Abstract Rock varnishes are μm-thin, dark, manganese(Mn)-rich crusts that accrete in the order of few μm/ka on weathering-resistant lithologies. Although these crusts can form in all climates, they are best known in arid to semi-arid settings. Aeolian dust is understood as a major contributor to the distinct trace metal and REE enrichments in rock varnish. However, the exact proportions of abiotic and biotic formation mechanisms that may explain the oxidation-reactions of Mn2+ to Mn4+, present as Mn oxyhydroxides in the varnish, are still a matter of ongoing debate. We present here the first systematic study of trace element enrichment processes between the uppermost layer of the varnish sequence and their adjacent 50 μm dust fractions across a selection of 41 major and trace elements. This approach is used to investigate samples from three fully arid deserts: the An Nafud in Saudi Arabia, the Negev in Israel, and the Mojave in the USA, which are compared to the significantly different environment of the semi-arid Knersvlakte in South Africa. A new in situ trace element analysis protocol was developed to perform femto- and nanosecond sector-field LA-ICP-MS microanalyses at high-spatial resolution, which allows us to measure the most recent varnish layers for their comparison with recent dust. In agreement with previous studies, all varnishes are enriched in Mn, Pb, Ce, Co, Ba, Zn, Ni, and the Rare-Earth Elements (REE). Here we demonstrate that fine ( 50 μm has low trace element mass fractions, and enrichment patterns plotting distinctly away from varnish and fine dust. Based on these geochemical patterns, our results indicate a general enrichment mechanism from fine dust to varnish. Previous studies suggested dust to play an integral role in providing the trace elements that are incorporated into the varnish by pH-Eh fluctuations in short-term rain and fog events. We amalgamate and refine previous growth models by providing direct evidence that leaching of about 10% of the Mn and other trace elements from clay minerals in rain or fog droplets at pH ~5 and subsequent scavenging on varnish surfaces at pH ~8 leads integrated over time to the distinct enrichment patterns of the varnish, while initial Mn oxyhydroxide formation is suggested to follow pathways of metal oxide mediated photo-catalysis. For the semi-arid occurrence in the Knersvlakte we present a distinct growth model as both environment, varnish, and dust composition differ significantly from the arid settings. Here, thick, metallic-looking varnish occurs mainly on the rim of quartz pebbles, lacks microlamination, and likely has upscaled growth processes. Among other aspects, we suggest a more complex interplay between photo-catalysis, nocturnal condensation events on quartz pebbles with subsequent water trapping at the rock-soil-atmosphere interface, due to a salic top-soil layer, to mainly account for these differences.

中文翻译：

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对岩石清漆和相邻矿物粉尘组分关系的地球化学见解

摘要 岩漆是 μm 薄、深色、富含锰 (Mn) 的结壳，在抗风化岩性上以几 μm/ka 的量级增生。尽管这些地壳可以在所有气候条件下形成，但它们在干旱至半干旱环境中最为人所知。风成尘被认为是岩石清漆中明显的微量金属和稀土元素富集的主要贡献者。然而，可以解释 Mn2+ 氧化反应为 Mn4+（在清漆中以 Mn 羟基氧化物形式存在）的非生物和生物形成机制的确切比例仍然是一个持续争论的问题。我们在此首次系统地研究了清漆序列的最上层与其相邻的 50 μm 灰尘部分之间的微量元素富集过程，其中包括 41 种主要和微量元素。这种方法用于调查来自三个完全干旱的沙漠的样本：沙特阿拉伯的 An Nafud、以色列的 Negev 和美国的 Mojave，将它们与南非半干旱的 Knersvlakte 的显着不同环境进行比较。开发了一种新的原位微量元素分析协议，以高空间分辨率执行飞秒和纳秒扇形场 LA-ICP-MS 微量分析，这使我们能够测量最新的清漆层，以便与最近的灰尘进行比较。与之前的研究一致，所有清漆都富含 Mn、Pb、Ce、Co、Ba、Zn、Ni 和稀土元素 (REE)。在这里，我们证明细 (50 μm) 具有较低的微量元素质量分数，并且富集模式明显远离清漆和细尘。基于这些地球化学模式，我们的结果表明从细尘到清漆的一般富集机制。先前的研究表明，灰尘在通过短期雨雾事件中的 pH-Eh 波动提供微量元素方面发挥着不可或缺的作用。我们通过提供直接证据表明约 10% 的 Mn 和其他微量元素从 pH ~5 的雨滴或雾滴中从粘土矿物中浸出，随后在 pH ~8 的清漆表面清除导致随着时间的推移整合并改进先前的生长模型清漆的不同富集模式，而最初的 Mn 羟基氧化物形成被认为遵循金属氧化物介导的光催化途径。对于 Knersvlakte 的半干旱地区，我们提出了一个独特的增长模型，即环境、清漆、和灰尘成分与干旱环境有显着差异。在这里，厚的、金属外观的清漆主要出现在石英卵石的边缘，缺乏微层压，并且可能具有升级的生长过程。在其他方面，我们建议光催化、石英卵石上的夜间冷凝事件与随后在岩石 - 土壤 - 大气界面处捕获的水之间存在更复杂的相互作用，主要是由于土壤表层土壤层，主要解释了这些差异。