A Melt Inclusion Study on Volatile Abundances in the Lunar Mantle

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Geochimica et Cosmochimica Acta 249 (2019) 17–41 www.elsevier.com/locate/gca

A melt inclusion study on volatile abundances in the lunar mantle

Peng Ni (倪鹏) a,⇑, Youxue Zhang (张有学) a, Sha Chen (陈沙) a, Joel Gagnon b

a Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109-1005, USA b Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada

Received 22 December 2017; accepted in revised form 23 December 2018; Available online 08 January 2019

Abstract

Earth’s Moon was thought to be highly depleted in volatiles due to its formation by a giant impact. Over the last decade, however, evidence has been found in apatites, lunar volcanic glass beads, nominally anhydrous minerals and olivine-hosted melt inclusions, to support a relatively ‘‘wet” Moon. In particular, based on H2O/Ce, F/Nd, and S/Dy ratios, recent melt inclusion (MI) work estimated volatile (H2O, F, and S) abundances in lunar rocks to be similar to or slightly lower than the terrestrial depleted mantle. Uncertainties still occur, however, in whether the limited numbers of lunar samples studied are representative of the primitive lunar mantle, and whether the high H2O/Ce ratio for pyroclastic sample 74220 is due to local heterogeneity. In this paper, we report major element, trace element, volatile, and transition metal data in MIs for 5 mare basalt samples (10020, 12040, 15016, 15647 and 74235) and a pyroclastic deposit (74220). With our new lunar MI data, H2O/Ce ratios are still found to vary significantly among different lunar samples, from 50 for 74220, to 9 for 10020, 3 for 74235, 1.7 to 0.9 for 12008, 15016, and 15647, and 0.5 for 12040. H2O/Ce ratios for these samples show positive correlation with their cooling rates, indicating a possible effect of post-eruptive loss of H on their H2O/ Ce variations. It is evident that most other lab and lunar processes, including loss of H2O during homogenization, mantle partial melting, magma evolution, and ingassing during or post eruption are unlikely the causes of high H2O/Ce variations among different lunar samples. By comparing ratios of F/Nd, S/Dy, Zn/Fe, Pb/Ce, Cs/Rb, Rb/Ba, Cl/K, Na/Sr, Ga/Lu, K/Ba, and Li/Yb between 74220 and other lunar samples, the possibility of 74220 originating from a volatile-enriched heterogeneity in the lunar mantle can also be excluded. With all the above considerations, we think that the H2O/Ce ratio for 74220 best represents the pre-degassing lunar basaltic melt and primitive lunar mantle, either because it was formed by a rapid eruption process, or it was sourced from a deeper part of the lunar mantle that experienced less degassing H2O loss during lunar magma ocean crystallization. With an H2O/Ce ratio of 50, the primitive lunar mantle is estimated to contain 84 ppm H2O. Comparing volatile abundances in melt inclusions, glassy embayments, and glass beads in 74220 yields the following volatility trend for volcanic eruptions on the lunar surface: H2O Cl Zn Cu F>S Ga Cs > Rb Pb > Na > K Li. Using the melt inclusion data obtained thus far, the volatile depletion trend for the Moon from a MI perspective is estimated. Our results show that most of the volatile elements in the lunar mantle are depleted relative to the bulk silicate Earth by a factor of 2 to 20, however, a good correlation between half condensation temperature and the volatile depletion trend is not observed. The relatively flat pattern for the lunar volatile depletion trend requires a lunar formation model that can reconcile the abundances of these volatiles in the lunar mantle. Ó 2018 Elsevier Ltd. All rights reserved.

Keywords: Moon; Melt inclusions; Volatiles; Water; Lunar mantle

⇑ Corresponding author at: Geophysical Laboratory, Carnegie Institute of Washington, Washington, DC 20015, USA. E-mail address: [email protected] (P. Ni). https://doi.org/10.1016/j.gca.2018.12.034 0016-7037/Ó 2018 Elsevier Ltd. All rights reserved. 18 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41

1. INTRODUCTION (Hauri, 2013). However, it turned out that the Giant Impact Hypothesis is resilient and flexible enough to Early geochemical data and other considerations led to accommodate the new discoveries. For example, one sug- the hypothesis that the Moon was formed by a giant impact gested solution is that right after the Giant Impact, there between the proto-Earth and a large (probably Martian was a gas disk enveloping the newly formed proto-Moon, size) planetary body (e.g. Hartmann and Davis, 1975; and dissolution of H species (mostly OH) from the gas disk Cameron and Ward, 1976). In the past ten years, magmatic into the lunar magma ocean is enough to establish lunar water has been detected and reported in variable types of H2O abundance (Pahlevan et al., 2016; Sharp, 2017). lunar samples, including volcanic glass beads (e.g. Saal Another suggested solution is that the upper parts of the et al., 2008, 2013; Chen et al., 2015), apatites (e.g. Boyce Moon-forming disk are dominated by an atmosphere of et al., 2010, 2014; McCubbin et al., 2010a,b; Greenwood heavy atoms or molecules, leading to inefficient diffusion- et al., 2011; Barnes et al., 2014; Tartese et al., 2014), limited H loss, allowing the Moon to retain H2O anorthosites (Hui et al., 2013; Hui et al., 2017), and (Nakajima and Stevenson, 2018). A third solution is olivine-hosted melt inclusions (hereafter referred to as asteroidal/cometary bombardment during the lunar magma MIs, e.g. Hauri et al., 2011, 2015; Chen et al., 2015; Ni ocean stage that replenished H2O in the lunar mantle et al., 2017a). Among these types of samples, lunar melt (Hauri et al., 2015, 2017; Barnes et al., 2016). These devel- inclusions are able to provide direct evidence of high H2O opments highlight the importance of establishing the abun- concentrations in the pre-eruptive magma (up to dances of not only H2O but also other volatiles in the Moon 1410 ppm, Hauri et al. 2011). In addition, based on H2O/ in setting stringent constraints on the origin of the Moon. Ce, F/Nd, and S/Dy ratios in lunar MIs, Chen et al. In this paper, we extend previous lunar melt inclusion (2015) estimated the primitive lunar mantle to contain studies to a broader collection of lunar mare basalt and 110 ppm H2O, 5.3 ppm F, and 70 ppm S, similar to or pyroclastic deposit samples (10020, 12040, 15016, 15647, slightly lower than the terrestrial depleted mantle. Despite 74220, and 74235) to better understand volatile abundances the powerful role of lunar MIs in supporting a relatively in the lunar mantle. One main purpose is to address the ‘‘wet” Moon, uncertainties still exist in lunar MI studies primitive H2O/Ce ratio for the Moon. Furthermore, in that complicate the interpretation of volatile abundances addition to volatile elements (H2O, F, Cl, S) that are typi- in the lunar mantle. In particular, Chen et al. (2015) found cally analyzed in lunar melt inclusions, effort was also made that H2O/Ce ratios are much higher for natural MIs from to precisely measure moderately volatile elements (Li, Na, 74220 than homogenized MIs from other lunar samples. K, Cu, Zn, Ga, Rb, Cs, and Pb) for a systematic compar- To rule out the possible complexity of diffusive H2O loss ison between 74220 and other lunar samples, as well as a from MIs during homogenization in the lab, Ni et al. broader understanding of the volatile depletion trend for (2017a) conducted a systematic study and concluded that the Moon, to provide further constraint on the origin of such an effect is minimal if the homogenized MIs are larger the Moon. than 45 mm in diameter. After excluding the effect of diffusive H2O loss during homogenization, the large variations 2. SAMPLE PREPARATION AND METHODS between 74220 and other lunar samples still exist. On the other hand, 74220 is currently the only lunar sample with 2.1. Lunar samples studied high H2O/Ce ratios. This sample, however, has been known as a ‘‘volatile-rich” pyroclastic deposit since its return by Olivine-hosted melt inclusions in three low-Ti basalts the Apollo 17 mission (e.g. Meyer et al., 1975; Butler and (12040, 15016 and 15647), two high-Ti basalts (10020 and Meyer, 1976; Moynier et al., 2006; Paniello et al., 2012). 74235), and one pyroclastic deposit (74220) investigated Therefore, some studies (Albarede et al., 2013; Albarede in this work are briefly described below (Lunar Sample et al., 2015) argued that 74220 might be a local anomaly Compendium unless otherwise noted). A summary of their originated from a volatile-enriched part of the lunar mantle, bulk compositions is also shown in Table 1. More detailed which should not be used to represent the bulk Moon. To background information about these samples can be found resolve the controversy, it is necessary to investigate melt from the Lunar Sample Compendium: https://www-cura- inclusions from a larger collection of lunar samples, and tor.jsc.nasa.gov/lunar/lsc/index.cfm. to verify whether 74220 is sourced from a local heterogene- 10020,49: A fine-grained (with an average grain size of ity in the lunar mantle. 200 mm) low-K, high-Ti ilmenite basalt containing olivine The discovery of primitive H2O in the Moon is having a phenocrysts with a composition of Fo77-59 with partially significant impact on the Giant Impact Hypothesis for its glassy MIs (Chen et al., 2015). origin. Before 2013, the high H2O concentration was found 12040,199: A slowly cooled low-Ti olivine basalt with an in mare basalts younger than 3.9 Ga, which permits a average grain size of 1 mm and olivine compositions rang- ‘‘bone-dry” Moon at the time of formation from the Giant ing from Fo63 to Fo45 and averaging Fo58. Evidence of Impact because there was sufficient time for H2Otobe accumulation of olivine has been reported (e.g. Walter gradually added after the Giant Impact (Hauri, 2013; et al., 1971; Walker et al., 1976; Bombardieri et al., 2005), Tartese and Anand, 2013). At the time Hui et al. (2013) which indicates that the whole rock composition of published results that the lunar magma ocean contained 12,040 might not be representative of its parental liquid. hundreds of ppm H2O, it was thought to be difficult to 15016,47: A highly vesicular (50% vesicles in volume) maintain the Giant Impact Hypothesis for Moon formation and medium-grained olivine-normative basalt returned Table 1 Whole rock data, major elements, volatile elements, non-volatile trace elements, and transition metal data for all the melt inclusions and glass beads analyzed in this study. Moderately volatile element concentrations by laser-ablation ICP-MS are reported separately in Table 2.

Lunar 10020 12040 15016 15421 15647 74220 sample # Type Whole Natural Whole Homogenized Whole Homogenized Glass Whole Homogenized Whole Natural Embayment Glass rock* rock* rock* bead rock* rock* bead Homo T N/A 1210 1200 1180 1140 1160 1160 1270 1300 1287 1300 1280 1280 N/A (°C)

Homo t N/A 10 8 10 10 10 10 2 10 2 10 10 2 N/A (min) Sample # NMI5 OL13 OL15a OL16b OL36b OL41 OL43 OL8 OL10 OL12a OL19 GL7 OL3 OL6 OL11 OL8bEmb OldOL2Emb GL5 Host ol 69 51 54 55 44 57 47 65 70 57 65 60 44 81 80 80 Fo# Host ol 402 324 596 515 274 153 553 228 517 223 253 368 443 250 330 394 dia(mm) MI dia 23 34 70 22 28 31 35 25 29 31 49 45 40 62 .N ta./Gohmc tCsohmc ca29(09 74 19 17–41 (2019) 249 Acta Cosmochimica et Geochimica / al. et Ni P. (mm)

SiO2 (wt 38.57 54.57 41.68 44.88 41.57 48.03 46.28 46.49 45.43 43.56 48.01 48.3 45.12 44.26 45.79 44.4 46.83 42.12 38.57 38.3 39.13 38.83 39.23 %)

TiO2 10.69 3.6 2.66 4 6.17 2.79 3.08 3.68 4.21 2.09 2.13 1.94 2.13 2.04 0.4 2.44 1.76 3.24 8.81 10.29 11.73 9.77 8.78 Al2O3 9.23 13.46 7.45 7.93 7.9 9.26 8.42 10.6 9.4 8.07 9.77 9.12 9.2 8.15 7.15 9 7.74 6.71 6.32 8.09 7.57 6.14 5.50 FeO 19.34 5.55 23.05 25.71 26.04 22.47 25.34 20.33 24.28 23.29 17.22 19.04 23.64 24.5 20.37 22.36 24.26 32.82 22.04 22.75 22.79 23.18 22.56 MnO 0.27 0.09 0.27 0.25 0.27 0.27 0.27 0.23 0.25 0.28 0.21 0.24 0.25 0.34 0.27 0.28 0.26 0.35 0.3 0.31 0.31 0.3 0.28 MgO 7.89 1.68 16.40 6.4 6.81 4.71 4.34 5.94 5.28 11.76 7.87 8.6 6.01 8.1 16.8 10.04 8.52 4.53 14.44 6.68 6.04 10.97 14.72 CaO 11.25 19.23 8.03 8.95 8.74 10.91 9.42 11.1 9.3 9.33 11.31 10.41 10.04 9.05 8.17 9.65 8.96 8.16 7.68 9.97 10.22 8.07 7.05

Na2O 0.39 0.56 0.21 0.26 0.32 0.33 0.36 0.46 0.39 0.25 0.26 0.24 0.27 0.35 0.11 0.22 0.23 0.24 0.36 0.38 0.42 0.31 0.31 K2O 0.06 0.09 0.04 0.06 0.18 0.05 0.09 0.1 0.14 0.04 0.04 0.03 0.05 0.04 0.01 0.04 0.05 0.08 0.09 0.07 0.08 0.06 H2O – 434 – 13.4 14.2 7.1 14.4 12.2 14.0 – 12.0 12.0 15.1 30.1 – – 11 15.4 – 994 20 – (ppm) F – 108.0 – 385 210 149 501 578 331 – 63.2 48.0 57.3 41.4 – – 27.7 82.9 – 45.1 8.3 – P – 709 – 539 951 347 419 498 291 – 311 286 397 281 – – 227 382 – 122.8 119.9 – † S – 495 – 867 2788 960 1113 1269 926 – 907 834 905 833 – – 745 1507 – 602 375 – Cl – 10.98 – 3.83 10.75 4.29 5.18 7.38 3.61 – 3.44 3.19 3.5 3.17 – – 2.44 5.57 – 3.44 0.39 – à Li – 10.2 – 8.6 4.7 – 5.1 4.8 4.5 – – 2.7 4.2 27.5 – – 4.0 6.2 – 11.5 10.1 – Na – 4472 – 2301 2676 – 3065 3484 3079 – – 2092 2031 2773 – – 1763 1996 – 3051 2877 – K – 843 – 377 1194 – 635 718 1037 – – 209 288 211 – – 319 580 – 529 568 – Sr – 175 – 96 188 – 128 145 116 – – 93 94 92 – – 85 105 – 221 261 224 – Y – 150.2 – 171.1 122.0 – 501.0 196.5 197.4 – – 29.3 28.8 32.0 – – 24.5 43.6 – 54.4 68.7 58.3 – Zr – 329 – 125 376 – 161 183 132 – – 101 116 106 – – 88 183 – 209 267 227 – Nb – 21.4 – 7.9 22.4 – 9.9 11.7 7.8 – – 7.2 7.0 6.4 – – 5.5 10.9 – 14.7 20.3 16.8 – Ba – 124 – 61 192 – 85 95 62 – – 42 58 50 – – 42 80 – 66 89 79 – La – 15.1 – 5.7 16.4 – 9.4 9.5 6.4 – – 5.3 6.2 6.3 – – 4.5 9.8 – 7.1 9.4 7.3 – Ce – 46.3 – 17.2 46.8 – 28.6 27.3 18.6 – – 15.7 19.3 17.4 – – 12.0 25.7 – 21.3 28.0 23.0 – Pr – 7.4 – 2.7 7.3 – 5.5 4.4 3.0 – – 2.0 2.2 2.8 – – 1.8 3.8 – 3.28 4.74 3.95 – Nd – 40.9 – 15.2 36.3 – 40.1 24.4 18.1 – – 12.2 12.0 13.3 – – 8.5 19.7 – 21 29 23 – Sm – 15.6 – 6.7 11.1 – 27.3 10.9 8.9 – – 4.1 2.7 3.7 – – 2.4 6.5 – 6.6 10.2 8.3 – Eu – 3.2 – 1.2 1.6 – 1.9 1.2 2.0 – – 0.7 3.2 0.7 – – 0.4 0.6 – 2.1 3.6 3.0 – Gd – 23.6 – 13.7 16.6 – 53.9 18.2 15.6 – – 4.8 6.3 6.9 – – 3.8 7.6 – 9.2 11.9 7.9 – Tb – 3.3 – 3.4 2.9 – 12.9 4.5 3.6 – – 0.9 1.0 0.8 – – 0.7 1.4 – 1.3 2.3 1.7 – Dy – 29.9 – 28.4 21.2 – 95.6 33.8 30.3 – – 6.4 6.3 5.7 – – 4.9 8.4 – 12.5 14.4 11.7 – Ho – 6.2 – 5.9 4.5 – 20.0 7.7 7.4 – – 1.2 0.9 1.5 – – 0.87 1.6 – 2.0 3.3 2.1 – Er – 20.2 – 19.8 13.5 – 52.0 20.9 21.6 – – 3.4 3.4 2.9 – – 2.3 4.8 – 5.4 7.6 6.2 – Tm – 2.7 – 2.4 1.6 – 5.8 2.6 3.5 – – 0.4 0.23 0.5 – – 0.31 0.60 – 0.8 1.2 0.8 – Yb – 16.6 – 17.0 11.0 – 36.8 15.7 21.8 – – 2.8 3.4 2.3 – – 1.8 3.7 – 5.5 6.9 5.0 – Lu – 2.8 – 2.3 1.8 – 4.8 2.4 3.0 – – 0.6 0.7 0.2 – – 0.43 0.44 – 0.7 1.0 1.0 – V – – – – 70.4 – 30 63 18.9 – – 141.6 133.5 115.8 – – 150.7 111 – – – – – Cr – – – – 2918 – 1735 2516 1136 – – 8835 6525 6439 – – 8941 4308 – – – – – Mn – – – – 3301 – 2129 2161 2222 – – 2860 2823 3833 – – 3123 3891 – – – – – Co – – – – 73 – 22 21 43 – – 28 46.5 58.4 – – 67 129 – – – – – Ni – – – – 21 – 2.1 1.1 2.2 – – – 15.2 56 – – 25.4 18 – – – – – Cu – – – – 52 – 16 15 24 – – 14.0 26.6 4.3 – – 28.5 72 – – – – – 0P ie l eciiae omciiaAt 4 21)17–41 (2019) 249 Acta Cosmochimica et Geochimica / al. et Ni P. 20

Lunar sample # 74235

Type Whole rock* Homogenized Natural Interstitial glass

Homo T (°C) 1285 1285 N/A N/A N/A N/A N/A N/A N/A N/A N/A

Homo t (min) 2 2 N/A N/A N/A N/A N/A N/A N/A N/A N/A Sample # OL2 OL4 NMI1a NMI1b NMI1b’ NMI3b NMI4 EMB1 EMB3 NMI5 NMI6 NMI7 NMI11 Matrix1a Matrix1b Matrix2@1 Matrix2@3 Matrix2@4 Host ol Fo# 72 72 72 72 72 72 75 72 72 71 71 69 71 Host ol dia(mm) 237 240 192 192 192 278 541 147 133 185 130 105 165 MI dia (mm) 51 45 21 36 36 14 58 45 37 14 12 35 24

SiO2 (wt%) 38.62 39.12 38.04 50.36 43.21 – 53.15 50.33 49.46 51.69 51.17 52.39 50.97 52.18 56.56 56.56 – – – TiO2 12.17 11.77 12.45 3.23 12.13 – 3.07 6.15 3.94 3.6 5.77 3.2 4.34 3.998 2.34 2.34 – – – Al2O3 8.61 8.45 8.18 13.46 11.17 – 14.15 12.89 12.58 12.94 13.55 13.95 12.59 13.51 16.15 16.15 – – – FeO 19.32 20.16 22.16 10.1 15.5 – 8.38 9.57 11.14 10.33 9 8.12 10.71 9.59 12.37 12.37 – – – MnO 0.28 0.27 0.31 0.216 – 0.14 0.14 – 0.17 0.15 0.14 0.19 0.19 0.18 0.18 – – – MgO 8.35 7.24 6.53 2.39 2.74 – 2.27 3.08 3.17 2.72 2.08 2.39 2.29 2.31 1.78 1.78 – – – CaO 10.7 10.21 9.95 17.4 13.4 – 17.22 17.12 17.21 16.72 17.37 17.98 17.03 16.73 9.8 9.8 – – –

Na2O 0.4 0.46 0.32 0.56 0.51 – 0.65 0.63 0.55 0.56 0.6 0.58 0.57 0.63 1.13 1.13 – – – K2O 0.07 0.07 0.06 0.05 0.1 – 0.11 0.1 0.11 0.09 0.09 0.1 0.09 0.11 0.186 0.186 – – – H2O (ppm) – 21.2 53.5 72.9 76.1 75.8 57.9 57.2 57 91 58.6 63.9 88.4 68 108 125 88 115 116 F – 46.4 48.9 65.1 64.0 68.1 54.8 51.0 53.9 54.8 62.0 57.9 75.7 88 100.1 138.1 91 108 109.1 P – 172 199 365 319 347 299 317 354 298 329 299 356 317 347 361 320 373 375 † S – 1526 1643 1490 952 754 763 1145 1566 1338 1037 810 1187 675 – 1138 ––– Cl – 3.70 – 3.8 3.64 3.81 3.23 3.52 3.66 4.8 4.15 3.69 4.06 3.38 5.5 6.3 5.2 6.8 7.28 à Li – 62.6 7.7 6.3 5.3 – – 6.1 7.0 8.8 – – – 11.8 11.6 12.7 11.5 11.3 10.2 Na – 4154 3667 3714 3488 – – 5363 5083 5012 – – – 5255 8008 7873 5156 5547 4760 K – 554 614 636 570 – – 885 869 981 – – – 812 1173 1137 785 897 735 Sr – 189 177 188 201 – – 245 237 214 – – – 241 371 320 312 288 285 Y – 99.2 102 111 128 – – 144.6 141 110 – – – 153 134.7 140.6 105.9 104.2 115.8 Zr – 306 254 270 321 – – 374 368 382 – – – 384 405 506 296 293 299 Nb – 28.0 28.1 29.5 24.9 – – 30.4 28.0 61 – – – 30.3 42.1 57.7 33.0 35.7 34.7 Ba – 89 65 91 95 – – 111 110 104 – – – 108 179 149 121 121 118 La – 6.1 6.5 7.4 9.0 – – 10.0 11.0 7.7 – – – 11.2 14.7 12.2 12.3 11.0 11.6 Ce – 22.2 25.9 27.6 29.8 – – 32.8 35.2 28 – – – 35.9 44.9 39.0 38.7 35.6 39.3 Pr – 4.0 4.4 4.8 5.8 – – 6.3 5.7 4.5 – – – 6.9 7.3 6.8 6.5 5.9 7.1 Nd – 24.3 23.6 29 34 – – 37.6 35 28 – – – 38 42.9 41.1 35.5 34.3 39 Sm – 9.6 8.4 11 13 – – 14.8 15 12 – – – 17 14.5 14.6 13.5 11.7 13.6 Eu – 0.9 3.0 2 0.8 – – 2.3 4 3 – – – 5 1.9 3.5 2.9 2.8 2.5 Gd – 14.9 12.0 16 17 – – 18.7 19 19 – – – 21 19.4 19.6 15.5 13.9 16.4 Tb – 2.5 2.7 3.4 4.0 – – 4.0 3.7 3.6 – – – 4.1 3.6 3.9 2.8 2.7 3.3 Dy – 19.6 19.6 23.8 26.0 – – 26.8 25.4 24 – – – 31.9 25.3 26.8 19.7 20.0 21.8 Ho – 4.1 3.9 4.9 5.7 – – 6.1 4.7 4.8 – – – 6.2 5.0 6.0 3.7 4.3 4.3 Er – 11.9 10.8 14.8 16.3 – – 15.6 16.6 13.3 – – – 17.0 15.2 16.6 12.1 11.8 12.9 Tm – 1.8 1.8 1.6 2.3 – – 2.1 2.3 2.0 – – – 2.3 2.2 2.17 1.8 1.9 2.0 Yb – 11.6 11.2 14.9 13.8 – – 14.9 14.2 12.0 – – – 17.0 15.4 16.7 12.3 11.9 12.9 Lu – 2.0 1.4 2.3 2.1 – – 2.0 2.6 2.4 – – – 2.7 2.0 2.6 2.0 1.7 2.1 V – 58.5 141 9.5 16.3 – – 25.0 26.6 52 – – – – 56 17.9 – – – Cr – 3298 5169 997 622 – – 1479 1206 2778 – – – – 2133 1596 – – – Mn – 3373 3223 1936 1475 – – 2746 1772 2451 – – – – 2912 3036 – – – Co – 7.6 23 10 23 – – 39 18 17 – – – – 17 27 – – – Ni – 1.1 0.3 0.6 2 – – 2 0.9 0.8 – – – – 0.7 0.8 – – – Cu – 7.9 10.6 4.3 4 – – 2.9 5 4 – – – – 11 7 – – – * Whole rock data are from Rhodes et al. (1974), Rhodes et al. (1976), Ryder and Schuraytz (2001), and Hallis et al. (2014). † S concentrations for natural MIs in 74235 could be affected by sulfide globules in the MIs. Italicized S concentrations are corrected by excluding the cycles affected by sulfides and are less reliable. à Italicized Li concentrations are for homogenized MIs, which have the concern of contamination during heating. Lithium concentrations for one MI in 74235 are evidently affected by contamination and hence marked in red. This MI is close to olivine surface, hence more easily affected by Li contamination and H loss during homogenization. P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 21 from Apollo 15. Olivine crystals identified in this sample concern of olivine reduction in our short homogenization have compositions ranging from Fo69 to Fo58. experiments (2 to 10 min). One small olivine crystal in 15647,22: A relatively coarse-grained olivine basalt from 74235 with visible reduction was discarded. By inserting Apollo 15 containing anhedral olivine crystals that are the crucible slowly, heating rate for the sample was con- <1 mm in diameter and have compositions of Fo62 to Fo45. trolled at <200 °C/min to prevent olivine cracking. The 74220,892: A soil sample containing abundant orange homogenization temperatures (1140 to 1300 °C) were cho- and black glass beads, and individual grains of olivine with sen to be near the liquidus of the mare basalts. The dura- compositions of Fo80. Hauri et al. (2011), Saal et al. tions (2–10 min) were chosen as a compromise to (2013), Chen et al. (2015), and Ni et al. (2017a). In this minimize H2O loss (short duration) and to equilibrate the study, one additional MI, two embayments, and a glass melt inclusions with the host olivine (long duration). After bead from 74220 are analyzed. homogenization, the crucible was quickly removed from the 74235,22: A vitrophyric high-Ti basalt from Apollo 17 furnace and immediately quenched in water. Direct mea- that contains thin blades of ilmenite and phenocrysts of oli- surements show that the crucibles are quenched to below vine within a matrix of glass and feathery minerals. This 200 °C in less than 20 seconds. The olivine grain was recov- sample represents a rapidly quenched volcanic liquid. Com- ered from the graphite crucible, polished to expose the melt positions of olivine crystals in 74235 are within a narrow inclusion interior, and pressed into an indium mount for range of Fo75-70. subsequent analyses. In addition to the lunar samples described above, one green glass bead from 15421 is also studied for comparison. 2.3. Analytical methods

2.2. Sample preparation methods Major element compositions of MIs and olivine crystals were determined using a CAMECA SX-100 electron micro- For 74220 and 15421, no crushing was required and probe at the University of Michigan, with a 10 nA focused glass beads or olivine crystals were directly picked. Each beam at the accelerating voltage of 15 kV. The analytical of the other mare basalt samples was gently crushed in a procedure is similar to that described in Ni et al. (2017a). stainless-steel crusher, and olivine grains were manually Volatile and trace element compositions for the MIs picked from the crushed samples using an optical micro- were determined using a CAMECA IMS 7-f GEO SIMS scope. Among the samples, a small number of MIs identi- at Caltech during four visits. A total of 33 elements was fied in 10020 and 74235 were partially glassy. In addition, analyzed for the MIs in three separate sessions: volatiles interstitial glasses with measurable sizes (>10 mmby (H reported as H2O, F, S, and Cl) and P, transition metals 10 mm) were found in the matrix of 74235. These samples (V, Cr, Mn, Co, Ni, and Cu), and other trace elements (Li, were directly polished to expose the target MI or interstitial Na, K, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, glass, and prepared into indium mounts for electron micro- Dy, Ho, Er, Tm, Yb and Lu). Analyses were conducted fol- probe (EMP), and secondary ion mass spectrometry lowing procedures as in Chen et al. (2015) and Ni et al. (SIMS) analyses. The glass beads from 74220 and 15421 (2017a), which are briefly described below. For any given (250 mm and 480 mm in diameter, respectively) were directly MI, volatiles were analyzed in the first session, followed polished to expose the center of the beads and then pressed by other trace elements, and then the transition metals, with into indium mounts for EMP and laser ablation inductively all three analytical sessions conducted during the same lab- coupled plasma mass spectrometry (LA-ICP-MS) analyses. oratory visit. All melt inclusions found in 12040, 15016, and 15647 are In the first session, 12C, 16O1H, 18O, 19F, 30Si, 31P, 32S, highly crystalline, requiring homogenization experiments to and 35Cl were sequentially analyzed using a 3–5 nA, 15- produce a homogeneous glass phase for EMP and SIMS mm-diameter Cs+ primary ion beam. A set of seven MPI- analyses. Therefore, homogenization experiments were con- DING glass standards (GOR128-G, GOR132-G, KL2-G, ducted on MIs from 12040, 15016, and 15647 in a gas- ML3B-G, StHs6/80-G, T1-G, ATHO-G, Jochum et al., mixing furnace. Crystalline inclusions from 74235 were also 2006) and a mid-ocean ridge basalt glass (MORB) was used homogenized using the same procedure. Each host olivine for calibration of H2O, F, Cl, P, and S. Concentrations of grain containing unexposed MIs was placed in a graphite H2O in the reference glasses are from Chen et al. (2015) crucible, and gradually inserted into the hot spot of the fur- and Ni et al. (2017a). For F, Cl, P, and S, reference values nace. The olivine crystal was protected during heating using for MPI-DING reference glasses from Jochum et al. (2006) a constant flow of 99.9999% pure N2 to avoid being oxi- were used. A synthetic basaltic glass, Et1 (Ni and Zhang, 16 1 dized by O2 or contaminated by H2O in the ambient atmo- 2016) with 12.8 ± 0.6 ppm H2O was used to assess O - 18 sphere. The use of a graphite crucible also ensures a H / O ratios for low H2O samples across different reducing environment (IW-1.9 to IW-2.6, Ni et al., indium blocks and during different analytical sessions. 2017a), slightly more reducing than that for mare basalts The sample blocks were placed in the high vacuum chamber (IW-1, Sato, 1976; Weitz et al., 1997). Such a reducing for one or two days prior to volatile measurements, and condition during homogenization is slightly below the sta- background 16O1H/18O ratios of 1 103 to 3 103 bility line for olivine (IW-0.3 to IW-1.1, Nitsan, 1974). were achieved. To make sure the beam was on the melt Reduction of the olivine, however, was found to be slow, inclusion instead of its host olivine, beam position was care- with the reduced layer being only 10 to 25 mm thick after fully adjusted based on 27Al16O secondary ion images 30 minutes at 1330 °C(Ni et al., 2017a), mitigating the before acquisition of each data point. Each position was 22 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 pre-sputtered for 60–120 s to remove possible surface con- the University of Windsor using a PhotonMachines Ana- tamination before the data acquisition was started. Twenty lyte Excite 193 nm, short-pulse-width (sub 4 ns), Ar-F exci- cycles of data were collected at each point and in each cycle, mer laser ablation system coupled with an Agilent 7900, and every ion species was counted for 1 s. Uncertainties are quadrupole ICP-MS. A laser beam with an energy density 2 approximately 20% (2r) for H2O and Cl, and approxi- of 5.19 J/cm , and a repetition rate of 25 Hz was used for mately 10% (2r) or lower for F, P and S, as indicated by the analyses. Spot size of the laser beam was 40 mm for analyses of standard glasses Et1 and GOR 128 under stable the melt inclusion and glass beads. The ellipsoidal-shaped vacuum and comparable analytical conditions. In addition, embayment, which measured 280 mm along the long axis, two analyses on the same MI (74235 NMI1b) yielded was analyzed using an array of 8 spots, each with a diame- results that are within 10% difference for H2O, F, P, and ter of 25 mm, situated along a linear traverse. Quadrupole Cl (Table 1), supporting the reproducibility of our analysis. dwell time was 10 ms per mass for 7Li, 9Be, 23Na, 60Ni, In the second session of analyses, twenty-two trace ele- 63Cu, 66Zn, 71Ga, 85Rb, 133Cs and 208Pb. Signal intensities ments (Li, Na, K, Sr, Y, Zr, Nb, Ba, La, Ce, Pr, Nd, Sm, for 44Ca and 25Mg were also measured during the analyses, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) were analyzed with 44Ca used as the internal standard to correct for differ- with a 10 to 14 nA O and 15 mm diameter primary ion ences in ablation rate between standard reference materials beam using the energy-filtering technique (c.f. Zinner and and unknowns, and 25Mg being used to monitor when the Crozaz, 1986). The position of trace element analysis for laser beam penetrated the MI and into the host olivine crys- each MI was carefully located on or near the previous spot tal. Additional details about the operating conditions of the of volatile measurements with the help of 27Al+ ion images. LA-ICP-MS can be found in Ni et al. (2017b). A time- In addition to ion species for the target trace elements, 28Si resolved signal for the LA-ICP-MS analysis of the melt was also monitored as the internal standard. Two glass inclusion (74220 OL11) is shown in Supplementary standards (NIST 610 and NIST 612, Pearce et al., 1997) Fig. S2. The laser beam penetrated the host olivine after from the National Institute of Standards and Technology 28 s of analysis on the MI, which is clearly resolved by were used for the trace element analyses. Average 2r ana- the increase in 25Mg and 60Ni counts, and the decrease in lytical errors given by the instrument are less than 10% 44Ca and 23Na counts in the spectra. A series of 5 MPI- for Li, Na, K, Sr, Y, Zr, Ba, and Ce, 10 to 40% for Nb, DING glasses (GOR128-G, GOR132-G, KL2-G, ML3B- La, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, and Yb, approxi- G, T1-G, Jochum et al., 2006) plus one NIST standard mately 50% for Tm and Lu, and approximately 100% for (NIST 614, Gao et al., 2002) were used for external calibra- Eu. Multiple analyses on NIST 612 during different sessions tion to bracket concentrations in the unknowns and correct reproduced concentrations to <10% difference for Li, K, Zr, for matrix effects. Calibration curves for all 10 elements are Nb, Sm, Eu, and Gd, and to <5% for the other trace ele- shown in Supplementary Fig. S3. Average 2r analytical ments analyzed. During one of the three visits, one addi- uncertainties are <10% for Na, Ni, and Ga, <15% for Li, tional NIST standard (NIST 614, Gao et al., 2002) and Cu, and Rb, <30% for Be and Zn, and <50% for Cs and two MPI-DING glass standards (GOR128-G and KL2-G, Pb. For a melt inclusion 74220 OL11, Li and Na were mea- Jochum et al., 2006) were used as secondary standards to sured by both SIMS (Ni et al., 2017a) and LA-ICP-MS, verify our analyses, and the results are within 2r analytical and the results are within 10% relative difference. errors compared to the reference values (Ni et al., 2017a). Six transition metal elements (V, Cr, Mn, Co, Ni, and 3. COMPOSITIONS OF THE MELT INCLUSIONS Cu) were analyzed in the third session using a 12 or AND GLASS BEADS 17 nA O primary ion beam that was 15 mm in diameter. A mass resolution power (MRP) of 6000 V was used in Homogenized melt inclusions in 15016, 12040, 15647, the analysis to separate target ion species from possible and 74235 are usually one single glassy phase (Fig. 1a interferences (e.g. 47Ti16O on 63Cu and 49Ti16O on and b), but may contain a bubble, or a particle of Fe metal, 65Cu). 28Si or 30Si was used as the internal standard, or occasionally a spinel crystal. One natural MI in 10020 and two isotopes for Ni (60Ni and 62Ni) and Cu (63Cu was identified to contain a glassy fraction that is large and 65Cu) were monitored to check for possible mass inter- enough (e.g. >10 mm) for SIMS analysis (Fig. 1c). Natural, ferences. Three MPI-DING glasses (GOR128-G, GOR132- partially glassy MIs found in 74235 are oval or elongated in G and T1-G), which are more similar to our MIs in major shape, and usually contain ilmenite crystals and element composition, were used as the standards for transi- submicrometer-sized sulfide globules. Major elements, vola- tion metal measurements, because strong matrix effects tiles, non-volatile trace elements, and transition metal con- were observed when analyzing Co, Ni and Cu (Supplemen- centrations measured for the homogenized and natural MIs tary Fig. S1). Average 2r analytical errors are 5% or lower are reported in Table 1, with whole rock data from pub- for V, Cr and Mn, approximately 10% for Co and Cu, and lished literature for comparison. approximately 30% for Ni. Repeated analyses on glass standard NIST 612 during different SIMS sessions reproduced 3.1. Major elements all transition metal concentrations to about 5%. Concentrations of Li, Be, Na, Ni, Cu, Zn, Ga, Rb, Cs, In general, major element compositions of the homoge- and Pb for one melt inclusion (74220 OL11), one embay- nized MIs are relatively close to their whole rock composi- ment (74220 OldOL2Emb), and two glass beads (74220 tions, except for 12040, whose whole rock composition has GL5 and 15421 GL7) were analyzed by LA-ICP-MS at been affected by olivine accumulation (e.g. Walter et al., P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 23

Fig. 1. Back-scattered electron images for two homogenized (a, b) and two natural (c, d) MIs in this study. (a) The homogenized MI 12040 OL41 is essentially one single glassy phase; (b) Homogenized MI 15016 OL10 contains a tiny Fe metal particle; (c) Natural partially glassy MI 10020 NMI5 contains a spinel and a plagioclase crystal; (d) Natural partially glassy MIs discovered in 74235 usually contain ilmenite crystals and submicrometer-sized sulﬁde globules.

1971; Walker et al., 1976; Bombardieri et al., 2005). Con- lowing reasons: a) inadequate homogenization tempera- centrations of SiO2 in the homogenized MIs range from ture/duration could result in residual crystal phases in the 39.1 wt% to 48.3 wt%, while MgO concentrations vary from MIs and cause the apparent KD to be higher or lower than 4.3 to 8.6 wt%. On the other hand, major element compo- the equilibrium value, depending on whether the residual sitions for glass in partially glassy MIs in 10020 and phase is more enriched in Mg or Fe; b) overheating can 74235 are significantly more evolved than their parental cause olivine dissolution into the MI, resulting in elevated rocks. For example, glass in a partially glassy MI (10020 KD values; c) post-entrapment loss of Fe through the host OL15) contains 54.6 wt% SiO2 and 1.7 wt% MgO, com- olivine can also result in an elevated apparent KD between pared to 40.8 wt% SiO2 and 7.0 wt% MgO in the whole MI and olivine (Danyushevsky et al., 2000; Bombardieri rock (Rhodes and Blanchard, 1980), indicating a high et al., 2005). As a quality control procedure, only homoge- degree of post-entrapment crystallization. Glass in partially nized MIs within a small range of KD values close to equi- glassy MIs in 74235 contains 43.2–52.4 wt% SiO2 and 2.1– librium (0.25–0.44 for low-Ti basalts, 0.21 to 0.37 for high- 3.2 wt% MgO compared to 38.6 wt% SiO2 and 8.4 wt% Ti basalts) were selected for SIMS analyses. Additional MgO in the whole rock (Rhodes et al., 1976). Post- homogenized MIs with KD values outside of the expected entrapment crystallization of ilmenite has a significant range were only measured by EMP, and the results are impact on TiO2 concentrations for MIs in 74235, resulting reported in Supplementary Table S1 as a reference. in as low as 3.1 wt% TiO2 in the glass of the natural MIs, compared to the whole rock with 12.2 wt% TiO2. Intersti- 3.2. Non-volatile trace elements tial glasses found in the matrix of 74235 are even more evolved in chemical composition, with approximately 56.6 Bulk silicate Earth (BSE) normalized rare earth element wt% SiO2, 1.8 wt% MgO, and 2.3 wt% TiO2. (REE) patterns of all lunar samples in this study are pre- To avoid possible complications caused by homogeniza- sented in the Supplementary Fig. S4. The patterns are rela- tion experiments on the melt inclusion composition, it is tively flat or bow-shaped, which differ from terrestrial important to check whether homogenized MIs reached basalts but are similar to other lunar mare basalts equilibrium with their host olivines. One way to evaluate (Shearer et al., 2006). Fig. S4 also shows that various lunar whether equilibrium is reached between an MI and its host samples studied here have more divergence in heavy REE olivine is to calculate the apparent KD values (KD = [Fe/ than in light REE, which is very different from terrestrial Mg]OL/[Fe/Mg]MI). According to Xirouchakis et al. basalts. Modeling of the lunar REE patterns (e.g. Nyquist (2001), the KD between olivine and silicate melt at equilib- et al., 1977, 1979) indicated that mare-basalts are derived rium is 0.33 for low-Ti basalts, and 0.28 for high-Ti from a fractionated, non-chondritic source, and are most basalts. Apparent KD values for the homogenized MIs likely mafic cumulates from the lunar magma ocean can depart from the equilibrium values for at least the fol- (Shearer et al., 2006). Therefore, the more divergent HREE 24 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 concentrations among different lunar basalts could be a fea- lunar magma ocean (Taylor et al., 2006). For the two ture inherited from their lunar mantle sources formed dur- high-Ti basalt samples (10020 and 74235), positive anoma- ing lunar magma ocean crystallization. Close examination lies for Ti and Nb are observed, which are attributed to of Sm/Dy and La/Yb ratios shows that melt inclusions in ilmenite addition during the formation of high-Ti basalts the same lunar sample can vary by a factor of 4 in these (Schearer et al., 2006; Chen et al., 2015). Natural partially REE ratios (Fig. S5). The large variations in Sm/Dy and glassy MIs in 10020 and 74235, however, show lower La/Yb ratios for an individual sample cannot be explained degrees of enrichment in Nb and Ti (sometimes depletion by simply crystal fractionation, as the commonly trivalent for Ti), indicating precipitation of ilmenite during post- REE, with a few exceptions (i.e., Ce4+ and Eu2+), are rela- entrapment crystallization (even though ilmenite cannot tively similar in compatibility. A more likely explanation is be seen in Fig. 1c). For sample 74235, the homogenized the mixing of melts with different degrees of partial melting MI shows essentially the same degrees of enrichment of during lunar basalt formation, similar to the findings by Nb and Ti as the whole rock (Fig. 2d). Shimizu (1998) and Slater et al. (2001) for terrestrial basalts. 3.3. Volatiles (H2O, F, Cl, and S) Spidergrams for all measured melt inclusions in this study are plotted in Fig. 2. In general, trace element pat- Concentrations of H2O, F, Cl, and S for all MIs and terns for MIs in 10020, 15016, 15647, and 74235 are consis- glasses measured in this study are plotted in Fig. 3. The tent with their whole rock compositions. Trace element highest H2O concentration determined in this study is concentrations for MIs in 12040, however, are significantly 434 ppm, which was found in the natural partially glassy elevated compared to the whole rock composition of 12040 MI in 10020. The other group of partially glassy MIs in (Fig. 2b), which might be caused by olivine accumulation in 74235 contained 51–88 ppm H2O. Interestingly, the more the whole rock (e.g. Walter et al., 1971; Walker et al., 1976; evolved interstitial glasses in the matrix of 74235 contain Bombardieri et al., 2005). Most samples studied in this 91 to 138 ppm of H2O, slightly higher than in the partially work show depletion in K, Sr, P, F, and Eu when normal- glassy MIs in 74235. The glasses in the matrix of 74235 also ized to BSE, which is typical for lunar samples. The deple- contain higher concentrations of non-volatile incompatible tion of K and F agrees with the volatile-depleted nature of elements than the MIs. These observations together indi- the Moon (discussed later). One exception is sample 12040, cate retention of H2O in the final stage of magma evolution whose MIs do not show obvious depletion in F compared for 74235. All the other homogenized MIs are relatively to terrestrial samples. Depletion of P and other siderophile dry, with less than 30 ppm H2O, similar to the low H2O elements in the Moon was explained by segregation of a concentrations measured in homogenized MIs in 12008 small Fe-rich core (Newsom, 1986). Depletion of Eu and and 15016 by Chen et al. (2015). The H2O/F ratio in natu- Sr, on the other hand, was explained by plagioclase removal ral 74220 MIs is similar to but other MIs have lower H2O/F from the mantle source of mare basalts (e.g. Taylor et al., ratios than terrestrial MIs and the depleted mantle. 2006). The widespread Eu anomaly in essentially all lunar In terms of other volatiles in the melt inclusions, Cl and mare basalts has been interpreted as support for a global S concentrations are in approximately the same range

Fig. 2. Spidergrams of all measured melt inclusions in this study. Whole rock data from Neal (2001) and Hallis et al. (2014) are plotted in black or blue solid circles. All other ﬁlled symbols are for homogenized MIs, while the open symbols are for natural unheated MIs. Data for interstitial glasses in 74235 are plotted as crosses in (d). BSE composition is from McDonough and Sun (1995). P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 25

Fig. 3. Comparison of H2O, F, Cl and S concentrations in lunar MIs from this study to those in lunar MIs and glass beads from the literature, and to those in terrestrial MIs. The concentrations and ratios in the depleted mantle (DMM, Salters and Stracke, 2004), CI chondrites and bulk silicate earth (CI and BSE, McDonough and Sun, 1995) are also plotted for comparison. Lunar data from the literature are plotted in grey, and terrestrial data are plotted in light grey to be distinguished from data obtained in this study. *74220 MI data are from Hauri et al. (2011), Chen et al. (2015) and Ni et al. (2017a). Data for ‘‘Other MIs” in the literature are from Chen et al. (2015). A compilation of literature lunar MI data is provided in Supplementary Table S2. Data for lunar glass beads are from Hauri et al. (2015). Volatiles data for the terrestrial MIs are from GeoRoc (http://georoc.mpch-mainz.gwdg.de/), except for the MORB MIs, which are from Saal et al. (2002).

(mostly 3 to 11 ppm Cl and 440 to 1500 ppm S) with (Fig. 3b). Fluorine concentrations in MIs in 10020, 15016, published lunar MI data for 74220 and other lunar samples, 15647, and 74235 are in the range of 30 to 140 ppm, and the Cl/S ratios are close to the depleted mantle which is also similar to previously published lunar MI data. 26 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41

Fluorine concentrations in MIs in 12040, however, are 150 12040 are higher than MIs from mid-ocean ridge basalt to 580 ppm, which are signiﬁcantly higher than MIs in (MORB) and similar to MIs in ocean island basalt (OIB), other lunar samples. MIs in 12040 yield elevated F/S ratios suggesting enrichment of F in the parental liquid of 12040. and lowered Cl/F ratios, as can be seen in Fig. 3b and c. Compared to terrestrial MIs, F concentrations for MIs in 3.4. Transition metals

Concentrations of first-row transition metals (V, Cr, Mn, Co, Ni, and Cu) in lunar MIs are plotted in Fig. 4 and compared with their whole rock and MORB compositions. As can be seen in the figure, transition metal concentration patterns for the MIs are often consistent with their associated whole rock compositions. For 12040, however, Ni concentrations in the MIs are lower than in the whole rock by almost two orders of magnitude (Fig. 4a). Such a difference is likely due to both post-entrapment crystallization of olivine from the MIs, and the presence of 25% modal olivine in the whole rock of 12040 (Walker et al., 1976). Concentrations of V, Cr and Mn in lunar whole rocks are close to 1 when normalized to BSE. When compared to MORB and OIB (shaded region in each panel of Fig. 4), there is typically significant Cr enrichment, minor Mn enrichment, and significant Cu depletion in lunar melt inclusions. The minor Mn enrichment is likely related to Fe enrichment in lunar basalts compared to terrestrial basalts, which is reflected in the constant Mn/Fe ratios. The significant Cr enrichment in lunar melt inclusions is likely due to the lower f(O2) on the Moon, so that Cr is less likely removed or buffered by spinel. Assuming the transition metal abundances in the MIs represent the pre-eruptive melt composition, the ubiquitous depletion of Cu in lunar melt inclusions is likely related to the volatile depletion of lunar rocks (discussed in more detail later). Significant Ni depletion and minor V depletion are also occasionally observed in lunar MIs, which is likely due to olivine fractionation. Among the transition metal elements measured in this study, Cu, Co and Ni are chalcophile (e.g. Rajamani and Naldrett, 1978), and their abundances could be affected significantly by sulfides. For the four lunar samples whose transition metal abundances were analyzed, sulfides were only observed in 74235. Sulfide crystallization caused large sulfur concentration variations in MIs in 74235 (Table 1) and resulted in significantly lower Cu concentrations compared to the whole rock (Fig. 4d). The concentrations of Co and Ni seem less affected by sulfide crystallization, with

3 Fig. 4. Transition metal concentrations in lunar MIs measured in this study compared to their concentrations in the whole rock and in terrestrial basaltic glasses. Whole rock data for the lunar samples are from Morrison et al. (1970), Compston et al. (1971), and Neal (2001). The red and blue shadowed regions show 1r variation in MORB and OIB data, respectively. Global MORB glass data are from PetDB (http://www.earthchem.org/petdb) and OIB glass data are from GeoRoc (http://georoc.mpch-mainz.gwdg.de/). Whole rock data in the databases are excluded to avoid the possible eﬀect of olivine accumulation on Ni concentrations. BSE concentrations are from McDonough and Sun (1995). The elements are sequenced so that MORB and OIB trend is smooth from slightly incompatible to highly compatible. P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 27

Table 2 Concentrations (in ppm) of moderately volatile elements for a melt inclusion (74220 OL11), an embayment (74220 OldOL2Emb) and a glass bead in 74220, as well as a green glass bead in 15421. Average concentrations reported in Hauri et al. (2015) for glass beads in 74220 are also included as a comparison. Half condensation temperatures (50% Tc) are from Lodders (2003). Element Zn Pb Cs Rb Na Ga K Cu Li

50%Tc (K) 726 727 799 800 958 968 1001 1037 1142 74220 OL11 8.4 0.23 0.04 0.77 2954 5.8 529 8.0 12.6 74220 OldOL2Emb* 1.6 0.21 n/a 0.57 2518 3.2 n/a 2.4 12.0 74220 GL5 1.5 0.15 0.02 0.47 2146 2.8 n/a 1.17 11.0 74220 (Hauri15) n/a n/a n/a n/a n/a n/a 515 n/a 11.78 GB/MI** 0.22 0.80 0.61 0.75 0.89 0.59 1.19 0.18 1.07 15421 GL7 0.36 0.037 b.d. 0.131 788 1.49 n/a 0.93 3.2 * Average of 8 analyses. ** GB/MI: ratio of concentration in the glass bead (GL5) to that in the melt inclusion (OL11) after a post-entrapment crystallization correction of 22.5%. Potassium concentration for the glass beads is from Hauri et al. (2015).

the complication of olivine crystallization, which could also which is consistent with expectations. From the figure, the deplete Co and Ni concentrations in the residual melt. volatility trend for volcanic eruption on the Moon can be Residual sulfide phases in the source mantle during partial categorized as: melting could control chalcophile element abundances (e.g. H O Cl Zn Cu F > S Ga Cs > Rb Peach et al., 1990). According to recent experiments by 2 Pb > Na > K Liðno lossÞ: Ding et al. (2018), however, mare basalts are under- saturated in sulfide during partial melting, thus eliminating The observed high degree of loss for Zn and Cu in glass the potential for residual sulfides to control the chalcophile beads and embayments compared to MIs suggests caution element signatures for mare basalts. in using literature Zn and Cu data to estimate their concentrations in pre-eruptive basalts. 3.5. Moderately volatile elements The above results are inconsistent with half condensation temperatures (Lodders, 2003), indicating that the Concentrations for moderately volatile elements mea- volatility scale for lunar volcanic eruptions is different from sured in 74220 OL11, 74220 OldOL2Emb and two glass the condensation sequence in a solar nebular. The above beads are summarized in Table 2. The orange and green relative volatility trend also differs from the volatility trend glass beads have similar K, Na, Cu, Zn, Ga, and Li abun- observed in recent experimental work by Norris and Wood dances compared to those reported in Albarede et al., (2017). At one bar and approximately the iron-wustite oxy- (2015) and Hauri et al. (2015). The melt inclusion contains gen buffer, the experimentally derived relative volatility higher abundances of Zn, Pb, Cs, Rb, Na, Ga, and Cu than trend was: Sb > Cd > Ge > Bi > Tl Ag > Sn > Cu the glass beads, indicating an effect of degassing loss on Pb > Zn > In > Cr > Ga (no loss). Both volatile solubility moderately volatile elements in glass beads. and diffusivity in lunar melts at near vacuum conditions seem to be the main control on volatility during basaltic 4. DISCUSSION eruptions. The lack of existing theory and experimental data in quantitatively explaining volatile degassing during 4.1. Volatile loss during volcanic eruptions on the lunar lunar volcanic eruptions indicate that more work is neces- surface sary to improve understanding of volatile behavior at lunar conditions. Loss of volatiles during lunar volcanic eruptions can be Copper and zinc concentrations measured in the melt examined by comparing glassy melt inclusions, glassy inclusion from 74220 are 8 ppm, which is higher than in embayments and glass beads in 74220, which experienced the glass beads (1 ppm), but significantly lower than pre- various degrees of degassing during eruption. Before mak- vious measurements of the bulk composition of glass bead ing the comparison, volatile concentrations in the glassy samples (30 ppm Cu and 300 ppm Zn, Wanke et al., MI and embayments were corrected for their degrees of 1973; Duncan et al., 1974). We infer that the concentrations post-entrapment crystallization (22.5% for 74220 OL11, in the melt inclusion are close to the pre-eruptive concentra- 24.1% for 74220 OL8bEmb and 8.6% for 74220 OldO- tions in this lunar basalt, and the concentrations in the glass L2Emb) following the procedure in Hauri et al. (2011). bead interior from micro-beam measurements demonstrate The corrected concentrations were then used to compare volatile degassing, whereas the high concentrations in bulk volatile abundances in the degassed glass beads and embay- glass bead samples likely reflect vapor condensates on bead ments to the relatively un-degassed melt inclusion from surfaces (Meyer et al., 1975), as supported by their rela- 74220 (Fig. 5). Almost all the volatiles measured in this tively light Cu and Zn isotopic compositions (Moynier study show increasing degrees of depletion from glassy melt et al., 2006). The Zn/Fe ratio in the MI from 74220 inclusions to glassy embayments and glass beads (Fig. 5), (OL11) is 4.7 105, which is about one order of magni- 28 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41

Fig. 5. Volatile element concentrations for glass beads, glassy embayments and a melt inclusion in 74220. Elements (H reported as H2O) on the x-axis are ordered by decreasing half condensation temperatures (Lodders, 2003) from left to right. The filled symbol is for the melt inclusion. Open symbols are for embayments and crosses are for glass beads. Data from the literature are plotted in grey symbols. Concentrations in the glassy embayments and melt inclusion are corrected for post-entrapment crystallization. tude higher than those for the orange glass beads (this study Stracke, 2004; Taylor and Wieczorek, 2014; Albarede and Albarede et al., 2015). The degassing loss of moderately et al., 2015; Chen et al., 2015; Hauri et al., 2015). Because volatile elements (e.g. Cu, Zn, Pb) in the glass beads indi- the lunar mantle sources for mare basalts are unlikely the cates that whole rock measurements of these elements for same as terrestrial mantle peridotite in composition (e.g. lunar basalts could also be affected by degassing loss. Nyquist et al., 1977; 1979), preferred volatile/refractory Therefore, using whole rock data of lunar basalts to discuss ratios for terrestrial studies might not be the best choices lunar mantle composition for moderately volatile elements for the Moon. Therefore, multiple ratios were tested for (e.g., O’Neill, 1991) may need reevaluation. each volatile element with existing lunar MI and basalt data, and the one with the best correlation in lunar data 4.2. Volatile abundances in the primitive lunar mantle was adopted, similar to the approach in Albarede et al. (2015). As a result, we focus on ratios of H2O/Ce, F/Nd, To obtain the volatile depletion trend for the Moon, it is Cl/K, S/Dy, Li/Yb, Na/Sr, K/Ba, Cu/Mn, Zn/Fe, Pb/Ce, necessary to estimate the abundances of multiple volatile Cs/Rb, Rb/Ba, and Ga/Lu in this study. Similar results elements, ideally covering a large range of volatilities, in were obtained using alternate ratios, such as Cl/Ba, Na/ the primitive lunar mantle. For studies on terrestrial Ho, K/La, Li/Dy, and Cu/Fe, which support our estima- basalts, volatile concentrations are often paired with non- tion based on the selected ratios. volatile trace elements that show similar geochemical behaviors during partial melting or crystal fractionation 4.2.1. H2O/Ce ratios to assess volatile abundances in their mantle source (e.g. Ratios of H2O/Ce have been employed in the literature Michael, 1988; Saal et al., 2002; Plank et al., 2009; for studying terrestrial mid-ocean ridge basalts (e.g. Koleszar et al., 2009). In this study, we employ a similar Michael, 1988; Saal et al., 2002), as well as lunar basalts method to estimate the abundances of H2O, F, Cl, S, Li, (Chen et al., 2015; Hauri et al., 2015). The H2O/Ce ratios Na, K, Cu, Zn, Pb, Cs, Rb, and Ga in the primitive lunar are roughly constant for terrestrial submarine MORB mantle. Estimates of the bulk silicate Moon composition MIs with a wide range of MgO concentrations (5–13 wt%, for these elements have been done previously using certain Fig. S6b), supporting the use of H2O/Ce ratios to compen- volatile/refractory ratios for mare basalts and volcanic glass sate for the effect of partial melting and igneous differenti- beads (e.g. O’Neill, 1991; Taylor and Wieczorek, 2014; ation. Note that the submarine samples erupted at Hauri et al., 2015; Albarede et al., 2015). Our estimation sufficiently high pressures and cooled rapidly, which sup- of their abundances, however, is mostly based on melt pressed H2O loss, whereas no such luxury is possible in inclusion data, with whole rock data for mare basalts and studying lunar samples. Nonetheless, lunar MIs also show volcanic glass beads as a reference. Compared to similar that the H2O/Ce ratio in a given sample is identical between work in the literature, our approach helps avoid possible the high-MgO homogenized MIs and the low-MgO glass in concerns related to data for volcanic glass beads and basalt partially glassy MIs: meaning that crystal fractionation whole rock measurements (e.g. degassing loss, surface itself does not change the H2O/Ce ratio (see later discus- contamination). sions on 10020 and 74235). The selection of appropriate volatile/refractory element For the homogenized MIs, there is a concern of diffusive pairs is based on: a) their relative degrees of compatibility H2O loss from the MIs during homogenization, which in the terrestrial mantle (e.g. Sun and McDonough, 1989; would lower the H2O/Ce ratios. In Chen et al., 2015, Zhang, 2014); and b) the use of such ratios in previous stud- homogenized lunar MIs show systematically lower H2O/ ies for terrestrial and lunar basalts (e.g. O’Neill, 1991; Ce compared to the natural MIs from 74220, promoting McDonough and Sun, 1995; Michael, 1995; Salters and such a concern. Ni et al. (2017a) conducted a series of P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 29 experiments on lunar melt inclusions, and concluded that from 4.3 to 6.8 wt%, and H2O/Ce ratio from 0.3 to 0.8 (this diffusive H2O loss during homogenization could be signifi- study). Most of the homogenized MIs have similar H2O cant within minutes, but the effect is small for large MIs concentrations of 11 to 14 ppm, and the variations in under most conditions. For simplification, homogenized H2O/Ce ratio are mostly due to their variable Ce concentra- MIs larger than 45 mm in diameter will be prioritized when tions (Table 1). Similar trends have been reported for ter- discussing H2O/Ce ratios. Below we briefly review all H2O/ restrial MIs from Iceland and Galapagos, possibly Ce data for lunar MIs in the literature and from this study, indicating H2O re-equilibration (Fig. S6, Koleszar et al., which are also plotted in Fig. 6a after excluding homoge- 2009; Neave et al., 2014). As a result, the largest MI with nized MIs smaller than 45 mm in diameter. a diameter of 70 mm turns out to have the lowest H2O/Ce H2O/Ce ratios in lunar MIs have been studied for four ratio of 0.3 because of its high Ce concentration. Therefore, low-Ti mare basalts (12008, 12040, 15016, and 15647), an average H2O/Ce ratio of 0.5 for all homogenized MIs is two high-Ti mare basalts (10020 and 74235), and one used to represent sample 12040. high-Ti pyroclastic deposit (74220) (Chen et al., 2015; For vesicular low-Ti basalt 15016, five homogenized Hauri et al., 2015; Ni et al., 2017a). All MI data from this MIs were investigated (Chen et al., 2015 and this study), study are summarized in Table 1, and a compilation of lit- with SiO2 ranging from 44 to 48 wt%, and MgO from 6.0 erature lunar MI data is provided in Supplementary to 9.4 wt%. H2O/Ce ratio in three MIs ranges from 0.8 to Table S2. 1.7 (this study). For the other two homogenized MIs, For low-Ti basalt 12008, six homogenized MIs have REE concentrations were not measured and hence H2O/ been studied, with SiO2 ranging from 41 to 49 wt%, MgO Ce ratios are unavailable. One large MI with a diameter from 6.8 to 10.7 wt%, and a narrow range of H2O/Ce ratios of 49 mm has a H2O/Ce ratio of 1.7. from 1.5 to 1.8 (Chen et al., 2015). One of the six homoge- For low- to very-low-Ti basalt 15647, two homogenized nized MIs with a diameter of 74 lm has an H2O/Ce ratio of MIs have been examined (this study). The whole rock con- 1.6. tains 10.0 wt% MgO, and the two homogenized MIs con- For low-Ti basalt 12040, six homogenized MIs have tain 8.5 and 4.5 wt% MgO, respectively, indicating been studied, with SiO2 ranging from 41 to 48 wt%, MgO various degrees of crystal fractionation. The H2O/Ce ratio

Fig. 6. Concentrations of volatile versus refractory elements in terrestrial submarine and lunar olivine-hosted melt inclusions. In (a), homogenized MIs data for MIs < 45 mm in diameter from this study and Chen et al. (2015) are excluded to reduce the concern of diffusive H loss during homogenization. Submarine MORB and OIB MI data are from GeoRoc and Michael (1995), Danyushevsky et al. (2000), Saal et al. (2002), and Dixon et al. (2002). Lunar MI and glass data from the literature are marked with stars and plotted in grey. Detailed sources for the lunar literature data can be found in Fig. 3. In the legend, ‘‘HMI” and ‘‘NMI” stand for homogenized and natural melt inclusions, respectively. 30 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 ranges from 0.9 to 0.6 (this study). One large MI (45 mmin This conclusion is also supported by the major element diameter) from this sample has a H2O/Ce ratio of 0.9. composition, because the interstitial glasses contain higher For high-Ti basalt 10020, data for H2O and Ce are avail- SiO2 (57 wt%) and incompatible elements compared to able for three MIs from Chen et al. (2015) and this study: the glassy parts of natural MIs (43 to 52 wt% SiO2) and two for partially glassy natural MIs, and one for a homog- are lower in Mg#. The fact that interstitial glasses from enized MI (with a diameter of 83 mm). As shown in Fig. 6a, 74235 preserved similar H2O/Ce ratios as the MIs is a bit the glassy parts of two natural MIs (on the high end of surprising and might suggest that the matrix of 74235 10020 group) are very similar in composition, with evolved as a closed system with respect to H2O at the final 50 ppm Ce, and 400 ppm H2O. The elevated Ce concen- stages of basalt solidification, when these MIs and intersti- trations in these two MIs from 10020 are consistent with the tial glasses formed. Compared to terrestrial MIs that often highly evolved nature of the melt inclusions, which contain contain weight percent levels of H2O, the low concentra- 55 wt% SiO2. The homogenized MI from 10020 contains tions (100 ppm) of H2O in 74235 are easier to preserve a factor of three lower H2O and Ce abundances (131 ppm in a lava. Even one meter of basalt lava on the Moon would H2O, 16.1 ppm Ce), and a similar H2O/Ce ratio (Fig. 6a). provide sufficient pressure to keep 100 ppm H2O under- The high consistency in H2O/Ce ratios for the three MIs saturated in the magma. Also, relatively rapid cooling (evi- from 10020 indicate that: (a) H2O was not lost from this denced by glass in the MIs and matrix) would help to pre- homogenized MI during heating; (b) the H2O/Ce ratio does vent H2O loss. One homogenized MI has a slightly lower not vary significantly even after 67% crystal fractionation. H2O/Ce ratio, as well as an elevated Li concentration, likely H2O/Ce ratio in the MIs of this sample is 9. because this MI is close to the surface of the olivine grain The pyroclastic deposit sample 74220 contains abundant (Supplementary Fig. S7), allowing effective exchange of orange glass beads originated from fire fountain eruptions H2O and Li between the MI and the environment during (Heiken et al., 1974), as well as olivine fragments bearing homogenization. Despite the large variations in major ele- essentially glassy MIs. Both the glass beads and especially ment compositions (38 to 57 wt% SiO2 and 1.8 to 6.5 wt the glassy MIs have been extensively studied (Saal et al., % MgO) for homogenized MIs, partially glassy MIs, and 2008; Chen et al., 2011; Hauri et al., 2011; Hauri et al., interstitial glasses, their H2O/Ce ratios all fall in a small 2015; Ni et al., 2017a) because glassy lunar MIs are rare. range from 1.6 to 3.3, indicating similar behaviors for The major oxide compositions of these glassy MIs in olivine H2O and Ce during crystal fractionation (Fig. 6a). The are similar to the glass beads or may be related to them H2O/Ce ratio of 1.6–3.3 for 74235 is approximately a factor through crystal fractionation (Hauri et al., 2011; Chen of 20 lower than MIs from 74220, and a factor of 3 lower et al., 2015; Ni et al., 2017a). Saal et al. (2008) reported than MIs from 10020. H2O in glass beads and Hauri et al. (2011) reported H2O Overall, H2O/Ce ratios vary by two orders of magnitude in MIs. Chen et al. (2015), Hauri et al. (2015), and Ni among different lunar samples, from 50 for 74220 to 9 for et al. (2017a) reported H2O/Ce ratios in natural and 10020, 3 for 74235, 1.6 for 12008 and 15016, 0.9 for homogenized MIs. As Ni et al. (2017a) pointed out, H2O 15647, and 0.5 for 12040. The variation in H2O/Ce ratios concentrations in natural MIs in 74220 show positive corre- has been reported previously by Chen et al. (2015), and lation with MI diameter, indicating post-eruptive H2O loss the authors argued that the naturally glassy MIs best main- from the MIs. Therefore, H2O/Ce ratios recorded by the tain pre-eruptive H2O/Ce ratio due to rapid quenching largest natural MIs are more representative of the pre- compared to the homogenized crystalline MIs. On the other eruptive H2O/Ce ratio for 74220. Ni et al. (2017a) found hand, Albarede et al. (2015) argued that the high H2O/Ce that H2O concentrations in MIs in 74220 are systematically ratio in 74220 is a local feature and cannot be used to rep- different between those reported in Hauri et al. (2015) and resent the lunar mantle. With a wider range of sample stud- those reported in Chen et al. (2015) and Ni et al. (2017a), ied, and a larger set of elements measured in this study, the either due to heterogeneities among different subsamples high H2O/Ce ratio for 74220 is interpreted to be a better of 74220, or due to differences in analytical configuration representative for the primitive lunar mantle. A detailed and calibration. For consistency, here we adopt H2O/Ce discussion of the H2O/Ce ratio variation in lunar samples ratios from the work of Chen et al. (2015) and Ni et al. will be provided after going through all the volatile/refrac- (2017a), which reported H2O/Ce ratios between 25.9 and tory ratios. Using a BSE abundance of Ce (1.675 ppm, 55.5 for natural MIs, with an H2O/Ce of 50 for the largest McDonough and Sun, 1995), and a H2O/Ce ratio of 50 two MIs. for 74220, the primitive lunar mantle is estimated to contain For high-Ti basalt 74235, H2O and Ce concentrations in 84 ppm H2O. homogenized MIs, natural partially glassy MIs and interstitial glasses were measured using SIMS (this study). As men- 4.2.2. F/Nd ratios tioned earlier, the interstitial glasses in the matrix of 74235 Ni et al. (2017a) showed that loss of F, Cl or S from MIs contain higher H2O concentrations (88–125 ppm) than the during short-duration homogenization experiments is negli- natural partially glassy MIs (57–91 ppm). By comparing gible. Therefore, homogenized MIs are not distinguished them in the H2O-Ce plot (Fig. 6a), however, it can be seen from natural MIs in the discussion of F/Nd, Cl/K, or S/ that H2O/Ce ratios for the natural MIs and interstitial Dy ratios below. F/Nd ratios have been used to estimate glasses are very similar, and the relatively high H2O concen- F abundances in the lunar mantle by Chen et al. (2015) trations in interstitial glasses are due to their more evolved and Hauri et al. (2015). The same ratio is used here for con- compositions compared to the natural partially glassy MIs. sistency. Ratios of F/Nd for most MIs analyzed in this P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 31 study (Fig. 6b) fall in the range of 1.4 to 4, which is consis- Cl/K ratio, a Cl/K ratio of 0.008 for lunar MIs would tent with results from Chen et al. (2015). MIs from 12040, correspond to a concentration of 0.39 ppm Cl in primitive however, show much higher F/Nd ratios (6–25, 20 for lunar mantle sources (using a K abundance of 49.2 ppm most MIs), about the same as for BSE, DMM, and terres- based on K/La ratios, discussed later). trial OIB MIs (22, Chen et al., 2015). Plotting F versus REEs other than Nd does not remove the large scatter in 4.2.4. S/Dy ratios the data. MIs in 12040 show other compositional peculiar- Concentrations of S are plotted versus Dy for lunar MIs ities, including relatively high P, S and Cl concentrations in in Fig. 6d. Unlike H2O/Ce, F/Nd, and Cl/K in lunar sam- one MI (but P enrichment is not correlated with F enrich- ples, S/Dy ratios can be affected not only by degassing loss ment, indicating that apatite addition is not the explana- of S, but also sulfide segregation in the melt. For green and tion), and relatively high heavy REE concentrations in orange glass beads, the degassing effect is more dominant in another MI. The elevated F/Nd ratios in 12040 are an generating variations in the S/Dy ratios (Fig. 6d). For the enigma and might reflect enriched F in the source lunar natural partially glassy MIs and interstitial glasses, how- mantle of 12040 or secondary enrichment of F during the ever, sulfide saturation may play a more important role in formation of 12040. Using a BSE abundance of 1.25 ppm fractionating S/Dy ratios. For example, the homogenized Nd, a F/Nd ratio of 4 corresponds to a F abundance of MIs from 74235 have S/Dy ratios of 78 to 84, but S/Dy 5 ppm in the primitive lunar mantle based on most lunar ratios in natural MIs from the same sample can be fraction- samples studied. If the relatively high F/Nd ratio of 20 ated to values < 25. The low S/Dy ratio for the natural MI for MIs from 12040 is used, the lunar mantle would contain from 10020 (17) is also likely due to the formation of sul- 26 ppm F, which is very similar to BSE. Sample 12040 is fides, based on its highly evolved composition (Table 1). currently the only mare basalt with a high F abundance Because S/Dy ratios could be lowered due to the above similar to BSE. Because 12040 is thought to have been two factors, the highest S/Dy ratio for each sample is formed by olivine accumulation and might be genetically assumed to be more representative of its parental melt. related to other Apollo 12 basalts (e.g. 12018, 12004, For low-Ti basalts (12008, 12040, 15016 and 15647), the 12021 and 12051; Walter et al., 1971), it is recommended highest S/Dy ratio in each sample ranges from 118 to that future studies examine F/Nd ratios for MIs in these 179. For the high-Ti basalts (10020, 74220 and 74235), samples to verify the high F/Nd ratios measured for the highest S/Dy ratio in each sample ranges from 84 to 12040, and to better understand the formation of Apollo 95 (Fig. 7). This difference between low- and high-Ti mare 12 basalts. Even though there is large variation in F/Nd basalts in terms of S/Dy is about 20% to 50%, which needs ratio in MIs from 12040 and other lunar basalts, the total to be confirmed using more lunar MI data. If this difference variation of about one order of magnitude is still much is accurate, however, it might provide insights on the for- smaller than the two orders of magnitude variation in the mation of low- and high-Ti mare basalts (e.g. the mineral observed H2O/Ce ratio in MIs of lunar basalts. mode of sulfides in their source mantle). Assuming a BSE abundance of 0.67 ppm for Dy (McDonough and Sun, 4.2.3. Cl/K ratios 1995), the estimated S abundance in the source mantle In Fig. 6c, Cl is compared to K for lunar melt inclusions. would be 52 to 64 ppm for the high-Ti basalts, and 79 to Although Cl is commonly compared with K in the litera- 120 ppm for the low-Ti basalts. The estimated S abundance ture, Cl/K ratio in DMM (0.009; Salters and Stracke, for source mantle of high-Ti basalt is about a factor of 2 2004) is lower than in BSE (0.07; McDonough and Sun, lower than DMM (119 ppm, Salters and Stracke, 2004), 1995) by a factor of 8, meaning that this ratio is fraction- but the estimated S abundance for the low-Ti basalt is very ated significantly during magma processes on Earth and Cl similar to DMM. is more incompatible than K in the terrestrial mantle. Ter- restrial MI data are consistent with Cl being more incom- 4.2.5. Li/Yb, Na/Sr, K/Ba, and Cu/Mn ratios patible than K as evidenced by Cl/K ratios in MORB In this study, concentrations of moderately volatile ele- MIs being very close to the DMM value of 0.009, whereas ments Na (50% Tc = 958 K, Lodders, 2003), K (50% the OIB MIs from Samoa, Iceland, and Hawaii show higher Tc = 1001 K, Lodders, 2003), Li (50% Tc = 1135 K, and more variable Cl/K ratios, averaging 0.045 and with Lodders, 2003) and Cu (50% Tc = 1037 K, Lodders, 2003) values greater than 0.07. In the lunar MI data, Cl concen- were also measured by SIMS for most MIs, enabling us trations correlate well with K concentrations, indicating to estimate their abundances in the lunar mantle based on similar degrees of incompatibility during crystal fractiona- lunar MI data. For these elements, Li/Dy, Li/Yb, Li/Lu, tion and partial melting on the Moon. The Cl/K ratios Na/Ti, K/U, K/La, K/Ba, Cu/Sc, and Ti/Cu ratios have for lunar MIs are in a narrower range, between 0.0035 been used in the literature for their abundances in the ter- and 0.015, with an average of 0.009. The average Cl/K restrial or lunar mantle (O’Neill, 1991; McDonough and ratio of 0.009 for lunar MIs is similar to that in the Sun, 1995; Salters and Stracke, 2004; Hauri et al., 2015). DMM and is 8 times lower than the BSE ratio of 0.07 Here we adopt Li/Yb and K/Ba ratios from previous stud- (Fig. 5c). Estimation of the abundance of Cl in the lunar ies for Li and K. For Na, Na/Sr is used because a better mantle, however, is hindered by the fractionated Cl/K correlation between Na and Sr is observed for lunar data ratios between DMM and BSE. Assuming the source man- (Fig. 8a). For Cu, we use Cu/Mn instead of Cu/Sc because tle for mare basalts studied here did not experience signifi- Mn also has similar incompatibility compared to Cu, and cant depletion or enrichment processes that would alter the Sc data are not available in our dataset. In addition, ratios 32 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41

Fig. 7. S/Dy ratios in olivine-hosted melt inclusions plotted versus the TiO2 concentrations in their host rock. Host rock TiO2 concentrations are used because their concentrations in the melt inclusions could have been fractionated by post-entrapment crystallization. S/Dy ratios in each lunar sample can be signiﬁcantly lowered due to degassing or ilmenite fractionation. Data sources for the melt inclusions and glass beads are the same as in Fig. 6. References for host rock concentrations are listed in Table 1.

Fig. 8. Plots of moderately volatile elements versus refractory elements. (a) Na versus Sr; (b) K versus Ba. (c) Li versus Yb; (d) Cu versus Mn. Lunar MI data for Na and K in (a) and (b) are all measured by SIMS. In the legend, ‘‘HMI” stands for homogenized melt inclusions and ‘‘NMI” stands for natural melt inclusions. Terrestrial MORB data are from PetDB (http://www.earthchem.org/petdb). OIB data are from

GeoRoc (http://georoc.mpch-mainz.gwdg.de/), screened to those with 35 to 52 wt% SiO2, and 6 to 20 wt% MgO. Lunar basalt data are from the MoonDB (http://www.moondb.org) and Wiesmann (1975). Terrestrial MI data in (b) are from the same sources as noted in Fig. 6. *74220 MIs, orange and glass beads data are from Hauri et al. (2011), Chen et al. (2015) and Ni et al. (2017a). P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 33 such as Li/Dy, Na/Ho, K/La and Cu/Fe ratios are also used in the literature to estimate their abundances in the used to verify our estimation, and similar results were terrestrial or lunar mantle (Newsom, 1986; O’Neill, 1991; obtained. McDonough and Sun, 1995; Salters and Stracke, 2004; Lithium data for all homogenized MIs and copper data Taylor and Wieczorek, 2014; Albarede et al., 2015; Hauri for natural MIs from 74235 are excluded when estimating et al., 2015). Here we focus on Zn/Fe, Pb/Ce, Cs/Rb, and Li and Cu abundances in the lunar mantle. For Li, we Rb/Ba ratios for Zn, Pb, Cs, and Rb. For Ga, which shows noted that Li concentrations in homogenized MIs could similar degree of incompatibility compared to the heavy be affected by contamination from the crucible during REE (Zhang, 2014), multiple ratios are tested and Ga/Lu homogenization. In olivine, Li diffusivity is higher than H is selected for this study. diffusivity (Chakraborty, 2010), making it possible for Li Individual plots for Zn/Fe, Pb/Ce, Cs/Rb, Rb/Ba, and contamination to happen in relatively short time periods. Ga/Lu ratios of terrestrial basalt, lunar basalt and lunar For Cu, as discussed in Section 3.4, natural MIs from MI data are in Supplementary Fig. S8. Based on our melt 74235 contain visible sulfide grains, which could decrease inclusion measurement of Zn, Pb, Cs, Rb, and Ga concen- Cu concentrations in the residual glass significantly. There- trations for 74220, we get Zn/Fe = 4.7 105, Pb/ fore, these Li and Cu data are excluded to avoid Ce = 0.011, Cs/Rb = 0.052, Rb/Ba = 0.012, and Ga/ complications. Lu = 8.5, which are roughly within the range for lunar In general, ratios of Na/Sr, K/Ba, and Li/Yb for lunar basalt data (Supplementary Fig. S8). Assuming BSE-like MIs overlap well with the lunar basalt data (Fig. 8), indicat- concentrations of 6.26 wt% Fe, 1.675 ppm Ce, 6.6 ppm ing no significant loss of Na, K, and Li in lunar basalts rel- Ba, and 0.0675 ppm Lu for the lunar mantle ative to MIs. This conclusion is consistent with our results (McDonough and Sun, 1995), we obtain 2.9 ppm Zn, indicating less than 10% degassing loss of Na, K, and Li 0.018 ppm Pb, 0.079 ppm Rb, 0.0041 ppm Cs, and in orange glass beads and glassy embayments compared to 0.57 ppm Ga for the primitive lunar mantle. the melt inclusion from 74220 (Fig. 5). Ratios of Na/Sr in lunar MIs range from 11.5 to 30, with a geometric average 4.3. H2O/Ce variation among different lunar samples of 20 1.2 (1r error, Fig. 8a). Lunar MI K/Ba ratios are In Chen et al. (2015), large variations in melt inclusion mostly between 4.2 and 9.4, with a geometric average of H2O/Ce ratios between 74220 (with glassy MIs) and other 6.9 1.2 (Fig. 8b). Relatively more scatter is seen in the lunar samples (with mostly homogenized MIs) were first Li-Yb plot (Fig. 8c). Highly evolved interstitial glasses and noticed, which remain an uncertainty in interpreting H2O MIs from 74235 and 10020 have Li/Yb ratios a factor of 2 abundance for the lunar mantle. Ni et al. (2017a) studied to 3 lower than the less evolved green and orange glasses the effect of diffusive H loss during homogenization on and 74220 MIs. If Li/Yb data for the highly evolved glasses H2O/Ce variation and concluded that this effect is insignif- and MIs are excluded, the rest of the data gives approxi- icant compared to the range of variations observed. Hence, mately consistent Li/Yb ratios, which averaged 2.4 1.1 the H2O/Ce variation among different lunar samples is not an artifact due to the homogenization experiments, but a (Fig. 8c). Cu/Mn ratios in lunar MIs also correlate well with real signal possibly recording a complicated history that lunar basalt data, however, the scatter is more than one has shaped volatile behavior on the Moon. Albarede order of magnitude greater, with an average of 0.011 1.4 et al. (2015) interpreted the high H2O/Ce ratio for 74220 (Fig. 8d). The higher scatter in Cu/Mn ratios might be due to be a local feature that is not representative of the lunar to the higher volatility for Cu in lunar volcanism as con- mantle. While Chen et al. (2015) explained the H2O/Ce cluded above, but these ratios could also be affected by the ratio variation as a result of post-eruptive cooling loss of role of sulfides during lunar magma processes. H2O, and that the naturally glassy MIs best maintain pre- Assuming a BSE abundance for the refractory elements eruptive H2O/Ce ratio due to rapid quench. Based on our in the lunar mantle (19.9 ppm Sr, 6.6 ppm Ba, 0.441 ppm current understanding of Moon formation and evolution, Yb, and 1045 ppm Mn, McDonough and Sun, 1995; below we provide a detailed discussion on possible pro- Hauri et al., 2015), Na, K, Li, and Cu abundances in the cesses and reasons that could have caused heterogeneous lunar mantle can be calculated to be 398 ppm, 45.5 ppm, H2O/Ce distribution in different lunar igneous samples. 1.1 ppm, and 11 ppm, respectively. 4.3.1. Diffusive loss of H2O during cooling on the lunar 4.2.6. Zn/Fe, Pb/Ce, Cs/Rb, Rb/Ba and Ga/Lu ratios surface Analyses of Zn, Pb, Cs, Rb, and Ga concentrations in As shown by numerous hydration and dehydration lunar melt inclusions by SIMS is a challenging task due experiments on terrestrial (Hauri, 2002; Portnyagin et al., to their low abundances, low ionization rate, or strong mass 2008; Chen et al., 2011; Gaetani et al., 2012) and lunar interferences. Therefore, no data for these elements have MIs (Ni et al., 2017a), a melt inclusion is not a perfectly been previously reported for lunar MIs. In this study, how- closed system with respect to H2O. Rapid H diffusion ever, we were able to analyze these elements by LA-ICP- through the host olivine could potentially decrease H2O/ MS for one MI, and use the data to estimate Zn, Pb, Cs, Ce ratios in MIs during cooling on the lunar surface. One Rb and Ga abundances in the lunar mantle. implication of the effect of cooling on H2O/Ce variation For these elements, ratios of Zn/Sc, Zn/Fe, Pb/Ce, Cs/ among lunar samples is the correlation between H2O/Ce Rb, Cs/Ba, Cs/La, Rb/Ba, Rb/La, and Ga/Al have been ratio and the apparent cooling rate. Based on MI data 34 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 available so far, samples with more glassy MIs tend to have et al. (1974) does not affect our model significantly because higher H2O/Ce ratios. Sample 74220 has the highest H2O/ the range of estimated cooling rates (3600–20000 K/h) for Ce ratio (50), and most MIs found in this sample are 74220 is high enough to ensure limited H2O loss after glassy. For samples with intermediate H2O/Ce ratios (9 eruption. for 10020, 3 for 74235), some of the MIs found in these As shown in Fig. 9,H2O/Ce ratios for different lunar samples are partially glassy. While for samples with the samples are plotted versus their estimated cooling rates. lowest H2O/Ce ratios (1.7–0.5, 15016, 15647, 12040, and The H2O/Ce ratios show positive correlations with cooling 12008, this study and Chen et al., 2015), virtually all MIs rates in Fig. 9, in accordance with our observations based were found to be highly crystalline. on the occurrence of glassy melt inclusions. Qualitative For a more quantitative understanding of the role of cooling rate data are also consistent with Fig. 9. For exam- cooling rate on H2O/Ce variations, the diffusive H loss ple, 15016 cooled more rapidly than 12040 (Takeda et al., model from Ni et al. (2017a) is employed to model H2O/ 1975), and the H2O/Ce ratio in 15016 is about 2 times that Ce ratio decrease in lunar MIs as a function of cooling rate. in 12040. Also, 15016 has a higher H2O/Ce ratio compared In this model, a spherical melt inclusion is assumed to be at to 15647, and the latter likely cooled more slowly as dis- the center of a spherical olivine, and H2O inside the MI is cussed earlier. The only exception seems to be 10020, which gradually drained by diffusive loss of H across the host oli- likely cooled at a slower rate than 74235 as discussed ear- vine. Main parameters needed for this model include diam- lier, however, has a higher H2O/Ce ratio than 74235. Even eter of the olivine, diameter of the MI, boundary condition with this exception, the overall trend of higher H2O/Ce for H, and the temperature history. For the diameter of oli- ratio in MIs for more rapidly cooled lunar basalts is vine, a value of 200 mm is assumed for all the MIs. convincing. Although the actual diameter for the olivines could vary Modeled trends of H2O/Ce ratios in Fig. 9 show that the from 100 mm to 600 mm in different samples, Ni et al. decrease in H2O/Ce is highly dependent on cooling rate, (2017a) has shown that the effect of varying olivine diame- especially for the range of cooling rates that are typical ter is relatively small. For the diameter of MI, we assume a for mare basalts. Although our current model is limited range of 20 mmto60mm, which approximates most MIs by the evolution of H2O abundance in the host magma, studied in lunar samples (Table 1). Ideally, the boundary and a detailed understanding of their thermal history after condition for H should be the evolving H2O concentration eruption, it unambiguously shows that H2O/Ce in lunar in the host magma during degassing. But because such MIs can be efficiently decreased due to diffusive H loss information is unavailable, the extreme case with an exte- under typical time scales of mare basalt cooling. Therefore, rior concentration of zero H2O is assumed. The tempera- diffusive loss of H2O could potentially be an important pro- ture history of cooling is assumed to start at 1200 °Cor cess that caused large H2O/Ce variations in lunar MIs. 1300 °C, and cools asymptotically to the final temperature with a given initial cooling rate (q). Equations and other 4.3.2. Ingassing contamination that increased H2O/Ce in details about the model can be found in Ni et al. (2017a). 74220 during eruption In terms of the cooling rate for low-Ti basalts, 12008 Because an olivine-hosted melt inclusion is not a per- and 12040 have been estimated to have cooling rates of fectly closed system (e.g., Hauri, 2002; Portnyagin et al., 10–100 K/h, and < 15 K/h, respectively (Donaldson et al., 2008; Chen et al., 2011; Gaetani et al., 2012; Ni et al., 1975). For quantitative comparison and plotting, 15 K/h 2017a), there is also a possibility of H2O diffusing into the will be used for 12040. Quantitative cooling rates are not MIs just before eruption, thus resulting in a higher H2O/ available in the literature for 15016 or 15647. But Takeda Ce for 74220 compared to other lunar samples (Albarede et al. (1975) estimated that 15016 is one of the fastest cooled et al., 2015). This possibility, however, is not favored for samples among many Apollo basalts, and cooled more the following reasons: (1) For ingassing to occur, the melt rapidly than 12040. 15647 likely cooled at a slower rate inclusions would have had to be surrounded by a source compared to 15016 because this rock is a coarse-grained with high H2O fugacity. Such a wet environment with high basalt, whereas 15016 is a medium-grained basalt. H2O fugacity is common on Earth (such as hydrothermal For the two high-Ti basalts, 74235 was estimated to fluid, and alteration by liquid water), but unknown on the have a cooling rate of 150–250 K/h (Usselman et al., Moon. Contamination from water-rich carbonaceous chon- 1975), while quantitative cooling rate data have not been drites during eruption is also unlikely because such contam- reported for 10020. Based on the presence of glass in natu- ination would also result in elevated Zn, Cl, F, or Pb in ral MIs and absence of glass in the matrix, the cooling rate 74220, which is not observed (discussed further in 4.3.3 for 10020 is likely slightly lower than 74235, which has and in Fig. 10); and (2) more importantly, according to quenched glass in the MIs and also in the matrix. the modeling and experiments by Ni et al. (2017a) among The cooling rate of 74220 glass beads has been estimated others, smaller MIs are more vulnerable to ingassing and to be 20000 K/h by Uhlmann et al. (1974) and 6060 K/h degassing compared to larger MIs. If the MIs are affected by Hui et al. (2018), and that of MI-bearing olivine crystals by ingassing during eruption, a negative correlation has been estimated to be 3600 K/h by Ni et al. (2017a). between H2O content and MI size is expected. The observed Considering the model used here is for H loss from trend, however, is opposite to this expectation with smaller olivine-hosted MIs, the estimated cooling rate of MIs having lower H2O concentrations and lower H2O/Ce 3600 K/h for MI-bearing olivine crystals is adopted. ratio in 74220 (Ni et al., 2017a), indicating an effect of Using the higher cooling rate estimated by Uhlmann ‘‘degassing loss” instead of ‘‘ingassing gain” for H2O during P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 35

Fig. 9. Correlation between H2O/Ce ratio and sample cooling rate (q) for lunar MIs studied so far. Modeled H2O/Ce ratios as a function of cooling rate assuming an initial temperature (T0) of 1200 °C and 1300 °C for MIs with a diameter of 20–60 mm have been plotted in green and blue patches, respectively. Error bars for the cooling rate represent the range of estimation in the literature.

Fig. 10. Comparing volatile/refractory element ratios in MIs from 74220 to other samples. Besides H2O/Ce, MIs from 74220 show no sign of volatile enrichment relative to other lunar MIs or basalts. Error bars in the ﬁgure represent 1 standard deviation variation of the data. Glass beads data in the ﬁgure is from Hauri et al. (2015). Lunar basalt data are from MoonDB.

eruption. Therefore, it is unlikely that the high H2O/Ce densed volatiles, and gave elevated Zn, Cu, and Pb abun- ratio for 74220 is the result of late stage water ingassing. dances, as well as relatively low 238U/204Pb ratios, and light Cu and Zn isotopes (Tera and Wasserburg, 1976; 4.3.3. Is sample 74220 a volatile-enriched local Moynier et al., 2006). Such observations led to the concern heterogeneity? that 74220 was sourced from a volatile-enriched local As the lunar sample with the highest H2O/Ce ratios, heterogeneity in the lunar mantle (e.g. Paniello et al., some studies have proposed that 74220 could be originated 2012), however, these bulk measurements might not repre- from a volatile-enriched local heterogeneity inside the lunar sent the pre-eruptive chemical and isotopic compositions mantle, and therefore cannot be used to represent the for 74220. With more MI data for F, Cl and S, and Moon (e.g. Albarede et al., 2013, Albarede et al., 2015). in situ analyses of moderately volatile elements, it is possible If this is the case, sample 74220 should also show enriched to compare a larger set of volatile/refractory ratios for MIs abundances of other volatiles, such as F, Cl, S, Cu, Zn, Pb, in 74220 to other lunar MIs and lunar basalts to test the among others. In Chen et al. (2015), it was shown that MIs possibility of a local heterogeneity source for 74220. As from 74220 have similar F/Nd, Cl/Ba and S/Dy ratios com- plotted in Fig. 10, with the exception of H2O/Ce, volatile/ pared to MIs from lunar mare basalt samples. For the mod- refractory element ratios for MIs from 74220 are mostly erately volatile elements, previous bulk analyses were within error compared to MIs, glass beads, and mare almost always aﬀected by a thin coating of surface con- basalts from other lunar samples. The consistent ratios of 36 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41

S/Dy, Zn/Fe, Pb/Ce, F/Nd, Cs/Rb, Rb/Ba, Cl/K, Na/Sr, H2O/Ce variations in lunar samples are dominated by the Ga/Lu, K/Ba and Li/Yb between 74220 and other lunar extent of partial melting, a strong dependence between samples indicate that 74220 is unlikely originated from a H2O/Ce ratios and the major elements (MgO and FeO used volatile-enriched source in the lunar mantle, which justifies in here) is expected. Such a dependence, however, is absent the use of its MI data to estimate H2O abundance for the as can be seen in Fig. 11.InFig. 11, all MIs with less than Moon. 3.5 wt% MgO and 15 wt% FeO are natural glassy MIs in 10020 and 74235, which experienced significant post- 4.3.4. Partial melting of the lunar mantle and magma entrapment crystallization. Homogenized MIs in the same evolution on H2O/Ce samples contain much higher MgO and FeO, while preserv- As mentioned earlier, H2O/Ce ratios in lunar MIs are ing the same H2O/Ce ratios compared to the natural glassy used to estimate H2O abundances in the lunar mantle. This MIs. The effect of homogenization unambiguously shows is based on the assumption that H2O/Ce ratios in the melt that the low MgO and FeO concentrations in the natural do not change during mantle partial melting or crystal frac- MIs from 10020 and 74235 are the result of post- tionation, leading to the same H2O/Ce ratios in lunar MIs entrapment crystallization, not differences in the degree of compared to their mantle source. In addition, the La/Yb partial melting. and Sm/Dy ratios could vary by a factor of 4 in individual With the above considerations, we can exclude partial lunar samples (Fig. S5), possibly indicating the mixing of melting or magma evolution as factors controlling the magma produced by different degrees of partial melting observed H2O/Ce ratio variations in lunar samples. Further (Shimizu, 1998; Slater et al., 2001). Therefore, it is impor- studies on the behavior of H2O/Ce ratio during lunar tant to test whether our assumption of constant H2O/Ce igneous processes will be helpful to verify our conclusion. ratio during mantle partial melting and crystal fractionation is valid. 4.3.5. Degassing loss of H2O during magma ocean This assumption is well supported by terrestrial sub- crystallization marine MORB MI data, which show constant H2O/Ce The Moon is thought to have experienced a global ratios (variations within a factor of 2) over a wide range magma ocean stage, which produced a plagioclase flotation of major element concentrations (5 to 13 wt% MgO, crust as it solidified, and might have caused an overturn of Fig. S6). Such a wide range of MgO concentrations repre- the mantle due to a gravitationally unstable stratify (e.g. sents variable degrees of upper mantle partial melting and Elkins-Tanton et al., 2011). Degassing loss of volatiles dur- crystal fractionation. The constant H2O/Ce ratio for all ing lunar magma ocean crystallization could be quite signif- these submarine MORB MIs strongly supports correlated icant, especially before the formation of a plagioclase crust. H2O and Ce behaviors during these magma processes. When the plagioclase crust is thick enough, degassing loss When applied to lunar conditions, the H2O/Ce ratio is of volatiles would be sufficiently prohibited. During the late expected to behave the same way unless compatible mineral overturn, however, part of the solidified lunar mantle could hosts for H2O or Ce are involved in the process. This is have been exposed to the surface, resulting in further degas- unlikely the case because, most mineral phases discovered sing. Due to the complicated crystallization history of the on the Moon are incompatible with respect to H2Oor lunar magma ocean, it is possible for different parts of the Ce, except for rare minerals such as apatite, which only lunar mantle to have experienced varying degrees of degas- occur at late stages of magma crystallization (Heiken sing. An intuitive expectation is that the deeper part of the et al., 1991). lunar mantle could have experienced less degassing com- Other evidence supporting the assumption of constant pared to the shallower mantle. Experimental petrology H2O/Ce ratio during magma evolution is based on multiple studies (e.g. Longhi, 1992; Lee et al., 2009) indicate that measurements in two specific lunar samples: (1) In lunar pyroclastic glass deposits originate from a deeper source sample 10020, the H2O/Ce ratio in homogenized MIs with (1.6 to 3.0 GPa, 300 to 600 km) compared to the mare 6.5 wt% MgO and 18.1 wt% FeO is about the same as that basalts (0.7–2.0 GPa, 150–300 km). If the H2O/Ce varia- in the glassy parts (1.7–2.3 wt% MgO and 5.6–7.4 wt% tions in lunar MIs are truly representative of their mantle FeO) of partially glassy MIs (Fig. 6a, Chen et al., 2015 sources, it is possible that the pyroclastic glasses originate and this study); and (2) In lunar sample 74235, the highest from a less degassed part of the lunar mantle that better H2O/Ce ratio in homogenized MIs with 6.5–7.2 wt% MgO preserved the primitive H2O/Ce signal of the lunar magma and 20.2–22.2 wt% FeO is about the same as that in the par- ocean. tially glassy MIs (2.1–3.2 wt% MgO and 8.1–15.5 wt% With all the above considerations, we think the H2O/Ce FeO), as well as the glassy matrix (1.8 wt% MgO and ratio of 74220 (50) better represents the pre-eruptive 12.4 wt% FeO) (Fig. 6a, this study). Consistent H2O/Ce magma and the primitive lunar mantle. Based on this, we ratios in less evolved and more evolved MIs from the above estimate the primitive lunar mantle to have contained two samples indicate similar behavior of H2O and Ce dur- 84 ppm H2O. ing crystal fractionation. Both of the above evidences strongly support relatively 4.4. Volatile depletion trend for the Moon based on melt constant H2O/Ce ratios during mantle partial melting and inclusion data crystal fractionation processes. To further assess the effect of partial melting, H2O/Ce ratios are plotted versus MgO Based on previous discussions on volatile/refractory and FeO concentrations for all lunar MIs in Fig. 11.If ratios, abundances of volatile elements (H2O, F, Cl, S) P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 37

Fig. 11. Testing the effect of partial melting on lunar H2O/Ce ratios by plotting H2O/Ce versus (a) MgO and (b) FeO content for lunar MIs. In general, H2O/Ce ratios show no clear dependence on MgO and FeO content. MIs with low MgO and FeO concentrations are all partially glassy ones from 10020 and 74235 that were significantly affected by post-entrapment crystallization. Homogenized MIs in 10020 and 74235 have much higher MgO and FeO concentrations, but essentially the same H2O/Ce ratios compared to the natural MIs. In the legend, ‘‘HMI” and ‘‘NMI” stand for homogenized and natural melt inclusions, respectively. *One homogenized MI and one natural MI data in 10020 and one MI data in 12008 are from Chen et al. (2015); MI data in 74220 are from Ni et al. (2017a); glass beads data are from Hauri et al. (2015).

Table 3 Estimated volatile abundances in the primitive lunar mantle based on lunar MI data. Chlorine abundance is italicized because of the uncertainty in our approach using Cl/K ratios (discussed in Section 4.2.3).

Element H2OF ClS LiNaK Abundance (ppm) 84 5 0.39 73 1.1 398 45.5 Element Cu Zn Pb Rb Cs Ga Abundance (ppm) 11 2.9 0.018 0.57 0.0041 0.57

4 and moderately volatile elements (Na, K, Li, Cu, Zn, Pb, Tc temperatures being calculated at 10 bar total pressure, Cs, Rb, and Ga) for the primitive lunar mantle can be esti- which might not be applicable to the condensation of vola- mated based on lunar MI data (Table 3). In Table 3, the tiles on the Moon. Wang and Jacobsen (2016) argued that H2O/Ce ratio for 74220 is used to estimate H2O abundance their K isotope data, for example, are best explained by the in the lunar mantle. F/Nd ratios for 12040 are excluded condensation of the Moon under a pressure higher than when estimating F abundance. Sulfur abundance is esti- 10 bar after the Giant Impact, which is dramatically differ- mated using the geometric mean of maximum S/Dy ratios ent from the pressure used to calculate the 50% Tc temper- 4 for all lunar sample (S/Dy = 109 1.4), ignoring the poten- atures (10 bar H2, Lodders, 2003). Experimental work by Norris and Wood (2017) also shows that volatilities of ele- tial difference between low- and high-Ti basalts. ments during vapor-melt reaction at one atmosphere pres- If the estimated volatile abundances are compared to sure and 1300 °C could be significantly different from that those for the BSE, a volatile depletion trend for the Moon in a nebular gas at 10 4 bar. Hence, the trend for the can be obtained (Fig. 12). From Fig. 12, the inferred degree degrees of volatile element depletion may be used to con- of volatile depletion in the primitive lunar mantle relative to strain the physical conditions during Moon formation. the primitive terrestrial mantle can be ranked as follows Despite the uncertainties in predicting behavior of vola- based on the limited available data: tile elements during Moon formation based on their half

Zn>PbRb>CuGaNaCs>KF>H2OS>Li. condensation temperatures, one general feature in Fig. 12 is that the lunar mantle is not very depleted in highly vola- The degree of depletion is more than a factor of 10 for tile elements (e.g. 50% Tc < 800 K). This observation is in Zn, and is smaller for other elements. This order of volatile accordance with recent estimation of the bulk silicate Moon depletion is not correlated with the half condensation tem- composition by Hauri et al. (2015), except that our estima- perature (Lodders, 2003), the experimentally determined tion is mostly based on lunar MI data. Several models have volatility trend by Norris and Wood (2017), or the order been proposed to reconcile the existence of highly volatile of volatile depletion during volcanic eruptions on the lunar elements (especially H2O) in the lunar mantle since the dis- surface (Section 4.1). Our data also do not follow the lunar covery of high concentrations of intrinsic H2O in lunar volatile depletion trend suggested by Albarede et al. (2015), samples. These models might also be applicable in explain- especially for elements with 50% Tc lower than 800 K. The ing the lunar volatile depletion trend estimated in this lack of correlation between lunar volatile depletion and study. One possibility is that the Moon formed in a rela- 50% Tc temperatures can be partly explained by the 50% tively low-energy impact (Hauri et al., 2015), in which only 38 P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41

Fig. 12. Volatile depletion trend for the lunar mantle on a melt inclusion perspective. Half condensation temperatures are from Lodders

(2003). The composition of the bulk silicate earth is from McDonough and Sun (1995), except for H2O, which comes from H2O/Ce = 184 in Chen et al. (2015). Abundances of refractory elements used for estimation of volatile abundances in this study are assumed to be ‘‘BSE-like” in the lunar mantle. Error bars are 1r geometric standard deviation considering data variations in lunar MIs. For H2O, our preferred values for the Moon are plotted in solid symbols, while the related complications are plotted in open symbols and discussed in the text. The volatile depletion trend suggested in Albarede et al. (2015) is plotted in dashed line as a reference. part of the Moon-forming materials was volatile depleted. for one melt inclusion, two glassy embayments, and two The most significant weakness of this model is that it can- glass beads from 74220 and 15421. Using our new data not explain the indistinguishable stable isotopic composi- combined with literature data, H2O/Ce ratios are found tions between the Earth and Moon (Pahlevan, 2014). A to be correlated with the cooling rate of different lunar sam- second possibility is the incomplete loss of highly volatile ples, indicating an important role of degassing in causing elements during an energetic giant impact (e.g. Pahlevan large H2O/Ce variations among lunar samples. After ruling et al., 2016; Nakajima and Stevenson, 2018). Pahlevan out other possibilities such as loss of H2O during homoge- et al. (2016) provided a solution by formation of a gas disk nization, mantle partial melting, magma evolution, and surrounding the lunar magma ocean after the giant impact ingassing during eruption, we conclude that the H2O/Ce that would be capable of maintaining hundreds of ppm dis- ratio of 50 for 74220 is deemed more representative of solved H species in the magma ocean. Nakajima and the primitive lunar mantle. In addition, in situ analyses on Stevenson (2018) postulated that, if the upper parts of the MIs from 74220 show similar S/Dy, Zn/FeO, Pb/Ce, Moon-forming disk were dominated by heavy species, then F/Nd, Cs/Rb, Rb/Ba, Cl/K, Na/Sr, Ga/Lu, K/Ba, and the loss of H2O would be limited by diffusion, causing inef- Li/Yb ratios compared to other lunar basalts, making it ficient depletion of H2O in the disk. Another way to explain unlikely to be sourced from a volatile-enriched local the abundances of highly volatile elements in the Moon is heterogeneity in the lunar mantle. Previous studies regard- by asteroidal/cometary bombardment in the lunar magma ing 74220 as an anomalous lunar sample are based on bulk ocean before it fully solidified (Bottke et al., 2010; Elkins- analyses, which were significantly affected by surface- Tanton and Grove, 2011; Hauri et al., 2015; Barnes et al., correlated volatiles. 2016; Hauri et al., 2017). The last model is capable of A volatility trend for volcanic eruption on the lunar explaining the abundances of highly volatile elements on surface is obtained by comparing post-entrapment- the Moon, and also to reconcile isotopic compositions of crystallization corrected volatile abundances in melt inclu- sulfur, chlorine, hydrogen, and nitrogen in lunar samples sions, glassy embayments, and glass beads from 74220: H2 (Hauri et al., 2017). In general, explaining the observed O>Cl>F Cu Zn > S Ga > Cs > Rb Pb > Na > volatile depletion trend for the Moon requires multi- K Li. The estimated volatility trend is significantly differ- disciplinary work to better understand the behavior of vola- ent from the trend of half condensation temperatures of tile elements during the giant impact, existence of the lunar these elements and indicates that melt-vapor equilibrium magma ocean, as well as magma generation and eruption experiments applicable to lunar conditions are important on the Moon. to understand volatile behaviors during lunar volcanism. Combining lunar melt inclusion data obtained in this 5. CONCLUSIONS study and from the literature, a volatile depletion trend can be obtained for the Moon. This lunar volatile depletion In this paper we report volatile, major and trace ele- trend does not show obvious dependence on half condensa- ment, and transition metal data in olivine-hosted melt tion temperatures of the volatile elements and is also differ- inclusions from 5 mare basalt samples, including both ent from the volatility trend for volcanic eruptions on the high-Ti (10020 and 74235) and low-Ti basalts (12040, lunar surface. The lack of correlation indicates that the 15016, and 15647), and moderately volatile element data environment for volatile depletion for the Moon differs P. Ni et al. / Geochimica et Cosmochimica Acta 249 (2019) 17–41 39 from that for nebular condensation. The depletion trend Chakraborty S. (2010) Diffusion coefficients in olivine, wadsleyite inferred for the primitive lunar mantle will provide vital and ringwoodite. Rev. Mineral. Geochem. 72, 603–639. constraints on the Moon formation model, requiring vola- Chen Y., Provost A., Schiano P. and Cluzel N. (2011) The rate of tiles to be partially preserved during the Giant Impact, or water loss from olivine-hosted melt inclusions. Contrib. Min- replenished to the Moon before fully crystallization of the eral. Petrol. 162, 625–636. Chen Y., Zhang Y., Liu Y., Guan Y., Eiler J. and Stolper E. M. lunar magma ocean. (2015) Water, fluorine, and sulfur concentrations in the lunar mantle. Earth Planet. Sci. Lett. 427, 37–46. ACKNOWLEDGMENT Compston W., Berry H., Vernon M. J., Chappell B. W. and Kaye M. J. (1971) Rubidium-strontium chronology and chemistry of This research is supported by NASA grant NNX15AH37G. We lunar material from the Ocean of Storms. In Lunar and would like to thank NASA CAPTEM for providing the lunar sam- Planetary Science Conference Proceedings, p. 1471, [accessed ples, K. P. Jochum for providing the MPI-DING glass standards, March 26, 2017]. Yunbin Guan for his assistance in SIMS analyses, J. C. Barrette for Danyushevsky L. V., Della-Pasqua F. N. and Sokolov S. (2000) the assistance in LA-ICP-MS analyses, and Miki Nakajima for Re-equilibration of melt inclusions trapped by magnesian helpful discussions. We thank Francis Albarede and an anonymous olivine phenocrysts from subduction-related magmas: petro- reviewer for their constructive reviews that greatly improved the logical implications. Contrib. Mineral. Petrol. 138, 68–83. manuscript. And we thank Francis for his courtesy of sharing an Ding S., Hough T. and Dasgupta R. (2018) New high pressure * Excel file on lunar basalt data from Wiesmann (1975). The electron experiments on sulfide saturation of high-FeO basalts with microprobe Cameca SX100 used in this study was purchased using variable TiO2 contents – implications for the sulfur inventory of NSF grant EAR-9911352. the lunar interior. Geochim. Cosmochim. Acta 222, 319–339. Dixon J. 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