Title: Cosmic ray effects on the isotope composition of hydrogen and noble gases in lunar samples: Insights from Apollo 12018

Abstract

Exposure of rocks and regolith to solar (SCR) and galactic cosmic rays (GCR) at the Moon's surface results in the production of ‘cosmogenic’ deuterium and noble gas nuclides at a rate that depends on a complex set of parameters, such as the energy spectrum and intensity of the cosmic ray flux, the chemical composition, size, and shape of the target as well as the shielding depth. As the effects of cosmic rays on the D production in lunar samples remain poorly understood, we determine here the D content and noble gas (He-Ne-Ar) characteristics of nominally anhydrous mineral (olivine and pyroxene) grains and rock fragments, respectively, from different documented depths (0 to ≥4.8 cm) within Apollo olivine basalt 12018. Deuterium concentrations, determined by secondary ion mass spectrometry, and cosmogenic ³He, ²¹Ne, and ³⁸Ar abundances, measured by CO₂ laser extraction static mass spectrometry, are constant over the depth range investigated. Neon isotope ratios (²⁰Ne/²²Ne ≈0.86 and ²¹Ne/²²Ne ≈0.85) of the cosmogenic endmember are comparable to the theoretical signature of GCR-produced neon. These observations indicate that the presence of significant amounts of SCR nuclides in the studied sub-samples can be ruled out. Hence, D within the olivines and pyroxenes must have been predominantlymore » produced in situ by GCR-induced spallation reactions during exposure at the lunar surface. Comparison of the amount of D with the ²¹Ne (184 ± 26 Ma) or ³⁸Ar (193 ± 25 Ma) exposure ages yields a D production rate that is in good agreement with the value of (2.17 ± 0.11) × 10^–12 mol(g rock)^-1Ma^-1 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.« less

Authors:

Füri, Evelyn ;

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Zimmermann, Laurent; Deloule, Etienne; Trappitsch, Reto

Publication Date:

Sun Nov 01 00:00:00 EDT 2020

Research Org.:

Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)

Sponsoring Org.:

USDOE National Nuclear Security Administration (NNSA); European Research Council (ERC)

OSTI Identifier:

1656684

Alternate Identifier(s):

OSTI ID: 1657680

Report Number(s):

LLNL-JRNL-813331
Journal ID: ISSN 0012-821X; S0012821X20304945; 116550; PII: S0012821X20304945

Grant/Contract Number:  

AC52-07NA27344; 813331; 2739; 715028

Resource Type:

Published Article

Journal Name:

Earth and Planetary Science Letters

Additional Journal Information:

Journal Name: Earth and Planetary Science Letters Journal Volume: 550 Journal Issue: C; Journal ID: ISSN 0012-821X

Publisher:

Elsevier

Country of Publication:

Netherlands

Language:

English

Subject:

79 ASTRONOMY AND ASTROPHYSICS; hydrogen isotopes; noble gases; mare basalt; cosmic rays; cosmogenic nuclides; exposure age