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Cosmic ray effects on the isotope composition of hydrogen and noble gases in lunar samples: Insights from Apollo 12018
Earth and Planetary Science Letters ( IF 5.3 ) Pub Date : 2020-11-01 , DOI: 10.1016/j.epsl.2020.116550
Evelyn Füri , Laurent Zimmermann , Etienne Deloule , Reto Trappitsch

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 predominantly 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 Furi 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.

中文翻译:

宇宙射线对月球样本中氢和稀有气体同位素组成的影响:来自阿波罗 12018 的见解

摘要 岩石和风化层在月球表面暴露于太阳 (SCR) 和银河宇宙射线 (GCR) 会导致“宇宙成因”氘和惰性气体核素的产生速度取决于一组复杂的参数,例如宇宙射线通量的能谱和强度,目标的化学成分、大小和形状以及屏蔽深度。由于宇宙射线对月球样品中 D 产生的影响仍然知之甚少,我们在这里分别确定了名义上无水矿物(橄榄石和辉石)颗粒和岩石碎片的 D 含量和惰性气体(He-Ne-Ar)特征,来自阿波罗橄榄石玄武岩 12018 内不同记录深度(0 至 ≥4.8 厘米)。氘浓度,由二次离子质谱法测定,以及宇宙源 3He、21Ne 和 38Ar 丰度,通过 CO2 激光提取静态质谱法测量,在所研究的深度范围内是恒定的。宇宙起源端元的氖同位素比(20Ne/22Ne ≈0.86 和 21Ne/22Ne ≈0.85)与 GCR 产生的氖的理论特征相当。这些观察结果表明,可以排除所研究的子样品中存在大量 SCR 核素。因此,橄榄石和辉石中的 D 主要是在月球表面暴露期间通过 GCR 引起的散裂反应在原位产生的。将 D 的量与 21Ne (184 ± 26 Ma) 或 38Ar (193 ± 25 Ma) 暴露年龄进行比较,得出的 D 生成速率与 ( 2.17 ± 0.11 ) × 10 − 12 mol( g rock)−1Ma−1 来自 Furi 等人。(2017)。
更新日期:2020-11-01
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