# Mare basalt meteorites, magnesian-suite rocks and KREEP reveal loss of zinc during and after lunar formation

James M. D. Day, Elishevah M. M. E. van Kooten, Beda A. Hofmann, Frederic Moynier

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## To cite this version:

James M. D. Day, Elishevah M. M. E. van Kooten, Beda A. Hofmann, Frederic Moynier. Mare basalt meteorites, magnesian-suite rocks and KREEP reveal loss of zinc during and after lunar formation. Earth and Planetary Science Letters, 2020, 531, pp. 981-995. 10.1016/j.epsl.2019.115998. insu-03584810

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# Mare basalt meteorites, magnesian-suite rocks and KREEP reveal loss of zinc during and after lunar formation

James M.D Day¹,²*, Elishevah M.M.E. van Kooten², Beda A. Hofmann³, Frederic Moynier²

¹Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0244, USA

²Université de Paris, Institut de Physique du Globe de Paris, CNRS UMR 7154, Paris, France

³Naturhistorisches Museum Bern, Bernastrasse 15, CH-3005 Bern, Switzerland.

*Corresponding author: jmdday@ucsd.edu

Submitted to: *Earth and Planetary Science Letters*

Abstract: 369

Total Text: 6009

Figures: 7

Tables: 2

Supplementary Tables: 2

Keywords: Zinc; moon; magnesian-suite; KREEP; mare basalts; condensation; evaporation

## Abstract
29 Isotopic compositions of reservoirs in the Moon can be constrained from analysis of rocks generated during lunar magmatic differentiation. Mare basalts sample the largest lunar mantle 30 volume, from olivine- and pyroxene-rich cumulates, whereas ferroan anorthosites and magnesian- 31 suite rocks represent early crustal materials. Incompatible element enriched rocks, known as 32 ‘KREEP,’ probably preserve the last highly differentiated melts. Here we show that mare basalts, 33 including Apollo samples and meteorites, have remarkably consistent δ⁶⁶Zn values (+1.4 ±0.2‰) 34 and Zn abundances (1.5 ±0.4 ppm). Analyses of magnesian-suite rocks show them to be 35 characterized by even heavier δ⁶⁶Zn values (2.5 to 9.3‰) and low Zn concentrations. KREEP-rich 36 impact melt breccia Sayh al Uhaymir 169 has a nearly identical Zn composition to mare basalts 37 (δ⁶⁶Zn = 1.3‰) and a low Zn abundance (0.5 ppm). Much of this variation can be explained 38 through progressive depletion of Zn and preferential loss of the light isotopes in response to 39 evaporative fractionation processes during a lunar magma ocean. Samples with isotopically light 40 Zn can be explained by either direct condensation or mixing and contamination processes at the 41 lunar surface. The δ⁶⁶Zn of Sayh al Uhaymir 169 is probably compromised by mixing processes 42 of KREEP with mafic components. Correlations of Zn with Cl isotopes suggests that the urKREEP 43 reservoir should be isotopically heavy with respect to Zn, like magnesian-suite rocks. Current 44 models to explain how and when Zn and other volatile elements were lost from the Moon include 45 nebular processes, prior to lunar formation, and planetary processes, either during giant impact, 46 or magmatic differentiation. Our results provide unambiguous evidence for the latter process. 47 Notwithstanding, with the currently available volatile stable isotope datasets, it is currently 48 difficult to discount if the Moon lost its volatiles relative to Earth either during giant impact or 49 exclusively from later magmatic differentiation. If the Moon did begin initially volatile-depleted, 50 then the mare basalt δ⁶⁶Zn value likely preserves the signature, and the Moon lost 96% of its Zn 51
inventory relative to Earth and was also characterized by isotopically heavy Cl ($\delta^{37}\text{Cl} = \ge 8\text{‰}$).

Alternative loss mechanisms, including erosive impact removing a steam atmosphere need to be examined in detail, but nebular processes of volatile loss do not appear necessary to explain lunar and terrestrial volatile inventories.

# 1. Introduction

Compared with Earth, the Moon is poor in volatile compounds (e.g., H₂O, CO₂) and elements (e.g., K, Na, Cl, Zn, Rb, Sn). Comparison of volatile element data for these planetary bodies shows that the Moon has lower ratios of element pairs that behave similarly during igneous processes but have a more volatile numerator over denominator, including K/U and Rb/Sr (e.g., Wolf & Anders, 1980; Jones & Palme, 2000; O'Neill and Palme 2008; Day & Moynier, 2014). The extent of volatile depletion, however, remains an open question. At one extreme, moderately volatile elements are depleted by a factor of ten or more (O'Neill, 1991; Albarede et al., 2015), whereas melt inclusions within olivine grains from the Apollo 74220 pyroclastic glass indicate that the Moon may have as much water as Earth's depleted upper mantle (Hauri et al., 2011; Ni et al., 2019). For the moderately volatile elements, defined by their nebular condensation temperatures intermediate between 'common' silicate minerals (forsterite, enstatite) and troilite (Lodders, 2003), stable isotope ratios are fractionated by evaporation/condensation processes (e.g., Herzog et al. 2009; Sharp et al., 2010; Paniello et al., 2012a; Kato et al., 2015; Boyce et al., 2015; Wang
74 & Jacobsen, 2016; Day et al., 2017a; 2019; Kato & Moynier, 2017; Pringle & Moynier, 2017; 75 Wang et al., 2019).

76

77 While the extent of volatile depletion in the Moon is an important question, how and when 78 volatile element depletion occurred may be even more critical. This is because the extent of volatile 79 depletion is dependent on how and when the process took place, which has ‘planet-wide’ 80 consequences. As noted previously (O’Neill and Palme 2008; Day & Moynier, 2014), at least two 81 phases of volatile depletion occurred during Solar System formation. The first was in the solar 82 nebula (nebular depletion), prior to or during the formation of the first solids. Partial or exclusive 83 loss of volatiles from the Moon has been proposed in this way (Humayun & Clayton, 1995; Taylor 84 el al., 2006). The second volatile depletion event, or series of events, is associated with 85 planetesimal and planet formation (planetary depletion). Within planetary depletion, there are 86 several avenues for volatile loss, focused around melting either during collisional events (Paniello 87 et al., 2012a; Kato & Moynier, 2017; Pringle & Moynier, 2017), or during differentiation of the 88 planets or planetesimals (Sharp et al., 2010; Kato et al., 2015; Boyce et al., 2015; Dhaliwal et al., 89 2018).

90

91 One of the best-studied moderately volatile elements for examining volatile loss from the 92 Moon is zinc. Zinc has five stable isotopes, ⁶⁴Zn (48.6%), ⁶⁶Zn (27.9%), ⁶⁷Zn (4.1%), ⁶⁸Zn (18.8%) 93 and ⁷⁰Zn (0.6%) and the ⁶⁶Zn/⁶⁴Zn value is typically reported as the delta-notation (δ⁶⁶Zn = 94 [(⁶⁶Zn/⁶⁴Zn)sample/(⁶⁶Zn/⁶⁴Zn)standard]-1) × 1000) relative to the ‘Lyon’ Zn standard, JMC 3–0749 95 L (see Moynier et al., 2017). Zinc is moderately volatile with a low 50% nebular condensation
temperature (~700K) and a low bond-bond energy (e.g., Lodders, 2003; Albarède et al., 2015).
Mass-dependent stable isotopic fractionation of an element occurs during any exchange reaction
and is due to the difference in vibrational energy of the bonds formed by the different isotopes at
equilibrium or to the difference of motions of the isotopes under kinetic conditions (e.g., Urey,
1947). Zinc has been measured in partial melt products from the lunar interior as mare basalts and
bulk pyroclastic glass beads, some ferroan anorthosites (FAN) that are considered to make up the
crust, and a few magnesian-suite (MGS) rocks, which are thought to be intrusions within the crust.
Data from these samples has revealed a range of δ⁶⁶Zn values in lunar samples, from -13.7‰ to
+6.4‰, which have been interpreted to reflect the various processes of volatile loss and
condensation, largely due to differentiation processes (Moynier et al., 2006; Herzog et al., 2009;
Paniello et al., 2012a; Kato et al., 2015; Day et al., 2017a).

There are some restrictions in the existing lunar Zn isotope dataset that limits interpretation of
how, when and to what extent volatile depletion occurred in the Moon. First, only two MGS rocks
have been examined (dunite 72415 and norite 77215), and they both exhibit heavy δ⁶⁶Zn values
(Kato et al., 2015), making analysis of further samples to examine whether this is a ubiquitous
signature of the MGS important. Alternatively, it is possible that the MGS could show similar
ranges in δ⁶⁶Zn to the FAN (Kato et al., 2015). Second, Apollo 11, 12, 15 and 17 mare basalts have
been examined from the Moon, but these are all near-side samples from a restricted area. Mare
basalt meteorites are considered to come from different regions of the Moon compared with the
Apollo samples (e.g., Korotev, 2005), so analysis of these samples is important for assessing the
likelihood of homogeneity of δ⁶⁶Zn in lunar mantle source regions. Finally, no potassium-rare
earth element-phosphorous enriched samples (KREEP) have been examined for Zn isotopes. This
is an important lunar reservoir, as it represents the dregs of magmatic differentiation, likely from
a global magma ocean (Warren & Wasson, 1979). The δ<sup>66</sup>Zn composition of KREEP would
therefore constrain models of volatile loss during magma ocean crystallization processes. In this
study, we take advantage of a new Zn isotopic method that decreases the required sample size by
a factor of approximately ten to analyze the isotope and abundance data for four MGS rocks (15445
cataclastic anorthositic norite [CAN] clast, 15455 'B' norite clast, 78235 shocked norite, 76535
troctolite), four low-Ti (<4 wt.% TiO<sub>2</sub>) mare basalt meteorites (La Paz 02205, Northwest Africa
[NWA] 8632, NWA 479 and NWA 4734), as well as the first reported data on a KREEP-rich
sample; the impact melt breccia of Sayh al Uhaymir [SaU] 169. With these data, we examine the
loss of volatile Zn during processes acting prior to and during lunar formation.

## 2. Methods

Aliquots of powdered samples (50 to 250 mg) were digested in a mixture of ultra-pure
HF/HNO₃ in Teflon beakers for 48 hours on a hotplate at 140°C. The samples were then dried
down and Aqua Regia was added to dissolve the aliquots for another 24 hours on the hotplate. Zinc
purification was achieved using an improved and miniaturized anion-exchange chromatography
method, with a recovery of 99.99% (see Van Kooten & Moynier, 2019 for details). Samples (5 mg
aliquots of total sample solutions) were loaded in 1.5N HBr on 0.1 ml AG-1X8 (200-400 mesh)
ion-exchange columns and Zn was collected in 0.5N HNO₃. This purification step was repeated to
ensure purification of the Zn fraction. The procedural blank measured with samples was 0.2 ng,
and generally represents less than 4% of total measured Zn for samples, which was typically 5-10
ng (Van Kooten & Moynier, 2019). Zinc isotopic compositions were measured on the Thermo
Fischer Neptune Plus multi collector-inductively coupled plasma-mass spectrometer housed at the
Institut de Physique du Globe, Paris. The Faraday cups were positioned to collect ions on the masses 62, 63, 64, 65, 66, 67 and 68. Possible <sup>64</sup>Ni isobaric interferences were controlled and corrected by measuring the intensity of the <sup>62</sup>Ni peak. A solution containing 10 ppb Zn in 0.1 M HNO<sub>3</sub> was prepared for isotopic analysis. Isotopic ratios of Zn in all samples were analyzed using an Apex IR introduction system, combined with a 100 µl/min PFA nebulizer. One block of 30 ratios, in which the integration time of 1 scan was 8.3 seconds, was measured for each sample. The background was corrected by subtracting the on-peak zero intensities from a blank solution. The instrumental mass bias was corrected by bracketing each of the samples with standards.

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### 3. Results

Zinc isotope and abundance data for mare basalt meteorites (La Paz 02205, Northwest Africa [NWA] 479, NWA 4734 and NWA 8632) spans a remarkably limited range, with 0.59 ±0.01 ppm Zn and δ<sup>66</sup>Zn of 1.23 ±0.05‰ (Tables 1 and 2). These values are more restricted than the range of zinc abundance and isotopic compositions for Apollo mare basalts (δ<sup>66</sup>Zn = +1.9 to -5.4‰; Zn = 0.6 to 12.1 ppm), although mare basalts typically fall within a restricted range of δ<sup>66</sup>Zn compositions (Figure 1). Compared with bulk pyroclastic glass bead data, the Apollo mare basalts and mare basalt meteorites are typically isotopically heavier with respect to Zn and have lower abundances.

160

The Apollo magnesian-suite (MGS) rocks span a limited range of Zn concentrations (0.42 to 1.36 ppm), and are all isotopically heavy (δ<sup>66</sup>Zn = 2.46 to 9.27‰), confirming previous data showing that crustal rocks have the heaviest isotope compositions of Zn of all lunar igneous rocks
164 (Kato et al., 2015). Similarly, high δ<sup>66</sup>Zn has been measured in ferroan anorthosite 15415, with the
165 other FAN samples that have been measured being isotopically light (Fig. 1). The MGS rocks
166 examined in this study have been shown to be ‘pristine’, or free from impactor contamination
167 (Gros et al., 1976; Warren et al., 1980; Day et al., 2010), despite the evidence for cataclasis in
168 some MGS rocks (15445, as well as 77215 and 74215). The degree of shock and cataclasis of
169 samples is not correlated with either Zn concentration or isotopic composition.

170
171 We analyzed a portion of the 3.91 ± 0.01 Ga mafic impact melt breccia lithology of Sayh al
172 Uhaymir (SaU) 169 (Gnos et al., 2004). The ejection origin of this meteorite on the Moon has been
173 attributed to the Procellarum KREEP terrane based on enrichments of Th (32.7 ppm), U (8.6 ppm)
174 and K2O (0.54 wt.%) within it, and it is the most KREEP-rich lunar sample examined to date (Gnos
175 et al., 2004). Our analysis of Zn (0.47 ppm) and δ66Zn (1.30 ± 0.04‰) are essentially identical to
176 values obtained for the lunar mare basalt meteorites.

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178
4. Discussion

179

4.1 *Evaporation, condensation, mixing and contamination at the lunar surface*

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Lunar materials from the Apollo missions and as meteorites have been shown to form from the
same silicate reservoirs based on stable isotopes of refractory major rock-forming elements (e.g.,
O, Mg, Si, Fe; Spicuzza et al., 2007; Liu et al., 2010; Armytage et al., 2012; Sossi & Moynier,
2017; Sedaghatpour & Jacobsen, 2019). For moderately volatile elements like zinc, the isotopic
compositions of lunar crustal samples reflect effects of evaporation, condensation, mixing and
contamination at the lunar surface (Moynier et al., 2006; Herzog et al., 2009; Paniello et al., 2012a;
Kato et al., 2015; Day et al., 2017a), as well as the Zn-bearing phases present in samples. For the MGS rocks and FAN 15415, evaporation of isotopically light zinc from their parental melts that exceeded those for mare basalt parental melts is required to explain both the low Zn contents and high $\delta^{66}\text{Zn}$ (>2.5‰) of these samples. This process differs from that which formed the Zn-rich (>10 ppm) regolith and regolith breccias, where impact gardening and spallation of isotopically light Zn occurred over >100 Ma in the lunar surface environment (Moynier et al. 2006; Herzog et al., 2009) (Fig. 2). Prior work has shown that evaporation processes during a magma ocean operating throughout much/all of the Moon offers the most likely explanation for variable Zn (and other volatile element) losses in magmatically-derived rocks (e.g., Day & Moynier, 2014; Boyce et al., 2015; Dhaliwal et al., 2018).

Modelling the effect of zinc isotope evaporation is possible through empirically derived fractionation factors that are closer to unity (>0.999) than theoretical values (Day et al., 2017b; Wimpenny et al., 2019). Acknowledging that there was no single mantle source for MGS rocks, we take the approach of modelling evaporation processes in them as a unified process, with an initial starting composition of 30 ppm Zn and $\delta^{66}\text{Zn}$ = 1.25‰. The $\delta^{66}\text{Zn}$ was chosen to reflect a source similar to the mare basalt source (e.g., Paniello et al., 2012a). The high content of Zn in the source of MGS rocks is due to the progressive Zn enrichment that counters loss through evaporation and that was likely to have occurred in a magma ocean (Dhaliwal et al., 2018). The bulk partition coefficient for Zn under terrestrial conditions is typically approximated as close to unity ($D_{\text{mantle-melt}}\sim 1$), with limited fractionation of Zn isotopes during fractional crystallization (e.g., Chen et al., 2013; Doucet et al., 2016), indicating that MGS rocks should derive from a mantle reservoir that is more Zn-rich than for the mare basalts. The low Zn measured in mare
basalts (1-2 ppm) are consistent with previous estimates of a bulk Moon Zn content (~2 ppm; O'Neill, 1991). The choice of Zn concentration in the source is difficult to estimate without knowing the lunar bulk partition coefficient, but any concentration between ~10 and 30 ppm for the MGS parental melts leads to broadly similar outcomes to the models shown in **Figure 2**. However, higher estimated concentrations in the source (>30 ppm) or lower $\delta^{66}\text{Zn}$ (e.g., 0.3‰ as in chondrites or Earth) require lower fractionation factors and a poorer fit to the data.

The variable $\delta^{66}\text{Zn}$ of MGS rocks implies inhomogeneous evaporative loss of zinc amongst these samples, either reflecting different extents of evaporation, varying efficiency of evaporative fractionation, or heterogeneous initial Zn in their parental sources. These results are consistent with more than one mantle source of MGS samples, and for the possibility of mixing between early LMO olivine, plagioclase and KREEP reservoirs (e.g., Shearer et al., 2015). The variability in Zn isotopic compositions in the MGS rocks and in FAN 15415 are most consistent with inheritance of these signatures from variable evaporative loss during the late stages of a magma ocean, and after formation of the mare basalt source reservoirs, where higher $\delta^{66}\text{Zn}$ values are expected provided that the resultant evaporated Zn condensates could be effectively lost (Dhaliwal et al., 2018). The CAN clast of 15455 (9.3‰), cataclastic dunite 72415 (6.3‰) and 15415 (4.2‰) would represent products from the most extremely fractionated and latest crystallized melts in this model.

Some of the isotopically light condensate Zn from lunar degassing remains on the lunar surface today. Analysis of the 'Rusty Rock' 66095 has illustrated how magmatic outgassing, during
231 impact-related or volcanic events on the Moon, led to significant condensation of isotopically light
232 vapors into the impact melt breccia (Day et al., 2017a; 2019). Similarly, volatile condensation onto
233 the lunar surface can explain how late-stage differentiates in the Moon evolved to higher δ<sup>66</sup>Zn
234 than mare basalt sources. Modelled condensate compositions match δ<sup>66</sup>Zn values measured for the
235 Zn-rich ‘Rusty Rock’ 66095 (Fig. 2). Mixing of evaporated reservoirs and condensates on the
236 lunar surface also matches trends observed in δ<sup>66</sup>Zn and Zn content for the FAN, further
237 reinforcing the concept that volatile loss during the magma ocean was facilitated by the formation
238 of the lunar crust. Stabilization and thickening of the cooling lunar crust would inevitably have led
239 to the formation of a sink for condensate volatiles, separating them from the magma ocean, and
240 enabled rapid depletion of volatile contents in the hot molten rocks, without the requirement for
241 other volatile-loss mechanisms.

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## 4.2 Zinc isotope composition of mare basalt sources

244 With five exceptions (10017, 12005, 12018, 14053, 15016), mare basalts have a range of δ<sup>66</sup>Zn
245 values from 0.8 to 1.9‰ (Fig. 3). As originally suggested by Paniello et al. (2012a) and quantified
246 in the presented models, mixing and contamination from isotopically light Zn on the lunar surface
247 explains the isotopically light basalts reported previously. Mixing and contamination of between
248 10 and 50% condensate Zn matching that measured in 66095 provides a good approximation for
249 the Zn compositions of these anomalous mare basalts. These results are supported by leaching
250 experiments on 14053 which show that the silicate fraction of the sample was identical to the
251 majority of mare basalts at ~1.4‰ (Day et al., 2017a). Contamination of lunar mare basalts could
252 have occurred through two mechanisms. First, Zn could have been assimilated during ascent of
253 magmas and eruption on the lunar surface, where mare basalts would be highly sensitive to
254 assimilation of volatile phases due to low magmatic Zn contents (<2 ppm) versus high contents in
255 condensates (e.g., 300 ppm in 66095). Second, Zn addition could have occurred through partial
256 condensation directly onto samples during or after they had crystallized. This latter process is
257 possibly analogous to the pyroclastic glass beads 15426, 74001 and 74220, that experienced
258 directed condensation onto the bead surfaces, during or after their formation. The surface areas of
259 the beads, combined with evidence for Zn phases on the exteriors of the beads (Ma & Liu, 2019)
260 and high Zn abundances correlated to these outer surfaces (e.g. Herzog et al. 2009), implies that
261 the beads were effective ‘traps’ of condensate volatile species.

262

263 It has been suggested that volatile loss of Cl occurred during eruption and crystallization of
264 lava flows on the lunar surface (Sharp et al., 2010). To examine whether Zn isotopes were
265 fractionated during eruption, we compare mare basalt δ<sup>66</sup>Zn values with petrological proxies of
266 outgassing and lava-flow differentiation. We compare texture, presence or absence of vesicles and
267 vugs, and bulk-rock MgO content. The presence of vesicles in basaltic rocks indicates outgassing
268 of gas species, while coarser-grained lava samples might be associated with prolonged outgassing
269 processes at the lunar surface, or relate to suites of samples from the Apollo landing sites
270 originating from the same lava flow. Magnesium oxide (MgO) is an indicator of differentiation, as
271 this compound is compatible in early crystallizing silicate mineral phases, including olivine,
272 orthopyroxene and clinopyroxene. Relationships of MgO with Zn and δ<sup>66</sup>Zn, therefore, might be
273 expected to relate to outgassing prior to, during and after eruption during lava flow differentiation
274 processes in Apollo mare basalt suites (e.g., Rhodes & Hubbard, 1973; Neal & Taylor, 1992;
275 Schnare et al., 2008). While some mare basalt samples, specifically some of the mare basalt
276 meteorites, have quantitative textural data associated with them (Day & Taylor, 2007), textural
277 descriptions in the literature for Apollo mare basalts are often qualitative, being described as fine,
278 medium or coarse grained. Mare basalt samples also have a range of textures from porphyritic
279 (e.g., 15058, 15499), to poikilitic (e.g., 70135), ophitic or aphanitic. Acknowledging current
280 petrological complexities, we defined samples where average grain sizes are reported as <0.5 mm
281 as fine-grained, samples with grain sizes of >2 mm as coarse grained, and intermediate samples as
282 medium-grained (Table S1).

283
284 Comparison of δ<sup>66</sup>Zn with presence or absence of vesicles shows that they are not correlated
285 (Fig. 4). Mare basalts with the most negative δ<sup>66</sup>Zn (<-1‰) are medium to fine-grained, but
286 otherwise there is no relationship of grain-size with mare basalt δ<sup>66</sup>Zn values. Nor do the Zn
287 isotope compositions track changes with magmatic differentiation. Low-Ti mare basalts have
288 uniformly low Zn abundances (<2 ppm) and there is no correlation between MgO and δ<sup>66</sup>Zn (Fig.
289 5). High-Ti basalts show a wide range of δ<sup>66</sup>Zn and Zn content for a limited range of MgO, likely
290 due to higher compatibility of Zn within spinel and oxide phases, but otherwise no relationships
291 between δ<sup>66</sup>Zn and MgO exist. While these lines of evidence do not rule out minor degassing of
292 Zn during lava flow emplacement processes, or for degassing of other volatiles, such as H or Cl,
293 they demonstrate that this is not the dominant process for δ<sup>66</sup>Zn variations in mare basalts. Instead,
294 these comparisons provide evidence that the average δ<sup>66</sup>Zn of mare basalts reflect that of their
295 mantle source reservoirs within the Moon, as suggested previously (Paniello et al., 2012a; Kato et
296 al., 2015). We confirm this with new mare basalt meteorite data that suggest a homogeneous mare
297 basalt mantle source reservoir for zinc. Removing the five samples with negative δ<sup>66</sup>Zn from the
compilation, mare basalts as a whole are characterized by a δ<sup>66</sup>Zn value of 1.40 ± 0.08‰ (Table 2). There are variations in δ<sup>66</sup>Zn between high-Ti mare basalts (1.50 ±0.15‰) and low-Ti mare sources (1.37 ±0.15‰), but any significant differences that may occur are outside current levels of analytical precision.

## 4.3 The zinc isotope composition of KREEP and relations to H and Cl

The potassium-rare earth element-phosphorous enriched (KREEP) reservoir has been shown to exist dominantly in the nearside Procellarum KREEP Terrane (Jolliff et al., 2000). Numerous Apollo 12 and 14 samples, MGS rocks, and some mare basalt meteorites (e.g., LaPaz mare basalts) have KREEP signatures and similar inter-element ratios of incompatible elements, implying a single KREEP reservoir, sometimes referred to as urKREEP (Warren & Wasson, 1979). This reservoir is postulated to have formed during crystallization of a lunar magma ocean, where elements that are incompatible in silicate minerals (olivine, pyroxene, plagioclase) would be predicted to concentrate in residual melts. Given that many moderately volatile elements and compounds like water are incompatible, this reservoir should be the most volatile-rich reservoir in the Moon. This appears to be the case for Cl, which is estimated to have ~28 times higher abundances relative to mare basalts (Boyce et al., 2018).

To date, no pristine urKREEP sample has been discovered, and all KREEP-rich rocks have Mg-numbers (atomic ratios of Mg/Mg + Fe) too high to have formed as direct late-stage magma ocean products (e.g., Snyder et al., 1992). KREEP-rich rocks with high Mg-numbers can be explained by the incorporation of an urKREEP signature, via mixing or contamination, into partial
melts from early-formed olivine and pyroxene-rich cumulate reservoirs (e.g., Warren & Wasson, 1979; Snyder et al., 1992; Shearer et al., 2015). The SaU 169 impact melt breccia is no exception to this rule; while it is among the most KREEP-rich samples reported to date, being enriched in incompatible trace elements by factors of 1.5 to 1.8 relative to the KREEP composition reported by Warren & Wasson (1979), it is also Mg-rich (Mg-number = 70; Gnos et al., 2004). SaU 169 has similar Zn isotope and abundance systematics to mare basalts (Table 1); this differs from the prediction that KREEP should be heavier for δ⁶⁶Zn based on models of magma ocean processes (Dhaliwal et al., 2018). We suggest two possible reasons for this discrepancy and favor the latter explanation. First, the similarity of δ⁶⁶Zn in KREEP compared with mare basalts could reflect no change in δ⁶⁶Zn during magma ocean processes, contrary to models. Second, the δ⁶⁶Zn of SaU 169 is not reflective of pure urKREEP, but is rather reflective of mixing with mafic components during its petrogenesis.

Boyce et al. (2015) and Barnes et al. (2016) have argued for KREEP mixing into mare basalt sources to explain apatite δ³⁷Cl values, and have made this association from extrapolations with MGS rocks, including 76535 and 78235, which are isotopically heavy with respect to Cl (δ³⁷Cl = >30‰) and that have fractionated La/Lu and La/Sm, consistent with an urKREEP signature. These MGS samples also have high δ⁶⁶Zn (3.1 to 3.5‰), so if KREEP was partially responsible for the variations in moderately volatile isotopic compositions, one might also expect to see a correlation with Zn isotopes in mare basalts and a KREEP signature. Abundance ratios of La and Lu are useful for assessing possible urKREEP mixing with mare basalts, because urKREEP is considered to have high La (230 ppm) and Lu (7.8 ppm), with La/Lu of 29.5, versus KREEP-free mare sources with lower La (1.1 ppm) and Lu (0.26 ppm) and a low La/Lu ratio (4.2) (Boyce et al., 2018). Mare
basalt La/Lu ratios are not correlated with Zn isotopic composition (Table S1), with Apollo 17 basalts with low La/Lu (~4) having broadly similar δ⁶⁶Zn to Apollo 15 basalts with the highest La/Lu (~18). This result indicates that admixtures of KREEP and a mare basalt source are not required to explain δ⁶⁶Zn values in mare basalts. Consequently, this suggests that KREEP addition does not control Zn isotopic variations in mare basalts, or that KREEP has the same δ⁶⁶Zn value to the mare basalt source.

In contrast, evidence supporting a higher δ⁶⁶Zn value for KREEP than that measured in SaU 169 comes from comparisons of Zn with H and Cl isotope compositions. Comparison of Zn with Cl and H is not intuitive as H and Cl abundances tend to be measured within apatite, where they can be preferentially partitioned into the mineral structure (Ca₅[PO₄]₃[Cl,F,OH]), whereas Zn is likely sited within S-rich phases and silicate minerals in lunar rocks. Indeed, analysis of the Rusty Rock, 66095, has demonstrated decoupling of Cl, Cu and Fe from Zn isotope compositions, largely due to condensation effects on the sample (Day et al., 2017a; 2019). In contrast, magmatic phases are likely to trap the H, Cl and Zn compositions within mare basalts and HMS rocks making comparisons between these elements possible. Positively correlated relationships for Cl and Zn have previously been reported for eucrite meteorites (Sarafian et al., 2017; Barrett et al., 2019). This trend has been interpreted to reflect degassing of ZnCl₂ in an H-poor environment in the eucrite parent body (4 Vesta).

We find no relationship between H and Zn isotope ratios (Fig. 6a). However, we note a relationship of increasing δ³⁷Cl with increasing δ⁶⁶Zn from mare basalts to MGS rocks (Fig. 6b).
The Zn-Cl isotope lunar trend is not well-developed and is steeper than the eucrite data trend. For a given Cl isotope composition, Zn isotopes are less fractionated in lunar samples than in eucrite meteorites, implying possible differences in extents and style of degassing. Furthermore, for mare basalts, there is a restricted range in Zn (δ⁶⁶Zn = 1.4‰), as well as consistently heavy Cl isotope compositions (δ³⁷Cl = 8 to 15‰). KREEP impact melt breccia SaU 169 lies within this range (δ⁶⁶Zn = 1.3‰, δ³⁷Cl = 9‰). This composition likely reflects the dominant role of a mafic component in the sample, consistent with the high MgO (12.2 wt.%) and bulk composition of the impact melt breccia itself (Gnos et al., 2004).

The most extremely heavy δ⁶⁶Zn from the Moon is for the CAN clast in 15455 (9.3‰), versus for basaltic eucrite DaG 945 (13.5‰) that is the most extreme equivalent from Vesta. These comparisons suggest that greater loss of Zn occurred on Vesta compared with the Moon, which would confirm previous measurements of K/U and Rb/Sr ratios, and with planetary body size (Day & Moynier, 2014). Estimates for maximum chlorine loss via degassing are similar for the Moon and Vesta (Fig. 6), which makes this isotope proxy of moderately volatile element loss distinct from those of δ⁶⁶Zn, K/U or Rb/Sr. These results suggest contrasting degassing processes for Cl and Zn, with Zn loss on Vesta possibly occurring as ZnCl₂ (e.g., Barrett et al., 2019). The lunar urKREEP composition is an important composition to measure for Zn isotopes to examine volatile loss models for Moon and Vesta. Based on KREEPy impact breccia SaU 169, the KREEP reservoir would have δD, δ³⁷Cl and δ⁶⁶Zn within the range of mare basalts, but these values are significantly lower in δD and δ³⁷Cl than for the classic KREEP basalt 15382/6. Magma ocean models predict urKREEP to have δ⁶⁶Zn higher than mare basalt sources (Dhaliwal et al., 2018). To test if KREEP
does have an elevated Zn isotope composition, and if degassing processes on Vesta and Moon were similar, measurement of KREEP basalt 15382/6 will be critical given the high δ³⁷Cl (22 ±8‰; Barnes et al., 2016) of apatite grains within the sample.

## 4.4 Early loss of Zn and Cl through crust-breaching impacts or before lunar formation?

The significant difference between δ⁶⁶Zn values of MGS rocks and mare basalts provide unequivocal evidence that Zn was lost, and its isotopes fractionated, during evaporative fractionation processes on the Moon. The most likely cause of Zn loss between the formation of the mare basalt sources and the crystallization of MGS rocks was during magma ocean differentiation, supporting models of progressive loss of volatiles from the lunar interior during magma ocean degassing (Dhaliwal et al. 2018), and the elevated δ⁶⁶Zn and δ³⁵Cl values in MGS rocks. On the other hand, the homogenous δ⁶⁶Zn values of mare basalt sources suggest a large-scale process for their generation, either through magma ocean processes (Kato et al., 2015; Dhaliwal et al., 2018), or prior to or during the giant impact (Paniello et al., 2012a). The bulk silicate Earth (BSE) value for δ⁶⁶Zn is between 0.16 and 0.3‰ (Chen et al., 2013; Wang et al., 2017; Sossi et al., 2018), and its Zn content is estimated at 55 ppm (McDonough & Sun, 1995), whereas the δ⁶⁶Zn and Zn content for Cl chondrites are ~0.3‰ and 310 ppm (Moynier et al., 2017).
As mentioned previously, mare basalt sources have δ⁶⁶Zn of 1.40 ± 0.08‰, and Zn contents as low as 1-2 ppm. At face value, the BSE is depleted by more than 80% of Zn compared with chondrites, assuming it formed from similar sources, without significantly fractionating Zn isotopes. Part of the BSE Zn loss can be explained by core formation as under conditions relevant to Earth's core differentiation Zn is slightly siderophile and the bulk Earth may have ~ 114 ppm
409 (Mahan et al. 2017). Conversely, the reservoir of mare basalts has lost significantly more Zn (99% 410 relative to chondrites, 96% relative to Earth), and has experienced evaporative fractionation of Zn 411 isotopes, suggesting that the lunar Zn depletion is due to planetary processes.

412
413 Understanding whether the difference in Zn abundances and isotopic compositions between 414 the Moon and Earth occurred through volatile-loss of material forming the Moon in a giant impact 415 (Fig. 7), or whether it was wholly lost after this event during magma ocean degassing and other 416 processes on the Moon, is important for models to explain the formation of the Moon, including 417 volatile loss during a lunar ‘synestia’ (Lock et al., 2018), from a lunar disk (Nakajima & Stevenson, 418 2018), or from the Moon itself (Dhaliwal et al. 2018). Dhaliwal et al. (2018) showed that during 419 magma ocean processes, volatile loss could occur throughout the cooling of the magma ocean, but 420 that the volatile condensates required a loss-mechanism. The ‘problem’ of how to separate 421 isotopically light condensate Zn from the melt residue during magma ocean processes on the Moon 422 can be at least partially resolved by condensation onto the cooling lunar surface (Day et al., 2017a), 423 or to the Earth when the early forming Moon was at the Roche limit (Sossi et al. 2018). 424 Condensation of volatiles on the forming crust acts to effectively remove the isotopically light Zn 425 condensate and would indicate that Zn isotope fractionation might have been initially slow during 426 the early stages of lunar differentiation when early-formed crust as only semi-permanent, 427 foundering into the lunar interior. Later, as the crust became stable and permanent, rapid evolution 428 and complexity in Zn isotope compositions of crustal rocks could have occurred.

429
Another likely mechanism for volatile loss might include impacts to the Moon during lunar magma ocean formation and after dissipation of a nascent atmosphere surrounding the Moon (Fig. 7), as has been suggested previously (Barnes et al., 2016). Retention of impactor mass has been shown to be quite low, of the order of 20 to 30%, and large impactors striking the Moon were likely to have been numerous, with as many as 300 impact basins of initial diameters of 300 km or more up to ~4.1 Ga (Zhu et al., 2019). These features imply that erosive-loss of a nascent atmosphere on the Moon was likely and may have occurred as many as 300 times or more between the formation of the Moon and ~4.1 Ga. However, the likelihood of erosive loss of isotopically light condensates from the Moon is limited for two reasons. First is the effectiveness and quantity of volatile species that can be retained as a steam gas atmosphere above a molten magma ocean on the Moon, prior to erosive impacts. Second, is the requirement that loss of Zn by impact erosion must have affected the Moon across the entire formation of mare basalt sources, from low-Ti to high-Ti compositions, during a magma ocean (Fig. 7). To fully investigate this problem, combined modeling efforts to estimate gas contents in a lunar steam atmosphere and to examine if impact-erosion can successfully remove this atmosphere are needed.

Alternatively, the homogeneity of mare basalt δ<sup>66</sup>Zn values might suggest a global signature of volatile element depletion inherited from the giant impact. Various studies have proposed loss of moderately volatile elements (K, Rb, Ga, Sn: Wang & Jacobson, 2016; Pringle & Moynier, 2017; Kato & Moynier, 2017; Wang et al., 2019) prior to the formation of the Moon, but few of them have been able to provide robust resolution of when volatile loss occurred, mainly due to the limited number of samples and associated lunar reservoirs analyzed. Arguments have typically relied on differences in the isotopic compositions of moderately volatile elements between lunar
and terrestrial samples. However, this approach cannot necessarily discriminate between volatile element loss prior to the formation of the Moon, from nebular volatile loss processes, during giant impact, or during or after the formation of the Moon.

With a large set of Zn isotope data it is possible to distinguish processes acting on volatile elements *after* the formation of the Moon. The heavy $\delta^{66}\text{Zn} (>2.5\text{‰})$ of pristine MGS and FAN samples, and the light isotopic compositions of some mare basalts and FAN ($<0\text{‰}$) can be explained by magmatic degassing and condensation of isotopically light Zn, respectively. If the Moon did experience moderately volatile element loss *prior to*, or *during* its formation, then the Zn isotope signature of that event would be recorded by the mare basalts at $1.4\text{‰}$. In this scenario, significant moderately volatile element loss and Zn isotope fractionation occurred to materials forming the Moon, leading to a 1.1 to 1.2‰ difference between terrestrial $\delta^{66}\text{Zn}$ (~0.16-0.3‰; Chen et al., 2013; Wang et al., 2017; Moynier et al. 2017; Sossi et al., 2018) and the composition of the Moon. Without data for urKREEP to examine the maximum extents of magma ocean degassing, and without collision models to examine if volatile elements could be effectively stripped from the Moon during its formation, it is impossible to say with confidence if this signature was inherited *prior to*, *during* or *after* the formation of the Moon. Notwithstanding, if mare basalts do record the Zn isotope signature during the formation of the Moon, then it would suggest that the Moon also inherited an isotopically heavy Cl ($\delta^{37}\text{Cl} = 8$ to $15\text{‰}$) signature from this event.

## 5. Conclusions and implications for volatile loss during moon and planet formation
New data are presented for mare basalt meteorites to show that they, and Apollo mare basalts, have remarkably consistent δ⁶⁶Zn values (+1.4 ±0.2‰) and low Zn (1.5 ±0.4 ppm) inherited from their mantle sources. New analyses of magnesian-suite rocks show them to be characterized by even heavier δ⁶⁶Zn values (2.5 to 9.3‰) and low Zn concentrations. Zinc isotope analysis of KREEP impact melt sample Sayh al Uhaymir 169 shows that it has a nearly identical composition to mare basalts (δ⁶⁶Zn = 1.3‰, 0.5 ppm [Zn]). We explain these variations through progressive depletion of Zn and preferential loss of the light isotopes in response to evaporative fractionation processes during lunar magmatic differentiation. Variations in δ⁶⁶Zn in samples to isotopically light values can be explained by condensation of isotopically light Zn on the lunar surface and mixing and contamination processes with magmas interacting with these surface reservoirs. The δ⁶⁶Zn of KREEP defined by Sayh al Uhaymir 169 is likely to be compromised by mixing processes with a mafic component. Correlations of Zn with Cl isotopes suggests that the KREEP reservoir should also be isotopically heavy, like the magnesian-suite rocks, and analysis of 15382/15386 will be essential to test this hypothesis.

Current models to explain how and when Zn and other volatile elements were lost from the Moon include nebular processes, prior to the Moon's formation, and planetary processes, either during the formation of the Moon in a giant impact, or during magmatic differentiation. Our results provide unambiguous evidence for the latter process and imply that nebular processes are not required to explain the excessive depletion in volatile elements in the Moon relative to Earth. On the other hand, with the currently available volatile stable isotope datasets, it is currently difficult to say with certainty whether the Moon lost its volatiles relative to Earth either during giant impact or from later magmatic differentiation. If the Moon did begin initially volatile-depleted, then the
mare basalt δ⁶⁶Zn value likely preserves that signature, in which case the Moon lost 96% of its

zinc inventory relative to Earth at that time and was also characterized by isotopically heavy Cl

(δ³⁷/³⁵Cl = ≥8‰).

501

## 502 Acknowledgements

Constructive and helpful reviews by J. Barnes and an anonymous are gratefully acknowledged.

We thank the lunar subcommittee of NASA CAPTEM, and the Meteorite Working Group for

provision of Apollo and Antarctic meteorites, respectively. 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. SaU

169 was recovered during a joint Omani-Swiss search campaign supported by the Public Authority

for Mining, Sultanate of Oman, and the Swiss National Science Foundation. This work was

supported by the NASA Emerging Worlds program (NNX15AL74G) and an IPGP Visiting

Professor position to JD. FM acknowledges funding from the European Research Council under

the H2020 framework program/ERC grant agreement #637503 (Pristine), as well as financial

support of the UnivEarthS Labex program at Sorbonne Paris Cité (ANR-10-LABX-0023 and

ANR-11-IDEX-0005-02), and the ANR through a chaire d'excellence Sorbonne Paris Cité. EK

acknowledges funding from the European Union's Horizon 2020 research and innovation

programme under the Marie Skłodowska-Curie Grant Agreement No 786081.

518

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693

Figures and Figure Captions

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<figure><img src="image_3.png" /></figure>

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**Figure 1** – Histogram of zinc isotope compositions for lunar igneous samples. Data are from this study (mare basalt meteorites and magnesian suite rocks), Moynier et al. (2006), Paniello et al. (2012a) and Kato et al. (2015).
<figure><img src="image_4.png" /><figcaption>**Figure 2** – Zinc isotope composition versus content for lunar crustal samples. Orange bar represents the estimated range in δ<sup>66</sup>Zn for the mare basalt source, from Paniello et al. (2012a). Rayleigh distillation models of evaporation assume 30 ppm as the initial source composition, with fractionation factors (α) from Day et al. (2017b), to more extreme values. Condensation models represent sources with between 10-30 ppm initial Zn using the same fractionation factors. Mixing and contamination line in increments of 10% models the effect of adding a Zn composition observed in the ‘Rusty Rock’ 66095 with a pristine mare basalt composition and is after Day et al. (2017a). Large symbols represent new data from the study, with published data from Herzog et al. (2009), Kato et al., (2015) and Day et al. (2017a). Error bars are smaller than symbols.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 3** - Zinc isotope composition versus content for mare basalts and pyroclastic glasses. Solid line and bar represent the estimated range in δ<sup>66</sup>Zn for the mare basalt source, from **Table 2**. Condensation and mixing/contamination models are the same as those calculated from Day et al. (2017a) and Figure 2. Data are from this study (Low-Ti mare basalt meteorites and SaU 169), Paniello et al. (2012a) and Kato et al. (2015). Error bars are smaller than symbols.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 4** – Relationships of δ<sup>66</sup>Zn for (a) vesiculated or vuggy versus non-vesicular mare basalts and (b) versus mare basalt grain size. Data are given in **Table S1**.</figcaption></figure>

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<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Figure 5** – Zinc isotope compositions (a) and abundances (b) for lunar mare basalts as a function of MgO content, and Zn isotopes versus La/Lu (c). Data for Zn are from this study, Paniello et al. (2012a), Kato et al. (2015), and for MgO content and La/Lu are from the lunar compendium and reported in **Table S1**. Error bars are smaller than symbols.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 6** – Plots of (a) hydrogen and (b) chlorine versus zinc isotope composition for lunar samples and eucrite meteorites. Gray bar represents the hydrogen and chlorine isotope composition of KREEP basalt 15386; Zn isotope data has not been obtained for this sample. Zinc isotope data sources are the same as those for Figure 2, with lunar hydrogen and chlorine isotope data from the compilation in McCubbin et al. (2015), and H, Cl and Zn isotope data for eucrites from Paniello et al. (2012b), Sarafian et al. (2017) and Barrett et al. (2019) and reported in **Table S2**. Error bars are smaller than symbols, unless they are shown.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 7** – Scenarios to explain the δ⁶⁶Zn and δ³⁷Cl of mare basalt mantle sources. In (a) the giant impact that created the Earth and Moon leads to volatile loss from the Moon, possibly to Earth or to space, resulting in a dichotomy in moderately volatile element abundances and isotopic compositions (c.f., Lock et al., 2018). In (b) impact erosion from glancing impacts to the Moon (cf. Zhu et al., 2019) lead to loss of impactor mass and of volatile elements. Multiple impacts result in progressive depletion in the volatile elements and preferential enrichment in the heavier isotopes of zinc (inset schematic).</figcaption></figure>
<table><caption>Table 1: Zinc isotopes and abundances in mare basalt meteorites and Mg-suite rocks</caption><thead><tr><th>Sample</th><th>Type</th><th>Zn ppm</th><th>δ<sup>66</sup>Zn</th><th>2SD</th><th>δ<sup>66</sup>Zn</th><th>2SD</th><th>δ<sup>66</sup>Zn</th><th>2SD</th></tr></thead><tbody><tr><td rowspan="3">78235, 156</td><td rowspan="3">Mg-Suite</td><td>Shocked Norite</td><td></td><td>3.40</td><td></td><td>5.30</td><td></td><td>6.90</td><td></td></tr><tr><td></td><td>3.61</td><td></td><td>5.66</td><td></td><td>7.3</td><td></td></tr><tr><td><strong>Average</strong></td><td><strong>0.42</strong></td><td><strong>3.51</strong></td><td><strong>0.30</strong></td><td><strong>5.48</strong></td><td><strong>0.51</strong></td><td><strong>7.10</strong></td><td><strong>0.57</strong></td></tr><tr><td rowspan="3">15455, 391</td><td rowspan="3">Mg-Suite</td><td>Norite CAN clast</td><td></td><td>9.23</td><td></td><td>14.27</td><td></td><td>18.74</td><td></td></tr><tr><td></td><td>9.31</td><td></td><td>14.52</td><td></td><td>18.89</td><td></td></tr><tr><td><strong>Average</strong></td><td><strong>0.67</strong></td><td><strong>9.27</strong></td><td><strong>0.11</strong></td><td><strong>14.40</strong></td><td><strong>0.35</strong></td><td><strong>18.82</strong></td><td><strong>0.21</strong></td></tr><tr><td rowspan="3">15445, 333</td><td rowspan="3">Mg-Suite</td><td>Norite (Clast B)</td><td></td><td>2.25</td><td></td><td>3.92</td><td></td><td>5.09</td><td></td></tr><tr><td></td><td>2.66</td><td></td><td>4.46</td><td></td><td>5.7</td><td></td></tr><tr><td><strong>Average</strong></td><td><strong>0.83</strong></td><td><strong>2.46</strong></td><td><strong>0.58</strong></td><td><strong>4.19</strong></td><td><strong>0.76</strong></td><td><strong>5.40</strong></td><td><strong>0.86</strong></td></tr><tr><td>76535, 186</td><td>Mg-Suite</td><td>Troctolite</td><td><strong>1.36</strong></td><td><strong>3.14</strong></td><td></td><td><strong>5.03</strong></td><td></td><td><strong>6.58</strong></td><td></td></tr><tr><td rowspan="3">SAU 169</td><td rowspan="3">KREEP</td><td>Impact Melt</td><td></td><td>1.31</td><td></td><td>2.05</td><td></td><td>2.62</td><td></td></tr><tr><td>Breccia</td><td></td><td>1.28</td><td></td><td>1.91</td><td></td><td>2.52</td><td></td></tr><tr><td><strong>Average</strong></td><td><strong>0.47</strong></td><td><strong>1.30</strong></td><td><strong>0.04</strong></td><td><strong>1.98</strong></td><td><strong>0.20</strong></td><td><strong>2.57</strong></td><td><strong>0.14</strong></td></tr><tr><td rowspan="3">LAP 02205</td><td rowspan="3">Low-Ti Mare</td><td>Meteorite</td><td></td><td>1.28</td><td></td><td>1.76</td><td></td><td>2.56</td><td></td></tr><tr><td></td><td>1.26</td><td></td><td>1.98</td><td></td><td>2.56</td><td></td></tr><tr><td></td><td>1.27</td><td></td><td>1.86</td><td></td><td>2.55</td><td></td></tr><tr><td></td><td></td><td><strong>Average</strong></td><td><strong>0.58</strong></td><td><strong>1.27</strong></td><td><strong>0.02</strong></td><td><strong>1.87</strong></td><td><strong>0.22</strong></td><td><strong>2.56</strong></td><td><strong>0.01</strong></td></tr><tr><td rowspan="3">NWA 8632</td><td rowspan="3">Low-Ti Mare</td><td>Meteorite</td><td></td><td>1.16</td><td></td><td>1.65</td><td></td><td>2.39</td><td></td></tr><tr><td></td><td>1.12</td><td></td><td>1.69</td><td></td><td>2.23</td><td></td></tr><tr><td><strong>Average</strong></td><td><strong>0.61</strong></td><td><strong>1.14</strong></td><td><strong>0.06</strong></td><td><strong>1.67</strong></td><td><strong>0.06</strong></td><td><strong>2.31</strong></td><td><strong>0.23</strong></td></tr><tr><td rowspan="3">NWA 479</td><td rowspan="3">Low-Ti Mare</td><td>Meteorite</td><td></td><td>1.33</td><td></td><td>1.94</td><td></td><td>2.59</td><td></td></tr><tr><td></td><td>1.24</td><td></td><td>1.88</td><td></td><td>2.45</td><td></td></tr><tr><td></td><td>1.27</td><td></td><td>1.94</td><td></td><td>2.47</td><td></td></tr><tr><td></td><td></td><td><strong>Average</strong></td><td><strong>0.58</strong></td><td><strong>1.28</strong></td><td><strong>0.09</strong></td><td><strong>1.92</strong></td><td><strong>0.07</strong></td><td><strong>2.50</strong></td><td><strong>0.15</strong></td></tr><tr><td rowspan="3">NWA 4734</td><td rowspan="3">Low-Ti Mare</td><td>Meteorite</td><td></td><td>1.23</td><td></td><td>2.11</td><td></td><td>2.5</td><td></td></tr><tr><td></td><td>1.28</td><td></td><td>2.28</td><td></td><td>2.51</td><td></td></tr><tr><td><strong>Average</strong></td><td><strong>0.59</strong></td><td><strong>1.26</strong></td><td><strong>0.07</strong></td><td><strong>2.20</strong></td><td><strong>0.24</strong></td><td><strong>2.51</strong></td><td><strong>0.01</strong></td></tr></tbody></table>
<table><caption>Table 2: Zinc reservoir compositions in and on the Moon</caption><thead><tr><th></th><th>Zn ppm</th><th>2SE</th><th>δ<sup>66</sup>Zn</th><th>2SE</th><th>n</th></tr></thead><tbody><tr><td>Low-Ti Apollo Source(s)</td><td>1.1</td><td>0.2</td><td>1.37</td><td>0.12</td><td>14</td></tr><tr><td>Low-Ti Mare Meteorites</td><td>0.59</td><td>0.01</td><td>1.23</td><td>0.05</td><td>5</td></tr><tr><td>High-Ti Apollo Source(s)</td><td>2.4</td><td>0.9</td><td>1.50</td><td>0.15</td><td>12</td></tr><tr><td><i>Mare Sources</i></td><td></td><td></td><td>1.40</td><td>0.20</td><td></td></tr><tr><td>KREEP</td><td>0.47</td><td>-</td><td>>1.3</td><td>-</td><td>1</td></tr><tr><td>Pristine FAN</td><td>0.60</td><td>-</td><td>>4</td><td>-</td><td>1</td></tr><tr><td>Mg-Suite</td><td>0.67</td><td>-</td><td>2.4 to 9.3</td><td>-</td><td>1</td></tr><tr><td>Condensate</td><td>300</td><td>100</td><td>-13.7</td><td>0.3</td><td>3</td></tr><tr><td>Impact Gardened Material</td><td>27</td><td>5</td><td>5.1</td><td>0.9</td><td>7</td></tr></tbody></table>