Impact events were frequent in the early history of our Solar System, and the dynamics of planetary crust formation were, consequently, substantially different from the processes that dominate today. Mars, a planet with stagnant lid tectonics and a unique preservation of ancient surface terrains, provides an outstanding opportunity to investigate the early processes related to the formation and reshaping of the first crust. Northwest Africa (NWA) 7034 and paired meteorites (such as NWA 7533) are fragments of polymict, regolith breccia that provide a tangible record of the ancient, brecciated crust on Mars. Zircon and baddeleyite from NWA 7034/7533 record evidence for two events of intense crustal reworking at 4442 ± 17 and 4474 ± 10 million years ago (Ma) triggered by impacts, placing important constraints on the timing and the dynamics of early crust formation on Mars. To date, only few studies have focussed on the geochronology of the igneous clasts present within NWA 7034 and its pairs. Although these studies consistently report ancient ages (∼4.4 Ga) for basaltic, basaltic andesitic and monzonitic clasts, the associated precisions are generally too low to link the different lithologies with the two age peaks inferred from NWA 7034/7533 zircon and baddeleyite. Here, we conduct an isotopic and petrographic study of igneous clasts from NWA 7533 to shed further light on the timing and nature of crustal reworking in the early history of Mars. We show that six out of seven investigated igneous clasts, representing at least four distinct types, record undisturbed Lu-Hf isotope systematics that indicate contemporaneous formation. Together with two zircons hosted in basalt and basaltic andesite clasts, these igneous clasts yield an isochron age of 4440 ± 41 Ma (2SE, MSWD = 2.1). This isochron age is consistent with clast ages inferred from zircon U-Pb geochronology, and altogether the available age constraints for the lithic components in NWA 7533 indicate that they derive from the younger of the two peaks of intense crustal reworking on early Mars (4442 ± 17 Ma). The initial εHf values (the 176Hf/177Hf ratio in the sample normalised to that of the chondritic uniform reservoir at the time of crystallisation in parts per ten thousand) of the igneous clasts range between −2.07 and −0.74, consistent with crystallisation from enriched source melts deriving from impact-induced reworking of the crust. The mean Lu-Hf isotope composition of the igneous clasts constrains the timing of primordial crust formation and reveals planet formation and differentiation within the first 10 Myr of the history of the Solar System, in consistence with the conclusions in earlier reports. The results presented here suggest a 176Lu/177Hf ratio of ∼ 0.0135 or higher in the primordial martian crust.

中文翻译：

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从 NWA 7533 中火成岩碎屑的 Lu-Hf 同位素系统学推断出火星上地壳再加工的时间


撞击事件在太阳系的早期历史中很频繁，因此，行星地壳形成的动力学与今天主导的过程大不相同。火星是一颗拥有停滞的盖子构造和独特保存的古老地表地形的行星，为研究与第一地壳的形成和重塑相关的早期过程提供了绝佳的机会。西北非洲 （NWA） 7034 和成对的陨石（如 NWA 7533）是多云母、风化层角砾岩的碎片，为火星上古老的角砾岩地壳提供了有形的记录。来自 NWA 7034/7533 的锆石和白锆石记录了 1000 万年前 （马） 在 1000 万年前 （） ± 17 ± 4474 发生的两次强烈地壳返工事件的证据，对火星早期地壳形成的时间和动力学提出了重要限制。迄今为止，只有少数研究关注 NWA 7034 及其对中存在的火成碎屑的地质年代学。尽管这些研究一致报告了玄武岩、玄武岩安山岩和二长岩碎屑的古老年龄 （∼4.4 Ga），但相关的精度通常太低，无法将不同的岩性与从 NWA 7034/7533 锆石和白辉石推断的两个年龄峰值联系起来。在这里，我们对 NWA 7533 的火成岩碎屑进行了同位素和岩相学研究，以进一步阐明火星早期历史中地壳改造的时间和性质。我们表明，在调查的 7 个火成岩碎屑中，有 6 个，代表至少四种不同的类型，记录了未受干扰的 Lu-Hf 同位素系统学，表明同时期形成。这些火成岩碎屑与玄武岩安山岩碎屑中的两个锆石一起，产生了 4440 ± 41 马 （2SE， MSWD = 2.1）. 这个等时年龄与从锆石 U-Pb 地质年代学推断的碎屑年龄一致，总的来说，NWA 7533 中岩石成分的可用年龄限制表明，它们来自火星早期两个强烈地壳返工峰中较年轻的一个（4442 ± 17 马）。火成岩碎屑的初始 εHf 值（样品中的 176Hf/177Hf 比值与结晶时软骨均匀储层的比值标准化，以万分之几为单位）在 -2.07 和 -0.74 之间，与冲击诱导的地壳返工产生的富集源熔体结晶一致。火成岩碎屑的平均 Lu-Hf 同位素组成限制了原始地壳形成的时间，并揭示了太阳系历史上前 10 Myr 内的行星形成和分化，这与早期报告中的结论一致。此处介绍的结果表明，原始火星地壳中的 176Lu/177Hf 比值约为 ∼ 0.0135 或更高。