.8, Pt/Ir = 2.1 to 2.4, Ru/Ir = 1.7 to 2.1) show less variation. Significant heterogeneity occurs within MIL 090034 (Pd/Ir = 0.2 to 3.2, Pt/Ir = 1.1 to 5.9, Ru/Ir = 0.3 to 5.7), MIL 090070 (Pd/Ir = 0.2 to 2.7, Pt/Ir = 0.5 to 0.7, and Ru/Ir = 0.9 to 1.4), and MIL 090075 (Pd/Ir = 0.03 to 0.6, Pt/Ir = 0.7 to 2.3, and Ru/Ir = 0.5 to 4.7) (Table S8).

Ratios of

187Os/188Os

for ARB are between 0.1164 (MIL 090075) and 0.1284

(MIL 090036) (Figure 5). Sample MIL 090036 is distinct from MIL 090034/70/75 and shows more elevated

187Os/188Os

and, for one fragment, more elevated Ru, Pt, and Pd

compared to other samples. A powder of MIL 090070 and a fragment of MIL 090075 have low

187Os/188Os

study have lower

of 0.1177 and 0.1164, respectively. The ARB measured in this

187Os/188Os

than terrestrial primitive mantle (Meisel et al., 2001), and

the majority of measurements are less radiogenic than previously measured 187Os/188Os in Apollo 16 and Apollo 17 impact melt breccias (Puchtel et al., 2008; Fischer-Gödde and Becker, 2012). The majority of ARB have lower

187Os/188Os

compared to feldspathic

impactites, which range from 0.1278 – 0.1435.

5. Discussion 5.1 Heterogeneity within IMC and ARB

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Using HSE concentrations is an effective way to estimate contributions of exogeneous sources to impact melt rocks since lunar crustal rocks have low concentrations of these elements compared to chondrites (Day et al., 2010). Although HSE in lunar impact melt rocks and breccias can be dominantly exogenous in origin, siderophile elements such as S, Fe, Ni, and P can be a combination of lunar (endogenous) and meteoritic (exogenous) origin (Gleißner and Becker, 2017). Due to incompatible element behavior during magma ocean crystallization processes, KREEP-rich rocks contain high abundances of P, and so P in schreibersite, FeNi metal, and FeNiP assemblages could be partly endogenous. For example, Gleißner and Becker (2017) calculated nearly half the S and greater than 90% of the P in an impact melt containing 1% CI chondrite was originally derived from the Moon.

The partitioning behavior of the HSE is influenced by pressure, temperature and composition (Corrigan et al., 2009; Chabot et al., 2014 and references therein). For example, in the Fe-Ni system the solid metal/liquid metal partition coefficient can change for Ir by approximately three orders of magnitude depending on the S-content in the system (Jones and Drake, 1983). Notwithstanding the potential origins of P and S versus the HSE in IMC, noted above, Corrigan et al. (2009) found that for the majority of elements, the effect of P on solid metal/liquid metal partitioning is minor and suggested that S partitioning plays a greater role. It has previously been argued that, during cooling, sub-centimeter-scale fractional crystallization occurs between Fe-Ni metal and Fe-Ni-P-S liquid droplets in the silicate impact melt (James et al., 2007; Fischer-Gödde and Becker, 2012; Gleißner and Becker, 2017). Our observations of troilite-schreibersite-metal

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associations in IMC are important in this regard, as they not only demonstrate the veracity of this process, but also allow empirical estimation of fractionation between the three phases in IMC. From our restricted analyses, we find that metal/sulfide fractionation is of the order of 4-5 for Os and Ir, ~1.6 for Ru, Rh, Pt, Pd and Au, and as high as ~60 for Re. These fractionations may range significantly, and more analyses of troilite-schreibersite-metal associations would be useful to examine fractionation behavior of the HSE in impact melts.

The IMC fragments are elevated in Pd, Pt, and Ru compared to ARB meteorites and many (Norman et al., 2002; Puchtel et al., 2008; Sharp et al., 2014; Liu et al., 2015), but not all reported values for IMB (Gleißner and Becker, 2017). The IMC on 65035 has broadly chondritic Pd and Pt, and the elevated Pd and Pt abundances are likely because the HSE are compatible in the main carrier phase(s). There is significant heterogeneity within the analyses of IMC due to uneven distribution of troilite-schreibersite-metal blebs. The non-chondritic and elevated Ru/Ir of IMC and ARB meteorites might therefore reflect sampling of metal-sulfide phases that generated inter-element HSE fractionation. As noted previously, CI-chondrite normalized HSE abundances for fragments of ARB meteorites show a wider range of compositions than for powders analyzed from the same samples. Powders of MIL 090034 and MIL 090075 also have elevated Ru/Ir and contain low Ir abundances, indicating the ratio is controlled by Ir content in these two samples (Figure 6). That fragments of ARB meteorites exhibit more heterogeneity than powders has been observed in previous studies of IMB (Puchtel et al., 2008) and suggests an uneven distribution of metal and sulfide phases throughout these

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rocks. The observations of: (1) a combined meteoritic and lunar origin for P and S; (2) the presence of troilite-schreibersite-metal segregations and intergrowths in IMC, and; (3) HSE fractionation within sulfide and metal carrier phases, all support HSE fractionation that can occur at the <100 m scale to the scale of individual samples within lunar impact melt rocks.

5.2 Comparison of impact lithologies; melt coats, breccias and regolith breccias To fully constrain the origin of impact melt rocks, it is important to understand their precursor lithologies. This can be achieved through petrography of samples, with IMC clearly accumulating FAN materials. For the ARB, different lithologies and mineral fragment compositions indicate a diverse array of lunar crustal lithologies, dominated by a FAN component. There is also evidence within ARB clasts, that they preserve more than one impact event. Geochemical evidence comes from the different incompatible trace element compositions of impact melt rocks which are dominantly inherited from reservoirs in or on the Moon (Delano et al., 1981; Delano, 1991). In contrast, chondrites and other impactor materials are not unusually enriched in incompatible trace elements. The new Rb-Sr isotope systematics of IMC and FAN data fall within the limits of previously analyzed impact melt rocks analyzed by Reimold et al. (1985) and Norman et al. (2016). Ferroan anorthosite samples have lower 87Sr/86Sr and 87Rb/86Sr than the IMC, which suggests long term low Rb/Sr and an origin from a Feldspathic Highland Terrane or target. As noted previously, the IMC display higher abundances of incompatible trace elements compared to ARB meteorites, whereas impact melt breccias have higher

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abundances of the majority of incompatible trace elements relative to both of these rock types.

Dominant terranes on the surface of the Moon determined from orbital geochemical data are the Procellarum KREEP terrane, the lunar crust and mare basalts (e.g., Korotev et al., 2009). The Procellarum KREEP terrane is a distinct lunar province that is rich in K, REE, P, and other incompatible elements (Jolliff et al., 2000). The differing trace element compositions of ARB meteorites, IMC and impact melt breccias can be used to infer different inputs from different lunar terranes. A binary mixing model (Figure 7) between a KREEP composition from Warren and Wasson (1979) and the pristine FAN sample, 65325, implies that the majority of IMC and ARB meteorites require between 1 and 10% of a KREEP component to recreate their La and Yb abundances. Impact melt coat 65325 is the only exception and requires ~20% KREEP to explain its composition. Samples 60015, 65035, and MIL 090036 need closer to 10% to explain their compositions, while all other samples imply ~ 1% of a KREEP component.

The IMC from this study exhibit REE abundances ~100 × CI-Chondrites and were discovered near the Descartes Highlands and the Cayley Plains Formations. This likely suggests an Imbrium or Serenitatis origin for the target materials involved in these rocks, given the absence of KREEP-rich pre-cursor rocks in the vicinity of the Apollo 16 site. The IMC are therefore either young, reworked Cayley Plains material or extremely old and from the Cayley Plains Formation itself. Constraining the origin of the IMC is a challenging task without age constraints. Apollo 16 Low-Mg High-K Fra Mauro

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(lmHKFM) impact glasses analyzed by Delano et al. (2007) were determined to have an age of 3.73 ± 0.04 Ga. The preferred age from Delano et al. (2007) is close to the accepted age for the Imbrium basin using U-Pb dating of Apollo 14 samples, which is ~3.92 Ga (Snape et al., 2016). Barra et al. (2006) suggested that the Apollo 12 KREEP breccias were affected by a ~700-800 Ma event, either the Copernicus impact and ejecta or another impact event or series of impact events. A similar event could have reworked precursor lithologies and formed the IMC. In order to better determine which scenario is more likely, high-precision ages for IMC are required.

5.3 Models for the composition of impact melt rocks Heterogenous 187Os/188Os and HSE abundances in IMC suggest that fractionation of the HSE can occur even at small scales and masses (<0.1 g). In Figure 8, the calculations of the quantity of sulfide, schreibersite and metal are shown for IMC, lunar ARB meteorites, and impact melt breccias assuming all the HSE are hosted within the sulfide, schreibersite, and metal grains, with calculations made using data presented in Table 2. Of the HSE bearing phases, we found that IMC have an average of ~25% schreibersite, ~36% sulfide, and ~37% metal (determined from ImageJ analysis of 2D XRay maps and back-scatter electron images). The models (Figure 8) imply that ~0.01-2 % sulfide, schreibersite, and metal grains must be present in the IMC to recreate the whole rock compositions. Sample 65035 has anomalously high Pd and Pt abundances for one glass analysis, which cannot be explained by our model. These anomalous abundances could reflect another sulfide or metal phase(s) present in the sample that was not observed during petrographic study of the samples or analyzed by LA-ICP-MS due to

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our sampling size restriction (>100 m). Lunar ARB meteorites require less sulfide, schreibersite, and metal grains to explain their whole rock compositions, consistent with our petrographic observations for limited sulfide and metal phases in samples. The model implies that the majority of ARB and previously measured impact melt rocks (Puchtel et al. 2008; Fischer-Gödde & Becker 2012; Sharp et al. 2014; Liu et al. 2015; Gleißner and Becker, 2017; Gleißner and Becker, 2019) can be explained by 0.1-1% and 0.01-1% metal, schreibersite and sulfide grains, respectively.

As discussed previously, in order to derive the compositions of materials striking the Moon, numerous studies have utilized the compositions of impact melt rocks (e.g., Gros et al., 1976; Hertogen et al., 1977; Anders, 1978; Higuchi & Morgan, 1975; Norman et al., 2002; Puchtel et al., 2008; Fischer-Gödde and Becker, 2012; Sharp et al., 2014; Liu et al., 2015; Gleißner and Becker, 2017; Day et al., 2017a; Gleißner and Becker, 2019). The assumption of these studies is that the HSE measured in the samples come directly from impactor materials that are then incorporated into samples. Plots of data from recent studies of lunar impact melt breccias show generally positive trends of 187Os/188Os versus Pt/Ir, Pd/Ir and Ru/Ir (Figure 6). Fischer-Gödde and Becker (2012) found HSE ratios extending much higher than known chondrites in Apollo 16 impact melt rocks and noted the possible incorporation of two major HSE-rich materials; pre-existing, impactcontaminated granulites and a component resembling chemically evolved group IVA iron meteorites. Consequently, they proposed that both components became variably mixed during younger impact events.

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Our results for IMC, and metal and sulfide phases within them, suggests that some IMB compositions may reflect the processes noted by previous workers, as well as HSE fractionation during metal-sulfide-silicate differentiation processes. Physical evidence for the fractionation of the HSE in melt-sheets has been shown by the preservation of metal, schreibersite nuggets in 14286,11. (e.g., Warren 2012). Day et al. (2016) showed that mixing trends between lunar crustal compositions of FAN and magnesian suite rocks and a CI chondrite composition make lunar crustal rocks candidates for mixing to explain IMB compositions. However, given the percentage of impactor additions to lunar impact melt rocks estimated from the HSE (<5%), and the low HSE abundances of lunar crustal materials, some form of prior scavenging of the HSE in Fe–Ni metal and sulfides is required, to explain the relative mixing required for lunar impact melt rocks. This may be possible by the combination of lunar S and P to form metal-sulfide assemblages in some impact melt rocks and an ‘R-factor’ type process (Sulfide liquid content = [HSEcontent of silicate magma

× D[R+1]))/(R+D]), where the high partition coefficients of the HSE into

sulfide versus silicate, combined with the high mass ratio of silicate melt to sulfide melt available can lead to significant enrichment from otherwise low HSE abundance sources (e.g., Day et al., 2016).

In melt sheets with low silicate to sulfide melt ratios (100 to 1000:1) versus those with high silicate to sulfide melt ratios (10000 to 100000:1) and assuming elevated partition coefficients for the HSE between silicate and sulfide (in this example, KD = 30,000) the amount of enrichment in sulfides can go from 0.01 to 2.3 ppm in melt sheets with initial abundances of the HSE as low as 100 ppt. Sulfide scavenging and HSE

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fractionation during this type of process would therefore effectively obscure original impactor signatures, especially in large impact melt sheets. Collectively, these results, suggest that HSE fractionation during formation or melt-evolution of IMB may be significant in some instances and that HSE inter-element ratios in lunar impact melt rocks may not always preserve impactor compositions with fidelity.

5.4 Implications for the compositions of impactors striking the Moon The HSE have been used to trace late accretion material, and impact melt rocks have been shown to contain chondritic and iron meteorite-like signatures (Higuchi and Morgan, 1975; Hertogen et al., 1977; Warren et al., 1989; Korotev 1994; Norman et al., 2002; Puchtel et al., 2008; Fischer-Gödde and Becker, 2012; Sharp et al., 2014; Liu et al., 2015). Previous studies have used homogenized sample powders (Norman et al., 2002) and fragments of impact melt rocks in order to determine HSE ratios in impactors (Puchtel et al., 2008; Gleißner and Becker, 2019). A result of this study is that HSE interelement ratios in IMB may also be fractionated due to differing partitioning into segregated metal-sulfide phases. This inter-element fractionation leads to the cautionary statement that this process may also have affected Re/Os at the time of IMB formation and therefore influence present day

187Os/188Os

ratios. Most IMB as well as lunar pre-

cursor rocks (e.g., FAN) and likely impactor compositions (e.g., chondrites, iron meteorites), however, tend to have low initial 187Os/188Os

187Re/188Os,

meaning that measured

are unlikely to be strong affected by inter-element fractionation.

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The prior evidence for IMB suggests that the impactors striking the Earth and Moon were distinct from chondrite groups collected and analyzed on Earth today (e.g., Puchtel et al., 2008). Some analyses of both IMC and ARB meteorites in this study are outside the range of previously measured lunar impact melt rocks. The difference in composition between IMB, IMC and ARB offers the possibility that there was an earlier population of impactors striking the Moon with distinct HSE compositions compared to modern-day chondrite meteorites. Generally, 4.2-3.8 Ga IMB have similar to higher 187Os/188Os

than ordinary or enstatite chondrites. In one interpretation, addition of

differentiated impactor components with elevated Re/Os and

187Os/188Os,

such as iron

meteorites, and their subsequent mixing with lunar lithologies and previous impactor materials, led to the compositions of some IMB (Fischer-Gödde and Becker, 2012; Gleißner and Becker, 2017; Gleißner and Becker, 2019). By contrast, younger ARB meteorites are characterized by

187Os/188Os

ratios similar to modern carbonaceous

chondrites, implying a change in the composition of impactors striking the Moon with time.

Alternatively, it is possible that different regions of the Moon were struck by different impactor populations. In this scenario, KREEP-rich mafic impact melt rocks from the Apollo sites contain a differentiated impactor component, whereas feldspathic impactites sampled as meteorites contain more primitive components. A spatial distribution of distinct impactor populations is permissible since ARB meteorites sample regions of the lunar surface distinct from the Procellarum KREEP Terrane (e.g., Joy et al., 2014; Calzada-Diaz et al., 2017; Martin et al., 2017). Further examination of lunar

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meteorites and of samples collected from the Feldspathic Highland Terrane and South Pole Aitken Basin will enable examination of the temporal and spatial variability of impactors striking the Moon.

6. Conclusions Lunar impact rocks provide valuable information about the composition of impactors striking the crust after the Moon’s differentiation. New petrography, trace element, major element, and HSE abundances,

187Os/188Os

and

87Sr/86Sr

data for impact

melt coats and anorthositic regolith breccia meteorites show distinct differences when compared to Apollo impact melt breccias. Impact melt coats are heterogenous due to troilite-schreibersite-metal associations within them and anorthositic regolith breccias are also heterogenous with respect to HSE abundances. These samples have distinct Ru/Ir and Pd/Ir compared to impact melt breccias, but similar

87Sr/86Sr.

Iron-Ni metal and

sulfide segregation processes may have been important in HSE fractionation within some impact melt sheets on the Moon during sulfide or metal scavenging of the HSE meaning that some impact melt rocks may not retain information on the compositions of impactors striking the Moon with fidelity. Nonetheless, we observe a distinct difference in the Os isotope compositions of younger anorthositic regolith breccias and earlier impact melt breccias and coats. The low

187Os/188Os

ratios of anorthositic regolith breccia meteorites

versus the more radiogenic compositions of impact melt breccias might infer a change from ordinary chondrite-like and/or differentiated bodies, to carbonaceous chondrite-like impactors with time. Alternatively, or in addition, Apollo impact melt rocks derived from Procellarum KREEP Terrane may sample a different impactor population to the

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Feldspathic Highlands Terrane or South Pole Aitken Basin, where samples are currently only sampled as meteorite finds.

Acknowledgements Completion of this manuscript is dedicated to our colleague, Lawrence A. Taylor. Constructive and helpful reviews from David van Acken and Phillip Gleißner are gratefully acknowledged, along with editorial handling by Christian Koeberl. We thank the Meteorite Working Group and NASA CAPTEM for provision of samples. US Antarctic meteorite samples are recovered by the Antarctic Search for Meteorites (ANSMET) program which has been funded by NSF and NASA and characterized and curated by the Department of Mineral Sciences of the Smithsonian Institution and Astromaterials Curation Office at NASA Johnson Space Center. Support for this work came from the NASA Emerging Worlds (NNX15AL74G) and Solar System Workings programs (NNX16AR95G). References Albarède F. (2009) Volatile accretion history of the terrestrial planets and dynamic implications. Nature 461, 1227-1233. Albrecht A., Herzog G., Klein J., Middleton R., Schultz L., Weber H., Kallemeyn G. and Warren P. (1995) Trace Elements, Al26 and Be10, and Noble Gases in Lunar Rock 14286. Lunar and Planetary Science Conference: 25th, 1007. Anders, E. (1978) Procrustean science: Indigenous siderophiles in the lunar highlands, according to Delano and Ringwood. Proceedings of the Lunar and Planetary Science Conference: 8th, 161-184. Barra F., Swindle T.D., Korotev R.L., Jolliff B.L., Zeigler R.A. and Olson E. (2006) 40Ar/39Ar dating of Apollo 12 regolith: Implications for the age of Copernicus and the source of nonmare materials. Geochim. Cosmochim. Acta. 70, 6016-6031. Basilevsky A. T., Neukum G. and Nyquist L. (2010) The spatial and temporal distribution of lunar mare basalts as deduced from analysis of data for lunar 34

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meteorites. Planet. Space Sci., 58, 1900-1905. Becker H., Horan M.F., Walker R.J., Gao S., Lorand J.-P. and Rudnick R.L. (2006) Highly siderophile element composition of the Earth’s primitive upper mantle: Constraints from new data on peridotite massifs and xenoliths. Geochim. et Cosmochim. Acta 70, 4528-4550. Birck J. L., Roy Barman M. and Capmas F. (1997) Re-Os isotopic measurements at the femtomole level in natural samples. Geostand. Newsl. 20, 19-27. Bogard D.D., Garrison D.H. and Nyquist L.E. (2000) Argon-29-Argon-40 ages of Lunar Highland rocks and meteorites. Lunar and Planetary Science Conference: 31st, 1138. Borisov A., Palme H. and Spettel B. (1994) Solubility of palladium in silicate melts: implications for core formation in the Earth. Geochim. Cosmochim. Acta 58, 705-716. Brandon A. D., Puchtel I. S., Walker R. J., Day J. M. D., Irving A. J. and Taylor L. A. (2012) Evolution of the martian mantle inferred from the 187Re-187Os isotope and highly siderophile element abundance systematics of shergottite meteorites. Geochim. Cosmochim. Acta. 76, 206-235. Calzada-Diaz A., Joy K.H., Crawford I.A. and Strekopytov S. (2017) The petrology, geochemistry, and age of lunar regolith breccias Miller Range 090036 and 090070: Insights into the crustal history of the Moon. Meteorit. Planet. Sci. 52, 3-23. Chabot N.L., Wollack E.A., McDonough W.F. and Ash R. (2014) The effect of light elements in metallic liquids on partitioning behavior. Lunar and Planetary Science Conference: 44th, 1165. Chou C.-L. (1978) Fractionation of siderophile elements in the Earth’s upper mantle. Proceedings of the Lunar and Planetary Science Conference: 9th. 219-230. Cohen A. S. and Waters F. G. (1996) Separation of osmium from geological materials by solvent extraction for analysis by thermal ionisation mass spectrometry. Anal. Chim. Acta. 332, 269-275. Cohen B.A, Swindle T.D. and Kring D.A. (2005) Geochemistry and 40Ar-39Ar geochronology of impact-melt clasts in feldspathic lunar meteorites: Implications for lunar bombardment history. Meteorit. Planet. Sci. 40, 755-777. Cooper G., Kimmich N., Belisle W., Sarinana J., Brabham K. and Garrel L. (2001) Carbonaceous meteorites as a source of sugar-related organic compounds for the early Earth. Nature 414, 879-883. Corrigan C.M., Chabot N.L., McCoy T.J., McDonough W.F., Watson H.C., Saslow S.A. andAsh R. (2009) The iron-nickel-phosphorous system: effects on the distribution of 35

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trace elements during the evolution of iron meteorites. Geochim. Cosmochim. Acta. 73, 2674-2691. Day J.M.D., Floss C., Taylor L.A., Anand M. and Patchen A.D. (2006) Evolved mare basalt magmatism, high Mg/Fe feldspathic crust, chondritic impactors and the petrogenesis of Antarctic lunar meteorites Meteorite Hills 01210 and Pecora Escarpment 02007. Geochim. Cosmochim. Acta 70, 5957-5989. Day J.M.D., Pearson D.G. and Taylor L.A. (2007) Highly siderophile element constraints on accretion and differentiation of the Earth-Moon system. Science 315, 217-219. Day J.M.D., Walker R.J., James O.B. and Puchtel I.S. (2010) Osmium isotope and highly siderophile element systematics of the lunar crust. Earth Planet. Sci. Lett. 289, 595605. Day J.M.D and Walker R.J. (2015) Highly siderophile element depletion in the Moon. Earth. Planet. Sci. Lett., 423, 114-124. Day J.M.D. (2016) Siderophile Elements. Encyclopedia of Geochemistry. In: White W. (eds) Encyclopedia of Geochemistry. Encyclopedia of Earth Sciences Series. Springer, Cham. Day J.M.D., Brandon A.D. and Walker R.J. (2016) Highly Siderophile Elements in Earth, Mars, the Moon, and Asteroids. Reviews in Mineralogy and Geochemistry 81, 161– 238. Day, J.M.D., Moynier, F. and Shearer, C.K. (2017a). Late-stage magmatic outgassing from a volatile-depleted Moon. Proceedings of the National Academy of Sciences, 114, 9547-9551. Day J.M.D, Walker R.J. and Warren J.M. (2017b) 186Os-187Os and highly siderophile element abundance systematics of the mantle revealed by abyssal peridotites and Osrich alloys. Geochim. Cosmochim. Acta. 200, 232-254. Day J. M. D., Maria-Benavides J., McCubbin F. M. and Zeigler R. A. (2018) The potential for metal contamination during Apollo lunar sample curation. Meteorit.Planet. Sci. 53, 1283-1291. Delano J. W., Lindsley D.H. and Rudowski R. (1981) Glasses of impact origin from Apollo 11, 12, 15 and 16: Evidence for fractionational vaporization and mare/highland mixing. Proceedings of the Lunar and Planetary Science Conference: 13th, A159A170. Delano J.W. (1991) Geochemical comparison of impact glasses from lunar meteorites ALHA81005 and MAC 88105 and Apollo 16 regolith 64001. Geochim. Cosmochim. Acta. 55, 3019-3029. 36

McIntosh et al.

Lunar Impactor Compositions

Delano J.W., Zellner N.E.B., Barra F., Olson E., Swindle T.D., Tibbetts N.J. and Whittet D.C.B. (2007) An integrated approach to understanding Apollo 16 impact glasses: Chemistry, isotopes, and shape. Meteorit. Planet. Sci. 42, 993-1004. Dixon J.R. and Papike J.J. (1975) Petrology of anorthosites from the Descartes region of the moon: Apollo 16. Proceedings of the Lunar Science Conference: 6th, 263-291. Ebihara M., Wolf R., Warren P.H. and Anders E. (1992) Trace elements in 59 mostly highland moon rocks. Proceedings of the Lunar and Planetary Science Conference: 22nd,417-426. Fischer-Gödde M., Becker H. and Wombacher F. (2010) Rhodium, gold and other highly siderophile element abundances in chondritic meteorites. Geochim. Cosmochim. Acta. 74, 356-379. Fischer-Gödde M. and Becker H. (2012) Osmium isotope and highly siderophile element constraints on ages and nature of meteoritic components in ancient lunar impact rocks. Geochim. Cosmochim. Acta 77, 135-156. Fischer-Gödde M. and Kleine T. (2017) Ruthenium isotopic evidence for an inner Solar System origin of the late veneer. Nature 541, 525-527. Gallant M., Gladman B. and Cuk M. (2009) Current bombardment of the Earth-Moon system: emphasis on cratering asymmetries. Icarus. 202, 371-382. Gleißner P. and Becker H. (2017) Formation of Apollo 16 impactites and the composition of late accreted material: Constraints from Os isotopes, highly siderophile elements and sulfur abundances. Geochim. Cosmochim. Acta. 200, 1-24. Gleißner P. and Becker H. (2019) Origin of lunar fragmental matrix breccias – Highly siderophile element constraints. Meteorit. Planet. Sci. 54, 2006-2026. Goderis S., Paquay F. and Claeys P. (2012) Projectile identification in terrestrial impact structures and ejecta material. Impact cratering: Processes and products. In: Osinski G.R., Pierazzo E. (eds) Impact cratering: Processes and products. Wiley-Blackwell. Gooley R., Brett R. and Warner J. (1973) Crystallization history of metal particles in Apollo 16 rake samples. Proceedings of the Lunar Science Conference: 3rd, 799-810. Gros J., Takahashi H., Hertogen J., Morgan J.W. and Anders E. (1976) Composition of the projectiles that bombarded the lunar highlands. Proceedings of the Lunar Science Conference: 7th, 2403-2425.

37

McIntosh et al.

Lunar Impactor Compositions

Hertogen J., Janssens M.-J., Takahashi H., Palme H. and Anders E. (1977) Lunar basins and craters: evidence for systematic compositional changes of bombarding population. Proceedings of the Lunar Science Conference: 8th, 17-45. Higuchi H. and Morgan J.W. (1975) Ancient meteoritic component in Apollo 17 boulders. Proceedings of the Lunar Science Conference: 6th, 1625-1651. Horan M.F., Walker R.J., Morgan J.W., Grossman J.N. and Rubin A. (2003) Highly siderophile elements in chondrites. Chem. Geol. 196, 5-20. James O.B. and Lindstrom M.M. (1991) Apollo 16 Dimict Breccias: II. Sample 65035. Lunar and Planetary Science Conference: 24th, 637-638. Jolliff B.L., Korotev R.L. and Haskin L.A. (1991) A ferroan region of the lunar highlands as recorded in meteorites MAC 88104 and MAC 88105. Geochim. et Cosmochim Acta. 55, 3051-3071. Jolliff B.L., Gillis J.J., Haskin L.A., Korotev R.L. and Wieczorek M.A. (2000) Major lunar crustal terranes: Surface expressions and crust-mantle origins. J. of Geophys. Res.105, 4197-4216. Joy K.H., Crawford I.A., Russell S.S. and Kearsley A.T. (2010) Lunar meteorite regolith breccias: An in situ study of impact melt composition using LA-ICP-MS and implications for the composition of the lunar crust. Meteorit. Planet. Sci. 45, 917-946. Joy K.H., Crawford I., Huss G., Nagashima K. and Taylor G. (2014) An unusual clast in lunar meteorite MacAlpine Hils 88105: A unique lunar sample or projectile debris? Meteorit. Planet. Sci. 49, 677-695. Kimura K., Lewis R.S. and Anders E. (1974) Distribution of gold and rhenium between nickel-iron and silicate melts: implications for the abundance of siderophile elements on the Earth and Moon. Geochem. Cosmochim. Acta 38, 683-701. Koeberl C. (2014) The geochemistry and cosmochemistry of impacts. Planets, Asteroids, Comets and The Solar System. In: Davis, A.M. (eds) Planets, Asteroids, Comets and the Solar System. Treatise on Geochemistry. Elsevier. Korotev R.L. (1994) Compositional variation in Apollo 16 impact-melt breccias and inferences for the geology and bombardment history of the Central Highlands of the Moon. Geochim. Cosmochim. Acta 58, 3931-3969. Korotev R. L., Jolliff B. L. and Carpenter P. K. (2011) Miller Range Feldspathic Lunar Meteorites. Lunar Planet. Sci. Conf. 42, 1999. Kring D.A. and Cohen B.A. (2002) Cataclysmic bombardment throughout the inner solar system 3.9-4.0 Ga. J. of Geophys. Res. 107, 5009. Doi:10.1029/2001JE001529. 38

McIntosh et al.

Lunar Impactor Compositions

Lindstrom M.M., Wentworth S.J., Martinez R.R., Mittlefehldt D.W., McKay D.S., Wang M.-S. and Lipschutz M.J. (1991) Geochemistry and petrography of the MacAlpine Hills lunar meteorites. Geochim. Cosmochim. Acta. 55, 3089-3103. Liu J., Sharp M., Ash R.D., Kring D.A. and Walker R.J. (2015) Diverse impactors in Apollo 15 and 16 impact melt rocks: evidence from osmium isotopes and highly siderophile elements. Geochim. Cosmochim. Acta 155, 122-153. Liu Y., Patchen A. and Taylor L.A. (2011) Lunar Highland breccias MIL 090034/36/70/75: A significant KREEP component. Lunar Planet. Sci. Conf. 42, 1261. Lucey P.G., Taylor G.J. and Malaret E. (1995) Abundance and distribution of Fe on the Moon. Science 268, 1150-1153. Mann U., Frost D.J., Rubie D.C., Becker H. and Audetat A. (2012) Partitioning of Ru, Rh, Pd, Re, Ir and Pt between liquid metal and silicate at high pressures and temperatures- Implications for the origin of highly siderophile element concentrations in the Earth’s mantle. Geochim. Cosmochim. Acta 84, 593-613. Martin D.J.P., Pernet-Fisher J.F., Joy K.H., Wogelius R.A., Morlok A. and Hiesinger, H. (2017) Investigating the shock histories of lunar meteorites Miller Range 090034, 090070, and 090075 using petrography, geochemistry, and micro-FTIR spectroscopy. Meteorit. Planet. Sci. 52, 1103-1124. McDonald I., Andreoli M.A.G., Hart R.J. and Tredoux M. (2001) Platinum-group elements in the Morokweng impact structure, South Africa: Evidence for the impact of a large ordinary chondrite projectile at the Jurassic-Cretaceous boundary. Geochim. Cosmochim. Acta 65, 299-309. Mehta S. and Goldstein J. (1980) Metallic particles in the glassy constituents of three lunar highland samples 65315, 67435, and 78235. Proceedings of the Lunar and Planetary Science Conference: 10th, 1713-1725. Meisel T., Walker R. J., Irving A. J. and Lorand J. P. (2001) Osmium isotopic compositions of mantle xenoliths: A global perspective. Geochim. Cosmochim. Acta. 65, 1311-1323. Melosh, H.J. (1996) Impact Cratering. A Geologic Process. Oxford Monographs on Geology and Geophysics 11, 254. Misra K.C. and Taylor L.A. (1975) Characteristics of metal particles in Apollo 16 rocks. Proceedings of the Lunar Science Conference:6th, 615-639.

39

McIntosh et al.

Lunar Impactor Compositions

Morbidelli A., Chambers J., Lunine J.I., Petit J.M., Robert F., Valsecchi G.B. and Cyr K.E. (2000) Source regions and timescales for the delivery of water to Earth. Meteorit. Planet. Sci. 35, 1309-1320. Morgan J.W. (1985) Osmium isotope constraints on Earth’s late accretionary history. Nature 317, 703-705. Morris R.V., See T.H. and Horz F. (1986) Composition of the Cayley Formation at Apollo 16 as inferred from impact melt splashes. Proceedings of the Lunar and Planetary Science Conference: 17th in J. of Geophys. Research 90, E21-E42. Moynier F., Day J. M. D., Okui W., Yokoyama T., Bouvier A., Walker R. J. and Podosek F. A. (2012) Planetary-scale strontium isotopic heterogeneity and the age of volatile depletion of early solar system materials. Astrophys. J. 758, 1-7. Neal C. R., Taylor L. A., Liu Y.-G. and Schmitt R. A. (1991) Paired lunar meteorites MAC88104 and MAC88105: A new “fan” of lunar petrology. Geochim. Cosmochim. Acta. 55, 3037-3049. Norman M.D. (2009) The lunar cataclysm: Reality or “Mythconception”. Elements 5, 2328. Norman M.D., Bennett V.C. and Ryder G. (2002) Targeting the impactors: siderophile element signatures of lunar impact melts from Serenatatis, Earth Planet. Sci. Lett. 202, 217-228. Norman M.D. and Roberts J. (2013) Metal particles in Apollo 17 impact melt breccias: Textures and highly siderophile element compositions. Lunar Planet. Sci. Conf., 44. 1802. Norman M.D., Taylor L.A., Shih C.-Y., Nyquist L.E. (2016) Crystal accumulation in a 4.2 Ga lunar impact melt. Geochim. Cosmochim. Acta 172, 410-429. Park J., Nyquist L.E., Shih C.-Y., Herzog G.F., Yamaguchi A., Shirai N., Ebihara M., Lindsay F.N., Delaney J., Turrin B. and Swisher III C. (2013) Late bombardment of the Lunar Highlands recorded in MIL 090034, MIL 090036, and MIL 090070 Lunar meteorites. Lunar Planet. Sci. Conf., 44. 2576. Puchtel I.S., Walker R.J., James O.B. and Kring D.A. (2008) Osmium isotope and highly siderophile element systematics of lunar impact melt rocks: Implications for the late accretion history of the Moon and Earth. Geochim. Cosmochim. Acta 72, 3022-3042. Reimold W.U., Nyquist L.E., Bansal B.M., Wooden J.L., Shih C.-Y., Wiesmann H. and Mackinnon I.D.R. (1985) Isotope analysis of crystalline impact-melt rocks from Apollo 16 stations 11 and 13. North Ray Crater. Proceedings of the Lunar Science Conference: 15th in J. of Geophys. Research 90, C431-C448. 40

McIntosh et al.

Lunar Impactor Compositions

Robert F. (2001) The Origin of Water on Earth. Science 293, 1056. Ryder G. and Norman M.D. (1980) Catalog of Apollo 16 rocks (2 vol.). Curator’s Office pub.: 52, JSC16904. See T.H., Hörz F. and Morris R.V. (1986) Apollo 16 impact-melt splashes: Petrography and major-element composition. J. of Geophys. Research: Solid Earth 91, E3-E20. Sharp M., Gerasimenki I., Loudin L., Liu J., James O.B., Puchtel I.S. and Walker R.J. (2014) Characterization of the dominant impactor signature for Apollo 17 impact melt rocks. Geochim. Cosmochim. Acta 131, 62-80. Snape J.F., Nemchin A.A., Grange M.L., Bellucci J.J., Thiessen F. and Whitehouse M.J. (2016) Phosphate ages in Apollo 14 breccias: Resolving multiple impact events with high precision U-Pb SIMS analyses. Geochim. Cosmochim. Acta 174, 13-29. Tait K.T. and Day J.M.D. (2018) Chondritic late accretion to Mars and the nature of shergottite reservoirs. Earth Planet. Sci. Lett. 494, 99-108. Terra F., Papanastassiou D.A. and Wasserburg G.J. (1974) Isotopic evidence for a terminal lunar cataclysm. Earth Planet. Sci. Lett. 22, 1-21. Tingle T.N., Tyburczy J.A., Ahrens T.J. and Becker C.H. (1992) The fate of organic matter during planetary accretion: preliminary studies of the organic chemistry of experimentally shocked Murchison meteorite. Origins of Life and Evolution of the Biosphere. 21, 385-397. Vaughan W.M., Head J.W., Wilson L. and Hess P.C. (2013) Geology and petrology of enormous volumes of impact melt on the Moon: A case study of the Orientale basin impact melt sea. Icarus 223, 749-765. Warren P. H. (2003) The Moon. In Treatise on Geochemistry (eds. Turekian K.K. and Holland H.D.). Elsevier, Amsterdam. Warren P.H. (1993) A concise compilation of petrologic information on possibly pristine nonmare Moon rock. American Mineralogist. 78, 360-376. Warren P.H. (1994) Lunar and Martian Meteorite Delivery Services. Icarus 2, 338-363. Warren P.H. (2012) Let’s get real: not every lunar rock sample is big enough to be representative for every purpose. Second Conf. Lunar Highlands Crust: 9034. Warren P.H. and Wasson J.T. (1977) Pristine nonmare rocks and the nature of the lunar crust. Proceedings of the Lunar Science Conference 8th, 2215-2235.

41

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Warren P.H. and Kallemeyn G.W. (1991a) The MacAlpine Hills lunar meteorite and implications of the lunar meteorites collectively for the composition and origin of the Moon. Geochim. Cosmochim. Acta. 55, 3123-3138. Warren P.H. and Wasson J. T. (1979) The origin of KREEP. Reviews of Geophysics. 17, 73-88. Warren P.H., Jerde E.A. and Kallemeyn G.W. (1989) Lunar meteorites: siderophile element contents and implications for the composition and origin of the Moon. Earth and Planet. Sci. Lett. 91, 245-260. Wasson J. T. (1999) Trapped melt in IIIAB irons; solid/liquid elemental partitioning during the fractionation of the IIIAB magma. Geochim. Cosmochim. Acta. 63, 28752889. Wasson J. T. and Choe W. H. (2009) The IIG iron meteorites: Probable formation in the IIAB core. Geochim. Cosmochim. Acta. 73, 4879-4890. Wasson J. T. and Richardson J. W. (2001) Fractionation frends among IVA iron meteorites: Constrast with IIIAB trends. Geochim. Cosmochim. Acta. 65, 951-970. Wasson J. T., Willis J., Wai C. M. and Kracher A. (1980) Origin of Iron Meteorite Groups IAB and IIICD. Zeitschrift fur Naturforsch. - Sect. A J. Phys. Sci. 35, 781-795. Yamamoto, T. (1985). Formation environment of cometary nuclei in the primordial solar nebula. Astronomy and Astrophysics, 142, 31-36. Zeigler R.A., Korotev R.L., and Jolliff B.L. (2012) Pairing relationships among feldspathic lunar meteorites from Miller Range, Antarctica. Lunar and Planetary Science Conference: 42, 2377. Zellner N. and Delano J. (2015) 40Ar/39Ar ages of lunar impact glasses: Relationships among Ar diffusivity, chemical composition, shape, and size. Geochim. Cosmochim. Acta 161, 203-218.

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Figures and Figure Captions

Figure 1 – Images of the glassy impact melt coating or ‘splashes’ on cataclastic anorthosite 60015 (116): (a, b) Reflected light images of differentiated FeNi metalschreibersite-sulfide assemblages. (c) Plane-polarized light image of impact melt coat from 60015 showing metal grains, sulfides, and reflective oxides, as well as cataclastic anorthosite fragments embedded in the glass. Backscatter electron image (d), X-ray S map (e), and P map (f) of differentiated FeNi metal-schreibersite-sulfide assemblage. Bright colors in (e) and (f) denote higher concentrations of S and P, respectively. These images show the typical association of FeNi in the center with schreibersite sandwiched between sulfide on the exterior of the assemblage. (g) Reflected light image of impact melt coating (IMC) in abrupt contact with interior cataclastic anorthosite. Major components are labelled.

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Figure 2 - Metal and schreibersite compositions within Apollo 16 impact melt coats measured in situ using LA-ICP-MS. (A) Carbonaceous Ivuna (CI) chondrite normalized abundances of phases in IMC 60015. (B) CI chondrite normalized abundances of phases

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in IMC 65035 and 65325. CI chondrite normalization from Horan et al. (2003).

Figure 3 - Carbonaceous Ivuna chondrite normalized trace element abundances for (A) impact melt coats and corresponding FAN samples and (B) lunar anorthositic regolith breccia meteorites. Published data for Apollo 17 impact melt breccias (MB) are from Norman et al. (2002). CI chondrite normalization from McDonough and Sun (1995). 45

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Figure 4 – Carbonaceous Ivuna (CI) chondrite normalized highly siderophile element (HSE) abundances for (A) Apollo 16 impact melt coats with published data for individual aliquot analyses of impact melt rocks are shown in grey (Norman et al. 2002; Puchtel et al. 2008; Fischer-Gödde and Becker, 2012; Sharp et al. 2014; Liu et al. 2015; Day et al., 2017a; Gleißner and Becker, 2017; Gleißner and Becker, 2019)and (B) anorthositic regolith breccia meteorites with published data for feldspathic and KREEP-poor

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impactites (Fischer-Gödde and Becker, 2012; Gleißner and Becker, 2019). CI chondrite normalization from Horan et al. (2003).

Figure 5 - Histogram of measured 187Os/188Os ratios for anorthositic regolith breccia meteorites (dark gray), impact melt coats (black), and impact melt rocks from Apollo 14 (Puchtel et al. 2008; Fischer-Gödde & Becker, 2012), Apollo 15 (Liu et al., 2015), Apollo 16 (Fischer-Gödde & Becker, 2012; Liu et al., 2015; Day et al., 2017a; Gleißner and Becker, 2017; Gleißner and Becker, 2019) and Apollo 17 (Puchtel et al., 2008; FischerGödde & Becker 2012; Sharp et al., 2014). Previous impact melt rock data are for individual aliquots. Average carbonaceous (grey dotted-dashed line), enstatite/ ordinary chondrite (black dashed line) 187Os/188Os from Walker et al. (2002) are shown. Terrestrial primitive mantle value from Meisel et al. (2001) is shown as a solid black line.

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Figure 6 - Plots of 187Os/188Os versus (A, B) Ru/Ir, (C, D) Pd/Ir, and (E,F) Pt/Ir for impact melt coats, lunar anorthositic regolith breccias, IMB, and chondrites. Previously published IMB data are from Puchtel et al. (2008), Fischer-Gödde & Becker (2012), Sharp et al. (2014), Liu et al. (2015), Gleißner and Becker, (2017), and Gleißner and Becker (2019). A single sample from Gleißner and Becker (2019) has an extreme composition and is not shown at the scale of the axes. Chondrite fields are from Horan et al. (2003). Note differences in scale for axes.

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Figure 7 – Mixing model of KREEP and the most pristine FAN composition in this study. In order to recreate the compositions of IMC and lunar anorthositic regolith breccia meteorites between 1-10% KREEP is needed, with the exception of IMC 65325 which requires ~18%. KREEP composition from Warren (2003).

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Figure 8 – Plots of (A) Re, (B) Ru, (C) Pd, (D) Pt, and (E) Os versus Ir in IMC, lunar anorthositic regolith breccia meteorites, and previously measured impact melt rocks (Norman et al., 2002; Puchtel et al., 2008; Fischer-Gödde & Becker 2012; Sharp et al. 2014; Liu et al. 2015; Gleißner and Becker 2017; and Gleißner and Becker 2019). The model is represented by the solid black line and CI-Chondrite (Horan et al., 2003) is represented by the black dotted line. In order to explain the whole rock HSE abundances in IMC ~0.01-2 wt.% of sulfide-metal-schreibersite is needed, while in order to explain lunar anorthositic regolith breccia meteorites whole rock HSE abundances ~0.01-1% of sulfide-metal-schreibersite is needed for the majority of samples. The pristine lunar crustal composition is assumed to be the following in ppt: Os = 1.4; Ir = 1.5; Ru = 6.8; Pd = 33; Re = 0.29; Pt = 16 from Day et al. (2010). 50

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Table 1: Rhenium-Os isotope and highly siderophile element abundance (in ppb) data for lunar impact melt coats and regolith breccia meteorites

Sam ple

Materi al

Mas s (g)

Re

2S D

Pd

2S D

Pt

2S D

Ru

2S D

Ir

2S D

O s

2S D

187Re/1 87Os

187Os/1

2SD

88Os

2SE

0.0088

0.00 7 0.00 02

2.38

0.06

0.1288 5 0.1297 3 0.1189 4

1.59

0.04 0.00 8

0.1287 0.1302

0.00 007 0.00 009 0.00 009 0.00 11 0.00 06

0.0098

0.00 6 0.00 05 0.00 02

0.1283 2 0.1293 3 0.1366 2

0.00 008 0.00 014 0.00 009

0.901

0.02 3

0.1258 5

0.00 030

Lunar Impact Melt Coats 6503 5, 192

Glass Glass Glass

6532 5, 24

Glass Glass

6001 5, 791

Glass Glass Glass

MIL 0900

Powd er

0.02 13 0.04 11 0.04 49 0.01 23 0.01 18

2.4 2 0.0 6 0.3 4 2.0 3 2.6 2

0.3 7 0.0 1 0.0 1 0.5 9 0.0 1

42 .2 22 9 50 .8 41 .3 11 6

4. 4 0. 4 0. 4 7. 4 0. 4

66. 5 32 7 69. 3 74. 2 90. 2

0. 7

0.02 44 0.04 30 0.04 81

1.1 7 0.0 9 0.0 2

0.3 1 0.0 1 0.0 1

2. 76 22 .4 20 .8

2. 4 0. 4 0. 4

51. 9 48. 0 27. 3

0. 7 1. 0 0. 5

1. 0

0.16 35

0.3 74

0.0 07

1. 9 1. 6 0. 10 3. 2 1. 6

39 .2 35 .0 0. 70 6. 1 42 .0

0. 8 0. 7 0. 06 1. 3 0. 2

1. 6 0. 1 0. 1

22 .8 21 .0 10 .8

0. 7 0. 4 0. 2

Lunar Regolith Breccia Meteorites 0. 2.8 0. 4.2 0. 2.3 0. 2. 4 1 13 6 02 5 07 00

0. 04

7 1. 4 1. 3 1. 8

63 27. 7 2.1 8

21 0. 5 0. 50

73 66. 9

34 0. 5

33. 7 18. 7 3.2 4 33. 2 35. 9

43 38. 6 24. 5

18 0. 5 0. 5

23. 2 23. 4 11. 6

0.297

0.301 0.247 0.0196

51

McIntosh et al.

Lunar Impactor Compositions

34, 22

MIL 0900 36, 14

MIL 0900 70, 15

MIL 0900 75, 15

Powd er Frag ment Frag ment

0.15 32 0.15 53 0.14 54

0.1 3 0.1 5 0.1 3

0.0 5 0.0 1 0.0 1

2. 0 0. 5 0. 3

0. 6 0. 4 0. 4

3.6 7 2.6 4 1.4 1

0. 10 0. 08 0. 08

3.5 7 3.5 1 1.7 2

2. 4 0. 5 0. 5

0.6 2 1.9 2 1.2 4

0. 22 0. 10 0. 10

1. 79 2. 32 2. 17

0. 11 0. 02 0. 02

Powd er Powd er Frag ment Frag ment

0.15 67 0.10 42 0.10 36 0.11 87

0.6 74 0.4 0 0.4 2 0.0 4

0.0 08 0.0 8 0.0 1 0.0 1

7. 0 7. 9 23 .9 1. 4

0. 5 0. 9 0. 4 0. 4

11. 00 10. 42 58. 30 12. 62

0. 14 0. 15 0. 08 0. 08

10. 13 10. 6 46. 7 10. 7

0. 02 5. 2 0. 5 0. 5

6.5 2 5.7 0 25. 89 7.3 5

0. 08 0. 40 0. 10 0. 10

6. 72 5. 83 33 .1 7. 40

0. 13 0. 16 0. 03 0. 02

Powd er Powd er Frag ment Frag ment

0.15 82 0.12 06 0.14 97 0.04 93

0.3 74 0.1 6 0.1 2 0.1 0

0.0 08 0.0 6 0.0 1 0.0 1

0. 8 2. 0 7. 9 2. 0

0. 4 0. 7 0. 4 0. 4

2.9 8 3.4 0 2.0 8 0.7 7

0. 13 0. 13 0. 08 0. 08

4.4 1 3.5 5 2.6 2 2.2 5

0. 02 2. 8 0. 5 0. 5

3.6 6 3.5 7 2.9 6 1.6 0

0. 08 0. 33 0. 10 0. 10

2. 09 3. 12 2. 07 2. 35

0. 04 0. 14 0. 02 0. 06

Powd er Powd er

0.12 20 0.05 45

0.3 41 0.1 3

0.0 10 0.1 0

0. 5 0. 7

0. 4 0. 8

2.8 1 2.8 6

0. 17 0. 28

5.2 8 5.8 3

0. 03 5. 4

4.0 3 1.2 5

0. 10 0. 58

1. 80 1. 86

0. 04 0. 29

0.355 0.316 0.293

0.484 0.330 0.061 0.027

0.863 0.253 0.287 0.206

0.915 0.350

0.00 9 0.00 8 0.00 7

0.1253 2 0.1233 9 0.1222 5

0.00 013 0.00 013 0.00 011

0.01 2 0.00 8 0.00 2 0.00 1

0.1262 2 0.1255 8 0.1284 0 0.1257 1

0.00 021 0.00 007 0.00 009 0.00 007

0.02 2 0.00 6 0.00 7 0.00 5

0.1233 5 0.1176 5 0.1249 9 0.1221 6

0.00 010 0.00 011 0.00 008 0.00 018

0.02 3 0.00 9

0.1245 0 0.1229 7

0.00 018 0.00 027

52

McIntosh et al.

MAC 8810 5, 180

Lunar Impactor Compositions

Frag ment Frag ment

0.15 98 0.19 36

1.0 5 2.8 9

0.0 1 0.0 1

1. 1 0. 9

0. 4 0. 4

2.9 8 30. 53

0. 08 0. 08

3.5 6 13. 6

0. 5 0. 5

2.1 5 25. 84

0. 10 0. 10

2. 37 54 .3

0. 02 0. 01

Powd er Powd er Frag ment Frag ment

0.12 26 0.10 14 0.20 00 0.20 41

0.3 2 0.3 5 0.2 7 0.3 2

0.0 6 0.0 1 0.0 1 0.0 1

6. 4 4. 4 3. 1 5. 4

0. 8 0. 4 0. 4 0. 4

7.2 2 6.8 9 6.1 4 5.9 3

0. 13 0. 08 0. 08 0. 08

6.7 3 6.7 8 5.6 5 7.0 7

3. 3 0. 5 0. 5 0. 5

4.2 3 5.7 4 3.9 5 4.1 5

0. 33 0. 10 0. 10 0. 10

2. 03 5. 55 4. 18 3. 83

0. 14 0. 03 0. 01 0. 01

2.12 0.256

0.754 0.301 0.310 0.400

0.05 0.00 6

0.1252 3 0.1163 9

0.00 015 0.00 007

0.01 9 0.00 8 0.00 8 0.01 0

0.1234 0 0.1235 0 0.1252 8 0.1272 4

0.00 024 0.00 008 0.00 007 0.00 039

Abundance uncertainties reflect propogated blank correction errors relating to the measured blank in the appropriate sample cohort as the two standard deviation. Because of the range in the quantitites of HSE present among samples,uncertainties in the abundance data are variable, with the magnitude of uncertainties largely reflecting blank/sample ratios. Uncertainty in the 187Re/188Os reflects the propogated errors of the Os and Re abundance uncertainties as the 2 standard deviation (SD). Uncertainty of 187Os/188Os is the two sigma error from in-run statistics during analysis.

53

McIntosh et al.

Lunar Impactor Compositions

Table 2: Trace element data (in ppm) for metal, sulfide and schreibersite in lunar impact melt coats Sample

Phase

Co

Ni

Cu

Mo

Ru

Rh

Pd

W

Re

Os

Ir

Pt

Au

60015 60015, 805

Metal

2946

73016

234

8.1

3.30

0.86

3.56

9.24

0.30

1.80

2.24

5.40

1.13

60015, 805

Metal

1596

50615

5427

5.3

1.37

0.16

1.84

1.80

0.21

0.36

0.44

1.16

0.53

60015, 805

Metal

3279

89692

407

9.0

5.10

1.18

5.33

17.4

0.25

2.76

3.01

7.78

0.93

60015, 116

Metal

1125

21408

520

2.2

0.91

0.09

1.54

7.33

0.06

0.21

0.39

0.30

60015, 116

Metal

733

9788

780

5.1

55.1

9.63

1.18

4.10

2.26

31.2

29.4

75.6

1.16

60015, 116

Metal

3186

49386

386

4.4

2.74

0.62

2.61

29.0

0.13

1.48

1.56

3.83

0.50

60015, 116

Metal

2386

84926

387

4.6

2.98

0.72

3.55

3.31

0.15

0.66

1.22

3.81

1.30

60015, 805

Sulfide

2008

58637

2361

6.0

2.71

0.78

4.25

6.82

0.12

1.16

0.91

2.81

2.76

60015, 805

Schreibersite

2211

55073

283

4.9

3.59

0.78

4.10

10.6

0.21

2.16

1.80

4.18

0.80

60015, 116

Schreibersite

1968

49493

284

2.4

1.33

0.30

2.44

4.09

0.04

0.66

0.60

1.26

0.57

0.04

0.01

0.13

0.69

0.06

0.09

0.77

0.47

0.81

2.00

0.03

0.10

65035 65035

Metal

117

991

1128

7.9

65035

Metal

123

1512

1211

7.2

65035

Schreibersite

134

1918

944

0.9

0.05

0.31

1.22

65035

Schreibersite

183

1459

1236

1.8

0.01

0.11

20.9

0.06

65035

Schreibersite

149

1296

1069

0.8

0.10

0.01

0.29

4.92

0.05

1.41

0.09

3.16

3.47

0.05

64.2

0.10

1.25

5.25

6.29

0.07

0.06

1.54

1.02

0.36

0.20

30.3

0.35

1.27

0.13

65325 65325

Metal

969

18460

110

3.2

0.36

54

McIntosh et al.

Lunar Impactor Compositions

65325

Metal

1157

38944

1486

1.6

0.58

0.08

1.11

1.39

0.02

0.06

0.18

0.29

0.44

65325

Metal

1423

42553

1704

3.2

0.32

0.27

2.98

1.53

0.08

0.19

0.15

2.24

1.38

65325

Metal

651

3897

43154

494

2.30

4.98

94.3

107

3.86

7.81

33.0

26.6

65325

Schreibersite

3792

108208

21251

19.6

1.45

3.92

28.3

0.16

0.18

0.32

6.14

19.7

2.45

55

McIntosh et al.

Lunar Impactor Compositions

Table 3: Trace element data (in ppm) for lunar impact melt coats and lunar regolith breccia meteorites Sampl e ID

MAC881 05

MIL0900 70, 15

MIL0900 75, 15

MIL0900 36, 14

MIL0900 34, 22

Rock Type Mass (g)

Regolith Breccia

Regolith Breccia

Regolith Breccia

Regolith Breccia

Regolith Breccia

0.04633

0.05104

0.04863

0.05186

0.05267

3.11 1.16 8.88 1298 18.22 589 497

3.29 1.44 116 6.95 838 12.71 420 381

3.65 1.51 148 7.25 890 13.16 437 393

9.71 2.01 567 9.87 3579 16.88 504 464

3.93 1.36 125 7.48 867 13.62 447 392

12.56 96.70 3.52 6.94 3.61 0.52

27000 8.35 45.88 2.65 8.04 3.31 0.36

26700 8.15 36.07 2.08 14.51 3.37 0.34

46400 16.57 139.8 3.01 13.43 4.67 0.68

27700 8.60 42.62 2.24 3.90 3.21 0.34

Li B P Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge

65325, 24 Impact Melt Coat 0.0095 3

65325, 24

11.16 12.78 1700 10.67 3581 23.16 945 585 13600 0 45.47 800 11.85 425.1 4.65 1.54

3.60 21.82 1691 3.98 132 4.83 37 68

FAN 0.0124

5983 1.85 26.60 10.86 103.6 3.96 0.16

65035, 192 Impact Melt Coat 0.0353 8

65035, 192 FAN 0.0119 0

60015, 791 Impact Melt Coat 0.0257 7

7.46 1.56 417 5.09 1410 14.10 563 407 11700 0 42.81 776 4.51 4.50 3.47 0.63

5.38 2.74 314 2.22 271 5.18 98 122 56100 27.27 383 2.68 35.47 4.00 0.50

62255

62255

FAN 0.063 1

FAN 0.049 2

7.77 1.80 409 7.41 1987 19.53 818 595

2.13

2.11

14.7 0.65 220 2.54 55 45.9

12.6 0.62 208 2.42 52 43.5

170000 71.96 1112 6.84 6.80 3.06 1.00

4476 3.58 4.35 0.46

4216 2.90 3.45 0.44

4.46 0.01

4.16 0.01

56

McIntosh et al.

Se Rb Sr Y Zr Nb Mo Sn Te Cs Ba La Ce Pr Nd Sm Eu Eu/Eu* Gd Tb Dy Ho Er Tm Yb Lu

0.11 0.72 147.0 9.25 33.38 2.42

Lunar Impactor Compositions

0.65

0.13 0.48 181.4 7.70 28.47 1.92 0.05 4.79

0.14 0.60 183.7 9.04 33.37 2.06 0.03 5.01

0.21 2.75 241.8 37.5 159.0 9.73 0.04 5.31

0.15 0.61 179.2 7.60 25.83 1.74 0.02 4.19

0.04 29.94 2.37 6.27 0.85 3.82 1.13 0.76 1.91 1.31 0.23 1.61 0.34 0.99 0.15 0.96 0.13

0.02 30.12 2.04 6.44 0.73 3.27 0.94 0.89 2.66 1.10 0.20 1.35 0.29 0.83 0.12 0.81 0.11

0.03 33.03 2.45 6.94 0.86 3.89 1.11 0.89 2.25 1.30 0.23 1.59 0.33 0.97 0.14 0.93 0.13

0.16 141.9 12.12 33.06 4.30 19.21 5.26 1.55 0.84 5.97 1.02 6.76 1.39 3.88 0.55 3.51 0.49

0.03 27.87 1.96 5.30 0.71 3.21 0.92 0.85 2.60 1.08 0.19 1.31 0.27 0.81 0.12 0.77 0.11

1.69 6.44 193.4 67.4 273.6 16.9 1.10 89.35 0.01 0.44 235 21.17 56.69 7.59 33.56 9.46 1.42 0.43 10.66 1.86 12.55 2.56 7.31 1.05 6.45 0.95

3.64 0.70 254.0 0.56 5.34 0.35 0.59 183.3 0.05 0.27 429 0.24 0.65 0.08 0.33 0.10 1.01 28.91 0.11 0.02 0.12 0.02 0.05 0.01 0.05 0.01

0.24 1.97 190.5 26.5 108.1 6.67 0.09 7.88 0.01 0.12 83.25 8.00 21.63 2.90 13.06 3.56 1.04 0.83 4.08 0.70 4.60 0.98 2.74 0.39 2.54 0.35

0.44 1.16 203.3 3.24 13.89 0.81 0.11 16.37 0.01 0.14 22.43 1.08 2.84 0.37 1.67 0.45 0.84 5.34 0.52 0.09 0.56 0.12 0.34 0.05 0.30 0.04

0.34 2.56 180.6 37.3 152.2 9.48 0.13 8.96 0.01 0.14 117.5 11.40 30.44 4.05 18.26 5.12 1.11 0.62 5.75 1.01 6.68 1.40 3.94 0.56 3.60 0.50

0.03 0.01 161.3 0.64 9.32 0.03 0.02 0.04

0.01 155.9 0.60 8.83 0.03 0.02 0.04

0.001 13.76 0.24 0.62 0.08 0.38 0.10 1.17 33.03 0.11 0.02 0.11 0.02 0.06 0.01 0.06 0.01

0.001 13.44 0.22 0.60 0.08 0.37 0.10 1.14 32.87 0.11 0.02 0.10 0.02 0.06 0.01 0.06 0.01

57

McIntosh et al.

Hf Ta W Pb Th U 87Rb/86 Sr 2SD 87Sr/86 Sr 2SE

0.86 0.12 0.06 0.48 0.43 0.10

Lunar Impactor Compositions

0.70 0.10 0.06 0.31 0.39 0.14

0.81 0.10 0.05 0.28 0.46 0.14

3.83 0.48 0.22 0.86 1.90 0.47

0.63 0.09 0.05 0.16 0.35 0.09

6.90 0.89 0.58 2.59 3.73 0.96

0.12 0.10 0.53 3.06 0.25 0.09

2.62 0.32 0.14 0.47 1.33 0.33

0.33 0.05 0.42 0.67 0.18 0.05

3.72 0.45 0.28 0.37 1.94 0.47

0.0963 0.0048 0.7044 05 0.0000 04

0.0080 0.0004 0.6992 21 0.0000 04

0.0299 0.0015 0.7010 13 0.0000 02

0.0164 0.0008 0.6995 34 0.0000 02

0.0410 0.0020 0.7018 64 0.0000 02

0.19 0.01 0.01 0.32

0.19 0.01 0.01 0.31

0.01

0.01

58

McIntosh et al.

Lunar Impactor Compositions

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

59