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首页> 外文期刊>Contributions to Mineralogy and Petrology >Genesis of zeolites in the Neapolitan Yellow Tuff: geological, volcanological and mineralogical evidence
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Genesis of zeolites in the Neapolitan Yellow Tuff: geological, volcanological and mineralogical evidence

机译：那不勒斯黄凝灰岩中沸石的成因：地质，火山学和矿物学证据

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摘要

The study proposes a model by which a thick succession of volcanic tuffs can be zeolitized by alteration of pyroclastic material in the presence of sufficient eruptive water and at temperatures close to water vapour condensation. In the case of phreatomagmatic products, the model simplifies interpretation of problematic deposits that exhibit pronounced vertical and lateral variation in lithification grade. A major feature of the model is that thick zeolitized tuffs can be formed during emplacement of pyroclastic products, in marked contrast to later alteration in an open hydrologic system. Geological, volcanological and mineralogical data for the Neapolitan Yellow Tuff, a widespread trachytic pyroclastic deposit outcropping around Campi Flegrei (Southern Italy), have been used to infer the physicochemical conditions that determined mineral genesis. This tuff shows a reduction in lithification grade towards the base, top and with distance from the vent and very variable zeolitization within the lithified portion. We suggest that during initial emplacement the erupted products chilled against the ground, inhibiting zeolite crystallization. During rapid deposition of the thick, wet succession thermal insulation allowed the persistence of elevated temperatures for a time sufficient for enhancement of hydration-dissolution processes in the volcanic glass. The highly reactive alkali-trachytic glass quickly buffered the acid pH of the system, favouring phillipsite crystallization followed by chabazite nucleation. The variable zeolite content reflects fluctuating emplacement conditions (e.g. changes in water content and temperature). Cooling of the upper and relatively thin distal deposits inhibited the zeolitization process, thereby preserving the primary unlithified deposit.
机译：这项研究提出了一个模型，通过该模型可以在存在足够喷水的情况下和接近水蒸气冷凝的温度下通过改变火山碎屑物质来将一系列厚厚的火山凝灰岩沸石化。对于发磁产品，该模型简化了对有问题的沉积物的解释，这些沉积物在石化级别上表现出明显的垂直和横向变化。该模型的主要特点是，在放置火山碎屑产物期间，可以形成较厚的沸石凝灰岩，这与以后在开放水文系统中的变化形成鲜明对比。那不勒斯黄凝灰岩的地质，火山学和矿物学数据已被用来推断决定矿物成因的理化条件，那不勒斯黄凝灰岩位于坎皮弗莱格雷（意大利南部）附近，分布广泛，呈线状火山碎屑沉积。该凝灰岩显示，朝向底部，顶部以及与排气孔的距离，以及在固化部分内的沸石变化非常大，固化程度降低。我们建议在初始放置期间，喷出的产品在地面上冷却，从而抑制了沸石的结晶。在快速沉积厚的，连续的湿热绝热层期间，高温持续存在的时间足以增强火山玻璃中的水化溶解过程。高反应性的碱-trachytic玻璃可快速缓冲系统的酸性pH，有利于磷灰石结晶，然后菱沸石成核。可变的沸石含量反映了变化的进料条件（例如水含量和温度的变化）。上部和较薄的远端沉积物的冷却抑制了沸石化过程，从而保留了主要的未石化沉积物。

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