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Cosmic Ray Effects on the Isotope Composition

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Cosmic Ray Effects on the Isotope Composition Cosmic ray effects on the isotope composition of hydrogen and noble gases in lunar samples: Insights from Apollo 12018 Evelyn Füri, Laurent Zimmermann, Etienne Deloule, Reto Trappitsch To cite this version: Evelyn Füri, Laurent Zimmermann, Etienne Deloule, Reto Trappitsch. Cosmic ray effects on the isotope composition of hydrogen and noble gases in lunar samples: Insights from Apollo 12018. Earth and Planetary Science Letters, Elsevier, 2020, 550, pp.116550. 10.1016/j.epsl.2020.116550. hal- 02926751 HAL Id: hal-02926751 https://hal.archives-ouvertes.fr/hal-02926751 Submitted on 1 Sep 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. 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Distributed under a Creative Commons Attribution - NonCommercial - NoDerivatives| 4.0 International License Earth and Planetary Science Letters 550 (2020) 116550 Contents lists available at ScienceDirect Earth and Planetary Science Letters www.elsevier.com/locate/epsl Cosmic ray effects on the isotope composition of hydrogen and noble gases in lunar samples: Insights from Apollo 12018 ∗ Evelyn Füri a, , Laurent Zimmermann a, Etienne Deloule a, Reto Trappitsch b a Centre de Recherches Pétrographiques et Géochimiques, Université de Lorraine, CNRS, F-54000 Nancy, France b Lawrence Livermore National Laboratory, Nuclear and Chemical Sciences Division, 7000 East Ave, L-231, Livermore, CA 94550, USA a r t i c l e i n f o a b s t r a c t Article history: Exposure of rocks and regolith to solar (SCR) and galactic cosmic rays (GCR) at the Moon’s surface Received 19 May 2020 results in the production of ‘cosmogenic’ deuterium and noble gas nuclides at a rate that depends on Received in revised form 19 August 2020 a complex set of parameters, such as the energy spectrum and intensity of the cosmic ray flux, the Accepted 21 August 2020 chemical composition, size, and shape of the target as well as the shielding depth. As the effects of Available online xxxx cosmic rays on the D production in lunar samples remain poorly understood, we determine here the D Editor: F. Moynier content and noble gas (He-Ne-Ar) characteristics of nominally anhydrous mineral (olivine and pyroxene) Keywords: grains and rock fragments, respectively, from different documented depths (0 to ≥4.8 cm) within Apollo hydrogen isotopes olivine basalt 12018. Deuterium concentrations, determined by secondary ion mass spectrometry, and 3 21 38 noble gases cosmogenic He, Ne, and Ar abundances, measured by CO2 laser extraction static mass spectrometry, 20 22 21 22 mare basalt are constant over the depth range investigated. Neon isotope ratios ( Ne/ Ne ≈0.86 and Ne/ Ne cosmic rays ≈0.85) of the cosmogenic endmember are comparable to the theoretical signature of GCR-produced neon. cosmogenic nuclides These observations indicate that the presence of significant amounts of SCR nuclides in the studied sub- exposure age samples can be ruled out. Hence, D within the olivines and pyroxenes must have been predominantly produced in situ by GCR-induced spallation reactions during exposure at the lunar surface. Comparison 21 38 of the amount of D with the Ne (184 ± 26 Ma) or Ar (193 ± 25 Ma) exposure ages yields a D −12 −1 −1 production rate that is in good agreement with the value of (2.17 ± 0.11) × 10 mol(g rock) Ma from Füri et al. (2017). These results confirm that cosmic ray effects can substantially alter the hydrogen isotope (D/H) ratio of indigenous ‘water’ in returned extraterrestrial samples and meteorites with long exposure ages. © 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 1. Introduction to the lunar interior (e.g., Anand et al., 2014; Barnes et al., 2016; Desch and Robinson, 2019; Füri et al., 2014; Greenwood et al., The hydrogen isotope ratio is the key indicator for plan- 2011; Hui et al., 2017; Robinson et al., 2016; Saal et al., 2013; etary water origin(s) because different solar system reservoirs Sharp, 2017; Singer et al., 2017; Tartèse and Anand, 2013). How- (solar, chondritic, cometary) have characteristic D/H signatures ever, the D/H ratio of mantle-derived samples does not always (e.g., Alexander, 2017; McCubbin and Barnes, 2019). Volcanic reflect the hydrogen isotope composition of the lunar mantle glass beads and olivine-hosted melt inclusions therein, the phos- source. In addition to magmatic processes (e.g., degassing; Saal et phate mineral apatite, and nominally anhydrous minerals in var- al., 2013; Tartèse and Anand, 2013), solar wind (SW) implantation ious rock types returned from the Moon by the Apollo missions and cosmic ray induced spallation reactions – triggered by solar record a wide range of δD values (where δD[] =[(D/H)sample/ (SCR) and galactic cosmic rays (GCR) that can penetrate lunar mat- −6 ter to depths of a few centimeters or several meters, respectively [(D/H)SMOW − 1] × 1000, with (D/H)SMOW = 155.76 × 10 ; Hage- mann et al., 1970), between ≤−500 and ≥+1000 (see Mc- (Reedy and Arnold, 1972) – can modify the D/H signature of in- Cubbin et al., 2015 for a review), which have been interpreted digenous ‘water’ (i.e., H, H2, and/or H2O) in lunar rocks, minerals, to reflect hydrogen or water contributions from multiple sources, and volcanic glasses. such as the solar nebula, carbonaceous chondrites, and/or comets, Since the Moon is an airless body and has no global magnetic field, SW particles, including protons and noble gas ions, are im- planted into the top few tens of nanometers of all rocks or regolith * Corresponding author. grains exposed to the lunar surface environment (e.g., Hashizume E-mail address: [email protected] (E. Füri). et al., 2000). Nonetheless, a contribution of SW-implanted hydro- https://doi.org/10.1016/j.epsl.2020.116550 0012-821X/© 2020 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2 E. Füri et al. / Earth and Planetary Science Letters 550 (2020) 116550 gen to the measured water abundances and D/H ratios can gen- ent depths (0 to ≥4.8 cm) within Apollo olivine basalt 12018 by erally be ruled out because all recent hydrogen isotope measure- SIMS. In parallel, we analyzed the noble gas (He-Ne-Ar) contents ments of lunar samples have been carried out in situ by secondary and isotope ratios of bulk rock fragments by CO2 laser extraction ion mass spectrometry (SIMS), in the interior of lunar volcanic static mass spectrometry to quantify the abundances of cosmo- 3 21 38 glass beads (Füri et al., 2014) or within mineral grains and melt genic noble gas nuclides ( He, Ne, Ar) at each depth and to inclusions that were never directly exposed to SW irradiation (e.g., constrain the irradiation conditions and duration. This combined Barnes et al., 2014, 2013; Boyce et al., 2010; Hui et al., 2017; data set permits to determine if depth-dependent shielding vari- Robinson et al., 2016; Saal et al., 2013; Tartèse et al., 2013; Tartèse ations result in significant inter-sample differences in the rate of and Anand, 2013). In contrast, cosmic ray produced (‘cosmogenic’ cosmogenic D production within Apollo 12018. or ‘spallogenic’) deuterium is expected to contribute significantly to the D/H ratio of water-poor lunar samples that experienced long 2. Samples and analytical techniques exposure to cosmic rays at the surface of the Moon (Füri et al., 2017). Apollo sample 12018 is a medium-grained, low-Ti olivine basalt Knowledge of the cosmogenic D production rate (P D ) and the (supplementary Table S1; Papike et al., 1976) – also described as cosmic ray exposure (CRE) age of the sample of interest is critical olivine dolerite (Cuttitta et al., 1971; Kushiro et al., 1971) or gab- for correcting measured D/H ratios for the cosmogenic contribu- bro (Megrue, 1971) –, composed of approximately 70% large olivine tion, and, ultimately, for determining the source(s) of lunar water. and pyroxene crystals set in a variolitic matrix (Walter et al., 1971). −12 −1 −1 To this date, a P D value of 0.92 to 1 × 10 mol(g rock) Ma , Ten chips were allocated for this study by NASA’s Curation and derived by Merlivat et al. (1976) and Reedy (1981), has been used Analysis Planning Team for Extraterrestrial Materials (CAPTEM); in most studies of lunar samples (e.g., volcanic glasses, melt inclu- these chips were extracted from different documented depths (0 sions, apatites, plagioclase), irrespective of their chemical composi- to ≥4.8 cm; supplementary Table S2) along a slab cut through the tion. However, the P D value depends on the abundance of various middle of the rock that was originally 8 × 6 × 6cm in size (Fig. 1). target elements (O, Mg, Si, Fe, Al), the size and shape of the object, The chips were gently crushed in an agate mortar to obtain indi- and its exposure history (Reedy, 1981). Furthermore, Greenwood vidual olivine and pyroxene grains for hydrogen isotope analyses et al. (2018)argued that mare basalt 70215 analyzed by Merlivat by SIMS as well as small rock fragments for noble gas analyses by et al.
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