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Shock-induced H loss from pyroxene and maskelynite in a Martian meteorite and the mantle source δD of enriched shergottites
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2021-10-28 , DOI: 10.1016/j.gca.2021.10.020
J.-M. Dudley 1 , A.H. Peslier 1 , R.L. Hervig 2
Affiliation  

Assessing the water abundance and hydrogen isotopic signature (δD) of the Martian interior dictates our understanding of the formation of inner solar-system planets, the origin of their volatiles, Martian volcanic history, and the potential for life-bearing environments on the surface of the red planet. Although several Martian meteorites, representing the planet's crust, have been analyzed before for this assessment, little is known about the effect of shock on recorded hydrogen (H) in their mineral phases. Here, hydrogen contents and isotopes are measured by secondary ion mass spectrometry (SIMS) in an enriched olivine-phyric shergottite, Larkman Nunatak (LAR) 06319, containing impact-melted zones. Systematic 100 μm-long traverses in pyroxene and maskelynite grains reveal decreases of hundreds of µg/g HO and increases in δD of thousands of ‰ towards the contact with impact-melted zones, which is interpreted as H diffusive loss during shock-melting. Diffusion modeling reveals that temperatures high enough to permit H diffusion following shock were maintained near the impact-melted zone for a few minutes. By comparison, the interior of pyroxenes > 200 μm away from impact-melted zones have some of the highest H content with 170–480 µg/g HO and the lowest δD with ∼ 300‰. The latter values, obtained on the most Mg-rich, i.e. earliest crystallized pyroxenes, are used to estimate that the enriched shergottite mantle source contains 300–1000 µg/g HO and has a δD of ∼ 300‰. This δD is similar to that of depleted shergottite and nakhlite mantle sources, but higher than Earth’s upper mantle, suggesting slightly different water source materials for the two planets. The enriched shergottite mantle source has ∼ 10 times more water than that inferred for the depleted shergottite source and for Earth’s upper mantle. The high water content and wide range of δD in olivine (from 90 µg/g HO and 2700‰ to 1350 µg/g HO and −14‰) is interpreted as overprinting by a combination of Martian and terrestrial surface alteration. Finally, the high δD recorded in the impact-melt produced glass (3350–4700‰), its moderate water content (100–230 µg/g HO), and the presence of vesicles, are likely the result of incorporation of Martian surficial material (ice and atmospheric gases) and degassing during shock melting. This study shows that shock can induce H loss from minerals, accompanied by > 1000‰ δD increases. Additionally, although it confirms that the Martian mantle may be heterogeneous in its water content, it implies that the Martian mantle is homogeneous within uncertainties for δD.

中文翻译:

火星陨石中辉石和马氏长石中冲击引起的 H 损失以及富集的六角辉石的地幔源 δD

评估火星内部的水丰度和氢同位素特征(δD)决定了我们对太阳系内部行星的形成、其挥发物的起源、火星火山历史以及火星表面存在生命环境的潜力的理解。红色星球。尽管之前已经对代表火星地壳的几颗火星陨石进行了分析,但对于冲击对其矿物相中记录的氢 (H) 的影响知之甚少。在此,通过二次离子质谱 (SIMS) 测量了含有冲击熔化区的富集橄榄石-镁橄榄石 Larkman Nunatak (LAR) 06319 中的氢含量和同位素。对辉石和马氏长石晶粒进行 100 μm 长的系统扫描,揭示了与冲击熔化区接触处的 H2O 减少了数百 µg/g H2O,而 δD 增加了数千 ‰,这被解释为冲击熔化过程中的 H 扩散损失。扩散模型表明,冲击熔化区附近的温度足以允许 H 在冲击后扩散几分钟。相比之下,距冲击熔化区 > 200 μm 的辉石内部具有最高的 H 含量(170–480 µg/g H2O)和最低的 δD(约 300‰)。后者的值是在最富镁(即最早结晶的辉石)上获得的,用于估计富集的镁橄榄石地幔源含有 300–1000 µg/g H2O,δD 约为 300‰。该 δD 与贫化的六角辉石和钠辉石地幔源的 δD 相似,但高于地球上地幔,表明这两颗行星的水源材料略有不同。富集的六角辉石地幔源所含的水量比推断的贫乏的六角辉石源和地球上地幔的水多约 10 倍。橄榄石中的高含水量和宽范围的δD(从90 µg/g H2O和2700‰到1350 µg/g H2O和-14‰)被解释为火星和陆地表面蚀变共同造成的叠印。最后,冲击熔融生产的玻璃中记录的高 δD(3350–4700‰)、适度的含水量(100–230 µg/g H2O)以及囊泡的存在,很可能是火星表面物质掺入的结果(冰和大气气体)和冲击熔化过程中的脱气。这项研究表明,冲击会导致矿物中的 H 损失,并伴有 > 1000‰ δD 增加。此外,虽然它证实了火星地幔的含水量可能是不均匀的,但这意味着火星地幔在 δD 的不确定性范围内是均匀的。
更新日期:2021-10-28
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