Available online at www.sciencedirect.com ScienceDirect 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 hetero- geneity 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 erup- tion 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 esti- mated. 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 rec- oncile 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 dif- fusive 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.