Modern atmospheric signatures in 4.4 Ga Martian meteorite NWA 7034
Modern atmospheric signatures in 4.4 Ga Martian meteorite NWA 7034
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DOI：
10.1016/j.epsl.2014.05.008
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发表时间：
2014-08-15
影响因子：
5.3
通讯作者：
Agee, C. B.
中科院分区：
文献类型：
--
作者：
Cartwright, J. A.;Ott, U.;Agee, C. B.
NWA7034火星玄武岩角砾岩的年龄与4.4Ga相近，代表了一种全新的火星陨石类型。然而，由于与其他火星陨石类型(包括其反常的氧同位素比率)相比，NWA7034的独特组成，稀有气体分析--火星陨石鉴定的关键工具--对于确认其火星起源非常重要。在这里，我们报告了NWA7034的第一个惰性气体结果，它表明存在一个类似于当前火星大气的被捕获的成分。这种被捕获的成分在成分上也类似于在更年轻的雪长石中发现的被捕获的气体(类似于150-600 Ma)。我们对样品的形成年龄为1.6Ga(K-Ar)，类似于170 Ma(U-Th/He)，比Rb-Sr(2.1Ga)和Sm-Nd(4.4Ga；锆石类似4.4Ga)观测到的事件要年轻得多。然而，我们的K-Ar年龄类似于U-Pb锆石数据中的1.7Ga的扰动，这可能暗示这两个计时器都受到了共同的过程或事件的扰动。约170 Ma的U-Th/He年龄可能与此时放射性成因He-4的完全丧失有关，并与大多数钙钛矿的结晶年龄相似。虽然这可能是巧合，但它可能表明同一事件同时导致雪尔果特陨石的形成和NWA7034热变质作用。至于宇宙射线的暴露年龄，我们最喜欢的年龄是5 Ma，这超出了其他火星陨石群的范围，可能表明有明显的抛射事件。NWA7034显示了在BR上捕获中子的证据，这导致了Kr同位素Kr-80和Kr-82的上升。这些升高的丰度表明存在显著的屏蔽，可能与较大的流星体大小有关，和/或与雷石器时代起源的屏蔽有关。我们也对Ar和Xe数据应用了类似的中子俘获修正，这进一步完善了现代大气成分的可能性，尽管这种修正仍然是推测的。宇宙产生率和惰性气体数据与流星体半径>；50厘米一致。Xe数据中的裂变贡献很明显，有证据表明NWA7034同时含有U-238和Pu-244派生的裂变Xe组分。如果NWA7034中的气体被困在其古老的火成岩地层中，这将表明从4.4Ga到现在，火星大气几乎没有演化。然而，由于NWA7034是一种具有多种岩性的风化角砾岩，其成分与古谢夫土壤具有很强的相似性，因此捕获大气气体的时间和并入尚不清楚。有迹象表明，事件的重置时间类似于1.5-2.1Ga，大气成分可能在角砾岩形成期间并入--可能在亚马孙纪，尽管它也可能在从地表喷出时并入。(C)2014爱思唯尔B.V.保留所有权利。
The NWA 7034 Martian basaltic breccia, dated at similar to 4.4 Ga, represents an entirely new type of Martian meteorite. However, due to the unique make-up of NWA 7034 compared to other Martian meteorite types (including its anomalous oxygen isotope ratios), noble gas analyses - a key tool for Martian meteorite identification - are important to confirm its Martian origin. Here, we report the first noble gas results for NWA 7034, which show the presence of a trapped component that resembles the current Martian atmosphere. This trapped component is also similar in composition to trapped gases found in the much younger shergottites (similar to 150-600 Ma). Our formation ages for the sample suggest events at similar to 1.6 Ga (K-Ar), and similar to 170 Ma (U-Th/He), which are considerably younger than those observed by Rb-Sr (2.1 Ga), and Sm-Nd (4.4 Ga; zircons similar to 4.4 Ga). However, our K-Ar age is similar to a disturbance in the U-Pb zircon data at similar to 1.7 Ga, which could hint that both chronometers have been subjected to disturbance by a common process or event. The U-Th/He age of similar to 170 Ma could relate to complete loss of radiogenic He-4 at this time, and is a similar age to the crystallisation age of most shergottites. While this may be coincidental, it could indicate that a single event is responsible for both shergottite formation and NWA 7034 thermal metamorphism. As for cosmic ray exposure ages, our favoured age is 5 Ma, which is outside the ranges for other Martian meteorite groups, and may suggest a distinct ejection event. NWA 7034 shows evidence for neutron capture on Br, which has caused elevations in Kr isotopes Kr-80 and Kr-82. These elevated abundances indicate significant shielding, and could relate to either a large meteoroid size, and/or shielding in relation to a regolithic origin. We have also applied similar neutron capture corrections to Ar and Xe data, which further refine the likelihood of a modern atmospheric component, though such corrections remain speculative. Cosmogenic production rates and noble gas data are consistent with a meteoroid radius of >50 cm. Fission contributions are clear in the Xe data, with evidence to suggest that NWA 7034 contains both U-238 and Pu-244 derived fission Xe components. If the gas in NWA 7034 was trapped at its ancient igneous formation, this would suggest little evolution of the Martian atmosphere between similar to 4.4 Ga and present day. However, as NWA 7034 is a regolith breccia with multiple lithologies and a strong compositional similarity to Gusev soils, the timing and incorporation of trapped atmospheric gases is unclear. With hints of resetting events at similar to 1.5-2.1 Ga, the atmospheric component may have been incorporated during breccia formation - possibly in the Amazonian, though it could also have been incorporated on ejection from the surface. (C) 2014 Elsevier B.V. All rights reserved.