Shock-induced H loss from pyroxene and maskelynite in a Martian meteorite and the mantle source δD of enriched shergottites
Shock-induced H loss from pyroxene and maskelynite in a Martian meteorite and the mantle source δD of enriched shergottites
复制标题
火星陨石中辉石和马氏辉石中冲击引起的 H 损失以及富集的 Shergottites 的地幔源 δD
DOI：
10.1016/j.gca.2021.10.020
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发表时间：
2022
影响因子：
5
通讯作者：
Hervig, R.L.
中科院分区：
文献类型：
--
作者：
Dudley, J.-M.;Peslier, A.H.;Hervig, R.L.
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、挥发物的起源、火星火山的历史以及这颗红色星球表面可能存在生命的环境的理解。尽管一些代表火星地壳的火星陨石在此之前已经被分析过，但对于冲击对其矿物相中记录的氢(H)的影响知之甚少。在这里，氢的含量和同位素是用次级离子质谱（SIMS）测量的，在一个富含橄榄石的辉长石，Larkman Nunatak (LAR) 06319中，包含了撞击熔化带。辉石和掩斑岩颗粒在系统的100 μm长的横截面上，与冲击熔化区接触的H2O减少了数百μ g/g， δD增加了数千‰，这解释为冲击熔化过程中H的扩散损失。扩散模型显示，在撞击熔化区附近保持足够高的温度，使H在冲击后扩散几分钟。相比之下，距离冲击熔化区> 200 μm的辉石岩内部H含量最高，为170 ~ 480µg/g H2O， δD最低，为~ 300‰。后一个值是在最富镁的辉石岩上获得的，即最早结晶的辉石岩，用来估计富集的辉石岩地幔源含有300 - 1000µg/g H2O， δD为~ 300‰。这一δD与枯竭的辉长岩和钠长石地幔源相似，但高于地球上地幔，表明两行星的水源物质略有不同。富辉长岩地幔源的含水量比贫辉长岩源和地球上地幔推断的含水量多约10倍。橄榄石的高含水量和δD范围（从90µg/g H2O和2700‰到1350µg/g H2O和- 14‰）解释为火星和地球表面蚀变的叠加作用。最后，撞击熔融玻璃的δD值高（3350 ~ 4700‰），含水量适中（100 ~ 230µg/g H2O），且存在囊泡，可能是冲击熔融过程中火星表面物质（冰和大气气体）的掺入和脱气的结果。研究表明，冲击可引起矿物中H的损失，并伴有bbb10 ~ 1000‰δD的增加。此外，虽然它证实了火星地幔的含水量可能是不均匀的，但它意味着火星地幔在δD的不确定性范围内是均匀的。
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 H2O 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 H2O 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 H2O 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 H2O and 2700‰ to 1350 µg/g H2O 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 H2O), 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.